Solution-Processed Organic Field-Effect Transistors and Unipolar

Effect of dipole layer on the density-of-states and charge transport in organic thin film transistors. Ling Li , Nianduan Lu , Ming Liu. Applied Physi...
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Langmuir 2007, 23, 13223-13231

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Solution-Processed Organic Field-Effect Transistors and Unipolar Inverters Using Self-Assembled Interface Dipoles on Gate Dielectrics Cheng Huang,† Howard E. Katz,*,† and James E. West‡ Department of Materials Science and Engineering and Department of Electrical and Computer Engineering, G. W. C. Whiting School of Engineering, Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed August 6, 2007. In Final Form: October 2, 2007 Self-assembled monolayers (SAMs) of polarized and nonpolarized organosilane molecules on gate insulators induced tunable threshold voltage shifting and current modulation in organic field-effect transistors (OFETs) made from solution-deposited 5,5′-bis(4-hexylphenyl)-2,2′-bithiophene (6PTTP6), defining depletion-mode and enhancementmode operation. p-Channel inverters were made from pairs of OFETs with an enhancement-mode driver and a depletion-mode load to implement full-swing and high-gain organic logic circuits. The experimental results indicate that the shift of the transfer characteristics is governed by the built-in electric field of the SAM. The effect of surface functional groups affixed to the dielectric substrate on the grain appearance and film mobility is also determined.

Introduction Organic field-effect transistors (OFETs) based on organic semiconductor (OSC) thin films show promise as building blocks for flexible electronics such as bendable displays, smart cards, radio frequency identification tags, chemical sensors, wearable electronics, skins for robotics, and other applications.1-3 Techniques such as ink-jet printing, transfer printing, and roll-to-roll processing of soluble organic semiconductor oligomers and polymers enable the realization of low-cost and large-area manufacturing of plastic integrated circuits.2,3 While complementary circuits composed of n-type and p-type transistors are desirable because of high signal integrity and low power consumption, most of the best developed organic semiconductors for use in circuits are p-type materials.4-7 Many n-type organic semiconductors are easily degraded in the atmosphere, though progress is continuing.8 For circuits comprising OFETs with different turn-on voltages, on/off current ratios, or carrier * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Materials Science and Engineering. ‡ Department of Electrical and Computer Engineering. (1) (a) Klauk, H. Organic Electronics, Materials, Manufacturing and Applications; Wiley-VCH: Weinheim, 2006. (b) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (c) Kagan, C. R.; Andry, P. Thin-Film Transistors; Marcel Dekker: New York, 2003. (d) Singh, T. B.; Sariciftci, N. S. Annu. ReV. Mater. Res. 2006, 36, 199. Also see Chem. ReV. 2007, 107, number 4, special issue. (2) (a) Berggren, M.; Nilsson, D.; Robinson, N. D. Nat. Mater. 2007, 6, 3. (b) Sirringhaus, H. AdV. Mater. 2005, 17, 2411. (c) Klauk, H. Nat. Mater. 2007, 6, 397. (d) Hines, D. R.; Ballarotto, V. W.; Williams, E. D.; Shao, Y.; Solin, S. A. J. Appl. Phys. 2007, 101, 024503. (3) (a) Katz, H. E. Chem. Mater. 2004, 16, 4748. (b) Katz, H. E.; Li, W.; Lovinger, A. J. Synth. Met. 1999, 102, 897. (c) Katz, H. E.; Siegrist, T.; Lefenfeld, M.; Gopalan, P.; Mushrush, M.; Ocko, B.; Gang, O.; Jisrawl, N. J. Phys. Chem. B 2004, 108, 8567. (d) Zielke, D.; Hu¨bler, A. C.; Hahn, U.; Brandt, N.; Bartzsch, M.; Fu¨gmann, U.; Fischer, T.; Veres, J.; Ogier, S. Appl. Phys. Lett. 2005, 87, 123508. (e) Song, D. H.; Choi, M. H.; Kim, J. Y.; Jang, J.; Kirchmeyer, S. Appl. Phys. Lett. 2007, 90, 053504. (4) (a) Crone, B. K.; Dodabalapur, A.; Lin, Y.-Y.; Filas, R. W.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521. (b) Meijer, E. J.; de Leeuw, D. M.; Setayesh, S.; Van Veenendaal, E.; Huisman, B.-H.; Blom, P. W. M.; Hummelen, J. C.; Scherf, U.; Kadam, J.; Klapwijk, T. M. Nat. Mater. 2003, 2, 678. (5) (a) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D. V.; Theiss, S. D. Appl. Phys. Lett. 2003, 82, 3964. (b) Singh, T. B.; Senkarabacak, P.; Sariciftci, N. S.; Tanda, A.; Lackner, C.; Hagelauer, R.; Horowitz, G. Appl. Phys. Lett. 2006, 89, 033512. (c) Anthopoulos, T. D.; Setayesh, S.; Smits, E.; Coelle, M.; Cantatore, E.; de Boer, B.; Blom, P. W. M.; de Leeuw, D. M. AdV. Mater. 2006, 18, 1900.

