High Improvement in Conversion Efficiency of mu c-SiGe Thin-Film Solar Cells with Field-Enhancement Layers

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Volume 2012, Article ID 817825,5pages doi:10.1155/2012/817825

Research Article

High Improvement in Conversion Efficiency of

µc-SiGe Thin-Film

Solar Cells with Field-Enhancement Layers

Shu-Hung Yu,

1

_{Wei Lin,}

1

_{Yu-Hung Chen,}

2

_{and Chun-Yen Chang}

1

1_{Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 30010, Taiwan}

2_{Photovoltaic Technology Division, Green Energy and Environment Research Laboratories, Industrial Technology Research Institute,}

Hsinchu 31040, Taiwan

Correspondence should be addressed to Chun-Yen Chang,[email protected]

Received 20 January 2012; Accepted 19 February 2012 Academic Editor: David Lee Phillips

Copyright © 2012 Shu-Hung Yu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The improved performance for hydrogenated microcrystalline silicon-germanium (μc-Si1−xGex:H,x∼0.1) p-i-n single solar cells

with hydrogenated microcrystalline silicon (μc-Si:H) field-enhancement layers (FELs) is demonstrated for the first time. The fill factor (FF) and conversion eﬃciency (η) increase by about 19% and 28% when the thickness of the μc-Si FEL is increased from 0 to 200 nm, it is attributed to the longer hole life-time and enhanced electric field in theμc-Si0.9Ge0.1:H layer. Therefore,

we can successfully manufacture high-performanceμc-SiGe:H solar cells with the thickness of absorbers smaller than 1 μm by conducting FELs. Moreover, the simulation tool is used to simulate the current-voltage (J-V) curve, thus we can investigate the carrier transport in the absorber ofμc-Si0.9Ge0.1:H solar cells with diﬀerent EFLs.

1. Introduction

In order to enhance the infrared absorption of thin-film solar cells, many efforts have been developed such as textured substrates or alloy absorbers. Textured substrates efficiently scatter incoming light and increase the optical length in absorbers [1]. However, rough morphology of the substrates can induce many cracks in absorption layers and damage the photocarrier transport [2]. The alternative methods, that is, high absorption coefficient alloy absorbers such as hydrogenated microcrystalline silicon-germanium ( μc-Si1−xGex:H), have been developed for thin-film solar cells

[3]. It has been reported that light absorption coeﬃcient of

μc-Si1−xGex:H films can be enhanced about from 4×102to

4×103_cm−1_{(@ 900 nm) when the Ge content is increased}

from 0 to 60 at. % [4,5]. Moreover, Takuya Matsui et al. have demonstrated that a micromorph tandem cell with a μc-Si1−xGex:H (x = 0.1) bottom cell can reveal an

initial conversion eﬃciency of 11.2% [6]. However, the incorporation of Ge may induce strain defects, dangling bond defects [7,8], and acceptor-like states [4], hence it can result in reduced electric field in the absorbers.

Here, hydrogenated microcrystalline silicon (μc-Si:H) field-enhancement layers (FELs) are developed to improve the performance ofμc-Si1−xGex:H p-i-n single solar cells for

the first time, and we optimize the thickness of the FEL based on the conversion eﬃciency. Moreover, Atlas device simulator from SILVACO company is used to simulate the current-voltage curve of thin film solar cells. We analyze the band diagram and electric field in the device, the simulation results demonstrate that the FEL can enhance the hole life-time and electric field in theμc-Si0.9Ge0.1:H absorber.

2. Device Fabrication

In this study, superstrate-typeμc-Si1−xGex:H single cells are

fabricated on Asahi type-U glass substrates by a 40 MHz plasma enhanced chemical vapor deposition (PECVD) system. The single cell consists of Asahi type-U glass/ ZnO:Ga(GZO)/p-μc-Si:H/μc-Si1−xGex:H buﬀer

layer/i-μc-Si0.9Ge0.1:H/μc-Si:H FEL/n-μc-Si:H/GZO/Ag. The schematic

diagram is shown in Figure 1. The buﬀer layer is graded intrinsicμc-Si1−xGex:H and its thickness is about 75 nm, it is

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Table 1: Material parameters.

