Measuring the extent of dynamical polaron formation and delocalization in photocatalysts Scott Cushing California Institute of Technology, USA The role of polarons in metal oxide photocatalysts and photoelectrodes is now accepted as a major limiting factor to overall performance. Transient X-ray and XUV spectroscopy have proven that polarons dynamically form in the excited as well as ground state to limit transport. Now the search is on for how off-stoichiometric, atomically substituted, and doped materials can be used to eliminate or reduce polaron formation. In this talk we will discuss how our recent theory and experimental advances are giving new insight into excited state small polaron formation. In particular, we use an excited state approximation to the Bethe-Salpeter equation to interpret transient XUV measurements of small polaron formation in α-Fe 2 O 3 versus CuFeO 2 . We measure that the addition of Cu orbitals in the valence band lead to a dynamic screening and renormalization of the polaron radius. Whereas in α-Fe 2 O 3 small polarons are formed immediately and persist until carrier recombination, in CuFeO 2 the polaron is compensated on the picosecond timescale by a coherent lattice relaxation and charge neutralization in the first and second nearest neighbors. This results in a higher free carrier to polaron ratio and increased carrier lifetimes. The results are the first to measure how a small polaron can be dynamically compensated to form a mid- to-large scale polaron. The results therefore give insight into how materials can be designed that better screen polaron formation for improved photocatalysis and photoelectrochemistry.
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