Integrated Design for High-Efficiency Copper Zinc Tin Sulfide Solar Cells: Harnessing Mg-Alloyed Buffer and Advanced Interface Engineering

The promise of high-efficiency copper zinc tin sulfide or CZTS-based solar cells is hindered by critical challenges such as detrimental defects and problematic interfaces. The conventional ZnS/CZTS junction exhibits a large conduction band offset, leading to a reduced short-circuit current density (JSC), while the open-circuit voltage (VOC) remains consistently low. Additionally, the use of a toxic CdS layer raises serious environmental and disposal concerns. To address these issues and eliminate degradation at the CZTS/Mo interface, this study explores performance enhancement through systematic optimization of material parameters, including defect density, acceptor concentration, interface quality, electron affinity, and alloying ratio. We introduce TiN as a back contact due to its high reflectance and favorable work function and NiO as an effective hole transport layer. Our analysis reveals that defect density within NiO has a minor effect on device efficiency, whereas increasing acceptor concentration significantly improves built-in potential and band bending, enhancing charge transport. However, the CZTS/NiO interface suffers from severe recombination losses due to lattice mismatch. To further improve performance, we investigate Mg-alloyed ZnS as a buffer layer. By tuning the Mg alloying ratio, we effectively tailor its electrical and optical properties, reducing conduction band offset and minimizing interface losses. The interplay between electron affinity and Mg concentration is shown to be critical for optimal band alignment. Through this integrated approach, we achieve a power conversion efficiency of 18.95% at 300 K using an Mg alloying ratio of 0.45. This work marks a significant step toward high-efficiency, environmentally friendly CZTS solar cells, eliminating reliance on toxic Cd and detrimental MoSx contacts.