High-rate deposition of microcrystalline silicon p-i-n solar cells in the high pressure depletion regime

Hydrogenated microcrystalline silicon films (mu c-Si:H) deposited at high deposition rates (similar to 2 nm/s) by means of the very-high-frequency (VHF) deposition technique in the high pressure depletion regime have been integrated into single junction p-i-n solar cells. It is demonstrated that mu c-Si:H solar cells can be optimized using a twofold approach. First the bulk properties, deposited under steady-state plasma conditions, are optimized by monitoring the presence of crystalline grain boundaries in mu c-Si:H. These hydrogenated crystalline grain boundaries can easily be detected via the crystalline surface hydrides contribution to the narrow high stretching modes by infrared transmission spectroscopy. The crystalline grain boundaries suffer from postdeposition oxidation which results in a reduced red response of the solar cell. The absence of these crystalline surfaces in an as-deposited mu c-Si:H matrix reflects the device grade microcrystalline bulk material. Second, the prevention of silane backdiffusion from the background during the initial growth is a necessity to deposit a uniform mu c-Si:H phase over the entire film thickness. The initial growth is optimized while preserving the optimized bulk properties deposited under steady-state conditions, using initial profiling of plasma parameters such as the silane flow and the VHF power density. Solar cell devices with efficiency of 8.0% at a mu c-Si:H deposition rate of 2.0 nm/s are obtained using the presented approach. (C) 2008 American Institute of Physics.