Parameters μc-Si0.9Ge0.1:H μc-Si:H

FEL=0 nm FEL=100 nm FEL=200 nm FEL=300 nm

Optical gap (eV) 1.24 1.3

Elec. mob. (cm2_/Vs) ₂₀ ₃₀

Hole mob. (cm2_/Vs) ₂ ₂

Con. Band DOS (cm−3₎ ₁_×₁₀20 ₁_×₁₀20

Val. Band DOS (cm−3) 1×1020 ₁_×₁₀20

Hole life-time (s) 5.0×10−10 _1.2_×₁₀−9 _1.7_×₁₀−9 _8.0_×₁₀−10 ₁_×₁₀−7

Tail states density factor:

Con. Band (cm−3/eV) 5.0×1019 _5.2_×₁₀19 _5.3_×₁₀19 _5.6_×₁₀19 _1.0_×₁₀19

Val. Band (cm−3_/eV) _5.0_×₁₀19 _5.2_×₁₀19 _5.2_×₁₀19 _5.3_×₁₀19 _1.0_×₁₀19

Tail states charact. energy:

Con. Band (eV) 0.027 0.027

Val. Band (eV) 0.045 0.045

Accepter-like Gaussian defect states:

Below con. band (eV) 0.4 0.4

Decay energy (eV) 0.2 0.2

DOS (cm−3_/eV) _7.0_×₁₀16 _7.0_×₁₀16 _7.0_×₁₀16 _7.1_×₁₀16 _1.0_×₁₀15

Donor-like Gaussian defect states:

Above val. band (eV) 0.45 0.45

Decay energy (eV) 0.2 0.2

DOS (cm−3/eV) 7.0×1016 _1.0_×₁₀15 Ag GZO Asahi type U GZO Main layer (1 μm) FEL (_μc-Si:H) (0−→300 nm) p-layer (μ-Si:H) (925−→625 nm)

Buﬀer layer (μc -Si1₋xGex:H)

(75 nm)

i-layer (_{μc -Si0.9}Ge_0.1:H)

n-layer (μc-Si:H)

Figure 1: Schematic diagram of theμc-Si0.9Ge0.1:H p-i-n solar cell

with the gradedμc-Si1−xGex:H buﬀer layer and μc-Si:H FEL.

rate ([GeH4]) from 0 to 4 sccm under a constant SiH4 gas

flow rate ([SiH4]) of 17 sccm. The i-μc-Si0.9Ge0.1:H absorber

is deposited at a fixed [GeH4] of 5 sccm and an [SiH4] of

17 sccm. The μc-Si:H FEL is grown at a constant [SiH4]

of 20 sccm and its thickness is varied from 0 to 300 nm. The main layer of solar cells consists of the buﬀer layer, the absorber, and the FEL and the total thickness is kept 1μm.

The Raman spectra are performed by Confocal Raman Microscope (HOROBA, LabRAM HR) at room temperature in the backscattering configuration. The source light is Helium-Neon (HeNe) laser emitting at a wavelength of 632.8 nm. The Ge content of μc-Si1−xGex:H films can be

identified from the Raman spectra. Edge isolation is con-ducted by the laser scriber to define the device area of 1.0 cm2_{. The current-voltage (J-V) characteristics of solar}

cells are measured under an Air Mass 1.5 Global (AM 1.5 G) spectrum with an irradiation of 100 mW/cm2 _{by a solar}

simulator.

We use Atlas to calculate internal electrical characteristics of solar cells in the one-dimension (perpendicular to the substrate). The simulation is based on J-V measurement results. Here, Newton method is used to solve Poisson’s and continuity equations for the steady state. The carrier occupation and recombination in the forbidden gap states are considered by the Shockley-Read-Hall (SRH) recombi-nation model.Table 1shows the basic parameters in the μc-Si_0.9Ge_0.1:H andμc-Si:H layers.

