- Email: [email protected]
Organic electroluminescent device with R-G-B emission
Thin Solid Films 331 (1998) 89±95
Organic electroluminescent device with R-G-B emission Yutaka Ohmori*, Norio Tada, Akihiko Fujii, Hiroshi Ueta, Takumi Sawatani, Katsumi Yoshino Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 Japan
Abstract Many research workers have concentrated their attention to realize high performance organic EL devices. Particularly, multicolor electroluminescent (EL) devices which emit red (R), green (G) and blue (B) lights are attractive for ¯at panel full color display application. This paper shows several types of multicolor EL devices which show the multicolor emission by changing the applied electric ®eld or by applying the opposite polarity of the applied ®eld. The multicolor emission EL devices which show three different colors (R-G-B) are composed by stacking a two-color-emission part on a single-color-emission part. The emission color can be modulated by applying different polarity of applied ®eld to the two-color part, and by applying various voltages to the single-color part, independently. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Organic electroluminescent diodes; Multicolor emitting diodes; Dye molecules; Multilayer structure
1. Introduction
mer materials, (a) poly(2,5-dialkoxy-p-phenylene vinylene)
Organic electroluminescent (EL) devices have attracted great interest because of their potential applicability to fullcolor ¯at panel displays, for example, due to wide spectral range, low driving voltage, light weight or other characteristics for portable display devices. Since highly ef®cient electroluminescence was achieved by Tang and Van Slyke [1], various EL devices were demonstrated in order to obtain high brightness, long life time [2], white color emission [3,4] and multicolor emission [5±15] for realizing display applications. Multicolor organic EL devices are classi®ed into four types; type I: dye dispersed polymer emissive device [5], type II: single quantum-well layer inserted in the emissive layer device [6], type III: two emissive layers separated by a carrier blocking layer device [7±11], type IV: two emissive parts stacked on a transparent electrode device [13,14]. We have realized the multicolor organic EL devices of these four types. In this paper, we discuss the emission characteristics of these multicolor EL devices, especially the full color EL device which can emit completely different colors of R-G-B emission. 2. Layer structure of multicolor EL device Fig. 1 shows the molecular structures of ¯uorescent poly* Corresponding author Tel.: 181-6-879-7758, fax: 181-6-879-7774; e-mail: [email protected].
Fig. 1. Molecular structures of ¯uorescent dye materials used in multicolor EL devices. (a) poly (2,5-dialkoxy-p-phenylene vinylene) (RO-PPV), (b) poly(3-alkylthiophene) (PAT), (c) 8-hydroxyquinoline aluminum (Alq3), (d) N,N 0 -diphenyl-N,N 0 -(3-methylphenyl)-1,1 0 -biphenyl-4,4 0 -diamine (TPD) and (e) poly(9,9-dialkyl¯uorene) (PDAF), (f) N,N 0 -bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedicarboximide (BPPC), (g) 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP).
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(RO-PPV), (b) poly(3-alkylthiophene) and (e) poly(9,9dialkyl¯uorene) (PDAF), and low molecular dye materials, (c) 8-hydroxyquinoline aluminum (Alq3), (d) N,N 0 -diphenyl-N,N 0 -(3-methylphenyl)-1,1 0 -biphenyl-4,4 0 -diamine (TPD), (f) N,N 0 -bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BPPC), (g) 1,2,3,4,5-pentaphenyl-1,3cyclopentadiene (PPCP), used for multicolor emission devices. Multicolor emission diodes have been realized by several methods, the device structures of which are schematically shown in Fig. 2. The multicolor emission EL diodes are classi®ed into four groups. The dye doped polymer diodes are classi®ed as type I and is shown in Fig. 2a. Type II is a single quantum well structure diode which consists of Alq3 single-layer in the TPD barrier layers, and is shown in Fig. 2b. Type III is two-color-emission diodes with two emissive layers separated by a carrier blocking layer, into which emissive layers carriers are injected by applying the opposite polarity of the applied electric ®eld. The device structures of type III are shown in Fig. 2(c). Type IV is a three-color-emission diode, which is composed of a single-color-emission part on a two-color-emission part in the type III device. The diode shown in Fig. 2d emits
three different colors, red (R), green (G), and blue (B), by changing a polarity of an applied electric ®eld to a two-color part and changing an applied ®eld to a single-color part of the diode. The vapor deposited magnesium (Mg) containing indium (In) was used as a cathode, and indium-tin oxide (ITO) was used as an anode and acted as transparent electrode for all types of diodes. In order to obtain high ef®cient injection into organic materials, low work function metals are used as a cathode such as Mg. In some cases, aluminum (Al) electrodes are also used as a cathode. Since Mg is easily oxidized by air, a mixture of Mg with In metal (Mg/In) was co-evaporated at a high vacuum under 10 24 Pa onto the organic ®lms. The active area of the EL diodes was 2 mm 2. In the device shown in Fig. 2a, type I, dye doped RO-PPV or PAT is spin-coated onto an ITO transparent electrode. Alq3 is doped into polymer materials by using a mixture of a toluene solution of Alq3 and RO-PPV. The layer thickness of the dye doped polymer materials are about 100 nm. All the devices shown in Fig. 2b±d, type II±IV, are fabricated by organic molecular beam deposition (OMBD). The
Fig. 2. Schematic description of multicolor emission EL devices. (a) Type I (dye dispersed polymer device), (b) type II (single quantum well device), (c) type III (polarity change type two-color emission device) and (d) type IV (a single-color-emission part stacked on a two-color-emission part device).
