Coumarin-based donor–π–acceptor organic dyes for a dye-sensitized solar cell: photophysical properties and electron injection mechanism

Abstract

© 2015, Springer-Verlag Berlin Heidelberg. The electronic structure and photophysical properties of five coumarin-based donor–π–acceptor (D–π–A)-type organic dyes for a dye-sensitized solar cell (DSSC) which were recently developed have been investigated using the time-dependent density functional theory and the symmetry-adapted cluster configuration interaction method. Theoretical calculations including the solvent effect in state-specific and linear-response scheme reproduced the experimental UV–Vis absorption spectra of these dyes satisfactorily. The π-spacers, thiophene and thiophene–phenylene mixed units, affect the planarity of the molecular structures which is relevant to the photophysical properties and charge polarization. Energy levels of the frontier orbitals and charge separation were analyzed, and the thiophene linker was found to be effective for the electron injection in DSSC. The adsorption of these dyes on the TiO2anatase (101) surface and the electron injection mechanism were also investigated using a dye–(TiO2)38cluster model employing PBE and TD-CAM-B3LYP calculations, respectively. The adsorption energies of these dyes were estimated to be ~14 kcal/mol, indicating strong adsorption of dye to a TiO2surface by carboxylate group. The possible direct electron injection mechanism was suggested in the present coumarin-based D–π–A dyes in a dye–TiO2interacting system.