Aromatic-Amine-Containing Polymers for Organic Electroluminescent

Chapter 25

Aromatic-Amine-Containing Polymers for Organic Electroluminescent Devices

Downloaded by STANFORD UNIV GREEN LIBR on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch025

J. Kido, G. Harada, M. Komada, H. Shionoya, and K. Nagai Department of Materials Science and Engineering, Graduate School of Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan

We investigated the suitability of aromatic amine-containing polymers as active layers in organic electroluminescent devices. Polymers used in this study include poly(N-vinylcarbazole) (PVK), poly(N-substituted methacrylamide)s, poly(methacrylate) and poly(arylene ether)s. The device structures are a double-layer-type and a single-layer type. The double-layer-type devices consist of a polymer hole transport layer and an electron-transporting emitter layer. High luminance of over 14,000 cd/m was observed for some double-layer devices using the polymer hole transport layer. Single-layer devices with dye-dispersed PVK emitted white light with a luminance of over 4000 cd/m . This demonstrates that dye-dispersed polymer systems are quite useful to obtain white light. 2

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Organic electroluminescent (EL) devices are the subject of study by many researchers because of their potential application as light-emitting devices which operate at low drive voltages. These devices are injection-type, in which carriers, such as electrons (radical anions) and holes (radical cations), are injected into the organic emitter layer where they recombine. It is, therefore, necessary for the component organic materials to possess carrier-transporting properties as well as fluorescence properties. Although electroluminescence in organic molecules have been known since the 1960's (7), a major breakthrough was made in the 80's by Tang and VanSlyke who developed a device exhibiting practical brightness (2). Their device consisted of a double layer structure with an organic hole transport layer and a luminescent metal complex layer. The hole transport layer plays an important role in transporting holes and blocking electrons, thus preventing electrons from moving into the electrode without recombining with holes. Aromatic diamine derivatives, such as yV^V-diphenyl^,^V-bis(3-methylphenyl)-l,r-biphenyl-4,4'-diamine (TPD), have been widely used as a hole transport layer because of their high hole mobilities (10" cm /Vs) and good 3

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© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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film-forming properties (3-5). In these devices, degradation is partly caused by the crystallization of the organic molecules (6,7), which destroys the contact between organic layers. Therefore, the use of a less crystalline polymer is desirable to minimize the degradation of the device. In addition, the use of polymers will simplify the device fabrication processes because solution casting methods can be used to fabricate organic thin layers. In this paper, some of the aromatic amine-containing polymers synthesized in our group are introduced and the devices using these polymers are summarized. Examples of the devices using these polymers as a hole transport layer as well as an emitter layer are discussed. Experimental Section The structures of the polymers used are shown in Figure 1. Side-chain-type polymers, except poly(N-vinylcarbazole) (PVK), were synthesized by radical polymerizations of the corresponding monomers, and main-chain-type polymers were synthesized by the nucleophilic substitution reactions of the corresponding difluoro compounds and diphenols. All the polymers were purified at least three times by reprecipitation into methanol. The details of the syntheses will be published elsewhere. The structure of double-layer-type devices is an anode/polymer/emitter layer/cathode (Figure 2a). Single-layer type was an anode/polymer/cathode (Figure 2b). The emitter material in the double-layer device is tris(8quinolinolato)aluminum(III) complex, Alq, which has an electron drift mobility of 10" cm /Vs, and has been used as an emitter layer in organic EL devices (2, 8-10). Alq was synthesized from aluminum trichloride and 8-quinolinol using piperidine as a base, and was purified by recrystallization from Λ^/V-dimethylformamide/ethanol. The anode was ITO (indium-tin-oxide) that is coated on a glass substrate, having a sheet resistance of 15 Ω/Π, and the cathode was magnesium-silver (10:1). The polymer layer was formed by spin coating or dip coating. Alq was vacuum deposited at 2xl0" Torr onto the polymer layer with a thickness of 700 Â. Finally a 2000-Â-thick magnesium and silver (10:1) layer was deposited on the Alq layer surface as the top electrode at 5x1ο Torr. The deposition rates for Alq was 3 À/s, and for magnesium:silver 10-11 Â/s. During the evaporation process, the thickness and evaporation rate were monitored with a thickness monitor (ULVAC CRTM 5000) having a quartz oscillator. The actual thicknesses of the layers were measured with a Sloan Dektak 3ST surface profiler. The emitting area was 0.5x0.5 cm . In single-layer devices, the emitter layer was fabricated by the dip coating of the polymer layer. Dichloroethane solutions containing PVK, having a molecular weight of 150,000 purchased from Kanto chemical Ltd., and several dopant dyes were prepared and dip coated onto an ITO coated glass substrate. The thickness of the PVK layer was ca. 1000 À. Then, a magnesium and silver cathode was codeposited. Ionization potentials (Ip), or Highest Occupied Molecular Orbital (HOMO), of the polymers and the vacuum deposited films of organic dyes were determined from the wavelength dependence of photoemission of electrons using Riken Keiki AC-1, and the energy gap values, Eg, were determined from the lower energy threshold of the electronic absorption spectra of the thin films of the materials. Then, pseudo electron 5

