Exploiting Conformational Dynamics of Structurally Tuned Aryl-Substituted Terpyridyl Ruthenium(II) Complexes to Inhibit Charge Recombination in Dye-Sensitized Solar Cells

Abstract

To explore the impact of dye structure on photoinduced interfacial electron-transfer (ET) processes, a series of systematically tuned 4′-aryl-substituted terpyridyl ruthenium­(II) complexes have been studied in TiO2 film and dye-sensitized solar cell (DSSC) device settings. Structural tuning is achieved by the introduction of methyl substituents at the ortho positions of a ligand aryl moiety. Solar power conversion efficiencies are measured, and these values are deconstructed to better understand the fundamental processes that control light-to-current conversion. Injection yields are identified as the primary factor limiting efficiencies, due in large part to significant nonradiative decay pathways in these bis-terpyridyl Ru­(II) systems. Encouragingly, the addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design strategy for improving solar conversion efficiency in solar cells and water-splitting devices.

Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics

“…Furthermore, recent studies have shown that charge recombination to oxidized dyes is sensitive to the bridge that separates them from the surface or the orientation of the surface linker. 39 , 40 Indeed, specific bridge mediated pathways to oxidized dyes have been identified. 37 Taken together, these findings indicate that the observed rate constants do, at least partially, report upon the interfacial electron transfer rate constant(s).…”

“… 32 – 38 It is possible that diffusion does rate limit interfacial electron transfer in some cases, however this does not explain the reported driving force dependencies or the recent observation of specific interfacial electron transfer pathways at sensitized TiO 2 interfaces. 39 , 40 …”

Dihedral‐Angle‐Controlled Crossover from Static Hole Delocalization to Dynamic Hopping in Biaryl Cation Radicals

“…[1][2][3][4] These measurements have shown that under coherent transport mechanism the charge-transfer rates scale as the square of the electronic coupling in biaryl compounds, and since the electronic coupling varies with the interplanar dihedral angle (f)b etween the aryl groups,t he chargetransfer rates follow as quared cosine trend with f. [5][6][7][8] Importantly,t he residence time for the charge onto biaryls is expected to be negligible in coherent charge transport using break-junction techniques. The implication of this finding in non-coherent charge-transfer rates is being investigated.…”

“…[1][2][3][4] These measurements have shown that under coherent transport mechanism the charge-transfer rates scale as the square of the electronic coupling in biaryl compounds, and since the electronic coupling varies with the interplanar dihedral angle (f)b etween the aryl groups,t he chargetransfer rates follow as quared cosine trend with f. [5][6][7][8] Importantly,t he residence time for the charge onto biaryls is expected to be negligible in coherent charge transport using break-junction techniques. [1][2][3][4] These measurements have shown that under coherent transport mechanism the charge-transfer rates scale as the square of the electronic coupling in biaryl compounds, and since the electronic coupling varies with the interplanar dihedral angle (f)b etween the aryl groups,t he chargetransfer rates follow as quared cosine trend with f. [5][6][7][8] Importantly,t he residence time for the charge onto biaryls is expected to be negligible in coherent charge transport using break-junction techniques.…”

“…The implication of this finding in non-coherent charge-transfer rates is being investigated.Biaryl compounds represent the smallest building blocks for exploration of the fundamental properties of the charge transfer in p-conjugated molecular wires.F or example,t he angular dependence of the charge transfer in biaryls has been probed by conductance measurements using break-junction techniques. [1][2][3][4] These measurements have shown that under coherent transport mechanism the charge-transfer rates scale as the square of the electronic coupling in biaryl compounds, and since the electronic coupling varies with the interplanar dihedral angle (f)b etween the aryl groups,t he chargetransfer rates follow as quared cosine trend with f. [5][6][7][8] Importantly,t he residence time for the charge onto biaryls is expected to be negligible in coherent charge transport using break-junction techniques. [2,4] Unlike the break-junction techniques,t he non-coherent charge transfer through abiaryl linkage would involve afinite residence time onto the biaryl and would depend on the interplay between the electronic coupling and the structural reorganization.…”