Efficient Thin-Film Polycrystalline-Silicon Solar Cells Based on Aluminium-Induced Crystallization
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0989-A24-03
Efficient Thin-Film Polycrystalline-Silicon Solar Cells Based on Aluminium-Induced Crystallization Ivan Gordon, Lode Carnel, Dries Van Gestel, Guy Beaucarne, and Jef Poortmans Solar Cell Technology, IMEC, Kapeldreef 75, Leuven, B-3001, Belgium
ABSTRACT Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could lower the price of photovoltaic electricity substantially. At the MRS conference in 2006, we presented a pc-Si solar cell with an efficiency of 5.9% that had an absorber layer made by aluminum-induced crystallization (AIC) of amorphous silicon followed by high-temperature epitaxial thickening. The efficiency of this cell was mainly limited by the current density. To obtain higher efficiencies, we therefore need to implement an effective light trapping scheme in our pc-Si solar cell process. In this work, we describe how we recently enhanced the current density and efficiency of our cells. We achieved a cell efficiency of 8.0% for pc-Si cells in substrate configuration. Our cell process is based on pc-Si layers made by AIC and thermal CVD on smoothened alumina substrates. The cells are in substrate configuration with deposited a-Si heterojunction emitters and interdigitated top contacts. The front surface of the cells is plasma textured which leads to an increase in current density. The current density is further enhanced by minimizing the back surface field thickness of the cells to reduce the light loss in this layer. Our present pc-Si solar cell efficiency together with the fast progression that we have made over the last few years indicate the large potential of pc-Si solar cells based on the AIC seed layer approach. INTRODUCTION The current high price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon thin films on inexpensive substrates. Due to the recent silicon feedstock shortage, prices of silicon solar modules have increased. A silicon thin-film technology could lead to cheaper modules by the use of less silicon material and by the implementation of monolithic module processes. A technology based on polycrystalline-silicon thin films with a grain size between 1 µm and 1 mm (pc-Si), seems particularly promising since it combines the low-cost potential of a thin-film technology with the high efficiency potential of crystalline silicon [1]. State-of-the-art pc-Si mini-modules based on solid-phase crystallization (SPC) with efficiencies close to 10% and open-circuit voltages (Voc) around 500 mV per cell have recently been reported by CSG Solar AG [2]. At the MRS conference in 2006, we presented promising solar cell results that were obtained on pc-Si films made by aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) followed by high-temperature epitaxial thickening [3]. The AIC process leads to very thin pc-Si seed layers with a typical grain size in the range of 5-20 µm [4]. Absorber layers for solar cells can be made by epitaxial thickening of these AIC layers [3,
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