mobilities, it is often necessary to pattern different OSCs to act as the active materials in the contrasting OFETs. In other cases, circuits were made from a single OSC. These unipolar circuits are made up of all enhancement-mode,5 ambipolar, or depletionmode transistors.6 Circuit complexity is increased, and the swing is limited.7 A level-shifting design with an integrated level-shift stage would be required to adjust the threshold voltage of the driver transistors to make the inverter characteristics more symmetrical, and/or more transistors would be added to allow circuits to operate over a wide range of threshold voltages.6a,7 At the very least, device dimensions would have to be different for OFETs made of the same materials to approximately match the contrasting output characteristics. Modification or design of the gate dielectric is a promising route for improving and tuning the function of OFETs and process simplification for organic circuits.9 Recently, we have demonstrated that gate dielectrics with permanently stored space charges enable controllable shift of the threshold voltage and current modulation in OFETs and inverter circuit tuning.9a,10 An external (6) (a) Klauk, H.; Gundlach, D. J.; Jackson, T. N. IEEE Electron DeVice Lett. 1999, 20, 289. (b) Klauk, H.; Halik, M.; Zschieschang, U.; Eder, F.; Schmid, G.; Dehm, C. Appl. Phys. Lett. 2003, 82, 4175. (c) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. AdV. Mater. 2002, 23, 1717. (d) Brown, A. R.; Pomp, A.; Hart, M.; de Leeuw, D. M. Science 1995, 270, 972. (e) Gelinck, G. H.; Geuns, T. C. T.; de Leeuw, D. M. Appl. Phys. Lett. 2000, 77, 1487. (7) Gelinck, G. H.; Huitema, H. E. A.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; Putten, J. B. P. H.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B.; Meijer, E. J.; Benito, E. M.; Touwslanger, F. J.; Marsman, A. W.; Rens, B. J. E.; Leeuw, D. M. Nat. Mater. 2004, 3, 106. (8) (a) Bao, Z. N. Organic Field-Effect Transistors; CRC Press: Boca Raton, FL, 2007. (b) Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y.-Y.; Dodabalapur, A. Nature 2000, 410, 478. (c) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. Angew. Chem. 2004, 43, 6363. (d) Facchetti, A. Mater. Today 2007, 10, 28. (9) (a) Katz, H. E.; Hong, X. M.; Dodabalapur, A.; Sarpeshkar, R. J. Appl. Phys. 2002, 91, 1572. (b) Facchetti, A.; Yoon, M.-H.; Marks, T. J. AdV. Mater. 2005, 17, 1705. (c) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 4678. (d) Halik, M. In Organic Electronics, Materials, Manufacturing and Applications; Klauk, H., Ed.; Wiley-VCH: Weinheim, 2006. (e) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schu¨tz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963. (f) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445, 745. (g) Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E.C.-W.; Ho, P.K.-H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 195. (10) (a) Huang, C.; Katz, H. E.; West, J. E. AdV. Funct. Mater. 2007, 17, 142. (b) Huang, C.; West, J. E.; Katz, H. E. J. Appl. Phys. 2006, 100, 114512. (c) Mushrush, M.; Facchetti, A.; Lefenfeld, M.; Katz, H. E.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 9414.

10.1021/la702409m CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007

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electric field charging process is necessary for organic transistor and circuit tuning either by the existing electrodes to apply voltage across the dielectric-semiconductor bilayer after complete fabrication of the devices, or by noncontact corona charging of dielectrics before the deposition of OSC and source-drain electrodes. Ferroelectric or ferroelectret polymers have been also utilized as gate dielectrics for OFETs and memory elements.11 In these cases, stability is governed by the stability of the polarized state. Here, as an alternative, we utilize the permanent dipole polarization of directionally aligned organosilane self-assembled monolayers (SAMs) chemically grafted onto gate dielectrics to define threshold voltages in solution-deposited OFETs and circuits. The built-in electric field of a dipolar SAM induces mobile charge carriers in OFET channels and causes the threshold voltage shift, which has been investigated by some groups several times.12 In other words, if the electrostatic SAM dipole field easily controls the “doping” level to change the carrier concentration and modulate the electronic properties of the organic semiconductor layer, this method enables controllable threshold voltage shifting and thus current modulation of OFETs and logical operation, without the need for external electric field poling in our current research work. We present a detailed study of the solution deposition of the organic semiconductor 5,5′-bis(4-hexylphenyl)-2,2′-bithiophene (6PTTP6) on SAM-modified gate dielectrics and the influence of the SAMs on the morphology and electronic properties of 6PTTP6 thin films. We also demonstrate solution-processed p-channel metal oxide semiconductor (PMOS)-like inverters on gate dielectrics modified with SAMs of nonpolarized and polarized organosilane molecules, composed of p-channel OFETs with an enhanced-mode driver and a depletion-mode load, respectively. Thus, a pair of accumulation-mode and depletionmode OFETs can be deposited from the same soluble and airstable OSC, and full-swing unipolar inverters can be realized by these pairs of OFETs with the same channel dimensions. Experimental Section Gate Dielectric Surface Treatment and Modification. Figure 1 shows molecular structures of the studied organosilane coupling agents along with their names and abbreviations. Phenyltrimethoxysilane (C6H5Si(OCH3)3, PTS) was purchased from Aldrich and used as received to form a nearly unpolarized self-assembled monolayer. Three fluorinated organosilanes as starting molecules with different chain lengths and dipole moments were used as received for polarized self-assembled monolayers: Trichloro(3,3,3-trifluoropropyl)silane (CF3(CH2)2SiCl3, FPTS) and trichloro(1H,1H,2H,2H-perfluorooctyl)silane (CF3(CF2)5(CF2)2SiCl3, FOTS) were purchased from Aldrich; trichloro(1H,1H,2H,2H-perfluorodecyl)silane (CF3(CF2)7(CH2)2SiCl3, FDTS) was purchased from Oakwood Products, Inc. The substrates were heavily doped p-type silicon wafers (Si-Tech Inc.), used as a common gate for OFETs, with a 300-nm-thick wet thermally (11) (a) Naber, R. C. G.; Tanase, C.; Blom, P. W. M.; Gelinck, G. H.; Marsman, A. W.; Touwslager, F. J.; Setayesh, S.; De Leeuw, D. M. Nat. Mater. 2005, 4, 243. (b) Singh, Th. B.; Marjanovic´, N.; Matt, G. J.; Sariciftci, N. S.; Schwo¨diauer, R.; Bauer, S. Appl. Phys. Lett. 2004, 85, 5409. (c) Nguyen, C. A.; Lee, P. S.; Mhaisalkar, S. G.; Org. Electron. 2007, 8, 415. (d) Baeg, K.-J.; Noh, Y.-Y.; Ghim, J.; Kang, S.-J.; Lee, H.; Kim, D.-Y. AdV. Mater. 2006, 18, 3179. (e) Graz, I.; Kaltenbrunner, M.; Keplinger, C.; Schwoediauer, R.; Bauer, S.; Lacour, S.; Wagner, S. Appl. Phys. Lett. 2006, 89, 073501. (f) Scharnberg, M.; Zaporotchenko, V.; Adelung, R.; Faupel, F.; Pannerman, C.; Diekmann, T.; Hilleringmann, U. Appl. Phys. Lett. 2007, 90, 013501. (g) Scott, J. C.; Bozano, L. D. AdV. Mater. 2007, 19, 1452. (12) (a) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y. Nat. Mater. 2004, 3, 317. (b) Takeya, J.; Nishikawa, T.; Takenobu, T.; Kobayashi, S.; Iwasa, Y.; Mitani, T.; Goldmann, C.; Krellner, C.; Batlogg, B. Appl. Phys. Lett. 2004, 85, 5078. (c) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431. (d) Jang, Y.; Cho, J. H.; Kim, D. H.; Park, Y. D.; Hwang, M.; Cho, K. Appl. Phys. Lett. 2007, 90, 132104.

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Figure 1. Molecular structures of the studied organosilane coupling agents: PTS, phenyltrimethoxysilane; FPTS, trichloro(3,3,3-trifluoropropyl)silane; FOTS, trichloro(1H,1H,2H,2H-perfluorooctyl)silane; FDTS, trichloro(1H,1H,2H,2H-perfluorodecyl)silane. grown silicon dioxide (SiO2) insulating layer (Process Specialties), used as the gate dielectric. The wafers were successively cleaned in hot acetone and hot isopropanol in an ultrasonic bath and then with a piranha solution (70 vol % sulfuric acid/30 vol % hydrogen peroxide (Caution! Extremely corrosiVe and organic-reactiVe!)) for 20 min, and finally they were thoroughly rinsed with deionized water. Oxygen plasma cleaning (Harrick Scientific) was further used for the activation of the SiO2 surface before depositing the selfassembled layers. Thin layers of silane coupling agents were added to the SiO2 surface by exposure of the cleaned wafer to the silane as a 1 vol % solution in hot anhydrous toluene for 10 min. The monolayers were then baked on a hot plate for 1 h at 150 °C. The treatment process was optimized for PTS and was applied in the same way for the other organosilanes. Solution Deposition of Organic Semiconductor Thin Films. The organic semiconductor oligomer 5,5′-bis(4-hexylphenyl)-2,2′bithiophene (6PTTP6) was prepared using the standard method and purified by recrystallization. It was chosen because it typically exhibits high p-type carrier mobility, air stability, solution processibility, and thin-film quality.10c The 6PTTP6 organic semiconductor thin films were deposited by solution casting from a 0.04 w/v % xylene solution. The solution from which the film grew was contained inside rectangles of 1-2 cm2 bounded by a fluorinated polymer (3M Novec EGC-1700), which was not wetted or dissolved by xylene. Thin films of 6PTTP6 were deposited by applying 6PTTP6 in xylene dropwise onto a substrate maintained at the temperature of 115 °C. They were kept on a hot plate and under an inverted glass chamber where excess xylene was allowed to evaporate around the films, reducing turbulence and slowing the solvent evaporation. Organic semiconductor thin films with gold luster were formed in 1 min. Materials Surface and Interface Characterization. The surface morphology of gate dielectrics was determined in the tapping mode with an atomic force microscope (AFM; NanoScope IIIa, Digital Instruments). The surface potential differences USAM between PTSSiO2 and other fluorinated SAMs were measured by using an electrostatic force microscope (EFM) modified from a NanoScope IIIa scanning probe microscope using a 0.01-0.25 Ω‚cm antimony (n) doped Si cantilever. The hydrophobic or hydrophilic surfaces of gate dielectrics were analyzed by measuring water contact angles through a model 100-00 Contact Angle goniometer. The grain and thin film growth of organic semiconductor films were observed through an Olympus SZX12 optical microscope and an AFM. Organic Transistors and Inverters Fabrication and Testing. Top-contact source and drain electrodes of OFETs were thermally deposited on top of solution-deposited 6PTTP6 thin films, under a pressure of 1 × 10-6 mbar in a vacuum evaporator (Edwards/E306A Coating Systems). The gold source and drain contacts are 50 nm in thickness, defining a channel length L of 50 µm and a width W of 2 mm through a shadow mask. The unipolar inverters were formed by connecting two individual OFETs on separate substrates: one was fabricated on a gate dielectric with PTS, and the other with a

