Effect of Electron-Phonon Coupling on the Color Purity of Two- Dimensional Ruddlesden-Popper Hybrid Lead Iodide Perovskites

Two-dimensional (2D) lead halide perovskites have become a class of promising luminescent materials due to their excitonic nature and excellent environmental stability. However, it seems counterintuitive that the 2D perovskites possess extremely narrow emission, while at the same time they have been reported to reveal strong electron-phonon coupling, which generally leads to significant emission broadening. Here, the main factors affecting the electron-phonon coupling of 2D perovskites are investigated to interpret the observed light emission with full width at half maximum as small as 12.7 nm (similar to 58 meV) at room temperature (RT). We demonstrate that the electron-phonon coupling in 2D perovskites is structure-dependent, and coupling with the phonon mode in the organic layer rather than the inorganic one plays an important role in the broadening of emission. This scenario is evidenced by temperature-dependent photoluminescence spectroscopy and structural change of 2D perovskites with various bulky organic cations (L-series) and different numbers of inorganic layers (n-series). Our results suggest that the electron-phonon coupling in L-series 2D perovskites could be weak below 250 K, which leads to a small broadening, thus mainly contributing to high color purity at RT. While for the n-series, the increase of inorganic layer number induces a monotonic increase of the line width broadening in the low-temperature region, which reduces the color purity at RT.