Exciton–Phonon Coupling and Vibronic Emission Structure in 2D Perovskite Thin Films with Naphthylmethylamine Spacers

Exciton–phonon coupling (EPC) plays a key role in the photophysics of 2D hybrid perovskites (2DHPs) and greatly affects their optoelectronic properties. Compared with excitons coupled with phonons of metal halide lattices, our knowledge of EPC with phonons of organic cations is still limited. In this work, we conducted systematic temperature-dependent photoluminescence (PL) analysis on I- and Br-based 2DHPs with 1-NMA or 2-NMA cations (NMA = naphthylmethylamine). For I-based (1-NMA)2PbI4 and (2-NMA)2PbI4, moderate EPC with a Pb–I lattice phonon (∼20 meV) was observed. Intriguingly, Br-based 2DHPs exhibit strong cation-dependent PL spectra, i.e., (1-NMA)2PbBr4 exhibits a vibronic structure extending to the low-energy side, while a hot-exciton vibronic structure extending to the high-energy side was observed for (2-NMA)2PbBr4. The latter was attributed to excitons coupled with a ∼50 meV phonon residing on 2-NMA cations. Our work revealed the critical role of both organic cations and halide anions in EPC in 2DHPs, which may provide inspiration for designing high-performance 2DHP optoelectronic devices.