AbstractLead–halide perovskites have attracted intense scientific attention as a consequence of their favourable properties for applications in optoelectronic devices, most notably as absorbing layers in solar cell devices. Despite this focus, understanding the mechanisms that govern some of their fundamental charge transport properties remains a challenge. It is broadly accepted that the interaction between charge carriers and polar optical phonons is primarily responsible for determining such properties as charge carrier mobilities and hot carrier cooling times. The simplest description of this interaction, given by the Fröhlich Hamiltonian, describes the interaction between an electron in a parabolic band and a single dispersionless optical phonon branch in the limit of long phonon wavelengths, but it is not adequate for calculating mobilities and cooling times with an acceptable degree of accuracy. This has led to a variety of proposals of additional mechanisms, or extensions to the simple Fröhlich Hamiltonian, that may be required for the accurate calculation of charge carrier dynamics.
In this thesis, charge carrier mobilities and transient hot carrier distributions are calculated by finding exact solutions to the semiclassical Boltzmann transport equation using BoltMC, an implementation of the ensemble Monte Carlo technique that was developed in collaboration with William Saunders and Matthew Wolf for the purposes of this thesis. BoltMC makes use of PPMD, a modern performance-portable high-level framework, to ensure efficient, scalable calculations.
Charge carrier mobilities are calculated to assess the effects of several extensions to the simple Fröhlich electron–phonon interaction, from which the following conclusions are drawn: (i) while the formation of large polarons in lead–halide perovskites has noticeable effects on room temperature mobilities, these effects are more limited than has been suggested in some parts of the recent literature; (ii) contrary to conclusions made elsewhere in the literature, coupling to multiple effective optical phonon branches has virtually no impact on mobilities at room temperature; (iii) the screening of Fröhlich interactions by the presence of free charge carriers affects mobilities at carrier densities that are typical of many experimental measurements.
Finally, by showing that carrier–carrier scattering can occur on a timescale similar to that of scattering between charge carriers and polar optical phonons, it is suggested that that assigning a well-defined temperature to evolving hot carrier distributions may not always be possible. Carrier–carrier scattering rates are found to have consequences for calculations of hot carrier cooling times in lead–halide perovskites.
|Date of Award
|22 Jun 2022
|Alison Walker (Supervisor), Matthew Wolf (Supervisor) & Benjamin Morgan (Supervisor)