Investigation of solution processed poly(4,4-dioctylcyclopentadithiophene) thin films as transparent conductors
Introduction
Conjugated polymers are emerging as the basis of a number of new technologies, including plastic electronics, in which the intrinsic properties of the polymers are critical in determining device performance [1]. In plastic electronics the use of polymer based dielectrics, conductors and semiconductors has attracted considerable interest [2] and for the manufacture of simple electronic devices by direct write techniques such as ink-jet printing, solution processability from common organic solvents is essential [3]. Thiophene based polymers have been extensively used as the semiconducting layer in organic field effect transistors and when doped as the contact electrodes [4]. In general the high conductivity of doped polythiophene films can be attributed to the formation of a charge transfer complex between the electron rich thiophene ring and the oxidative dopant [5]. Polyethylenedioxythiophene (PEDOT) doped with polystyrenesulfonic acid (PSS) has recently found application in organic light emitting devices, where it provides an ohmic contact to the organic semiconductor. PEDOT-PSS films are transparent in the visible region of the spectrum and stable in the doped form [6]. Neutral PEDOT is difficult to process as it is insoluble and essentially intractable. In the doped form PEDOT-PSS can be processed from aqueous colloidal dispersions to give thin films of the quality required for application in plastic electronics and PLEDs [6].
Cyclopentadithiophene polymers are a class of highly conjugated polythiophenes in which adjacent thiophene rings are bridged by a carbon centre. Polycyclopentadithiophenes can be directly grown by anodic coupling of cyclopentadithiophene monomers and they can be doped to high conductivity with a low absorption in the visible region of the spectrum [7].
This contribution reports the optical and electronic properties of solution processed films of poly(4,4-dioctylcyclopentadithiophene) (see Fig. 1) that are doped with iodine and with DDQ. These films show limited absorption in the visible region of the spectrum and an electrical conductivity comparable to that of PEDOT-PSS films.
Section snippets
Results and discussion
Poly(4,4-dioctylcyclopentadithiophene) (see Fig. 1) was prepared by solution polymerisation of the corresponding monomer with iron(III) chloride [8], [9]. Dedoping treatment with hydrazine gave a solution processable, neutral polymer. The iron content of the polymer was reduced to <200 ppm by filtration through a layer of silica gel. The neutral polymer was of high molecular weight (Mn=45 000) and exhibits good solubility in chloroform (∼30 g/l) but limited solubility in tetrahydrofuran and
Experimental details
Poly(4,4-dioctylcyclopentadithiophene) was prepared, de-doped and extensively purified according to previously reported procedures.[9]. Chloroform was purchased from Fisher chemicals (HPLC grade). Thin films were obtained by spin coating filtered solutions (0.8 g/100 ml) of polymer or from dispersions of doped polymer (0.5 g/100 ml), over 100 nm thick interdigitated gold electrodes, supported on glass, at 2000 rpm for 1 min in a clean room atmosphere. The films were then allowed to dry in a vacuum
Acknowledgements
Avecia Ltd., the EPSRC (Organic Materials for Electronics Consortium, see www.omec.org.uk) and The Royal Society are gratefully acknowledged for funding this project.
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