the influence of synthesis temperature on the ... · iii depth discussion and investigation of two...

The Influence of Synthesis Temperature on the

Crystallographic and Luminescent Properties of

NaYF4-Based Upconverters and Their Application to

Amorphous Silicon Photovoltaics

By

Daniel Owen Faulkner

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Graduate Department of Materials Science & Engineering

University of Toronto

© Copyright by Daniel Owen Faulkner, 2012

ii

The Influence of Synthesis Temperature on the Crystallographic and Luminescent Properties of

NaYF4-Based Upconverters and Their Application to Amorphous Silicon Photovoltaics

Daniel Owen Faulkner

Doctor of Philosophy

Department of Materials Science & Engineering

University of Toronto, 2012

Abstract

There are several factors which conspire to limit the efficiency of solar cells. One of these is the fact

that a solar cell is unable to absorb photons of energy less than the band gap of the semiconductor

from which it is made; in the case of some high-band gap materials such as amorphous silicon – the

model system used in this study – this can mean that as much as 50% of the solar spectrum is

unusable. Upconversion phosphors – materials which can, by way of two or more successive photon

absorptions, convert low energy (typically near infrared) light into high energy (typically visible) light

– offer a potential solution to this problem as they can be used to convert light, which would

otherwise be useless to the cell, into light which can be used for power generation. In this thesis we

work towards the application of NaYF4-based upconverters to enhanced efficiency amorphous

silicon (a-Si) photovoltaic power generation. We begin by synthesizing these upconverters at a range

of temperatures and studying the crystallographic and spectroscopic properties of the resulting

materials, elucidating heretofore undocumented trends in their luminescence and crystallography,

including the effect of synthesis temperature on upconversion intensity, crystallite size, and lattice

parameter. We also investigate the emission quantum yield of these materials, beginning with an in

iii

depth discussion and investigation of two methods for recording absolute quantum yields. We

demonstrate that the quantum yields of the materials may vary by a factor of over 100, depending on

the synthesis conditions. After we have fully characterized these properties we turn our attention to

the application of these materials to amorphous silicon solar cells, for which we provide a proof of

concept by demonstrating the effect of upconversion luminescence on the photoconductance of an

a-Si film. We conclude by developing a roadmap for future improvements in the field.

iv

For Lynn.

v

Acknowledgements

I thank my supervisors – professors Geff Ozin, Nazir Kherani, and Doug Perovic for their guidance

and support over the course of the project. I also thank the many people who taught me the various

techniques used to conduct the work in this thesis; Srebri Petrov for powder x-ray diffraction and

Rietveld analysis; Abbas Hosseini, Shujie Lin, Greg Scholes, Venkat Venkataramanan and Georg

von Freymann for help with many optical and spectroscopic issues encountered and overcome. And

I thank the Ozinites past and present including Sue Mamiche who keeps the lab running smoothly;

JP Nijjer for his work as an undergraduate summer student with me in 2011; Zaheen Sadeq, Jordan

Thomson, Jeffrey McDowell, Navid Soheilnia, Bryan Lau and Min Guan for edifying discussions.

Financial support by the Ontario Research Fund – Research Excellence program under the auspices

of the High Efficiency Silicon Photovoltaics research project and by the Department of Materials

Science and Engineering at the University of Toronto is gratefully acknowledged.

vi

Publications

The work described in this thesis and related work conducted during my PhD in collaboration with

other researchers has been published, submitted for publication, or drafted for submission as:

Henderson, E. J.; Shuhendler, A. J.; Prasad, P.; Baumann, V.; Maier-Flaig, F.; Faulkner, D. O.;

Lemmer, U.; Wu, X. Y.; Ozin, G. A., Colloidally Stable Silicon Nanocrystals with Near-Infrared

Photoluminescence for Biological Fluorescence Imaging, Small, 2011

Wang, W.; Faulkner, D.; Moir, J.; Ozin, G. A., The Effect of Solvent in Evaporation-induced Self-assembly:

A case study of Benzene Periodic Mesoporous Organosilica, Science China Chemistry, 2011

Mastronardi, M.; Maier-Flaig, F.; Faulkner, D.; Henderson, E.; Kubel, C.; Lemmer, U.; Ozin, G. A.,

Size-Dependent Absolute Quantum Yields for Size-Separated Colloidally-Stable Silicon Nanocrystals, Nano

Letters, 2011

Thomson, J. W.; Wang, X.; Hoch, L.; Faulkner, D.; Petrov, S.; Ozin, G. A., Discovery and Evaluation

of a Single Source Selenium Sulfide Precursor for the Synthesis of Alloy PbSxSe1-x Nanocrystals, Journal of

Materials Chemistry, 2012

Faulkner, D. O.; Petrov, S.; Perovic, D. D.; Kherani, N. P.; Ozin, G. A., Absolute Quantum Yields in

NaYF4:Er,Yb Upconverters – Synthesis Temperature and Power Dependence, Journal of Materials Chemistry,

2012

Faulkner, D. O.; McDowell, J. J.; Price, A. J.; Perovic, D. D.; Kherani, N. P.; Ozin, G. A.,

Measurement of Absolute Photoluminescence Quantum Yields Using Integrating Spheres – Which Way to Go?,

Laser & Photonics Reviews, 2012

Faulkner, D. O.; Petrov, S.; Perovic, D. D.; Kherani, N. P.; Ozin, G. A., Dependence of the

Crystallographic and Luminescence Properties of Rare Earth Doped NaYF4 Upconverters on Synthesis Temperature,

submitted

Faulkner, D. O.; Mahtani, P.; Perovic, D. D.; Ozin, G. A.; Kherani, N. P., Upconversion Amplified

Photoconductivity in an Amorphous Silicon Film - Roadmap of Discovery to Practicality, in preparation

vii

Contents

Figures ............................................................................................................................................................... xii

Tables ................................................................................................................................................................. xv

Symbols & Acronyms .................................................................................................................................... xvii

1 Introduction & Background..................................................................................................................... 1

1.1 Motivation ............................................................................................................................................. 1

1.2 Upconversion & Upconverters .......................................................................................................... 2

1.2.1 Theoretical Background on the Upconversion Process ...................................................... 2

1.2.1.1 APTE Effect (Energy Transfer Upconversion (ETU)) .............................................. 4

1.2.1.2 Two Step Absorption (Excited State Absorption (ESA)) .......................................... 4

1.2.1.3 Co-Operative Sensitization ............................................................................................. 4

1.2.1.4 Co-Operative Luminescence........................................................................................... 5

1.2.1.5 Light Absorption & Emission ........................................................................................ 5

1.2.1.5.1 Selection Rules for Electronic Transitions ................................................................ 6

1.2.1.6 Energy Transfer in Upconversion .................................................................................. 9

1.2.2 Upconverting Materials .......................................................................................................... 13

1.2.2.1 Considerations When Designing an Upconverter ..................................................... 13

1.2.2.2 Spectroscopy of NaYF4:Er,Yb ..................................................................................... 14

1.3 Prior Art on Upconverters in Solar Cells ....................................................................................... 16

1.3.1 Theoretical Work on UCPV .................................................................................................. 16

1.3.2 Experiments on Silicon-Based Solar Cells .......................................................................... 20

1.3.3 Experiments on Non-Silicon-Based Solar Cells ................................................................. 23

1.3.4 Plasmonic/photonic Studies on Standalone Upconverters .............................................. 25

1.3.4.1 Photonic Enhancement of Upconversion .................................................................. 25

1.3.4.2 Plasmonic Enhancement of Upconversion ................................................................ 27

viii

1.4 Outline of Thesis ................................................................................................................................ 28

1.5 References ........................................................................................................................................... 29

2 Dependence of the Crystallographic and Luminescent Properties of NaYF4-Based

Upconverters on Preparation Temperature ................................................................................................. 34

2.1 Introduction ........................................................................................................................................ 35

2.1.1 Introduction to Rietveld Analysis ......................................................................................... 37

2.2 Experimental ....................................................................................................................................... 40

2.3 Results & Discussion ......................................................................................................................... 42

2.3.1 Scanning Electron Microscopy (SEM) of NaYF4:Er,Yb .................................................. 42

2.3.2 Crystallographic Characterization ......................................................................................... 46

2.3.3 Spectroscopic Characterization ............................................................................................. 53

2.4 Conclusions ......................................................................................................................................... 57

2.5 References ........................................................................................................................................... 58

3 A Comparison of Two Common Methods for the Measurement of Absolute

Photoluminescence Quantum Yields Using Integrating Spheres ............................................................. 61

3.1 Introduction ........................................................................................................................................ 62

3.2 Theoretical Discussion of AQY Measurement Methods ............................................................. 63

3.2.1 The Two-Measurement Method ........................................................................................... 63

3.2.2 The Three-Measurement Method ........................................................................................ 66

3.3 Results .................................................................................................................................................. 68

3.3.1 Experimental ........................................................................................................................... 69

3.3.2 Experimental Results .............................................................................................................. 70

3.4 Conclusions ......................................................................................................................................... 74

3.5 References ........................................................................................................................................... 74

4 Control of Absolute Quantum Yields in NaYF4:Er,Yb Upconverters - Temperature and Power

Dependence ...................................................................................................................................................... 76

ix

4.1 Introduction ........................................................................................................................................ 76

4.2 Experimental ....................................................................................................................................... 79


4.4 Conclusions ......................................................................................................................................... 88

4.5 References ........................................................................................................................................... 88

5 Upconversion Amplified Photoconductivity in an Amorphous Silicon Film - Roadmap of

Discovery to Practicality ................................................................................................................................. 91

5.1 Introduction ........................................................................................................................................ 91

5.2 Experimental ....................................................................................................................................... 92


5.3.1 Characterization of the a-Si Film in the Absence of Upconverters ................................ 95

5.3.2 Characterization of the a-Si film in the Presence of Upconverters ................................. 98

5.3.3 Estimation of the Photo-Generated Carrier Density due to Absorption of

Upconverted Photons .......................................................................................................................... 100

5.3.4 Modeling the Relationship between the Absorption Spectrum of an Upconverter and

its Potential Use in Solar Photovoltaic Power Generation............................................................. 101

5.4 Conclusions ...................................................................................................................................... 106

5.5 References ........................................................................................................................................ 106

6 Conclusions & Future Work ............................................................................................................... 109

6.1 Conclusions ...................................................................................................................................... 109

6.2 Future Work ..................................................................................................................................... 109

Appendix A: Proof-of-Concept Upconversion Display Device ............................................................ 112

A.1 Introduction ....................................................................................................................................... 112

A.2 Results ................................................................................................................................................. 113

A.3 Conclusions ........................................................................................................................................ 116

Appendix B: Spectroscopic Term Symbols .............................................................................................. 117

x

Appendix C: LabVIEW VIs Created for the Work Undertaken in this Thesis .................................. 119

C.1 Monochromator and Detector Control Software ......................................................................... 120

C.1.1 Monochromator and Detector Control Software – Front Panel ........................................ 120

C.1.1.1 Monochromator and Detector Control Software – Front Panel – Wavelength-

Dependent Signal ............................................................................................................................ 120

C.1.1.2 Monochromator and Detector Control Software – Front Panel – Wavelength-

Dependent Signal ............................................................................................................................ 121

C.1.2 Monochromator and Detector Control Software – Block Diagram .................................. 124

C.2 CCD Spectrometer Quantum Yield Software ............................................................................... 126

C.3 Upconversion Quantum Yield Calculation Software ................................................................... 128

Appendix D: Potential Sources of Error present in this Work .............................................................. 130

D.1 Materials Variations .......................................................................................................................... 130

D.2 Optical Errors and Aberrations ...................................................................................................... 130

D.3 Electronic Variations ........................................................................................................................ 131

D.4 Software Errors ................................................................................................................................. 132

xi

Figures

Figure 1.1. Solar spectrum showing region in which upconversion may lead to efficiency

enhancements. .................................................................................................................................................... 2

Figure 1.2. An overview of upconversion processes including addition de photon pars transferts

d’energie (APTE) effect. Adapted from (1) with permission from the American Chemical Society. ... 3

Figure 1.3. Left: Upconversion transitions in NaYF4:Er,Yb. Coloured arrows indicate upconversion

emission, wavy arrows are vibrational relaxation processes (phonon emission), dotted lines indicate

energy transfer. Adapted from (7) with permission from Elsevier. Right: Typical upconversion

emission spectrum from NaYF4:Er,Yb under 972 nm laser excitation. .................................................. 14

Figure 1.4. Left: Schematic of a back-face upconverting solar cell. Middle: Schematic energy level

diagram for an upconverting solar cell, with the upconverter adding an effective intermediate state

mid-band gap. Right: Equivalent circuit for upconverter solar cell: C1: Solar cell; C2-C4:

Upconverter; C2: Upconversion emission; C3, C4: Sequential excitations of upconverter.

Reproduced from (10) the American Institute of Physics. ........................................................................ 17

Figure 1.5. Proposed Upconversion-silver nanomaterial for plasmonic enhancement of

upconversion. Reproduced from (15). .......................................................................................................... 19

Figure 1.6. Photocurrent–voltage (I–V) characteristics of 980 nm laser-driven a-Si:H photovoltaic

cells without and with different kinds of NaYF4:Yb/Er/Gd nanorods. (a) Nanorods with Au

nanoparticles, (b) nanorods with Au shells, (c) naked nanorods and (d) the reference solar cell.

Figure and caption reproduced from (20). ................................................................................................... 23

Figure 1.7. A typical example of an opal film – a face-centred cubic array of silica spheres which

exhibits diffraction peak as visible wavelengths due to the periodic arrangement of monodisperse

spheres leading to regions of constructive and destructive optical interference. Adapted from (27)

with permission from the American Chemical Society. ............................................................................. 26

Figure 2.1. Typical upconversion spectrum from NaY0.78Yb0.2Er0.02F4 showing visible upconversion

peaks under 972 nm laser excitation. ............................................................................................................ 36

Figure 2.2. Phases of NaYF4. Left: cubic Fm3m. Right: hexagonal P63/m. Green: F-; red/orange:

Y3+/Yb3+/Er3+/Na+. The pink polyhedra outline the nearest-neighbor co-ordination sphere around

each metal ion. .................................................................................................................................................. 37

Figure 2.3. Schematic of the optical system used to study upconversion luminescence: 1 light from a

fibre-coupled 972 nm diode laser is collimated by a lens held in a clamp and focused with a 5 cm

xii

focal length 1 inch diameter biconvex lens onto 2 an upconverting sample in a melting point tube

held in a clamp. Upconverted light is collected and collimated by 3 a 10 cm focal length 2 inch

diameter plano-convex lens and focused into 4 a monochromator coupled to a photomultiplier tube

by a 20 cm focal length 2 inch diameter plano-convex lens. ..................................................................... 41

Figure 2.4. SEM image of a sample of NaYF4:Er,Yb prepared at 250 °C............................................. 42






Figure 2.10. SEM image of a sample of NaYF4:Er,Yb prepared at 700 °C. .......................................... 46

Figure 2.12. Phase compositions of NaYF4 (top), NaY0.98Er0.02F4 (middle) and NaY0.78Yb0.2Er0.02F4

(bottom) prepared at different temperatures, fitted by Rietveld analysis of PXRD patterns. .............. 49

Figure 2.13. Cubic (left) and hexagonal (right) grain sizes measured by Rietveld refinement. Note

that to maintain a reasonable scale, the maximum cubic size fitted was 450 nm and the maximum

hexagonal was 4500 nm. ................................................................................................................................. 51

Figure 2.14. Cubic a (top), hexagonal a (middle) and hexagonal c (bottom) parameters for NaYF4-

based materials prepared at different temperatures, fitted by Rietveld analysis. Due to poor fitting at

high temperatures due to its low abundance, the cubic phase is only fitted up to 475 °C. .................. 52

Figure 2.16. FTIR Spectra of NaY0.78Yb0.2Er0.02F4 prepared at 250, 400 and 700 °C, as indicated..... 55

Figure 3.1. 1 Collimated 365 nm light from an LED is focused by a 10 cm focal length 2 inch

diameter plano-convex lens (some light is incident on the outside of the sphere and does not enter

it, and is not shown in the diagram) into 2 an integrating sphere with a rotating sample holder

containing a sample for study (sample location indicated by the dotted line). Outputted light is fibre-

coupled into 3 a CCD grating spectrometer. ............................................................................................... 70

Figure 4.1. 1 Light from a fibre-coupled 972 nm diode laser is focused into 2 an integrating sphere

with a rotating sample holder (sample is held inside the centre of the sphere and its location is

indicated by the dotted line) allowing for direct and indirect excitation. Light output from the sphere

is coupled to a 1 mm diameter optical fibre coupled to 3 a collimating lens held in a clamp. The

collimated output is split at 4 a beamsplitter with one beam being attenuated by an OD 1.5 neutral

density filter, focused into a 200 micron optical fibre by a 2.5 cm focal length 1 inch diameter

xiii

biconvex lens, and coupled into 5 a CCD grating spectrometer. The other beam passes through 6 a

filter wheel and is focused by a 5 cm focal length 1 inch diameter biconvex lens into 7 a

monochromator coupled to a PMT. ............................................................................................................. 80

Figure 4.2. PXRD patterns for samples of bulk NaYF4:Er,Yb prepared at different temperatures, as

indicated. Reference patterns for both cubic (Fm3m) and hexagonal (P63/m) phases are also shown.

............................................................................................................................................................................ 82

Figure 4.3. Phase composition of NaYF4:Er,Yb as a function of synthesis temperature, fitted by

Rietveld refinement. Each synthesis was repeated three times and the results of all fittings are

shown. ................................................................................................................................................................ 83

Figure 4.4. Typical NaYF4:Er,Yb emission spectrum under 972 nm excitation, showing the main

transitions giving rise to visible upconversion emission. The peak heights are proportional to the

number of photons contributing to each wavelength, and are produced by multiplying the measured

intensity (which is proportional to power) by the wavelength, as is required for the measurement of

absolute quantum yields. ................................................................................................................................. 84

Figure 4.5. Red and green emission absolute quantum yields of samples of NaYF4:Er,Yb prepared

at different temperatures and recorded under 380 mW of 972 nm laser excitation. The inset shows

the region from 250-475 °C in greater detail. .............................................................................................. 85

Figure 4.6. FTIR spectra of NaYF4:Er,Yb prepared at 250, 400 and 700 °C, as indicated. ................ 86

Figure 4.7. Incident power dependence of the absolute quantum yields of red and green emissions

for a sample of NaYF4:Er,Yb prepared at 700 °C. ..................................................................................... 87

Figure 5.1. Schematic of the experimental design used in this investigation. ........................................ 94

Figure 5.2. Attenuation spectrum of the a-Si film used in these experiments. ..................................... 95

Figure 5.3. Dark I-V characteristics of the a-Si film used in this study. ................................................ 96

Figure 5.4. AM 1.5 I-V characteristics of the a-Si film used in this study. ............................................ 97

Figure 5.5. Top: I-V characteristics for a-Si film coupled with samples of doped and undoped

NaYF4 prepared at various temperatures, as indicated, excited at 972 nm. The suffix ‘b’ is used to

indicate a blank (undoped, i.e. non-upconverting) sample. Bottom: Photoconductance of the a-Si

film coupled with samples of doped and undoped NaYF4 prepared at various temperatures, and of

various luminescence intensities, as indicated. The units of luminescence are arbitrary. ...................... 99

Figure A.1. Red, green, and blue emission from the upconversion film selected by use of an

appropriate colour filter………………………………………………………………………….115

Figure A.2. Upconversion emission spectra for the upconversion composite used in this study; a:

xiv

unfiltered emission; b: emission via red filter; c: emission via green filter; d: emission via blue filter.

The instrument sensitivity used for recording spectra b, c, and d was 10x higher than that for a, and

the y-axis values for the spectra are not quantitatively comparable. The apparent broad peak

appearing beyond 600 nm in spectra c and d is a consequence of the relatively low response of the

system in this range resulting in increased error when the calibration is applied, and should be

neglected. 116

Figure C.1. Front panel of monochromator-based spectroscopy software: control panels for

recording wavelength (top) and time (bottom) dependent signals. ....................................................... 123

Figure C.2. Queued message handler structure used from controlling spectroscopy software,

showing the event structure used for handling user inputs. The numbered structures are 1 while

loop, 2 case structure, 3 event structure. ................................................................................................... 125

Figure C.3. Front panel for quantum yield measurement software, showing typical spectra taken

during the measurement. ............................................................................................................................. 127

Figure C.4. Front panel of the software used for calculating quantum yields from spectra recorded

from monochromator-based and Maya spectrometers. .......................................................................... 129

xv

Tables

Table 3.1. Quantum yield data for four different samples recorded under two experimental

geometries using the two experimental methods. ....................................................................................... 71

Table 5.1. Charge carrier densities estimated electronically from measured conductances, using an

estimated mobility value of 1 cm2 V-1 s-1. .................................................................................................. 101

Table A.1. Advantages and disadvantages of various display technologies………………………113

xvi

Symbols & Acronyms

2MM – two measurement method (of recording absolute quantum yields)

3MM – three measurement method (of recording absolute quantum yields)

AFM – atomic force microscope

AM1.5 – air mass 1.5 solar spectrum

APTE – addition de photon pars transferts d’energie

(A)QY – (absolute) quantum yield

a-Si(:H) – (hydrogenated) amorphous silicon

BS – beamsplitter

DSSC – dye-sensitized solar cell

EQE – external quantum efficiency

ESA – excited state absorption

ETU – energy transfer upconversion

FTIR – Fourier transform infrared spectroscopy

IQE – internal quantum efficiency

IS – integrating sphere

I-V – current-voltage

LED – light-emitting diode

MEH-PPV – poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylenevinylene]

ND – neutral density

n-UC – nanocrystalline upconverter

OD – optical density

PMT – photomultiplier tube

PXRD – powder X-ray diffraction

QTH – quartz tungsten halogen

R6G – rhodamine 6G

RE – rare earth

RF PECVD – radio-frequency plasma-enhanced chemical vapor deposition

SEM – scanning electron microscopy

TFA – trifluoroacetate

UC – upconverter/upconversion

xvii

UCPV – upconversion photovoltaic

VI – virtual instrument

– second derivative with respect to the position of electron (nucleus) i

∫…dτ – integral over all space

A(B) – Einstein co-efficient for spontaneous (stimulated) transitions

A – fraction of photons absorbed by sample on initial incidence with it, in AQY measurement

c – speed of light

e – electron charge

ℰ(c) – electric field in vacuum (crystal)

E(e/n) – (electronic/nuclear) energy/excitation energy

Eg – band gap energy

f – fraction of photons directly incident on sample

Fa(E) – shape of activator absorption band

fs(E) – shape of sensitizer emission band

G - conductance

g – gerade (even) symmetry

g – degeneracy

– Planck’s constant divided by 2π

ℋ(e/n) – (electronic/nuclear) Hamiltonian

H1 – interaction Hamiltonian in Dexter theory

Hk – full width at half maximum of diffraction peak k

I – current

Ik – intensity of diffraction peak k

J – total angular momentum

jk – multiplicity of diffraction peak k

Jsc – short-circuit current

L – orbital angular momentum

La,b,c – area under laser peak in empty, indirect, direct measurements in AQY determination

Lk – Lorentz factor of diffraction peak k

M – transition moment integral

me(n) – electron (nucleus) mass

xviii

n – refractive index

n – number of photons involved in upconversion process

n – charge carrier density

Pa,b,c – area under photoluminescence peak in empty, indirect, direct measurements in AQY

determination

Psa – sensitizer-activator energy transfer probability

Qa(s) – area under absorption band of activator (sensitizer)

R – nuclear separation vector

R – nuclear separation distance

r – electron position relative to nucleus

S – spin angular momentum

Sk – structure factor of diffraction peak k

u – ungerade (odd) symmetry

V – potential energy

V – voltage

Wi – weight function applied to diffraction point i

yi – intensity of diffraction point i

β – exponent in power dependence of upconversion luminescence

Γ – irreducible representation

η – quantum yield

κ – dielectric constant

μ – fraction of photons absorbed after initially being scattered in AQY measurement

- (electronic/nuclear) dipole moment operator

σ – conductivity

σ2 – variance

τ – lifetime

ψ(e,n,o,s,i,f) – (electronic/nuclear/orbital/spin/initial/final) wavefunction

1

1 Introduction & Background

1.1 Motivation

It is widely agreed that one of the greatest challenges currently facing humanity is the increasing

demand for energy and the dwindling supply of traditional sources of it such as fossil fuels. Perhaps

the most promising route to satisfying humanity’s growing need for energy is the successful

adoption of solar power on a global scale. Although great progress has been made in this field, there

is still much ground to be covered, and solar photovoltaic power still only accounts for a tiny

fraction of the power used by humans. Widespread application faces a number of limitations and a

significant reduction in the cost, typically measured in $/W, is necessary to make solar power truly

competitive. One way to decrease this $/W figure is to increase the efficiency of the solar cell

(provided that can be achieved without a significant cost increase). One of the main factors limiting

the efficiency of solar cells is the presence of sub-band gap light which ordinarily passes through a

cell and is, in effect, wasted. By using an upconverter, a material which is able to sequentially absorb

multiple photons and combine their energy into the emission of a single higher energy photon, we

can alleviate some of these losses and effectively increase the spectral range over which a solar cell

responds; this concept was the core motivation for this work, and is illustrated graphically in Figure

1.1. Although similar work had previously been performed, the typical choice of cell, crystalline

silicon, seemed to be a poor one as the band gap does not maximize the potential utility of the

upconverting phosphors. By focusing on the use of amorphous silicon (a-Si) we hoped to give a

stronger demonstration of the application of upconverters to high efficiency solar power. As the

project evolved, it came to include more fundamental crystallographic and spectroscopic studies of

the materials – we focus on NaYF4 doped with Er and Yb – themselves after we noted a striking

temperature dependence of the luminescent properties of these materials which had not previously

been investigated in detail. In the following sections we provide background information and prior

art on the material addressed in this thesis.

