Nanometer-scale study of resistance on CdTe solar cell devices

We report a nanometer-resolution resistance mapping across the junction of a CdTe solar cell by using scanning spreading resistance microscopy. The multiple device layers were identified by the resistance mapping. The nonuniform resistance in the CdTe layer reflects the nonuniformity of the doping in the CdTe. A high-resistance region close to the junction in the CdTe side as a result of carrier depletion was measured. With a forward bias voltage applied to the device, we observed the decrease of the resistance in the depletion region and the movement of the depletion region toward the CdS/CdTe interface as a result of the carrier injection. We compared the resistance of the device in the dark to that of the device in illumination. With the atomic force microscope laser illumination, the resistance in the deep depletion region decreased and the resistance across the entire device became relatively uniform. The results illustrate that under illumination, photo-excited carriers dominate the device over the carriers in the thermoequilibrium state and the carriers injected by the bias voltage to the device.