Exciton Self-Trapping for White Emission in 100-Oriented Two-Dimensional Perovskites via Halogen Substitution

Low-dimensional organic–inorganic hybrid lead halides have opened up a new frontier in single-component phosphors for white emission, which stems from self-trapped excitons (STEs), where STE states are dependent on lattice deformation, involving interactions between an inorganic skeleton and organic cations to consequently affect electron–phonon coupling. Herein, to decouple the crystal structure dominator on emission mechanisms, we employ the protonated benzimidazole as organic cations to synthesize two 100-oriented two-dimensional (2D) perovskites with Br^– or Cl^– as halogen anions, separately. Interestingly, even with a similar single layered crystal structure that is almost distortion-free in an inorganic octahedral framework, the two as-synthesized perovskites show distinct emission mechanisms. The underlying halogen regulatory mechanism is unveiled. In addition to changing the lattice deformation energy and self-trapping energy of STEs, the halogen substitution results in a 10-fold enhancement in electron–phonon coupling that affects STE dynamics. Therefore, this suggests a general design principle to tailor electron–phonon coupling in low-dimensional perovskites for broadband white emission.