Open questions after 20 years of CuInS2 research

In the early 1990s, CuInS2 thin film solar cells with >10% efficiency had been developed. Since then, they are limited by an open-circuit voltage which is too low for the band gap of CuInS2. Recombination at the CdS/CuInS2 interface was made responsible for this shortcoming. This was concluded from two experimental results: a conduction band cliff found at the buffer/absorber interface and activation energy of the saturation current being smaller than the absorber band gap energy. However, replacing the CdS buffer layer in the Mo/CuInS2/buffer/ZnO heterostructure solar cell with wide gap buffers did not lead to substantially higher Voc. Also the activation energy was unaltered. In this paper, we discuss interface and bulk aspects of CuInS2 and Cu(In,Ga)S2 cells, try to give a consistent picture and make suggestions for novel experiments. Copyright (c) 2012 John Wiley & Sons, Ltd.