Browsing by Author "Shrestha, SK"

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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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    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.