High-efficiency heterojunction solar cells on crystalline germanium substrates

Stand-alone heterojunction (HJ) solar cells demonstrated on crystalline germanium (c-Ge) substrates are proposed for usage as the bottom cells of tandem-junction solar cells in various thin-film solar cell technologies. The emitter of the HJ solar cells is formed by growing thin layers of highly doped hydrogenated microcrystalline silicon (μc-Si) and further passivated by growing thin layers of hydrogenated amorphous silicon (a-Si). The μc-Si and a-Si layers are grown in the same reactor using plasma-enhanced chemical vapor deposition (PECVD) at temperatures close to 200°C. The quality of the c-Ge surface passivation by μc-Si and a-Si has a direct impact on the electrical performance of the HJ solar cells. Conversion efficiencies of 5.9% and 7.2% have been achieved for stand-alone c-Ge solar cells on n-type and p-type c-Ge substrates, respectively. These conversion efficiencies are well-comparable with the conversion efficiencies reported for conventional homojunction solar cells fabricated at temperatures as high as 600°C. © 2014 SPIE.