Abstract
Metal-halide perovskites have emerged as highly promising semiconductors with excellent optoelectronic properties. This Perspective outlines how the dynamic response of the ionic lattice affects key electronic properties such as exciton binding energies and charge-carrier mobilities in hybrid perovskites. Such links are shown to derive from the frequency-dependence of the dielectric function, which is governed by contributions from electronic interband transitions, polar vibrations of the metal-halide sublattice, organic cation collective reorientations, and ionic movement. The influence of each of these contributions to charge-carrier screening and carrier-lattice interactions is discussed, which allows for general trends with material composition to be revealed. Overall, this Perspective highlights the challenges and questions arising from the peculiar combination of a soft polar metal-halide sublattice interspersed with rotationally mobile dipolar molecules that is encountered in hybrid metal-halide perovskites.
Highlights
Metal-halide perovskites have emerged as highly promising semiconductors with excellent optoelectronic properties
While organic A-cations may leave a vibrational footprint in the infrared part of the spectrum that arises from internal molecular vibrations,[4−9] the polar modes of the metal-halide sublattice are found at much lower THz frequencies.[5,8−12] In the 100 GHz range, collective reorientations of the A-cation may onset,[13−17] while at very low frequencies (
The dynamic response of the lattice directly contributes to the dielectric environment experienced by charge carriers in metal-halide perovskites
Summary
Frequencies, each subsequent electronic response contributes an additional background offset to the overall value of the dielectric function. One recent suggestion[85] has been that lattice anharmonicity should be taken into account for metal-halide perovskites, given that they are mechanically relatively soft[86] and exhibit low-energy features in their Raman spectra, which have been attributed to anharmonic polar fluctuations.[87] Since the Fröhlich model is based on harmonic approximations, such anharmonic contributions could potentially give rise to modifications to the expected temperature-dependence and values of the charge-carrier mobility. Absolute charge-carrier mobility values derived from this model appear to form reasonable upper limits when compared to the spread of available experimental data, and trends with compositional substitutions, e.g., along the halide series, or when swapping lead for tin, reproduce actual observations well It is still an interesting open question to which extent additional effects, such as lattice anharmonicity and temperature-dependent ionic screening, need to be added for a more complete description of charge-carrier mobilities.
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