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Recent Advances in Semiconductor Performance and Printing Processes for Organic Transistor-Based Electronics Howard E. Katz† Bell Laboratories-Lucent Technologies, 600 Mountain Avenue 1D-249, Murray Hill, New Jersey 07974 Received February 12, 2004. Revised Manuscript Received August 17, 2004
The past two years have been a time of dramatic growth in the field of organic and printable electronics. This review summarizes the most recent advances in the design, application, and understanding of semiconducting materials relevant to this technology. Pentacene, other organic molecular solids, solution-deposited oligomers and polymers, and higher mobility inorganic and nanostructured materials are discussed. In addition, scientific investigations of single-crystal properties and contact barriers are covered, and the most advanced organicbased circuits are pointed out.
Introduction The field known as “organic” or “plastic” electronics is centered on field effect transistor (FET)-based circuits mounted on large-area and/or flexible substrates. In some applications, these circuits are associated with other devices made from soft materials, such as lightemitting diodes, electrophoretic display pixels, microfluidic channels, and passive electrical elements. While charge carrier transport in organic semiconductors has been studied for decades,1 a few research groups produced the main body of work from the time of the initial organic transistors with potentially usable mobilities,2 through the first replications and refinements of these results,3,4 and the opening up of the field to multiple compound classes and device geometries. This work has been extensively reviewed, most notably and comprehensively by Dimitrakopoulous and Malenfant,5 and also in two recent monographs.6,7 Numerous earlier reviews are also available. In the last two years, there has been a notable acceleration in the development of the field. The purpose of this brief review is to describe the many new discoveries brought to light during this two-year time frame, after the previous reviews had been written. Not only have performance parameters greatly increased, but also new fabrication approaches have been demon† Present address: Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, 102 Maryland Hall, Baltimore, MD 21218. E-mail:
[email protected]. (1) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford Science Publications: New York, 1999. (2) Garnier, F.; Horowitz, G.; Peng, X. Z.; Fichou, D. Synth. Met. 1991, 45, 163-171. (3) Katz, H. E.; Torsi, L.; Dodabalapur, A. Chem. Mater. 1995, 7, 2235-2237. (4) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem. Soc.1993, 115, 8716-8721. (5) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99-117. (6) Gamota, D. R.; Brazis, P.; Kalyanasundaram, K.; Zhang, J. Printed Organic and Molecular Electronics; Kluwer: New York, 2003. (7) Kagan, C. R.; Andry, P. Thin Film Transistors; Marcel Dekker: New York, 2003.
strated, a much wider range of materials has been employed, and our fundamental understanding of organic field-effect transistors (OFETs) has grown. It is also important to note the much broader representation of research groups contributing to the progress. Before launching the technical discussion of OFETS, the concept of mobility needs some introduction. A typical FET layout is shown in Figure 1. Charge carrier mobility is the quantity, usually designated by the letter µ, that defines the current flowing through an FET at a given set of device dimensions and applied voltages above certain threshold values. There are two standard equations from which mobility can be determined, one for the case where the gate-source voltage is significantly greater than the drain-source voltage, the socalled “linear regime”, and the other for the opposite case, known as the “saturation” regime.8
Linear regime: I ) (W/L)Ciµ(Vg - Vt)Vd Saturation regime: I ) (W/2L)Ciµ(Vg - Vt)2 Where I is current measured from the drain contact, W and L are channel width and length, respectively, Ci is capacitance of the gate dielectric per unit area, and V is voltage: gate and drain each relative to the source, and threshold voltage, referring to the critical gate turnon voltage. The left side of the graph in Figure 1 is the linear regime, while the flat parts of the curves on the right side are in saturation regime. Amorphous silicon, the material to which organic semiconductors are most often compared, has a generally accepted measured mobility in the range of 0.1-1 cm2/Vs. A value >0.01 cm2/Vs is considered the minimum acceptable value for any conceivable application, though practically speaking, most anticipated applications such as display drivers (8) Sze, S. M. Semiconductor Devices, Physics, and Technology; John Wiley & Sons: New York, 1985; p 490.
10.1021/cm049781j CCC: $27.50 © 2004 American Chemical Society Published on Web 10/02/2004
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Figure 2. Drain current as a function of gate bias for pentacene TFT on polymer-coated alumina. W/L was 15, and drain voltage was -40 V. The gate dielectric capacitance was 25 nF/cm2. Reprinted with permission from ref 14. Copyright 2003 American Institute of Physics.
