ALL-ORGANIC FLEXIBLE AND TRANSARENT AMBIPOLAR FETs WITH ORGANIC BULK HETEROJUNCTIONS
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1029-F09-20
ALL-ORGANIC FLEXIBLE AND TRANSARENT AMBIPOLAR FETs WITH ORGANIC BULK HETEROJUNCTIONS Piero Cosseddu1,2, Annalisa Bonfiglio1,2, Ingo Salzmann3, Jurge P. Rabe3, and Norbert Koch3 1 Electrical and Electronic Engineering, University of Cagliari, Piazza d'Armi, Cagliari, 09123, Italy 2 S3 nanoStructures and bioSystems at Surfaces, CNR-INFM, Via Campi 213/A, Modena, 41100, Italy 3 Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, Berlin, 12489, Germany ABSTRACT We report on the realization of flexible and transparent all-organic Ambipolar Field Effect Transistors. The devices were assembled on a flexible plastic foil, i.e. Mylar®, and the contacts were realized with poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography, MicroContact Printing (µCP). As semiconductor layer we used organic bulk heterojunctions of pentacene/C60 realized either by co-deposition of the two different molecules or by a double layer structure in which pentacene was used as buffer layer at the interface with the gate dielectric. Interestingly, all devices (co-deposited and double layer), measured in air, worked in accumulation mode as ambipolar OFETs, however some interesting differences between the two structures can be pointed out. Supported by Atomic Force Microscopy, we demonstrated that growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to improve n-type conduction by two orders of magnitude. This work is particularly interesting because on one hand it confirms the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials; moreover, it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. The flexibility and transparency of the final OFET structure and the simple low cost fabrication technique employed pave the way for an economic mass production of large area transparent “Plastic Electronics”.
INTRODUCTION Recently, ambipolar organic field effect transistors (OFETs) have been a research focus due to their potential application in organic integrated circuits (ICs) [1-3]. The simultaneous transport of both types of carriers could lead to a simplification in the design of complementary logic circuits, allowing at the same time to reduce power dissipation, to increase noise margins and achieve high operational stability. One of the main characteristics of almost all organic semiconductors is that, despite their undoped state, often only a single carrier transport, either holes (in most cases) [4, 5] or electrons [6-8], can be achieved. In several cases, the work function of metals employed for contact fabrication dictated the polarity of charge tr
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