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Figure 2. Principle for the directionally aligned silane self-assembled molecules or permanent dipolar molecules on SiO2 gate dielectrics by chemical grafting or fixing of one end of the molecular chains: (a) nonpolar and hydrophobic surface with PTS on SiO2; (b) hydrophilic surface of control SiO2; and (c) dipolar and hydrophobic surface with FDTS on SiO2. fluorosilane; both were covered by 6PTTP6 and had gold as the source and drain electrodes. In the unipolar inverters, the channel dimensions for the transistor load and driver remained the same with L ) 50 um and W ) 2 mm. The electrical characterization of discrete OFETs was performed using an Agilent 4155C semiconductor parameter analyzer with coaxial cables and probes. The Agilent I/CV 2.1 Lite automation software controlled the measurements. The mobility values were derived from the drain-source currents in the saturation region from devices in the central portion of solutiondeposited films and were reproducible from film to film. The gate dielectric capacitance per unit area (C′ox ) 11.0 nF cm-2) was measured using an Agilent 4282A precision LCR meter at a frequency of 1 kHz and a level of 0.5 V. Quasistatic voltage transfer characteristics of inverters were measured using an Agilent 4155C parameter analyzer with coaxial cables and probes as well as a digital voltmeter. All the electrical measurements were carried out in ambient air (relative humidity ≈ 22-40%, temperature ) 18-25 °C).

Results and Discussion Solution-Deposited 6PTTP6 on Self-Assembled Dipole Gate Dielectrics. Figure 2 shows the schematic for the growth of organosilane self-assembled monolayers on the SiO2 surfaces. These organosilane chemical modifiers alter the surface properties of the gate dielectric. AFM images in Figure 3 show the surface morphologies of SAM-modified SiO2. The images in Figure 4 show their water contact angles. The control SiO2 surface is relatively flat (surface root-mean-square (rms) ) 0.175 nm), and the contact angle is 39°, indicating that the control SiO2 surface is hydrophilic with hydrophilic reactive groups such as -OH on the surface. After the treatment with PTS, the surface became somewhat rougher (rms ) 0.220 nm), and the water contact angle was 91°, indicating that the SiO2 surface has become hydrophobic. There are slightly aligned patterns shown in the AFM images in Figure 3a and b. We believe that these are artifacts originating from the AFM tip scanning on a relatively smooth surface, but they did not influence the surface morphology comparison and surface roughness measurements. When the SiO2 surfaces were treated with fluorinated dipolar FDTS organosilanes with long chain molecules, the surfaces became even rougher (rms ) 0.325 nm), and the water contact angles reached 111°, even more hydrophobic. The chemical grafting or fixing of one end of the fluorosilane molecular chains results in polarized self-assembled monolayers, in which the permanent dipole

moments on the SiO2 surface spontaneously point to the same direction, as has been seen in other self-assembled monolayers.13 During solution deposition of organic semiconductors on gate dielectric surfaces, the atmospheric turbulence and solventvapor equilibration could influence the molecular packing, crystallization, and finally the growth of the thin films of organic materials in 6PTTP6.3c Therefore, the growth of 6PTTP6 thin films was under this “closed environment”, where, in the presence of a saturated xylene atmosphere, evaporation from the film is somewhat reversible, atmospheric turbulence is kept to a minimum, and the grains could be larger and more tightly packed. Both the optical micrographs and AFM topographic images in Figure 5 show 6PTTP6 grain growth on three different SiO2 surfaces when deposited in a xylene-saturated, reduced turbulence, “closed” environment”: (a) the surface modified with shortchain nonpolar PTS silane; (b) no surface modification; and (c) the surface modified with long-chain FDTS fluorosilane. Compared to the grains grown on the control SiO2 surface, larger and more tightly packed 6PTTP6 grains occur on the PTSmodified SiO2 surface, while smaller and more loosely packed grains occur on the FDTS-modified SiO2 surface. This is consistent with the better compatibility of organic semiconductors with a phenyl-rich surface.14 However, dewetting was observed when 6PTTP6 molecules were deposited on the fluorinated surfaces, though not as much as on the fluorinated polymer boundary.3c,10c Electronics properties of the OSC are also influenced by these organosilane self-assembled monolayers. Figure 6 shows the transfer characteristics of field-effect transistors in which the 6PTTP6 organic semiconductor solution was deposited on organosilane SAM gate dielectrics. The best overall electronic performance occurs when 6PTTP6 is deposited on the PTSmodified SiO2 surface.14 Compared to the measured hole carrier mobility µp of 0.02-0.04 cm2 V-1 s-1, on/off current ratio Ion/ (13) (a) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (b) Jaworek, T.; Neher, D.; Wegner, G.; Wieringa, R. H.; Schouten, A. J. Science 1998, 279, 57. (c) Ko¨rner, H.; Shiota, A.; Bunning, T. J.; Ober, C. K. Science 1996, 272, 252. (14) (a) Park, Y. D.; Lim, J. A.; Lee, H. S.; Cho, K. Mater. Today 2007, 10, 46. (b) Salleo, A.; Chabinyc, M. L.; Yang, M. S.; Street, R. A. Appl. Phys. Lett. 2002, 81, 4383. (c) Kim, D. H.; Park, Y. D.; Jang, Y.; Yang, H.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S.; Chang, T.; Chang, C.; Joo, M.; Ryu, C. Y.; Cho, K. AdV. Funct. Mater. 2005, 15, 77. (d) Yagi, I.; Tsukagoshi, K.; Aoyagi, Y. Appl. Phys. Lett. 2005, 86, 103502. (e) Kumaki, D.; Yahiro, M.; Inoue, Y.; Tokito, S. Appl. Phys. Lett. 2007, 90, 133511. (f) McDowell, M.; Hill, I. G.; McDermott, J. E.; Bernasek, S. L.; Schwartz, J. Appl. Phys. Lett. 2006, 88, 073505. (g) Kumaki, D.; Ando, S.; Shimono, S.; Yamashita, Y.; Umeda, T.; Tokito, S. Appl. Phys. Lett. 2007, 90, 053506.

