Critical Interface Defect Density Governing Thermodynamic Losses and Photovoltaic Performance in FASnI₃ Perovskite Solar Cells

Abstract

Lead-free formamidinium tin iodide (FASnI₃) perovskite solar cells are promising alternatives to lead-based devices, but their performance is limited by interface defect-assisted recombination and thermal instability. Although SCAPS-1D has been widely used to optimize these devices, the combined effects of operating temperature and interface defect density on both photovoltaic and thermodynamic performance remain insufficiently understood. In this study, SCAPS-1D was employed to investigate the coupled influence of operating temperature (300–400 K) and SnO₂/FASnI₃ interface defect density (10⁸–10¹⁸ cm⁻²) on a Front contact/SnO₂/FASnI₃/CuI/Back contact perovskite solar cell under AM1.5G illumination (100 mW cm⁻²). Conventional photovoltaic parameters (Voc, Jsc, FF, and PCE) were evaluated alongside power loss and a normalized Entropy-Generation Index (EGI) to quantify irreversible energy dissipation. The optimized baseline device achieved a Voc of 1.00 V, Jsc of 28.52 mA cm⁻², FF of 70.81%, and PCE of 20.30%. The device exhibited good thermal tolerance, maintaining efficiencies close to 20% over the investigated temperature range, with only a 3.13% reduction between the maximum efficiency and that at 400 K. In contrast, interface defect density had a much stronger impact on performance. The PCE remained nearly constant below 10¹⁵ cm⁻² but declined rapidly at higher defect densities because of enhanced Shockley–Read–Hall recombination. Corresponding increases in power loss and changes in the normalized EGI identified 10¹⁵ cm⁻² as the critical SnO₂/FASnI₃ interface defect threshold. These findings highlight interface quality as the dominant factor governing the efficiency and thermodynamic stability of lead-free FASnI₃ perovskite solar cells.