Structure and dynamics in photovoltaic metal hydrides

Abstract

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% [1]. 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 from 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 [2], however, there is little experimental data to confirm this. TiH1.65 has been measured using X-ray powder diffraction and inelastic neutron scattering whereby it was found that this sample had a phonon band gap of 65 meV [3]. We present here further X-ray powder diffraction and ineleastic neutron scattering data on powder samples of TiH2 and TiH1.5 whereby we show the correlation of phonon band gap with hydrogen content. © The authors.