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Blue light emitting diode with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB)
Synthetic Metals 117 (2001) 227±228
Blue light emitting diode with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) Jung-Hyun Kima, Sokwon Noha, Kwanju Kimb, Sung-Taek Lima, Dong-Myung Shina,* a
b
Department of Chemical Engineering, Hong-Ik University, 72-1 Sangsu-dong Mapo-gu, Hong-ik University, Seoul 121-791, South Korea Department of Mechanical Engineering, Hong-Ik University, 72-1 Sangsu-dong Mapo-gu, Hong-ik University, Seoul 121-791, South Korea
Abstract Blue electroluminescence (EL) devices with the structure ITO/poly(N-vinylcarbazole) (PVK):1,1,4,4-tetraphenyl-1,3-butadiene (TPB):triphenylamine (TPA)/Al have been studied. PVK doped with TPB was used for hole transport layer (HTL) and emitting layer (EML). The triphenylamine contributed to the small band width of blue emission. TPB is known as a blue-emitting material. In addition, the wavelength of blue emission observed from PVK:TPB (3 mol%) is almost 450 nm, which is the recommended blue emission wavelength. In this work, a TPA doping effect on full-width at half maximum (FWHM) observed from TPB:TPA-doped PVK devices is demonstrated. To investigate the role of TPB at the ITO/PVK:TPB:TPA layer interface, the morphology of the PVK:TPB:TPA layer on ITO was studied with atomic force microscopy (AFM). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroluminescence; Blue; FWHM; PVK; TPB; TPA
1. Introduction The process technology of organic light emitting diode (OLED) structures has been developed for the realization of low cost, low weight full-color ¯at panel displays due to the opportunity of fabrication on a large scale. During recent years, much progress has been made in areas of new charge transport materials, device ef®ciency, and reliability [1±3]. Despite these important developments, there are still some obstacles to be overcome for full-color OELD. For full-color display, it is necessary to realize pure red, green and blue. So the full-width at half maximum (FWHM) of the spectra at the primary colors should not exceed about 50 nm for ef®cient color mixing. And luminance intensity should be high. In this paper, blue electroluminescence (EL) devices with the structure ITO/poly(N-vinylcarbazole) (PVK):1,1,4,4-tetraphenyl-1,3-butadiene (TPB):triphenylamine (TPA)/Al have been studied. 2. Experimental PVK doped with TPB was synthesized as reported previously [4]. TPB:TPA-doped PVK was spin-cast on the ITO glass. Aluminum electrodes were vacuum-deposited under a pressure of 10ÿ5 Torr. A programmable Keithley 238 SMU electrometer was used as a voltage source and current * Corresponding author. Tel.: 82-2-320-1652; fax: 82-2-334-5842. E-mail address: [email protected] (D.-M. Shin).
measurement equipment. This SMU was controlled by computer. To measure the intensities of EL and photoluminescence (PL), we used Perkin Elmer LS50B. 3. Results and discussion Fig. 1 shows the molecular structure of materials used in this study. Most of them, i.e. every single device, showed pure blue luminescence. From the EL devices with different blend materials, the characteristics of FWHM are shown in Figs. 2 and 3. Due to the contribution of TPA, FWHM is signi®cantly reduced compared to that observed from PVK:TPB (3 mol%) of both PL and EL [5]. It may be attributed to the quenching effect of TPA molecules on the reduced vibrational effect of TPA. In Figs. 2 and 3, it is observed that the device of PVK:TPB:TPA (1 mol%) has the highest EL and PL intensities. EL and PL intensities seem to be related to the concentration of TPA (Table 1). TPA also enhanced the emission intensity. This experiment shows that PVK:TPB emits pure blue with an FWHM of 85 nm which is signi®cantly reduced to about 65 nm by addition of TPA. As shown in Fig. 4, the morphology of interface is not pro®table for achieving high ef®ciency. The turn-on voltages of devices were around 10±15 V which was the characteristic voltage for PVK devices (Fig. 5).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 3 7 0 - 2
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J.-H. Kim et al. / Synthetic Metals 117 (2001) 227±228
Fig. 4. AFM image of the PVK:TPB:TPA interface. Fig. 1. Structures of (a) poly(N-vinylcarbazole) (PVK), (b) triphenylamine (TPA) and (c) 1,1,4,4-tetraphenyl-1,3-butadiene (TPB).
