High Electronic Coupling between Cu Complexes and Oxidized Dyes Confirmed by Measurements of Driving Force Dependent...

We confirm fast regeneration kinetics between copper complexes and oxidized organic dyes and the major contribution of electronic coupling (H_DA). The highest efficiency of dye-sensitized TiO₂ solar cells has been shown by employing Cu complex redox couples. Various groups have reported a fast regeneration rate of oxidized dyes by Cu complexes giving a low driving force attributed to low reorganization energy (λ), but the effect of H_DA has not been evaluated. The values of H_DA and λ can be derived from driving force dependent transient absorption (TA) measurements. However, analyzing TA decay using Cu complexes is not trivial because accelerated recombination by the presence of Cu²⁺ complexes and biphasic TA decay often complicates the analysis. Here we employ 16 Cu¹⁺ and Co²⁺ complexes and two dyes. To simplify the system, i.e., making a minimal electrolyte system, Cu²⁺ and Co³⁺ complexes and a common additive of 4-tert-butylpyridine are not used. From the driving force dependent TA decays of oxidized dyes by both Cu¹⁺ and Co²⁺ complexes, λ for the combination of the Cu complexes and dyes is found to be about 0.15 eV lower than that of Co complexes. Approximately 3 to 5 times higher H_DA values of Cu complexes than those of Co complexes are obtained, which is the dominant factor for faster rates. The values vary with the structure of the molecules, showing the possibility of increasing the H_DA values further. The higher H_DA values of a Cu complex than that of a Co complex are also reproduced by quantum chemical calculations.