Ordering Rigid Rod Conjugated Polymer Molecules for High

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Ordering Rigid Rod Conjugated Polymer Molecules for High Performance Photoswitchers Huanli Dong,†,§ Hongxiang Li,† Erjing Wang,†,§ Zhongming Wei,†,§ Wei Xu,† Wenping Hu,*,† and Shouke Yan*,‡ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids and State Key Laboratory of Polymer Physics & Chemistry, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China, and Graduate UniVersity of CAS, Beijing 100039, P. R. China ReceiVed August 12, 2008. ReVised Manuscript ReceiVed September 9, 2008 Molecules of a rigid rod conjugated polymer, a derivative of poly(para-phenylene ethynylene)s with thioacetyl end groups (TA-PPE), were well aligned by drop-casting the polymer solution onto the friction-transferred poly(tetrafluoroethylene) substrates. TA-PPE molecules were found to be exactly oriented with their conjugated backbones along the PTFE sliding direction. Photoresponse characteristics based on the uniaxially ordered film were significantly improved compared to those of devices with the disordered film. For example, the switch on/off ratio of the photoswitchers with aligned molecules was as high as 330-400, while that of devices without alignment was only 8-12. It was due to the efficient carrier transport along the highly aligned polymer films, in which the molecules of TA-PPE oriented along the carrier transport direction of the devices.

Since the discovery of conducting polymers in 1977, conjugated polymers have been extensively studied in electronic and optoelectronic devices due to their unusual electrical and optical properties and easy solution processing for large-area device fabrication with any substrates.1-3 However, the polymer devices fabricated by spin coating usually exhibit low device performance (e.g., photoresponse devices and field-effect transistors) due to the random orientation of polymer molecules in the spin-coated films. It is well-known that polymer molecules are expected to exhibit intrinsically anisotropic properties because of the electronic delocalization along the conjugated backbones of the polymer chains,4-8 so that the intrachain mobility of carriers (electrons or holes) on an isolated polymer chain was 4 orders higher than that of the films.9 Therefore, an important research direction to improve the performance of polymer devices is to establish optimal structure and ordered films of polymers.9-15 Various techniques have been applied to align the polymer backbones, such as directly stretching or rubbing polymer films,7,16,17 epitaxial growth on a rubbed substrate or using a nanoimprinting process,14,15,18,19 Langmuir-Blodgett (LB) technique,20-22 with magnetic/electric field or photoirradiation * To whom correspondence should be addressed. E-mail: [email protected] (W.H.); [email protected] (S.Y.). † Key Laboratory of Organic Solids. ‡ State Key Laboratory of Polymer Physics & Chemistry. § Graduate University of CAS. (1) Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Bum, P. L.; Holmes, A. B. Nature (London) 1990, 347, 539. (2) Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (3) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (4) Grell, M.; Bradley, D. D. C. AdV. Mater. 1999, 11, 895. (5) Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C. Nature (London) 1998, 392, 261. (6) Amundson, K. R.; Sapjeta, B. J.; Lovinger, A. J.; Bao, Z. Thin Solid Films 2002, 414, 143. (7) Jandke, M.; Strohriegl, P.; Gmeiner, J.; Bru¨tting, W.; Schwoerer, M. AdV. Mater. 1999, 11, 1518. (8) Chen, X. L.; Bao, Z.; Sapjeta, B. J.; Lovinger, A. J.; Crone, B. AdV. Mater. 2000, 12, 344. (9) Hoofman, R. J. O. M.; de Haas, M. P.; Siebbeles, L. D. A.; Warman, J. M. Nature (London) 1998, 392, 54. (10) Warman, J. M.; de Haas, M. P.; Dicker, G.; Grozema, F. C.; Piris, J.; Debije, M. G. Chem. Mater. 2004, 16, 4600.

