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Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes
Synthetic Metals 159 (2009) 1460–1463
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes Jonghee Lee, Jeong-Ik Lee, Hye Yong Chu ∗ Convergence Components & Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2009 Received in revised form 26 March 2009 Accepted 28 March 2009 Available online 9 May 2009 Keywords: Organic light-emitting diodes (OLEDs) Phosphorescent Double emissive layer structures (D-EMLs) Roll-off
a b s t r a c t The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N -dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2 ) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m2 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting devices (OLEDs) have been received considerable attention for both display and lighting applications due to their high light-emitting performances [1–3]. Especially, electrophosphorescent organic light-emitting diodes (PHOLEDs) using green and red phosphorescent dyes have reached almost 100% internal quantum efficiency by efficient utilizing both singlet and triplet excitons [3]. However, there are still several challenges for highly efficient blue PHOLEDs because they require wider bandgap materials for both host and dopant than those for green or red PHOLEDs. There have been many approaches for developing efficient blue PHOLEDs [4–10]. An endothermic host–guest energy transfer by using (4,4 -N,N -dicarbazole) biphenyl (CBP) and iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2 ) picolinate (FIrpic) system was reported by Adachi et al. [4]. Incorporation of high triplet energy host – N,N -dicarbazolyl-3,5-benzene (mCP) or 4,4 -bis(9-carbazolyl)-2,2 -dimethyl-biphenyl (CDBP) enabled effective exothermic energy transfer, and resulted in further enhancement of blue OLEDs performance [5–6]. A stepwise doping structure of blue PHOLEDs has been also reported [7]. On the other side, direct charge trapping on the blue dopants by using ultrawide band-gap hosts was also demonstrated [8–10]. Previously, we reported a peak external quantum efficiency (EQE) over 20% by employing the blue light emitting FIrpic doped
∗ Corresponding author. Tel.: +82 42 860 6031; fax: +82 42 860 5202. E-mail address: [email protected] (H.Y. Chu). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.03.026
in ultrawide band-gap host UGH3 with the adequate interlayers [10]. Despite this encouraging report, the efficiency of the blue PHOLEDs at a high current density region is quite limited due to severe roll-off (the decrease of efficiency with increasing current density) [11] in electro-phosphorescent devices. In the previous work, the EQE of FIrpic-based device showed significant reduction of 9.5% at 10 mA/cm2 which is almost half the value of the maximum efficiency [10]. This roll-off of the quantum efficiency was understood by the triplet–triplet annihilation (TTA) or triplet–polaron annihilation (TPA) in the emissive layer with heavy doping ratio [12–13]. In this paper, blue PHOLEDs performances at a high current density have been investigated with double emissive layer structures (D-EMLs) based on a hole-transport-type host and an electron-transport-type ultrawide band-gap host. With the D-EMLs structure, we have successfully achieved the distribution of exciton generation region in the emissive layer by controlling of each EML thickness, therefore obtained highly enhanced blue PHOLEDs performance as well as reduced roll-off at high current density region.
2. Experimental A series of organic light-emitting devices in the current study were made using the configuration (Device A): indium tin oxide (ITO)/NPB (40 nm)/EML-1 (x nm)/EML-2 (y nm)/Bphen (50 nm)/LiF (1 nm)/Al (120 nm); x (EML-1) + y (EML-2) = 30 as shown in Fig. 1. For the device with D-EMLs, the x-nm-thick mCP layer doped with 7% FIrpic for the EML-1 and the ynm-thick UGH3 layer doped with 10% FIrpic for the EML-2,
J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463
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characteristics were measured with a current/voltage source/ measure unit (Keithley 238) and a Minolta CS-100.
