Development of D-π-A Fluorescent Dyes with a 3-Pyridyl Group as Electron-Withdrawing Anchoring Group for Dye-Sensitized Solar Cells

D-pi-A and (D-pi-)(2)A fluorescent dyes (OUY-1 and OUY-2) with a 3-pyridyl group as an electron-withdrawing anchoring group have been newly developed as photosensitizers for dye-sensitized solar cells (DSSCs). We found that both dyes OUY-1 and OUY-2 were adsorbed onto a TiO2 electrode through the formation of pyridinium ions and/or hydrogen bonding of the pyridyl group with the Bronsted-acidic sites on the TiO2 surface. However, D-pi-A dye NI-6, with a 4-pyridyl group, was predominantly adsorbed onto the TiO2 surface through coordinate bonding between the pyridyl group and the Lewis-acidic sites on the TiO2 surface. The photovoltaic performance of DSSC based on dye OUY-1 is comparable to that of the dye NI-6. On the other hand, the photovoltaic performance of DSSC based on dye OUY-2 is much lower than that of the dye OUY-1. This work revealed that the binding mode of D-pi-A dyes with pyridyl groups on the TiO2 surface can be changed by control of the basicity and electron density of pyridyl groups.