Fig. 5. Luminance vs. voltage curve observed for ITO/PVK:TPB:TPA/Al. Fig. 2. PL spectra observed from ITO/PVK:TPB:TPA/Al.
The low EL ef®ciency, 0.0006, may be due to the bad morphology at the PVK:TPB:TPA interface which was observed with atomic force microscopy (AFM). Acknowledgements This work was supported in part by the Ministry of Information and Communication, Korea (Grant No. Cl98-5170) and Korea Research Foundation. References
Fig. 3. EL spectra of ITO/PVK:TPB:TPA/Al. Table 1 FWHM of PL and EL observed from PVK:TPB and PVK:TPB:TPA devices Structures
PL (nm)
EL (nm)
PVK:TPB (3 mol%) PVK:TPB:TPA (1 mol%) PVK:TPB:TPA (3 mol%) PVK:TPB:TPA (7 mol%)
84.928 67.604 66.752 66.492
85.3 67.604 63.139 67.811
[1] Y. Hamada, J. Sano, M. Fufita, Y. Nisho, K. Shihata, Jpn. J. Appl. Phys. 32 (1993) L514. [2] Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yotemoto, S. Kawami, K. Intai, Appl. Phys. Lett. 56 (1994) 807. [3] Y. Hamada, J. Sano, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 (1995) L824. [4] Zhi-lin Zhang, et al., in: S. Miyata, H. Singh Nalwa (Eds.), Organic Electroluminescent Materials and Devices, Gordon and Breach, London, 1997. [5] P. Suppan, Chemistry and Light, The Royal Society of Chemistry, London, 1994, p. 220.
Blue light emitting diode with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) Jung-Hyun Kima, Sokwon Noha, Kwanju Kimb, Sung-Taek Lima, Dong-Myung Shina,* a
b
Department of Chemical Engineering, Hong-Ik University, 72-1 Sangsu-dong Mapo-gu, Hong-ik University, Seoul 121-791, South Korea Department of Mechanical Engineering, Hong-Ik University, 72-1 Sangsu-dong Mapo-gu, Hong-ik University, Seoul 121-791, South Korea
Abstract Blue electroluminescence (EL) devices with the structure ITO/poly(N-vinylcarbazole) (PVK):1,1,4,4-tetraphenyl-1,3-butadiene (TPB):triphenylamine (TPA)/Al have been studied. PVK doped with TPB was used for hole transport layer (HTL) and emitting layer (EML). The triphenylamine contributed to the small band width of blue emission. TPB is known as a blue-emitting material. In addition, the wavelength of blue emission observed from PVK:TPB (3 mol%) is almost 450 nm, which is the recommended blue emission wavelength. In this work, a TPA doping effect on full-width at half maximum (FWHM) observed from TPB:TPA-doped PVK devices is demonstrated. To investigate the role of TPB at the ITO/PVK:TPB:TPA layer interface, the morphology of the PVK:TPB:TPA layer on ITO was studied with atomic force microscopy (AFM). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroluminescence; Blue; FWHM; PVK; TPB; TPA
1. Introduction The process technology of organic light emitting diode (OLED) structures has been developed for the realization of low cost, low weight full-color ¯at panel displays due to the opportunity of fabrication on a large scale. During recent years, much progress has been made in areas of new charge transport materials, device ef®ciency, and reliability [1±3]. Despite these important developments, there are still some obstacles to be overcome for full-color OELD. For full-color display, it is necessary to realize pure red, green and blue. So the full-width at half maximum (FWHM) of the spectra at the primary colors should not exceed about 50 nm for ef®cient color mixing. And luminance intensity should be high. In this paper, blue electroluminescence (EL) devices with the structure ITO/poly(N-vinylcarbazole) (PVK):1,1,4,4-tetraphenyl-1,3-butadiene (TPB):triphenylamine (TPA)/Al have been studied. 2. Experimental PVK doped with TPB was synthesized as reported previously [4]. TPB:TPA-doped PVK was spin-cast on the ITO glass. Aluminum electrodes were vacuum-deposited under a pressure of 10ÿ5 Torr. A programmable Keithley 238 SMU electrometer was used as a voltage source and current * Corresponding author. Tel.: 82-2-320-1652; fax: 82-2-334-5842. E-mail address: [email protected] (D.-M. Shin).