3. Results and discussion
Fig. 1. Schematic diagrams of the structures of the blue PHOLEDs.
respectively, were used as emissive layers. 4,4 -bis[N-(1-nathyl)-Nphenyl-amino]biphenyl (NPB), and bathocuproine(2,9-dimethyl4,7-diphenyl-1,10-phenanthroline) (Bphen) were used as the hole-transporting layer (HTL), and the electron transporting layer (ETL), respectively. We have also introduced undoped UGH3 interlayer (5 nm) into the EML/ETL interface in the Device A for further improvements of blue PHOLEDs performance (Device B). Detailed functionality of this interlayer will be discussed in Section 3. Chemical structures and energy level diagrams for the materials used in this study were shown in Fig. 2. ITO was cleaned with the standard oxygen plasma treatment. The OLED grade materials were purchased and used without further purification. All organic layers were deposited in a high vacuum chamber below 5 × 10−7 Torr and thin films of LiF and Al were deposited as a cathode electrode. The OLEDs were transferred directly from vacuum into an inert environment glove-box, where they were encapsulated using a UV-curable epoxy, and a glass cap with a moisture getter. The electroluminescence spectrum was measured using a Minolta CS1000. The current–voltage (J–V) and luminescence–voltage (L–V)
Four types of blue PHOLEDs were fabricated based on the Device A structure. Device A-1 and A-4 containing single emitting layer (S-EML) of UGH3 and mCP host for control devices and Device A2 and A-3 containing a double emitting layer (D-EML) of UGH3 and mCP host with different thicknesses were prepared (Fig. 1). The reported triplet energy gap of UGH3 and mCP hosts were 2.9 and 3.5 eV, respectively. Therefore, triplet energy gaps of both hosts are higher than that of FIrpic dopant (2.62 eV), in which effective confinement of dopant triplets is expected. However, as shown in Fig. 2, there were big differences in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels between the mCP and the UGH3 host, 0.9 and 0.4 eV, respectively. Moreover, the mCP which is a carbazole based material, is a hole-transport-type host [7], while the UGH3 is slightly electron-transport-type host [10]. Based on these carriertransporting ability and the energy level differences between two hosts, the hole/electron recombination would be mainly occurred at the mCP/UGH3 interface. Current density–voltage–luminance (J–V–L) characteristics of the Device A are shown in Fig. 3. Devices with the mCP or mCP/UGH3 hosts, Device A-2, A-3 and A-4, showed almost the same driving voltage while Device A-1 which contains only the UGH3 host exhibited higher driving voltage. This is attributed to the huge hole injection barrier (2.8 eV) from the NPB to the UGH3 layer in case of Device A-1. The driving voltages of the devices at 100 cd/m2 were 7.2, 5.3, 5.4, and 5.4 eV, respectively. Fig. 4 shows the electroluminescence spectra of the Device A. All Device A exhibited a similar maximum luminescence wavelength, near 468 nm, which is originated from the triplet emission of FIrpic dopant; however Device A-1 with only UGH3 host exhibited another broad shoulder around 430 nm which could be assigned to the NPB emission [10,14,15]. The huge hole injection barrier from the NPB to the UGH3 (2.8 eV) and low injection barrier from the UGH3 to the NPB (0.4 eV) might be resulted in hole accumulation and electron overflow. In other words, exciton confinement is not achieved in the Device A-1, thus the performance of this blue OLEDs is inefficient.
Fig. 2. Chemical structures and energy level diagrams for the materials tested in this study.
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J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463 Table 1 Device characteristics of blue PHOLEDs in this study. Device
A-l A-2 A-3 A-4 B a b c
At 10 mA/cm2
At peak EQE (%)a
P.E. (lm/W)b
C.E. (cd/A)c
EQE (%)a
P.E. (lm/W)b
C.E. (cd/A)c
1.20 13.3 13.8 9.99 16.4
0.86 18.5 19.2 12.2 19.6
2.17 24.5 25.7 18.8 31.10
1.05 11.5 12.1 9.47 12.8
0.87 10.4 10.6 8.31 10.6
1.9 21.2 22.5 17.8 24.3
EQE is external quantum efficiency. P.E. is power efficiency. C.E. is current efficiency.
