Origins of Contrast in Near-Field Scanning Optical Microscopy Photovoltage Images of Cu(In,Ga)Se2 Solar Cells

ABSTRACTPhotovoltage images of Cu(In,Ga)Se2 solar cells taken using near-field scanning optical microscopy (NSOM) show reduction of photoresponse along grain boundaries. Two factors might be responsible for this contrast. The first is that the topographic changes associated with the grains could cause the NSOM tip to move further from the sample, resulting in a reduction in photoresponse. The second possibility is that the contrast is due to real physical properties of the sample, e.g. higher recombination velocities near grain boundaries. We show convincing evidence that the major contribution to the contrast in the photovoltage images is due to the latter.