Air-processed high-quality additive-free FAPbI₃ enabling efficient and reproducible perovskite solar cells

The optoelectronic properties and crystallization behavior of formamidinium lead iodide (FAPbI₃) perovskites are highly sensitive to processing conditions, particularly moisture during annealing. Although trace humidity can promote the conversion of intermediate phases to the photoactive α-FAPbI₃ phase and suppress δ-FAPbI₃ impurities, most reported strategies rely on additives and antisolvent engineering to regulate crystallization. In this context, the fabrication of high-quality FAPbI₃ films without additives and antisolvent treatment remains rarely reported due to the difficulty in controlling phase purity and film morphology. Here, we report a humidity-mediated ambient-air annealing strategy to fabricate neat FAPbI₃ films via a two-step sequential deposition method. This approach enables the formation of high-phase-purity, highly crystalline, large-grained, and defect-minimized perovskite layers without additives, antisolvents, or strict atmospheric control. By systematically tuning the relative humidity during annealing, we found that 33% relative humidity (RH) yields optimal film quality compared with films annealed at 39% and 54% RH or processed inside a nitrogen glovebox (O₂ < 14 ppm, H₂O < 0.1 ppm). Consequently, n-i-p-structured PSCs achieve a power conversion efficiency of 19.51%, outperforming glovebox-processed devices (16.53%), while also exhibiting reduced hysteresis and improved operational stability.