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layer structure was fabricated by OMBD using separate Knudsen cells (K-cells) at a background pressure of about 10 25 Pa. The powders of ¯uorescent dyes were loaded into separate K-cells, and then the cells were subsequently heated to their sublimation temperatures, and deposited onto an ITO coated glass substrate. For the optical measurement, the layer structure was composed on quartz substrates. The typical deposition rates were 2±3 nm/min. The layer thickness of the deposited material was determined in situ by using an oscillating quartz thickness monitor. Al and Mg/ In electrodes were vapor deposited at a background pressure of about 10 25 Pa. The type IV device shown in Fig. 2d consists of ITO coated glass substrate, ¯uorescent dye layers, a half-transparent Al electrode, ¯uorescent dye layers, and an Mg/In cathode in the order. The device consists of two parts, i.e. a two-color-emission part and a single-color-emission part. Electrical properties such as current±voltage characteristics and the emission spectra were measured by a conventional method, under DC, pulsed or AC conditions, which we reported previously [11]. The measurements were carried out at liquid nitrogen temperature (77 K). At room temperature, the devices showed similar emission characteristics to those measured at 77 K, but the emission intensity is not so high as that at 77 K. 3. Emission characteristics of multicolor EL device The type I multicolor emission diode was simply obtained by doping dye materials into polymer materials [5] as shown in Fig. 2a. The diodes show two different colors, which are composed with Alq3 doped RO-PPV or PAT. As shown in Fig. 3, Alq3 doped poly(3-octadecylthiophene) (PAT (n 18)) at its molar ratio of 10:15, where the molar ratio
Fig. 3. EL spectra from type I device as a function of driving voltage.
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is calculated at their monomer, shows emission from PAT at 630 nm and that from Alq3 at 520 nm. At the low driving voltage, emission comes mainly from PAT, however, the emission from Alq3 increased compared with that from PAT with increasing applied voltage. In the case of Alq3 doped RO-PPV (n 8) with molar ratio of 3:7, emission comes mainly from RO-PPV at 600 nm in the case of low driving voltage, which increases from Alq3 at 520 nm with increasing applied voltages. The similar emission color change was realized by doping low molecular dye materials into emissive polymers, i.e. PPCP doped PAT or RO-PPV. The emission mechanism is explained as follows. Under the low driving voltage, the injected carriers are mainly localized in the low energy gap materials and the excitons recombine with each other to emit light. With increasing driving voltage, carriers are injected to higher energy materials to recombine in the higher energy sites. In the case of Alq3 doped PAT diode, the emission mainly occurs at PAT under low driving voltage and then Alq3 emits light at higher voltage. As a result, the emission color changes from red to green in the Alq3 doped PAT device with increasing applied voltage. The type II device consists of a single quantum well of Alq3 (1 nm thick) sandwiched by TPD barrier layers [6] as shown in Fig. 2b. The thin ®lm Alq3 layer emits green light at 520 nm, however, the barrier layer of TPD also emits blue light at 410 nm. The layer thickness, d and y are important parameters, and the device parameters shown in Fig. 4 are chosen as d 20 nm and y 120 nm. At the low driving voltage, the emission intensities from Alq3 and TPD are the same, however, emission from the TPD increases compared with that from Alq3 with increasing applied voltage. In this device, the intensity ratio of green light at 520 nm and blue light at 410 nm changes with changing the applied voltage. The reason for the emission spectrum changes depending on the applied voltage is explained as follows. The thin layer of Alq3 emits green light in spite of the thin layer thickness, because carriers are injected and con®ned in the Alq3 well layer. With increasing injection voltage, electrons are injected not only to the Alq3 well layer but also to the TPD barrier layers since the Alq3 well layer is set 20 nm from the cathode, thin enough for electrons to transport electrons to the Alq3 layer. As a result, both Alq3 and TPD layers can emit light depending on the driving voltage. Multicolor emitting diodes which are operated by alternating the polarity of the applied ®eld [8] are realized by the device structure shown in Fig. 2c. The device consists of an ITO-coated glass substrate/BPPC emission layer (30 nm)/ TPD electron blocking layer (40 nm)/poly(9,9-dialkyl¯uorene (PDAF) emission layer (30 nm)/Al electrode. In this device, poly(9,9-dihexyl¯uorene) was used as PDAF (n 6). The organic thin ®lms were fabricated by OMBD onto ITO-coated glass substrates. EL spectra of the device driven in positive and negative bias conditions are shown in Fig. 5a. In the case of positive bias condition, blue light at the maximum peak of 470 nm
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Fig. 4. EL spectra from type II device as a function of driving voltage.
originated from PDHF was emitted from the device. On the other hand, in the negative biased condition, it emits red
Fig. 6. Current±voltage (I±V), EL intensity±voltage (L±V) characteristics and EL intensity±current (L±I) characteristic of the two-color emission part in the type IV device. (a) I±V and L±V characteristics, (b) L±I characteristics.
