Effects of Ultra-Thin Copper Layer on the Performance and Stability of CdTe/CdS Solar Cells

CdTe solar cells have shown high efficiency well above 20%. Despite different copper-free back contacts have been successfully introduced this is still required for very high efficiency devices. On the other hand an excessive amount of copper results in degradation of the device and a suitable concentration is very important for a good trade off between conversion efficiency and stability. A study on how the amount of copper at the back contact affects the performances and the stability of CdTe thin film solar cells is presented. CdTe cells with 0.1 nm Cu layer are compared to devices with 2 nm Cu and the electrical properties are measured. The formation of Cu-Te compounds has been analysed and degradation of cells made with thick and thin Cu layer has been recorded. With different copper thicknesses, different approaches are needed for high efficiencies; moreover devices degrade with different mechanisms.