(Invited) How to Mitigate Excited State Polaron Formation in Transition Metal Oxide Photocatalysts

We measure how lattice versus electronic coupling in transition metal oxide photocatalysts can mitigate photoexcited polaron formation. Despite well-aligned energetics for photocatalysis, transition metal oxide materials have limited carrier mobilities due to the formation of polarons. The coupling of photoexcited charge carriers with phonons in the crystal lattice causes carrier localization and results in transport that relies on site-to-site hopping. Models that predict polaron formation are limited to ground-state predictions, which limits the exploration of new photocatalysts. Small polaron formation rates have been measured to be independent of defects, dopants, surface treatments, and other material tuning parameters. Our transient extreme ultraviolet (XUV) measurements show the first hints about how electronic correlations can be used to counter polaron formation for increased transport. For example, in CuFeO 2 , we add Cu layers between FeO 6 octahedra, which increases photoexcited transport compared to α-Fe2O3. In ErFeO 3 , where Er frustrates FeO 6 octahedra, coherent Fe-Fe charge hopping due to correlated electron interactions slows polaron formation. We frame our discussion in terms of the Hubbard-Holstein Hamiltonian, which we are slowly proving can be used to guide materials design rules for non-polaronic photocatalysts using only ground-state DFT parameters.