Journal of the Japanese Association for Crystal Growth

To manufacture crystalline silicon solar cells, high purity silicon with the purity of 99.9999% or higher is required. Between 2006 and 2008 when PV power generation rapidly developed, there emerged a serious shortage of high purity silicon supply. The issue of the supply shortage is being solved with a variety of countermeasures. Amid these circumstances, the PV power generation has been receiving expectations for further development as a future source of energy. Consequently, securing supply and cost reduction of high purity silicon continue to be critical issues, which may affect the development of solar cells. This paper focuses on technologies to supply high purity silicon for solar cells and the current status of efforts to stabilize the supply and demand status of high purity silicon for solar cells.

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Generally, Si multicrystals, which are grown by a casting method using a crucible, contain many grain boundaries (GBs) and crystal grains with various orientations. Since the grain size has increased as a result of improving in the growth technique, instead of GBs, sub-grain boundaries (sub-GBs) have become major defects acting as recombination centers for photogenerated carriers. In this paper, study of sub-GBs in Si multicrystals is comprehensively reviewed with the authors' current results.

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For accurate prediction of carbon and oxygen impurities in multicrystalline silicon material for solar cells, global simulation of coupled oxygen and carbon transport in a unidirectional solidification furnace was implemented. Both the gas flow and silicon melt flow were considered. Five chemical reactions were included during the transportation of impurities. The simulation results show good agreement with experimental data.

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Thin film silicon solar cells offer potential advantages over wafer-based silicon solar cells in terms of cost, large area and less silicon consumption. In the last decade, solar cell efficiency has progressed markedly by applying hydrogenated microcrystalline silicon (μc-Si:H) as an infrared-absorbing material in double junction tandem solar cells. Until now, great efforts have been made for the growth of high-quality μc-Si:H by means of plasma-enhanced chemical vapor deposition at high growth rates. Several design criteria for device grade μc-Si:H are found in terms of crystalline volume fraction, crystallographic orientation, grain size and grain boundary microstructure. For further progress in conversion efficiency of thin film silicon solar cells, the addition of a third component cell to the tandem device, i.e. triple junction, is required based on a multi band-gap approach, allowing more efficient use of the solar spectrum. Hydrogenated microcrystalline Si_<1-x>Ge_x (μc-Si_<1-x>Ge_x:H) alloys have been developed as a new low-band-gap absorber that extends the spectral sensitivities of solar cell into longer infrared wavelengths at the thinner absorber thickness.

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