solar cell nanotechnology (tiwari/solar) || photon management in rare earth doped nanomaterials for...

223

Atul Tiwari, Rabah Boukherroub, and Maheshwar Sharon (eds.) Solar Cell Nanotechnology, (223–240) 2014 © Scrivener Publishing LLC

9

Photon Management in Rare Earth Doped Nano materials for Solar Cells

Jiajia Zhou1, Jianrong Qiu1,2,*

1State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, China

2State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China

AbstractIn the last decade, innovations in photonic material design and large-area nanostructure fabrication technique have brought a new era of high- effi ciency solar cells. Photon management approach containing up-conversion (UC) and down-conversion (DC) has received considerable attention as an effi -cient way to improve the photoelectric conversion effi ciency of solar cells by breaking through the Shockley-Queisser effi ciency limits. In this chapter, we introduce the basic aspects of the photon management approach, typical experimental results, and discuss the suitability and existing problems of UC and DC nanomaterials for solar cells.

Keywords: Up-conversion, down-conversion, rare earth, nanomaterials, solar cells

9.1 Introduction

Solar cells are devices that absorb energy from sunlight to produce electricity and are regarded as a key technology for a sustainable energy supply. Due to the increased demand for renewable energy sources, the manufacturing of solar cells has advanced considerably in recent years. However, it is well known that the low photoelec-tric conversion effi ciency has limited the extensive use of solar cells

*Corresponding author: [email protected]

224 Solar Cell Nanotechnology

up to now. There are many types of approaches to improve the con-version effi ciency, in which the photon management strategy based on the manipulation of the energy distribution of solar irradiation shows various advantages such as ease of implementation in solar cells without structural change, environmental friendliness, and low cost, etc. Benefi ting from the abundant energy levels of rare earth (RE) ions, it is convenient to fabricate RE-doped nanomateri-als for enhancement of conversion effi ciency of solar cells. In this chapter, we would like to introduce the basic aspects of the photon management approach for solar cells and discuss the suitability of the up-conversion (UC) and down-conversion (DC) nanomaterials for this goal.

9.2 Basic Aspects of Solar Cell

9.2.1 Mechanism of Effi ciency Limitation

Shockley and Queisser applied the concept of detailed balancing to estimate the effi ciency limits of solar cells for the fi rst time [1]. The principle of detailed balance dictates that at thermal equilibrium, every photon absorption event must be countered by a photon emission event, with the balance holding at every frequency and solid angle. As the solar cells always operate far from thermal equi-librium, the principle of detailed balance cannot be directly applied to it. However, Shockley and Queisser recognized that the emis-sion spectrum away from thermal equilibrium is different from the emission spectrum at equilibrium by only a scaling factor, and this recognition aided the understanding of fundamental effi ciency lim-its of solar cells. Based on the calculations, the Shockley-Queisser maximum theoretical effi ciency was found to be 30% for an energy gap of 1.1ev and a radiative recombination fraction (fc) of 1. The upper limit of the conversion effi ciency of solar cell is caused by the intrinsic energy loss among the generation, trapping, recombi-nation, and transport of electron-hole pairs throughout the semi-c onducting material and within the contact electrodes after photon incidence. A typical p-n junction band diagram with related energy loss mechanisms is shown in Figure 9.1 [2]. The two major loss mechanisms that need to be overcome to improve the photoelectric effi ciency are lattice thermalization (process �) and transparency to sub-band gap photons (proce ss �). Further loss mechanisms

Photon Management 225

contain recombination of photoexcited e-h pairs (process �), junc-tion loss (process �) and contact voltage loss (process �).

9.2.2 EQEs of Solar Cells

All of the above mentioned energy loss processes constrain the response of each solar cell in a featured wavelength region in accor-dance with the bandgap of the absorbing material. The wavelength-dependent response of a device can be described by its external quantum effi ciency (EQE), defi ned as the ratio of the number of electron-hole pairs generated to the number of photons incident on the front surface of the cell.

