Ionic Impurities in Poly(vinyl alcohol) Gate Dielectrics and Hysteresis Effects in Organic Field Effect Transistors
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Ionic Impurities in Poly(vinyl alcohol) Gate Dielectrics and Hysteresis Effects in Organic Field Effect Transistors Martin Egginger1, Mihai Irimia-Vladu2, Reinhard Schwödiauer2, Andreas Tanda3, Siegfried Bauer2, and Serdar Niyazi Sariciftci1 1 LIOS - Linz Institute for Organic Solar Cells, Johannes Kepler University Linz, Altenbergerstr. 69, Linz, 4040, Austria 2 SOMAP - Soft Matter Physics, Johannes Kepler University Linz, Altenbergerstr. 69, Linz, 4040, Austria 3 plastic electronic, Rappetsederweg 28, Linz, 4040, Austria ABSTRACT Poly(vinyl alcohol) (PVA) is a water based dielectric often used as a coating layer in paper industry. Due to its water solubility PVA is also interesting as gate insulator in organic field effect transistors. Depending on the preparation of the PVA gate, transistors with and without hysteresis can be produced, with applications in organic electronic circuits or memory elements. In the production of PVA, a major side product is sodium acetate, an ionic salt not completely removed during industrial purification. Such ionic impurities likely influence the hysteresis in PVA based organic field effect transistors. While a hysteresis is desirable in memory elements, it is unwanted in transistors for electronic circuits. Ways to prepare transistors with a desired transfer characteristic are described, for example by using electronic grade products directly from the producer of PVA, or by emplyoing PVA purified by means of dialysis. Measurements are performed with metal-insulator-metal (MIM) structures and organic field effect transistors (OFETs), where Buckminsterfullerene C60 is employed as organic semiconductor. INTRODUCTION Solution based organic materials may be employed in roll to roll processing of organic field effect transistors [1-2]. Poly(vinyl alcohol) (PVA), a water based dielectric, is interesting as gate insulator in such devices. Depending on the preparation conditions, PVA based transistors have been produced with and without hysteresis in the transistor characteristics [3]. In transistors with as received PVA hysteresis effects are usually observed, while the hysteresis is significantly reduced when chemically cleaned PVA is used. Ionic impurities have been recently proposed to be the source of the hysteresis in PVA based transistors [3-5]. The assumption of mobile ions causing hysteresis in OFETs seems plausible, since polymers as well as many inorganic dielectrics are often hosts of mobile ions. It has been demonstrated for example that mobile Na+ ions may diffuse from an underlying substrate into organic semiconductors, like pentacene or poly(3-hexylthiophene) under the influence of an applied voltage. Thereby they cause an increase of the current through the semiconductor and simultaneously a current-voltage hysteresis [6]. The finding that mobile ions in semiconductors alter their electrical performance is also not new: it has been widely investigated in the 50’s and 60’s of the last century [7-10]. p-n junction devices, called flexodes, with a variable I-V c
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