- Email: [email protected]
Origin of mechanical strain sensitivity of pentacene thin-film transistors
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 Organic Electronics xxx (2013) xxx–xxx 1
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 5 6
Origin of mechanical strain sensitivity of pentacene thin-film transistors
3 4 7
Q1
8
a
9 10 11 12
Humboldt-Universität zu Berlin, Institut für Physik, Newtonstr. 15D-12489 Berlin, Germany 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 b
14 13 15
a r t i c l e
1 3 7 0 18 19 20 21 22 23 24 25 26 27 28 29
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,⇑
i n f o
a b s t r a c t
Article history: Received 20 November 2012 Received in revised form 4 February 2013 Accepted 22 February 2013 Available online xxxx
Q3
We report on bending strain-induced changes of the charge carrier mobility in pentacene organic thin-film transistors employing a combined investigation of morphological, structural, 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.
Keywords: Pentacene Strain OFET Bending experiment Morphology
31 32 33 34 35 36 37 38 39 40 41 42 43
44 45
1. Introduction
46
Thin films of conjugated organic molecules are subject of intense research due to their applicability in novel (opto-)electronic devices, including organic thin-film transistors (OTFTs). Pentacene (PEN) is the prototypical holeconducting material with notably high charge carrier mobilities of up to 5.5 cm2/V s [1–3] in p-type OTFTs. One key advantage of organic electronic devices is the possibility to produce flexible all-organic OTFTs with the functional organic semiconductor films deposited on flexible plastic foils like MylarÒ 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
47 48 49 50 51 52 53 54 55 56 57 58
Q2
⇑ Corresponding authors. Tel.: +49 1786905049 (V. Scenev). E-mail addresses: [email protected] hu-berlin.de (J.P. Rabe).
(V.
Scenev),
rabe@physik.
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 electrical 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 mobility were proposed to be due to morphological changes of the PEN layers under mechanical strain and/or the activation 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 order 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 report 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
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 2 78 79 80 81 82 83 84 85
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
properties of the active layer, as determined by a combination of scanning force microscopy (SFM) and specular Xray diffraction (XRD). We identify the deterioration of the metal electrodes to be responsible for irreversibilities in the device characteristics, while the observed modifications of the active organic layer remain reversible for deformations up to 10%, which suggests using soft organic electrodes for future improved sensing applications.
86
2. Experimental
87
OTFTs were fabricated on 200 lm thin and flexible polyethylenetherephthalate (PET) substrates. A gold layer was vacuum-deposited on the substrates as gate electrode. Polyvinylalcohol (PVA) with ammonium dichromate (AD) salt (cross-linking agent) was deposited from water solution as gate dielectric by spin-coating and subsequent UV-curing for cross-linking. Gold source and drain electrodes (Fig. 1a) were patterned by thermal deposition through a shadow mask. Pentacene (PEN) was used as organic semiconductor. PEN films were evaporated at a base pressure <107 mbar; the film thickness and the deposition rate (0.5 nm/min) were monitored by a quartz crystal microbalance placed next to the sample; the device is sketched in Fig. 1a. A special apparatus for defined bending of the OTFT (Fig. 1c) was developed to be suitable for use in both SFM and specular XRD experiments. XRD measurements were carried out at the beamline W1 at DESY– HASYLAB (Hamburg, Germany) with a primary beam energy of 10.5 keV, using the bending apparatus (Fig. 1c) as sample holder for in situ experiments, and a standard setup (PVA samples fixed planar on a Si wafer) for reference
88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
measurements. The electrical and morphological characterization of the PEN layers under deformation was carried out in situ under ambient conditions. For electrical characterization a Keithley 2636A sourcemeter was used. The surface topography was imaged with SFM (Nano Wizard, JPK instruments). The PEN film strain was calculated with the model of beam buckling (Fig. 1b), which allows to calculate the film strain, e, directly from the bending radius [16], as demonstrated before [9,17]. The bending radius was calculated taking into account the homogeneous bending of the PET foil. This setup allowed to strain PEN films up to 10%, with the upper limit defined by the smallest accessible radius of curvature and the thickness of the PET substrate. All electrical device characterization, XRD and SFM measurements, were carried out under ambient conditions.
