Thin Film Solar Technology II

PROCEEDINGS OF SPIE

Thin Film Solar Technology II Alan E. Delahoy Louay A. Eldada Editors

1–4 August 2010 San Diego, California, United States Sponsored and Published by SPIE

Volume 7771 Proceedings of SPIE, 0277-786X, v. 7771 SPIE is an international society advancing an interdisciplinary approach to the science and application of light.

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Contents v vii

SESSION 1 7771 02

SESSION 2

Conference Committee From nanotechnology to efficient organic and hybrid solar cells (abstract only) J. T. Yardley, Columbia Univ. (United States) and Columbia Energy Frontier Research Ctr. (United States) A-SI AND NC-SI PHOTOVOLTAICS Light trapping effect from randomized textures of Ag/ZnO back reflector on hydrogenated amorphous and nanocrystalline silicon based solar cells [7771-01] B. Yan, G. Yue, L. Sivec, J. M. Owens, K. Lord, J. Yang, S. Guha, United Solar Ovonic, LLC (United States); C.-S. Jiang, National Renewable Energy Lab. (United States) THIN FILM C-SI AND POLY-SI SOLAR CELLS

7771 06

Thin-film monocrystalline-silicon solar cells based on a seed layer approach with 11% efficiency [7771-05] I. Gordon, Y. Qiu, D. Van Gestel, J. Poortmans, IMEC (Belgium)

7771 07

A new laser patterning technology for low cost poly-Si thin film solar cells [7771-06] S.-W. Lee, Y.-J. Lee, Y.-H. Lee, J.-K. Chung, D.-J. Kim, TG Solar Corp. (Korea, Republic of)

SESSION 3

ANALYSIS AND CHARACTERIZATION OF SOLAR THIN FILMS AND MODULES

7771 08

Stabilization of electrical parameters of thin-film modules under controlled conditions [7771-07] Ü. Aksünger, D. Philipp, M. Köhl, K.-A. Weiβ, Fraunhofer-Institut für Solare Energiesysteme (Germany)

7771 09

Scatter metrology of photovoltaic textured surfaces [7771-08] J. C. Stover, E. L. Hegstrom, ScatterMaster LLC (United States)

SESSION 4 7771 0E

GROWTH AND PATTERNING OF THIN FILMS FOR SOLAR CELLS Direct inkjet patterning for series connection of silicon thin film solar cells [7771-13] Y. J. Lee, S. W. Lee, Y. H. Lee, D. J. Lee, M. S. Hwang, D. J. Kim, TG Solar Corp. (Korea, Republic of); K. S. Lim, Korea Advanced Institute of Science and Technology (Korea, Republic of)

iii

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Thin-film monocrystalline-silicon solar cells based on a seed layer approach with 11% efficiency I. Gordon, Y. Qiu, D. Van Gestel, and J. Poortmans imec, Kapeldreef 75, B-3001 Leuven, Belgium

ABSTRACT Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. Almost half of the price of wafer-based silicon solar modules is currently due to the cost of the silicon wafers themselves. Using crystalline-silicon thin-film as the active material would substantially reduce the silicon consumption while still ensuring a high cell-efficiency potential and a stable cell performance. One way to create a crystalline-silicon thin film on glass is by using a seed layer approach in which a thin crystalline-silicon layer is first created on a non-silicon substrate, followed by epitaxial thickening of this layer. In this paper, we present new solar cell results obtained on 10-micron thick monocrystalline-silicon layers, made by epitaxial thickening of thin seed layers on transparent glass-ceramic substrates. We used thin (001)-oriented silicon single-crystal seed layers on glass-ceramic substrates provided by Corning Inc. that are made by a process based on anodic bonding and implant-induced separation. Epitaxial thickening of these seed layers was realized in an atmospheric-pressure chemical vapor deposition system. Simple solar cell structures in substrate configuration were made from the epitaxial mono-silicon layers. The Si surface was plasma-textured to reduce the front-side reflection. No other light trapping features were incorporated. Efficiencies of up to 11% were reached with Voc values above 600 mV indicating the good electronic quality of the material. We believe that by further optimizing the material quality and by integrating an efficient light trapping scheme, the efficiency potential of these single-crystal silicon thin films on glass-ceramics should be higher than 15%. Keywords: Solar cell, crystalline, silicon, thin-film, glass-ceramic, seed layer

1. INTRODUCTION Solar modules made from thin-film crystalline-silicon layers of high quality on glass substrates could lower the price of photovoltaic electricity substantially. Around one third of the price of wafer-based silicon solar modules is currently due to the cost of the silicon wafers themselves. Using crystalline-silicon thin-film as the active material provides numerous advantages: First, compared to the wafering process, the silicon consumption can be minimized. Secondly, the high crystalline quality of the silicon layer should ensure a high cell-efficiency potential. Thirdly, crystalline silicon has a demonstrated stable performance over time and irradiation. Finally, the knowledge about silicon wafer physics can be easily transferred and applied to crystalline silicon thin films. Various research centers and companies are currently exploring the potential for photovoltaics of polycrystalline-silicon thin films [1]. The difficulty of this approach is to obtain a sufficiently high material quality and to achieve an effective passivation of the numerous grain boundaries. However, the utilization of a monocrystalline-silicon thin film instead of a polycrystalline-silicon layer should enable simpler device processes, higher process yield and a single-crystal-like efficiency due to the intrinsic higher quality of the material. In this paper, we present solar cell results obtained on 10-micron thick monocrystalline-silicon layers on transparent glass-ceramic substrates. These layers were made by epitaxial thickening of thin monocrystalline-silicon seed layers on glass-ceramics.

2. EXPERIMENTAL Corning’s spinel glass-ceramic (code 9664), which has a strain point of 910°C and matches the coefficients of thermal expansion (CTE) of silicon, has been selected as a transparent substrate for this study. The high strain point of the glassceramic enables the utilization of high-temperature thermal chemical vapor deposition (CVD) to realize the silicon Thin Film Solar Technology II, edited by Alan E. Delahoy, Louay A. Eldada, Proc. of SPIE Vol. 7771, 777106 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.860385

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epitaxial deposition. The high temperature CVD process potentially provides two main advantages: it achieves the growth of high quality epitaxial layers leading to enhanced electronic properties of the silicon absorber layers and, at the same time, it provides a deposition rate above 1 micron per minute which is an important parameter in view of achieving a high throughput. In a previous work, the spinel glass-ceramic substrates have been demonstrated to be compatible with epitaxial silicon deposition at temperatures as high as 1130°C [2]. Previously, these substrates were used to realize thinfilm polycrystalline-silicon solar cells based on aluminium-induced crystallization of amorphous silicon (AIC) to form a thin seed layer and epitaxy by thermal CVD to form the absorber layer [2]. In the present work, 300nm thin (001)-oriented silicon single-crystal layers were transferred to glass-ceramic substrates to study the effect of high quality seed layers on the solar cell performance. These silicon seed layers were provide by Corning Inc. and were transferred onto the glass-ceramic substrates using Corning’s proprietary process described in detail elsewhere [3]. Prior to introducing the material into the CVD reactors, contamination analyses were performed using reflection X-ray fluorescence (TXRF) and secondary ion mass spectroscopy (SIMS) after annealing of the samples at temperatures comparable to those used in the CVD reactors. The thickening of the silicon single-crystal seed layers has been realized in two different reactors: in a low-pressure chemical vapor deposition (LPCVD) system and in an atmospheric-pressure chemical vapor deposition (APCVD) system. The LPCVD system used is a batch-type system in which up to twenty 12.5 x 12.5 cm2 wafers can be processed simultaneously. This system operates at low pressure (~ 1 mbar) and at a standard deposition temperature of 1000˚C. Silane (SiH4) is used as a precursor with H2 as dilution gas and diborane (B2H6) as dopant gas. The silicon deposition rate is typically around 100 nm per minute. To limit the processing time, epitaxial layers with a total thickness of around 3 micrometer were grown. The APCVD system used is a single-wafer system that works with wafers of eight-inch diameter. This system operates at atmospheric pressure and at a standard deposition temperature of 1130˚C. Trichlorosilane (HSiCl3) is used as the precursor with H2 as dilution gas and diborane (B2H6) as dopant gas. The deposition rate is typically around 1.5 μm per minute. Layers with a p+/p structure were grown consisting of 2 micrometer p+ (~5x1019 cm-3) and 2 to 8 micrometer p (1016 cm-3).

Emitter contacts Base contacts ITO a-Si Absorber layer (p) BSF layer (p+) Seed layer Glass-ceramic substrate Figure 1. Schematic drawing (not in scale) of the simple solar cell test structure used in this work.


The defect density of the epitaxial layers was checked by applying a Schimmel etch directly to the grown layers, after which the defects are visible by optical or electron microscope [4].

Intrinsic barrier layer

Glass ceramic

Silicon single crystal

Figure 2. TEM picture of the mono-Si layer on glass-ceramic after epitaxial thickening (cross-section). After epitaxial deposition, some of the layers were made into simple solar cells with a-Si heterojunction emitters and interdigitated top contacts (see Figure 1). All cells were in substrate configuration, had a cell size of 1 cm2, and did not have any light trapping features besides a plasma-textured front surface to reduce the front surface reflection. The emitters were formed by deposition of thin double layers of undoped and P-doped a-Si using the same PECVD system. The thickness of the heterojunction emitter was around 16 nm. As anti-reflective coatings, indium tin oxide (ITO) layers were deposited by rf-sputtering. The ITO layers are conductive to provide a conductive path towards the metal contacts for the carriers collected in the amorphous emitter. Finally, contacts were formed by photolithography and wet chemical etching in combination with metal evaporation.

10 μm Figure 3. Optical microscope picture of a mono-Si epitaxial layer after defect etching.


3. RESULTS AND DISCUSSION 3.1 Seed layer characterization X-Ray diffraction measurements confirm that the transferred silicon films on glass-ceramic are (001)-oriented silicon single crystals. The bonding has been measured to be very strong [3]. The unpolished silicon seed layers on glassceramic samples present a characteristic micrometric roughness due to the exfoliation step. In addition to the silicon single crystal layer, a barrier layer between the glass-ceramic and silicon layer is intrinsically created during the anodic bonding process (see Figure 2). This barrier layer, identified as a silica layer, is due to the depletion of mobile glass components at the glass-silicon interface during the anodic step. During the epitaxial step, this intrinsic barrier layer prevents the contamination of the silicon layer by the glass-ceramic components. 3.2 Epitaxial deposition The epitaxial growth using the LPCVD system at 1000°C resulted in epitaxial layers of around 3 microns thickness with a defect density of around 107 cm-2. The defect types are both stacking faults and dislocations. The epitaxial growth using the APCVD system at 1130°C resulted in epitaxial layers of up to 10 microns thickness with a much lower defect density in the order of 105 cm-2. The processing temperature seems to have a direct effect on the epitaxial layer quality. In addition, we found that the prebake time, i.e. the time the samples were kept at 1130°C in flowing hydrogen prior to the start of the epitaxial deposition, has a direct effect on the defect density. A very short prebake time in the order of 30 seconds led to epitaxial layers with a defect density between 5x106 and 107 cm-2 while a prebake time of 10 minutes led to epitaxial layers with a much lower defect density of around 105 cm-2 (Figure 3). In contrast to the APCVD system, longer prebake times did not result in lower defect densities for the LPCVD system. This may be related to the lower processing temperature or to the lower working pressure of the LPCVD. More growth experiments are needed to fine tune the epitaxial processing parameters to the spinel glass-ceramic material which has fundamentally different properties than a silicon wafer. However, the prebake time at 1130°C cannot be extended for too long due to glassceramic deformation as a result of material compaction that starts to be noticeable after 15 minutes of processing at 1130°C in the APCVD system (Figure 4).

Figure 4. Material presenting warpage after 15 minutes exposure at 1130°C.


3.3 Solar cell results Simple solar cell structures (Figure 1) were made from the epitaxial mono-silicon layers grown by APCVD on the glassceramic substrates. The measured efficiencies of the different samples reached values of up to 11%. We investigated layers with different thicknesses. All layers received the same plasma texturing process that removed around 1.5 microns of the original p-layer thickness. Table 1 summarizes the solar cell results of the investigated layers. The best single cell efficiency was 11%. The Voc values above 600 mV illustrate the good material quality of these thin monocrystalline-Si layers on glass-ceramic substrates. The Jsc values are still relatively low due to the absence of advanced light trapping structures in these cells and the fact that the cells are in substrate configuration and do not have a back reflector incorporated. Table 1. Averaged cell results for mono-Si samples with a different p-layer thickness

Original p-layer thickness microns

Jsc -2

mA cm

Voc

Fill factor

Efficiency

mV

%

%

8

24.3

598

74

10.8

6

23.1

600

71

9.9

4

19.9

613

73

8.9

2

16.3

576

61

5.7

4. CONCLUSIONS Our solar cell results obtained on 10-micron thick monocrystalline-silicon layers on transparent glass-ceramic substrates are presented. These single crystal layers were made by epitaxial thickening of thin monocrystalline-silicon seed layers on glass-ceramics. Although the used cell structure is very simple, silicon single-crystal solar cells on glass-ceramic showed efficiencies of up to 11%. By further optimizing the minority carrier diffusion length and by integrating an efficient light trapping scheme, the efficiency potential of these single-crystal silicon thin films on glass-ceramics with a thickness between five to ten microns should be higher than 15%.

5. ACKNOWLEDGEMENTS The authors would like to thank Alexandre Mayolet, Donnell Walton and Ta-Ko Chuang of Corning for supplying the seed layers. The authors are also very grateful to Kris Van Nieuwenhuysen and Srisaran Venkatachalam for their help with the epitaxial depositions and defect etching.


REFERENCES [1] Keevers, M., Young, T., Schubert, U., Green, M., “10% efficient CSG minimodules”, Proceedings of the 22nd European Photovoltaic Solar Energy Conference, 1783-1786 (2007). [2] Gordon, I., Carnel, L., Van Gestel, D., Beaucarne, G., Poortmans, J., Pinckney, L., Mayolet, A., “Thin-film polycrystalline-silicon solar cells on glass-ceramics”, Proceedings of the 22nd European Photovoltaic Solar Energy Conference, 1993-1996 (2007). [3] Dawson-Elli, D., Kosik Williams, C., Couillard, J., Cites, J., Manley, R., Fenger, G., Hirschman, K. “Demonstration of High Performance TFTs on Silicon-on-Glass (SiOG) Substrate”, ECS Transactions 8(1), 223-228 (2007). [4] Schimmel, D., “Defect etch for Silicon evaluation”, Journal of Electrochemical Society 126, 479-483 (1979).