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Figure 4. Water droplets on the surface of (a) PTS on SiO2; (b) control SiO2; and (c) FDTS on SiO2. Deionized water was used.

Figure 3. Tapping mode AFM images of the surface morphology of SAM-modified SiO2: (a) PTS on SiO2; (b) control SiO2; and (c) FDTS on SiO2.

Ioff of 30 000, and off current Ioff of 0.1 nA for the 6PTTP6 transistor on the control SiO2 gate dielectric, the 6PTTP6 larger grains (∼10 µm as shown in Figure 5d) and closed packed grain boundaries from the PTS-modified surface result in a drastic improvement of transistor performance: µp ) 0.04-0.08 cm2 V-1 s-1, Ion/Ioff ) 6 000 000, and Ioff ) 1 pA (at high gate voltages (>40 V above the threshold voltage), the contact resistance limits the output currents). However, dewetting of 6PTTP6 on a fluorinated hydrophobic surface results in the decrease in the

magnitude of the hole mobilties and on/off current ratios: µp ) 0.01-0.03 cm2 V-1 s-1, Ion/Ioff ) 1 000, and Ioff ) 2 nA for a FPTS-modified surface; µp ) 0.01-0.02 cm2 V-1 s-1, Ion/Ioff ) 300, and Ioff ) 3 nA for a FOTS-modified surface; and µp ) 0.01 cm2 V-1 s-1, Ion/Ioff )100, and Ioff ) 7 nA for a FDTS-modified surface. It was found that the longer the molecular chains of the fluorosilanes, the more serious the deterioration of the transistor performance and charge transport, which is related to the surface morphology from 6PTTP6 molecular packing, grain growth, and thin film growth on these different fluorosilanes. Nevertheless, the threshold voltage shift induced by the fluorosilanes was useful for circuit assembly, as shown below. Threshold Voltage Shift of OFETs on Self-Assembled Dipole Gate Dielectrics. Figure 6 shows the threshold voltage shift of 6PTTP6 transistors on surface-functionalized gate dielectrics. Compared to the threshold voltage of -1 V for OFETs on the control gate dielectric, there is a slight negative shift of the threshold voltage to -5 V for the OFET on the neutral PTSmodified gate dielectric, which results in a typical enhancementmode transistor and a completely off state at zero-gate bias condition. Obvious positive shifts of the threshold voltage occur for 6PTTP6 OFETs on polar fluorosilane-modified gate dielectrics: +45 V for FPTS-modified SiO2; +70 V for FOTS-modified

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Figure 5. Optical micrographs (a-c) and corresponding AFM topographic images (d-f) of 6PTTP6 grains, grin boundaries, and thin films from 6PTTP6 in xylene dropwise onto the surfaces: (a and d) PTS on SiO2; (b and e) control SiO2; and (c and f) FDTS on SiO2.

SiO2; and +80 V for FDTS-modified SiO2. This significant positive threshold voltage shift is related to the electrostatic dipole field from fluorosilane self-assembled layers. As schematically shown in Figure 7a, polarization from directionally aligned, ultrathin permanent dipoles results when those fluorosilane molecules are grafted to SiO2 surfaces. The grafted fluorosilane SAM dipole field results in the positive charge accumulation in the transistor channels at zero-gate bias. For a SAM with a permanent electric dipole field inserted between the gate insulator and 6PTTP6 semiconductor, the situation is as illustrated in Figure 7b. The built-in electric field of the SAM induces mobile charge carriers and causes the threshold voltage shift.12 To further understand the influence of the electrostatic dipole field on the threshold voltage shift, we specially compared the results of OFET-measured threshold voltage shifts Vth1 and threshold voltage shifts predicted from electrostatic measurements Vth2. The measured threshold voltage shift Vth1 of SAM-modified OFETs was obtained from the transfer characteristics as shown in Figure 6. The mobile charge carrier density induced by the SAM (Q′SAM) can be estimated using12c

Q′SAM ) C′oxVth )

0ox V d2 th

(1)

where C′ox is the silicon dioxide capacitance per unit area (electron charge e ) 1.6 × 10-19 C), d2 is the thickness of the silicon dioxide, Vth is the SAM-induced threshold voltage shift, and ox and 0 are the permittivities of the silicon dioxide ()3.9) and free space ()8.85 × 10-12 F/m), respectively. Ideally, Vth is expressed equally by Vth1 and Vth2. The mobile charge carrier density induced by the SAM was also estimated15 using

