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

Microscopic 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 V-oc increase was reduced by hydrogen dilution profiling. When the dilution profiling was sufficiently steep, no light-induced V-oc increase was observed. Instead, the V-oc 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 V-oc 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 V-oc increase is observed. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim