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Chem. Mater. 2000, 12, 2542-2549
Statistical Copolymers with Side-Chain Hole and Electron Transport Groups for Single-Layer Electroluminescent Device Applications Xuezhong Jiang† and Richard A. Register* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544
Kelly A. Killeen‡ and Mark E. Thompson Department of Chemistry, University of Southern California, Los Angeles, California 90089
Florian Pschenitzka and James C. Sturm Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 Received August 18, 1999. Revised Manuscript Received June 19, 2000
New statistical copolymers with bipolar carrier transport abilities were synthesized through free radical copolymerization of N-vinylcarbazole (NVK, hole-transport monomer) with either of two substituted styrenes containing oxadiazole groups, which serve as electron transport monomers: 2-phenyl-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, PVO, and 2-(4tert-butylphenyl)-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, BVO. In all cases, the charge transport moieties exist in side groups, and carrier transport proceeds by hopping. Copolymerization yields homogeneous statistical copolymers of widely variable composition and thus tunable carrier transport properties; the copolymers are transparent in the visible region and form good films. Compared with systems where the oxadiazole units are incorporated by simply blending a small-molecule oxadiazole into poly(N-vinylcarbazole), the glass transition temperatures of these copolymers are high, and there is no possibility for the oxadiazole units to phase-separate through recrystallization. The glass transition temperatures for the copolymers show positive deviations from a harmonic mixing rule, suggesting some interaction between the NVK and BVO residues; however, blends of the homopolymers show limited miscibility at best, indicating that copolymerization is essential to produce a homogeneous material. Incorporating the oxadiazole units reduces the hole transport ability of these copolymers somewhat relative to NVK homopolymer, but singlelayer dye-doped devices emitting blue, green, and orange light fabricated from these copolymers all showed good efficiency.
Introduction Electroluminescence (EL) from small organic molecules1 and polymers2 forms the basis for their use in light-emitting diodes (LEDs). An LED produces light via the recombination of electrons and holes, injected from electrodes on opposite sides of the film. To achieve high luminescence efficiency, the electron and hole currents must be balanced, implying similar injection and transport behavior for the two types of carriers. However, typical organic molecules and polymers are not simultaneously good conductors for both electrons and holes. Heterostructured devices, where additional charge transport layers are inserted between the emission layer and the electrodes to facilitate charge injection and trans* To whom correspondence should be addressed. † Present address: Department of Materials Science and Engineering, Box 352120, University of Washington, Seattle, WA 98195. ‡ Present address: Lexmark International Inc., 400 Circle Road NW, Lexington, KY 40550. (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackey, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539.
port, are one means for improving device efficiency. Heterostructures also allow for insertion of hole- or electron-blocking layers adjacent to the emission layer to maximize recombination of carriers. However, multilayer devices are especially difficult to produce from polymers, as the layers are typically deposited by spincoating, and only in unusual cases is it possible to choose a solvent that will dissolve the material being deposited and yet will not dissolve the previously deposited layers.3-7 Another advantage of the single-layer device architecture is that the emitter can be incorporated in a patterned way after deposition of the carrier transport layer, such as via ink-jet printing of dye-containing solutions.8 (3) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. (4) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahart, R. E.; Ba¨ssler, H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551. (5) Buchwald, E.; Meier, M.; Karg, S.; Po¨sch, P.; Schmidt, H. W.; Strohriegl, P.; Riess, W.; Schwoerer, M. Adv. Mater. 1995, 7, 839. (6) Pei, Q.; Yang, Y. Chem. Mater. 1995, 7, 1568. (7) Strukelj, M.; Papadimitrakopoulos, F.; Miller, T. M.; Rothberg, L. J. Science 1995, 265, 1969.
