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.