Radiative recombination and electron-phonon coupling in lead-free CH3NH3SnI3 perovskite thin films

Solution-processed metal halide perovskites are attracting significant attention as high-performance photovoltaic materials. The electron-phonon interactions in lead halide perovskites are currently in the focus of interest because these interactions play an important role to the observed superior material properties and high device performance. Here, we investigated the electron-phonon coupling in lead-free $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ thin films by analyzing the temperature-dependence of the photoluminescence (PL) spectra. The PL linewidth shows a superlinear dependence on the temperature, which suggests that the electron--longitudinal-optical (LO) phonon interaction is the dominating broadening mechanism of the optical transitions in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ near room temperature. Furthermore, from the analysis of the low-energy tail of the PL spectra by using the van Roosbroeck-Shockley relation, we determined the Urbach energy below the band gap and the optical-phonon energy that contributes to the optical absorption. This phonon energy coincides with the LO phonon energy determined from the PL width analysis. Our work quantitatively verifies that the electrons in tin halide perovskites efficiently couple with optical phonons, and demonstrates that their intrinsic optical properties at room temperature are comparable to those of lead halide perovskites implemented in high-efficiency solar cells.