An atomistic picture of the active interface in dye sensitized solar cells

Abstract

The objective of this work was to contribute to a detailed understanding of the atomistic structure and the processes of the active interface in dye sensitized solar cells (DSSC) using computational chemistry methods. This study
deals with a prototypical setup used in high performance DSSC, consisting of anatase nanocrystals, the cis-bis(4,4'-dicarboxy-2,2'-bipyridine) dithiocyanato ruthenium(II) dye (N3) and the iodide/triodide redox couple in acetonitrile
as liquid electrolyte. Using force eld based molecular dynamics (FF-MD), the structure of the anatase acetonitrile interface is studied. A strong interaction of the rst layer of acetonitrile with the (101) surface of anatase is found which passivates the surface. In combination with the strong
dipole of acetonitrile, the ordering of the rst layer propagates up to 12A into the bulk solvent and leads to a reduced self diusion constant in this region. From density functional theory (DFT) calculations, a pH dependent equilibrium for the preferred binding mode of the dye on the anatase surface is found, and could be veried by attenuated total re ection (ATR)-IR experiments.
Furthermore, a chain-like self-assembly via hydrogen bonds between neighboring dyes is possible for the determined binding mode.
Next to these structural insights, the regeneration mechanism of the N3 dye has been determined by ab initio molecular dynamics. This mechanism is based on an interaction of the thiocyanate ligand with iodide or diiodide, forming weakly bound intermediate complexes. The predicted formation of a N3-diiodide complex on the anatase surface has been veried afterwards using ATR-IR experiments. Furthermore, the concentration prole of the
redox couple close to the anatase surface has been computed. An inhomogeneous distribution of iodide was found with a maximum concentration in the region of the thiocyanate ligand, explaining the fast regeneration in real
devices. Finally, by systematic studies of system size, solvation eects and density functionals the minimal requirements for unbiased calculations of electronic and excited state properties have been determined. This setup has then been used to compute the excitation spectrum of the N3-dye on the surface and to estimate a lower bound for the electron injection rate of the N3-dye.