investigation of metal oxides thin films developed by pvd ... · investigation of metal oxides thin...

Investigation of Metal Oxides Thin

Films Developed by PVD System for

Perovskite Solar Cells

Submitted by: Fawad Ali

Master of Engineering

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Science and Engineering Faculty

Queensland University of Technology

2019

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cells i

Keywords

Perovskite Solar Cells, Metal Oxide Thin Films, Electron Transport Materials, Hole

Transport Materials, PVD, Sputtering, e-Beam Evaporation, Low Temperature

Processing, Charge Transporting Materials, Stable ETL and HTL, Substrate

Temperature, Oxygen Vacancy, Band Alignment, Hysteresis, PSCs Stability,

SCAPS, Simulation, Inorganic Hole Transport Materials.

ii Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

Abstract

Organic lead halide based perovskite solar cells (PSCs) have attracted tremendous

attention due to high power conversion efficiency (PCE). The is due to broad range

of light absorption, ambipolar nature and long electron and hole diffusion length of

the perovskite. However, the high processing temperature, and the high cost and low

stability of the organic charge transport materials have prevented the

commercialization of the device. The existing electron transport materials (ETM) are

either expensive and/or require high processing temperature that complicates the

processing procedure. The hole transporting materials (HTM) are not stable and

expensive and have low conductivity which is usually improved by doping. The

mismatch in the band alignment is another problem for the Charge Transporting

Materials (CTMs) which reduces the performance of devices and cause high

hysteresis due to charge accumulation at the interfaces. These electron and hole

transport layers (ETL and HTL) are, mostly deposited using solution-based processes

without good control over the film quality and uniformity over larger area. A better

interface between CTL and perovskite can help in charge injection from perovskite

to ETL. Introduction of stable and inexpensive charge transport layers that can be

processed at low temperature using a robust deposition technique would enhance the

chance commercialization of the PSCs.

This research investigates stable and inexpensive ETL and HTL inorganic metal

oxides thin films (SnO2, WO3-x and MoOx) by Physical Vapour Deposition

(sputtering and e-beam). Tuning the electronic and optical properties by changing the

experimental parameters without any doping and insertion of an extra layer is

investigated in this study. The effect of deposition parameters on the structural,

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsiii

morphological and composition properties of the thin films and its effects on the

performance of perovskite solar cells have also been reported.

First, the oxygen vacancies were tuned in SnOx thin films by magnetron sputtering at

different substrate temperatures. The oxygen deficient SnOx displayed a better device

performance with high Voc. In this part an interface modifying approach was

demonstrated by band alignment of charge transporting layer with perovskite

material and hence enhancing the performance of the PSCs. The oxygen vacancies

were tuned by heating the substrate to different temperatures during deposition until

a better band alignment was found at 250 ºC. Furthermore, a room temperature

deposition of WO3-x films as ETL for perovskite solar cells was conducted using e-

beam evaporation of WO3 at room temperature under high vacuum. This created

WO3-x thin film with high numbers of oxygen vacancies confirming optical and

electrical properties suitable for PSCs. These vacancies lead to improved device

performance due to better properties and reduced device hysteresis compare to the

stoichiometric WO3. Finally, MoOx was deposited by e-beam technique and

investigated as hole transport layer for perovskite solar cells using solar cell

capacitance simulator software (SSCSS). The simulation showed that MoOx can

work as HTL in both the inverted and regular structures of the perovskite solar cells.

In short thin film metal oxides were developed as ETL and HTL for PSCs using PVD

technique. These inexpensive and stable materials can be potential candidate in

reducing the cost of PSCs and increasing the lifetime of the device.

iv Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

List of Publications

1. Fawad Ali, Ngoc Duy Pham, H. Jonathan Bradford, Nima Khoshsirat, Ken

Ostrikov, John M. Bell, Hongxia Wang and Tuquabo Tesfamichael, Tuning of

Oxygen Vacancy in sputter-deposited SnOx films for Enhancing the Performance of

Perovskite Solar Cells, ChemSusChem, Jul 20 2018 (IF: 7.4).

DOI: http://dx.doi.org/10.1002/cssc.201801541

2. Fawad Ali, Ngoc Duy Pham, Ken Ostrikov, John M. Bell, Hongxia Wang and

Tuquabo Tesfamichael, Low Hysteresis Planar Perovskite Solar Cells WO3-x

Electron Transporting Layer Deposited at Room Temperature, Submitted to ACS

Applied Energy Materials

3. Fawad Ali, Ngoc Duy Pham, Ken Ostrikov, John M. Bell, Hongxia Wang and

Tuquabo Tesfamichael Prospects of e-beam evaporated Molybdenum Oxide as a

Hole Transport Layer for Perovskite Solar Cells Journal of Applied Physics, vol.

122, p. 123105, 2017 (IF:2.17)

DOI: https://doi.org/10.1063/1.4996784

4. Tengfei Qiu, Bin Luo, Fawad Ali, Esa Jaatinen, Lianzhou Wang, Hongxia Wang,

Metallic nanomesh with disordered dual-size apertures as wide-viewing-angle

transparent conductive electrode ACS Applied Materials & Interfaces, 8 (35), 22768-

22773 (IF: 8.09)

DOI: http://doi.org/10.1021/acsami.6b08173

http://dx.doi.org/10.1002/cssc.201801541

https://doi.org/10.1063/1.4996784

http://doi.org/10.1021/acsami.6b08173

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsv

5. Shengli Zhang , Harri Dharma Hadi, Ying Wang, Baolai Liang, Vincent Tiing

Tiong, Fawad Ali, Yi Zhang, Tuquabo Tesfamichael, Lydia H. Wong, and Hongxia

Wang, A Precursor Stacking Strategy to Boost Open-Circuit Voltage of Cu2ZnSnS4,

Thin-Film Solar Cells IEEE Journal of Photovoltaics, Vol. 8, NO. 3, May 2018

(IF: 4.4)

DOI: http://doi.org/10.1109/JPHOTOV.2018.2813264

6. Nima Khoshsirat, Fawad Ali, Vincent Tiing Tiong, Motabah Amjadpori, Hongxia

Wang Mehnaz Shafiei and Nunzio Motta*, Optimization of Mo/Cr bilayer back

contact for thin film solar cell application Beilstein J. Nanotechnol.2018,9,2700–

2707

DOI: doi:10.3762/bjnano.9.252

http://doi.org/10.1109/JPHOTOV.2018.2813264

vi Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

Table of Contents

Keywords .................................................................................................................................. i

Abstract .................................................................................................................................... ii

List of Publications .................................................................................................................. iv

Table of Contents .................................................................................................................... vi

List of Figures ....................................................................................................................... viii

List of Tables .......................................................................................................................... xii

List of Abbreviations .............................................................................................................. xii

Statement of Original Authorship ....................................................................................... xviii

Acknowledgements ............................................................................................................... xix

Chapter 1: Introduction............................................................................................. 1

1.1 Background ........................................................................................................................ 1

1.2 Device Structure ................................................................................................................. 5

1.3 Research Problems, Aims and Objectives .......................................................................... 7

1.4 Significance ........................................................................................................................ 9

1.5 Thesis Outline .................................................................................................................. 10

1.6 References ........................................................................................................................ 12

Chapter 2: Literature Review ................................................................................. 17

2.1 Perovskite materials ......................................................................................................... 17

2.2 Working Principle of Perovskite Solar Cells .................................................................... 18

2.3 Perovskite Solar Cell Structure ........................................................................................ 20

2.4 Charge Transporting Layers (CTLs) ................................................................................ 23

2.4.1 Electron Transport Layer (ETL) .................................................................................... 24

2.4.2 Hole Transport Layer (HTL) ......................................................................................... 32

2.5 Deposition Techniques for Charge Transport Layers (CTLs) .......................................... 35

2.5.1 Sputtering and e-beam Techniques for Perovskite Solar Cells ..................................... 37

2.6 References ........................................................................................................................ 40

Chapter 3: Research Methodology ......................................................................... 51

3.1 Working Principle of Sputtering and E-beam evaporation .............................................. 51

3.2 ETL and HTM metal oxide thin films deposition ............................................................ 54

3.3 Perovskite deposition ....................................................................................................... 56

3.4 Spiro-OMeTAD (HTL) deposition .................................................................................. 56

3.5 Characterization................................................................................................................ 56

3.6 Simulation of Perovskite Solar Cells ................................................................................ 58

3.7 Reference .......................................................................................................................... 58

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsvii

Chapter 4: Tuning of Oxygen Vacancy in sputter-deposited SnOx films for

Enhancing the Performance of Perovskite Solar Cells ......................................... 60

4.1 Introduction .......................................................................................................................64

4.2 Results and discussion ......................................................................................................66

4.3 Conclusion ........................................................................................................................77

4.4 Experimental Section ........................................................................................................77

4.5 Acknowledgement ............................................................................................................81

4.6 References .........................................................................................................................83

Supporting Information ...........................................................................................................87

Chapter 5: Low Hysteresis Planar Perovskite Solar Cells using WO3-x Electron

Transporting Layer Deposited at Room Temperature......................................... 95

5.1 Introduction .......................................................................................................................99

5.2 Results and discussion: ...................................................................................................102

5.3 Conclusion ......................................................................................................................113

5.4 Experimental Section ......................................................................................................113

5.5 Acknowledgement ..........................................................................................................116

5.6 References: ......................................................................................................................117

Supporting Information .........................................................................................................122

Chapter 6: Prospects of e-beam Evaporated Molybdenum Oxide as Hole

Transport Layer for Perovskite Solar Cells ........................................................ 127

6.1 Introduction .....................................................................................................................131

6.2 Experimental ...................................................................................................................133

6.3 Results and Discussions ..................................................................................................136

6.4 Conclusion ......................................................................................................................151

6.5 Acknowledgement ..........................................................................................................152

6.6 References .......................................................................................................................153

Chapter 7: Conclusions and Recommendation for Future Work ..................... 157

7.1 Conclusions .....................................................................................................................158

7.2 Future Recommendations ...............................................................................................160

Appendix ................................................................................................................. 163

viii Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

List of Figures

Figure 1.1 Classification of different generations of solar cells [3]. ............................ 2

Figure 1.2 Best research-Cell Efficiencies [7] ............................................................. 3

Figure 1.3 Solar efficiency of Silicon and Perovskite based Solar Cells. .................... 5

Figure 1.4 Device architecture of mesoporous (a) and planar (b) perovskite based

solar cell. ...................................................................................................................... 6

Figure 2.1 Typical crystal structure (unit cell) of perovskite material. ...................... 18

Figure 2.2 Schematic of the basic working principle of perovskite solar cells. ......... 19

Figure 2.3 Schematic diagram of photo-generated charge transfer and recombination

process in perovskite solar cells [5]. .......................................................................... 19


solar cell. .................................................................................................................... 20

Figure 2.5 Device architecture of regular n-ip (a) and inverted p-i-n (b) perovskite

based solar cell. .......................................................................................................... 22

Figure 2.6 Schematic of the reduction of surface recombination by passivating the

trap states by incorporation of PCBM layer [35]. ...................................................... 26

Figure 2.7 Perovskite solar cells using low temperature ZnO as ETL on flexible PET

substrate [49]. ............................................................................................................. 28

Figure 2.8 J-V curve of low temperature SnO2 based perovskite with minimized

hysteresis [63]. ........................................................................................................... 30

Figure 2.9 Solution based low temperature processed WOx thin film ETL showing

lower Voc and higher Jsc values as compare to TiO2 based ETL for PSCs [67]. ........ 31

Figure 2.10 Spin coated TiO2 with irregular film thickness, poor contact with

substrate (FTO) and discontinuous areas [108].......................................................... 36

Figure 2.11 TiO2 blocking layer deposited by (a) sputtering and (b) spin coating

technique for PSCs [112]. .......................................................................................... 38

Figure 3.1 Schematic diagram of sputtering deposition process. ............................... 52

Figure 3.2 Schematic diagram of e-beam evaporation technique .............................. 53

Figure 3.3 (a) Sputtering system (PVD 75 Kurt J. Lesker) (b) inside chamber of

sputtering showing four targets. ................................................................................. 55

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsix

Figure 4.1 a) UV-Visible transmittance spectra and b) (αhv)2 vs hv plot c) UPS data

and d) conduction band position of SnOx thin films deposited at various substrate

temperatures based on UPS and UV-visible spectrum. ............................................. 69

Figure 4.2 XPS analysis of the deposited SnOx thin films. (a)-(c) show representative

wide survey, Sn 3d and O 1s core level spectra, respectively, and (d) shows the

temperature dependent stoichiometry of SnOx thin films (1<x<2). .......................... 71

Figure 4.3 (a) ESR spectra of room temperature and 250 ºC deposited SnOx thin

films and (b) schematics showing SnOx structure with oxygen vacancies. .............. 73

Figure 4.4 Schematic diagram of perovskite solar cell device with SnOx thin films

used as ETL, (b) Cross-sectional SEM image of actual perovskite solar cells device

and (c) J–V curves of the device scanned under reverse voltage. The inset in (c) is the

device performance for the different SnOx thin films and, (d) External quantum

efficiency of the cell with SnOx deposited at 250 ºC. ................................................ 73

Figure 4.5 Nyquist plots of PSCs with SnOx ETL deposited at room temperature and

250 ºC under light, inset shows the equivalent circuit model for data fitting. ........... 76

Figure S4.1 XRD spectrum of SnOx thin films deposited at RT and 250 ºC showing

amorphous characteristics in both films. .................................................................... 87

Figure S 4.2 Raman spectroscopy of SnOx films deposited at RT and 250 ºC. For

comparison the spectrum of the glass substrate is shown. ......................................... 88

Figure S4.3 SEM surface morphology of SnOx thin films deposited at different

substrate temperatures (RT-250 ºC). For comparison the surface morphology of the

perovskite absorber deposited on two different SnOx films (RT and 250 ºC) is shown

in Figure S4.6. ............................................................................................................ 89

Figure S4.4 Fermi edge (EVBM) region of SnOx thin films deposited at different

substrate temperatures (RT-250 ºC) obtained using UPS measurements. .................. 90

Figure S4.5 Energy cut-off region of SnOx thin films deposited at different substrate

temperatures (RT-250 ºC). ......................................................................................... 91

Figure S4.6 SEM surface morphology of perovskite absorber deposited on two

different SnOx films (room temperature and 250 ºC). ................................................ 92

Figure S4.7 AFM images (5 × 5 μm2) showing the morphology and RMS surface

roughness of SnOx thin films deposited at room temperature and 250 ºC. ................ 92

Figure S4.8 Current-voltage (J-V) curve of PSC device in both reverse (Rev) and

forward (Fwd) scans for SnOx thin films as ETL deposited at room temperature and

250 ˚C. ........................................................................................................................ 93

Figure 5.1 Transmittance spectra, (b) (αhν)1/2 vs hν plot, (c) He-I UPS spectra, inset

in Figure c is the fermi-edge region and (d) band energy alignment of ETLs with

perovskite light absorbing material, for the room temperature deposited and post-

annealed WO3-x thin film samples............................................................................ 103

x Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

Figure 5.2 High resolution XPS spectra of as-deposited and annealed WO3-x thin

films shows (a) W 4f, (b) O 1s, (c) C 1s core levels with fits to the spectral

envelopes. ................................................................................................................. 105

Figure 5.3 (a) EPR spectra and (b) conductivity and resistivity of WO3-x thin films

deposited at room temperature (RT) and annealed at 300 ºC. .................................. 106

Figure 5.4 Schematic diagram of perovskite solar cell, b) Cross-sectional SEM image

of actual perovskite solar cells device, c) Current-voltage (J-V) curve of PSC at both

reverse (Rev) and forward (Fw) scan, and d) external quantum efficiency (EQE) of

perovskite solar cell for both the WO3-x-RT and WO3-x-300 °C thin films. ............. 109

Figure 5.5 (a) PL spectra and (b) Nyquist plots of PSCs under light using as-

deposited and post-annealed WO3-x ETL. For comparison the PL of the perovskite

absorber is also shown. ............................................................................................. 110

Figure S5.1 Survey spectra of the as-deposited (RT) and post-annealed (300 ºC)

WO3-x thin films. ...................................................................................................... 122

Figure S5.2 Thickness of as-deposited and annealed WO3-x measured by stylus

profilometer. ............................................................................................................. 123

Figure S5.3 Statistic from four batches of as-deposited and annealed WO3-x ETLs

based PSCs. .............................................................................................................. 125

Figure 6.1 Raman spectra of MoOx thin films deposited by e-beam evaporation at

different substrate temperatures (RT, 100 ºC and 200 ºC)…………………………132

Figure 6.2 XPS of MoOx thin films deposited by e-beam evaporation at different

substrate temperatures (RT, 100 ºC and 200 ºC). ..................................................... 139

Figure 6.3 XPS spectra of MoOx thin films at different substrate temperatures

showing high resolution scans of (a) C 1s, (b) O 1s and (c) Mo 3d core levels with

synthetic fits to the spectral envelopes. .................................................................... 140

Figure 6.4 Micrograph of MoOx thin films deposited at room temperature (a, d), 100 ºC (b, e) and 200 ºC (c, f). (a, b, c) are HIM images having a scale bar of 1 μm and

(d, e, f) are AFM images scanned over 2 μm × 2 μm. ............................................. 141

Figure 6.5 Transmittance spectra and (b) (αhv)2 vs hv plot of 100 nm thick MoOx

thin films deposited at various substrate temperatures. ............................................ 143

Figure 6.6 Schematic diagram of (a) inverted structure and (b) regular structure of a

perovskite solar cell used in SCAPS simulation. ..................................................... 146

Figure 6.7 Open circuit voltage (Voc), short-circuit current density (Jsc), current

recombination (Jrec), fill factor (FF) and efficiency of PSC as a function of MoOx

layer thickness in (a) regular and (b) inverted PSC structure. The room temperature

deposited bandgap energy (3.75 eV) of MoOx films is considered. ........................ 147

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsxi

Figure 6.8 Quantum efficiency of Perovskite solar cell using MoOx films with

different thicknesses in (a) regular structure and (b) inverted structure. The room

temperature deposited bandgap energy (3.75 eV) of MoOx films is considered. .... 149

Figure 6.9 (a) Open circuit voltage (Voc), short-circuit current density (Jsc), current

recombination (Jrec), fill factor (FF), efficiency, and (b) quantum efficiency of PSC as

a function of MoOx bandgap energy in the inverted structure for a film thickness of

100 nm. ..................................................................................................................... 150

Figure 6.10 Quantum efficiency (QE) of PSC using MoOx as HTM in inverted

structure of PSC. Inset shows the optimized efficiency of the PSC in the inverted

structure using film thickness of 50 nm. .................................................................. 151

Figure 0.1 Cross-sectional SEM images of planar perovskite solar cells with (a) TiO2

layer prepared by E-beam and (b) TiO2 layer prepared by spray-pyrolysis method.

.................................................................................................................................. 163

Figure 0.2 (a) Current Density -Voltage (J-V) characteristics of planar structure

under AM 1.5G illumination, (b) stability of e-beam and sprayed device with time.

.................................................................................................................................. 164

xii Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

List of Tables

Table 4.1 Transmittance, bandgap energy and Sn to Oxygen ratio of SnOx films

deposited at various substrate temperatures. .............................................................. 69

Table 4.2 Extracted EIS parameters of perovskite solar cells measured under 1 sun

illumination at open circuit voltage. ........................................................................... 75

Table 5.1 Oxygen to tungsten ratio and electronic properties of RT deposited and

post-annealed WO3-x thin films. ............................................................................... 106

Table 5.2 Reverse and forward scan photovoltaic I-V parameters of PSCs using the

as-deposited and post-annealed WOx-3 films as ETL. .............................................. 109


illumination at open circuit voltage. ......................................................................... 112

Table S5.1 Electrical properties (conductivity and resistivity) of as-deposited and

annealed WO3-x thin films using four-point-probe………………………………...117

Table S5.2 Series and shunt resistance for perovskite solar cells using as-deposited

and annealed WO3-x as ETL from J-V curve………………………………………117

Table 6.1 Input parameters obtained from this experiment and various reference

papers for SCAPS simulation of PSC using MoOx as HTM [29]. ........................... 136

Table 6.2 Transmittance, atomic ratio of O:Mo, surface roughness and bandgap

energy of MoOx thin films deposited at different substrate temperatures. ............... 144

List of Abbreviations

BEP Band Edge Parameter

AFM Atomic Force Microscope

ALD Atomic Layer Deposited

MRF Magnetic Resonance Field

BCP bathocuproine

Bphen bathophenanthroline

C Capacitance

CB Conduction Band

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsxiii

CdTe Cadmium Telluride

CIGS Copper-Indium-Gallium-Di-selenide

CIS Copper Indium Selenide

CTL Charge Transport layer

CZTS Copper Zinc Tin Sulphur

CZTS Copper Zinc Tin Sulphur

DC Direct Current

DMF Dimethyl Formamide (DMF) and 78 mg of

DMSO Dimethyl Sulfoxide

DSSC Dye Sensitised Solar Cells

e-beam Electron Beam

Ec Conduction Band

EF Fermi Level

Eg Bandgap

EIS Electrochemical Impedance Spectroscopy

EPR/ESR Electron paramagnetic resonance/ Electron spin spectroscopy

EQE External Quantum Efficiency

ETL Electron Transport Layer

eV Electron Volt

Ev Valence Band

F Farad

FF Fill Factor

FTO Fluorine doped Tin Oxide

FWHM Full Width at Half Maximum

h Plank’s Constant

xiv Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

HCl Hydrochloric acid

HIM Helium Ion Microscopy

HOMO Highest Occupied Molecular Orbit

HTL Hole Transport layer

I Intensity

IPCE Incident Photon-to-Current Efficiency

Jrec Recombination Current Density

Jsc Current Density

J-V Current-Voltage

KPFM Kelvin Probe Force Microscopy

LUMO Lowest Unoccupied Molecular Orbit

Mono c-Si Monocrystalline Silicon

NA Acceptor Density

Nc Density of Charge at Conduction Band

ND Donor Density

Ns Number of unpaired Electrons

Nt Defect Density

Nv Density of Charge at Valence Band

OPV Organic Solar cells

P3HT poly(3 hexylthiophene-2,5-diyl)

PCBM [6,6]-phenyl-C61-butyric acid methyl ester

PCE Power Conversion Efficiency

PEDOT:PSS poly(3,4-ethylenedioxythiophene) polystyrene sulphonate

PL Photoluminescence

Poly c-Si Polycrystalline Silicon

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsxv

PSCs Perovskite Solar Cells

PTAA poly[bis(4 phenyl)(2,4,6-trimethylphenyl)amine]

PV Photovoltaics

PVD Physical Vapour Deposition

R Resistance

Rrec Recombination Resistance

Rs Sheet resistance

RT Room Temperature

SCAPS Solar cell Capacitance Simulator Software

SEM Scanning Electron Microscope

Spiro-OMeTAD) 2,2’,7,7tetrakis(N,N-pdimethoxyphenylamino)-9,9’-

pirobifluorene;

TBP tert-butylpyridine

TCO Transparent Conducting Oxide

UPS Ultraviolet Photoelectron Spectroscopy

V Volt

VB Valence Band

VBM Valance Band Maximum

Vo Oxygen Vacancy

Voc Open Circuit Voltage

XPS X-ray photoelectron spectroscopy

XRD X-ray Diffraction

α Alpha (absorption coefficient)

εr Relative Permittivity

ν Frequency of Light

xvi Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

χ Electron Affinity

µB Bohr magneton

µm Micrometre

µn Electron Mobility

µp Hole Mobility

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsxvii

xviii Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the

best of my knowledge and belief, the thesis contains no material previously

published or written by another person except where due reference is made.

