The underlying synergistic mechanism of co-solvents to fabricate high-quality FAPbI3 perovskite films

Abstract

The quality of perovskite layers is a critical factor in determining the performance of perovskite solar cells (PSCs). α-FAPbI₃ is a promising absorber material, but fabricating high quality metastable α-FAPbI₃ thin films remains challenging due to poor control over intermediate phases during deposition. Although co-solvent strategies have been identified as an effective method to control these intermediate phases, the underlying mechanism is not fully understood. In this study, we systematically investigate the synergistic effects of DMSO and NMP co-solvents on perovskite mesophase formation, combining theoretical and experimental insights into intermediate phase control and α-FAPbI₃ crystallization. Using in situ photoluminescence (PL) monitoring during spin-coating, together with density functional theory (DFT) calculations and Fourier-transform infrared (FTIR) spectroscopy, we reveal how solvent competition modulates intermediate complex formation and evolution. The synergistic effect of these solvents was finally disclosed, which effectively governs the formation and evolution of the intermediate phases. The optimized co-solvent system promotes the transformation of intermediates into α-FAPbI₃ at lower thermodynamic transition temperatures, yielding high phase purity and low defect density. As a result, PSCs fabricated using this approach achieved a power conversion efficiency (PCE) of 22.80% with improved stability. The insights gained from this work provide a rational framework for solvent selection in the fabrication of high-quality perovskite films for advanced photovoltaics.