inducing alignment, and so forth.23,24 The friction transfer technique initially proposed by Wittmann and Smith in 199125 is an easy way of processability and is applicable to the orientation of a wide variety of materials, such as inorganic molecules,26 small molecules,27-30 and polymers.6,8,31,32 Here, we would like to extend the method for the orientation of rigid rod conjugated polymer molecules of poly(para-phenylene ethynylene)s (PPEs).33-39 Actually, although principally any method mentioned above is not exclusive for the alignment of PPEs, the (11) Prins, P.; Grozema, F. C.; Schins, J. M.; Savenije, T. J.; Patil, S.; Scherf, U.; Siebbeles, L. D. A. Phys. ReV. B 2006, 73, 045204. (12) Chen, Y. S.; Meng, H.-F. Phys. ReV. B 2002, 66, 035202. (13) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature (London) 1999, 401, 685. (14) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406. (15) Zheng, Z.; Yim, K.-H.; Saifullah, M. S. M.; Welland, M. E.; Friend, R. H.; Kim, J.-S.; Huck, W. T. S. Nano Lett. 2007, 7, 987. (16) Dyreklev, P.; Berggren, M.; Inganas, O.; Andersson, M. R.; Wennerstrom, O.; Hjertberg, T. AdV. Mater. 1995, 7, 43. (17) Derue, G.; Coppe´e, S.; Gabriele, S.; Surin, M.; Geskin, V.; Monteverde, F.; Lecle`re, P.; Lazzaroni, R.; Damman, P. J. Am. Soc. Chem. 2005, 127, 8018. (18) Lieser, G.; Oda, M.; Miteva, T.; Meisel, A.; Nothofer, H.-G.; Scherf, U.; Neher, D. Macromolecules 2000, 33, 4490. (19) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. AdV. Mater. 1999, 11, 671. (20) Rikukawa, M.; Rubner, M. F. Langmuir 1994, 10, 519. (21) Xu, G.; Bao, Z.; Groves, J. T. Langmuir 2000, 16, 1834. (22) Cimrova, V.; Remmers, M.; Neher, D.; Wegner, G. AdV. Mater. 1996, 8, 146. (23) Oguma, J.; Kawamoto, R.; Goto, H.; Itoh, K.; Akagi, K. Synth. Met. 2001, 119, 537. (24) Mas-Torrent, M.; den Boer, D.; Durkut, M.; Hadley, P.; Schenning, A. P. H. J. Nanotechnology 2004, 15, S265. (25) Wittmann, J. C.; Smith, P. Nature (London) 1991, 352, 414. (26) Misaki, M.; Chikamatsu, M.; Tanigaki, N.; Yamashita, M.; Ueda, Y.; Yase, K. AdV. Mater. 2005, 17, 297. (27) Brinkmann, M.; Wittmann, J. C.; Barthel, M.; Hanack, M.; Chaumont, C. Chem. Mater. 2002, 14, 904. (28) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2003, 15, 495. (29) Brinkmann, M.; Graff, S.; Straupe´, C.; Wittmann, J. C.; Chaumont, C.; Nuesch, F.; Aziz, A.; Schaer, M.; Zuppiroli, L. J. Phys. Chem. B 2003, 107, 10531. (30) Tanaka, T.; Honda, Y.; Ishitobi, M. Langmuir 2001, 17, 2192. (31) Tanigaki, N.; Yase, K.; Kaito, A.; Ueno, K. Polymer 1995, 36, 2477. (32) Damman, P.; Dosie`re, M.; Wittmann, J. C. Macromolecules 1997, 30, 8386.

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Figure 2. (a) Sliding a PTFE rod at constant pressure against the substrates held at approximately 280 °C for obtaining an aligned PTFE layer. (b) TA-PPE films were obtained by drop-casting TA-PPE tetrahydrofuran solution (about 5 mg/mL) onto the oriented PTFE layer.

Figure 1. Molecular structure, HOMO, and LUMO of TA-PPE molecules, n ≈ 100.

