3416 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 10, OCTOBER 2011 Fully Deformable Organic Thin-Film Transistors With Moderate Operation Voltage Piero Cosseddu, Andrea Piras, and Annalisa Bonfiglio Abstract—Highly flexible and mechanically stable organic thin- film transistors (OTFTs) have been fabricated using a very simple approach. Very thin parylene C films have been realized and employed as a freestanding gate dielectric for the realization of OTFTs. We demonstrate that this approach can be employed for scaling down the operating voltage in OTFTs and complementary inverters just by lowering the final free-standing film thickness. Moreover, it has also been demonstrated that the device electrical behavior is scarcely sensitive to mechanical deformation, opening the way for the realization of highly flexible foldable electronics. Index Terms—Flexible devices, organic electronics, organic- semiconductor (OSC) thin-film transistors (TFTs). I. I NTRODUCTION R ECENTLY, there has been an increasing demand for advanced electronic systems that could be adapted to dif- ferent shapes and functional contexts. In particular, this require- ment is common to fields as wearable electronics and, more in general, ambient intelligence in which everyday objects (such as garments or any other object of common use) are endowed with new functions, which are far beyond their original ones (e.g., protection for garments). For this kind of applications, it is extremely interesting to develop innovative technologies, which are able to allow designers to get rid of the current constraints imposed by more traditional electronic technology. Bending and stretching abilities are certainly among the most desirable features that these technologies should exhibit. Organic semiconductors (OSCs), joining together the electri- cal properties of semiconductor materials (which exhibit gen- erally lower mobility than inorganic semiconductors, although these are constantly improving) and the physical properties of organic compounds (e.g., process ability from a liquid phase and mechanical flexibility), are paving the way for a wide range of possible applications such as flexible displays, photo- voltaic modules, and smart tags [1]–[3]. Very recently, several groups also have demonstrated the possibility of employing Manuscript received February 7, 2011; revised May 31, 2011; accepted July 1, 2011. Date of publication August 4, 2011; date of current version September 21, 2011. This work was supported in part by the European Commission VII Framework Programme under Project “Roboskin” Contract 231500. The work of P. Cosseddu was supported by Regione Autonoma della Sardegna under PO Sardegna FSE 2007-2013, L.R.7/2007 “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna” CRP Prot. No. 1399/207. The review of this paper was arranged by Editor J. Kanicki. The authors are with the Department of Electrical and Electronic Engi- neering, University of Cagliari, 09123 Cagliari, Italy, and also with the S3 Nanostructures and Biosystems at Surfaces, CNR-INFM, 41100 Modena, Italy. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2161763 OSC-based thin-film transistors (OTFTs) for the realization of biochemical and mechanical sensors [4]–[11]. In 2003 [12], we already introduced a highly flexible free- standing structure for the realization of OTFTs; we employed a freestanding poly(ethylene therephtalate) (PET) film (Mylar) that not only acts as mechanical support but also as the gate dielectric of the device. We showed that such a configuration can be employed for a considerably wide range of applications, particularly in the field of biochemical sensing [6], [9], [12], [13]. Unfortunately, Mylar is characterized usually by high surface roughness [14] and, most importantly, is not available in very thin films. Both these characteristics have a big impact in the electrical performances of this structure, limiting carrier mobility and particularly impeding the low-voltage operation of the OTFT devices. In this paper, we report on the fabrication of a relatively low- voltage highly flexible free-standing OTFTs and their employ- ment for the realization of full-swing complementary inverters. These devices have been realized on a very thin freestanding parylene C film. Interestingly, these free-standing devices can be fabricated in thin films (total thickness down to 400 nm) using a simple procedure. Parylene C is a well-known insulating polymer, which is able to form transparent pinhole-free confor- mal coatings with excellent dielectric and mechanical proper- ties. It has been already employed for the realization of OTFTs with good results [15]. The flexible OTFTs described here have been tested using different OSCs, i.e., pentacene (Sigma Aldrich), pentacene 6,13-Bis(triisopropylsilylethynyl) (TIPS) (Sigma Aldrich), and N1400 (Polyera), in order to demonstrate that highly flexible and mechanically robust OTFTs and com- plementary inverters can be easily obtained. More noticeably, due to the low thickness and mechanical features of the insu- lating layer, the electrical characteristics of these devices do not change significantly upon the application of mechanical deformation. Therefore, within the limits of deformability of the substrate, they are strain-sensitive. II. DEVICE FABRICATION The parylene layers were deposited from the vapor phase using a Labcoater 2 SCS PDS 2010. The vapor deposition process begins when a raw material (di-para-xylylene) is heated to 175 ◦ C in a vacuum system until the material sublimes. In the gaseous state at 690 ◦ C, the dimer decomposes to form a gaseous monomer (para-xylylene). The gas then flows into a deposition chamber containing the substrates to be coated. Upon encountering the substrate, the gas converts into a solid polymer state poly(para-xylylene), which takes the form of a 0018-9383/$26.00 © 2011 IEEE