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Effective design of blue organic electroluminescent devices by introducing functional monomeric layers
Materials Science and Engineering B85 (2001) 96 – 99 www.elsevier.com/locate/mseb
Effective design of blue organic electroluminescent devices by introducing functional monomeric layers Jae-Ho Choi a, Sin-Hye Jung a, Soon-Ki Kwon b, Won-Jei Cho a, Chang-Sik Ha a,* b
a Department of Polymer Science and Engineering, Pusan National Uni6ersity, Pusan 609 -735, South Korea Department of Polymer Science and Engineering, Gyungsang National Uni6ersity, Jinju city, Gyungsang Namdo 660 -701, South Korea
Received 24 July 2000; received in revised form 7 September 2000
Abstract Blue organic electroluminescent devices (OELDs) of a multi-layered structure were fabricated and their device performance was investigated. A distyryl biphenyl arylene derivative was synthesized as a blue emitting material. To improve thermal stability of the monomeric hole-transporting emissive material, poly(bisphenol A-co-4-nitrophthalic anhydride-co-l,3-phenylene diamine) was used as a matrix. For more effective design of the devices, poly(styrene sulfonate) doped poly(3,4-ethylenedioxythiophene),2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine) and tris(8-quinolinolato)aluminum (Alq3) were introduced as a buffer layer, a hole-blocking layer, and an electron-injection layer, respectively. The OELDs showed bright green color when Bathocuproine layer was not applied. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Blue organic electroluminescent devices; Hole-transporting emissive material; Bathocuproine
1. Introduction Electroluminescence (EL) is a phenomenon that has been seen in a wide range of semiconductors and organic solids, for the generation of light [1]. Organic electroluminescent devices (OELDs) are of considerable interests in the applications to flat panel displays because of their high luminance efficiency, low driving voltage, and tunable colors [2,3]. The EL mechanism is followed through the recombination of holes and electrons injected from anode and cathode, respectively. Recombination of the carriers in the organic active layer produces excitons; energy is transformed into light through the radiative decay of the excited states. The EL efficiency could be increased by improving band match and choosing materials with high radiative recombination possibility [4]. In this work, we fabricated a multi-layered blue OELD by introducing functional monomeric layers as a buffer layer, a hole-blocking layer (HBL), and an electron-injection layer. Poly(styrene sulfonate) doped
* Corresponding author. Fax: + 82-51-5144331. E-mail address: [email protected] (C.-S. Ha).
poly(3,4-ethylene dioxythiophene; PEDOT/PSS) was used as a buffer layer, where the PEDOT has been doped with polystyrene sulfonic acid in order to enhance the conductivity of the film [5]. 2,9-Dimethyl-4,7diphenyl-1,10-phenanthroline (Bathocuproine) was also introduced as a HBL. It was reported that the Bathocuproine can be used as a very effective electrontransport material because of its high electron mobility [6]. Since Bathocuproine is a small, rigid and planar molecule with extended p-electron, the high electron mobility of Bathocuproine may be originated from short hopping length for electron-transport. Naka et al. [6] found that the electrons in Bathocuproine are about two orders of magnitude more mobile than in Alq3. The use of Bathocuproine as a HBL instead of as an electron-transport layer has been attemped very recently [11]. Also, we used a new distyryl biphenyl arylene (DBA) derivative, which was synthesized by the Wittig and Suzuki reaction, as a blue emitting material. Details of the synthesis of DBA were described elsewhere [7]. The purpose of this work is to investigate the performance of the blue OELDs containing DBA, as a new blue emitting material, coupled with Bathocuproine as a HBL (Fig. 1).
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 3 7 - 2
J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
2. Experimental We fabricated ITO (100 nm)/PEDOT:PSS (15 nm)/ DBA-dispersed PEI (70 nm)/Bathocuproine (50 nm)/ Alq3 (40 nm)/Al (80 nm) structured devices. To improve thermal stability of the devices, a fully aromatic polyimide, poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine; PEI) was incorporated as a matrix for binding DBA derivative [8,13,14]. Aqueous dispersion of PEDOT/PSS was spin-coated
97
onto an ITO glass substrate at 2850 rpm for 2 min. Soft-baking process was performed in order to remove solvent and stabilize the film. A solution of PEI and DBA blend was prepared using dimethylformamide as a solvent. The weight ratio of DBA to PEI and overall solid concentration were 50/50 and 4 wt.%, respectively. The solution was spin-coated onto PEDOT/PSS layer at 2850 rpm for 2 min. Then Bathocuproine, Alq3 and aluminum were sequentially evaporated under 2×10 − 5 Torr. via the thermal vacuum deposition method.
Fig. 1. The structures of the organic materials used for the blue emitting device; DBA, PEDOT/PSS and Bathocuproine.
Fig. 2. The UV –VIS absorption and PL spectra of DBA and EL spectrum of the device.
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J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
3. Results and discussion
Fig. 3. (a) Voltage vs. current density relationship of the device. (b)Voltage vs. luminance relationship of the device.
Fig. 4. The energy level diagrams of the blue emitting device.
The photoluminescence (PL) and EL of the film were measured by high-sensitive fiber optic spectrometer (S2000, Ocean Optic. Inc.) with 2048-element linear CCD-array silicon detector. The current– voltage–luminance (I –V– L) characteristics of the device were measured by KEITHLEY 6517 electrometer.
Fig. 2 shows the absorption and PL spectra of a DBA film. In the film, DBA has an absorption peak at 350 nm and a fluorescence peak at 470 nm. The large Stokes shift of 120 nm caused by vibrational relaxation suggests that the equilibrium molecular configurations are different between the singlet excited state and the ground state. We could observe the blue EL spectrum of the device at 480 nm, taken at 15 V DC. It was found that Bathocuproine can play a role of an effective hole-blocking material, to make recombination of holes and electrons occur in the emitting layer. The hole-blocking role of Bathocuproine can be proven by the fact that a bright green color (515 nm) was observed when Bathocuproine layer was not used. The position and the shape of the emission spectrum almost coincide with its fluorescence spectrum, indicating that the EL takes place in the excited singlet-state of DBA [9]. Fig. 3 shows the I–V–L relationship. It is seen that the turn-on voltage was 8 and 21 V DC, in the presence and absence of the PEDOT/PSS layer, respectively. The device design with the PEDOT/PSS layer has been reported to bring about a reduction of the barrier height at the anode/organic solid interface [5,10]. Fig. 4 shows the energy level diagrams of the device. Highest occupied molecular orbital levels of ITO, PEDOT/PSS, DBA, and Bathocuproine are 4.8, 5.0, 5.25, and 6.1 eV, respectively, whereas lowest unoccupied molecular orbital levels of DBA, Bathocuproine, Alq3, and aluminum are 2.3, 2.4, 2.6, and 4.2 eV, respectively [9,11]. The energy levels of DBA were determined by conventional cyclovoltammetry method. The energy level diagrams clearly show why the device performance was improved when the PEDOT/PSS layer was introduced [12]. Brown et al. [5] reported that EL efficiency was increased by 30% on average, and turn-on voltage was decreased about 50%, after the insertion of the PEDOT/PSS buffer layer. The holes injected from ITO and the electrons injected from aluminum move into the DBA layer respectively. The holes, however, cannot move over the energy barrier between DBA and Bathocuproine, because of the effective hole-blocking property of the Bathocuproine. Therefore, the recombination area is in the DBA layer, and hence blue emission was observed from DBA [10]. Kijima et al. [11] achieved bright blue emission, 10 000 cd m − 2 at 9 V DC and low driving voltage, 3 V DC when Bathocuproine was incorporated as a HBL. More detailed studies on the effect of device structures on the device performances including luminescence efficiency as well as lifetime are now underway and will be reported elsewhere.
J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
99
wishes to thank the Centre of Integrated of Molecular Systems, POSTECH, Korea.
4. Conclusions We propose a new device system for blue emission. DBA derivative was used as a blue emitting material. For band gap alignment, a metallic polymer PEDOT/ PSS was introduced as a buffer layer. PEI was used as a matrix for binding a monomeric emitting material to improve thermal stability of the device. And we chose Bathocuproine as a hole-blocking material to prevent holes from moving into the Alq3 layer, thus leading to the recombination of holes and electrons occur in the emitting layer. We measured the optoelectronic characteristics of the blue emitting devices. The optical absorption peak and the fluorescence peak of a DBA film were observed at 350 and 470 nm, respectively. The large Stokes shift of 120 nm, which has been typically found in organic solids was recognized. A blue EL spectrum of the device was observed at 480 nm, and we saw blue color through the transparent opposite side of ITO glass substrate. Bright green color was observed at 515 nm when Bathocuproine layer was not used.
