Semitransparent Wide Bandgap Cu2ZnGe(S,Se)4 Thin‐Film Solar Cells: Role of the Sulfurization Process

Abstract

Semitransparent solar cells are very attractive due to the increasing integration in daily life. Kesterite‐type based thin‐film solar cells stand out because of its environmentally benign composition and outstanding stability. Herein, the influence of the back contact (Mo/V2O5/FTO or Mo/FTO) and thickness of Cu2ZnGe(S,Se)4 (CZGSSe) absorber layer, grown by sulfurization of coevaporated CZGSe, is investigated. To increase the transparency, thinner absorber layers with higher bandgap energy are produced. A double sulfur gradient through the CZGSSe layer with a considerable S content near the back contact and the formation of Mo(S,Se)2 phase at the back interface is detected for an absorber of only 400 nm thickness. Efficiencies of 3.1% and 2.7% are achieved for 1.2 μm CZGSSe‐based devices with Eg of 1.73 and 1.86 eV, respectively, while enabling transmittance values higher than 20% in the near‐infrared (NIR). The highest transmittance, 40% in the NIR, is achieved for the 400 nm CZGSSe‐based solar cells with Eg of 2.1 eV; however, a significant reduction of these devices’ performance is obtained due to the presence of ZnS secondary phase and a different back‐contact interface formation. This work presents the first promising semitransparent CZGSSe solar cells, opening new paths of applications.