Q′SAM ) -C′SAMUSAM ) -

0SAM USAM d1

(2)

where C′SAM is the SAM layer capacitance per unit area, d1 is the thickness of the SAM which is close to the length of the silane chain L assuming the silane does not significantly incline (which is likely not correct, as discussed later), USAM is the surface potential difference between PTS-SiO2 and other fluorinated SAMs which were measured by using an EFM, and SAM and (15) (a) Sugimura, H.; Hayashi, K.; Saito, N.; Nakagiri, N.; Takai, O. Appl. Surf. Sci. 2002, 188, 403. (b) Saito, N.; Hayashi, K.; Sugimura, H.; Takai, O.; Nakagiri, N. Surf. Interface Anal. 2002, 34, 601. (c) Ofir, Y.; Zenou, N.; Goykhman, I.; Yitzchaik, S. J. Phys. Chem. B 2006, 110, 8002. (d) Palermo, V.; Palma, M.; Samorı`, P. AdV. Mater. 2006, 18, 145. (e) Palermo, V.; Liscio, A.; Palma, M.; Surin, M.; Lazzaroni, R.; Samori, P. Chem. Commun. 2007, 3326. (f) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. ReV. B 1996, 54, 14321.

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Figure 6. (a) Drain current (IDS) versus gate voltage (VGS) transfer characteristics and (b) square root of drain current versus gate voltage at drain voltage (IDS) ) -50 V of 6PTTP6 transistors fabricated on a SiO2 gate insulator treated with fluorosilanes: FPTS, FOTS, and FDTS, compared to those from the neutral organosilane (PTS), and control SiO2.

0 are the permittivities of the SAM layer (∼2.5) and free space, respectively. Therefore, the SAM electrostatically induced threshold voltage shift Vth2 was given by

Vth2 ) -

SAMd2 U oxd1 SAM

(3)

In addition, compared to USAM measured by using an EFM, the surface potential difference between PTS-SiO2 and other fluorinated SAMs could be also estimated by 15a-c

USAM ) -

µSAM ASAMSAM0

(4)

where µSAM is the effective dipole moment of the SAM expressed in units of debye (1 D ) 3.33564 × 10-30 C‚m) and ASAM is the effective cross-sectional area of the SAM molecules, that is, the area occupied by each molecule. Figure 8 shows the surface electrostatic potential images and contrast of the SAM layers relative to SiO2. In our case, the effective dipole moment µ ) 2.91 D and the molecular chain L ) 0.43 nm for the FPTS SAM. The measured surface potential USAM for FPTS relative to PTS is -50 mV. Therefore, the FPTS electrostatically induced threshold voltage shift Vth2 is +22.4 V as derived from eq 3,

Figure 7. (a) Schematic cross section of an organic transistor with a p-type organic semiconductor thin film deposited on a dipolar SAM-modified gate insulator. The arrows represent directionally aligned, ultrathin permanent dipolar molecular layers. (b) Schematic energy level diagram suggested for OFETs with dipolar SAMmodified gate insulators. For an untreated SiO2 surface, the vacuum levels Evac of the SiO2 and the 6PTTP6 are aligned and no band bending occurs; the permanent dipole field of the SAM shifts the surface potential, which has the same effect as applying a gate voltage. They shift the gate electrode’s Fermi level EF toward higher energies and bend the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the 6PTTP6. The grafted fluorosilane SAM dipole field results in the positive charge accumulation in the transistor channels at the zero-gate bias. d1 is the thickness of the SAM, and d2 is the thickness of gate insulator.

which may be compared to the measured value of +45 V. Substituting -50 mV into eq 4 gives an effective molecular surface density (or the dipole density NSAM) for the FPTS SAM of ∼1.14 × 1013/cm2, or 1 molecule/8.76 nm2. The dipole moment µ ) 3.30 D and the molecular chain L ) 1.20 nm for FOTS. The measured surface potential USAM for FOTS relative to PTS is -200 mV. For FOTS, the electrostatically induced threshold voltage shift Vth2 is +32.0 V as derived from eq 3, which may be compared to the measured Vth1 of +70 V. Substituting -200 mV into eq 4 gives an effective molecular surface density for the FOTS SAM of ∼4.02 × 1013/cm2, or 1 molecule/2.48 nm2. The dipole moment µ ) 3.41 D and the molecular chain L ) 1.34 nm for FDTS. The measured surface potential USAM for FDTS relative to PTS is -300 mV. The FDTS electrostatically induced threshold voltage shift Vth2 is +43.1 V as derived from eq 3, which may be compared to an observed value Vth1 of +80 V. Substituting -300 mV into eq 4 gives an effective molecular surface density for the FDTS SAM of ∼5.83 × 1013/cm2, or 1 molecule/1.71 nm2. Table 1 summarizes the electrostatic and electronic parameters at organic semiconductor-fluorinated SAM gate dielectric interfaces resulting from electrostatic measurements. Figure 9 relates the apparent threshold voltage shifts Vth1 measured from the transistor transfer characteristics and those Vth2 calculated from the measured surface potential (relative to PTS) of SAM-modified SiO2. The observed shifts of the OFET transfer characteristics can be correlated to the surface potential and electrostatic dipole field of the SAM layer next to the transistor

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Langmuir, Vol. 23, No. 26, 2007 13229

