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 (η) values under illumination of simulated solar light irradiation (100 mW/cm²) 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 η value (2.2%) was obtained for a DSSC using the [Cu(dmp)₂] ^2+/+ redox couple under the light irradiation of 20 mW/cm² intensity, where a higher open-circuit voltage of the cell was attained as compared to that of the conventional I₃^-/I^- couple.