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Role of mixed hole transport layer with exciton blocking properties in phosphorescent organic light-emitting diodes
Synthetic Metals 159 (2009) 568–570
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Role of mixed hole transport layer with exciton blocking properties in phosphorescent organic light-emitting diodes Sung Hyun Kim a , Jyongsik Jang a , Kyoung Soo Yook b , Jun Yeob Lee b,∗ a b
School of Chemical and Biological Engineering, Seoul National University, Shinlim-dong, Kwanak-gu, Seoul 151-742, Republic of Korea Department of Polymer Science and Engineering, Dankook University, Hannam-dong, Yongsan-gu, Seoul 140-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 August 2007 Received in revised form 3 November 2008 Accepted 27 November 2008 Available online 8 January 2009 Keywords: Mixed hole transport layer High efficiency Exciton blocking
a b s t r a c t Improved efficiency in green phosphorescent organic light-emitting diodes using a composite hole transport layer (HTL) with hole transport and exciton blocking function was investigated. Mixed layer of (4,4 -N,N -dicarbazole)biphenyl (CBP) and N,N -di(1-naphthyl)-N,N -diphenylbenzidine (NPB) was used as a HTL and quantum efficiency could be enhanced from 4.3% to 8.1% at 100 cd/m2 . Best performances could be obtained in the device with 50% CBP and 50% NPB in the HTL. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Light-emitting efficiency of phosphorescent organic lightemitting diodes (PHOLEDs) can be higher than fluorescent organic light-emitting diodes (OLEDs) by a factor of four [1], but PHOLEDs need extra exciton blocking layers to maximize device performances compared with fluorescent OLEDs [2–5]. Exciton blocking layers can be inserted between light-emitting layer (EML) and electron transport layer [2,3] or it can be placed between hole transport layer (HTL) and EML depending on host materials [4,5]. Electron transport type host materials require exciton blocking layer with electron-blocking function, while hole transport type host materials need exciton blocking layer with hole blocking properties. 4,7-Diphenyl-1,10-phenanthroline (Bphen) and 3-phenyl-4-(1 naphthyl)-5-phenyl-1,2,4-triazole have been known as electron transport type triplet host materials and there have been a few studies about using electron or exciton blocking layer in these devices [4–6]. 4,4 ,4 -Tris(N-carbazolyl)-triphenylamine could be used as exciton blocking layer in Bhen:Tris(2-phenylpyridine) iridium(Ir(ppy)3 ) devices and fac-Tris(1-phenylpyrazolato-N,C2 ) iridium(Ir(ppz)3 ) with its lowest unoccupied molecular orbital (LUMO) level of 1.7 eV and triplet energy level of 3.1 eV was efficient as an electron and exciton blocking material [4,6]. However, the use of additional exciton blocking layer in OLEDs makes device structure complicated because the exciton blocking layer should be
∗ Corresponding author. Tel.: +82 2 793 4102; fax: +82 2 709 2614. E-mail address: [email protected] (J.Y. Lee). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.11.020
inserted between HTL and an emitting layer. Therefore, a HTL with exciton blocking function can be efficient as one layer can play a role of the HTL and an exciton blocking layer. To fabricate a device with combined properties of two hole transport materials, mixed hole transport layer systems have been developed [7,8]. N,N -di(1-naphthyl)-N,N -diphenylbenzidine (NPB) was mixed with copper phthalocyanine and high quantum efficiency of 8.7% was obtained in sky blue fluorescent devices compared with 5.3% of standard device [7]. Hole mobility was controlled by mixed hole transport layer and hole–electron charge balance was improved. In addition to this result, long lifetime in fluorescent devices by mixed hole transport layer was reported [8]. In this work, we studied the use of composite HTL as an exciton blocking layer as well as a HTL layer to get high efficiency in green PHOLEDs. NPB and (4,4 -N,N -dicarbazole)biphenyl (CBP) were co-deposited and device performances of green PHOLEDs were investigated according to relative content of NPB and CBP.
