Abstract
Variations in local strain, defect densities, and composition of hybrid metal-halide perovskites have been reported to create heterogeneous energy landscapes in thin films, which impact charge-carrier diffusion and recombination dynamics. Here, we employ one- and two-photon transient absorption spectroscopy to selectively probe the dynamics of charge carriers from surface and bulk regions of methylammonium lead bromide thin films. Differences in the transient absorption spectra indicate that an energy gradient of approximately 100 meV is formed between the higher band-gap surface and lower band-gap bulk regions. Thus, during their lifetime, photoexcited carriers move away from the surface to recombine in the bulk, where our experiments detect long-lived charge populations despite the significant band splitting that has conventionally been assumed to inhibit efficient radiative recombination. Supported by first-principles calculations, we demonstrate that bright emission can still arise from the bulk with states that occupy a wide range of momenta in the vicinity of the band extrema, which show strong dipole transitions. Our results report that photoexcitations in the hybrid perovskites avoid defect-rich surface regions, and that particularly strong emission is generated from accumulated excitation populations in the bulk.
- Received 14 December 2023
- Revised 14 May 2024
- Accepted 2 July 2024
DOI:https://doi.org/10.1103/PRXEnergy.3.033001
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Hybrid halide perovskites show impressive performance for optoelectronic applications, which raises key questions on the direct/indirect nature of the bandgap, their unexpected defect tolerance, and physical mechanism of bright luminescence. Combining theory and experiment, the authors show that the nature of the bandgap of hybrid halide perovskites spatially varies from a more direct surface region to a slightly indirect bulk. They find that this spatial energy level gradient diffuses carriers away from the defect-rich surface and into the bulk region, where they can avoid traps. Efficient bright emission is realized in the bulk of thin films, where Rashba-split bands provide strong radiative recombination due to a wide momentum range carrier occupation. These insights lay the foundation for the design of optimized interfaces and trap passivation in solar cells and light emitting diodes toward maximizing efficiencies.