Improved Thin Film Transistor (TFT) performance using fractionated Poly-3-hexylthiophene (P3HT)
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Improved Thin Film Transistor (TFT) performance using fractionated Poly-3-hexylthiophene (P3HT) Munira Raja§, Giles Lloyd§, Naser Sedghi§, Rafaella di Lucrezia*, Simon J. Higgins* and W. Eccleston§ § Department of Electrical Engineering and Electronics, *Department of Chemistry University of Liverpool, Liverpool L69 3BX, UK ABSTRACT By using the Trznadel method, it has been possible to increase molecular weight, the regio-regularity, and probably to decrease the residual catalyst of poly-3-hexylthiophene thin films. The drift mobility of holes, normal to the surface of cast films, in air, has been found using Schottky diodes, and field-effect mobility has been measured with Thin-Film Transistors. Three types of film have been studied using the two methods of assessing mobility. The as-synthesised films are compared with those that have been fractionated. The third set of films involves doping with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The doped films show a field effect mobility of 0.2 cm2V-1s-1: all others being lower. Field effect mobility is approximately two orders of magnitude greater than that in the bulk normal to the plane of the film. Doping levels in the films are found to be similar, probably because of the process conditions. INTRODUCTION Thin-Film Transistor (TFT) circuits are, in principle, capable of being incorporated into both large area and low cost applications. The potential of such applications was improved considerably with the achievement of improved carrier mobility values under controlled conditions in regio-regular polyalkylthiophenes [1]. Such materials are soluble in common solvents and can, therefore, provide circuitry over larger areas than is economic with silicon. Obvious applications are solar panels, flat panel displays, and the replacement of costly medical X-ray film. 3D circuits also become a possibility and so, therefore, does the realisation of high interconnect hard-wired neural networks. Pixel drivers for displays are also a strong possibility, as is a range of smart credit-card sized circuits, on plastic, with applications in commercial and domestic security, and finance. The kinds of costs that are compatible with disposability are a further incentive. In fact the range of applications is very extensive, with circuit configurations that are fairly simple, if new in concept. Polymer Quality and Electrical Assessment Materials quality and supply has emerged as a major issue. Commercial material proved to have variable head-to-tail counts between batches: a factor that we believe to be endemic with poly-3-alkylthiophenes synthesised by the Rieke Method [2]. From the point of view of circuits this is a major difficulty since it leads to parameter spreads and, therefore, a reduction in performance. This phenomenon also became apparent in the Liverpool field emission work [3,4] where very low thresholds seen with the early batches of material gradually increased. The problem became very serious after the third batch, and was assisted, but not cured, by a change of solv
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