Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO₂ as a prototypical system, we use ab initio nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds. Polaron migration among Ti atoms is strongly correlated with quenching through an electron-hole (e-h) recombination process. At low temperature, the polaron is localized on a single Ti atom and polaron quenching occurs within several nanoseconds. At increased temperature, as under solar cell operating conditions, thermal phonon excitation stimulates the hopping and delocalization of polarons, which induces fast polaron quenching through the e-h recombination within 200 ps. Our study proves that e-h recombination centers can be formed by photoexcited polarons, which provides new insights to understand the efficiency bottleneck of photocatalysis and photovoltaics in TMOs.