Electronically active layers and interfaces in polycrystalline devices: Cross-section mapping of CdS/CdTe solar cells

Electronic mapping of cross sections of a polycrystalline device, the $\mathrm{n-}\mathrm{CdS}\mathrm{/p-}\mathrm{CdTe}$ solar cell, show that the photovoltaic and metallurgical junctions coincide to within experimental resolution (50 nm), which rules out both type conversion of CdS and buried homojunctions. Compositional analysis of the CdS supports this. Mapping was done using scanning capacitance, complemented by scanning Kelvin probe microscopy. Our results explain why a high-resistance transparent conducting oxide layer is needed as contact to the CdS for successful device operation. They define limits on inputs for modeling performance of these devices.