Metastability of defect states is a phenomenon behind many peculiar properties of metal-halide (MH) perovskite semiconductors. For example, irradiation by light can lead to photoluminescence (PL) enhancement, the material self-heals after degradation, solar cells recover their performance after staying in dark. These effects are assigned to instability of concentration of the non-radiative recombination centres. The usual picture is that concentration of the defects and even their type depend on the history of the sample, illumination conditions, environment and so on. In other words, these concentrations are not fixed and can be influenced by external factors. Creation and annihilation of defects and their chemical transformation occur on very different time scales. Due to very large number of defects in the typical sample volume probed by traditional methods individual contributions of each non-radiative recombination centre is averaged over time and space. However, the “top of the iceberg” of this plethora of processes can be nicely seen and studied at the individual defect level due to PL intensity fluctuations of individual nano crystals as well as grains in films.¹

The origin of PL blinking is fluctuations of the non-radiative decay rate. Literature is filled with drastically diverse estimations of the defect concentrations in MH perovskites ranging from 10¹⁰ to 10¹⁷ cm^-3. These concentrations correspond to one defect per cube with sizes from 5000 to 20 nm. A typical grain in a perovskites film of 400 x 400 x 400 nm³ would contain 64 defects if their concentration is 10¹⁵ cm^-3, while a crystal 100 x 100 x 100 nm³ would contain one defect only. Note that it is not necessary that all these defects are effective non-radiative recombination centres, that is why the actual concentration (number N) of the effective non-radiative centres per crystal/grain can be even smaller. Small averaged number of defects per crystal leads to a large statistical fluctuation of this number resulting in different PL brightness of individual grains as well as PL intensity fluctuations when N is changing over time.

This simple consideration is well supported by many reports where PL fluctuations of perovskite crystals of very different sizes (up to micrometres) have been observed. PL micro-spectroscopy providing spatial resolution of 500 nm and, in particular, techniques inspired by single molecule spectroscopy suit very well to study defect metastability. One can see an analogy between a TV screen (representing a whole solar cell) and a one pixel of the TV active matrix (one grain of the film). By investigating the behaviour of small grains individually, we are able to observe and rationalize fundamental properties behind solar cells and other devices operation, life and failure.

I will talk about of PL micro-spectroscopy of individual crystals, discuss the defect dynamics as a function of temperature,² light irradiation and external electric field³ and implications of the very small number of strong non-radiative centres per crystal (N is an integer number, it is “digital”, not a continuous variable) on optical and electrical properties of metal halide perovskites and interpretation of experimental results.

(1)         Merdasa, A.; et al, ACS Nano 2017, 11 (6), 5391–5404.

(2)         Gerhard, M.; et al, Nat. Commun. 2019, 10 (1), 1698.

(3)         Chen, R.; Li, J.; et al, Adv. Opt. Mater. 2019, 1901642