2

Figure 1.1. Solar spectrum showing region in which upconversion may lead to efficiency enhancements.

1.2 Upconversion & Upconverters

In this section we first present an introduction and some theoretical background on various

upconversion processes, before discussing design considerations to be made by those trying to

design optimized upconverting systems, and finally discussing the spectroscopy of NaYF4:Er,Yb, the

material of focus in this study.

1.2.1 Theoretical Background on the Upconversion Process

In the simplest terms, ‘upconversion’ refers to any process in which low energy light, for example

that in the near-infrared region of the spectrum is converted to higher energy, often visible or

ultraviolet light. This may occur via a number of mechanisms, which can be grouped into two main

categories:

3

1. Processes in which multiple photons must interact with the upconverting material

simultaneously

2. Processes in which non-simultaneous interaction may result in upconversion

In the former category are processes such as two-photon absorption and second harmonic

generation. A defining feature of these phenomena is that there is no real intermediate state to which

excitation can occur. The result of this is that two (or more) photons must interact with the material

at the same instant, necessitating the use of extremely high excitation intensities, typically achievable

only with high-powered pulsed lasers. In the latter category are processes such as those studied in

this work, in which initial excitation to a real intermediate state allows for the use of relatively low

intensity excitation, typically from a continuous wave laser. Although this is still orders of magnitude

more intense than sunlight, particularly in the spectral regions of interest, it is many orders of

magnitude less intense than the pulsed lasers needed to excite simultaneous absorption processes,

and although materials have not yet been developed which display high efficiency under solar

illumination, there is no fundamental reason why this should not be possible. In this section of the

thesis, we present an introduction to the various non-simultaneous upconversion processes,

particularly concentrating on the process of energy transfer upconversion (ETU) which is dominant

for the materials studied herein, and also that of excited state absorption (ESA), both of which are

illustrated, along with a variety of other upconversion processes, in Figure 1.2.

Figure 1.2. An overview of upconversion processes including addition de photon pars transferts d’energie

(APTE) effect. Adapted from (1) with permission from the American Chemical Society.

The various processes illustrated in this figure are briefly described below:

4

1.2.1.1 APTE Effect (Energy Transfer Upconversion (ETU))

In the addition de photon pars transferts d’energie (APTE) effect, also commonly referred to as the

ETU process, incident light is absorbed by a species – the sensitizer – and then the absorbed energy

is transfered to an emitting species – the activator. Upon the transfer, the sensitizer becomes de-

excited and it (or another sensitizer within the vicinity of the activator) is free to absorb another

photon and again transfer its energy to the activator. Sequential absorption and transfer of energy

may occur, allowing progressively higher and higher energy states to be reached, before

upconversion emission occurs. The sensitizer may be the host lattice, or a dopant which may be the

same or a different species as the activator.

1.2.1.2 Two Step Absorption (Excited State Absorption (ESA))

In ESA, the entire upconversion process occurs on a single ion (or atom, etc.) by way of sequential

absorption events. In many cases there is competition between ESA and ETU, and the dominant

mechanism may depend on the concentration of the various active species(2). The two processes

may be distinguished either by studying the power dependence of the luminescence intensity(2) or

by monitoring their rise and decay times(3) – because of the energy transfer steps which are much

slower than electronic excitations, ETU typically display much longer time constants.

1.2.1.3 Co-Operative Sensitization

A prerequisite for ETU to occur is that the activator must have an energy level lying at roughly the

same (or slightly below) the energy of the excited state of the sensitizer which is being used. If no

such state exists then energy transfer cannot occur. However, it may be that the activator has an

excited state which could be reached through the transfer of two (or more) quanta of excitation

from multiple sensitizers. This process is referred to as co-operative sensitization, and has been

reported in a number of species co-doped with ytterbium and terbium(1).

5

1.2.1.4 Co-Operative Luminescence

Co-operative luminescence is a process in which two excited ions in close proximity combine their

excitation energies into the emission of a single high energy photon. This process relies on

wavefunction overlap between two ions and requires very close proximity (<1 nm). It has been

observed in a number of ytterbium-doped materials(1).

The intensity of luminescence from these processes generally decreases in the order presented

above. In general, ETU and ESA are relevant to the practical application of upconverters and the

system that we focus on in this work, NaYF4:Er,Yb, primarily displays an ETU mechanism. This

process may be considered to involve three sequential sub-processes: light absorption, energy

transfer, and light emission. We will give a brief overview of these three processes in the following

sub-sections.

1.2.1.5 Light Absorption & Emission

In the non-simultaneous upconversion processes of interest for photovoltaic applications, a photon

whose energy will ultimately contribute to the emission of an upconverted photon must be absorbed

by some material, most typically a rare earth ion immobilized in a spectroscopically inert solid host,

often a metal oxide, fluoride or chloride. When discussing upconversion we are typically considering

a process that emits visible photons and involves excitation between different electronic energy

levels (which will also be accompanied by vibrational transitions). Likewise, emission events involve

similar transitions. A typical upconversion process relies on f-f transitions – that is, transitions of

electrons within the 4f sub-shell – we begin with a review of electronic selection rules, which tell us

that in an idealized system such transitions would be forbidden. We must caution, however, that due

to significant spin-orbital coupling exhibited by f-electrons, along with the influence of the solid

lattice, many of the transitions which occur in the systems of interest in this study are forbidden

according to the simplest form of the selection rules which do not consider these influences.

Nonetheless, a review of the basic concept of spectroscopic selection rules is important and a good

starting point for an understanding of the subject.

6

1.2.1.5.1 Selection Rules for Electronic Transitions

The derivation of spectroscopic selection rules is most commonly conducted within the framework

of the Born-Oppenheimer approximation, which states that as the movement of electrons is so

much more rapid than the movement of nuclei, electronic transitions may be considered to occur

without any change in the position of the nuclei. This allows us to decouple the electronic and

nuclear Hamiltonians and to treat them as two separate, independent problems, with the

eigenfunctions and eigenvalues of the electronic Hamiltonians yielding the electron orbitals and

energy levels, and the solutions to the nuclear Hamiltonian giving the vibrational energy levels.

Mathematically, the effect is that when considering the electronic states we go from the full

Hamiltonian:

ℋ

∑

∑

(1.1)

to:

ℋ

∑

(1.2)

where:

me (mn) are the masses are of the electrons (nuclei),

the subscript i (j) ranges over all electrons (nuclei) in the system, and

V is the potential energy of the system, consisting of Coulombic interactions of three types:

electron-electron, electron-nucleus, and nucleus-nucleus.

That is, we have removed the second term which describes the motion of the nuclei, as we have

assumed these to be stationary over the timeframe of interest. This second term can be treated

separately and leads to the vibrational states of the system. As we have separated the Hamiltonian

into two parts, it is natural to also separate the wavefunction:

7

(1.3)

So that the Schrodinger equation becomes:

ℋ ℋ ℋ ℋ ℋ

(1.4)

The transition between two spectroscopic states may occur when an electric dipole interacts with a

time-dependent field (e.g. that produced by light) and in this case the probability of a transition

depends on the transition moment integral:

∫

(1.5)

where:

The subscript f (i) represents the final (initial) state of the system, and

represents the dipole moment operator.

Transitions may also be mediated by magnetic dipoles, and higher order electric and magnetic terms,

but generally electric dipole transitions are the strongest. This is not always true however, and it is

not uncommon in the solid phase for these other transitions to be strong, indeed we will see that if

the electric dipole model were rigorously true that the transitions which give rise to upconversion

would not occur. The dipole moment operator may be separated into electronic and nuclear terms,

and the Born-Oppenheimer approximation may again be applied:

∫ ∫

∫

∫ ∫

∫ ∫

∫ ∫

8

(1.6)

where the orthogonality of the electronic wavefunctions has been used to remove the term on the

second line. Of the two remaining terms, the first represents the overlap between the initial and final

vibrational states (note that as these states are within different electronic energy levels they will not

be orthogonal to each other – vibrational states are only orthogonal to those within the same

electronic energy level) and is known as the Franck-Condon factor – it tells us that the probability of

a transition between two states is increased when there is greater overlap between their

wavefunctions – and the second term, containing the electronic wavefunction and the electric dipole

operator, is the basis for the derivation of electronic selection rules. The electronic wavefunction

accounts for both the movement of the electrons within their orbits and for their spins, and in the

simplest case, which is a good approximation for many smaller atoms and molecules, we can

separate the orbital and spin wavefunctions into terms o and s, just as we separated the electronic

and nuclear wavefunctions, in which case the transition moment integral becomes:

∫ ∫

∫

(1.7)

the second term is the root of the spin selection rules and the third term leads to the orbital selection

rules (note that the dipole operator does not act upon the spin states). A transition is therefore

forbidden if either of these terms is zero, and this allows us to derive the following selection rules:

As spin states are orthogonal, the second term will be zero unless the two spin wavefunctions are

equal. This means that the overall electron spin may not change during a transition.

The second term will be non-zero if the direct product contains the

totally symmetric representation of the point group of the molecule (or other species) under

consideration. This gives rise to the Laporte selection rule in centrosymmetric systems: the totally

symmetric representation will by definition have ‘g’ (gerade or even) symmetry (it will be an even

function) whereas the components of the dipole moment operator which transform as the Cartesian

co-ordinates x, y, z, will have ‘u’ (ungerade or odd) symmetry. This means that any transition between

orbitals of the same parity is forbidden, as u × u × u = u and g × u × g = u, and therefore the d-d

9

and f-f transitions (for example) are forbidden by this rule. The reality however is significantly more

complex than these rules imply, due to a number of factors including:

‘Vibronic transitions’ in which a change in vibrational state also occurs that may alter the

symmetry of the system so as to allow transitions which are otherwise forbidden.

Although we have only considered electric dipole-mediated transitions, it is possible for

spectroscopic transitions to occur via coupling with magnetic dipoles, and higher order

electric and magnetic multipoles. In the cases of heavier elements and solids these can be a

significant source of absorption and emission.

Orbital hybridizations occur in molecules, complex ions, and solids, meaning that it is not

really correct to consider a transition as being strictly ‘f-f’ or ‘d-d’; due to some degree of

hybridization with another orbital of a different parity, accordingly the Laporte selection rule

may be relaxed somewhat.

The approximation that the spin and orbital wavefunctions can be completely separated is

often not a very good one, particularly for the heavier atoms which are often used in

luminescent materials. When this separation is not valid, there is said to be ‘spin-orbit

coupling’ and this can lead to the presence of transitions which would otherwise be

forbidden, either on spin or orbital grounds.

In upconverters, which are typically rare earth-based solids, the transitions giving rise to

upconversion, which are typically f-f, become somewhat allowed due to a combination of these

different influences; in particular the effects of spin-orbit coupling and higher-order multipole-

mediated transitions can become significant. Although vibronic transitions may occur, upconversion

lattices generally have relatively low vibrational frequencies which will reduce their effect, and orbital

hybridization of the 4f orbitals is generally minimal due to their small size.

1.2.1.6 Energy Transfer in Upconversion

In many upconversion systems (and indeed many luminescent materials in general) energy transfer

occurs between an absorbing species (the sensitizer) and an emitting species (the activator). The

theory of energy transfer was originally treated by Förster(4) who considered electric dipole-dipole

mediated transfer processes only, and extended by Dexter(5) who treated the possibility of higher-

10

order multipoles and exchange interactions contributing to the energy transfer process. As we

know, the transitions occurring in many of the upconversion systems of interest to us are forbidden

according to the simple electric dipole-dipole theory, and therefore the more advanced treatment by

Dexter is necessary to understand the interactions taking place. The Dexter theory of energy transfer

is outlined in this section, in the form described in the original paper.

In the Dexter theory, the probability of energy transfer between two states – an excited state on a

sensitizer and a ground (or excited – if there is sequential excitation as in upconversion) state on an

activator – depends on the overlap between the wavefunctions describing the two states, much as

the probability of an absorption or emission between two states depends on the degree of overlap

between their wavefunctions. The transfer probability per unit time, Psa, is given by:

∑∑

∫ ∫ ∫

|⟨

⟩ |

(1.8)

where:

I (F) indicates the initial (final) state of the system, with the summations accounting for the

possibility of degeneracy (multiple potential transitions contributing to the transfer),

A prime indicates an excited state,

gs’ (ga) is the degeneracy of the excited (ground) state of the sensitizer (activator),

E is the electronic excitation energy wa’-wa=ws’-ws (w indicates an energy), and

p(w) is the probability of being in a state of a particular energy, w.

The Hamiltonian, H1, describes the interactions between the electrons on the sensitizer, S, with

those on the activator, A. By expanding it as a Taylor series of Coulombic terms of different orders

we see the origin of the various multipole transfer mechanisms:

{

}

11

(1.9)

where:

R is the separation between the nuclei of the sensitizer and activator,

κ is the dielectric constant of the host (note that this is the only way in which the nature of the host

material is accounted for in this theory),

rs (ra) is the sum of position vectors of the electrons on the sensitizer (activator) relative to the

position of the nucleus, and

e is the electronic charge.

The term above is the dipole-dipole term; successive terms account for dipole-quadrupole,

quadrupole-dipole, quadrupole-quadrupole, etc. interactions.

By inserting the dipole-dipole term into the expression for Psa, and averaging over all possible

separations R, we may derive an expression for the total dipole-dipole transfer probability per unit

time:

∑∑∫ {∫

|⟨

⟩| }

{∫ |⟨ ⟩| }

(1.10)

Fortunately, this expression can be re-written in terms of spectroscopic properties of the system of

interest which may be measured experimentally. By making use of the Einstein A and B coefficients

to describe the rates of absorption and emission and substituting into this equation, we finally find:

(

)∫

(1.11)

where:

12

Qa is the area under the absorption band of the activator,

n is the refractive index of the host material,

τs is lifetime of the excited state of the sensitizer,

ℰ (ℰc) is the electric field strength in the vacuum (crystal),

fs(E) describes the shape of the sensitizer’s emission band, and

Fa(E) describes the shape of the activator’s absorption band.

All of the above may be evaluated spectroscopically or otherwise approximated easily.

What has been covered so far is very similar to the work originally presented by Förster. Dexter’s

contribution was to extend this treatment to higher order terms. If the expansion of H1 is now

extended we begin to derive terms describing energy transfer via higher order multipole interactions,

and again these can be found in terms of experimentally measurable parameters. The dipole-

quadrupole transfer probability, for example, can be shown to be given by:

(

)∫

(1.12)

where α=1.266.

Of note is that for dipole-quadrupole transfers, the transfer probability falls off as R-8, whereas for

dipole-dipole transfers it diminshes as R-6. In general for any transfer mediated by multipoles of

order n and m on the sensitizer and activator, the transfer probability will scale with R-(n+m+2). This

treatment may be extended to higher orders as desired by further expanding the Hamiltonian and

using successive terms to compute the transfer probabilities.

Another factor treated by Dexter, and ignored by Förster, is the influence of exchange effects on the

transfer probabilities, and this gives rise to the possibility of transfers occurring simultaneously with

a change in the spin state of an electron. Crucially, exchange interactions do not in general forbid

any transitions on symmetry grounds, and are therefore far less restrictive than multipole-multipole

transfers.

13

1.2.2 Upconverting Materials

1.2.2.1 Considerations When Designing an Upconverter

As discussed above, in general the three key components of an upconverting system are the

sensitizer, which absorbs incoming photons, the activator, which emits upconverted light, and the

host material into which these are immobilized. In some cases, such as in the case of erbium doped

NaYF4 which we consider in this work, the sensitizer and activator may be identical (in this case

erbium) and in other cases, such as those of NaYF4 doped with both erbium and ytterbium or

thulium and ytterbium, the roles may be taken by different species (in these cases ytterbium is the

sensitizer and erbium or thulium the activator).

Due to the lanthanide contraction limiting the interaction of the 4f electrons with other electrons,

the spectroscopic properties, and in particular the positions of the absorption and emission peaks, of

lanthanides tend to be relatively constant. For this reason, it is, in principle, simple to design a

material to upconvert light of wavelength x to wavelength y, by following a few simple ‘rules’:

The sensitizer must be able to absorb at wavelength x.

The activator must emit at wavelength y.

The activator must have an energy level suitable to accept energy transfer from the

sensitizer, ideally at a wavelength close to (possibly slightly longer than) x as a good energy

match facilitates efficient energy transfer.

The radiative lifetimes of the excited states of the sensitizer and activator must be long

(~milliseconds) to maximize the chance of the activator receiving another quantum of

energy before emitting, and to minimize the chance of the sensitizer decaying radiatively

rather than by energy transfer to the activator.

The host material must have low-energy vibrational modes, as higher energy modes increase

the probability of de-excitation by energy loss to the lattice as heat as opposed to by energy

transfer or emission. As a rough guide, vibrational relaxation is often favoured when the

energy gap is less than six times the phonon energy(6). As a caveat, the vibrational energy of

the host should not be too low, as higher vibrational energy serves to broaden the energy

levels(6) and can help to increase the chance of energy transfer, especially in the case of an

imperfect energy match. As mentioned above, in practice many rare earth-containing oxides,

14

fluorides and chlorides make good host materials, although it is very difficult to make

quantitative predictions about the likely efficiency of a particular material.

1.2.2.2 Spectroscopy of NaYF4:Er,Yb

Many systems have been reported to display upconversion, including those based on transition

metals, rare earths, and organic molecules. In the previous section we discussed some basic ‘rules’

for the design of an upconverting system to give an overview of some of the factors which may

affect the efficiency of an upconverting material. In this section we will provide an introduction to

the spectroscopy of NaYF4:Er,Yb – a common upconverting material and the focus of this study.

Reference to spectroscopic term symbols will be made in the section; an explanation of these

symbols is provided in Appendix A. The energy level diagram for NaYF4:Er,Yb is shown in Figure

1.3, along with a typical emission spectrum for this material, collected under continuous wave 972

nm laser excitation.

Figure 1.3. Left: Upconversion transitions in NaYF4:Er,Yb. Coloured arrows indicate upconversion

emission, wavy arrows are vibrational relaxation processes (phonon emission), dotted lines indicate energy

transfer. Adapted from (7) with permission from Elsevier. Right: Typical upconversion emission spectrum

from NaYF4:Er,Yb under 972 nm laser excitation.

400 500 600 700

Wavelength /nm

4S3/2, 2H11/2 →4I15/2

2H9/2 →4I15/2

15

Of note is that two of the transitions indicated involve a change in spin state – this is a consequence

of the strong spin-orbital coupling which commonly exists in these systems, and which adds

significant complication to the understanding of their spectroscopy. The diagram illustrates the

energy transfer schemes leading to the three main upconversion emissions, which are also shown in

the spectrum; these occur in the red, green, and violet regions of the spectrum. The red and green

emission processes primarily involve the transfer of two quanta of energy, with the difference in

wavelength resulting from a difference in phonon emission. The violet peak results primarily from a

three-photon process and tends to be significantly weaker than the other two. Interesting, and in

contrast to many other emission processes, this material shows a decrease in luminescence at low

(~5 K) temperatures and an increase in the number of photons corresponding to each process. This

is attributed to the fact that at very low temperatures the Yb 2F5/2 state rapidly relaxes to the lowest

energy state in its multiplet, which is unable to transfer energy to the Er 4I15/2 state and must instead

transfer to the lower-lying Er 4I13/2, with which it has a relatively poor energy match(8).

Other rare earths, most notably holmium and thulium, have also been shown to display

upconversion emission, and a number of these species are discussed in more depth in ref. 1. The

reasons for the particular choice of NaYF4:Er,Yb as the focus of our study are:

1. There was already a well established literature on this material giving a strong foundation to

the study

2. The energy levels of Yb3+ and Er3+ are highly favourable for pairing with amorphous silicon,

our solar cell material of choice

3. Erbium is one of the less expensive rare earths and so this material was one of the more

economical choices available. Although NaYF4:Tm,Yb has been investigated the cost of

thulium is considerably higher than that of erbium which makes this a much less economical

choice (in addition to the fact that the upconversion efficiency of this material is significantly

lower).

16

1.3 Prior Art on Upconverters in Solar Cells

In this section we present a review of work in the literature which discusses the application of

upconversion to photovoltaic power generation. We begin with a look at theoretical work in the

area, then explore experimental demonstrations on various types of solar cell, and discuss light-

confinement strategies in development to improve upconversion efficiency, some of which have

already been demonstrated in an upconverting photovoltaic (UCPV) system.

1.3.1 Theoretical Work on UCPV

A number of groups have tackled UCPV from a theoretical standpoint, from those who have made

simple calculations on solar cells coupled to ideal upconverters, to those who have sought to

introduce more physically realistic parameters simulating real systems with real losses, and those who

have performed simulations on various ways of enhancing upconversion efficiency, most notably

through coupling to plasmonic structures, and coupling these enhanced upconverters to solar cells.

In this section we present a survey of the work performed in this area, beginning with the simplest

and most idealized cases and moving towards the more complex.

In 2002, the theoretical efficiency enhancement available from UCPV was calculated by Trupke et

al.(10). They used a detailed balance approach and demonstrated a theoretical efficiency of 63.2%

for concentrated sunlight and 47.6% for non-concentrated, at band gaps of ~2 eV, as compared to

33.7% for a conventional 1.1 eV cell. Because the primary factor influencing the applicability of

upconversion to a particular solar technology is the band gap of the cell, the potential for a given

solar technology to benefit by pairing with an upconverter may be determined by considering the

band gap of the cell. Technologies which due to their wide band gaps are appealing for pairing with

upconverters include:

Amorphous silicon, band gap ~1.7 eV

Copper zinc tin sulfide, band gap 1.5 eV

Copper gallium sulfide, band gap 1.7 eV

Cadmium selenide, band gap 1.7 eV

17

Cadmium telluride, band gap 1.5 eV

The upconversion of low energy photons results in a blue-shift of the optimum band gap and an

increase of the per-photon energy harvested by the cell. Trupke et al. modeled a simplified and

idealized upconverter and upconverter-solar cell device using an equivalent circuit, as shown in

Figure 1.4.

Figure 1.4. Left: Schematic of a back-face upconverting solar cell. Middle: Schematic energy level diagram for

an upconverting solar cell, with the upconverter adding an effective intermediate state mid-band gap. Right:

Equivalent circuit for upconverter solar cell: C1: Solar cell; C2-C4: Upconverter; C2: Upconversion emission;

C3, C4: Sequential excitations of upconverter. Reproduced from (10) the American Institute of Physics.

