Highly efficient green light-emitting diodes with aluminium cathode

ELSEVIER

Synthetic

Metals 84 (1997)

615618

Highly efficient green light-emitting diodes with aluminium cathode Do-Hoon

Hwanga, Sung Tae K.imbfc, Hong-Ku Sh.imd, Andrew B. Holmes ale, Stephen C. Morattic and Richard H. Friendb aUniversity Chemical Laboratory, Department of Chemistry, Lensfield Road, Cambridge CB2 IEW, UK bCavendish Laboratory, Department of Physics, Madingley Road, Cambridge CB3 OHE, UK ‘Melville Laboratory for Polymer Synthesis, Department of Chemistry, Pembroke Street, Cambridge CB2 3RA, UK dDepartment of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea

Abstract A novel silyl-substituted solvent processible poly(l,4-phenylenevinylene) (PPV) derivative, poly(2-dimethyloctylsilyl-1,4phenylenevinylene) (DMOS-PPV) and copolymers of DMOS-PPV and poly(2-methoxy-5-ethylhexyloxy-l,4-phenylenevinylene) (MEH-PPV) have been synthesized by the dehydrohalogenation route, and the light-emitting properties of these polymers have been studied. Electroluminescent devices were fabricated with these polymers as emitting layers, and IT0 and Al as anode and cathode, respectively. A single layer device using DMOS-PPV as the emissive layer showed 0.2 % internal quantum efficiency. Single layer devices using the copolymers showed internal quantum efficiencies between 0.1 - 0.02 % depending on the copolymer compositions. Keywords:

1.

PoIy(phenylene

viny&e)

and derivatives; Electroluminescence,

Introduction

Light-emitting polymers have been extensively investigated in recent years since Burroughes et al. fist reported a green light-emitting diode (LED) using poly(l,4-phenylene vinylene) (PPV) as an emitting layer [l-4]. Organic polymer LEDs have many advantages for the development of a largearea visible light-emitting display, such as good processibility, low operation voltage, fast response time, and color tunability over the full visible range by control of the HOMO-LUMO band gap of the emissive layer. PPV has been most widely used as the emissive layer in light-emitting diodes, and has been prepared through a thermal elimination process from water- [S] or organic solvent-soluble precursor polymers [6-g]. Several organic solvent soluble PPV derivatives have been developed in order to improve processibility [g-11]. In this paper, we report the synthesis of new silyl-substituted soluble PPV derivatives, poly(2dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS-PPV) and copolymers of DMOS-PPV and MEH-PPV, poly(DMOSPV-coMEHPV)s. Single layer EL devices have been fabricated using these polymers as the emissive layers. The synthetic route is outlined in Scheme I. 2. Results

and

Discussion

The polymers DMOS-PPV and poly(DMOSPV-co-MEHPV)s were totally soluble in common organic solvents such as chloroform, tetrahydrofuran and toluene. The compositions of the copolymers were determined by ‘H-NMR. The polymer compositions are very similar to the feed monomer ratios, suggesting that the two monomers have almost equal 03794779/97/$17.00 0 1997 Elsevier PII SO379-6779(96)04076-3

Science S-A. All ri$htsreserved

Diodes

reactivity to polymerization. Molecular weights of the copolymers were determined by GPC using polystyrene as a calibration standard. Table 1 shows the compositions, molecular weights and polydispersities of the copolymers. Schema

1

SL BrL&Br

+

“+C, H&O

KO’Bu,

THF

616

D.-H. Hwang

Table 1. Compositions, Polymers

molecular

Met&

84 (1997

weights, polydispersities

lOa-poly(DMOSPV-coMEHpv)

Feed Monomer Ratio (DMOSPVMEHPV) Polymer Composition Molecular

et al. /Synthetic

and polymer yields of the copolymers

88a-poly(DMOSPV-coWHW)

40a-poly(DMOSPV-comm)

10 : 1

Weight

615-618

1 : 10

1:l

90: 10

60 : 40

12 : 88

82,000

93,000

58,000

4.3

4.8

5.3

65 %

72 %

Ww)

Polydispersity Polymer Yield

80 %

aThese numbers stand for the mole % of MEHPV

250

300

3.50

400

450

Wavelength Figure 1. UV-VIS

500

550

600

units incorporated

in the copolymers.

460520

650

560600

(nm)

spectra of the polymers

Figure 1 shows the UV-VIS spectra of DMOS-PPV, poly(DMOSPV-co-MEHPV)s and MEH-PPV. DMOS-PPV shows a slightly narrower absorption band than unsubstituted PPV. The absorption maximum and edge of the DMOS-PPV are at about 414 nm and 500 run, respectively. In the copolymers, the absorption maxima and band edges shifted to longer wavelength as the proportion of MEHPV units into copolymers increased. Figure 2 shows the PL emission spectra of the polymer films. DMOS-PPV shows an emission maximum at about 520 nm which corresponds to the green region, The emission maxima of the copolymers shifted to longer wavelength as the proportion of MEHPV units into copolymers increased. 88-Poly(DMOSPV-co-MEHPV) shows an emission maximum at about 630 M-I which corresponds to an orange-red color. The absolute photoluminescence quantum efficiency for a solid film of DMOS-PPV was 60 %. By comparison, the reported PL efficiencies of PPV and MEHPPV are 27 % and 15 %, respectively [12]. In the copolymers, the PL efficiencies decrease as the proportion of MEHPV units into copolymers increases. Figure 3 shows the current density-electric field characteristics measured for a typical ITO/DMOS-PPV/Al device with film thickness of 700 A. The forward current density increases with increasing forward bias field and the

