Ultrafast Electronic Dynamics Observation in Energy Storage Materials
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Thin film electrodes are able to enhance energy storage performance by their microscale thickness while ensuring mechanical flexibility. Transition metal oxides are typical electrode materials with unique properties on charge carriers, among them tricobalt tetraoxide possesses a band gap of near-infrared excitation, widely applied in photocatalysis, optoelectronics, and so forth. However, the lack of research on charge carrier dynamics and photo-accelerated charging of tricobalt tetraoxide restricts the application of this class of materials for thin film electrode energy storage devices. This work summarized the basic concept of absorption spectroscopy and raised one scheme for building a transient absorption spectrum experimental setup. Afterward, steady-state and transient absorption spectroscopy were employed to identify the optical transition in tricobalt tetraoxide, then photoexcited free charge carriers and their lifetime was analyzed. Based on transient spectroscopy, all free charge carriers observed had a long lifetime of >10 ns, surpassing the general value. It is inferred that the abnormally long charge carrier lifetime is due to small polaron formation by the observed phenomenon and literature research. In addition to the experimental findings, first principles calculation was implemented to validate the result by steady-state absorption spectrum, and deviations of the calculated absorption spectra from the experiment caused by low calculation accuracy and the implementation of Hubbard correction were found, the calculation result still plays a role of guidance to the experiments. Based on the small polaron formation observed in the experiments of tricobalt tetraoxide thin film, this work proposes the application potential of tricobalt tetraoxide thin film electrode for photo-accelerated charging-energy storage devices and prospects the future research directions.