Room‐Temperature Partial Conversion of α‐FAPbI 3 Perovskite Phase via PbI 2 Solvation Enables High‐Performance Solar Cells

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI 3 hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI 3 , is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI 3 , requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI 2 into the α‐FAPbI 3 perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI 2 precursor from its different states are compared. α‐FAPbI 3 forms spontaneously and efficiently at room temperature from P 2 (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI 2 ) to 17.23(±0.28)% (from solvated PbI 2 ) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA + cation without the need for Cs + or mixed halide formulation.