Solution Processed Self-Assembled Monolayer Gate Dielectrics for Low-Voltage Organic Transistors
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1114-G09-02
Solution Processed Self-Assembled Monolayer Gate Dielectrics for Low-Voltage Organic Transistors James Ball1, Paul H. Wöbkenberg1, Florian Colléaux1, Floris B. Kooistra2, Jan. C. Hummelen2, Donal D. C. Bradley1 and Thomas D. Anthopoulos1 1 Imperial College London, Blackett Laboratory, Prince Consort Road, London, SW7 2BZ, U.K. 2 Zernike Institute for Advanced Materials and Stratingh Institute of Chemistry, University of Groningen, Groningen, Netherlands. ABSTRACT Low-voltage organic transistors are sought for implementation in high volume low-power portable electronics of the future. Here we assess the suitability of three phosphonic acid based self-assembling molecules for use as ultra-thin gate dielectrics in low-voltage solution processable organic field-effect transistors. In particular, monolayers of phosphonohexadecanoic acid in metal-monolayer-metal type sandwich devices are shown to exhibit low leakage currents and high geometrical capacitance comparable to previously demonstrated self-assembled monolayer (SAM) type dielectrics [1, 2] but with a higher surface energy. The improved surface energy characteristics enable processing of a wider range of organic semiconductors from solution. Transistors based on a number of solution-processed organic semiconductors with operating voltages below 2 V are also demonstrated. INTRODUCTION Intense research efforts into solution processable materials for organic field-effect transistors (OFETs) have driven device performance to levels approaching the requirements for commercialization [3]. The advantages of solution processable systems include ease of patterning on large areas at room temperature which can potentially be applied to high throughput and flexible [4] electronic applications. Much of the research effort has focused on understanding and improving the field-effect mobility of charge carriers in the active organic semiconductor layer for device development. However, high voltage operation of OFETs is a further hindrance to the practical implementation of this technology. Therefore, a reduction in operating voltage and hence power consumption is crucial for portable (i.e. battery powered) applications where organic devices are likely to be competitive. A successful approach to surmounting the problem has involved the use of solution processable self-assembled monolayers (SAMs) as the gate dielectric. Ultra-thin SAM layers, in comparison to conventional dielectrics, yield an increased geometrical capacitance (Ci) enabling the accumulation of high charge carrier densities at reduced gate bias. There are several earlier examples of SAM based devices. Reports commonly employ alkyltrichlorosilanes, as used by Collet et al. [5], or alkylphosphonic acids, as used by Klauk et al. [1], as the gate insulation layer. However, the majority of examples of low-voltage OFETs based on SAM dielectrics utilize evaporated organic semiconductors for the active layer. Demonstrations of solution processed semiconductors on SAMs are scarce [2, 6, 7]. In several instances
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