Stabilizing Intrinsic Phase of Phase-Pure FAPbI3 by Organic Molten-Salt Molecular Bridges

Abstract

Phase-pure formamidinium lead iodide (FAPbI3) is a highly promising absorber for perovskite solar cells (PSCs) but suffers from intrinsic phase instability. Here, we report a buried interface molecular bridge using formamidinium trifluoroacetate (FATFA) to stabilize the phase of phase-pure FAPbI3. The TFA− anion affords bidirectional coordination: the carbonyl oxygen passivates oxygen vacancies in SnO2 while the –CF3 terminus lowers the surface energy of the perovskite (100) facet. Moreover, the octahedral distortion was significantly suppressed by the coordination between TFA− and [PbI6]⁴− octahedra, which strengthens the buried interface and establishes an ordered nucleation template that guides periodic lattice assembly. These coupled effects reconstruct the crystallization pathway, alleviate residual strain, and suppress defect formation. Devices incorporating FATFA achieve a power conversion efficiency of 25.14% and retain 85% of their initial efficiency after 1,000 h of continuous maximum power point operation.