CdTe thin film solar cells with copper iodide as a back contact buffer layer

•A CuI buffer layer not only assists hole transport but also acts as an electron reflector at back contact.•The band alignment at the CuI/CdTe interface demonstrates that CuI has a well-matched band alignment with CdTe.•The performance of the CdTe cells with a CuI buffer layer are improved.•The application of a CuI buffer layer in the back contact improves CdTe solar cell device stability.