Origin of mechanical strain sensitivity of pentacene thin-film transistors V. Scenev a,⇑ , P. Cosseddu b,c , A. Bonfiglio b,c , I. Salzmann a , N. Severin a , M. Oehzelt d , N. Koch a,d , J.P. Rabe a,⇑ a Humboldt-Universität zu Berlin, Institut für Physik, Newtonstr. 15D-12489 Berlin, Germany b University of Cagliari, Dep. of Electrical and Electronic Engineering, Piazza d’Armi, 09123 Cagliari, Italy c CNR–Institute of Nanoscience, Centre S3 via Campi 213A, I-41100 Modena, Italy d Helmholtz Zentrum Berlin für Materialien und Energie–BESSY II, D-12489 Berlin, Germany article info Article history: Received 20 November 2012 Received in revised form 4 February 2013 Accepted 22 February 2013 Available online 14 March 2013 Keywords: Pentacene Strain OFET Bending experiment Morphology abstract We report on bending strain-induced changes of the charge carrier mobility in pentacene organic thin-film transistors employing a combined investigation of morphological, struc- tural, and electrical properties. The observed drain current variations are reversible if the deformation is below 2%. The morphology and structure of the active pentacene layer is investigated by scanning force microscopy and specular synchrotron X-ray diffraction, which show that bending-stress causes morphological rather than structural changes, modifying essentially the lateral spacing between individual pentacene crystallites. In addition, for deformations >2% the rupture of source and drain gold electrodes is observed. In contrast to the metal electrodes, the modification of the organic layer remains reversible for deformations up to 10%, which suggests the use of soft and flexible electrodes such as graphene or conducting polymers to be beneficial for future strain sensing devices. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. 1. Introduction Thin films of conjugated organic molecules are subject of intense research due to their applicability in novel (opto-)electronic devices, including organic thin-film tran- sistors (OTFTs). Pentacene (PEN) is the prototypical hole- conducting material with notably high charge carrier mobilities of up to 5.5 cm 2 /V s [1–3] in p-type OTFTs. One key advantage of organic electronic devices is the pos- sibility to produce flexible all-organic OTFTs with the func- tional organic semiconductor films deposited on flexible plastic foils like Mylar Ò or polyethylenetherephthalate (PET) as substrate [4]. Through their intrinsic flexibility, OTFTs can be applied as mechanical strain-sensing devices [5,6] exhibiting significant advantages over conventional types of sensors: they can be processed under ambient conditions and are generally inexpensive to fabricate [4–11]. For sensing strain, reversible changes in the electri- cal characteristics of OTFTs were employed, including drain current, charge carrier mobility, threshold voltage, and contact resistance for deformations up to 1–2% [5,6,9,12,13]. In particular, the reported changes in mobil- ity were proposed to be due to morphological changes of the PEN layers under mechanical strain and/or the activa- tion of trap states in the PEN/electrode interface region [5,6,9,14,15]. However, up to now there has been no direct experimental evidence to support these suggestions. In or- der to enable targeted research for improving current OTFTs for future reliable sensing applications, further experimental work is needed to complete the microscopic picture of strain-impact on OTFT performance. Here we re- port a comprehensive electrical characterization of flexible OTFT devices under applied bending-stress, relating the device characteristics to morphological and structural 1566-1199/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.02.030 ⇑ Corresponding authors. Tel.: +49 1786905049 (V. Scenev, J.P. Rabe). E-mail addresses: vscenev@aol.com (V. Scenev), rabe@physik. hu-berlin.de (J.P. Rabe). Organic Electronics 14 (2013) 1323–1329 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel