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

Direct and stable alpha-phase formation of formamidinium lead triiodide (FAPbI(3)) perovskite serves as an indispensable prerequisite for constructing stable and efficient perovskite solar cells. However, the alpha-phase formation is highly limited by molecular additives. Here, we report a strategy for directly fabricating stable alpha-phase of FAPbI(3) 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 center dot center dot center dot 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.

Automated summary

This study reports a strategy for directly fabricating stable alpha-phase of FAPbI(3) perovskite by controlling the precursor solution on the molecular level using methylammonium acetate (MAAc) ionic liquid. The results show that the strong interaction between Ac- and Pb2+ enables the relaxation of Pb-I bonds and allows the effective insertion of formamidinium iodide (FAI) into the perovskite framework. The resulting photovoltaic cell exhibits high efficiency and long-term stability.