Investigations into Current Modulation Mechanisms in Low Operating Voltage Organic Thin Film Transistors and Their Relat
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Investigations into Current Modulation Mechanisms in Low Operating Voltage Organic Thin Film Transistors and Their Relationship to the Materials Employed Daniel Elkington1, Xiaojing Zhou1, Warwick Belcher1, Paul Dastoor1 1 Centre for Organic Electronics, Department of Physics, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia ABSTRACT Systematic studies have been conducted on the electrical characteristics of poly(3hexylthiophene)-based organic thin film transistors (OTFTs). The OTFTs have been characterized at low-operating voltages and deductions have been made regarding the current modulation mechanisms involved. Irreproducibility of transfer characteristics in these devices beyond a certain gate voltage, as well as a slow time-dependant component to drain current at certain gate voltages, indicates electrochemical changes occurring in the device during operation. It is hoped that this work can help to improve the understanding of OTFTs of this type and, in turn, their performance in the future. INTRODUCTION OTFTs which operate at low voltages have the advantage of combining all the benefits of organic electronics, such and solution processability and potential low cost, without the impractical high operating voltages required by many organic transistors (sometimes several tens of volts) [1]. They could be particularly useful as an electronic transducer in applications where sensitivity at low voltages is paramount to successful performance, such as biosensors. Sandberg et al.[2,3], introduced the concept of a low-operating voltage OTFT with a hygroscopic insulator – poly(vinylphenol) (PVP) – which was labelled a HIFET (hygroscopic insulator field effect transistor). Here, we have further investigated similar devices with a view to understanding more clearly the mechanisms involved in current modulation. There are several other varieties of organic transistors which operate at low voltages, frequently with better electrical performance than the devices presented here, however their fabrication typically involves processes such as thermal evaporation, thermal cross-linking or some critical inorganic components [4,5]. The aim of researching the proceeding types of devices lies in the desire to fabricate all-solution-processed OTFTs in a low-energy process in air. EXPERIMENTAL DETAILS Devices are fabricated on pre-patterned indium-tin-oxide (ITO)-on-glass substrates. Device architecture and the materials used are shown in Figure 1. ITO forms the source and drain electrodes and the channel length is 20ȝm while the channel width is 3mm. P3HT (from Lumtec) is spin-cast onto these substrates at 2000rpm for 60 seconds from chloroform solution. This layer is then manually patterned and the device is treated at 45°C for 10 minutes to remove
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any excess solvent. The PVP (from Sigma-Aldrich) layer is deposited from ethanol solution by the same spin conditions and the layer is again manually patterned. The device then is treated at 85°C for 10 minutes – again to remove any excess solvent. Fina
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