Spin dynamics of bulk and quasi-two-dimensional metal halide based perovskite semiconductors
This dissertation presents the ultrafast analysis of the electronic states within solution processed bulk and quasi-two-dimensional (quasi-2d) metal halide perovskites. Specifically, of their spin dependencies on lattice and many-body interactions. Many of these results were obtained in close collaboration with others, as specified throughout the dissertation. First, we report that spin-dependent exciton interactions modify the exciton energy and induce spin depolarisation in the quasi-2d Ruddlesden-Popper hybrid metal-halide perovskite Butylammonium Formamidinium Lead Iodide, BA2FAPBI7. Experiments were carried out jointly between Dr Ravichandran Shivanna and me. For carrier densities above ~1015 cm-3, the room temperature exciton depolarisation rate increases with exciton density, showing that many-body interactions outcompete the precessional motional narrowing spin relaxation that was previously reported for lower carrier densities. Therefore, we observe that room temperature spin lifetimes are largest, ~3.2 ps, at an operating carrier density of ~1015 cm-3. We also demonstrate the dynamic circular dichroism that arises from a photoinduced polarisation in the exciton distribution between total angular momentum states. We rationalise this dichroism by a polarisation-dependent exciton-exciton interaction energy that splits co and counter polarised states by 25 meV. In the second part of the thesis, we analyse spin relaxation at cryogenic temperatures, and investigate how the coupling between excitons and the soft ionic crystal lattice impacts the spin relaxation of exciton states in BA2FAPbI7. Faraday rotation measurements were obtained by Dr Soumen Ghosh and Dr Franco V. A. Camargo in close collaboration with me. We report that photoexcitation with energy in excess of the exciton absorption peak increases the spin lifetime by two orders of magnitude at 77K. Our hypothesis is that such optical control of the spin relaxation mechanism arises from the strong coupling between excitons and the optically excited phonons, which form polarons at low temperatures that experience a different spin depolarisation mechanism. This new spin-control phenomenon highlights the special role of exciton-lattice interactions on the spin physics in the layered perovskites, and the potential to use different wavelengths of light to encode spin information. In the third section of this thesis, we explore the potential of a two-photon transient absorption technique to analyse the change in optoelectronic properties as a function of film depth and measurably determine the impact of the direct-indirect nature of electronic states in hybrid metal-halide perovskites on charge-carrier dynamics. Experimental work and subsequent analysis in this section was completed jointly between Dr Thomas Winkler and me. We selectively excite surface and bulk states of bulk MAPbBr3 and employ one and two-photon transient absorption spectroscopy to reveal the energetic landscape. We show that the surface region is greater in band gap energy and shows reduced Rashba-type band splitting. Carriers diffuse away from trap-rich surface states and recombine efficiently within the bulk, despite the significant band splitting that has been argued to inhibit efficient radiative recombination. First first-principles calculations performed by Dr Xie Zhang resolve this apparent controversy and are included within the thesis to demonstrate how bright emission arises from the bulk.
2d material, perovskite, spin, transient absorption, transient grating
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