Novel Electroluminescent Conjugated Polyelectrolytes Based on

Jan 28, 2004 - After treatment with the bromoethane, all signals for the methylene (−CH2N, −NCH2CH3) and methyl (−NCH3) groups adjacent to the n...
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Chem. Mater. 2004, 16, 708-716

Novel Electroluminescent Conjugated Polyelectrolytes Based on Polyfluorene Fei Huang, Hongbin Wu, Deli Wang,† Wei Yang, and Yong Cao* Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China Received July 18, 2003

Alternating copolymers poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7(9,9-dioctylfluorene)] (P1) and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt1,4-phenylene] (P3) were synthesized by the palladium-catalyzed Suzuki coupling reaction and their quaternized ammonium polyelectrolyte derivatives (P2, P4) were obtained through a postpolymerization treatment on the terminal amino groups. The resulting conjugated polyelectrolytes (P2, P4) are soluble in polar solvents such as methanol, DMF, and DMSO while P4 is a water-soluble blue-emitting conjugated polyelectrolyte. The electrochemical and photophysics properties of the resulting copolymers were fully investigated. Both the neutral amino-functionalized polyfluorenes (P1, P3) and the quaternized ammonium salt functional conjugated polyelectrolytes derivatives (P2, P4) were used as the emitting layers in device fabrication. All these polymers show even higher external quantum efficiencies (QE) with the high work-function metal cathode such as Al than with the low work-function (Ba) cathode. The maximal external quantum efficiencies of the diodes are respectively 0.38%, 0.16%, 0.07%, and 0.09% for P1, P2, P3, and P4 with an Al cathode. A possible mechanism of self-assembly of dipole alignment of polar polymer in the polymer/cathode interface was proposed. We have shown that such polymers can be used as an electron-injection layer, which can significantly enhance device performance of light-emitting polymers with high work-function metals such as Al. ITO/PEDT/MEHPPV/P1/Al devices show an external quantum efficiency greater than 2%, as high as that by using a Ba/Al cathode.

Introduction Since the first PLED (polymer light-emitting diode) was made in 1990,1 fluorescent conjugated polymers have attracted much attention because of their potential applications in flat panel displays. Extensive interdisciplinary research has been performed by scientists all over the world to develop high-efficiency, long lifetime, and good color-purity light-emitting polymers. Presently, conjugated polymers are used for a variety of optoelectronic applications such as light-emitting diodes,2 photovoltaic devices,3 and field-effect transistors.4 Since electrical, optical, electrochemical, and optoelectronic properties of conjugated polymers can be modified by environmental stimulus, they can be used as sensory materials to detect chemical (chemosensors) or bioactive species (biosensors).5 Among the variety of optoelectronic conjugated polymers such as poly(phenylene * Corresponding author. E-mail: [email protected]. † Permanent address: Department of Electrical & Computer Engineering, University of California, San Diego, LaJolla, CA 92093-0407. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (2) (a) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science 1995, 269, 1086. (b) Herold, M.; Gmeiner, J.; Schworer, M. Acta Polym. 1994, 451, 392. (c) Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982. (d) Burns, P. L.; Holmes, A. B.; Kraft, A.; Bradley, D. D. C.; Brown, A. R.; Friend, R. H.; Gywer, R. W. Nature 1992, 356, 47. (3) (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (b) Wohrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129. (4) (a) Lovinger, A. J.; Rothverg, L. J. J. Mater. Res. 1996, 11, 1581. (b) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem. Soc. 1998, 120, 207.

vinylene) (PPV), poly(p-phenylene) (PPP), and polyfluorene (PF),6 conjugated polyelectrolytes derivatives based on these polymers were also widely investigated7 because of their unique characteristic different from that of pristine “neutral ”polymers.8 Water-soluble conjugated polyelectrolytes can be used as highly sensitive materials in biosensors for aqueous environments in a living body.9 The ionic groups of conjugated polyelectrolytes can offer some new application opportunities in LEDs. For example, it can be used as the active layers (5) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537. (b) Faı¨d, K.; Leclerc, M. Chem. Commun. 1996, 2761. (6) (a) Feast, W. J.; Tsibouklis, J.; Pouwer, K. L.; Groenendaal, L.; Meijer, E. W. Polymer 1996, 37, 5017. (b) Holmes, A. B.; Kraft, A.; Grimsdale, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 402. (7) (a) Shi, S.; Wudl, F. Macromolecules 1990, 23, 2119. (b) Peng, Z.; Xu, B.; Zhang, J.; Pan, Y. Chem. Commun. 1999, 1855. (c) Fujii, A.; Sonoda, T.; Yoshino, K. Jpn. J. Appl. Phys. 2000, 39, L249. (d) Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411. (e) Child, A. D.; Reynolds, J. R. Macromolecules 1994, 27, 1975. (f) Balanda, P. B.; Ramey, M. B.; Reynolds, J. R. Macromolecules 1999, 32, 3970. (g) Rulkens, R.; Schulze, M.; Wegner, G. Makromol. Rapid. Commun. 1994, 15, 669. (h) Wittemann, M.; Rehahn, M. Chem. Commun. 1998, 623. (i) Liu, B.; Yu, W. L.; Lai, Y. H.; Huang, W. Chem. Commun. 2000, 551. (j) Stork, M.; Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Adv. Mater. 2002, 14, 361. (k) Liu, B.; Yu, W. L.; Lai, Y. H.; Huang, W. Macromolecules 2002, 35, 4975. (l) Li, C. J.; Slaven, W. T.; John, V. T.; Banerjee, S. Chem. Commun. 1997, 1569. (m) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (n) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321. (o) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466. (p) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 12389. (q) Pinto, M. R.; Reynolds, J. R.; Schanze, K. S. Polym. Prepr. 2002, 43, 139. (r) Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446. (8) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 1293.