The quantum and power efficiencies of the Device A are shown in Fig. 5 and summarized in Table 1. The quantum efficiency of Device A-1 with only UGH3 single emissive layer is quite low because of those reasons mentioned above. However, Device A-2 and A-3 with D-EML show remarkably improved OLED performances, which is even much higher than that of Device A-4 with only mCP sin-
gle emissive layer (the peak EQE of Device A-1, A-2, A-3, and A-4 were 1.2, 13.3, 13.8, and 9.99%, respectively). This improved performance of Device A-2 and A-3 with D-EMLs can be explained by the distribution of triplet exciton recombination region at the two emissive layers. The differences of the energy levels (both HOMO and LUMO) and charge carrier-transporting ability (both hole and electron) between two hosts of the mCP and the UGH3 might lead to the hole/electron recombination at the mCP/UGH3 interface. In the cases of S-EMLs devices, Device A-1 and A-4, the recombination zones might be positioned at shallow region of the HTL/EML interface (Device A-1) or of the EML/ETL interface (Device A-4) due to the preferred charge-transport ability of both host materials [7,10]. The high density of triplet excitons would lead to inferior triplet quenching process. On the other side, in the cases of D-EMLs devices (Device A-2 and A-3), we could control the recombination region by the optimization of both emissive layer thicknesses. Accordingly, we could enhance the charge balance within emissive layer as well as reduce exciton quenching possibility [16,17]. Furthermore, we investigated the addition of an interlayer between the EML and the ETL interface to fabricate Device B based on the results of Device A-3 which exhibited the best performance among Device A. We have introduced undoped UGH3 interlayer (5 nm) at the EML/ETL interface to provide effective confinement of excitons/holes within the EML as we previously reported highly efficient blue OLEDs with UGH3 single emissive layer [10]. The insertion of an interlayer resulted in a lower current density compared with the device without an interlayer because electron injection encountered some difficulty, which led to the effective confinement of excitons/holes and good charge balance. As expected, improved performance was successfully obtained in Device B compared with Device A-3 as shown in Fig. 6. The peak
Fig. 5. External quantum efficiency versus current density characteristics of Device A (inset: power efficiency versus brightness).
Fig. 6. External quantum efficiency versus brightness characteristics of device with S-EML (Ref. [10]) and Device B with D-EML (inset: external quantum efficiency versus current density).
Fig. 3. Current density–voltage–luminescence (J–V–L) characteristics of Device A tested in this study.
Fig. 4. Normalized electroluminescence spectra of Device A tested in this study.
J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463
external quantum efficiency of Device B with interlayer was 16.4% at 5.5 V. Moreover, Device B showed better device performance in high luminance region than the device with UGH3 single EML reported in the previous paper, although the maximum efficiency of Device B is slightly lower, which could be explained by the significantly reduced roll-off. Less than 24% drop of the external quantum efficiency at 10 mA/cm2 was found, while more than 53% roll-off was observed with UGH3 single host device. (Current density of half external quantum efficiency (J0 ) for UGH3 single EML device, and double EML Device B with interlayer were 7.9 and 49 mA/cm2 , respectively.) The external quantum efficiency (EQE = 10.0%) of Device B at high brightness of 5000 cd/m2 was two times higher than that (4.5%) of the previous single EML device. 4. Conclusion We have demonstrated that blue phosphorescent OLEDs performance at high current region was significantly improved by using D-EMLs with the hole-transport-type host (mCP) and the electro transport-type host (UGH3). Moreover, this D-EMLs exhibited highly enhanced performance and reduced roll-off at high brightness (EQE = 10.0% at 5000 cd/m2 ) compared to conventional blue PHOLEDs with a single EML. We attributed this enhancement to the distribution of exciton generation zone along with confinement of exciton within emissive layer. Acknowledgment We gratefully acknowledged Prof. Jun Yeob Lee (Department of Polymer Science and Engineering of Dankook University) for fruitful
