Mixed-Solvent Crystallization Expands the Chemical Space of 2D Perovskites and Enables Phonon-Limited Exciton Transport

The advancement of two-dimensional (2D) halide perovskite optoelectronics relies on synthetic methods that enable new material discovery and the growth of high-quality crystals with a low defect density. Despite the vast chemical space of potential organic cations, many remain incompatible with conventional aqueous solution synthesis, which leads to thermodynamically stable non-2D-perovskite structures or introduces chemical reactivity issues. Here, we present a room-temperature evaporation-induced crystallization approach using a mixed organic-aqueous solvent system for the exploratory synthesis of 2D perovskites. This method is compatible with an extended range of organic cations, enabling the facile discovery of 25 new 2D perovskites, including 16 that are either kinetically stable or inaccessible with the conventional method. The expanded 2D perovskite library reveals the critical role of intermolecular interactions between the organic cations in stabilizing the kinetically stable 2D perovskite structures. Moreover, the as-grown crystals exhibit near-intrinsic free exciton emission with significantly reduced defect emissions and exciton diffusion coefficients enhanced by up to 5-fold, approaching the intrinsic transport limits governed by exciton-phonon scattering. This work not only broadens the design space of organic cations for 2D perovskites but also establishes a general synthetic strategy for growing high-quality 2D perovskite crystals, which are essential for high-performance optoelectronics.