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Chlorine effect on- electroluminescence of Tb complexes
Synthetic Metals 111–112 Ž2000. 113–117 www.elsevier.comrlocatersynmet
Chlorine effect on- electroluminescence of Tb complexes Young Kwan Kim a,) , Sang-Woo Pyo b, Don Soo Choi a , Hyun Sue Hue c , Seung Hee Lee c , Yun-Kyoung Ha d , Han Sung Lee b, Jung Soo Kim b, Woo Young Kim e a b
Department of Chemical Engineering, Hongik UniÕersity, Seoul, South Korea Department of Electrical Engineering, Hongik UniÕersity, Seoul, South Korea c Department of Industrial Chemistry, Hongik UniÕersity, Seoul, South Korea d Department of Basic Science, Hongik UniÕersity, Seoul, South Korea e LCD DiÕision, Hyundai Electronics Industries, Ichon, South Korea
Abstract Terbium complexes are known to be the green-light-emitting materials. Electroluminescent ŽEL. characteristics of terbium complexes containing phenanthroline ŽPhen. and 3-chlorophenanthroline ŽCl-Phen. as a ligand were evaluated, where the device structure of ITOrTPDrTb complexesrAlq 3 or Bebq 2rLi:Al was used. It was found that terbium complex containing Cl-Phen shows higher luminance efficiency compared to terbium complex containing Phen. Measurements of oxidation and reduction potential of these complexes have been carried out by using cyclic voltammetric method in order to explain the difference in luminance efficiency. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Terbium complex; Organic electroluminescence; Cyclic voltammetric measurement
1. Introduction Electroluminescent ŽEL. devices based on organic materials have been of great interest due to their possible applications for large-area flat-panel displays. They are attractive because of their capability for multi-color emission, and low operation voltage w1–3x. The fluorescent properties of lanthanide chelate complexes have been known long w4x since these materials have been used as an active medium in dye lasers w5x. EL materials for the whole visible spectrum have been found and the brightness, as well as the device performance, are sufficient for most potential applications w6,7x. One of the problems still remaining is how to obtain narrow emission bands at the three primary colors — red, green, and blue — with sufficient luminance efficiencies for multi-color display applications. White-light organic electroluminescent devices ŽOELDs. can be realized using a combination of three emitter layers w8x or fabricating a multi-mode microcavity device w9x, but there is still no single fluorescent material emitting white light. An approach to realize such device characteristics is to use active layers of lanthanide
)
Corresponding author. E-mail address: [email protected] ŽY.K. Kim..
complexes with their inherent extremely sharp emission bands instead of commonly known organic dyes. In general, organic molecular compounds show the emission due to their p–p ) transitions resulting in luminescence bandwidths of about 80–100 nm. Spin statistic estimation leads to an internal quantum efficiency of dye-based EL devices limited to 25%. On the contrary, the fluorescence of lanthanide complexes is based on an intramolecular energy transfer from the triplet of the organic ligand to the 4 f energy states of the ion. Therefore, theoretical internal quantum efficiency is principally not limited w10x. In this study, multi-layer structures of OELD using TbŽACAC. 3 ŽPhen. and TbŽACAC. 3 ŽPhen-Cl. as an emissive layer were fabricated and their electrical characteristics such as EL, and J–V curves were investigated, where the device structure of ITOrTPDrTb complexesrAlq 3 or Bebq 2r Li:Al was used. Ionization potential ŽIP. and electron affinity ŽEA. of these terbium complexes, TPD, Alq 3 , and Bebq 2 were measured by cyclic voltammetric method.
2. Experimental In this study, glass substraterITOrTb complexesrAlq 3 or Bebq 2rLi:Al structures were fabricated by evaporation
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 1 9 - 7
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method, where TPD was used as a hole-transporting material, terbium complexes as emitting materials, Alq 3 or Bebq 2 as electron-transporting materials. The molecular structures of these materials are shown in Fig. 1. A schematic cross-section of the light-emitting device is also illustrated in Fig. 2. The thickness of TPD, terbium complex, and Alq 3 or Bebq 2 is fixed as 30, 30 and 30 nm, respectively. The electrical characteristics of these devices
Fig. 2. A schematic diagram of OELDs used in this study.
were measured by Keithley 238 electrometer controlled by a PC. The luminance of these devices was measured by The Minolta CS-100. Details on the measurements of
Fig. 1. Molecular structures of TbŽACAC. 3 ŽPhen., TbŽACAC. 3 ŽPhen-Cl., TPD, Alq 3 , and Bebq 2 .