10.1021/cm034650o CCC: $27.50 © 2004 American Chemical Society Published on Web 01/28/2004

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in LEDs through layer-by-layer self-assembly.10 Another advantage of conjugated polyelectrolytes in PLED application is that environment-friendly solvents such as water or alcohols can be used in device fabrication. Water-soluble conjugated polyelectrolytes were first reported in 3-substituted polythiophenes11 and then developed into the PPV-,7a-7c PPP-,7d-7h poly(phenylene ethylene) (PPE)-,7l-7r and polyfluorene (PF)-based7i-7k polymers. Among them, the water-soluble anionic and cationic PPP and PPE derivatives have been synthesized and widely investigated. There are only a few reports on the research of PF-based polyelectrolytes. The first polyfluorene-based polyelectrolytes was reported by Liu et al.;7i it is a copolymer of fluorene units and ammonium-functionalized phenylene units synthesized through the Suzuki coupling reaction. Stork et al.7j investigated energy transfer from water-soluble anionic MPS-PPV oligomer poly(2,5-methoxy-propyloxysulfonate phenylenevinylene) to the cationic poly(trimethylamoniumhexyl-fluorenephenylenes) tetraiodines in the presence of surfactant. The first LED based on a conjugated polyelectrolyte was reported by Cimrova et al.12 They used a sulfonated PPP (PPPSO3Na or PPPSO3NC14H29(CH3)3) as the emission layer in a single-layer device with the configuration ITO/PPPSO3Na (or PPPSO3NC14H29(CH3)3)/Al. The external quantum efficiencies of single-layer devices were in the range of 0.5-0.8% with counterions H+ or Na+. Baur et al.13 reported the fabrication of LEDs with the configuration ITO/[((-)PPP/polycation)n]/Al using layer-by-layer self-assembly of cationic and anionic PPP derivatives as the active layer. The highest external quantum efficiency of their devices was 0.01%. Fujii reported a device which emits yellow light (540 nm) at an operating voltage of 25 V by using carboxypentoloxysubstituted PPVs.7e Thunemann et al.14 investigated the influence of cationic counterions on the emission properties in poly(1,4-phenyleneethynylene carboxylate) polyelectrolytes. The peak wavelength of the electroluminescence shifts from 430 to 515 nm for single-layer lightemitting diodes (LEDs) when the counterions were changed from tetraethylammonium to sulfonium ions. No efficiencies data were reported in these reports. To the best of our knowledge, so far, no electroluminescence (EL) properties of polyfluorene polyelectrolytes have been reported. In this paper we synthesized two kinds of polyfluorenes with amino end groups on the side chains and their conjugated polyelectrolytes derivatives through a postpolymerization treatment. The effect of the induced ionic groups on the optical and electronic properties of the conjugated copolymers was investigated. Light(9) (a) Chen, L.; Mcbranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287. (b) Wang, D. L.; Gong, X.; Heeger, P. S.; Rininsland, F.; Bazan, G. C.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 49. (c) Song, X.; Wang, H. l.; Shi, J.; Park, J. W.; Swanson, B. I. Chem. Mater. 2002, 14, 2342. (10) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7101. (11) (a) Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858. (b) Pickup, P. J. Electroanal. Chem. 1987, 225, 273. (12) Cimrova, V.; Schmidt, W.; Rulkens, R.; Schulze, M.; Meyer, W.; Neher, D. Adv. Mater. 1996, 8, 585. (13) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater. 1998, 10, 1452. (14) Thunemann, A. F.; Ruppelt, D. Langmuir 2001, 17, 5098

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emitting devices were fabricated by using the obtained polymers. To our knowledge, this is the first report on the electroluminescence properties of polyfluorene electrolytes. It is shown that the ionic or the strong polar groups attached to polyfluorene side chains have a great influence on their light-emitting performance. All these polymers show high external quantum efficiencies (QE) when the high work-function metal such as Al was used as a cathode. We have shown that when such polymers are used as an electron-injection layer spin-coated on top of conventional light-emitting polymers in combination with an Al cathode, devices with external quantum efficiency as high as that with a Ba/Al cathode can be obtained. Experimental Section Measurement and Characterization. The 1H and 13C NMR spectra were collected on a Varinan Inova 500 or Bruker DRX 400 spectrometer in deuterated chloroform solution operating respectively at 500 MHz (for 1H) and 100 MHz (for 13 C), with tetramethylsilane as reference. Number-average (Mn) and weight-average (Mw) molecular weights were determined by a Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve of polystyrene standards. Elemental analyses were performed on a Vario EL Elemental Analysis Instrument (Elementar Co.). UV-visible absorption spectra were recorded on a HP 8453 UV-Vis spectrophotometer. The PL quantum yields were determined in an Integrating sphere IS080 (Labsphere) with 325-nm excitation of HeCd laser (Mells Griot). EL efficiency and brightness were carried out with a calibrated silicon photodiode. PL and EL spectra were recorded on a Instaspec IV CCD spectrophotometer (Oriel Co.). Cyclic voltammetry was carried out on a CHI660A electrochemical workstation with platinum or ITO glass electrodes at a scan rate of 50 mV/s against a calomel reference electrode with nitrogen-saturated solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile (CH3CN). Thermogravimetric analyses (TGA) were conducted on a NETZSCH TG 209 under a heating rate of 10 °C/min and a nitrogen flow rate of 20 mL/min. LED Fabrication and Characterization. Polymers were dissolved in chloroform (for neutral polymers) or N,N-dimethyl formamide (DMF) (for polyelectrolytes) and filtered through a 0.45-µm filter. Patterned indium tin oxide (ITO)-coated glass substrates were cleaned with acetone, detergent, distilled water, and 2-propanol and subsequently in an ultrasonic bath. After treatment with oxygen plasma, 150 nm of poly(3,4ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (Baytron-P 4083, Bayer AG) or poly(vinylcarbazole) (PVK, Aldrich) from 1,1,2,2,-tetrachloro ethane solution was spin-coated onto the substrate followed by drying in a vacuum oven at 80 °C for 8 h. A thin film of electroluminescent polymer was coated onto the anode by spin-casting inside a drybox. The film thickness of the active layers was around 70 nm, as measured with an Alfa Step 500 Surface Profiler (Tencor). Ba and Al layers were vacuum-evaporated on the top of an EL polymer layer under a vacuum of 1 × 10-4 Pa. Device performances were measured inside a drybox. For a double-layer device with copolymers of this study as an electron-injection layer, a thin layer (ca. 5 nm) in methanol (with a few drops of acetic acid) solution was spin-coated on top of the MEH-PPV layer followed by deposition of 200-nm Al as a cathode. Current-voltage (I-V) characteristics were recorded with a Keithley 236 source meter. EL spectra were recorded by an Oriel Instaspec IV CCD Spectrograph. Luminance and external quantum efficiencies were determined by a calibrated photodiode. Materials. All manipulations involving air-sensitive reagents were performed under an atmosphere of dry argon. All reagents, unless otherwise specified, were obtained from Aldrich, Acros, and TCI Chemical Co. and used as received. All the solvents used were further purified before use.

