Investigation on the coordination between methylpyridine additives and the [Cu(dmp)₂]^2+/+ redox couple and its improvement towards the stability of the dye-sensitized solar cells

In dye-sensitized solar cells (DSSCs), the use of copper(ii/i) complex redox couples enables a high open-circuit voltage of greater than 1.0 V. However, the widely used electrolyte additive, 4-tert-butylpyridine (TBP), poisons the copper(ii) complex, resulting in poor device stability. In this study, a series of pyridine derivatives 2-methylpyridine (2MP), 3-methylpyridine (3MP), 4-methylpyridine (4MP), and 3,5-dimethylpyridine (35DMP) were studied as alternatives to TBP for solving or mitigating the TBP-induced performance degradation. The coordination between the additives and copper(ii) complex was extensively studied using ultraviolet-visible spectrophotometric titration, cyclic voltammetry, and H-1-nuclear magnetic resonance spectroscopy. The moiety position on the pyridine ring was found to greatly affect the electrochemical properties of the redox couple. The 35DMP penta-coordinated copper(ii) was found to least affect the electrochemical activity at the counter electrode and ionic diffusion in the bulk electrolyte. A DSSC with a 35DMP additive outperforms its TBP-based counterparts in not only power conversion efficiency but also long-term stability.