2. Experimental Device structure used in this experiment was indium tin oxide (ITO, 150 nm)/N,N -diphenyl-N,N -bis-[4-(phenyl-m-tolylamino)-phenyl]-biphenyl-4,4’-diamine (DNTPD, 60 nm)/HTL (30 nm)/PH1:Ir(ppy)3 (30 nm, 5% doping)/biphenoxy-bi(8hydroxy-3-methylquinoline) aluminum (Balq, 5 nm)/tris(820 nm)/LiF(1 nm)/Al hydroxyquinoline) aluminium (Alq3 , (200 nm). Four devices with different HTL structure were fabricated to investigate the effect of NPB:CBP composite layer on device performances. CBP concentration in composite HTL was
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varied as 0%, 25%, 50%, and 75% and each device was abbreviated as CBP0, CBP25, CBP50, and CBP75 according to CBP content. PH1 was supplied from Merck Co. and it has a spirobifluorene type backbone structure with electron transport properties because of spirobifluorene units. Triplet bandgap of PH1 was 2.4 eV and highest occupied molecular orbital (HOMO) and LUMO were 5.9 eV and 2.8 eV, respectively. Energy level of organic materials was obtained by cyclovoltametric measurements and surface analyzer. Current density–voltage–luminance characteristics of the devices were measured with Keithley 2400 source measurement unit and PR 650 spectrophotometer. 3. Results and discussion Hole transport layer in PHOLEDs with electron transport type host materials is required to have additional exciton blocking layer to block triplet exciton diffusion and quenching [4–6]. However, common HTL such as NPB cannot play a role of exciton blocking HTL because of narrow triplet bandgap of NPB [4]. Therefore, composite layer of NPB and CBP was developed because it can have merits of both NPB and CBP. NPB has merits as a HTL such as high hole mobility of 10−3 cm2 /V s order from our time-of-flight measurement and low hole injection energy barrier of 0.4 eV with DNTPD. CBP is advantageous in that it has wide triplet bandgap of 2.6 eV for green triplet exciton blocking [9] and moderate hole mobility [10]. Therefore, composite HTL of NPB and CBP can be beneficial to get excellent hole transporting and exciton blocking properties in green PHOLEDs. Fig. 1 shows current density–voltage curves of composite HTL devices with different CBP content. Current density of CBP25 was similar to that of standard device with NPB HTL, while it was decreased at high CBP content above 50%. Current flow in the device was dominated by NPB at low CBP content, while it was determined by CBP at high CBP content, indicating that continuous phase contribute greatly to current density in the composite HTL devices [11,12]. Holes are injected into NPB in CBP0 and CBP25 devices and current density typically follows NPB device characteristics. However, holes are injected into CBP in CBP75 and current flow is dominated by CBP because CBP is a continuous phase. There is a large hole injection energy barrier of 0.8 eV between DNTPD (5.1 eV) and CBP (5.9 eV), limiting hole injection from DNTPD to CBP. In addition, HOMO level of NPB (5.5 eV) is lower than that of CBP by 0.4 eV, acting as a deep hole trap in HTL. Therefore, hole injection and transport are very difficult in CBP75 device. Compared with CBP75, CBP50 shows current density–voltage characteristics of both NPB and CBP because both materials are present equally.
Fig. 1. Current density–voltage curves of green phosphorescent devices with different HTL structures.
Fig. 2. Luminance–voltage curves of green phosphorescent devices with different HTL structures.
Luminance–voltage curves of composite HTL devices are presented in Fig. 2. Luminance was almost constant up to CBP content of 50% and then it was decreased sharply at high CBP content. CBP50, which showed lower current density in current density–voltage curves than CBP0 and CBP25, exhibited similar luminance value, implying high efficiency in CBP50. Quantum efficiency–luminance plots of composite HTL devices are shown in Fig. 3. Quantum efficiency was greatly improved in CBP50, while other devices showed similar quantum efficiency value. The high quantum efficiency in CBP50 devices can be explained by efficient hole transport and exciton blocking effect of composite HTL. CBP50 composite HTL transports holes efficiently through NPB and CBP even though it is not as efficient as NPB. In addition, holes in CBP material can be injected easily to PH1 layer because there is no energy barrier between CBP and PH1. Therefore, hole accumulation at the interface between HTL and PH1 layer can be reduced due to effective hole injection from HTL to PH1 by CBP. In addition, triplet exciton quenching by NPB in green PHOLEDs can also be depressed as CBP can block exciton quenching of Ir(ppy)3 . Triplet bandgap of CBP is 2.6 eV compared with 2.3 eV of NPB and it can block exciton quenching of Ir(ppy)3 with triplet bandgap of 2.4 eV. As recombination zone of typical green PHOLEDs is placed near HTL, it can be expected that triplet exciton blocking by CBP contribute to high efficiency. The triplet exciton blocking effect was confirmed with photoluminescence (PL) measurement of devices (Fig. 4). PL intensity of emitting layer can give information about triplet quenching effect because PL would be weakened by triplet exciton quenching. The PL measurement was carried out by irradiating a light with an excitation wavelength of 450 nm. A 30-nm thick film of the emitting
Fig. 3. Current efficiency–luminance curves of green phosphorescent devices with different HTL structures.
