Determining Composition–Structure–Property Relationships in Quaternary Perovskitoids Through Megalibraries

ABSTRACT Understanding how composition governs structure and function is essential for designing functional materials, however, establishing these relationships remains prohibitively slow for materials with massive and tunable compositional spaces, such as Bi and Sb‐based perovskitoids. Here, a novel megalibrary‐based synthetic methodology has been developed and used to rapidly identify and characterize quaternary perovskite derivatives across an expansive compositional space. Parylene templates were used to synthesize one million compositionally distinct, positionally encoded Cs 3 Sb 2−x Bi x Br 9−y I y microcrystals on 2 × 2 cm 2 chips with 20 µm periodicity. These megalibraries enabled the systematic investigation of structural and optoelectronic evolution across this compositional space using micro‐area x‐ray diffraction, position‐resolved absorbance spectroscopy, scanning electron microscopy, and confocal fluorescence microscopy. An unusual non‐monotonic bandgap shift was observed with increasing Br content, reaching a minimum at 33% Br. This behavior is attributed to the formation of halide‐segregated heterostructures, consisting of low‐dimensional, I‐rich shells surrounding Br‐rich layered cores. Phase separation was most pronounced in Sb‐rich compositions, which were found to stabilize heterostructure formation. This study demonstrates how megalibraries enable the simultaneous evaluation of key optoelectronic parameters, including bandgap and phase purity, revealing complex, composition‐dependent structure‐function relationships in perovskitoids and providing a versatile platform for accelerating the discovery of optoelectronic‐relevant materials.