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Synthesis. 2,7-Dibromofluorene (1)15 and 2,7-bis(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3)16 were prepared according to the published procedures and have been described elsewhere.17 2,7-Dibromo-9,9-bis(3′-(N,N-dimethylamino)propyl)fluorene (2). To a stirred mixture of 2,7-dibromofluorene (4 g, 12 mmol) and 60 mL of dimethyl sulfoxide (DMSO) under nitrogen were added tetrabutylammoium bromide (80 mg) and 8 mL of a 50 wt % aqueous solution of sodium hydroxide. Twenty-milliliters DMSO solution of 3-dimethylaminopropylchloride hydrochloride (5 g, 32 mmol) was added dropwise to the mixture. The reaction mixture was stirred at room temperature for 6 h and then was diluted with 50 mL of water, to dissolve all salts. The product was extracted with ether (3 × 100 mL) and the combined organic layer was washed with 10% NaOH (aq) (2 × 100 mL), water (3 × 100 mL), and brine (1 × 100 mL). The solution was dried over MgSO4, filtered, and stripped of solvent by vacuum evaporation to yield a crude solid. The crude solid was recrystallized from MeOH/H2O to afford 2 (3.1 g, 51%) as white crystals. 1H NMR (500 MHz, DMSO-d6): δ 7.82-7.80 (d, 2H, fluorene ring), 7.69 (s, 2H, fluorene ring), 7.56-7.53 (d, 2H, fluorene ring), 2.04-2.00 (t, 4H, -CH2N), 1.92-1.88 (m, 16H, -NCH3, -CH2-), 0.60-0.52 (m,4H, -CH2-). 13C NMR (100 MHz, CDCl3): δ 152.37, 139.50, 130.75, 126.51, 122.04, 121.58 (C-fluorene ring), 60.01 (-CH2N), 55.70 (C9-fluorene ring), 45.78 (-NCH3), 37.96 (-CH2-), 22.42 (-CH2-). Element Anal. Calcd for C23H30Br2N2: C, 55.89; H, 6.12; N, 5.67. Found: C, 55.76; H, 6.12; N, 5.60. Poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (P1). 2,7-Dibromo-9,9bis(3′-(N,N-dimethylamino)propyl)fluorene (2) (0.248 g, 0.500 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9dioctylfluorene (3) (0.321 g, 0.500 mmol), tetrakis(triphenylphosphine)palladium [(PPh3)4Pd(0)] (10 mg), and several drops of Aliquat 336 were dissolved in a mixture of 3 mL of toluene and 2 mL of 2 M Na2CO3 aqueous solution. The mixture was refluxed with vigorous stirring for 3 days under an argon atmosphere. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol. The precipitated material was recovered by filtration through a funnel. The resulting solid material was washed for 24 h using acetone to remove oligomers and catalyst residues (0.28 g, 77%). 1H NMR (500 MHz, CDCl3): δ 7.82-7.80 (m, 4H, fluorene ring), 7.68-7.63 (m, 8H, fluorene ring), 2.20-1.95 (m, 24H, -CH2N, -NCH3, H-alkyl), 1.18-1.11 (m, 24H, H-alkyl), 0.81-0.77 (m, 10H, H-alkyl). 13C NMR (100 MHz, CDCl3): δ 152.20, 151.60, 140.50, 126.81, 121.84, 120.45, 60.18, 55.43, 45.69, 38.29, 32.18, 30.49, 29.66, 24.31, 22.99, 14.46. Elem. Anal. Calcd for C52H70N2: C, 86.43; H, 9.69; N, 3.88. Found: C, 85.38; H, 9.70; N, 3.76. Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] Dibromide (P2). A 100-mL flask with a magnetic stirring bar was charged with the polymer P2 (100 mg) dissolved in 40 mL of THF. To this solution was added bromoethane (2 g, 18 mmol) and 10 mL of DMSO. The reaction mixture was stirred at 50 °C for 5 days. THF and extra bromoethane were evaporated. The polymer was precipitated by the addition of about 80 mL of ethyl acetate, collected by centrifugation, washed with chloroform and THF, and dried overnight in a vacuum at 80 °C (71 mg, 55%). 1H NMR (500 MHz, DMSO-d6): δ 8.06-7.97 (m, 4H, fluorene ring), 7.92-7.87 (m, 8H, fluorene ring), 3.153.12 (m, 8H, -CH2N, -NCH2CH3), 2.90-2.78 (m, 16H, -NCH3,), 2.56-2.53 (m, 6H), 2.20 (m, 4H), 1.03 (m, 32H), 0.77-0.73 (m, 8H). Elem. Anal. Calcd for C52H70N2‚1.6C2H5Br‚2H2O: C, 69.92; H, 8.70; N, 2.93. Found: C, 69.47; H, 8.22; N, 3.14. (15) (a) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R. WO 99 05184, 1997. (b) Lee, J. K.; Klaerner, G.; Miller, R. D. Chem. Mater. 1997, 11, 11083. (16) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30, 7686. (17) (a) Hou, Q.; Xu, Y. S.; Yang, W.; Yuan, M.; Peng, J. B.; Cao, Y. J. Mater. Chem. 2002, 12, 2887. (b) Yang, R. Q.; Tian, R. Y.; Yang, W.; Hou, Q.; Cao, Y. Macromolecules, 2003, 36, 7453.

