The effect of a post-activation annealing treatment on thin film CdTe device performance

The cadmium chloride activation treatment of cadmium telluride solar cells is essential for producing high efficiency devices. The treatment has many effects but the most significant is the complete removal of stacking faults in the cadmium telluride grains and the diffusion of Chlorine along the grain boundaries of the device. Chlorine decorates all cadmium telluride and cadmium sulphide grain boundaries and also builds up along the CdTe/CdS junction. . This paper reveals that by annealing devices to temperatures of 400ºC to 480 ºC for times ranging from 30 to 600 seconds in moderate vacuum results in the re-appearance of stacking faults and the removal of Choline from the grain boundaries. STEM analysis confirms the re-appearance of the stacking faults and SIMS and EDX confirm the removal of chlorine from the grain boundaries. This directly corresponds to a lowering in cell efficiency. The study provides further evidence that CdCl2 diffusion and certain microstructural defects directly affect the performance of cadmium telluride photovoltaic devices.