Correlating cobalt redox couples to photovoltage in the dye-sensitized solar cell

We report a series of structurally analogous cobalt mediators related to [Co-bpy]Z (bpy = 2,2'-bipyrimidine, Z = 2+ or 3+) to demonstrate a linear relationship between the redox potential of the Co(iii/ii)-based redox couple (Emed) and open-circuit voltage (VOC) of the DSSC. The Emed values vary from 0.42 to 1.07 V vs. NHE depending on the number of nitrogen atoms and the presence of tert-butyl substituents on the ligand. A 64-mV gain in photovoltage was calculated for every +100 mV shift in Emed. Differences in the mediator sizes, diffusion coefficients, light absorption profiles, and spin state configurations for the complexes were not significant and therefore not expected to contribute to changes in the VOC. A decrease in the photocurrent, downward shift in quasi-Fermi level (EF,n) and shorter electron lifetime (Tn) with increasingly positive Emed were instead attributed to enhanced electron recombination from the TiO2 film to oxidized mediator species in the electrolyte.