Chemically and morphologically distinct grain boundaries in Ge-doped Cu2ZnSnSe4 solar cells revealed with STEM-EELS

Abstract

Critical to the future development of Cu2ZnSnSe4 (CZTSe) materials is a comprehensive understanding of the underlying nanoscale mechanisms responsible for reduced performance. Investigating these mechanisms is challenging since they arise on the nanoscale, yet manifest themselves over macroscopic regions. Here, we present an analytical study combining Scanning Transmission Electron Microscopy (STEM), sample preparation, and hyperspectral Electron Energy Loss Spectroscopy (EELS) mapping techniques to meet this challenge. We apply our method to a Ge-doped CZTSe sample with a measured efficiency of 10.1%, revealing that its microstructure is dominated by two distinct types of grain boundaries. The first type appears in the upper half of the absorber separating large grains. These are Cu-enriched, Se-poor, and have varying amounts of O. The second type of grain boundary is largely parallel to the substrate and appears predominately in the lower half of the absorber where the Cu/Zn ratio of the kesterite material is slightly lower. These grain boundaries contain voids and Sn oxide nanoparticles, exhibit high concentrations of Na, Cd, and S, and Cu assumes a higher valence state. We conclude with a discussion on the nature of and possible technological implications of these grain boundaries in this system.