21. Hydrogenated Microcrystalline Silicon Thin Films

We review μc-Si:H material’s properties, deposition techniques, and applications in solar cells. The focus is on the issues that limit μc-Si:H solar cell performance. Because of unintentional impurity incorporation, μc-Si:H shows an N-type conductivity, especially in the early days, μc-Si:H was used only as the doped layers. Meier et al. used VHF-PECVD technique with reduced impurity levels and made first μc-Si:H solar cell in 1994. Since then the research and development of μc-Si:H solar cells have attracted a great attention, and a significant progress has been made for using μc-Si:H as the bottom cell in multijunction structures. Compared to a-Si:H, μc-Si:H structure is much complicated with nanometer sized grains, grain boundaries, amorphous tissues, and microvoids. The complexity in the structure leads to complicated material properties, such as electronic structure, optical absorption, carrier transport, and stability. The material’s property affects the solar cell performance significantly. The material structure changes with the film thickness under a constant deposition condition, which influences the solar cell performance. For solar cell applications, it has been found that the best μc-Si:H material should be compact with a low defect density and a crystalline volume fraction around 50% in the whole absorber layer. In order to obtain a high photocurrent density, an effective light trapping is achieved using textured substrates. However, a high textured substrate causes a degradation of material quality. A thick μc-Si:H up to 5 μm requires a high rate deposition. Here, we will discuss the techniques for resolving the μc-Si:H issues and the approaches for achieving the high quality materials and high efficiency μc-Si:H solar cells.