108
3. Results
124
Transfer curves (i.e., drain current (ID) versus gate voltage (VG) characteristics) of the devices were recorded for different degrees of strain. The bending-stress was released after acquiring transfer characteristic for a particular strain in order to verify the reversibility of the process (i.e., if the current recovers to the initial value). We find, as expected [9], a decrease in drain current as function of device deformation (Fig. 2a). For deformations of up to 1.7%, straining of the device causes reversible variations of the transfer curves. Starting from 1.7% ID does not recover to the initial value of the pristine device upon bending-stress relief. Transfer characteristics exhibit a pronounced hysteresis with the hysteresis loop-area increasing with increasing
125
Fig. 1. (a) Schematic representation of the top-contact OTFT architecture. (b) Model used to calculate PEN film strain e; R denotes the radius of curvature, t the device thickness. Red and dashed green lines indicate the upper and neutral plane, respectively. Strain of PEN film is assumed to be equal to the strain of the beam surface. (c) The bending apparatus with the OTFT device in place. (d) Scheme of the device under bending stress with the SFM-tip; electrical contacts are indicated for the drain-current sensitivity measurements in the deformed OTFT and for the in situ morphological investigations on both electrodes and active layer by SFM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
109 110 111 112 113 114 115 116 117 118 119 120 121 122 123
126 127 128 129 130 131 132 133 134 135 136 137
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
3
Fig. 2. Electrical characteristics of a representative OTFT device. (a) Transfer curves recorded during successive device bending and relief of bending stress after each deformation (indicated with e = 0%) with VD = 60 V. (b) Transfer curves representing the increase of the hysteresis loop area with increasing deformation (with VD = 60 V). (c) Dependency of the hysteresis loop area versus bending deformation. (d) Device strain sensitivity of the drain current at 60 V gate voltage versus deformation; the red line denotes the linear fit within the range, where the device sensitivity is reversible; the green line indicates the transition from reversible to irreversible variation of drain current. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165
deformation (Fig. 2b and c). This is attributed to charge carrier trapping enhanced by the strain. The effect of bending strain induced activation of trap states in the active layer and its relationship to the hysteresis in transfer characteristics of OTFTs has already been studied and attributed to the intrinsic properties of the organic semiconductor [6,15]. The sensitivity of ID to the strain is determined by plotting the difference between pristine and bended device currents (recorded with 60 V drain voltage) normalized to the pristine value versus strain (Fig. 2d). Our electrical measurements reveal that the strainsensitivity of PEN OTFT devices scales almost linearly with strain for deformations up to 1.7% (Fig. 2a). Beyond this value we find that the device becomes less sensitive to strain (Fig. 2b). With our experimental set up it was possible to obtain the device sensitivity up to 7.5% mechanical stress. The finding that after a certain degree of deformation the current does not recover to its initial value is clear evidence that irreversible processes occurred in the device structure. To shed light on this issue we performed morphological and structural investigations of the deformed device, with particular attention on the active layer and the electrodes. SFM height images of unstrained devices reveal uniformly distributed grains with lateral grain sizes of few hundreds of nm (Fig. 3a–d), typical for PEN films grown on PVA by vacuum deposition [18]. Straining the device up to 1.3% did not cause any noticeable variation in PEN
film morphology (Fig. 3b–d). Due to the distortions of the SFM images, which are inherent to the SFM instrumentation including piezo creep, the strain of 1.3% could not be detected directly. The PEN film morphology on PVA substrates remained qualitatively alike for strains beyond 2%, and the impact of bending was clearly reflected in the topography images (Fig. 4b), revealing an increased distance between remote morphological features for higher values of induced surface strain (Fig. 4c). Moreover, the radius of curvature could be directly estimated from the cross-section analysis of the SFM micrograph (see Supporting Information, Fig. S1) to (1.1 ± 0.1) mm. Since the bending radius was measured [16,19] to be 1 mm (see Supporting Information, Fig. S2), the value from cross-section analysis is in very good agreement. However, the resolution of the SFM imaging was not sufficient to clearly reveal whether individual grains became strained or whether the distance between grains (along grain boundaries) increased. Therefore we performed specular XRD measurements to verify straining of PEN crystallites as described further below. In contrast to PEN films on PVA, we detected the formation of cracks within the gold electrodes on top of PEN film for strains larger than 2% (Fig. 4d and e). The cracks persisted after release of strain. To investigate the impact of strain on the PEN film structure, we performed specular XRD measurements of PEN films in situ deformed up to 3.3% (Fig. 4f). For the un-
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 4
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
Fig. 3. (a) SFM topography of PEN film. (c) Zoom-in before straining; (d) an in situ 1.3% strained PEN film. (b) Provides a cross-section analysis of the SFM height images carried out on PEN films under deformations of up to 1.3% to allow for a comparison of the lateral distance between two distinctive positions (marked with arrows in (c)). Arrows in (c) correspond to arrows in the cross-sections (b), respectively. Also for deformations of up to 1.3% no significant changes in PEN films were found compared to the pristine device. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Representative SFM micrographs of (a) a pristine PEN layer, and (b) an in situ 10% strained PEN film, together with (c) the corresponding crosssections; arrows in (a) and (b) correspond to arrows in the cross-sections. (d) SFM micrograph of the top gold electrode, demonstrating crack formation upon deformation of the gold electrode; (e) transition region (interface) of electrode (left)/pentacene (right). (f) Specular XRD data in the range of the (001)reflection of the PEN thin-film phase (lattice spacing d (001) = 1.54 nm) recorded as function of in situ applied bending-stress e; qz denotes the vertical 0 momentum transfer; the peak at 0.46 A Å1 (on the black curve) labels a reflection assigned to the substrate; the peak intensity decreases with the degree of deformation due to geometrical reasons in the experiment (i.e., less area being illuminated by the X-ray beam). Insets: (left) sketch of the (inclined) molecular orientation in PEN thin-film phase with respect to the PVA substrate (illustrated by yellow triangles); (right) specular XRD data (ex situ) for a pristine sample and the same sample after releasing a stress of 4.6%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