Huang et al. Poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-1,4-phenylene] (P3). The mixture of (2) (0.9 g, 1.8 mmol), 1,4-phenyldiboronic acid (0.3 g, 1.8 mmol), PdCl2 (dppf) (25 mg), and K2CO3 (1.0 g, 7.3 mmol) in 30 mL of THF and 10 mL of water were degassed and stirred at 80 °C for 3 days. The mixture was extracted with chloroform, concentrated, and the desired polymer was precipitated from methanol (0.54 g, 72%).1H NMR (500 MHz, DMSO-d6): δ 8.06-7.97 (m, 4H, fluorene ring), 7.92-7.87 (m, 8H, fluorene ring), 3.15-3.12 (m, 8H, -CH2N, -NCH2CH3), 2.90-2.78 (m, 16H, -NCH3,), 2.562.53 (m, 6H), 2.20 (m, 4H), 1.03 (m, 32H), 0.77-0.73 (m, 8H). 13 C NMR (100 MHz, CDCl3): δ 152.36, 151.59, 140.78, 140.64, 140.18, 139.49, 130.80, 129.23, 128.59, 127.95, 127.45, 126.63, 126.57, 122.03, 121.74, 121.64, 120.63, 60.24, 55.40, 45.70, 38.34, 22.38. Elem. Anal. Calcd for C29H34N2: C, 84.88; H, 8.29; N, 6.83. Found: C, 83.75; H, 7.76; N, 6.71. Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-1,4-phenylene] Dibromide (P4). According to the procedure for P2, the 100 mg of P3 was treated with bromoethane in DMSO/THF (1:4). Once there were some precipitations observed, some of the water was added to the solution to dissolve the precipitation. After the reaction was stirred at 50 °C for 5 days, ether was added and the mixture was extracted with water. The aqueous layer was concentrated and the residue was precipitated from THF and dried overnight in a vacuum at 80 °C to yield P4 (107 mg, 69%). 1H NMR (500 MHz, DMSO-d6): δ 8.06-7.99 (m, 4H), 7.85 (m, 4H), 7.71 (m, 2H), 3.14 (m, 8H, -CH2N, -NCH2CH3), 2.91-2.82 (m, 16H, -NCH3), 2.57-2.53 (m, 4H,), 1.04 (m, 10H). Elem. Anal. Calcd for C29H34N2‚1.8C2H5Br‚H2O: C, 62.67; H, 6.89; N, 4.49. Found: C, 62.52; H, 6.81; N, 4.53.

Results and Discussion Synthesis and Characterization. The synthesis route is shown in Scheme 1. The monomer 3 was synthesized by reaction between 2,7-dibromofluorene and 3-dimethylaminopropylchloride hydrochloride in a two-phase mixture of water and DMSO in the presence of excess NaOH. The neutral polymers were synthesized by a Suzuki coupling reaction. The P1 was synthesized in a mixture of toluene and aqueous sodium carbonate solution containing about 1 mol % Pd(PPh3)4 by stirring at about 80 °C for 3 days in a nitrogen atmosphere. A few drops of Aliquat 336 were added as the phase transfer catalyst. For the polymer P2, a water-soluble palladium catalyst, Pd(dppf)Cl2 (dppf ) 1,1′-bis(diphenylphosphino)ferroene),7f,7j was used and the reaction was carried out in aqueous THF by vigorously stirring at the refluxing temperature for 3 days in a nitrogen atmosphere. The neutral polymers were quaternized by treatment with bromoethane. Conversion of the P1 to P2 was achieved by treatment with bromoethane in a mixture of DMSO and THF at 50 °C for 5 days. The P4 was obtained from the quaternization of P3 at almost the same condition except that a small amount of water was added to dissolve the reaction mixture. All the monomers and polymers have been characterized and verified by NMR and by elemental analysis. It is difficult to obtain clear 1H NMR spectra of monomer 1 when CDCl3 was used as the solvent, though it has very clear 13C NMR (100 MHz,) spectra in CDCl3 solution. When DMSO-d6 was used as the solvent, monomer 1 has much better 1H NMR spectra in which the 1H of the methyleneamino groups (-CH2N) can be distinguished from the methylamino groups (-NCH3). For the neutral polymers P1 and P3, the signals corresponding to the methyleneamino and methylamino

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Scheme 1. Synthetic Route of the Monomer and Polymers

groups mixed together with the peaks for the methylene (-CH2-) groups adjacent to the 9-position carbon atoms of the fluorene rings, in the region around 2.20-1.95 ppm for P1 and 2.22-1.97 ppm for P3, respectively. After treatment with the bromoethane, all signals for the methylene (-CH2N, -NCH2CH3) and methyl (-NCH3) groups adjacent to the nitrogen atoms shift to the lower field region. It is difficult to obtain a full quaternization for the conversion of the neutral polymers into quaternized salt. A typical degree of quaternization for P2 and P4 is about 80% and 90%, respectively, as estimated by the results of elemental analysis. We note also that there is always some water remaining in the quaternized polymers;7f,7k even the copolymers were dried in a vacuum at 150 °C for long time. The amount of water in the polymers was estimated at about 4% and 3% for P2 and P4, respectively, by thermogravimetric analysis (TGA). Figure 1 shows the thermograms of these four

polymers. The temperature was ramped from 25 to 800 °C under a nitrogen atmosphere. All the polymers have fair thermal stability. The onset of degradation temperature for P1, P2, P3, and P4 is about 250, 200, 275, and 268 °C, respectively. The main decomposition of the neutral polymers happened when the side chains began to cleave, whereas for the quaternized salts, there are two main degradation processes due to the loss of ethyl bromide and the cleavage side chains, respectively.7f The quaternized polymers also have a small amount of water loss at a lower temperature. These thermal properties were very similar to those of the other conjugated polymer with terminal amino or ammonium side groups.7f,7k The solubility of the quaternized polymers is different from their neutral precursors. The neutral polymers P1 and P3 are readily soluble in common organic solvents such as THF, chloroform, toluene, and xylene, but insoluble in DMSO and water. The polymer P1 has a

