pubs.acs.org/Langmuir © 2009 American Chemical Society
Organic Field Effect Transistor Using Pentacene Single Crystals Grown by a Liquid-Phase Crystallization Process Yasuo Kimura* and Michio Niwano Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577
Naohiko Ikuma, Kenichi Goushi, and Kingo Itaya WPI Advanced Institute for Materials Research (WPI-AIMR), Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Aoba-yama 04, Aoba-ku, Sendai 980-8579, Japan Received February 23, 2009. Revised Manuscript Received March 15, 2009 Nearly perfect pentacene single crystals with wide terraces several micrometers in width were grown by crystallization from a pentacene-containing trichlorobenzene solution. Organic field-effect transistors (OFETs) were fabricated with the pentacene single crystals and characterized for their electrical properties. The field effect mobility was found to be in the range of 0.4-0.6 cm2/V 3 s, which is comparable to that of OFETs fabricated with pentacene single crystals prepared by a physical vapor-phase growth method. The results described in this paper clearly demonstrate that the crystallization of organic semiconductors from solution is a promising chemical method for device processing of OFETs.
Pentacene is one of the most intensively investigated organic semiconductors for organic field effect transistors (OFETs).1-6 Vacuum evaporation technique has long been used for the preparation of thin films of pentacene.1-3 However, the electrical performance of OFETs depends strongly on the crystal structure and morphology of the organic active layer. It is well-known that the existence of grain boundaries and defects in the organic active layer deteriorates the electrical performance as a result of the lowering of the mobility of carriers. Indeed, it was reported that OFETs fabricated with an evaporated thin film of rubrene exhibited a low field effect mobility, 1.24 10-4 cm2/V 3 s,7 while bulk rubrene single crystals show a much higher carrier mobility in the range of 3-15 cm2/V 3 s.8 The OFETs based on single crystals instead of polycrystalline or amorphous materials are of great interest for their intrinsic charge transport properties and for their limitations in electrical performance. Single crystals of rubrene and pentacene have recently been investigated for the application of OFETs. However, those single crystals were almost exclusively prepared by the physical vapor-phase growth method.9 We recently reported that nearly perfect single crystals of pentacene can be grown from a trichlorobenzene solution.4 Molecularly flat and extraordinarily wide terraces, extending over *Corresponding author: Phone: +81-22-217-5502. Fax: +81-22-217-5503. E-mail:
[email protected]. (1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99 –117. (2) Sekitani, T.; Someya, T. Jpn. J. Appl. Phys. 2007, 46, 4300–4306. (3) Shibata, K.; Wada, H.; Ishikawa, K.; Takezoe, H.; Moria, T. Appl. Phys. Lett. 2007, 90, 193509. (4) Sato, K.; Sawaguchi, T.; Sakata, M.; Itaya, K. Langmuir 2007, 23, 12788– 12790. (5) Butko, V. Y.; Chi, X.; Lang, D. V.; Ramirez, A. P. Appl. Phys. Lett. 2003, 83, 4773–4775. (6) Takeya, J.; Goldmann, C.; Haas, S.; Pernstich, K. P.; Ketterer, B.; Batlogg, B. J. Appl. Phys. 2003, 94, 5800–5804. (7) Park, Se-W.; Hwang, J. M.; Choi, J.-M.; Hwang, D. K.; Oh, M. S.; Kim, J. H.; Im, S. Appl. Phys. Lett. 2007, 90, 153512. (8) 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. (9) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Cryst. Growth 1998, 187, 449–454.
Langmuir 2009, 25(9), 4861–4863
the width of more than a few micrometers with monomolecular steps, were consistently observed on a single crystal of pentacene by using noncontact atomic force microscopy (AFM). The results of that study strongly encouraged us to evaluate the hole-transport property of nearly perfect single crystals of pentacene. The pentacene used in this investigation was purchased from Tokyo Chemical Industry Co., Ltd., and it was purified by repeating temperature-gradient vacuum sublimation at a pressure lower than 10-6 Torr several times. Because of the easy oxidation of pentacene in air,10 the purified pentacene was placed in a Pyrex glass tube in a nitrogen glovebox, and the tube was connected to a standard vacuum line as described previously.11 Triply distilled trichlorobenzene was thoroughly degassed by repeating five freezing-pumping-thawing cycles to prevent oxidation of pentacene molecules. We confirmed the stability of the fully degassed pentacene solution by using ultraviolet-visible spectroscopy.4 The concentration of pentacene was typically 2-3 mg/mL. The glass tube with the degassed pentacene solution was sealed off from the vacuum line, and then it was heated up to 200 °C in a temperature-controlled oil bath. It was then slowly cooled (0.1 °C/h) with a programmable temperature controller. Figure 1a shows a typical noncontact AFM image of the surface of a pentacene single crystal obtained from the trichlorobenzene solution. The scan area was 4 4 μm2. Figure 1b shows the cross-sectional profile along the white arrow in Figure 1a. The step height was about 1.6 nm, which was in good agreement with the previously reported monomolecular step height of a pentacene single crystal.12 The step lines in Figure 1a are seen to be nearly straight and parallel to each other. The average width of molecularly flat terraces is about 1 μm in the direction of the arrow, while the distance between two terrace edges measured in the direction of 90° to the arrow sign is longer than the side of the scanned area (4 μm). These results strongly indicate that (10) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem. Mater. 2004, 16, 4980–4986. (11) Itaya, K.; Kawai, M.; Toshima, S. J. Am. Chem. Soc. 1978, 100, 5996–6002. (12) Campbell, R. B.; Robertson, J. M.; Trotter, J. Acta Crystallogr. 1961, 14, 705–711.
Published on Web 3/31/2009
DOI: 10.1021/la900647y
4861
Letter
Figure 3. The gate voltage dependence of the saturation drain current (Idsat) of the pentacene single-crystal OFET derived from the output characteristics shown in Figure 2. The mobility ( μFE) and threshold voltage (Vth) were estimated at 0.4 cm2/V 3 s and -1.6 eV from the slope and the x-intercept of the Vg - (Idsat)1/2 characteristics, respectively.
Figure 1. (a) Typical AFM image of a 4 4 μm2 area of the surface of a pentacene single crystal recrystallized in trichlorobenzene, and (b) cross-sectional profile along the white arrow in the AFM image.
Figure 2. Output characteristics of the OFET fabricated using a pentacene single crystal grown by the liquid-phase crystallization method. The figure on each curve indicates the applied gate voltage (Vg). The channel width and length are 1 mm and 50 μm, respectively.
well-defined single crystals were formed in trichlorobenzene. In our previous work, each pentacene molecule was clearly identified on the terrace of the pentacene single crystal produced in trichlorobenzene by means of frequency modulation AFM (FM-AFM), which suggested that the single crystal formed in trichlorobenzene was nearly perfect.4 We fabricated a bottom-contact-type OFET using a pentacene single crystal produced in trichlorobenzene. A heavily borondoped p-type silicon (Si) wafer (
