Exciton–Phonon Coupling and Low Energy Emission in 2D and Quasi-2D BA₂MA_<i>n</i>–1Pb_<i>n</i>I_3<i>n</i>+1 Thin Films with Improved Phase Purity

Abstract

Phonon scattering with photogenerated excitons and free charges greatly affects optoelectronic properties of metal halide perovskites and governs their emission line width. Benefiting from the improved phase purity, we are able to analyze exciton–phonon coupling in 2D and quasi-2D BA2MAn–1PbnI3n+1 (n = 1–3) thin films using temperature-dependent photoluminescence (PL) spectroscopy. The layer thickness (n value) dependent coupling of free excitons with both acoustic and longitudinal optical (LO) phonons was extracted quantitatively by fitting the temperature-dependent PL line width and band gap. The low energy emissive signatures below free excitons at low temperature might belong to the emission of self-trapped excitons and bounded excitons in structural defects. Our findings provide a systematic picture for the layer thickness (n value) dependent exciton–phonon coupling in 2D and quasi-2D perovskite thin films and could be helpful for improving the optoelectronic performance of devices made by Ruddlesden–Popper perovskite thin films