Charge carrier transport in poly(2,5-dialkoxy-p-phenylene ethynylene)s Akshay Kokil a , Irina Shiyanovskaya b , Kenneth D. Singer a,b,* , Christoph Weder a a Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA b Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7079, USA Received 22 July 2002; received in revised form 3 October 2002; accepted 6 November 2002 Abstract The charge carrier transport in poly[2,5-dioctyloxy-1,4-diethynyl-phenylene-alt-2,5,-bis(2 0 -ethylhexyloxy)-1,4-phenylene], a poly(p- phenylene ethynylene) (PPE) derivative, was investigated by time-of-flight (TOF) measurements. The charge transport characteristics in this material are ambipolar, and the transport is dispersive, with high electron (2:2  10 3 cm 2 V 1 s 1 ) and hole (1:8  10 3 cm 2 V 1 s 1 ) mobilities at room temperature and at low field (3:1  10 4 V cm 1 ). The mobility strongly depends on the field strength, and is observed to decrease with increasing bias. The positional and energetic disorder parameters were calculated from the temperature and field dependencies of the charge mobility using a Gaussian disorder transport formalism. The positional disorder was found to be larger than the energetic disorder over the entire experimental temperature range, leading to the observed negative field dependence of the carrier mobility. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Charge carrier transport; Poly(p-phenylene ethynylene); Electron; Hole In the past three decades, p-conjugated semiconducting polymers have attracted significant interest, since these materials may combine the processability and outstanding mechanical properties of polymers with the exceptional, readily tailored electronic and optical properties of func- tional organic molecules [1]. The potential use of these materials in light-emitting diodes (LEDs) [2], field-effect transistors (FETs) [3], photorefractive devices [4], and photovoltaic cells [5], have motivated the development of synthesis and processing methods of conjugated polymer materials with unique, field-responsive properties [6]. Among a variety of materials platforms, PPE derivatives (Fig. 1) have attracted the attention of a number of research groups [7]. While the nonlinear optical properties of PPEs have been carefully studied [8], and the photoluminescence of these materials has been exploited in sensors [9] and other devices [10], the investigation and exploitation of the semi- conducting properties of PPEs have, somewhat surprisingly, received relatively little attention [11–13]. Only recently, PPEs have been demonstrated [14] to be more useful in polymer LEDs, than the original experiments had indicated [15]. In connection with the investigation of organometallic polymer networks based on a PPE derivative and Pt (0) [16], the carrier mobilities in poly[2,5-dioctyloxy-1,4-diethynyl- phenylene-alt-2,5,-bis(2 0 -ethylhexyloxy)-1,4-phenylene] (EHO-OPPE, Fig. 1) [17], a soluble PPE derivative have recently attracted our own interest. In this letter, we present a detailed study of the charge transport properties of this material, which is representative of this family of conjugated polymers. The charge transport in EHO-OPPE was investigated using TOF measurements on indium-tin-oxide (ITO)/poly- mer/gold sandwich structures [18]. In this technique, a short light pulse incident on the polymer through a semitranspar- ent electrode creates a thin sheet of charge carriers and, depending on the polarity of the electric field E applied between the electrodes, electrons or holes are driven across the sample. The absorption depth of the optical excitation is small compared to the sample thickness L (for the present polymer ca. 1 mm at the wavelength of interest [17]) and the duration of the optical pulse is short compared to the transit time t tr of the charge carriers. Thus, using m ¼ L=t tr E the carrier mobility m can be obtained from the displacement photocurrent transients. The polymer sample used in the present study was of a number-average molecular weight, M n , of about 10,000 g mol 1 , and was synthesized according to standard procedures [17]. ITO/EHO-OPPE/gold sand- wich structures were produced by casting a toluene solution of the polymer (10 mg ml 1 ) onto ITO-coated glass-slides (EHC, Japan), drying the resulting films in vacuo at 408 for at least 12 h (L ¼ 6:512 mm) and depositing a 3 mm  3 mm Synthetic Metals 138 (2003) 513–517 * Corresponding author. Present address: Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106- 7079, USA. Tel.: þ1-216-368-4017; fax: þ1-216-368-4671. E-mail address: kds4@po.cwru.edu (K.D. Singer). 0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01241-9