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

Abstract

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-tertbutylpyridine (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 TiO 2 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.

First Report of Chenodeoxycholic Acid–Substituted Dyes Improving the Dye Monolayer Quality in Dye‐Sensitized Solar Cells

“…Perhaps most strikingly, the increase in V OC from C 3 to C 3 –CDCA was measured at 60 mV. In other studies, the use of longer alkoxy chains has produced the same V OC ‐enhancing behavior, and thus we conclude that the CDCA substituent blocks electron recombination from TiO 2 to the electrolyte and thereby improves the photovoltage. This is supported by a slight improvement in electron lifetimes (see Figure c).…”

The Performance‐Determining Role of Lewis Bases in Dye‐Sensitized Solar Cells Employing Copper‐Bisphenanthroline Redox Mediators

“…Our data experimentally confirms the theoretical prediction of a pentacoordinated complex by Saygili et al based on DFT calculations. [ 62 ] This coordination has major implications for the redox chemistry and hence the performance of [Cu(dmp) 2 ] +/2+ as a redox couple in DSCs which will be discussed in further detail later.…”

“…Using density functional theory (DFT) calculations, Saygili et al predicted a coordination of tBP to the [Cu(dmp) 2 ] 2+ complex and have suggested that this coordination induces the shift in rest potential with respect to the tBP‐free solution. [ 62 ] The induced shift in redox potential caused by coordination with electrolyte additives has been speculated to be a crucial factor for the success of these redox mediators, by significantly reducing the driving force for recombination and therefore allowing for the high open‐circuit voltage ( V OC ) values reported. Very recently, Kannankutty et al studied the coordination of tBP to [Cu(dmp) 2 ] 2+ using absorption and 1 H‐NMR spectroscopy and concluded that [Cu(dmp) 2 (TBP)(MeCN) x ] 2+ (with x = 0, 1) is the most likely species in the electrolyte mixture.…”

Copper Coordination Complexes for Energy-Relevant Applications

“…The electrochemical properties of copper complexes strongly rely on the chelating ligands, since the coordination sphere impacts the redox potentials and the reorganisation between Cu(I) and Cu(II) ions [28]. For example, incorporation of bulky groups into neighboring sites of the chelating atoms exerts a remarkable steric hindrance effect to the Cu(I)/Cu(II) redox shuttle for Cu(I) complexes [11,29], which lowers the reorganisation energies and therefore the structural changes between copper I and II, which subsequently leads to more efficient charge transfer between the sensitizer and the redox species [30].…”