10.1021/cm990535v CCC: $19.00 © 2000 American Chemical Society Published on Web 07/26/2000
Statistical Copolymers
Another approach to improved efficiency retains the single-layer device structure but blends charge injection/ transport molecules into a polymer matrix. A successful example9,10 blends small-molecule oxadiazoles, which are good electron transporters, into poly(N-vinylcarbazole), PVK, a good hole transporter with a high glass transition temperature.11,12 The electron-withdrawing character of the 1,3,4-oxadiazole ring in derivatives such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, PBD,9,10 facilitates electron injection and transport. Fabrication of such devices is straightforward, and good efficiencies and wide color tunability have been demonstrated.9,10,13 The main concern with this approach is device stability: the level of added transport molecule is normally quite high, possibly leading to its phase separation by recrystallization and degraded device performance.7 Polymers containing oxadiazole moieties in either the main chain5,6 or a side group7,14,15 have also been synthesized, but the hole-blocking nature of the oxadiazole group16 makes such materials most useful as the electron transport or emissive layers in heterostructured devices. Simply blending such polymers with PVK in an attempt to balance the charge carrier density would likely lead to phase separation more severe than with the PVK:PBD blends. Phase separation can be suppressed by covalently attaching the hole and electron transport moieties to the same polymer. Kido et al.17 synthesized a strictly alternating copolymer of triarylamine (hole-transporting) and oxadiazole (electron-transporting) monomers, where the active groups were both present in the polymer backbone, and demonstrated this polymer’s use in both undoped and dye-doped LEDs. Galvin and coworkers incorporated oxadiazole units into derivatives of poly(p-phenylenevinylene), PPV, either in the main chain18,19 or in side groups.20 Since the holes in PPV move through backbone conjugation, placing the oxadiazole units in the main chain (where they are in conjugation with the PPV units) produces a higher device efficiency.19 However, even the PPV derivatives with oxadiazoles in the side chain showed greatly improved efficiencies, and related materials are still being developed.21,22 The closest analogue to the PVK:PBD blends would be a statistical (“random”) copolymer prepared from N-vinylcarbazole and a monomer bearing a pendant (8) Hebner, T. R.; Sturm, J. C. Appl. Phys. Lett. 1998, 73, 1775. (9) Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys. Lett. 1995, 67, 2281. (10) Wu, C. C.; Sturm, J. C.; Register, R. A.; Tian, J.; Dana, E. P.; Thompson, M. E. IEEE Trans. Electron Devices 1997, 44, 1269. (11) Kido, J.; Hongawa, K.; Okuyama, K. Appl. Phys. Lett. 1993, 63, 2627. (12) Jiang, X. Z.; Liu, Y. Q.; Song, X. Q.; Zhu, D. B. Synth. Met. 1997, 87, 175. (13) Wu, C. C.; Sturm, J. C.; Register, R. A.; Thompson, M. E. Appl. Phys. Lett. 1996, 69, 3117. (14) Cacialli, F.; Li, X.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1995, 75, 161. (15) Li, X. C.; Cacialli, F.; Gilles, M.; Gru¨ner, J.; Friend, R. H.; Holmes, A. B.; Moratti, S. C.; Yong, T. M. Adv. Mater. 1995, 7, 898. (16) Brocks, G.; Tol, A. J. Chem. Phys. 1997, 106, 6418. (17) Kido, J.; Harada, G.; Nagai, K. Chem. Lett. 1996, 161. (18) Peng, Z.; Bao, Z.; Galvin, M. E. Adv. Mater. 1998, 10, 680. (19) Peng, Z.; Bao, Z.; Galvin, M. E. Chem. Mater. 1998, 10, 2086. (20) Bao, Z.; Peng, Z.; Galvin, M. E.; Chandross, E. A. Chem. Mater. 1998, 10, 1201. (21) Chung, S. J.; Kwon, K. Y.; Lee, S. W.; Jin, J. I.; Lee, C. H.; Lee, C. E.; Park, Y. Adv. Mater. 1998, 10, 1112. (22) Chen, Z.-K.; Meng, H.; Lai, Y.-H.; Huang, W. Macromolecules 1999, 32, 4351.
Chem. Mater., Vol. 12, No. 9, 2000 2543
Figure 1. Structures of the monomers and polymers used in this work. The copolymers (CPVKPVO and CPVKBVO) are statistical copolymers; that is, the two monomer units shown are distributed essentially randomly along the chain.