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Fig. 4. Photoluminescence spectra of green phosphorescent devices with different HTL structures.
blue EL peak is due to light emission from NPB and CBP. Similar emission spectra were observed in all devices around 515 nm, while blue emission spectra were different depending on CBP content. Four devices could be classified into two groups, CBP0 and CBP25 with rather strong NPB emission and CBP50 and CBP75 with weak NPB and CBP emission. The blue emission in CBP50 and CBP75 was blue shifted compared with that of CBP0 and CBP25 because of CBP emission as well as NPB emission. Relatively strong blue emission from NPB was observed in CBP0 and CBP25 devices as holes are accumulated at the interface between NPB and PH1 and they can recombine with electrons injected from PH1 to NPB layer. The hole accumulation at the interface is due to high energy barrier and poor hole transport in PH1 emission layer. Hole transport in PH1 host is limited due to its poor hole transport properties and hole hopping between Ir(ppy)3 sites is difficult at low doping concentration. However, CBP can reduce hole accumulation at the interface and can shift recombination zone of holes and electrons from HTL side to light-emitting layer, resulting in decrease of NPB and CBP emission. Both NPB and CBP emission peaks were observed in CBP50 and CBP75 devices. Therefore, it can be inferred from EL data that high efficiency in composite HTL devices is partly due to depressed NPB and CBP emission originated from shift of recombination zone from HTL side to EML side. 4. Conclusions In summary, composite HTL of NPB and CBP improved lightemitting efficiency of green PHOLEDs by 90%. Extra blue emission from HTL was depressed by using composite HTL structure. CBP50 which have 50% NPB and 50% CBP in HTL showed the best performances as a composite HTL. References
Fig. 5. Electroluminescence spectra of green phosphorescent devices with different HTL structures.
layer with the exciton blocking layer was analyzed. PL intensity of green emission was increased according to the content of CBP in hole transport layer, indicating that exciton quenching near hole transport layer was reduced by mixing CBP with NPB. The origin of high efficiency in composite HTL devices can be confirmed with electroluminescence (EL) spectra of composite HTL devices (Fig. 5). All devices showed a peak maximum between 513 nm and 515 nm and weak blue emission around 450 nm region. The main peak corresponds to Ir(ppy)3 emission in PH1 host, while
[1] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999) 2095. [2] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [3] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [4] V. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B. Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [5] X. Zhou, D.S. Qin, M. Pfeiffer, J. Blochwitz, A. Werner, J. Crechsel, B. Maennig, K. Leo, M. Bold, P. Erk, H. Hartmann, Appl. Phys. Lett. 81 (2002) 4070. [6] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D’Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, New J. Chem. 26 (2002) 1171. [7] C. Liao, M. Lee, C. Tsai, C.H. Chen, Appl. Phys. Lett. 86 (2005) 203507. [8] Y. Kim, E. Oh, H. Lim, C. Ha, Appl. Phys. Lett. 88 (2006) 043504. [9] M.A. Baldo, S.R. Forrest, Phys. Rev. B 62 (2000) 10958. [10] C. Adachi, R. Kwong, S.R. Forrest, Org. Electron. 2 (2001) 37. [11] J.Y. Lee, J. Ind. Eng. Chem 14 (2008) 676. [12] J.Y. Lee, J. Ind. Eng. Chem 14 (2008) 792.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Role of mixed hole transport layer with exciton blocking properties in phosphorescent organic light-emitting diodes Sung Hyun Kim a , Jyongsik Jang a , Kyoung Soo Yook b , Jun Yeob Lee b,∗ a b
