Alicyclic Lewis Bases Enable 100 °C-Processed α-FAPbI3 Inverted Perovskite Solar Cells

Formamidinium lead iodide (α-FAPbI3) perovskite, despite its ideal bandgap, requires high-temperature (≥150 °C) annealing for the δ-to-α phase transformation, hindering its application in low-temperature processed inverted perovskite solar cells (PSCs), particularly on flexible substrates. Herein, we demonstrate that the alicyclic Lewis base caprolactam (CPL) effectively drives the complete δ-to-α phase transformation of FAPbI3 at a reduced temperature of 100 °C. Experiments and theoretical calculations reveal that CPL lowers the phase transition energy barrier and retards crystallization kinetics via strong Pb2+ coordination and controlled desorption. This strategy yields highly crystalline, fully phase-converted α-FAPbI3 films with improved morphology, reduced defect densities, and enhanced optoelectronic properties. Consequently, CPL-modified inverted PSCs achieve a champion power conversion efficiency (PCE) of 25.64% with good operational stability. The low-temperature processability also enables high-performance flexible PSCs with a PCE of 23.73%. This work presents an additive engineering approach for realizing efficient and stable α-FAPbI3 PSCs through low-temperature manufacturing.