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Figure 1. Thermal gravimetric analysis of P1-P4 in a nitrogen atmosphere.

better solubility in common nonpolar organic solvents than P3 because of its long alkyl substitutions in the 9-position of the fluorene unit. After quaternization, the resulted polymers P2 and P4 are insoluble in THF and chloroform but completely soluble in DMSO, methanol, and DMF. P2 is almost insoluble in water probably because of its long alkyl side chain in the fluorene unit. With use of a comonomer without side chains (1,4phenyldiboronic acid), the resulting P4 has good solubility in water (about 2 mg/mL). Both the neutral and the quaternized polymers have good solubility in the aque-

Huang et al.

ous acetic acid or its mixture with other organic solvents such as methanol due to a weak interaction between the nitrogen atoms in amino and ammonium groups in the side chain and the acetic acid.7k The number molecular weight (Mn) estimated by gel permeation chromatography (GPC) against the polystyrene standard with THF as an eluent was about 14 000 with a polydispersity of 1.4 for P1 and 7800 with a polydispersity of 1.7 for P3. It is difficult to obtain the molecular weight of the P2 and P4 by GPC measurement due to the polymer aggregation on the column fillers induced by the ionic groups on the polymer side chain.7d Optical and Electrochemical Properties. Figure 2 shows UV-visible absorption and PL spectra of the four polymers in the solution of several solvents in a concentration of 1 × 10-5 M based on the polymer repeat unit. For the neutral precursor polymers P1 and P3 as displayed in Figure 2a and Figure 2c, respectively, both absorption and PL spectra of P1 (Figure 2a) and P3 (Figure 2c) are similar in chloroform and THF. P1 shows the absorption maximum at 389 nm in chloroform and 391 nm in THF. PL spectra of P1 polymer in both chloroform and THF solution are peaked at 419 nm with a vibronic shoulder at 441 nm. P3 has the main absorption peak at 371 nm in chloroform and 373 nm in THF and the PL emission maximum at 409 nm with a vibronic shoulder around 430 nm in both solvents. Unlike neutral polymers P1 and P3, the electronic spectra of polyelectrolytes P2 (Figure 2b) and P4

Figure 2. UV-vis absorption and PL emission spectra in chloroform and THF solution. (1 × 10-5 M based on polymer repeat unit): (a) P1; (b) P2; (c) P3; (d) P4.

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Figure 3. UV-vis absorption, PL and EL spectra in solid-state films: (a) P1; (b) P2; (c) P3; (d) P4.

(Figure 2d) are significantly dependent on the solvent, showing a red shift with an increase in solvent polarity. The absorption peaks of P2 in methanol, DMF, and DMSO are observed at 388, 396, and 400 nm, respectively. The corresponding PL spectrum is peaked at 416, 422, and 424 nm with the vibronic shoulder around 437, 445, and 449 nm, respectively. Absorption peaks of P4 in methanol, DMSO, and water are 375, 382, and 386 nm, respectively. The corresponding PL spectrum is peaked at 407, 413, and 416 nm with the vibronic shoulder around 428, 433, and 437 nm, respectively. The remarkable increased red shift in both absorption and PL spectra for P2 and P4 with increasing polarity of solvent can be explained by the increase of planarity of the polymer main chain due to increased repulsion between positive ammonium groups in the side chain between the adjacent fluorene units in the polar solvents. Another possible explanation for such red shift is the increase in chain aggregation of the polymer main chain of the polyelectrolytes in the polar solvents. Tan et al.7r reported that PPE-SO3- (sulfonated polyphenylene ethynylene) showed PL spectra with a maximum peak at 447 nm and distinct vibronic shoulders, which is typical for nonaggregated PPEs in good solvents. By contrast, in the polar solvent, PPE-SO3- shows a broad PL peak with a maximum moved to 550 nm, which is typical for excimer-like photoluminescence. The authors7r suggested that the tendency for aggregation of polymer electrolyte in a strong polar solvent is due to the increased face-to-face π-stacking between phenylenes in the adjacent chains.8 In our case, aggregation of positive electrolytes in the polar solvent does not seem to be the

case, for we observed exactly the same absorption and PL profiles for neutral precursor and polyelectrolytes in both nonpolar and polar solvents (Figure 2a-d). Excimer-like PL emission has not been observed either in the polar solvent or in the solid-state spectra (see Figure 3). Thus, the red shift of absorption and PL emission spectra of polyelectrolytes in polar spectra must be related to the increase in the chain conjugation. Furthermore, we note that such a red shift does not decrease or disappear in very dilute solution (down to 10-7 mol/L). If this is due to chain aggregation, the dissociation of chain aggregation will happen in a very dilute solution; thereby, the blue shift in absorption and PL spectra with the dilution should be expected. The UV-visible absorption, electrochemical, and photoluminescence properties of these polymers in solid film are summarized in Table 1. Transparent and uniform films of the polymers were prepared on a quartz substrate by spin-casting from their chloroform solutions (for P1 and P3) or methanol solutions (for P2 and P4) at room temperature. The main absorption peaks of these polymers were respectively 390, 400, 382, and 388 nm for P1, P2, P3, and P4, as shown in Table 1 and Figure 3. All these polymers emit intense blue light when excited by the UV light. The PL spectra of the polymer films are respectively peaked at 425, 431, 420, and 425 nm for P1, P2, P3, and P4, with the relative PL quantum yields of 37.3%, 4.7%, 18.5%, and 8.3%. It can be found that the absorption and the PL emission spectra for the quarternized polymers are slightly redshifted (around 10-15 nm, compare Figures 2 and 3) compared with their neutral precursor polymers. PL