Figure 1. Schematic of an organic thin film transistor with contacts in green, gate lead in blue, dielectric in violet, and semiconductor in red. The layout shown is “bottom contact”, meaning that the upper pair of contacts, the source and drain, are at the bottom of the semiconductor film; placement of the source and drain on top of the semiconductor film gives the “top contact” configuration. Also shown are typical plots of drain current versus drain voltage for a set of gate voltages. Reprinted from ref 48. Copyright 2003 American Chemical Society.
will require mobility at least as high as that of amorphous silicon. Organic LEDs driven by organic transistors meeting this requirement have been demonstrated.9 Higher Mobility Molecular Crystals: Pentacene Pentacene continues to receive the most attention by far among candidate molecular solid semiconductors. Its thin films consistently display higher mobilities than any other stable, sublimable organic solid, especially when evaluated on a variety of dielectric substrates and over a range of gate voltages. Beginning with the dramatic announcement several years ago of pentacene mobility significantly exceeding 0.1 cm2/Vs,10 steady progress has been made. In the past year, pentacene has been deposited by the Infineon group on polymeric gate dielectrics coating glass or plastic substrates with mobility of 0.7 cm2/Vs, subthreshold swing of 1-2 V/decade, and high/low current ratio approaching one million.11 The mobility was even higher on flatter substrates, up to 3 cm2/Vs on poly(vinylphenol) polymer and copolymer.12 The best current performance for a pentacene thin film has been obtained at 3M Company using alumina as the gate dielectric material with a (9) Kitamura, M.; Imada, T.; Arakawa, Y. Appl. Phys. Lett. 2003, 83, 3410-3412. (10) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron Device Lett. 1997, 18, 606-608. (11) Klauk, H.; Halik, M.; Zschieschang, U.; Eder, F.; Schmid, G.; Dehm, C. Appl. Phys. Lett. 2003, 82, 4175-4177. (12) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259-5263.
Figure 3. Photomicrograph of an RFID circuit using the OFETs in Figure 2. The upper right portion is an enlargement of the left end of a ring oscillator located at the bottom. The width of the image is approximately 2.5 mm. Reprinted with permission from ref 14. Copyright 2003 American Institute of Physics.
hydrophobic phosphonic acid monolayer coating.13 Mobilities of 2 cm2/Vs, on/off ratio above one million, and subthreshold slope of 2-3 V/decade are reproducibly obtained. However, much higher mobility values have been obtained in “exceptional” cases, higher even than those observed in single-crystal devices (see below). The pentacene/alumina system has been used to demonstrate transponder action in a circuit relevant to the design of radio frequency identification (RFID) tags14 (Figures 2 and 3). A system of shadow masks was used to pattern the materials. The growth mechanism of pentacene films has been studied at the nanoscopic level, and various growth (13) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. Y. P. J. Phys. Chem. B 2003, 107, 58775881. (14) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D. V.; Theiss, S. D. Appl. Phys. Lett. 2003, 82, 3964-3966.
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Figure 4. AFM image of pentacene multilayer crystallites on Si (001). The height difference between features is between 1 and 1.5 nm, suggesting that each represents a single molecular layer step. Reprinted from ref 15. Copyright 2003 American Chemical Society.
