Trapped Together: How a Second Phase Induces Extrinsic Self-Trapping Leading to Enhanced Emitting States in 2D Perovskites

Two-dimensional (2D) lead halide (R2PbX4, with X = Br, Cl, and R = organic ammonium cation) perovskites (PSK) are promising materials for the generation of white light due to self-trapping events leading to broadband photoluminescence (PL). In the mixed-halide series NEA2Pb(Br1–nCln)4 (NEA = 2-naphthyl 2-ethylammonium), we systematically investigated defect effects on optical properties. We focused on Cl-rich samples, which showed enhanced broad emission, and compared the chloride-only component with two mixtures, n = 0.7, which exhibits one phase, and n = 0.5, including a small quantity of an additional Br-rich phase. We combined X-ray diffraction with steady-state and time-resolved optical studies and found prolonged PL and recombination lifetimes for n = 0.5 > 0.7 > 1, demonstrating the enhancing effect of defect-induced extrinsic self-trapping on emitting states.