Effect of Tetrahydroquinoline Dyes Structure on the Performance of Organic Dye-Sensitized Solar Cells

Eleven novel donor acceptor π-conjugated (D-π-A) org. dyes were engineered and synthesized as sensitizers for the application in dye-sensitized solar cells (DSSCs). The electron-donating moieties are substituted tetrahydroquinoline, and the electron-withdrawing parts are cyanoacrylic acid group or cyanovinylphosphonic acid group. Different lengths of thiophene-contg. conjugation moieties (thienyl, thienylvinyl, and dithieno[3,2-b;2',3'-d]thienyl) are introduced to the mols. and serve as electron spacers. Detailed study on the relation between the dye structure, photophys. and photoelectrochem. properties, and performance of DSSCs is described here. The bathochromic shift and increase of the molar absorptivity of the absorption spectrum are achieved by introduction of larger conjugation moiety. Even small structural changes of dyes result in significant changes in redox energies and adsorption manner of the dyes on TiO2 surface, affecting dramatically the performance of DSSCs based on these dyes. The higher performances are obtained by DSSCs based on the rigid dye mols., C2 series dyes (Figure 1), although these dyes have lower light absorption abilities relative to other dyes. A max. solar-to-elec. energy conversion efficiency (η) of 4.53% is achieved under simulated AM 1.5 irradn. (100 mW/cm2) with a DSSC based on C2-2 dye (Voc = 597 mV, Jsc = 12.00 mA/cm2, ff = 0.63). D. functional theory (DFT) calcns. were performed on the dyes, and electron distribution from the whole mols. to the anchoring moieties occurred during the HOMO-LUMO excitation. The cyanoacrylic acid groups or cyanovinylphosphonic acid group are essentially coplanar with respect to the thiophene units, reflecting the strong conjugation across the thiophene-anchoring groups.