194 195 196 197 198 199
strained device (Fig. 4f, black curve) the strong 0peak at a value of vertical momentum transfer (qz) of 0.41 Å A1 corresponds to the (0 0 1) reflection of the PEN thin-film polymorph [20]. This indicates that the PEN molecules adopt a standing orientation within the film, however, slightly inclined by ca. 6° with respect to the PVA substrate normal
(see yellow triangles in the left inset of Fig. 4f), and that the molecules are arranged in a herringbone motif typical for this specific polymorph [20]. Recently, bending-stressdriven phase transitions were suggested upon flattening of inward and outward bent PEN devices [21]. To observe such phase transitions upon deformation of the present
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
200 201 202 203 204 205
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx 206 207 208 209 210 211 212 213
214 215 216 217 218 219 220 221 222 223
224 225 226 227
PEN/PVA films, we performed both in situ specular XRD on films deformed up to 3.3% (Fig. 4f), and ex situ specular XRD on a film after the release of strain as high as 4.6% (Fig. 4f, right inset). Most notably, in no case a shift of the (0 0 1)-peak maximum occurred, which would, however, be expected if a phase transition to any other PEN polymorph [22–25] would take place (expected change in vertical momentum transfer of the (0 0 1) reflection: 0 Dqz P 0.02 A Å1), and if, therefore, the lateral intermolecular distance would accordingly change. Note that, apart from phase transitions, relevant stress-related changes of the intermolecular distance can be excluded within this deformation range, as they typically occur upon pressures in the GPa region, which is reported in related high-pressure investigations [26]. Note, that the elastic modulus of PEN film is 15 GPa and this large value is due to the polycrystalline nature of PEN film [27]. In particular, already a change in lattice spacing by 2.5% would correspond to Dqz 0.01 Å1 and would, thus, be well captured by our experiment. This is also true for even higher deformations of up to 10%, as indicated by SFM micrographs and the correspond-
5
ing cross-section analyses (Fig. 5) comparing the morphology of the same PEN area before (Fig. 5a and c) and after (Fig. 5b and d) stress release, where no significant changes in morphology are observed. Note that the identity of the investigated areas in the two micrographs is clearly confirmed by characteristic morphological features present in both the micrographs (Fig. 5a and b) and in the respective cross-sections (Fig. 5c and d). Moreover, both micrographs exhibit the same morphological features, which demonstrates that the PEN film completely recovers to its initial state, even after the present severe mechanical deformation.
228
4. Discussion
240
The electrical measurements agree well with previous reports, where both spherical [5,6] and cylindrical [9,28] deformations showed similar dependencies of the electrical response on mechanical strain, i.e., a reversible variation of transfer curves for small strain values and irreversible changes for large strain. Our present study reveals two distinct deformation regimes of the flexible
241
Fig. 5. SFM micrographs of (a) a pristine PEN film and (b) a flattened PEN film after deformation to 10%. Corresponding cross-sections taken along white line between two positions marked with arrows in the SFM micrographs of (c) a pristine and (d) flattened PEN film after deformation to 10%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
229 230 231 232 233 234 235 236 237 238 239
242 243 244 245 246 247
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 6
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
301
OTFTs: reversible deformation up to 10% occurs for PEN on PVA substrates, while an irreversible deterioration of the gold electrodes is observed starting at a strain value of 2%. Note that, in full agreement with this observation, thin gold electrodes have been previously reported to deteriorate for strains exceeding 2% [29]. As it has been studied elsewhere [29,30], the electrical resistance of thin gold films decreases with mechanical strain. Moreover, very recently, A.N. Sokolov and coworkers reported about irreversible changes observed in OTFTs upon mechanical deformation. In particular, it was found that for surface strain higher than 1.3%, source and drain electrodes buckling and cracking starts to appear. This issue typically led to a decrease of the electrodes conductivity that induced a permanent decrease of the OTFT output current [31]. Therefore, we propose that the observed irreversible variation of transfer curves is mainly due to the deterioration of the gold electrodes. The reversibility of both the transfer curve variation and the film topography until gold electrode deterioration starts infer that the PEN film itself sustains large strains of up to, at least, 10%. Most importantly, our study reveals that PEN films might be used in devices to sense substantially larger strains than previously reported. From these findings, hence, a design strategy for future improved organic sensors clearly arises: using soft, flexible and mechanically stable materials like graphene or conductive polymers might enable the full reversibility of strain-related measures in reliable future sensor devices. Furthermore, our study allows proposing a well founded model for the reason of the observed reversibility of transfer curves variations. One may speculate of tensile stress having two distinct effects on the PEN thin film: (i), the lateral size of the pentacene crystals might become stretched, therefore modifying the PEN microstructure, which might be detrimental to charge transport, or (ii), the distance between individual neighboring PEN crystallites might be increased, therefore reducing the tunneling probability of charge carriers in the process of charge transport through the grain boundaries. From the SFM and XRD data in our present study the related fundamental process can now be clearly identified as (ii): tensile stress leads to an increase in inter-grain distance while leaving the crystal structure unaffected, as no phase transitions were observed. It is well established that hopping transport, in which a conducting path from the source to the drain electrode is formed by coupled grain chains oriented in random directions [9] rather than band transport dominates conduction in polycrystalline thin organic films [9,32,32]. The increase in distance between crystalline PEN grains translates into an increase in potential barrier for (thermally activated) tunneling between grains, which, therefore, reduces the overall charge carrier mobility in bended PEN thin film OTFTs and, thus, fully explains the observed PEN-OTFT strain sensitivity.
