Spatial photoluminescence heterogeneity is a frequently recognized phenomenon in lead halide perovskite thin films meant for efficient photovoltaic devices. Deep trap states associated with local variation in structure, composition and strain are the main players dictating this photoluminescence heterogeneity and the device performance. These trap states can appear as nanoscale clusters and majorly reside in the grain boundaries.¹ Despite notable progress in understanding the fundamental aspects of these traps such as origin, location, and distribution, real time response of these trap states remains elusive. Temporal photoluminescence heterogeneity or intensity fluctuation introduced by the traps can be a major loss channel in the film and would require detailed investigation. In this context several questions that can arise are: (i) Are these photoluminescence quenching traps static or metastable? (ii) what are the quenching domains of these traps? (iii) Do they have any correlation with grain size and grain boundaries? (iv) Do they have any working timescale? (v) How does the surrounding environment influence formation and annihilation of these traps?

We observed large photoluminescence fluctuation in MAPbI₃ thin films when investigated in real time. The fluctuation varies as a function of grain size (500 nm-several µm) and surrounding environment. This fact indicates presence of metastable traps which can deplete significant amount of photogenerated carriers even in good quality films. Unlike spatially isolated crystals, proximity of the grains and crystallites in thin films provide a mixed photoluminescence signal making the trapping dynamics and carrier recombination more complex for individual grains. We have developed an advanced photoluminescence mapping technique based on correlation of the intensity fluctuation which divide the image into microscale clusters having high intra-cluster and low inter-cluster correlation, respectively. Size of the clusters changes as the grain size or the surrounding environment changes from air to inert. By correlating these clusters with electron microscopy image, we obtain important insight about the microscale quenching domain of these metastable traps. Power spectral density estimation of the intensity fluctuation of each cluster reveals existence of different types of metastable traps with timescales ranging from hundreds of milliseconds to tens of seconds.² Distinctly different response time of these quenching traps are noted when grains size is varied, and porosity is introduced into the sample. This approach demonstrates a methodology to understand several structural (quenching domain, grain size dependence) and dynamical aspects (metastability and working timescale) of the photoluminescence quenching traps in any thin films.