Fig. 3. Current density Ž J . –bias voltage Ž V . characteristics for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117
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ionization potential and electron affinity of these materials using the cyclic voltammetric method will be published elsewhere w11x.
3. Results and discussion Current density Ž J . –bias voltage Ž V . characteristics for the device structures of glass substraterITOrTb complexesrAlq 3rLi:Al and glass substraterITOrTb complexesrBebq 2rLi:Al are shown in Fig. 3a and b, respectively. It was shown in Fig. 3 that the turn-on voltage of the device containing TbŽACAC. 3 ŽPhen. was lower than that of the device containing TbŽACAC. 3 ŽCl-Phen. regardless of electron transport material. The log Ž J . –log Ž V . characteristics for the device structures of glass substraterITOrTb complexesrAlq 3r Li:Al and glass substraterITOrTb complexesrBebq 2r Li:Al are also shown in Fig. 4a and b, respectively. As shown in Fig. 4, it was found that the current density J is proportional to V w9,10x over nine decades of the current, which is consistent with the trapped-charge-limited ŽTCL. conduction characteristics. The current density Ž J . –bias voltage Ž V . characteristics for terbium-complex-based OELD are shown in Fig. 4. These current density Ž J . –bias Fig. 5. The luminance Ž L. –current density Ž J . for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
Fig. 4. The log Ž J . –log Ž V . characteristics for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
voltage Ž V . curves can be interpreted by trap-chargelimited current ŽTCLC. mechanism. At low voltages, the density of injected carriers in these devices is low. The conductance is due to terbium complex’s own carriers. For trap levels located at a single energy, this trap-filled limit ŽTFL. is an extremely sharp transition, at which the current directly switches to the space-charge-limited current ŽSCLC.. The more gradual increase, as observed in Fig. 4, points to a distribution of trap-level energies. The log Ž J . –log Ž V . characteristic in Fig. 4 is well-described using an exponential distribution of traps. The luminance Ž L. –current density Ž J . characteristics of OELDs with various device structures are shown in Fig. 5. The maximum luminance of approximately 95 cdrm2 from the ITOrTPDrTbŽACAC. 3 ŽCl-Phen.rAlq 3rLi:Alr Al and ITOrTPDrTbŽACAC. 3 ŽCl-Phen.rBebq 2rLi:Alr Al structures were obtained at the current density of 0.65 and 0.52 Arm2 , respectively, which is relatively low because Tb complexes have a poor carrier transport property in the solid state. From this figure, it was found that the device using TbŽACAC. 3 ŽCl-Phen. and Bebq 2 as a emissive layer and an electron transporting layer, respectively, has the highest luminance at the same current density. In Fig. 6a and b, the luminous efficiencies for the device structures of glass substraterITOrTb complexesr
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Alq 3rLi:Al and glass substraterITOrTb complexesr Bebq 2rLi:Al are shown, respectively. The external luminous efficiencies of the OELDs with various device structures as a function of the bias voltage are shown in Fig. 6. The OELDs using TbŽACAC. 3 ŽCl-Phen. as an emitting layer have higher external luminous efficiency in comparison to that of TbŽACAC. 3 ŽPhen.. And the OELDs using Bebq 2 as an electron-transporting layer has higher external luminous efficiency than those using Alq 3 . Therefore, the OELD using TbŽACAC. 3 ŽCl-Phen. and Bebq 2 as an emitting layer and an electron-transporting layer has achieved the external luminous efficiency up to 0.03 lmrW. Using ionization potential and electron affinity values of various materials obtained by cyclic voltammetric method, the schematic energy band diagrams for four device structures used in this study are shown in Fig. 6. Under forward bias conditions, holes are injected from ITO into the valence band of the TPD layer and electrons from Li:Al into the conduction band of the Alq 3 or Bebq 2 layer. Injected holes can migrate into the Tb complex layer through the TPD layer and some electrons can migrate into the Tb complex layer over the injection barrier at the
Fig. 7. Schematic energy band diagrams of four devices structures with energy levels obtained by cyclic voltammetric method.
interface of Tb complexesrAlq 3 or Bebq 2 layer Ž0.155 eV:TbŽACAC. 3 ŽPhen., 0.13 eV:TbŽACAC. 3 ŽCl-Phen.. and can move the Tb complex layer through the electron-transporting layer. It was therefore found in Fig. 7 that the injection process of the device having TbŽACAC. 3 ŽClPhen. was easier than that of the device having TbŽACAC. 3 ŽPhen. as an emissive in spite of the higher turn-on voltage.
4. Conclusions The multi-layer thin-film OELDs containing Tb complexes as green-light-emitting materials with different electron-transporting materials were fabricated. In this study, it was found that the luminous efficiency of the device containing TbŽACAC. 3 ŽCl-Phen. was higher than that of the device containing TbŽACAC. 3 ŽPhen. in spite of the higher turn-on voltage, which was explained by using energy band diagram of two devices.
Acknowledgements This work was fully supported by Ministry of Information and Communication ŽMIC..
References
Fig. 6. The luminous efficiency for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substrater ITOrTb complexesrBebq 2 rLi:Al.
w1x C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 Ž1987. 913. w2x Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Tkahashi, K. Shibaya, Appl. Phys. Lett. 71 Ž1997. 3338. w3x S. Miyata, Organic Electroluminescent Materials and Devices, Gordon and Breach Publishers, 1997. w4x S.P. Sinha, Complexes of the Rare Earth, Pergamon, London, 1996.
Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117 w5x R. Reisteld, C.K. Joergensen, Lasers and Excited States of Rare Earths, Springer, Berlin, 1977. w6x J. Kido, M. Kimura, K. Nagai, Chem. Lett. 657 Ž1990. 47. w7x J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 192 Ž1993. 30. w8x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. Lett. 35 Ž1990. 394.
117
w9x A. Dodabalapur, L.J. Rothberg, T.M. Miller, Appl. Phys. Lett. 65 Ž1994. 2308. w10x S. Dirr, H. Johannes, J. Schobel, D. Ammermann, A. Bohler, W. Kowalsky, W. Grahn, SID ’97 Digest Ž1997. 778. w11x In preparation.
Chlorine effect on- electroluminescence of Tb complexes Young Kwan Kim a,) , Sang-Woo Pyo b, Don Soo Choi a , Hyun Sue Hue c , Seung Hee Lee c , Yun-Kyoung Ha d , Han Sung Lee b, Jung Soo Kim b, Woo Young Kim e a b
Department of Chemical Engineering, Hongik UniÕersity, Seoul, South Korea Department of Electrical Engineering, Hongik UniÕersity, Seoul, South Korea c Department of Industrial Chemistry, Hongik UniÕersity, Seoul, South Korea d Department of Basic Science, Hongik UniÕersity, Seoul, South Korea e LCD DiÕision, Hyundai Electronics Industries, Ichon, South Korea
Abstract Terbium complexes are known to be the green-light-emitting materials. Electroluminescent ŽEL. characteristics of terbium complexes containing phenanthroline ŽPhen. and 3-chlorophenanthroline ŽCl-Phen. as a ligand were evaluated, where the device structure of ITOrTPDrTb complexesrAlq 3 or Bebq 2rLi:Al was used. It was found that terbium complex containing Cl-Phen shows higher luminance efficiency compared to terbium complex containing Phen. Measurements of oxidation and reduction potential of these complexes have been carried out by using cyclic voltammetric method in order to explain the difference in luminance efficiency. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Terbium complex; Organic electroluminescence; Cyclic voltammetric measurement
1. Introduction Electroluminescent ŽEL. devices based on organic materials have been of great interest due to their possible applications for large-area flat-panel displays. They are attractive because of their capability for multi-color emission, and low operation voltage w1–3x. The fluorescent properties of lanthanide chelate complexes have been known long w4x since these materials have been used as an active medium in dye lasers w5x. EL materials for the whole visible spectrum have been found and the brightness, as well as the device performance, are sufficient for most potential applications w6,7x. One of the problems still remaining is how to obtain narrow emission bands at the three primary colors — red, green, and blue — with sufficient luminance efficiencies for multi-color display applications. White-light organic electroluminescent devices ŽOELDs. can be realized using a combination of three emitter layers w8x or fabricating a multi-mode microcavity device w9x, but there is still no single fluorescent material emitting white light. An approach to realize such device characteristics is to use active layers of lanthanide
)
Corresponding author. E-mail address: [email protected] ŽY.K. Kim..
