Flexible, organic, ion-sensitive field-effect transistor

APPLIED PHYSICS LETTERS 86, 103512 共2005兲

Flexible, organic, ion-sensitive field-effect transistor Andrea Loi and Ileana Manunza INFM-University of Cagliari, Department of Electric and Electronic Engineering, Italy

Annalisa Bonfiglio INFM-University of Cagliari, Department of Electric and Electronic Engineering, Italy and INFM-S3 NanoStructures and BioSystems at Surfaces, Modena, Italy

共Received 22 September 2004; accepted 11 January 2005; published online 4 March 2005兲 Organic ion-sensitive field-effect transistors assembled on flexible plastic films have been fabricated. A thin Mylar™ foil acts both as substrate and gate dielectric. The active layer is vacuum-sublimed on one side of the foil, prepatterned with bottom-contact Au source and drain electrodes. The opposite side of the insulating film is in contact with an electrolytic solution that together with a reference electrode forms an ionic gate. A sensitivity of the device to the pH of the electrolyte solution has been observed. Thanks to the flexibility of the substrate and the low cost of the employed technology, this device opens the way for flexible sensors that can be employed in a variety of innovative applications. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1873051兴 Organic materials, based on conjugated organic small molecules and polymers, offer the opportunity to produce devices on large-area, low-cost, plastic substrates.1 So far, great progresses have been made in the field of optoelectronic devices, like organic light-emitting diodes 共OLEDs兲 共Ref. 2兲 and for switching functions by means of organic field effect transistors 共OFETs兲.3 So far, only few examples of organic semiconductor based field effect sensors have been presented,4–6 but none of them fully exploits the favorable mechanical properties of organic semiconductors. Nevertheless, these findings are attractive as they open a possibility to fabricate alternative, low cost, sensor devices. Recently, a fully flexible structure for field effect devices has been produced.7 The main advantage of this structure is that it is assembled starting from a flexible insulating film, similarly than in Ref. 8, but without any substrate. Thanks to this feature, it is possible to expose the gate side of film to an external medium; this is normally impossible for structures assembled on a substrate. In this way, it is in principle possible to realize with a fully flexible structure a function similar to that of a silicon based ion sensitive field effect transistor9 共ISFET兲. Ion sensitivity in silicon based FETs results from the presence on the surface of the insulating layer of specific sites for H+ ions in the electrolytic solution.10 Starting from this principle, several examples of 共bio兲sensors have been developed, based on the possibility of functionalizing the surface of the insulating layer with molecular layers with specific binding properties for the 共bio兲molecules dissolved in the medium to monitor.11 In principle, with respect to the silicon based structures, organic field effect sensors have several advantages, as the low cost of the technology and the possibility to achieve mechanically flexible structures but on the other hand still require high voltages to give measurable currents. Nevertheless, this issue is less relevant, as the mobility of organic semiconductors is continuously increasing towards values comparable with those of amorphous silicon,12,13 making realistic the possibility to have, in a near future, devices working at low voltages. Ion sensitive organic FETs 共ISOFETs兲 fabricated on plastic substrates could open the way to the fabrication of flexible devices for

solution monitoring and for a number of innovative applications 共as, for example, smart food packages兲 that are not possible at present for silicon based devices. In this letter we report on the fabrication of an ion sensitive field effect device based on pentacene films grown on flexible plastic structures. Pentacene films have been considered as active material because of their high hole FET mobility 关up to 0.1 cm2 / V s 共Ref. 12兲兴 and their good behavior in flexible field effect structures.7,14 Figure 1 shows the structure of the device. A 900-nm-thick Mylar™ sheet 共Du Pont兲, adapted to a plastic frame, has been employed as substrate and gate insulator 共dielectric constant of 3.3兲. The Mylar™ sheet has a dielectric rigidity of 105 V / cm that allows to apply a gate bias sufficiently high to induce a field-effect in the organic semiconductor. Bottom-contact Au source and drain electrodes have been patterned on one side of the dielectric using a standard photolithographic technique whilst the opposite side of the Mylar film is exposed to the electrolytic solution where an Ag/ AgCl reference electrode is immersed. Experimental details on the fabrication of source and drain contacts have been reported elsewhere.7 Gold source and drain electrodes with W / L = 250 共W and L are the channel width and length, respectively兲, with L = 25 ␮m, have been used. Prior to organic deposition, the substrate has been cleaned with acetone, washed with deionized water, and dried with a nitrogen flux. Pentacene 共Sigma Aldrich兲 has been used as received. Pentacene films with a nominal thickness of 50 nm

