Design Rules for Quantitative Control of Intragrain Planar Defects in Halide Perovskites

ABSTRACT Intragrain planar defects limit carrier transport and operational stability in halide perovskite photovoltaics, yet quantitative rules to control their formation are still lacking. Here defect‐specific design rules are established for two dominant planar defects—{112} t ferroelastic twins and {111} c twins/stacking faults—in MA 1‐x Gua x PbI 3 thin films (MA = methylammonium, Gua = guanidinium) by rigorously dose‐budgeted low‐dose transmission electron microscopy. Independent variation of A‐site composition and annealing conditions decouples the roles of tetragonality ( t ; pseudo‐cubic c / a ), grain size, and crystallization kinetics. {112} t twin density increases with tetragonality and grain size but is insensitive to crystallization kinetics, whereas {111} c defects emerge at reduced tetragonality and remain grain‐size independent. These correlations define a processing window ( t ≈ 1.005–1.010 for submicron grains) that simultaneously suppresses both defect types. Solar cells fabricated within this window exhibit longer carrier lifetimes and higher power‐conversion efficiencies, supporting the importance of intragrain planar‐defect control.