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  Skiba, E.J.; Buckner, H.B.; Lee, C.; McKnight, G.; Wallick, R.F.; van der Veen, R.; Ertekin, E.; Perry, N.H.: UV-Driven Oxygen Surface Exchange and Stoichiometry Changes in a Thin-Film, Nondilute Mixed Ionic Electronic Conductor, Sr(Ti,Fe)O_3-d. Journal of the American Chemical Society 146 (2024), p. 23265-23277

10.1021/jacs.4c05764

Abstract:
Enabling light-controlled ionic devices requires insight into photoionic responses in technologically relevant materials. Mixed-conducting perovskites containing nondilute Fe-serving as electrodes, catalysts, and sensors─can support large, electronically accommodated excursions in oxygen content, typically controlled by temperature, bias, and gas atmosphere. Instead, we investigated the ability of low-fluence, above-bandgap illumination to adjust oxygen stoichiometry and drive oxygen fluxes in nondilute Sr(Ti1–xFex)O3–x/2+δ (x = 0.07, 0.35) thin films with high baseline hole concentrations. Films’ optical transmission at 2.8 eV was used as a probe of oxygen stoichiometry in the range ∼100–500 °C. We compared pO2-step-driven and UV (3.4 eV)-step-driven visible optical transmission relaxations in films, finding that the time constants and activation energies of the relaxations were consistent with each other and thus with oxygen-surface-exchange-limited kinetics. Blocking oxygen exchange at the solid–gas interface with a UV-transparent capping layer resulted in no UV-induced optical relaxations. These results demonstrate that above-bandgap illumination can increase oxygen content in nondilute compositions through oxygen flux into the solid from the gas. First-principles simulations of defect formation enthalpies indicate that oxygen vacancies are energetically less favorable under steady-state illumination owing to shifts in quasi-Fermi levels. A larger 2.8 eV-optical response to UV illumination in x = 0.07 vs x = 0.35 samples was further investigated through ultrafast transient spectroscopy, where it was found that the x = 0.07 sample exhibits a slower carrier recombination. Together, these results suggest potential design principles for materials supporting large stoichiometry changes under above-gap illumination: (1) long excited carrier lifetimes and (2) highly charged, rather than neutral, defects/associates.