12041
2007, 111, 12041-12044 Published on Web 10/03/2007
Reinvestigation of Carrier Transport Properties in Liquid Crystalline 2-Phenylbenzothiazole Derivatives Keiji Tokunaga,†,‡ Hiroaki Iino,†,§ and Jun-ichi Hanna*,†,§ Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, and JST-CREST, 4259 Nagatsuta Midori-ku, Yokohama 226-8503, Japan, and Research & DeVelopment Center, Dai Nippon Printing Co., Ltd., 250-1 Wakashiba Kashiwa, Chiba 277-0871, Japan ReceiVed: July 19, 2007
We have reinvestigated the charge carrier transport properties in a liquid crystal of 2-(4′-heptyloxyphenyl)6-dodecylthiobenzothiazole (7O-PBT-S12), for which the electronic conduction was first established in rodlike liquid crystals and for which the highest hole mobility in the smectic A (SmA) phase ever achieved was reported. We found that 7O-PBT-S12 exhibited three crystal phases, one of which appeared in a limited temperature range of 10° just below the phase transition temperature from the SmA phase. In this crystal phase, nondispersive transient photohole currents were observed in time-of-flight experiments, and its hole mobility was determined to be 8 × 10-3 cm2/Vs, slightly higher than that reported previously in the SmA phase. For the SmA phase, however, the hole mobility was 1 × 10-4 cm2/Vs. Furthermore, we established the electron transport in the SmA phase of purified 7O-PBT-S12, whose mobility was the same as the hole mobility in that phase. In order to confirm generality of the new findings in 7O-PBT-S12, we investigated the carrier transport properties of its derivative having a short hydrocarbon chain, 2-(4′-heptyloxyphenyl)6-butylthiobenzothiazole (7O-PBT-S4), and obtained comparable results. The present results correct a mistake in the previous report and give an idea of what a typical mobility in the SmA phase is. On the basis of these results, we discuss what determines the charge carrier mobility in smectic mesophases.
Introduction The establishment of fast hole conduction in the discotic columnar phase of hexapentyloxytriphenylene (H5T) in 19931 gave the final answer to a long historical inquiry of the electronic conduction in the liquid crystal started in the 1960s. Furthermore, it was discovered in a smectic mesophase of a rodlike liquid crystal, 2-(4′-heptyloxyphenyl)-6-dodecylthiobenzothiazole (7O-PBT-S12),2 in 1997 also, although its conduction had been believed to be ionic for a long time. Since then, liquid crystals have been recognized to be a new type of organic semiconductors exhibiting self-organization. In this decade, the general features of charge carrier transport properties in the liquid crystals including ambipolar and nonPoole-Frenckel types of charge transport and phase-dependent mobility were clarified,3 and their carrier transport mechanism was understood in a framework of hopping transport among narrowly distributed localized states.4 Nowadays, various device applications of liquid crystals have been proposed, including organic thin film transistors,5,6 organic light emitting diodes,7-9 and organic solar cells.10,11 As for the materials, various new liquid crystals exhibiting a high mobility up to 1 cm2/Vs are synthesized, which includes hexabenzocolonenes,12 phthalocyanines,13 and quaterthiophenes.14 * To whom correspondence should be addressed. E-mail: hanna@ isl.titech.ac.jp. Phone: +81-45-924-5176. Fax: +81-45-924-5188. † Tokyo Institute of Technology. ‡ Dai Nippon Printing Co., Ltd. § JST-CREST.
