Non-linear charge transport in polythiophene under high AC field
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0889-W08-01.1
Non-linear charge transport in polythiophene under high AC field
J. Obrzut, T. Psurek, C. K. Chiang and D. M. DeLongchamp National Institute of Standards and Technology, Gaithersburg, MD, U.S.A.
ABSTRACT Complex impedance and conductivity were measured for regioregular poly(3hexylthiophene) (P3HT) at alternating current (AC) voltages using a waveform technique. The waveforms were Fourier transformed from time domain to frequency domain and analyzed at fundamental and higher order harmonic frequencies. It was found that the impedance of the semi-conducting P3HT decreases with increasing electric field strength. The non-linear charge transport is dominated by a third harmonic response that originates from extended polarizability of π-type electronic states. The third order non-linear conductivity can be used to quantify the effect of an electric field on the conduction mechanism and to correlate the intrinsic charge carriers mobility with molecular structure.
INTRODUCTION Semi-conducting conjugated polymers have been the focus of many studies due to their potential applications in organic field effect transistors (OFET), and light emitting diodes [1]. In particular, regioregular P3HT shows reasonable charge carrier mobility, which has been attributed to the self organization of this material [2, 3]. An analytical one-dimensional hopping model developed for semi-conducting polymers predicts strong dependence of conductivity on electric field [4]. Since in typical OFETs the thickness of the semi-conducting layer is in the range of about 100 nm, non-linear dielectric and conductivity effects can be induced by the operating electric field, which is typically larger than 106 V/cm. A convenient method of probing the field-induced non-linear characteristics is to measure the higher-order harmonic components of the non-linear polarization under a sinusoidal alternating (AC) electric field. The amplitude of the harmonics reflects the magnitude of the non-linear effect. This methodology was pioneered by Furukawa [5] to measure non-linear dielectricity in ferroelectric polymers. The alternating current response in non-linearly conducting composite materials has been analyzed by Gu [6]. Several authors have extended this technique recently and applied it to study fundamental molecular dynamics in non-linear composites, dipolar fluids and molecular dielectrics [7, 8]. In this paper, we describe measurements of complex impedance and non-linear conductivity by recording and analyzing AC waveforms at their fundamental frequency, and at higher order harmonic frequencies. In particular, we are seeking to discover a correlation between the charge carrier transport mechanism and non-linear conductivity in organic semiconductors. We have examined and demonstrated this effect for semi-conducting regioregular poly(3-hexylthiophene).
Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.
0889-W08-01.2
EXPERIMENTAL DETAILS Materials: Poly(3-hexylthiophe
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