Distribution of Local Open-Circuit Voltage on Amorphous and Nanocrystalline Mixed-Phase Si:H and SiGe:H Solar Cells

Local open-circuit voltage (V_oc) distributions on amorphous and nanocrystalline mixed-phase silicon solar cells were measured using a scanning Kelvin probe microscope (SKPM) on the p layer of an n-i-p structure without the top ITO contact. During the measurement, the sample was illuminated with a laser beam that was used for the atomic force microscopy (AFM). Therefore, the surface potential measured by SKPM is the sum of the local V_oc and the difference in workfunction between the p layer and the AFM tip. Comparing the SKPM and AFM images, we find that nanocrystallites aggregate in the amorphous matrix with an aggregation size of ∼0.5 μm in diameter, where many nanometer-size grains are clustered. The V_oc distribution shows valleys in the nanocrystalline aggregation area. The transition from low to high V_oc regions is a gradual change within a distance of about 1 μm. The minimum V_oc value in the nanocrystalline clusters in the mixed-phase region is larger than the V_oc of a nc-Si:H single-phase solar cell. These results could be due to lateral photo-charge redistribution between the two phases. We have also carried out local V_oc measurements on mixed-phase SiGe:H alloy solar cells. The magnitudes of V _oc in the amorphous and nanocrystalline regions are consistent with the J-V measurements.