Electrical Properties of CdTe/CdS and CdTe/SnO2 Solar Cells Studied with Scanning Kelvin Probe Microscopy

We measured the distribution of the electric potential and electric field on cross sections of working CdTe/CdS solar cells using SKPM. We analyzed the cells biased at different voltages (reverse and forward bias) and were able to identify different regions inside the devices. On standard cells, the main potential drop was around the CdTe/CdS junction and there was some drop inside the CdTe. As we increased the reverse bias, the region of the CdTe with the potential drop became wider, showing the expansion of the depletion region. The maximum of the electric field, which locates the p-n junction, occurred at the apparent CdTe/CdS interface. However, there was a sharp peak in the electric field at this location, which was correlated to an interdiffusion layer between CdTe and CdS. Results for cells without the CdS film were similar, but the strong electric field at the junction was not present. In this case, the maximum of the electric field was located at the CdTe/SnO₂ junction.