Direct and stable α-phase formation via ionic liquid solvation for formamidinium-based perovskite solar cells

Direct and stable α-phase formation of formamidinium lead triiodide (FAPbI₃) perovskite serves as an indispensable prerequisite for constructing stable and efficient perovskite solar cells. However, the α-phase formation is highly limited by molecular additives. Here, we report a strategy for directly fabricating stable α-phase of FAPbI₃ perovskite by controlling the precursor solution on the molecular level using methylammonium acetate (MAAc) ionic liquid. We found that a strong interaction between Ac⁻ and Pb²⁺ via C=O···Pb chelation enables the relaxation of Pb–I bonds and thus allows the effective insertion of formamidinium iodide (FAI) into soft Pb–I framework. Molecular dynamics simulations also show that the precursor solution has undergone an exchange of coordination between I⁻ and Ac⁻ to provide the reaction sites for FAI. With a one-step, antisolvent-treatment-free, and air-processing approach regardless of humidity, the corresponding photovoltaic cell exhibits an efficiency close to 24% along with long operational stability over 1,000 h.