Exciton Localization and Enhancement of the Exciton–LO Phonon Interaction in a CsPbBr₃ Single Crystal

Abstract

This is the first study of lead halide perovskites (LHPs) that compares the coupling strengths between longitudinal optical (LO) phonons and free or localized excitons. In this research, we measured the excitation density and temperature dependence of the photoluminescence (PL) spectra in the range of 4-300 K in a CsPbBr 3 single crystal. The PL peak energy of the free excitons monotonically blueshifts when the temperature increases, and so does the localized excitons until the temperature reaches 80 K. Above 80 K, however, the PL peak energy of the localized excitons redshifts when the temperature increases. We analyzed this unique temperature dependence using the configuration coordinate model, focusing on the exciton -LO phonon interaction. We showed that this unique temperature dependence can be exhibited as the localized excitons couple with the LO phonons with enhanced strength and have larger relaxation energy than the free excitons.

Fröhlich interaction dominated by a single phonon mode in CsPbBr3

“…Models involving defect-localized charges have been used extensively to explain strong Fröhlich interactions in nanostructures 44 , 56 , 70 – 74 , as well as in the bulk 53 , 63 . Localization is also known to enhance Fröhlich coupling in bulk CsPbBr 3 75 , thus supporting the implementation of such a model in describing our observations. The broken lattice periodicity at a defect enables the relaxation of wave vector conservation rules, thereby enabling carrier–phonon coupling with k > 0 63 .…”

Free and self-trapped exciton emission in perovskite CsPbBr3 microcrystals

“…These results t well with the features of FE emission. 12,24,26,27 Therefore, peak-2 can be undoubtedly attributed to FE recombination.…”

“…22 Emission spectra with three peaks were assigned to two BEs and an FE emission. 23,24 Even without observation of two or more luminescence peaks, the low-temperature PL spectrum of CsPbX 3 usually reveals an asymmetric tail in the low-energy side of the FE peak, implying the existence of some weak components. 12,[24][25][26][27] In some reports the weak subcomponent was suggested to be from the trap states in the band gap induced by either defects, e.g.…”

Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape

“…Therefore, it is challenging to map the temperature dependence of mini-strain. Considering the mini-strain is determined by the relative changes in spatial position, it can be speculated that the uniform temperature distribution has a much smaller influence on strain than the defects and impurities [68]. Consequently, we temporarily ignore the influence of temperature on mini-strain.…”