Band Alignment Engineering of Cu‐Based Hole Transport Layers for High‐Efficiency CZTS Thin‐Film Solar Cells: A SCAPS‐1D Simulation Analysis

The practical efficiency of Cu 2 ZnSnS 4 (CZTS) thin‐film solar cells remains constrained by unfavorable band alignment, high defect densities, and inefficient charge extraction at interfaces. This study numerically investigates and optimizes CZTS‐based solar cells by evaluating four Cu‐based hole transport layers (HTLs)—Cu 2 O, CuO, CuI, and CuSCN—to minimize recombination losses and enhance photovoltaic performance. The proposed device architecture (ITO/SnS 2 /CZTS/HTL/Pt) was simulated using the SCAPS‐1D platform under standard illumination conditions (AM 1.5G, 1000 W/m 2 , 300 K). Key parameters including layer thickness, doping concentration, defect density, operating temperature, and resistive losses were systematically optimized. The results reveal that Cu 2 O exhibits the most favorable energy band alignment and carrier transport properties, yielding the highest simulated power conversion efficiency (PCE) of 34.35%, with an open‐circuit voltage of 1.192 V, short‐circuit current density of 32.12 mA cm −2 , and fill factor of 89.7%. Maintaining low defect density, moderate absorber thickness (~1.5 μ m), and high shunt resistance proved essential for optimal device stability and performance. These findings highlight the critical role of interface engineering and HTL optimization in CZTS‐based photovoltaics, offering valuable design guidelines for future experimental validation and practical device fabrication.