In this case the upconverter is electronically isolated from the cell so that its only possible

interaction with it is via photon emission, and the upconverter has broad ground and upper excited

states (‘valence’ and ‘conduction bands’ although the material is not necessarily a semiconductor)

separated by an energy, Eg, equal to the band gap of the semiconductor used in fabricating the cell

and a single narrow intermediate state, such that when light of energy E strikes the cell, if E1<E<E2

a transition from valence band to intermediate state occurs, if E2<E<Eg then a transition from

intermediate state to conduction band occurs, and if E>Eg then the photon is absorbed by the solar

cell itself. The authors considered the solar cell as having a refractive index n=3.6, assumed perfect

absorption and emission for the possible transitions, and modeled the system as being acted upon by

blackbodies at 6000 K (solar radiation) and 300 K (background radiation). Interestingly, it was

demonstrated that the maximum efficiency possible, 63.2% was achievable in the case of maximum

concentration of the light emitted by the upconverter, and that the maximum efficiency at the

18

highest possible concentration of the incident light is slightly lower, at 61.4%. For the non-

concentrated system, it was found that an efficiency of 47.6% could be achieved, if the intermediate

state was given a finite width such that relaxation within an ‘intermediate band’ could occur. The

maximum efficiency without allowing for this relaxation was not reported. This work was extended

in 2008 by Badescu(11, 12) who moved away from a perfectly idealized system and took into

account various other factors including reflection losses, imperfect electron-hole generation,

imperfect absorption, and imperfect upconversion efficiency. He also considered the question of

whether an upconverter mounted to the front of the cell could be beneficial, as the previous work

had considered only the possibility of mounting it on the back. He concluded that in any realistic

case a front-mounted upconverter would be detrimental to cell performance and that for gains to

occur it must be mounted to the back of a bifacial (as in, a cell which is able to utilize light which is

incident from either front or rear sides) solar cell. As expected his implementation of more realistic

parameters lead to a decrease in the predicted efficiencies. Perhaps most interestingly, he predicted

that with unconcentrated light it would be detrimental to cell performance to couple it to an

upconverter, even with the converter mounted behind the cell, due to the resulting reduced

efficiency of the cell’s back-reflector, but that under concentrated sunlight improvements could be

expected and at a concentration factor of 100 he predicted an increase of efficiency of a ‘realistic’

system from ~24 to ~29%, and at 1000 suns from ~25 to ~36%. For these numbers an

upconversion efficiency of 100% was assumed. Although the case of realistic upconversion

efficiency with concentrated light was not treated (and it should be remembered that due to the

nature of the upconversion process, the efficiency is expected to increase with the concentration

factor) it was shown that for unconcentrated light a realistic upconverter would lead to a decrease in

overall efficiency. As it became clear that existing upconverters have little potential for improving

solar cell efficiency when modeled as-is, a number of groups turned their attentions to simulating

different manners is which upconversion efficiency could potentially be increased, perhaps most

notably via the use of light concentration techniques involving slow light effects in photonic crystals,

or enhancement due to proximity with metallic plasmonic resonances. In 2011, Johnson et al.(13)

reported the concept of using slow light in a one-dimensional photonic crystal (multilayer mirror) to

amplify upconversion intensity and presented calculations on the potential electric field

enhancement achievable for various numbers of layers. Although they demonstrated theoretical field

enhancements in 40-bilayer structures of over 18 times, they noted the inherent trade off with this

method that higher enhancement at one wavelength comes at the cost of greater reflection at others,

19

although this is potentially avoidable using specially engineered structures allowing for slow light

over a relatively broad range of wavelengths(14). Unfortunately, however, this study did not present

any calculations relating to upconversion or solar cells, and merely mentioned the concept and

performed calculations on field strengths in 1D photonic crystals, so the true potential of this

approach is so far untested by experiment or theory. Nonetheless, if a suitable broadband slow light

structure can be engineered in order to maintain the demonstrated field enhancement over a wider

range of wavelengths then this approach may have some practical benefits.

A second approach which has been treated theoretically by Atre et al.(15) is the use of a metal

plasmonic nanostructure as a means of generating a field enhancement to boost upconversion

efficiency. They model a spherical upconverting particle surrounded by a three-dimensional crescent

shaped metallic shell, where a gap in the shell acts as a point of concentration for field intensity. This

is compared with an upconverter encapsulated in a solid metal shell and also a lone upconverter with

no plasmonic enhancement. The proposed structure is shown in Figure 1.5.

Figure 1.5. Proposed Upconversion-silver nanomaterial for plasmonic enhancement of upconversion.

Reproduced from (15) with permission from the Institute of Physics and Prof J. A. Dionne.

These calculations demonstrated an effective enhancement factor (which the authors described as

‘conservative’) of ~100, for upconverted light coupled into the cell, versus an upconverter without

the plasmonic enhancement. Crucially, it was shown that the orientation of the plasmonic structure

20

with respect to the cell is relatively unimportant when calculating the total expected power coupled

into the cell. This would be a major advantage from a practical perspective as it would allow for

random coating via a means such as spin- or dip-coating rather than requiring a precise technique to

maintain alignment. Also of note is that both of these techniques – photonic and plasmonic – allow

for higher order resonances to enhance the emission as well as the absorption.

Now that we have provided an overview of the theoretical work on UCPV, we will review the

experimental results that have been gathered in the field. This will be presented in three parts:

1. Experiments on silicon-based solar cells

2. Experiments on other solar cells

3. Plasmonic/photonic enhancement experiments on standalone upconverters

1.3.2 Experiments on Silicon-Based Solar Cells

In 2005, Shalav et al.(16) coupled an NaYF4:Er3+ upconverter to a bifacial silicon solar cell, and

excited it at a wavelength of 1523 nm and a power of 5.1 mW. An internal quantum efficiency (IQE)

of 3.8% and an external quantum efficiency (EQE) of 2.5% were reported. Although this work did

not attempt to estimate the achievable benefits under AM1.5 light, the authors concluded that it

constituted a promising proof of principle. A similar study was reported again by Shalav et al.(6) in

2007 using the same material, but this time greater attention was paid to low intensity light - a quartz

tungsten halogen (QTH) bulb was used for generating this in order to mimic exposure to the solar

spectrum. A peak EQE of 5x10-4% at 1523 nm was reported under monochromated QTH light,

with an IQE of 8x10-4%; under laser excitation similar efficiencies to those previously reported were

observed. Using this experimental quantum efficiency data, the researchers were able to estimate a

theoretical solar cell efficiency enhancement under maximum (46200x) solar concentration of

~0.08%, which they conclude is not a practically realistic solution. As well as the inherently low

efficiency of the upconversion process (which is mitigated significantly by solar concentration, due

to the dependence of the quantum yield on the excitation intensity, discussed in chapter 4) the

authors point to the relatively low absorption bandwidth, and note that even for a 100% efficient

UC process, the maximum expected PV efficiency enhancement would be ~2.4%, if the wavelength

range of the study were the only one to be used. It should also be noted that due to the strong

dependence of upconversion intensity on incident power, it is likely that if the absorption could be

21

broadened resulting in the capture of more light by the upconverter, that the conversion efficiency

would increase also. Fischer et al.(17) added to these studies with further investigations of silicon

solar cells coupled to upconverters under laser excitation, and also investigated the various loss

mechanisms within the device including the non-linear upconversion efficiency, and imperfect

absorption by the upconverter.

Similar studies have also recently been reported on amorphous silicon. In contrast to crystalline

silicon which has a band gap around 1100 nm, and tends to be coupled with upconverters excited at

~1550 nm, amorphous silicon typically has a band gap (to the extent that one can truly be defined

for an amorphous material) in the region of ~700 nm – as defined by the onset of strong

absorption, typically by fitting a Tauc plot – and when coupled to an upconverter, a-Si cells are

typically excited in the region of 980 nm. The blue-shifted band gap of a-Si relative to c-Si is closer

to the ideal band gap for an upconverter coupled solar cell, in the ideal case of high upconverter

efficiency. De Wild et al.(18) published a study using NaYF4:Er,Yb to enhance the efficiency of

amorphous silicon cells via emissions in the red, green and blue regions of the visible spectrum.

Under laser excitation this work showed a clear gain in the response of the cell to 20 W/cm2 980 nm

light, with an effective doubling of the resulting current density from 5 to 10 A/cm2. However,

under broadband illumination, the UCPV cell performed worse than an a-Si cell coupled to a white

back reflector, due to the poorer reflectivity of the UC laser as compared with the reflector. This

observation underlines the need to achieve high upconversion efficiencies before they will be of

practical use in solar power generation, given that there are sacrifices in other areas of cell design

that must naturally be made, the reflector being one such area. De Wild et al. extended this work(19)

with an investigation into the resulting solar cell EQE on the upconversion excitation intensity,

demonstrating a strong dependence of the response of a UCPV cell to the excitation intensity, but

little response for a control cell without the upconverting layer, again demonstrating the importance

of high excitation intensity and also serving to better separate the contributions from upconversion

and sub-band gap absorption by the cell.

In light of studies published on plasmonic enhancement of upconversion, (see section 1.3.4.2) in

2012, Li et al.(20) reported, so far as we are aware, the only published study to date which

experimentally demonstrates plasmonic enhancement of UCPV, by using hexagonal-phase

NaYF4:Er,Yb,Gd nanorods to improve the NIR response of a-Si cells. In contrast to the other work

discussed here so far, this study made use of nano-sized upconverters (n-UC) as the UC material.

22

Whilst these materials typically have much lower emission intensity than their bulk counterparts the

authors point to their advantage when attempting to fabricate flexible UCPV devices, which was

noted as a goal in this work. It will also be seen later that n-UC materials have also attracted some

attention for their potential to be directly integrated into dye-sensitized solar cells. Despite

previously reported studies which indicated that coating of an upconverter onto the front of a

cell(11) will not lead to overall efficiency enhancement under AM1.5 light, in this proof-of-concept

work the upconverters were coated onto the front surface of the cell by spin-coating, although SEM

imaging demonstrated that they did not fully cover the surface of the cell. Under 1.1 W excitation by

a 980 nm laser, the authors demonstrated a current of 0.25 mA from n-UC coupled cells; 0.45 mA

from Au-coated-n-UC cells and 1.1 mA from cells coated with n-UC, coated with Au nanoparticles;

as shown in Figure 1.6. The authors attributed these gains to the enhancement of upconversion by

the plasmon resonances of the gold nanoparticles/shells, although no data was presented on the

plasmon resonances of the gold in their system so it is difficult to attribute this gain with certainty.

23

Figure 1.6. Photocurrent–voltage (I–V) characteristics of 980 nm laser-driven a-Si:H photovoltaic cells

without and with different kinds of NaYF4:Yb/Er/Gd nanorods. (a) Nanorods with Au nanoparticles, (b)

nanorods with Au shells, (c) naked nanorods and (d) the reference solar cell. Figure and caption reproduced

from (20) with permission of the Institute of Physics and Prof S. M. Huang.

1.3.3 Experiments on Non-Silicon-Based Solar Cells

The first report of an experimental UCPV device was made in 1996 by Gibart et al.(21) using an

Er3+, Yb3+-doped vitroceramic and reported a 2.5% efficiency under 1 W of 890 nm laser light

illuminating a cell area of 3.9 mm2. Despite this observation the authors concluded that the

efficiency was too low to be of practical use. Aside from this study, the majority of work reported on

non-silicon systems has used dye-sensitized solar cells (DSSCs) as the model system. Because

organic dyes used in these cells typically absorb most strongly in the green region of the spectrum

and show very little absorption into the NIR, they are excellent candidates for pairing with

upconverters as they can, in principle, take advantage of significant gains from the inclusion of an

upconverter. As mentioned in section 1.3.1 the ideal band gap for a solar cell increases from ~1.3 eV

in the absence of an upconverter to ~2 eV in the presence of one. In 2010, Shan and

Demopoulos(22) published a study in which they investigated a number of different UC-DSSC

device architectures, using a nanocrystalline TiO2-LaF3:Er,Yb composite material and including an

FTO/transparent TiO2/UC–TiO2/scattering TiO2/Pt/FTO architecture in which the transparent

TiO2 and UC-TiO2 composites contained TiO2 particles in the 10-15 nm range, and the scattering

TiO2 contained particles in the 350-450 nm range, from which they reported promising results. A

sharp decrease in the green luminescence was observed from the UC material integrated into the cell

as compared with an external control, indicating good absorption of upconverted light by the N719

ruthenium dye which was used in the experiment, and verifying the Er3+, Yb3+ couple as a promising

upconverting system for use in enhanced-efficiency DSSC applications. Under AM1.5 excitation an

increase in the photocurrent of 2.3% was observed, which was attributed to upconversion. This

result should be interpreted with a modicum of caution however, as a 2.3% increase in photocurrent

(which should translate to a roughly equal increase in the number of captured photons) is

significantly above any published theoretical value, particularly for an unconcentrated cell. It was

also not made clear in this work whether the authors fabricated multiple copies of each architecture

or just one, and therefore one cannot be sure as to the statistical significance of their result, but we

24

suspect that it is low. Nonetheless, the fact that the incorporation of the upconverter was apparently

not detrimental to the electronic performance of the cell is promising for the future integration of

upconverters into these devices.

In 2011, Xie et al.(23) investigated an upconverter-coupled DSSC system in which the TiO2 itself

was doped with Er3+ and Yb3+, and combined with undoped TiO2 to form a composite electrode.

Under NIR excitation, a clear improvement in the I-V characteristics of the cell was observed

relative to a regular, non-UC DSSC. Under simulated AM1.5 light, an increase in short circuit

current (Jsc) and overall efficiency were observed at relatively low upconverter loadings, but at higher

loadings a decrease was observed. This behavior was attributed to a number of different effects:

1. Upconversion of the NIR light to visible, resulting in the conversion of a greater number of

photons

2. A raising of the Fermi level in the UC-TiO2 layer due to replacement of Ti4+ with RE3+ (rare

earth 3+) ions in the UC material

3. The larger size of the UC-TiO2 vs TiO2 (~70 vs 20 nm, respectively) allowed them to act as a

scattering layer, increasing the path length of light in the cell and thereby increasing

absorption

4. At higher doping levels the action of RE3+ ions as recombination centres becomes the

dominant effect and a decrease in efficiency is observed.

Although the authors hypothesized on the presence of these effects, no attempt was made to

quantitatively separate the contributions from each (we suspect that this would be extremely

challenging to achieve in practice). Nonetheless, given that a maximum efficiency increase of 13.6%

(relative) was observed, it seems likely that the upconversion process was not the dominant

mechanism responsible for improving the efficiency of the cells. This work was further supported by

a study published by Wu et al.(24) in 2012, which used YF3:Er,Yb as an internal upconverting layer

and reported very similar effects and conclusions, including the effect of the upconverter on the

Fermi level of the photoanode and the resulting increase in photovoltage.

25

Also in 2011, Shan et al.(25) followed their previous work with a new investigation which used β-

NaYF4:Er,Yb nanocrystals as an external (i.e. placed outside of the cell) back-scattering

upconversion layer. The external placement of the layer electronically de-coupled it from the cell and

prevented any unwanted processes such as recombination that might have occurred due to the

presence of an internal layer. As has been mentioned for other studies, the UC layer was thought to

act both as an upconverter and also as a reflector/light scatterer, effectively increasing the path

length of light passing into the cell, and increasing absorption via this effect. With this structure,

relative efficiency and Jsc increases of 10.2% and 15.4%, respectively, were observed. The authors

commented that most of this increase was likely due to improved light trapping by scattering, and

attribute ~1% of the gain to the upconversion process. Unfortunately, the origin of this value was

not explained and it is significantly higher than had been predicted theoretically, and must therefore

be viewed with some caution. Also of note is that the authors did not prepare an undoped non-

luminescent NaYF4 scattering layer to use as a control material which would likely have assisted

attempts to quantitatively define the contribution of upconversion to the enhanced efficiency.

A number of groups have attempted to demonstrate enhancement of upconversion via light

intensity increases using nanoscale plasmonic or photonic structures, which is of course very

important for practical applications of upconversion to solar power given the extremely strong

dependence of the luminescence on the intensity of the excitation, and the relatively low efficiency

of upconversion processes overall. In the following section we provide an overview of some of the

experimental work which has been done in this area.

1.3.4 Plasmonic/photonic Studies on Standalone Upconverters

1.3.4.1 Photonic Enhancement of Upconversion

In 2002, Markowicz et al.(26) studied the enhancement of two-photon-excited emission using a

photonic crystal by incorporating Coumarin 503 dye into a polystyrene opal, Figure 1.7. A

significant decrease in emission intensity was observed in the region of the photonic stop gap, along

with an increase at the band edges, which is consistent with theory and with other experimental

observations on optical processes coupled to photonic crystals. In contrast to the upconversion

26

processes targeted for solar cell applications, in two-photon-excited emission two exciting photons

are absorbed at the same instant in time and so the photon density (light intensity) required to

achieve this is extremely high, many orders of magnitude higher than is needed for upconversion

processes which allow for non-simultaneous absorption. Nonetheless, it was an important

demonstration, and some years passed before researchers turned their attentions to enhanced

upconversion processes of the sort being targeted for use in solar power generation.

Figure 1.7. A typical example of an opal film – a face-centred cubic array of silica spheres which exhibits

diffraction peak as visible wavelengths due to the periodic arrangement of monodisperse spheres leading to

regions of constructive and destructive optical interference. Adapted from (27) with permission from the

American Chemical Society.

In 2010, experimental work began to appear on the photonic enhancement of these upconversion

processes. Zhang et al.(28) reported formation of NaYF4:Er,Yb nanocrystals into an inverse opal

structure as a means to modify their upconversion properties, and reported that by overlapping the

opals’ stop gaps with the various emission bands of the upconverter, they were able to suppress

emission by as much as a factor of 3.5. An increased excited state lifetime was also demonstrated via

fluorescence decay measurements, presumably due to the suppression of radiative decay pathways.

In 2011, Yang et al.(29-32) reported similar studies in which they used Er, Yb doped bismuth

titanate (Bi4Ti3O12), calcium titanate (CaTiO3) lead lanthanum titanate (Pb0.9La0.1TiO3) and Er-doped

ytterbium phosphate (YbPO4) inverse opals – which were chosen as host materials for their high

27

refractive index – to suppress upconversion emission, again reporting a maximum decrease in

emission by a factor of 3.5.

1.3.4.2 Plasmonic Enhancement of Upconversion

Plasmonic enhancement of upconversion was demonstrated in 2010 in studies by Schietinger et

al.(33) and Zhang et al.(34) both of whom demonstrated control of upconversion luminescence

from NaYF4-based phosphors (Er, Yb doped in the former case and Er, Tm in the latter). In the

first of these studies, Schietinger et al. used an atomic force microscope (AFM) coupled to a

confocal microscope to manipulate the positions of gold and NaYF4:Er,Yb nanocrystals relative to

each other, and investigated the dependence of their luminescence behavior on the proximity of the

two varieties of particles to each other, as well as the effects of having multiple Au particles in close

proximity to a single upconverter. They were able to show significant enhancement, up to a factor of

4.8 for the green emission and 2.7 for the red, and showed that the luminescence rise and decay

times of an upconverting nanoparticle were decreased in the presence of an Au particle. In their

study, Zhang et al. grew upconverting nanocrystals and attached Au nanoparticles to them. They

were able to then use these attached particles as seeds to grow a gold shell around the upconverting

particles, and demonstrated that initial attachment of the gold particles could enhance upconversion

emission by up to a factor of 2.6 due to surface plasmon-mediated light intensity enhancement at the

upconverting particles and also more efficient emission of upconverted light via coupling to surface

plasmon modes of the gold particle. When a gold shell was grown, however, suppression of

emission by a factor of ~3.5 was seen, and that was attributed to scattering of the 980 nm excitation

light by the gold shell. It is interesting to note that this quenching by growth of a gold shell is in

contrast to the previously discussed work by Li et al.(20) whose work on gold plasmonic

enhancement of upconversion for amorphous silicon solar shells showed an increase in emission

upon growth of a gold shell; one possible explanation for this disparity is that the work of Li et al.,

in which a gold shell was grown directly onto a UC nanorod resulting in a smoother shell than that

produced by Zhang et al. which relied on seeding of the gold shell via initial nanocrystal attachment,

likely leading to a rougher and more scattering surface. The morphology of the upconverters may

also have had an influence – Li et al. worked with large nanorods roughly 50 nm across and 300 nm

long, whereas Zhang et al. used flatter nanoplates of approx 150 nm diameter (depth not given).

28

Similar work was reported in 2011 by Liu et al.(35) who again investigated plasmonic enhancement

by coupling of gold nanocrystals to the surface of NaYF4:Yb,Tm upconverting nanoplates. In this

study, the dependence of the plasmonic enhancement on the excitation power density was also

investigated and it was found that at low power densities of ~5 W cm-2 an enhancement factor of as

much as 100 could be achieved, which decreased significantly at higher excitation intensities,

possibly due to the decrease in the exponent describing the power dependence(2) of upconversion

emission which occurs at higher incident powers, as upconversion becomes a more dominant

emission mechanism. Remarkable in this case, and in contrast to other studies is that the authors

consistently observed increased upconversion rise and decay times relative to a gold-free system,

indicating that plasmonic enhancement of the emission was not likely the dominant upconversion

process – the authors concluded that local field enhancements of the incident excitation were likely

responsible for the observed behavior. More control was added to this area of study by Paudel et

al(36). in 2011, who used a 2D patterned array of gold nanopillars as a support for upconversion

nanoparticles, and demonstrated significant dependence of the emission intensity on the proximity

of the upconverters to the gold nanopillars.

Most recently, an interesting twist was reported by Wu et al. in an abstract at the 2012 MRS Spring

Conference. They reported using silver nanorods with controlled aspect ratios to achieve plasmonic

enhancement of both emission and excitation wavelengths by overlap with the transverse and

longitudinal plasmon modes(37), respectively; a principle which is very similar to the theoretical

work from the same group using 3D silver nanocrescents which was discussed above. Although the

work has not yet been extended to solar power, the group was clear about their intention to do so in

the future.

1.4 Outline of Thesis

The work detailed in this thesis builds on previous work in the field by looking more closely at the

dependence of an upconverter’s ultimate applicability to a-Si photovoltaics on its synthesis

conditions and resulting materials properties.

29

Chapters 2 & 4 are concerned with investigating the dependence of the properties of NaYF4-based

upconverters on their synthesis temperature – observing the crystallographic and spectroscopic

(characterized by overall luminescence intensity as well as emission quantum yields) behavior of

materials synthesized at temperatures ranging from 250 to 700 °C. In chapter 3 we introduce the

measurement of quantum yields in more detail before embarking upon the study of the quantum

yields of upconverters, and we provide an in-depth comparison of two different methods of

performing these measurements, both of which are prevalent in the literature. When preparing to

undertake our quantum yield study we were faced with making a choice between these two

protocols, and with nothing in the literature to guide us as to the correct choice we thought it

prudent to investigate each in more detail. Finally, in chapter 5 we take the materials that we have

synthesized and studied in chapters 2-4 and apply them to amorphous silicon to investigate their

compatibility for solar power generation. Realizing that although we have made progress there is still

much to be done in order to produce a practical UCPV system we provide an outline of what such a

realistic system might entail and then end by summarizing the overall conclusions of this work and

discussing some still unanswered questions.

In the first of four appendices we provide an overview of a proof-of-concept upconversion display

device which we developed based on the use of coloured filters and a red, green, and blue emitting

upconversion material. This data is included as an appendix because, although it does not relate

directly to UCPV which is the focus of the thesis, we believe that it is an interesting and potentially

important application. The following three appendices present descriptions of spectroscopic term

symbols, which are used in describing the spectroscopic behavior of upconverters; details of three

LabVIEW programs that were written to run the experiments detailed in the thesis; and finally a

discussion of potential sources of error throughout the work.

1.5 References

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104, 139 (Jan, 2004).

30

2. M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, M. P. Hehlen, Power dependence of

upconversion luminescence in lanthanide and transition-metal-ion systems. Physical Review B 61, 3337

(Feb 1, 2000).

3. L. Gomes et al., Energy transfer rates and population inversion of 4I11/2 excited state of Er3+

investigated by means of numerical solutions of the rate equations system in Er:LiYF4 crystal. Journal

of Applied Physics 106, (Nov 15, 2009).

4. T. Förster, Zwischenmolekulare energiewanderung und fluoreszenz. Annalen Der Physik 2, 55

(1948, 1948).

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Tm3+ and/or Yb3+. Journal of Luminescence 117, 1 (Mar, 2006).

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up-conversion process in NaYF4 : Er3+,Yb3+. Journal of Luminescence 114, 53 (Jul, 2005).

9. B. Oregan, M. Gratzel, A low-cost, high-efficiency solar-cell based on dye-sensitized

colloidal TiO2 films. Nature 353, 737 (Oct 24, 1991).

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31

14. J. Li, T. P. White, L. O'Faolain, A. Gomez-Iglesias, T. F. Krauss, Systematic design of flat

band slow light in photonic crystal waveguides. Optics Express 16, 6227 (Apr, 2008).

15. A. C. Atre, A. Garcia-Etxarri, H. Alaeian, J. A. Dionne, Toward high-efficiency solar

upconversion with plasmonic nanostructures. Journal of Optics 14, (Feb, 2012).

16. A. Shalav, B. S. Richards, T. Trupke, K. W. Kramer, H. U. Gudel, Application of NaYF4 :

Er3+ up-converting phosphors for enhanced near-infrared silicon solar cell response. Applied Physics

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electrical characterization. Journal of Applied Physics 108, (Aug 15, 2010).