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Table 1. UV-Vis Absorption, Electrochemical, and Photoluminescence Properties and Electroluminescence of the Polymers (in Solid Films) photoluminescence

polymers

λabsmax (nm)

optical band gapa (eV)

P1 P2 P3 P4

390 400 382 388

2.91 2.85 2.96 2.92

a

Eox (V)

Ered (V)

HOMO (eV)

LUMO (eV)

Egap (eV)

λPLmax (nm)

1.21 1.25 1.30 1.33

-2.26 -2.22 -2.28 -2.29

-5.61 -5.65 -5.70 -5.73

-2.14 -2.18 -2.12 -2.11

3.47 3.47 3.58 3.62

425, 449 431, 455 420, 444 425, 447

QPLmax (%)

electroluminescence λELmax (nm)

37.3 4.7 18.5 8.3

515 433, 492 535 442, 497

Determined from absorption onset. Table 2. Device Performances of the Polymers device performancesa

polymers P1

P2

P3

P4

Figure 4. Cyclic voltammograms of the polymers films coated on platinum electrodes (P1, P3) or on ITO glass (P2, P4) in 0.1 mol/L Bu4NPF6, CH3CN solution.

quantum yields of the quaternized polymers are significantly reduced in comparison with the neutral precursors. We also note that almost exact PL profiles for P2 and P4 are observed both in solid-state films (Figure 3b,d) and in the dilute nonpolar solvent (methanol) (Figure 2b,d). The fact that no excimer-like PL emission was observed in solid-state film typical for other polyelectrolytes7r,8 indicates that interchain interaction between fluorene chains is weak in such polyelectrolytes in results of strong repulsion between positively charged ammonium side groups. The electrochemical behavior of the polymers was investigated by cyclic voltammetry (CV). The CV was performed in a solution of Bu4NPF6 (0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature under the protection of argon. A platinum electrode (for P1 and P3) or an ITO glass (for P2 and P4) was coated with a thin polymer film and was used as the working electrode. A Pt wire was used as the counter electrode, and a calomel electrode was used as the reference electrode. Two reduction waves can be observed for all these four polymers (Figure 4). The first oxidation peak occurred at around 0.93-1.00 V and corresponding first reduction peak occurred respectively at around -1.90 to -2.07 V. These peaks can be attributed to the oxidation and reduction of the terminal amino or ammonium groups.7k,18 The second oxidation and reduction features appeared respectively at 1.2-1.3 and 2.2-2.3 V. They are attributed to the oxidation and reduction potentials for the polymer main chains. The HOMO and (18) Zotti, G.; Zecchin, S.; Schiavon, G.; Berlin, A. Macromolecules 2001, 34, 3889.

anode buffer PEDOT PEDOT PVK PVK PEDOT PEDOT PVK PVK PEDOT PEDOT PVK PVK PEDOT PEDOT PVK PVK

current voltage density lumin. cathode (V) (mA/cm2) (cd m-2 ) QE % Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al Ba/Al Al

25.8 28.0 24.0 21.1 4.0 6.0 3.3 5.5 21.5 19.0 30 29.4 10.0 4.1 11.2 10.0

187 176 35 67 172 530 33 141 253 141 144 343 154 33 41 176

13 21 24 22 102 160 39 130 20 21 29 137 75 150 17 91

0.01 0.02 0.12 0.38 0.01 0.05 0.06 0.16 0.01 0.03 0.02 0.07 0.04 0.07 0.07 0.09

a Device structure: ITO/anode buffer/polymer/cathode, active area 0.15 cm2.

LUMO levels calculated according to an empirical formula, EHOMO ) -e(Eox + 4.4) (eV) and ELUMO ) -e(Ered + 4.4) (eV),19 are also listed in Table 1. Because the postpolymerization treatment does not change the polymers main chain structure, there are no obvious changes in the HOMO and LUMO levels between the neutral polymers (P1, P3) and the quarternized ones (P2, P4). From the electrochemical data we estimate that the band gap is around 3.47-3.62 eV for these polymers, which are significantly larger than the optical band gap of 2.85-2.96 eV estimated from absorption onset. A significant difference between gaps determined electrochemically and optically for poly(9,9-dioctylfluorenes) were previously reported by Janietz et al.,20 no detailed explanation about the discrepancy between optical and elechtrochemical gaps was provided in this paper.20 Electroluminescence Properties. The electroluminescence properties of these polymers are investigated. Both the neutral precursor and the quaternized polyelectrolytes were used as the emitting layer in a double-layer light-emitting device with the configuration ITO/PEDOT (or PVK)/polymer/Al (or Ba/Al). Device performance was listed in Table 2. We note that the device efficiency was significantly enhanced for all copolymers when PVK coated on top of the ITO surface was used as an anode buffer in replace of the PEDOT. (19) Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 87, 53. (20) Janietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.; Ibnbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 2453.