302
5. Conclusions
303
The underlying mechanism of bending-strain sensitivity observed for PEN OTFTs was investigated by combining in situ electrical characterization with morphological and
248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
304 305
structural investigations. Our results indicate that the bending-strain sensitivity of PEN OTFTs is due to changes in the film morphology and that the correlated increase in spacing between PEN crystallites is responsible for the observed decrease in drain current, which is found to be reversible for small deformations. From our study, the irreversibility in electrical characteristics for deformations larger than 2% is recognized to be due to crack formation in gold source- and drain electrodes. In contrast to irreversible changes in the metal layers, PEN itself is found to be fully reversibly modified even for extreme deformations of up to 10%, which identifies the organic/metal interface as bottleneck of the investigated OTFTs and suggests the use of soft flexible electrodes for future strain-sensing devices.
306
Acknowledgements
321
We thank Wolfgang Caliebe (DESY–HASYLAB), Tatjana Djuric and Armin Moser (both TU-Graz) for experimental support. Financing through the DFG (Germany) and the Austrian Science Fund (FWF) project P21094-N20 is gratefully acknowledged.
322
Appendix A. Supplementary material
327
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2013.02.030.
328
References
331
[1] T. Hasegawa, J. Takeya, Science and Technology of Advanced Materials 10 (2009) 024314. [2] Y. Yamashita, Science and Technology of Advanced Materials 10 (2009) 024313. [3] N. Koch, ChemPhysChem 8 (2007) 1438. [4] P. Cosseddu, A. Bonfiglio, I. Salzmann, J.P. Rabe, N. Koch, Organic Electronics 9 (2008) 191. [5] I. Manunza, A. Sulis, A. Bonfiglio, Applied Physics Letters 89 (2006) 143502. [6] I. Manunza, A. Bonfiglio, Biosensors & Bioelectronics 22 (2007) 2775. [7] G. Darlinski et al., Journal of Applied Physics 97 (2005) 093708. [8] H. Kawaguchi, T. Someya, T. Sekitani, T. Sakurai, Ieee Journal of SolidState Circuits 40 (2005) 177. [9] T. Sekitani et al., Applied Physics Letters 86 (2005) 073511. [10] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [11] T. Someya et al., Proceedings of the National Academy of Sciences of the United States of America 102 (2005) 12321. [12] P. Cosseddu, A. Piras, A. Bonfiglio, Ieee Transactions on Electron Devices 58 (2011) 3416. [13] P. Cosseddu, S. Milita, A. Bonfiglio, Ieee Electron Device Letters 33 (2012) 113. [14] F.C. Chen, T.D. Chen, B.R. Zeng, Y.W. Chung, Semiconductor Science and Technology 26 (2011) 034005. [15] T. Lindner, G. Paasch, S. Scheinert, Journal of Applied Physics 98 (2005) 114505. [16] J.K.F. Yau, N. Savvides, C.C. Sorrell, Physica C 266 (1996) 223. [17] T. Sekitani et al., Journal of Non-Crystalline Solids 352 (2006) 1769. [18] T.B. Singh et al., Advanced Materials 17 (2005) 2315. [19] A. Jedaa, M. Halik, Applied Physics Letters 95 (2009) 103309. [20] S. Schiefer, M. Huth, A. Dobrinevski, B. Nickel, Journal of the American Chemical Society 129 (2007) 10316. [21] C. Yang et al., Applied Physics Letters 92 (2008) 243305. [22] R.B. Campbell, J. Trotter, J.M. Robertson, Acta Crystallographica 14 (1961) 705. [23] D. Nabok et al., Physical Review B 76 (2007) 235322. [24] S.C.B. Mannsfeld, A. Virkar, C. Reese, M.F. Toney, Z.N. Bao, Advanced Materials 21 (2009) 2294.
332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
307 308 309 310 311 312 313 314 315 316 317 318 319 320
323 324 325 326
329 330
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx 369 370 371 372 373 374 375
[25] D. Holmes, S. Kumaraswamy, A.J. Matzger, K.P.C. Vollhardt, Chemistry–A European Journal 5 (1999) 3399. [26] M. Oehzelt et al., Physical Review B 74 (2006) 104103. [27] D. Tahk, H.H. Lee, D.Y. Khang, Macromolecules 42 (2009) 7079. [28] T. Sekitani, I. Shingo, Y. Kato, T. Someya, Japanese Journal of Applied Physics Part 1-Regular Papers Brief Communications & Review Papers 44 2841 (2005).
7
[29] S.P. Lacour, S. Wagner, Z.Y. Huang, Z. Suo, Applied Physics Letters 82 (2003) 2404. [30] T. Li, Z.Y. Huang, Z. Suo, S.P. Lacour, S. Wagner, Applied Physics Letters 85 (2004) 3435. [31] A.N. Sokolov, Y.D. Cao, O.B. Johnson, Z.A. Bao, Advanced Functional Materials 22 (2012) 175. [32] Y. Takamatsu, T. Sekitani, T. Someya, Applied Physics Letters 90 (2007) 133516.
