Photoinduced Electron Injection from Ru(dcbpy)₂(NCS)₂ to SnO₂ and TiO₂\nNanocrystalline Films

Abstract

Photoinduced electron injection from the sensitizer Ru(dcbpy)₂(NCS)₂ (RuN3) into SnO₂ and TiO₂ nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO₂ (1/55 fs^-1) than to SnO₂ (1/145 fs^-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO₂ singlet, whereas for SnO₂ triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from ∼1 ps (2.5 ps for SnO₂) to ∼50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.