Hopping Transport and Voltage Induced Metal-Insulator Transition in Polythiophene Field Effect Transistors
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Hopping Transport and Voltage Induced Metal-Insulator Transition in Polythiophene Field Effect Transistors Anoop S. Dhoot1,2, Guangming Wang1,2, Daniel Moses1,2, and Alan J. Heeger1,2 1 Center for Polymers and Organic Solids, University of California, Santa Barbara, California, 93106 2 Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California, 93106
ABSTRACT We have studied the carrier transport in regio-regular polythiophene field effect transistors (FETs) by four-probe measurements of the steady-state channel conductance from room temperature to 4.2 K. At high gate voltage (constant total carrier density, n = 5×1012 cm-2) and at low temperatures, we find that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition. In the insulating regime, the carrier transport is well described by phonon assisted hopping in a disordered Fermi Glass (with Coulomb interactions between the hopping charge carrier and the charge left behind). At the highest gate voltages and at sufficiently high source-drain voltages, the data imply a zero-temperature transition from disordered insulator to metal.
INTRODUCTION Since the discovery of metallic conduction in a chemically doped conjugated polymer [1], there has been interest in realising the metallic regime in pristine conducting polymers. One possible approach is to use the gate voltage (Vg) and capacitance in field-effect transistors (FETs) to induce a sufficiently high density of carriers to reach the metallic regime in pristine polymer devices. Carrier transport in semiconducting polymers is generally limited by the spatial disorder attributable to randomly distributed conjugated polymer segments [3]. For devices with a lot of polymer disorder, the carrier transport proceeds by temperature activated hopping between localized sites, and carrier mobilities typically decrease strongly with decreasing temperature, with only a minor decrease in the activation energy for increasing Vg (carrier density) [4]. Eventually, typical of an insulator, high tunnelling barriers suppress the conductivity in disordered conducting polymers at low temperatures. The state of the art has progressed rapidly since early demonstrations of the field effect in polymer FETs [5], largely due to improvements in the molecular structure of polymers and the processing of polymer films. Recent reports of polymer carrier mobilities that are comparable to amorphous silicon underscore the improved performance of devices. Consistently good device performance is achieved using regioregular poly(3hexylthiophene-2,5′-diyl) (RR-P3HT) as the semiconducting polymer with field-effect mobilities as high as µ = 0.1-0.2 cm2/Vs [6, 7]. Inclusion of regioregular head-to-tail polymer side chains
[6] and optimisation of the polymer deposition [7] has been largely responsible for the improved carrier mobility. X-ray diffraction studies of P3HT-based devices reveal the formation of selforganised nanocrystalline regions [8] that consist of
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