Theoretical studies of the suitability of imidazoly-pyridine as bidentate ligands for copper redox couples in the Dye Sensitized Solar Cell (DSSC) application

Copper complex redox couples have emerged as alternative redox mediator to iodide/triiodide redox couple for DSSC devices. The copper redox couples enable high and tunable voltage in DSSC devices. However, the Cu(II) species in a typical copper redox couples are strongly electrophilic towards injected electrons in TiO ₂ thereby promoting recombination and decreases the cell performance. Although 4-tertbutylpyridine (TBP) passivation of TiO ₂ surface reduces the extent of the recombination, its interaction with Cu(I) complex in a copper redox couples may be detrimental to the cell performance. Therefore, rational selection of ligands of the copper complexes is critical in obtaining high voltage and performance. In this work, Density Functional Theory (DFT) was adopted to screen four imidazoly-pyridine bidentate ligands: 2-(1-methyl-4,5-dihydro-1H-imidazol-2-yl)pyridine (MDIP); 2-(1-methyl-1H-imidazol-2-yl)pyridine (MIP); 2-(1,4,5-trimethyl-1H-imidazol-2-yl)pyridine (TMIP) and 1-methyl-2-(pyridin-2-yl)-1H-benzo[d]imidazole (MPBI). Calculated binding constants of interaction of 4-tertbutylpyridine (TBP) with the imidazoly-pyridine copper complexes indicated Cu(I) complexes of MDIP and MIP interact strongly with TBP. This has adverse effects on device voltage, thus, making MDIP and MIP ligand unsuitable. But Cu(I) complexes of TMIP and MPBI interact very weakly with TBP, indicating they are suitable ligands for copper complex redox couples for DSSC application.