Lead halide perovskites (HaPs) have been intensively reinvestigated over the past decade. Their attractive device-level properties are partially attributed to the purported tolerance of the electronic structure against structural and chemical defects. However, challenges remain in verifying the existence of defect tolerance and, if present, precisely defining what it is and elucidating its origin(s). Theory and computation have led experiment thus far, though the calculation of quantities such as defect energetics depends critically on parameters such as which exchange-correlation functional is used in DFT calculations. Hence, experimental support is urgently required but progress has been hindered by variations in sample quality, material susceptiblity to beam damage, the lack of suitable atomic-level probes and more.

Here we use bulk-sensitive, element-/orbital-/symmetry-selective high-resolution X-ray spectroscopy of the PbBr₆ octahedra, in prototypical APbBr₃ perovskites in single crystal form, to show which portions of the electronic structure are or are not defect tolerant to photodegradation. The organic perovskites show gradual photoinduced changes to the bromine 4p σ-π energy separation and lead 6d crystal field splitting in the conduction band, which is not observed with CsPbBr₃. We find evidence for photoinduced self-n-doping in CsPbBr₃, not observed with the organic perovskites. Changes to the crystal field splitting are mostly reversed with self-healing in MAPbBr₃. Our work reveals the pivotal role of the A-cation in defect formation and self-healing, in a regime that does not involve degradation into the precursors.

While photodegradation of the organic perovskites leads to internal structural distortion of the PbBr₆ octahedra and changes to the conduction band electronic structure, band edge electron dynamics may not be affected significantly as the bromine K-edge absorption onsets, associated with the band edge states, are relatively unaffected. Bromine valence-to-core X-ray emission spectroscopy reveals that the distribution of bromine 4p states near the valence band edge are unaffected by photodegradation, supporting the existence of A-cation-independent defect tolerance in the valence band electronic structure.

Our findings clarify the underpinnings of defect tolerance in halide perovskites and has implications for all beam-based measurements of HaPs, electron dynamics, energy level alignment at device interfaces and defect calculations.

Wolfgang Caliebe

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