Acknowledgements The authors thank to Prof. Soon-Ki Kwon for supplying monomeric emitting materials. This study was financially supported by the Korea Research Foundation under grant number: 1998-001-E01751. CSH
.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 12. [2] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [3] 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 347 (1990) 539. [4] C.W. Tang, S.A. VanSlyke, C.H. Chen, Appl. Phys. Lett. 65 (1989) 1. [5] T.M. Brown, J.S. Kim, R.H. Friend, F. Cacialli, Appl. Phys. Lett. 75 (1999) 1679. [6] S. Naka, H. Okada, H. Onnagawa, Appl. Phys. Lett. 76 (2000) 197. [7] Japanese Patent 6-207170 (1994). [8] J.H. Choi, S.H. Jung, Y.H. Kim, D.C. Shin, S.K. Kwon, W.J. Cho, C.S. Ha, Mol. Cryst. Liq. Cryst. 349 (2000) 123. [9] T.P. Nguyen, P. Jolinat, P. Destruel, R. Clergereaux, J. Farenc, Thin Solid Films 325 (1998) 175. [10] F. Cacialli, J.S. Kim, T.M. Brown, J. Morgado, M. Granstrom, R.H. Friend, G. Gigli, R. Cingolani, L. Favaretto, G. Barbarella, R. Daik, W.J. Feast, Synth. Met. 109 (2000) 7. [11] Y. Kijima, N. Asai, S. Tamura, Jpn. J. Appl. Phys. 38 (1999) 5274. [12] Y.Z. Wang, R.G. Sun, F. Meghdadi, G. Leising, A.J. Epstein, Appl. Phys. Lett. 74 (1999) 3613. [13] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, Mol Cryst. Liq. Cryst 316 (1998) 301. [14] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 345.
Effective design of blue organic electroluminescent devices by introducing functional monomeric layers Jae-Ho Choi a, Sin-Hye Jung a, Soon-Ki Kwon b, Won-Jei Cho a, Chang-Sik Ha a,* b
a Department of Polymer Science and Engineering, Pusan National Uni6ersity, Pusan 609 -735, South Korea Department of Polymer Science and Engineering, Gyungsang National Uni6ersity, Jinju city, Gyungsang Namdo 660 -701, South Korea
Received 24 July 2000; received in revised form 7 September 2000
Abstract Blue organic electroluminescent devices (OELDs) of a multi-layered structure were fabricated and their device performance was investigated. A distyryl biphenyl arylene derivative was synthesized as a blue emitting material. To improve thermal stability of the monomeric hole-transporting emissive material, poly(bisphenol A-co-4-nitrophthalic anhydride-co-l,3-phenylene diamine) was used as a matrix. For more effective design of the devices, poly(styrene sulfonate) doped poly(3,4-ethylenedioxythiophene),2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine) and tris(8-quinolinolato)aluminum (Alq3) were introduced as a buffer layer, a hole-blocking layer, and an electron-injection layer, respectively. The OELDs showed bright green color when Bathocuproine layer was not applied. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Blue organic electroluminescent devices; Hole-transporting emissive material; Bathocuproine
1. Introduction Electroluminescence (EL) is a phenomenon that has been seen in a wide range of semiconductors and organic solids, for the generation of light [1]. Organic electroluminescent devices (OELDs) are of considerable interests in the applications to flat panel displays because of their high luminance efficiency, low driving voltage, and tunable colors [2,3]. The EL mechanism is followed through the recombination of holes and electrons injected from anode and cathode, respectively. Recombination of the carriers in the organic active layer produces excitons; energy is transformed into light through the radiative decay of the excited states. The EL efficiency could be increased by improving band match and choosing materials with high radiative recombination possibility [4]. In this work, we fabricated a multi-layered blue OELD by introducing functional monomeric layers as a buffer layer, a hole-blocking layer (HBL), and an electron-injection layer. Poly(styrene sulfonate) doped
* Corresponding author. Fax: + 82-51-5144331. E-mail address: [email protected] (C.-S. Ha).
poly(3,4-ethylene dioxythiophene; PEDOT/PSS) was used as a buffer layer, where the PEDOT has been doped with polystyrene sulfonic acid in order to enhance the conductivity of the film [5]. 2,9-Dimethyl-4,7diphenyl-1,10-phenanthroline (Bathocuproine) was also introduced as a HBL. It was reported that the Bathocuproine can be used as a very effective electrontransport material because of its high electron mobility [6]. Since Bathocuproine is a small, rigid and planar molecule with extended p-electron, the high electron mobility of Bathocuproine may be originated from short hopping length for electron-transport. Naka et al. [6] found that the electrons in Bathocuproine are about two orders of magnitude more mobile than in Alq3. The use of Bathocuproine as a HBL instead of as an electron-transport layer has been attemped very recently [11]. Also, we used a new distyryl biphenyl arylene (DBA) derivative, which was synthesized by the Wittig and Suzuki reaction, as a blue emitting material. Details of the synthesis of DBA were described elsewhere [7]. The purpose of this work is to investigate the performance of the blue OELDs containing DBA, as a new blue emitting material, coupled with Bathocuproine as a HBL (Fig. 1).
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 3 7 - 2
J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
2. Experimental We fabricated ITO (100 nm)/PEDOT:PSS (15 nm)/ DBA-dispersed PEI (70 nm)/Bathocuproine (50 nm)/ Alq3 (40 nm)/Al (80 nm) structured devices. To improve thermal stability of the devices, a fully aromatic polyimide, poly(bisphenol A-co-4-nitrophthalic anhydride-co-1,3-phenylene diamine; PEI) was incorporated as a matrix for binding DBA derivative [8,13,14]. Aqueous dispersion of PEDOT/PSS was spin-coated
97
onto an ITO glass substrate at 2850 rpm for 2 min. Soft-baking process was performed in order to remove solvent and stabilize the film. A solution of PEI and DBA blend was prepared using dimethylformamide as a solvent. The weight ratio of DBA to PEI and overall solid concentration were 50/50 and 4 wt.%, respectively. The solution was spin-coated onto PEDOT/PSS layer at 2850 rpm for 2 min. Then Bathocuproine, Alq3 and aluminum were sequentially evaporated under 2×10 − 5 Torr. via the thermal vacuum deposition method.
Fig. 1. The structures of the organic materials used for the blue emitting device; DBA, PEDOT/PSS and Bathocuproine.
Fig. 2. The UV –VIS absorption and PL spectra of DBA and EL spectrum of the device.
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J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
3. Results and discussion
Fig. 3. (a) Voltage vs. current density relationship of the device. (b)Voltage vs. luminance relationship of the device.
Fig. 4. The energy level diagrams of the blue emitting device.
The photoluminescence (PL) and EL of the film were measured by high-sensitive fiber optic spectrometer (S2000, Ocean Optic. Inc.) with 2048-element linear CCD-array silicon detector. The current– voltage–luminance (I –V– L) characteristics of the device were measured by KEITHLEY 6517 electrometer.
