Abstract:

New organic dyes that contain variable lengths of conjugation, featuring oligothiophene and arylamines at the 2- and 3-position, have been synthesized. These compounds are characterized by photophysical, electrochemical, and theoretical computational methods. Nanocrystalline TiO₂-based dye-sensitized solar cells were fabricated using these molecules as light-harvesting sensitizers. The overall efficiencies of the sensitized cells range from 4.11 to 6.15%, compared to a cis-di(thiocyanato)-bis(2,2′-bipyridyl)-4,4′-dicarboxylate ruthenium(II)-sensitized device (7.86%) fabricated and measured under similar conditions. The devices made from these compounds have higher open-circuit voltage (V_OC) compared to oligothiophene congeners with arylamines at the 2-position only. The hydrophobic segment at the 3-position appears to help retarding the charge transfer from the conduction band of TiO₂ to the electrolyte, I₃⁻. Supplementary studies of the transient photovoltage and electrochemical impedance are in support of retarded charge transfer of TiO₂ (e⁻) with both oxidized sensitizer and I₃⁻. Computation on a pair of compounds with and without arylamines at 3-position indicates that the former has larger charge transfer efficiency at the electronically excited state.