18. J. de Wild, A. Meijerink, J. K. Rath, W. G. J. H. M. van Sark, R. E. I. Schropp, Towards

upconversion for amorphous silicon solar cells. Solar Energy Materials and Solar Cells 94, 1919 (Nov,

2010).

19. J. de Wild, J. K. Rath, A. Meijerink, W. G. J. H. M. van Sark, R. E. I. Schropp, Enhanced

near-infrared response of a-Si:H solar cells with beta-NaYF4:Yb3+ (18%), Er3+ (2%) upconversion

phosphors. Solar Energy Materials and Solar Cells 94, 2395 (Dec, 2010).

20. Z. Q. Li et al., Core/shell structured NaYF4:Yb3+/Er3+/Gd3+ nanorods with Au

nanoparticles or shells for flexible amorphous silicon solar cells. Nanotechnology 23, (Jan 20, 2012).

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substrate-free GaAs solar cells using two-photon up-conversion. Japanese Journal of Applied Physics Part

1-Regular Papers Short Notes & Review Papers 35, 4401 (Aug, 1996).

22. G.-B. Shan, G. P. Demopoulos, Near-Infrared Sunlight Harvesting in Dye-Sensitized Solar

Cells Via the Insertion of an Upconverter-TiO2 Nanocomposite Layer. Advanced Materials 22, 4373

(Oct 15, 2010).

23. G. Xie et al., Application of upconversion luminescence in dye-sensitized solar cells. Chinese

Science Bulletin 56, 96 (Jan, 2011).

24. J. Wu et al., Enhancement of the Photovoltaic Performance of Dye-Sensitized Solar Cells by

Doping Y0.78Yb0.20Er0.02F3 in the Photoanode. Advanced Energy Materials 2, 78 (Jan, 2012).

32

25. G.-B. Shan, H. Assaaoudi, G. P. Demopoulos, Enhanced Performance of Dye-Sensitized

Solar Cells by Utilization of an External, Bifunctional Layer Consisting of Uniform beta-

NaYF4:Er3+/Yb3+ Nanoplatelets. Acs Applied Materials & Interfaces 3, 3239 (Sep, 2011).

26. P. Markowicz et al., Enhancement of two-photon emission in photonic crystals. Optics Letters

27, 351 (Mar 1, 2002).

27. S. Wong, V. Kitaev, G. A. Ozin, Colloidal crystal films: Advances in universality and

perfection. Journal of the American Chemical Society 125, 15589 (Dec, 2003).

28. F. Zhang, Y. Deng, Y. Shi, R. Zhang, D. Zhao, Photoluminescence modification in

upconversion rare-earth fluoride nanocrystal array constructed photonic crystals. Journal of Materials

Chemistry 20, 3895 (2010, 2010).

29. Z. Yang et al., Color Tunable Upconversion Emission in Yb, Er Co-Doped Bismuth Titanate

Inverse Opal. Journal of the American Ceramic Society 94, 2308 (Aug, 2011).

30. Z. Yang et al., Significant reduction of upconversion emission in CaTiO3: Yb, Er inverse

opals. Thin Solid Films 519, 5696 (Jun 1, 2011).

31. Y. Zhengwen et al., Photonic band gap and upconversion emission properties of Yb, Er co-

doped lead lanthanum titanate inverse opal photonic crystals. Applied Physics a-Materials Science &

Processing 103, 995 (Jun, 2011).

32. Z. Yang et al., Effect of photonic bandgap on upconversion emission in YbPO4:Er inverse

opal photonic crystals. Applied Optics 50, 287 (Jan 20, 2011).

33. S. Schietinger, T. Aichele, H.-Q. Wang, T. Nann, O. Benson, Plasmon-Enhanced

Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals. Nano Letters 10, 134 (Jan, 2010).

34. H. Zhang et al., Plasmonic Modulation of the Upconversion Fluorescence in NaYF4:Yb/Tm

Hexaplate Nanocrystals Using Gold Nanoparticles or Nanoshells. Angewandte Chemie-International

Edition 49, 2865 (2010, 2010).

35. N. Liu, W. Qin, G. Qin, T. Jiang, D. Zhao, Highly plasmon-enhanced upconversion

emissions from Au@beta-NaYF4:Yb,Tm hybrid nanostructures. Chemical Communications 47, 7671

(2011, 2011).

33

36. H. P. Paudel et al., Enhancement of Near-Infrared-to-Visible Upconversion Luminescence

Using Engineered Plasmonic Gold Surfaces. Journal of Physical Chemistry C 115, 19028 (Oct 6, 2011).

37. B. Pietrobon, M. McEachran, V. Kitaev, Synthesis of Size-Controlled Faceted Pentagonal

Silver Nanorrods with Tunable Plasmonic Properties and Self-Assembly of These Nanorods. Acs

Nano 3, 21 (Jan, 2009).

34

2 Dependence of the Crystallographic and

Luminescent Properties of NaYF4-Based

Upconverters on Preparation Temperature

The work described in this chapter has been submitted for publication as:

D. O. Faulkner, S. Petrov, D. D. Perovic, N. P. Kherani, G. A. Ozin, Dependence of the

Crystallographic and Luminescence Properties of Rare Earth Doped NaYF4 Upconverters on

Synthesis Temperature

We begin our discussion of the experimental work in this thesis with a description of the synthesis,

and crystallographic and spectroscopic characterization of NaYF4-based upconverters. The

development of upconverting nanocrystals for use in solar power and biological imaging, amongst

other applications, has become a topic of extreme interest in recent years. Despite this, the

luminescence efficiency of these nanocrystals is known to lag behind that of the corresponding bulk

materials by as much as two orders of magnitude. We investigate the temperature dependence of the

crystallographic and spectroscopic properties of NaYF4-based upconverters from 250 °C – a typical

nanocrystal synthesis temperature – up to 700 °C – a typical bulk synthesis temperature – and

investigate the changes which occur over this range. It is demonstrated that the formation of the

hexagonal phase, commonly targeted for its superior luminescence, is not in itself sufficient to

guarantee high upconversion efficiency. Rather, our results reveal that the situation is significantly

more complex than previously thought with parameters such as preparation temperature and particle

size potentially having as much if not more influence that the crystal phase itself. Full understanding

of these effects promises to lead to the development of upconverting nanocrystals with brightnesses

rivaling their bulk counterparts.

35

2.1 Introduction

Upconverters(1) – materials which are able to non-simultaneously absorb multiple low energy

photons and combine their energies resulting in the emission of high energy photons – have been at

the centre of a resurgence in interest in recent years. These materials are attracting a great deal of

attention for a number of potential applications in fields including solar power(2-5), bioimaging(6-8),

and display(9, 10) technologies. Although they have been known for many years(11), much of the

research originally performed was concerned with the spectroscopy(12) of the materials in bulk form

synthesized at high temperatures (≥700 °C), and much of the current attention is on forming

nanoparticles at relatively low temperatures(13) (~300 °C or less). What has not been extensively

studied is the transition between these two regimes: the differences and changes that occur in the

properties of these materials when prepared under different temperatures and the relationship that

these structural changes may have to the resulting luminescence. As it is known that the high

temperature bulk materials are two to three orders of magnitude brighter(14) and display similarly

higher quantum yields(15) than the low temperature nanoparticles, it is of significant interest to

understand the changes occurring over this range, as this might allow advances including the

development of strategies for the preparation of ultra-luminescent upconverting nanoparticles, or

may provide insight into new strategies which can be used for developing new and more efficient

upconverting compounds, or allow the production of highly efficient materials at low temperatures,

which would decrease their manufacturing cost, and improve their economic viability. Herein we

further investigate this dramatic dependence of the brightness of NaYF4-based upconverting

materials on their preparation temperature. Furthermore, in contrast to other reported syntheses of

bulk NaYF4 and its upconverting derivatives, we use a synthetic procedure – based on a common

nanoparticle synthesis(16, 17) – which uses relatively non-hazardous trifluoroacetate precursors and

which may be conducted in ambient air without the need for specialized equipment that is

commonly called for when working with fluorides. An aspect of this synthetic method is the

possibility of carbon contamination in the products leading to decreased luminescence and indeed

this possibility and its use as a potential way to study the decreased luminescence typically seen in

upconverting nanocrystals is one reason that we chose to pursue this particular synthetic approach.

This and the matter of other impurities will be discussed in the context of an FTIR study in part

2.3.2 of this article and we will show that such contamination does not appear to be a significant

factor.

36

In particular, we study the temperature dependence of the properties of NaY0.98Er0.02F4 and

NaY0.78Yb0.2Er0.02F4 , with undoped (non-luminescent) NaYF4 as a control. A typical example of the

upconversion spectrum, along with the transitions responsible for these emissions, is shown in

Figure 2.1. The peak labels show the transitions in the Er3+ ions which are responsible for the

emission observed; in the case of the material doped solely with Er3+, it is the erbium ions

themselves which absorb the exciting radiation, whereas in the Yb3+-doped material, the excitation is

absorbed primarily by Yb3+ sensitizer ions which non-radiatively transfer the excitation energy to

Er3+ activator ions via a multipole-coupling mechanism(1, 18), which then emit upconverted light, as

was discussed in section 1.2.

Figure 2.1. Typical upconversion spectrum from NaY0.78Yb0.2Er0.02F4 showing visible upconversion peaks

under 972 nm laser excitation.

400 500 600 700

Wavelength /nm

4S3/2, 2H11/2 →4I15/2

2H9/2 →4I15/2

37

Of particular interest in this study is the phase composition of the resulting material; NaYF4 and the

derivatives studied herein exist in two crystal forms; a cubic alpha phase, and a hexagonal beta

phase, shown in Figure 2.2.

Figure 2.2. Phases of NaYF4. Left: cubic Fm3m. Right: hexagonal P63/m. Green: F-; red/orange:

Y3+/Yb3+/Er3+/Na+. The pink polyhedra outline the nearest-neighbor co-ordination sphere around each

metal ion.

2.1.1 Introduction to Rietveld Analysis

One of the key techniques used for the work presented in this thesis is powder x-ray diffraction

(PXRD) and Rietveld analysis of the resulting PXRD patterns. A thorough introduction to the

principles of single crystal and powder diffraction is available via many sources, and is widely taught

(at least to some level) in undergraduate courses; however relatively little information, particularly in

the way of a less technical introduction, is available on how to actually analyze these data. In this

section we present a brief overview of the principles of Rietveld analysis for the fitting of PXRD

patterns.

The Rietveld method was introduced by Hugo Rietveld in two seminal papers(19, 20) in the late

1960s. He originally developed it for use in neutron diffraction experiments, but it is equally

38

applicable to X-ray data. Although many programs and algorithms have been developed for

performing Rietveld analysis, some with a great many parameters and complexities, at the heart of

the method is a simple least-squares fitting technique which attempts to optimize the fit between

observed data and some calculated model. Typically, the model begins with a set of peaks of a

certain position and intensity, and by adjusting these as well as the peak widths and shapes it is

possible to account for factors including particle size, lattice parameters, texture, and strain effects.

The goal of the method is to minimize, for each point yi in a diffraction pattern, the quantity:

∑ { }

(2.1)

where:

Wi is a weight function, commonly set equal to 1/σ(yi)2 – the reciprocal of the variance of the

intensity yi. This ensures that a greater statistical weight is given the points with lower variance

(which are presumably more accurate), and

obs (calc) indicates the observed (calculated – from the model) data.

The refinement may be made as simple or as complex as desired through the introduction of various

parameters, which may either be kept constant as constraints or refined. One such definition of the

yi which may be used in the model is to treat it as some intensity function Ik multiplied by a Gaussian

peak shape (although these days it is often common to use a linear combination of Gaussian and

Lorentzian shapes to account for both homogeneous and heterogeneous broadening processes)

where the index k indicates a peak as opposed to a discrete observation (i.e. the same intensity

function is applied across an entire peak, although of course a different function could be applied to

each data point if so desired). For example, Ik may take the form:

(

√

√ )

(2.2)

where:

39

Sk is the structure factor, describing the expected intensity of a single peak, all other considerations

aside,

jk is the multiplicity of a given peak,

Lk is the Lorentz factor which accounts for the fact that different peaks intersect the Ewald sphere

for different amounts of time depending on their 2θ values, and

Hk is the full-width at half-maximum of the peak k.

The quantity in brackets is a normalization coefficient.

Additional parameters may be introduced to account for other effects, if desired.

Despite the considerable power of the Rietveld method, some caution is needed. Partly due to the

freedom which comes from the availability of so many parameters for refinement, it can be relatively

easy to obtain a good fit with data which does not make sense, or to fall into the common trap of

having too much trust in information because it came from a computer program. There will often be

many ways to obtain a good fit for data, but these will not necessarily be correct. Excellent reviews

describing some of the dangers of fitting PXRD data have been published by Toby(21) and

Weidenthaler(22). In his paper, which discussed the use of various factors commonly used to

measure the quality of a Rietveld fit Toby warns: “there is no simple way to distinguish a good fit from one

that is just plain wrong based on R factors or other discrepancy values” and goes on to discuss a number of

issues that may arise when performing Rietveld analysis. In particular he discusses cases in which

high quality data with a long collection time and low variance give a statistically worse fit than poor

quality data with high variance, because the larger variance of the low quality data is better able to

‘absorb’ inconsistencies between the calculated and observed data, whereas the higher quality data is

not able to account for these with its low variance. In her paper, Weidenthaler takes a more

experimental perspective and discusses the effect of various complications such as particle size

distribution, strain, texture, and sample displacement – all of which can, in principle, be accounted

for by Rietveld analysis, but nonetheless an understanding of their significance is important,

particularly as knowledge of the likely presence of such complications in a given sample may be

40

cause for one to regard their fitted data with more skepticism than would perhaps otherwise be

warranted.

2.2 Experimental

Yttrium, erbium and ytterbium oxides and sodium trifluoroacetate were ordered from Acros. A

mixture of these compounds containing the appropriate ratios of metal atoms to yield either NaYF4,

NaY0.98F4:Er0.02, or NaY0.78F4:Er0.02Yb0.20 was weighed out and dissolved in ~50% aqueous

trifluoroacetic acid (Acros). After this, excess water and trifluoroacetic acid were evaporated to yield

a solid precipitate of metal trifluoroacetates; this precipitate was the precursor used in the syntheses

described herein. The final products were formed by placing 1.5 g of the appropriate precursor into

a quartz boat, placing the boat into a quartz tube and heating under ambient air to the desired

temperature. A heating rate of 300 °C hr-1 was used, and samples were held at the final treatment

temperature for 2 hours, after which the furnace was automatically switched off and the sample

cooled in air. In all cases the sample was allowed to cool to below 100 °C before being removed

from the furnace. All syntheses were repeated three times.

Crystallographic analysis was performed on a Siemens D5000 fitted with a Kevex detector, and

PXRD patterns were analyzed by Rietveld refinement using TOPAS software.

A schematic of the system used to measure upconversion spectroscopy is shown in Figure 2.3.

Spectroscopic data was collected using an Oriel photomultiplier tube (PMT) fitted to a ¼ m Oriel

Cornerstone model 74100 monochromator. The sample was excited with a 500 mW, 975 nm, (actual

measured output power 380 mW, center wavelength 972 nm) diode laser from Edmund Optics. The

signal from the PMT was amplified with a Stanford Research Systems SR570 current-voltage

amplifier and the system controlled with LabVIEW software which was developed in-house, the

operation of which is discussed in Appendix B.

41

Figure 2.3. Schematic of the optical system used to study upconversion luminescence: 1 light from a fibre-

coupled 972 nm diode laser is collimated by a lens held in a clamp and focused with a 5 cm focal length 1

inch diameter biconvex lens onto 2 an upconverting sample in a melting point tube held in a clamp.

Upconverted light is collected and collimated by 3 a 10 cm focal length 2 inch diameter plano-convex lens

and focused into 4 a monochromator coupled to a photomultiplier tube by a 20 cm focal length 2 inch

diameter plano-convex lens.

1

2

3

4

42

2.3 Results & Discussion

In the following sections we first present crystallographic and then spectroscopic data on the

materials studied, and then relate the two data sets in terms of possible explanations for the

luminescence behavior in terms of the crystallographic properties of the material.

2.3.1 Scanning Electron Microscopy (SEM) of NaYF4:Er,Yb

To complement the crystallographic and spectroscopic studies presented later in this chapter, we

also undertook SEM imaging studied of NaYF4:Er,Yb samples prepared at different temperatures

from 250 to 700 °C. Figures 2.4-2.10 show the evolution of the microscopic appearance of these

materials with synthesis temperature. As the materials are prepared at progressively higher

temperatures we can see that adjacent particles begin to neck and sinter together forming much large

networks as opposed to the distinct particles or aggregates present at lower temperatures. At high

temperature faceting begins to occur which particle faces gradually becoming smoother and flatter

and angles more pronounced until at 700 °C we are able to see many crystallites with clear faceting.

Figure 2.4. SEM image of a sample of NaYF4:Er,Yb prepared at 250 °C.

In this image small crystallites have assembled into larger aggregates but it is clear that the

morphology of the material is extremely rough and it appears to consist largely of separate particles

in the sub-100 nm range.

43


Necking and sintering has begun to occur by 325 °C and we can see that some of the aggregates are

beginning to smooth out as solid-state diffusion occurs and the individual particles sinter together.


Sintering is continuing to occur at 400 °C, and the individual grains appear to have grown

significantly in size relative to those at 250 °C.

44


At 475 °C sintering is very clear and many of the individual particles have necked together to form

extended networks.


45

At 550 °C we observe the beginnings of faceting, with the appearance of sharper angles along the

crystal boundaries and the flattening of particle faces as the material approaches thermodynamic

equilibrium through the preferential exposure of certain crystal facets.


Sintering and faceting continue to progress at 625 °C.

46


At 700 °C very pronounced faceting is present, with many crystallites displaying sharp angles and flat

crystal faces. Also of note is the significant increase in the apparent particle size relative to those

prepared at low temperatures.

Overall the temperature behavior of the particles shown in these micrographs is in line with what

would be expected from classical theories of crystal growth.

2.3.2 Crystallographic Characterization

In the analysis of our crystallographic data we concentrate on the following properties:

1. the phase composition of the resulting materials (percentage cubic vs hexagonal)

2. the typical sizes of the diffracting domains of each phase (although we remind the reader to

beware that when one phase is present in low abundance, as the cubic phase typically is at

high temperatures, that it is very difficult to perform accurate peak fitting and therefore

some data should be viewed with caution)

3. the lattice parameters for the phases (cubic a and hexagonal a and c).

47

Figure 2.11. PXRD patterns of NaY0.78Yb0.2Er0.02F4 prepared at various temperatures, as indicated. Cubic and

hexagonal reference patterns are also shown.

Figure 2.11 shows typical examples of PXRD data collected for the materials under study at various

preparation temperatures. The particular material shown in this case is NaY0.78Yb0.2Er0.02F4 although

they all exhibit very similar diffraction patterns. It can be seen that as the synthesis temperature is

increased there is an initial decrease in the percentage of the cubic phase, which is most directly

evidenced in the decrease of the intensity of the peak at 28° 2θ. The crystallinity of the hexagonal

phase also showed a marked improvement, best evidenced by the change in profile of the two peaks

at 53-54°. Towards the top end of our temperature range we see the re-emergence of the cubic

phase, in accordance with the known phase behavior of the system(23). Due to the similarity in their

diffraction patterns we cannot rule out the possibility that YOF or some similar yttrium oxyfluoride

24 29 34 39 44 49 54

2Θ /degrees

700 C

625 C

550 C

475 C

400 C

325 C

250 C

hexagonal

cubic

48

species is forming in this regime. Indeed, the cubic peaks in some of the 700 °C samples appear to

be made of two distinct phases and so these were fitted with both YOF and α-NaYF4. This behavior

was previously reported in (23), in which they ascribed the formation of these extra peaks to a new

phase, ‘X’, for which a structure was not proposed at that time, but which we believe is most likely

to be some form of yttrium oxyfluoride. The reason that we did not extend our investigation beyond

700 °C is that above this temperature no significant increase in brightness as compared with those

prepared at 700 °C was seen, and also that impurities started to become evident which would have

required a more specialized reaction system to eliminate than was available. Also of note are the

presence of impurity peaks in the 24-28° range which are due to YF3 contamination – this

disappeared at high temperatures – and at 39° which was due to NaF, and which again was only

evident at lower temperatures. Finally, the strong agreement of intensities between the measured and

reference peaks indicates that texturing is not present and this is further supported by the SEMs

presented above. Rietveld analysis using TOPAS diffraction software was used to fit the PXRD

patterns recorded for all samples produced in this study.

49

Figure 2.12. Phase compositions of NaYF4 (top), NaY0.98Er0.02F4 (middle) and NaY0.78Yb0.2Er0.02F4 (bottom)

prepared at different temperatures, fitted by Rietveld analysis of PXRD patterns.

0

20

40

60

80

100

200 300 400 500 600 700

% C

om

po

siti

on

Cubic

Hexagonal

0

20

40

60

80

100

200 300 400 500 600 700

% C

om

po

siti

on

0

20

40

60

80

100

200 300 400 500 600 700

% C

om

po

siti

on

Preparation Temperature /°C

50

Figure 2.12 shows the cubic/hexagonal phase composition of the various samples (the numbers do

not add up exactly to 100% due to the use of NaF, YF3 and YOF impurity phases in fitting some

patterns). As mentioned previously, at 700 °C there is evidence of an impurity phase which we

believe to be YOF. It was extremely difficult to fully assign and separate the fitting of these two

phases and therefore we caution that the reported phase abundances for cubic-NaYF4 at high

temperatures may be prone to higher errors than the lower temperature values. Nonetheless, a clear

trend is observed: hexagonal phase is strongly favored with increasing temperature until the

reappearance of the cubic phase begins at the upper end of the scale.

Figure 2.13 shows the variation in the sizes of the diffracting domains for the cubic and hexagonal

phases, determined by Rietveld refinement of PXRD data. Once again, caution should be used when

interpreting the values for the cubic phase at high temperatures as the low abundance and overlap

with possible YOF peaks make it difficult to fit with confidence – this is evidenced by the wide

range of values seen for the cubic phase in the high temperature regime. In particular the data

indicates a decrease in cubic domain size at 700 °C, which we believe is most likely due to poor

fitting because of the overlap between the cubic phase and YOF peaks. For the cubic phase the

constraints used on the fitting parameters were such that the maximum size was 450 nm, whereas

for the hexagonal phase the maximum size was 4.5 µm. The quantitative reliability of size analysis by

PXRD analysis at these large sizes is low, but they can be useful as a qualitative guide to the trends

in crystallite size. Despite these limitations, the data clearly demonstrate a significant increase in

average particle size with increasing temperature and that the typical hexagonal domain size is

approximately an order of magnitude larger than the cubic for a given temperature, which is

consistent with observations made on nanoparticle systems using syntheses which yield a mixture of

phases(24).

51

Figure 2.13. Cubic (left) and hexagonal (right) grain sizes measured by Rietveld refinement. Note that to

maintain a reasonable scale, the maximum cubic size fitted was 450 nm and the maximum hexagonal was

4500 nm.

In Figure 2.14 we show the variation of cubic and hexagonal lattice parameters with synthesis

temperature. Due to the difficulty in achieving a good fit for phases with low abundance, the cubic

phase is not shown above 475 °C. The data shows that the lattice parameters are largely unchanged

by the substitution of 2% of yttrium for erbium, and that they decrease when ytterbium is included

into the lattice, which is expected given the smaller size of the Yb3+ cation relative to Y3+ (0.858 vs

0.893 Å respectively). Of particular note is the trend in the hexagonal lattice parameters in which the

aspect ratio (c/a) of the unit cell appears to vary in an almost sinusoidal manner with the preparation

temperature – the cell appears to become wider at 325 °C before elongating up to ~600 °C and then

again relaxing to intermediate dimensions at 700 °C.

0

100

200

300

400

500

200 300 400 500 600 700

Ave

rage

Do

mai

n S

ize

/n

m


NaYF4

NaYF4:Er

NaYF4:Er,Yb

0

1000

2000

3000

4000

5000

200 300 400 500 600 700

Ave

rage

Do

mai

n S

ize

/n

m


52

Figure 2.14. Cubic a (top), hexagonal a (middle) and hexagonal c (bottom) parameters for NaYF4-based

materials prepared at different temperatures, fitted by Rietveld analysis. Due to poor fitting at high

temperatures due to its low abundance, the cubic phase is only fitted up to 475 °C.

