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

Abstract

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 ∼90% of its initial efficiency after 500 hours of maximum power point loading under constant light irradiation at ∼55 °C in an N₂ 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.