Stable alpha/delta phase junction of formamidinium lead iodide perovskites for enhanced near-infrared emission

Although formamidinium lead iodide (FAPbI(3)) perovskite has shown great promise in the field of perovskitebased optoelectronic devices, it suffers the complications of a structural phase transition from a black perovskite phase (alpha-FAPbI(3)) to a yellow non-perovskite phase (delta-FAPbI3). Generally, it is pivotal to avoid delta-FAPbI(3) since only alpha-FAPbI(3) is desirable for photoelectric conversion and near-infrared (NIR) emission. However, herein, we firstly exploited the undesirable delta-FAPbI(3) to enable structurally stable, pure FAPbI(3) films with a controllable alpha/delta phase junction at low annealing temperature (60 degrees C) through stoichiometrically modified precursors (FAI/PbI2 = 1.1-1.5). The alpha/delta phase junction contributes to a striking stabilization of the perovskite phase of FAPbI(3) at low temperature and significantly enhanced NIR emission at 780 nm, which is markedly different from pure alpha-FAPbI(3) (815 nm). In particular, the optimal alpha/delta 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 alpha-FAPbI(3) (0.7%). The present study opens a new approach to realize highly stable and efficient emitting perovskite materials by utilizing the phase junctions.