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discussion, and Ms. K.-I. Song and Ms. S.J. Lee (ETRI) for assistance of EL device experiments. This work was supported by the future technology development program of MOCIE/ITEP [2006-10028439, OLED Lighting]. References [1] M.A. Baldo, D.F. O’Brien, Y. You, Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [2] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [3] C. Adachi, M.A. Baldo, M.E. Thomposon, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [4] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082. [5] R.J. Homes, S.R. Forrest, Y.-J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422. [6] S. Tokito, T. Iijima, Y. Suzuri, H. Kira, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 569. [7] J. Lee, J.-I. Lee, K.-I. Song, S.J. Lee, H.Y. Chu, Appl. Phys. Lett. 92 (2008) 133304. [8] R.J. Holmes, B.W. D’Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson, Appl. Phys. Lett. 83 (2003) 3818. [9] Y. Zheng, S.-H. Eom, N. Chopra, J. Lee, F. So, J. Xue, Appl. Phys. Lett. 92 (2008) 223301. [10] J. Lee, J.-I. Lee, K.-I. Song, S.J. Lee, H.Y. Chu, Appl. Phys. Lett. 92 (2008) 203305. [11] J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, J.-J. Kim, Appl. Phys. Lett. 90 (2007) 223508. [12] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. [13] S. Reineke, K. Walzer, Karl Leo, Phys. Rev. B 75 (2007) 125328. [14] V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W. D’Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [15] S.H. Kim, J. Jang, J.Y. Lee, Appl. Phys. Lett. 90 (2007) 223505. [16] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, Appl. Phys. Lett. 93 (2008) 113301. [17] X. Zhou, D.S. Qin, M. Pfeiffer, J. Blochwitz-Nimoth, A. Werner, J. Drechsel, B. Maennig, K. Leo, Appl. Phys. Lett. 81 (2002) 4070.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes Jonghee Lee, Jeong-Ik Lee, Hye Yong Chu ∗ Convergence Components & Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2009 Received in revised form 26 March 2009 Accepted 28 March 2009 Available online 9 May 2009 Keywords: Organic light-emitting diodes (OLEDs) Phosphorescent Double emissive layer structures (D-EMLs) Roll-off
a b s t r a c t The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N -dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2 ) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m2 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting devices (OLEDs) have been received considerable attention for both display and lighting applications due to their high light-emitting performances [1–3]. Especially, electrophosphorescent organic light-emitting diodes (PHOLEDs) using green and red phosphorescent dyes have reached almost 100% internal quantum efficiency by efficient utilizing both singlet and triplet excitons [3]. However, there are still several challenges for highly efficient blue PHOLEDs because they require wider bandgap materials for both host and dopant than those for green or red PHOLEDs. There have been many approaches for developing efficient blue PHOLEDs [4–10]. An endothermic host–guest energy transfer by using (4,4 -N,N -dicarbazole) biphenyl (CBP) and iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C2 ) picolinate (FIrpic) system was reported by Adachi et al. [4]. Incorporation of high triplet energy host – N,N -dicarbazolyl-3,5-benzene (mCP) or 4,4 -bis(9-carbazolyl)-2,2 -dimethyl-biphenyl (CDBP) enabled effective exothermic energy transfer, and resulted in further enhancement of blue OLEDs performance [5–6]. A stepwise doping structure of blue PHOLEDs has been also reported [7]. On the other side, direct charge trapping on the blue dopants by using ultrawide band-gap hosts was also demonstrated [8–10]. Previously, we reported a peak external quantum efficiency (EQE) over 20% by employing the blue light emitting FIrpic doped
∗ Corresponding author. Tel.: +82 42 860 6031; fax: +82 42 860 5202. E-mail address: [email protected] (H.Y. Chu). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.03.026
in ultrawide band-gap host UGH3 with the adequate interlayers [10]. Despite this encouraging report, the efficiency of the blue PHOLEDs at a high current density region is quite limited due to severe roll-off (the decrease of efficiency with increasing current density) [11] in electro-phosphorescent devices. In the previous work, the EQE of FIrpic-based device showed significant reduction of 9.5% at 10 mA/cm2 which is almost half the value of the maximum efficiency [10]. This roll-off of the quantum efficiency was understood by the triplet–triplet annihilation (TTA) or triplet–polaron annihilation (TPA) in the emissive layer with heavy doping ratio [12–13]. In this paper, blue PHOLEDs performances at a high current density have been investigated with double emissive layer structures (D-EMLs) based on a hole-transport-type host and an electron-transport-type ultrawide band-gap host. With the D-EMLs structure, we have successfully achieved the distribution of exciton generation region in the emissive layer by controlling of each EML thickness, therefore obtained highly enhanced blue PHOLEDs performance as well as reduced roll-off at high current density region.
