Publication date: 15th December 2025
The physical origin of the exceptional optoelectronic properties of halide perovskites has been the subject of intensive investigations. Various microscopic mechanisms have been proposed to explain the defect tolerance of these materials. These mechanisms are often assumed to be linked to the soft and polar lattice of HPs. However, further experimental investigations at the microscale are necessary to clarify the interactions between defects, the lattice, and charge carriers. HPs single crystals present a very low defect density and superior optoelectronic properties compared to polycrystalline thin films.[1][2] They represent the best platform for studying the intrinsic properties of HPs.[3][4] At very low temperatures, the presence of defect states associated with energy levels within the bandgap can be evidenced directly through their photoluminescence.
Low-temperature luminescence spectroscopy of high-quality single crystals revealed the presence of ultra-sharp emission lines (< 500 µeV at 4 K). A detailed analysis based on steady-state and time-resolved spectroscopy shows that these lines can be assigned to donor-acceptor pairs (DAP). Photoluminescence excitation spectroscopy reveals the existence of a highly efficient excitation pathway for these defect states, located below the bandgap and the free excitonic transition. The ultra-sharp emission lines present significant temporal fluctuations in their emission energy. Statistical analysis of the spectral diffusion (SD) revealed the existence of photoinduced spectral jumps. The spectral fluctuations present a transition from a Gaussian to a Lorentzian distribution with increasing excitation power. Remarkably, the lack of antibunching demonstrates that the emission originates from an ensemble of defect states. Additionally, synchronicity is observed in the SD of multiple lines, suggesting a coupling of an ensemble of defect states with slow correlated lattice deformations. These results provide new insights into the nature of defect states and their interactions with the lattice.
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