Conjugated Polymer Blends as Emitting Layer for White Light LED

Chapter 11

Conjugated Polymer Blends as Emitting Layer for White Light LED Show-An Chen and En-Chung Chang

Downloaded by MONASH UNIV on May 23, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch011

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China

Studies on white light organic light emitting diodes with organic molecules and polymers as emitting materials are reviewed. The white light can be obtained i n the following ways: (1) single-emitting materials, such as metal complex o f 2(2-hydroxyphenyl) benzothiazole with zinc (Zn(BTZ) ) and emeraldine base poly-aniline; (2) poly(vinyl carbazole) ( P V K ) doped with red, green and blue emitting organic materials; (3) three emitting organic layers each emitting red, green and blue lights; (4) polymer blends, i n which the major and minor (or trace) components are blue and red light emitting materials, respectively; (5) bilayer with good mixing at the interface, in which exciton and exciplex emissions occur. 2

1. Introduction Organic light emitting diodes ( O L E D ) have drawn great attention for they process several advantages over the conventional L E D s , such as lower power consumption, large area and easy to fabricate and over the liquid crystal display ( L C D ) such as wide view angle, lower cost, and no need o f backlight. O L E D s involve the two categories i n accordance with their emitting materials: O M L E D with organic molecular dye as emitting materials and polymer L E D ( P L E D ) with conjugated polymers as emitting materials. Extensive studies on O L E D began from the discovery o f the thin device: I T O / d i a m i n e / A l q / M g : A g prepared by thermal evaporation by Tang and Vanslyhe (7) in 1987; this device has the brightness 1000 cd/m at the bias 10 V, and external quantum efficiency 1 %. Later the Cambridge group (2) i n 1990 found that poly (phenylene vinylene) ( P P V ) can be used as light-emitting layer for L E D s , which can emit light as bright as 500 cd/m at 8 V (film thickness about 1000 A ) when using calcium as the cathode material. Introducing side groups at the phenylene ring and/or vinylene unit allow a control o f the color of the emitted light by 3

2

2

©1999 American Chemical Society

In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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providing a blue (or red) shift as electron withdrawing (or donating) groups are introduced, i n addition to an increase in solubility. A side chain that imparts a steric hindrance to the conjugation could also lead to a blue shift. Considerable efforts on the molecular design o f conjugated polymers for tuning the color o f emitted light through adjustment of the bandgap has been attempted. Other conjugated polymers that have been found to impart the luminescent property are poly (p-phenylene)s and polythiophenes (3-7). O L E D can be fabricated to emit light with various colors from red to blue covering the entire visible range. It can also emit white light by use o f a proper single emitting material or by use o f a polymer blend or multi-emitting layers each emitting different colors. The chemical structures o f the materials involved i n this article are listed on Figure 1. Such O L E D s have wide range of applications as displays and backlight sources for L C D , flat television screens, personal computer displays and wristwatch. This paper reports a survey on the research works i n this area. 2. White light from a single emitting material Hamada et. al. (1996) (8) have synthesized a metal complex o f 2-(2hydroxyphenyl) benzothiazole ( B T Z ) with zinc (Zn) to yield Z n ( B T Z ) which can emit greenish-white light from the device I T O / T P D / Z n ( B T Z ) / M g I n (Figure 2), i n which T P D behaves as a hole transport layer ( H T L ) . Its chromaticity coordinates are x=0.246 and y=0.364, and brightness over 10,000 cd/m at the bias 8 V . 2

2

2

Our laboratory (1996) (9) has found that the emeraldine base polyaniline (PAn) can emit nearly white light covering the full range o f visible light (380750 nm) (Figure 3) from the device with ITO coated glass as the hole ejector and deposited aluminum (or magnesium) thin film as the electron ejector. It is found that the phase with reduced repeat units (amine form) can emit white light, while the phase with oxidized repeat units (quinoid form) has no contribution to the emitting light. The turn-on voltages for eye-observable light intensity are 13 V and 6 V for the L E D s with aluminum and magnesium electrodes, respectively at the thickness o f the emitting layer 800 A . The E L spectrum covering the range 300-750 nm is much broader than that o f its P L spectrum, 350-510 nm. 3. White light from multilayer O M L E D O M L E D s have been fabricated to emit white light i n two ways, one is by dispersing organic dyes (that can emit various colors o f light) i n polymer as the emitting layer, and the other is by depositing several emitting layers, each emitting different colors o f light. K i d o and coworkers (1994) (10) have constructed a white light emitting device using P V K doped with various dyes T P B (5 mole %, blue light), coumarin 6 (0.3 mole %, green light) and D C M 1 (0.2 mole %, orange light) as the emitting layer and using T A Z and A l q as E T L . The structure o f the device so constructed is ITO/doped P V K / T A Z / A l q / M g : A g ; its E L spectrum is broad and covers the active visible range having the characteristic wavelengths: 450 ( from T P B ) , 510 (from coumarin 6) and 550 (from D C M 1 ) as shown in 3