Signature:

Date: February 2019

QUT Verified Signature

Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar Cellsxix

Acknowledgements

First and foremost, I would like to express my sincere gratitude to my Principal

supervisor Dr. Tuquabo Tesfamichael for the continuous support of my research, for

his motivations and patience throughout my PhD study. His guidance always helped

me in the research and writing of this thesis. He was always available for discussion,

support and help. I could not have imagined having a better advisor and mentor for

my PhD study.

Beside him I would like to thank Associate Professor Hongxia Wang for sharing her

knowledge and guidance. Her continues support to my research and critical and

constructive comments polished my knowledge and research skills. She has always

been very cooperative during my whole study.

Also, I would like to thank Prof. Ken Ostrikov for motivation he gave me and for his

encouragement to face the challenges and mentoring how to deal with it. I would like

to thank him for his help in revising my papers and necessary suggestions to better

this study. I would like to thank Prof. John Bell for giving critical comments on my

papers and mentoring.

I would like to thank QUTPRA for providing me the opportunity to study at QUT.

I would like to acknowledge all the technical research staff in CARF (operated by the

Institute for Future Environments (QUT)), especially Dr. Peter Hines, Dr. Sanjleena

Singh, Dr. Josh L. Duffin, Tony Raftery, Dr. Henry Spratt and all those who helped

me with my research characterization.

xx Investigation of Transparent Metal Oxides for Improving the Performance and Stability of Perovskite Solar

Cells

Thanks go to all my fellow labmates, colleagues and friends for stimulating

discussion and for all the fun that we have had in the last few years which made

research valuable, and time enjoyable and memorable here in Brisbane.

I must also thank my friends outside QUT, here in Brisbane, in South Korea and in

Pakistan whom I didn’t devote as much time as they deserved.

Last but not the least, I would like to thank my parents (Sayyed Ali Shah and Shakira

Shah) my brother Jawad Shah and all cousins for their spiritual support throughout

my PhD and in life in general. I don’t have enough words to thank my parents. I

dedicate this PhD to my parents.

Introduction 1

Chapter 1: Introduction

1.1 Background

The increasing energy demands and environmental concerns due to the use of fossil

fuels in the 21st century have motivated researchers and policy makers to explore

clean and environmental friendly renewable sources of energy. Apart from

environmental pollution the limited resources of fossil fuels are going to be

consumed soon and therefore alternative sources of energy are required for future

energy demands. These alternative sources of energy to overcome the energy crisis

include hydroelectricity, tides, geothermal, wind and solar energy. Among all these

sources of energy, solar energy is the centre of interest due to the enormous amount

of energy provided by the sun. Also, solar energy has the most extractable potential

with the least environmental effects which can meet stringent energy needs [1]. The

sun provides much more energy per day than the energy consumption requirement of

the current population for the whole year. Photovoltaics (PV) provide a direct

conversion of the incident solar radiation into electricity. This process does not have

any side product such as noise and pollution which makes the PV technology a

robust, reliable and long lasting renewable source of energy.

The first working photovoltaic device demonstrated in the 1950s with the efficiency

of 3% using silicon. Today the performance of such devices is rapidly increasing

with efficiency as high as 25% [2]. The first major boost in research and

development on solar cells received from the space industry in the 1960s. These are

solar cells which were more expensive than solar cells we have today. The main

attention on photovoltaics occurred after the oil crisis in 1970s. In this era the

photovoltaics were investigated and promoted as an alternative energy resource to

2 Chapter 1: Introduction

overcome the energy crisis. It was quickly recognized that the PV can supply power

to "remote" areas and hence prompted to terrestrial photovoltaics industry. Today

photovoltaics are classified in three generations based on materials used as shown in

Figure 1.1.

Today photovoltaics are found to be one of the effective technologies in overcoming

the shortage of energy. Solar cells are divided into three different generations on the

basis of materials used as shown in Figure 1.1.

Figure 1.1 Classification of different generations of solar cells [3].

The up to date efficiencies of the different generation solar cells are shown in Figure

1.2. The first generation of photovoltaics are the silicon based solar cells. Silicon

based solar cells have high power conversion efficiency but the high volume of

materials and manufacturing cost are the major issues with this type of solar cells.

Silicon based solar cells are divided into monocrystalline (Mono c-Si),

polycrystalline (Poly c-Si) and amorphous silicon cells. The second generation of

solar cells are thin film solar cells including CIGS, CdTe and CIS. These second

generation thin film solar cells, couldn’t meet the requirements due to the use of

Introduction 3

indium and tellurium etc. The third generation of solar cells consisting of dye

sensitized solar cells (DSSCs), Copper zinc tin sulphide (CZTS), organic solar cells

(OPVs), quantum dot solar cells and perovskite based solar cells (PSCs) are the most

promising photovoltaic technology due to their high efficiency at low processing

cost. These third generation solar cells are mostly solution-processed using organic

semiconductors, hybrid composites, or inorganic semiconducting materials. A key

role has been played by the solution-processed dye by producing photo-generated

current. However, the low range of power conversion efficiency (PCE) for DSCs,

OPVs and CZTS has limited their commercialization. For widely use of the third

generation of solar cells, a technology that produces durable, high efficiency and low

cost solar cells is needed [4-6].

Figure 1.2 Best research-Cell Efficiencies [7]

Currently perovskite solar cells have become the focus of research, due to the

tremendous optical and electrical properties of the perovskite materials. The perovskite

semiconducting material has attracted the attention of scientists and researchers

because of its low binding energy [7], long diffusion length, long carrier life time

and a strong light absorption in broad absorption range from visible to near infrared


spectrum with a direct tuneable band gap of 1.2-2.7 eV [8-10]. Since the introduction

of organic lead halide perovskite semiconductor photovoltaic device by Miyasaka

and his group in 2009, a huge progress has been made in the design and optimization

of perovskite solar cells [11]. High efficiency perovskite solar cells (PSCs) can be

produced at low cost using simple processing methods [12, 13]. As perovskites have

excellent light absorbing property, they require a thin layer of about 300-500 nm

which minimizes the material cost [14]. Furthermore, by introducing low

temperature processable inorganic metal oxide engineering the chemical composition

of the perovskite materials can alter a range of properties including optical and

electronic properties that are useful for enhancing the performance and stability of

the solar cells.

Today PSCs have shown high power conversion efficiencies (PCE) of over 23% [2,

11, 13, 15-17]. As shown in Figure 1.3, a sharp increase in the efficiency of PSCs is

observed over very short period of time. In 2011 the Park group improved the

perovskite solar cells efficiency from its initial value of 3.8% to 6.5 %. In 2012 the

collaborated work of Gratzel and Park increased the efficiency further to 9.7% [8].

Yang’s group reported 19.3% efficiency for planar structure in 2015. Furthermore

Seok group have certified efficiency of 20.1% in 2015 [18]. Recently the efficiency

reached to 23.2% for perovskite solar cells in 2018 [13]. However, for widely use of

this type of solar cells, a technology that produces durable and low cost that is

competitive with the Si solar cell technology is needed.

Introduction 5

Figure 1.3 Solar efficiency of Silicon and Perovskite based Solar Cells.

Taken from “Enabling Breakthroughs in Solar Technology”

1.2 Device Structure

A typical perovskite solar cell employs perovskite material as a light absorbing layer

which is sandwiched between an electron transport layer (ETL) on a transparent

conducting oxide (TCO) glass and a hole transporting layer (HTL), coated with metal

back contact electrode. There are two basic structures for this device, one with the

inclusion of a mesoporous TiO2 layer and hence known as mesoporous structure

(Figure 1.4a) and the other without the mesoporous layer known as planar structure

(Figure 1.4b). In the mesoporous device the perovskite is infiltrated to the

mesoporous TiO2 scaffold. Snaith and co-worker substituted the mesoporous TiO2

with insulating Al2O3 and the device was still working quite well [19]. This gives an

indication the perovskite can be used both as light harvesting material and electron

transporting layer and hence the PSC device can be completed without the

mesoporous layer. High efficiency of over 23% has been achieved using the

mesoporous device structure [13]. However, the high sintering temperature required

for the mesoporous layer not only increases the processing time but also the cost of

the cell production [20]. The planar structure has a simplified device fabrication


process, with reduced material cost and sintering temperature. A 20% efficiency has

been reported using the planar structure which justifies the removal of the

mesoporous layer as shown in Figure 1.4 b [21]. This is because electrons can be

easily collected by the electron transport layer without the incorporation of

mesoporous layer as charge diffusion length of perovskite materials [20].


solar cell.

For achieving high efficiency PSC device, the selection of charge (electron and hole)

transport materials is very important. These charge transport materials must have

excellent charge transport property and an energy level matching with the light

absorbing perovskite material. In addition, the ETL must be stable in air and

moisture and should be processable at low temperature to increase the durability of

the perovskite solar cells and simplify the fabrication process. Various organic hole

and electron transport layers have been incorporated into perovskite depending on

the type of perovskite (Iodide, Bromide or Chloride) and the device structure, namely

regular, n-i-p or inverted p-i-n. The commonly used organic p-type hole transport

materials are poly(3,4-ethylenedioxythiophene) polystyrene sulphonate

(PEDOT:PSS); 2,2’,7,7 tetrakis(N,N-pdimethoxyphenylamino)- 9,9’-spirobifluorene

(spiro-OMeTAD); poly[bis(4 phenyl)(2,4,6-trimethylphenyl)amine] (PTAA); and

Introduction 7

poly(3 hexylthiophene-2,5-diyl) (P3HT). The n-type metal oxide based on TiO2 is

commonly used as the electron transport layer because of the high band gap and high

transmittance. Similarly, organic n-type electron transport layer based on [6,6]-

phenyl C61 butyric acid methyl ester (PCBM); C60; and their derivatives are also

employed [22-26]. Inorganic metal oxide charge transport materials can effectively

help improving the stability of perovskite solar cells against moisture. Recently,

some research groups are replacing the organic p-type materials with inorganic

materials [27, 28]. It is well known that the metal oxides semiconductors have

superior oxygen and moisture stability and higher charge mobility than the above

mentioned organic charge transport materials [29].

1.3 Research Problems, Aims and Objectives

Perovskite based hybrid solar cells emerged in the last decade as potential alternative

devices for the development of PV technology. However, important issues need to be

solved before perovskite based solar cells become a commercialized product.

Stability of perovskite and use of toxic lead (Pb) heavy matal are big challenges for

commercialization of perovskite solar cells. Also, high processing temperature and

lower conductivity of the inorganic metal oxide used as ETL and poor stability and

high materials cost of the organic HTL are some of the main challenges. The

currently used ETMs need high temperature post-treatment to increase the

crystallinity and conductivity. Similarly, the organic HTMs have extensively used in

high efficiency PSCs which are often expensive and un-stable upon exposing to the

ambient environment. Therefore using high temperature processable ETLs and

organic charge extraction layer may cause problems in the future commercialization

of PSCs. In this regard, low temperature processable, stable and low cost ETL and

HTL materials are required for PSCs. The stability and performance of perovskite


solar cells can also be affected by the underlying n-type layers. It is proposed that

optimization of the stoichiometry and morphology of ETLs using appropriate

processing methods would enhance the stability and performance of perovskite solar

cells. Low temperature processable metal oxides such as WO3 and SnO2 may also be

tuned to obtained good electron transport properties and comparable energy levels

that can substitute TiO2 blocking layer and as electron transport materials to enhance

the overall performance of PSCs. Inorganic p-type metal oxide semiconductors such

as NiO, CuOx, and MoO3 are the most suitable replacement as hole transport

materials due to their excellent chemical stability, higher charge mobility and high

transparency. Different metal oxides such as MoO3 and NiO as HTM have been

explored for perovskite solar cells.

The objective of this study is to explore new, stable and inexpensive ETL that can be

processed at low temperatures. For example one of the aims is to study ETL that can

be processed at low temperature using PVD (sputtering and e-beam evaporation).

Once the suitability of SnOx as ETL using sputtering process is explored, then a

thorough and systematic study of the ETL will be performed. Similar approach will

be used for the other materials. The research will further investigate the optical and

electrical properties of the ETL by tuning the composition of the material for

achieving high PCE. Similarly, this study will explore HTL metal oxides to replace

the expensive and unstable organic hole transport layer. This research strategy will

not only provide stable and high performance solar cell device but will also reduce

the cost of the material. This research aims to modify the properties of different

metal oxides as ETL and HTL by tuning their electronic bands and align with the

band of the perovskite absorber and thereby improve the performance of the PSC

device.

Introduction 9

The research challenges of the PSCs are summarized below:

Un-stability of the perovskite solar cell devices

High processing temperature of the electron transporting layer

Band energy level mismatch between the perovskite absorber and the charge

transporting layers

Limitation of using robust and industrial viable method of deposition for the

CTLs

Poor stability of the organic hole transporting layer

The specific aims of this proposed research are:

To investigate metal oxide charge transport layer

Study ETLs which can be processed at low temperature

Optimize the properties of the ETLs

Apply new metal oxide HTLs to perovskite solar cells

Exploring PVD (sputtering & e-beam) techniques for deposition of these materials

1.4 Significance

Ideal charge transport layer must have a high conductivity, well-matched energy

level to that of the perovskite materials, high transmittance, lower cost and low

processing temperature. Incorporating new metal oxide based ETL which can be

deposited at low temperature can reduce the processing cost and time of the PSCs.

This will also open a window for flexible and tandem solar cells. Similarly tuning the

electrical and optical properties of metal oxide based ETL by varying the

experimental parameters provide a huge potential for achieving high performance

PSCs. Also, finding a good band alignment of charge transport later with perovskite

layer is essential for better device performance. A scalable and industrial viable

technique is needed for the deposition of metal oxide charge transport layers for

perovskite solar cells applications. Organic hole conducting materials are often

applied for high efficient perovskite solar cells (PSC), which are often expensive and

have relatively low hole mobility. The commonly used organic hole transport

materials are not stable for perovskite based solar cells and remained an issue for the


long term [11, 30]. For example, the acidic nature of Poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), has high tendency to

absorb water. High materials cost and poor stability of organic HTM on exposure to

air and moisture is a big problem for the device stability. Inorganic hole conducting

materials such as p-type metal oxides (MoOx, NiO) are found to be alternative

materials. They are chosen due to the higher charge mobility, high transparency in

the visible region, good chemical stability and various selections in terms of the

valency band (VB) energy level [18]. Therefore, this study could contribute to

solving the challenges of band alignment mismatch (chapter 4), optimizing the

properties of ETLs processed at low temperature (chapter 5) poor stability of

perovskite solar cells (chapter 6). This may help in enhancing the performance of

device and lowering the processing temperature and cost of perovskite solar cells.

1.5 Thesis Outline

Chapter 1 describes a brief introduction of the thesis describing the different types of

energy resources and their effects on the environment. The different generation of

PV technology and their advantages and limitation today are also discussed. In this

chapter the research challenges in the perovskite based solar cells have been

discussed and the roles of inorganic metal oxides to ease these challenges are

discussed.

Chapter 2 of this thesis will give a comprehensive survey of the literature review on

perovskite based solar cells and the importance of metal oxide semiconductors as

electron and hole transporting materials. First, the basic structure of perovskite

materials and the architecture of the perovskite based solar cells are described. This

is followed by an in-depth review of the different types of electron and hole

transporting layers including their strength and weaknesses have been discussed. The

Introduction 11

chapter finishes by highlighting the exciting use of low temperature processable

metal oxide semiconducting materials for perovskite solar cells.

Chapter 3 is the research methodology and experimental procedure. The typical

preparation method used for the manufacturing of perovskite solar cells and the thin

film metal oxide deposition using PVD methods are outlined. Thin film

characterization techniques and device characterization are also described in full

detail in this chapter.

In Chapter 4 the effect of oxygen vacancies in SnOx on the optical and electrical

properties of SnOx and the performance of PSCs are investigated. A detailed study on

the effect of experimental parameters (substrate temperature) on the composition and

properties of SnOx developed by sputtering method were conducted in this study.

This work has been published in the Journal ChemSusChem.

Chapter 5 follows the outcome achieved from chapter 4 and discusses some

alternative method for creating oxygen vacancies at low temperature for performance

enhancement of PSCs. In this chapter, a room temperature oxygen deficit metal

oxide (WO3) as ETL by e-beam evaporation has shown significant effect in

improving the performance of the PSC device with reduce hysteresis. The work in

this chapter has been submitted for publication to ACS Applied Energy Materials.

Chapter 6 focused on metal oxides as HTL. Development of suitable ETL (SnOx and

WOx) with improved performance alone is not sufficient for the commercialization

of PSCs as the HTL is also an important component of the device. Thus a p-type

metal oxide (MoOx) as HTL was investigated. First a simulation using solar cell

capacitance simulator software (SCAPS) was carried out to find the potential of the

material and its best deposition parameters. Simulation showed an encouraging result


and the MoOx was deposited at different substrate temperatures using e-beam

evaporation. This work is published in the Journal of Applied Physics.

The thesis is concluded in chapter 7 and the future prospect of this work is also

discussed.

In the appendix part performance and stability of perovskite solar cells using TiO2

deposited via e-beam evaporation and spray-pyrolysis are compared. It is shown that

a similar performance is achieved for both device but the device using e-beam

evaporated TiO2 ETL having high stability due to the presence of oxygen vacancies.

1.6 References

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"Solvent engineering for high-performance inorganic–organic hybrid

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[3] K. Ranabhat, L. Patrikeev, A. Antal'evna-Revina, K. Andrianov, V.

Lapshinsky, and E. Sofronova, "An introduction to solar cell technology,"

Istrazivanja i projektovanja za privredu, vol. 14, pp. 481-491, 2016.

[4] T. C. Sum and N. Mathews, "Advancements in perovskite solar cells:

photophysics behind the photovoltaics," Energy Environ. Sci., vol. 7, pp.

2518-2534, 2014.

[5] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T.

Leijtens, et al., "Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in

an Organometal Trihalide Perovskite Absorber," Science, vol. 342, pp. 341-

344, 2013.

[6] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, et al.,

"Long-Range Balanced Electron and Hole Transport Lengths in Organic-

Inorganic CH3NH3PbI3," Science, vol. 342, pp. 344-347, 2013.

[7] K. Tanaka, T. Takahashi, T. Ban, T. Kondo, K. Uchida, and N. Miura,

"Comparative study on the excitons in lead-halide-based perovskite-type

crystals CH3NH3PbBr3 CH3NH3PbI3," Solid State Communications, vol. 127,

pp. 619-623, 2003.

Introduction 13

[8] H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, et al.,

"Lead iodide perovskite sensitized all-solid-state submicron thin film

mesoscopic solar cell with efficiency exceeding 9%," Sci Rep, vol. 2, p. 591,

2012.


"Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-


[10] A. Abrusci, S. D. Stranks, P. Docampo, H. L. Yip, A. K. Jen, and H. J.

Snaith, "High-performance perovskite-polymer hybrid solar cells via

electronic coupling with fullerene monolayers," Nano Lett, vol. 13, pp. 3124-

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[11] K. T. Akihiro Kojima, Yasuo Shirai, and Tsutomu Miyasaka, "Organometal

halide perovskites as visible-light sensitizers for photovoltaic cells," J. AM.

CHEM. SOC., vol. 131, pp. 6050-6051, 2009.

[12] B.-W. P. Woon Seok Yang, Eui Hyuk Jung, Nam Joong Jeon, Young Chan

Kim, Dong Uk Lee, Seong Sik Shin, Jangwon Seo, Eun Kyu Kim, and S. I. S.

Jun Hong Noh, "Iodide management in formamidinium-lead-halide–based

perovskite layers for efficient solar cells," Science, vol. 356, pp. 1376–1379,

2017.

[13] N. J. Jeon, H. Na, E. H. Jung, T.-Y. Yang, Y. G. Lee, G. Kim, et al., "A

fluorene-terminated hole-transporting material for highly efficient and stable

perovskite solar cells," Nature Energy, 2018.

[14] F. Huang, Y. Dkhissi, W. Huang, M. Xiao, I. Benesperi, S. Rubanov, et al.,

"Gas-assisted preparation of lead iodide perovskite films consisting of a

monolayer of single crystalline grains for high efficiency planar solar cells,"

Nano Energy, vol. 10, pp. 10-18, 2014.

[15] J. T. M. M. Lee, T. Miyasaka, T. N. Murakami and H. J. Snaith,, "Low-

temperature processed meso-superstructured to thin-film perovskite solar

cells," CScience, vol. 338, pp. 643-647, 2012.

[16] M. Liu, M. B. Johnston, and H. J. Snaith, "Efficient planar heterojunction

perovskite solar cells by vapour deposition," Nature, vol. 501, pp. 395-398,

2013.

[17] M. A. Green, A. Ho-Baillie, and H. J. Snaith, "The emergence of perovskite

solar cells," Nat Photon, vol. 8, pp. 506-514, 2014.

[18] L. Wiegrebe, "An autocorrelation model of bat sonar," Biological

Cybernetics, vol. 98, pp. 587-595, 2008.

[19] J. T. Michael M. Lee, Tsutomu Miyasaka, Takurou N. Murakami, Henry J.

Snaith1, "Efficient Hybrid Solar Cells Based on Meso-Superstructured

Organometal Halide Perovskites," Science, vol. 338, 2012.


[20] L. Meng, J. You, T. F. Guo, and Y. Yang, "Recent Advances in the Inverted

Planar Structure of Perovskite Solar Cells," Acc Chem Res, vol. 49, pp. 155-

65, 2016.

[21] Huanping Zhou1, Qi Chen1, Gang Li, Song Luo, Tze-bing Song, Hsin-Sheng

Duan, et al., "Interface engineering of highly efficient perovskite solar

cells.pdf," vol. 345, pp. 542-546, 2014.

[22] M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, et al., "A

fast deposition-crystallization procedure for highly efficient lead iodide

perovskite thin-film solar cells," Angew Chem Int Ed Engl, vol. 53, pp. 9898-

903, 2014.

[23] Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, et al., "Efficient, high

yield perovskite photovoltaic devices grown by interdiffusion of solution-

processed precursor stacking layers," Energy & Environmental Science, vol.

7, p. 2619, 2014.

[24] B. Conings, L. Baeten, C. De Dobbelaere, J. D'Haen, J. Manca, and H. G.

Boyen, "Perovskite-based hybrid solar cells exceeding 10% efficiency with

high reproducibility using a thin film sandwich approach," Adv Mater, vol.

26, pp. 2041-6, 2014.

[25] J. Y. Jeng, Y. F. Chiang, M. H. Lee, S. R. Peng, T. F. Guo, P. Chen, et al.,

"CH3NH3PbI3 perovskite/fullerene planar-heterojunction hybrid solar cells,"

Adv Mater, vol. 25, pp. 3727-32, 2013.

[26] P. W. Liang, C. Y. Liao, C. C. Chueh, F. Zuo, S. T. Williams, X. K. Xin, et

al., "Additive enhanced crystallization of solution-processed perovskite for

highly efficient planar-heterojunction solar cells," Adv Mater, vol. 26, pp.

3748-54, 2014.

[27] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, et al., "CuSCN-Based

Inverted Planar Perovskite Solar Cell with an Average PCE of 15.6%," Nano

Lett, vol. 15, pp. 3723-8, 2015.

[28] A. S. Subbiah, A. Halder, S. Ghosh, N. Mahuli, G. Hodes, and S. K. Sarkar,

"Inorganic Hole Conducting Layers for Perovskite-Based Solar Cells," The

Journal of Physical Chemistry Letters, vol. 5, pp. 1748-1753, 2014.

[29] M.-H. Li, P.-S. Shen, K.-C. Wang, T.-F. Guo, and P. Chen, "Inorganic p-type

contact materials for perovskite-based solar cells," J. Mater. Chem. A, vol. 3,

pp. 9011-9019, 2015.

[30] Q. Jiang, X. Sheng, B. Shi, X. Feng, and T. Xu, "Nickel-Cathoded Perovskite

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25883, 2014.