School of Chemical and Biological Engineering, Seoul National University, Shinlim-dong, Kwanak-gu, Seoul 151-742, Republic of Korea Department of Polymer Science and Engineering, Dankook University, Hannam-dong, Yongsan-gu, Seoul 140-714, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 August 2007 Received in revised form 3 November 2008 Accepted 27 November 2008 Available online 8 January 2009 Keywords: Mixed hole transport layer High efficiency Exciton blocking
a b s t r a c t Improved efficiency in green phosphorescent organic light-emitting diodes using a composite hole transport layer (HTL) with hole transport and exciton blocking function was investigated. Mixed layer of (4,4 -N,N -dicarbazole)biphenyl (CBP) and N,N -di(1-naphthyl)-N,N -diphenylbenzidine (NPB) was used as a HTL and quantum efficiency could be enhanced from 4.3% to 8.1% at 100 cd/m2 . Best performances could be obtained in the device with 50% CBP and 50% NPB in the HTL. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Light-emitting efficiency of phosphorescent organic lightemitting diodes (PHOLEDs) can be higher than fluorescent organic light-emitting diodes (OLEDs) by a factor of four [1], but PHOLEDs need extra exciton blocking layers to maximize device performances compared with fluorescent OLEDs [2–5]. Exciton blocking layers can be inserted between light-emitting layer (EML) and electron transport layer [2,3] or it can be placed between hole transport layer (HTL) and EML depending on host materials [4,5]. Electron transport type host materials require exciton blocking layer with electron-blocking function, while hole transport type host materials need exciton blocking layer with hole blocking properties. 4,7-Diphenyl-1,10-phenanthroline (Bphen) and 3-phenyl-4-(1 naphthyl)-5-phenyl-1,2,4-triazole have been known as electron transport type triplet host materials and there have been a few studies about using electron or exciton blocking layer in these devices [4–6]. 4,4 ,4 -Tris(N-carbazolyl)-triphenylamine could be used as exciton blocking layer in Bhen:Tris(2-phenylpyridine) iridium(Ir(ppy)3 ) devices and fac-Tris(1-phenylpyrazolato-N,C2 ) iridium(Ir(ppz)3 ) with its lowest unoccupied molecular orbital (LUMO) level of 1.7 eV and triplet energy level of 3.1 eV was efficient as an electron and exciton blocking material [4,6]. However, the use of additional exciton blocking layer in OLEDs makes device structure complicated because the exciton blocking layer should be
∗ Corresponding author. Tel.: +82 2 793 4102; fax: +82 2 709 2614. E-mail address: [email protected] (J.Y. Lee). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.11.020
inserted between HTL and an emitting layer. Therefore, a HTL with exciton blocking function can be efficient as one layer can play a role of the HTL and an exciton blocking layer. To fabricate a device with combined properties of two hole transport materials, mixed hole transport layer systems have been developed [7,8]. N,N -di(1-naphthyl)-N,N -diphenylbenzidine (NPB) was mixed with copper phthalocyanine and high quantum efficiency of 8.7% was obtained in sky blue fluorescent devices compared with 5.3% of standard device [7]. Hole mobility was controlled by mixed hole transport layer and hole–electron charge balance was improved. In addition to this result, long lifetime in fluorescent devices by mixed hole transport layer was reported [8]. In this work, we studied the use of composite HTL as an exciton blocking layer as well as a HTL layer to get high efficiency in green PHOLEDs. NPB and (4,4 -N,N -dicarbazole)biphenyl (CBP) were co-deposited and device performances of green PHOLEDs were investigated according to relative content of NPB and CBP.
