title: Preferred Molecular Orientation of Coumarin 343 on TiO2 Surfaces: Application to Dye-Sensitized Solar Cells.
abstract: The dye···TiO2 interfacial structure in working electrodes of dye-sensitized solar cells (DSCs) is known to influence its photovoltaic device performance. Despite this, direct and quantitative reports of such structure remain sparse. This case study presents the application of X-ray reflectometry to determine the preferred structural orientation and molecular packing of the organic dye, Coumarin 343, adsorbed onto amorphous TiO2. Results show that the dye molecules are, on average, tilted by 61.1° relative to the TiO2 surface, and are separated from each other by 8.2 Å. These findings emulate the molecular packing arrangement of a monolayer of Coumarin 343 within its crystal structure. This suggests that the dye adsorbs onto TiO2 in one of its lowest energy configurations; that is, dye···TiO2 self-assembly is driven more by thermodynamic rather than kinetic means. Complementary DSC device tests illustrate that this interfacial structure compromises photovoltaic performance, unless a suitably sized coadsorbant is interdispersed between the Coumarin 343 chromophores on the TiO2 surface.
authors.affiliation: †Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.|‡Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia.
authors.forename: Jonathan
authors.initials: J
authors.lastname: McCree-Grey
authors.affiliation: †Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.|§Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Cole
authors.affiliation: ‡Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales 2234, Australia.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Evans
keywords: Coumarin 343; dye-sensitized solar cells; molecular orientation; reflectometry