376 377 378 379 380 381 382 383 384
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
No. of Pages 7, Model 3G
13 March 2013 Organic Electronics xxx (2013) xxx–xxx 1
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel 5 6
Origin of mechanical strain sensitivity of pentacene thin-film transistors
3 4 7
Q1
8
a
9 10 11 12
Humboldt-Universität zu Berlin, Institut für Physik, Newtonstr. 15D-12489 Berlin, Germany 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 b
14 13 15
a r t i c l e
1 3 7 0 18 19 20 21 22 23 24 25 26 27 28 29
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,⇑
i n f o
a b s t r a c t
Article history: Received 20 November 2012 Received in revised form 4 February 2013 Accepted 22 February 2013 Available online xxxx
Q3
We report on bending strain-induced changes of the charge carrier mobility in pentacene organic thin-film transistors employing a combined investigation of morphological, structural, 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.
Keywords: Pentacene Strain OFET Bending experiment Morphology
31 32 33 34 35 36 37 38 39 40 41 42 43
44 45
1. Introduction
46
Thin films of conjugated organic molecules are subject of intense research due to their applicability in novel (opto-)electronic devices, including organic thin-film transistors (OTFTs). Pentacene (PEN) is the prototypical holeconducting material with notably high charge carrier mobilities of up to 5.5 cm2/V s [1–3] in p-type OTFTs. One key advantage of organic electronic devices is the possibility to produce flexible all-organic OTFTs with the functional organic semiconductor films deposited on flexible plastic foils like MylarÒ 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
47 48 49 50 51 52 53 54 55 56 57 58
Q2
⇑ Corresponding authors. Tel.: +49 1786905049 (V. Scenev). E-mail addresses: [email protected] hu-berlin.de (J.P. Rabe).
(V.
Scenev),
rabe@physik.
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 electrical 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 mobility were proposed to be due to morphological changes of the PEN layers under mechanical strain and/or the activation 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 order 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 report 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
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 2 78 79 80 81 82 83 84 85
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
properties of the active layer, as determined by a combination of scanning force microscopy (SFM) and specular Xray diffraction (XRD). We identify the deterioration of the metal electrodes to be responsible for irreversibilities in the device characteristics, while the observed modifications of the active organic layer remain reversible for deformations up to 10%, which suggests using soft organic electrodes for future improved sensing applications.
86
2. Experimental
87
OTFTs were fabricated on 200 lm thin and flexible polyethylenetherephthalate (PET) substrates. A gold layer was vacuum-deposited on the substrates as gate electrode. Polyvinylalcohol (PVA) with ammonium dichromate (AD) salt (cross-linking agent) was deposited from water solution as gate dielectric by spin-coating and subsequent UV-curing for cross-linking. Gold source and drain electrodes (Fig. 1a) were patterned by thermal deposition through a shadow mask. Pentacene (PEN) was used as organic semiconductor. PEN films were evaporated at a base pressure <107 mbar; the film thickness and the deposition rate (0.5 nm/min) were monitored by a quartz crystal microbalance placed next to the sample; the device is sketched in Fig. 1a. A special apparatus for defined bending of the OTFT (Fig. 1c) was developed to be suitable for use in both SFM and specular XRD experiments. XRD measurements were carried out at the beamline W1 at DESY– HASYLAB (Hamburg, Germany) with a primary beam energy of 10.5 keV, using the bending apparatus (Fig. 1c) as sample holder for in situ experiments, and a standard setup (PVA samples fixed planar on a Si wafer) for reference
88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
measurements. The electrical and morphological characterization of the PEN layers under deformation was carried out in situ under ambient conditions. For electrical characterization a Keithley 2636A sourcemeter was used. The surface topography was imaged with SFM (Nano Wizard, JPK instruments). The PEN film strain was calculated with the model of beam buckling (Fig. 1b), which allows to calculate the film strain, e, directly from the bending radius [16], as demonstrated before [9,17]. The bending radius was calculated taking into account the homogeneous bending of the PET foil. This setup allowed to strain PEN films up to 10%, with the upper limit defined by the smallest accessible radius of curvature and the thickness of the PET substrate. All electrical device characterization, XRD and SFM measurements, were carried out under ambient conditions.
