Blue Copper Model Complexes with Distorted Tetragonal Geometry Acting as Effective Electron-Transfer Mediators in Dye-Sensitized Solar Cells

The electron self-exchange rate constants of blue copper model complexes, (-)-sparteine-N,N'(maleonitriledithiolato-S,S')copper (Cu(SP)(mmt))(0/)(-), bis(2,9-dimethy-1,10-phenanthroline)copper (Cu(dmp)(2)(2+/+)), and bis(1,10-phenanthroline)copper (Cu(phen)(2)(2+/+)) have been determined from the rate constants of electron transfer from a homologous series of ferrocene derivatives to the copper(II) complexes in light of the Marcus theory of electron transfer. The resulting electron self-exchange rate constant increases in the order: Cu(phen)(2)(2+/+) < Cu(SP)(mmt)(0/)(-) < Cu(dmp)(2)(2+/+), in agreement with the order of the smaller structural change between the copper(II) and copper(I) complexes due to the distorted tetragonal geometry. The dye-sensitized solar cells (DSSC) were constructed using the copper complexes as redox couples to compare the photoelectrochemical responses with those using the conventional I(3)(-)/I(-) couple. The light energy conversion efficiency (eta) values under illumination of simulated solar light irradiation (100 mW/cm(2)) of DSSCs using Cu(phen)(2)(2+/+), Cu(dmp)(2)(2+/+), and Cu(SP)(mmt)(0/)(-) were recorded as 0.1%, 1.4%, and 1.3%, respectively. The maximum eta value (2.2%) was obtained for a DSSC using the Cu(dmp)(2)(2+/+) redox couple under the light irradiation of 20 mW/cm(2) intensity, where a higher open-circuit voltage of the cell was attained as compared to that of the conventional I(3)(-)/I(-) couple.