Organo-metal halide perovskites (OMHPs) have demonstrated semiconducting properties of remarkably high quality. However, for further progress it is necessary to understand and control non-radiative decay processes, which depend strongly on fabrication and storage conditions. Here, we study non-radiative decay in methyl-ammonium lead iodide nanocrystals by employing methods of single molecule spectroscopy in a temperature range between room temperature and 77 K. Due to the small size of the nanocrystals (50-100 nm), the properties of individual non-radiative centers become apparent.

Looking at individual nanocrystals, we find highly diverse PL enhancement behavior upon cooling and only the ensemble-averaged trend reproduces a slope, which can be described by an Arrhenius-law, as often reported for OMHP films in the literature. From this we infer that the rate of non-radiative decay in OMHPs depends strongly on the local concentration of non-radiative centers, whereas the common practice of relating the PL intensity solely to the ratio between excitons and free charges seems to be oversimplified.

A certain fraction of very efficient luminescence quenchers (‘super traps’ [1,2]) leads to PL intensity fluctuations, also referred to as ‘blinking’, because of their ability to switch between a passive and an active state on time scales up to several seconds, while another fraction of quenchers is considered as persistent in time. Interestingly, blinking of the nanocrystals reduces remarkably upon cooling due to reduction of the relative switching amplitudes and decreasing switching rates. Thus, we assume that the PL enhancement has actually distinct origins: Reaching the quenchers at low temperatures via diffusion and/or capture of charges by non-radiative centers may be inhibited by thermal barriers, and also switching of the quencher from passive to the active state seems to require a certain amount of energy. We employ a simple model to simulate PL blinking in order to estimate the underlying switching times, which cannot be directly accessed in the experimental blinking transients, because they typically contain contributions of several super traps.

Overall, our study reveals, for the first time, a microscopic view on the luminescence quenching processes and the distinct phenomena contributing to the PL enhancement upon cooling. Moreover, the temperature-dependent study of PL blinking provides mechanistic insight into the switching behavior of super traps and will hopefully help to unravel their chemical nature in future work.

[1] Y. Tian et al., Nano Lett. 2015, 15, 1603

[2] A. Merdasa et al., ACS Nano 2017, 11, 5391