Collective Spectral Diffusion of Defect Luminescence in CH₃NH₃PbI₃ Halide Perovskites

Solution-processed halide perovskites (HPs) have demonstrated exceptional optoelectronic performance and a range of unusual physical properties. Many of their intriguing characteristics can be attributed to the interactions among the dynamic polar lattice, charge carriers, and point defects. Here, microphotoluminescence (μPL) spectroscopy is used to investigate the spectral diffusion (SD) of ultrasharp emission lines (<500 μeV at 4 K) associated with defect states in the prototypical 3D perovskite MAPbI₃. Temporal spectral fluctuations are typically considered a hallmark of single nano-object emission and are absent in ensemble-averaged luminescence. Contrary to this expectation, the absence of photon antibunching demonstrates that the observed spectral diffusion arises from the collective behavior of an ensemble of emitters. The magnitude of the spectral fluctuations varies from hundreds of μeV to the meV range. The time traces show continuous spectral jitter and discrete spectral jumps. The frequency of spectral jumps increases with excitation power, and statistical analysis shows a transition from Gaussian to Lorentzian statistics with an increasing excitation density. Furthermore, correlated spectral fluctuations are observed among multiple sharp emission lines. The energy spacing and excitation power dependence demonstrate that the synchronous lines are not phonon sidebands but instead arise from distinct defect states. The temporal correlations indicate that multiple defect states are coupled to the same local fluctuations of their nanoenvironment. These results suggest the existence of domains of slow, correlated lattice deformations that induce a shared perturbation of the optical transitions. Overall, these findings provide new insights into the defect-lattice interactions in HPs.