Effect of 1-Substituted 2-(Pyridin-2-yl)-1H-Benzo[d]imidazole Ligand-Coordinated Copper and Cobalt Complex Redox Electrolytes on Performance of Ru(II) Dye-Based Dye-Sensitized Solar Cells.

A comparative study has been attempted on 1-substituted 2-(pyridin-2-yl)-1H-benzo[d]imidazole ligand-coordinated copper and cobalt metal complex electrolytes Cu+/2+[nbpbi]2(PF6-)1/2, Cu+/2+[npbi]2(PF6-)1/2, Co2+/3+[nbpbi]3(PF6-)2/3, and Co2+/3+[npbi]3(PF6-)2/3 in dry acetonitrile coupled with both N3 and N719 dyes in dye-sensitized solar cell (DSSC) devices. Impressively, the copper metal sites coordinated with ligands nbpbi (L1) and npbi (L2) shift the redox potential about 190-200 mV and pave the way to achieve remarkably higher power current efficiency, which is clarified with cyclic voltammetry, electrochemical impedance spectrum, electron lifetime, and quasi Fermi-level experimental results. Overall efficiencies of 4.99, 4.82, 3.26, and 3.19% under 1 sun conditions (100 mW cm-2) were obtained for Cu+/2+[nbpbi]2(PF6-)1/2 and Cu+/2+[npbi]2(PF6-)1/2 electrolytes coupled with the sensitizers (N3 and N719 dyes), which are considerably higher than those acquired for devices containing the cobalt electrolytes. These results signify a record for copper complex-based electrolytes coupled with ruthenium dyes in liquid DSSCs. In particular, bulky acceptor 4-nitro benzyl moiety-substituted 2-(pyridin-2-yl)-benzimidazole (on the N-H position) (ligand L1)-coordinated copper metal complex electrolytes achieved higher efficiency, approaching a suitable redox potential of 0.68 V versus NHE. At the same time, the napthyl moiety-substituted 2-(pyridin-2-yl)-benzimidazole (ligand L2)-coordinated copper metal complex electrolytes showed less redox potential due to its donating nature. It was determined that the Jsc and PCE increment of the devices consisting of Cu+/2+[nbpbi]2(PF6-)1/2 electrolytes was mainly attributed to various factors such as higher chemical capacitance, larger charge, longer electron life time, a downward shift in the quasi Fermi level of TiO2, the slow recombination process, and fast dye regeneration. These results make easily tunable metal complexes bearing a new sort of 1-substituted 2-(pyridin-2-yl)-1H-benzo[d]imidazole ligand-based electrolytes as very promising copper electrolytes for further improvements of extremely efficient liquid DSSCs.