3


165

CH,

CH,

QQ

Diamine

Zn(BTZ)

Akfe

2

* -P Downloaded by MONASH UNIV on May 23, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch011

C=CH-CH=c'

CH,

>

HjC

TPD

o

B

TPB

2 ^

Coumarin 6

-'tcC^

-ogoo

DCMl

Nile Red

TAZ

CO N^O

H,CO

p-EtTAZ Figure 1(a).

OCH,

NAPOXA

The chemical structures o f the organic molecular materials involved i n this article. Diamine: diamine derivative A l q : tris(8-hydroxyquinoline)aluminum Z n ( B T Z ) : bis(2-(2-hydroxyphenyl)benzothiazolate)zinc TPD:N,N'-diphenyl-N,N -bis(3-mathlphenyl)-l ,1 '-biphenyl4,4'-diamine 3

2

,

T P B : 1,1,4,4,-tetraphenyl-l ,3-butadiene Coumarin 6: 3-(2-benzothiazolyl)-7-(diethylamino)coumarin D C M 1: 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4//-pyran TAZ: 3-(4'-tert-butylphenyl)-4-phenyl-5-(4' '-biphenyl-1,2,4triazole N i l e Red: N i l e Blue A oxazone p - E t T A Z : 1,2,4-triazole derivative N A P O X A : 2-naphthyl-4,5-bis(4-methoxy-phenyl)-l ,3-oxzole

Continued on next page. In Semiconducting Polymers; Hsieh, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

166

PPV

PAn CH

r

3

H H 1

HC 3

c=o 1 OCH

PMOT

PVK CH 8


17

3

PMMA

17

^O(CH ) 2

OC,oH

C,H

n

1

0

O-^^CH=CH-^y-CH=CHH^^)-^

2l

ether-PPV

C O-CNPPV 1 0

PC12H25 cCH, CH, C-O' CHj

+m3 PN

Durel

C120-PPP

Figure 1(b). The chemical structures o f the polymers involved i n this article. PPV: poly(phenylene vinylene) PAn: polyaniline PMMA: poly(methyl mathacrylatte) PVK: poly(N-vinylcarbazole) PMOT: poIy(3-methyl-4-octylthiophene) PCHT: poly(3-cyclohexylthiophene) PTOPT:poly[3-(4-octylphenyl)-2,2 -bithiophene] PSA: tristilbene amne PPDB: poly(perylene-co-diethylbenxene) ,

m-LPPP: methyl substituted laddertype poly(paraphenylene) C10O-CNPPV: cyano-substituted poly(2,5didecyloxy-p-phenylene vinylene) Ether-PPV: ether-type phenylene vinylene based copolymer PN: polynobornene Durel: polyarlate Durel C12)-PPP: poly(2-dodecyloxy-/?-phenylene



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800

Wavelength (nm) Figure 2.

300

E L spectrum for I T O / T P D / Z n ( B T Z ) / M g : I n . 2

400

500

600

700

800


E L spectra for I T O / P A n / M g / A g and I T O / R - P A n / M g / A g .


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Figure 4 (curve a). The turn-on voltage o f the device is 4V, and has the brightness 50 cd/m at 10 V and 3400 cd/m at 14 V . Later (1995), they have also prepared a white light emitting device in the same way, the dopants used are T P B , coumarin 6, D C M 1 and N i l e red (Figure4 curve b) (11). Their device has the brightness 4100 cd/m at 20 V. However, effects o f the applied voltage on the color o f emitted light have not been reported. 2

2

2

K i d o and co-workers (1995) (12) have prepared a multi-emitting-layer white light O M L E D having the structure ITO/TPD(400 A)/p-EtTAZ(30 A)/Alq (50 A)/Nile Red-doped (1 mol %)Alq (50 A)/Alq3(400 A)/Mg:Ag, in which red, green and blue ( R G B ) emitting materials are included (Figure4 curve c) and this device has the brightness 2200 cd/m at the bias 16 V . Strukelj et al (13,14) i n 1996 have added the blue light emitting materials (2-naphthyl-4,5-bis(4-methoxy-phenyl)-l,3-oxzole, N A P O X A ) to the H T L , bis(triphenyl)diamein ( T A D ) , and to the E T L tris(8-hydroxy quinoline) aluminum (Alq3) and in addition, the red light emitting material D C M 1 to the latter to yield the device, I T O / T P D / N A P O X A ( b l u e ) / A l q / D C M l ( 0 . 5 % , r e d ) doped A l q / A I Q / L i : A l , which emits white light (Figure 5). This device has the brightness, 4000 cd/m at 15 V and external quantum efficiency greater than 0.5 %. 3

