Interpretation of the photoluminescence decay kinetics in metal halide perovskite nanocrystals and thin polycrystalline films

Abstract

One of distinguishing features of metal halide perovskites is their long (up to microseconds) photoluminescence (PL) lifetimes, which are regularly observed for this class of semiconductors in spite of their direct gap origin. It is difficult to explain this contradiction in the framework of usual two-level Jablonski photophysical diagram because the absorption coefficient (i.e., the oscillator strength of the direct optical transition) is too high in perovskites to hold such long PL lifetimes. In this paper, we describe practical steps how the PL decay kinetics of perovskites in the forms of (1) passivated nanocrystals and (2) thin multicrystalline films can be described. In case of nanocrystals, the three-level delayed luminescence model is described by including shallow non-quenching traps providing multiple trapping and de-trapping of carriers and thus essentially lengthening the observed PL lifetime. In the case of perovskite thin films limited by interfacial recombination, the PL decay kinetics is usually determined by the non-radiative recombination on the film surfaces and can be satisfactorily described in terms of one-dimensional diffusion equation. The limits of applicability of such approaches are discussed.

Engineering Sr-doping for enabling long-term stable FAPb1−xSrxI3 quantum dots with 100% photoluminescence quantum yield

“…The PL decay profile of 100% PLQYs does not exhibit single-exponential decay behavior, a fact that could be explained by the trapping detrapping-processes with shallow defects in the perovskite conduction band, as it has been previously reported. 60 Accordingly, we suggest that Sr modification also plays a main role in the recombination kinetics of PQDs. Yao et al reported that Sr 2+ can provide radiative energy levels near or into the conduction band (CB) states from CsPbI 3 QDs.…”

Direct Observation of Shallow Trap States in Thermal Equilibrium with Band‐Edge Excitons in Strongly Confined CsPbBr3 Perovskite Nanoplatelets

“…The radiative recombination rate corroborates the previous values reported in the literature, [ 39,45,46 ] and the two trapping rates are also within the range expected based on numerous studies. [ 30,39,40 ] These findings indicate that trapping into the shallow trap states is relatively fast and can fully explain the difference observed between the TA and FLUPS signals. Second, the energy difference Δ E between S and X 1 states is small (<k B T ), as expected.…”

“…This may explain the extended PL lifetimes that have been reported previously in the literature. [ 39,40,47 ] Additionally, we have also demonstrated the importance of capping ligands to prevent the formation of deep trap states. Desorption of the ligand from the surface due to sample aging increases the number of deep trap states and directly influences the PLQY of the NPls.…”

“…This model has already been reported by Chirvony et al to explain the formation of long-lived PL kinetics in PNCs. [39,40] Second, we consider that the shallow trap states, S, and the lowest excited state, X 1 , are in thermal equilibrium and the population distribution between these two states follows a Maxwell-Boltzmann distribution. Furthermore, since the energy difference between S and X 1 is considered to be less than the thermal energy k B T, the trapping into the shallow trap states is reversible, and the back transfer rate constant, k −1 , is related to that of the trapping process, k 1 , via Equation (1).…”

New Synthetic Route of Ultrapure Alkylammonium Iodides for Perovskite Thin Films of Superior Optoelectronic Properties

“…The TRPL and TRMC measurements were carried out with different light intensities, both being significantly smaller than 1 sun, where carrier dynamics are dominated by trapping processes, which can make direct comparison of these two techniques difficult. [ 43 ]…”