Theoretical investigation of regeneration mechanism of the metal-free sensitizer in dye sensitized solar cells

Abstract

The regeneration mechanism of the oxidized metal-free sensitizer reacting with electrolyte containing iodide/triiodide(I−/I3-) as a redox mediator in dye-sensitized solar cells (DSSC) has been investigated by the density functional theory (DFT). Reported mechanisms which are usually discussed in ruthenium sensitizer-based DSSC, single iodide process (SIP) and two iodide process (TIP), are theoretically evaluated in the metal-free sensitizer containing carbon chains with heteroatoms (S atoms). Calculation indicates that TIP plays a critical role when regeneration reactions occur in different reaction sites of the cationic dye from the view of charges and energetics. While, reduction reactions at two special sites, where iodide ion reacts with S atom of the carbon chain in the direction of being perpendicular to the nearby benzene plane, are more likely to proceed along with the SIP. The occurrence of the SIP lies in a fast inner-sphere electron transfer (ISET) process, which is usually caused by the partially desolvated in acetonitrile when solvent effects are considered. In addition, calculated Mulliken spin densities also imply that partial charges are transferred from the iodide ion to dye+. Once the second iodide participates, oxidized dye is reduced completely. Besides I−, reactions involving I3− and I2− are also taken into consideration due to the complexity of the regeneration process. This exploration on the regeneration mechanism of the metal-free sensitizer confirms the reported regeneration process and provides assistance for further research in the regeneration process of the metal-free sensitizer.