The normalized EQE spectra of typical single-junction cells are shown in Figure 9.2 and Figure 9.3 [3]. In Figure 9.2, the narrow bandgap solar cells with the absorbing materials like CuInGaSe2 (CIGS), mult icrystalline silicon (mc-Si), monocrystalline silicon (c-Si), etc., show their featured response to the wavelength range from 300 nm to 1200 nm. “Miasole CIGS” presents the CIGS mod-ule with 13.8% effi ciency per m2 fabricated by Miasole and mea-sured by National Renewable Energy Laboratory (NREL) [4]. “Kyocera mc-Si” presents a multi crystalline silicon module with an effi ciency record of 17.3% for 1.3 m2 aperture area fabricated by Kyocera and measured by the Japanese National Institute of Advanced Industrial Science and Technology (AIST). “Mitsubishi mc-Si” also relates to multicrystalline silicon cell, where an effi -ciency increase to 19.3% has been confi rmed by AIST for 218 cm2 area fabricated by Mitsubishi Electric [5]. Another result of “ZSW

Figure 9.1 Loss processes in a single-junction solar cell: (1) lattice thermalization loss, (2) transparency, (3) recombination loss, (4) junction loss, and (5) contact voltage loss [2].

Energy

Electron(e*)

Photonabsorption

Hole (h+)

p-typematerial

n-typematerial

ConductionBand (CB)

ValenceBand (VB)

1

1

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3

5

2

4

5


CIGS” is the improvement of a small area (0.50 cm2) CIGS cell fab-ricated by Zentrum für Sonnenenergie-und Wasserstoff-Forschung (ZSW), Stuttgart to 20.1% effi ciency as measu red by Fraunhofer Institute for Solar Energy Systems (FhG-ISE). “UNSW c-Si” pres-ents the monocrystalline cell with an effi ciency approaching 25% as fabricated and measured by the University of New South Wales (UNSW) [6]. The wide bandgap solar cells, including GaAs, amor-phous Si, dye-sensitized and organic solar cells, also can work well

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Wavelength, nm

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E, %

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Miasole CIGSKyocera mc-Simitsubishi mc-SiZSW CIGSUNSE c-Si

1200

Figure 9.2 Normalized EQE spectra of fi ve narrow bandgap solar cells, which were fabricated by different organizations [3].

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Wavelength, nm

No

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E, %

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ISE GaAs

OERLIKEN a-Si

Sony dye sensitised

Solamer organic

Figure 9.3 Normalized EQE spectra of several wide bandgap solar cells, which were fabricated by different organizations [3].


in the whole visible region, but their absorption is generally limited to a maximum wavelength of ~900 nm (Figure 9.3).

9.2.3 Photon Management Approaches to Enhance the Effi ciency of Solar Cell

The absorption materials of solar cells limit their usable EQE in a certain wavelength region. In the face of such a problem, pho-ton management approaches containing up-conversion, down- conversion and down-shifting have been proposed for use in wide and effi cient solar energy harvesting [2, 7, 8]. Figure 9.4 plots the AM1.5G spectrum, the fraction of this light that can be absorbed by a bulk Si device, and the fractions available for up-conversion and down-conversion. Silicon cells with a bandgap of 1.12 eV own a response which approaches the ideal in the intermediate wave-lengths from 400 nm to 1100 nm, as shown in Figure 9.5. At short wavelengths below 400 nm, the covered glass absorbs most of the light and the cell response is very low. Hence, DC and DS, which offer the wavelength shifting ability from high frequency to low, could be used to overcome poor solar cell performance to UV/blue light. However, in general, benefi ting from the DC principle that two-photon emission occurs for a single-photon absorption and the effi ciency approaches 200%, the maximum cut-off wavelength of DC for silicon solar cells is always set to 550 nm according to double energy of Si bandgap, as labeled with shadow in visible

0.20.0

0.5

1.0

1.5

0.6 1.0 1.4 1.8 2.2

Maximum fractionavailable for DC:

149 W/m2

Maximum fractionavailable for UC:

164 W/m2

Maximumfraction

effectivelyutilised by Si:

468 W/m2

Wavelength, λ (mm)

Sp

ectr

al ir

rad

ian

ce (

kW/m

2 /mm

)

Figure 9.4 AM1.5G spectrum showing the fraction that is currently absorbed by a thick silicon device and the additional regions of the spectrum that can contribute towards up-and-down conversion [2].