methods have been explored. Conventional high-vacuum sublimation of pentacene onto clean surfaces gives shallow pyramids with terraces of molecular thickness and breadths of tens of micrometers15 (Figure 4). These broad islands fuse to make substantially continuous films, even to the point of crystal axis registry.16 Theoretical models have been used to explain this growth habit, and to point out ways to ensure the favorable, continuous, high-mobility morphology over the alternative, three-dimensional crystallite morphology where the islands are poorly connected and grain boundaries limit the mobility.17,18 There is new evidence that the flatness of the substrate is correlated to the film continuity and the morphology, probably to a greater extent than is the chemical functionality on the surface.19 Transistors built from single two-dimensional multilayer crystallites grown at elevated temperature on poly(methyl methacrylate) gate dielectric have mobility >1 cm2/Vs.20 This is much higher than the mobility of polycrystalline pentacene film grown at ambient temperature, or of crystallite devices only two pentacene monolayers thick. In a separate study, the mobility appears to decrease as the pentacene layer becomes very thick.21 This decrease was attributed to resistance from a top contact to the channel, rather than a real difference in channel mobility. Deep level traps seem to form in pentacene films, possibly because of chemical impurities.22 Even when (15) Weidkamp, K. P.; Hacker, C. A.; Schwartz, M. P.; Cao, X. P.; Tromp, R. M.; Hamers, R. J. J. Phys. Chem. B 2003, 107, 11142-11148. (16) Laquindanum, J.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem. Mater. 1996, 8, 2542-2544. (17) Verlaak, S.; Steudel, S.; Heremans, P.; Janssen, D.; Deleuze, M. S. Phys. Rev. B 2003, 68. (18) Luo, Y.; Wang, G. H.; Theobald, J. A.; Beton, P. H. Surf. Sci. 2003, 537, 241-246. (19) Knipp, D.; Street, R. A.; Volkel, A. R. Appl. Phys. Lett. 2003, 82, 3907-3909. (20) Wang, G. Z.; Luo, Y.; Beton, P. H. Appl. Phys. Lett. 2003, 83, 3108-3110.
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the pentacene was carefully purified and contact resistance was carefully minimized, mobility in one singlecrystal device study never exceeded 0.5 cm2/Vs.23 Shallow interfacial traps were proposed as the limiting feature. In a related but independent study, similar mobilities were obtained on single-crystal devices.24 Furthermore, using an experimentally derived activation energy for the charge carriers in these latter devices and several mathematical assumptions, it was determined that as few as 0.4% of the carriers are actually contributing to the channel current as “free carriers”. The mobility of just those carriers was predicted to be approximately 75 cm2/Vs, indicating that control of interfacial energy levels could result in huge increases in observed mobility in both single crystals and thin films. Process modifications are being investigated for special applications. For example, mobility anisotropy in the plane of the device can be obtained by depositing the pentacene on a rubbed poly(vinyl alcohol) alignment layer, though the absolute mobilities were low.25 Laserinduced evaporation of pentacene has been studied because of its compatiblity with an overall dry printing process for plastic circuits.26 A photodefinable acrylate has been used as the gate dielectric to offer another patterning alternative.27 Other High-Mobility Molecular Crystals Although generally not as impressive electrically as pentacene, alternative conjugated organic molecular crystals continue to be synthesized and examined as semiconductors. It is conceivable that some of these compounds will offer some attribute, such as threshold voltage, stability, transparency, or processability, that is superior to that of pentacene for a particular application. Molecular structures of representative compounds are shown in Figure 5. Compounds 2,3,9,10-tetramethylpentacene28 and naphthacene (tetracene, 2,3-benzanthracene),29 closely related to pentacene, have mobilities >0.3 and >0.1 cm2/Vs, respectively, when sublimed onto substrates at controlled, optimized temperatures. A partially conjugated, diaza version of pentacene had mobility 2 cm2/Vs.115 The partial pressure of oxygen during vacuum sputtering controls the intrinsic conductivity as well as the mobility of the material. Zinc oxide offers the additional advantage of high transparency. Surface-patterned chemical bath deposition of another II-VI compound, cadmium sulfide, gives films with mobility 0.1-1 cm2/Vs and outstanding on/off ratios of 10 million.116 Another promising approach to inorganic semiconductors applied to plastic electronics is to grow well-known inorganics such as silicon and cadmium sulfide as nanowires, and form aligned films on dielectrics. This separates the high-temperature process of synthesizing the material from the ambient-condition processes that are compatible with flexible substrates and simple printing. Mobilities that had been reported in the hundreds for individual nanowires have indeed been realized by these same materials in thin film form.117 Conclusion An explosive growth in the interest and accomplishments in the field of organic and plastic electronics has occurred in the last two years. Superficially, the key criteria of high mobility and acceptable dynamic range have been met for a number of applications. However, numerous questions remain that are just beginning to be addressed in a rigorous way. Contacts and dielectrics need to be optimized further to allow low voltage operation and avoid undue resistive losses. Generally applicable packaging is needed to stabilize the devices against the environment. Materials need to be shown to be usable in parallel printing operations. The reproducibility of electrical parameters, especially of the higher mobility materials, needs to be kept within tolerances defined by circuit requirements. Finally, from an economic perspective, the processes associated with plastic electronics must be run so that they remain significantly less expensive than those associated with the established and ever more efficient silicon electronics industry. Note Added in Revision The field of organic semiconductors has progressed rapidly and intensely since this manuscript was first prepared. In the interest of completeness and the best (112) Chen, Z. H.; Du, X.; Du, M. H.; Rancken, C. D.; Cheng, H. P.; Rinzler, A. G. Nano Lett. 2003, 3, 1245-1249. (113) Krupke, R.; Hennrich, F.; von Lohneysen, H.; Kappes, M. M. Science 2003, 301, 344-347. (114) Chattopadhyay, D.; Galeska, L.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 3370-3375. (115) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, G. Appl. Phys. Lett. 2003, 82, 1117-1119. (116) Meth, H.; Zane, S. G.; Sharp, K. G.; Agrawal, S. Thin Solid Films 2003, 444, 227-234. (117) Duan, X. F.; Niu, C. M.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. Nature 2003, 425, 274-278.
