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

Specific interactions of the I − /I 3 − 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, have been studied by means of density functional theory (DFT) with the focus on the charge transfer process involving {dye + I − } adducts; computations had been performed with a series of density functionals (gradient-corrected density functional BP86, and the hybrid density functionals B3LYP, MPW1K, B3PW1K, and MPW1PW91). Different pathways leading to {dye + I − } adducts have been studied. First, mechanistic insights into the interaction of I − with Ru III (dcbpy) 2 (NCS) 2 via an SCN − ligand directly giving rise to Ru II (dcbpy) 2 (NCS) 2 I⌉ 0 have been obtained with the distinctive S−I bonding. Second, the binding of I − to the N3 dye cation via I − −dcbpy interactions has been analyzed. We also report experimental 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 has been identified. Finally, geometries and electronic structures of plausible intermediates have been computationally analyzed based on an inner-sphere interaction between the metal center and the redox mediator, including a two-step regeneration reaction: Ru III (dcbpy) 2 (NCS) 2 ⌉ + + I − → Ru III (dcbpy) 2 (NCS)I⌉ + + SCN −, followed by the interaction of a second I − with the intermediate Ru III (dcbpy) 2 (NCS)I⌉ + complex. Conclusive evidence of a charge-transfer process that gives rise to the regenerated Ru II complex, where I − interacts with the intermediate Ru III (dcbpy) 2 (NCS)I⌉ + complex has been identified.