Formamidinium-Solvent Intermediates Mediate Self-Seeding Crystallization for Efficient Inverted α-FAPbI3 Perovskite Solar Cells

Abstract Formamidinium lead iodide (FAPbI3) perovskites are promising for high-performance solar cells; however, the low surface energy of self-assembled monolayers (SAMs) impedes α-phase nucleation in inverted devices. Here, we report a formamidinium (FA)-solvent intermediate-mediated self-seeding crystallization strategy to enable quality α-phase FAPbI3. By employing N-formylmorpholine (NFM) as a non-toxic solvent, (FA···NFM)PbI3 intermediates are formed, thereby lowering the phase-transition barrier and promoting the spontaneous formation of α-phase nuclei at the early stage of crystallization. These preformed nuclei serve as intrinsic seeds, facilitating rapid nucleation and orientated crystal growth. Consequently, NFM-based devices achieve efficiencies of 26.41% and 26.56% using 2-methoxyethanol (2-Me) and N,N-dimethylformamide (DMF) as primary solvents, respectively, with a 1 cm2 device reaching 25.47%. The unencapsulated devices retain 85% and 90% of their initial efficiencies after 1200 h illumination and 1400 h thermal aging at 85 °C, respectively.