Local Current Flow in Amorphous and Nanocrystalline Mixed-Phase Silicon Solar Cells

Local current flow in amorphous and nanocrystalline mixed-phase n-i-p silicon solar cells is measured using conductive atomic force microscopy (C-AFM) and correlated to the material structure from Raman measurement. The C-AFM images show that the current is very low over the entire surface of the fully amorphous region. High current spikes are observed in the mixed-phase region, where the current spike corresponds to aggregations of nanocrystallites. The size of the nanocrystalline aggregations is on the order of a half micrometer in diameter. The density of the current spike increases from the mixed phase to the heavily nanocrystalline regions. A thick amorphous silicon buffer layer inserted between the p and i layers significantly reduced the magnitude of the current spike. The C-AFM measurements suggest that the mixed-phase cells can be considered as a two-phase parallel-connected diode structure, consistent with our previously proposed model.