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possible timeliness, a few additional highlights are presented below. New condensed ring chalcogenide and vinylene compounds have shown high hole mobilities. 2,6-Diphenylbenzodithiophene and -selenophene were sublimed to form films with FET mobilities on the order of 0.1-0.2 cm2/Vs.118 A single crystal of symmetrical dithienotetrathiofulvalene has a mobility of 1.4 cm2/Vs, rivaling that of single-crystal pentacene (2 cm2/Vs).119,120 A surprisingly stable phenylenevinylene oligomer was sublimed onto a substrate heated at 150 °C, and produced a mobility of 0.26 cm2/Vs.121,122 Progress has also been made in n-channel semiconductor devices. The fascinating compound perfluoropentacene was synthesized (a feat in itself), the herringbone crystal structure was solved, and electron mobilities up to 0.1 cm2/Vs were obtained.123 The material was paired with conventional pentacene for the construction of bipolar transistors and high-gain inverters. Perylenetetracarboxylic diimide compounds continue to be developed as n-channel materials as well.124 Conventional pentacene continues to be the workhorse organic semiconductor for circuit prototypes, with examples too numerous to mention here. Experimentation on single crystals and theoretical projections put the attainable mobility for “perfect” pure, crystalline pentacene well above 10 cm2/Vs.125,126 Mobilities of this magnitude (15 cm2/Vs) have also been obtained on rubrene single crystals.127 Finally, two intriguing molecular structures have been described for soluble semiconductors. An a6T derivative with a pair of bulky, unsymmetrical, and removable side chains was deposited as a precursor film and then converted to a semiconductor film with mobility of 0.05 cm2/Vs.128 A regioregular poly(dialkylquaterthiophene) based on symmetrical subunits formed a lamellar film with mobility of about 0.1 cm2/Vs with greater stability to oxygen than the commonly used poly(3-hexylthiophene).129 CM049781J (118) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. J. Am. Chem. Soc. 2004, 126, 5084-5085. (119) Bromley, S. T.; Mas-Torrent, M.; Hadley, P.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 6544-6545. (120) Mas-Torrent, M.; Durkut, M.; Hadley, P.; Ribas, X.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 984-985. (121) Gorjanc, T. C.; Levesque, I.; D’Iorio, M. J. Vacuum Sci. Technol. A 2004, 22, 760-763. (122) Gorjanc, T. C.; Levesque, I.; D’Iorio, M. Appl. Phys. Lett. 2004, 84, 930-932. (123) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 81388140. (124) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Frisbie, C. D.; Ewbank, P. C.; Mann, K. R.; Miller, L. L. J. Appl. Phys. 2004, 95, 6396-6405. (125) Jurchescu, O. D.; Baas, J.; Palstra, T. T. M. Appl. Phys. Lett. 2004, 84, 3061-3063. (126) Minari, T.; Nemoto, T.; Isoda, S. J. Appl. Phys. 2004, 96, 769772. (127) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644-1646. (128) Murphy, A. R.; Frechet, J. M. J.; Chang, P.; Lee, J.; Subramanian, V. J. Am. Chem. Soc. 2004, 126, 1596-1597. (129) Ong, B. S.; Wu, Y. L.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378-3379.