2. Experimental A series of organic light-emitting devices in the current study were made using the configuration (Device A): indium tin oxide (ITO)/NPB (40 nm)/EML-1 (x nm)/EML-2 (y nm)/Bphen (50 nm)/LiF (1 nm)/Al (120 nm); x (EML-1) + y (EML-2) = 30 as shown in Fig. 1. For the device with D-EMLs, the x-nm-thick mCP layer doped with 7% FIrpic for the EML-1 and the ynm-thick UGH3 layer doped with 10% FIrpic for the EML-2,
J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463
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characteristics were measured with a current/voltage source/ measure unit (Keithley 238) and a Minolta CS-100.
3. Results and discussion
Fig. 1. Schematic diagrams of the structures of the blue PHOLEDs.
respectively, were used as emissive layers. 4,4 -bis[N-(1-nathyl)-Nphenyl-amino]biphenyl (NPB), and bathocuproine(2,9-dimethyl4,7-diphenyl-1,10-phenanthroline) (Bphen) were used as the hole-transporting layer (HTL), and the electron transporting layer (ETL), respectively. We have also introduced undoped UGH3 interlayer (5 nm) into the EML/ETL interface in the Device A for further improvements of blue PHOLEDs performance (Device B). Detailed functionality of this interlayer will be discussed in Section 3. Chemical structures and energy level diagrams for the materials used in this study were shown in Fig. 2. ITO was cleaned with the standard oxygen plasma treatment. The OLED grade materials were purchased and used without further purification. All organic layers were deposited in a high vacuum chamber below 5 × 10−7 Torr and thin films of LiF and Al were deposited as a cathode electrode. The OLEDs were transferred directly from vacuum into an inert environment glove-box, where they were encapsulated using a UV-curable epoxy, and a glass cap with a moisture getter. The electroluminescence spectrum was measured using a Minolta CS1000. The current–voltage (J–V) and luminescence–voltage (L–V)
Four types of blue PHOLEDs were fabricated based on the Device A structure. Device A-1 and A-4 containing single emitting layer (S-EML) of UGH3 and mCP host for control devices and Device A2 and A-3 containing a double emitting layer (D-EML) of UGH3 and mCP host with different thicknesses were prepared (Fig. 1). The reported triplet energy gap of UGH3 and mCP hosts were 2.9 and 3.5 eV, respectively. Therefore, triplet energy gaps of both hosts are higher than that of FIrpic dopant (2.62 eV), in which effective confinement of dopant triplets is expected. However, as shown in Fig. 2, there were big differences in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels between the mCP and the UGH3 host, 0.9 and 0.4 eV, respectively. Moreover, the mCP which is a carbazole based material, is a hole-transport-type host [7], while the UGH3 is slightly electron-transport-type host [10]. Based on these carriertransporting ability and the energy level differences between two hosts, the hole/electron recombination would be mainly occurred at the mCP/UGH3 interface. Current density–voltage–luminance (J–V–L) characteristics of the Device A are shown in Fig. 3. Devices with the mCP or mCP/UGH3 hosts, Device A-2, A-3 and A-4, showed almost the same driving voltage while Device A-1 which contains only the UGH3 host exhibited higher driving voltage. This is attributed to the huge hole injection barrier (2.8 eV) from the NPB to the UGH3 layer in case of Device A-1. The driving voltages of the devices at 100 cd/m2 were 7.2, 5.3, 5.4, and 5.4 eV, respectively. Fig. 4 shows the electroluminescence spectra of the Device A. All Device A exhibited a similar maximum luminescence wavelength, near 468 nm, which is originated from the triplet emission of FIrpic dopant; however Device A-1 with only UGH3 host exhibited another broad shoulder around 430 nm which could be assigned to the NPB emission [10,14,15]. The huge hole injection barrier from the NPB to the UGH3 (2.8 eV) and low injection barrier from the UGH3 to the NPB (0.4 eV) might be resulted in hole accumulation and electron overflow. In other words, exciton confinement is not achieved in the Device A-1, thus the performance of this blue OLEDs is inefficient.