108
3. Results
124
Transfer curves (i.e., drain current (ID) versus gate voltage (VG) characteristics) of the devices were recorded for different degrees of strain. The bending-stress was released after acquiring transfer characteristic for a particular strain in order to verify the reversibility of the process (i.e., if the current recovers to the initial value). We find, as expected [9], a decrease in drain current as function of device deformation (Fig. 2a). For deformations of up to 1.7%, straining of the device causes reversible variations of the transfer curves. Starting from 1.7% ID does not recover to the initial value of the pristine device upon bending-stress relief. Transfer characteristics exhibit a pronounced hysteresis with the hysteresis loop-area increasing with increasing
125
Fig. 1. (a) Schematic representation of the top-contact OTFT architecture. (b) Model used to calculate PEN film strain e; R denotes the radius of curvature, t the device thickness. Red and dashed green lines indicate the upper and neutral plane, respectively. Strain of PEN film is assumed to be equal to the strain of the beam surface. (c) The bending apparatus with the OTFT device in place. (d) Scheme of the device under bending stress with the SFM-tip; electrical contacts are indicated for the drain-current sensitivity measurements in the deformed OTFT and for the in situ morphological investigations on both electrodes and active layer by SFM. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
109 110 111 112 113 114 115 116 117 118 119 120 121 122 123
126 127 128 129 130 131 132 133 134 135 136 137
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
3
Fig. 2. Electrical characteristics of a representative OTFT device. (a) Transfer curves recorded during successive device bending and relief of bending stress after each deformation (indicated with e = 0%) with VD = 60 V. (b) Transfer curves representing the increase of the hysteresis loop area with increasing deformation (with VD = 60 V). (c) Dependency of the hysteresis loop area versus bending deformation. (d) Device strain sensitivity of the drain current at 60 V gate voltage versus deformation; the red line denotes the linear fit within the range, where the device sensitivity is reversible; the green line indicates the transition from reversible to irreversible variation of drain current. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165
deformation (Fig. 2b and c). This is attributed to charge carrier trapping enhanced by the strain. The effect of bending strain induced activation of trap states in the active layer and its relationship to the hysteresis in transfer characteristics of OTFTs has already been studied and attributed to the intrinsic properties of the organic semiconductor [6,15]. The sensitivity of ID to the strain is determined by plotting the difference between pristine and bended device currents (recorded with 60 V drain voltage) normalized to the pristine value versus strain (Fig. 2d). Our electrical measurements reveal that the strainsensitivity of PEN OTFT devices scales almost linearly with strain for deformations up to 1.7% (Fig. 2a). Beyond this value we find that the device becomes less sensitive to strain (Fig. 2b). With our experimental set up it was possible to obtain the device sensitivity up to 7.5% mechanical stress. The finding that after a certain degree of deformation the current does not recover to its initial value is clear evidence that irreversible processes occurred in the device structure. To shed light on this issue we performed morphological and structural investigations of the deformed device, with particular attention on the active layer and the electrodes. SFM height images of unstrained devices reveal uniformly distributed grains with lateral grain sizes of few hundreds of nm (Fig. 3a–d), typical for PEN films grown on PVA by vacuum deposition [18]. Straining the device up to 1.3% did not cause any noticeable variation in PEN
film morphology (Fig. 3b–d). Due to the distortions of the SFM images, which are inherent to the SFM instrumentation including piezo creep, the strain of 1.3% could not be detected directly. The PEN film morphology on PVA substrates remained qualitatively alike for strains beyond 2%, and the impact of bending was clearly reflected in the topography images (Fig. 4b), revealing an increased distance between remote morphological features for higher values of induced surface strain (Fig. 4c). Moreover, the radius of curvature could be directly estimated from the cross-section analysis of the SFM micrograph (see Supporting Information, Fig. S1) to (1.1 ± 0.1) mm. Since the bending radius was measured [16,19] to be 1 mm (see Supporting Information, Fig. S2), the value from cross-section analysis is in very good agreement. However, the resolution of the SFM imaging was not sufficient to clearly reveal whether individual grains became strained or whether the distance between grains (along grain boundaries) increased. Therefore we performed specular XRD measurements to verify straining of PEN crystallites as described further below. In contrast to PEN films on PVA, we detected the formation of cracks within the gold electrodes on top of PEN film for strains larger than 2% (Fig. 4d and e). The cracks persisted after release of strain. To investigate the impact of strain on the PEN film structure, we performed specular XRD measurements of PEN films in situ deformed up to 3.3% (Fig. 4f). For the un-
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 4
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
Fig. 3. (a) SFM topography of PEN film. (c) Zoom-in before straining; (d) an in situ 1.3% strained PEN film. (b) Provides a cross-section analysis of the SFM height images carried out on PEN films under deformations of up to 1.3% to allow for a comparison of the lateral distance between two distinctive positions (marked with arrows in (c)). Arrows in (c) correspond to arrows in the cross-sections (b), respectively. Also for deformations of up to 1.3% no significant changes in PEN films were found compared to the pristine device. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Representative SFM micrographs of (a) a pristine PEN layer, and (b) an in situ 10% strained PEN film, together with (c) the corresponding crosssections; arrows in (a) and (b) correspond to arrows in the cross-sections. (d) SFM micrograph of the top gold electrode, demonstrating crack formation upon deformation of the gold electrode; (e) transition region (interface) of electrode (left)/pentacene (right). (f) Specular XRD data in the range of the (001)reflection of the PEN thin-film phase (lattice spacing d (001) = 1.54 nm) recorded as function of in situ applied bending-stress e; qz denotes the vertical 0 momentum transfer; the peak at 0.46 A Å1 (on the black curve) labels a reflection assigned to the substrate; the peak intensity decreases with the degree of deformation due to geometrical reasons in the experiment (i.e., less area being illuminated by the X-ray beam). Insets: (left) sketch of the (inclined) molecular orientation in PEN thin-film phase with respect to the PVA substrate (illustrated by yellow triangles); (right) specular XRD data (ex situ) for a pristine sample and the same sample after releasing a stress of 4.6%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
194 195 196 197 198 199
strained device (Fig. 4f, black curve) the strong 0peak at a value of vertical momentum transfer (qz) of 0.41 Å A1 corresponds to the (0 0 1) reflection of the PEN thin-film polymorph [20]. This indicates that the PEN molecules adopt a standing orientation within the film, however, slightly inclined by ca. 6° with respect to the PVA substrate normal
(see yellow triangles in the left inset of Fig. 4f), and that the molecules are arranged in a herringbone motif typical for this specific polymorph [20]. Recently, bending-stressdriven phase transitions were suggested upon flattening of inward and outward bent PEN devices [21]. To observe such phase transitions upon deformation of the present
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
200 201 202 203 204 205
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx 206 207 208 209 210 211 212 213
214 215 216 217 218 219 220 221 222 223
224 225 226 227
PEN/PVA films, we performed both in situ specular XRD on films deformed up to 3.3% (Fig. 4f), and ex situ specular XRD on a film after the release of strain as high as 4.6% (Fig. 4f, right inset). Most notably, in no case a shift of the (0 0 1)-peak maximum occurred, which would, however, be expected if a phase transition to any other PEN polymorph [22–25] would take place (expected change in vertical momentum transfer of the (0 0 1) reflection: 0 Dqz P 0.02 A Å1), and if, therefore, the lateral intermolecular distance would accordingly change. Note that, apart from phase transitions, relevant stress-related changes of the intermolecular distance can be excluded within this deformation range, as they typically occur upon pressures in the GPa region, which is reported in related high-pressure investigations [26]. Note, that the elastic modulus of PEN film is 15 GPa and this large value is due to the polycrystalline nature of PEN film [27]. In particular, already a change in lattice spacing by 2.5% would correspond to Dqz 0.01 Å1 and would, thus, be well captured by our experiment. This is also true for even higher deformations of up to 10%, as indicated by SFM micrographs and the correspond-
5
ing cross-section analyses (Fig. 5) comparing the morphology of the same PEN area before (Fig. 5a and c) and after (Fig. 5b and d) stress release, where no significant changes in morphology are observed. Note that the identity of the investigated areas in the two micrographs is clearly confirmed by characteristic morphological features present in both the micrographs (Fig. 5a and b) and in the respective cross-sections (Fig. 5c and d). Moreover, both micrographs exhibit the same morphological features, which demonstrates that the PEN film completely recovers to its initial state, even after the present severe mechanical deformation.
228
4. Discussion
240
The electrical measurements agree well with previous reports, where both spherical [5,6] and cylindrical [9,28] deformations showed similar dependencies of the electrical response on mechanical strain, i.e., a reversible variation of transfer curves for small strain values and irreversible changes for large strain. Our present study reveals two distinct deformation regimes of the flexible
241
Fig. 5. SFM micrographs of (a) a pristine PEN film and (b) a flattened PEN film after deformation to 10%. Corresponding cross-sections taken along white line between two positions marked with arrows in the SFM micrographs of (c) a pristine and (d) flattened PEN film after deformation to 10%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
229 230 231 232 233 234 235 236 237 238 239
242 243 244 245 246 247
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 6
V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx
301
OTFTs: reversible deformation up to 10% occurs for PEN on PVA substrates, while an irreversible deterioration of the gold electrodes is observed starting at a strain value of 2%. Note that, in full agreement with this observation, thin gold electrodes have been previously reported to deteriorate for strains exceeding 2% [29]. As it has been studied elsewhere [29,30], the electrical resistance of thin gold films decreases with mechanical strain. Moreover, very recently, A.N. Sokolov and coworkers reported about irreversible changes observed in OTFTs upon mechanical deformation. In particular, it was found that for surface strain higher than 1.3%, source and drain electrodes buckling and cracking starts to appear. This issue typically led to a decrease of the electrodes conductivity that induced a permanent decrease of the OTFT output current [31]. Therefore, we propose that the observed irreversible variation of transfer curves is mainly due to the deterioration of the gold electrodes. The reversibility of both the transfer curve variation and the film topography until gold electrode deterioration starts infer that the PEN film itself sustains large strains of up to, at least, 10%. Most importantly, our study reveals that PEN films might be used in devices to sense substantially larger strains than previously reported. From these findings, hence, a design strategy for future improved organic sensors clearly arises: using soft, flexible and mechanically stable materials like graphene or conductive polymers might enable the full reversibility of strain-related measures in reliable future sensor devices. Furthermore, our study allows proposing a well founded model for the reason of the observed reversibility of transfer curves variations. One may speculate of tensile stress having two distinct effects on the PEN thin film: (i), the lateral size of the pentacene crystals might become stretched, therefore modifying the PEN microstructure, which might be detrimental to charge transport, or (ii), the distance between individual neighboring PEN crystallites might be increased, therefore reducing the tunneling probability of charge carriers in the process of charge transport through the grain boundaries. From the SFM and XRD data in our present study the related fundamental process can now be clearly identified as (ii): tensile stress leads to an increase in inter-grain distance while leaving the crystal structure unaffected, as no phase transitions were observed. It is well established that hopping transport, in which a conducting path from the source to the drain electrode is formed by coupled grain chains oriented in random directions [9] rather than band transport dominates conduction in polycrystalline thin organic films [9,32,32]. The increase in distance between crystalline PEN grains translates into an increase in potential barrier for (thermally activated) tunneling between grains, which, therefore, reduces the overall charge carrier mobility in bended PEN thin film OTFTs and, thus, fully explains the observed PEN-OTFT strain sensitivity.