region in Figure 9.4. At long wavelengths, the responses of Si cell fall back to zero, which means the optical transparent property of silicon in the wavelength region longer than 1200 nm. To overcome this limitation, UC is available through absorbing the light above 1100 nm and converting it into emission around 1000 nm. It should be noted that compared to conventional c-Si type of narrow band-gap solar cells, wide bandgap solar cells would benefi t much more from incorporation of an up-converting layer due to the dominant transmission losses. Specifi cally, up to now, most of the up-con-verting layers were based on Yb3+ absorbing (around 1000 nm) and were followed with emission in wavelength region shorter than 900 nm, which is more appropriate for wide bandgap cells according to their EQE spectra (Figure 9.3).

9.3 Up-Conversion Nanomaterials for Solar Cell Application

9.3.1 Principles of Photon Up-Conversion

Up-conversion is known as an anti-Stokes process coupled with simultaneous two-photon absorption (TPA) and second-harmonic generation (SHG) [10]. In general, UC could be defi ned as a process

00

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0.8

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Wavelength (mm)

Sp

ectr

al r

esp

on

se (

A/W

)

0.6 0.8

Ideal cell

Measured cell

1 1.2hcEg

λ =

Figure 9.5 The ideal and measured spectral response of a silicon solar cell under glass [9].


that refers to a system absorbing two or more photons, combining their energies, and emitting one higher energy photon. Typical clas-sifi cation of UC includes excited state absorption (ESA) and energy transfer up-conversion (ETU) through the use of physically existing intermediary energy states of RE ions [10, 11]. Recently, a new UC process named energy migration up-conversion (EMU) was pro-posed by Wang et al. which involves the use of four types of RE ions and a core-shell design [12]. All of the above mentioned anti-Stokes processes have been simplifi ed and described in Figure 9.6 [12].

9.3.2 Spectroscopy Analysis and Application Demonstration

Trivalent erbium (Er3+) is an ideal candidate for single wavelength NIR up-conversion due to its ladder of nearly equally spaced energy levels that are multiples of the 4I15/2 →

4I13/2 (~1540 nm) transition, as shown in Figure 9.7(a) [13, 14]. G d2(MoO4)3: Er3+ nanophosphors, Figure 9.7(b) exhibits its SEM image, and have been proposed as potential luminescent materials to enhance silicon solar-cell NIR response by Zhang et al. [15]. Upon excitation with low-energy near-infrared photons provided by a tunable laser ranging 1510–1565 nm, intense up-converted emissions at 545, 665, 800, and 980 nm have been achieved with conversion effi ciencies of 0.12%, 0.05%, 0.83%, and 1.35%, respectively. In Figure 9.7(c), colloidal LiYF4: Er3+ nanocrys-tals (NCs) with bright green emission upon 1490 nm excitation have been reported by Prasad et al. [16]. The total up-conversion quantum yield (UCQY) has been measured to be as high as 1.2 ± 0.1%, which is almost 4 times higher than the UCQY for the most effi cient UC NCs

(a) (b) (c) (d) (e)

TPA SHG ESA ETU EMU

nx

III II III III IVI

Figure 9.6 Simplifi ed energy level diagrams depicting the anti-Stokes processes.


reported to date. Transparent glass containing YF3 NCs, which could be clearly observed by transmission electron microscopy (TEM) and high resolution TEM as shown as Figure 9.7(c,d), also has been inves-tigated as Er3+ host to generate up-converted emissions [17]. The typical emission spectrum of YF3:Er3+ NCs under 1530 nm laser exci-tation and the absorption spectrum of NCs in the wavelength range of 1400–1650 nm are shown in Figure 9.7(f) [17].

