Engineering CZTS thin film properties via Ag and Ge co-doping

Ag–Ge co-doping in Cu 2 ZnSnS 4 (CZTS)-based optoelectronic devices is important because it enables effective tuning of the material’s structural, electronic, and optical properties, leading to improved crystalline quality, controlled defect states, and enhanced overall device performance. This study investigates the impact of Ag–Ge co-doping on CZTS thin films deposited by sputtering and sulfurized under controlled conditions. Structural characterization by XRD and Raman spectroscopy confirmed kesterite phase formation for all samples, with 20% Ge doping inducing a minor GeS 2 secondary phase. SEM analysis revealed that co-doping enhanced grain growth, increasing the average grain size to ~ 0.68 μm. Optical studies showed a bandgap widening from ~ 1.4 to ~ 1.5 eV and a photoluminescence band shift from ~ 1.32 to ~ 1.36 eV, consistent with reduced band tailing and lower defect densities. Hall measurements indicated that while Ag doping decreased the carrier concentration from ~ 10²⁰ 10 20 to ~ 10¹⁷ 10 17 cm⁻³ cm - 3 , Ge doping slightly increased it to ~ 10¹⁸ 10 18 cm⁻³ cm - 3 , reflecting modifications in defect chemistry. Devices fabricated from co-doped films exhibited improved performance, with efficiencies rising from 2.91% for the reference CZTS to 5.29% for the ACZTS-Ge10 sample. Higher Ge content (20%) led to a decrease in efficiency (4.34%) due to secondary phase formation and increased series resistance. These findings demonstrate that Ag–Ge co-doping effectively tunes the structural, optical, and electronic properties of CZTS films, offering a promising approach to enhance the performance of CZTS-based solar cells.