The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
a-Si Related Photovoltaics III
Chair: Christiane Becker, Helmholtz-Zentrum Berlin
Development of a-Si Solar Cells using "Liquid Si Printing"
1. Device Solutions Center, R&D Division, Panasonic Corporation, 3-1-1 Yagumo-naka-machi, Moriguchi City, Osaka 570-8501, Japan
2. Japan Advanced Institute of Science and Technology, 1-1, Asahidai, Nomi City, Ishikawa 923-1292, Japan
3. Japan Science and Technology Agency, ALCA, 1-1 Asahidai, Nomi City, Ishikawa 923-1292, Japan
4. Japan Science and Technology Agency, ERATO, Shimoda Nano-Liquid Process Project, 1-1 Asahidai, Nomi City, Ishikawa 923-1292, Japan
Cost-effective solar cells require thin-film silicon; however, vacuum processes such as plasma-CVD and sputtering are high-cost steps in photovoltaic module manufacture. To solve this problem, "Liquid-Si Printing (LSP)" is being developed: this is a non-vacuum process that consists of printing Liquid-Si followed by solidification by pyrolysis. Liquid-Si contains polydihydrosilane (–(SiH2)n–) that has been synthesized using cyclopentasilane.
Printed a-Si:H has been applied to TFT fabrication . LSP a-Si:H films have been reported to show relatively high photoconductivities, and have worked in a-Si solar cells , although at a conversion efficiency of below 2%. Printed a-Si:H thus has potential for use in photovoltaic layers.
Our studies showed the a-Si:H films and the a-Si solar cells printed using LSP to have very good photo-stability . After light soaking at an intensity of 500 mW/cm2 at 25°C for 180 min, the light-induced degradation ratios of the photoconductivity of the a-Si:H film and conversion efficiency of the a-Si:H solar cell with the printed a-Si:H were lower than half of those seen in a-Si:H fabricated by plasma-CVD. Measurements of IR absorption spectra revealed low hydrogen contents (CH), due to the high temperatures of over 400°C used in the pyrolyzing process. This low CH value appears to make a-Si:H printed using LSP resistant to light soaking.
One of the problems with a-Si:H printed by LSP is the missing link between film properties and cell performance. We found this to be partly attributable to thermal damage to the under layer during pyrolysis at over 400°C. To prevent thermal damage, we improved the p-i-n junction structures. We also improved the properties of printed a-Si:H materials. As a result, we achieved a conversion efficiency of 3.1% by stacking with non-doped a-Si:H, fabricated using plasma-CVD.
Keywords: amorphous Si, solar cells, stability, liquid Si, cyclopentasilane
 T. Shimoda, Y. Matsuki, M. Furusawa, T. Aoki, I. Yudasaka, H. Tanaka, H. Iwasawa, D. Wang, M. Miyasaka, and Y. Takeuchi: Nature 440 (2006) 783
 T. Masuda, N. Sotani, H. Hamada, Y. Matsuki, and T. Shimoda: Appl. Phys. Lett. 100 (2012) 253908
 H. Murayama, T. Ohyama, I. Yoshida, A. Terakawa, T. Masuda, K. Ohdaira, and T. Shimoda: Thin Solid Films to be published
Towards High-efficiency Thin Film Silicon Solar Cells on Nanopillar-based Superstrates
1. Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin Film Electronics Laboratory (PVlab), Rue A.-L. Breguet 2, CH-2000 Neuchâtel, Switzerland
2. Nanotechnology & Life Sciences Division, CSEM SA, CH-2002 Neuchâtel, Switzerland
3. Photovoltaics Division, CSEM SA, CH-2002 Neuchâtel, Switzerland
With a substrate based on nanopillars with a high aspect ratio, a photovoltaic device with an optically thick but electrically thin absorber layer can be obtained. This is particularly interesting for solar cells based on hydrogenated amorphous silicon (a-Si), since high current densities can then be reached while keeping an efficient collection of the photogenerated carriers, even after light-induced degradation. Yet, one key challenge to address in order to obtain high-efficiency a-Si cells on high-aspect ratio nanopillars is the obtaining of a conformal silicon layer by plasma-enhanced chemical vapor deposition (PECVD). We will present recent advances aiming at integrating high-aspect-ratio nanopillars (i.e. with a ratio nanopillar height over pitch of the nanopillar structures of over 1:1) in high-efficiency a-Si based solar cells. We will in particular show how a conformal coating on high-aspect-ratio nanopillars can be obtained for the a-Si layers—still maintaining high electrical quality—thanks to adapted nanopillar fabrication and well-chosen PECVD conditions during the a-Si-layer growth.
