Static and Dynamic Optical Properties of La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub>: The Effects of A‑Site and Oxygen Stoichiometry
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Abstract
Perovskite
oxides are a promising material class for photovoltaic
and photocatalytic applications due to their visible band gaps, nanosecond
recombination lifetimes, and great chemical diversity. However, there
is limited understanding of the link between composition and static
and dynamic optical properties, despite the critical role these properties
play in the design of light-harvesting devices. To clarify these relationships,
we systemically studied the optoelectronic properties in La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub> epitaxial films, uncovering the effects of A-site cation substitution
and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry
was used to measure static optical properties, revealing a linear
increase in absorption coefficient at 1.25 eV and a red-shifting of
the optical absorption edge with increasing Sr fraction. The absorption
spectra can be similarly tuned through the introduction of oxygen
vacancies, indicating the critical role that nominal Fe valence plays
in optical absorption. Dynamic optoelectronic properties were studied
with ultrafast transient reflectance spectroscopy, revealing similar
nanosecond photoexcited carrier lifetimes for oxygen deficient and
stoichiometric films with the same nominal Fe valence. These results
demonstrate that while the static optical absorption is strongly dependent
on nominal Fe valence tuned through cation or anion stoichiometry,
oxygen vacancies do not appear to play a significantly detrimental
role in the recombination kinetics