Amorphous SiGe: H for High Performance Solar Cells

A preliminary study has been carried out on electrical and optical properties of amorphous silicon-germanium mixed films (a-SiGe: H) deposited by a glow discharge plasma reaction. As a germanium content in the film increases, an optical absorption coefficient increases and an optical gap energy E g opt. decreases. Growth rate of a-SiGe: H films increases with the germanium content in the film and reaches to the value for pure a-Ge: H films about 3 times larger than that of a pure a-Si: H film. Film properties were examined by dark- and photoconductivity, MOS FET transistor characteristics, and photoluminescence. Photoconductivity and carrier mobilities decreases drastically when the mole ratio GeH 4 /(GeH 4 +SiH 4 ) exceeds 25%. Solar cells of various structures using a-SiGe: H and a-Si: H films were fabricated and compared their photovoltaic properties with that of a simple p-i-n type a-Si: H solar cell. Tandem structure with a-Si: H p-i-n and a-SiGe: H p-i-n cells shows a broadened spectral response at long wave length region and an open circuit voltage of 1.13 V, about twice of a single cell.