Chemical Modulation of α-FAPbI3 Perovskite Solar Cells: The Dual Substitution Role of CsSCN for Enhanced Stability and Efficiency

Formamidinium lead triiodide (FAPbI3) is a highly promising semiconductor for efficient and thermally stable perovskite solar cells (PSCs). However, its photoactive black α-phase often transitions to the photoinactive yellow δ-phase below 150 °C, degrading photovoltaic performance. Here, we introduce cesium thiocyanate (CsSCN) as a dual-functional additive into the perovskite precursor solution to stabilize the α-phase, enhance phase purity, and improve device efficiency. X-ray diffraction (XRD) confirms CsSCN promotes pure α-phase formation, while molecular dynamics (MD) simulations reveal that co-doping with Cs+ and SCN− ions yields a more stable lattice structure. CsSCN-modified PSCs achieve a peak power conversion efficiency (PCE) of 22.91 % and retain over 90 % of their initial performance after 1000 h under continuous illumination. These improvements stem from larger grain sizes (up to 1.5 μm), enhanced film uniformity, and reduced defects, which collectively improve charge transport. By addressing phase instability and degradation, CsSCN offers a robust strategy to advance the efficiency and durability of perovskite solar cells, paving the way for their commercial viability.