Semi-transparent wide band gap Cu2ZnGe(S,Se)4 thin-film solar cells: Role of the sulfurization process

Semi-transparent solar cells are very attractive due to the increasing integration into our daily life. Kesterite-type based thin-film solar cells stand out because of its environmentally-benign composition and outstanding stability. Here, 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 co-evaporated CZGSe, is investigated. To increase the transparency, thinner absorber layers with higher band gap 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 E _g of 1.73 and 1.86 eV respectively, while enabling transmittance values higher than 20 % in the NIR. The highest transmittance, 40 % in the NIR, is achieved for the 400 nm CZGSSe-based solar cells with E _g 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 semi-transparent CZGSSe solar cells, opening new paths of applications.