Interfacial ferroelectricity unlocks stable formamidinium-based perovskites.

Pure iodide formamidinium (FA) based-perovskite has emerged as highly promising candidates for perovskite photovoltaics, but it remains challenging to achieve long-term phase-stabilized FA-based perovskites. Herein, we present a physics-driven strategy of interfacial ferroelectricity, achieved by the integration of ferroelectric CsMnBr3 nanocrystals (NCs) into FA-based perovskites. The ferroelectric field generated by these NCs promotes FA+ cation ordering, modulates Pb-I framework, and enhances the structural regulation of the perovskite lattice. This synergistically increases the kinetic barrier for the undesired Pb-I octahedral transformation and raises the energy barrier for ion migration. The resulting perovskite materials exhibit high structure stability, enabling perovskite solar cell (PSC) minimodule to retain 99% of its initial efficiency after 1000 hours' stability testing under 85% relative humidity at 85 °C. Owing to the improvement at the interface, the PSCs yield an efficiency of 26.62% (certified 26.40%), and the minimodules reach 24.67% (certified 23.23%). This work presents an effective approach to achieving high-performance, long-term stable perovskite optoelectronic devices through interfacial ferroelectric engineering.