Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by Aluminum-Induced Crystallization and Thermal
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0910-A23-04
Thin-Film Polycrystalline-Silicon Solar Cells on Ceramic Substrates Made by AluminumInduced Crystallization and Thermal CVD Ivan Gordon, Dries Van Gestel, Lode Carnel, Kris Van Nieuwenhuysen, Guy Beaucarne, and Jef Poortmans IMEC VZW, Kapeldreef 75, B-3001 Leuven, Belgium ABSTRACT A considerable cost reduction could be achieved in photovoltaics if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) layers. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening is an effective way to obtain largegrained pc-Si layers with excellent properties for solar cells. To obtain efficient solar cells, the electronic quality of the pc-Si material obtained by AIC has to be optimized and the cell design has to be adapted to the material. In this paper, we report on pc-Si solar cells made by AIC in combination with thermal CVD on ceramic alumina substrates. We made pc-Si solar cells on alumina substrates that showed Voc values up to 533 mV and efficiencies up to 5.9%. This is the highest efficiency ever achieved with pc-Si solar cells on ceramic substrates where no (re)melting of silicon was used. We demonstrate that the quality of the pc-Si material can be improved drastically by reducing the substrate roughness using spin-on oxides. We further show that a-Si/c-Si heterojunctions lead to much higher Voc values than diffused homojunctions. A cell concept that incorporates spin-on oxides and heterojunction emitters is therefore best suited to obtain efficient pc-Si solar cells on alumina substrates. INTRODUCTION The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on cheap foreign substrates. At present the semiconducting material in crystalline-silicon solar modules accounts for 30% to 50% of the total module price, while most of the used silicon material only serves as mechanical support. A solar cell technology based on thin crystalline-silicon films could achieve a large cost reduction by the use of much less silicon material in combination with cheap supporting substrates. Polycrystalline-silicon films with grain sizes between 1-10 µm seem particularly good candidates to obtain low-cost thin-film solar cell modules [1]. We recently showed that aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) followed by epitaxial thickening is an effective way to obtain good pc-Si solar cells [2]. Ideally, the grains of pc-Si films used for solar cells should have a low aspect ratio. However, small grains with an average size well below 1 µm are usually obtained when pc-Si is deposited directly onto foreign substrates. In contrast, aluminum-induced crystallization of amorphous silicon leads to very thin pc-Si layers with average grain sizes around 10-20 µm [3]. These layers are too thin and too highly doped to be used as absorber layers but instead they can be used as seed layers to obtain much thicker layers with the desired doping level by epitaxia
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