Revealing Charge Dynamics of Cu₂O Thin Film via Transient Absorption Spectroscopy and Numerical Modeling in Picosecond to Microsecond Time Scale

Understanding the mechanism of charge dynamics of photocatalytic materials is the key to design and optimize more efficient materials for diverse renewable energy applications such as solar water splitting, solar CO₂ conversion, environmental remedy, etc. In this study, the charge dynamics of Cu₂O thin film, a low band gap photocatalyst, is unraveled by the combination of ultra-fast (picosecond) and nanosecond transient absorption spectroscopy (TAS) techniques.

For TAS measurements, different excitation energies, are utilized at above, near, and below the band gap: 3.5 eV, 2.5 eV, 2.2 eV, and 1.9 eV. Four time constants are resolved, 0.5 picoseconds, 40 picoseconds, 10 nanoseconds and 2 microseconds, which confirms the necessity of exploring the charge dynamics on a broad time scale. The slowest time constant, [endif]--> is most likely due to the decay of conduction band electrons to the low-lying energy states whereas [endif]--> could come from the recombination of electrons and holes in the donor and acceptor energy states, respectively. The defect states are probably responsible for the slow relaxation of electrons (10 nanoseconds and 2 microseconds) to the valence band. Based on the previous theoretical and experimental results, a compelling energy diagram with a rate equation-based numerical model has been proposed to successfully explain both the spectroscopy- and time-dependent behaviors observed. This understanding can help us to better design Cu₂O-based photocatalysts.