title: Ultrafast electron and hole transfer dynamics of a solar cell dye containing hole acceptors on mesoporous TiO2and Al2O3.
abstract: The stability of dye cations against recombination with conduction band electrons in mesoporous TiO2electrodes is a key property for improving light harvesting in dye-sensitised solar cells. Using ultrafast transient broadband absorption spectroscopy, we monitor efficient intramolecular hole transfer in the solar cell dye E6 having two peripheral triarylamine acceptors. After photoexcitation, two hole transfer mechanisms are identified: a concerted mechanism for electron injection and hole transfer (2.4 ps) and a sequential mechanism with time constants of 3.9 ps and 30 ps. This way the dye retards unwanted recombination with a TiO2conduction band electron by quickly moving the hole further away from the surface. Contact of the E6/TiO2surface with the solvent acetonitrile has almost no influence on the electron injection and hole transfer kinetics. Fast hole transfer (2.8 ps) is also observed on a "non-injecting" Al2O3surface generating a radical cation-radical anion species with a lifetime of 530 ps. The findings confirm the good intramolecular hole transfer properties of this dye on both thin films. In contrast, intramolecular hole transfer does not occur in the mid-polar organic solvent methyl acetate. This is confirmed by TDDFT calculations suggesting a polarity-induced reduction of the driving force for hole transfer. In methyl acetate, only the relaxation of the initially photoexcited core chromophore is observed including solvent relaxation processes of the electronically excited state S1/ICT.
authors.affiliation: Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany. oum@chemie.uni-siegen.de lenzer@chemie.uni-siegen.de.
authors.forename: Mirko
authors.initials: M
authors.lastname: Scholz
authors.affiliation: Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany. oum@chemie.uni-siegen.de lenzer@chemie.uni-siegen.de.
authors.forename: Oliver
authors.initials: O
authors.lastname: Flender
authors.affiliation: Department of Chemistry-Ångström Laboratory, Center of Molecular Devices, Uppsala University, Box 523 751 20, Uppsala, Sweden. gerrit.boschloo@kemi.uu.se.
authors.forename: Gerrit
authors.initials: G
authors.lastname: Boschloo
authors.affiliation: Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany. oum@chemie.uni-siegen.de lenzer@chemie.uni-siegen.de.
authors.forename: Kawon
authors.identifier: http://orcid.org/0000-0001-6137-2236
authors.initials: K
authors.lastname: Oum
authors.affiliation: Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany. oum@chemie.uni-siegen.de lenzer@chemie.uni-siegen.de.
authors.forename: Thomas
authors.identifier: http://orcid.org/0000-0002-0766-709X
authors.initials: T
authors.lastname: Lenzer