Although formamidinium lead iodide (FAPbI3) perovskite has shown great promise in the field of perovskite-based optoelectronic devices, it suffers the complications of a structural phase transition from a black perovskite phase (α-FAPbI3) to a yellow non-perovskite phase (δ-FAPbI3). Generally, it is pivotal to avoid δ-FAPbI3 since only α-FAPbI3 is desirable for photoelectric conversion and near-infrared (NIR) emission. However, herein, we firstly exploited the undesirable δ-FAPbI3 to enable structurally stable, pure FAPbI3 films with a controllable α/δ phase junction at low annealing temperature (60 °C) through stoichiometrically modified precursors (FAI/PbI2 = 1.1–1.5). The α/δ phase junction contributes to a striking stabilization of the perovskite phase of FAPbI3 at low temperature and significantly enhanced NIR emission at 780 nm, which is markedly different from pure α-FAPbI3 (815 nm). In particular, the optimal α/δ phase junction with FAI/PbI2 = 1.2 exhibited preferable long-term stability against humidity and high PLQY of 6.9%, nearly 10-fold higher than that of pure α-FAPbI3 (0.7%). The present study opens a new approach to realize highly stable and efficient emitting perovskite materials by utilizing the phase junctions.
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