Inkjet printing of transparent, flexible, organic transistors Laura Basiricò a, ⁎, Piero Cosseddu a , Beatrice Fraboni b , Annalisa Bonfiglio a a Department of Electric and Electronic Engineering, University of Cagliari, Piazza D'Armi, 09123 Cagliari, Italy and S3 nanoStructures and bioSystems at Surfaces, CNR-INFM, via Campi 213A, I-41100 Modena, Italy b Department of Physics, Alma Mater Studiorum, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy abstract article info Available online 5 May 2011 Keywords: Inkjet printing Organic semiconductor Conductive polymers PEDOT:PSS Two different types of all-organic, transparent transistors, namely Organic Thin Film Transistors (OTFTs) and Organic Electrochemical Transistors (OECTs), were fabricated on transparent, flexible plastic substrates by means of inkjet printing. In OTFTs the source, drain and gate electrodes were inkjet printed using a poly (3,4ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS ) solution, while a thermally sublimated layer of Parylene C acted as gate dielectric. Two kinds of organic semiconductors were used as active layers: 6,13-bis(triisopropylsilylethynyl) pentacene for p-type and N1400 for n-type OTFTs. For OECTs, all electrodes were also realized by inkjet printing deposition of a PEDOT:PSS solution. Electrical output characteristics of both kinds of devices are reported, demonstrating that the performances of our devices may be compared to those of transistors fabricated employing different realization techniques. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In flexible electronics based on organic semiconductors, devices as Organic Field Effect Transistors, Organic Light Emitting Diodes, Organic Solar Cells are fabricated building up multilayered structures formed by thin films of different materials [1–3]. For instance, in organic field effect devices, the active layer has a thickness of a few nanometers, and the possibility of using flexible transparent substrates usually permits to preserve the optical transparency of such thin films. The only layers which may compromise the transparency of the assembly are metal electrodes, namely gate, source and drain. In the majority of cases, such electrodes are fabricated employing metals able to guarantee a good coupling (in terms of work functions) with the organic semiconductor. In particular for source and drain, the main criterion used to select the metal is the height of the hole (for p-type transistors) or electron (for n-type transistors) injection barriers. Unfortunately, metals are not transparent, even when they are in the form of very thin films. Therefore it is interesting to investigate alternative materials as conductive polymers which, besides transparency, have also the advantage of having better characteristics of mechanical flexibility and easier processing. Inkjet printing of polymers is with no doubt the most promising technique to easily fabricate, at low temperatures, low cost, transparent and flexible devices in the field of Organic Electronics. Inkjet printing shows several advantages compared to other more traditional deposition techniques; first of all, it is based on an additive process i.e. only the material actually needed (a few picoliters) is deposited, in contrast with subtractive processes, as photolithography, where a homogeneous layer of material is deposited by spin coating over the whole substrate and the final pattern is achieved by subtracting excess material. This feature, together with the fact that no photolithographic masks are required, limits manufacturing costs of inkjet printing. Moreover, inkjet printing is a non- contact technique, which means that the substrate comes into contact only with those materials that compose the device, so that risks of possible damage and contamination are reduced [4]. In addition to that, high printing rate makes this technology suitable for mass production of all-plastic electronics. In this paper we demonstrate how inkjet printing of thin, semitransparent, conducting polymer films, can be employed for the realization of all organic complementary transistors with good electrical performances. 2. Experimental details Organic Thin Film Transistors (OTFTs) have been assembled on a transparent and flexible poly(ethyleneterephtalate) (PET, Goodfellow) sheet 175 μm thick (in Fig. 1 a digital photo of the OTFTs realized is reported). Before assembling the devices, PET sheets were cleaned by 15 min ultrasonic baths first in acetone and afterwards in isopropyl alcohol. Then the substrates were washed in deionized water and dried under nitrogen flow. All devices were fabricated using a bottom- gate/bottom-contact configuration, where gate, source and drain contacts were realized in air using a Dimatix 2800 drop on demand inkjet printer. Fujifilm Dimatix Materials Printer (DMP) 2800 uses a piezoelectric Drop On Demand technology (DOD), in which the deformation of a piezoelectric crystal induced by a voltage stimulus generates a single ink droplet ejection from printhead nozzle [5]. The DOD technology, compared to continuous-mode inkjet printing, allows Thin Solid Films 520 (2011) 1291–1294 ⁎ Corresponding author. E-mail address: laura.basirico@diee.unica.it (L. Basiricò). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.188 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf