Conjugated oligomers with various ratios of cyclopentadithiophene (CPDT) to benzothiadiazole (BT) repeating units are reported. These oligomers can be polymd. to high mol. wt. polymers (Mn>100 k) by oxidative polymn. using iron(iii)chloride. The electronic properties of these materials were examd. by cyclic voltammetry and UV-vis spectroscopy and the results compared to those calcd. using d. functional theory (DFT). Polymers with optimum ratios of CPDT : BT (2 : 1) units show hole mobilities in excess of 10-2 cm2 V-1 s-1 as the active layer in org. field effect transistors (OFETs). Bulk heterojunction org. 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. [on SciFinder(R)] ; Copyright (C) 2012 American Chemical Society (ACS). All Rights Reserved. CAPLUS AN 2011:1590346(Journal; Online Computer File) 10.1039/c1jm12449h