A Solid–Solid Reaction Growth Strategy for Organic–Inorganic Perovskite Films Based on Superoversaturated Intermediate Phase

Vapor deposition technology is a core candidate for perovskite solar cell (PSC) industrialization due to its superior repeatability and scalability. However, diffusion limitations of the top-down vapor–solid (VS) reaction induce critical issues, such as nonstoichiometric films, precursor residue, and uneven crystallization, which severely impede device performance enhancement. Herein, we innovatively propose a bottom-up solid–solid (SS) reaction strategy based on a superoversaturated intermediate phase (SOIP). By embedding formamidinium iodide (FAI)-rich SOIP as the amine salt donor in the bottom layer, the SS reaction between SOIP and upper PbI2 effectively eliminates PbI2 residue in conventional VS-derived films. The “inside-out” crystallization mode optimizes film composition uniformity, crystallization quality, and buried interface contact, while reducing trap density and suppressing nonradiative recombination. The resultant p-i-n devices deliver champion power conversion efficiencies (PCEs) of 20.69% for 0.148 cm2 and 19.70% for 1 cm2, with excellent uniformity and scalability on 10 × 10 cm2 substrates. Unencapsulated devices retain 89.9% of initial efficiency after 1350 h aging in dark conditions (20 ± 5 °C, 20 ± 5% relative humidity). This work provides a technical pathway for high-quality large-area perovskite films and facilitates PSC industrialization.