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Abstract The intriguing functionalities of emerging quasi‐2D metal halide perovskites (MHPs) have led to further exploration of this material class for sustainable and scalable optoelectronic applications. However, the chemical complexities in precursors—primarily determined by the 2D:3D compositional ratio—result in uncontrolled phase heterogeneities in these materials, which compromises the optoelectronic performances. Yet, this phenomenon remains poorly understood due to the massive quasi‐2D compositional space. To systematically explore the fundamental principles, herein, a high‐throughput automated synthesis‐characterization workflow is designed and implemented to formamidinium (FA)‐based quasi‐2D MHP system. It is revealed that the stable 3D‐like phases, where the α‐FAPbI₃surface is passivated by 2D spacers, exclusively emerge at the compositional range (35–55% of FAPbI₃), deviating from the stoichiometric considerations. A quantitative crystallographic study via high‐throughput grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) experiments integrated with automated peak analysis function quickly reveals that the 3D‐like phases are vertically aligned, facilitating vertical charge conduction that can be beneficial for optoelectronic applications. Together, this study uncovers the optimal 2D:3D compositional range for complex quasi‐2D MHP systems, realizing promising optoelectronic functionalities. The automated experimental workflow significantly accelerates materials discoveries and processing optimizations that are transferrable to other deposition methods, while providing fundamental insights into complex materials systems. 

Abstract 2D‐on‐3D (2D/3D) perovskite heterostructures present a promising strategy to realize efficient and stable photovoltaics. However, their applicability in inverted solar cells is limited due to the quantum confinement of the 2D‐layer and solvent incompatibilities that disrupt the underlying 3D layer, hampering electron transport at the 2D/3D interface. Herein, solvent‐dependent formation dynamics and structural evolution of 2D/3D heterostructures are investigated via in situ X‐ray scattering. It is revealed that solvent interaction with the 3D surface determines the formation sequence and spatial distribution of quasi‐2D phases withn= 2–4. Isopropanol (IPA) reconstructs the perovskite into a PbI₂‐rich surface, forming a strata with smallernfirst, followed by a thinner substratum of largern. In contrast, 2,2,2‐Trifluoroethanol (TFE) preserves the 3D surface, promoting the formation of uniformly distributed largerndomains first, and smallernlast. Leveraging these insights, Dion–Jacobson perovskites are used with superior charge transport properties and structural robustness to fabricate 2D/3D heterostructures dominated byn≥ 3 and engineer a favorable energy landscape for electron tunneling. Inverted solar cells based on 3‐Aminomethylpyridine and TFE achieve a champion efficiency of 23.60%, withV_ocand FF of 1.19 V and 84.5%, respectively, and superior stabilities witht₉₄of 960 h under thermal stress.