Ageing of CdTe Devices by Copper Diffusion

CdTe solar cells are widely used in industrial production and they currently have the lowest cost per Watt available in the market thanks to its simple and scalable technology. One of the main engineering challenges for these devices is to provide a suitable back contact for CdTe due to its high electron affinity which requires a material with a very high work function. Solar cells with copper based back contacts have shown the highest efficiencies, however it is well known that their performance reduces with time, mainly connected with diffusion of Cu through the absorber. Working conditions (i.e. light intensity, temperature and applied bias) can dramatically affect the degradation speed. In order to study the impact of the bias on the Cu diffusion, then on performance degradation, three different kinds of stress were applied on identical CdTe solar cells with Cu/Au back contact (Dark without bias, light-temperature without bias, light temperature with bias). The degraded devices have been periodically analyzed by currentvoltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy. A detailed analysis of defect characterization and distribution has been addressed.