Thin film solar cells based on microcrystalline silicon–germanium narrow-gap absorbers

We report on the improved performance of p–i–n junction solar cells incorporating hydrogenated microcrystalline silicon–germanium (μc-Si1−xGex:H) absorber i-layers prepared by low-temperature (∼200 °C) plasma-enhanced chemical vapor deposition. While the optical absorption increases with Ge content, the photocarrier transport in the solar cells for x>0.2 is dominated by the carrier recombination due to the increased dangling bond defects and the illumination-induced field distortion in the i-layer. In contrast, the solar cells with smaller Ge contents (x∼0.2) exhibit better carrier collection characteristics with extended infrared sensitivities even higher than those of double-thickness μc-Si:H solar cells. As a result, we have achieved a 6.3% efficiency using a 1-μm-thick μc-Si0.8Ge0.2:H i-layer.