Effect of imidazole-based additives on the voltage of dye sensitized solar cells with Cu(II/I) redox mediators

Abstract

Acetonitrile solutions of Cu(tmby)₂²⁺ exhibit aging-dependent changes of UV–Vis spectra, which were attributed to partial reduction to Cu(tmby)₂⁺. This effect is boosted by the addition of imidazole bases, i.e., N-methyl-benzimidazole (NMB) or 5‑chloro-1-ethyl-2-methyl-imidazole (CEMI). These bases have only small effect on the diffusion coefficients of Cu(tmby)₂²⁺, but downshift the redox potential of Cu(tmby)₂^2+/+ by ca. 0.1–0.2 V. The addition of CEMI to the electrolyte solution decreases the flatband potential of the TiO₂ anatase (101) face. The corresponding position of conduction band minimum is upshifted by 0.19 or 0.26 eV, depending on the counterion type, Li⁺ or Na⁺, respectively. The formal redox potentials of Cu(tmby)₂^2+/+ in the CEMI-modified electrolyte solution, interrelated with the respective flatband potentials in TiO₂, provide a refined estimate of the maximum accessible open-circuit photovoltage of DSSC.