2. Experimental Device structure used in this experiment was indium tin oxide (ITO, 150 nm)/N,N -diphenyl-N,N -bis-[4-(phenyl-m-tolylamino)-phenyl]-biphenyl-4,4’-diamine (DNTPD, 60 nm)/HTL (30 nm)/PH1:Ir(ppy)3 (30 nm, 5% doping)/biphenoxy-bi(8hydroxy-3-methylquinoline) aluminum (Balq, 5 nm)/tris(820 nm)/LiF(1 nm)/Al hydroxyquinoline) aluminium (Alq3 , (200 nm). Four devices with different HTL structure were fabricated to investigate the effect of NPB:CBP composite layer on device performances. CBP concentration in composite HTL was
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varied as 0%, 25%, 50%, and 75% and each device was abbreviated as CBP0, CBP25, CBP50, and CBP75 according to CBP content. PH1 was supplied from Merck Co. and it has a spirobifluorene type backbone structure with electron transport properties because of spirobifluorene units. Triplet bandgap of PH1 was 2.4 eV and highest occupied molecular orbital (HOMO) and LUMO were 5.9 eV and 2.8 eV, respectively. Energy level of organic materials was obtained by cyclovoltametric measurements and surface analyzer. Current density–voltage–luminance characteristics of the devices were measured with Keithley 2400 source measurement unit and PR 650 spectrophotometer. 3. Results and discussion Hole transport layer in PHOLEDs with electron transport type host materials is required to have additional exciton blocking layer to block triplet exciton diffusion and quenching [4–6]. However, common HTL such as NPB cannot play a role of exciton blocking HTL because of narrow triplet bandgap of NPB [4]. Therefore, composite layer of NPB and CBP was developed because it can have merits of both NPB and CBP. NPB has merits as a HTL such as high hole mobility of 10−3 cm2 /V s order from our time-of-flight measurement and low hole injection energy barrier of 0.4 eV with DNTPD. CBP is advantageous in that it has wide triplet bandgap of 2.6 eV for green triplet exciton blocking [9] and moderate hole mobility [10]. Therefore, composite HTL of NPB and CBP can be beneficial to get excellent hole transporting and exciton blocking properties in green PHOLEDs. Fig. 1 shows current density–voltage curves of composite HTL devices with different CBP content. Current density of CBP25 was similar to that of standard device with NPB HTL, while it was decreased at high CBP content above 50%. Current flow in the device was dominated by NPB at low CBP content, while it was determined by CBP at high CBP content, indicating that continuous phase contribute greatly to current density in the composite HTL devices [11,12]. Holes are injected into NPB in CBP0 and CBP25 devices and current density typically follows NPB device characteristics. However, holes are injected into CBP in CBP75 and current flow is dominated by CBP because CBP is a continuous phase. There is a large hole injection energy barrier of 0.8 eV between DNTPD (5.1 eV) and CBP (5.9 eV), limiting hole injection from DNTPD to CBP. In addition, HOMO level of NPB (5.5 eV) is lower than that of CBP by 0.4 eV, acting as a deep hole trap in HTL. Therefore, hole injection and transport are very difficult in CBP75 device. Compared with CBP75, CBP50 shows current density–voltage characteristics of both NPB and CBP because both materials are present equally.
Fig. 1. Current density–voltage curves of green phosphorescent devices with different HTL structures.
Fig. 2. Luminance–voltage curves of green phosphorescent devices with different HTL structures.
Luminance–voltage curves of composite HTL devices are presented in Fig. 2. Luminance was almost constant up to CBP content of 50% and then it was decreased sharply at high CBP content. CBP50, which showed lower current density in current density–voltage curves than CBP0 and CBP25, exhibited similar luminance value, implying high efficiency in CBP50. Quantum efficiency–luminance plots of composite HTL devices are shown in Fig. 3. Quantum efficiency was greatly improved in CBP50, while other devices showed similar quantum efficiency value. The high quantum efficiency in CBP50 devices can be explained by efficient hole transport and exciton blocking effect of composite HTL. CBP50 composite HTL transports holes efficiently through NPB and CBP even though it is not as efficient as NPB. In addition, holes in CBP material can be injected easily to PH1 layer because there is no energy barrier between CBP and PH1. Therefore, hole accumulation at the interface between HTL and PH1 layer can be reduced due to effective hole injection from HTL to PH1 by CBP. In addition, triplet exciton quenching by NPB in green PHOLEDs can also be depressed as CBP can block exciton quenching of Ir(ppy)3 . Triplet bandgap of CBP is 2.6 eV compared with 2.3 eV of NPB and it can block exciton quenching of Ir(ppy)3 with triplet bandgap of 2.4 eV. As recombination zone of typical green PHOLEDs is placed near HTL, it can be expected that triplet exciton blocking by CBP contribute to high efficiency. The triplet exciton blocking effect was confirmed with photoluminescence (PL) measurement of devices (Fig. 4). PL intensity of emitting layer can give information about triplet quenching effect because PL would be weakened by triplet exciton quenching. The PL measurement was carried out by irradiating a light with an excitation wavelength of 450 nm. A 30-nm thick film of the emitting
Fig. 3. Current efficiency–luminance curves of green phosphorescent devices with different HTL structures.
