Incorporation of Fused Tetrathiafulvalenes (TTFs) into Polythiophene Architectures: Varying the Electroactive Dominance of the TTF Species in Hybrid Systems Rory Berridge, ² Peter J. Skabara,* ,² Cristina Pozo-Gonzalo, ² Alexander Kanibolotsky, ² Jan Lohr, ² Joseph J. W. McDouall, ² Eric J. L. McInnes, ² Joanna Wolowska, ² Christoph Winder, ‡ N. Serdar Sariciftci,* ,‡ Ross W. Harrington, § and William Clegg § School of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K., Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler UniVersity Linz, Linz A-4040, Austria, and School of Natural Sciences (Chemistry), UniVersity of Newcastle, Newcastle upon Tyne NE1 7RU, U.K. ReceiVed: December 12, 2005 A novel polythienylenevinylene (PTV) and two new polythiophenes (PTs), featuring fused tetrathiafulvalene (TTF) units, have been prepared and characterized by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroelectrochemistry. All polymers undergo two sequential, reversible oxidation processes in solution. Structures in which the TTF species is directly linked to the polymer backbone (2 and 4) display redox behavior which is dictated by the fulvalene system. Once the TTF is spatially removed from the polymer chain by a nonconjugated link (polymer 3), the electroactivity of both TTF and polythiophene moieties can be detected. Computational studies confirm the delocalization of charge over both electroactive centers (TTF and PT) and the existence of a triplet dication intermediate. PTV 4 has a low band gap (1.44 eV), is soluble in common organic solvents, and is stable under ambient conditions. Organic solar cells of polymer 4:[6,6]- phenyl-C 61 butyric acid methyl ester (PCBM) have been fabricated. Under illumination, a photovoltaic effect is observed with a power conversion efficiency of 0.13% under AM1.5 solar simulated light. The onset of photocurrent at 850 nm is consistent with the onset of the π-π absorption band of the polymer. Remarkably, UV-vis spectroelectrochemistry of polymer 4 reveals that the conjugated polymer chain remains unchanged during the oxidation of the polymer. Introduction The design and synthesis of novel conjugated oligomer and polymer architectures continues to attract great attention in the field of organic semiconductors. The most critical advances focus on improved properties toward specific device applica- tions, such as high mobilities for field effect transistors (OFETs), 1-4 high values of luminescence efficiencies for organic light emitting diodes (OLEDs), 5-7 control of absorption wave- length for OLEDs and electrochromic materials, 8-11 as well as low band gaps 12-17 and charge separation of excited states for organic photovoltaics. 18-20 Organic conjugated oligomers and polymers exhibit semi- conducting behavior and photo/electroactivity in the main chain of the materials. In this respect, the incorporation of secondary redox-active units into conjugated structures is a quandary. Does the additional electroactive species impart its electroactive character into a hybrid state, or does it simply act independently? There are several examples of conjugated materials that feature electroactive units tethered to the polymer backbone, and these include polythiophenes with ferrocene, 21 phthalocyanine, 22 transition metal complexes, 23 viologens, 24,25 fullerenes, 26,27 and anthraquinones. 28,29 Our own interests in this area are based on polythiophene-tetrathiafulvalene (TTF) hybrid materials. Al- though several groups have succeeded in attaching TTFs to polythiophene, 30-33 the fulvalene units are held by nonconju- gated links, which render the TTFs electroactively independent. In a recent paper, 34 we presented the synthesis and properties of a thieno-TTF polymer in which the two heterocyclic units are fused together (1). In this work, we have shown that the TTF species is dominant as the redox-active component and essentially nullifies the p-doping ability of the polythiophene backbone and precludes the possibility of polaron/bipolaron delocalization within the chain. From this viewpoint, one can question the rationale for designing such a structureswhy limit the use of an organic semiconductor by switching off its * To whom correspondence should be addressed. E-mail: peter.skabara@ strath.ac.uk (P.J.S.). Present address: WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland (P.J.S.). ² University of Manchester. ‡ Johannes Kepler University Linz. § University of Newcastle. 3140 J. Phys. Chem. B 2006, 110, 3140-3152 10.1021/jp057256h CCC: $33.50 © 2006 American Chemical Society Published on Web 02/02/2006