Fig. 5. EL spectra from type III device dependent on polarity of applied ®eld (a) and frequency dependence of alternative current (b).
light from the BPPC layer, the emission peak was observed at 640 nm. That is, the device could emit both blue and red light, which could be selected by inverting the polarity of the applied ®eld. The dips of emission at 500 and 530 nm correspond to the absorption by BPPC in the forward bias condition. This result indicates that the blue emission occurred at the interface of the PDHF layer and the Al electrode and was emitted through the BPPC layer. In the case of the negative bias condition, holes are injected from Al to the HOMO (highest occupied molecular orbitals) state of PDHF, and electrons are injected from ITO to the LUMO (lowest unoccupied molecular orbitals) state of BPPC. Holes injected to the HOMO of PDHF migrate in
Y. Ohmori et al. / Thin Solid Films 331 (1998) 89±95
Fig. 7. I±V, L±V characteristics and L±I characteristic of the single-color emission part in the type IV device. (a) I±V and L±V characteristics, (b) L±I characteristics.
the PDHF and TPD layers and reach the BPPC layer. Electrons injected from ITO to the LUMO state of BPPC are con®ned in the BPPC layer, and cannot move into the TPD layer since the barrier at the BPPC/TPD interface is so high that they cannot move into the PDHF layers. Consequently, excitons are formed and con®ned in the BPPC layer and the recombination takes place in the BPPC layer for the EL emission. Therefore, the red light from BPPC comes out in the negative bias condition. This fact shows that the emission occurred only from the layer near the electron injecting electrode. The TPD layer plays a role of an electron blocking layer in the EL device. The introduction of the electron blocking layer between two emissive layers makes it possible to control the emission region in each side of the electron
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blocking layer by changing the polarity of the applied voltage. As a consequence, the light color is optional between two colors depending on the polarity of the applied ®eld. Fig. 5b shows the EL spectra under AC biased condition as a function of frequency of the applied AC ®eld. These spectra are normalized by BPPC emission peak intensity at 640 nm. The applied ®eld waveform is an AC rectangularwave (voltage pulses of 121 and 218 V in height and duty of 50 %). Under an AC biased condition of 500 Hz, both blue and red light was observed. Applying AC ®eld of higher frequency, however, the blue light originating from PDHF was suppressed compared with the red emission from BPPC. That is, the emission color can be gradually modi®ed by sweeping only the frequency of the applied AC ®eld, keeping the pulse height of AC ®eld as it is. It should be mentioned that the total EL intensity of each emissions at various frequencies were not below 50% of the emission intensity in the low frequency of 500 Hz. A similar multicolor emitting diode [7] could be realized by using a superlattice structure of 1.5-nm thick PDHF layers and 1.0-nm thick BPPC layers, which are constructed on the ITO anode, and then 80-nm thick PDHF layers on the superlattice structure are terminated by the Mg/In cathode. The layer thickness of the superlattice structure is 40 nm. When the cathode is negatively biased, the PDHF layer emits blue light at 470 nm. In the opposite case, as the ITO electrode is negatively biased, the BBPC layers emit red light at 640 nm. In the device structure, electrons are con®ned by the superlattice structure of BPPC and PDHF in the reverse bias condition. On the other hand, in the forward bias condition, electrons are injected to the PDHF layer and recombine with holes in the PDHF layer. Electrons are only injected near the layer to the cathode, i.e. PDHF layer or BPPC/PDHF superlattice layer. The emission spectra are similar to those with the device shown in Fig. 5a. A three different color emitting diode is realized by the device structure shown in Fig. 2d. First, to form the two colors of red and blue emission part, BPPC (25 nm) as a red emissive layer, TPD (40 nm) as an electron blocking layer, PPCP (30 nm) as a blue emissive layer, Alq3 (5 nm) as an electron transporting layer, and a half-transparent Al electrode were deposited onto an ITO electrode. The Alq3 layer was inserted between the Al electrode and the PPCP layer, so as to increase electron injection from the Al electrode to the PPCP, because the LUMO state of Alq3 is lower than that of PPCP. Next, to form the green single-coloremission part, TPD (40 nm) as a hole transporting layer, Alq3 (40 nm) as a green emissive layer, and a Mg/In cathode were deposited on the Al electrode. The half-transparent Al electrode, the transmittance of which is about 17% at 515 nm of the green emissive wavelength, plays the roles of cathode to the two-color-emission part and anode to the green emission part, simultaneously. Forward bias conditions are de®ned as the case in which the ITO electrode is positively biased against the Al elec-
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green emission part, the device emits green light originated from Alq3 with an emission peak at 520 nm. The dips of the emission at 490 and 530 nm correspond to the absorption by BPPC layer, because the light comes through the BPPC layer which exists between the Alq3 layer and the window of the ITO electrode. The device can emit three colors of red, green, and blue light by switching the polarity and by changing the biasing part. The EL intensities of the R-G-B spectrum are comparable with respect to the bias ®eld. The green emission is the strongest compared with other blue and red emissions, however, we can control the total intensity of the emission by modulating the strength of the applied voltage and by controlling the pulse width of the injection current.
