Distance and Driving Force Dependencies of Electron Injection and Recombination Dynamics in Organic Dye-Sensitized Solar Cells

A series of dyes based on a triphenylamine donor and a rhodanine acetic acid anchor/acceptor for solar cell application has been studied with regards to electron injection and recombination kinetics using femtosecond transient absorption. The series contains three dyes, with estd. electron transfer distances ranging from 17.2 to 11.0 Å, and which have shown significant differences in energy conversion efficiencies. The injection and recombination kinetics were studied in the near-IR region where electrons in the conduction band of the TiO2 are suggested to absorb. For all dyes, the injection rate is larger than (200 fs)-1 which implies a quant. injection efficiency. Surprisingly, the subsequent recombination reaction has a rate that increases with increasing linker length. On the other hand, this behavior is consistent with the concomitant decrease in driving force for this series of dyes. Moreover, the lifetimes show exponential distance dependence when cor. for driving force and reorganization energy, which indicates a superexchange interaction between the electrons in TiO2 and the radical cations of the dyes. A dependence on probe wavelength of the attenuation factor was found, giving a β value of 0.38 Å-1 at 940 nm and 0.49 Å-1 at 1040 nm. The difference is suggested to be due to the difference in electronic coupling between fully sepd. dye cations and injected electrons vs. geminate electron-hole pairs. Addn. of tert-butylpyridine, which from previous work is known to cause a substantial drop in the incident photon-to-current-efficiency values for the studied dyes, was found to decrease the amt. of long-lived electrons in the TiO2 without affecting the injection rate.