Hot carrier transfer processes in nonstoichiometric titanium hydride

Abstract

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 TiHx (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 TiHx (1< x <2) thin film by transient absorption, and estimate the carrier cooling time in this material.