The large spectral overlap between Yb3+/Er3+ and the effi cient ETU for this lanthanide couple, makes Yb3+/Er3+ a widely inves-tigated up-conversion couple in a variety of host materials. The Yb3+ ion absorbs around 980 nm and transfers the energy from the 2F5/2 level to the 4I11/2 level of Er3+. Subsequent energy transfer from a second excited Yb3+ ion to Er3+ (4I11/2), excites Er3+ ion to the 4F7/2 excited state. After multiphonon relaxation to the lower lying 4S3/2 and 4F9/2 states, green and red emissions are observed. In 2010, Shan and Demopoulos reported for the fi rst time the application of an Yb3+/Er3+ co-doped LaF3-TiO2 layer to fabricate a “triple layer” working electrode on fl uorine-doped tinoxide (FTO) glass for a

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Gd2 (MoO4)3:Er3+ LiYF4:Er3+

YF3:Er3+200 nm

200 nm

Figure 9.7 (a) Three-step UC process between two Er3+ ions. (b) SEM image of Gd2(MoO4)3:Er3+ nanophosphors. (c) TEM image of LiYF4:10% Er3+ NCs; inset shows the digital photograph of UC in 1 wt% colloidal LiYF4:10% Er3+ NCs under unfocused laser excitation at 1490 nm of 4 W/cm2. (d) TEM and (e) HRTEM images of Er3+: YF3 NCs embedded transparent glass; inset of (d) shows the digital photograph of the bulk sample. (f) UC emission spectrum of Er3+: YF3 NCs under 1530 nm laser excitation; inset shows the absorption spectrum of NCs in the wavelength range of 1400–1650 nm.


dye-sensitized solar cell (DSSC), as shown in Figure 9.8. In their investigation, 0.40 V of Voc and 0.036 mA of Isc were obtained under illumination of a 980 nm fi ber laser (fi ber core diameter: 105 μ m) with 2.5 W power supply [18]. In 2013, Liang et al. for the fi rst time employed the highly uniform bifunctional core/double-shell-struc-tured b-NaYF4: Er3+,Yb3+@SiO2@TiO2 hexagonal sub-microprisms for high- performance dye-sensitized solar cells [19]. Due to the scatter-ing effect of the sub-micrometer dimension and the surface protect-ing effect of the amorphous SiO2 insulating layer, at a certain mixing mass ratio of 10%, an effi ciency of 8.65% was obtained, which is 120% higher than that of a device based on bare NaYF4:Er3+, Yb3+ crystals, and an enhancement of about 10.9% was achieved when compared to the reference cell. Furthermore, to enhance the NIR light harvest-ing, the fi rst demonstration of photoelectrochemical electrodes with up-conversion nanoparticles embe dded in porous photonic crystals, which render the nanoparticles spatially positioned in close proxim-ity to the CdSe quantum dots, was reported by Su et al. [20]. In their report, the hetero-nanostructured photoanode exhibits a photocur-rent of 0.02 mA upon near-infrared laser excitation.

Recently, for more effi cient improvement on solar cell effi ciency by the up-conversion approach, Zou et al. reported that using an organic near-infrared dye as an antenna for the b-NaYF4:Yb, Er nanoparticles could dramatically enhance (by a factor of ∼3,300) the up-conversion as a result of increased absorptivity and overall

Near infrared lightUp-converter (UV)

Visible light

Dye absorption

PhotocurrentSemiconductor film

Electron production

Scattering TiO2 layer

UC-TiO2 layer + Dyer

Transparent TiO2 layer + Dyes

FTO layer

Glass substrate

(I)

(II)

Figure 9.8 The structure (II) and mechanism of UC-TiO2-based work electrode [18].


broadening of the absorption spectrum of the up-converter [21]. Consequently, a tendency named “up-conversion goes broadband,” which promotes the nanofabrication of rare earth ions doped struc-ture, was proposed by Liu et al. [22]. Er3+:NaGdF4@Ho3+:NaGdF4@NaGdF4 active-core/active-shell/inert-shell NCs, which can extend the NIR wavelength excitable range for UC emissions, were success-fully fabricated for the fi rst time by Chen et al. [23]. However, for the above mentioned methods for broadening of the absorption spec-trum, intrinsic shortages exist. For example, the poor photostabil-ity of dye and the unavoidable detrimental energy transfer between co-doped rare earth ions. Thus, a new approach named multi-color excitation based on the GSA, ESA, and phonon-coupled absorption of RE ions was proposed by our group, which not only broadened the absorption spectrum of up-converter but also greatly enhanced the up-conversion due to the most effi cient ETU mechanism.

