Lead‐Free MaSnI ₃ /Sb ₂ S ₃ Heterojunction Solar Cell with Power Conversion Efficiency Approaching 30%: A SCAPS‐1D Simulation Study

The pursuit of efficient and environmentally sustainable photovoltaic technologies has intensified interest in lead‐free perovskite solar cells (PSCs). This study presents a comprehensive theoretical study on a novel heterojunction device architecture integrating two active, nontoxic absorber layers: methylammonium tin iodide (MASnI 3 ) and antimony trisulfide (Sb 2 S 3 ). Using the SCAPS‐1D simulator under AM1.5G illumination, we systematically investigate the impact of critical device parameters, including layer thickness, doping concentrations, defect densities, and series resistance, on the device's optoelectronic performance. The optimized architecture, FTO/TiO 2 /MASnI 3 /Sb 2 S 3 /Spiro‐OMeTAD/back‐electrode, achieves a remarkable power conversion efficiency (PCE) of 30.84%, with a photocurrent density ( J SC ) of 29.08 mA/cm 2 , an open‐circuit voltage ( V OC ) of 1.22 V, and a fill factor (FF) of 87.15%. The inclusion of a 200 nm Sb 2 S 3 layer not only broadens the absorption spectrum, especially in the longer wavelength region, but also enhances charge extraction through favorable band alignment. Additionally, the role of different low‐cost hole transport layers was assessed (MoS 2 , Cu 2 O x , NiO x , and MoO x ), outperforming the conventional materials like Spiro‐OMeTAD in terms of PCE and FF. Our results demonstrate that precise control of absorber's thickness, suppression of interfacial defects, and reduction of series resistance are key elements to achieving high‐efficiency lead‐free PSCs. Further analysis reveals that reducing interfacial defects and series resistance is crucial for maximizing efficiency while maintaining low defect density in the absorber layer is vital for device reliability.