Impact of Orientational Glass Formation and Local Strain on Photo-Induced Halide Segregation in Hybrid Metal-Halide Perovskites.

Band gap tuning of hybrid metal-halide perovskites by halide substitution holds promise for tailored light absorption in tandem solar cells and emission in light-emitting diodes. However, the impact of halide substitution on the crystal structure and the fundamental mechanism of photo-induced halide segregation remain open questions. Here, using a combination of temperature-dependent X-ray diffraction and calorimetry measurements, we report the emergence of a disorder- and frustration-driven orientational glass for a wide range of compositions in CH3NH3Pb(Cl x Br1-x )3. Using temperature-dependent photoluminescence measurements, we find a correlation between halide segregation under illumination and local strains from the orientational glass. We observe no glassy behavior in CsPb(Cl x Br1-x )3, highlighting the importance of the A-site cation for the structure and optoelectronic properties. Using first-principles calculations, we identify the local preferential alignment of the organic cations as the glass formation mechanism. Our findings rationalize the superior photostability of mixed-cation metal-halide perovskites and provide guidelines for further stabilization strategies

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

Data for: How Exciton Interactions Control Spin-Depolarisation in Layered Hybrid Perovskites

Contains Faraday rotation, Circularly polarised transient absorption, time-resolved photoluminescence and absorption data on the 2D perovskite FAMAPbI7. Also contains pre-review PDF's of manuscript and SI

Research data supporting: Long-lived polarization memory in the electronic states of lead-halide perovskites from local structural dynamics

Experimental dat

Control of crystal symmetry breaking with halogen substituted benzylammonium in layered hybrid metal-halide perovskites

International audienceLayered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen para-substituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC6H4CH2NH3)2PbI4 (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single crystal and thin film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine- and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and non-linear optical applications

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation.

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA2FAPbI7 using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control

Research data supporting: Long-lived polarization memory in the electronic states of lead-halide perovskites from local structural dynamics

Experimental dat

Research data supporting "Impact of orientational glass formation and local strain on photo-induced halide segregation in hybrid metal-halide perovskites"

Research data supporting the publication. Data was collected using X-ray diffraction, heat capacity measurements, density functional theory calculations and photoluminescence spectroscopy