9.4 Down-Conversion Nanomaterials for Solar Cell Application

9.4.1 Principles of Photon Down-Conversion

Down-conversion, which is also named as quantum cutting, is well known through the photon conversion in Eu3+-doped LiGdF4, where two visible photons can be emitted by Eu3+ through an effi cient two-step energy transfer from Gd3+ to Eu3+ with a quantum effi ciency that approaches 200% upon excitation of Gd3+ with a high-energy photon [24]. In principle, DC is a process related to the phosphor that could generate two or more low-energy photons for each incident high-energy photon absorbed. For solar cell, the absorption of the two or more low-energy photons emitted by the DC converter, which is usually located on the front surface of a conventional single-junction solar cell, leads to the generation of more than one electron-hole pair in the solar cell per incident high-energy photon [8].

The concept of DC is illustrated (Figure 9.9) with two types of ions, I and II, with hypothetical energy level schemes [24]. Type I is an ion for which emission from a high energy level can occur. Type II is an ion to which energy transfer takes place. For NIR quantum cut-ting applied for solar cells, the type II ion usually is focused on Yb3+

ion, and the type I ion as a donor should be with a level that matches with Yb3+’ level, i.e., an energy level with twice the energy difference


to 2F5/2 → 2F7/2 transition of Yb3+. There are two types of donor includ-

ing those with and without an i ntermediate level at approximately 10000 cm-1, which is in accordance with the fi rst-order and second-order energy transfer, respectively. Evaluation of the Dieke diagram reveals that potential donors are Pr3+, Nd3+, Ho3+, Er3+, Tb3+, and Tm3+ ions, in which the fi rst four have the intermediate levels. Also, Eu2+, Ce3+, Yb2+ ions are new kinds of donors with f-d transition nature, which exhibit broad absorption bands in the UV/VIS region and ligand controlled bandgap [25–29]. The energy transfer between Eu2+, Ce3+, and Yb2+ ions and Yb3+ usually takes place through the cooperative down-conversion mechanism due to the lack of inter-mediate level form NIR to UV of these donors.

9.4.2 Experimental and Spectroscopy Analysis

NIR down-conversion with prospects for increasing the energy effi ciency of crystalline Si solar cell was fi rst reported by Vergeer

(a) (b) (c) (d)

vis

vis vis vis visvis

vis

vis1

11 1

1

1

2

I I I III II II II

Figure 9.9 Energy level diagrams for two (hypothetical) types of lanthanide ions

(I and II) showing the concept of down-conversion. Type I is an ion for which

emission from a high energy level can occur. Type II is an ion to which energy

transfer take s place. (a) Quantum cutting on a single ion I by the sequential

emission of two visible photons. (b) The possibility of quantum cutting by a

two-step energy transfer. In the fi rst step (indicated by �), a part of the excitation

energy is transferred from ion I to ion II by cross-relaxation. Ion II returns to the

ground state by emitting one photon of visible light. Ion I is still in an excited

state and can transfer the remaining energy to a second ion of type II (indicated

by �), which also emits a photon in the visible spectral region, giving a quantum

effi ciency of 200%. (c) and (d)The remaining two possibilities involve only

one energy transfer step from ion I to ion II. This is suffi cient to obtain visible

quantum cutting if one of the two visible photons can be emitted by ion I [24].


et al. in YbxY1-xPO4:Tb3+, where luminescence decay curves of the 5D4 emission from Tb3+ show an excellent agreement with simulations based on cooperative energy transfer via dipole-dipole interaction [30]. In 2008, Ye et al. demonstrated the NIR quantum cutting with the maximum quantum effi ciency approaching 155% in Tb3+, Yb3+ co-doped transparent glass ceramics containing CaF2 nanocrys-tals (inset of Figure 9.10(a)) [31]. As shown in Figure 9.10(a), the

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420 440 460 480 570 600 630 660 690 720 950 10001050Wavelength (nm) Wavelength (nm)

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Emission EmissionExcitation

1 G4→

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(a)

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PLE PL (λex = 482 nm)Pr3+: 3p0→

