Abstract
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 <sub>n</sub> > 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 <sup>-2</sup> cm <sup>2</sup> V <sup>-1</sup> s <sup>-1</sup> 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 = 1:1) and also shows a moderate improvement in OPV performance. © 2012 The Royal Society of Chemistry.