Figure 8. Surface electrostatic potential images and contrast of the SAM layers (relative to SiO2). Table 1. Summarized Electrostatic and Electronic Properties at Organic Semiconductor-Fluorinated SAM Gate Dielectric Interfaces Resulting from Electrostatic and Electronic Parameter Measurementsa SAM interface

µSAM Debye

d1 nm

USAM (mV)

Vth2 (V)

Vth1 (V)

ASAM (nm2)

NSAM (1/cm2)

Q′SAM (1/cm2)

NSAM/Q′SAM

FPTS-SiO2 FOTS-SiO2 FDTS-SiO2

2.91 3.30 3.41

0.43 1.20 1.34

-50 -200 -300

+22.4 +32.0 +43.1

+45 +70 +80

8.76 2.48 1.71

1.14 × 1013 4.02 × 1013 5.83 × 1013

3.09 × 1012 4.81 × 1012 5.50 × 1012

4 8 11

aµ SAM is the calculated dipole moment (in Debye) of the SAM, d1 is the calculated thickness of the SAM, USAM is the measured surface potential difference between PTS-SiO2 and the listed fluorinated SAMs (which were measured by an EFM), Vth2 is the SAM electrostatically induced threshold voltage shift (calculated from eq 3), Vth1 is the extracted apparent threshold voltage shift from the transistor transfer characteristics, Vth is the experimental SAM-induced threshold voltage shift, ASAM is the effective area occupied by each molecule (in nm2 molecule-1), NSAM is the dipole density (per square centimeter), Q′SAM is the calculated mobile charge carrier density induced by the SAM, and NSAM/Q′SAM is the effective SAM molecule number corresponding to about one induced mobile hole.

channel. While observed shifts are of the same order of magnitude as predicted shifts, they are invariably larger, and many reflect changes in mobility or doping density that are morphological or chemical, rather than electrostatic, in nature.12c In addition, if we assume tilting of the silanes, effectively decreasing d1, then Vth2 as calculated with eq 3 would proportionally increase. Current Modulation, Unipolar Inverters, and Circuit Tuning on Self-Assembled Dipole Gate Dielectrics. The performance of the solution-processed OFETs with SAMmodified gate dielectrics is sufficient to allow transistor current modulation and circuit tuning and the construction of simple unipolar inverters and logic gates.16 We demonstrated inverters based on nonpolar and dipolar SAMs on SiO2 as gate dielectrics. As shown in Figures 10a and 6a, the transistor configurations and the transfer characteristics, respectively, the p-channel OFET with nonpolar PTS SAM-modified SiO2 exhibits a slight negative threshold voltage shift, which means operation in the enhancement-mode, while the p-channel OFET with polar fluorosilane(16) (a) Sedra, A. S.; Smith. K. C. Microelectronic Circuits, 5th ed.; Oxford University Press: New York, 2003. (b) Huang, C.; Katz, H. E.; West, J. E. AdV. Funct. Mater. 2007, 17, 142.

Figure 9. Relationship between the apparent threshold voltage shifts Vth1 measured from the transistor transfer characteristics and those Vth2 calculated from the measured surface potential of SAM-modified SiO2 (relative to PTS-SiO2).

modified SiO2 exhibits a positive threshold voltage shift, which means operation in the depletion-mode and effectively maintain-

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Huang et al.

Figure 10. (a) Schematic configuration of the organic p-channel unipolar inverter based on dipolar and neutral SAMs, which comprised one transistor on fluorinated SAM-modified SiO2 as the depletion-mode load and one transistor on neutral SAM-modified SiO2 as the enhancement-mode driver. The channel dimensions for both OTFTs were identical. (b) Equivalent electric circuit layout of the unipolar inverter. VIN is input voltage, VOUT is output voltage, and VDD is the supply voltage.

ing the “on” state. To illustrate this capability, we built a solutiondeposited 6PTTP6 transistor inverter with an enhancement-mode OFET driver and a depletion-mode OFET load utilizing SAMmodified OFETs. Figure 10 shows the PMOS-like inverter configuration and the corresponding equivalent electric circuit layout of the unipolar inverter. In this inverter, the enhancementmode OFET is on the nonpolar PTS-modified gate dielectric, and the depletion-mode OFET is on the polar fluorosilanemodified gate dielectric. The channel dimensions for both OFETs were the same in both cases and equal to L ) 50 µm and W ) 2 mm. The gates for two the OFETs were separated by using two different heavily doped wafer substrates. Figure 11a shows the measured characteristics of the enhancement-mode (driver) OFET and also the I-V curves for the depletion-mode (load) OFETs at VG ) 0. The simulated output voltage as a function of the input voltage of the inverter (without level shifting) based on data from Figure 11a is shown in Figure 11b. Figure 12 shows the quasistatic voltage transfer characteristics of 6PTTP6 p-channel unipolar inverters with identical channel lengths and widths of the transistors on PTS- and fluorosilane-modified gate dielectrics. When the drive voltage (VDD) and the input voltage (VIN) are biased negatively, the inverter works in the third quadrant of the output voltage (VOUT) versus VIN plot, exhibiting a maximum voltage gain of 6 and distinct flat levels, a nearly full swing characteristic. Thus, with only two simple OFETs, the minimum output voltage goes down to 0 V, which is much more difficult using only enhancement-mode or depletion-mode transistors. At the low frequency used (10 mHz), there exists some hysteresis, and the arrows indicate that the measurement and hysteresis direction of the inverter output characteristics are anticlockwise, which is mainly excess charge injection from the gate dielectrics under a high gate voltage and a long charging time.1a,17 It is believed that the hysteresis will become less with increased switching speed. In addition, the reduction of the transistor operation voltage could reduce the inverter hysteresis due to lower gate leakage current, charge injection, and charging.16

Conclusions In summary, we have demonstrated solution-processed OFETs and unipolar logic gates built on nonpolar and polarized SAM(17) Brown, A. R.; Jarret, C. P.; de Leeuw, D. W.; Matters, M. Synth. Met. 1997, 88, 37.