3H6

Pr3+: 3p0→3F3,4

Yb3+: 2F5/22→F7/22

X = 0

X = 0.1

X = 0.2

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X = 1.0

X = 1.5

Fi gure 9.10 (a) Emission spectra of glass ceramics with different Yb3+

concentrations under excitation at 484 nm (solid line in red: 0 mol % Yb3+,

blue: 4 mol % Yb3+, green: 6 mol % Yb3+). Inset s hows the TEM image of glass

ceramic containing CaF2 nanocrystals. (b) Left: Excitation spectra of Tm3+ 651 nm

emission monitored in 0.5Tm3+ single doped glass ceramic (blue line) and of Yb3+

1016 nm emission monitored in 0.5Tm3+-8Yb3+ co-doped glass ceramic (green

line). Right: Emission spectra of 0.5Tm3+ single doped (black linne) and 0.5Tm3+-

8Yb3+ co-doped (red line) glass ceramics under the excitation of 468 nm. Inset

shows the TEM image of glass ceramic containing LaF3 nanocrystals. (c) TEM

image and the corresponding selected area electron diffraction pattern of the

glass ceramic containing β-YF3 nanocrystals. (d) Photoluminescence excitation

(PLE) spectra of the Pr3+: 3P0 →

3H6 emission (605 nm, dotted) and the Yb3+: 2F

5/2 →

2F7/2

emission (976 nm, solid) in the glass ceramic; Visible-NIR PL spectra of the

glass ceramics with different Yb3+ concentrations (x%) on excitation at 482 nm.


emission bands of Tb3+: 5D4 → 7FJ transitions decreased with increas-

ing Yb3+ concentrations from 0, 4, to 6 mol % under 484 nm exci-tation, while a sharp peak at 980 nm and a broader peak at 1016 nm of Yb3+: 2F5/2 →

2F7/2 transition increased. A similar cooperative DC process with a maximum quantum effi ciency value of 162% was also observed in Tm3+-Yb3+ codoped glass ceramics contain-ing LaF3 nanocrystals (inset of Figure 9.10(b)) [32]. Figure 9.10(b) depicts the typical excitation and emission spectra of Tm3+-Yb3+ ion pair with energy transfer. More effi cient DC with optimal quantum effi ciency close to 200% involving the emission of two near-infra-red photons for each blue photon absorbed was realized by Chen et al. in transparent glass ceramics with embedded Pr3+/Yb3+: b- YF3 nanocrystals (inset of Figure 9.10) [33]. In the PLE spectra shown in Figure 9.10(d), intense excitation bands centered at 482, 467, and 441 nm corresponding to Pr3+: 3H4 →

1I6, 3Pj(j=0,1,2) transitions were

measured by monitoring both the Pr3+: 3P0 → 3H6 transition at 605

nm and the Yb3+: 2F5/2 → 2F7/2 transition at 976 nm, verifying the exis-

tence of energy transfer from Pr3+ to Yb3+. PL spectra show the Yb3+ concentration dependent emission bands of Pr3+ in visible region and Yb3+ in NIR region.

9.4.3 Evaluation

EQE evaluation of down-converter covered solar cell wafers was fi rst demonstrated by our group in Pr3+-Yb3+ ion pair [34]; 0.4Pr3+/1Yb3+ (mol %) doped oxyfl uoride glass (60SiO2–20Al2O3–20CaF2) and host glass with size of 2 cm×2 cm×2 mm are covered on Si solar cell wafers. The EQE values of the cell covered by the doping glass are lower than that of cell covered by the host glass in the visible and NIR regions, as shown in Figure 9.11(a). From the difference of the two curves shown in Figure 9.11(b) it was easy to see that absorption of the light due to the doping ions is dominant rather than the expected improvement of EQE in the absorption wavelength region, due to DC of rare earth ions. Quantitative anal-ysis of EQE values, absorbance and solar irradiation fl ux in various wavelength regions (Figure 9.11(c)) implied that the DC process may contribute to the increase in the EQE, though the increasing effect is not high enough to compete with the absorption effect. Unfortunately, there are no more positive results that report down-conversion of rare earth ion pair greatly improve the solar cell EQE at this time.