The morphology of the nanopillars is expected to play a major role on the quality of the a-Si layer subsequently grown; whereas sharp tips are harmless for the PECVD silicon layer, a tapered shape with rounded bottoms is required to prevent the formation of pinching points leading to "cracks". To obtain a dense array of 1 μm high nanopillars with a suitable shape, we developed a process based on nanosphere lithography combined with adequate etching steps. The nanopillar array is then replicated by nano-imprint lithography into a UV-curable lacquer on glass. Concerning the PECVD process conditions, we investigated several different deposition regimes, to identify the key elements leading to a conformal coating. No hydrogen dilution (pure SiH4 plasma), relatively high temperature (230°C), and low pressure (0.4 mbar) were identified as favorable conditions to such a conformal growth. The separate influence of each parameter will be studied and presented at the conference.
Our best result obtained up to now on high-aspect-ratio nanopillars in the superstrate configuration is an 8.4% initial efficiency a-Si cell (Voc = 0.843V; FF = 64.1%; Jsc = 15.6 mA/cm2), using hydrogenated indium oxide (IOH) and zinc oxide (ZnO) as front and back electrodes. This is, to our knowledge, amongst the highest reported efficiencies for this type of device. Reference cells on a flat glass + IOH and on our state-of-the-art ZnO superstrate showed an efficiency of 7.1% (0.851V; 66.5%; 12.7 mA/cm2) and 10.9% (0.842V; 73.3%; 17.6 mA/cm2) respectively. Importantly, scanning electron microscope images and analysis of the interference fringes in total absorptance measurements both indicate that the a-Si cell is in average twice thinner on nanopillar-based superstrates (~300 nm in total) than on flat or state-of-the-art superstrates (600 nm), explaining the relatively low current density on nanopillars. A lower light-induced degradation rate is however expected; this will be elucidated by the conference time. Further developments will focus on increasing the nanopillar density (by lowering the pitch from 1 μm to 500 nm) to increase the current density, and on adjusting the front-electrode properties, both in the flat and nanopillar cases, to recover a good FF.
Keywords: nanopillars; amorphous silicon solar cells; conformal growth; light trapping; light-induced degradation
Light Absorption and Carrier Separation in Radial p-i-n Junction Solar Cells
1. School of Electronics Science and Engineering, Nanjing University, 210093, Nanjing, China
2. Laboratoire de Physique des Interfaces et Couches Minces (LPICM), CNRS, Ecole Polytechnique, 91128 Palaiseau, France
Radial junction (RJ) solar cells built over a silicon nanowires (SiNWs) forest demonstrate extraordinary light trapping performance, which allows the use of a thin absorber layer for optimized carrier separation. Applying this advanced concept to develop a new generation of Si thin film solar cells has the potential to suppress the light-induced degradation and boost the overall cell efficiency. In a RJ Si thin film solar cell, multiple layers of intrinsic and doped a-Si:H layers are deposited consecutively over a matrix of thin SiNWs, which serve as one of the PIN electrodes for carrier extraction. The SiNWs matrix is grown via a high-throughput plasma-enhanced vapor-liquid-solid (VLS) process, catalyzed by a group of low-melting point metals including tin (Sn), indium (In) and bismuth (Bi) [1–3], enabling a low temperature growth of SiNWs (in the range of 240°C to 400°C) on top of low-cost glass or foil substrates. Based on this approach, we have recently demonstrated radial junction a-Si:H solar cells showing a power conversion efficiency higher than 8%.
Further device optimization requires a profound and comprehensive understanding of the light trapping, absorption and carrier separation in this virtually 3D multilayer radial junction structure. Maximization of the effective light absorption in the active i-layer within a vertical radial p-i-n junction layout has to be realized against the loss in inactive n+-layer and TCO layers. In addition, the facing electrodes in a radial p-i-n configuration have intrinsically asymmetrical areas, causing nonuniform built-in field distribution across the absorber i-layer. This effect becomes more prominent when the ratio of i-layer thickness over the SiNW radius increases, thus bringing in an important geometry factor for the optimal structure of a RJ solar cell. However, except several pioneering works on the theoretical understanding of light trapping among crystalline Si pillars, there has been no study dedicated to the optical response in RJ a-Si:H thin film solar cells.
Here, we will address both the optical absorption and electrical carrier separation in the RJ a-Si:H solar cell by establishing a comprehensive simulation model with the aid of COMSOL packages. Based on a set of realistic optical properties for each material in the RJ structure and the exact calculation of electrical field distribution in the asymmetrical PIN junction, we are be able to simulate simultaneously the optical response and electrical performance of the radial junction solar cells, and by combining these two important aspects, to predict or provide a guideline for an optimal device configuration of the single junction radial junction a-Si:H solar cell. In addition, these modeling results will be compared to the experimental external quantum efficiency of actual RJ solar cells, revealing the true benefit of the strong light trapping among nanowire forest and the critical information for future parametric optimization.
Keywords: silicon nanowire, Si thin film solar cell, optical response simulation
 L. Yu, F. Fortuna, B. O'Donnell, T. Jeon, M. Foldyna, G. Picardi, and P. Roca i Cabarrocas, Nano Lett., 12 (2012) 4153–4158
 L. Yu, B. O'Donnell, M. Foldyna, and P. Roca i Cabarrocas, Nanotechnology, 23 (2012) 194011
 P.-J. Alet, L. Yu, G. Patriarche, S. Palacin, and P. Roca i Cabarrocas, J. Mater. Chem., 18 (2008) 5187–5189
High Performance Solar Cell Fabricated on Flattened SnO2/ZnO Substrate for Full Spectrum Splitting Solar Cell Application
1. Department of Physical Electronics, Tokyo Institute of Technology
2. Photovoltaic Research Center (PVREC), Tokyo Institute of Technology 2-12-1-NE-15, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
In recent years of development and worldwide PV market growth, an increasing of the conversion efficiency with low investment and manufacturing cost is major issues. To realize this prerequisite, spectrum splitting type thin-film solar cell is satisfied these factors. We have already developed a spectrum splitting type thin-film solar cell using hydrogenated amorphous silicon (a-Si:H) fabricated on ZnO substrate and Copper Indium Gallium Diselenide Cu(In1–xGax)Se2 (CIGS) as the top and bottom cells and we have achieved a conversion efficiency of 22% at the splitting wavelength of 620 nm.
To further increase the performance of this spectrum splitting type solar cell, the top cell (a-Si:H) with high Voc and good response at the short wavelength has to be developed. It is generally known that the hazy of TCO affects the cell Voc substantially. So in this study we consider how to smoothen the TCO surface by Ar plasma treatment in order to obtain higher Voc. Furthermore, to achieve better response at the short wavelength, TCO with wider band gap such as SnO2 can be considered instead of ZnO. However, since SnO2 can be easily damaged by hydrogen plasma, we have studied the application of multilayer SnO2/ZnO substrate. Here, commercial VU-type SnO2 :F glass substrate manufactured by Asahi Glass has been used as the main TCO while ZnO layers fabricated by MOCVD and sputtering method have been used as covering layers. The ZnO thickness has been fixed to be less than 100 nm.
a-Si:H solar cells with a 1.9 eV band gap i-layer and an area of 0.086 cm2 have been fabricated on these SnO2/ZnO substrates. Solar cells have the cell structure of glass / SnO2:F / ZnO / p-a-SiC:H(2.0 eV) / buffer / i-a-Si:H(1.9 eV, 300 nm) / n-μc-SiO:H / Ag / Al. Low deposition temperature as low as 150°C and high H2 dilution conditions have been employed to obtain the i-layer with wider band gap. Here, in order to improve the Voc, the TCO surface has been smoothened by Ar plasma treatment and the Ar treatment time has been optimized to achieve the best performance.
As the results, the cell performance with Voc = 0.96 V, Jsc = 13.5 mA/cm2, FF = 0.61 and Eff. = 7.16% has been achieved on commercial VU-type SnO2 substrate while the better one with Voc = 0.964 V, Jsc = 14.5 mA/cm2, FF = 0.72 and Eff. = 10.06% has been obtained on flattened SnO2/ZnO substrate. Here the flattened substrate has been treated by 10 min. Ar plasma. Overall cell performance has been improved with SnO2/ZnO substrate especially FF. This result shows that TCO glass substrate coated with SnO2/ZnO and flattened by Ar plasma treatment is a promising TCO substrate for a-Si:H top cell of splitting type solar cell.
Keywords: high Voc , amorphous silicon, thin film solar cell, VHF-PECVD, TCO