摘要:
Green light-emitting diodes from poly(2-dimethyloctylsilyl-1,4=phenylenevinylene) Do-Hoon Hwang,a Sung Tae Kirn,b,c Hong-Ku Shim,d Andrew B. Holrnes,*aTc Stephen C. MorattiC and Richard H. Friendb a University Chemical Laboratory, Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW Cavendish Laboratory, Department of Physics, Madingley Road, Cambridge, UK CB3 OHE Melville Laboratory for Polymer Synthesis, Department of Chemistry, Pembroke Street, Cambridge, UK CB2 3RA Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701,Korea A novel silyl-substituted solvent processable poly( 1,4-phe- nylenevinylene) (PPV) derivative, poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) is synthesized by the dehydrohalogenation route from 2-dimethyloctylsilyl-1,4-bis(bromomethyl)benzene, and the light-emitting properties of the polymer are studied; single layer electroluminescent devices (ITO/polymer/Ca or Al) exhibit an emission max- imum at 520 nm with internal quantum efficiency in the range 0.243%. Light-emitting polymers have been extensively investigated in recent years since the Cambridge group first reported a green light-emitting diode (LED) using poly( 1,4-phenylenevinylene) (PPV) as an emitting layer.14 Organic polymer LEDs have many advantages for the development of a large-area visible light-emitting display, because of the good processability, low operation voltage, fast response time and colour tunability over the full visible range by control of the HOMO-LUMO bandgap of the emissive layer.PPV has been most widely used as the emissive layer for the light-emitting diodes, and has been prepared through a thermal elimination process from a water-5 or organic-soluble precursor polymer.”* Several organic sol- vent soluble PPV derivatives have been developed in order to improve processability.9-11 Recently, Zhang et al. reported the improved quantum efficiency in green polymer light-emitting diodes with a silyl-substituted soluble PPV derivative, poly(2- cholestanoxy-5-thexylsilyl-174-phenylenevinylene) (CS-PPV).lI They reported12 that CS-PPV showed high quantum efficiency with an air-stable aluminium electrode by adding an electron transporting molecular dopant, 2-(4-biphenyl)-5-(4-tert-butylphenyl)- 1,3,4-0xadiazole (PBD).13314 The effects of silicon substitution on the luminescence properties are of interest in the field of polymer LEDs, and here we report the synthesis of a new silyl-substituted soluble PPV derivative, poly(2-dimethyloctylsil yl-174-phenylenevinylene) (DMOS-PPV). Single layer electroluminescent (EL) devices have been fabricated using this polymer as the emissive layer. The synthetic route is outlined in Scheme 1. Silylation of the Grignard reagent derived from 1 afforded the silyl derivative 2 which after radical bromination gave the dibromo-compound 3.1. Dehydrohalogenation condensation polymerization afforded DMOS-PPV 4.15916 DMOS-PPV 4 is completely soluble in common organic solvents such as chloroform, tetrahydrofuran and toluene without evidence of gel formation. Fig.1 shows the UV-VIS, photoluminescence (PL) and EL spectra of the DMOS-PPV film. DMOS-PPV 4 shows a slightly narrower absorption band compared with PPV. The absorption maximum and edge of the DMOS-PPV are at ca. 414 and 500 nm, respectively, at room temperature. These positions are blue- shifted compared with those of the unsubstituted PPV (420 and 530 nm, respectively), presumably owing to the steric effect of the bulky dimethyloctylsilyl group. DMOS-PPV 4 shows an emission maximum at ca. 520 nm which corresponds to the green region. The absolute photo- luminescence quantum efficiency for a solid film of DMOS- PPV was 60%.By comparison, the reported PL efficiencies of PPV and MEH-PPV are 27 and 15%, re~pective1y.l~ Fig.2 shows the current density-electric field characteristics measured for a typical ITODMOS-PPV/Al device with film thickness of 700 A. The forward current density increases with increasing forward bias field and the curve shows typical diode r-Br \F- M e e M e Mg, THF * M C8HI7SiMe&I e e M e 1 45% 2 48% Si-KOBut,THF 6* 87% CH=CH 3 4 Scheme 1 200 300 400 500 600 700 800 A/nm Fig. 1 UV-VIS (crosses), PL (open circles) of DMOS-PPV film and EL (solid circles) of the ITODMOS-PPV/A1 device Chem. Commun., 1996 2241 characteristics. The voltage dependence of emission intensity from the device shows that light emission becomes observable at a bias of about 15 V at a current density of 0.93 mA cm-2.The devices showed reproducible internal quantum efficiencies of 0.2% (ca. 0.05% external efficiency).