How the amount of copper influences the formation and stability of defects in CdTe solar cells

With a 22.1% efficiency record and the successful results in terms of production yield, CdTe based thin film solar cells are today a competing technology with traditional silicon solar cells. Despite different copper-free back contacts have been applied, Cu is present in all the most performing CdTe devices. On the other hand, it is well known that Cu is a fast diffuser in CdTe, and it heavily influences the devices degradation; thus controlling its concentration is very important. In this paper a study of the influence of copper quantity on the performance of the devices and stability at the back contact is presented. CdTe cells fabricated with a 0.1 nm thick Cu layer are compared to devices fabricated with 2.0, 1.0 and 0.5 nm thick Cu layers. The amount of copper affects the performance and aging of the samples. Moreover an inversion of the bias dependency (solar cells in open circuit or in short circuit under current flow), during the aging, occurs in samples containing a copper layer below a certain thickness, suggesting that another degradation mechanism predominates.