Introduction 15

Literature Review 17

Chapter 2: Literature Review

2.1 Perovskite materials

Those materials possessing the crystalline structure of Calcium Titanium Oxide

(CaTiO3) are known as perovskite materials. The basic crystal structure of perovskite

materials with a chemical formula of ABX3 is shown in Figure 2.1. As discussed in

chapter 1, because of the extraordinary electrical and optical properties of

perovskites, these materials have attracted a tremendous attention in the past few

years. The high optical absorption coefficient (as high as 104 cm-1)[1], tunable

bandgap [2] and low exciton binding energy [3] and a long carrier mobility of up to 1

µm [4] make perovskite materials the best choice for photovoltaic applications.

Perovskite materials used for photovoltaic applications are hybrid organic and

inorganic metal halide compounds consisting of organic ammonium cations such as

CH3NH3+ (MA+) or NH2CHNH2

+ (FA+), inorganic cations such as Pb2+ or Sn2+ and

halogen anions Cl-, Br- or I-. In the molecular structure shown in Figure 2.1, the

organic cation is represented with A site, the inorganic cation with B and the anion

positioned at X.

18 Chapter 2: Literature Review

Figure 2.1 Typical crystal structure (unit cell) of perovskite material.

2.2 Working Principle of Perovskite Solar Cells

The different types of perovskite solar cells have been described in chapter 1. The

light absorbing perovskite layer is sandwiched between ETL and HTL of the solar

cell. As shown in Figure 2.2, upon exposure of the solar cell to sunlight, the

perovskite absorbs the light to produce the excitons (electrons and holes). These

excitons then form free carriers because of the difference in the binding energy of

perovskite materials and generate current. These generated free electrons and holes

are then separated at the ETL and HTL interfaces by the respective electron and hole

transporting layers. Electrons from perovskite material are then transferred to

electron transport layer (ETL) and holes are transferred to hole transporting layer

(HTL). Finally, the electrons are collected by TCO from ETL and hole collected by

metal back


Figure 2.2 Schematic of the basic working principle of perovskite solar cells.

electrode. The TCO and metal back electrode are connected to create a photocurrent

in the outer circuit. Due to the high carrier mobility and long diffusion length of the

perovskite materials, the PSCs have superior photovoltaic performance.

Figure 2.3 Schematic diagram of photo-generated charge transfer and recombination

process in perovskite solar cells [5].

As explained by Marchioro et al. the charge transport is achieved by the charge

separation at the ETL/perovskite and perovskite/HTL interfaces and charge injection


to ETL and HTL from perovskite (process i and ii in Figure 2.3) [5]. Process i and ii

are the required charge transfer procedures. At the same time, undesirable processes

which are detrimental to the performance of the perovskite solar cells also occur.

These undesirable processes include exciton annihilation (process iii), non-radiative

recombination, reverse transmission of electrons and holes (processes iv and v) and

the carrier recombination at ETL/perovskite interface (process vi). This whole

process of charge transport in the ETL/perovskite/HTL contributes to the

performance of the PSCs.

2.3 Perovskite Solar Cell Structure

Because of the ambipolar nature of the perovskite, various architectures are possible

for perovskite solar cells. Basically two device structures are constructed, the

mesoporous structure [6] and the planar hetero-structure [7]. Both the mesoporous

and planar structures are shown in Figure 2.4 a & b, respectively. The performance

of the solar cells can be improved by efficient separation of the charges, then

transporting the charges to respective charge transporting layer and efficiently

collected them at the electrodes.


solar cell.


Because of the large specific surface area (~1000 m2/g) and high porosity, the

mesoporous structure has been intensively used in perovskite solar cells [8]. The

efficiency of the perovskite solar cells is increased by allowing the light absorbing

layer to have good adhesion with the mesoporous charge transport layer which

increases the light receiving area of the light absorbing layer. By introducing a

mesoporous layer the perovskite can infiltrate into it and can have a better contact

and larger area which helps in the charge generation and mobility. The most used

mesoporous material for perovskite solar cells is TiO2, where the perovskite

penetrates into the pores and forms an interconnected layer. All solid state perovskite

solar cells with a mesoporous architecture was reported by Kim et al. and Burschka

et al. achieving efficiencies of 9.7 % and 15.0%, respectively [6, 9]. The PCE of the

mesoporous perovskite solar cell increased to 20% at the end of 2014 [10]. Recently

the highest efficiency of 23.2% on laboratory scale was reported by Jaemin

Lee & Jangwon Seo using TiO2 based mesoporous architecture [11]. High PCE

values have been achieved using mesoporous architecture, however, the high

sintering temperature required for TiO2 mesoporous layer is a barrier to the

commercialization of perovskite solar cells on flexible device and low cost solar cells

[9, 12, 13]. Also, the high processing temperature adds complications to the process

by increasing the processing time and cost of making the solar cell devices. Snaith

and co-worker substituted the mesoporous TiO2 with insulating Al2O3 and the device

was still working quite well [12]. This give an indication the perovskite can be used

both as light harvesting material and electron transporting layer and hence the PSC

device can be completed without the mesoporous layer. After the realization of the

ambipolar nature and the longer diffusion length of perovskite materials, more and

more interest was developed into the planar hetrojunction perovskite solar cells [1,

https://www.nature.com/articles/s41560-018-0200-6#auth-9




4]. These properties of perovskite materials opened the possibilities of removing the

high temperature sintered mesoporous layer and make a simple planar solar cell

structure, which can be processed at lower temperature [7, 14-16]. A planar

hetrojunction perovskite solar cell without the mesoporous layer is shown in Figure

2.4b. Planar devices are basically of two types, the regular structure (n-i-p) and

inverted structure (p-i-n) depending on the order on the transporting layers [17]. In n-

i-p structure a TCO is coated with an ETL followed by perovskite and then an HTL

and finally a metal electrode as shown in Figure 2.5a while in a p-i-n structure TCO

is coated with a HTL then perovskite which is followed by ETL and a metal back

contact as shown in Figure 2.5b. Regular planar structures are the most explored

PSCs due to the high performance compared to the inverted structures and the ease

of fabrication and simpler architecture.

Figure 2.5 Device architecture of regular n-i-p (a) and inverted p-i-n (b) perovskite

based solar cell.

There are many factors effecting the device performance including a better light

absorption by perovskite layer and charge collection by charge transporting layers.

The most crucial role in achieving high open-circuit voltage and high current density

is played by optimally aligning the electronic levels between perovskite light

absorbing layer and charge transport layers (ETL&HTL). A non-optimised interface

badly effect the device performance and therefore a better band alignment between


the perovskite and charge transport layer must be made to achieve high performance

[118]. Among the various parameters affecting the device performance, charge

injection at the interfaces through energy band edge match between the perovskite

light absorber and the ETL/HTL is critical. Thus, it can be stated that band edge

matching is one of the helpful predictors of device performance as discussed in

chapter 4 of the thesis.

2.4 Charge Transporting Layers (CTLs)

Charge transporting layers (both ETL and HTL) play a vital role in the performance

of perovskite solar cells. CTLs extract either holes or electrons and inject them to the

respective electrodes. Therefore, an ideal CTL must have a good energy level

matching with the perovskite layer to facilitate the photogenerated charges and

reduces the charge loss [18-21]. Also, a high optical transmission is required so that

maximum amount of light passes through to the perovskite layer. A high charge

mobility of the CTL is critical to efficiently transport the charge to respective

electrodes. Similarly a compact layer with full coverage and pinhole free layer is

required to ensure excellent charge transport and minimize any recombination. The

CTLs should have a better interface contact with the perovskite layer. CTLs should

also have an optimal thickness, which should be thick enough to give full coverage

and thin enough to reduce the series resistance (Rs) which can help in achieving high

fill factor (FF) and hence high device performance. Additionally CTLs help to avoid

the diffusion of metal ions, oxygen and moisture to perovskite, and make them stable

in air. Various organic and inorganic charge transport layers as ETL and HTL have

been reported in literature [9, 22-25]. Various strategies have been applied to

improve the properties of charge transport layers and hence the device performance.


2.4.1 Electron Transport Layer (ETL)

The basic purpose of electron transport layer is to create an electron-selective contact

with perovskite layer to enhance the extraction efficiency of electrons from the light

absorbing perovskite layer and blocking the hole. This can reduce the carrier

recombination and enhance the carrier’s separation and performance of the device.

The basic selection criteria for electron transport layer include an n-type large

bandgap semiconductor that is transparent to visible light having higher carrier

mobility and electrical conductivity to transport electrons effectively. Also, the

semiconductor should have a good band alignment with the perovskite material.

Furthermore, it should be low cost and processed at low temperature. Finally, the

ETL need to be stable in ambient condition upon exposure to the environment.

Different organic and inorganic electron transport materials have been used for

perovskite solar cells including TiO2, SnO2, PCBM, PEDOT:PSS, Spiro-OMeTAD

[26-29].

2.4.1.1 Organic Electron Transport Layer

The commonly used organic electron transport layer is fullerene and its derivative

i.e. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or C60. An organic electron

transport layer is the best choice for inverted PSCs, because of its low temperature

processability, good electron mobility and suitable band alignment with the

perovskite material [15, 30]. For organic electron transport layer usually an extra

buffer inter layer is introduced for a better energy alignment. Usually a

bathocuproine (BCP) or bathophenanthroline (Bphen) is used for tuning the work

function of the electrode [31, 32]. PCBM is the most used organic electron transport

layer in PSCs because of its high electrical conductivity (~10-7 Scm-1) [33]. Also

because PCBM has suitable surface work function and appropriate band alignment


with perovskite layer, it is very effective in extracting electron at the

PCBM/perovskite interface. The effective electron extraction is also because of the

large energy offset (0.4 eV) between the lowest unoccupied molecular orbit (LUMO)

of CH3NH3PbI3 (−3.9 eV) with that of PCBM (−4.3 eV) [28, 34]. Hysteresis is one of

the biggest challenges for perovskite solar cells which is believed to be because of

the charge traps on the surface of the perovskite. PCBM can help to cure these traps

and reduce the current voltage hysteresis for PSCs. Huang and his team reported that

the traps present on the surface of perovskite can be passivated by depositing

fullerene on the top of perovskite and eliminate hysteresis effect as shown in Figure

2.6 [35]. The performance of the cell is increased and the hysteresis is reduced with

the introduction of fullerene to the PSCs [36-39]. The performance of the device can

be further improved by changing the structure of PCBM [21]. However, organic

charge transport materials are expensive and are not stable upon exposure to the

environment. The poor stability of these materials are not suitable for practical

applications in ambient condition due to their degradation by humidity and oxygen

[17].


.

Figure 2.6 Schematic of the reduction of surface recombination by passivating the

trap states by incorporation of PCBM layer [35].

2.4.1.2 Inorganic Electron Transport Layer

Because of the poor stability and higher material cost of the organic electron

transport layer, an alternative ETL is required for perovskite solar cells. Inorganic

metal oxides, having wide bandgap, high electron mobility and deeper conduction

band with better stability and lower material cost can be a better alternative for the

organic electron transport material. TiO2 is the most used electron transport inorganic

metal oxide material for perovskite solar cells. TiO2 has been highly utilised in all

high performing perovskite solar cells [40, 41]. However, the high processing

temperature (>450 ˚C) of TiO2 leads to high production cost and process


complication which restricts its use for flexible devices and limits the chances of

commercialization [42, 43]. Also, the decomposition of perovskite material by the

TiO2 upon exposure to ultraviolet light is another issue of TiO2 ETL. In addition the

bulk electron mobility of TiO2 (<1 cm2 V-1s-1) is lower than most of its counterparts

[44]. Due to the low electron mobility the recombination rate increases and the

device performance decreases [45]. Some low temperature processed TiOx based

ETL with a power conversion efficiency of 17.6% have been introduced [46].

However, the poor stability of the TiOx based perovskite solar cells under UV light

encounters a problem [47]. To overcome these problems and achieve commercially

competitive PSC device alternative metal oxides such as ZnO, SnO2, WO3, CeOx and

ZrO2 have been used as alternative ETL to TiO2 for perovskite solar cells. Although

these various metal oxide ETLs have been used as replacement for TiO2, but the

processing temperature, poor chemical stability and mismatch of band alignment

between the perovskite and the ETL are still some of the challenges.

ZnO has a higher bandgap and high electron mobility than TiO2 which can be a

better alternative as electron transporting layer for perovskite solar cells [48]. ZnO

thin films can be fabricated at low temperature for larger area perovskite solar cells.

Both nanorods and nanoparticles of ZnO have been used as ETL with power

conversion efficiencies of 11.13% and 15.7%, respectively [49, 50]. Kumar et al.

used chemical bath deposition technique to fabricate ZnO nano-rods on a flexible

substrate and achieved 2.6% efficiency [51].


Figure 2.7 Perovskite solar cells using low temperature ZnO as ETL on flexible PET

substrate [49].

Liu and Kelly fabricated low temperature perovskite solar cell using ZnO

nanoparticles achieving 15.7% efficiency on FTO substrate and 10% on flexible

substrate as shown in Figure 2.7 [49]. Similarly ZnO quantum dots with a higher

conductivity than pristine ZnO and TiO2 have also been used as ETL for perovskite

solar cells [52]. Doping of ZnO also helped in further improving the properties of

ZnO. ZnO doped with Al, Mg and Li have been reported with improved electron

mobility [53-55]. However, the chemical instability of ZnO is another issue which

can cause degradation of the perovskite [56]. The reverse decomposition of ZnO at

the ZnO/perovskite interface (because of the low temperature deposited ZnO) and

bandgap mismatch are the major drawbacks of ZnO used as ETL.

Recently, SnO2 has attracted attention as the ETL for perovskite solar cells, and

appeared the most promising alternative to TiO2. Due to the higher electron mobility

of bulk SnO2 (240 cm2 V-1s-1) and high conductivity the electron transport efficiency

increases and this can help in limiting recombination losses. Similarly the wide


optical bandgap of the material (3.6-4.1 eV) confirms high transmittance over a wide

spectral range [27]. Therefore, most of the light can pass through the SnO2 and

absorbed by the perovskite light absorbing material. The wide optical bandgap

energy of SnO2 allows a higher chance of band alignment with the perovskite

material. Also SnO2 is processable at low temperature and this can reduce the

complexity of high temperature processing. Low processing temperature reduces

fabrication cost and can reduce the complexity of the process. Low process

temperature is also useful in the development of flexible devices and tandem solar

cells. The basic purpose for high temperature sintering is to get crystalline films and

better conductivity. SnO2 has much higher conductivity and photo-stability than TiO2

which can also help in the perovskite solar cell device stability. Therefore, the above

properties make SnO2 a better alternative to TiO2 as ETL to be used in perovskite

based solar cells.

SnO2 as ETL has already been used in organic and dye sensitised solar cells with

minimum success [57, 58]. In perovskite solar cells SnO2 was introduced in 2015

achieving 10.18% PCE [59]. Song et al. improved the performance of the SnO2 base

PSC to 13% utilising low temperature compact SnO2 with 30 days stability in

ambient environment [60]. The efficiency of the low temperature solution-based

nano-crystalline SnO2 based PSC device was further improved to 16% by Weijun Ke

et al. who produced better conductivity of SnO2 and suitable band alignment with

perovskite, [22]. Fand and co-workers further improved the efficiency to 17.21% by

producing the SnO2 by thermal decomposition of SnCl2.2H2O [20]. Hagfeldt and his

co-worker achieved power conversion efficiency of 18% with high Voc of 1.19 eV

using atomic layer deposited (ALD) of SnO2 [61]. J. You et al. got a 19.9%

efficiency with no hysteresis using high quality SnO2 deposited at 150 ˚C as shown


in Figure 2.8 [62]. A further work from Hagfeldt group increased the power

conversion efficiency to 20.7% using a double-layered SnO2 using spin coating and

chemical bath deposition (CBD) techniques [63]. You’s group has also pushed the

efficiency to 21.6% by using finely controlled SnO2 based PSC [62]. Even though

high efficiency has been achieved by increasing the charge mobility and matching

band alignment of the SnO2 ETL with perovskite using the above mentioned

techniques, the bigger challenge is that these techniques are either expensive or the

results are not reproducible. A new technique to make ETL and perovskite without

doping ETL and putting an extra layer is required to further simplify device

fabrication process. Also, the present deposition methods described above do not

provide good uniformity over a larger area.

Figure 2.8 J-V curve of low temperature SnO2 based perovskite with minimized

hysteresis [63].

Another low temperature processable potential ETL is WO3. WO3 has high stability

in corrosive environment with wide bandgap energy (2.7-3.9 eV) and can be a

replacement for TiO2. WO3 has high electron mobility (10-20 cm2V-1s-1) and high

stability in moisture and ambient environment with low material cost. Because of its


wide bandgap most of the light will transmit through this material and reach the light

absorbing perovskite layer. Stoichiometric WO3 prepared by solution method and

processed at 150 °C was reported by Wang et al. In their report they compared WOx

with TiO2 as ETL and showed that the WOx based device have same current density

as TiO2 based device but with lower Voc as shown in Figure 2.9 [64]. The lower Voc

in the WOx based device is believed to be because of the charge accumulation at the

ETL/perovskite interface due to imbalanced charge transfer. Various techniques have

been used to improve the Voc of the WO3 based PSCs. Wang et al. mixed amorphous

TiO2 with WOx to be used as ETL for PSC and achieved 17.47% PCE using low

processing temperature (150 °C) [65]. In another study Niobium doped WOx was

prepared by solution method and used as ETL for flexible solar cell achieving

15.65% PCE [66]. Further modification is required for WO3 based ETL to compete

with the SnO2 based ETL for PSCs.

Figure 2.9 Solution based low temperature processed WOx thin film ETL showing

lower Voc and higher Jsc values as compare to TiO2 based ETL for PSCs [67].


2.4.2 Hole Transport Layer (HTL)

Hole transporting layers are the key component for achieving high performance

perovskite solar cells. The main role of the HTL is the extraction and transportation

of photogenerated holes from the light absorbing perovskite material to the back

electrode material so as to limit charge recombination [68, 69]. The general

requirements for a successful hole transporting material include a better energy level

alignment with the perovskite material, high hole mobility and good conductivity so

that the charges are effectively transported, and it should have a better photo and

thermal stability and should have low material cost. Various organic and inorganic

hole transport materials such as MoO3, NiO, CuO, Cu2O, V2O3, Spiro-OMeTAD,

P3HT, and PEDOT:PSS have been used for perovskite solar cells [27, 70-72].

2.4.2.1 Organic Hole Transport Layer

In conventional high efficiency perovskite solar cells many organic hole transporting

materials have been used [11, 32, 69, 73, 74]. The most successful organic HTM for

perovskite solar cells with efficiency over 20% is spiro-OMeTAD [11, 75]. Another

common group of HTMs consisting of organic materials is conjugated conducting

polymers [76-80]. The best performance of 22.1% is shown by Poly(triarylamie)

(PTAA) [40]. Even though high performance has been achieved with these organic

HTMs, but due to the complicated synthesis process the price of these materials is

too high and severely affects the PSC device prospects in large scale application.

Also, due to the doping of hygroscopic lithium salt, the stability of the device is in

question. Another organic hole transport layer which has been widely used in

inverted perovskite solar cells is PEDOT:PSS [72, 81]. Due to the acidic nature and

hygroscopic behaviour of the PEDOT:PSS, the material is harmful for long-term

stability of the perovskite solar cells. Therefore, hole transport materials should have


low cost and higher stability in ambient environment for the commercialization of

perovskite solar cells.

2.4.2.2 Inorganic Hole Transport Layer

Inorganic metal oxide p-type semiconductors such as nickel oxide (NiO), copper

oxide (CuO2, CuO) and molybdenum oxide (MoO3) are promising alternatives to

organic hole transport material for perovskite solar cells. The lager bandgap, high

hole mobility and higher conductivity, better stability, suitable energy level matching

with perovskite and low cost of these inorganic metal oxides make promising

alternatives as HTMs for perovskite solar cells. Cuprous and cupric oxides (Cu2O,

CuO) have the bandgaps of 2.1-2.6 and 1.3-2.0 eV respectively and hole mobility

exceeding 100 cm2 V-1 s-1 [82, 83]. Cu2O has been widely used as HTL due to the

abundance of the material, low material cost, high absorption coefficient and its

nontoxic nature of [84, 85]. Both Cu2O and CuO were utilised as HTL by Zuo and

Ding using low temperature processing (250 ˚C) [86]. Chatterjee and Pal used ionic

layer adsorption and reaction method for the deposition of Cu2O and utilised as

HTM for inverted perovskite solar cells having efficiency of 8.3% [87]. Wu and co-

workers used sputter deposited Cu by annealing in air at 250 ˚C for an hour as an

HTM for PSC and obtained 11% PCE using a 5 nm thin layer [88]. PCE of 17% was

obtained by Bian and co-workers using CuOx and further improved to 19% by

modification of the perovskite layer. They claim a higher stability of the PSCs using

CuOx based HTM [89]. In regular perovskite structures Cu2O as HTM is used by

Ahmadi et al. achieving maximum PCE of 8.7% [71].

Nickel oxide (NiO) is another wide bandgap (3.6 eV) p-type metal oxide

semiconductor with high stability and low cost [83]. NiO has been widely used in

dye-sensitized and organic solar cells [90-92]. Chen and co-workers applied the


solution processed NiOx to perovskite solar cells and studied the effect of ultraviolet

ozone (UVO) treatment which resulted in better surface wettability and obtained

7.8% PCE [93]. Further work on NiO as HTM for perovskite was done by Tang and

Yang and achieved PCEs of 7.6% and 9.1%, respectively. With further modification

of NiO by doping with Cu the efficiency of NiO based PSC was further improved.

Jen and co-workers used Cu:NiOx in inverted perovskite devices and obtained a high

PCE of 15.4% [94]. To achieve high crystallinity and conductivity they annealed the

Cu:NiO thin films above 400 ˚C. The same group further developed low temperature

(150 ˚C) process and obtained higher conductivity than the high temperature sintered

sample and hence higher PCE of 17.7% [29]. Similarly Mg2+ and Li+ doped NiO

developed by spray pyrolysis method was utilized by Han and his group achieving a

high PCE of 18.3% [95]. Recently they have achieved 19.6% PCE for a larger area

of over 1 cm2 [96]. Seok et al. used pulsed laser deposited NiO post-annealed at 200

˚C and got 17.3% efficiency [70]. An ultrathin atomic layer deposited (ALD) NiO

has also been used as HTM and achieved 16.4% efficiency [97]. Jen and co-workers

used nano-crystalline NiOx and obtained 17.6% efficiency. Zhou et al. used nano-

crystal of NiOx in regular and inverted structures showing PCE of 9.5% and 15.9%,

respectively [98]. Recently Panagiotis Lianos et al. have incorporated nanoparticles

of NiO to carbon electrode and an improved stability of the PSC has been observed

[99, 100]. These results justify NiO as a potential HTL for perovskite solar cells.

MoO3 is another potential inorganic semiconductor that is utilized in organic solar

cells due to low cost, high transmittance, excellent stability and good charge mobility

[101]. MoOx has been used with spiro-OMeTAD and PEDOT:PSS to improve the

performance of perovskite solar cells [102, 103]. The first time in 2006 Yang’s group

reported MoO3 can effectively replace PEDOT:PSS [104]. An ultrathin MoOx has

https://www.sciencedirect.com/science/article/pii/S0008622317310072#%21


been used as cathode buffer by Schropp et al. for a perovskite solar cells [105]. Kim

et al. used MoOx as an interfacial layer and obtained a high Voc of 1.02 V from

vacuum deposited perovskite solar cell [106]. Sun et al. have developed flexible

perovskite solar cell using MoO3/Au semitransparent electrode [107].

