Interactions of the N3 dye with the iodide redox shuttle: Quantum chemical mechanistic studies of the dye regeneration in the dye-sensitized solar cell

The iodide/triiodide redox couple plays a unique role in the dye-sensitized solar cell (DSSC). It is a necessary and unique part of every highly efficient DSSC published to date; alternative redox couples do not perform nearly as well. Hence, a detailed molecular-level understanding of its function is desirable. A density-functional theory (DFT) study has been carried out on the kinetic and thermodynamic aspects of the dye regeneration mechanism involving the iodide/triiodide redox couple and the prototypical N3 dye in the DSSC. The intermediate complexes between the oxidized dye and iodide have been identified. These are outer-sphere complexes of the general formula [dye ⁺⋯I^-]. Solvent effects are seen to play a critical role in the thermodynamics, whereas relativistic spin-orbit effects are less important. Both the kinetic and thermodynamic data reveal that the formation of complexes between [dye⁺⋯I^-] and I^- is the rate limiting step for the overall dye regeneration process. The regeneration of the neutral dye proceeds with the liberation of I-2; processes involving atomic iodine or I^- are inferior, both from thermodynamic and kinetic considerations. The overall dye regeneration reaction is an exothermic process.