Fig. 2 shows the absorption and PL spectra of a DBA film. In the film, DBA has an absorption peak at 350 nm and a fluorescence peak at 470 nm. The large Stokes shift of 120 nm caused by vibrational relaxation suggests that the equilibrium molecular configurations are different between the singlet excited state and the ground state. We could observe the blue EL spectrum of the device at 480 nm, taken at 15 V DC. It was found that Bathocuproine can play a role of an effective hole-blocking material, to make recombination of holes and electrons occur in the emitting layer. The hole-blocking role of Bathocuproine can be proven by the fact that a bright green color (515 nm) was observed when Bathocuproine layer was not used. The position and the shape of the emission spectrum almost coincide with its fluorescence spectrum, indicating that the EL takes place in the excited singlet-state of DBA [9]. Fig. 3 shows the I–V–L relationship. It is seen that the turn-on voltage was 8 and 21 V DC, in the presence and absence of the PEDOT/PSS layer, respectively. The device design with the PEDOT/PSS layer has been reported to bring about a reduction of the barrier height at the anode/organic solid interface [5,10]. Fig. 4 shows the energy level diagrams of the device. Highest occupied molecular orbital levels of ITO, PEDOT/PSS, DBA, and Bathocuproine are 4.8, 5.0, 5.25, and 6.1 eV, respectively, whereas lowest unoccupied molecular orbital levels of DBA, Bathocuproine, Alq3, and aluminum are 2.3, 2.4, 2.6, and 4.2 eV, respectively [9,11]. The energy levels of DBA were determined by conventional cyclovoltammetry method. The energy level diagrams clearly show why the device performance was improved when the PEDOT/PSS layer was introduced [12]. Brown et al. [5] reported that EL efficiency was increased by 30% on average, and turn-on voltage was decreased about 50%, after the insertion of the PEDOT/PSS buffer layer. The holes injected from ITO and the electrons injected from aluminum move into the DBA layer respectively. The holes, however, cannot move over the energy barrier between DBA and Bathocuproine, because of the effective hole-blocking property of the Bathocuproine. Therefore, the recombination area is in the DBA layer, and hence blue emission was observed from DBA [10]. Kijima et al. [11] achieved bright blue emission, 10 000 cd m − 2 at 9 V DC and low driving voltage, 3 V DC when Bathocuproine was incorporated as a HBL. More detailed studies on the effect of device structures on the device performances including luminescence efficiency as well as lifetime are now underway and will be reported elsewhere.
J.-H. Choi et al. / Materials Science and Engineering B85 (2001) 96–99
99
wishes to thank the Centre of Integrated of Molecular Systems, POSTECH, Korea.
4. Conclusions We propose a new device system for blue emission. DBA derivative was used as a blue emitting material. For band gap alignment, a metallic polymer PEDOT/ PSS was introduced as a buffer layer. PEI was used as a matrix for binding a monomeric emitting material to improve thermal stability of the device. And we chose Bathocuproine as a hole-blocking material to prevent holes from moving into the Alq3 layer, thus leading to the recombination of holes and electrons occur in the emitting layer. We measured the optoelectronic characteristics of the blue emitting devices. The optical absorption peak and the fluorescence peak of a DBA film were observed at 350 and 470 nm, respectively. The large Stokes shift of 120 nm, which has been typically found in organic solids was recognized. A blue EL spectrum of the device was observed at 480 nm, and we saw blue color through the transparent opposite side of ITO glass substrate. Bright green color was observed at 515 nm when Bathocuproine layer was not used.
Acknowledgements The authors thank to Prof. Soon-Ki Kwon for supplying monomeric emitting materials. This study was financially supported by the Korea Research Foundation under grant number: 1998-001-E01751. CSH
.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Nature 397 (1999) 12. [2] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [3] 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 347 (1990) 539. [4] C.W. Tang, S.A. VanSlyke, C.H. Chen, Appl. Phys. Lett. 65 (1989) 1. [5] T.M. Brown, J.S. Kim, R.H. Friend, F. Cacialli, Appl. Phys. Lett. 75 (1999) 1679. [6] S. Naka, H. Okada, H. Onnagawa, Appl. Phys. Lett. 76 (2000) 197. [7] Japanese Patent 6-207170 (1994). [8] J.H. Choi, S.H. Jung, Y.H. Kim, D.C. Shin, S.K. Kwon, W.J. Cho, C.S. Ha, Mol. Cryst. Liq. Cryst. 349 (2000) 123. [9] T.P. Nguyen, P. Jolinat, P. Destruel, R. Clergereaux, J. Farenc, Thin Solid Films 325 (1998) 175. [10] F. Cacialli, J.S. Kim, T.M. Brown, J. Morgado, M. Granstrom, R.H. Friend, G. Gigli, R. Cingolani, L. Favaretto, G. Barbarella, R. Daik, W.J. Feast, Synth. Met. 109 (2000) 7. [11] Y. Kijima, N. Asai, S. Tamura, Jpn. J. Appl. Phys. 38 (1999) 5274. [12] Y.Z. Wang, R.G. Sun, F. Meghdadi, G. Leising, A.J. Epstein, Appl. Phys. Lett. 74 (1999) 3613. [13] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, Mol Cryst. Liq. Cryst 316 (1998) 301. [14] H. Lim, H. Park, J.G. Lee, Y. Kim, W.J. Cho, C.S. Ha, SPIE Proc. 3281 (1998) 345.