measurement equipment. This SMU was controlled by computer. To measure the intensities of EL and photoluminescence (PL), we used Perkin Elmer LS50B. 3. Results and discussion Fig. 1 shows the molecular structure of materials used in this study. Most of them, i.e. every single device, showed pure blue luminescence. From the EL devices with different blend materials, the characteristics of FWHM are shown in Figs. 2 and 3. Due to the contribution of TPA, FWHM is signi®cantly reduced compared to that observed from PVK:TPB (3 mol%) of both PL and EL [5]. It may be attributed to the quenching effect of TPA molecules on the reduced vibrational effect of TPA. In Figs. 2 and 3, it is observed that the device of PVK:TPB:TPA (1 mol%) has the highest EL and PL intensities. EL and PL intensities seem to be related to the concentration of TPA (Table 1). TPA also enhanced the emission intensity. This experiment shows that PVK:TPB emits pure blue with an FWHM of 85 nm which is signi®cantly reduced to about 65 nm by addition of TPA. As shown in Fig. 4, the morphology of interface is not pro®table for achieving high ef®ciency. The turn-on voltages of devices were around 10±15 V which was the characteristic voltage for PVK devices (Fig. 5).
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 3 7 0 - 2
228
J.-H. Kim et al. / Synthetic Metals 117 (2001) 227±228
Fig. 4. AFM image of the PVK:TPB:TPA interface. Fig. 1. Structures of (a) poly(N-vinylcarbazole) (PVK), (b) triphenylamine (TPA) and (c) 1,1,4,4-tetraphenyl-1,3-butadiene (TPB).
Fig. 5. Luminance vs. voltage curve observed for ITO/PVK:TPB:TPA/Al. Fig. 2. PL spectra observed from ITO/PVK:TPB:TPA/Al.
The low EL ef®ciency, 0.0006, may be due to the bad morphology at the PVK:TPB:TPA interface which was observed with atomic force microscopy (AFM). Acknowledgements This work was supported in part by the Ministry of Information and Communication, Korea (Grant No. Cl98-5170) and Korea Research Foundation. References
Fig. 3. EL spectra of ITO/PVK:TPB:TPA/Al. Table 1 FWHM of PL and EL observed from PVK:TPB and PVK:TPB:TPA devices Structures
PL (nm)
EL (nm)
PVK:TPB (3 mol%) PVK:TPB:TPA (1 mol%) PVK:TPB:TPA (3 mol%) PVK:TPB:TPA (7 mol%)
84.928 67.604 66.752 66.492
85.3 67.604 63.139 67.811
[1] Y. Hamada, J. Sano, M. Fufita, Y. Nisho, K. Shihata, Jpn. J. Appl. Phys. 32 (1993) L514. [2] Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada, Y. Yotemoto, S. Kawami, K. Intai, Appl. Phys. Lett. 56 (1994) 807. [3] Y. Hamada, J. Sano, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 (1995) L824. [4] Zhi-lin Zhang, et al., in: S. Miyata, H. Singh Nalwa (Eds.), Organic Electroluminescent Materials and Devices, Gordon and Breach, London, 1997. [5] P. Suppan, Chemistry and Light, The Royal Society of Chemistry, London, 1994, p. 220.