Fluorinated Phenethylammonium and Pre-Annealing Static Stage Enable Intermediate-Mediated Stepwise Crystallization in...

Tin-based perovskite solar cells (Sn-PSCs) are fundamentally limited by rapid crystallization, which induces defective morphologies and severe Sn2+ oxidation. Here, we report a combination of 4-fluorophenethylammonium (FPEA) and a pre-annealing static stage (PASS) for Sn-PSCs. While excessive FPEA generates a significant trade-off between grain growth and film compactness, our systematic optimization reveals that an optimal low concentration of FPEA combined with PASS simultaneously mitigates this trade-off. In situ optical spectroscopy reveals an intermediate-mediated stepwise crystallization process, in which a gradual and partial transition from the intermediate to the perovskite phase occurs during PASS, accompanied by effective crystal growth. Subsequent thermal annealing promotes grain reconstruction and healing together with completion of the phase transition. X-ray photoelectron spectroscopy confirms that the electron-withdrawing fluorine atoms modulate the [SnI6]4- electronic landscape and dramatically suppress Sn2+ oxidation. Consequently, the optimized FPEA0.04FA0.96SnI3 films exhibit improved crystalline quality, enlarged grains, and reduced energetic disorder (FA: formamidinium). Their devices deliver a power conversion efficiency of 9.29% (reference: 7.99%) with markedly improved operational stability. This work establishes a clear mechanistic framework and provides a robust design principle for high-performance tin-based perovskites.