302
5. Conclusions
303
The underlying mechanism of bending-strain sensitivity observed for PEN OTFTs was investigated by combining in situ electrical characterization with morphological and
248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
304 305
structural investigations. Our results indicate that the bending-strain sensitivity of PEN OTFTs is due to changes in the film morphology and that the correlated increase in spacing between PEN crystallites is responsible for the observed decrease in drain current, which is found to be reversible for small deformations. From our study, the irreversibility in electrical characteristics for deformations larger than 2% is recognized to be due to crack formation in gold source- and drain electrodes. In contrast to irreversible changes in the metal layers, PEN itself is found to be fully reversibly modified even for extreme deformations of up to 10%, which identifies the organic/metal interface as bottleneck of the investigated OTFTs and suggests the use of soft flexible electrodes for future strain-sensing devices.
306
Acknowledgements
321
We thank Wolfgang Caliebe (DESY–HASYLAB), Tatjana Djuric and Armin Moser (both TU-Graz) for experimental support. Financing through the DFG (Germany) and the Austrian Science Fund (FWF) project P21094-N20 is gratefully acknowledged.
322
Appendix A. Supplementary material
327
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2013.02.030.
328
References
331
[1] T. Hasegawa, J. Takeya, Science and Technology of Advanced Materials 10 (2009) 024314. [2] Y. Yamashita, Science and Technology of Advanced Materials 10 (2009) 024313. [3] N. Koch, ChemPhysChem 8 (2007) 1438. [4] P. Cosseddu, A. Bonfiglio, I. Salzmann, J.P. Rabe, N. Koch, Organic Electronics 9 (2008) 191. [5] I. Manunza, A. Sulis, A. Bonfiglio, Applied Physics Letters 89 (2006) 143502. [6] I. Manunza, A. Bonfiglio, Biosensors & Bioelectronics 22 (2007) 2775. [7] G. Darlinski et al., Journal of Applied Physics 97 (2005) 093708. [8] H. Kawaguchi, T. Someya, T. Sekitani, T. Sakurai, Ieee Journal of SolidState Circuits 40 (2005) 177. [9] T. Sekitani et al., Applied Physics Letters 86 (2005) 073511. [10] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [11] T. Someya et al., Proceedings of the National Academy of Sciences of the United States of America 102 (2005) 12321. [12] P. Cosseddu, A. Piras, A. Bonfiglio, Ieee Transactions on Electron Devices 58 (2011) 3416. [13] P. Cosseddu, S. Milita, A. Bonfiglio, Ieee Electron Device Letters 33 (2012) 113. [14] F.C. Chen, T.D. Chen, B.R. Zeng, Y.W. Chung, Semiconductor Science and Technology 26 (2011) 034005. [15] T. Lindner, G. Paasch, S. Scheinert, Journal of Applied Physics 98 (2005) 114505. [16] J.K.F. Yau, N. Savvides, C.C. Sorrell, Physica C 266 (1996) 223. [17] T. Sekitani et al., Journal of Non-Crystalline Solids 352 (2006) 1769. [18] T.B. Singh et al., Advanced Materials 17 (2005) 2315. [19] A. Jedaa, M. Halik, Applied Physics Letters 95 (2009) 103309. [20] S. Schiefer, M. Huth, A. Dobrinevski, B. Nickel, Journal of the American Chemical Society 129 (2007) 10316. [21] C. Yang et al., Applied Physics Letters 92 (2008) 243305. [22] R.B. Campbell, J. Trotter, J.M. Robertson, Acta Crystallographica 14 (1961) 705. [23] D. Nabok et al., Physical Review B 76 (2007) 235322. [24] S.C.B. Mannsfeld, A. Virkar, C. Reese, M.F. Toney, Z.N. Bao, Advanced Materials 21 (2009) 2294.
332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030
307 308 309 310 311 312 313 314 315 316 317 318 319 320
323 324 325 326
329 330
ORGELE 2039
No. of Pages 7, Model 3G
13 March 2013 V. Scenev et al. / Organic Electronics xxx (2013) xxx–xxx 369 370 371 372 373 374 375
[25] D. Holmes, S. Kumaraswamy, A.J. Matzger, K.P.C. Vollhardt, Chemistry–A European Journal 5 (1999) 3399. [26] M. Oehzelt et al., Physical Review B 74 (2006) 104103. [27] D. Tahk, H.H. Lee, D.Y. Khang, Macromolecules 42 (2009) 7079. [28] T. Sekitani, I. Shingo, Y. Kato, T. Someya, Japanese Journal of Applied Physics Part 1-Regular Papers Brief Communications & Review Papers 44 2841 (2005).
7
[29] S.P. Lacour, S. Wagner, Z.Y. Huang, Z. Suo, Applied Physics Letters 82 (2003) 2404. [30] T. Li, Z.Y. Huang, Z. Suo, S.P. Lacour, S. Wagner, Applied Physics Letters 85 (2004) 3435. [31] A.N. Sokolov, Y.D. Cao, O.B. Johnson, Z.A. Bao, Advanced Functional Materials 22 (2012) 175. [32] Y. Takamatsu, T. Sekitani, T. Someya, Applied Physics Letters 90 (2007) 133516.
376 377 378 379 380 381 382 383 384
Please cite this article in press as: V. Scenev et al., Origin of mechanical strain sensitivity of pentacene thin-film transistors, Org. Electron. (2013), http://dx.doi.org/10.1016/j.orgel.2013.02.030