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(dmp)(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.