Computational studies of the interactions of I^- and I₃^- with TiO₂ clusters: Implications for dye-sensitized solar cells

First principle density-functional theory calculations have been carried out on the interaction of I^- and I₃^- with TiO₂ anatase surfaces, modeled by finite clusters that range in size from 48 to 180 atoms. The total energy per TiO₂ unit and the HOMO-LUMO gaps decrease with increasing the size of the clusters. Both redox species (I^- and I₃^-) are strongly adsorbed on the TiO₂ surface with the adsorbtion of I^- being stronger. Adsorption of triiodide leads to its dissociation. The positions of the HOMO and LUMO of the adsorbed systems shift negatively from their respective cluster values. Solvation effects have been modeled using the CPCM model. Introducing solvent reduces the shifting of HOMO and LUMO. Implications for dye-sensitized solar cells (DSSC) are discussed. Both the HOMO-LUMO shifting and the strong adsorption might affect the performance of the cell. Graphical Abstract: I₃^- dissociates upon adsorption but I^- adsorbs strongly[Figure not available: see fulltext.]