Air Stable, Amorphous Organic Films and their Applications to Solution Processable Flexible Electronics

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Air Stable, Amorphous Organic Films and their Applications to Solution Processable Flexible Electronics Janos Veres, Simon Ogier, Stephen Leeming, Beverley Brown and Domenico Cupertino AVECIA, Hexagon House, P.O. Box 42, Manchester, M9 8ZS, U.K. ABSTRACT The rapidly expanding field of organic semiconductors for display and low-cost electronic applications requires materials, which not only have high mobility but also benefit from solution processability and environmental stability. In this paper we present a new class of solution coatable organic materials with excellent stability to air and light. Spin-coated FET devices operate at ambient conditions without encapsulation and show p-type field-effect mobilities of 2 x 10-3 cm2V-1s-1 and on/off ratios greater than 104. Thin films can be deposited from common organic solvents onto a variety of substrates. These films are mechanically robust and can withstand temperatures in excess of 100 °C without significant changes in electrical performance. FET switching and transient characteristics at higher frequencies are also discussed. These types of materials should find applications in many areas of flexible electronics.

INTRODUCTION During the past decade there has been tremendous progress in the development of organic thin film transistors (OFETs) [1,2,3]. Recent developments in the use of materials such as pentacene, polythienylenevinylene (PTV) and regioregular poly(3-hexylthiophene) (RR-3PHT) as the active components for OFETs has meant that the technology is increasingly considered for the fabrication of low-cost and/or large area flexible electronic circuits [1,2,3]. However, these materials have significant disadvantages for practical/commercial applications. For example, pentacene has the best performance in OFETs, but only when deposited by vacuum sublimation. RR-3PHT can be solution processed but is extremely sensitive to the deposition conditions and is rapidly doped by air oxidation. New high performance materials are therefore required which can be readily synthesised, are liquid processable and which are stable in devices. Our fundamental approach has been to look for chemistries that are more likely to yield sufficient stability in air, without the need for encapsulation, and try to explore how to maximise their mobility. We report here on our progress towards developing a new class of p-type semi-conducting materials based on arylamine chemistry. These materials can be synthesised to very high purity and have good solubility characteristics allowing them to be spin coated from a solution phase. The materials have excellent stability in air and light and as a consequence show little change in performance in the device under ambient conditions over several months. An additional benefit of the materials is that they exhibit little hysteresis in cycling. The general formula of our polyarylamine (PTAA) organic semiconductor is presented in Figure 1. This material has enormous versatility in the substitution X, Y and end capping Z. By varying the pendant group

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