All-Printed Transistors on Nano Cellulose Substrate
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All-Printed Transistors on Nano Cellulose Substrate Tomi Hassinen, Ari Alastalo, Kim Eiroma, Tiia-Maria Tenhunen, Vesa Kunnari, Timo Kaljunen, Ulla Forsström and Tekla Tammelin VTT Technical Research Centre of Finland, Tietotie 3, Espoo, FI-02044 Finland ABSTRACT We report fully-printed top-gate-bottom-contact organic thin-film transistors using substrates prepared from cellulose nanofibers and commercially available printing inks to fabricate the devices. Gravure printing was used to coat the substrate with a polymer resist to decrease the surface roughness and close the surface. Transistor structures were fabricated using inkjet printing for conductors and gravure printing for the dielectric and semiconducting layers. The obtained transistor performance is compared to that of similar transistors on plastic substrate. INTRODUCTION Printing is an emerging fabrication paradigm for low-cost large-area electronics. When the expected market growth of printed, possibly disposable, electronic applications takes place, the concern for environmental loading will also strengthen and call for biodegradable solutions. For this purpose, plastic substrates are not an optimal choice. Paper would be a desirable substrate for printed electronics because it is a much more environmentally friendly choice than plastics. However, basic paper itself is not well suited for printed electronics for which high resolution and thin films are needed because the roughness and porosity of paper are typically high compared to plastics. Furthermore, chemical, mechanical and thermal stability of paper is typically not as good as that of plastics. Some of these limitations can be circumvented by using added coating layers. On the other hand, special paper-like substrates can be made with modern materials and techniques such as composite substrates using micro / nanofibrillated cellulose (MFC/NFC) with inorganic fillers [1]. Also without fillers, by adjusting the fibril size and film processing, the substrates can be made very dense, smooth and transparent and they can be used for solution-processed and printed electronics [2-4]. In this work, fully-printed top-gate-bottom-contact organic thin film transistors (TFT) were fabricated on cellulose nanofibril (CNF) substrate using commercial electronics inks. The substrate was produced by a roll-to-roll process [5]. Gravure printing was first used to coat the CNF film with a polymer resist to smooth and close the surface. Inkjet (IJ) and gravure printing was then used to fabricate the conducting, dielectric and semiconducting device layers. Transistor performance was characterized for mobility, threshold voltage, on/off ratio, and bias stress stability. The TFT performance is compared to that of similar transistors fabricated on plastic PET substrate. MATERIALS AND METHODS The TFT device structure of this work is shown in Figure 1 together with a photograph of fabricated devices. In addition to devices on the CNF substrate, reference devices were made on PET film using also other than printing processes
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