Fig. 8. EL spectra from the stacked R-G-B emission device (type IV) as a function of applied voltages.
trode in the two-color emission part and as that in which the Al electrode is biased against the Mg/In electrode in the green emission part, whereas reverse bias conditions are the opposite case for the type IV device. Current±voltage (I±V) and EL intensity±voltage (L±V) characteristics of the two-color-emission part are shown in Fig. 6a. The current increases superlinearly with increasing applied voltage in both the forward and reverse bias conditions in the twocolor-emission part. Emission starts to increase at 15 V, and EL intensity increases proportionally with increasing injection current in both bias conditions as shown in Fig. 6b. The L±V characteristics of the two-color emission part show nearly symmetrical behavior with respect to the bias ®elds, however, the emission in the forward bias condition is stronger than that in the reverse one. Blue light is rather stronger than red under the same injection current in the two-color part. I±V and L±V characteristics, and L±I characteristics of the single-color (green) emission part are shown in Fig. 7a,b, respectively. The current and emission intensity increased only in the forward bias condition with increasing applied voltage. The emission starts to increase at 10 V in the forward bias case. The emission from the green emission part is stronger than that of the two-color emission part due to the high emission ef®ciency of Alq3. The EL spectra from the stacked R-G-B emission device (type IV) are shown in Fig. 8 as a function of driving voltage. In the forward bias condition in the two-color emission part, the device emits blue light at its peak emission of 450 nm originating from PPCP. On the other hand, it emits red light at its peak emission of 640 nm from BPPC in the reverse bias case. The emission color changes depending on the polarity of applied voltage because the emission region is restricted only to the layer near the electron injecting electrode by the electron blocking layer of TPD. The mechanism of the emission color change is similar to the type III device. In the case of forward bias condition for the
4. Conclusions Multicolor EL devices with four kinds of device structure have been discussed. Single layer or single-quantum-well devices can be modulated the emission color by controlling the applied ®eld, and the multilayer devices with a carrier blocking layer can also emit multicolor by changing the polarity of the applied ®eld. A multicolor device which emits red, green, and blue light has been realized by stacking a single-color-emission part on a two-color-emission part. The emission color can be controlled by changing the polarity of the applied ®eld to the two-color-emission part and controlling applied voltage to the single-coloremission part, separately. The color can be continuously selected by modulating the applied voltage and by controlling the pulse width of the injection current to the emission part. Acknowledgements Part of this work was supported by the Support Center for Advanced Telecommunications Technology Research. References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [2] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160. [3] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [4] J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [5] M. Uchida, Y. Ohmori, T. Noguchi, T. Ohnishi, K. Yoshino, Jpn. J. Appl. Phys. 32 (1993) L921. [6] A. Fujii, M. Yoshida, Y. Ohmori, K. Yoshino, Jpn. J. Appl. Phys. 34 (1995) L499. [7] M. Yoshida, A. Fujii, Y. Ohmori, K. Yoshino, Jpn. J. Appl. Phys. 35 (1996) L397. [8] M. Yoshida, A. Fujii, Y. Ohmori, K. Yoshino, Appl. Phys. Lett. 69 (1996) 734. [9] M. Hamaguchi, K. Yoshino, Appl. Phys. Lett. 69 (1996) 143. [10] Y. Yang, Q. Pei, Appl. Phys. Lett. 68 (1996) 2708. [11] N. Tada, A. Fujii, Y. Ohmori, K. Yoshino, IEEE Trans. Electron Devices 44 (1997) 1234.
Y. Ohmori et al. / Thin Solid Films 331 (1998) 89±95 [12] V. Bulovic, G. Gu, O. E. Burrows, S.R. Forrest, M.E. Thompson, Nature 380 (1996) 29. [13] P.E. Burrows, S.R. Forrest, S.P. Sibley, M.E. Thompson, Appl. Phys. Lett. 69 (1996) 2959.
