Aging Effects on the Exciton Relaxation and Diffusion Processes in CsPbBr3 Nanocrystals

Fully inorganic perovskite nanocrystals (NCs) have been widely investigated due to their potential as very interesting active materials for several types of photonic and optoelectronic devices. Despite several experiments designed to investigate the basic emission properties of these NCs, a clear and complete understanding of their photophysics is still missing. In this work, temperature-dependent steady state and time-resolved photoluminescence (PL) measurements are used to investigate the nature of the emitting states, the origin of the excitation relaxation dynamics and the effects of aging upon long exposure to wet air for thin films of CsPbBr3 NCs prepared by coprecipitation. It is demonstrated that both free excitons and localized excitons contribute to the NC emission and that electron traps within the conduction band, approximate to 14 meV and approximate to 80 meV above the band edge, determine thermal emission quenching. Moreover, it is shown that the non-exponential PL relaxation dynamics are due to short-range energy migration within a disordered distribution of localized states between 10 and 100 K, with activation of a second, long-range diffusion process at higher temperatures. It is also demonstrated that aging determines the variations in defect levels, exciton-phonon coupling and exciton relaxation dynamics. The results substantially improve the current understanding of the basic photophysics of CsPbBr3 NC films and of the aging effects and are expected to provide a useful guide for future characterization of other similar materials.

Automated summary

In this work, the photophysics of CsPbBr3 nanocrystal films were investigated using temperature-dependent steady state and time-resolved photoluminescence measurements. It was found that both free excitons and localized excitons contribute to the nanocrystal emission, and electron traps within the conduction band play a crucial role in thermal emission quenching. Additionally, non-exponential photoluminescence relaxation dynamics were attributed to short-range energy migration within a disordered distribution of localized states at low temperatures and a long-range diffusion process at higher temperatures. Ageing effects were also observed, which resulted in variations in defect levels, exciton-phonon coupling, and exciton relaxation dynamics. These findings significantly enhance our understanding of the photophysics of CsPbBr3 nanocrystal films and ageing effects, and could guide the characterization of similar materials in the future.