Therefore, these inorganic metal oxides have the potential to be used as hole

transporting materials for perovskite solar cell devices to improve the stability of the

devices and reduce the cost by replacing the expensive organic HTM. This could

represent another step closer to the commercialization of perovskite solar cells.

2.5 Deposition Techniques for Charge Transport Layers (CTLs)

Various methods have been used for the deposition of the CTLs (ETL and HTL),

including solution based, thermal evaporation and physical vapour deposition

techniques. Each of these methods have their own advantages and limitations but the

physical vapour deposition have the greatest average for deposition ETL and HTL

inorganic metal oxides discussed in the previous sections. For most of the high

efficiency devices spin coating technique is used for the deposition of CTLs,

however, this method have several shortcomings. The spin-coated films have a poor

film quality and imprecise control of the film thickness. Also, uniformity of the film

is another issue over the lager area of substrate. Even though a very smooth film can

be produced but the thickness is not consistent from centre to edges of the sample.

This uneven thickness can cause charge recombination and affect the performance of

PSCs as shown in Figure 2.10 [108]. Therefore these techniques have limitations for

large scale manufacturing and commercialization of PSCs [52].


Figure 2.10 Spin coated TiO2 with irregular film thickness, poor contact with

substrate (FTO) and discontinuous areas [108].

In an attempt to make compact and pinholes free thin films, sol-gel and dip-coating

methods have also been applied but they were facing the same problems. Even

though electro-deposition has a better control of the thickness, the films are not

completely pinhole free. Atomic layer deposition techniques (ALD) has firm control

over the film thickness, uniformity and surface roughness and the films can be fully

covered with a small thickness of ~10 nm. Wo et al. used ALD deposited ETL and

have observed a major improvement in the device performance as compared to the

solution based spray pyrolysis method [109]. ALD has a good control over film

thickness uniformity and quality but the higher cost limits its proof of concept.

Therefore, a large scale industrial method for the preparation of charge transport

layers (ETL and HTL) of desired properties (electronic, electrical, optical, crystal

structure, etc), morphology (surface roughness, film uniformity, etc) with low

manufacturing cost for high efficiency PSCs is highly needed. To achieve these

outcomes advance physical vapour deposition techniques such as sputtering and e-

beam evaporation methods have been explored in this research work.

Sputtering and e-beam are industrially viable and well established techniques. These

advance deposition techniques have a full control over film thickness, quality,

uniformity and composition over a large area [26, 27, 110]. Also, as surface


morphology plays a vital role in determining better CTLs properties and so as device

performance so a better control of the morphology and composition is needed. The

composition of the film can also play a role in changing the energy level and hence a

best energy level matching can be achieved by tuning the proper composition. Also,

a suitable technique is the one with which the low temperature deposition is possible.

Various experimental parameters can control the electrical and optical properties of

the films and thus engineered the CTLs according the demands of applications [27,

111]. Also, various HTL can be deposited on perovskite materials without damaging

the perovskite and without needing any post annealing. Therefore, for large scale

production and commercialization of perovskite sputtering and e-beam techniques

are extremely important for the deposition of thin films CTLs.

2.5.1 Sputtering and e-beam Techniques for Perovskite Solar Cells

PVD (sputtering and e-beam) are well-established technology for large scale

production of thin films having a good controlled over the film uniformity

composition and altering the properties for variable applications, still there are few

reports on PVD deposited thin films for perovskite solar cells. Apart from these

advantages there are very few reports on PVD deposited charge transport layers for

PSCs application. Sputtered TiO2 as ETL is reported in literature and claimed better

performance for the device due to better uniformity and compactness of the film.

Hong et al. deposited TiO2 with different techniques (sputtering, spin coating, sol

gel, etc) and compared the surface morphology and performance for PSCs [112]. In

their study it is shown that sputter deposition of TiO2 (see Figure 2.11) gives a better

surface coverage and uniformity for all thicknesses and hence a better device

performance. Jin et al. obtained a high Jsc of 24.19 mA/cm2 and an efficiency of

17.25% with sputtered TiO2 ETL deposited at room temperature [113]. Wook et al.


also reported a similar current density value for sputtered TiO2 compact layer in their

study [114].

Figure 2.11 TiO2 blocking layer deposited by (a) sputtering and (b) spin coating

technique for PSCs [112].

Similarly Sharp et al. deposited sub-stoichiometric TiO2 with oxygen vacancies

using e-beam evaporation [26]. The native defects were produced by depositing TiO2

in an oxygen deficient environment. Efficiency 19% with good stability was reported

due to deep-level defects in the TiO2 film. A similar results were obtained in our

work where an e-beam deposited TiO2 with reduce amount of oxygen displayed a

stable device performance for over 20 days in ambient environment. The results are

shown in the appendix. These results are clear evidence that PVD deposition can be a

better alternative to the solution based processes.

Similarly, sputter deposited SnO2 has been used successfully for various

applications. K. Bouras et al. has deposited SnO2 by sputtering whose electrical and

optical properties were improved by tuning the deposition temperature for CIGS

application. It was found that high electrical and optical properties were obtained at

300 °C [115]. Fang et al. investigated room temperature RF magnetron sputtering for

deposition of SnO2 thin films. The effect of oxygen partial pressure on electrical and

optical properties were investigated where a high transmittance of over 80% in the

(a) (b)


visible region and good conductivity of 9.1 x 10-4 Ωcm at 2% oxygen partial pressure

[116]. A. Alhuthali et al. investigated the effect of post-annealing temperature on the

optical properties and found that the bandgap reduced with increasing the annealing

temperature [117]. These results are suggest that sputtering is a powerful and more

reliable technique for deposition of thin film metal oxides for various application.

Still there was not a single report on sputtered SnO2 for perovskite solar cells.


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Research Methodology 51

Chapter 3: Research Methodology

This chapter describes the design adopted by this research to achieve the aims and

objectives stated in Chapter 1. The first section of this chapter discusses the

methodology and basic working principle of sputtering and e-beam evaporation used

in the study for the deposition of metal oxide thin films. The second section details

the preparation of perovskite and organic hole transporting materials. The third

section lists all the characterization techniques used for characterization of the thin

films and analysing the device performance.

3.1 Working Principle of Sputtering and E-beam evaporation

Physical vapour deposition (PVD) including magnetron sputtering and electron beam

(e-beam) evaporator techniques were used for the deposition of metal oxide thin

films.

Sputtering is a physical vapour deposition technique conducted under low/medium

vacuum in a chamber containing inert gas usually Argon (Ar). The chamber is first

evacuated to a high vacuum to achieve high purity of the film before argon gas is

introduced to the chamber. A negative charge is applied to the target material (which

is to be deposited on the substrate). Free electrons from the negative charge eject

electrons from the argon gas after collision due to same charge repulsion. The inert

gas (Ar) atoms become positively charged ion after losing the electrons creating

plasma. These positively charged ions are attracted to the negative charge applied to

the target materials with high speed which causes ejection of atoms from the target

surface due to high energy of collision. The ejected atoms move through the vacuum

and deposit on the substrate in the form of thin films. For depositing metal oxide thin

films either a metal oxide target is used or a metal target using reactive sputtering

52 Chapter 3: Research Methodology

technique is applied. In reactive sputtering oxygen is introduced to the chamber

during deposition. This oxygen gas chemically reacts with the ejected atoms from the

target before depositing on to the substrate. The oxygen gas becomes ionized and

reactive in the presence of plasma inside the sputtering chamber as a result of high

collision energy. A sputtering system with a heater can be advantageous to enhance

the quality and stoichiometry of the films. Sputtering system with multiple sources

can allow deposition of multilayer or mixed thin films. The sputtering process with a

single source is shown by the schematic diagram in Figure 3.1. As shown in Figure

3.1 the ionized gas is bombarded on the sputtering target where the atoms are ejected

from the target. The ejected atoms are represented by grey dots while argon ions are

represented by green dots. A substrate shutter protect the substrate from unwanted

materials deposition. The shutter is the removed when the deposition starts. The

substrate continuously rotates as shown by the arrow sign in Figure 3.1.

Figure 3.1 Schematic diagram of sputtering deposition process.


Electron beam evaporation (e-beam) is a physical vapour deposition technique for

deposition of thin films in a chamber at high vacuum. A strong electron beam is

produced using tungsten filament at high voltage and directed onto a crucible

through electromagnetic control commonly curving the beam by 270 degree as

shown in Figure 3.2. The e-beam hits a target material (evaporant) in the crucible and

caused to evaporate. Unlike in sputtering, the process in e-beam evaporation is

conducted in high vacuum and produces high purity (low contamination) of the thin

films. E-beam evaporation system with multiple rotary pockets can allow deposition

of multilayer thin films. There is a continuous flow of coolant beneath the crucibles

during operation. The evaporation rate is controlled through the electron beam’s

power. A quartz crystal monitor is installed inside the chamber to measure the film

thickness. A heater may be used to enhance the quality and stoichiometry of the

films. E-beam evaporation can be used for a wide range of applications to coat high

purity metals and metal oxides thin films.

Figure 3.2 Schematic diagram of e-beam evaporation technique [1].


3.2 ETL and HTM metal oxide thin films deposition

Magnetron sputtering (PVD 75 K.J. Lesker with 4 targets of 50 mm diameter) was

used to produce metal oxide thin films. High purity (99.99%) metal target using

oxygen as reactive sputtering or tin oxide target (99.99% purity) was used. For

deposition of SnOx thin films the chamber was evacuated to a base pressure of less

than 1 x 10-6 Torr. Argon gas (99.99% purity) was then introduced into the chamber

and maintained at the desired pressure for sputtering. Different target powers ranging

from 30-100W DC were used for the deposition of the SnOx thin films. The optimum

power of 60W was choosed for this study. Also, different thicknesses ranging from

20-100nm were used having 60 nm as optimum thickness for this thickness. Various

working pressures from 3 to 10 mTorr was used to for sputtering process. During

deposition the substrate was continuously rotating to ensure uniform coating. For the

reactive sputtering oxygen gas (99.99% pure) was introduced to the chamber. The

ratio of argon to oxygen can be varied to get different composition of tin oxide. Ar/O

ratio was 80/20. The film composition and thickness can also be changed by

increasing the target power and deposition time, respectively. The sputtering system

is shown in Figure 3.3a while Figure 3.3b shows the sputtering chamber consisting of

four targets.


Figure 3.3 (a) Sputtering system (PVD 75 Kurt J. Lesker) (b) inside chamber of

sputtering showing four targets.

MoOx and WO3-x thin films were deposited using electron beam evaporation

technique (PVD 75 Kurt J. Lesker) from MoO3 and WO3 pellets (99.9% purity),

respectively. For the films deposited at high, the substrate was preheated to the

desired temperature and maintained at that temperature throughout the deposition.

After deposition the samples were allowed to naturally cool in the vacuum chamber

to less than 50 ˚C before they were taken out of the chamber for characterization.

Before each deposition, the substrates were cleaned thoroughly with acetone, ethanol

and then dried with nitrogen gas. The chamber was evacuated to a base pressure of

less than 1 x 10-6 Torr. The films were deposited at a deposition rate of 1Å/s as

controlled by quartz crystal monitor. During deposition the substrate was

continuously rotating at 10 rpm in order to ensure uniform and homogenous coatings

fully covering the substrate.

The metal back contact (Au/Ag) is also deposited by e-beam evaporation at 1 Å/sec

deposition rate.


The motivation for these techniques have been discussed in respective chapters (4-6),

where it’s advantages over other methods are explained.

3.3 Perovskite deposition

Methylammonium lead tri-iodide (MAPbI3) perovskite film was deposited from a

precursor solution of a lead iodide (PbI2) and methylammonium iodide (MAI). To

prepare perovskite solution, a mixture of 461 mg of PbI2 and 159 mg MAI, was

dissolved in 650 mg of dimethyl formamide (DMF) and 78 mg of dimethyl sulfoxide

(DMSO) at room temperature for more than 4 hours. The solution was then filtered

with a syringe (filter size 0.22 µm) before deposition. The prepared perovskite

solution was deposited by spin coating. A 60 µL solution was dropped onto the

substrate and rotated at 4000 rpm for 30 s. A 0.5 mL of diethyl ether was dropped

onto the spinning substrate after 12 seconds of rotation. The deposited perovskite

layer was then dried in two steps on the hot plate. At first the film was heated at 65

ºC for two minutes and then annealed at 100 ºC for another 2 minutes.

3.4 Spiro-OMeTAD (HTL) deposition

The hole transporting layer (HTL) of Spiro-OMeTAD layer was prepared by

dissolving 72 mg of Spiro-OMeTAD powder into 1 mL chlorobenzene, this was

followed by adding of 18 µL Li-TFSI (520 mg/mL in acetonitrile) and 28.5 µL 4-

tert-butylpyridine (TBP). The prepared Spiro-OMeTAD solution was deposited by

spin coating at 4000 rpm for 35 seconds.

3.5 Characterization

The surface morphology of the thin film was characterized by scanning electron

microscope (SEM). Field emission scanning electron microscope (FESEM JOEL

7001F) using a 5 KV accelerating voltage. Scanning Kelvin Probe Force Microscopy


(KPFM) (Oxford instrument, Asylum Research) was performed to measure the

surface roughness using a NSG-03 Pt coated cantilever at room temperature. The

composition of the films was analysized using X-ray photoelectron spectroscopy

(XPS) was carried out using Kratos Axis Supra with a monochromatic Al Kα source

(1486.7 eV) and 160 eV energy of the survey scan. CasaXPS was used for the

quantification of the XPS data. To find the energy levels (valance and

conduction band positions) ultraviolet photoemission spectroscopy (UPS)

measurements were taken on the cleaned surfaces using the same XPS

instrument with a He I source (21.22 eV) and an analyzer pass energy of 20

eV. The film thickness was calculated by stylus profilometer and further

confirmed by an ellipsometry. Transmittance of the thin films was measured

using Cary 5000 UV-Vis-NIR spectrometer from 200-1100 nm wavelength

range, using a 150 mm integrating sphere. The oxygen vacancies in the metal

oxide thin films were detected by XPS and are confirmed by electron spin

resonance (ESR or EPR) using a Magnetech MiniScope MS400 system

operating at X band with a microwave power of 20 mW and modulation

amplitude of 1.20 mT. The perovskite solar cells performance was measured

by a solar simulator (Oriel Sol3A, Newport) equipped with 450 W Xenon lamp. The

performance of perovskite solar cells was measured under irradiation of 100

mW/cm2 (AM1.5) provided by a solar simulator (Oriel Sol3A, Newport) equipped

with 450 W Xenon lamp. The measurement were done in both reverse and forward

direction (by applying current and by applying voltage) with a dwell time of

100msec at a scan rate of 0.14 V/s using an active area of 0.09cm2. For each condition

16-20 samples were made in different batches to measure the reproducibility. The

values obtained from I-V measurements are up to four decimal points which we


rounded to two decimal points. The external quantum efficiency (EQE) of the device

was measured using a quantum efficiency system (IQE 200B, Newport) under AC

mode. The impedance spectroscopy (IS) of the PSCs was measured with an

electrochemical workstation (VSP BioLogic) at open circuit condition under 1 sun

illumination in a frequency range from 1 MHz to 100 mHz. An AC voltage with

perturbation amplitude of 20 mV was applied on the sample in the IS measurement.

3.6 Simulation of Perovskite Solar Cells

Solar cell capacitance simulator software (SCAPS) was used for the simulation of

perovskite solar cells. SCAPS is a 1D simulation software for thin film solar cells

but as the structure of perovskite solar cells resembles the thin film solar cells, so

with some minor modification the software can be used for PSCs as well. SCAPS

was developed in Computer and Informatics faculty of university of Gent in

Belgium. The input parameters used for the simulation are taken from literature and

experiment values.

3.7 Reference

[1] M. E. A. Hussein, "Fabrication and Characterization of GaN based Nanowire

for Photoelectochemical Water Splitting Applications," 2015.

60 Chapter 4: Tuning of Oxygen Vacancy in sputter-deposited SnOx films for Enhancing the Performance of


Chapter 4: Tuning of Oxygen Vacancy in sputter-deposited SnOx

films for Enhancing the Performance of Perovskite Solar Cells

Fawad Ali, Ngoc Duy Pham, H. Jonathan Bradford, Nima Khoshsirat, Ken Ostrikov,

John M. Bell, Hongxia Wang* and Tuquabo Tesfamichael*

Corresponding author email: [email protected]

[email protected]

The authors listed below have certified that:

1. they meet the criteria for authorship in that they have participated in the

conception, execution, or interpretation, of at least that part of the

publication in their field of expertise;

2. they take public responsibility for their part of the publication, except

for the responsible author who accepts overall responsibility for the

publication;

3. there are no other authors of the publication according to these criteria;

4. potential conflicts of interest have been disclosed to (a) granting

bodies, (b) the editor or publisher of journals or other publications, and

(c) the head of the responsible academic unit, and

5. they agree to the use of the publication in the student’s thesis and its

publication on the QUT’s ePrints site consistent with any limitations

set by publisher requirements.

In the case of this chapter: Contributor Statement of contribution

Fawad Ali Designed and conducted the experiments, analyze

the data and wrote the manuscript. Also drawing

the structure for front cover of the journal.

Ngoc Duy Pham Assistance with EIS measurement and discussion

Jonathan Bradfod Helped with XPS and UPS data acquisition and analysis

Nima Khoshsirat Helped in structure design in Vesta software

Ken Ostrikov Manuscript revision and supervision

John M. Bell Manuscript revision and supervision

Hongxia Wang Manuscript revision and supervision

Tuquabo Tesfamichael Manuscript revision and supervision

Principal Supervisor Confirmation

I have sighted email or other correspondence from all Co-authors confirming their

certifying authorship. (If the Co-authors are not able to sign the form please forward

their email or other correspondence confirming the certifying authorship to the RSC).

Name Signature Date

mailto:[email protected]


http://www.mopp.qut.edu.au/F/F_01_03.jsp

Tuning of Oxygen Vacancy in sputter-deposited SnOx films for Enhancing the Performance of Perovskite Solar

Cells 61

Tuning of Oxygen Vacancy in sputter-deposited SnOx films for

Enhancing the Performance of Perovskite Solar Cells

This chapter was originally accepted for publication in Journal ChemSusChem in

2018. This paper addresses the use of sputter-deposited SnOx as ETL for perovskite

solar cells. The role of oxygen vacancies was found to be crucial for tuning the

electrical and optical properties of SnOx thin film and on the performance of

perovskite solar cells.

ChemSusChem 2018, 11, 3096 –3103

DOI: http://dx.doi.org/10.1002/cssc.201801541

http://dx.doi.org/10.1002/cssc.201801541



ABSTRACT:

This work demonstrates the effect of oxygen vacancy of SnOx thin films on the

performance of perovskite solar cells. Various SnOx films with different amount of

oxygen vacancy were deposited by sputtering at different substrate temperatures (25-

300 ºC). The transmittance of the films decreases from 82% to 66% with increasing

the deposition temperature from 25 ºC to 300 ºC. Both XPS and ESR have confirmed

that higher density of oxygen vacancies were created within the SnOx film at high

substrate temperature, which caused narrowing of the SnOx bandgap from 4.1 eV (25

ºC) to 3.74 eV (250 ºC). Combined measurements of UPS and UV-VIS spectroscopy

have shown an excellent conduction band position alignment between the

methylammonium lead iodide perovskite layer (3.90 eV) and the SnOx electron

transport layer deposited at 250 ºC (3.92 eV). As a result, a significant enhancement

of Voc from 0.82 V to 1.0 V was achieved, resulting in an increase of power

conversion efficiency of the perovskite solar cells from 11% to 14%. This research

demonstrates a facile approach for controlling oxygen vacancies of SnOx thin films to

achieve desirable energy alignment with the perovskite absorber layer for enhanced

device performance.


Cells 63

Graphical Abstract



4.1 Introduction

Perovskite solar cells (PSCs) using organometal lead halide perovskite have emerged

as one the most attractive and promising solar cells to deliver cost-effective solar

electricity in the future. The energy conversion efficiency of PSC has increased from

3.8% [1] in 2009 to over 22% in 2016 [2, 3] which was obtained using a simple

solution processing method. The unprecedented progress is attributed to the

outstanding optical and electrical properties of the perovskite compound such as

large charge carrier mobility, high optical absorption coefficient in visible light

spectrum, low trap density [4]-[5], a tunable direct band gap and long charge carrier

diffusion length [4, 6, 7].

In a typical perovskite solar cell, the absorber layer is sandwiched between electron

transport layer (ETL) and hole transport layer (HTL). The photo-generated charges

are injected into and then transported through the respective charge selective layers

(ETL and HTL) in the device. As we know an efficient charge injection from the

perovskite absorber layer to the charge selective layers is essential to obtain high

power conversion efficiency of the PSCs. One of the prerequisite for charge injection

to occur is the energy band edge match between the perovskite light absorber and the

ETL/HTL. Specifically, the conduction band (Ec) of perovskite should be higher than

the Ec of ETL while the valence band (Ev) of the perovskite should be lower than that

of HTL. An energy level mismatch of the charge selective layers and the adjacent

perovskite layer can have direct influence on the open circuit voltage (Voc) and short

circuit current density, thus device performance of the PSCs [8-12].

To date, a majority of research for higher device performance has been devoted to

optimizing the morphology, crystallinity, grain size and composition of perovskite


Cells 65

layer [13, 14]. However, optimal properties of ETL and HTL for charge transfer and

extraction are equally important for high performing PSCs. SnO2 is one of the most

widely used electron transport layer in PSCs. Theoretically, the Ec of SnOx (Ec = 4.5

eV vs vacuum) based ETL is not ideal for efficient charge injection from the

perovskite such as MAPbI3 (Ec = 3.90 eV vs vacuum). The large energy offset

between them can lead to energy loss, which in turn reduces the Voc of the PSCs. In

order to reduce the energy offset between the SnOx and perovskite for improved

efficiency, modification of the electronic properties of SnOx have been reported. For

example, fullerene was used as surface coating on ETL to enhance electron

extraction ability of SnO2 [15]. Doping of ETL was also applied to get good bandgap

alignment with adjacent perovskite materials. Bai et al. improved the energy level

alignment in PSCs by doping SnO2 with Sb and obtained a PCE of 17.7% with open

circuit voltage of 1.06 V [16, 17]. Recently Jung et al. reported the effect of post

annealing temperature of SnO2 on the device performance, showing that 250 ºC

demonstrated the highest performance of 19%[18]. Nevertheless, the process of

doping, introduction of an extra layer, or post annealing heat treatment could only

increase the complication of device fabrication.

It is an effective approach to modify the electrical properties of metal oxide

semiconductors such as charge carrier mobility and bandgap energy by creating a

small amount of oxygen vacancy [19, 20]. Bandgap narrowing of ZnO by

introducing oxygen vacancy was reported by Huang et al. [20]. Similarly the

electrical conductivity of WO3 has been improved by creation of oxygen vacancy

within the material [21]. These vacancies cause defect band which help to increase

the conductivity [19]. By considering the role of SnOx thin films as ETL in PSCs, it

https://pubs.acs.org/author/Huang%2C+Baibiao



is desirable to explore the effect of SnOx oxygen vacancies on the performance of

PSCs.

SnO2 based ETL film for PSCs is normally obtained by solution based deposition

methods such as spin coating and spray pyrolysis of precursor solutions of SnO2

followed by annealing. High device efficiency over 21% have been achieved using

spin-coated SnO2 ETL film [18]. Sputtering deposition is a well-established

technology for large scale production of thin film materials with controlled properties

for variable applications such as thin film solar cells and other electronic devices

[22]. It has advantages of low manufacturing cost, good control over film properties

and uniformity, which is important for device reproducibility. The crystal structure,

morphology, composition and thickness of the deposited films can be altered by

controlling the deposition conditions such as working pressure, in-situ substrate

temperature, reactive gas and sputtering power. In spite of these merits, use of

sputtering for fabrication of SnOx based ETL for PSCs has not yet been reported.

