Microcrystalline silicon-germanium as a low-gap material for solar cells

Microcrystalline silicon-germanium (muc-SiGe) is investigated as a low-gap material for stacked solar cells. muc-SiGe films were prepared by PECVD (13.56 MHz) from a SiH4, GeH4 and H-2 mixture. The optical absorption coefficient becomes 10 times larger than that for microcrystalline silicon (muc-Si) by increasing the Ge content to 50 at.%. muc-SiGe solar cells are demonstrated with 30% Ge content and 5000 A thickness, and a conversion efficiency of 4.3% (the highest value in muc-SiGe solar cells so far) and short-circuit current density (Isc) of 28 mA/cm(2) are achieved. A 20% of collection efficiency is observed at 1000 nm. This Isc value is larger than that of the best pc-Si solar cells with 2 pm thickness. It is suggested that the muc-SiGe provides much thinner microcrystalline solar cells with higher efficiency by utilizing longer wavelength light.