Synthesis Of Triphenylamine-based Dyes With A Pyridine Unit As π-bridge And Study On Their Photovoltaic Properties

The present energetic and environmental crises have stimulated the interest in searching renewable energy sources. Solar energy has induced more and more attention due to its cleanliness and not by regional restrictions. Dye-sensitized solar cells (DSSC) certainly appeared as one of the most promising devices for converting the solar energy and therefore have attracted significant attention since the first report by Gratzel in1991. In1996, McLendon et al. reported the first metal-free organic dye coumarin (C343). Although the efficiency was below1%, it excited the research interests in metal-free organic dyes due to their many advantageous features, such as high molar extinction coefficients, easily synthesized molecular structure and relatively low cost. The first triphenylamine-based dye with a conversion efficiency of3.3%was reported in2004by Kitamura. Recently the highest conversion efficiency of triphenylamine-based dyes used in solar cells has been improved to10.3%. A lot of studies have been focused on the development of conjugate chains in the molecules of triphenylamine-based dyes, such as vinyl, phenyl, thiophenyl, triple bonds and so on. Until now, to the best of our knowledge, a pyridine bridge in the molecule of D-π-A type of pure organic dyes has not been reported.In this paper, four triphenylamine-based organic isomer dyes bridged with a pyridine-containing π-conjugation and a cyanoacrylic acid unit as electron acceptor have been designed and synthesized firstly. The structures of the synthesized dyes were confirmed by1H NMR,13C NMR and MS-EI.Secondly, the phenomenon of isomers of synthetic dyes molecules was preliminary inquiried.Thirdly, the photophysical, electrochemical properties and photovoltaic performances were studied by UV-Vis absorption, emission, cyclic voltammograms and the linear sweep voltammetry.Fourthly, the effects of both (E)-and (Z)-isomer conformations upon the photophysical and photovoltaic properties of the dyes were studied. The results showed that the absorption spectrum and molar extinction coefficient of (Z)-isomer dyes are higher in comparison with that of (E)-isomer, and the ground-state oxidation potential and the excited-state oxidation potential of both (E)-and (Z)-isomer dyes are suitable for electrons transfer in DSSC. Furthermore, the frontier molecular orbital level of the dyes was calculated under density functional theory (DFT) calculation, the calculated results indicated that efficient intramolecular charge transfer occurred in dyes.At last, the effects of both (E)-and (Z)-isomer conformations upon the photovolatic performances of the dyes were studied. On the basis of optimized conditions, the solar-to-electrical energy conversion efficiencies (η) of (Z)-isomer dyes are higher than those of (E)-isomer dyes.