title: A hybrid electron donor comprising cyclopentadithiophene and dithiafulvenyl for dye-sensitized solar cells.
abstract: Two new photosensitizers featured with a cyanoacrylic acid electron acceptor (A) and a hybrid electron donor (D) of cyclopentadithiophene and dithiafulvenyl, either directly linked or separated by a phenyl ring, were synthesized and characterized. Both of them undergo two reversible oxidations and strongly absorb in the visible spectral region due to a photo-induced intramolecular charge-transfer (ICT) transition. To a great extent, the electronic interaction between the D and A units is affected by the presence of a phenyl spacer. Without a phenyl ring, the D unit appears more difficult to oxidize due to a strong electron-withdrawing effect of the A moiety. In sharp contrast, the insertion of the phenyl ring between the D and A units leads to a broken π-conjugation and therefore, the oxidation potentials remain almost unchanged compared to those of an analogue without the A group, suggesting that the electronic coupling between D and A units is relatively weak. As a consequence, the lowest-energy absorption band shows a slight hypsochromic shift upon the addition of the phenyl spacer, indicative of an increased HOMO-LUMO gap. In turn, the direct linkage of D and A units leads to an effective π-conjugation, thus substantially lowering the HOMO-LUMO gap. Moreover, the application in dye-sensitized solar cells was investigated, showing that the power conversion efficiency increases by the insertion of the phenyl unit.
authors.affiliation: Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Gleb
authors.initials: G
authors.lastname: Sorohhov
authors.affiliation: Laboratory of Photonics and Interfaces, Institute of Chemical Science and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1050 Lausanne, Switzerland.
authors.forename: Chenyi
authors.initials: C
authors.lastname: Yi
authors.affiliation: Laboratory of Photonics and Interfaces, Institute of Chemical Science and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1050 Lausanne, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Grätzel
authors.affiliation: Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Silvio
authors.initials: S
authors.lastname: Decurtins
authors.affiliation: Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
authors.forename: Shi-Xia
authors.initials: SX
authors.lastname: Liu
keywords: Knoevenagel reaction; donor–acceptor systems; dye-sensitized solar cells; electrochemistry; intramolecular charge transfer; tetrathiafulvalene