Mitigating sub-bandgap transmission losses through Cu alloying and gradient engineering in CdTe solar cells: A numerical simulation study

In this study, we numerically investigate and optimize (Cd ∗ 1 − x Cu ∗ x )Te solar cells using SCAPS-1D, focusing on performance comparison between ungraded and compositionally graded absorber structures. Cu incorporation at the Cd site is modeled as an effective bandgap tuning mechanism, reducing the bandgap from 1.52 eV (CdTe) to 1.40 eV (5% Cu), thereby extending absorption and improving carrier generation. The effects of absorber thickness (0.1– 2 μ m ) and shallow acceptor density ( 1 0 12 – 1 0 18 cm −3 ) are systematically analyzed. The optimized ungraded device achieves a peak PCE of 28.17%, with V ∗ OC = 1 . 09 V , J ∗ SC = 29.15 mA/cm 2 , and FF = 88.48% under idealized conditions considering only SRH recombination. Linear and parabolic compositional grading profiles are further explored, showing comparable efficiencies with improved carrier collection due to grading-induced internal electric fields. Inclusion of radiative and Auger recombination reduces the efficiency to ∼ 25.6–25.7%, mainly through a reduction in V OC . Defect analysis of the optimized parabolically graded device shows strong sensitivity to bulk defect density, while interface defects have minimal impact. These results demonstrate that controlled Cu incorporation and compositional grading provide an effective strategy for improving CdTe solar cell performance and offer useful guidance for future experimental realization.