640600

720760

Wavelength Figure 2. Photoluminescence

600

(nm)

spectra of the polymers.

curve shows typical diode 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 cmw2. The devices showed reproducible internal quantum efficiencies of 0.2 %. In the devices from copolymers, the EL efficiencies decreased as the MEHPV units in copolymers increased.

g a g .g $E E E

5 0

0.3 0.3 0.2 0.2 j 0.1

7 5

. .

f g E 5

. ; : 0

5 “cd&

15

WV)

20

0.05 0

Electric

Field

Figure 3. I-V and V-L characteristics I’IO/DMOS-PPV/Al.

(V/cm)

of the single layer device

D.-H.

Table 2. Absorption

maximum,

et al. /SyntheticMetals

PL, EL efficiencies 10aPoly(DMOSPVco-MEHPV)

DMOS-PPV

Polymers

Hwang

84 (1997)

40aPoly(DMOSPV-coMEHPV)

414

nm

420

nm

444nm

PL emission maximum PL Efficiency

523

nm

567

nm

600

EL Efficiency Turn-on

Voltages

41%

0.2 %

0.1 %

15 v

12v

617

and turn-on voltages of the devices from the polymers

UV Amax

60%

615-618

464 nm

21 % 0.03

%

7V

aThese numbers stand for the mole % of MEHPV

88aPoly@MOSPVCO-MEHPV)

units incorporated

nm

618 run 18 % 0.02

%

6V

MEH-PPV

500

nm

626

nm

15 % 0.02

%

4v

in the copolymers.

Synthesis of 2-dimethyloctylsilyl-1,4-bis(bromo methyl) benzene. To dimethyloctyl-p-xylene (4 g, 14.5 mmol) in carbon tetrachloride (40 mL) was added Nbromosuccinimide (5.34 g, 30.0 mmol) and benzoyl peroxide as initiator. The reaction mixture was heated to reflux at 90 OC for 5 h under a nitrogen atmosphere. After evaporation of the solvent, a yellowish oil was obtained. Chromatography on a silica column using hexane as eluent gave the brominated product as a colorless oil (3.01 g, 48 %). lH-NMR (CDC13, 200 MHz) : 6 7.48 (lH, s), 7.43 (2H, s), 4.61 (2H, s), 4.48 (2H, s), 1.43-1.19 (12H, m), 0.99-0.81 (5H, m), 0.42 (6H, s) [Found C, 49.4; H, 6.8. C18H30Br2Si requires C, 49.78; H, 6.96 %] 0

5

10

Voltage

15

20

(V)

Figure 4. L-V characteristics of the single layer devices (ca. 70 nm film thickness) using(a) 88-, (b) 40- and (c) lopoly(DMOSPV-co-MEHPV) The turn-on voltages of the devices using the copolymers decreases as the MEHPV units in copolymers increase. Figure 4 shows the L-V characteristics of the devices using copolymers. The absorption maximum, PL, EL efficiencies and turn-on voltages of the polymers are summarized in Table 3.

Experimental

Synthesis of dimethyloctylsilyl-p-xylene. To 2bromo-p-xylene (10 g, 54.03 mmol) in anhydrous THF was slowly added clean magnesium tunings (3.9 g, 160.43 mmol) after initiation by 5 mol % of 1,2-dibromoethane at 70 OC. When the magnesium turnings had been completely consumed, chlorodimethyloctylsilane (13.41 g, 64.84 mmol) was added. The mixture was heated to reflux for 3 h, and the reaction was quenched with dilute aqueous HCl solution. The THF layer was separated, washed with water several times, dried and the solvent was removed on the rotary evaporator. The residue was vacuum distilled to give the silyl product (6.72 g, 45 %). b.p. 120-121 OC at 0.4 mm Hg. lH-NMR (CDC13, 200 MHz) : 6 7.28 (lH, s), 7.09 (2H, s), 2.43 (3H, s), 2.34 (3H, s), 1.411.14 (12H, m), 0.97-0.75 (5H, m), 0.33 (6H, s) [Found C,: 76.0; H,ll.S. C18H32Si requires C, 78.18 ; H,11.35 %]