Figure 11. (a) Load lines of the depletion load inverters. The blue curve represents the drain current (IDS) versus the drain-source voltage (VDS) output characteristic of an enhancement-mode transistor driver with 6PTTP6 OSC deposited on the PTS SAM-modified SiO2, and the data are taken in descending VGS mode from 0 to -50 V in steps of 2 V. The black, red, and green curves represent the load lines for the depletion transistor loads with 6PTTP6 OSC on the FPTS, FOTS, and FDTS SAM-modified SiO2, respectively (VGS ) 0 V for black, red, and green curves). (b) The fitting results of the voltage transfer characteristics and the corresponding gains of the PMOS-like unipolar inverter circuits based on an enhancement-mode transistor driver and a depletion-mode transistor load on SAM-modified SiO2. The solid curves, the fitting data from Figure 8a, are the voltage transfer characteristics, and the dashed curves are the corresponding gains.

modified gate dielectrics. The use of polarized gates with fluorosilane SAMs enables the threshold voltage shift and current modulation of organic semiconductor thin films as well as circuit tuning. It is worth noting that the permanent polarization from SAMs avoids the excess external electric field charging and poling during inverter fabrication processes. The investigation of organic semiconductor-dielectric interface chemistry indicates that both the morphology and electronic properties of organic semiconductors are influenced by silane SAMs on gate dielectrics: silane molecules alter the organic semiconductor molecular packing and thin film formation and finally their charge transport and carrier mobility, and the electrostatic dipole field of silane results in their threshold voltage shifting and current modulation. The unprecedented ability to pattern electrostatic properties prior to semiconductor and electrode solution deposition and to form full-swing unipolar inverters with constant OFET channel

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Langmuir, Vol. 23, No. 26, 2007 13231

stamping, UV photolithography, e-beam writing, and scanning probe lithography.15a,18 In addition, dipolar orientation-on-demand films can be patterned by surface-grafting polymerization.13,19 Our present results suggest that the organic transistors and inverters can be tuned over a broad range by selecting from a large number of both soluble gate dielectrics9 and air-stable organic semiconductors.3,8,20

Figure 12. Measured quasistatic voltage transfer characteristics and the corresponding gains of the PMOS-like unipolar inverter circuit (f ) 10 mHz) based on an enhancement-mode transistor driver and a depletion-mode transistor load on SAM-modified SiO2. The channel dimensions for both OFETs were identical. The solid curves are the voltage transfer characteristics, and the dashed curves are the corresponding gains. The arrows indicate that the hysteresis direction of the inverter output characteristics is anticlockwise, which is mainly from excess charge injection from the gate dielectrics. The supply voltage VDD is -50 V.

dimensions and from a single organic semiconductor material suggests alternatives for process simplification of electronic components in single-component organic electronics. The SiO2 substrates could be replaced with compatible and patternable gate dielectric films. SAMs on the surfaces can be micropatterned by physical vapor deposition through a shadow mask, micro-

Acknowledgment. This work was supported by the National Science Foundation (ECS 0601356), the Department of Energy through Los Alamos National Laboratory (DE-FG01-05ER0501), and the Johns Hopkins Whiting School of Engineering startup fund and postdoctoral fellowship. We are grateful to Nina Markovic and Guoqiang Xia at Johns Hopkins for setup and assistance with EFM measurements. We wish to thank A. G. Andreou, D. H. Reich, K. J. Stebe, and Kedar D. Deshmukh at Johns Hopkins and Arved C. Hu¨bler and Kay Reuter at the Institute for Print and Media Technology, Chemnitz Technical University, Germany for technical assistance and helpful discussions. The authors also thank C. A. Richter and D. Gundlach from NIST for stimulating discussions. LA702409M (18) (a) Tien, J.; Xia, Y.; Whitesides, G. M. In Thin Films; Ulman, A., Ed.; Academic Press: London, 1998; Vol. 24, p 227. (b) Morita, M.; Koga, T.; Otsuka, H.; Takahara, A. Langmuir 2005, 21, 911. (c) Bhatnagar, P.; Mark, S. S.; Kim, I.; Chen, H.; Schmidt, B.; Lipson, M.; Batt, C. A. AdV. Mater. 2006, 18, 315. (d) Liu, G. Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (19) (a) Rutenberg, I. M.; Scherman, O. A.; Grubbs, R. H.; Jiang, W.; Gargunkel, E.; Bao, Z. N. J. Am. Chem. Soc. 2004, 126, 4062. (b) Kratzmu¨ller, T.; Appelhans, D.; Braun, H.-G. AdV. Mater. 1999, 11, 555. (c) Wang, Y.; Chang, Y. C. AdV. Mater. 2003, 15, 290. (20) (a) Salleo, A. Mater. Today 2007, 10, 38. (b) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (c) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. J. Am. Chem. Soc. 2007, 129, 4112. (d) Sauve´, G.; McCullough, R. D. AdV. Mater. 2007, 19, 1822.