Understanding the mechanism of charge dynamics of photocatalytic materials is the key to design and optimize more efficient materials for many renewable energy applications such as solar water splitting, solar CO<sub>2</sub> conversion, environmental remedy, etc. In this study, the charge dynamics of CuO thin films, a strongly correlated low band gap semiconductor, is unraveled by the combination of ultrafast (picosecond) and nanosecond TRS techniques. The TAS spectra show invariant features under above bandgap and below bandgap excitations in both picosecond and nanosecond time scales, which indicate that the unique and complicated in energy levels and transitions in CuO. In particular, a broad-band depletion region associated with two depletion transitions at ~ 2.2 eV (bandgap) and ~ 2.7 eV, has been observed, indicating that the CuO thin film possesses a higher energy conduction band. This is consistent with the observation of a second linear region in the Tauc plot. Three time constants, τ1~0.3ps, τ2~150 ns, τ3~2µs, show that the charge dynamics is dominated by three very different time scales. Furthermore, the existence of band tails <i>via</i> Urbach tail analysis reveals that defects play an very important role. These defect states are proposed to be responsible for the long delay (τ2 and τ3) of electron transitions to the valence band. Based on previous theoretical and experimental results, a compelling energy diagram with a rate-equation-based numerical model has proposed to successfully explain both the spectroscopy- and time-dependent behaviors observed. This understanding can help us to better design CuO based photocatalysts.

Understanding the charge dynamics in CuO thin films using time-resolved spectroscopies

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