Herein, we develop sputter-deposited SnOx films with controlled oxygen vacancy for

planar structured PSCs. The optimum thickness of the film was 60 nm. Tuning of

the oxygen vacancy in the film through control of in-situ substrate temperature has

been achieved to modify the optical and electronic properties of the SnOx films. At

the optimal substrate deposition temperature, we developed SnOx film with desirable

energy level that matches the conduction band of methylammonium lead iodide

(MAPbI3) based perovskite. As a result, the Voc of the PSC was increased leading to

enhanced efficiency.

4.2 Results and discussion

Figure 4.1a shows the spectral transmittance of SnOx thin films deposited at different

in situ substrate temperatures. The film deposited at room temperature is highly


Cells 67

transparent in the visible and near infrared wavelength range (weighted transmittance

> 80%). A sharp drop of the transmittance below 380 nm is consistent with the

characteristics of tin oxide material. At higher substrate temperature the

transmittance of the SnOx film is gradually reduced from 79% at 250 ºC to about

66% at 300 ºC. The film obtained at 300 ºC has low transmittance and is highly

undesirable for its application in PSCs because it can block significant amount of

light reaching the perovskite layer. Therefore we decided not to consider it for

further investigation.

The bandgap energy of the SnOx films was calculated by using the equation below

for a direct bandgap material:

(αhν)2 = A(hν-Eg)

where α is the absorption coefficient, A is the band edge parameter, h is the Plank

constant, and ν is the frequency of light. The plot produced from this relation is

known as the Tauc plot and gives a linear behaviour in the high energy region. Eg of

the films is then determined by fitting the linear region of the plot to zero

transmittance. Figure 4.1b shows the Tauc plot of the SnOx films deposited at

different temperatures. It is found that the Eg value of the SnOx films deposited at

room temperature and at 100 ºC is similar with Eg = 4.12 eV. This value decreases to

3.91 eV and further to 3.74 eV for the films deposited at 200 ºC and 250 ºC,

respectively. The details of optical transmittance and bandgap of the deposited SnOx

film are shown in Table 4.1. The decrease in the bandgap energy with increasing in-

situ substrate temperature is attributed to the oxygen vacancy introduced by sputter

deposition. Oxygen vacancies can be donor centres and act as n-type dopants [20,

23]. The increase of oxygen vacancies leads to overlap of the donor orbitals. For

higher concentration of oxygen vacancies, the defect band broadens sufficiently so



that the gap between the conduction band and defect band disappear, resulting in

band gap narrowing [19]. Oxygen vacancies can also contribute free electrons to the

conduction band, increasing the film conductivity and charge transport properties

[24, 25].

The bandgap energies of the sputter-deposited tin oxide films in this work (4.12-3.74

eV) are higher than the bandgap of crystalline SnO2 (Eg = 3.6 eV )[26] but

comparable with the Eg of sputtered SnO2 (Eg = 3.5-4.2 eV) reported in the literature

[27]. As the Eg changes with the increasing of deposition temperature, therefore we

expect the composition of the deposited tin oxides is non-stoichiometric. To confirm

this, we measured the chemical composition of the films deposited at different

temperatures by XPS. The atomic ratio of Sn:O for all the samples are shown in

Table 4.1. From the table, clearly there is a deficiency of oxygen in the deposited

SnOx (1< x <2) films. The Sn:O atomic ratio of the films deposited at room

temperature and at 100 ºC is 1:1.96, which is slightly lower than the stoichiometric

ratio of SnO2. The result agrees well with previous reports [23]. A significant

amount of oxygen deficiency is obtained in the films deposited at higher substrate

temperatures. As shown in Table 4.1 and Figure 4.2d, the amount of oxygen in the

SnOx is nearly constant up to 100 ºC and beyond this temperature the amount

decreases continuously. Grazing incident XRD and Raman measurements show no

characteristic peaks of the SnOx film and this may be due to the very thin layer (~60

nm) and low crystallinity of the films (Figure S4.1, S4.2). Also, the SEM surface

morphology (Figure S4.3) of SnOx thin films deposited at various substrate

temperatures show almost the same among the different samples.


Cells 69

Figure 4.1 a) UV-Visible transmittance spectra and b) (αhv)2 vs hv plot c) UPS data

and d) conduction band position of SnOx thin films deposited at various substrate

temperatures based on UPS and UV-visible spectrum.

Table 4.1 Transmittance, bandgap energy and Sn to Oxygen ratio of SnOx films

deposited at various substrate temperatures.

SnOx Optical

Transmittance (%)

Bandgap Energy

(Eg)

Sn:O

RT 82 4.12 1:1.96

100 ºC 82 4.12 1:1.94

200 ºC 81 3.91 1:1.80

250 ºC 79 3.74 1:1.71

300 ºC 66 - -



The effect of oxygen vacancies on the valence and conduction band (Ev and Ec)

positions of SnOx was investigated by combined measurements of ultraviolet

photoelectron spectroscopy (UPS) and UV-VIS spectroscopy. As shown in Figure

4.1c, the valence band maxima position (EVBM) in the UPS measurement (Figure

S4.4) for all the SnOx films is 3.5 eV. The energy cut off is at 17.06 eV for samples

deposited at RT, 100 ºC and 200 ºC, while for the sample deposited at 250 ºC it is at

17.1 eV (Figure S4.5). The Ev is determined to be 7.66 eV for the SnOx film

deposited at room temperature, 100 ºC and 200 ºC and 7.62 eV for the film deposited

at 250 ºC. The conduction band position was determined based on the valence band

energy position and the bandgap energy values of individual samples. As shown in

Figure 4.1d, Ec of the SnOx film has been shifted downward from 3.54 eV for the

sample deposited at RT to 3.92 eV for the SnOx deposited at 250 ºC. The slightly

lower Ec of the SnOx deposited at 250 ºC (3.92 eV) relative to the perovskite absorber

(3.90 eV) gives a favourable energy offset and driving force for electron injection

from perovskite to the SnOx ETL. The improved energy alignment in turn benefits

the device performance as shown below.

The effect of the in situ substrate temperature on the chemical environment of Sn and

oxygen was investigated using XPS. Figure 4.2a-c shows XPS survey and high

resolution Sn 3d and O 1s core level spectra for the SnOx thin films deposited at 250

ºC. We observe that the core level peaks of Sn 3d5/2 and Sn 3d3/2 of all the samples

deposited at different substrate temperatures can be fitted with a single component at

486.7 eV and 495.1 eV, respectively, corresponding to Sn-O bonds (shown in Figure

4.2c). These binding energies are found to be matching well with the energies

reported for SnO2 [28, 29]. It is noted that, although the peaks can be fitted by a


Cells 71

single component, Sn4+ (binding energy Sn 3d5/2 = 487.2 eV and Sn 3d3/2 =495.6 eV)

and Sn2+ (Sn 3d5/2 = 486.5 eV and Sn 3D3/2 =494.8 eV) do not exhibit significant

splitting in the Sn 3d peaks and therefore both states may exist in the samples [30] .

Although previous report showed a shift in binding energy of the Sn 3d peaks

depending on the stoichiometry [28], in this work there was no significant shift of the

Sn 3d5/2 and Sn 3d3/2 due to the narrow range of stoichiometry observed in the

samples. Figure 4.2c shows the high resolution O 1s core level peak deconvoluted

into components corresponding to Sn-O (530.5 eV) and C-Ox (531.9 eV). The area of

Sn-O contribution of the O 1s core level was used to correct the ratio of Sn:O

calculated from the survey spectra.

Figure 4.2 XPS analysis of the deposited SnOx thin films. (a)-(c) show representative

wide survey, Sn 3d and O 1s core level spectra, respectively, and (d) shows the

temperature dependent stoichiometry of SnOx thin films (1<x<2).



Electron spin spectroscopy (ESR) was carried out to confirm the oxygen vacancies of

the deposited SnOx films. Typically, metal oxides are known to have oxygen

vacancies with three different charge states: 𝑉𝑂0, 𝑉𝑂

+and 𝑉𝑂++.[31] In SnO2, 𝑉𝑂

+ state is

reportedly ESR active owing to an unpaired electron.[32] The ESR spectra of the

samples (deposited at RT and 250 ºC) are shown in Figure 4.3a. The g-value (2.006)

which was calculated from the resonance magnetic field is nearly equal to the g-

value (2.008) of a free electron [33] (𝑔 = ℎ𝑣 𝜇𝐵𝐵⁄ , ℎ = 6.626 × 10−34 𝐽 𝑠, 𝜇𝐵 =

9.274 × 10−24 𝐽 𝑇−1, 𝑣 = 9.5 𝐺𝐻𝑧, and B is the magnetic resonance field of ESR).

The intensity of the ESR peak is proportionate to the oxygen vacancy content in

SnOx,[32] and therefore, as seen in Figure 4.3a the SnOx thin film deposited at 250 °C

has relatively high content of oxygen vacancies. Also, a broad ESR peak is reported

for amorphous materials whereas a sharp peak has been observed in crystalline

materials [34]. The peaks in Figure 4.3a confirms the amorphous nature of SnOx

films deposited at RT. Clearly the crystallinity of the film deposited at 250 ºC is

improved. A schematic showing the oxygen vacancies (Vo) in the SnOx structure

with the dotted circles representing the Vo is illustrated in Figure 4.3b.


Cells 73

Figure 4.3 (a) ESR spectra of room temperature and 250 ºC deposited SnOx thin

films and (b) schematics showing SnOx structure with oxygen vacancies.

Figure 4.4 Schematic diagram of perovskite solar cell device with SnOx thin films

used as ETL, (b) Cross-sectional SEM image of actual perovskite solar cells device

and (c) J–V curves of the device scanned under reverse voltage. The inset in (c) is the

device performance for the different SnOx thin films and, (d) External quantum

efficiency of the cell with SnOx deposited at 250 ºC.

To investigate the effect of oxygen vacancies and bandgap alignment on the

performance of device, planar structured PSCs composed of

FTO/SnOx/CH3NH3PbI3/Spiro-OMeTAD/Au have been fabricated. The schematic

diagram of the device structure and cross-sectional SEM image are shown in Figure

4.4a and Figure 4.4b, respectively. The thickness of the different SnOx films

deposited on FTO glass at various substrate temperatures is about 60 nm. The

MAPbI3 perovskite film of around 400 nm thick is composed of large grains with

grain sizes ranging from 100 nm to 500 nm. The Spiro-OMeTAD based HTL

deposited on the perovskite film has thickness around 200 nm. The device structure



is completed by depositing a 100 nm gold film using e-beam evaporator. The J-V

characteristic of the PSCs with different SnOx based ETL is shown in Figure 4.4c. It

is found that the device performance has been enhanced from 11% to 14% as the in

situ substrate temperature of SnOx increases from room temperature to 250 ºC,

respectively. In particular, it is observed that the improved device performance is

mainly attributed to the increase of Voc of the device because Jsc of the PSCs remains

similar (Jsc = ~20.5 mA/cm2) regardless of the substrate temperature of SnOx. The

Voc of the PSC is significantly improved from 0.82 V for the SnOx deposited at room

temperature to ~1.0 V for the sample made at 250 ºC. The Voc of the PSC with SnOx

film deposited at room temperature and 100 ºC is similar but a dramatic enhancement

is observed beyond this deposition temperature. The change in Voc follows the same

trend as that of the band energy alignment of the SnOx thin films. The films

deposited at room temperature and 100 ºC have higher energy offset leading to lower

Voc of 0.82 V. The energy offset of the film deposited at 200 ºC is reduced due to

reduction in the bandgap energy which means the conduction band shifts downward

and closer to the conduction band position of the perovskite absorber as shown in

Figure 4.1d. This shows improved energy band alignment leading to increased Voc of

0.93 V. For the SnOx film deposited at 250 ºC, the ideal energy band matches very

well with that of the perovskite and therefore the electron injection from perovskite

to ETL is favoured and a higher Voc of about 1.0 V is achieved. In this study we have

not optimised the properties of the MAPbI3 perovskite light absorbing layer and

hence a relatively low device performance (14%) was obtained. However, it is clear

from this work that the oxygen vacancies in the SnOx film plays a significant role in

tuning the conduction band position to enhance the performance of the PSC by

reducing the energy offset and adjusting the energy alignment between the ETL and


Cells 75

absorber layer of the device. The external quantum efficiency (EQE) plot of the PSC

using SnOx film deposited at 250 ºC as ETL is shown in Figure 4.4d. The broad EQE

spectrum of the device is around 80% across the wavelength range of 400-700 nm.

The calculated Jsc (20.5 mA/cm2) from the EQE agrees well with the Jsc value

obtained from J-V curve for all the samples.

The impedance spectroscopy (IS) was carried out at open circuit condition under 1

sun illumination to investigate the carrier recombination resistance of the PSCs with

SnOx deposited at different temperatures. Figure 4.5 shows the Nyquist plot of the

PSCs with SnOx films deposited at RT and at 250 ºC, respectively. The inset in

Figure 4.5 shows the equivalent circuit used to fit the Nyquist plots. In the equivalent

circuit, Rs is associated with the sheet resistance of the FTO which is the same for

both devices while Cg and Cs are ascribed to the geometric property of perovskite

layer and capability of ion accumulation at perovskite interface, respectively. In

addition, R1 and R3 are resistive components at low and high frequency region and

the sum of the two, R1 + R3, is associated with the charge recombination resistance

(Rrec) at the interface of perovskite/SnOx based ETL[13, 35, 36]. The fitted results of

experimental data are shown in Table 2.


illumination at open circuit voltage.

ETLs Rs

( cm2)

Cg

(F/cm2)

R3

( cm2)

Cs

(F/cm2)

R1

( cm2)

RT 1.66 7×10-7 0.7 0.045 4

250 ºC 1.63 7×10-7 0.45 0.002 8.4



Clearly the PSC with higher oxygen vacancies of SnOx has a two-fold higher

recombination resistance R1+R3 (Rrec = 8.85 ohm.cm2) compared to the one

deposited at room temperature (with low oxygen vacancies) (Rrec = 4.7 ohm.cm2).

The higher Rrec should be associated with the improved energy alignment for

efficient charge injection, higher carrier mobility and better electrical conductivity of

SnOx deposited at 250 ºC. The improved charge injection can mitigate the effect of

charge accumulation at the interface of perovskite/SnOx (Cs, of the RT sample is

0.0045 F/cm2 and 0.002 F/cm2 for 250 ºC), which can cause higher recombination in

the device. This interpretation is in good agreement with the increase of Voc observed

with device using SnOx ETL deposited at 250 ºC. Therefore, the improved energy

alignment and lower charge recombination rate governs the higher photovoltaic

performance of the PSC.

Figure 4.5 Nyquist plots of PSCs with SnOx ETL deposited at room temperature and

250 ºC under light, inset shows the equivalent circuit model for data fitting.


Cells 77

4.3 Conclusion

We have developed non-stoichiometric SnOx (x=1.96-1.71) thin films with

controlled optical and electronic properties by sputter-deposition at different in situ

substrate temperatures (RT to 300 ºC) for use as ETL in perovskite solar cells. It is

found that the film deposited at 250 ºC possess higher transmittance (79%), lower

bandgap energy (3.74 eV) and higher oxygen vacancy. In addition, the energy

alignment between the conduction band position of this film (3.92 eV) and the

MAPbI3 perovskite layer (3.90 eV) is excellent, which results in dramatic

improvement of Voc value of the PSC device (Voc=1.0 V) compared to the SnOx

made at room temperature (Voc =0.83 V). A PCE of 14 % was obtained from the

reverse scan data of this device, which is a great enhancement from 11% for the

device fabricated using room temperature SnOx film. The study of impedance

spectroscopy shows that the PSC with SnOx film having higher oxygen vacancies has

a higher interfacial recombination resistance (8.85 ohm.cm2) than the one with low

oxygen vacancies (4.7 ohm.cm2). This work demonstrates manipulation of the

conduction band position of sputter-deposited SnOx film by tuning oxygen vacancy

to achieve favourable energy alignment between the ETL and perovskite layer for

higher energy conversion efficiency of PSCs.

4.4 Experimental Section

All materials were purchased from Sigma-Aldrich and used as received

without further purification or processing unless otherwise stated.

Methylammonium lead tri-iodide (MAPbI3) perovskite films were prepared

based on Lewis acid-base adduct approach, as described in our previous report

[14]. In brief, a mixture of 461 mg of PbI2, 159 mg of methylammonium

iodide (MAI) (Dyesol), was dissolved in 78 mg of dimethyl sulfoxide (DMSO)



and 650 mg of dimethyl formamide (DMF) at room temperature, to prepare

MAPbI3 perovskite precursor solution. A syringe filter (pore size: 0.22 µm)

was used to filter the prepared MAPbI3 precursor solution prior to the

deposition of perovskite film. Hole transporting material solution was prepared

by mixing 72.3 mg of 2,2’,7,7’-Tetrakis-(N,N-di-4-methoxyphenylamino)-

9,9’-spirobifluorene (Spiro-OMeTAD) (Borun New Material), 28.8 µL of 4-

tert-butylpyridine, and 17.5 µL of Bis(trifluoromethane)sulfonimide lithium

(Li-TFSI) solution (720 mg of Li-TFSI in acetonitrile) in 1 mL of

chlorobenzene.

Device fabrication

Solar cells were fabricated on fluorine-doped tin oxide (FTO) coated glass substrate

(Nippon Electric Glass, 15 /). The substrate was patterned through partial removal

of FTO via chemical etching using 35.5 wt% HCl and zinc powder. Then a 5%

Decon-90 detergent and a mixture of acetone, isopropanol and ethanol were used to

clean the substrate for 20 mins in an ultrasonic bath, respectively. Prior to use, the

substrate was treated with ultraviolet-Ozone for 30 mins to fully remove organic

solvent residuals. An electron transport layer based on SnOx film (~60 nm) was

deposited by a sputtering system (KJ Lesker) at in-situ substrate temperature ranging

from room temperature to 300 °C. A Sn target was used for deposition and oxygen

gas was used as a reactive gas. For the films deposited at 100 °C, 200 °C, 250 °C and

300 °C, the substrate was preheated to the set temperature and maintained at that

temperature during the deposition. The chamber for the sputtering was first

evacuated to a base pressure of less than 2 x10-6 Torr before the sputtering. During

the deposition, the substrate was continuously rotating at 10 rpm in order to ensure a


Cells 79

uniform and homogenous coating of the substrate. After the deposition, the samples

were taken out once the temperature of the sputtering chamber cooled down to less

than 50 °C. After this, the SnOx coated FTO substrates were treated under UV-ozone

for 20 mins before being transferred to an Ar-filled glove box. MAPbI3 layer (~400

nm) was deposited onto the prepared SnOx layer by spin coating at 4000 rpm for 30

s. During the spin-coating, 0.5 mL of diethyl ether was dropped onto the center of the

spinning substrate 18 s prior to the end of the program. The deposited perovskite

layer was then dried at 65 ºC for 2 mins and then annealed at 100 ºC for 2 mins. After

this, a hole-transport layer of Spiro-OMeTAD (~200 nm) was deposited onto the

perovskite layer using the prepared HTM solution at 4000 rpm for 30 s. The device

fabrication was completed by depositing a 100 nm gold layer as a back contact using

e-beam evaporation (KJ Lesker) in high vacuum (<10-6 Torr).

Characterization

The morphology of the samples was measured by field emission scanning electron

microscope (FESEM JOEL 7001F) at an acceleration voltage of 5 kV. The

thickness of the films was measured by a stylus profilometer and further

confirmed by an ellipsometry. X-ray photoelectron spectroscopy (XPS)

measurements were taken using a Kratos Axis Supra with a monochromatic Al

Kα source (1486.7 eV). Ar gas cluster etching was performed to remove

adventitious carbon from the sample until the C 1s peak was no longer visible

in the survey spectra. Survey scans were taken at analyzer pass energy of 160

eV, and Sn 3d, O 1s, and C 1s high resolution scans were acquired at 20 eV

pass energy. The binding energy scale of all spectra was calibrated by a rigid

shift of all spectra to align adventitious carbon in the C 1s core level to 284.8

eV. XPS data was analyzed for quantification by using CasaXPS software. The



XPS peaks of high resolution spectra were fitted by using Voigt functions with

a Shirley background. The stoichiometry was determined from the elemental

sensitivity corrected ratio of O 1s to Sn 3d peak intensities in the survey

spectrum, adjusting for the C-Ox species seen in the high resolution O 1s core

level. Ultraviolet photoemission spectroscopy (UPS) measurements were taken

on the cleaned SnOx surfaces using the same XPS instrument with a He I

source (21.22 eV) and an analyzer pass energy of 20 eV. The binding energy

scale of these spectra was calibrated by a rigid shift of the spectra to align the

Fermi level of an electrically contacted Au sample to 0 eV. The transmittance

of the SnOx films on glass substrate was measured using Cary 5000 UV-Vis-

NIR spectrophotometer with a 150 mm integrating sphere. The measurements

were performed in the wavelength range 200 to 1100nm at a near-normal angle

of incidence. A Teflon coating (BaSO4) was used as a 100% reference. From

these measurements, the weighted solar transmittance for AM1.5 and optical

bandgap of the films were obtained. Electron spin resonance (ESR)

measurements were performed at room temperature to detect defects of the

samples using a Magnetech MiniScope MS400 system operating at X band

with a microwave power of 20 mW and modulation amplitude of 1.20 mT. The

performance of perovskite solar cells was measured under irradiation of 100

mW/cm2 (AM1.5) provided by a solar simulator (Oriel Sol3A, Newport) equipped

with 450 W Xenon lamp. The measurement were done in both reverse and forward

direction (by applying current and by applying voltage) with a dwell time of

100msec at a scan rate of 0.14 V/s. A quantum efficiency system (IQE 200B,

Newport) was used to conduct the external quantum efficiency (EQE) measurement

under AC mode. The impedance spectroscopy (IS) of the PSCs was measured with


Cells 81

an electrochemcial workstation (VSP BioLogic) at open circuit condition under 1 sun

illumination in a frequency range from 1 MHz to 100 mHz. An AC voltage with

perturbation amplitude of 20 mV was applied on the sample in the IS measurement.

The top-view of SnOx and cross-sectional images of the device were taken were

taken using a field emission scanning electron microscope (FSEM JOEL 7001F) at

5kV acceleration voltage. To determine the crystal structure of SnOx thin films X-ray

diffraction (Rigaku SmartLab) was used with a monochromatic CuKα (λ = 0.154

nm) as an excitation source. A Renishaw inVia Raman spectrometer was used to

determine the chemical structure and crystalline state of the SnOx films using

Renishaw frequency doubled NdYAG laser excitation source of wavelength 532

nm. To avoid local heating of the samples, a low power of about 5 mW was

applied to the samples. A Raman shift between the wavenumber 200 to 1200 cm-1

was measured.

4.5 Acknowledgement

The first author is indebted for QUTPRA scholarship and financial support. H.W.

acknowledges the financial support by the Australian Research Council (ARC)

Future Fellowship (FT120100674) and the Queensland government (Q-CAS). This

research was mainly done at the Institute for Future Environments (IFE) Central

Analytical Research Facility (CARF) at QUT. Access to CARF is supported by

generous funding from the Science and Engineering Faculty (QUT). We also thank

Dr. Joseph Fernando for helping in the ESR data acquisition.

Keywords: SnOx, sputtering, substrate temperature, oxygen vacancy, band

alignment.


Cells 83

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Cells 87

Supporting Information

Tuning Oxygen Vacancies in sputtering-deposited SnOx films for Enhancing the

Performance of Perovskite Solar Cells

Fawad Ali, Ngoc Duy Pham, H. Jonathan Bradford, Nima Khoshsirat, Ken Ostrikov,

John M. Bell, Hongxia Wang*, Tuquabo Tesfamichael*

School of Chemistry, Physics and Mechanical Engineering,

Science and Engineering Faculty, Queensland University of Technology


[email protected]

Figure S4.1 XRD spectrum of SnOx thin films deposited at RT and 250 ºC showing

amorphous characteristics in both films.





Figure S 4.2 Raman spectroscopy of SnOx films deposited at RT and 250 ºC. For

comparison the spectrum of the glass substrate is shown.


Cells 89

Figure S4.3 SEM surface morphology of SnOx thin films deposited at different

substrate temperatures (RT-250 ºC). For comparison the surface morphology of the

perovskite absorber deposited on two different SnOx films (RT and 250 ºC) is shown

in Figure S4.6.



Figure S4.4 Fermi edge (EVBM) region of SnOx thin films deposited at different

substrate temperatures (RT-250 ºC) obtained using UPS measurements.


Cells 91

Figure S4.5 Energy cut-off region of SnOx thin films deposited at different substrate

temperatures (RT-250 ºC).



Figure S4.6 SEM surface morphology of perovskite absorber deposited on two

different SnOx films (room temperature and 250 ºC).

Figure S4.7 AFM images (5 × 5 μm2) showing the morphology and RMS surface

roughness of SnOx thin films deposited at room temperature and 250 ºC.


Cells 93

Figure S4.8 Current-voltage (J-V) curve of PSC device in both reverse (Rev) and

forward (Fwd) scans for SnOx thin films as ETL deposited at room temperature and

250 ˚C.

The Hysteresis of the PSCs using SnOx deposited at various substrate temperatures

was investigated by measuring the device in the reverse and forward scan direction

as shown in Figure 3. The hysteresis Index (HI) is calculated by using the following

equation:

HI = (PCErev – PCEfw)/PCErev

Where PCErev and PCEFwd are the power conversion efficiency in the reverse and

forward scan directions, respectively. Even though the hysteresis of the PSC using

SnOx films deposited at 250 ˚C reduced significantly as compared to PSC using room

temperature deposited SnOx (as shown in Figure S8) but still not completely

eliminated. One of the factors for hysteresis is known to be caused by charges

accumulation at the ETL/Perovskite interface (as Perovskite/HTM interface is the

same for all devices). As observed in Table 2 in the manuscript the capacitance of the

device using SnOx deposited at 250 ˚C is much lower as compare to the device using



room temperature deposited SnOx indicating lower charge accumulation at

ETL/perovskite interface and lower hysteresis for PSC. The PSC using SnOx ETL

deposited at 250 ˚C having a better conduction band alignment and help a better flow

of charges as compare to room temperature deposited SnOx where the conduction

band is not matching very well with the conduction band of perovskite. Also, as

shown in the EIS results (Figure 5) the recombination resistance increased two folds

for the film deposited at 250 ˚C. This improved charge injection with higher

recombination resistance mitigates the charge accumulation at the interface and

hence reduce the hysteresis for PSC using SnOx deposited at 250 ºC.

Low Hysteresis Planar Perovskite Solar Cells using WO3-x Electron Transporting Layer Deposited at Room

Temperature 95

Chapter 5: Low Hysteresis Planar Perovskite Solar Cells using

WO3-x Electron Transporting Layer Deposited at Room

Temperature


1 they meet the criteria for authorship in that they have participated in the



2 they take public responsibility for their part of the publication, except


publication;

3 there are no other authors of the publication according to these criteria;

4 potential conflicts of interest have been disclosed to (a) granting



5 they agree to the use of the publication in the student’s thesis and its



In the case of this chapter: Contributor

Statement of contribution

Contributor Statement of contribution

Fawad Ali Designed and conducted the experiments, analaize

the data and wrote the manuscript.

Ngoc Duy Pham Assistance with EIS measurement and discussion









Name Signature Date


96 Chapter 5: Low Hysteresis Planar Perovskite Solar Cells using WO3-x Electron Transporting Layer Deposited

at Room Temperature

Low hysteresis Planar Perovskite Solar Cells using WO3-x Electron

Transporting Layer Deposited at Room Temperature

The following paper has been submitted for publication in ACS Applied Energy

Materials and is currently under revision. As described in chapter 4, creating oxygen

vacancies is beneficial for ETL and enhances the performance of PSCs. In this paper

oxygen-vacant WO3-x thin films were produced by e-beam evaporation and a reduced

J-V hysteresis with high device performance was observed for as deposited WO3-x

based PSCs as compare to annealed WO3-x.


Temperature 97

Room Temperature Deposited WO3-x Films as Electron Transporting Layer for

Low Hysteresis Planar Perovskite Solar Cells

Fawad Ali, Ngoc Duy Pham, Ken Ostrikov, John M. Bell, Hongxia Wang* and

Tuquabo Tesfamichael*



2 George Street, Brisbane, 4000, QLD Australia

*Corresponding author: Phone: +61-7-31381988

Fax: +61-7-31381516


[email protected]

ABSTRACT

Perovskite solar cells utilize metal oxide thin films as electronic transport for high

performance devices. These electronic transport metal oxides are generally processed

at higher temperatures. In this research we report room temperature processed WO3-x

thin film as electron transport layer for high performance and low hysteresis device.

High oxygen deficient WO3-x film was deposited at room temperature using e-beam

evaporation in high vacuum condition. For comparison, the amount of oxygen

vacancies was reduced by post-annealing of the as-deposited WO3-x films at 300 oC

for 1 hour in air. XRD and Raman measurements showed no WO3-x characteristic

peak of both the as-deposited and annealed films. From XPS and EPR, the as-

deposited film shows large amount of oxygen vacancies compared to the post-

annealed film. The bandgap of the post-annealed film increases due to reduced

conductivity and thus a reduction in the device performance, mainly because of the

low Voc and high current-voltage hysteresis in the forward and reverse scans. The

perovskite solar cell device developed using the room temperature deposited electron

transport WO3-x layer has shown low current-voltage hysteresis. This device

achieved a power conversion efficiency of 10.3% and hysteresis index of 2.1%. This

work demonstrates the feasibility of WO3-x film as electron transport layer for high



98 Chapter 5: Low Hysteresis Planar Perovskite Solar Cells using WO3-x Electron Transporting Layer Deposited

at Room Temperature

efficiency perovskite solar cell with reduced hysteresis fabricated at low temperature

using industrially viable e-beam evaporation method.

Graphical Abstract


Temperature 99

5.1 Introduction

Perovskite solar cells (PSCs) using organo-metal lead halides perovskites as light

absorber are at the centre of attention in the photovoltaic research community due to

their low cost, ease of fabrication and higher power conversion efficiency (PCE).

The low binding energy [1], optimal direct tunable bandgap of 1.2 1.6 eV, long

diffusion length, long carrier life time [2-4] (800 nm) have made perovskite the most

attractive material for optoelectronic devices including solar cells, light emitting

diodes, etc. The efficiency of PSC has increased from 3.8% [5] in 2009 to over 22%

in 2016 [6, 7] thanks to the advancement of material synthesis approaches and device

architecture engineering.

In a typical perovskite solar cell, the absorber layer is sandwiched between an

electron transport layer (ETL) and a hole transport layer (HTL). The photo-generated

charge carriers in the perovskite are extracted through the charge selective ETL and

HTL. Metal oxides such as TiO2 and SnO2 have been extensively used as electron

transport layer for high performance PSCs [6-10]. TiO2 is the most common ETL for

PSCs. In order to get high crystallinity and conductivity, TiO2 need to be annealed at

high temperature (450 °C) [6, 11]. Similarly, precursor solution of SnO2 requires

post-annealing at temperature above 180 °C. The requirement for high annealing

temperature not only adds complexity and cost in the manufacturing process of the

device, but also halts further development of flexible PSCs and tandem solar cells.

To overcome these issues, alternative metal oxide semiconducting materials have

been investigated as electron transport material. Tungsten oxide (WO3) is n-type

semiconducting material with high electron mobility (10-20 cm2V-1s-1) and wide

bandgap energy (2.7-3.9 eV). It has high stability against moisture, low material cost

and can be made at low temperature. The high electrical conductivity and

100 Chapter 5: Low Hysteresis Planar Perovskite Solar Cells using WO3-x Electron Transporting Layer

Deposited at Room Temperature

comparable optical transmittance to TiO2, means WO3 is a potential ETL for

perovskite solar cells. In spite of these extraordinary properties of WO3, compared to

TiO2 and SnO2, there is much less reports on perovskite solar cells using WO3 based

ETL. Currently, the PSC using WO3 as ETL has low energy conversion efficiency

(less than 10%) and the device normally showed high hysteresis due to the

imbalanced charge transport at the ETL/perovskite and perovskite/HTL interfaces.

Wang et al. reported PSC using WOx as ETL processed at 150 °C. The device

showed higher circuit-current density (Jsc) than the device using TiO2, but much

lower open-circuit voltage (Voc) (0.71 eV), leading to lower power conversion

efficiency (PCE) [12]. The lower Voc was explained by the inherent charge

recombination in the WO3. In order to overcome this problem a hybrid ETL

consisting of amorphous WOx coated TiOx (TiOx-WOx) processed at 150 °C was

developed. Using this hybrid ETL an impressive PSC efficiency of 17.47% was

observed [13]. On the other hand, Nb doped WOx ETL processed at low temperature

(120 °C) for flexible PSC with efficiencies of 13.14% was developed [14]. These

results suggest that WO3 has a great potential as low temperature processed ETL for

PSCs.

Although various methods have been reported for synthesis of WO3 thin films

including evaporation [15, 16], sol-gel [17], sputtering [18-21] and chemical vapour

deposition[22], the WO3 for PSCs application has mostly been produced through

solution based spin-coating processing method [13, 14]. The problem with the

solution based method is that a post annealing treatment is required to remove the

solvent and to decompose the precursors into WO3. Also uniformity of the film

quality by spin coating over large area is another issue. In addition, a large hysteresis

is often reported with WO3 based PSC, which is caused by imbalanced charge


Temperature 101

extraction at the interface of perovskite/ETL and perovskite/HTL. In the literature a

small amount of oxygen vacancies can increase the electrical conductivity of WO3

[23]. Different studies have reported that creating oxygen vacancies in WO3 can

affect the conductivity, crystallinity and charge transport properties of the material

[20, 24-26]. These vacancies also cause defect band which help in increasing the

conductivity by reducing the bandgap [25, 27], as demonstrated in our previous work

of sputtering-deposited SnO2 films and in the work by others [27]. Liao et al. have

showed that the conductivity of WO3 decreases with decreasing the amount of

oxygen vacancies [28]. Similarly for photoelectrical conversion an improved

photoelectric conversion efficiency was attributed to the oxygen vacancies in the

WO3 thin film [29].

Usually oxygen vacancies are produced by doping or post-thermal treatments in

limited oxygen atmosphere which increase the processing cost and also complicated

the process. A simple one step method for controlling oxygen vacancies is desired

which helps to speed up the process of device fabrication at lower cost.

In this study we report a room temperature deposited WO3-x thin film by electron-

beam evaporation in high vacuum. Electron beam (e-beam) evaporation is a versatile

and robust technique for deposition of uniform metal oxide films over a large surface

area with good control over film quality and composition. Also, oxygen vacancies

can be created in the WO3 through control of oxygen environment which can be

beneficial in device performance improvement. The WO3-x exhibited high optical and

electrical properties, large oxygen vacancies with wide bandgap and high electron

mobility. For comparison we also annealed the as-deposited film at 300 °C to reduce

the oxygen vacancies and compare their performance in perovskite solar cells. The

as-deposited WO3-x as ETL has demonstrated a much higher Voc and FF compared to



the annealed WO3-x film, leading to energy conversion efficiency over 10% under

AM1.5 one sun illumination. Most importantly, the current-voltage hysteresis of the

as-deposited WO3-x film was almost eliminated compared to the annealed sample.

5.2 Results and discussion:

Figure 5.1a shows spectral transmittance of the as-deposited and post-annealed WO3-

x thin films. The films are highly transparent in the visible and near infrared

wavelength and their transmittance sharply drops in the ultraviolet spectral

wavelength. The transmittance of the post-annealed film at 300 °C decreases slightly

and its absorption edge shifts towards a shorter wavelength suggesting increased

bandgap [30]. The weighted optical transmittance of both films in the wavelength

range of 400-1100 nm is above 75%. The bandgap energy of the films was calculated

using the relation for an indirect band material of WO3-x:

(αhν)1/2 = A(hν-Eg)


constant, and ν is the frequency of light. The plot produced from this relation is

known as the Tauc plot. The Eg of the films is then calculated by extrapolating the

linear region of the Tauc plot to zero. The bandgap for the as-deposited WO3-x is 3.84

eV, which increased to 3.91 eV after annealing at 300 °C in air as shown in Figure

5.1b and Table 5.1. The larger values of the bandgap energies observed in this study

are thought to be the result of quantum size effect due to small crystal sizes and

amorphous nature of WO3-x [31, 32]. In our previous studies we have observed that

the bandgap energy of MoO3-x [30] thin films decreases after annealing in vacuum.

This is because the donor orbitals overlap with the increase of oxygen vacancies.

When the concentration of oxygen vacancies is high, the defect band broadens to an


Temperature 103

extent that the gap between the conduction band and defect band disappeared and the

band gap reduced [25].

Figure 5.1 Transmittance spectra, (b) (αhν)1/2 vs hν plot, (c) He-I UPS spectra, inset

in Figure c is the fermi-edge region and (d) band energy alignment of ETLs with

perovskite light absorbing material, for the room temperature deposited and post-

annealed WO3-x thin film samples.

The valence band maximum (Evb) and conduction band minimum of the WO3-x

(annealed and as-deposited) films were determined by a combined measurement of

UPS and UV-visible. The fermi level was estimated to be –5.12 eV for both the as-

deposited and annealed WO3-x.

The EVB of the films is –8.22 eV which is determined by subtracting the VBM (fermi

edge which is 3.1 eV as shown in the inset in Figure 5.1c) from the fermi level EF (–

5.12 eV):

EVB = EF – VBM= –5.12 eV – 3.1 eV = –8.22 eV.

The conduction band position was then calculated by adding the bandgap energy

value to the valance band and we get ECB = –4.38 eV for the room temperature



deposited WO3-x thin film and ECB = –4.31 eV for the post-annealed film. Eg was

calculated from the Tauc plot as shown in Figure 5.1b. The schematic diagram of the

conduction band positions of WO3-x thin films and perovskite are shown in Figure

5.1d. The deeper conduction band position of the as-deposited WO3-x film relative to

the perovskite creates a favourable energy alignment for faster electron injection

from the conduction band of perovskite to the conduction band of the WO3-x ETL

[33].

The chemical composition of the WO3-x thin film was investigated by XPS. Figure

(S5.1) contains the survey spectra of the as-deposited and post-annealed samples.

The corresponding high resolution W 4f, O 1s and C 1s core level spectra are shown

in Figure 5.2. In both samples the W 4f core level consists of a single component at

36.0 eV (see Figure 5.2a) which is in agreement with the peak positions reported for

tungsten oxide [16, 34]. The corresponding O 1s can be seen in the Figure 5.2b at

531.0 eV. The O 1s core level also has peaks at 531.9 eV and 533.1 eV which have

been ascribed to adventitious O-C and adsorbed H2O, respectively.

The ratio of oxygen to tungsten for each sample was calculated based on the XPS.

The room-temperature sample was found to be sub-stoichiometric with a O:W ratio

of 2.79 ± 0.06, whereas the sample annealed at 300 °C is nearly stoichiometric WO3

with a O:W ratio of 2.91 ± 0.06 as shown in Table 5.1. In contrast, despite the non-

stoichiometric composition of WO3-x, we find that the data can be fitted with peaks

of the same line-shape and FWHM indicating the presence of only the W(VI) state

(see Figure 5.2(a)).


Temperature 105

Figure 5.2 High resolution XPS spectra of as-deposited and annealed WO3-x thin

films shows (a) W 4f, (b) O 1s, (c) C 1s core levels with fits to the spectral

envelopes.

Electron paramagnetic resonance (EPR) was carried out to measure the oxygen

vacancies of the as-deposited and post-annealed samples as shown in Figure 5.3a.

The g-value (2.004) calculated (calculation shown in supporting information) from

the resonance magnetic field is almost equal to the g-value (2.008) of a free electron

[35]which corresponds to oxygen vacancy. It is known that the peak intensity

increased with increasing content of oxygen vacancies [24, 30]. The higher peak

intensity of the as-deposited sample than the annealed sample supports the XPS

result that more oxygen vacancies are present in the sample deposited at room

temperature.



Figure 5.3 (a) EPR spectra and (b) conductivity and resistivity of WO3-x thin films

deposited at room temperature (RT) and annealed at 300 ºC.

Table 5.1 Oxygen to tungsten ratio and electronic properties of RT deposited and

post-annealed WO3-x thin films.

ETL O/W Conductivity

(S/cm)

Bandgap (Eg)

(eV)

ECB

(eV)

WO3-x RT 2.79 0.034 3.84 – 4.31

WO3-x 300 ºC 2.91 0.012 3.91 – 4.38

The relative number of spins (Ns) of unpaired electrons (in other words the oxygen

vacancy) participating in the resonance is calculated by using the formula below

[36];

Ns α I (ΔH)2 Equation (1)

In equation (1), I is the intensity, and ΔH is the width of the EPR line. From this

equation the resultant Ns value calculated for the as deposited WO3-x is 1.53 x 108

which is much higher compared to annealed WOx (Ns = 3.6 x 107). This result is in

good agreement with the XPS result, which shows a decrease in oxygen vacancy

after annealing at 300 ˚C. Also, a broad EPR peak is observed for amorphous

materials while a sharp peak is observed in crystalline materials [37]. The peaks in

Figure 5.3a support the amorphous nature of the as-deposited and annealed WO3-x

thin films. In order to see the effect of oxygen vacancies on the electrical properties


Temperature 107

of WO3-x thin films, four-point-probe measurement were carried out. The resistivity

of the as-deposited WO3-x film increased significantly after post-annealing the film at

300 °C in air as shown in Figure 3b, leading to reduced conductivity (Table S5.1).

The reduction in conductivity of WO3-x thin films is due to the reduction in the

amount of oxygen vacancies as reported in the literature [38].

PSC with a structure of FTO/WO3/CH3NH3PbI3/Spiro-OMeTAD/Au were fabricated

using 60 nm thick WO3-x film as ETL as shown in schematic in Figure 5.4a. The

cross-sectional SEM image of the device is shown in Figure 4b. The J-V curve for

the PSC using the 3003-x-RT and WO3-x-300 °C film at both reverse and forward scan

is shown in Figure 5.4c. The active area of the devices was 0.1256 cm2 which was

controlled with a black masked for the J-V measurement. The PCE of the device

using the as-deposited WO3-x-RT ETL is 10.3 % for reverse scan and 10.08% for

forward scan. Both the reverse and forward scans have the same Jsc value of 18

mA/cm2, while the Voc is slightly reduced from 0.87 to 0.86 V, from reverse scan to

forward scan, respectively (see Figure 5.4c). The fill factor (FF) of the device based

on WO3-x –RT is 65.5 % and 64.5 % for the reverse and forward scans, respectively.

Statistic of the photovoltaic performance is shown in Figure S5.3. It is noted that the

device using the as-deposited WO3-x-RT film has very little current-voltage

hysteresis in the reverse and forward scans. In contrast the device with post-annealed

WO3-x-300 °C film shows a reduced performance mostly because of much lower Voc

and higher current-voltage hysteresis in the forward scan compared to the reverse

scan measurements. We need to emphasize that although the power conversion

efficiency of the PSCs in this work is much lower than the high efficiency PSCs

reported in literature, which normally used SnO2 or TiO2 as electron transport layer

(ETL) and has better energy alignment between the ETL and the perovskite film,



leading to high Voc of around 1.1 V. [39], However preparation of SnO2 or TiO2

based ETL requires high temperature annealing. The efficiency of the PSCs using the

as-deposited WO3-x-RT film in this work is already one of the best according to

literature on PSC using only tungsten oxide as ETL [12, 40, 41]. The series

resistance (Rs) and shunt resistance (Rct) of both devices were calculated based on

the J-V plot (Shown in Table S5.2). We have found the WO3-x -RT based device has

lower series resistance (Rs = 6.1 Ωcm2) and larger Voc (Voc = 0.87 V) compared to

the WO3-x -300 ˚C (Rs = 7.4 Ωcm2, Voc = 0.82 V in reverse scan and Voc = 0.66 for

forward scan). This explains the higher FF of the WO3-x -RT based device than the

WO3-x -300 ˚C based device.

Figure 4d, shows the external quantum efficiency (EQE) for the best performing

devices using WO3-x-RT and WO3-x-300 °C films. About 80 % of EQE spectrum is

shown in the range from 400 to 760 nm for the WO3-x-RT based PSC. The integrated

Jsc value obtained by IPCE for device using WO3-x-RT (18.1%) is in good agreement

with the experimental value, whereas the Jsc obtained from IPCE (17 mA/cm2) for

the device using WO3-x-300 °C is much lower than the experimental value (18.7

mA/cm2).


Temperature 109

Figure 5.4 Schematic diagram of perovskite solar cell, b) Cross-sectional SEM image

of actual perovskite solar cells device, c) Current-voltage (J-V) curve of PSC at both

reverse (Rev) and forward (Fw) scan, and d) external quantum efficiency (EQE) of

perovskite solar cell for both the WO3-x-RT and WO3-x-300 °C thin films.

Table 5.2 Reverse and forward scan photovoltaic I-V parameters of PSCs using the

as-deposited and post-annealed WOx-3 films as ETL.

ETL Scan

direction

Jsc

(mA/cm2) Voc (V) FF

Efficiency

(%)

WO3-x -RT Reverse 18 0.87 65 10.3

WO3-x -RT Forward 18 0.86 63 10.08

WO3-x -300 ºC Reverse 18.7 0.82 60 9.3

WO3-x -300 ºC Forward 18.7 0.66 59 7.5

Clearly the hysteresis increases after the as-deposited film is annealed at 300 °C in

air. The bandgap of the as-deposited WO3-x increases from 3.84 eV to 3.91 eV after

annealing, which means that the conduction band of the annealed film gets closer to

the conduction band of the perovskite as shown in Figure 1b. There is not much

difference in the Jsc of the reverse and forward measurements using the annealed

WO3-x-300 °C. However, the Voc of the device is affected by scan direction



dramatically as shown in Table 5.2 with larger value (0.82 V) in the reverse scan

compared to the forward (0.66 V) scan. As shown in the XPS results, the as-

deposited WO3-x is oxygen deficient and by annealing in air at 300 °C reduces the

oxygen vacancies. Oxygen vacancies contribute free electrons to the conduction band

and increase the film conductivity and electron transport properties [42]. As shown in

Figure 3b the electrical resistivity of the tungsten oxide film is increased due to

reduced oxygen vacancies in the film [26]. Also, it is reported by Gillet et al. that

the defect band caused by oxygen vacancies improves the conductivity by reducing

the bandgap [25]. The increase in conductivity improves the electron extraction

efficiency which contributes to lower hysteresis in the as-deposited WO3-x based PSC

[43] (as shown in Table 5.2). Our results indicated that increased oxygen vacancies in

WO3-x thin films have proven benefit for enhanced PSC device performance and

reduction in current-voltage hysteresis.

Figure 5.5 (a) PL spectra and (b) Nyquist plots of PSCs under light using as-

deposited and post-annealed WO3-x ETL. For comparison the PL of the perovskite

absorber is also shown.

In order to further elucidate the effect of oxygen vacancies on charge transfer

efficiency at the interface between perovskite and WO3-x ETLs, steady-state

photoluminescence was conducted for glass/perovskite, glass/WO3-x-


Temperature 111

300°C/perovskite and glass/WO3-x-RT/perovskite. As shown in Figure 5.5a a

photoemission peak at 780 nm is observed for all the samples which is originated

from the perovskite material. When the perovskite absorbing layer is interfaced with

the WO3-x, a clear quenching of the PL is observed. It is known that a decrease of PL

intensity can be caused by non-radiative recombination and/or interfacial charge

transfer, leading to reduction of electron-hole pairs which can release photon when

recombines. Since all the perovskite films were made under the same condition and

we did not observe noticeable morphological change of the film, we assume the

properties of the perovskite should be largely the same. Therefore the contribution of

non-radiative recombination on the PL should be the same as well. We have found

that the reduction of the PL intensity is consistent with enlarged energy offset

between the conduction band of the perovskite and the conduction band of the WO3-x

which provides the driving force for charge injection from the perovskite to the

adjacent WO3-x. As shown in Figure 5.1d, the driving force for charge injection

between the perovskite and the WO3-x -RT is 0.48 eV, which is 70 mV higher than

the driving force for charge transfer between the perovskite and WO3-x -300 ˚C.

Therefore we believe the decrease of PL should be mainly due to interfacial charge

transfer between the perovskite and the WO3-x based ETL.

Hysteresis in a perovskite solar cell is caused by charge accumulation at the interface

between perovskite and electron transport materials. To investigate the carrier

recombination resistance of PSC using different WO3-x based ETL, impedance

spectroscopy at open circuit voltage under 1 sun illumination was carried out. Figure

5.5b shows Nyquist plot of PSC using the WO3-x-RT and WO3-x-300 °C. The

equivalent circuit of the Nyquist plot is shown in the inset of Figure 5b. The series

resistance is represented by Rs in the equivalent circuit. The geometric capacitance of



bulk material and surface, which reflects ion accumulation at the perovskite interface

are represented by Cg and Cs, respectively. The sum of the resistive components, R1

and R2 is associated with the recombination resistance (Rrec) at the interface of WO3-

x/perovskite. The fitted results from experimental date are shown in Table 5.3.


illumination at open circuit voltage.

ETLs Rs (

cm2)

Cg

(F/cm2)

R3 (

cm2)

Cs

(F/cm2)

R1 (

cm2)

WO3-x -RT 1.54 3.8×10-7 6.8 0.0005 11.3

WO3-x -300 ºC 1.55 3.8×10-7 2.8 0.002 8.7

As shown in Table 5.3, the recombination resistance i.e. R1 + R3 for PSC using the

as-deposited (Rrec= 18.1 ( cm2)) WO3-x is almost 50% higher than the PSC using the

annealed WO3-x (11.5 ( cm2)). This higher recombination resistance can be

associated with the higher conductivity of the as-deposited WO3-x thin film. Also, the

high recombination resistance at the ETL/perovskite interface might be the reason for

low hysteresis [44, 45]. The charge recombination at the ETL/perovskite interface is

mitigated by the improved charge injection (Cs of RT WO3-x is 5 x 10-4 F/cm2 and 2 x

10-3 F/cm2 for annealed WO3-x). The higher conductivity and lower conduction band

position of the as-deposited WO3-x could be the driving force for better charge

extraction from the perovskite material. Hysteresis index (HI) of the PSC which is

defined by the following equation:

HI = (PCErev – PCEfw)/PCErev Equation (2)

HI was calculated for both the as-deposited and post-annealed WO3-x samples. The

calculated HI using the WO3-x-RT ETL is 2.1% which is much lower than the WO3-x-

300 °C ETL (25%). The higher conductivity and charge transport property of the as-


Temperature 113

deposited WO3-x with oxygen vacancies governs superior device performance with

low J-V hysteresis.

5.3 Conclusion

In this study we report a room temperature deposited WO3-x thin film by electron-

beam evaporation in high vacuum. Electron beam (e-beam) evaporation is a versatile

and robust technique for deposition of uniform metal oxide films over a large surface

area with good control over film quality and composition. Also, oxygen vacancies

can be created in the WO3 through control of oxygen environment which can be

beneficial in device performance improvement. The WO3-x exhibited high optical and

electrical properties, large oxygen vacancies with wide bandgap and high electron

mobility. For comparison we also annealed the as-deposited film at 300 °C to reduce

the oxygen vacancies and compare their performance in perovskite solar cells. The

as-deposited WO3-x as ETL has demonstrated a much higher Voc and FF compared to

the annealed WO3-x film, leading to energy conversion efficiency over 10% under

AM1.5 one sun illumination. Most importantly, the current-voltage hysteresis of the

as-deposited WO3-x film was almost eliminated compared to the annealed sample.

5.4 Experimental Section

The materials used for experiment were purchased from Sigma-Aldrich and

used as received, unless otherwise stated. For the preparation of

Methylammonium lead tri-iodide (MAPbI3) perovskite films an Lewis acid-

base adduct approach was used, details of which are described in the previous

reports [9, 46, 47].In Brief, a mixture of PbI2 and methylammonium iodide

(MAI) (Dyesol), using 461 mg and 159 mg each respectively, was dissolved in

78 mg of dimethyl sulfoxide (DMSO) and 650 mg of dimethyl formamide

(DMF) at room temperature, for the preparation of MAPbI3 perovskite



precursor solution. The MAPbI3 solution was then filtered using syringe filter

(pore size: 0.22 µm) prior to use for deposition of film. Spiro-OMeTAD based

HTM solution was prepared by using 72.3 mg of 2,2’,7,7’-Tetrakis-(N,N-di-4-

methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) (Borun New

Material), 28.8 µL of 4-tert-butylpyridine, and 17.5 µL of

Bis(trifluoromethane)sulfonimide lithium (Li-TFSI) solution (720 mg of Li-

TFSI in acetonitrile) in 1 mL of chlorobenzene.

Device fabrication

Solar cells were fabricated on fluorine-doped tin oxide (FTO) coated glass (Nippon

Electric Glass, 15 /) as substrate. The substrate was patterned through partial

removal of FTO via etching using 35.5 wt% HCl and zinc powder. Then a 5%

Decon-90 detergent and a mixture of acetone, isopropanol and ethanol were used to

clean the substrate for 20 mins in an ultrasonic bath, respectively. Prior to use, the

substrate was treated with ultraviolet for 30 mins to fully remove organic solvent

residuals. WO3-x thin films were developed using electron beam evaporation

technique (PVD 75 Kurt J. Lesker) in high vacuum (<10-6 Torr) from WO3 pellets

(99.9% purity). The films were deposited at a deposition rate of 1 A/s as

controlled by the quartz crystal monitor. Some samples were post annealed at

300 °C in air. During deposition the substrate was continuously rotating at 10 rpm in

order to ensure uniform and homogenous coatings fully covering the substrate. All

the films had a nominal thickness of 60 nm. The prepared WO3-x films were treated

in a UV-ozone for 20 mins before being transferred to an Ar-filled glove box.

MAPbI3 layer (~350 nm) was deposited onto the prepared WO3-x layer at 4000 rpm

for 30 s. During spin-coating, 0.5 mL of diethyl ether was dropped on the center of


Temperature 115

the spinning substrate 18 s prior to the end of the program. The perovskite layer was

then dried at 65 ºC for 2 mins and then annealed at 100 ºC for 2 mins. A ~200 nm

Spiro-OMeTAD was deposited onto the perovskite layer as hole-transport layer from

the prepared HTM solution at 4000 rpm for 30 s. The device fabrication was

completed by depositing a 100 nm gold layer as a back contact using e-beam

evaporation in high vacuum (<10-6 Torr).

Characterization

Field emission scanning electron microscope (FESEM JOEL 7001F) was used at an

acceleration voltage of 5 kV to monitor the top and cross-sectional views of the

samples. The thickness of the films was measured by a stylus profilometer and

further confirmed by ellipsometry.

X-ray photoelectron spectroscopy (XPS) data was acquired from a Kratos Axis Supra

with a monochromated Al Kx-ray source (h = 1486.7 eV). Survey spectra of the

samples were taken at analyser pass energy of 160 eV, and high resolution XPS

spectra were acquired at 20 eV pass energy. XPS spectrum was analysed using

CasaXPS software version 2.3.17PR.1.1. All spectra were aligned by a rigid shift of

the adventitious C-C signal to 284.8 eV. High resolution W 4f, O 1s and C 1s spectra

were fitted using Voigt functions, enforcing the correct 4:3 area ratio for the W 4f7/2

and W 4f5/2 peaks [48].

Ultraviolet photoemission spectroscopy (UPS) measurements of the valence

band were taken on the cleaned WO3-x surface to measure the valence band of

the material using He I source (21.22 eV) and an analyzer pass energy of 20

eV. The binding energy scale of these spectra was calibrated by a rigid shift of

the spectra to align the Fermi level of an electrically contacted Au sample to 0

eV.



The optical transmittance of the WO3-x films on glass substrate was measured

using Cary 5000 UV-Vis-NIR spectrophotometer with a 150 mm integrating

sphere in the wavelength range 200 to 1100 nm. The electrical properties i.e.

the conductivity and resistivity of the samples were measured 4-point probing

system. Electron spin resonance (EPR) measurement was performed using a

Magnetech MiniScope MS400 system. The power conversion efficiency (PCE) of

the perovskite solar cells was measured under irradiation of 100 mW/cm2 (AM1.5)

provided by a solar simulator (Oriel Sol3A, Newport) equipped with 450 W Xenon

lamp. A quantum efficiency system (IQE 200B, Newport) was used to conduct the

IPCE measurement under AC mode. Impedance spectrum (EIS) of the PSCs was

obtained with an electrochemical workstation (VSP BioLogic Science Instruments)

under 1 sun illumination in a frequency range from 1 MHz to 100 mHz. An AC

voltage with perturbation amplitude of 20 mV was applied in the (EIS) measurement.

5.5 Acknowledgement

The first author is indebted for QUTPRA scholarship and financial support. H.W.

acknowledges the financial support by the Australian Research Council (ARC)

Future Fellowship (FT120100674) and the Queensland government (Q-CAS). The

author will like to thank Dr. Joseph for his help in the EPR characterization and

Jonathan for helping in the XPS data analysis. This research was mainly done at the

Institute for Future Environments (IFE) Central Analytical Research Facility (CARF)

at QUT. Access to CARF is supported by generous funding from the Science and

Engineering Faculty (QUT).

Keywords: WO3-x thin film, Oxygen vacancy, e-beam evaporation, hysteresis,

perovskite solar cells


Temperature 117

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Supporting Information

Room Temperature Deposited WO3-x Films as Electron Transporting Layer for

Low Hysteresis Planar Perovskite Solar Cells

Fawad Ali, Ngoc Duy Pham, Ken Ostrikov, John M. Bell, Hongxia Wang* and

Tuquabo Tesfamichael*





Fax: +61-7-31381516


[email protected]

Figure S5.1 Survey spectra of the as-deposited (RT) and post-annealed (300 ºC)

WO3-x thin films.




Temperature 123

Table S5.1 Electrical properties (conductivity and resistivity) of as-deposited and

annealed WO3-x thin films using four-point-probe

ETLs Resistance (Ω) Sheet Resistivity

(Ω /square)

Volume

Resistivity (Ω-

cm)

Conductivity

(Siemen/cm)

WOx-RT 390498 1460072 29 0.034

WOx-300 ºC 1103330 4125350 82.5 0.012

Figure S5.2 Thickness of as-deposited and annealed WO3-x measured by stylus

profilometer.

Table S5.2 Series and shunt resistance for perovskite solar cells using as-deposited

and annealed WO3-x as ETL from J-V curve.

Measurement Voc (V) Jsc

(mA/cm2) Fill Factor Efficiency

Series

Resistance

(Ωcm2)

Shunt

Resistance

(Ωcm2)

WOx-RT 0.87 18.0 65 10.3369 6.1 1782

WOx-300 ºC 0.82 18.7 60 9.268 7.4 724



Calculation of g value;

(𝑔 = ℎ𝑣 𝜇𝐵𝐵⁄ , ℎ = 6.626 × 10−34 𝐽 𝑠, 𝜇𝐵 = 9.274 × 10−24 𝐽 𝑇−1, 𝑣 = 9.5 𝐺𝐻𝑧,

and B is the magnetic resonance field of ESR)

The relative number of spins (Ns) unpaired electrons (in other words the oxygen

vacancy) participating in the resonance is calculated by using the formula;

Ns α I (ΔH)2 Equation (1)

From this equation the resultant Ns value calculated for the as deposited WOx is

much higher (1.53 x 108) as compared to annealed WOx (3.6 x 107). Also, this result

is in good agreement with the XPS result, which shows an increase in oxygen content

after annealing.


Temperature 125

Figure S5.3 Statistic from four batches of as-deposited and annealed WO3-x ETLs

based PSCs.

Figure S5.3 shows the Voc, Jsc, FF and efficiency of perovskite solar cells

using WO3-x deposited at room temperature and annealed at 300 ºC. Four

samples were used for each condition to see the variation in device

performance. As shown in Figure S5.3 the PSCs with room temperature

WO3-x have high Voc, FF and efficiency as compare to the annealed

samples.

Prospects of e-beam Evaporated Molybdenum Oxide as Hole Transport Layer for Perovskite Solar Cells 127

Chapter 6: Prospects of e-beam Evaporated Molybdenum Oxide as

Hole Transport Layer for Perovskite Solar Cells


1 they meet the criteria for authorship in that they have participated in the



2 they take public responsibility for their part of the publication, except


publication;

3 there are no other authors of the publication according to these criteria;

4 potential conflicts of interest have been disclosed to (a) granting



5. they agree to the use of the publication in the student’s thesis and its



In the case of this chapter:

Contributor Statement of contribution

Fawad Ali Designed and conducted the experiments, analyze

the data and wrote the manuscript..

Nima Khoshsirat Helped in drawing the structure

J. Lipton Duffin Helped with XPS data analysis









Name Signature Date


128 Chapter 6: Prospects of e-beam Evaporated Molybdenum Oxide as Hole Transport Layer for Perovskite

Solar Cells

Prospects of e-beam evaporated Molybdenum Oxide as a Hole

Transport Layer for Perovskite Solar Cells

This chapter was originally accepted for publication in Journal of Applied Physics in

2017. In this combined experimental and empirical simulation, MoOx was found to

be alterative and a better candidate as HTL for PSCs to replace the expensive organic

HTM. The film is found to be suitable in both the regular and inverted architectures

of the PSCs. The purpose of this study was to find whether MoOx can be use as

alternative HTM for perovskite solar cells. Experimental validation is needed to

confirm these results.

Journal of Applied Physics, vol. 122, p. 123105, 2017.


Prospects of e-beam evaporated Molybdenum Oxide as a Hole

Transport Layer for Perovskite Solar Cells

F. Ali, N. Khoshsirat, J. L. Duffin, H. Wang, K. Ostrikov, J.M. Bell and T.

Tesfamichael*





Fax: +61-7-31381516

email: [email protected]

Abstract

Perovskite solar cells have emerged as one of the most efficient and low cost

technologies for delivery of solar electricity due to their exceptional optical and

electrical properties. Commercialization of the perovskite solar cells is, however,

limited because of the higher cost and environmentally sensitive organic hole

transport materials such as Spiro-OMETAD and PEDOT:PSS. In this study, an

empirical simulation was performed using Solar Cell Capacitance Simulator software

to explore MoOx thin film as an alternative hole transport material for perovskite

solar cells. In the simulation, properties of MoOx thin films deposited by electron

beam evaporation technique from high purity (99.99%) MoO3 pellets at different

substrate temperatures (room temperature, 100 °C and 200 °C) were used as input

parameters. The films were highly transparent (>80%) and have low surface

roughness (≤ 2 nm) with bandgap energy ranging between 3.75 eV to 3.45 eV.

Device simulation has shown that the MoOx deposited at room temperature can work

in both the regular and inverted structures of the perovskite solar cell with a

promising efficiency of 18.25%. Manufacturing of the full device is planned in order

to utilize the MoOx as an alternative hole transport material for improved

performance, good stability and low cost of the perovskite solar cell.



Solar Cells

Keywords: Perovskite solar cell; inorganic hole transport material; electron beam

evaporation; MoOx thin films; SCAPS Solar cell Simulation


6.1 Introduction

Perovskite solar cells (PSCs) are one of the promising technologies in photovoltaics

that have encouraged the world’s ambition towards solar energy utilization at low

cost. Due to the low cost and simple solution procedure, PSCs have revolutionized

the field of photovoltaic research. The efficiency of PSC has increased from 3.8% [1]

in 2009 to more than 20% in 2016 [2]. Despite the high efficiency of the solar cell,

the hole transport materials (HTM) used are mostly limited to organic compounds

including 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene

(spiro-OMeTAD)[3] and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

(PEDOT:PSS). These materials are expensive and have poor stability upon exposure

to the environment [4, 5]. The acidic nature of PEDOT:PSS [6] and the hygroscopic

nature of the additives used in the Spiro-OMeTAD decreases the long term stability

of PSCs [7]. In practice, high cost and poor environmental stability are the main

limitations for large scale applications of the PSCs. Metal oxides will be a better

replacement to overcome these challenges. Metal oxide thin films have been widely

used in various applications such as display devices, optical small window, gas

sensors and photovoltaics [8] and they can be a better candidate as HTM for solar

cells [9, 10]. Inorganic hole transport materials such as NiO, Cu2O, CuOx and VOx

have been incorporated into PSC as a replacement of the existing organic HTMs [4,

11] due to their higher stability and lower materials cost than organic materials [12].

MoOx is one of the potential materials not only for solar cells but also gas sensors

and light-emitting diodes [13]. Recently, MoO2 nanoparticles were used as a hole

transport layer with efficiency of 15%, with very low hysteresis and high device

stability [14]. Similarly, MoOx has been used as a hole selective contact for silicon

solar cell [15]. Xiao et al., use MoOx as a hole transport layer for perovskite solar


Solar Cells

cells but only achieved low efficiency of 5.9% [16]. Hou et al., used bilayer

MoOx/PEDOT:PSS to improve the performance and stability of the solution

processed perovskite solar cells [9]. Chen et. al., used thermally evaporated MoOx in

inverted structure of the perovskite solar cell achieving 13% efficiency and good

stability [10]. These results suggest that MoOx is a promising material as an

alternative HTM.

Different techniques have been used for the preparation of MoOx thin films with

amorphous and crystalline structure. Films can be deposited using solution-based

methods and vacuum deposition techniques. The solution methods include spray

pyrolysis [17], sol-gel process [18], electrodeposition and spin coating [19] while the

vacuum methods are thermal evaporation [20], sputtering [21] and electron beam

evaporation (e-beam) [22]. Solution processing is usually easy and cheap; however,

there is less control over the film properties, leading to poor reproducibility. Vacuum

deposition however has good control over the film properties and reproducibility.

Also, a wide range of experimental parameters are available which can tailor the

structure, morphology, composition and other properties of the films according to the

required applications.

In this study empirical simulation using solar cell capacitance simulator software

(SCAPS) was applied to investigate MoOx as an alternative HTM for perovskite

solar cells. MoOx thin films deposited at different substrate temperatures by electron

beam evaporation technique were considered for the simulation. Helium Ion

Microscopy (HIM) and Atomic Force Microscopy (AFM) were used to study the

surface morphology of the films. The surface roughness was measured by AFM and

confirmed using stylus profilometer. The film thickness was measured by both

stylus profilometer and ellipsometry. The chemical composition was investigated


using X-Ray Photoelectron Spectroscopy (XPS), and the crystalline nature of the

films was determined using Raman spectroscopy. The optical properties of the films

were characterized using UV-Vis-NIR spectrometry. From these measurements, the

thickness and bandgap energy (Eg) of the MoOx thin films were used as input

parameters in the empirical simulation analysis to study the films as an alternative

HTM layer in both the inverted and regular structures of the PSC.

6.2 Experimental

Thin Film Deposition

MoOx thin films were deposited on a glass substrate using electron beam evaporation

technique (PVD 75 Kurt J. Lesker) from MoO3 pellets (99.9% purity). The films

were deposited at different substrate temperatures (room temperature, 100 °C and

200 °C). For the films deposited at 100 °C and 200 °C, the substrate was preheated to

the desired temperature and maintained at that temperature throughout the

deposition. Before each deposition, the glass was cleaned thoroughly with acetone,

ethanol and then dried with nitrogen gas. The chamber was evacuated to a base

pressure of less than 1 x 10-6 Torr. As expected the pressure slightly increased to 4 x

10-5 Torr during the film deposition due to the vapour of the MoO3. The films were

deposited at a deposition rate of 1Å/s as controlled by quartz crystal monitor. All the

films had a nominal thickness of 100 nm. During deposition the substrate was

continuously rotating at 10 rpm in order to ensure uniform and homogenous coatings

fully covering the substrate. After deposition the samples were allowed to naturally

cool in the vacuum chamber to less than 50 °C before they were taken out of the

chamber for characterization.


Solar Cells

2.2 Thin Film Characterization

Various techniques were used to characterize the as-deposited MoOx thin films. The

surface morphology of the films was characterized by Atomic Force Microscope

(AFM) and Helium Ion Microscope (HIM). An NT-MDT Solver P47 scanning probe

microscope (NT-MDT Co., Moscow, Russia) operated in semi-contact mode with

"Golden" Si cantilevers was used in the AFM measurement. The nominal tip

diameter was 10 nm. High resolution micrographs were obtained using Zeiss Orion

HIM at 25 kV with a 0.3 pA blank current. HIM is found to be the preferred

technique for characterizing the as-deposited MoOx thin films without using

conductive coating. Any charging caused by the insulating properties of the films can

be compensated using a flood gun having a beam of electrons which compensate for

the accumulated ion beam charge. The thickness of the films was measured by stylus

profilometer and confirmed by ellipsometry. X-ray photoelectron spectroscopy was

performed using a Kratos Axis Supra with aluminium Al Kα X-ray radiation (h =

1486.7 eV). Wide survey scans were acquired using analyzer pass energy of 160 eV,

high resolution scans of the O 1s, C 1s, and Mo 3d regions were performed using a

pass-energy of 20 eV to better discriminate the sub-structure of the spectral lines. In

all cases the binding energy scale was corrected by a rigid shift to align the peak of

the C 1s core level to 284.8 eV, corresponding to adventitious carbon. Quantification

was performed using the CasaXPS software version 2.3.17PR1.1, using the

appropriate element sensitivity library for the Kratos instrument. Carbon present on

the surface was assumed to be adventitious in nature, and as such a fraction of the

measured oxygen signal was associated to the surface contamination, using the

method of Payne et. al [23]. Mo 3d and O 1s spectra were fitted with a series of

Voigt line shapes, using area constraints across Mo and O chemical states to enforce


the expected 3:1 and 5:2 intensity ratios for Mo (VI) and Mo (V). While the Mo (V)

line shape is known to be somewhat more complicated than a simple Voigt function

due to multiple splitting [24]. We have found that our analysis using simple

mathematical models is self-consistent. The mean ratio for the total O:Mo ratio

determined from high resolution analysis yielded the same result as found using the

total intensity of O:Mo from the survey spectrum.

The chemical structure and crystalline state of the MoOx films were studied using

Renishaw inVia Raman spectrometer. A Renishaw frequency doubled NdYAG laser

excitation source of wavelength 532 nm was used. To avoid local heating of the

samples, a low power of about 5 mW was applied to the samples. A Raman shift

between the wavenumber 200 to 1200 cm-1 was measured. The transmittance of the

MoOx films on glass substrate was measured using Cary 5000 UV-Vis-NIR

spectrophotometer with a 150 mm integrating sphere. The measurements were

performed in the wavelength range 300 to 2500 nm at a near-normal angle of

incidence. The measured transmittance values were subtracted from the base (zero)

signal. A Teflon coating was used as a 100% reference. From these measurements

the weighted solar transmittance for A.M. 1.5 and optical bandgap of the films were

obtained.

Simulation of Perovskite Solar Cell

SCAPS simulator was used for the simulation of the perovskite solar cell device

using MoOx as HTM. SCAPS software is developed at Electronics and Information

Systems (ELIS), University of Gent which is modelled under an AM 1.5 light

spectrum [25]. It is very well-known simulation software in thin film solar cells as

well as solar cells that have planar structure [26]. In this work, a planar structure

consisting of regular (FTO/TiO2/CH3NH3PbI3/MoOx/Au) and inverted


Solar Cells

(FTO/MoOx/CH3NH3PbI3/PCBM/Au) configuration were used for the simulation of

the perovskite solar cell. Input parameters used for the simulation of the device were

obtained from experimental results of this work and literature as shown in Table 6.1.

Here, NA and ND denote acceptor and donor densities, εr is relative permittivity, χ is

electron affinity, Eg is bandgap energy, µn and µp are mobility of electron and hole

and Nt is defect density[27] [28]. The other parameters not mentioned in the Table

are the effective density of charge at conduction band (NC) and at valence band (NV)

which have values of 2 × 1018 and 1.8 × 1019, respectively [28].

Table 6.1 Input parameters obtained from this experiment and various reference

papers for SCAPS simulation of PSC using MoOx as HTM [29].

*IDL1: Interface Defect Layer between ETL/Perovskite *IDL2: Interface Defect Layer between Perovskite/HTL **This thickness range includes the experimental value of MoOx (100 nm) *exp: Experimental results of this work

6.3 Results and Discussions

Structural properties

The Raman spectra of the MoOx thin films deposited at different substrate

temperatures is shown in Figure 6.1. Three sharp Raman peaks obtained at 665, 820,

and 995 cm-1 belong to orthorhombic α-phase of molybdenum oxide [30]. Such

phase is normally obtained after high temperature annealing (450 °C). The film


deposited at room temperature has peaks of α-phase with very weak Raman intensity

which is the characteristic of amorphous films [31]. The peak intensity, however,

increases with increasing substrate temperature and this shows that the films

deposited at higher substrate temperatures are dominated by the crystalline properties

[31]. The Raman band at 995 cm-1 is assigned to the terminal oxygen (Mo=O)

stretching mode, while the 820 cm-1 to the doubly-connected bridge-oxygen

(Mo−O−Mo) stretching mode and the 665 cm-1 to the triply connected bridge-oxygen

(Mo3−O) vibration [32]. The peak at 820 cm-1 has high sharpness for the films

deposited at 100 °C and 200 °C which indicates that the corresponding vibrational

modes are due to highly ordered structure. The corner-sharing chains of MoO6

octahedra may be visualized in the α-phase which shares the edge with two similar

chains to form the MoO3 stoichiometry. Each octahedron of MoO3 has one unshared

oxygen atom, two oxygen atoms are common to two octahedra and three oxygens are

in part-shared edges and common to the three octahedra [31] which referred as α-

phase [33].


Solar Cells

Figure 6.1 Raman spectra of MoOx thin films deposited by e-beam evaporation at

different substrate temperatures (RT, 100 ºC and 200 ºC). In reference [30, 31], the

peaks were assigned to α-phase of MoO3.

To observe the effect of substrate temperature on the chemical characteristics of

MoOx thin films, XPS analysis was conducted. Figure 6.2 shows a series of survey

and high resolution (Mo 3d, O 1s and C 1s) spectra of the MoOx thin films deposited

at different substrate temperatures. The films produced at room temperature consist

of pure MoO3, as they exhibit oxygen to molybdenum ratio of 3:1. This implies that a

pure stoichiometric film is deposited from the electron beam evaporator at room

temperature. High resolution scans of the Mo 3d and O 1s core level confirm this

finding, where the Mo 3d doublet can be fitted with a single doublet pair with the

expected 3:2 intensity ratio between the 3d5/2 and 3d3/2 states. By contrast, the films

deposited at elevated temperatures (100 ºC and 200 ºC) show a significant reduction

of Mo(V) state, with films deposited at 100 ºC containing 75% Mo (V) and films

deposited at 200 ºC containing 76% Mo (V) as shown in Figure 6.3. This analysis


points to an overall oxygen to molybdenum ratio of 2.7:1 for the sample deposited at

200 oC.

Figure 6.2 XPS of MoOx thin films deposited by e-beam evaporation at different

substrate temperatures (RT, 100 ºC and 200 ºC).

The high resolution Mo 3d and O 1s scans in Figure 6.3 are fitted based on an

assumed 5:2 and 3:1 oxygen weighting of the Mo (V) and Mo (VI) components

respectively. The spectral weight of oxygen was adjusted for the presence of C-Ox

species in adventitious carbon [23], as well as ascribing high-binding energy spectral

weight to adsorbed water. As explained by Bulpett et al., that the conduction band of

MoO3 consists of empty 4d and 5s states while the valence band consists of oxygen

2p state. At higher substrate temperature the transition from oxygen 2p to an empty

Mo6+ 4d level will give rise to the incorporation of lower valency Mo5+ ion in the

lattice [34].


Solar Cells

Figure 6.3 XPS spectra of MoOx thin films at different substrate temperatures

showing high resolution scans of (a) C 1s, (b) O 1s and (c) Mo 3d core levels with

synthetic fits to the spectral envelopes.

3.2 Morphological Properties

Figure 6.4 shows the surface morphology of the MoOx thin films deposited at room

temperature, 100 ºC and at 200 ºC characterized using HIM (Figure 4(a-c)) and AFM

(Figure 6.4(d-f)). Both films are homogenous with no cracks or pits and have smooth

surface morphology covering the substrate uniformly as observed from the HIM

micrographs in Figure 6.4 (a-c). There is usually an increase in grain size with

increasing deposition temperature [31], which is not obvious in the HIM and AFM

images of this work. However, the surface roughness of the films slightly decreases

with increasing the deposition temperature as observed from the AFM images in


Figure 6.4(d-f) and Table 6.2. These surface roughness values were confirmed by the

stylus profilometer and AFM.

Figure 6.4 Micrograph of MoOx thin films deposited at room temperature (a, d), 100 ºC (b, e) and 200 ºC (c, f). (a, b, c) are HIM images having a scale bar of 1 μm and

(d, e, f) are AFM images scanned over 2 μm × 2 μm.

3.3 Optical Properties

Figure 6.5a shows the spectral transmittance of the 100 nm MoOx films deposited at

different substrate temperatures. The films are highly transparent in the visible and

near-infrared wavelength region and their transmittance sharply dropped in the

ultraviolet wavelength. With increasing deposition temperature the absorption edge

shifts towards the longer wavelength. From the Raman spectra, the film deposited at

room temperature has amorphous behaviour and this property can also be observed

from the optical spectra in Figure 6.5a having low interference fringes. The strong

coloration of the films deposited at high temperature is due to oxygen ion vacancies,

which promotes the reduction of oxygen atoms in the oxide structure which is


Solar Cells

confirmed from the XPS data [35]. The weighted optical transmittance of all the

films in the solar wavelength is more than 80% as shown in Table 6.2 with slightly

higher value observed from the amorphous film. This amorphous film has slightly

higher transmittance in the infrared compared to the crystalline MoOx films. The

interference fringes would also have effect on the variation in the weighted solar

transmittance.


Figure 6.5 Transmittance spectra and (b) (αhv)2 vs hv plot of 100 nm thick MoOx

thin films deposited at various substrate temperatures.


Solar Cells

The bandgap energy (Eg) of the MoOx films (100 nm) was calculated using the

relation [35]:

(αhν)2 =A(hν− Eg) (1)


constant and ν is the frequency of light. The plot produced from this relation is

known as Tauc plot and gives a linear behaviour in the high energy region. The Eg of

the films is then calculated by fitting the linear region of the plot to zero. Figure 5(b)

shows the (αhν) vs photon energy (hν). The estimated Eg value of MoOx deposited at

room temperature is 3.75 eV (see Table 2). This value decreases to 3.45 eV when the

film was deposited at 200 °C. The decrease in bandgap energy is attributed to the

oxygen vacancies which enable to capture electrons and act as a donor centres [36].

The deficiency of oxygen is confirmed by XPS result which shows that the atomic

percentage of oxygen is reduced with the increase in substrate temperature (see Table

6.2). The bandgap energy of MoOx obtained in this work (3.45-3.75 eV) are found to

be within the values of e-beam deposited molybdenum oxide films reported in the

literature (3.1-3.7 eV) [37]. These experimentally obtained Eg values of MoOx for a

100 nm thick film are used in the SCAPS simulation to predict the efficiency of the

regular and inverted structures of the PSC. In the simulation the MoOx film thickness

was varied between 50 to 250 nm that included the experiment film thickness in

order to observe the effect of film thickness on the various parameters of the solar

cells.

Table 6.2 Transmittance, atomic ratio of O:Mo, surface roughness and bandgap

energy of MoOx thin films deposited at different substrate temperatures.


Simulation of Perovskite Solar Cell with MoOx as HTM

Empirical simulation analysis using SCAPS simulation software was performed to

study the performance of perovskite solar cell with MoOx as an HTM layer. The

schematic diagram of the inverted and regular structures used in the simulation are

shown in Figure 6.6(a) and Figure 6.6(b), respectively. A typical perovskite solar cell

configuration adopts a solid planar heterojunction p-i-n structure where the

perovskite material in the regular structure is sandwiched between electron transport

layer (ETL) (TiO2) and HTM (MoOx). In the regular structure the HTM is applied on

the top of the perovskite material, thus deposition of MoOx at only room temperature

is suitable as the perovskite material is sensitive to elevated temperature (more than

100 °C). In the inverted structure, the HTM (MoOx) is applied on the top of fluorine

doped tin oxide (FTO) and PCBM is typically used as ETL and allows high substrate

temperature deposition of the MoOx thin films. FTO coated glass is used as a

transparent conductive oxide and Au serves as a back metal contact. Thus using the

experimentally obtained bandgap energy (3.75 eV) and film thickness (100 nm) of

MoOx, the solar cell efficiency for both the regular and inverted structures are found

to be 18.21% to 17.85%, respectively.


Solar Cells

Figure 6.6 Schematic diagram of (a) inverted structure and (b) regular structure of a

perovskite solar cell used in SCAPS simulation.

An optimum thickness of the HTM with full surface coverage is very important for

the device performance. Figure 6.7 shows the effect of MoOx thickness on the open

circuit voltage (Voc), short-circuit current density (Jsc), recombination current density

(Jrec), fill factor (FF) and efficiency for both the regular (Figure 6.7a) and inverted

(Figure 6.7b) structures. As observed in the figure the Jsc slightly decreases with the

increase in HTM (MoOx) thickness while the Voc remains unchanged. This decrease

in Jsc can be linked to the increase in recombination current as shown in the Figure

6.7(a). The total recombination current density (Jrec) increases with the increase of

HTM thickness which causes a reduction in Jsc and overall efficiency of the solar

cell. The same trend is followed in both regular and inverted structures. This increase

in recombination current density is due to longer distance that photo-generated

carriers should transport to be driven out while their life-time and mobility are kept

constant. So a minimum of MoOx layer thickness that gives full layer coverage to

minimize recombination in HTM layer is desirable. Also, in the inverted structure

(Figure 6.7b) where the light enters through MoOx side, the FF decreased with the

increase of MoOx thickness due the increase in recombination current density and an

increase in light absorption. From the HIM images in Figure 6.4, the 100 nm thick


MoOx films deposited at different substrate temperatures have fully covered the

substrate. By varying the MoOx layer thickness from 50 to 250 nm for the highest

bandgap energy of MoOx obtained in Table 6.2, the efficiency of the PSC only

slightly decreases from 18.25% to 18.15% for the regular structure (Figure 6.7a) and

from 17.88% to 17.70% for the inverted structure (Figure 6.7b). The best cells with

50 nm thicknesses have shown high Voc and Jsc values of 1.03 V and 22.3 mA/cm2

for the regular structure, and 0.97 V and 22.7 mA/cm2 for the inverted structure with

fill factor of 78.7 and 82.5, respectively.

Figure 6.7 Open circuit voltage (Voc), short-circuit current density (Jsc), current

recombination (Jrec), fill factor (FF) and efficiency of PSC as a function of MoOx

layer thickness in (a) regular and (b) inverted PSC structure. The room temperature

deposited bandgap energy (3.75 eV) of MoOx films is considered.

The quantum efficiency (QE) is one of the important parameters that describes the

quality of light absorption, charge transfer and collection of a solar cell. As shown in

Figure 6.8 the QE for the (a) regular and (b) inverted structures (Eg=3.75 eV)

decreases with the increase of HTM thickness in the wavelength region below 400

nm and this is more prominent in the regular structure of the solar cell. The decrease


Solar Cells

in QE at the lower wavelength can be due to the recombination in the MoOx layer

and/or the light absorption of MoOx layer. The carriers which are generated by the

high energy photons (> bandgap energy of MoOx) don’t contribute to the quantum

efficiency because they cannot reach the back contact of the inverted structure PSC

due to their limited life time and mobility leading to enhanced recombination in the

MoOx layer. Smaller bandgap of MoOx means the photons at wavelength less than

400 nm can be absorbed by the MoOx layer, which in turn reduces the effective light

absorption of the perovskite light absorber and thus the QE. The higher the MoOx

thickness, the higher is the light absorption by the HTM of the inverted structure and

the lower the QE.


Figure 6.8 Quantum efficiency of Perovskite solar cell using MoOx films with

different thicknesses in (a) regular structure and (b) inverted structure. The room

temperature deposited bandgap energy (3.75 eV) of MoOx films is considered.

As discussed before, the experimental results have confirmed that the increase in

substrate temperature causes an increase in the crystallinity, a decrease in the

bandgap energy along with the overall oxygen to molybdenum ratio and surface

roughness of the MoOx thin film. Thus the effect of MoOx bandgap energy on the

PSC performance for the inverted structure has been investigated using SCAPS

simulation. As shown in Figure 9 (a) for the 100 nm layer of MoOx, the Jsc increases


Solar Cells

from 22.3 mA/cm2 to 22.6 mA/cm2, while the Voc remains constant with increasing

MoOx bandgap. The recombination current density decreases with the increase of

bandgap energy. The increase in Jsc governs an overall increase in the efficiency of

the simulated PSC device to 17.85% as shown in Figure 6.9(a). From the above

analysis, deposition of MoOx films at higher substrate temperature causes a reduction

in the bandgap energy which reduces the simulated Jsc. Variation in QE with

different MoOx bandgap energy (100 nm layer of MoOx) is shown in Figure 6.9(b).

An increase in QE with the increase of the MoOx bandgap energy is observed at the

lower wavelength region. However, with increasing deposition temperature an

increase in both crystallinity and defect density of the film and a reduction in surface

roughness were observed. These properties which are useful for enhancing the

performance of the solar cell were not considering in the SCAPS simulation.

Figure 6.9 (a) Open circuit voltage (Voc), short-circuit current density (Jsc), current

recombination (Jrec), fill factor (FF), efficiency, and (b) quantum efficiency of PSC as

a function of MoOx bandgap energy in the inverted structure for a film thickness of

100 nm.

As shown in the simulated results the variation in bandgap energy of MoOx as HTM

only slightly affects the efficiency of the PSC. However, as observed in the


experiment, deposition of MoOx thin films at higher substrate temperature (e.g.

200oC) gives rise to oxygen deficiency and enhanced crystallinity of the films which

would be expected to improve the performance and stability of the PSC [38]. From

the calculation, thinner MoOx film of about 50 nm thick that can completely cover

the surface of the substrate is found to be beneficial for enhanced QE. Using these

conditions, the performance of the PSC was determined as shown in Figure 6.10 with

optimum efficiency of 17.82% and high QE.

Figure 6.10 Quantum efficiency (QE) of PSC using MoOx as HTM in inverted

structure of PSC. Inset shows the optimized efficiency of the PSC in the inverted

structure using film thickness of 50 nm.

6.4 Conclusion

SCAPS simulation software was used to determine MoOx as an alternative HTM in

perovskite solar cells. The HTM layer thickness and bandgap energy obtained

experimentally were used to study the device performance. Empirical simulation

results indicate that a larger HTM thickness gives a lower efficiency due to higher

recombination current (Jrec) whereas the bandgap energy has slight effect on the

efficiency of the PSC. Using the properties of electron beam evaporated MoOx film


Solar Cells

deposited at room temperature, the modelling shows an efficiency of 18.25% for the

regular structure and 17.88% for the inverted structure. The MoOx deposited at

higher substrate temperature (200 oC) is suitable for the inverted structure of PSC

with promising efficiency of 17.82%. The overall simulated results suggest that

MoOx thin film is a potential hole transport inorganic material in perovskite solar cell

and can replace the organic hole transport materials with improved performance,

stability and lower cost. Experimental results of the MoOx films indicated a reduction

in bandgap energy, oxygen composition and surface roughness but enhanced

crystallinity with increasing substrate temperature. Device fabrication using MoOx as

HTM is needed to examine the effect of these parameters with the change of

substrate temperature on the performance of perovskite solar cells.

6.5 Acknowledgement

The first author is indebted for QUTPRA scholarship and financial support. H.W

acknowledges the financial support by Australian Research Council (ARC) Future

Fellowship (FT120100674) and Queensland government (Q-CAS). This research

was mainly done at the Institute for Future Environments (IFE) Central Analytical

Research Facility (CARF) at QUT. Access to CARF is supported by generous

funding from the Science and Engineering Faculty (QUT). We also thank Dr. Barry

Wood (University of Queensland, Australia) for XPS data acquisition and Mr

Akshay Prakash for contributing to the experimental data.


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Conclusions and Recommendation for Future Work 157

Chapter 7: Conclusions and Recommendation for Future Work

158 Chapter 7: Conclusions and Recommendation for Future Work

7.1 Conclusions

At the outset in chapters 1-3, an introduction and a critical literature review about

perovskite solar cells, its structure, working principle and challenges were discussed.

Then various types of electron and hole transport layers were reviewed along their

advantages and limitations. Finally different techniques for the deposition of metal

oxide charge transport layers were described along with their limitations and

alternative industrially robust methods.

In chapter 4 low temperature deposited SnOx as ETL was investigated. Non-

stoichiometric SnOx thin films with oxygen vacancies were deposited by sputtering

at different substrate temperatures to be used as ETL for perovskite solar cells. By

reducing the electronic band energy mismatch between the SnOx and perovskite and

alignment of their conduction bands resulting in the increase of Voc and device

performance from 0.8 V to 1 V and 11% to 14%, respectively. It was also verified

that the PSC using SnOx film with higher oxygen vacancies has higher recombination

resistance which ease the transport of charges from the perovskite to the ETL and

hence a reduced J-V hysteresis. A new approach was adopted in this chapter for band

alignment between the ETL and perovskite, by tuning the oxygen vacancies which

has proven to be beneficial for enhancing the device performance. The overall results

showed that creating oxygen vacancies in SnOx and conduction band alignment of

ETL and absorber layer are found to be a convenient strategy for improving the PSC

device performance.

In chapter 5, oxygen deficient WO3-x thin films were produced in oxygen deficient

environment by e-beam evaporation in high vacuum at room temperature as ETL for

perovskite solar cells. Some of the WO3-x films were annealed at 300 ºC in air and

reduced the oxygen vacancies. The high conductivity and higher recombination


resistance of the WO3-x film with oxygen vacancies (as-deposited film) used as ETL

achieved better performance and lower hysteresis as compared to the film with low

oxygen vacancy (annealed film). This result showed that a room temperature

deposited WO3-x can be a promising ETL for PSC including flexible and tandem

solar cells that require low processing temperatures.

Finally in chapter 6, MoOx thin films were deposited by e-beam evaporation at

different substrate temperatures which showed a change in the bandgap energy.

Characterization of MoOx showed a reduction in oxygen composition and bandgap

while increase in crystallinity with increasing substrate temperature, which is

beneficial for the device performance. Using SCAPS simulation the MoOx was

proven as a potential alternative to the organic HTL owing to its high bandgap

energy, better stability and lower material cost.

Overall, various metal oxides including SnOx, WO3-x, and MoOx were investigated

and utilized as charge transport layers for perovskite solar cells. Their electronic,

electrical and optical properties were altered by changing experimental parameters

and improved band alignment with the perovskite. As described in chapter 1, a

suitable CTL must be stable and low cost and should provide high charge transfer to

enhance the performance of the PSC device. These are the basic requirements for

commercialization of the PSCs. This research study investigated stable metal oxide

thin films as charge transporting layers (ETL and HTM) by depositing industrially

viable and inexpensive PVD technique and achieved promising results for perovskite

solar cells.


7.2 Future Recommendations

As described in chapter 6 that e-beam evaporated MoOx showed promising

results via SCAPS simulation as HTL for PSCs. Now a logical next step will

be making PSC utilizing MoOx as HTL in both regular and inverted

structures.

Secondly, other p-type metal oxides HTL such as NiO, need to be

investigated using the same modification treatment as described in chapter 4

and 5 and incorporated to perovskite solar cells as a replacement to

PEDOT:PSS and Spiro-OMeTAD.

Fabricating PSCs using inorganic metal oxides as ETL and HTL should be of

next step for long term stability of the PSCs and reducing materials cost.

Because the current challenge for metal oxide as HTL is the high temperature

sintering process and this issue can be addressed by utilizing the PVD system.

Fabrication of flexible perovskite solar cell devices is only possible at low

processing temperature of the ETL and HTM and this can be achieved using

the PVD systems.

Appendix 163

Appendix

Effect of e-beam evaporated TiO2 compact layer on the performance and

stability of perovskite solar cells

In planar PSCs structures compact TiO2 plays a very important role in preventing

carrier recombination at the interface and carrier mobility. Therefore, pinhole free

and compact TiO2 layer contribute significantly to the development of high

performance PSCs. In this part of the experiment we deposited TiO2 films with

spray-pyrolysis and PVD (E-beam) on FTO coated glass, to see the effect of TiO2

films deposited by different methods on perovskite cell performance and stability.

Perovskite absorbing layer is then deposited onto both compact layers using spin

coating as mentioned above.

Figure 0.1 Cross-sectional SEM images of planar perovskite solar cells with (a) TiO2

layer prepared by E-beam and (b) TiO2 layer prepared by spray-pyrolysis method.

Figure 1 shows SEM cross-section view of PSCs layers with TiO2 layer produced by

e-beam Figure1a and by spray-pyrolysis method Figure1b. The perovskite layer

deposited on e-beam coated TiO2 sample appeared uniform throughout and no voids

at the TiO2-perovskite interface, whereas the spray coated film does not produce a

uniform perovskite film and some voids are present at the interface between TiO2

and perovskite layers.

164 0Appendix

The I-V characteristic of the device is shown in Figure 2a. An average PCE of 7.4%

was obtained for both the devices on day 2. The e-beam samples Jsc was slightly

higher than the sprayed sample. The highest Voc of 1.0 V was obtained for spray

sample which reduced with time. The average Voc for both sample were around 0.88

V. The samples were kept in glovebox for the first few days and the efficiency was

tested on different days. The e-beam sample retained almost 90% of the highest

efficiency for 16 days while the sprayed sample efficiency reduced to 25% with time

as shown in Figure 2b. The device prepared with e-beam TiO2 is more stable.

Figure 0.2 (a) Current Density -Voltage (J-V) characteristics of planar structure

under AM 1.5G illumination, (b) stability of e-beam and sprayed device with time.

An important factor in the high stability of the e-beam device is the quality of the

compact layer. The full surface coverage provides a better resistance to short

circuiting, which can help in improving the Jsc. The good coverage and uniform

nature of the e-beam TiO2 will restrict the recombination and increase the Voc. A

further study will be conducted to fully characterize and understand the composition

and morphology of both TiO2 layers.

(a) (b)

investigation of metal oxides thin films developed by pvd ... · investigation of metal oxides thin...

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