Origin of the Kinetic Heterogeneity of Ultrafast Light-Induced Electron Transfer from Ru(II)-Complex Dyes to Nanocrystalline Semiconducting Particles

Abstract

Interfacial electron transfer from a molecular dye to a semiconductor is a keystone process in the conversion of light into electricity in dye-sensitized solar cells. The most successful devices developed so far are based on the sensitization of nanocrystalline titanium dioxide by ruthenium polypyridyl complexes. The ultrafast electron injection from the widely used RuII(dcbpy)2(NCS)2 dye in particular has been intensely studied. Several research groups, including ours, have found that this reaction apparently proceeds with a fast sub-100 fs phase, followed by a slower kinetic component with a time constant of 0.7–100 ps and accounting for 16–65% of the total yield. No convincing explanation has been provided for a clear understanding of the origin of this non-exponential kinetic behavior. In this contribution we show that aggregation of dye molecules at the interface is actually responsible for the slow kinetic component of the interfacial electron transfer. A thorough control of the dissolution of the dye and of its adsorption onto nanocrystalline oxide films allowed the reduction of the portion of dye excited states that react within the slow compartment and even made the latter completely disappear. In the absence of dye aggregates, femtosecond pump-probe studies of the sensitizer's oxidized state appearance yielded a rate constant for charge injection >5×1013 s?1, corresponding to an electron transfer time of less than 20 fs.