Tuning the Electronic and Optical Properties of Cu2ZnSn1−xGexS4 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 Cu₂ZnSn_1−xGe_xS₄ using density functional theory combined with hybrid functional calculations. Alloying Cu₂ZnSnS₄ 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 Cu₂ZnSn_1−xGe_xS₄ 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 Cu₂ZnSn_1−xGe_xS₄ solar cells, highlighting the potential of Ge alloying for enhancing PV efficiency.