Performance enhancement of CZTS solar cells via Cu2O HTL and Cd0.4Zn0.6S buffer layer: a numerical study

Copper-zinc tin sulfide (Cu 2 ZnSnS 4 or CZTS), a quaternary semiconductor with superior optoelectronic properties, has emerged as a promising absorber material for thin-film photovoltaic applications. In this study, a high-efficiency CZTS solar cell was numerically investigated using SCAPS-1D, emphasizing the roles of Cu 2 O as a hole transport layer (HTL) and Cd 0.4 Zn 0.6 S as a tunable buffer layer. The simulated device structure—ITO/AZO/Cd 0.4 Zn 0.6 S /CZTS/Cu 2 O/Mo—was systematically optimized by varying the thickness and bandgap of each functional layer. The best performance was achieved with a 0.01 µm-thick Cu 2 O HTL (2.2 eV bandgap) and a 0.01 µm-thick Cd 0.4 Zn 0.6 S buffer (2.98 eV bandgap), enabling efficient hole extraction, high transparency, and suppressed interfacial recombination. Under these optimized conditions, the device exhibited an open-circuit voltage (V oc ) of 0.867 V, short-circuit current density (J sc ) of 43.424 mA/cm 2 , Fill Factor (FF) of 82.23 %, and a remarkable power conversion efficiency (PCE) of 31.18 %. Additionally, the influence of operating temperature on solar cell performance was evaluated in the range of 280 K to 360 K. The results revealed a notable reduction in the PCE, decreasing from 32.19 % to 27.57 %. This degradation in efficiency is primarily attributed to the temperature-induced increase in reverse saturation current, which adversely impacts the V oc and overall device performance. The interface defect studies revealed severe performance degradation, with efficiency reduced to 21.75 % at a defect density of 1 × 10 20 cm −3 at the CZTS/Cd 0.4 Zn 0.6 S junction. These findings underscore the critical importance of HTL/buffer layer engineering, interface defect control, and thermal management in advancing high-efficiency, thermally stable, and environmentally benign CZTS-based solar cells.