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

Abstract

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 (E_med) and open-circuit voltage (V_OC) of the DSSC. The E_med 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 E_med. 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 V_OC. A decrease in the photocurrent, downward shift in quasi-Fermi level (E_F,n) and shorter electron lifetime (T_n) with increasingly positive E_med were instead attributed to enhanced electron recombination from the TiO₂ film to oxidized mediator species in the electrolyte.