Computational studies on the interactions among redox couples, additives and TiO2: implications for dye-sensitized solar cells

One of the major and unique components of dye-sensitized solar cells (DSSC) is the iodide/triiodide redox couple. Periodic density-functional calculations have been carried out to study the interactions among three different components of the DSSC, i.e. the redox shuttle, the TiO2 semiconductor surface, and nitrogen containing additives, with a focus on the implications for the performance of the DSSC. Iodide and bromide with alkali metal cations as counter ions are strongly adsorbed on the TiO2 surface. Small additive molecules also strongly interact with TiO2. Both interactions induce a negative shift of the Fermi energy of TiO2. The negative shift of the Fermi energy is related to the performance of the cell by increasing the open voltage of the cell and retarding the injection dynamics (decreasing the short circuit current). Additive molecules, however, have relatively weaker interaction with iodide and triiodide.