Improving the performance of kesterite solar cells by solution germanium alloying

Cation substitution is an effective strategy to regulate the defects/electronic properties of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) absorbers and improve the device photovoltaic performance. Here, we report Ge alloying kesterite Cu₂Zn(Sn,Ge)(S,Se)₄ (CZTGSSe) via a solution approach. The results demonstrate that the same chemical reaction of Ge⁴⁺ to Sn⁴⁺ ensures homogeneous Ge incorporation in the whole range of concentrations (from 0 to unit). Ge alloying promotes grain growth and linearly enlarges the absorber band gap by solely raising the conduction band minimum, which maintains a "spike" conduction band offset at the heterojunction interface until 15% alloying concentration and thus facilitates effective charge carrier collection. A promising efficiency of 11.57% has been achieved at 15% Ge alloying concentration with a significant enhancement in open-circuit voltage and fill factor. By further 10% Ag alloying to improve the absorber film morphology, a champion device with an efficiency of 12.25% has been achieved without an antireflective coating. This result emphasizes the feasibility of achieving homogeneous and controllable Ge alloying of kesterite semiconductors through the solution method, paving the way for further improvement and optimization of device performance.