9.5 Conclusion and Outlook

In this chapter, fundamentals of the photon management approaches relying on up-conversion and down-conversion mechanisms for solar cells have been presented. Rare earth ions doped nanomate-rials, including nanocrystals and NCs embedded glass ceramics, have been used as spectral converters owing to their advantages like easy structuralization with different matrixes and weak parti-cle scattering. Spectros copy analysis of up-conversion implied that it is effi cient to convert the NIR energy to visible energy with the help of Yb3+ sensitization. As a result, in their studies researchers favor using Yb3+-Er3+ doped up-converter to enhance the effi ciency of wide bandgap solar cells, especially for dye-sensitized solar

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Figu re 9.11 (a) EQE values of silicon solar cell covered with the host glass and P1(0.4Pr3+/1Yb3+ doped glass). (b) The difference of the EQE values of silicon solar cell covered with the host glass and P1, and the absorbance difference between the host glass and P1. (c) The integral areas of the EQE difference and the absorbance difference in three different regions: 410–510 nm, 550–650 nm and 850–1100 nm. The insets give the quantitative integral areas normalized to the middle region and the solar spectrum (AM1.5) [34].


cells. One of the major challenges in using up-converter for solar cell is how to change the excitation source from intense laser light to natural solar light. Down-conversion, which is regarded as an effi cient approach for using the above high-energy bandgap, has been widely investigated in spectroscopy from narrow-band sensi-tization with f-f transition RE ions to broadband sensitization with f-d transition RE ions. However, little effort has been given to the practical demonstration of a down-converter embedded in a solar cell. In an attempt to achieve the practical application purpose of photon management in rare earth ions doped nanomaterials, mate-rial form and effi ciency of the spectral conversion process is impor-tant. Thus, we propose that the following issues be considered in future work.

9.5.1 Solution-Processable Nano-Coating for Broadband Up-Converter or Down-Converter

The thickness of the converter, which affects the energy loss in the optical transmission process, plays an important role in the effi ciency of the spectral conversion. Solution-processable nano- coating is considered to be a possible way to control the thickness of the converter. For broadband UC or DC, dyes or quantum dots could be introduced as the sensitizer for rare earth ions. In 2010, Yan et al. successfully synthesized the CdSe/NaY F4:Yb, Er nanohet-erostructures using a seeded-growth method [35]. Benefi ting from the strong coupling between CdSe and NaYF4 in the nanohetero-st ructures, as shown in the TEM images in Figure 9.12, suffi cient

50 nm

(a) (b)

3 nm

0.32 nm

0.35 nm

CdSe(001)

NaYF4:Yb,Er(111)

Figu re 9.12 (a) TEM and (b) HRTEM images of CdSe/NaYF4:Yb, Er nanoheterostructures [35].


energy transfer from rare earth ions to quantum dots was observed. This implies that such nanoheterostructures may also prove useful in the realization of broadband down-converter owing to the strong absorbance of CdSe in UV/Visible region and the energy transfer possibility between quantum dots and REs. Besides, these nano-sized structures obtained in wet chemical methods are easy to be surface modifi ed and fi nally dispersed in various solutions.

9.5.2 Effi cient Photon Management Using Nanoplasmonic Effect

It is well known that metal nanostructures (i.e. plasmonics sub-strates) can effi ciently collect light and enhance the light intensity in their vicitniy due to surface plasmon resonances, and these effects have been widely used for enhancing various optical processes, such as the Raman scattering, down-conversion luminescence, and up-conversion luminescence [36–45]. For instance, in 2012, Zhang et al. reported a 310-fold UC enhancement uniformly over a large area, and an 8-fold reduction in the luminescence decay time were observed in NaYF4:Yb3+/Er3+ co-doped nanocrystals using a 3D plasmonic nanoantenna architecture: disk-coupled dots-on-pillar antenna array [45]. Similarly, Li et al. have also investigated the UC nanomaterials with Au nanostructures applied to fl exible amor-phous silicon solar cells, where 16-fold to 72-fold improvement of the short-circuit current under 980 nm illumination compared to a cell without up-converters were obtained [46]. Thus, it is possible to improve the luminescence effi ciency of the spectral converters using nanoplasmonic effect, which can ultimately lead to an effec-tive photon management process.

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