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Fig. 4. Photoluminescence spectra of green phosphorescent devices with different HTL structures.
blue EL peak is due to light emission from NPB and CBP. Similar emission spectra were observed in all devices around 515 nm, while blue emission spectra were different depending on CBP content. Four devices could be classified into two groups, CBP0 and CBP25 with rather strong NPB emission and CBP50 and CBP75 with weak NPB and CBP emission. The blue emission in CBP50 and CBP75 was blue shifted compared with that of CBP0 and CBP25 because of CBP emission as well as NPB emission. Relatively strong blue emission from NPB was observed in CBP0 and CBP25 devices as holes are accumulated at the interface between NPB and PH1 and they can recombine with electrons injected from PH1 to NPB layer. The hole accumulation at the interface is due to high energy barrier and poor hole transport in PH1 emission layer. Hole transport in PH1 host is limited due to its poor hole transport properties and hole hopping between Ir(ppy)3 sites is difficult at low doping concentration. However, CBP can reduce hole accumulation at the interface and can shift recombination zone of holes and electrons from HTL side to light-emitting layer, resulting in decrease of NPB and CBP emission. Both NPB and CBP emission peaks were observed in CBP50 and CBP75 devices. Therefore, it can be inferred from EL data that high efficiency in composite HTL devices is partly due to depressed NPB and CBP emission originated from shift of recombination zone from HTL side to EML side. 4. Conclusions In summary, composite HTL of NPB and CBP improved lightemitting efficiency of green PHOLEDs by 90%. Extra blue emission from HTL was depressed by using composite HTL structure. CBP50 which have 50% NPB and 50% CBP in HTL showed the best performances as a composite HTL. References
Fig. 5. Electroluminescence spectra of green phosphorescent devices with different HTL structures.
layer with the exciton blocking layer was analyzed. PL intensity of green emission was increased according to the content of CBP in hole transport layer, indicating that exciton quenching near hole transport layer was reduced by mixing CBP with NPB. The origin of high efficiency in composite HTL devices can be confirmed with electroluminescence (EL) spectra of composite HTL devices (Fig. 5). All devices showed a peak maximum between 513 nm and 515 nm and weak blue emission around 450 nm region. The main peak corresponds to Ir(ppy)3 emission in PH1 host, while
[1] M.A. Baldo, M.E. Thompson, S.R. Forrest, Pure Appl. Chem. 71 (1999) 2095. [2] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [3] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [4] V. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B. Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [5] X. Zhou, D.S. Qin, M. Pfeiffer, J. Blochwitz, A. Werner, J. Crechsel, B. Maennig, K. Leo, M. Bold, P. Erk, H. Hartmann, Appl. Phys. Lett. 81 (2002) 4070. [6] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D’Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, New J. Chem. 26 (2002) 1171. [7] C. Liao, M. Lee, C. Tsai, C.H. Chen, Appl. Phys. Lett. 86 (2005) 203507. [8] Y. Kim, E. Oh, H. Lim, C. Ha, Appl. Phys. Lett. 88 (2006) 043504. [9] M.A. Baldo, S.R. Forrest, Phys. Rev. B 62 (2000) 10958. [10] C. Adachi, R. Kwong, S.R. Forrest, Org. Electron. 2 (2001) 37. [11] J.Y. Lee, J. Ind. Eng. Chem 14 (2008) 676. [12] J.Y. Lee, J. Ind. Eng. Chem 14 (2008) 792.