Influence of dye sensitizers on charge dynamics in SnO₂ nanoparticles probed with THz spectroscopy

The development of ways to harness solar energy is important in order to mitigate increasing energy demands. Water-splitting dye sensitized photoelectrochemical cells (WS-DSPECs) are one viable option to do this. The photoanode material where water oxidation occurs is not well studied. The photoanode generally consists of a wide bandgap metal oxide sensitized with a dye to capture a large region of the visible spectrum.

In a recent publication by our group, we studied the influence of the dye on charge carrier dynamics and photoconductivity in the nanoparticles (NPs). We calculated the frequency resolved photoconductivity of SnO₂ sensitized with either a ruthenium polypyridyl complex or a porphyrin dye ranging in thickness from 3.0 to 12.7 µm. The sensitizers differ in absorption coefficients, resulting in varying optical penetration depths of 15 µm and 1 µm, respectively. Additionally, charge transport was measured using optical pump terahertz probe (OPTP) spectroscopy. Upon photoexcitation of a sensitized metal oxide material, electrons are injected from the dye into the metal oxide, where these carriers absorb and reflect THz radiation.

We find that the injection dynamics are very similar for both dyes, but the trapping dynamics strongly depend on the dye. We conclude that this dependence is linked to dye-induced surface states, electrostatic environment, and defects, which increase carrier trapping at the surface. Frequency-dependent measurements confirm that the dye influences the carrier dynamics in the NPs. Further insight into the conductivity and recombination rates will aid in the development of WS-DSPECs.