Solvent-induced surface state passivation reduces recombination in semisquarylium dye-sensitized solar cells

Abstract

The semisquarylium dye SY1T that is strongly bound to the surface of nanocrystalline TiO2 experiences very fast back-electron transfer of injected electrons to the SY1T cation, when the TiO2/SY1T interface is surrounded by ultrahigh vacuum. However, when located in methoxypropionitrile (MPN), which is frequently used as electrolyte solvent in dye-sensitized solar cells, the back-electron transfer is significantly retarded. Results are obtained both for picosecond and microsecond time scales using transient absorption spectroscopy. As solvent-induced interfacial energy level shifts can be excluded as possible cause, the role of TiO2 surface states in the beneficial retardation process is investigated. Highly surface sensitive synchrotron-induced photoelectron spectroscopy exhibits high densities of surface states on the pristine nanocrystalline TiO2 (nc-TiO2) surfaces. While SY1T dye-sensitization from a SY1T solution in tetrahydrofuran saturates about 30% of the surface states, the subsequent in-situ adsorption of MPN molecules at the TiO2/SY1T interface leads to further reduction by more than 50% of the remaining surface states. It is concluded that the saturation of TiO2 surface states hampers the otherwise efficient recombination of injected electrons with the SY1T dye cation.