Signatures of photogenerated small polarons in transition metal oxides

Photogenerated polaron formation in transition metal oxides has significant impacts on their photophysical properties. A polaron is a quasi-particle that forms upon the coupling of a charge-carrier (electron or hole) and a lattice vibration (phonon). Because of the implications photogenerated polarons have for the application of transition metal oxides as photoactive materials, it is critical to develop a fundamental understanding of the processes by which polaronic excited states form, as well as their behavior. Although our understanding of polarons began almost a century ago with the description by Landau of an electron localizing in an ideal crystal, spectroscopic and computational advancements have enabled a distinction between polarons that form via charge-injection pathways, whereby only one charge-carrier present, and photoexcitation, where there is an electron/hole pair. Here, we review recent progress in understanding photogenerated polarons in transition metal oxide materials, with a focus on α-Fe₂O₃ (hematite) and TiO₂, in addition to ternary oxide materials. Polaron formation mechanisms are further explored in the context of static lattice defects as we compare compositionally-pure materials to those with vacancies, dopants, or antisite defects. Finally, we discuss how specific advancements in the fundamental understanding of photogenerated polarons will enable unique photo-applications and direct material engineering.