10.1021/jp079538i CCC: $37.00
However, guiding principles have not ever been established to design a liquid crystal to achieve a high mobility, except for the simple idea of extending the aromatic π-conjugation in the core. In order to get insight into what determines the charge carrier mobility in a particular mesophase, we reinvestigated a 2-phenylbenzothiazole derivative of 7O-PBT-S12 in which remarkable charge transport properties were reported. It shows the highest hole mobility of 5 × 10-3 cm2/Vs ever achieved in the smectic A (SmA) phase. This mobility is 5 times higher than that of H5T in spite of being the least ordered smectic mesophase, and it is 20 times higher than that of the SmA phase in typical rodlike liquid crystals such as 2-phenylnaphthalene derivatives.15 In addition, there is a question about the transient photocurrent in the SmA phase of 7O-PBT-S12 reported previously. The transient photocurrent shows a slow photocurrent decay after the transit time and an exponential decay for negative carriers, as shown in Figure 1 and its inset. Thus, we have reinvestigated the charge carrier transport properties of 7O-PBT-S12 and its derivative having a short hydrocarbon chain, 2-(4′-heptyloxyphenyl)-6-butylthiobenzothiazole (7O-PBT-S4), in detail. As a result, we have obtained new findings that lead to the exact understanding of charge carrier transport properties in the SmA phase of 2-phenylbenzothiazoles. In this report, we describe the phase transition behaviors and hole transport properties in 7O-PBT-S12 and its derivative. Furthermore, we report the electron transport in their SmA © 2007 American Chemical Society
12042 J. Phys. Chem. B, Vol. 111, No. 42, 2007
Figure 1. Positive and negative (inset) transient photocurrents of a 28 µm thick cell in the SmA phase of 7O-PBT-S12 at an electric field of 1 × 105 V/cm and 95 °C. This figure is reprinted with permission from Funahashi and Hanna.2 Copyright 1997 The American Physical Society.
Figure 2. DSC chart of 7O-PBT-S12 at a cooling rate of 10 °C/min and a heating rate of 10 °C /min and texture images under a polarized optical microscope at (a) 95 °C and (b) 85 °C.
phases and discuss what determines the carrier mobility in the smectic mesophases. 7O-PBT-S12 and 7O-PBT-S4 were synthesized as reported elsewhere2 and purified by silica column chromatography and recrystallization from hexane repeatedly. The phase transition behavior was determined by differential scanning calorimetry (DSC, Shimazu DSC-60) and X-ray diffraction (XRD, Rigaku RAD-2B diffractometer with Cu KR radiation) studies. The charge carrier transport properties were studied by time-of-flight experiments. Results and Discussion 7O-PBT-S12 exhibited four phase transitions, as is clearly shown in the DSC chart in Figure 2. According to the texture images observed under a polarized optical microscope, the phase appearing in the temperature range from 99.7 to 87.9 °C on cooling and from 91 to 100.9 °C on heating shows a typical fanlike texture, indicating the SmA phase, as shown in Figure 2a. This agrees with the description of the SmA phase in the
Letters
Figure 3. XRD patterns of 7O-PBT-S12 at (a) 95 °C, (b) 85 °C, and (c) 65 °C.
original report, which appeared in a temperature range of 10°, that is, from 90 to 100 °C. We found a new phase next to the SmA phase appearing at a limited temperature range of about 10° also, that is, from 87.9 to 77 °C on cooling and from 81 to 91 °C on heating. The texture image of this phase shows microstructures in the domains derived from the fanlike textures of the SmA phase, as is clearly seen in the photoimage in Figure 2b. In addition, the phase transition to the new phase shows a large heat balance of 6 kcal/ mol both from the SmA phase on cooling and to the SmA phase on heating. This is good evidence that this new phase results from an obvious change in crystallographic structure when the phase transition takes place both from and to the SmA phase. Furthermore, a small heat balance of 0.4-2.2 kcal/mol both from and to the next ordered phase indicates that its crystallographic structure is not so different from that of this new phase. Judging from these facts, we indicate that both this new phase and another ordered phase next to it are crystal phases. Figure 3 shows typical XRD patterns of 7O-PBT-S12 at temperatures of 95, 85, and 65 °C, corresponding to the SmA phase and the two ordered phases discussed above. The XRD pattern at 95 °C in Figure 3a shows only a peak at the small diffraction angle of 2.32°, corresponding to the molecular length of 7O-PBT-S12, that is, 37 Å. This proves that the phase at 95 °C is the SmA phase, as identified in the previous report. On the other hand, the XRD patterns at 85 and 65 °C in Figure 3b and c, which are for the new ordered phases appearing next to the SmA phase, exhibit several peaks at large diffraction angles: the former pattern shows highly ordered diffraction corresponding to a molecular length of tilted 7O-PBT-S12 and the latter to a molecular length of 7O-PBT-S12 in addition to new peaks originating from a highly ordered molecular alignment. These prove that these new phases are crystalline. We characterized charge carrier transport properties in all of the phases appearing in the temperature range from 40 up to 120 °C. Transient photocurrents at the temperature range lower than 77 °C showed just an exponential decay, indicating that the photogenerated charges were extinct probably due to trapping at the deep states attributed to the grain boundaries. On the other hand, we observed nondispersive transient photocurrents for positive carriers in the new phase at a temperature range of 10° between 80 and 90 °C, as shown in
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Figure 4. (a) Positive and negative (inset) transient photocurrents at the various applied voltages at 88 °C. The cell thickness was 25 µm. (b) Positive and negative (inset) transient photocurrents at the various applied voltages at 96 °C. The cell thickness was 9 µm. (c) The charge carrier mobilities for positive (open circles) and negative (closed circles) carriers as a function of temperature.
