A series of cyclopentadithiophene based polymers have been synthesized by oxidative polymerisation or palladium-catalyzed coupling reactions. Polymers have been prepared with backbones containing benzothiadiazole, benzoxadiazole, 2,2′-bithiophene, phenyl and fluorene units. The backbone structure is shown to have a large effect on the film morphology, and the optical and electrochemical properties in solution and the solid state. We show that the polymer composition can be used to tune the band gap over the entire visible spectrum. Spin-coated films of these polymers have been used as the active layer of organic field effect transistors and hole mobilities up to 3.7 × 10⁻³ cm² V⁻¹ s⁻¹ have been measured under ambient conditions.

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 cm² 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 = 1 : 1) and also shows a moderate improvement in OPV performance.