5.49

5.50

5.51

5.52

5.53

200 300 400 500 600 700

Latt

ice

Par

ame

ter

/A

NaYF4

NaYF4:Er

NaYF4:Er,Yb

5.97

5.97

5.98

5.98

5.99

5.99

200 300 400 500 600 700

Latt

ice

Par

ame

ter

/A

3.51

3.52

3.53

200 300 400 500 600 700

Latt

ice

Par

ame

ter

/A


53

2.3.3 Spectroscopic Characterization

Now we turn our attention to the spectroscopic properties of the materials, and in particular to the

dependence of their luminescence intensities on their synthesis temperature. Figure 2.15 shows the

variation in total luminescence intensities (found by integrating the area underneath the

luminescence spectrum of each sample from 400-700 nm) across all samples. The y-axes on the two

graphs can be directly compared, and it is evident that, as expected, the Yb3+-containing samples are

more strongly luminescent than those based solely on Er3+, most likely due to the higher absorption

co-efficient of Yb3+(25) as well as to the higher concentration of absorbing species in this material. It

is also interesting to note that the luminescence intensity of the Yb3++Er3+ doped samples appears

to show a greater dependence on the processing temperature than do the pure Er3+ doped samples,

which tend to exhibit very low luminescence except at the highest temperatures when their

brightness increases extremely rapidly, in contrast to the Yb3++Er3+ doped materials which show a

more gradual rise with processing temperature. It may be possible to explain this behavior in terms

of the two competing modes of upconversion emission: energy transfer upconversion (ETU)

involving absorption by an Er3+ (in the case of the Er3+ only material) or Yb3+ (in the case of the co-

doped material) sensitizer and transfers to an Er3+ activator; and excited state absorption (ESA)

involving sequential absorption by Er3+ ions without energy transfer. In principle both of these

modes can occur in both of these materials, but it is generally accepted that ETU is the predominant

mode in the Yb3++Er3+ doped material. There are, however, reports of competition between ETU

and ESA in Er3+ doped materials(26), and it is possible that some improved quality of the lattice at

high temperatures is allowing more efficient energy transfer and leading to increased upconversion

emission by activating emission via ETU.

54

Figure 2.15. Integrated upconversion luminescence intensities of NaY0.98Er0.02F4 and NaY0.78Yb0.2Er0.02F4

prepared at different temperatures.

As previously mentioned, the luminescence of samples prepared at temperatures beyond 700 °C did

not increase appreciably beyond what is shown in this figure, and at higher temperatures there was

formation of significant amounts of other impurities (which can be seen to be emerging in Figure

2.2 at 700 °C). Although a decrease in brightness at high temperature following formation of a cubic

phase has been reported at highly elevated temperatures(4), this was not readily accessible with the

apparatus used in this study. Another point to note is the significant variation in the luminescence of

nominally identical samples, particularly the Er3+ doped samples synthesized at 700 °C whose

luminescence intensities span a very wide range (which was consistent on repeated analysis of the

samples in question) underlining the sensitivity of the luminescence of these materials and difficulty

in achieving precise control over it.

10

100

1000

10000

100000

200 300 400 500 600 700

log

(To

tal U

C In

ten

sity

/a.

u.)


NaYF4:Er,Yb

NaYF4:Er

55

In addition to studying the trends in luminescence, we were interested in probing for any potential

impurities which may have been present in our samples and which may lead to decreased

luminescence. For example, given that our reactions began with a carbon-containing precursor it was

important to investigate whether or not there was evidence of carbon contamination in the final

product as this could lead to quenching and contribute to the observed luminescence trend. Figure

2.16 shows FTIR spectra for NaY0.78Yb0.2Er0.02F4 samples prepared at 250, 400 and 700 °C.

Figure 2.16. FTIR Spectra of NaY0.78Yb0.2Er0.02F4 prepared at 250, 400 and 700 °C, as indicated.

It is evident from the figure that the sample prepared at 250 °C contains significant contamination,

in particular showing a series of peaks in the region of 850 cm-1 that are likely due to C-C bonds

from residual carbon following the decomposition of the trifluoroacetate precursors. There is also a

significant broad peak in the region of 3350 cm-1 which we ascribe to the presence of trapped HF

produced in the decomposition, and this peak overlaps a broader, less intense, peak which is present

in all three samples and which we believe is most likely due to the presence of OH groups, which

may either be bound to the surface or may isomorphously replace F in the lattice. Given the similar

intensity of this peak in all samples and the decreasing surface/volume ratio with temperature, we

0

10

20

30

40

50

60

70

350135023503350

Tran

smis

sio

n /

%

Wavenumber /cm-1

250 C

400 C

700 C

56

believe the latter explanation to be more reasonable. By 400 °C the supposed HF and C-C impurities

appear to have been removed and the FTIR spectra of the samples produced at 400 and 700 °C are

remarkably similar. Given this observation and the demonstrated trends in luminescence, along with

the fact that we observed no sign of carbon in the PXRD patterns, and no change in luminescence

upon prolonged treatment in air plasma, we conclude that product contamination due to

decomposition products from the precursor material is unlikely to contribute significantly to the

observed luminescence behaviour. Unfortunately we were not able to extend the range of our scan

beyond that shown due to instrumental limitations but it is interesting to note that the peak around

350 cm-1, which may in part be due to lattice vibration(12), but also likely contains a contribution

from the KBr used in forming the FTIR pellet, appears in the 700 °C sample to be significantly

weaker than that in the 250 or 400 °C samples. Although caution must be used when drawing

conclusions about the relative strength of scattering from FTIR spectra, particularly in this case

when there are two likely contributors, given that the baseline transmission in the samples are

comparable and the intensities of the 3350 cm-1 OH peaks in the 400 and 700 °C samples are also

comparable, this is suggestive of the possibility of weaker lattice phonon modes in the high

temperature samples. One potential explanation for decreased vibrational intensity is the presence of

defects which could block the propagation of lattice vibrations; however, such defects may be

expected to be annealed out at high temperatures, and their presence may lead to the formation of

trap states which could lead to luminescence quenching. For this reason we are cautious to suggest

this effect as the reason for the observed luminescence trend, but it is worthy of further

investigation to rule it out.

Now that we have presented the data gathered in this study, we will attempt to form some

hypotheses and draw conclusions about possible reasons for the observed spectroscopic properties

of the materials studied in terms of their crystallography. From comparing the crystallographic and

spectroscopic data at different temperatures we observe two interesting trends:

Firstly, the crystal phase itself does not appear to strongly influence luminescence, at least in the

temperature range studied. It has been reported that cubic phase produced at higher temperatures

shows decreased luminescence as compared with the hexagonal phase(4). However, we observe very

little change in luminescence on increasing the synthesis temperature from 250 to 400 °C, despite

57

the fact that this corresponds to a conversion of roughly 30% cubic phase to almost 100%

hexagonal phase. This result contrasts with many nanocrystals syntheses which explicitly seek to

yield the hexagonal phase for its perceived luminescence advantages. Furthermore, on increasing the

temperature to 700 °C we see a very large increase in luminescence, but relatively little change in the

crystal phase. This is strongly suggestive that control of the phase of the material alone is not

sufficient to guarantee efficient upconversion.

Secondly, the luminescence appears to increase strongly with increasing particle size; little increase in

the size of hexagonal crystallites produced between 250 to 400 °C is observed, and little increase in

luminescence is seen over this range, also. Above this temperature the size of hexagonal crystallites

increases dramatically, as does the luminescence. Although a similar trend is also observed for the

cubic phase, we are hesitant to draw any conclusions here due to its low abundance. It appears that

luminescence is strongly correlated to the size of the hexagonal crystallites present. Interestingly,

however, we have conducted experiments in which samples of the hexagonal phase prepared at 700

°C were ground down to a mean particle size (fitted by Rietveld refinement) of ~100 nm, without

any resulting decrease in luminescence intensity. It was even possible to create colloidal dispersions

of these particles for a period of hours. This suggests the possibility that there is some inherent

crystallographic property of the high temperature material as compared with the low temperature,

beyond merely particle size effects. This would also be expected as the work of Dexter(18) – which

describes the relevant multipole-multipole energy transfer processes in these materials predicts a

1/Rn dependence of the energy transfer probability, where R is the sensitizer-activator separation

and n depends on the orders of the multipoles involved in the transfer (6 for dipole-dipole, 8 for

dipole-quadrupole, 10 for quadrupole-quadrupole etc.) – predicts energy transfer efficiency to drop

very rapidly with increasing ionic separation, and thus it is unlikely that a large particle size would be

favored on these grounds.

2.4 Conclusions

In summary, we have demonstrated a new approach to synthesis of bulk NaYF4 and its derivatives

and used this to investigate the temperature dependence of the crystallographic and spectroscopic

58

properties of these materials. Perhaps most interestingly, we have demonstrated that upconversion

luminescence intensity displays extremely strong temperature dependence, varying by up to two

orders of magnitude for a given material. These findings should be of great interest to those

involved in the field, in particular to researchers working on upconverting nanoparticles which tend

to be made at comparatively low temperatures (300 °C or below) as our results suggest that through

appropriate treatment and novel high temperature synthetic routes it may be possible to yield

upconverting nanoparticles with intensities greatly exceeding the current state of the art. Although

we have sought to shed some light on the behavior of these materials and will be continuing these

investigations, as yet we are not able to offer a strong explanation for the observed trend in

luminescence in terms of the atomic properties of these materials. What is it, at other length scales,

which causes a sample synthesized at 400 °C to be so much less luminescent than one synthesized at

700 °C, for instance? We believe that this work will serve to inspire other researchers to pursue this

goal, and that understanding the underlying structural reasons for the behavior will lead to

significant practical advances and ultimately the viable application of upconversion to fields such as

photovoltaics and bioimaging.

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21. B. H. Toby, R factors in Rietveld analysis: How good is good enough? Powder Diffraction 21,

67 (Mar, 2006).

22. C. Weidenthaler, Pitfalls in the characterization of nanoporous and nanosized materials.

Nanoscale 3, 792 (2011, 2011).

23. M. D. Mathews, B. R. Ambekar, A. K. Tyagi, J. Kohler, High temperature X-ray diffraction

studies on sodium yttrium fluoride. Journal of Alloys and Compounds 377, 162 (Sep 8, 2004).

24. F. Wang et al., Simultaneous phase and size control of upconversion nanocrystals through

lanthanide doping. Nature 463, 1061 (Feb 25, 2010).

25. Z. Q. Li, Y. Zhang, S. Jiang, Multicolor Core/Shell-Structured Upconversion Fluorescent

Nanoparticles. Advanced Materials 20, 4765 (Dec, 2008).



(Feb 1, 2000).

61

3 A Comparison of Two Common Methods for

the Measurement of Absolute

Photoluminescence Quantum Yields Using

Integrating Spheres

The work described in this chapter has been submitted for publication as:

D. O. Faulkner, J. J. McDowell, A. J. Price, D. D. Perovic, N. P. Kherani, and G. A. Ozin,

Measurement of Absolute Photoluminescence Quantum Yields Using Integrating Spheres – Which

Way to Go?

Quantum yield – the efficiency of conversion of absorbed photons to emitted photons – is one of

the key measures of the utility of a luminescent material. Measuring this quantity for a sample which

is highly scattering or displays anisotropic emission, such as a powder or thin film, can be

challenging and commonly requires the use of an integrating sphere – a hollow diffusely reflective

sphere – to average out this anisotropy. There are two common methods (the so-called ‘two-

measurement method’ and ‘three-measurement method’) for conducting these experiments, but no

detailed comparison of the two has previously been presented. As part of our work we desired to

investigate the quantum yields of our upconverting materials, but were faced with two different

methods and no explanation about which would be more applicable. We therefore set out to

conduct a theoretical and experimental comparison of the two methods, and this is documented in

this chapter.

62

3.1 Introduction

The determination of photoluminescence quantum yield(1) (QY), that is, the number of photons

emitted per absorbed photon for a luminescent material, is a topic of significant interest to

researchers working on luminescent materials such as conjugated polymers(2), semiconductor

nanocrystals(3, 4), and upconversion phosphors(5). The ability to probe the efficiency of

luminescence from a particular material is an invaluable tool when choosing luminescent materials

for use in technologies such a biological imaging, dye lasers, and displays. Of particular interest,

especially for thin film and powder samples which often display significant emission anisotropy is

the determination of absolute quantum yield (AQY) using an integrating sphere as this allows the

determination of the emission quantum yield of any sample using a single, simple, set of

measurements. There are two protocols which are commonly employed to make AQY

measurements in this fashion – one requiring two separate measurements(6), and the other requiring

three(7) – but despite the importance of these measurements and the apparent differences between

these two methods, to the best of our knowledge no detailed study of the relative merits and pitfalls

of the two has been presented. Here we present a comprehensive examination and comparison of

these two methods, which we hope will assist other researchers in understanding the subtleties of

these measurements. Before we begin with a discussion of the theory behind the two methods and a

theoretical examination of the difference between them, we outline the difference between a number

of related concepts which may be referred to by terms such as ‘quantum yield’ and ‘quantum

efficiency’:

Quantum yield/photoluminescence quantum yield/fluorescence quantum yield: the probability of photon

emission from an excited state. It is important to realize that absorption does not (in principle at

least, although experimentally it may do) affect these values, because they are concerned with the

probability of emission after absorption has already occurred. It is possible to have a brightly

luminescent material with a low quantum yield, or, conversely, a material which displays very faint

luminescence despite having a very high quantum yield.

Absolute/relative quantum yield: as above, an absolute quantum yield is recorded without the need for a

reference sample, and is typically made using an integrating sphere. A relative quantum yield is

measured relative to a standard of known quantum yield – often and organic dye – and then scaled

63

accordingly. Absolute quantum yields have the advantage of being applicable to any sample, whereas

a relative measurement requires a suitable standard with similar spectroscopic properties to be valid.

Internal/external quantum efficiency (I/EQE): these terms are most commonly used when characterizing

the electronic properties of a solar cell and refer to the ability of the cell to extract energy from an

excited electron or from a photon, respectively. The EQE will always be less than the IQE in a

conventional cell because the conversion of photons to electrons will always occur with an efficiency

less than unity. There are, however, some technologies under development that aim to produce

more than one excited electron from each incident photon, and these may have EQE>IQE if

successful.

It is also important to note that the quantum efficiency will in general be different from the power

or energy conversion efficiency due to the (anti-)Stokes’ shifts typically encountered in these

processes.

3.2 Theoretical Discussion of AQY Measurement Methods

In all cases when making quantum yield measurements we are interested in measuring the number of

photons (or rather, as we work with ratios, in measuring some quantity directly proportional to said

number, knowing that the constants of proportionality will cancel) contained within a specific peak.

As a typical spectrometer measures a quantity proportional to the incident power, it is necessary to

multiply the measured intensity at a given wavelength by that wavelength in order to arrive at a value

proportional to the number of photons (for example, a properly calibrated spectrometer should

measure the same signal if exposed to 1 mW/s of light at either 300 nm or 600 nm, but it will take

twice as many 600 nm photons as 300 nm photons to deliver that power).

3.2.1 The Two-Measurement Method

In the two-measurement method (2MM), one records a spectrum with the excitation beam incident

inside the empty sphere which acts as a ‘baseline’ and another with the sample within the sphere. In

each case the spectrum is multiplied at each value by the wavelength as described above and then

64

the peaks are integrated to yield values proportional to the number of photons in the excitation

peak, L, and photoluminescence peak, P. If we use the subscript a for the empty sphere

measurement, and the subscript b for the measurement with the sample then the expected areas of

each peak in terms of the initial excitation La are approximately:

(3.1-3.4)

where:

f is the fraction of photons in the incident beam which directly hit the sample,

A is the fraction of these photons which are absorbed by the sample,

μ is the fraction of photons which do not directly hit the sample but which are eventually absorbed

by it after one or more scattering events, and

η is the photoluminescence quantum yield.

We can then examine the meaning of the terms in the expressions for Lb and Pb:

In Lb, the first term, f(1-A)La, represents photons which hit the sample on their initial pass into the

sphere, but which are not absorbed by it (they may be scattered, reflected, or transmitted). The

second term, (1-f)(1-μ)La, represents photons which do not initially hit the sample, and are not

absorbed by it at a later time (note the assumption throughout that the only change to the potential

photon loss mechanisms is the introduction of the sample, it is assumed that a photon which is not

absorbed by the sample has the same probability of ultimately being detected in both

measurements).

65

In Pb, the first term represents photons which are emitted after an excitation photon has hit the

sample and been absorbed by it on its initial entry into the sphere, before any scattering events. The

second term accounts for photons emitted due to absorption of a photon that initially did not hit

the sample but was absorbed at a later time. Note the implicit assumption that η is identical in both

of these processes, which essentially amounts to assuming that η is independent of the irradiance,

which is a strong assumption for single-photon processes but less accurate for multi-photon

processes such as second harmonic generation and excited state absorption whose efficiencies are

likely to be highly dependent on the intensity of light incident on the sample.

The simplicity of this method comes at the cost of neglecting a contribution to the quantum yield:

photons which initially hit the sample, are not absorbed at that time (they are reflected, scattered, or

transmitted) but are absorbed at a later time; and also of simplifying the expression for Lb by

omitting a factor (1- μ) in the term f(1-A)La which should read f(1-A)(1- μ)La to account for the

possibility of photons which are initially reflected/scattered/transmitted by the same and then being

absorbed at a later time. These two simplifications essentially amount to assuming that the photons

which are directly incident on the sample have their ultimate fate – detection, absorption, or loss by

other means – determined by their first interaction with the sample.

Combining all of these areas, multiplying out, and cancelling, we find that the absolute quantum

yield is given by:

(3.5)

Of note is that in this case, the result is independent of the value of f. This may provide an

experimental simplification, particularly if a spatially broad excitation source or very small sample is

used.

66

3.2.2 The Three-Measurement Method

In contrast, the three measurement method (3MM) takes account of all possible contributions to the

emission and excitation peaks, but we find that in this method it is – in principle at least, although

we shall show later that in practice the requirement is not as strict as it might seem – necessary that

we are able to conduct a measurement with f=1, that is, with all photons being directly incident

upon the sample before scattering. In reality, this is likely not possible as many samples are likely to

be contained, for example in a cuvette, which will naturally reflect some fraction of the incident

light, and cause f to be reduced. We present the derivation with a general f parameter and then show

that it reduces to an easily manageable form in the case f=1.

In the 3MM, the measurements (a, b, and c, respectively) made are:

The excitation source incident inside the empty sphere.

The sample inside the sphere, but not directly in the path of the excitation beam.

The sample inside the sphere and directly in the beam path.

(Note that measurement a along with either b or c would be sufficient for the 2MM)

In each case the expected areas under the peaks are (where the meanings of the symbols are the

same as defined in the 2MM section):

(3.6-3.11)

67

Note that we assume that the second experiment can be controlled such that f=0, if this is not the

case then we must use terms fb and fc to denote the different fractions of photons which are directly

incident on the sample. It should, however, be straightforward to ensure that essentially no light is

directly incident on the sample in the second experiment so this case is not discussed here but is, in

theory at least, a simple extension; the difficulty arises when trying to ensure that every photon is

directly incident on the sample in the third experiment.

Again, we can consider the meaning of all terms in these equations. The single term in Lb, (1-μ)La

accounts for photons which are not initially incident on the sample (recall that none are in this

experiment) and are not subsequently absorbed by the sample. In Lc, the term f(1-A)(1-μ)La

represents photons which are initially incident on the sample, and are not absorbed by it upon

incidence or at a later time (the possibility of absorption at a later time was neglected in the 2MM as

explained above), and the term (1-f)(1-μ)La accounts of photons which are not initially incident on

the sample and are not absorbed at a later time. In Pb, ημLa represents emission due to absorption of

photons which are initially scattered by the walls of the sphere and are later absorbed. In Pc, the term

fAηLa accounts for emission due to photons which are initially incident on the sample and absorbed

by it at that time; (1-f)μηLa is emission due to photons which are not initially incident on the sample,

but which are absorbed by it at a later time; and f(1-A)μηLa accounts for emission due to photons

which are initially incident on the sample, not absorbed by it at that time, but are absorbed at a later

time (this contribution is neglected in the 2MM).

Combining all of these terms we find that the AQY is given by:

(3.12)

where the absorption co-efficient, A is given by:

(3.13)

68

In the special case, f=1, as was used in the original derivation by de Mello & Friend(7), these

equations reduce to:

(3.14)

and:

(3.15)

Although this method is in principle more accurate, it suffers from the problem that it is necessary

to know the value of f, which, especially given the non-uniform distribution of light beams, is likely

to be extremely difficult to measure or calculate with high accuracy in a typical lab. Using a laser or

other focused light source to bring the excitation beam to a single spot on the sample and also to

avoid any direct excitation in the second experiment is likely to provide a reasonable approximation

to the ideal f=1 case, although to achieve high accuracy, losses due to, for instance, reflection from a

cuvette, should be accounted for.

3.3 Results

In this section we compare AQY values found via these two methods for four different samples –

100 µM and 1 µM solutions of rhodamine 6G (R6G) in ethanol, a solid powder of erbium doped

NaYF4, and a thin film of poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) –

and demonstrate that despite their theoretical differences the methods give remarkably similar

results and are both relatively insensitive to changes in the experimental geometry. It is important to

note though that for many materials, particularly those of current interest, there are no widely

accepted values of the quantum yield, and it is not possible to say for certain which method gives a

result that is closer to the true value. Rather, by understanding the principles underlying each

69

method and their respective practical considerations, we hope that researchers will be able to make

the appropriate choice of method for their purpose.

3.3.1 Experimental

Rhodamine 6G was purchased from Sigma and diluted with absolute ethanol. Erbium doped NaYF4

was synthesized by thermal decomposition of erbium, sodium, and yttrium trifluoroacetate salts at

725 °C in a tube furnace in air. MEH-PPV was purchased from American Dye Source and spin-

coated onto a glass substrate from a solution in toluene to form a film. Polyfluorene was produced

by Jeffrey McDowell, a PhD student in the Ozin group, according to a synthesis developed in-house

which will be reported elsewhere, and spin coated onto a glass substrate from a toluene solution to

form a film. The optical set up used in the experiments is shown schematically in Figure 3.1. The

samples were excited with a 365 nm Thorlabs M365L2-C2 LED equipped with a collimating lens,

and a 10 cm focal length, 2 inch diameter, plano-convex lens was used to focus the beam onto the

sample in the ‘focused beam’ experiments. The ‘collimated beam’ experiments were performed

without this extra lens. Samples were placed into a custom-made Teflon integrating sphere from

Gigahertz Optik equipped with a moveable sample holder to allow for both direct and indirect

excitation and interchangeable sample holders to allow analysis of samples on glass substrates and in

cuvettes. Light from within the sphere was collected with a 1 mm diameter optical fibre from Ocean

Optics, and coupled into a Maya 2000 USB spectrometer from Ocean Optics. The system was

calibrated using a calibrated quartz tungsten halogen source from Oriel. Signals were analyzed and

quantum yields calculated with LabVIEW software which was produced in-house, and which is

discussed in Appendix B.

70

Figure 3.1. 1 Collimated 365 nm light from an LED is focused by a 10 cm focal length 2 inch diameter

plano-convex lens (some light is incident on the outside of the sphere and does not enter it, and is not shown

in the diagram) into 2 an integrating sphere with a rotating sample holder containing a sample for study

(sample location indicated by the dotted line). Outputted light is fibre-coupled into 3 a CCD grating

spectrometer.

3.3.2 Experimental Results

The four samples had their quantum yields measured in two different experimental geometries: with

the incident beam tightly focused onto the sample (an approximation to the f=1 case assumed for

the direct measurement in the 3MM) and without the focusing lens, with the light from the

collimated LED merely aimed at the sample but not focused onto it resulting in a reduced value of f

(in both cases the assumption in the indirect experiment, that very little light is directly incident on

1

23

71

the sample, should be satisfied). Due to the practical difficulty of doing so, we were unable to

quantify this reduction exactly. The results are summarized in Table 3.1.

Table 3.1. Quantum yield data for four different samples recorded under two experimental geometries using

the two experimental methods.

R6G 100 µM

(EtOH solution)

R6G 1 µM

(EtOH solution)

NaYF4:Er

(solid powder)

MEH-PPV

(film on glass)

Focused

beam

3MM

2MM

97.3

97.2

12.4

15.9

1.1

1.3

15.7

15.9

Collimated

beam

3MM

2MM

97.7

97.9

13.6

18.5

1.2

1.4

14.0

14.1

It is clear that despite the theoretical differences between the two methods there is relatively little

experimental difference. Although the dilute rhodamine solution and the powder samples do show

relatively large variations, the luminescence of these samples was very low, and significant

background noise was present, making an accurate determination of QY unlikely. This is clearly

underlined by the recorded values for the R6G 1 µM solution, which are significantly below the

accepted literature values of 95%(8), and which the two methods both reproduce well for the more

concentrated solution. This observation underlines to need to approach quantum yield

measurements in general with a degree of caution, as a poor signal-to-noise ratio can have a

significant effect on the recorded value, often far more so than the choice of experimental protocol.

