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Excited State Processes in Perovskite Films and Nanostructures

Perovskite semiconductors show amazingly clean semiconductor behaviour for solution-processed materials, leading to highly efficient photovoltaics. However, their ultimate efficiency is limited to~28% by the same thermalisation losses that constrain any single-junction semiconductor, where energy above the bandgap is dissipated as heat [Richter et al., Nature Commun., 8, 376 (2017)]. Recent results in the literature [Li et al., Nature Commun. 8, 14350 (2017)] and from our own group suggest that carrier relaxation is significantly slowed in perovskite nanoparticles, which opens up the possibility to extract the energy of these carriers before relaxation occurs. In this project we will study carrier relaxation using transient absorption and transient photoluminescence spectroscopy, and will investigate whether multiple exciton generation (MEG) processes can compete with carrier relaxation when the sample is excited at more than twice the bandgap. Cambridge has world-leading facilities for ultrafast transient spectroscopy, with signal-to-noise levels that allow measurements at excitation intensities that are relevant to solar conditions. After initial measurements to learn the technique, the project will focus on extending the excitation and detection range to energies high enough to reach the MEG threshold (>2Eg), whilst also developing lower-gap perovskite nanoparticles which will bring the threshold down into a range that will be useful for applications. This will allow a comprehensive study of relaxation and MEG processes as a function of particle size, composition, excitation energy, excitation density and temperature. Based on these results, later stages of the project will involve forming nanoparticle-based photovoltaic devices (which are currently less well-developed in the literature than bulk devices), to investigate whether additional carriers from MEG can be extracted. If MEG turns out to be hard to achieve then the focus will move to hot-carrier extraction effects in perovskite films which may also provide an efficiency boost in appropriately structured device architectures. The work will build on our recent successful efforts to achieve quantum efficiencies >100% through MEG in devices based on PbSe and PbTe nanoparticles [see e.g. Davis et al., Nature Commun., 6, 8259 (2015)]. For further information on the project and how to apply, please email Felix Deschler, fd297@cam.ac.uk

For more information on what to expect as a CDT-PV student then please see our CDT-PV Handbook.