Modeling the effect of ionic additives on the optical and electronic properties of a dye-sensitized TiO2 heterointerface: absorption, charge injection and aggregation

Abstract

We present a joint experimental and theoretical study with the aim of investigating the individual effects of Li-TFSI and EMIM-TFSI additives on the optical and charge transfer properties of D102 dye in solution as well as at the dye-sensitized TiO₂ interface. Experimental results show that while the spectral shifts are negligible for the dye in solution, when moving to the TiO₂ film in air the addition of both lithium and EMIM salts clearly gives rise to a slight red-shift with the appearance of a lower-energy shoulder in the absorption spectrum. Computational modelling confirms the weak tendency to have stable dye/additive complexes in solution and predicts appreciable spectral red-shifts as a consequence of the interaction of the dye with Li⁺ and EMIM⁺ cations. We also predict a strong effect of the additives on the electronic coupling between the dye's LUMO and the TiO₂ conduction band states, reflecting on the calculated injection rates. Further, by modeling the formation and the optical response of selected dye/additive aggregate models, we find a general broadening of the absorption band, accounting for the experimentally observed lower-energy shoulder in the D102 absorption spectrum recorded on TiO₂ films where Li and EMIM salts are added.