Graded bandgap design in CZTS solar cells: comparative modeling of gradient architectures for enhanced photovoltaic performance

Abstract Bandgap grading is a promising strategy to suppress recombination losses and enhance carrier collection in kesterite (Cu 2 ZnSnS 4 , CZTS) solar cells. This work presents a detailed SCAPS-1D simulation study comparing front, back, and double grading configurations to optimize device performance. The optimized double-graded (OPT-DG) structure, incorporating high bandgap regions at both the front and rear interfaces, delivers the best performance with the photovoltaic parameters Voc (1.19 V), Jsc (25.10 mA cm −2 ), FF (85.09%) and an efficiency of 25.46%. This configuration enhances the built-in electric field, suppresses interface recombination, and improves charge separation compared to single-graded structures. Spatial analyses of carrier generation and recombination confirm that optimized double-graded effectively minimizes interfacial losses and maintains strong field uniformity across the absorber. Beyond an optimal absorber thickness, efficiency improvements stabilize, highlighting the importance of bandgap tailoring and interface defect control. These findings demonstrate that optimized double bandgap grading is a key design pathway toward achieving high-efficiency, earth-abundant CZTS solar cells.