Abstract
The hot carrier solar cell (HCSC) is one of the advanced concepts that have the potential to provide higher light conversion efficiency than a traditional Si solar cell. The HCSC aims at collecting high-energy carriers before they completely thermalise in order to reduce the energy loss due to carrier relaxation. In order to achieve this, a material with fairly slow carrier thermalisation speed is required to be the absorber of a HCSC. It is theoretically predicted that slow carrier thermalisation can result from suppression of Klemens Decay (an interaction between optical phonons and acoustic phonons) by a large phononic gap which likely exists in a binary compound with one heavy atom and one light atom. Binary hydrides are promising candidates for the materials of HCSC absorber. Among them, titanium hydride (TiHx) is specifically attractive due to its reasonable costs, uncomplicated fabrication process and relatively mild chemical features. This thesis is to study characteristic properties, phononic properties and carrier thermalisation of TiHx. A bulk TiH1.65 sample and a TiHx thin film sample have been prepared for inelastic neutron scattering (INS) experiments and ultrafast transient absorption (TA) experiments respectively due to different sample requirement of the two experiments. The INS results show a large phononic band gap enough to supress Klemens decay. The measured phonon DoS matches well with the phonon modes of TiH2 at the low energy range, but has slight red-shift at high energy range. A TiHx thin film has roughly the same hydrogen content and crystal structure as the bulk TiH1.65 sample, proven by XRD results. Ultrafast transient absorption (TA) measurements are used to study the electron thermalisation time of TiHx. Despite the strong noise affecting the results, considerably long carrier cooling time is calculated by ΔOD decay time fitting. The observed long carrier cooling time in titanium hydride is supportive to the expectation that its large phononic band gap could suppress the Klemens decay and cause slow carrier thermalisation.