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Figure 1. Structures of polymers used.

Cathode '

Electron Transport ^^Layer Hole Transport Layer ^ITO Anode ^Glass Substrate

— Cathode Bipolar Layer •

ITO Anode Glass Substrate

Figure 2. Structures of (a) double-layer and (b) single-layer EL devices.


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affinities (Ea), or Lowest Unoccupied Molecular Orbital (LUMO), were determined from the Ip values (HOMO) and Eg values. Luminance was measured with a luminance meter Topcon B M 8 at room temperature. EL spectra were taken with an optical multichannel analyzer (Hamamatsu Photonics PMA 10). Electronic absorption spectra were taken with a Shimadzu 2200A ultraviol et-visible spectrophotometer.


Double-Layer-Type Devices Side-Chain-Type Polymers. We synthesized two types of side-chain-type polymers, polymethacrylamide (77) and polymethacrylate, both containing triphenylamine or tetraphenyldiamine as functional groups. As illustrated in Figure 3, hole transport in these polymers is due to hole movement along the triphenylamine moieties and hopping between chains. In Figure 4, fluorescence spectra of some of side-chain-type polymers are shown. Triphenylamine-containing polymers, PTPAMAc and PTPAMAm, possess a fluorescence peak at around 390 nm, which originates from the triphenylamine moieties. It has been known that, in poly(N-vinylcarbazole), carbazole excimers are formed, and emission originates from the excimer, which causes spectral shift to longer wavelength (12). It has also been known that such excimer-forming sites serve as holetrapping sites, which lowers the hole mobility in the PVK film (73). Since such excimer formation is negligible in these triphenylamine-containing polymers, it can be assumed that there is no hole traps by excimer sites. Using these polymers, double-layer-type devices with a structure of ITO / polymer / Alq / Mg:Ag were fabricated. The thickness of the polymer and the Alq layer are 200 Â and 700 Â, respectively. When operated in a continuous dc mode for a forward bias with ITO at positive polarity, the devices exhibited bright green emission originating from the Alq layer. A photograph of the device is shown in Figure 5A. This suggests injection of holes from ITO to the polymer layer, and hole transport through the polymer layer to the emitter layer. Luminance increased with increasing injection current, and the driving voltage is dependent on the polymer used. Luminance levels of the device using polymethacrylamide, PTPAMAm, are higher than those of the device using polymethacrylate, PTPAMAc, at the same drive voltages; for example, the maximum luminance of 4,800 cd/m is reached at 15 V for PTPAMAm(77), but 4,600 cd/m is reached at 18 V for PTPAMAc. The higher luminance observed for the PTPAMAm system is due to higher current densities compared with the PTPAMAc system. Such difference in current density may be due to higher hole drift mobility in PTPAMAm than in PTPAMAc, because the Ip value of the PTPAMAm is the same as that of PTPAMAc, 5.6 eV. It has been reported for molecularly doped polymers that hole mobility depends on the polymer matrix and that the glass transition temperature (Tg) as well as the dielectric constant of the polymer are important factors in the hopping transport (14). It can be concluded that polymers with amide bonds are more suitable than polymers with ester bonds. The barrier height for hole injection from ITO to polymer can be lowered by using polymers with lower ionization potentials; thus, driving voltage can be lowered. For example, tetraphenyldiamine-containing polymers, PTPDMA, have Ip of ca. 5.4 eV which is smaller than that of PTPAAm, 5.6 eV. It is, therefore, expected that 2


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e

Figure 3. Hole transport along triphenyl amine moieties.

Wavelength (nm)

Figure 4. Photoluminescence spectra of (A) PTPAMAm, (B) PTPAMAc and (C) o-Me PTPDMA films.




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Figure 5. Photographs of (A) a green-light-emitting device and (B) a blue-lightemitting device.


