Beneficial Role of a Bulky Donor Moiety in pi-Extended Organic Dyes for Mesoscopic TiO2 Sensitized Solar Cells

This paper reports the spectroscopic and electrochemical characterization of two pi-extended D-A-pi-A organic dyes, G1 and G2, along with a comparison with their donor-free congeners, DTB-T and DTB-B. A collection of measurements has been carried out to provide insight into the different processes (injection, regeneration, and recombination) involved in the photovoltaic conversion. The study aims, in particular, at rationalizing how the presence of a bulky triarylamine donor group, along with the modification in the chemical structure of the pi-bridge, influences the operative cycle of dye-sensitized solar cells. We demonstrated that the donor group contributes to the formation of a more robust organic layer uniformly covering the TiO2 surface, thus decreasing unfavorable charge recombination processes. In addition, the presence of the thiophene sulfur atom in one of the dye pi-extensions (G1), by favoring interactions with the triiodide in the electrolyte, triggers detrimental recombination pathways. We conclude from these observations that G2, the sensitizer with a non-thiophene p-extension, possesses the highest photovoltaic performances among the investigated sensitizers.