Molecular photovoltaic devices based on nanocrystalline TiO₂ are prepared by using two dye sensitizers (see figure). Their relative efficiencies are greatly dependent on the molecular structures of the dyes. Two new organic dyes have been synthesized and used as efficient light-harvesting materials in molecular photovoltaic devices. These dyes are based on conjugated thienylvinylene units, with FL-4 consisting of a four-unit thienylvinylene oligomer and its homologue FL-7 which additionally incorporates the electron-donating triphenylamine unit (TPA) into its structure. Upon light excitation both dyes show efficient electron injection into the TiO₂ conduction band and slow back electron transfer to the oxidized dye. In fact, for FL-7, the back electron transfer dynamics are slower owing to efficient hole transfer to the TPA moiety situated further from the semiconductor surface. However, the electron recombination kinetics with the oxidized electrolyte for both FL-4 and FL-7 in dye-sensitized solar cells are faster than for devices made using the ruthenium dye N719. We believe that this is a serious limiting factor for devices based on oligothiophenes which, despite showing higher molecular extinction coefficients in the vis-NIR region of the solar spectrum, still cannot challenge the light-to-energy conversion efficiency of N719 or other ruthenium polypyridyl complexes.
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