Synthesis of poly(2-dimethyloctylsilyl-1,4-phe -nylenevinylene). A solution of 2-dimethyl-octylsilyl1,4-bis(bromomethy1) benzene (0.5 g, 1.15 mmol) in dry THF (20 mL) was degassed by purging with nitrogen at room temperature. To the stirred solution was added dropwise, over 20 min at 20 ‘C, a degassed solution of 95 % potassium tertbutoxide (0.58 g, 5.17 mmol) in dry THF (20 mL). The reaction mixture became progressively green and viscous during the addition. The highly viscous reaction mixture was stirred at 20 OC under an atmosphere of argon for 16 h, after which it was poured onto methanol (300 mL) with stirring. The resultant yellow precipitate was filtered, dissolved in the minimum quantity of THF, and then reprecipitated by pouring onto methanol (300 mL) and dried in a drying pistol under vacuum at 20 OC. The polymer yield was 87 % (0.27 g). GPC measurement of this polymer with polystyrene as the calibration standard showed a Mw of 1.1 x lo6 Da and polydispersity index of 7.2. [Found , 77.5; H, 10.1. C18H28Si requires C, 79.30; H,10.40 %] FT-IR (NaCl) Vmax 2955, 2922, 2853, 1729, 1468, 1377, 1251, 1137, 1066, 961, 836 cm-l. The copolymers were synthesized by the same method using a mixture of 2-dimethyloctylsilyl-1,4-bis(bromomethy1) benzene and 2-methoxy-5-ethylhexyloxy-1,4-bis(chloro -methyl) benzene by changing feed monomer ratios. Device Fabrication and Measurement. The DMOSPPV (20 mg) was dissolved in chloroform (4 mL) and then filtered through a 0.45 mm filter to remove particle impurities.

618

D.-H.

Hwang

et al. /SyntheticMetals

The polymer solution was deposited as a film on ITO-coated glass using a spin-coater at 2000-2500 rpm, depending on the desired thickness (70-100 nm). Electron-injecting aluminum or calcium electrodes were deposited onto the surface of the polymer film by vacuum evaporation at pressures below 7 x IO6 mm Hg. The Al electrode was typically 200 nm thick and the Ca electrode consisted of calcium (cu. 200 nm) and a protecting layer of aluminum (cu. 50 run). The film thickness was measured with a Dektak-II surface profiiometer. The active emissive area defined by the cathode was about 4 mm2. All the devices with the Al electrode were fabricated in air while those with Ca were processed in a nitrogen-filled glove box. Ultraviolet-visible absorption spectra were measured with a commercial Perkin-Elmer-19 spectrophotometer. Photoluminescence and electroluminescence spectra were recorded using a single-grating CCD spectrometer (Oriel Instaspec IV). All the device measurements were carried out in vacuum (about 0.1 mmHg). Acknowledgement. We thank the Korea Science and Engineering Foundation and the British Council (D.H.H), the European Commission (Brite Euram Project, BRE2-CT930592 ‘PolyLED’), the Engineering and Physical Sciences Research Council (UK), and LG Electronics (S.T.K) for financial support. References [I] Burroughes, J.H.; Bradley, D.D.C.; Brown, A.R.; Marks, R.N.; Mackay, K.; Friend, R.H.; Burn, P.L.; Holmes, A.B. Nature 347 (1990) 539.

84 (i997/

615618

[2] Greenham, NC.; Moratti, SC.; Bradley, D.D.C.; Friend, R.H.; Holmes, A.B. Nuture 365 (1993) 628. [3] Gustafsson, G.; Cao, Y.; Treaty, G.M.; Klavetter, F.; Colaneri, N.; Heeger, A.J. Nature 357 (1992) 477. [4] Burn, P.L.; Holmes, A.B.; Kraft, A.; Bradley, D.D.C.; Brown, A.R.; Friend, R.H.; Gymer, R.W. Nature 356 (1992) 47. [S] Wessling, R.A.; Zimmerman, R.G. U.S. Pat. No. 3 401 152 (1968) and No. 3 706 677 (1972). [6] Louwet, F.; Vanderzande, D.; Gelan, J.; Mullens, J. Macromolecules, 28 (1995) 1330. ---. [7] Burn, P.L.; Bradley, D1D.C.f Friend, R.H.; Halliday, D.A.; Holmes, A.B.; Jackson, R.W.; Kraft, A. J. Chern. Sot. Perkin Trans. 1 (1992) 3225. [8] Son, S.; Dodabalapur, A.; Lovinger, A.J.; Galvin, M.E. Science 269 (1995) 376. [9] Wudl, F.; Allemand, P.M.; Srdanov, G.; Ni, Z.; McBranch, D. in Materials for Nonlinear Optics: Chemical Perspectives (eds. Marder, S.R.; Sohn, J.E.; Stucky, G.D.) (ACS Symposium Series 455 (1991) 638) [lo] Wudl, F.; HBger, S.; Zhang, C.; Pakbaz, K.; Heeger, A.J. Polym. Prepr. 34 (1993) 197. [ll] Higer, S:; McNamara, J.J.; Schricker, S.; Wudl, F. Chem. Mater. 6 (1994) 171. [12] Greenham, NC.; Samuel, I.D.W.; Hayes, G.R.; Philips, R.T.; Kessener, Y.A.R.R.; Moratti, SC.; Holmes, A.B.; Friend, R.H. Chem. Phys. Lett. 241 (1995) 89. \--

-I