Electron Injection Dynamics from Ru Polypyridyl Complexes to ZnO Nanocrystalline Thin Films

Abstract

Electron injection dynamics from Ru(dcbpyH2)2(NCS)2 (RuN3) and (Bu4N)2Ru(dcbpyH)2(NCS)2 (N719) to nanocrystalline ZnO thin films were studied using transient IR spectroscopy. The injection dynamics using 400 nm excitation were found to be biphasic, consisting of a distinct <100 fs component (14−20%) and slower nonexponential rise that is fitted by a biexponential function with time constants of ∼20 ps and ∼200 ps. The fast component amplitude decreases to ∼5% at 530 nm excitation. Dye agglomeration was shown not to affect injection dynamics, although it reduced injection yield, suggesting the agglomerates do not contribute to electron injection within 1 ns. A two-state injection model, previously used to describe injection dynamics in TiO2, was proposed for ZnO. The fast and the slow components were attributed to injection from unthermalized and relaxed excited states, respectively. Using this model, the injection time scales for these states were estimated to be 1.5 and 150 ps, respectively, both of which are an order of magnitude slower than corresponding rates to TiO2. Possible reasons for the different injection dynamics in these semiconductors are discussed.