Contact effects in organic thin-film transistor sensors Luisa Torsi * , Francesco Marinelli, M. Daniela Angione, Antonio Dell’Aquila, Nicola Cioffi, Elvira De Giglio, Luigia Sabbatini Dipartimento di Chimica, Università degli Studi di Bari, via Orabona 4, 70126 Bari, Italy article info Article history: Received 20 October 2008 Received in revised form 12 November 2008 Accepted 13 November 2008 Available online 30 November 2008 PACS: 72.80.Le 73.40.Cg 85.30.Tv 87.85.fk Keywords: Thin films Transistors Chemo-bio sensors Conducting polymers Molecular electronics abstract Contact effects in organic thin-film transistors (OTFTs) sensors are here investigated specif- ically respect to the gate field-induced sensitivity enhancement of more than three orders of magnitude seen in a DHa6T OTFT sensor exposed to 1-butanol vapors. This study shows that such a sensitivity enhancement effect is largely ascribable to changes occurring to the transistor channel resistance. Effects, such as the changes in contact resistance, are seen to influence the low gate voltage regime where the sensitivity is much lower. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The scientific and technological driving force towards the development of performing conducting polymer (CP) based solid state sensors is still very strong despite this field has been initiated more than 30 years ago [1]. Re- cently, organic thin-film transistor (OTFT) sensors have risen the interest of the scientific community for their enhanced level of performance [2–6]. In this configuration, highly repeatable responses were measured by properly gate biasing the device [2,3] whereas broad chemical selec- tivity was conferred either by covalently bound side groups [7] or by means of CP blends [8]. Besides, an impor- tant assessment of their selectivity capabilities was achieved with a chiral OTFT, that exhibited field-effect amplified sensitivity allowing detection of optical isomers in the tens part-per-million (ppm) concentration range [9], i.e. with a three order of magnitude sensitivity improvement [10,11]. This report aims to demonstrate that the sensing pro- cess, in particular the field-induced sensitivity enhance- ment, can be largely ascribed to changes of the channel resistance R ch , eventually ruling out dominating contribu- tions from of the contact resistance (R c ) or leakage current variations. The results reported can also shed light on the origin of the contact resistance in OTFTs. 2. Experimental methods 2.1. Organic thin-film transistors fabrication The transistors used for this study have a bottom-gate structure and consist of a highly n-doped silicon wafer 1566-1199/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2008.11.009 * Corresponding author. E-mail address: torsi@chimica.uniba.it (L. Torsi). Organic Electronics 10 (2009) 233–239 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel