Experimental and theoretical insights into the incorporation of MACl in FAPbI3 perovskite films

Abstract

In this study, we investigate how methylammonium chloride (MACl) affects the formation and optoelectronic properties of FAPbI3 perovskite films. The perovskite precursor molarity was systematically varied (1.46-1.80 M) while keeping the MACl additive level constant at 20 mol% with respect to FAPbI3, and a chloride-rich condition (1.2 M with 50 mol% MACl) was additionally examined. Using SEM, XRD, FTIR, and steady-state/time-resolved photoluminescence together with density functional theory (DFT), we establish a direct correlation between MACl-driven crystallization, facet-selective growth, and the resulting structural and optoelectronic properties. XRD reveals enhanced alpha-phase purity and strengthened (001) preferential orientation upon MACl addition, without any significant or systematic peak shift beyond minor lattice variations. EDS confirms residual chlorine retention after annealing. DFT rationalizes these findings through a dual-ion compensation mechanism, in which MA+-induced lattice contraction and Cl--induced expansion nearly cancel, preserving the lattice framework while stabilizing the alpha-FAPbI3 phase. These results provide mechanistic insight into MACl-assisted stabilization and clarify the structural origin of performance improvements in FA-based perovskite solar cells.