In this study, a theoretical model is used to investigate the factors influencing the decreased efficiency of CZTS (copper zinc tin sulfide) solar cells. The investigation focuses on how device performance is impacted by various loss processes, including CZTS bulk defects, CdS/CZTS interface defects, temperature effects, and series and shunt resistance. The proposed model takes into account various recombination processes, including tunneling-enhanced recombination, Auger recombination, Shockley-Read-Hall (SRH) recombination, CZTS/CdS interface recombination, and non-radiative recombination. Significant agreement is attained when this model is tested against experimental results. Furthermore, a thorough understanding of the predominant recombination mechanism in CZTS solar cells is explored by illuminating the relationship between reverse dark current, minority lifetime and defect concentration. Our analysis shows that generating efficiencies below 8% is largely dependent on both bulk and interface recombination mechanisms. To boost the efficiency of the investigated CZTS solar cell, we suggest two structures with Nb2O5 and beta-Ga2O3 as competent electron transport layer (ETL) materials. By using the proposed model as the fitness function for the non-dominated sorting genetic algorithm (NSGA II) technique, we optimize both structures. The NSGA-II optimization suggests that Nb2O5 and beta-Ga2O3 are viable alternatives to CdS, with the optimized devices showing improved efficiency compared to conventional designs. Specifically, the Nb2O5-based device achieves short-circuit current density (JSC) of 20.95 mA/cm2, open-circuit voltage (VOC) of 0.68 V, and a fill factor (FF) of 65.32%. The beta-Ga2O3-based device demonstrates JSC of 20.95 mA/cm2, VOC of 0.81 V, and an FF of 67.86%. These enhancements are attributed to optimal band alignment at the interface, resulting in reduced resistance and improved efficiency. The optimized configurations demonstrate efficiency of 9.32% and 11.5%, respectively, highlighting their potential for advancing kesterite-based solar cells. This innovative approach could potentially lead to higher-efficiency Cd-free CZTS solar cells in the future.