successful cases are really rare,5,33,34 especially for the use of the friction transfer technique. Previously, we have synthesized a derivative of PPEs with thioacetyl end groups (TA-PPE, Figure 1) and found that it exhibited excellent optoelectronic properties.40-43 Here, we will use this TA-PPE as the candidate to carry out our study for the alignment of the rigid rod conjugated polymer by the friction transfer technique and study the photoresponse characteristics of the aligned polymer films. From Figure 1, it is obvious that the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) of TA-PPE are both located exactly along the rigid rod backbone of the polymer molecule; therefore, the alignment of the polymer molecules is actually to align the carrier transport channels for efficient carrier transport. It is definitely important for the fabrication of high performance optoelectronic devices based on this conjugated polymer. Prior to the poly(tetrafluoroethylene) (PTFE) deposition, all the substrates were successively cleaned with pure water, hot acetone, hot concentrated sulfuric acid-hydrogen peroxide solution (concentrated sulfuric acid/hydrogen oxide ) 2:1), pure water, and pure ethanol. Oriented PTFE thin films were then prepared by the friction transfer method as described elsewhere25 by sliding a PTFE rod at a constant pressure against the substrates held at approximately 280 °C (Figure 2a). In the predeposited PTFE, the PTFE chains are aligned along the sliding direction. Finally, TA-PPE films were obtained by drop-casting TA-PPE tetrahydrofuran solution (about 5 mg/mL) onto the oriented PTFE layer (Figure 2b) in a closed jar. Here, drop-casting was adopted (33) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. Science 1998, 279, 835. (34) Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 9942. (35) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605. (36) Pschirer, N. G.; Miteva, T.; Evans, U.; Roberts, R. S.; Marshall, A. R.; Neher, D.; Myrick, M. L.; Bunz, U. H. F. Chem. Mater. 2001, 13, 2691. (37) Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt, H. W.; Montali, A.; Feldman, K.; Smith, P.; Weder, C. AdV. Funct. Mater. 2001, 11, 41. (38) Chu, Q.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2002, 35, 7569. (39) Xu, Y.; Berger, P. R.; Wilson, J. N.; Bunz, U. H. F. Appl. Phys. Lett. 2004, 85, 4219. (40) Hu, W.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Torimitsu, K. Appl. Phys. Lett. 2004, 85, 115. (41) Hu, W.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Han, C.; Torimitsu, K. Phys. ReV. B 2004, 69, 165207. (42) Hu, W.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Liu, Y.; Zhu, D.; Torimitsu, K. J. Am. Chem. Soc. 2005, 127, 2804. (43) Hu, W.; Jiang, J.; Nakashima, H.; Luo, Y.; Kashimura, Y.; Chen, K.; Shuai, Z.; Furukawa, K.; Lu, W.; Liu, Y.; Zhu, D.; Torimitsu, K. Phys. ReV. Lett. 2006, 96, 027801.

Figure 3. AFM height (left) and phase (right) images of single frictiontransferred PTFE layer (a) and bilayer TA-PPE/PTFE films (b) on glass substrate. The region identified by arrows is the bare glass with only TA-PPE films.

instead of spin-coating because it was found that the order of our TA-PPE films obtained by spin-coating was not ideal, which was probably due to the rapid volatilization of the solvent during the spin-coating process, in which the molecules of the polymer had insufficient time to epitaxy themselves on the predeposited PTFE films. It was expected that drop-casting could be better to control the velocity of solvent volatilization. The orientation of the films was investigated by polarized optical microscopy (POM, Olympus BH-2; Panasonic 230 CCD), scanning electron microscopy (SEM, Hitachi S-4300 SE), and atomic force microscopy (AFM, Nanoscopy IIIa). Photoresponse characteristics of the devices were recorded with a Keithley 4200 SCS instrument and a Micromanipulator 6150 probe station in a clean and metallic shielded box at room temperature in air. AFM measurements were performed on the PTFE and TAPPE/PTFE bilayer films to observe the surface morphologies (as shown in Figure 3). The thickness of our friction-transferred PTFE film was about 10-12 nm estimated from the uncovered region (which was relatively bright in phase image corresponding to the stiffer surface of glass), and the root-mean-square (rms) roughness was around 2.5 nm with fully extended filaments of 20-30 nm in width (Figure 3a). This PTFE layer was used as the alignment substrate for subsequent TA-PPE deposition, and the corresponding AFM height (left) and phase (right) images of bilayer TA-PPE/PTFE film are shown in Figure 3b. It is

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Figure 4. (a) Polarized optical micrograph of aligned TA-PPE films on oriented PTFE of glass substrates under the 45° of polarizer with respect to the sliding direction, (b) SEM image of the aligned films, (c) polarized UV-vis absorption, and (d) photoluminescence spectrum of TA-PPE films on a PTFE layer. The solid and dashed lines represent parallel and perpendicular to the PTFE sliding direction, respectively.