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devices using PTPDMAs will have lower driving voltages than those using PTPAAm. Figure 6 shows the luminance-current density-voltage curves for a device with a o-Me PTPDMA/Alq. The maximum luminance of ca. 20,000 cd/m is reached at 14 V (75), and the external quantum efficiency of 1.1% photons/electron is observed at 10 V, which is similar to the value observed for the EL device using a small molecular weight aromatic amine and Alq(2). It was also found that the position of the methyl substituent, ο- or /?-, is not critical in these polymers although performance of the devices using vacuum-deposited tetraphenyldiamine derivatives depends highly on the position of the substituent (76). Downloaded by STANFORD UNIV GREEN LIBR on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch025

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Main-Chain-Type Polymers. In double-layer-type devices having a TPD-doped polymer and Alq, the device lifetime highly depends on the Tg of the host polymer. It has been reported that polymers with a higher Tg provide long device lifetimes (77). Among the polymer used as the host matrix, poly (ether sulfone) with a Tg of ca. 220 °C exhibits the longest device lifetime, which is 30 fold longer relative to that of a device having a vacuum deposited TPD. Therefore, the use of a polymer having a high Tg would give stable devices. Since poly(arylene ether)s are known to be one of the thermally stable engineering plastics (18), we designed polymers having arylene ether structures and hole-transporting tetraphenyldiamine units, and synthesized poly(arylene ether sulfone) (PTPDES) (79) and poly(arylene ether ketone) (PTPDEK) (79, 20), shown in Figure 1. As expected, PTPDES and PTPDEK exhibit high Tgs, 190 °C and 173 °C, respectively. Figure 7 displaysfluorescencespectra of the polymers. PTPDES exhibits an emission peak at 420 nm, originating from the diamine moiety. On the other hand, PTPDEK shows a broad emission peak at 500 nm, which indicates that the diamine moieties form exciplex with benzophenone moieties. Such exciplex formation is well known for molecular TPD, and various electron accepting molecules, such as 1,3,4-oxiadiazole derivatives, form exciplex with TPD. It was also found that Ip of PTPDES is lower, 5.5 eV, than that of PTPDEK, 5.6 eV. The higher Ips of these polymers compared with that of molecular TPD is attributed to the electron-withdrawing groups, such as sulfone and ketone groups, of the polymers. Figures 8 and 9 show luminance-voltage curves and current density-voltage curves for polymer (200À) /Alq (700Â) devices. The maximum luminance of 14,000 cd/m is reached at 14 V for PTPDES/Alq, while that of 9,400 cd/m is reached for PTPDEK/Alq. Compared with the device with PTPDES, the driving voltage is higher for the device with PTPDEK, which may be due to the slightly higher Ip of PTPDEK (5.6 eV) than that of PTPDES (5.5 eV). In this case, the barrier height for hole injection from ITO to polymer layer is higher for PTPDEK. It is also likely that the exciplex formation sites act as hole-trapping sites and consequently hole mobility in PTPDEK is decreased, resulting in the higher driving voltages. 2

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Single-Layer-Type Devices Single-layer devices have the simplest device structure, and the fabrication process is simpler than that of the multilayer type. In these devices, both electrons and holes should be injected to the emitter layer. To this end, we have used dye-dispersed polymer systems in which polymers are molecularly dispersed with carrier-transporting



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Bias voltage Figure 6. Luminance-voltage, current density-voltage characteristics of o-Me PTPDMA (200Â) / Alq (700Â) devices.

700

Wavelength (nm) Figure 7. Photoluminescence spectra of (A) PTPDES and (B) PTPDEK films.


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Bias voltage

Figure 9. Luminance-current density characteristics of polymer/Alq devices, (circles) PTPDES and (triangles) PTPDEK.




