Cyclopentadithiophene-benzothiadiazole oligomers and polymers; synthesis, characterisation, field-effect transistor and photovoltaic characteristics† Masaki Horie, * ac Jeff Kettle, bc Chin-Yang Yu, dc Leszek A. Majewski, e Shu-Wei Chang, a James Kirkpatrick, f Sachetan M. Tuladhar, g Jenny Nelson, g Brian R. Saunders h and Michael L. Turner * c Received 31st May 2011, Accepted 29th September 2011 DOI: 10.1039/c1jm12449h Conjugated oligomers with various ratios of cyclopentadithiophene (CPDT) to benzothiadiazole (BT) repeating units are reported. These oligomers can be polymerised to high molecular weight polymers (M n > 100k) by oxidative polymerisation using iron(III)chloride. The electronic properties of these materials were examined by cyclic voltammetry and UV-vis spectroscopy and the results compared to those calculated using density functional theory (DFT). Polymers with optimum ratios of CPDT : BT (2 : 1) units show hole mobilities in excess of 10 2 cm 2 V 1 s 1 as the active layer in organic field effect transistors (OFETs). Bulk heterojunction organic photovoltaic (OPV) devices of these polymers with PCBM as the electron acceptor show a power conversion efficiency of 2.1% when processed in the absence of any additives. The reported OFET performance is significantly higher than the parent PCPDTBT alternating copolymer (CPDT : BT ¼ 1 : 1) and also shows a moderate improvement in OPV performance. 1. Introduction Conjugated polymers have demonstrated tremendous potential for applications: the active layers of organic light-emitting diodes (OLEDs), 1 field-effect transistors (OFETs) 2,3 and photovoltaic devices (OPVs). 4,5 One of the important advantages of conju- gated macromolecules is the ability of these materials to be processed from solution. This enables the fabrication of elec- tronic devices such as OPVs by low cost, large area, high throughput production techniques such as gravure, inkjet and screen printing 6,7 or slot-die coating. 8 The performance of bulk- heterojunction (BHJ) 9 based OPV devices has rapidly improved recently, 4,5 as increased structural diversity of organic materials and improved processing protocols have led to record power conversion efficiencies in excess of 8%. 10 The most extensively investigated semiconducting polymers for application in OPVs are derivatives of polythiophenes. 11 In these polymers the introduction of fused ring systems such as cyclopentadithiophenes (CPDTs) (the homopolymer, PCPDT, is depicted in Chart 1), 12–14 thienothiophenes, 15 dithienopyrroles 16 and dithienosiloles 17 leads to extended conjugation and enhanced intermolecular stacking. Furthermore the introduction of acceptor units such as benzothiadiazole (BT) into these polymers leads to higher performance in OFETs and OPVs. 18–20 The charge transfer between donor and acceptor units leads to a reduction in the optical band gap 21 and more effective light harvesting in OPV devices. For example, the onset for absorption of poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b 0 ]dithio- phene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (PCPDTBT) (Chart 1) is around 1.4 eV and this polymer has been used to fabricate OPV devices in combination with fullerene derivatives with power conversion efficiencies of up to 5.5%. 19 Effective capture of solar light in OPVs requires polymer films that are thick enough to absorb all incident photons with ener- gies above the band gap (>100 nm). In general to attain these film Chart 1 a Frontier Research Center on Fundamental and Applied Sciences of Matters, Department of Chemical Engineering, National Tsing-Hua University, 101, Sec. 2, Kuang-Fu Road, Hsin-Chu, 30013, Taiwan. E-mail: mhorie@mx.nthu.edu.tw b School of Electronic Engineering, Bangor University, Dean st., Bangor, Gwynedd, LL57 1UT, Wales, UK c Organic Materials Innovation Centre, School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. E-mail: michael. turner@manchester.ac.uk d Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei, Taiwan 106 e School of Electrical and Electronic Engineering, University of Manchester, Manchester, M60 1QD, UK f Oxford Martin School, University of Oxford, UK g Department of Physics, Imperial College London, London, UK h School of Materials, University of Manchester, Grosvenor St, Manchester, M1 7HS, UK † Electronic supplementary information (ESI) available. See DOI: 10.1039/c1jm12449h This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 381–389 | 381 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 381 www.rsc.org/materials PAPER