apparent that the TA-PPE film has covered most of the PTFE layer and formed good orientation on the PTFE substrate, and the phase contrast image of this bilayer film (Figure 3b, right) shows more clearly the resulting well-aligned TA-PPE film on the PTFE layer with some subtle oriented structure, while the TA-PPE film (∼100 nm) exhibits homogeneous morphology with short “columns” on the bare glass region identified by the green arrows. The orientation of the films was further investigated by polarized optical microscopy (POM). Under the 45° of polarizer with respect to the sliding direction, all the aligned films exhibited obvious birefringence (as shown in Figure 4a), while when the film was rotated with the polarizer parallel/perpendicular to the sliding direction, the film was in complete extinction (not shown here). This result indicated that highly oriented films of TA-PPE were obtained significantly aligned along the direction of PTFE (because the PTFE component was so thin (here, it is about 10-12 nm) that it did not contribute significantly to the observed birefringence). The corresponding SEM image (Figure 4b) further confirmed the alignment of the rigid rod polymer molecules with the long-range order of the columnar-like structures uniaxially aligned in the sliding direction of PTFE. Additionally, since some typical vibration transitions are sensitive to the direction of the polarization of the incident light,15,30,44,45 polarized UV-vis absorption can be used to further estimate the alignment degree of TA-PPE films on PTFE substrate. As shown in Figure 4c, it can be clearly seen that the intensity of the transition peaks was much higher under polarized light along the sliding direction than that under polarized light perpendicular to the sliding direction. The dichroic ratio R ) A|/A⊥ of the absorption coefficient is about 3.0 at 488 nm (2.54 eV), which originates from the π-π* transition in the polymer molecules. This value suggested that the π-π* transition dipoles were exactly aligned parallel to the sliding direction; that is, the TA-PPE backbone was aligned along the sliding direction because the π-π* (44) Erb, T.; Raleva, S.; Zhokhavets, U.; Gobsch, G.; Stu¨hn, B. Thin Solid Films 2004, 450, 97. (45) Gurau, M. C.; Delongchamp, D. M.; Vogel, B. M.; Lin, E. K.; Fischer, D. A.; Sambasivan, S.; Richter, L. J. Langmuir 2007, 23, 834.

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Figure 5. (a) Photoswitchers of aligned TA-PPE films, (b) photoswitchers of TA-PPE films without alignment, (c,e) photoresponse and photoswitch behavior of TA-PPE films with alignment, and (d,f) photoresponse and photoswitch behavior of TA-PPE films without alignment.

transition was polarized parallel to the conjugated main-chain axis.46 Taking all of the above into consideration, it is concluded that fairly good alignment of TA-PPE films was obtained with the main chains parallel to the PTFE sliding direction. This is further confirmed by the polarized photoluminescence spectrum (Figure 4d). The same sample was fixed on a fluorescence spectrophotometer with a sheet polarizer inserted between the sample and the detector so as to be parallel or perpendicular to the sliding direction. The resultant polarization ratio of P ) I|/I⊥ is around 3.2∼3.6 at 503 and 535 nm, respectively, suggesting the identical result of PPE molecular alignment along the PTFE sliding direction. This facile, easily operated method for the fabrication of aligned polymer films in a large area indicated its potential application in optoelectronic devices. It is attractive for the application of highly ordered TA-PPE films in optoelectronic devices. Considering the oriented polymer films were based on the PTFE layer, that is, at the surface of the PTFE layer, one of the best choices is to use surface activated devices such as photoswitchers to examine the effect of the aligned films. As we know, photoswitchers are working based on photogenerated free carriers, such as electrons or holes. The absorbed light of the active film (here, it is TA-PPE) generates huge excitons, and then the excitons dissociate into free electrons and holes. After that, the free electrons and holes diffuse in the TA-PPE film and are finally collected by the anode and cathode. The efficiency of devices depends on the light absorbance, exciton dissociation, electron and hole transport, and collection by electrodes. Assuming other conditions are constant, the uniaxially aligned TA-PPE film on the PTFE orientation layer should be prospective for efficient electron and hole transport for electrode collection, which finally leads to the improvement of the device performance of photoswitchers. The device architecture is shown in Figure 5a with top-contacted Au electrodes and with a channel length of 100 µm and width of 4.82 mm. For comparison, devices (46) Comoretto, D.; Dellepiane, G.; Marabelli, F.; Cornil, J.; dos Santos, D. A.; Bre´das, J. L.; Moses, D. Phys. ReV. B 2000, 62, 10173.