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low molecular weight additives to balance carrier injection (21-23). In one of the devices, hole-transporting PVK was molecularly doped with 30 wt% of electrontransporting 1,3,4-oxadiazole derivative (PBD) (23). PBD is known to be electrontransporting (24) and has been used as a dopant in single-layer devices (25). Because PBD has a fluorescence peak at ca. 390 nm, a blue emitting fluorescent dye, 1,1,4,4tetraphenyl-l,3-butadiene (TPB), having afluorescencepeak at 430-450 nm was used as an emitting center in order to obtain blue emission. The molecular structures of the materials used are shown in Figure 10. With 3 mol% TPB doped and 30 wt% PBD doped to PVK, the device emitted pure blue light originally from TPB, having a peak at around 440-450 nm. A photograph of the device is shown in Figure 5B. A luminance as high as 450 cd/m was achieved at 18 V (23), which is the highest value observed from single-layer blue light-emitting devices. Since the emitted light can be tuned with ease by dispersing organic dyes of the desired emitting color (26, 27), white light can be obtained by using several dyes (27). The color of the emitted light of this single-layer device was tuned to white by adding severalfluorescentdyes with a differentfluorescentcolor. In addition to blue-emitting TPB,fluorescentdyes such as green-emitting Coumarin 6, yellow-emitting DCM 1 and orange-emitting Nile Red were added as emitting centers. By adjusting concentrations of these dyes, white electroluminescence was obtained as shown in Figurel 1 A. Four peaks from the dopants were clearly seen: TPB at 440 nm, Coumarin 6 at 490 nm, DCM 1 at 520 nm, and Nile Red at 580 nm (23). Luminance-voltage curve of the device is shown in Figure 12. Maximum luminance of 4100 cd/m was reached at 20 V, which is the highest value ever reported for white-light-emitting organic EL devices. Such high luminance was realized because of the low concentration of the dopant dyes, which minimize the concentration quenching of the dopant fluorescence. When the dopant concentrations are higher, PBD (30 wt%), TPB (3 mol%), coumarin 6 (0.08 mol%), DCM 1 (0.04 mol%), Nile Red (0.03 mol%), peaks originating from TPB, coumarin 6, DCM 1 become weaker relative to Nile Red, Figure 1 IB. In this case, the color of the emitted light is yellow. When the concentrations are PBD (30wt%), TPB (4 mol%), coumarin 6 (0.4 mol%), DCM 1 (0.2 mol%), Nile Red (0.15 mol%), the emitted light is orange-red mostly from Nile Red, Figure 11C. These results indicate that energy transfer among dopant dyes is quite efficient and that the concentrations of each dopants should be appropriately low in order to obtain emission from all the dopants. There are at least two possible mechanisms available for dopant excitation. One is energy transfer from the carrier transporting molecules (carbazole or PBD, or both) to the dopants via the Forster type resonance energy transfer (28). In this case, the excited energy is transferred from the host to the dopants that have appropriately lower excited energy levels relative to that of the host. The other is the carrier trapping mechanism, in which dopants serve as carrier trap and provide recombination sites (29). The excitation mechanism of this type becomes highly possible when the following conditions are met; that is, Ip of dopant is lower than that of host, and electron affinity (Ea) of dopant is higher than that of host. In the case of dye-dispersed PVK, the orders of Ip and Ea values are: Ip, TPB (6.0 eV) > PBD = BBOT (5.9 eV) > PVK (5.8 eV) > DCM 1 (5.6 eV) > Coumarin 6 (5.5 eV) > Nile Red (5.4 eV), and Ea, Nile Red = DCM 1 (3.5 eV) > Coumarin 6 = TPB (3.2 eV) > BBOT (3.0 eV) > PBD (2.4 eV) > 2

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Figure 10. Structures of dopant dyes used.

400

500

600

700

Wavelength (nm)

Figure 11. EL spectra for ITO/dye-dispersed PVK (1000Â)/Mg:Ag devices. PVK is molecularly dispersed with (A) 30 wt% PBD, 3 mol% TPB, 0.04 mol% coumarin 6, 0.02 mol% DCM 1, 0.015 mol% Nile Red, (B) 30 wt% PBD, 3 mol% TPB, 0.08 mol% coumarin 6, 0.04 mol% DCM 1, 0.03 mol% Nile Red, (C) 30 wt% PBD, 4 mol% TPB, 0.4 mol% coumarin 6, 0.2 mol% DCM 1, 0.15 mol% Nile Red. J = 20 mA/cm . 2



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Bias Voltage (V) Figure 12. Luminance-voltage characteristics of white-light-emitting ITO/dyedispersed PVK/Mg:Ag device.