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300 400 500 600 700 Si-Ge Raman int ensit y (a.u.) Si-Si Raman shift (cm−1) [GeH4]/[SiH4]=10/17 [GeH4]/[SiH4]=8/17 [GeH4]/[SiH4]=6/17 [GeH4]/[SiH4]=3/17 [GeH4]/[SiH4]=0/17 (a) 0 2 4 6 8 10 0 6 12 18 24 Ge c o nt ent (at. %) Hydrogen-diluted (10%) [GeH4] (sccm) (b)

Figure 2: (a) Raman spectra ofμc-Si1−xGex:H films deposited at diﬀerent [GeH4]/[SiH4] ratios. (b) The Ge content ofμc-Si1−xGex:H films

deposited at diﬀerent [GeH4] estimated from Raman spectra.

3. Results and Discussion

Raman spectra of μc-Si1−xGex:H deposited at diﬀerent gas

flow rates ([GeH4]: 0–10 sccm, [SiH4]: 17 sccm) are

pre-sented inFigure 2(a). When the Ge content ofμc-Si1−xGex:H

films is increased, the main peak (ω_Si−Si) corresponding to

the Si-Si transverse optical (TO) mode in the crystalline phase will gradually be lower than 520 cm−1 _{and the peak}

near 400 cm−1_{attributed to Si-Ge bond will be apparent. The}

relation between the Ge content andω_Si−Siis depicted as [6]:

ωSi−Si= 520–70 x.Figure 2(b)shows the nearly linear relation

between the Ge content ofμc-Si1−xGex:H films and the GeH4

gas flow rate, hence we can estimate the Ge content of the absorber at about 10 at. %.

J-V measurement and simulation curves ofμc-Si_0.9Ge_0.1: H solar cells with diﬀerent μc-Si:H FELs are shown in

Figure 3. The inserted table shows the J-V parameters of

μc-Si0.9Ge0.1:H solar cells. When the thickness of the FEL is

widened from 0 to 200 nm, the open-circuit voltage (Voc)

is increased from 0.433 to 0.453 V and the short-circuit current density (Jsc) is enhanced from 20.8 to 21.5 mA/cm2.

In addition, the fill factor (FF) is enhanced from 47.3 to 56.4% and conversion eﬃciency (η) is enhanced from 4.3 to 5.5%. Quantum eﬃciency (QE) spectra of μc-Si0.9Ge0.1:H solar cells (FEL= 0, 200 nm) at a reverse bias of 0 and−0.5 V are shown in Figure 4. Under a reverse bias of 0 and

−0.5 V, QE spectra of theμc-Si0.9Ge0.1:H solar cell without FEL are divided apparently. However, the μc-Si0.9Ge0.1:H solar cell with the 200 nm FEL exhibits more matching QE spectra in the wavelength range of 500–800 nm. This result implies that theμc-Si0.9Ge0.1:H solar cell equipping with the

FEL has the enhanced electric field in the μc-Si0.9Ge0.1:H layer. However, the FF and η decrease to 53.4% and 5.1% when the FEL rises to 300 nm, but Voc and Jsc do not

suﬀer severely. Consequently, we found that 200 nm is the optimized thickness of the FEL for solar cell.

0 0.1 0.2 0.3 0.4 0.5 0 4 8 12 16 20 24 Voltage (V) FEL=0 nm FEL=100 nm FEL=200 nm FEL=300 nm C u rr ent densit y (mA/cm 2) 0 0.433 20.8 47.3 4.3 100 0.448 21.4 53.8 5.2 200 0.453 21.5 56.4 5.5 300 0.444 21.3 53.4 5.1 Thickness (nm) Voc(V) FF (%) η (%) Jsc (mA/cm2₎

Figure 3: J-V measurements ofμc-Si0.9Ge0.1:H solar cells with

μc-Si:H FEL varied from 0 to 300 nm.