oxadiazole unit. Unlike PPV, PVK transports holes through a hopping mechanism between the side-chain carbazole units,23 so there should be little advantage to putting the oxadiazole units in the backbone. Statistical copolymerization (e.g., by a free-radical mechanism) offers a flexible and straightforward route to polymers having any desired composition, and thus tunable carrier transport properties, from two suitable monomers. However, such polymers have not been synthesized previously, possibly because of the difficulty in preparing monomers that will polymerize by a common mechanism and with comparable reactivity. Heischkel and Schmidt24 produced block copolymers having PVK and oxadiazole blocks (the latter via post-polymerization grafting) via living cationic polymerization. Statistical copolymers would be difficult to generate by this approach, given the large differences in reactivities between monomers in ionic polymerizations. Here, we report the synthesis of new copolymers (structures shown in Figure 1) that incorporate carbazole and oxadiazole groups through free radical copolymerization of N-vinylcarbazole, NVK, with two oxadiazole-bearingmonomers: 2-phenyl-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole, PVO, and 2-(4-tertbutylphenyl)-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4oxadiazole, BVO. The oxadiazole monomers are parasubstituted styrenes; styrene and NVK copolymerize readily.25 Experimental Section Monomer and Polymer Synthesis. Benzonitrile, 4-methoxybenzonitrile, 3-methoxybenzonitrile, p-anisoyl chloride, (23) Pai, D. M. J. Chem. Phys. 1970, 52, 2285. (24) Heischkel, Y.; Schmidt, H. W. Macromol. Chem. Phys. 1998, 199, 869. (25) Hart, R. Makromol. Chem. 1961, 47, 143.
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Chem. Mater., Vol. 12, No. 9, 2000
Jiang et al.
Scheme 1. Synthetic Route to PVO and BVO Monomers
4-tert-butylbenzoyl chloride, and 4-vinylbenzyl chloride (Aldrich) were used as received. NVK and azobis(isobutyronitrile), AIBN (Aldrich), were recrystallized from methanol twice before use. The synthetic route to PVO and BVO monomers is depicted in Scheme 1. 4-Methoxyphenyltetrazole:26 A mixture of 4-methoxybenzonitrile (20.0 g, 150 mmol), sodium azide (14.65 g, 225 mmol), and ammonium chloride (12.05 g, 225 mmol) in N,N-dimethylformamide (DMF, 150 mL) was heated at 100 °C for 20 h. After cooling to room temperature the mixture was poured into 1.2 L of water and acidified with dilute HCl. The resulting white precipitate was collected, washed with water (4 × 100 mL), and dried in a vacuum at 60 °C. Yield: 18.58 g (70%). 1H NMR (CDCl3): 3.70 (s, 3 H); 7.11 (d, 2 H, JHH ) 9); 8.26 (d, 2 H, JHH ) 9); 14.7 (br, 1 H). MS m/z ) 176 (M+). Phenyltetrazole was prepared by refluxing benzonitrile (20.0 g, 194 mmol), sodium azide (18.91 g, 291 mmol), and ammonium chloride (15.56 g, 291 mmol) in DMF (150 mL). Yield: 25.21 g (90%). 1H NMR (acetone-d6): 7.58 (m, 3 H); 8.12 (dd, J ) 7, J ) 3, 2 H); 15.70 (br, 1 H). 2-(4-tert-Butylphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole: 26 4-tert-Butylbenzoyl chloride (19.4 mL, 116 mmol) was added to a solution of 4-methoxyphenyltetrazole (18.50 g, 105 mmol) in pyridine (150 mL), and the mixture was refluxed for 2 h under argon. Water (300 mL) was added to the cooled reaction mixture affording a white precipitate. The precipitate was isolated, washed with water (3 × 100 mL), and dried in a vacuum at 60 °C. Yield: 31.03 g (96%). 1H NMR (CDCl3): 1.37 (s, 9 H); 3.89 (s, 3 H); 7.03 (d, 2 H, JHH ) 9); 7.54 (d, 2 H, JHH ) 9); 8.06 (t, 4 H, JHH ) 9). 2-(4-Methoxyphenyl)-5-(phenyl)-1,3,4-oxadiazole was prepared by the reaction of p-anisoyl chloride (12.84 g, 75.3 mmol) and phenyltetrazole (10.00 g, 68.4 mmol) in pyridine (100 mL). Yield: 15.96 g (92%). 1H NMR (pyridine-d5): 3.71 (s, 3 H); 7.11 (d, J ) 9, 2 H); 7.49 (m, 3 H); 8.18 (d, J ) 9, 4 H). 2-(4-tert-Butylphenyl)-5-(4-hydroxyphenyl)-1,3,4-oxadiazole: Boron tribromide (30.7 mL, 324 mmol) was vacuum transferred into a suspension of 2-(4-tert-butylphenyl)-5-(4methoxyphenyl)-1,3,4-oxadiazole (20.0 g, 65 mmol) in CH2Cl2 (200 mL) at -196 °C. The reaction mixture was warmed to 0 °C and stirred under argon for 3 h and then quenched by the slow addition of Et2O (75 mL) at 0 °C followed by addition of water (100 mL). The resulting white precipitate was isolated, washed with water (2 × 100 mL) and CH2Cl2 (3 × 100 mL), and then dried in a vacuum at 60 °C. The organic layer of the (26) Bettenhausen, J.; Strohriegl, P. Adv. Mater. 1996, 8, 507.