Wavelength (nm) Figure 13. Normalized EL spectra of ITO/dye-dispersed PVK (1000À)/Mg:Ag devices. PVK is molecularly dispersed with 30 wt% PBD, 3 mol% TPB, 0.04 mol% coumarin 6, 0.02 mol% DCM 1, 0.015 mol% Nile Red. (A) J = 10 mA/cm , (B) J = 20 mA/cm , (C) J = 200 mA/cm . 2

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PVK (2.3 eV). Comparing Ip and Ea of the carrier transporters and the dopants, we can assume that TPB is mainly excited by the energy transfer because of its higher Ip than those of PVK and PBD. Holes are not likely trapped at TPB sites. In contrast, Nile Red can also be excited by the carrier trapping because it serves as a deep hole trap as well as an electron trap due to its low Ip and high Ea relative to those of PVK and PBD. Similarly, both energy transfer and carrier trapping can be operative for Coumarin 6 and DCM 1. In addition to these mechanisms, energy transfer among dopants should not be neglected. Spectral variations in the white-light-emitting device under different current density are observed as shown in Figure 13. The emission peak of Nile Red is relatively higher at the current density below 20 mA/cm , and there is no significant difference in the spectrum at the current densities over 20 mA/cm . This result indicates that carriers are trapped at Nile Red, and the excitation of Nile Red by the carrier trapping mechanism is pronounced at low current density levels. At higher current density levels, the energy transfer mechanism dominates the excitation of the dopants because of the saturation of the carrier trapping sites. 2

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Conclusions In conclusion, we have demonstrated the application of aromatic amine-containing polymers in the fabrication of EL devices. Tetraphenyldiamine-containing polymers, PTPDMA and PTPDES, are quite useful as a hole transport layer, providing high luminance in bilayer devices. Hole-transporting PVK serves as an emitter layer in single-layer devices when it is molecularly dispersed with electron-transporting molecules. By using several emitting centers in these devices, bright white light can be obtained.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Pope, M.; Kallmann, H. P.; Magnante, P. J. Chem. Phys. 1963, 38, 2042. Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. Adachi, C.; Tokito, S.; Tsutsui, T.; Saito, S. Jpn. J. Appl. Phys. 1988, 27, L269. Kido, J.; Nagai, K.; Ohashi, Y. Chem. Lett. 1990, 657. Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332. Han, E.; Do, L.; Yamamoto, N.; Niidome, Y.; Fujihira, M. Chem. Lett. 1994, 969. Han, E.; Do, L.; Fujihira, M. Chem. Lett.1995, 57. Hosokawa, C.; Tokailin, H.; Higashi, H.; Kusumoto, T. Appl. Phys. Lett. 1992, 60, 1220. Kido, J.; Nagai, K.; Okamoto, Y.; Skotheim, T. Appl. Phys. Lett. 1991, 59, 2760. Kido, J.; Ohtaki, C.; Hongawa, K.; Okuyama, K.; Nagai, K. Jpn. J. Appl. Phys. 1993, 32, L917. Kido, J.; Harada, G.; Nagai, K. Kobunshi Ronbunshu 1995, 52, 216. Itaya, Α.; Okamoto, K.; Kusabayashi, S. Bull. Chem. Soc. Jpn. 1977, 50, 22.


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16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.


Yokoyama, M.;Akiyama, K.; Yamamori, N.; Mikawa, H.; Kusabayashi, S. Polymer J. 1985, 17, 545. Borsenberger, P. M. J. Appl. Phys., 1990, 68, 5188. Kido, J.; Komada, M.; Harada, G.; Nagai, K. Polym. Adv. Technol. 1995, 6, 703. Adachi, C.; Nagai, K.; Tamoto, N. Appl. Phys. Lett. 1995, 66, 2679. Uemura, T.; Kimura, H.; Okuda, N.; Ueba, Y. The 56th Autumn Meeting of The Japan Society of Applied Physics, Extended Abstracts, No.3, 1029 (1995). Hedrick, J. L.; Labadie, J. W. Macromolecules 1988, 21, 1883. Kido, J.; Harada, G.; Nagai, K. Polym. Adv. Technol. 1996, 7, 31. Kido, J.; Harada, G.; Nagai, K. Polym. Prep. Jpn. 1995, 44, 1848. Kido, J.; Kohda, M.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1992, 61, 761. Kido, J.; Kohda, M.; Hongawa, K.; Okuyama, K.; Nagai, K. Mol. Cryst. Liq. Cryst. 1993, 227, 277. Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys. Lett. 1995, 67, 2281. Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1989, 55, 1489. Mori, Y.; Endo, H.; Hayashi, Y.Oyo Buturi 1992, 61, 1044. Kido, J.; Hongawa, K.; Nagai, K.; Okuyama, K. Macromol. Symp. 1994, 84, 81. Kido, J.; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys. Lett. 1994, 64, 815. Tang, C. W.; VanSlyke, S. Α.; Chen, C. H. J. Appl. Phys. 1989, 65, 3610. Utsugi, K.; Takano, S. J. Electrochem. Soc. 1992, 139, 3610.


Aromatic-Amine-Containing Polymers for Organic Electroluminescent

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