complexes with their inherent extremely sharp emission bands instead of commonly known organic dyes. In general, organic molecular compounds show the emission due to their p–p ) transitions resulting in luminescence bandwidths of about 80–100 nm. Spin statistic estimation leads to an internal quantum efficiency of dye-based EL devices limited to 25%. On the contrary, the fluorescence of lanthanide complexes is based on an intramolecular energy transfer from the triplet of the organic ligand to the 4 f energy states of the ion. Therefore, theoretical internal quantum efficiency is principally not limited w10x. In this study, multi-layer structures of OELD using TbŽACAC. 3 ŽPhen. and TbŽACAC. 3 ŽPhen-Cl. as an emissive layer were fabricated and their electrical characteristics such as EL, and J–V curves were investigated, where the device structure of ITOrTPDrTb complexesrAlq 3 or Bebq 2r Li:Al was used. Ionization potential ŽIP. and electron affinity ŽEA. of these terbium complexes, TPD, Alq 3 , and Bebq 2 were measured by cyclic voltammetric method.
2. Experimental In this study, glass substraterITOrTb complexesrAlq 3 or Bebq 2rLi:Al structures were fabricated by evaporation
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 3 1 9 - 7
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Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117
method, where TPD was used as a hole-transporting material, terbium complexes as emitting materials, Alq 3 or Bebq 2 as electron-transporting materials. The molecular structures of these materials are shown in Fig. 1. A schematic cross-section of the light-emitting device is also illustrated in Fig. 2. The thickness of TPD, terbium complex, and Alq 3 or Bebq 2 is fixed as 30, 30 and 30 nm, respectively. The electrical characteristics of these devices
Fig. 2. A schematic diagram of OELDs used in this study.
were measured by Keithley 238 electrometer controlled by a PC. The luminance of these devices was measured by The Minolta CS-100. Details on the measurements of
Fig. 1. Molecular structures of TbŽACAC. 3 ŽPhen., TbŽACAC. 3 ŽPhen-Cl., TPD, Alq 3 , and Bebq 2 .
Fig. 3. Current density Ž J . –bias voltage Ž V . characteristics for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117
115
ionization potential and electron affinity of these materials using the cyclic voltammetric method will be published elsewhere w11x.
3. Results and discussion Current density Ž J . –bias voltage Ž V . characteristics for the device structures of glass substraterITOrTb complexesrAlq 3rLi:Al and glass substraterITOrTb complexesrBebq 2rLi:Al are shown in Fig. 3a and b, respectively. It was shown in Fig. 3 that the turn-on voltage of the device containing TbŽACAC. 3 ŽPhen. was lower than that of the device containing TbŽACAC. 3 ŽCl-Phen. regardless of electron transport material. The log Ž J . –log Ž V . characteristics for the device structures of glass substraterITOrTb complexesrAlq 3r Li:Al and glass substraterITOrTb complexesrBebq 2r Li:Al are also shown in Fig. 4a and b, respectively. As shown in Fig. 4, it was found that the current density J is proportional to V w9,10x over nine decades of the current, which is consistent with the trapped-charge-limited ŽTCL. conduction characteristics. The current density Ž J . –bias voltage Ž V . characteristics for terbium-complex-based OELD are shown in Fig. 4. These current density Ž J . –bias Fig. 5. The luminance Ž L. –current density Ž J . for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
Fig. 4. The log Ž J . –log Ž V . characteristics for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substraterITOrTb complexesrBebq 2 rLi:Al.