Figure 4a, which we identified to be a crystal phase. They resemble the photocurrent shape in Figure 1 very much, in terms of a weak shoulder and a slow decay after a transit time. The mobility was determined to be 8 × 10-3 cm2/Vs, which is comparable to the hole mobility determined for the SmA phase in the previous report. This mobility increases as the temperature goes down. However, this happened only in cooling from the SmA phase. In heating from the crystal phase, we could not see any transient photocurrent indicating the carrier transit. This hysteresis of hole transport properties is good evidence that this phase is not the mesophase, in which the charge transport is hardly affected by domain boundaries and shows no hysteresis.16-18 In addition, the carrier transport for negative carriers was not observed in this phase,19 as shown in the inset of Figure 4a, as is often the case of polycrystalline thin films of liquid crystals. All of these results coincide very much with those for the SmA phase, as shown in Figure 1. In contrast, we observed the nondispersive transient photocurrent having a sharp shoulder and a fast decay after the transit time in the temperature range for the SmA phase, that is, from 90 to 100 °C, as shown in Figure 4b. This photocurrent shape is quite different from the previous one, as shown in Figures 1 and 4a. The mobility for the positive carriers was determined to be 1 × 10-4 cm2/Vs, which is 1/50 of the mobility previously reported. Interestingly, we observed nondispersive transient photocurrents for negative charges also in the SmA phase, which have two shoulders: one is at the same time range for the positive carriers and another at a slower time range. We attribute the fast transit to the electronic conduction and the other to the ionic conduction.20 The electron mobility was determined to be 1 × 10-4 cm2/Vs comparable to the hole mobility. Both hole and electron mobilities depend on neither the temperature nor the electric field, which is very common in the electronic conduction in the liquid crystalline mesophases.14,21 In the isotropic phase, the transient photocurrents were also nondispersive, and the mobility was of the order of 10-5 cm2/ Vs as previously reported. In order to confirm the present results in the SmA phase of 7O-PBT-S12, we investigated the carrier transport properties of its derivative having a short hydrocarbon chain, 7O-PBT-
J. Phys. Chem. B, Vol. 111, No. 42, 2007 12043 S4. We found that electrons and holes were transported in the temperature range from 75 to 109 °C for its SmA phase, and the mobilities were determined to be 1 × 10-4 cm2/Vs for both electrons and holes. These are consistent with the results on the charge mobilities in the SmA phase of 7O-PBT-S12. Therefore, we come to a conclusion that the charge carrier transport in the SmA phase of 7O-PBT-S12 and its derivative is basically ambipolar and its mobility is 1 × 10-4 cm2/Vs for both electrons and holes, and that the high mobility of 5 × 10-3 cm2/Vs reported previously for the SmA phase is attributed to the mobility in the crystal phase that appears at a limited temperature range of 10° just below the SmA phase. Recently, we have reported the feasibility of controlling the grain boundaries in the polycrystalline thin films of liquid crystals.19 The grain boundaries in the polycrystalline thin films run along the columns in the discotic materials and along the molecular layers in the calamitic mesophases, when the films are formed so as to preserve the molecular orientation of the mesophase. The nondispersive hole transport in the crystal phase of 7O-PBT-S12 formed in cooling from the SmA phase is due to less grain boundaries running across the conduction channels. In fact, this nondispersive hole transport in this crystal phase vanishes once the phase transition to the next ordered phase takes place, as described above. As for the mobility in the SmA phase of 7O-PBT-S12, there is nothing too remarkable about it after the mobility is corrected to be 1 × 10-4 cm2/Vs. It is comparable to the mobilities in the SmA phases of 2-phenylnaphthalene derivatives14 and in the SmC phases of terthiophene derivatives21 reported previously. This fact indicates that the carrier mobility in the smectic mesophases is governed by what the molecular alignment is within the smectic layer, which determines the molecular distances. According to our accumulated results on the charge carrier mobilities in the mesophases, we may say that the variation of mobility in a particular mesophase is within 1 order of magnitude from material to material, where the guiding principle to design a molecule has not been established. Conclusion We have reinvestigated the phase transition behavior and charge carrier transport properties of the 2-phenylbenzothiazole derivative of 7O-PBT-S12, in which the first electronic conduction was reported in calamitic liquid crystals. As a result, we found that the charge carrier transport in the SmA phase is ambipolar and characterized by a mobility of 1 × 10-4 cm2/ Vs. The high mobility of 5 × 10-3 cm2/Vs previously reported for the SmA phase is attributed to a crystal phase next to the SmA phase, which was overlooked in the previous report. Therefore, the charge carrier transport properties of 2-phenylbenzothiazole derivatives is usual for the SmA phase, and the charge carrier mobility in the SmA phase of calamitic liquid crystals is of the order of 10-4 cm2/Vs, which is determined by the average molecular distances in the smectic layer. Acknowledgment. This study was supported in part by Grant-in-Aid for Scientific Research sponsored by the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.; Schuhmacher, P.; Siemensmeyer, K. Phys. ReV. Lett. 1993, 70, 457460. (2) Funahashi, M.; Hanna, J. Phys. ReV. Lett. 1997, 78, 2184-2187. (3) Iino, H.; Hanna, J. Optoelectron. ReV. 2005, 13, 295-302.
12044 J. Phys. Chem. B, Vol. 111, No. 42, 2007 (4) Ohno, A.; Hanna, J. Simulated carrier transport in smectic mesophase and its comparison with experimental result. Appl. Phys. Lett. 2003, 82, 751-753. (5) Breemen, A. J. J. M.; Herwig, P. T.; Chlon, C. H. T.; Sweelssen, J.; Schoo, H. F. M.; Setayesh, S.; Hardeman, W. M.; Martin, C. A.; Leeuw, D. M.; Valeton, J. J. P.; Bastiaansen, C. W. M.; Broer, D. J.; Popa-Merticaru, A. R.; Meskers, S. C. J. J. Am. Chem. Soc. 2006, 128, 2336-2345. (6) Funahashi, M.; Zhang, F.; Tamaoki, N. AdV. Mater. 2007, 19, 353358. (7) Kogo, K.; Goda, T.; Funahashi, M.; Hanna, J. Appl. Phys Lett. 1998, 73, 1595-1597. (8) Tokuhisa, H.; Era, M.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 26392641. (9) Seguy, I.; Destruel, P.; Bock, H. Synth. Met. 2000, 111, 15-18. (10) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1122.
Letters (11) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Friend, R. H.; MacKenzie, J. D. Physica. 2002, E14, 263-267. (12) Craats, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. AdV. Mater. 1999, 11, 1469-1472. (13) Iino, H.; Takayashiki, Y.; Hanna, J.; Bushby, R. J. Jpn. J. Appl. Phys. 2005, 44, L1310-L1312. (14) Funahashi, M.; Hanna, J. AdV. Mater. 2005, 17, 594-597. (15) Funahashi, M.; Hanna, J. Appl. Phys. Lett. 1997, 71, 602-604. (16) Maeda, H.; Funahashi, M.; Hanna, J. Mol. Cryst. Liq. Cryst. 2000, 346, 183-192. (17) Maeda, H.; Funahashi, M.; Hanna, J. Mol. Cryst. Liq Cryst. 2001, 366, 369-376. (18) Zhang, H.; Hanna, J. Appl. Phys. Lett. 2004, 85, 5251-5253. (19) Iino, H.; Hanna, J. Jpn. J. Appl. Phys. 2006, 45, L867-L870. (20) Iino, H.; Hanna, J. J. Phys. Chem. B 2005, 109, 22120-22125. (21) Funahashi, M.; Hanna, J. Appl. Phys. Lett. 2000, 76, 2574-2576.
