Improved light trapping in thin-film silicon solar cells via alternated n-type silicon oxide reflectors

We have investigated light trapping of p–i–n type hydrogenated amorphous silicon (a-Si:H) single-junction and p–i–n–p–i–n type a-Si:H/hydrogenated microcrystalline silicon (μc-Si:H) double-junction solar cells by adopting the oxygen-content graded or the oxygen-content alternated hydrogenated n-type silicon oxide (SiOx:H) reflectors. The graded n-type SiOx:H back reflector effectively increases the optical path length of the p–i–n type a-Si:H single-junction solar cells due to the refractive index grading. Moreover, the alternated n-type SiOx:H back reflector comprising of highly hydrogen-diluted n-type a-Si:H (n-a-Si:H) sublayers having a high refractive index and highly hydrogen-diluted n-type a-SiOx:H sublayers having a low refractive index provides the further improvement for the optical path length of the p–i–n type a-Si:H single-junction solar cells due to the multiple reflection. Furthermore, the developed alternated n-type SiOx:H reflector is suitable for the back reflector as well as the intermediate reflector of the p–i–n–p–i–n type a-Si:H/μc-Si:H double-junction solar cells. As a result, the high initial efficiency (η) of 13.1% and stabilized η of 11.5% are achieved. The considerably thin (30–45 nm) alternated n-type SiOx:H reflectors can be easily prepared using the in situ plasma enhanced chemical vapor deposition (PECVD) technique. Since the alternated n-type SiOx:H reflector can avoid the lateral shunting problem during the monolithic series integration of segments, it is a promising option for cost effective mass production of large-area thin-film silicon solar modules.