Material analysis of PVD-grown indium sulphide buffer layers for Cu(In,Ga)Se2-based solar cells

This paper is devoted to an X-ray photoelectron spectroscopy (XPS) study of Cu(In,Ga)Se2 (CIGSe)/In2S3 structures. The indium sulphide layers are grown by physical vapor deposition (PVD) in which indium and sulfur are evaporated on the substrates at a temperature Ts. This as-deposited thin films are then heated at 200 °C for 1 min. A 12.4% efficiency champion cell has been achieved using this process. The XPS study reveals that copper diffuses from the chalcopyrite absorber towards the indium sulphide layer during this synthesis process. The amount of copper strongly depends on Ts; the higher Ts, the more copper is diffused. This observation is then correlated with the solar cell performance to conclude that a significant Cu-diffusion inhibits the formation of a high-quality junction between the Cu(In,Ga)Se2 and the buffer layer.