Figure 5(a) shows the band diagram in the main layer of the solar cell in thermodynamic equilibrium. Because the optical band gap of Si:H is higher than one of μc-Si0.9Ge0.1:H, there must be a valance-band discontinuity in the p-i interface and the hole extraction might be block. Gradedμc-Si1−xGex:H buﬀer layers are conducted to reduce

this energy discontinuity and enhance the hole transport. Around the interface between the μc-Si0.9Ge0.1:H and the

μc-Si:H FEL, the energy barrier seen by hole carriers can

repel it and reduce recombination probability near the back side of the main layer. The simulation results based on the Shockley-Read-Hall (SRH) recombination model show that hole life-time can be enhanced from 5×10−10_{to 1.7}_×₁₀−9_s

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0 20 40 60 80 100 Quantum efficiency (%) 400 600 800 1000 Wavelength (nm) FEL=0 nm Vb=0 V Vb=0.5 V (a) 400 600 800 1000 0 20 40 60 80 100 Quantum efficiency (%) Wavelength (nm) FEL=200 nm Vb=0 V Vb=0.5 V (b)

Figure 4: (a) QE spectra of theμc-Si0.9Ge0.1:H solar cell without FEL and (b) with FEL of 200 nm at a reverse bias of 0 and−0.5 V.

L ev el position (eV ) 0 0.2 0.4 0.6 0.8 1 1.2 Position (μm) −1.6 −1.2 −0.8 −0.4 0 0.4 0.8 1.2 1.6 EC EV FEL=0 nm FEL=100 nm FEL=200 nm FEL=300 nm Buﬀer Layer= 75 nm FEL= 0−→300 nm μc-Si0.9Ge_0.1=925−→625 nm (a) 0 0.2 0.4 0.6 0.8 1 1.2 Elect ric field ( V /cm) Position (μm) 106 105 104 Buﬀer Layer= 75 nm FEL= 0−→300 nm μc-Si0.9Ge0.1=925−→625 nm FEL=0 nm FEL=100 nm FEL=200 nm FEL=300 nm (b)

Figure 5: (a) The band diagram and (b) the electric field in the main layer of the solar cell with diﬀerent μc-Si:H FEL in thermodynamic equilibrium.

shows the electric field of the main layer in thermodynamic equilibrium. When the thickness of the FEL is increased from 0 to 200 nm, the electric field in the μc-Si0.9Ge0.1:H layer

can be enhanced significantly. This is because thatμc-Si:H has lower defect density thanμc-Si0.9Ge0.1:H and the field-screening eﬀect related to trapped charges can be reduced eﬃciently. The enhanced electric field in the μc-Si0.9Ge0.1:H layer is useful for photocarrier extraction. The electric field in the μc-Si0.9Ge0.1:H layer can be increased further when we widen the FEL from 200 to 300 nm. However, the electric field in the thick FEL is decreased significantly. Additionally, we observe that hole life-time is reduced to 8×10−10_{s when}

the FEL is 300 nm. This result is probably due to the poor crystallinity in a thinner μc-Si0.9Ge0.1:H layer. Hence, we speculate that diminished hole life-time and reduced electric field in the thick FEL result in the degraded performance of the solar cell.

4. Conclusion

High-performance μc-Si0.9Ge0.1:H single cells with the

absorber smaller than 1μm are achieved by equipping

with μc-Si:H FELs. We use μc-Si:H FELs and graded

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transport in theμc-Si0.9Ge0.1:H layers. The Ge composition

ofμc-Si1−xGex:H films can be evaluated quantitatively from

Raman spectra. When we widen the thickness of the FEL from 0 to 200 nm,Vocis enhanced from 0.433 to 0.453 V and

Jscis enhanced from 20.8 to 21.5 mA/cm2. Moreover, FF and

η approximately exhibit 19% and 28% enhancements. It is

attributed to the improved electric field and longer hole life-time in theμc-Si1−xGex:H layer. However, FF andη are both

decreased when the thickness of the FEL exceeds 200 nm. This is due to poor crystallineμc-Si0.9Ge0.1:H layer and the

reduced electric field in the thick FEL. There is a trade-oﬀ between the enhanced electric field in the absorber and the reduced one in the FEL.

Acknowledgment

This work was supported by the National Science Council of Taiwan under Grant NSC 100-3113-E-009-004, and Bureau of Energy, Ministry of Economic Aﬀairs, Taiwan, under Grant no. A455DR1110.

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High Improvement in Conversion Efficiency of mu c-SiGe Thin-Film Solar Cells with Field-Enhancement Layers

Research Article