Processing and Characterization of Efficient Thin-Film Polycrystalline Silicon Solar Cells

The price of photovoltaic electricity could be lowered substantially if efficient solar cells could be made from polycrystalline-silicon (pc-Si) thin films on inexpensive substrates. A promising way to make pc-Si layers for solar cells is to use aluminum-induced crystallization (AIC) of amorphous silicon in combination with thermal CVD. To obtain efficient pc-Si solar cells, the material quality has to be optimized and cell processes different from those applied for standard bulk-Si solar cells have to be developed. In this paper, we present our best pc-Si solar cells so far and we discuss some key features of our solar cell process. We also discuss the structural and electronic quality of our pc-Si layers. We obtained efficiencies of up to 8% on polycrystalline-silicon solar cells made by AIC in combination with thermal CVD on alumina substrates. By replacing conventional diffused emitters by a-Si/c-Si heterojunction emitters, much higher V-oc values were obtained. The use of plasma texturing led to higher current densities due to an enhanced coupling of light into the pc-Si layers. The efficiency of our pc-Si solar cells is limited by the presence of a high intragrain defect density in our layers. A detailed TEM study revealed that most of these defects are already present in the AIC seed layers prior to epitaxial thickening. To improve the material quality of our pc-Si layers, we will therefore need to improve the seed layer quality.