This is further complicated by the fact that very few materials have well studied and well accepted

values of their fluorescence quantum yield, but perhaps the best way to try to ensure accuracy is to

calibrate against a known standard such as rhodamine 6G, using the minimum concentration

necessary (in order to minimize potential re-absorption of emission, leading to a decrease in the

measured quantum yield) to get the experimental system in use to yield a value close to the accepted

72

one, and then to prepare the sample of interest in such a way that its luminescence intensity is

comparable to that of the chosen standard. We do not recommend the practice of simply scaling

recorded yields according to the measured value for a known standard, especially if they display

significantly different luminescence intensities or peak positions.

From the data presented here it seems that both methods are equally applicable to the determination

of absolute quantum yields. Although the 3MM can be fooled if one grossly violates the f=1

assumption, this is not a situation which is likely to arise in a realistic setting, indeed despite this

assumption the 3MM was found in practice to be relatively insensitive to beam positioning, within a

reasonable range. Nonetheless, further experiments showed that the 2MM yielded consistent results

even in cases approaching f=0 – provided this still allowed for sufficient luminescence signal – as

expected from the theoretical outline provided above. Another interesting possibility of the 2MM,

which so far as we are aware has not previously been reported, is the measurement of time-

dependent quantum yields via an automated system for recording luminescence spectra at periodic

intervals. Although this would in principle also be possible for the 3MM, the need to reposition the

sample to record both direct and indirect spectra for each measurement would render it significantly

more challenging. In order to demonstrate this technique with the 2MM, we monitored the time

dependence of the fluorescence quantum yield of a polyfluorene film under 365 nm excitation under

both air and nitrogen, as shown in Figure 3.2.

73

Figure 3.2. Time-dependent variation in absolute quantum yield for a film of polyfluorene under 100

mW, 365 nm LED excitation in atmospheres of nitrogen and air.

The figure clearly shows the decrease in quantum yield which occurs very rapidly when the polymer

is exposed to UV in the presence of air, and also illustrates its high stability to UV under nitrogen.

The small oscillations in AQY from value to value are artifacts of the measurement system, and the

difference between the y-intercepts of the two curves is likely due to rapid decay of quantum yield

on initial exposure to UV in air, before the first measurement was taken. Time-dependent quantum

yield is an excellent way to monitor changes in an emitting material, especially as the quantum yield

of a sample is an intrinsic quantity of the material in question, in contrast to other potential probes

such as luminescence peak intensity which is an extrinsic parameter and so potentially less reflective

of changes occurring on a per-emitter basis.

60

100

0 50 100 150 200 250 300

Ab

solu

te Q

uan

tum

Yie

ld /

%

Time /s

Nitrogen Exposed

Air Exposed

74

3.4 Conclusions

In conclusion, we have elucidated the theoretical and practical difference between two common

methods of determining absolute quantum yields using integrating spheres. We have shown that

despite these differences the values yielded by the two methods tend to be in excellent agreement,

and we hope that this will serve to reassure and educate any researchers who may be unsure about

which method is appropriate for their use. We do not, of course, discount the possibility that there

may be certain specialized applications in which the difference between the two methods may be

manifest, but for any ‘standard’ conditions we conclude that there is unlikely to be a significant

difference between the two. We have also demonstrated the interesting possibility of using the 2MM

to conduct time-dependent absolute quantum yield measurements, and we expect that this will be of

interest to scientists working, for instance, on next generation lighting and solar cell technologies,

fields in which knowledge of the photo-degradation of materials is critical.

3.5 References

1. J. N. Demas, G. A. Crosby, Measurement of photoluminescence quantum yields – review.

Journal of Physical Chemistry 75, 991 (1971).

2. N. C. Greenham et al., Measurement of absolute photoluminescence quantum efficiencies in

conjugated polymers. Chemical Physics Letters 241, 89 (Jul 14, 1995).

3. A. M. Munro, I. Jen-La Plante, M. S. Ng, D. S. Ginger, Quantitative study of the effects of

surface ligand concentration on CdSe nanocrystal photoluminescence. Journal of Physical Chemistry C

111, 6220 (May, 2007).

4. Y. Ebenstein, T. Mokari, U. Banin, Fluorescence quantum yield of CdSe/ZnS nanocrystals

investigated by correlated atomic-force and single-particle fluorescence microscopy. Applied Physics

Letters 80, 4033 (May 27, 2002).



75

6. L. Mangolini, D. Jurbergs, E. Rogojina, U. Kortshagen, Plasma synthesis and liquid-phase

surface passivation of brightly luminescent Si nanocrystals. Journal of Luminescence 121, 327 (Dec,

2006).

7. J. C. deMello, H. F. Wittmann, R. H. Friend, An improved experimental determination of

external photoluminescence quantum efficiency. Advanced Materials 9, 230 (Feb, 1997).

8. R. F. Kubin, A. N. Fletcher, Fluorescence quantum yields of some rhodamine dyes. Journal of

Luminescence 27, 455 (1982, 1982).

76

4 Control of Absolute Quantum Yields in

NaYF4:Er,Yb Upconverters - Temperature and

Power Dependence

The work described in this chapter has been published as:

D. O. Faulkner, S. Petrov, D. D. Perovic, N. P. Kherani, G. A. Ozin, Absolute quantum yields in

NaYF4:Er,Yb upconverters – synthesis temperature and power dependence. Journal of Materials

Chemistry (2012). DOI: 10.1039/C2JM33457G

We conclude our characterization of the upconverting materials studied herein with an investigation

of the emission quantum yields of these materials, including the influence of excitation power, and

synthesis temperature. Following the discussion in the previous chapter, we elect to use the three

measurement method, although both would lead to the same conclusions. To give theoretical

grounding to this discussion we discuss a simple model of absolute quantum yields for multiphoton

processes. We show that the absolute quantum yields may vary by almost three orders of magnitude,

depending on the synthetic conditions. In keeping with the results of chapter 2, we also demonstrate

that formation of the hexagonal phase of this material is not in itself sufficient to guarantee high

upconversion efficiency.

4.1 Introduction

Upconverters(1, 2), which allow the conversion of near infrared to visible light via non-simultaneous

multiphoton absorption, have attracted much attention in recent years for their potential

applications to solar power(3, 4), biological imaging(5-7), and display technologies(8, 9). Although

77

the luminescence properties of these materials has been well studied(10), very little characterization

of their emission quantum yields(11-13), a critical measure when considering likely practical

applications, has been reported. Notable previous work in the field includes studies by Page(14) and

Boyer(15); whereas Page et al. presented a very in depth and specific model which is only applicable

to a particular material and must be adapted for all applications, we discuss our results in the context

of a simpler and more general model which should be extensible to many systems without

modification. In particular their work did not discuss the -1 form of the power dependence which

is a central focus of the model that we use. Boyer et al.’s work, whilst setting a precedent for the

investigation of upconversion quantum yields using an integrating sphere, concentrates on a

significantly different system (nanocrystals vs. bulk materials and size vs. temperature dependence)

to the one investigated in this work, and furthermore did not investigate the power dependence of

the quantum yield. We begin by discussing a simple theoretical treatment of the quantum yields of

multiphoton processes and an investigation of the dependence of the quantum yields of

NaYF4:Er,Yb upconverters on excitation power and synthetic temperature.

Emission quantum yield (η) is defined by:

(4.1)

Assuming (as should be valid in the case of relatively low incident powers when saturation is unlikely

to occur) the number of photons absorbed by a material is linearly proportional to the number of

photons incident upon it, and hence to the irradiance of the sample, we have:

(4.2)

As discussed in ref. (16) there are a number of regimes in which multiphoton processes may occur,

and most generally we expect a dependence of the form:

(4.3)

78

where the exponent β depends on the number of photons involved in the process, n, as well as the

efficiency of upconversion, the mechanisms of upconversion and decay, and various other

environmental factors. Combining the preceding equations it can be seen that in general:

(4.4)

However, in the simplest case of upconversion being a minor pathway relative to decay to a lower

lying state, which applies in our case despite the relatively very high upconversion yields

demonstrated, and has been shown to apply(10) for NaYF4:Er,Yb in cases similar to that described

herein, the emission intensity resulting from a n-photon process is proportional to the nth power of

the irradiance, i.e. β=n. Accordingly, for a n-photon process we have:

(4.5)

(this situation is analogous to the treatment of chemical reactions of different orders by classical

kinetics, except in this case the ‘reactants’ and ‘products’ are low and high energy photons,

respectively).

And hence:

(4.6)

We therefore expect that in contrast to the constant quantum yield observed for single-photon

processes, for an n-photon upconversion process in NaYF4:Er,Yb (and any other material or process

in which this power dependence applies) the quantum yield will depend on the (n-1)th power of the

irradiance (incident power).

79

4.2 Experimental

The setup used for the AQY measurements is shown schematically in Figure 4.1. A movable holder

and fibre-coupled integrating sphere (IS) were used for holding the sample and a fibre-coupled laser

was used for excitation. The output from the IS was fibre-coupled into an optical system which

splits the beam (BS) sending part of the signal into an Oriel ¼ m 74100 monochromator and Oriel

77341 photomultiplier tube for analysis of the upconversion emission and part through a neutral

density filter (OD 1.5) (to prevent saturation of the detector) and into a fibre-coupled Ocean Optics

Maya 2000 USB spectrometer, for analysis of the laser signal. The response of the system was

calibrated in the range 300-1100 nm using an Oriel 63358 quartz tungsten halogen spectral

irradiance standard. For the power-dependent study, the laser power was measured with a Spectra

Physics 407A TC thermopile detector. The only spectrum measured with the PMT for each

measurement was that with the sample directly in the beam, as no upconversion emission was

detectable for any indirectly excited sample. The system was controlled and the data collected and

processed with LabVIEW software that was created in-house, and which is discussed in Appendix

B.

80

Figure 4.1. 1 Light from a fibre-coupled 972 nm diode laser is focused into 2 an integrating sphere with a

rotating sample holder (sample is held inside the centre of the sphere and its location is indicated by the

dotted line) allowing for direct and indirect excitation. Light output from the sphere is coupled to a 1 mm

diameter optical fibre coupled to 3 a collimating lens held in a clamp. The collimated output is split at 4 a

beamsplitter with one beam being attenuated by an OD 1.5 neutral density filter, focused into a 200 micron

optical fibre by a 2.5 cm focal length 1 inch diameter biconvex lens, and coupled into 5 a CCD grating

spectrometer. The other beam passes through 6 a filter wheel and is focused by a 5 cm focal length 1 inch

diameter biconvex lens into 7 a monochromator coupled to a PMT.

1

2 3

4

5

6

7

81

NaYF4:Er,Yb upconverters were synthesized via thermal decomposition of 1.5 g of a mixed metal

trifluoroacetate (TFA) precursor with an Na:Y:Yb:Er ratio of 1:0.78:0.2:0.02, based on a common

nanoparticle preparation(17, 18). The decompositions were performed in a quartz boat and tube in a

tube furnace in ambient air, and the formation of oxides was not observed, although some small

amount of impurity yttrium oxyfluorite phase may exist at 700 °C; this and other crystallographic

properties of the samples was discussed in chapter 2. Samples were heated at a rate of 300 °C per

hour, held at the final synthesis temperature for 2 hours and then cooled in air. The TFA precursor

was produced by dissolving yttrium, ytterbium, and erbium oxides and sodium TFA in excess

aqueous trifluoroacetic acid (all purchased from Acros) under reflux. After dissolution, excess water

and trifluoroacetic acid were evaporated off and the precursor collected as a white solid.

Approximately 40 g of precursor was produced and this same batch was used for all samples in the

study. Before use the precursor was ground to a powder to maximize its homogeneity.

82


Figure 4.2 shows PXRD patterns for samples prepared at 75 °C increments from 250 to 700 °C,

which were collected with a Siemens D5000 diffractometer using Cu Kα radiation.

Figure 4.2. PXRD patterns for samples of bulk NaYF4:Er,Yb prepared at different temperatures, as

indicated. Reference patterns for both cubic (Fm3m) and hexagonal (P63/m) phases are also shown.

It can be seen that the samples all contain a mixture of both cubic and hexagonal phases. Each

synthesis was completed in triplicate, analyzed by PXRD, and the relative abundances of the cubic

and hexagonal phases in each were fitted using TOPAS software and are shown in Figure 4.3.

83

Figure 4.3. Phase composition of NaYF4:Er,Yb as a function of synthesis temperature, fitted by Rietveld

refinement. Each synthesis was repeated three times and the results of all fittings are shown.

It is evident that all samples consist primarily of the hexagonal phase. This is an extremely important

observation, as it is common wisdom that the hexagonal phase is the preferred phase for

upconversion, but our results will clearly show that the reality is far more complicated than this, and

that formation of the hexagonal phase does not in itself guarantee efficient luminescence. The re-

emergence of the cubic phase is becoming apparent at 700 °C as is expected from the known high-

temperature phase behavior of this material(19).

A typical upconversion emission spectrum for NaYF4:Er,Yb is shown in Figure 4.4, and the peaks

are labeled with the term symbols of the states that lead to these emissions.

84

Figure 4.4. Typical NaYF4:Er,Yb emission spectrum under 972 nm excitation, showing the main transitions

giving rise to visible upconversion emission. The peak heights are proportional to the number of photons

contributing to each wavelength, and are produced by multiplying the measured intensity (which is

proportional to power) by the wavelength, as is required for the measurement of absolute quantum yields.

The red and green emission peaks, appearing from roughly 640-680 and 510-565 nm, respectively,

are studied in this work. The violet emission around 400-420 nm is often extremely weak and we

were unable to collect high quality spectra for it from many of our samples, so we excluded it from

the present study.

Absolute emission quantum yields, recorded using an integrating sphere according to the method by

de Mello et al.(11) using samples in powder form, are summarized for both the red and green

emissions in Figure 4.5. The use of an integrating sphere allows the characterization of the quantum

yields of inhomogenous samples such as powders and thin films, and also minimizes the effect of

potential differences in sample geometry that may arise due to, for example, different crystal size

85

distributions leading to one sample displaying greater scattering than another. As was discussed in

the previous chapter the use of the two or three measurement methods was not expected to yield a

significant difference, and it was found that they both yielded similar values when used to examine

the quantum yields of upconverters. The three measurement method was chosen for the study

although both would have led to identical conclusions.

Figure 4.5. Red and green emission absolute quantum yields of samples of NaYF4:Er,Yb prepared at

different temperatures and recorded under 380 mW of 972 nm laser excitation. The inset shows the region

from 250-475 °C in greater detail.

It is evident that the quantum yields of these materials display extremely strong temperature

dependence, with almost three orders of magnitude separating the samples at either extreme. These results

underline the necessity of high-temperature processing when trying to achieve upconverters with

high luminescence efficiency. One of the key limiting factors of upconverting nanoparticles

preventing their deployment into a range of applications is their limited brightness.

86

It is worth underlining that from a phase perspective all of the samples are very similar –

predominantly hexagonal phase. This highlights the fact that simple phase control is not sufficient to

achieve high upconversion efficiency and that a more detailed understanding of the material

properties is required. We are currently actively investigating potential causes of the difference in

luminescence intensity displayed by samples prepared under different conditions. One simple

explanation which was discussed in more depth in chapter 2 is any effect of possible carbon

contamination from the precursor and isomorphous replacement of hydroxide for fluoride in the

lattice. Figure 4.6 shows FTIR spectra of samples prepared at 250, 400 and 700 °C, and

demonstrates that whilst the 250 °C sample does contain some of these kinds of impurities, the

appearance of the 400 and 700 °C samples is nearly identical in FTIR, despite the luminescence

properties of the 250 and 400 °C samples being almost identical and the 700 °C being orders of

magnitude brighter. In addition, the absence of any apparent amorphous background – which may

indicate the presence of carbon – in the PXRD, and the fact that no change in luminescence was

seen after treatment in air plasma for 90 minutes to remove potential residual carbon contaminants,

indicates that this is unlikely to be responsible for the observed trend in luminescence. Possible

explanations which we are investigating include a difference in defect structure or surface structure

and morphology.

Figure 4.6. FTIR spectra of NaYF4:Er,Yb prepared at 250, 400 and 700 °C, as indicated.

0

10

20

30

40

50

60

70

350135023503350

Tran

smis

sio

n /

%

Wavenumber /cm-1

250 C

400 C

700 C

87

In addition to probing the dependence of the quantum yield on synthesis temperature, we also

investigated its dependence on the excitation power, and the results of this study are shown in

Figure 4.7.

Figure 4.7. Incident power dependence of the absolute quantum yields of red and green emissions for a

sample of NaYF4:Er,Yb prepared at 700 °C.

The red and green upconversion emission processes for this material under 972 nm excitation are

primarily two-photon emissions(10), and so from the equation for the power dependence of the

quantum yield of an n-photon process derived above we would expect to see a linear relationship

between emission quantum yield and excitation power. We can see from Figure 4.7 that this

behavior is satisfied very well for both emissions; in particular extrapolation to zero incident power

yields y-intercept values below a quantum yield of 1%, which is consistent with the expected

observation of a zero intercept. Although we would ideally seek to extend the range of incident

powers investigated to cover a wider range, due to the relatively poor efficiency of integrating

y = 0.02x + 0.27R² = 0.97

y = 0.01x + 0.72R² = 0.95

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300 350

Ab

solu

te Q

uan

tum

Yie

ld /

%

Incident Power /mW

88

spheres at collecting emitted light this was not possible; nonetheless the evidence we have been able

to collect appears to experimentally validate the model we have presented, and we conclude that a

linear relationship between absolute quantum yield and excitation power is present as expected. The

strong dependence of emission quantum yield on excitation power observed here further underlines

the necessity of high excitation power density when working with these materials, and this is a key

reason why upconverter-coupled solar cells have not yet realized their promise.

4.4 Conclusions

In conclusion, we have demonstrated that the upconversion quantum yield of NaYF4:Er,Yb displays

a significant dependence on the synthesis temperature, and that this cannot be explained simply in

terms of the crystal phase of the material. We believe that this is likely to be of interest to the many

researchers working to realize practical applications of this material. Additionally, we have

investigated the AQYs of two-photon red and green emission processes from NaYF4:Er,Yb and

have discussed this in terms of a simple and widely applicable model. This result is likely to be of

interest both to those studying the practical applications of upconverters and to those interested

more broadly in multiphoton processes. The power dependence of the quantum yield demonstrated

herein would likely not just manifest itself in upconversion processes but also in multiphoton

photochemical processes and may be of interest to those studying processes such as sunlight-driven

water-splitting and solar fuel generation.

4.5 References


104, 139 (Jan, 2004).

2. F. Wang, X. G. Liu, Recent advances in the chemistry of lanthanide-doped upconversion

nanocrystals. Chemical Society Reviews 38, 976 (2009).

89





2010).

5. D. K. Chatteriee, A. J. Rufalhah, Y. Zhang, Upconversion fluorescence imaging of cells and

small animals using lanthanide doped nanocrystals. Biomaterials 29, 937 (Mar, 2008).

6. S. F. Lim et al., In vivo and scanning electron microscopy imaging of upconverting

nanophosphors in Caenorhabditis elegans. Nano Letters 6, 169 (Feb, 2006).

7. S. Sivakumar, P. R. Diamente, F. C. J. M. van Veggel, Silica-coated Ln3+-doped LaF3

nanoparticles as robust down- and upconverting biolabels. Chemistry-a European Journal 12, 5878 (Jul

24, 2006).

8. G. S. Maciel, A. Biswas, R. Kapoor, P. N. Prasad, Blue cooperative upconversion in Yb3+-

doped multicomponent sol-gel-processed silica glass for three-dimensional display. Applied Physics

Letters 76, 1978 (Apr, 2000).

9. S. Q. Xu, H. P. Ma, D. W. Fang, Z. X. Zhang, Z. H. Jiang, Tm3+/Er3+/Yb3+-codoped

oxyhalide tellurite glasses as materials for three-dimensional display. Materials Letters 59, 3066 (Oct,

2005).



11. J. C. deMello, H. F. Wittmann, R. H. Friend, An improved experimental determination of

external photoluminescence quantum efficiency. Advanced Materials 9, 230 (Feb, 1997).

12. J. N. Demas, G. A. Crosby, Measurement of photoluminescence quantum yields – review.

Journal of Physical Chemistry 75, 991 (1971).

13. C. Wuerth, M. Grabolle, J. Pauli, M. Spieles, U. Resch-Genger, Comparison of Methods and

Achievable Uncertainties for the Relative and Absolute Measurement of Photoluminescence

Quantum Yields. Analytical Chemistry 83, 3431 (May 1, 2011).

90

14. R. H. Page et al., Upconversion-pumped luminescence efficiency of rare-earth-doped hosts

sensitized with trivalent ytterbium. Journal of the Optical Society of America B-Optical Physics 15, 996 (Mar,

1998).





(Feb 1, 2000).

17. J. C. Boyer, F. Vetrone, L. A. Cuccia, J. A. Capobianco, Synthesis of colloidal upconverting

NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide

trifluoroacetate precursors. Journal of the American Chemical Society 128, 7444 (Jun, 2006).

18. J.-C. Boyer, L. A. Cuccia, J. A. Capobianco, Synthesis of colloidal upconverting NaYF4:

Er3+/Yb3+ and Tm3+/Yb3+ monodisperse nanocrystals. Nano Letters 7, 847 (Mar, 2007).

19. A. Grzechnik, P. Bouvier, W. A. Crichton, L. Farina, J. Kohler, Metastable NaYF4 fluorite at

high pressures and high temperatures. Solid State Sciences 4, 895 (Jul, 2002).

91

5 Upconversion Amplified Photoconductivity in

an Amorphous Silicon Film - Roadmap of

Discovery to Practicality

The work described in this chapter has been drafted for publication as:

D. O. Faulkner, P. Mahtani, D. D. Perovic, G. A. Ozin, N. P. Kherani, Upconversion Amplified

Photoconductivity in an Amorphous Silicon Film - Roadmap of Discovery to Practicality

In the previous chapters we have discussed the synthesis and characterization of upconverters at a

range of temperatures. In this chapter we investigate the practical applications of these materials by

using them to increase the photoconductance of an amorphous silicon film under 972 nm excitation.

In contrast to other studies which have typically used solar cells as the model system and only a

single upconverter, our use of a variety of samples and the fact that we directly study the

photoconductance enhancement allows us to gain a deeper understanding of the influence of the

upconverter on the potential for photovoltaic power generation. Finally, we present simple modeling

of a solar cell coupled to an upconverter with various spectral properties and lay out a roadmap for

future improvements in the field.

5.1 Introduction

The development of higher efficiency photovoltaic devices which can utilize a larger portion of the

solar spectrum is of great importance to modern society. A number of routes to this are under

investigation, including photon upconversion(1-3), intermediate-band solar cells(4, 5), and multi-

junction systems(6, 7). In this chapter we investigate the application of upconversion to amorphous

92

silicon (a-Si) photovoltaics. At the simplest level, the application of upconverters to increased solar

cell efficiency is concerned with increasing the photocurrent which flows when the cell is exposed to

light due to the absorption of upconverted photons resulting in excitation of a greater number of

electrons than would have been excited had the upconverter been absent. In order to demonstrate

this principle we investigate the photoconductance enhancement displayed by a thin film of

hydrogenated intrinsic amorphous silicon under continuous 972 nm laser excitation in the presence

and absence of an NaYF4:Er,Yb(8) upconverter. We demonstrate that the film, of band gap

approximately 650 nm which does not in itself absorb 972 nm light, shows a significant increase in

photoconductance under excitation by this wavelength in the presence of an upconverter, a clear

proof of concept of the potential for upconverters to improve the efficiency of a-Si photovoltaics.

We investigate the dependence of the enhancement on the luminescence intensity of the

upconverter, and demonstrate a strong correlation. Despite this progress, and the progress of other

groups working in the area, significant advances are still required before the application of

upconversion to photovoltaics becomes commercially viable; in the latter part of this chapter we

present a simple model for enhanced efficiency upconversion that allows us to suggest routes of

investigation that may be beneficial to achieving this goal of commercially viable upconversion-

enhanced photovoltaics.

