The way to panchromatic copper(I)-based dye-sensitized solar cells: co-sensitization with the organic dye SQ2

We report the first example of n-type dye-sensitized solar cells (DSCs) co-sensitized with a copper(I)-based sensitizer and an organic dye. The heteroleptic copper(I) dye [Cu( 3 )( 1 )] + ( 3 = anchoring ligand ((6,6'-dimethyl-[2,2'-bipyridine]-4,4'-diyl)bis(4,1-phenylene))bis(phosphonic acid), 1 = ancillary ligand 2-(6-methylpyridin-2-yl)thiazole) was combined with the commercially available squaraine derivative SQ2 . By prudent matching of the external quantum efficiency (EQE) maxima arising from the two dyes in complementary parts of the visible spectrum, we have achieved the highest photoconversion efficiency reported for a copper-based DSC (65.6% relative to N719 set at 100%). This confirms the potential for the use of Earth-sustainable copper as the basis of sensitizers in DSCs. A combination of J–V measurements ( J = current density, V = voltage), EQE spectra and electrochemical impedance spectroscopy has been used to optimize and understand the effective use of the co-sensitized DSCs. We have shown that the sequences in which the photoanodes of the n-type DSCs are exposed to [Cu( 3 )( 1 )] + and SQ2 , and the times that the electrodes are immersed in the respective dye baths, critically influence the overall performance of the DSCs. A degree of aggregation of the SQ2 molecules on the electrode surface is important in terms of achieving panchromatic light-harvesting of the co-sensitized DSCs, but excessive aggregation is detrimental