Effect of hydrogen dilution profiling on the microscopic structure of amorphous and nanocrystalline silicon mixed‐phase solar cells

Abstract

AbstractMicroscopic structure and solar cell performance in hydrogenated mixed‐phase thin film silicon (Si:H) solar cells are studied. The samples were made with RF glow discharge with different hydrogen dilution profiles. The material properties were measured with Raman, X‐TEM, AFM, and C‐AFM. Several interesting phenomena are observed. First, the cone‐structured nanocrystalline aggregations were formed when a constant hydrogen dilution was used. Second, no uniform block‐like (or cylinder‐like) structured nanocrystalline clusters were observed even when hydrogen dilution profiling was optimized for this purpose. Instead, tree‐like structured nanocrystalline clusters were formed and embedded in the intrinsic layer. Third, the magnitude of light‐induced Voc increase was reduced by hydrogen dilution profiling. When the dilution profiling was sufficiently steep, no light‐induced Voc increase was observed. Instead, the Voc decreased after light‐soaking regardless of the crystalline volume fraction. In addition, AFM and C‐AFM showed that this type of mixed‐phase material has hill‐like surface structure, where the hills correspond to nanocrystalline clusters. The local current density in hill‐like areas was much higher in the samples made with constant hydrogen dilution than those using hydrogen dilution profiling. For the samples with a very steep hydrogen dilution profiling, the local forward current density is very low. Based on our previous model, the light‐induced Voc increase depends on the formation of the current path in the nanocrystalline cluster areas. When a steep hydrogen dilution profiling is used, the tree‐like nanocrystalline clusters are isolated and embedded in the intrinsic layer, therefore, no high current paths are formed and no light‐induced Voc increase is observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)