Optimization of n–i–p protocrystalline SiGe:H thin film solar cells for application in thin film multijunction solar cells

Abstract

In developing n–i–p structured narrow band gap protocrystalline SiGe:H thin film solar cells, an S-shape was encountered in the cells’ current–voltage characteristics, which was not present in the curves of p–i–n cells that incorporated the same i-layer material. With the help of a carefully designed series of annealing experiments, the S-shape was found to consist of two barriers, one of which is located at the p/i interface while the other one is at the ITO/p front contact. Further investigations using thin layer Raman spectroscopy and activation energy measurements pointed out that the barriers are mainly due to a drifting of the μc-Si p-layer deposition from optimized conditions. By optimizing the p-layer deposition and with the use of post-deposition annealing of the entire cell structure, an efficiency of 8.7% was achieved for a 1.55eV band gap protocrystalline SiGe:H n–i–p cell on an Asahi U-type substrate coated with a Ag/ZnO back reflector.