Abstract
Halide perovskites show unusual thermalization kinetics for above-bandgap photoexcitation. We explain this as a consequence of excess energy being deposited into discrete large polaron states. The crossover between low-fluence and high-fluence “phonon bottleneck” cooling is due to a Mott transition where the polarons overlap (n ≥ 1018 cm–3) and the phonon subpopulations are shared. We calculate the initial rate of cooling (thermalization) from the scattering time in the Fröhlich polaron model to be 78 meV ps–1 for CH3NH3PbI3. This rapid initial thermalization involves heat transfer into optical phonon modes coupled by a polar dielectric interaction. Further cooling to equilibrium over hundreds of picoseconds is limited by the ultralow thermal conductivity of the perovskite lattice.
Highlights
A key challenge in the device physics of photovoltaic materials is understanding where the above-bandgap photon energy goes and how to control it
A fundamental material limit is how far the carriers move in the active photovoltaic layer before cooling to thermal equilibrium
There is growing literature on the kinetics of carrier cooling in halide perovskites.[3−9] The behavior has been linked to a “phonon bottleneck” at high fluence and more generally to the formation and stability of polaronic charge carriers
Summary
The common experimental choice of 400 nm (3.1 eV) excitation is problematic in terms of interpreting the data. We have shown how effective mass theories of excitons and polarons informed by first-principles calculations can be combined to describe the physical processes behind the slow hot carrier cooling rates observed for halide perovskites. From an interpretation of the density at which the polarons start to overlap, we indicate that significant changes in the photophysics should occur when n ≥ 1018 cm−3 This corresponds to the observed transition region between lowfluence “high-energy photoluminescence” and high-fluence “hot-phonon bottleneck” regimes.[8] We have underlined the unusual electronic structure of hybrid halide perovskites possessing a second conduction band at +2.5 eV above the valence band and, caution careful interpretation of photophysics data when pumping with photon energies > 2.5 eV. Data files and Jupyter notebooks outlining the calculation steps are available as a repository on GitHub at https://github.com/ WMD-group/hot-carrier-cooling
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