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[14] P.E. Burrows, G. Gu, V. Bulovic, Z. Shen, S.R. Forrest, M.E. Thompson, IEEE Trans. Electron Devices 44 (1997) 1188. [15] Y.Z. Wang, D.D. Gebler, D.K. Fu, T.M. Swager, A.J. Epstein, Appl. Phys. Lett. 70 (1997) 3215.
Organic electroluminescent device with R-G-B emission Yutaka Ohmori*, Norio Tada, Akihiko Fujii, Hiroshi Ueta, Takumi Sawatani, Katsumi Yoshino Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 Japan
Abstract Many research workers have concentrated their attention to realize high performance organic EL devices. Particularly, multicolor electroluminescent (EL) devices which emit red (R), green (G) and blue (B) lights are attractive for ¯at panel full color display application. This paper shows several types of multicolor EL devices which show the multicolor emission by changing the applied electric ®eld or by applying the opposite polarity of the applied ®eld. The multicolor emission EL devices which show three different colors (R-G-B) are composed by stacking a two-color-emission part on a single-color-emission part. The emission color can be modulated by applying different polarity of applied ®eld to the two-color part, and by applying various voltages to the single-color part, independently. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Organic electroluminescent diodes; Multicolor emitting diodes; Dye molecules; Multilayer structure
1. Introduction
mer materials, (a) poly(2,5-dialkoxy-p-phenylene vinylene)
Organic electroluminescent (EL) devices have attracted great interest because of their potential applicability to fullcolor ¯at panel displays, for example, due to wide spectral range, low driving voltage, light weight or other characteristics for portable display devices. Since highly ef®cient electroluminescence was achieved by Tang and Van Slyke [1], various EL devices were demonstrated in order to obtain high brightness, long life time [2], white color emission [3,4] and multicolor emission [5±15] for realizing display applications. Multicolor organic EL devices are classi®ed into four types; type I: dye dispersed polymer emissive device [5], type II: single quantum-well layer inserted in the emissive layer device [6], type III: two emissive layers separated by a carrier blocking layer device [7±11], type IV: two emissive parts stacked on a transparent electrode device [13,14]. We have realized the multicolor organic EL devices of these four types. In this paper, we discuss the emission characteristics of these multicolor EL devices, especially the full color EL device which can emit completely different colors of R-G-B emission. 2. Layer structure of multicolor EL device Fig. 1 shows the molecular structures of ¯uorescent poly* Corresponding author Tel.: 181-6-879-7758, fax: 181-6-879-7774; e-mail: [email protected].
Fig. 1. Molecular structures of ¯uorescent dye materials used in multicolor EL devices. (a) poly (2,5-dialkoxy-p-phenylene vinylene) (RO-PPV), (b) poly(3-alkylthiophene) (PAT), (c) 8-hydroxyquinoline aluminum (Alq3), (d) N,N 0 -diphenyl-N,N 0 -(3-methylphenyl)-1,1 0 -biphenyl-4,4 0 -diamine (TPD) and (e) poly(9,9-dialkyl¯uorene) (PDAF), (f) N,N 0 -bis(2,5-di-tertbutylphenyl)-3,4,9,10-perylenedicarboximide (BPPC), (g) 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP).
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(RO-PPV), (b) poly(3-alkylthiophene) and (e) poly(9,9dialkyl¯uorene) (PDAF), and low molecular dye materials, (c) 8-hydroxyquinoline aluminum (Alq3), (d) N,N 0 -diphenyl-N,N 0 -(3-methylphenyl)-1,1 0 -biphenyl-4,4 0 -diamine (TPD), (f) N,N 0 -bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BPPC), (g) 1,2,3,4,5-pentaphenyl-1,3cyclopentadiene (PPCP), used for multicolor emission devices. Multicolor emission diodes have been realized by several methods, the device structures of which are schematically shown in Fig. 2. The multicolor emission EL diodes are classi®ed into four groups. The dye doped polymer diodes are classi®ed as type I and is shown in Fig. 2a. Type II is a single quantum well structure diode which consists of Alq3 single-layer in the TPD barrier layers, and is shown in Fig. 2b. Type III is two-color-emission diodes with two emissive layers separated by a carrier blocking layer, into which emissive layers carriers are injected by applying the opposite polarity of the applied electric ®eld. The device structures of type III are shown in Fig. 2(c). Type IV is a three-color-emission diode, which is composed of a single-color-emission part on a two-color-emission part in the type III device. The diode shown in Fig. 2d emits
three different colors, red (R), green (G), and blue (B), by changing a polarity of an applied electric ®eld to a two-color part and changing an applied ®eld to a single-color part of the diode. The vapor deposited magnesium (Mg) containing indium (In) was used as a cathode, and indium-tin oxide (ITO) was used as an anode and acted as transparent electrode for all types of diodes. In order to obtain high ef®cient injection into organic materials, low work function metals are used as a cathode such as Mg. In some cases, aluminum (Al) electrodes are also used as a cathode. Since Mg is easily oxidized by air, a mixture of Mg with In metal (Mg/In) was co-evaporated at a high vacuum under 10 24 Pa onto the organic ®lms. The active area of the EL diodes was 2 mm 2. In the device shown in Fig. 2a, type I, dye doped RO-PPV or PAT is spin-coated onto an ITO transparent electrode. Alq3 is doped into polymer materials by using a mixture of a toluene solution of Alq3 and RO-PPV. The layer thickness of the dye doped polymer materials are about 100 nm. All the devices shown in Fig. 2b±d, type II±IV, are fabricated by organic molecular beam deposition (OMBD). The
Fig. 2. Schematic description of multicolor emission EL devices. (a) Type I (dye dispersed polymer device), (b) type II (single quantum well device), (c) type III (polarity change type two-color emission device) and (d) type IV (a single-color-emission part stacked on a two-color-emission part device).
Y. Ohmori et al. / Thin Solid Films 331 (1998) 89±95
layer structure was fabricated by OMBD using separate Knudsen cells (K-cells) at a background pressure of about 10 25 Pa. The powders of ¯uorescent dyes were loaded into separate K-cells, and then the cells were subsequently heated to their sublimation temperatures, and deposited onto an ITO coated glass substrate. For the optical measurement, the layer structure was composed on quartz substrates. The typical deposition rates were 2±3 nm/min. The layer thickness of the deposited material was determined in situ by using an oscillating quartz thickness monitor. Al and Mg/ In electrodes were vapor deposited at a background pressure of about 10 25 Pa. The type IV device shown in Fig. 2d consists of ITO coated glass substrate, ¯uorescent dye layers, a half-transparent Al electrode, ¯uorescent dye layers, and an Mg/In cathode in the order. The device consists of two parts, i.e. a two-color-emission part and a single-color-emission part. Electrical properties such as current±voltage characteristics and the emission spectra were measured by a conventional method, under DC, pulsed or AC conditions, which we reported previously [11]. The measurements were carried out at liquid nitrogen temperature (77 K). At room temperature, the devices showed similar emission characteristics to those measured at 77 K, but the emission intensity is not so high as that at 77 K. 3. Emission characteristics of multicolor EL device The type I multicolor emission diode was simply obtained by doping dye materials into polymer materials [5] as shown in Fig. 2a. The diodes show two different colors, which are composed with Alq3 doped RO-PPV or PAT. As shown in Fig. 3, Alq3 doped poly(3-octadecylthiophene) (PAT (n 18)) at its molar ratio of 10:15, where the molar ratio
Fig. 3. EL spectra from type I device as a function of driving voltage.
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is calculated at their monomer, shows emission from PAT at 630 nm and that from Alq3 at 520 nm. At the low driving voltage, emission comes mainly from PAT, however, the emission from Alq3 increased compared with that from PAT with increasing applied voltage. In the case of Alq3 doped RO-PPV (n 8) with molar ratio of 3:7, emission comes mainly from RO-PPV at 600 nm in the case of low driving voltage, which increases from Alq3 at 520 nm with increasing applied voltages. The similar emission color change was realized by doping low molecular dye materials into emissive polymers, i.e. PPCP doped PAT or RO-PPV. The emission mechanism is explained as follows. Under the low driving voltage, the injected carriers are mainly localized in the low energy gap materials and the excitons recombine with each other to emit light. With increasing driving voltage, carriers are injected to higher energy materials to recombine in the higher energy sites. In the case of Alq3 doped PAT diode, the emission mainly occurs at PAT under low driving voltage and then Alq3 emits light at higher voltage. As a result, the emission color changes from red to green in the Alq3 doped PAT device with increasing applied voltage. The type II device consists of a single quantum well of Alq3 (1 nm thick) sandwiched by TPD barrier layers [6] as shown in Fig. 2b. The thin ®lm Alq3 layer emits green light at 520 nm, however, the barrier layer of TPD also emits blue light at 410 nm. The layer thickness, d and y are important parameters, and the device parameters shown in Fig. 4 are chosen as d 20 nm and y 120 nm. At the low driving voltage, the emission intensities from Alq3 and TPD are the same, however, emission from the TPD increases compared with that from Alq3 with increasing applied voltage. In this device, the intensity ratio of green light at 520 nm and blue light at 410 nm changes with changing the applied voltage. The reason for the emission spectrum changes depending on the applied voltage is explained as follows. The thin layer of Alq3 emits green light in spite of the thin layer thickness, because carriers are injected and con®ned in the Alq3 well layer. With increasing injection voltage, electrons are injected not only to the Alq3 well layer but also to the TPD barrier layers since the Alq3 well layer is set 20 nm from the cathode, thin enough for electrons to transport electrons to the Alq3 layer. As a result, both Alq3 and TPD layers can emit light depending on the driving voltage. Multicolor emitting diodes which are operated by alternating the polarity of the applied ®eld [8] are realized by the device structure shown in Fig. 2c. The device consists of an ITO-coated glass substrate/BPPC emission layer (30 nm)/ TPD electron blocking layer (40 nm)/poly(9,9-dialkyl¯uorene (PDAF) emission layer (30 nm)/Al electrode. In this device, poly(9,9-dihexyl¯uorene) was used as PDAF (n 6). The organic thin ®lms were fabricated by OMBD onto ITO-coated glass substrates. EL spectra of the device driven in positive and negative bias conditions are shown in Fig. 5a. In the case of positive bias condition, blue light at the maximum peak of 470 nm
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Fig. 4. EL spectra from type II device as a function of driving voltage.
originated from PDHF was emitted from the device. On the other hand, in the negative biased condition, it emits red
Fig. 6. Current±voltage (I±V), EL intensity±voltage (L±V) characteristics and EL intensity±current (L±I) characteristic of the two-color emission part in the type IV device. (a) I±V and L±V characteristics, (b) L±I characteristics.
Fig. 5. EL spectra from type III device dependent on polarity of applied ®eld (a) and frequency dependence of alternative current (b).
light from the BPPC layer, the emission peak was observed at 640 nm. That is, the device could emit both blue and red light, which could be selected by inverting the polarity of the applied ®eld. The dips of emission at 500 and 530 nm correspond to the absorption by BPPC in the forward bias condition. This result indicates that the blue emission occurred at the interface of the PDHF layer and the Al electrode and was emitted through the BPPC layer. In the case of the negative bias condition, holes are injected from Al to the HOMO (highest occupied molecular orbitals) state of PDHF, and electrons are injected from ITO to the LUMO (lowest unoccupied molecular orbitals) state of BPPC. Holes injected to the HOMO of PDHF migrate in
Y. Ohmori et al. / Thin Solid Films 331 (1998) 89±95
Fig. 7. I±V, L±V characteristics and L±I characteristic of the single-color emission part in the type IV device. (a) I±V and L±V characteristics, (b) L±I characteristics.
the PDHF and TPD layers and reach the BPPC layer. Electrons injected from ITO to the LUMO state of BPPC are con®ned in the BPPC layer, and cannot move into the TPD layer since the barrier at the BPPC/TPD interface is so high that they cannot move into the PDHF layers. Consequently, excitons are formed and con®ned in the BPPC layer and the recombination takes place in the BPPC layer for the EL emission. Therefore, the red light from BPPC comes out in the negative bias condition. This fact shows that the emission occurred only from the layer near the electron injecting electrode. The TPD layer plays a role of an electron blocking layer in the EL device. The introduction of the electron blocking layer between two emissive layers makes it possible to control the emission region in each side of the electron
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blocking layer by changing the polarity of the applied voltage. As a consequence, the light color is optional between two colors depending on the polarity of the applied ®eld. Fig. 5b shows the EL spectra under AC biased condition as a function of frequency of the applied AC ®eld. These spectra are normalized by BPPC emission peak intensity at 640 nm. The applied ®eld waveform is an AC rectangularwave (voltage pulses of 121 and 218 V in height and duty of 50 %). Under an AC biased condition of 500 Hz, both blue and red light was observed. Applying AC ®eld of higher frequency, however, the blue light originating from PDHF was suppressed compared with the red emission from BPPC. That is, the emission color can be gradually modi®ed by sweeping only the frequency of the applied AC ®eld, keeping the pulse height of AC ®eld as it is. It should be mentioned that the total EL intensity of each emissions at various frequencies were not below 50% of the emission intensity in the low frequency of 500 Hz. A similar multicolor emitting diode [7] could be realized by using a superlattice structure of 1.5-nm thick PDHF layers and 1.0-nm thick BPPC layers, which are constructed on the ITO anode, and then 80-nm thick PDHF layers on the superlattice structure are terminated by the Mg/In cathode. The layer thickness of the superlattice structure is 40 nm. When the cathode is negatively biased, the PDHF layer emits blue light at 470 nm. In the opposite case, as the ITO electrode is negatively biased, the BBPC layers emit red light at 640 nm. In the device structure, electrons are con®ned by the superlattice structure of BPPC and PDHF in the reverse bias condition. On the other hand, in the forward bias condition, electrons are injected to the PDHF layer and recombine with holes in the PDHF layer. Electrons are only injected near the layer to the cathode, i.e. PDHF layer or BPPC/PDHF superlattice layer. The emission spectra are similar to those with the device shown in Fig. 5a. A three different color emitting diode is realized by the device structure shown in Fig. 2d. First, to form the two colors of red and blue emission part, BPPC (25 nm) as a red emissive layer, TPD (40 nm) as an electron blocking layer, PPCP (30 nm) as a blue emissive layer, Alq3 (5 nm) as an electron transporting layer, and a half-transparent Al electrode were deposited onto an ITO electrode. The Alq3 layer was inserted between the Al electrode and the PPCP layer, so as to increase electron injection from the Al electrode to the PPCP, because the LUMO state of Alq3 is lower than that of PPCP. Next, to form the green single-coloremission part, TPD (40 nm) as a hole transporting layer, Alq3 (40 nm) as a green emissive layer, and a Mg/In cathode were deposited on the Al electrode. The half-transparent Al electrode, the transmittance of which is about 17% at 515 nm of the green emissive wavelength, plays the roles of cathode to the two-color-emission part and anode to the green emission part, simultaneously. Forward bias conditions are de®ned as the case in which the ITO electrode is positively biased against the Al elec-
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green emission part, the device emits green light originated from Alq3 with an emission peak at 520 nm. The dips of the emission at 490 and 530 nm correspond to the absorption by BPPC layer, because the light comes through the BPPC layer which exists between the Alq3 layer and the window of the ITO electrode. The device can emit three colors of red, green, and blue light by switching the polarity and by changing the biasing part. The EL intensities of the R-G-B spectrum are comparable with respect to the bias ®eld. The green emission is the strongest compared with other blue and red emissions, however, we can control the total intensity of the emission by modulating the strength of the applied voltage and by controlling the pulse width of the injection current.