voltage Ž V . curves can be interpreted by trap-chargelimited current ŽTCLC. mechanism. At low voltages, the density of injected carriers in these devices is low. The conductance is due to terbium complex’s own carriers. For trap levels located at a single energy, this trap-filled limit ŽTFL. is an extremely sharp transition, at which the current directly switches to the space-charge-limited current ŽSCLC.. The more gradual increase, as observed in Fig. 4, points to a distribution of trap-level energies. The log Ž J . –log Ž V . characteristic in Fig. 4 is well-described using an exponential distribution of traps. The luminance Ž L. –current density Ž J . characteristics of OELDs with various device structures are shown in Fig. 5. The maximum luminance of approximately 95 cdrm2 from the ITOrTPDrTbŽACAC. 3 ŽCl-Phen.rAlq 3rLi:Alr Al and ITOrTPDrTbŽACAC. 3 ŽCl-Phen.rBebq 2rLi:Alr Al structures were obtained at the current density of 0.65 and 0.52 Arm2 , respectively, which is relatively low because Tb complexes have a poor carrier transport property in the solid state. From this figure, it was found that the device using TbŽACAC. 3 ŽCl-Phen. and Bebq 2 as a emissive layer and an electron transporting layer, respectively, has the highest luminance at the same current density. In Fig. 6a and b, the luminous efficiencies for the device structures of glass substraterITOrTb complexesr
116
Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117
Alq 3rLi:Al and glass substraterITOrTb complexesr Bebq 2rLi:Al are shown, respectively. The external luminous efficiencies of the OELDs with various device structures as a function of the bias voltage are shown in Fig. 6. The OELDs using TbŽACAC. 3 ŽCl-Phen. as an emitting layer have higher external luminous efficiency in comparison to that of TbŽACAC. 3 ŽPhen.. And the OELDs using Bebq 2 as an electron-transporting layer has higher external luminous efficiency than those using Alq 3 . Therefore, the OELD using TbŽACAC. 3 ŽCl-Phen. and Bebq 2 as an emitting layer and an electron-transporting layer has achieved the external luminous efficiency up to 0.03 lmrW. Using ionization potential and electron affinity values of various materials obtained by cyclic voltammetric method, the schematic energy band diagrams for four device structures used in this study are shown in Fig. 6. Under forward bias conditions, holes are injected from ITO into the valence band of the TPD layer and electrons from Li:Al into the conduction band of the Alq 3 or Bebq 2 layer. Injected holes can migrate into the Tb complex layer through the TPD layer and some electrons can migrate into the Tb complex layer over the injection barrier at the
Fig. 7. Schematic energy band diagrams of four devices structures with energy levels obtained by cyclic voltammetric method.
interface of Tb complexesrAlq 3 or Bebq 2 layer Ž0.155 eV:TbŽACAC. 3 ŽPhen., 0.13 eV:TbŽACAC. 3 ŽCl-Phen.. and can move the Tb complex layer through the electron-transporting layer. It was therefore found in Fig. 7 that the injection process of the device having TbŽACAC. 3 ŽClPhen. was easier than that of the device having TbŽACAC. 3 ŽPhen. as an emissive in spite of the higher turn-on voltage.
4. Conclusions The multi-layer thin-film OELDs containing Tb complexes as green-light-emitting materials with different electron-transporting materials were fabricated. In this study, it was found that the luminous efficiency of the device containing TbŽACAC. 3 ŽCl-Phen. was higher than that of the device containing TbŽACAC. 3 ŽPhen. in spite of the higher turn-on voltage, which was explained by using energy band diagram of two devices.
Acknowledgements This work was fully supported by Ministry of Information and Communication ŽMIC..
References
Fig. 6. The luminous efficiency for the device structures of Ža. glass substraterITOrTb complexesrAlq 3 rLi:Al and Žb. glass substrater ITOrTb complexesrBebq 2 rLi:Al.
w1x C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 Ž1987. 913. w2x Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Tkahashi, K. Shibaya, Appl. Phys. Lett. 71 Ž1997. 3338. w3x S. Miyata, Organic Electroluminescent Materials and Devices, Gordon and Breach Publishers, 1997. w4x S.P. Sinha, Complexes of the Rare Earth, Pergamon, London, 1996.
Y.K. Kim et al.r Synthetic Metals 111–112 (2000) 113–117 w5x R. Reisteld, C.K. Joergensen, Lasers and Excited States of Rare Earths, Springer, Berlin, 1977. w6x J. Kido, M. Kimura, K. Nagai, Chem. Lett. 657 Ž1990. 47. w7x J. Kido, K. Nagai, Y. Okamoto, J. Alloys Compd. 192 Ž1993. 30. w8x J. Kido, W. Ikeda, M. Kimura, K. Nagai, Jpn. J. Appl. Phys. Lett. 35 Ž1990. 394.
117
w9x A. Dodabalapur, L.J. Rothberg, T.M. Miller, Appl. Phys. Lett. 65 Ž1994. 2308. w10x S. Dirr, H. Johannes, J. Schobel, D. Ammermann, A. Bohler, W. Kowalsky, W. Grahn, SID ’97 Digest Ž1997. 778. w11x In preparation.