$ Fig. 3 shows the current Gensity-field characteristics of an ITO/DMOS/Ca device (700 A thickness). The threshold voltage of the device was about 11 V at a current density of 1.8 mA cm-2. The measured maximum internal quantum efficiency of the diode was 0.3% (ca. 0.1% external efficiency). These values compare favourably with the efficiency of the single layer green polymer LED reported by Son et aZ.8 Recently we have fabricated multilayer EL devices with DMOS-PPV and various charge-transporting materials, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-0xadiazole (PBD) or poly(aromatic oxadiazo1e)s.These devices showed highly improved quantum efficiencies using an aluminium cathode. The DMOS-PPV film has good processability and the high quantum efficiency may make it a good candidate for application in polymer LEDs. 0.35 r -0.30 0 0 a 0.25 -0 0 N -E 0.20E a -a E 0.75 '\ 0 5 10 15 20 0.10 -Voltage IV a -0 0.05 a 0.00 I 0 5 10 15 20 25 30 10-5 E/v cm-1 Fig. 2 Current density-electric field and light intensity-voltage (inset) characteristics of ITO/DMOS-PPV/Al device 2.5 r 0 -32.0 CrJ. 0=-. ._ rnc0 c11.5 --0 00 EE E 8 a 0E -\ 1.0 0_1'Y 20 nc; I-4 10-5E/v cm-' Fig. 3 Current density-electric field and light intensity-voltage (inset) characteristics of ITO/DMOS-PPV/Ca device We thank the Korea Science and Engineering Foundation and the British Council (D.H. H.), the European Commission (Brite Euram Project, BRE2-CT93-0592 'PolyLED'), the Engineering and Physical Sciences Research Council (UK) (Swansea mass spectrometry service, Daresbury databaselg), and LG Electron-ics (S. T. K.) for financial support. Footnotes t Selected spectroscopic data. 2, IH NMR (CDC13,200 MHz) 6 7.28 (1 H, s), 7.09 (2 H, s), 2.43 (3 H, s), 2.34 (3 H, s), 1.41-1.14 (12 H, m), 0.97-0.75 (5 H, m), 0.33 (6 H, s); 3, 'H NMR (CDC13, 200 MHz) 6 7.48 (1 H, s), 7.43 (2 H, s), 4.61 (2 H, s), 4.48 (2 H, s), 1.43-1.19 (12 H, m), 0.99-0.81 (5 H, m>,0.42 (6 H, s); DMOS-PPV 4, GPC (polystyrene standard) showed a M, of 1.1 x lo6 and polydispersity index of 7.2.(Found: C, 77.5; H, 10.05. Calc. C, 79.34; H, 10.36%). FTIR (NaC1) vmax/cm-I 2955, 2922, 2853, 1729, 1468, 1377, 1251, 1137, 1066,961, 836. 4 The internal efficiency is a factor of 2n2 larger than the external efficiency where n is the refractive index of the emissive layer.18 References 1 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Bum and A. B. Holmes, Nature, 1990, 347, 539. 2 N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend and A. B. Holmes, Nature, 1993, 365, 628. 3 G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri and A. J. Heeger, Nature, 1992, 357 477.4 P. L. Bum, A. B. Holmes, A. Kraft, D. D. C. Bradley, A. R. Brown, R. H. Friend and R. W. Gymer, Nature, 1992,356,47. 5 R. A. Wessling, J. Polym. Sci., Polym. Symp., 1985, 72, 55. 6 F. Louwet, D. Vanderzande, J. Gelan and J. Mullens, Macromolecules, 1995,28, 1330. 7 P. L. Bum, D. D. C. Bradley, R. H. Friend, D. A. Halliday, A. B. Holmes, R. W. Jackson and A. Kraft, J. Chem. SOC., Perkin Trans. I, 1992, 3225. 8 S. Son, A. Dodabalapur, A. J. Lovinger and M. E. Galvin, Science, 1995, 269, 376. 9 F. Wudl, P. M. Allemand, G. Srdanov, Z. Ni and D. McBranch, in Materials for Nonlinear Optics: Chemical Perspectives, ed. S. R. Marder, J. E. Sohn and G. D. Stucky, ACS Symp. Ser., 1991,45, 683. 10 F. Wudl, S. Hoger, C. Zhang, K.Pakbaz and A. J. Heeger, Polym. Prepr., 1993, 34, 197. 11 S. Hoger, J. J. McNamara, S. Schricker and F. Wudl, Chem. Muter., 1994, 6, 171. 12 C. Zhang, S. Hoger, K. Pakbaz, F. Wudl and A. J. Heeger, J. Electron. Muter., 1994, 23, 453. 13 P. L. Bum, A. B. Holmes, A. Kraft, A. R. Brown, D. D. C. Bradley and R. H. Friend, Muter. Res. SOC. Symp. Proc., 1992,247,447. 14 A. R. Brown, D. D. C. Bradley, J. H. Burroughes, R. H. Friend, N. C. Greenham, P. L. Bum, A. B. Holmes and A. Kraft, Appl. Phys. Lett., 1992, 61, 279. 15 H. G. Gilch and W. L. Wheelwright, J. Polym. Sci. A-1, 1966, 4, 1337. 16 W. S. Swatos and B. Gordon, Polym. Prepr., 1990,31, 505. 17 N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Philips, Y. A. R. R. Kessener, S. C. Moratti, A. B. Holmes and R. H. Friend, Chem. Phys. Lett., 1995, 241, 89. 18 N. C. Greenham, R. H. Friend and D. D. C. Bradley, Adv. Muter., 1994, 6, 491. 19 D. A. Fletcher, R. F. McMeeking and D. Parkin, J. Chem. 1nf. Comput. Sci., 1996, 36, 746. Received, 24th June 1996; Corn. 6l04364J 2242 Chem. Commun., 1996
ISSN:1359-7345
DOI:10.1039/CC9960002241
出版商:RSC
年代:1996
数据来源: RSC