FIG. 1. Structure of the device.

0003-6951/2005/86共10兲/103512/3/$22.50 86, 103512-1 © 2005 American Institute of Physics Downloaded 07 Mar 2005 to 192.167.164.69. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Appl. Phys. Lett. 86, 103512 共2005兲

FIG. 2. Output characteristics of a pentacene OFET on 900-nm-thick Mylar™ gate dielectric: Ids vs Vds 共a兲 and correspondent Ids vs Vgs 共b兲, both taken with a pH 7 solution.

have been grown by vacuum-sublimation at a nominal deposition flux of about 1 Å / s. Measurements of drain-source current 共Ids兲 versus drain–source and gate–source voltages have been carried out at room temperature in air, by means of a HP 4155 Semiconductor Parameter Analyzer. In order to avoid aging effects, all measurements have been performed immediately after pentacene deposition. Figure 2共a兲 shows the output characteristics of the device biased as a p-channel FET working in accumulation mode. This curve has been registered in presence of an electrolytic solution with a pH of 7 after the reference electrode has been left to stabilize in the same solution for about half a hour. This procedure has been done with the aim of obtaining more stable measurements. As can be seen, the device has the typical behavior of organic p-type field effect transistors, with increasing negative values of Ids with increasing negative Vds values and with a clear field effect induced by the Vgs voltage. 兩Ids,sat兩 increases with the increase of 兩Vgs兩 similarly to OFETs with a metallic gate. Figure 2共b兲 shows Ids as a function of Vgs, where it is possible to evaluate the threshold value of the device as the extrapolated intercept of the square root of Ids with the horizontal axis. It can be observed that the curve has a small hysteresis effect, that is normally observed also in devices with the metallic gate contact15 superposed to source and drain. This indicates that there is a parasitic capacitance effect due, in this case, to the charged layer at the interface between the insulating layer and the solution and probably also to charge trapping effects in the organic semiconductor 共possibly affected also by barrier injection effects at the metal–semiconductor interface and by bias stress of the device兲. Figure 3 shows Ids versus time while the pH value is kept constant 共at 7兲. The current is not

very stable, showing a capacitive behavior: after a time interval of more than 30 min 共value obtained in different measurements兲, the current stabilizes to a saturation value that depends on the pH value of the solution. Namely, as can be seen from Fig. 4 共taken by leaving a 30 min hold time after every pH variation兲, there is a decrease of the current with basic solutions and an increase with acid solutions that clearly indicate that there is an accumulation of ions at the interface between the insulating layer and the solution. When negative charge density is increased, as for basic solutions, holes accumulate in the channel in higher density than in the case of neutral or acid solutions. As a consequence, the 共negative兲 current recorded with fixed values of Vds and Vgs is higher. Conversely, in the case of acid solutions, the positive charge accumulated at the insulator/solution interface causes a decrease of the hole density in the channel and a 共negative兲 lower value of Ids. Figure 4 shows the curves taken at the saturation 共Vds = −90 V , Vgs = −50 V兲. The dependence of the transistor current on the pH value of the solution must be related to a variation of the threshold voltage of the device due to a charge variation at the solution-insulator interface that can be explained in the frame of the Gouy–Chapman–Stern theory10 for the behavior of the interface between a solid surface and an electrolytic solution. According to this theory, an insulator exposed to an aqueous solution interacts with H+ ions and causes a redistribution of the charge in the solution. When the structure formed by the insulator and the electrolytic solution is completed by a semiconductor layer located at the opposite side of the insulator, as in the case of MOSFET and OFET struc-