3


2

3

2

300

400

500

600

Wavelength (nm)

700

800

Figure 4. E L spectra for (spectrum a) ITO/dye-doped P V K / T A Z / A l q / M g : A g ( P V K is doped with 5 mol % T P B , 0.3 mol % Coumarin 6, and 0.2 mol % D C M 1), (spectrum b) ITO/dye-doped P V K / M g : A g ( P V K is doped with 30 wt % P B D , 3 mol % T P B , 0.04 mol % Coumarin 6, 0.02 mol % D C M 1, and 0.015 mol % N i l e Red), and (spectrum c) ITO/TPD/p-EtTAz/Nile Red-doped A l q / A l q / M g : A g .


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/A

/

\

^ Downloaded by MONASH UNIV on May 23, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch011

300

400

500

600

a 700

800

Vtevelength (nm) Figure 5. E L spectra

for

(spectrum

a)

ITO/TAD/NAPOXA/Alq/Al

and

(spectrum b) I T O / T A D / N A P O X A / A l q / A l q 3 + D C M 1/Alq/Al. 3

4. P o l y m e r blend as the emitting material Inganas and coworkers (1995) (15) have used the blend o f three polythiophene derivatives, which can emit red, green, and blue light respectively, as the emitting layer. This device so constructed allows a turning o f the color o f the emitting light by varying the applied voltage. A t the higher voltage 20 V , as all components are activated, the device can emit white light. The variation i n the color o f emitting light is resulted from the occurrence o f phase separation i n the blend, i n which the domain size o f each component is greater than the thickness o f the emitting layer causing that each component has its own passways for the charge carriers. They then introduced poly(methyl methacrylate) ( P M M A ) i n the blends o f the polythiophenes at the weight ratio, P M O T (blue): P C H T (green): P T O P T (red): P M M A (10:4:1:1), and obtained white light at 20 V having a quantum yield o f 0.4-0.6% (16). However, at the lower voltages 5 V and 12 V, the emission from the red light polymer (PTOPT) is dominant. Leising, M u l l e n and coworks (1997) (17,18) found that even by introduction only a very small amount about 1% o f red-light-emitting polymer, poly(perylene-co-diethylbenzene) ( P P D B ) , into the blue-light-emitting ladder type polymer, ladder type poly(paraphenylene) (m-LPPP), the P L and E L spectra appear as those from P P D B . This is due to the excitation energy transfer from m - L P P P to P P D B . The color o f the emitted light can be varied and, i n addition, the quantum yield can be increased by adjusting the amount o f P P D B . A s the amount o f P P D B is reduced to 0.05%, the device emits white light (as shown i n Figure 6), which does not vary with the applied voltage.


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3

300

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E L spectrum for I T O / m - L P P P : P P D B ( P P D B 0.05 % ) / A l .

We have studied E L spectra from the blends o f P C 1 0 with C O - C N P P V in 1998 (19). The devices with the blends (the weight ratio o f P C 1 0 / C O C N P P V , 1/1 and 4/1) emit light with various colors from red to yellow-orange light, depending on the applied voltage. When the ratio is 150/1, it can emit the light with broad spectrum near white light covering the entire visible region at all applied voltages (as shown i n Figure 7). Similar results are also obtained from the blend o f P C 1 0 / R O - P P V at the weight ratio, 14/1. l 0

10

Figure 7. E L spectrum for ITO/ether-PPV:C O-CNPPV(150:l)/AL 10

The optical properties o f the blend systems, P C 1 0 with C O - C N P P V , were further investigated using P L , photoluminescent excitation ( P L E ) and near field scanning optical microscopy ( N S O M ) . In these blends, a new emission in the P L and E L spectra appears, which can be attributed to the emission from an exciplex formation due to the energy transfer between the two polymers. The N S O M shows that the two components i n the blend system have good miscibility. A s the blend ratio 150/1 for the former blend, the minor component C , O - C N P P V is uniformly dispersed in the major component P C 1 0 having the domain size smaller than the film thickness o f the emitting layer. 1 0

0


171

A s a forward bias is applied, the charges can not pass through the domains o f C , O - C N P P V directly, and the light emitting from the component should be originated from the energy transfer. Thus, the color o f the emitting light from the device does not change with the applied voltage. Wendroff (1997) (20) found that as the emitting molecular material P S A in the blend o f polynobornene (PN) with polyarylate Durel at the weight ratio 1:2, its E L spectrum is broad and nearly white as shown i n Figure 8. This is a guest-host system with a chromophore as the guest molecule and a polymer blend as the host. P S A can form radical cations and is a hole transport material. Their P L spectra vary with host polymer, thus the host polymer blend can provide an emission of white light only at a particular composition. Downloaded by MONASH UNIV on May 23, 2013 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch011

