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.