Effect of Coordination Sphere Geometry of Copper Redox Mediators on Regeneration and Recombination Behavior in Dye-Sensitized Solar Cell Applications

The recombination of injected electrons with oxidized redox species and regeneration behavior of copper redox mediators are investigated for four copper complexes, [Cu(dmby)2]2+/1+ (dmby = 6,6′-dimethyl-2,2′-bipyridine), [Cu(tmby)2]2+/1+ (tmby = 4,4′,6,6′- tetramethyl-2,2′-bipyridine), [Cu(eto)2]2+/1+ (eto = 4-ethoxy-6,6′-dimethyl-2,2′-bipyridine), and [Cu(dmp)2]2+/1+ (dmp = bis(2,9-dimethyl-1,10-phenantroline). These complexes were examined in conjunction with the D5, D35, and D45 sensitizers, having various degrees of blocking moieties. The experimental results were further supported by density functional theory calculations, showing that the low reorganization energies, λ, of tetra-coordinated Cu(I) species (λ = 0.31–0.34 eV) allow efficient regeneration of the oxidized dye at driving forces down to approximately 0.1 eV. The regeneration electron transfer reaction is in the Marcus normal regime. However, for Cu(II) species, the presence of 4-tert-butylpyridine (TBP) in electrolyte medium results in penta-coordinated complexes with altered charge recombination kinetics (λ = 1.23–1.40 eV). These higher reorganization energies lead to charge recombination in the Marcus normal regime instead of the Marcus inverted regime that could have been expected from the large driving force for electrons in the conduction band of TiO2 to react with Cu(II). Nevertheless, the recombination resistance and electron lifetime values were higher for the copper redox species compared to the reference cobalt redox mediator. The DSC devices employing D35 dye with [Cu(dmp)2]2+/1+ reached a record value for the open circuit voltage of 1.14 V without compromising the short circuit current density value. Even with the D5 dye, which lacks recombination preventing steric units, we reached 7.5% efficiency by employing [Cu(dmp)2]2+/1+ and [Cu(dmby)2]2+/1+ at AM 1.5G full sun illumination with open circuit voltage values as high as 1.13 V.