APIVT-Grown Silicon Thin Layers and PV Devices

Large-grained (5-20 μm) polycrystalline silicon layers have been grown at intermediate temperatures of 750°-950°C directly on foreign substrates without a seeding layer by iodine vapor transport at atmospheric pressure with rates as high as 3 μm/min. A model is constructed to explain the atypical temperature dependence of growth rate. We have also used this technique to grow high-quality epitaxial layers on heavily doped CZ-Si and on upgraded MG-Si substrates. Possible solar cell structures of thin-layer polycrystalline silicon on foreign substrates with light trapping have been examined, compared, and optimized by two-dimensional device simulations. The effects of grain boundary recombination on device performance are presented for two grain sizes of 2 and 20 μm. We found that 104 cm/s recombination velocity is adequate for 20-μm grain-sized thin silicon, whereas a very low recombination velocity of 10³ cm/s must be accomplished in order to achieve reasonable performance for a 2-μm grain-sized polycrystalline silicon device.