Investigation of the electronic properties of cyclopentadithiophene polymers and copolymers
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Investigation of the electronic properties of cyclopentadithiophene polymers and copolymers Paolo Coppo* and Michael L. Turner* Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill, S3 7HF, Sheffield, UK. Domenico C. Cupertino and Stephen G. Yeates Avecia Research Centre, Hexagon Tower, Blackley, M9 8ZS, Manchester, UK. ABSTRACT Cyclopentadithiophene based homopolymers and copolymers have been investigated as semiconductor layers in field effect transistors. They generally show low hole mobilities due to a disordered amorphous structure in the solid state. FET mobility can be enhanced in the homopolymer, although this results in poor on/off ratio of the device. The low mobilities may be attributed to the presence of solubilizing alkyl chains normal to the long axis of the polymer which prevent the self assembly of the polymer into an ordered π-stacked configuration. INTRODUCTION Solution processable conjugated polymers have attracted considerable attention due to their possible application in low performance plastic circuits.[1] Tremendous recent interest has focused on understanding the mechanisms of charge transport in conjugated polymers, and it is clear that the electronic properties of thin films of these materials are strongly dependent on the degree of order achieved in the solid state. Conjugated polymers that form disordered amorphous thin films show low charge mobilities due to inefficient inter-chain transport of carriers.[2] To achieve higher charge mobilities in thin films of polymers, enhanced inter-chain order is required. This is seen for thin films of poly(3-alkylthiophenes) as these polymers can self assemble in the solid state when processed from solution.[3] In particular, the polymer chains are aligned such that the π-stacking direction is parallel to the substrate, allowing fast transport of the carriers in the transistor channel. The mobility of holes can be as high as 0.1 cm2/Vs in a field effect transistor.[4] Self-assembly from liquid crystal phases has also been observed for a dioctylfluorene-bithiophene copolymer (F8T2). This polymer can be aligned from the nematic phase and quenched into a quasi-monodomain that showed enhanced mobility, up to 0.01 cm2/Vs.[5] In this contribution, polycyclopentadithiophenes, a class of solution processable, thiophene based analogues of the polyfluorenes [6], are investigated as charge transport materials in FETs. RESULTS AND DISCUSSION: Dioctylcyclopentadithiophene homopolymer (1) was synthesised via oxidative polymerization using iron(III) chloride, according to previously reported procedures.[7] Two different samples, containing a different amount of iron impurities were tested. In the first sample (1a), 1050 ppm of iron were detected by ICP-AES analysis. A thin film of this material, processed by spin casting from a 0.8% chloroform solution, gave a FET mobility of 2 x 10-3 cm2/Vs. Further purification by filtration through a silica gel layer gave a second sample (1b)
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