Ultrafast Carrier Dynamics in Hematite Films: The Role of Photoexcited Electrons in the Transient Optical Response

Hematite (Fe2O3) is a promising earth-abundant, visible light absorber, and easily processable photocatalytic material. Understanding the dynamics of photogenerated electrons and holes in hematite and their optical signatures is crucial in designing hematite thin film devices such as photoanodes for water oxidation. We report carrier dynamics in hematite films as measured by ultrafast transient absorption spectroscopy (TA) with a pump pulse centered at 400 nm (3.1 eV) and a probe pulse spanning the visible range. We observe a small negative response for wavelengths shorter than 530 nm (2.34 eV) and a large positive response for longer wavelengths. We interpret the spectrally resolved TA data based on recent electronic band structure calculations, while accounting for excited state absorption, ground state bleach, and stimulated emission within the relevant bands. We propose that the origin of the positive TA response is absorption of the probe by photoexcited electrons within the conduction bands. This interpretation is consistent with features observed in the data, specifically an abrupt boundary near 530 nm, a diffuse edge at lower energy probes with a similar to 250 fs decay time characteristic of carrier relaxation, and slower decay components of similar to 5.7 and >670 ps characteristic of carrier recombination. We propose that the negative TA signal arises at short wavelengths where excited state absorption within the conduction bands is no longer possible and ground state bleach and stimulated emission dominate. This study will assist in understanding the origins of transient optical responses and their interpretation in hematite-based devices such as photoanodes.