Multiple-Exponential Electron Injection in Ru(dcbpy)₂(SCN)₂ Sensitized ZnO Nanocrystalline Thin Films

We have studied the electron injection dynamics of Ru(dcbpy)₂(SCN)₂ sensitized ZnO nanocrystalline thin films by femtosecond mid-IR absorption spectroscopy. Upon 400 nm excitation of the sensitizer, nonexponential electron injection kinetics from the sensitizer MLCT excited state to ZnO were observed by probing the absorption of injected electrons in the 2000 to 1900 cm^-1 region on the <1 ns time scale. A three-exponential fit to the data yields the following rise time constants and amplitudes (in parentheses): < 1ps (18%), 42ps (46%), and 450ps (36%). For films with increasing coverage of sensitizer molecules (optical density at 500 nm of 0.7, 2.3, 2.6), similar electron injection times were observed, while the injection yield decreases. This result suggests that aggregation or multilayer formation of sensitizer molecules reduces the quantum yield of electron injection to ZnO on the <1 ns time scale, and the observed multiexponential injection kinetics are attributed to the distribution of injection rates from the first monolayer of sensitizers. Possible reasons for the nonexponential injection are discussed. The injection dynamics are also compared with the very different injection dynamics observed for the same sensitizer on TiO₂.