Charge-Transport in Partially-Ordered Regioregular Poly(3-Hexylthiophene) Studied as a Function of the Charge-carrier De
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Charge-transport in partially-ordered regioregular poly(3-hexylthiophene) studied as a function of the charge-carrier density D.B.A. Rep1, B.-H. Huisman2, E.J. Meijer1,2, P. Prins1, and T.M. Klapwijk1 1 Department of Applied Physics and DIMES, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands 2 Philips Research Laboratories, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands ABSTRACT We demonstrate that charge-transport in partially-ordered thin films of poly(3hexylthiophene) is strongly dependent on the charge-carrier density in the film, which we relate to the localized nature of the charge carriers. We base our conclusions on measurements in which the charge-carrier density is varied by electrostatic means (in a transistor geometry) and by insitu thermal removal of oxygen dopants. For the doping levels investigated, we argue that the removal of oxygen does not induce structural changes in the conjugated polymer, and that it only shifts the Fermi-level of the film. INTRODUCTION The presence of disorder in semiconducting organic materials is known to severely influence the charge-transport properties of these materials. Charges become localized and move by hopping. Hopping conduction in disordered organic systems is often observed to be electricfield dependent and thermally-activated. Here we investigate these aspects of charge-transport and their dependence on the charge-carrier density in the organic material. This is done for regioregular poly(3-hexylthiophene) [RR P3HT]. RR P3HT forms partially-ordered thin films, in which ordered regions are embedded in a disordered matrix [1]. Recent results show that by increasing the charge-carrier density in RR P3HT, one can observe a change from semiconducting behavior to metallic behavior and even to superconducting behavior [2]. These results indicate the strong dependence of charge-transport in RR P3HT on the charge-carrier density. For other organic materials, we have also observed changes in the charge-transport properties with charge-carrier density [3]. Here we will utilize two methods to vary the chargecarrier density in RR P3HT while investigating the charge-transport properties for different densities. Both methods can be applied in-situ, and we will argue that they do not involve any significant structural rearrangement of the polymer material. Charge-transport is investigated as a function of temperature and electric field, and the results for different device geometries based on either (de)doping method will be compared and discussed in the framework of hopping conduction in an energetic distribution of localized states. DEDOPING BY THERMAL TREATMENT IN VACUO The first in-situ method to vary the charge-carrier density in the film, and thereby the position of the Fermi-level, is by making use of the interaction of RR P3HT with oxygen. We have prepared devices as shown in Fig. 1a, consisting of a glass substrate onto which a four-point geometry of gold electrodes is defined by photolithography. As a last step, the polymer film
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