5.2 Experimental

NaYF4:Er,Yb was used as an upconversion phosphor due to its potential to display bright

upconversion, and was produced via thermal decomposition of a mixed metal trifluoroacetate

precursor. A mixture of yttrium, erbium and ytterbium oxides and sodium trifluoroacetate (Acros)

containing a metal ion ratio of 1:0.78:0.2:0.02 Na:Y:Yb:Er was weighed out and dissolved in ~50%

aqueous trifluoroacetic acid (Acros) under reflux. Excess water and trifluoroacetic acid were

evaporated to yield a solid precipitate of metal trifluoroacetates; the precursor. The final products

were formed by placing 1.5 g of precursor into a quartz boat, placing the boat into a quartz tube and

heating under ambient air to the desired temperature. A heating rate of 300 °C/hr was used, and

samples were held at the final treatment temperature for two hours, after which the furnace was

automatically switched off and the sample cooled in air. A 300 nm thick intrinsic hydrogenated

93

amorphous silicon (a-Si) film was grown using Radio Frequency Plasma Enhanced Chemical Vapor

Deposition (RF PECVD) on a (Corning 1737 glass) substrate at a temperature of 200 oC using

silane gas (SiH4) maintained at a pressure of 175 mTorr with a flow rate of 30 sccm, and aluminum

contacts were attached by electron beam evaporation. The deposition of the a-Si film and Al

contacts were performed by Pratish Mahtani, a PhD student in the Kherani group. Electrical

contacts for conductivity measurements were made by evaporating co-planar Al contacts, 5 mm × 2

mm each and separated by a 0.7 mm gap; subsequently, copper conductors were electrically

connected to the Al contacts using silver paste, which in turn were connected to a Keithley 2400

sourcemeter for conductance measurements. The contact resistance between the a-Si film and Al

contacts was measured to be 5×10-5 Ω cm-2 for other samples deposited under identical conditions.

972 nm laser light was incident upon the region between the contacts and thick film samples of

NaYF4:Er,Yb prepared at different temperatures were placed behind this region, shown

schematically in Figure 5.1, so that upconverted light incident upon it would give rise to the

excitation of electrons to the conduction band and a concomitant increase in conductance. These

NaYF4:Er,Yb samples were approximately 0.5 mm thick and were prepared by securing a sample of

upconverting powder to a glass substrate using colorless adhesive tape.

94

Figure 5.1. Schematic of the experimental design used in this investigation.

Conductance was measured by scanning the voltage between the contacts from 0 to 10 V and

finding the gradient according to:

(5.1)

where V is the applied voltage, I is the measured current and G is the conductance. Due to

significant noise that was commonly present at lower applied voltages, only the range of 2 to 10 V is

shown and used for deriving the results presented.

Direction ofIncident Light

Glass Substrates

a-Si Film

Al Contacts

Copper Tape

Upconverter

AdhesiveTape

Silver Paste

95


5.3.1 Characterization of the a-Si Film in the Absence of

Upconverters

Prior to integrating the a-Si film with upconverting phosphors it was important to understand the

optical properties of the film itself. Therefore, the film was characterized by measuring its

absorption spectrum, dark conductivity, and photoconductance in the presence of simulated AM 1.5

light.

Figure 5.2. Attenuation spectrum of the a-Si film used in these experiments.

Figure 5.2 shows the absorption spectrum of the a-Si film used in these experiments, recorded

using a Perkin Elmer Lambda 900 UV/vis/NIR spectrometer. The spectrum has not been corrected

to account for the properties of the substrate and interfacial reflections, but nonetheless the onset of

absorption due to promotion of electrons across the band gap can be seen by extrapolation of the

absorption peak down to its intercept on the abscissa to be at approximately 650 nm. Smaller

oscillations in the spectrum at higher wavelengths are most likely due to optical interference effects,

0

1

2

3

400 600 800 1000

Att

en

uat

ion

(-

log 1

0(t

ran

smis

sio

n))

Wavelength /nm

96

although there may also be some trap states present within the band gap which may give rise to

small absorptions. Around 972 nm, the wavelength of interest, the absorption of the film can be

seen to be low, but the film displays significant absorption in the green region of the spectrum

overlapping the green emission of NaYF4:Er,Yb, and also appreciable absorption in the red (note

that absorption is plotted on a logarithmic scale). This indicates that a-Si is a strong candidate for

pairing with NaYF4:Er,Yb in upconversion-enhanced solar cells.

Figure 5.3. Dark I-V characteristics of the a-Si film used in this study.

Figure 5.3 shows the measured I-V behavior of the a-Si film in the dark. It is clear that there is

significant noise, and the low dark conductance (~4x10-12 S) is close to the limits of the instrument

used for the measurement. Nonetheless, the data clearly show that the intrinsic conductivity of the

film is low, as expected.

0

2E-11

4E-11

6E-11

8E-11

1E-10

1.2E-10

1.4E-10

2 4 6 8 10

Cu

rre

nt

/A

Applied Voltage /V

97

Figure 5.4. AM 1.5 I-V characteristics of the a-Si film used in this study.

In Figure 5.4 we can see that the conductance is greatly enhanced by almost four orders of

magnitude to 1.6x10-8 S by exposure to AM 1.5 light from a solar simulator, which is to be expected

given that a significant proportion of this light falls within the sub-650 nm range necessary to excite

electrons across the band gap in the amorphous silicon film.

Now that we have introduced the properties of the a-Si film when not coupled to an upconverter,

we will turn our attention to its behavior in the presence of NaYF4:Er,Yb upconverters prepared at

various temperatures (i.e. of various luminescence efficiencies as described in previous chapters).

0.0E+00

2.0E-08

4.0E-08

6.0E-08

8.0E-08

1.0E-07

1.2E-07

1.4E-07

1.6E-07

2 4 6 8 10

Cu

rre

nt

/A

Applied Voltage /V

98

5.3.2 Characterization of the a-Si film in the Presence of

Upconverters

Figure 5.5 shows measured current-voltage (I-V) characteristics for the film of a-Si when coupled

to samples of NaYF4:Er,Yb prepared at various temperatures and exposed to 972 nm laser light. An

incident power of 380 mW was used, of which approximately 50 mW was ultimately incident upon

the upconverter – the remainder being lost to reflection or absorption by the a-Si film or metal

contacts. A significant increase in gradient of the I-V curves, corresponding to an increase in

conductance due to the generation of charge carriers via the absorption of upconverted photons can

be seen when the film is paired with an upconverter fabricated at high temperatures, consistent with

the brighter luminescence displayed by these samples – the brightest sample used in this study,

prepared at 700 °C, displays approximately 60x greater luminescence intensity than the weakest.

Two blank samples were used, one prepared at 250 °C and one prepared at 700 °C. The two

different blanks were used because the samples prepared at lower temperature are darker in colour

than those prepared at high temperature and so are expected to be less efficient scatterers. By

plotting the conductances derived from this data, as well as the total luminescence signals measured

for each sample used in the study, also in Figure 5.5, we can clearly see a very close correlation

between the emitted upconversion intensity and the response on the a-Si film. Overall an increase in

the photoconductance by a factor of five is achieved, from 1 nS in the presence of a non-

upconverting or weakly upconverting sample to 5 nS in the presence of a brightly upconverting

sample. It is interesting to note that the minimum observed photoconductance of ~1 nS is

significantly higher than our measured dark conductance of 4 pS; we hypothesize that this is due to

the excitation of electrons to trap states within the band gap that may allow for some absorption of

972 nm light, or to thermal carrier generation via heating of the film and metal contacts due to

absorption of the laser light. Relative to the dark conductance, the maximum enhancement in

conductance on exposure to 972 nm laser light was approximately 1000 times.

99

Figure 5.5. Top: I-V characteristics for a-Si film coupled with samples of doped and undoped NaYF4

prepared at various temperatures, as indicated, excited at 972 nm. The suffix ‘b’ is used to indicate a blank

(undoped, i.e. non-upconverting) sample. Bottom: Photoconductance of the a-Si film coupled with samples

of doped and undoped NaYF4 prepared at various temperatures, and of various luminescence intensities, as

indicated. The units of luminescence are arbitrary.

0.E+00

1.E-08

2.E-08

3.E-08

4.E-08

5.E-08

2 4 6 8 10

Cu

rre

nt /

A

Voltage /V

700

625

550

475250700 b250 b325400

100

This data further demonstrates that NaYF4:Er,Yb is spectrally well-matched for coupling with a-Si

solar cells. We have therefore demonstrated that NaYF4:Er,Yb can be successfully used to generate a

photoresponse from a-Si, via the increase in excited charge carriers occurring due to the absorption

of upconverted light. In contrast to other published studies(9), we have investigated how the

synthesis conditions of the upconverter affect its potential for application to photovoltaic power

generation, and we believe that this new insight will be of interest to those working in the field.

5.3.3 Estimation of the Photo-Generated Carrier Density due to

Absorption of Upconverted Photons

When considering the utility of upconversion to enhancing the photoresponse of a-Si and,

ultimately, its use in enhanced efficiency solar cells it is useful to consider how the presence of the

upconverters affects the charge carrier density, as this helps to give an understanding of the effect

that is occurring on a microscopic scale. We may calculate this using the following equation:

(5.2)

where:

σ = conductivity

n= charge carrier density

e = electron charge

µ = carrier mobility

As mobility measurements on amorphous silicon are very difficult to carry out, and we have not

been able to conduct these experiments at the time of writing, we will use a value of 1 cm2 V-1 s-1,

which is representative of a typical electron mobility(10) for an a-Si film, measured using a field-

effect transistor configuration. Hole mobilities are usually around an order of magnitude lower, and

we will only consider electrons here. Starting with this value, we can estimate the resulting carrier

density as shown below in Table 5.1. Note that measured conductances have been converted to

conductivities using the dimensions of the illuminated portion of the a-Si film as 0.7 mm contact-

contact separation x 3 mm illuminated length x 300 nm film thickness.

101

Table 5.1. Charge carrier densities estimated electronically from measured conductances, using an estimated

mobility value of 1 cm2 V-1 s-1.

Sample Measured

Conductance /S

Calculated

Conductivity /S m-1

Charge carrier

density /cm-3

Ratio to dark

Characteristics

Dark 4.E-12 3.1E-09 2.0E+11 1

AM1.5 1.6E-08 1.2E-05 7.8E+14 4.0E+03

250 blank 8.8E-10 6.8E-07 4.3E+13 2.2E+02

700 blank 8.8E-10 6.8E-07 4.3E+13 2.2E+02

250 1.0E-09 7.8E-07 4.9E+13 2.5E+02

325 8.8E-10 6.8E-07 4.3E+13 2.2E+02

400 8.1E-10 6.3E-07 3.9E+13 2.0E+02

475 1.1E-09 8.3E-07 5.2E+13 2.7E+02

550 1.6E-09 1.2E-06 7.6E+13 3.9E+02

625 2.5E-09 1.9E-06 1.2E+14 6.1E+02

700 4.6E-09 3.5E-06 2.2E+14 1.1E+03

5.3.4 Modeling the Relationship between the Absorption Spectrum

of an Upconverter and its Potential Use in Solar Photovoltaic

Power Generation

Although progress has been made in the application of upconversion to solar power, there is still

significant progress that must be made before the application of upconversion to solar power is

realistic. We will now demonstrate simple modeling of the potential for upconversion enhancement

of solar power using materials with different spectral properties. We will model the upconverter as

102

having uniform absorption over a given range and as being coupled to a solar cell such that all

upconverted photons are incident on the cell, with each one delivering energy equal to the band gap

of the cell. We will also assume that upconversion occurs exclusively via a two photon mechanism.

We will consider the effects of changing the following parameters:

1. The spectral width of the absorption of the upconverter

2. The strength of the absorption

3. The central wavelength of the absorption

4. The photon quantum yield of the upconverter

We will choose a baseline system from which to start, which is designed to be approximately

representative of a real system comprising NaYF4:Er,Yb coupled to an a-Si solar cell and we will

demonstrate the predicted effects of varying the parameters of this baseline system, which will be:

1. An absorption width of 50 nm

2. An absorption of 40% of light incident within this window

3. A central wavelength of 980 nm

4. A photon quantum yield of 0.002%

5. A band gap of 700 nm

Our baseline absorption and quantum yield values are taken from measurements discussed in

chapter 4; we observe a maximum quantum yield for our brightest sample of ~20%, at an excitation

density of approximately 500 kW m-2, the solar spectrum contains 63 W m-2 in the 955-1005 nm

region which we are taking as our baseline, or about four orders of magnitude less intensity than was

present in our AQY experiment. We therefore expect, in accordance with a linear dependence of

two photon upconversion AQY on excitation power which was demonstrated in chapter 4, the

AQY to decrease by approximately this factor from ~20% to ~0.002%. The estimated AQY will be

scaled according to the number of photons in the absorption region throughout this modeling

process, again in accordance to the power dependence of the upconversion quantum yield. For the

baseline system we find that the maximum extra power that we can expect to be generated by the

cell is 7x10-4 W m-2. The solar power incident upon a 1 m2 region at peak insolation is 1 kW, with a

typical solar cell of ~10% efficiency therefore producing around 100 W m-2. Although any gain is

beneficial in theory, from an economic perspective both in terms of the costs in time and money, it

is impractical to implement systems which do not promise a significant improvement in power

103

generation, and we will assume that this practical cut-off occurs for an enhancement of 10 W m-2.

We will therefore adjust the parameters of our hypothetical upconverting material with a goal of

reaching or exceeding this value in order to present an outline of possible routes to making the

application of upconversion to high efficiency solar cells a realistic prospect.

Much of the work on upconverters relies on the use of Yb3+ sensitizers to absorb radiation, which

limits the region of light which can be upconverted to that around ~980 nm. Although we are

limited in our choices when it comes to using metal ions as sensitizers and activators, there is

progress being made elsewhere by groups working on developing organic molecules(11,12) and

inorganic complexes(13,14) that display upconversion properties, and it may be possible to adjust

the spectral properties of these upconverters to match certain design parameters. In Figure 5.6 we

consider the maximum potential photovoltaic power generation due to upconversion that our

hypothetical system could generate if the centre of the absorption were to be moved and all other

parameters kept equal. As we have chosen to set the band gap of the cell at 700 nm, we do not

consider the possibility of moving the absorption into the region below this value, as photons of

shorter wavelength than 700 nm can already be absorbed by the cell material. Also as we are only

considering a two photon process we will not consider the utilization of photons of greater than

1400 nm in wavelength, as this is the cut-off value for a two photon process to produce light of

wavelength 700 nm or less.

Figure 5.6. Left: The maximum power that could be generated by the solar cell as a function of the centre

wavelength of the absorption of the upconverter, all other parameters remaining equal. Right: The maximum

power that could be generated by the solar cell as a function of the width of the absorption of the

upconverter, all other parameters remaining equal.

104

The figure shows that the current wavelength of 980 nm is in a reasonable region for upconversion,

with an estimated increased power generation of 7x10-4 W m-2. There are however some other areas

that have significantly greater potential; the optimum wavelengths are ~730 nm and ~790 nm, both

of which would be expected to yield 1.4 mW m-2. A slight red-shift of the peak to around 1010 nm

would also be expected to boost the upconversion power to 9.6x10-4 W m-2. Although there is some

room for optimization of the position of the absorption of the upconverter, the achievable gain via

this route alone is still approximately four orders of magnitude less than the 10 W m-2 cut-off which

we have set as a limit of practical viability, and therefore simply shifting the absorption of the

upconverter will do little to meet the overall challenge. We will now consider the potential gain from

broadening the absorption of the upconverter, maintaining a central wavelength of 980 nm, also

shown in Figure 5.6.

Broadening of the absorption of the upconverter leads to an enhancement in the maximum power

that can be generated, up to 93 mW m-2 at an absorption width of 560 nm, corresponding to

upconversion of photons in the interval [700 nm, 1260 nm] which is chosen as the maximum for the

study to prevent overlap with the supra-band gap region shorter than 700 nm. Although this

represents a gain of approximately two orders over the baseline system, it is still two orders below

our goal of 10 W m-2. In order to gain this extra required power we must increase our quantum

efficiency which could be achieved in two ways:

1. Increase the ‘baseline’ AQY by developing new materials with higher quantum yields.

2. Use light concentration (e.g. solar concentration with a lens or

plasmonic(15,16)/photonic(17) enhancement of the local field in the vicinity of the

upconverters) to boost the excitation intensity incident on the upconverters, taking

advantage of the fact that their quantum yields scale with the intensity of light incident upon

them. This topic was reviewed in chapter 1.

Concentrator solar cells can reach light concentration of a factor of 100 or even 1000, which if

integrated with upconversion solar cells should lead to a quantum yield enhancement of a similar

amount, potentially leading to a practically viable photovoltaic upconversion system. This, of course,

is contingent on the development of new materials possibly through the use of organic molecules or

inorganic complexes or the development of new optimized rare earth-based systems using multiple

105

sensitizers, or a tandem upconverter system using different materials to upconverter different

spectral regions, analogous to the operation of a multi-junction solar cell.

Overall, we propose that an upconversion system viable for application to photovoltaic power

generation might possess the following characteristics:

A central absorption wavelength of 980 nm

An absorption width of 560 nm, possibly achieved by the development of new organic

upconverting materials which may display significantly broader absorptions than the peaks

present due to f-f transitions in lanthanide ions (although these materials will likely suffer

from a decrease in long-term stability relative to materials of the kind studied herein) the use

of multiple sensitizers, or the development of tandem upconverting layers with different

materials upconverting different spectral regions

A cell band gap of 700 nm

An absorption of 40% across the material’s active range

A quantum yield of ~2%, achievable via a solar concentration factor of 100 for this system, based

upon scaling the quantum yield of 0.002% estimated for a current realistic material with central

wavelength of 980 nm and absorption width of 50 nm (on going from a width of 50 nm to 560 nm

we approximately increase the intensity of usable photons by a factor of 10, which also leads to an

AQY enhancement by a factor of 10 because of the linear dependence of AQY on excitation

photon density which is then increased by another factor of 100 by solar concentration).

It should be noted, however, that this factor of 10 enhancement in AQY expected from the broader

absorption would not occur in a tandem upconverter made of several layers with narrow

absorptions as they would still individually only ‘see’ a narrow spectral range, so for this system

another factor of 10 would be necessary to achieve a realistic system. Nonetheless, our baseline

AQY value of 0.002% leaves significant room for improvement and it does not seem unrealistic that

a factor of 10 improvement upon it is possible with careful development of new upconverting

materials.

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5.4 Conclusions

In conclusion, we have demonstrated the application of NaYF4:Er,Yb to power generation by

amorphous silicon by showing a strong correlation between upconversion luminescence and

photoconductance, indicating that these two materials are spectrally well matched. In addition,

conscious that despite this progress there is still some distance to cover before upconversion-

enhanced photovoltaics (UCPV) are practically viable we have also developed a simple model of the

potential upconversion enhancement of a hypothetical solar cell and used this to outline some

potential routes that may be investigated with this goal in mind. At this time, practical realization of

UCPV is still some way off, but we expect that significant progress will be made in the future and

hope that our work can to some extent provide guidance to these efforts.

5.5 References

1. P. Gibart, F. Auzel, J. C. Guillaume, K. Zahraman, Below band-gap IR response of

substrate-free GaAs solar cells using two-photon up-conversion. Japanese Journal of Applied Physics Part

1-Regular Papers Short Notes & Review Papers 35, 4401 (Aug, 1996).



3. V. Badescu, A. M. Badescu, Improved model for solar cells with up-conversion of low-

energy photons. Renewable Energy 34, 1538 (Jun, 2009).

4. A. Luque, A. Marti, Increasing the efficiency of ideal solar cells by photon induced

transitions at intermediate levels. Physical Review Letters 78, 5014 (Jun, 1997).

5. A. Marti et al., Production of photocurrent due to intermediate-to-conduction-band

transitions: A demonstration of a key operating principle of the intermediate-band solar cell. Physical

Review Letters 97, (Dec 15, 2006).

6. R. R. King et al., 40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells.

Applied Physics Letters 90, (Apr 30, 2007).

107

7. W. Guter et al., Current-matched triple-junction solar cell reaching 41.1% conversion

efficiency under concentrated sunlight. Applied Physics Letters 94, (Jun 1, 2009).

8. N. Menyuk, J. W. Pierce, K. Dwight, NaYF4-YB,ER – Efficient upconversion phosphor.

Applied Physics Letters 21, 159 (1972, 1972).



2010).

10. L. Han, P. Mandlik, K. H. Cherenack, S. Wagner, Amorphous silicon thin-film transistors

with field-effect mobilities of 2 cm2/V s for electrons and 0.1 cm2/V s for holes. Applied Physics

Letters 94, (Apr 20, 2009).

11. A. Monguzzi, R. Tubino, F. Meinardi, Upconversion-induced delayed fluorescence in

multicomponent organic systems: Role of Dexter energy transfer. Physical Review B 77, (Apr, 2008).

12. A. Monguzzi, J. Mezyk, F. Scotognella, R. Tubino, F. Meinardi, Upconversion-induced

fluorescence in multicomponent systems: Steady-state excitation power threshold. Physical Review B

78, (Nov, 2008).

13. S. K. Sugunan, U. Tripathy, S. M. K. Brunet, M. F. Paige, R. P. Steer, Mechanisms of Low-

Power Noncoherent Photon Upconversion in Metalloporphyrin-Organic Blue Emitter Systems in

Solution. Journal of Physical Chemistry A 113, 8548 (Jul 30, 2009).

14. K. L. Wong, W. M. Kwok, W. T. Wong, D. L. Phillips, K. W. Cheah, Green and red three-

photon upconversion from polymeric lanthanide(III) complexes. Angewandte Chemie-International

Edition 43, 4659 (2004, 2004).

15. S. Schietinger, T. Aichele, H.-Q. Wang, T. Nann, O. Benson, Plasmon-Enhanced

Upconversion in Single NaYF4:Yb3+/Er3+ Codoped Nanocrystals. Nano Letters 10, 134 (Jan, 2010).

16. Z. Q. Li et al., Core/shell structured NaYF4:Yb3+/Er3+/Gd3+ nanorods with Au

nanoparticles or shells for flexible amorphous silicon solar cells. Nanotechnology 23, (Jan 20, 2012).

108

17. F. Zhang, Y. Deng, Y. Shi, R. Zhang, D. Zhao, Photoluminescence modification in

upconversion rare-earth fluoride nanocrystal array constructed photonic crystals. Journal of Materials

Chemistry 20, 3895 (2010, 2010).

109

6 Conclusions & Future Work

6.1 Conclusions

In the work presented in this thesis, we have successfully developed the synthesis of NaYF4-based

upconverters at various temperatures using a method which was previously unreported in the solid

state. After characterizing the materials produced via this route using powder X-ray diffraction,

luminescence spectroscopy, and absolute quantum yield studies, we have given a proof-of-concept

demonstration for the application of this material to amorphous silicon photovoltaics. In addition to

this, we have developed a simple model for the application of upconversion to solar power and used

this to produce a roadmap for future improvement in the field. Key findings and advances presented

herein include:

The observation of a temperature-dependent trend in luminescence and quantum yield for

NaYF4:Er,Yb upconverters spanning 2-3 orders of magnitude.

An in depth explanation and comparison of the two common methods of recording

quantum yields using integrating spheres.

The demonstration of the spectral compatibility of NaYF4:Er,Yb and amorphous silicon,

and in particular the demonstration of the strong correlation between the photoconductance

of the a-Si film and the brightness of the upconverter.

The development of a roadmap for future improvements in the field, outlining a practical

system which we believe is likely to be experimentally achievable using a combination of

optical and materials technologies.

6.2 Future Work

Despite these advances there is still significant progress to be made before UCPV, as well as

significantly more understanding of upconverters from a materials science perspective which is

needed before we can truly say that we understand the system. Although we have mentioned some

of these potential areas of investigation in previous chapters, we now summarize and briefly discuss

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what we believe are some of the key questions to be investigated and tasks to be achieved in this

field in the future:

There is very little understanding of the connection between the atomic structure of these

materials and their luminescence properties, in particular the influence of defects. The vast

majority of the understanding of upconverters has come via spectroscopic studies, and there

is relatively little understanding of their material properties beyond a simple understanding of

their phase behavior. Although we have sought to extend this somewhat by conducting an

investigation of both of these aspects together and attempting to gain some insight and form

some hypotheses about the relationship between NaYF4:Er,Yb’s crystallographic and

spectroscopic behavior, we have not been able to provide a satisfactory answer about what it

is, from a materials perspective, that determines the upconversion efficiency. Future

investigations of our materials at a microscopic level using high resolution microscopy and

electron diffraction techniques along with theoretical simulations may shed some light on

why it is that we see such a profound connection between the synthesis temperature and the

upconversion efficiency. One possible explanation which should be explored is that defects

such as stacking faults and dislocations may occur at low temperatures and these may

interfere with energy transfer and/or act a recombination centers. At higher temperatures

these may be annealed out, leading to more efficient luminescence. Interestingly, the

presence of the impurity phases which starts to become evident around 700 °C does not

seem to have a significant effect on the luminescence, although we hypothesize that due to

the relatively short range over which energy transfer occurs, these impurity phases, which are

presumably discrete and isolated from the luminescent phases, are unable to exert a

significant influence, in contrast to defects within the luminescent phases themselves.

