Cu2ZnSnSe4-Based Solar Cells With Efficiency Exceeding 10% by Adding a Superficial Ge Nanolayer: The Interaction Between Ge and Na

Abstract

Recently, beneficial effects of the incorporation of small amounts of Ge into Cu2ZnSnSe4 (CZTSe)-based solar cells have been reported, showing that the presence of Ge can enhance the crystalline properties of CZTSe, assisting the grain growth, leading to high-efficiency devices. In this study, we prepare CZTSe layers by a sequential process consisting of the sputtering of metallic stacks followed by a reactive annealing under Se atmosphere, previously adding different Ge nanolayers on top (from 0 to 50 nm). The present work is focused on the study of the interaction between germanium and sodium. As is widely known, Na is a very important dopant in kesterite, which plays an essential role in the doping level control. We demonstrate that during the annealing process, a Ge–Se liquid phase is formed which dissolves preferably Na-related phases modifying the content of this last element in the CZTSe absorber and impacting notably on the electrical properties of the layers and, concomitantly, on the performance of the devices. We support our Ge–Na interaction model with experiments using Na-free substrates, showing the importance of accurately controlling the Na content when Ge is used to increase the efficiency of CZTSe-based solar cells.