Polyelectrolytes Based on Polyfluorene

This fact is consistent with the electrochemical data from which HOMO levels were determined as ca. 5.7 eV (Table 1). A big hole injection barrier is expected between the copolymer and PEDOT:PSS, which has the HOMO level typically about 5.0 eV. 21 It is important to note that all these polymers show higher external quantum efficiencies (QE) when the high work-function metal like Al was used as the cathode. The maximum QEs for devices with an Al cathode from P1, P2, P3, and P4 are 0.38%, 0.16%, 0.07%, and 0.09% respectively, which were far more than QEs when the low work-function metal Ba was used as the cathode (Table 2). There were many works which reported that insulating polar or ionic species inserted between a metal electrode and a light-emitting layer could improve significantly electron injection from a high work-function cathode. High-efficiency organic and polymer LEDs with an Al cathode were reported for LiF and other metal fluoride and chloride22 and anionic surfactant.23 Significant enhancements in external quantum efficiencies for devices with an Al cathode with amino-substituted polyfluorene copolymer and corresponding ammonium salt-polyelectrolytes could be based on a similar mechanism where surface dipoles are aligned toward the Al cathode surface.24 According to this mechanism, the polar or ionic molecules under the influence of Al nearby produce dipole moments at the polymer/cathode interface. The large dipole moment remarkably decreases the surface potential of the aluminum and also leads to a significant reduction of the effective work function. The EL spectra of these polymers along with absorption and PL spectra in the solid state are presented in Figure 3. It is found that the EL spectra of P1 (Figure 3a) are very different from its PL spectra. The sharp peak at 425 nm with a vibronic shoulder around 449 nm in the PL spectra disappears in its EL spectra. At the same time, a new broad peak centered at 515 nm appears. A similar red shift of EL emission in comparison with that of PL emission was also observed for copolymer P3, for which EL spectra show a broad peak with a maximum at 535 nm, whereas its PL spectra are peaked at 420 nm with a vibronic shoulder around 444 nm (Figure 3c). In contrast to P1 and P3 neutral precursor, the EL spectra of polyelectrolytes P2 (Figure 2b) and P4 (Figure 2d) are more like the typical emission spectra for the aggregated polyfluorenes where the main EL peak coincides with PL emission and an additional broad shoulder peaked at 490 nm, which was assigned to excimer emission. We note that long wavelength components for P2 and P4 devices have different shapes and are peaked at shorter wavelengths than emission of P1 and P3 devices. Thus, it seems that EL spectra of neutral precursor and polyelectrolytes are of different origins. (21) Cao, Y.; Yu, G.; Zhang, C.; Menon, R.; Heeger, A. J. Synth. Met. 1997, 87, 171. (22) (a) Hung, L. S.; Tang, C. W.; Mason, M. G. Appl. Phys. Lett. 1997, 70, 152. (b) Yang, X.; Mo, Y.; Yang, W.; Yu, G.; Cao, Y. Appl. Phys. Lett. 2001, 79, 563. (23) (a) Cao, Y. U.S. Patent Application No. 08/888316, 1997. (b) Cao, Y.; Yu, G.; Heeger, A. J. Adv. Mater. 1998, 10, 917. (c) Cao, Y.; Yu, G.; Heeger, A. J. Synth. Met. 1999, 102, 881. (24) Shaheen, S. E.; Jabbour, G. E.; Morrell, M. M.; Kawabe, Y.; Kippelen, B.; Peyghambarian, N.; Nabor, M. F.; Shlaf, R.; Mash, E. A.; Armstrong, N. R. J. Appl. Phys. 1998, 84, 2324.

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Significant broadening and enhancement at long wavelength site in EL spectra for varieties of polyfluorene light-emitting polymers are common phenomena and investigated extensively.25-28 The enhancement of polyfluorene emission in the long wavelength region was attributed to the formation of an excimer due to strong interchain interaction between planar polyfluorenes chains.27,28 Other mechanisms such as hydrogen-bonding interaction25 and the defects formation in the polymers26 are also reported. In these cases, a very broad additional long wavelength emission can be observed in both PL and EL spectra.25-27 However, as discussed above for copolymers P1-P4 investigated in this paper, no obvious red shift and broadening has been observed in PL emission in the solid state films as well as in solution (Figure 2 and Figure 3). Both the neutral precursor and the quaternized polyelectrolytes emit a strong and pure blue light at excitation under 325-nm UV light. It is important to note that, in contrast with common polyfluorene homopolymer and copolymers, blue emission around 420 nm in PL emission disappears almost completely in EL spectra of P1 and P3 devices (Figure 3). A similar phenomenon has been reported by Bouillud et al. for an alternating copolymer of fluorenedi(3,4-ethilenedioxythiophene) (PEDOPF).27 A new peak centered at 588 nm was observed in the EL spectra for device configuration of ITO/PEDOPF/LiF/Ag, while PL emission peaked at 494 nm almost completely disappeared.27 Another famous example of such a type of PL and EL spectrum has been reported in the case of poly(vinylcarbazole) (PVK) single-layer devices. Nishiro et al. reported that single-layer PVK devices with structure ITO/PVK/Ca device emitted a reddish light with the maximum at 640 nm29 while PL emission peaked at 428 nm.30 No clear explanation about the process involved was given by the authors in these reports. Completely different PL and EL emissions usually implies that the PL and EL spectra originated from different lightemitting species or the recombination zone of PL and EL emissions are different. As was pointed out above, the high efficiencies reached with a high work-function Al cathode could have originated from self-assembled dipoles at the cathode/polymer surface. We speculate that the recombination zone for EL emission of ITO/P1 (or P3)/Al (or Ba/Al) devices should be located (or close to) in the interface area between the cathode and EL polymer. Significantly red-shifted EL emission for P1 and P3 devices is related to self-organization (selfassembly) of amino-substituted polyfluorene at the cathode interface. We note that long wavelength components for P2 and P4 devices have different shapes and are peaked at shorter wavelengths than emission of P1 and P3 devices. It seems that the recombination (25) Pei, J.; Liu, X. L.; Chen, Z. K.; Zhang, X. H.; Lai, Y. H.; Huang, W. Macromolecules 2003, 36, 323. (26) (a) List, E. J. W.; Guentner, R.; de Freitas, P. S.; Scherf, U. Adv. Mater. 2002, 14, 374. (b) Zojer, E.; Pogantsch, A.; Hennebicq, E.; Beljonne, D.; Bredas, J. L.; List, E. J. W. J. Chem. Phys. 2002, 117, 6794. (27) Bouillud, A. D.; levesque, I.; Tao, Y.; D’Iorio, M.; Beaupre, S.; Blondin, P.; Ranger, M.; Bouchard, J.; Leclerc, M. Chem. Mater. 2000, 12, 1931. (28) Yu, W.; Cao, Y.; Pei J.; Huang, W.; Heeger, A. J. Appl. Phys. Lett. 1999, 75, 3270. (29) Nishiro, H.; Yu, G.; Heeger, A. J.; Chen, T.-A.; Rieke, R. D. Synth. Met. 1995, 68, 243. (30) Von Seggern, H.; Schmidt, H.-W.; Heeger, A. J.; Kraabel, B.; Zhang, C. Synth. Met. 1995, 72, 185.

