Tuning the Electronic and Optical Properties of Cu₂ZnSn_1−<i>x</i>Ge_<i>x</i>S₄ Alloys for Photovoltaic Applications: A Hybrid Density Functional Theory and Device Simulation Approach

ABSTRACT In this study, we explore the electronic and optical properties of Cu 2 ZnSn 1− x Ge x S 4 using density functional theory combined with hybrid functional calculations. Alloying Cu 2 ZnSnS 4 with Ge and the formation of a band gap gradient are investigated as strategies to improve the efficiency of single‐junction photovoltaic (PV) devices and as top cells in tandem solar cells. Our findings reveal that increasing Ge concentration leads to a rise in the band gap, with a small bowing constant ( b ≈ 0.02 eV) indicating good miscibility of Ge in the host lattice. The electronic properties suggest that lower Ge incorporation may be optimal for PV applications. Additionally, device simulations were conducted to evaluate the impact of Cu 2 ZnSn 1− x Ge x S 4 layer thickness on device performance, with and without a back surface field. The integration of first‐principles calculations with SCAPS‐1D simulations offers a comprehensive framework for predicting the performance of Cu 2 ZnSn 1− x Ge x S 4 solar cells, highlighting the potential of Ge alloying for enhancing PV efficiency.