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based on the polymer films without molecular orientation (Figure 5b) were also fabricated and tested. The photoresponse characteristics of TA-PPE films with and without alignment under different power lights are shown in Figure 5c-f (the power of the irradiated light on the devices was tuned at 0-5.76 mW/cm2). It was obvious that the current of the devices with molecular alignment was as high as 2.65 nA at 20 V under 5.76 mW/cm2 (Figure 5c). It was much higher than that of the devices without molecular orientation (Figure 5d, only 0.12 nA at 20 V under 5.76 mW/cm2) under the same operational conditions. The photoconductivity of the aligned TA-PPE at 20 V under 5.76 mW/cm2 was estimated at ∼5.5 × 10-4 S/m. However, the devices without molecular orientation at the same conditions exhibited photoconductivity at ∼2.49 × 10-5 S/m. The photoconductivity of the films with perpendicular alignment was found to be even 1-2 orders lower than that of the devices without molecular orientation. On the other hand, with light on/off, both devices exhibited the capability of switching between high/low current states (Figure 5e and f; constant voltage, 10 V; light intensity, 5.76 mW/cm2). However, the switch on/off ratio of the devices with aligned polymer films reached 330∼400 (Figure 5e), which was several decades higher than that of devices without aligned polymer films (Figure 5f) (which showed an on/off ratio of only at about 8∼12). The high photoswitch ratio of the aligned polymer film devices could be assigned to several aspects. First, it is understandable that in these aligned films the carrier transport channels are highly ordered because of the rigidity of the molecules and the exact location of the HOMO and LUMO of the molecules on the backbones. In contrast to the case of the aligned polymer films, in devices without aligned films their carrier transport channels are highly disordered. Hoofman et al.9 suggested that the intrachain mobility of carriers on an isolated polymer chain was 4 orders higher than that of the films; that is, carrier transport along the polymer chain was 4 orders higher than that of the films. Therefore, it is reasonable to deduce that carrier transport in the aligned polymer films was much faster than that of films without alignment. Second, assuming the HOMO and LUMO of PPE at 6.3 and 3.9 eV,35 there exists energy barriers between the electrodes (Au, work function at ∼5.2 eV) and the polymer molecules, so that in dark conditions electrons and holes would be blocked by the energy barrier and the dark

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current would be very low. Under illumination, the photongenerated excitons were dissociated into free electrons and holes, with some of them possessing sufficient energy to jump over or tunnel through the energy barrier, resulting in the high current (on state) of the photoswitcher. The highly ordered structures, the low dark current, and the large photocurrent probably resulted in the large photoswitch ratio of the aligned polymer film devices. Our investigation here likely suggests a facile way to produce aligned films of the rigid rod conjugated polymer films of TAPPE for the fabrication of high performance photoswitchers. Finally, the stability of the devices should be mentioned because it is important for the practical application of polymer devices. Our preliminary results indicated that the devices showed good stability, and even after 15 months of keeping them in a normal dry box the devices still exhibited excellent photoswitch behavior although with obvious degradation. The degradation was probably because of the polymer itself or the degradation of the polymer/ electrode interface; further detailed investigation is under way. In conclusion, well-defined alignment of a rigid rod conjugated polymer, TA-PPE, was prepared by drop-casting onto the frictiontransferred PTFE substrates. The aligned films were investigated by atomic force microscopy, scanning electron microscopy, polarized optical microscopy, polarized UV-vis absorption, and photoluminescence spectra, which indicated that the TA-PPE polymer molecules were exactly oriented with their conjugated backbones along the PTFE sliding direction. Photoresponse characteristics based on this uniaxially ordered film were significantly improved over those of devices with disordered films. For example, the switch on/off ratio of the photoswitchers with aligned molecules was as high as 330-400, while that of devices without alignment was only 8-12, indicating efficient carrier transport in the highly aligned polymer films. This facile, easily operated method for the fabrication of aligned polymer films in large areas indicates its potential application in optoelectronic devices. Acknowledgment. The authors are thankful for the financial support from the National Natural Science Foundation of China, German-Chinese Transregio project, Ministry of Science and Technology of China, and Chinese Academy of Sciences. LA8026094