Ultrafast scattering of electrons in TiO₂

In photo-catalytic and solar energy conversion devices, the absorption of a photon results in the generation of electrons and holes, which, upon separation, can provide an electric potential or trigger chemistry. The efficiency of these devices is determined by the transport of the charges following photo-generation. In particular, for Ti02-based dye-sensitized solar cells, it has been demonstrated that the efficiency is limited by electron transport through TiO₂ nano-particles. Titanium dioxide (Ti02) is widely used in photo-catalysts and solar energy converters. A key factor determining the efficiency of these devices is the transport of photo-generated electrons. Electron mobility is determined by the scattering rate - the average frequency of momentum-changing collisions, inversely proportional to the electron mean free path. This chapter elucidates the nature and rate of electron scattering in single-crystal rutile TiO₂ using the contact-free technique of THz time-domain spectroscopy. Strong electron-phonon interactions result in the formation of polarons, electrons dressed by local lattice deformations, and electron-phonon scattering rates up to 10¹⁴ s^-1 at room temperature. The anomalous high scattering rates result in a low room temperature electron mobility of ~0.5 cm²/Vs, which sets the intrinsic limit for device performance.