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Figure 5. Current versus voltage (I-V) and light output of the LED of MEHPPV with a P1/Al cathode in comparison with Al and Ba/Al devices. Table 3. Device Performance from MEHPPV Using Polymer P1 as the ETL Layer and Al Cathode in Device Configuration ITO/PEDOT/MEHPPV/P1/Al EL polymer P1 MEHPPV MEHPPV MEHPPV

ETL

P1

cathode

V (V)

I (mA)

luminance (cd/m2)

QE (%)

Al Al Al Ba/Al

7.0 7.0 8.0 7.0

41.4 32.5 34.1 32.7

258 157 3004 2645

0.15 0.07 2.29 2.12

zone was moved from a polymer/cathode interface back to the P2 and P4 emitting region in these devices. It is not clear at this stage of experiment why neutral aminosubstituted polyfluorenes behave differently in the polymer/electrode interface. Electroabsorption and impedance studies can provide a clearer understanding of the interface of the polymer and electrode in these devices. Corresponding studies are in progress. If the dipole mechanism is correct, we can expect enhancement in device performance when a thin layer of these polymers is inserted between Al (or other high work-function metals) and conventional light-emitting polymers. Preliminary results indicate that the polyfluorene derivatives synthesized in this paper can be used as good electron-injection layers in polymer LEDs. Table 3 summarizes the preliminary results on multilayer devices with MEHPPV as an emitting layer and P1 as an electron injection layer (ETL) in device configuration: ITO/PEDT/MEHPPV/P1/Al. The results are encouraging. The device performance with the P1/ Al cathode shows similar high external efficiency as high as devices with low work-function metal Ba. A multilayer device emits the typical orange light for a MEHPPV single-layer device with a Ba cathode. Figure 5 compares representative IV curves and light output of the MEH-PPV with Al, Ba/Al, and P1/Al cathodes. As reported previously,23b current turn-on for an Al cathode occurs at around 1.1 V, corresponding to the difference between work functions of PEDOT and Al electrodes. Once a thin layer of P1 polymer was inserted between MEHPPV and Al cathode, current turn-on is moved to 1.6 V, similar to a Ba/Al device. This indicates that the barrier height at the MEHPPV and Al interface was significantly reduced. In the results of significantly improved electron injection, the external quantum efficiency increased from 0.07% for the MEHPPV/Al device to 2.3% for the MEHPPV/P1/Al device, for example, to levels comparable with that obtained using

Ba and Ca as the cathode (Table 3). Light output reached around 3000 cd/m2 at 8 V. A slightly higher operating voltage for the MEHPPV/P1/Al device is understandable considering the insulating character of the P1 copolymer. We note that this result might be a very important step in the efforts to look for replacement of air-sensitive metal (such as Ca and Ba) cathodes for polymer LEDs.23,32 A particular advantage of the use of copolymers synthesized in this study as an electron injection layer for polymer LEDs is that they can be processed from solvents such as methanol and water in which most conjugated light-emitting polymers are insoluble. This feature will allow one to avoid mixing between the light-emitting layer and the electroninjection layer that is a common and serious problem for fabrication of multilayer PLEDs by solution processing.31,32 The studies on devices from a variety of RGB emitting polymers by using all these amino- and ammonium-substituted copolymers as the ETL layer and high work-function metal cathodes are in progress, and the results and discussion on related mechanisms will be reported elsewhere. Conclusion A series of polyfluorene derivatives, the neutral amino-functionalized polyfluorenes (P1, P3) and the quaternized ammonium salt conjugated polyelectrolytes derivatives (P2, P4), were synthesized by Suzuki coupling polycondensation. All these polymers have a strong blue fluoresence under excitation by UV light in the solution and in the solid-state film. No excimer-like emission has been observed in PL spectra. Devices from the neutral amino-substituted precursor polymer (P1 and P3) and the quaternized ammonium salt polyelectrolytes (P2 and P4) show very different EL spectra. All devices show better efficiency with a high workfunction metal cathode than by using Ba/Al. The results are explained by the interface dipole formed between surfactant-like polyelectrolyte and its amino-substituted precursor and Al cathode. High-efficiency devices using such polymer inserted between an Al cathode and MEHPPV light-emitting layer support such interpretations. Polymer LEDs fabricated with the copolymer of this study as an electron-injection layer and aluminum as the cathode material can be enhanced to levels comparable with that obtained using Ba and Ca low work-function metals as the cathode. Polyelectrolyte and its neutral precursors synthesized in this study are promising candidates as a new type of electron-injection materials in polymer LEDs and displays. Acknowledgment. This work was supported by research grants from the National Natural Science Foundation of China (Project No. 90101019 and 50028302) and Ministry of Science and Technology, Project No. 2002CB613402 CM034650O (31) (a) Brown, A. R.; Bradley, D. D. C.; Burroughes, J. H.; Friend, R. H.; Greenham, N. C.; Holmes, P. L.; Kraft, A. Appl. Phys. Lett. 1992, 61, 2793. (b) Aratani, S.; Zhang, C.; Pakabaz, K.; Hoger, S.; Wudl, F.; Heeger, A. J. J. Electron. Mater. 1993, 22, 745. (c) Zhang, C.; Hoger, S.; Pakbaz, K.; Wudl, F.; Heeger, A. J. J. Electron. Mater. 1994, 23, 453. (32) Prutting, W.; Berleb, S.; Muckl, A. G. Org. Electron. 2001, 2, 1.