Photovoltaic and interfacial properties of heterojunctions containing dye-sensitized dense TiO2 and tri-arylamine derivatives

A series of solid-state heterojunctions comprising a dense TiO2 film electrode as an electron conductor, a ruthenium polypyridine complex (Ru(dcbpy)(2)(NCS)(2)) as a light-absorbing dye, and different triarylamine derivatives as hole conductors were prepared, and their photovoltaic properties as well as the molecular and electronic interfacial structures were investigated. The photovoltaic properties were compared to systems containing the hole conductors dissolved in an organic solvent as well as to a system containing a liquid electrolyte containing the iodide/tri-iodide redox couple. Two of the solid-state heterojunctions showed conversion efficiencies close to those of the system containing the iodide/tri-iodide redox couple, while one system was clearly less efficient. To explain the differences in photovoltaic properties the electronic and molecular interfacial structures of the solid-state heterojunctions were investigated by photoelectron spectroscopy (PES). By valence level PES the electronic energy levels highest in energy for the dye and the hole conductors were mapped, and the differences in energy matching partly explain the trends in photovoltaic properties. Differences in the molecular surface structure of the heterojunctions were also observed from the N 1s core level measurements. Specifically it was found that the smaller hole conductor, showing low photocurrent yield, is inserted into the dye layer.