A Study of the Interactions between I-/I3- Redox Mediators and Organometallic Sensitizing Dyes in Solar Cells

Specific interactions of the I-/I3- redox mediators with the reduced and oxidized dye, Ru(4,4'-dicarboxy-2,2'-bipyridyl)2(NCS)2, referred to as N3 or Ru(dcbpy)2(NCS)2, were studied by d. functional theory (DFT) with the focus on the charge transfer process involving {dye+ I-} adducts; computations had been performed with d. functionals (gradient-cor. d. functional BP86, and the hybrid d. functionals B3LYP, MPW1K, B3PW1K, and MPW1PW91). Different pathways leading to {dye+ I-} adducts were studied. First, mechanistic insights into the interaction of I- with RuIII(dcbpy)2(NCS)2 via an SCN- ligand directly giving rise to RuII(dcbpy)2(NCS)2I were obtained with the distinctive S-I bonding. Second, the binding of I- to the N3 dye cation via I--dcbpy interactions was analyzed. The authors also report exptl. and computational evidence that sheds light on the interaction of the redox mediator with bipyridyl moieties. Evidence for a charge transfer process in the presence of only one I- anion in the outer coordination sphere of the ruthenium center was identified. Finally, geometries and electronic structures of plausible intermediates were computationally analyzed based on an inner-sphere interaction between the metal center and the redox mediator, including a two-step regeneration reaction: [RuIII(dcbpy)2(NCS)2]+ + I- → [RuIII(dcbpy)2(NCS)I]+ + SCN-, followed by the interaction of a 2nd I- with the intermediate [RuIII(dcbpy)2(NCS)I]+ complex. Conclusive evidence of a charge-transfer process that gives rise to the regenerated RuII complex, where I- interacts with the intermediate [RuIII(dcbpy)2(NCS)I]+ complex was identified.