An improvement in the design of upconverting materials themselves is needed to open the

door to many of their potential applications. As we discussed in chapter 5, maximizing the

amount of light that the upconverter can utilize is of critical importance, particularly because

of the dependence of the quantum efficiency on the excitation photon concentration. The

design of new tandem systems (although these would not be expected to display this

concomitant AQY increase) or of systems able to make use of more complex energy transfer

pathways to capture the maximum amount of light are potential routes. A combination of

rare earth emitters and specifically engineered organic sensitizing molecules is one interesting

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possibility which may be used to yield an upconverter with the desired spectral properties.

Unfortunately, however, despite the amount of research which has gone into the area, there

has been relatively little progress made in recent times, and NaYF4:Er,Yb which was

discovered 40 years ago, is still widely believed to be the best upconversion material known.

It seems likely that, like so many of the greatest discoveries in science, it will take some

fortunate accident to illuminate whatever new research direction will ultimately lead to the

realization of this goal.

A cost-benefit analysis needs to be conducted to truly understand the potential impact of

upconversion and at what cost and efficiency gain it would become realistic. In order to

calculate this accurately a number of variables must be well understood:

The cost of manufacturing an upconverting photovoltaic device including the raw

materials cost of the rare earth components; the cost of processing these into an

upconverter including initial investments in equipment and ongoing running costs;

the cost of incorporating this material into a solar cell.

The realistic benefit that may be achieved taking into account factors such as non-

ideality of the upconverter and cell, as well as the effects of changes in weather such

as temperature and light exposure.

Overall, there is still much to be done before the full potential of these materials will be realized, but

we hope that our work will provide some help and guidance to those who continue to focus on this

problem in the future.

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Appendix A: Proof-of-Concept Upconversion

Display Device

A.1 Introduction

The development of new and novel display technologies is an increasingly competitive area, with

many companies and research groups working on perfecting and bringing to market technologies

such as OLED and laser TV. Although traditional CRT displays have largely been supplanted by

these thinner and lighter technologies, they still retain niche appeal in a number of sectors, in part

because of the excellent colour purity achievable via the use of metal, and in particular rare earth,

emitters, which tend to display sharp and narrow emission lines. In this work we present a proof of

concept demonstration for a full colour display based on upconversion emission from a 1:20

mixture of NaYF4:Er,Yb and NaYF4:Er,Tm excited by 972 nm laser light. The use of scanning lasers

as light sources in display technologies is an area of active commercial interest, with Mitsubishi

having launched the first laser television for the consumer market in 2008. Of particular note is the

thin form factor achieved by this display, comparable to that of modern LCD and plasma

technologies. In contrast to all other proposed upconversion display technologies that we are aware

of, which make use of multiple phosphors, our design utilizes only a single upconversion material

which may be simply deposited in a single layer by solution processing, and uses only a single laser

wavelength. Whereas other designs have used pixilated arrays of different upconverting materials or

different laser wavelengths and intensities to modulate the observed colour of upconversion

emission, our design uses a simple array of coloured filters to selectively pass the desired

wavelengths, the processing of which could easily be achieved in practice via inkjet printing, a far

less costly approach than the photolithographic patterning of upconversion layers or the use of

multiple separate laser scanning systems proposed by other designs.

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A.2 Results

Table A.1 summarizes the advantages and disadvantages of a number of display technologies, and

emphasizes that the design presented herein is competitive in a number of areas.

Table A.1. Advantages and disadvantages of various display technologies.

Advantages Disadvantages

Proposed

UC

Display

High colour purity – use of filters

allows better control over this

Single upconverter – solution

processable, no photolithography

required

Single laser wavelength – simpler

electronics required

Relative safety – NIR filters can be used

to prevent exposure of viewer to laser

radiation

Relative insensitivity of organics to NIR

light should allow use of organic inks

and polymer substrates

Laser-scanning system relatively

expensive

Relatively low efficiency; maximum

external quantum yield through filter

likely ~1% based on known

upconversion quantum yields and

observed attenuation by filters

Reported

UC

Displays

Lack of filter allows for higher external

quantum yields

Multiple UC materials and/or lasers

necessitates expensive fabrication

procedure and/or electronics

Lack of filter means emission colour

entirely determined by spectroscopy of

UC material which can be difficult to

control precisely

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OLED High efficiency

Very thin displays

Wide viewing angle

Can be used for flexible displays

Excellent colour quality

Expensive

Relatively short lifetime of organics,

particularly blue emitters which can lead

to colour degradation with time

Prone to damage by UV light

May suffer screen burn-in

Large displays unavailable

Plasma Very high contrast ratios

Wide viewing angle

Fast response time

High power consumption

Relatively large gaps between pixels are

visible

Early models prone to screen burn-in

Pixel size limitations prevents

manufacture of small screens

Fairly heavy

LCD Thin and light

Low power consumption

Can be made in very large sizes

Narrow viewing angle

Slow response time – prone to motion

blur

CRT Excellent response time

High contrast ratio and excellent

colours

Wide viewing angle

Very large, heavy displays

Commonly contain toxic elements such

as lead and cadmium

High power consumption

Screen burn-in possible

Impractical to make very large or very

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small displays

Laser TV Excellent colour

Low power consumption

Thin

Expensive

Possible safety risk from use of laser

beams

Our proof-of-concept device, produced by applying photographic gel filters to a thick film of the

upconversion composite described above, is shown in Figure A.1.

Figure A.1. Red, green, and blue emission from the upconversion film selected by use of an appropriate

colour filter.

Evident in the figure and further demonstrated in the spectra shown in Figure A.2 is the high

quality colour achievable via this method (the broad peaks appearing above 600 nm in the spectra

are artifacts of the measurement system and should be ignored). In particular the green and red

‘pixels’ exhibit very pure emission, with some green and red still present in emission from the blue

pixel, although this should be possible to correct for with the use of filter inks or coatings

specifically engineered for this application. Also evident in the photographs is the presence of some

diffuse green emission resulting from scattering of the emission within the upconverting layer; again

it should be possible to largely correct for this with either careful filter selection or the use of some

other optical technology such as refractive index matching to minimize scattering from within the

UC layer.

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Figure A.2. Upconversion emission spectra for the upconversion composite used in this study; a: unfiltered

emission; b: emission via red filter; c: emission via green filter; d: emission via blue filter. The instrument

sensitivity used for recording spectra b, c, and d was 10x higher than that for a, and the y-axis values for the

spectra are not quantitatively comparable. The apparent broad peak appearing beyond 600 nm in spectra c

and d is a consequence of the relatively low response of the system in this range resulting in increased error

when the calibration is applied, and should be neglected.

A.3 Conclusions

In conclusion, we have demonstrated a proof of concept device for the development of a high

colour-purity, full colour, upconversion display utilizing a single upconverting material and laser

wavelength. The display itself may be simply fabricated onto glass or plastic via a combination of

inkjet printing and spin/dip coating or roll-to-roll processing, and the use of a single laser

wavelength provides a significant simplification relative to other proposed and commercially

available laser-based displays. We believe that this work will be of significant interest to the many

parties involved in the development of next-generation display technologies.

400 500 600 700

Wavelength /nm

a

400 500 600 700

Wavelength /nm

b

400 500 600 700

Wavelength /nm

c

400 500 600 700

Wavelength /nm

d

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Appendix B: Spectroscopic Term Symbols

When considering the spectroscopy of a material it is important to understand and be able to easily

communicate about the electron energy levels that it possesses. Term symbols have been developed

as a simple and concise way to convey information about the properties of different electronic

energy levels within a material. There are different conventions used for describing the energy levels

of molecules and extended systems, and for describing those of atoms and ions. Because the energy

levels of metal ions in solids are very similar to those of the free ions (as opposed to in an organic

molecule where orbital hybridization leads to a significant change in the energy levels) the term

symbols used are the same as those used for a free ion, and hence these are the symbols explained

herein.

A term symbol takes the form:

(2S+1)LJ

(A.1)

where:

S is the total electron spin, found by summing the ms values for all electrons in partially occupied

shells (the total spin of a fully occupied or totally unoccupied shell is zero)

L is the total orbital angular momentum, found by summing the ml values for all electrons in

partially occupied shells (the total orbital angular momentum of a fully occupied or totally

unoccupied shell is zero). The values of L are labeled in the same fashion that the s, p, d, f,… atomic

orbitals are labeled according to their angular momentum, except with capital letters so for L=0 the

label is S, for L=1, P; etc.

J is a term which approximates the total angular momentum by combining the spin and orbital

terms. We say that it approximates the total because spin-orbital coupling may occur, which

complicates determination of the total angular momentum. The possible values of J are given by

L+S, L+S-1, …, |L-S| with the relative energies of each of these possible configurations being

118

determined according to Hund’s rules, which give a convenient way to determine the term symbol

of the ground (lowest energy) state of a species:

1. The ground state will have the highest value of S. This is because by maximizing the spin we

place electrons into as many different orbitals as possible, which reduces the extent to which

they screen out the electron-nucleus attraction.

2. For terms having the same S value, the one with the highest L will be lowest in energy, as

this tends to maximize the spatial separation between electrons.

3. If a shell is less than half full, the state with the lowest J lies lowest in energy, if it is more

than half full, the state with the highest J has the lowest energy.

As an example, consider the Er3+ ion with configuration [Xe]4f11.

↑ ↑ ↑ ↑↓ ↑↓ ↑↓ ↑↓

ml -3 -2 -1 0 1 2 3

This diagram shows the lowest energy state, as can be seen by application of Hund’s rules. The

values of S, L and J are 3/2, 6 and 15/2, respectively, and the term symbol is:4I15/2.

119

Appendix C: LabVIEW VIs Created for the Work

Undertaken in this Thesis

In order to conduct the work described in this thesis it was necessary to build a number of

experiments from scratch. Indeed, all of the upconversion and quantum yield studies presented were

conducted using experimental set ups that were designed and built in-house. A key component of

these experiments was the development of LabVIEW software to allow them to be easily controlled.

In this section we present an overview of three pieces of LabVIEW software which were created

during the course of the work presented herein:

1. Software to control our monochromator and record a spectrum or quantum yield using a

PMT or other monochromator-coupled detector.

2. Software to record a quantum yield using an Ocean Optic Maya fibre-coupled CCD

spectrometer

3. Software used for splicing together spectra from the PMT and the Maya and applying a joint

calibration to both, useful for either studying a wavelength range which is out of the range of

one detector, or when it is desirable to attenuate one signal (e.g. a laser) by some amount to

prevent detector saturation but not to attenuate another (e.g. a luminescence signal which is

likely to be much weaker).

For the first example we also present a description of the internal working of the program – it’s

block diagram – which should act as a reasonable introduction to the principles of LabVIEW

development and aid any future researchers who which to use these programs as a starting point for

the development of a more complex software suite. The program used for measuring I-V

characteristics in chapter 5 was developed by Navid Soheilnia in the Ozin group as part of a solar

cell project, and will not be discussed here.

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C.1 Monochromator and Detector Control Software

This software is used to control an Oriel Cornerstone monochromator and to collect a signal from a

detector such as a PMT. We begin with a discussion of the user interface or front panel and then

discuss the underlying program – the block diagram.

C.1.1 Monochromator and Detector Control Software – Front Panel

The front panel has two components and these are shown in Figure C.1. The front panel is divided

into two tabs, one of which records a ‘wavelength dependent signal’ and is used for recording a

spectrum, and the other which monitors a ‘time dependent signal’ at a single wavelength and is

useful when optimizing alignment.

C.1.1.1 Monochromator and Detector Control Software – Front Panel –

Wavelength-Dependent Signal

The panel is divided into several separate sections which serve various functions, for example the

top left section controls the parameters of the experiment, and the far right frame controls

measurement of quantum yields. Many of the controls and displays should be fairly self-explanatory

but some deserve an explanation:

From (nm), To (nm) and Step Size (nm) determine the range and resolution of the measurement.

For a typical upconversion spectrum these values were set to 400, 700 and 1, respectively, giving a

nominal resolution of 1 nm (although the actual resolution also depends on factors such as the

grating, the path length inside the monochromator, and the slit sizes). Samples per Point and

Sampling Rate (/s) are used to determine the amount of times a signal is read at a given

wavelength – the plotted value will be an average of this many individual measurements at a given

wavelength – and the frequency with which samples are read. The DAQ (data acquisition) card used

can support a sampling rate of over 106 s-1, although using a value that high is unlikely to confer

much additional benefit. The Grating, Shutter, and Port menus allow control of which diffraction

grating will be used for the experiment (there are three inside the monochromator, each suitable for

a different wavelength range), opening and closing of the electronic shutter on the monochromator

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(it must be opened to record a spectrum) and control of which of the two detector ports light is

directly towards. The bottom left panel is responsible for starting and stopping measurements, as

well as loading, saving and determining whether a calibration file is to be used, or whether a

background should be subtracted. Load Reference Spectrum is used for loading the spectrum of a

known source such as a NIST-calibrated lamp for use in creating a calibration file (a file which

shows the wavelength-dependent response of the system) and Load Calibration File can be used

for loading such a response file if one is available. Use Order Filters allows the operator to set a

number of wavelengths at which acquisition will stop so that a filter may be inserted to remove

higher order signals, for example a signal at 400 nm will lead to a second order peak at 800 nm,

which may obscure a peak in that range or otherwise lead to a misleading spectrum. Change

Detectors > 800 nm? is used as the PMT used in these experiments has a very poor response

beyond this range, although all this setting does is switch the monochromator ports and it is up to

the operator to adjust the electronics as necessary to collect from the second detector. The

integration panel below the spectrum integrate the spectrum over the indicated range and can be

used to compare luminescence signals from one sample to another. The function of the quantum

yield section on the far right is much the same as that in the dedicated quantum yield program which

will be discussed in section C.2.

C.1.1.2 Monochromator and Detector Control Software – Front Panel –

Wavelength-Dependent Signal

The time dependent signal tab is used to monitor the signal at a single wavelength over time. For

example, the procedure used to align upconversion luminescence measurements was to set the

monochromator to a wavelength of 540 nm and then adjust the positions of the optical elements so

as to obtain the maximum signal. As the alignment is improved the sensitivity setting of the Stanford

Research Systems SR570 current-voltage amplifier may need to be decreased so as to prevent

overloading.

123

Figure C.1. Front panel of monochromator-based spectroscopy software: control panels for recording

wavelength (top) and time (bottom) dependent signals.

124

C.1.2 Monochromator and Detector Control Software – Block

Diagram

The block diagram is where is actual programming in a LabVIEW virtual instrument (VI) is done.

The LabVIEW programming environment is graphical rather than text based and the ‘code’ consists

of icons representing data and operations linked together by wires which control the flow of this

data. The monochromator control software makes use of a control structure called a ‘queued

message handler’ which is shown in Figure C.2. The queued message handler contains two key

elements; a ‘while loop’ and a ‘case structure’ and in our case a third element, the ‘event structure’

which waits for an input from the user before initiating the case relevant to handling the particular

input (for example, the event structure will detect when the user presses the RECORD

SPECTRUM button and will the initiate the appropriate case in order to record a spectrum. The

cases are where the code detailing how to respond to each action are held; in some cases where the

operation is relatively simple such as the recording of the quantum yield, this may be achieved in a

single frame, in others which require more steps, this may take multiple frame, for example the

recording of a spectrum involves sequentially running the cases “Prepare Calibration”, “Find

Measurement Parameters”, “Calibrate Measurement”, “Go to Wavelength”, and “Acquire”. In

Figure C.2 the case “Setup” is shown and this case contains the event structure and awaits user

input. The event case shown in the figure is “Load Reference Spectrum” and in this case the input is

handled directly in the event structure as opposed to in another case, although either way is

reasonable. Outside of the case structure is the while loop, labeled 1 in Figure C.2. The while loop

repeats its operation over and over while a certain condition is met, typically while a STOP (in our

case STOP ALL) button is not pressed and it is used to keep the program running continuously

until it is stopped by the user, otherwise the case structure would execute only a single iteration

before stopping.

125

Figure C.2. Queued message handler structure used from controlling spectroscopy software, showing the

event structure used for handling user inputs. The numbered structures are 1 while loop, 2 case structure, 3

event structure.

126

C.2 CCD Spectrometer Quantum Yield Software

This software is used to record spectra from an Ocean Optics Maya 2000 USB CCD Spectrometer,

to process these and to use them to compute luminescence quantum yields according to both the

two and three measurement methods described in chapter 3. The front panel of this software is

shown in Figure C.3. Because the Maya spectrometer uses a grating to disperse light onto a CCD

array it is able to record spectra across its entire wavelength range at once. This makes control and

operation of it significantly simpler and faster than monochromator based systems, at the cost of

losing the ability to change detectors and having lower maximum resolution. The three (or two,

depending on the method, see chapter 3) measurements to be taken during the experiment are

achieved by clicking the Record Empty, Record Indirect and Record Direct buttons. Alignment

of the optics and signal optimization can be achieved by using the Acquire Continuously function

to monitor the signal in real time. In particular, it is important to ensure that the excitation intensity

is not so high as to saturate the detector, otherwise an artificially high quantum yield will be

recorded. The signal strength may be controlled by setting the Integration Time (ms) with a high

value meaning that the signal is acquired for a longer time and hence a higher signal will be recorded.

This can also reduce noise to some extent although this is better achieved by averaging, which is set

though the Spectra to Average setting – the values shown were typical of those used during an

experiment. The quantum yield may be calculated by clicking Calculate Quantum Yield and after

calculation the result of the 2MM is outputted as Quantum Yield (Kortshagen) and the 3MM as

Quantum Yield (Friend). The block diagram makes use of a very similar programming structure to

the spectroscopy software discussed above and we will not cover it in detail here.

127

Figure C.3. Front panel for quantum yield measurement software, showing typical spectra taken during the

measurement.

128

C.3 Upconversion Quantum Yield Calculation Software

The recording of upconversion quantum yields was complicated by the fact that the Maya

spectrometer would be saturated by the 972 nm laser peak before any detectable upconversion was

present. In order to get a strong upconversion signal by using a high intensity laser beam it was

therefore necessary to attenuate the laser somewhat before entry into the detector. At the same time,

attenuation of the upconversion would of course be undesirable as we wished to achieve the

strongest possible signal for it. We therefore used two different detectors – a PMT to detect

upconversion and the Maya to detect the laser, with a neutral density filter being used to attenuate

the laser peak. This meant that is was necessary to use the two different pieces of software described

in sections C.1 and C.2 to record the upconversion and laser spectra, respectively, and then to use a

third program to stitch together the data from these two spectrometers and apply a calibration to

both. The front panel of the program used for achieving this function is shown in Figure C.4. The

main feature of the front panel are the nine graphs, showing the empty, indirect and direct spectra

from the PMT, Maya and the overall stitched and calibrated spectra on the top, middle, and bottoms

rows, respectively. The From PMT, To PMT, From Maya, To Maya controls are used to set the

ranges over which the spectra from the PMT and Maya software should be loaded and these are

stitched together into single spectra by using the Stitch button. The From Ex, To Ex, From Em,

To Em controls are used to set the excitation and emission ranges over which the AQY should be

calculated. The Excitation Attenuation Factor control is used to account for the effect of the

neutral density filter which was used to attenuate the laser excitation, if no such filter is used it would

be set to 1, otherwise it should be set to 10OD where OD is the optical density of the filter. The

appropriate calibration may be applied by loading spectra using the functions in the bottom left

section, and then once a calibration has been applied the AQY can be calculated according to the

2MM and 3MM by pressing Calculate Quantum Yield. Again, the block diagram makes use of a

structure similar to the queued message handler described in C.1.

129

Figure C.4. Front panel of the software used for calculating quantum yields from spectra recorded from

monochromator-based and Maya spectrometers.

130

Appendix D: Potential Sources of Error present in

this Work

Any scientific study may naturally be expected to contain errors, and in this section we provide a

discussion of some of the errors, variations and limitations which may be present in this work, along

with, where appropriate, the measures taken to counteract them. We will discuss these potential

sources of variation in four categories:

1. Materials

2. Optics

3. Electronics

4. Software

D.1 Materials Variations

It has been shown that the materials studied herein exhibit a great deal of variation, indeed we have

shown that even for nominally identical materials the luminescences and quantum yields may vary by

a factor of two or more. Given that luminescence measurements are relatively highly reproducible, it

seems that materials variations from one sample to another is likely the most significant source of

variation.

D.2 Optical Errors and Aberrations

An ideal optical system will maintain a consistent response over the wavelength range of interest,

but in practice this is not the case due to a number of factors including: chromatic and spherical

aberrations of lenses, non-uniform absorption and transmission profiles of filters, integrating

spheres and optical fibres, non-uniform efficiencies of diffraction gratings and power meters. In

practice we attempt to account for these effect by applying a calibration using the standardized

131

calibration source (in our case a quartz tungsten halogen lamp) but even this method if not infallible.

The calibration spectrum provided with the lamp is recorded using a particular spectrometer

geometry at an excitation distance of 50 cm, and replicating this exact geometry is not always

practical for our purposes; for example the relatively low throughput of the integrating sphere

system necessitated moving the lamp closer to the sphere than the 50 cm calibration distance, and

the 1 cm circular aperture on the sphere is likely of significantly different size and shape to that used

when generating the calibration spectrum. Add to this the fact that a lamp for mounting inside of

the sphere was unavailable and that the calibration had to be recorded with the lamp external to the

sphere despite samples being mounted inside it, and it can be seen that there are many potential

error sources. Fortunately, testing of a rhodamine 6G reference sample yielded results very close to

the accepted values (see chapter 3) but the confirmation of the performance of a standard cannot be

taken as a guarantee of accuracy for the material of interest. This is particularly true in the case of

upconverters whose relatively unusual spectral properties are significantly different from all known

and well-characterized quantum yield standards. To add to this, the dependence of upconversion

quantum yield on excitation intensity leads to a significant sensitivity of measured quantum yield on

experimental alignment and any significant shifting of the alignment of the laser relative to the

sample would often lead to significant changes in observed quantum yield for the same sample.

Because of the sensitivity of the quantum yield to these effects, it is really the observed trend that is

of interest rather than the actual values themselves – if the alignment does not shift on exchanging

samples (and it should not) then any trend observed should be valid enough though there may be a

significant error on the actual values themselves.

D.3 Electronic Variations

The electronics used to recorded the data herein such as monochromators, amplifiers, sourcemeters,

spectrometers and detectors are themselves subject to potential errors. These may include

wavelength and calibration errors and non-linear responses and random noise. Measurements of the

positions of laser peak demonstrated good wavelength alignment and good agreement between the

Maya and monochromator-based spectrometers leads us to believe that this is not a significant

source of error. Signal noise may be a significant problem at very low intensities but fortunately we

132

did not work in many such situations during this study, and generally the signal/noise ratio was

evidently rather higher when recording spectra. One example of a result presented herein which is

subject to very significant noise is the dark I-V curve shown in figure 5.3 which is clearly suffering

from significant noise and also appears to be the victim of a systematic error due to the

measurement’s shift up the y-axis. Non-linearity is another potential source of error for which we

did our best to account by performing calibrations at a number of different intensities, but

nonetheless it is not possible to perform a calibration at every incident intensity of interest, and the

calibration source is very unlikely to be a true spectra match for the sample under study, and these

mismatches may lead to some error from this source.

D.4 Software Errors

The main source of software error encountered herein is likely to be that of Rietveld refinement

discussed in section 2.1.1 and mentioned several time throughout chapter 2 in the context of poor

fitting for low phase abundances. The section also provided a discussion of instances in which high

quality data may be fitted as ‘incorrect’ and poor data may be fitted as ‘correct’ (based on numerical

measured of fit accuracy). Of particular concern for the accuracy of Rietveld fitting are two cases,

which unfortunately both occur during our studies:

1. Low phase abundance – evident for the cubic phase in a number of the samples studied

2. Large particle size – evident for the hexagonal phase produced at high temperatures, which

can make accurate fitting of particle sizes extremely challenging.

the influence of synthesis temperature on the ... · iii depth discussion and investigation of two...

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