Development of a functionally separated D–π-A fluorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells

A functionally separated D–π-A dye OUK-3 with a pyrazyl group as an electron-withdrawing anchoring group and a carboxyl group as an additional anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells. The optical and electrochemical properties, adsorption states on TiO2 nanoparticles, and photovoltaic performances in dye-sensitized solar cells (DSSCs) were investigated. It was found that the maximum adsorption amount of dye adsorbed on the TiO2 electrode for OUK-3 is 3 times as much as that of the D–π-A dye sensitizer OUK-1 with a pyrazyl group as an electron-withdrawing anchoring group. Moreover, this work revealed that the DSSC based on the dye OUK-3 exhibits a higher photovoltage (Voc) value than the DSSC based on the dye OUK-1. On the basis of the FTIR spectra of the dyes adsorbed on TiO2 nanoparticles and the electrochemical impedance spectroscopy (EIS) analysis of DSSCs based on the two dyes OUK-1 and OUK-3, the differences of photovoltaic performances between the two dyes are discussed by taking into account the adsorption states of the dyes adsorbed on the TiO2 surface. This work demonstrates that functionally separated D–π-A dye sensitizers can achieve effective surface coverage of the TiO2 electrode due to their high adsorption ability onto the TiO2 electrode, leading to not only the improvement of light-harvesting efficiency (LHE), but also an increase in the number of injected electrons in the CB of TiO2, which is responsible for the higher Voc value of functionally separated D–π-A dye sensitizers.