Concentration effects on the performance of bis(diimine) copper(I) dyes in dye-sensitized solar cells

A critical factor that influences the performance of heteroleptic copper(I) bis(diimine) dyes in dye-sensitized solar cells (DSSCs) is shown to be the concentration of solution of the homoleptic copper(I) bis(diimine) complex used during the stepwise assembly of the dye on the semiconductor surface. The performance of the heteroleptic dye [Cu(1)(2)]+ (1 = ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid); 2 = 4,4′-(6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) is investigated as a function of the concentration of [Cu(2)2][PF6] which undergoes ligand exchange with TiO2-anchored 1. With 0.1 mM solutions of [Cu(2)2][PF6], optimal performance is reached on the day of fabricating the DSSC. Solar cells made using more concentrated solutions of [Cu(2)2][PF6] show a ripening effect and require increasing times (up to 4 days) to reach their optimal performance.