DOI: 10.1002/adem.200800050 Low-Voltage Organic Transistors Fabricated Using Reverse Gravure Coating on Prepatterned Substrates** By Nikolai J. Kaihovirta, Daniel Tobjörk, Tapio Mäkelä and Ronald Österbacka* The potential of plastic electronic devices has made organ- ic thin film transistors (OTFTs) an active topic of research for 20 years. Several reviews have summarized the state of art of printed electronic devices. [1–5] Accordingly, low-cost mass production demands proper roll-to-roll fabrication tech- niques, environmental-friendly materials (especially sol- vents), air stable materials, lower fabrication temperature as well as high yield of end-products. Direct printing methods such as inkjet printing (IJP) has been widely studied as a method for fabrication of patterned structures, e.g. elec- trodes, [6–10] and in some cases even semiconductor and insu- lator layers. [11–13] IJPs benefit lies in the possibility of only printing on desired areas, thus lowering material consump- tion, and the possibility to digitally change the print patterns. The disadvantages are, however, slow throughput, uneven layers and significant risk of clogging the nozzles. Other alter- natives for layer coating, such as reverse gravure (RG) coat- ing, must therefore be considered. The RG coating technique produces homogenous layers for a wide range of thicknesses and viscosities of inks. [14] In recent years various publications of all-printed OTFTs have emerged. [12,13,15,16] Although showing working devices, there are still some shortcomings both concerning the transis- tor characteristics and when considering low-cost mass-pro- duction. Some studies present e.g. fabrication steps with questionable roll-to-roll suitability, use of hazardous solvents and high baking temperatures. Furthermore, high driving voltages and poor transistor characteristics is often also a problem for practical realization. Nevertheless, it is evident that the research is going into commercialisation and it is only a matter of time until printed plastic electronic devices will appear on market in a bigger scale. Recently a hygroscopic insulator organic transistor (HIFET) was demonstrated. [17] The hygroscopic insulator en- hances the conductivity in the channel through ionic motion and thereby enables low-voltage operation of the transistor. An operational model was proposed [18,19] as well as some sim- ple applications based on HIFETs. [20] For printing purposes the advantages of the HIFET consist of using soluble poly- mers for all layers, flexible plastic substrate, thick insulator layer (1 – 2 lm) and air stable fabrication at low temperatures and operation in ambient atmosphere. Also, the insulating layer may be used for multifunctional purposes in applica- tions [20] which has been addressed as a way of reducing fabri- cation costs. [5] In this work we present the fabrication of HIFETs in room temperature utilizing three different methods on prepat- terned source-drain substrates. In the first method the semiconductor- and insulator-layers are spin-coated with a drop-cast gate electrode while the second method uses RG coating instead of spin-coating. The third method replaces the drop-cast gate electrode with an IJP counterpart on top of the RG coated layers. The thickness and homogeneity of the layers is analyzed and the transistors are electrically charac- terized. Experimental As organic semiconductor we used regioregular poly(3- hexylthiophene) (rr-P3HT) purchased from Plextronics. 1.0 wt% of the material was dissolved in toluene in ambient air. As the hygroscopic insulator 10 wt% poly(vinyl phenol) (PVP) from ChemFirst/DuPont was dispersed in ethylace- tate. Poly(ethylene terephthalate) (PET) -samples (Mylar ® A) with a thickness of 50 lm were cleaned with distilled water, acetone and isopropanol prior to vacuum evaporation of gold source- and drain-electrodes. The obtained gold thickness was 30 nm. The selected dimensions of the electrodes were W = 1.5 mm and L = 35 lm. Also L = 100 lm was success- fully tested. Equivalent flexible circuits are commercially available and hence easily adapted into roll-to-roll fabri- cation. For simplicity we chose to prepare them in our labora- tory. A tabletop Mini-Labo™ test coater (with the Micro-Gra- vure™ coating method) was used for coating of first rr-P3HT and then PVP onto prepatterned substrates. No post-baking of the layers was done. The engravings of the 20 mm diame- ter metal gravure rolls consisted of (trihelical) continuous COMMUNICATIONS 640 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2008, 10, No. 7 – [*] N. J. Kaihovirta, D. Tobjörk, Prof. R. Österbacka Center for Functional Materials and Department of Physics at Åbo Akademi University E-mail: rosterba@abo.fi Dr. T. Mäkelä Center for Functional Materials at Åbo Akademi University Porthansgatan 3 20500 Åbo, Finland [**] The authors wish to thank Dr. Kjell-Mikael Källman for help with AFM studies. This work is supported by Åbo Akademi Foundation through the Center of Excellence.