Special Chemistry Seminar w/Scott K. Cushing

The role of ultrafast, sub-nanosecond processes during transport is often overlooked in systems such as batteries and solar absorbers. Instead, the focus is on drift and diffusion on timescales ranging from milliseconds to longer. In this talk, we prove that charge carriers, such as electrons and ions, initially strongly couple to vibrational modes, the combination of which subsequently controls microscopic charge transport. We measure that electrons and ions couple with atomic motions in a correlated manner, and we relate these dynamics to macroscopic transport-limiting or enhancing behavior. Specifically, for solid-state ionic conductors, we find that the ion hopping on picosecond timescales is governed by a small number of strongly coupled THz vibrational modes. We measure the role of coupled ion-vibrational modes for both collective and isolated hopping pathways. For transition-metal oxides relevant to solar energy conversion, we study a range of materials to identify common patterns that describe how photoexcited electrons interact with lattice vibrations. These interactions lead to the formation of polarons - electrons accompanied by local lattice distortions - which limit charge mobility starting on femtosecond timescales. By mapping this behavior across materials, we establish a phase diagram for photoexcited polaron formation and uncover its quantum-mechanical origins using material-specific Hamiltonians. Overall, this work establishes new design rules for batteries and solar materials based on fundamental studies of ultrafast dynamics.