Realizing Phase-Pure (PEA)₂FAPb₂I₇ Perovskite Films by Inhibiting 3D Phase Formation

2D Ruddlesden-Popper (RP) layered metal-halide perovskites have garnered significant interest due to their unique photoelectric properties and excellent stability. However, 2D perovskite films prepared via solution process generally exhibit multi-quantum well structure with random well width distribution, which significantly impedes the carrier transportation. Herein, the synthesis of phase-pure 2D (PEA)(2)FAPb(2)I(7) (PEA = C8H11N, FA = CH4N2) perovskite thin films is reported in ambient environment achieved by suppressing the formation of the 3D perovskite phase. First, incorporation of MACl additive can effectively regulate the intermediate during crystallization, facilitating the formation of a metastable 3D MA(x)FA(1-x)PbI(y)Cl(3-y) phase. Second, precise control of environmental humidity is employed to further inhibit the formation of alpha-FAPbI(3) phase. As a result, the perovskite intermediate composed of predominantly metastable 3D phase and 2D n = 2 phase is achieved, which is eventually transformed into 2D n = 2 perovskite during annealing, leading to markedly enhanced phase purity. Photoconductive-type photodetectors based on the phase-pure 2D RP perovskite films are fabricated, demonstrating a high detectivity of over 9.4 x 10(12) Jones due to the improved carrier transportation. This study provides a comprehensive insight into the crystallization process of phase-pure 2D perovskites.