0

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Wavelength (nm)

Figure 8. E L spectrum for the device I T O / P S A : D u r e l : P N / A l We recently (1998) found that bilayer polymer light-emitting diodes with two blue light-emitting materials, poly(N-vinylcarbazole) ( P V K ) and poly(2dodecyl-p-phenylene) ( C 1 2 0 - P P P ) , can emit blue or white light, depending on the solvent used i n the fabrication o f the second layer, C 1 2 0 - P P P (21). If hexane (the nonsolvent for P V K ) is used, the device emits blue light as the single layer device with C 1 2 0 - P P P . However, i f toluene (the cosolvent for the two polymers) is used, the device emits white light originating from an exciplex emission at the bilayer interface in addition to the exciton emission from the C 1 2 0 - P P P (as shown in Figure 9). A t l o w temperatures, the intensity o f the exciplex emission drops and that o f the exciton emission becomes dominant.

LU

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Wavelength (nm) Figure 9. E L spectra for (spectrum a) ITO/PVK/C80-PPP(n-hexane)/Ca/Ag and (spectrum b) ITO/PVK/C80-PPP(toluene)/Ca/Ag.



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6. References 1. C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett. 1987, 51, 914. 2. J. H . Burroughes, D. D. C. Bradley, A. R. Brown, R. N . Marks, K. Mackay, R. H . Friend, P. L . Burns, A . B. Holmes, Nature, 1990, 347, 539. 3. D. Braun, A . J. Heeger, Kroemer-H, J. Elec. Mater., 1991, 20, 945. 4. G. Grem, G. Leditzky, B. Ullrich, G. Leising, Adv. Mater., 1992, 4, 36. 5. Y. Ohmori, K. Yoshino, M . Uchida, Jpan. J. Appl. Phys., 1991, 11B, 1941. 6. D. Braun, A . J. Heeger, Appl. Phys. Lett., 1991, 58, 1982. 7. J. Gruner, P. J. Hamer, R. H. Friend, H. -J. Huber, U. Scherf, and A. B. Holmes, Adv. Mater., 1994, 6, 748. 8. Y. Hamada, T. Sano, H . Fujii, Y. Nishio, H . Takahashi, K. Shibata, Jpan. J. Appl. Phys., 1996, 35, L1339. 9. S. A. Chen, K. R. Chuang, C. I. Chao, H . T. Lee, Synth. Met., 1996, 82, 207. 10. J. Kido, K. Hongawa, K. Okuyama, K. Nagai, Appl. Phys. Lett., 1994, 64, 815. 11. J. Kido, H . Shionoya, K. Nagai, Appl. Phys. Lett., 1995, 67, 2281. 12. J. Kido, M . Kimura, and K. Nagai, Science, 1995. 267, 1332. 3. M . Strukelj, R. H . Jordan, and A . Dodabalapur, J. Am. Chem. Soc., 1996, 118, 1213. 14. R. H . Jordan, A . Dodabalapur, M . Strukelj, and T. M . Miller, Appl. Phys. Lett., 1996, 68, 192. 15. M . Berggren, O. Inganas, G. Gustasson, J. Rasmusson, M . R. Andersson, T. Hjertberg, and O. Wennerstorm, Nature, 1994, 372, AAA. 16. M . Granstrom, and O. Inganas, Appl. Phys. Lett., 1996, 68, 147. 17. S. Tasch, E . J. W. List, C. Hochfilzer, G. Leising, P. Schlichting, U. Rohr, Y Geerts, U . Scherf, K. Mullen, Phys. Rev. B, 1997, 56, 4479. 18. S. Tasch, E . J. W. List, O. Ekstrom, W. Graupner, G. Leising, P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Mullen, Appl. Phys. Lett., 1997, 71, 2883. 19. S. A . Chen, E. C. Chang, K. R. Chuang, C. I. Chao, J. H . Hsu, P. K. Wei, W. S. Farm, Polym. Prepr., 1998, 39, 105; paper submitted to Macromolecule. 20. T. Christ, A . Greiner, R. Snder, V. Stumpflen, J. H . Wendorff, Adv. Mater., 1997, 9, 219. 21. C. I. Chao, S. A . Chen,Appl.Phys. Lett., 1998, 73, No. 4. 1


Conjugated Polymer Blends as Emitting Layer for White Light LED

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