Molecular Structure Determines Phase Selectivity in Quasi-2D Halide Perovskites

Controlling the phase distribution in quasi-two-dimensional (2D) halide perovskites (HPs) is a crucial bottleneck for bespoke manipulation of their optoelectronic functionalities. This mainly lies in the subtle interaction between molecular spacers and inorganic precursors during crystallization that steers phase synthesizability, which has been poorly explored. Here, we investigate the synthesis spaces of two distinct quasi-2D HPs incorporating phenylethylammonium (PEA) and octylammonium (OA)-possessing comparable molecular masses but different structures and chemical properties. Utilization of a high-throughput automated experimental workflow combined with a deep-learning analytic framework allows for fully uncovering the latent synthesis landscape and distinct crystallization behaviors across the two systems. Specifically, while PEA promotes rapid crystallization and the formation of multiple low-n phases, the OA system exhibits slower crystallization and a strong preference for single-phase growth. Such a selective and controlled behavior is found to originate from the structural instability of n=1 OA2PbI4 arising from alkyl chain-induced lattice distortion. These findings demonstrate how spacer molecular structure directs crystallization pathways and phase landscapes, which offer a guideline for the rational design of phase-controlled quasi-2D HPs from conventional nonequilibrium synthetic processes, thus benefiting future optoelectronic applications.