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

Microcrystalline silicon-germanium (/spl mu/c-SiGe) is investigated as a low-gap material for stacked solar cells. /spl mu/c-SiGe films were prepared by PECVD (13.56 MHz) from a SiH/sub 4/, GeH/sub 4/ and H/sub 2/ mixture. The optical absorption coefficient becomes 10 times larger than that for microcrystalline silicon (/spl mu/c-Si) by increasing the Ge content to 50 at.%. /spl mu/c-SiGe solar cells are demonstrated with 30% Ge content and 5000 /spl Aring/ thickness, and a conversion efficiency of 4.3% (the highest value in /spl mu/c-SiGe solar cells so far) and short-circuit current density (I/sub sc/) of 28 mA/cm/sup 2/ are achieved. A 20% of collection efficiency is observed at 1000 nm. This I/sub sc/ value is larger than that of the best /spl mu/c-Si solar cells with 2 /spl mu/m thickness. It is suggested that the /spl mu/c-SiGe provides much thinner microcrystalline solar cells with higher efficiency by utilizing longer wavelength light.