Progress and Challenges of Photovoltaic Applications of Silicon Nanocrystalline Materials
Nanocrystalline silicon (nc-Si) is a very important material for the fabrication of thin film solar cells, as this material combines the advantages of amorphous silicon (a-Si) and crystalline silicon (c-Si), namely, deposition over a large area, low-temperature processing, low material consumption, and high stability. The deposition rate of nc-Si has received much attention in recent years due to its importance as a rate-limiting step in the fabrication of multijunction solar cells using nc-Si as one of the bottom cells in combination with a-Si as the top cell. It is a scientific as well as technological challenge to obtain high-efficiency nc-Si cells at a deposition rate ≥5 nm/s. There are three areas where nc-Si faces competition: (1) the c-Si wafer that is getting thinner and thinner (<100 μm); (2) a-Si and, especially, its alloy a-SiGe; and (3) non-silicon materials, such as copper indium gallium diselinide (CuInGaSe2 or CIGS) and cadmium telluride (CdTe). CdTe-based modules with a system price of ~1$/Wp certainly seem to be the most attractive solar cells at present . CIGS is drawing a lot of attention due to its high efficiency, which touches almost 20%  on the laboratory scale.