Surface morphology, electronic defects and passivation strategies at the p-n junction of Cu(In,Ga)(S,Se)2 solar cells

Based on the high power conversion efficiencies and compatibility toward large-area deposition techniques, Cu(In,Ga)(S,Se)2 (CIGSSe) phototvoltaic absorbers are currently at the forefront of chalcopyrite thin-film solar cell technology. The performance of these solar cells is critically dependent on the properties of the interface between the p-type chalcopyrite absorber and the n-type buffer and window layers. Due to the complex defect physics of the chalcopyrites in general, the defect-electronic properties of the absorber surface is of particular concern. In this regard, the CIGSSe surfaces are considerably less understood compared with their S-free counterparts (e.g. Cu(In,Ga)Se2). In the present work, by applying high-resolution scanning probe techniques such as atomic force microscopy and scanning tunneling spectroscopy (STS), combined with electron backscatter diffraction, the morphology, the crystallographic orientation, and the defect electronic properties of CIGSSe thin-film surfaces were investigated. Our work highlights distinct differences as well as similarities between S-containing and S-free chalcopyrite thin films. Three types of features were found on the CIGSSe surface, which were found to be exclusively made of polar facets. This is different from S-free absorbers that are known to facet in both, polar and non-polar planes with distinct electronic properties. Defect density mapping using STS revealed a highly defective surface with significant lateral inhomogeneities. Furthermore, grain boundary band bending detected in S-free absorber surfaces was absent. However, similar to S-free absorbers, annealing under ultra-high vacuum conditions was found to electronically passivate the CIGSSe surface. Our results shed light on the fundamental properties of these S-containing chalcopyrite-type surfaces and demonstrate a valuable platform for further optimization of this promising solar cell technology.