Synthesis of Alkyl-functionalized Organic Dyes and Their Application to Dye Sensitized Solar Cells (DSSCs)

Ruthenium dye-based dye-sensitized solar cells (DSSCs) have received great deal of attraction from chemists and material scientists because of their over 10% of photon to current conversion efficiency. The high cost and much synthetic efforts of ruthenium dyes, nevertheless, has often been reported as problematic. Recently, organic dye-based DSSCs whose photon to current conversion efficiency approximates near 9% are suggested as an alternative. Organic dyes have several advantages compared to Ru dyes for DSSCs. For instance, organic dyes have larger absorption coefficients than Ru-based dyes which is attributed to an intramolecular π-π transition, that leads to an effective light harvesting properties. Organic dyes vary in structure and can be easily modified for molecular design, essential features for tuning the absorption characteristics of DSSCs. In addition, the use of organic dyes as DSSCs eliminates the requirement of large quantities of heavy metals which is an advantage considering escalating costs for metals and their limited availability. Generally, organic dyes consist of three parts: electrondonor, electron-acceptor, and spacer as a linker between the two. A variety of organic dyes with high efficiency have been synthesized to date, particularly, when electron donor material contributed to high efficiencies. Arylamine derivatives are reported as the electron donors mainly because these compounds make it easy to produce holes and to stabilize the complex through modification of the geometric structure in excited state. However, the synthesis of organic dyes with carbazole substituted electron donors and the performance of DSSC has been rarely done in spite of the similar properties of carbazole derivatives to those of arylamine in organic electronics. In addition, studies on increasing the efficiency of DSSC by controlling the spacer has been rarely explored, contrasting to the number of studies regarding increased efficiency of DSSC by varying the electron donors and/or acceptors. The previous studies suggest that including a thiophenesubstituted spacer shows a comparatively large value of molar extinction coefficient in the absorption spectra. Moreover, additional studies on spacer control of organic dyes are related to the effect of introduction of steric bulky groups. A bulky group introduced into the molecular framework does not only increase the electron life time (τe) of the dyes in the conduction band of TiO2 but also inhibit dyeaggregation. This results in an increase of open-circuit voltage (Voc). Such an approach was introduced by the Hara group, in which a bulky spacer, such as a hexylthiophene, was incorporated into the organic structure. We have previously synthesized a system containing thiophene analogs as a spacer, coupled with a phenyl-carbazole segment as an electron donor. Systematic investigation into the effect of bulky substituent in the spacers on efficiency of the DSSCs was carried out by molecular engineering. In this study, we report the synthesis of four new organic dyes, their photophysical characteristics and DSSCs performances.