Abstract
The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations. Using ultrafast pump-probe spectroscopy, we demonstrate that in state-of-the-art mixed-cation MHPs ultrafast photo-induced bandgap narrowing occurs with a linear to super-linear dependence on the excited carrier density ranging from 1017 cm−3 to above 1018 cm−3. Time-domain terahertz spectroscopy reveals carrier localization increases with carrier density. Both observations, the anomalous dependence of the bandgap narrowing and the increased carrier localization can be rationalized by photo-induced lattice fluctuations. The magnitude of the photo-induced lattice fluctuations depends on the intrinsic instability of the MHP lattice. Our findings provide insight into ultrafast processes in MHPs following photoexcitation and thus help to develop a concise picture of the ultrafast photophysics of this important class of emerging semiconductors.
Highlights
The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations
The decrease of μs with N may be related to the photo-enhanced nonlinearFröhlich coupling introduced by lattice fluctuations12,22. μdc is smaller than μs due to the Anderson localization
Μdc decreases faster than μs, since 1 − φφb decreases with N (Fig. 4b), indicating photo-enhanced Anderson localization and lattice fluctuations, consistent with the conclusion drawn from μs
Summary
The optoelectronic properties of metal-halide perovskites (MHPs) are affected by lattice fluctuations. Time-domain terahertz spectroscopy reveals carrier localization increases with carrier density Both observations, the anomalous dependence of the bandgap narrowing and the increased carrier localization can be rationalized by photo-induced lattice fluctuations. The Fröhlich model predicts a polaron mobility dependence on temperature of ~T−0.46 (for a range from 200 to 300 K), independent of multi-phonon coupling[18] This result is inconsistent with the ~T−3/2 dependence observed experimentally[19], which subsequently has been ascribed to large atomic displacements (lattice fluctuations)[11,20]. The thermally-activated lattice fluctuations[21] retard polaron transport by introducing nonlinearFröhlich coupling[22], while the random potential fields caused by lattice fluctuations favor quantum Anderson localization[20]. Both are potential mechanisms that can be responsible for the T−3/2-
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