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

Microcrystalline silicon-germanium (μc-SiGe) is investigated as a low-gap material for stacked solar cells. μc-SiGe films were prepared by PECVD (13.56 MHz) from a SiH₄, GeH₄ and H₂ mixture. The optical absorption coefficient becomes 10 times larger than that for microcrystalline silicon (μc-Si) by increasing the Ge content to 50 at.%. μc-SiGe solar cells are demonstrated with 30% Ge content and 5000 Å thickness, and a conversion efficiency of 4.3% (the highest value in μc-SiGe solar cells so far) and short-circuit current density (I_sc) of 28 mA/cm² are achieved. A 20% of collection efficiency is observed at 1000 nm. This I_sc value is larger than that of the best μc-Si solar cells with 2 μm thickness. It is suggested that the μc-SiGe provides much thinner microcrystalline solar cells with higher efficiency by utilizing longer wavelength light