Development of D–π–Cat fluorescent dyes with a catechol group for dye-sensitized solar cells based on dye-to-TiO2 charge transfer

D–π–Cat fluorescent dyes YM-1 and YM-2 with a diphenylamine moiety as the electron-donating group, a catechol (Cat) unit as the anchoring group and fluorene or carbazole as the π-conjugated system were designed and developed as a photosensitizer for type-II dye-sensitized solar cells (DSSCs), which have a direct electron-injection pathway from the dye to the conduction band (CB) of the TiO2 electrode by photoexcitation of the dye-to-TiO2 charge transfer (DTCT) bands. Furthermore, not only to gain insight into the influence of the molecular structure of D–π–Cat dyes on the appearance of a DTCT band and the electron-injection mechanism, but also to investigate the impacts of the DTCT characteristics of D–π–Cat dyes on the photovoltaic performances of DSSCs, a D–π–Cat fluorescent dye YM-3 with carbazole–terthiophene as the π-conjugated system was also synthesized. It was found that the D–π–Cat dyes possess a good light-harvesting efficiency (LHE) in the visible region due to a broad absorption band corresponding to DTCT upon binding to a TiO2 film. The incident photon-to-current conversion efficiency (IPCE) corresponding to the DTCT band for DSSCs based on YM-1 and YM-2 is higher than that for YM-3. This work indicates that the stabilization of the LUMO level and the expansion of the π-conjugated system by the introduction of a long π-bridge such as terthiophene on the Cat moiety can lead to an increase in the intramolecular charge transfer (ICT) excitation based on π → π* transition with a decrease in the DTCT characteristics, resulting in enhancement of an indirect electron-injection pathway from the excited dye to the CB of TiO2 by photoexcitation of the local band of the adsorbed dye on TiO2.