Small Polaron Dynamics in Transition Metal Oxide Photoelectrodes

The transition metal oxides are widely used in many optoelectronic applications, such as working as charge-transporting layers in solar cells and photoelectrodes in light-driven water-splitting. Considering the ionic bonding nature, the lattice of these oxides can be polarized by the charge carriers due to the strong electron-phonon coupling, giving rise to lattice distortion that fosters a local potential well. This lattice distortion together with the trapped carrier is regarded as a quasi-particle, i.e., a polaron. Here, the photocarrier dynamics in transition metal oxides were investigated using the transient absorption and time-resolved terahertz spectroscopies. We identified the spectral features of small polarons, self-trapped excitons, and free carriers and extracted their dynamics independently. We proposed a molecular model that could successfully explain the non-barrier self-trapping and quantitatively describe the temperature-dependent recombination of self-trapped excitons and the dopant effect on the photocarrier dynamics. Our model also suggested that the recombination of self-trapped excitons at room temperature was dominated by nuclear quantum tunneling. This model also predicted the micro-heterogeneity of the self-trapped species, and thus be able to quantitatively describe the Auger annihilation dynamics of self-trapped excitons.