filtrate was separated and dried over MgSO4. The solvent was removed in a vacuum, leaving pale peach solids which were combined with the original precipitate and extracted with a 2:1 mixture of acetone:pyridine (300 mL). The mixture was filtered, and water (300 mL) was added to precipitate the product from the filtrate. The white powder was collected, washed with water (3 × 100 mL), and dried in a vacuum at 60 °C. Yield: 13.40 g (70%). 1H NMR (acetone-d6): 1.36 (s, 9 H); 7.04 (d, 2 H, JHH ) 9); 7.64 (d, 2 H, JHH ) 9); 8.01 (d, 2 H, JHH ) 8); 8.06 (d, 2 H, JHH ) 8). 2-(4-Hydroxyphenyl)-5-(phenyl)-1,3,4-oxadiazole was prepared by the reaction of 2-(4-methoxyphenyl)-5-(phenyl)-1,3,4oxadiazole (5.00 g, 19.8 mmol) and BBr3 (18.7 mL, 198 mmol) in CH2Cl2 (100 mL). Yield: 2.93 g (62%). 1H NMR (pyridined5): 7.28 (d, J ) 9, 2 H); 7.48 (m, 3 H); 8.19 (m, 4 H); 12.6 (br, 1 H). 2-(4-tert-Butylphenyl)-5-{4-[(4-vinylphenyl)methoxy]phenyl}1,3,4-oxadiazole (BVO): Solid KOH (1.91 g, 34 mmol) was added to a solution of 2-(4-tert-butylphenyl)-5-(4-hydroxyphenyl)-1,3,4-oxadiazole (5.00 g, 17 mmol) and 4-vinylbenzyl chloride (5.18 g, 34 mmol) in DMF (75 mL), and the mixture was stirred for 6 h at room temperature. A colorless precipitate formed early in the bright yellow reaction mixture. Water (150 mL) was added to precipitate the product, which was isolated, washed with water (3 × 50 mL), and dried in a vacuum at 60 °C. Excess 4-vinylbenzyl chloride was removed by triturating the product in hexane (50 mL). The product was then washed with additional hexane (3 × 20 mL) and dried under a positive air flow. Yield: 6.72 g (96%). 1H NMR (CDCl3): 1.37 (s, 9 H); 5.14 (s, 2 H); 5.28 (d, 1 H, JHH ) 11); 5.78 (d, 1 H, JHH ) 18); 6.74 (dd, 1 H, JHH ) 18, JHH ) 11); 7.10 (d, 2 H, JHH ) 9); 7.43 (m, 4 H); 7.54 (d, 2 H, JHH ) 9); 8.05 (d, 2 H, JHH ) 9); 8.07 (d, 2 H, JHH ) 9). Melting point: 175 °C by DSC. 2-Phenyl-5-{4-[(4-vinylphenyl)methoxy]phenyl}-1,3,4-oxadiazole (PVO) was prepared by the reaction of KOH (0.94 g, 16.8 mmol), 2-(4-hydroxyphenyl)-5-(phenyl)-1,3,4-oxadiazole (2.00 g, 8.39 mmol), and 4-vinylbenzyl chloride (2.37 mL, 16.8 mmol) in DMF (50 mL). Yield: 2.97 g (100%). 1H NMR (CDCl3): 5.14 (s, 2 H); 5.28 (d, J ) 11, 1 H); 5.78 (d, J ) 18, 1 H); 6.74 (dd, J ) 18, J ) 11, 1 H); 7.10 (d, J ) 9, 2 H); 7.50 (m, 7 H); 8.11 (m, 4 H). Polymerization. Polymerizations were conducted in test tubes sealed with rubber septa, immersed in a water bath at 50 °C. Measured quantities of the solid NVK and PVO or BVO monomers and AIBN (0.03% w/w based on total monomer mass) were diluted to approximately 150 mg/mL in spectroscopic grade tetrahydrofuran (THF), and the solution was
Statistical Copolymers deoxygenated by sparging with nitrogen for about 40 s. For the small-scale, low-conversion polymerizations used to determine radical reactivity ratios, the total monomer mass was about 50 mg, and the polymerizations were allowed to proceed for about 4 h (
