Enhanced Luminescence Properties in Yellow Phase δ-CsPbI ₃ Crystals by Adjusting the Self-Trapped States

The development of efficient room-temperature broadband emitters based on self-trapped excitons in low-dimensional perovskites has been hindered by severe thermal quenching, which stems from insufficient thermal activation energy (Ea) and poorly optimized electron–phonon interactions. Here, we introduce a bromine-alloyed strategy in quasi-one-dimensional δ-CsPbI3 to enhance the self-trapped luminescence efficiency. Structural analysis reveals that Br incorporation selectively tunes the distortion of the PbX6 octahedra. This targeted lattice distortion acts as a unified design parameter: it concurrently deepens the potential wells for self-trapped excitons, thereby increasing Ea by more than an order of magnitude (from 7.4 to 117.2 meV) to suppress thermal quenching, while also optimally moderating the electron–phonon coupling strength (Huang–Rhys factor, S). Systematic compositional optimization shows that CsPb(I0.35Br0.65)3 achieves the optimal balance, with a large Ea (82.9 meV) for stability and a moderated S (∼40) for efficient radiative recombination. This work elucidates the structural origin for the synergistic tuning of photophysical parameters, providing quantitative design principles for tailoring efficient self-trapped emission in low-dimensional perovskites and advances their prospects for solid-state lighting and photonic applications.