Fig. 8. EL spectra from the stacked R-G-B emission device (type IV) as a function of applied voltages.
trode in the two-color emission part and as that in which the Al electrode is biased against the Mg/In electrode in the green emission part, whereas reverse bias conditions are the opposite case for the type IV device. Current±voltage (I±V) and EL intensity±voltage (L±V) characteristics of the two-color-emission part are shown in Fig. 6a. The current increases superlinearly with increasing applied voltage in both the forward and reverse bias conditions in the twocolor-emission part. Emission starts to increase at 15 V, and EL intensity increases proportionally with increasing injection current in both bias conditions as shown in Fig. 6b. The L±V characteristics of the two-color emission part show nearly symmetrical behavior with respect to the bias ®elds, however, the emission in the forward bias condition is stronger than that in the reverse one. Blue light is rather stronger than red under the same injection current in the two-color part. I±V and L±V characteristics, and L±I characteristics of the single-color (green) emission part are shown in Fig. 7a,b, respectively. The current and emission intensity increased only in the forward bias condition with increasing applied voltage. The emission starts to increase at 10 V in the forward bias case. The emission from the green emission part is stronger than that of the two-color emission part due to the high emission ef®ciency of Alq3. The EL spectra from the stacked R-G-B emission device (type IV) are shown in Fig. 8 as a function of driving voltage. In the forward bias condition in the two-color emission part, the device emits blue light at its peak emission of 450 nm originating from PPCP. On the other hand, it emits red light at its peak emission of 640 nm from BPPC in the reverse bias case. The emission color changes depending on the polarity of applied voltage because the emission region is restricted only to the layer near the electron injecting electrode by the electron blocking layer of TPD. The mechanism of the emission color change is similar to the type III device. In the case of forward bias condition for the
4. Conclusions Multicolor EL devices with four kinds of device structure have been discussed. Single layer or single-quantum-well devices can be modulated the emission color by controlling the applied ®eld, and the multilayer devices with a carrier blocking layer can also emit multicolor by changing the polarity of the applied ®eld. A multicolor device which emits red, green, and blue light has been realized by stacking a single-color-emission part on a two-color-emission part. The emission color can be controlled by changing the polarity of the applied ®eld to the two-color-emission part and controlling applied voltage to the single-coloremission part, separately. The color can be continuously selected by modulating the applied voltage and by controlling the pulse width of the injection current to the emission part. Acknowledgements Part of this work was supported by the Support Center for Advanced Telecommunications Technology Research. References [1] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913. [2] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160. [3] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [4] J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 67 (1995) 2281. [5] M. Uchida, Y. Ohmori, T. Noguchi, T. Ohnishi, K. Yoshino, Jpn. J. Appl. Phys. 32 (1993) L921. [6] A. Fujii, M. Yoshida, Y. Ohmori, K. Yoshino, Jpn. J. Appl. Phys. 34 (1995) L499. [7] M. Yoshida, A. Fujii, Y. Ohmori, K. Yoshino, Jpn. J. Appl. Phys. 35 (1996) L397. [8] M. Yoshida, A. Fujii, Y. Ohmori, K. Yoshino, Appl. Phys. Lett. 69 (1996) 734. [9] M. Hamaguchi, K. Yoshino, Appl. Phys. Lett. 69 (1996) 143. [10] Y. Yang, Q. Pei, Appl. Phys. Lett. 68 (1996) 2708. [11] N. Tada, A. Fujii, Y. Ohmori, K. Yoshino, IEEE Trans. Electron Devices 44 (1997) 1234.
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