Browsing by Author "Conibeer, G"

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    Anomalous structural evolution and glassy lattice in mixed‐halide hybrid perovskites
    (Wiley, 2022-05) Shahrokhi, S; Dubajic, M; Dai, ZZ; Bhattacharyya, S; Mole, RA; Rule, KC; Bhadbhade, MM; Tian, R; Mussakhanuly, N; Guan, X; Yin, Y; Nielsen, MP; Hu, L; Lin, CH; Chang, SLY; Wang, DY; Kabakova, IV; Conibeer, G; Bremner, S; Li, XG; Cazorla, C; Wu, T
    Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed‐halide hybrid perovskite single crystals of MAPbI3‐xBrx (MA = CH3NH3+ and x = 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed‐halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed‐halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties. © 2022 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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    Hot carrier transfer processes in nonstoichiometric titanium hydride
    (IOP Publishing, 2017-07-25) Wang, P; Iles, GN; Mole, RA; Yu, DH; Wen, X; Aguey-Zinsou, KF; Shrestha, SK; Conibeer, G
    The absorber of the hot carrier solar cell (HCSC) needs to have a considerably reduced hot carrier thermalisation rate, in order to maintain the photo-generated hot carriers for enough time such that they can be extracted. The slow carrier cooling effect is predicted in materials in which the phononic band gap is sufficiently large to block the Klemens decay. Binary compounds with a large mass ratio between the constituent elements are likely to have large phononic band gap. Titanium hydride is one of these binary compounds that has the potential to become an absorber of the HCSC. Whilst a large phononic gap has been observed in stoichiometric TiH2, it has not been experimentally confirmed for hydrogen deficient TiH x (where x < 2). In this article, we report the phonon density of states of TiH1.65 measured using inelastic neutron scattering and presented to clearly show the phononic band gap. We also present the carrier thermalisation process of a TiH x (1< x <2) thin film by transient absorption, and estimate the carrier cooling time in this material. © 2017 The Japan Society of Applied Physics.
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    Investigation of phonon dynamic in single crystal lead-halide perovskites by inelastic neutron scattering
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-19) Dubajic, M; Wang, D; Mahmud, A; Upama, M; Jia, X; Rule, KC; Srestha, S; Bremner, S; Conibeer, G
    The lead halide perovskite materials have recently risen to prominence for remarkably high photovoltaic efficiencies in polycrystalline materials that are highly defected [1] Some of the reasons for this good defect tolerance are the very low exciton binding energy and consequent highly de localized electrons and holes leading to high mobilities in these materials, coupled to low thermal conductance. Other recent work by our group has shown long lifetimes for hot carriers in a range of perovskites with organic lead iodide perovskites having the longest lifetimes [2]. In order to explain those rather promising physical properties, a closer investigation of phonon dynamics is needed. Al though ab initio simulations (DFT) can predict phonon dispersions to a reasonably accurate extent (comparison between different phonon modes) [2, 3], scaling their energies to actual phonon energies (particularly at high momenta near the zone edge) can be rather inaccurate. In order to obtain a detailed phonon dispersion to overcome the limits of the ab initio methods, scattering techniques can be used (as they offer full Brillouin zone mapping and are suitable for large single crystal samples). The thermal triple axis spectrometer (TAS) on TAIPAN at OPAL reactor at ANSTO was used with the aim to map phonon dispersion of single crystal Methyl Ammonium Lead Halide Perovskites, CH3NH3PbBr3 and CH3NH3PbI3 [4]. The alignment was performed so that we could scan through [h k l] and [ 0 k l] planes in reciprocal space for MAPbBr3 and MAPbI3 samples, respectively. Assuming the cubic space group, ABX3, for the MAPbBr3 sample, we were able to perform transverse and longitudinal scans along each high symmetry direction in the Brillouin zone (Г-X and Г-M) which would ensure the mapping of all phonon modes in the h k plane of the Brillouin zone. The full phonon dispersions that will be obtained with this method can be of great significance as by adding up to the already simulated data we will be able to have a deeper insight into the undergoing physics that is responsible for the previously observed properties (such as significantly extended hot carrier lifetimes) where phonon dynamics will play a significant role. © 2018 The Authors.
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    Structure and dynamics in photovoltaic metal hydrides
    (Australian Institute of Physics, 2018-01-30) Chea, K; Greaves, TL; Le, T; Rule, KC; Mole, RA; Wang, P; Shrestha, SK; Conibeer, G; Iles, GN
    Solar cell technology is an active area of research with the quest to improve the efficiency of solar cells to above the current value of 44%. Hot carrier solar cells are particular types of cells which may enable higher efficiencies to be obtained. However, these are only feasible where there is a sufficiently large band gap in the phonon dispersion of the bulk material to minimise energy loss to thermalisation, thus keeping the electrons ‘hot’. Binary compounds with a large mass difference between the two constituent atoms, and high level of crystal symmetry such as titanium hydride, can have such a gap in their phonon dispersion. Titanium hydride is an interesting photovoltaic material with a broad range of properties, which vary depending on the proportion of hydride present. Theoretical studies show TiH2 has a phonon band gap of 95 meV in the bulk phase, however, experimentally this compound exists as a powder because the hydrogenation process causes large stresses in the lattice which are strong enough to crack the bulk sample. For solar cell absorber materials, a bulk sample is preferred and these can be manufactured by hydrogenating very pure Ti metal. We have previously studied TiH1.65 using X-ray powder diffraction and inelastic neutron scattering and found that while the width of the acoustic and optical phonon bands is different from those of TiH2, it did have a phonon band gap of 65 meV i.e. large enough to block Klemens’ decay. We present here an extension of this work with Fourier Transform Infra-red (FTIR) and Raman spectroscopy, along with X-Ray Diffraction (XRD) data from the photovoltaic materials, TiH2 and ZrH2.