Structural Effects of the Donor Moiety on Reduction Kinetics of Oxidized Dye in Dye-Sensitized Solar Cells

One of the major factors influencing the regeneration rate of the oxidized dye in dye-sensitized solar cells (DSSCs) is the energy difference (Δ G ) between the levels of the dye’s HOMO and redox couple in the electrolyte. To investigate other factors that influence this process, we examined the effect of structural differences of donor moieties on the reduction rate of the oxidized dye of organic dyes that is composed of an acceptor unit, a π-conjugated linker unit, and a donor unit, including carbazole dye ( MK-1 ), triphenylamine dye ( MK-88 ), and coumarin dye ( MK-31 ). The DSSCs using MK-88 showed the fastest regeneration rate even though the Δ G was not the largest among the dye structures evaluated. The regeneration rates of all of the dyes were enhanced by reducing the number of adsorbed dyes. On the basis of the results, we attribute the fast regeneration of MK-88 to the large collision cross section of the oxidized dye, that is, the increased reduction rate to the larger exposure of the HOMO of the dyes to the redox species. The effect of the exposed surface area on the reduction rate was as large as the free-energy difference, suggesting a new design strategy for efficient sensitizers.