Modulation of nucleation and crystallization in PbI2 films promoting preferential perovskite orientation growth for efficient solar cells

Formamidinium (FA)-rich lead iodide perovskite solar cells (PSCs) are gaining popularity because of their excellent photovoltaic performance. Nevertheless, the FA-rich perovskites processed by a two-step sequential deposition method usually possess many non-radiative recombination sites, undesired crystal orientations, and too much excess lead iodide (PbI2) that can seriously deteriorate device performance. Here, we modulate the nucleation and crystallization of the PbI2 films by introducing pentafluoroanilinium trifluoromethanesulfonate (PFAT) into PbI2 precursor solutions. Such modulatory PbI2 films as porous templates allow the growth of perovskites with a preferential orientation, enlarged grains, reduced defects, and decreased PbI2 residues, owing to the reduced Gibbs free energy of PbI2 and the acceleration of perovskite formation. Ultimately, we derive FA-rich lead iodide PSCs with an impressive power conversion efficiency of 24.52%, which is among the highest values of two-step sequential deposition-processed PSCs. Furthermore, the unencapsulated device maintains similar to 90% of its initial efficiency after 500 hours of maximum power point loading under constant light irradiation at similar to 55 degrees C in an N-2 atmosphere. This study presents an effective way to improve the crystalline quality of perovskites by synchronously pre-modulating the morphology, nucleation, and crystallization of PbI2 precursor films.