Optimization of hole transport layers for Cu2FeSnS4 solar cells via SCAPS-1D simulation: Investigating the impact of interface defects on practical efficiency limits

The quaternary Cu₂FeSnS₄ (CFTS) chalcogenide garners significant interest as a sustainable alternative in solar cell applications due to its abundant and non-toxic composition. This study uses SCAPS-1D simulations to examine the performance of CFTS solar cells (ITO/HTL/CFTS (400 nm)/CdS (200 nm)/ZnO (10 nm)/Al) using three distinct hole transport layers (HTLs), namely NiO_x, Cu₂O, and CuI. The simulations led to a deeper understanding of their practical efficiency limits, considering the huge gap in the theoretical and experimental efficiency values reported earlier. The investigations reveal the precise mechanisms and the influence of hole transport layers on the device performance, specifically the bulk and interface defect densities. In addition, the other major aspects of CFTS solar cell performance, including the correlation between electric field, generation rate, and recombination rate are discussed. Our observations suggest that while identifying a suitable hole transport layer, it is imperative to consider these parameters, which are often overlooked in many numerical simulations, resulting in unrealistic theoretical efficiency values in contrast to the low efficiency observed in practical devices. Here, the optimized ITO/CuI/CFTS/CdS/ZnO/Al configuration demonstrated a maximum efficiency of 5.05 %, with a V_oc of 0.55 V, J_sc of 14.5 mA/cm², and FF of 61.8 %, which are in accordance with experimental values reported. Thus, the study here emphasizes the importance of considering the defect densities, electric field, generation rate, and recombination rate to bridge the gap between theoretical and practical efficiency values, which can significantly influence the design strategies to enhance the CFTS solar cell efficiency.