FIG. 4. Ids vs time curve taken with different pH values of the electrolytic solution. The curve has been obtained by leaving a hold time of 30 min after FIG. 3. Ids vs time curve taken with a pH value of 7 showing the time the change of the pH value. required to stabilize the current after the application of the voltages. Downloaded 07 Mar 2005 to 192.167.164.69. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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tures, the charge variation at the insulator/solution interface capacitively induces a redistribution of charge in the semiconductor. In field effect devices, this variation is directly detectable through the variation of the threshold voltage of the device. This working principle is valid both for MOSFETs and OFETs, as already demonstrated by Bartic et al. in Ref. 4, with the obvious differences due to the specific features of organic semiconductor based devices 共as low currents and high operating voltages兲. In conclusion, ion sensitive field effect transistors have been produced on fully flexible plastic films. The devices are based on pentacene films grown on Au contacts patterned on a 900-nm-thick Mylar™ gate dielectric. The electrical characteristics indicate that the device behaves as a typical p-channel transistor working in accumulation mode. Output current is modulated by the pH value of an electrolytic solution put in contact with one side of the insulating layer. Taking advantage of the full mechanical flexibility of the insulating sheet, attractive developments of the device structure can be envisaged. Indeed, smart electronic films with sensing properties can already be produced with this technique. At present, sensitivity to the presence of ions in the solution has been achieved without any treatment of the insulating layer. Work is in progress for obtaining properties of chemical selectivity by a proper functionalization of the insulating layer.

Professor S. Martinoia is gratefully acknowledged for helpful discussions. The authors acknowledge the EU-ISTFET program 共ARIANNE兲 and the Italian Ministry for Research program under project FIRB. This work is dedicated to the dear memory of Ornella Sanna. S. R. Forrest, Nature 共London兲 428, 911 共2004兲. T. Tsujimura, SID 2003 Technical Digest Vol. XXXIV, Book 1, 6 共2003兲. 3 C. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater. 共Weinheim, Ger.兲 14, 99 共2002兲. 4 C. Bartic, A. Campitelli, and S. Borghs, Appl. Phys. Lett. 82, 475 共2003兲. 5 L. Torsi, A. Dodabalapur, L. Sabbatini, and P. G. Zambonin, Sens. Actuators B 67, 312 共2000兲. 6 Z-T Zhu, J. T. Mason, R. Dieckmann, and G. Malliaras, Appl. Phys. Lett. 81, 4643 共2002兲. 7 A. Bonfiglio, F. Mameli, and O. Sanna, Appl. Phys. Lett. 82, 3550 共2003兲. 8 F. Garnier, R. Hajlaoui, A. Yassar, and P. Srivastava, Science 265, 1684 共1994兲. 9 P. Bergveld, Sens. Actuators B 88, 1-20 共2003兲. 10 M. Grattarola and G. Massobrio, Bioelectronics Handbook: MOSFETs, Biosensor, Neurons 共McGraw–Hill, New York, 1999兲. 11 P. Bergveld and A. Sibbald, Analytical and Biomedical Applications of ISFETs 共Elsevier, Amsterdam, 1988兲. 12 Y. Kato, S. Iba, R. Teramoto, T. Sekitani, T. Someya, H. Kawaguki, and T. Sakurai, Appl. Phys. Lett. 84, 3789 共2004兲. 13 H. Klauk, M. Halik, U. Zschieschang, G. Schmid, W. Radlik, and W. Weber, J. Appl. Phys. 92, 5259 共2002兲. 14 P. Cosseddu, A. Bonfiglio, A. Alessandrini, and P. Facci 共unpublished兲. 15 A. R. Brown, C. P. Jarrett, D. M. de Leeuw, and M. Matters, Synth. Met. 88, 37 共1997兲. 1 2

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