Fig. 2. Chemical structures and energy level diagrams for the materials tested in this study.
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J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463 Table 1 Device characteristics of blue PHOLEDs in this study. Device
A-l A-2 A-3 A-4 B a b c
At 10 mA/cm2
At peak EQE (%)a
P.E. (lm/W)b
C.E. (cd/A)c
EQE (%)a
P.E. (lm/W)b
C.E. (cd/A)c
1.20 13.3 13.8 9.99 16.4
0.86 18.5 19.2 12.2 19.6
2.17 24.5 25.7 18.8 31.10
1.05 11.5 12.1 9.47 12.8
0.87 10.4 10.6 8.31 10.6
1.9 21.2 22.5 17.8 24.3
EQE is external quantum efficiency. P.E. is power efficiency. C.E. is current efficiency.
The quantum and power efficiencies of the Device A are shown in Fig. 5 and summarized in Table 1. The quantum efficiency of Device A-1 with only UGH3 single emissive layer is quite low because of those reasons mentioned above. However, Device A-2 and A-3 with D-EML show remarkably improved OLED performances, which is even much higher than that of Device A-4 with only mCP sin-
gle emissive layer (the peak EQE of Device A-1, A-2, A-3, and A-4 were 1.2, 13.3, 13.8, and 9.99%, respectively). This improved performance of Device A-2 and A-3 with D-EMLs can be explained by the distribution of triplet exciton recombination region at the two emissive layers. The differences of the energy levels (both HOMO and LUMO) and charge carrier-transporting ability (both hole and electron) between two hosts of the mCP and the UGH3 might lead to the hole/electron recombination at the mCP/UGH3 interface. In the cases of S-EMLs devices, Device A-1 and A-4, the recombination zones might be positioned at shallow region of the HTL/EML interface (Device A-1) or of the EML/ETL interface (Device A-4) due to the preferred charge-transport ability of both host materials [7,10]. The high density of triplet excitons would lead to inferior triplet quenching process. On the other side, in the cases of D-EMLs devices (Device A-2 and A-3), we could control the recombination region by the optimization of both emissive layer thicknesses. Accordingly, we could enhance the charge balance within emissive layer as well as reduce exciton quenching possibility [16,17]. Furthermore, we investigated the addition of an interlayer between the EML and the ETL interface to fabricate Device B based on the results of Device A-3 which exhibited the best performance among Device A. We have introduced undoped UGH3 interlayer (5 nm) at the EML/ETL interface to provide effective confinement of excitons/holes within the EML as we previously reported highly efficient blue OLEDs with UGH3 single emissive layer [10]. The insertion of an interlayer resulted in a lower current density compared with the device without an interlayer because electron injection encountered some difficulty, which led to the effective confinement of excitons/holes and good charge balance. As expected, improved performance was successfully obtained in Device B compared with Device A-3 as shown in Fig. 6. The peak
Fig. 5. External quantum efficiency versus current density characteristics of Device A (inset: power efficiency versus brightness).
Fig. 6. External quantum efficiency versus brightness characteristics of device with S-EML (Ref. [10]) and Device B with D-EML (inset: external quantum efficiency versus current density).
Fig. 3. Current density–voltage–luminescence (J–V–L) characteristics of Device A tested in this study.
Fig. 4. Normalized electroluminescence spectra of Device A tested in this study.
J. Lee et al. / Synthetic Metals 159 (2009) 1460–1463
external quantum efficiency of Device B with interlayer was 16.4% at 5.5 V. Moreover, Device B showed better device performance in high luminance region than the device with UGH3 single EML reported in the previous paper, although the maximum efficiency of Device B is slightly lower, which could be explained by the significantly reduced roll-off. Less than 24% drop of the external quantum efficiency at 10 mA/cm2 was found, while more than 53% roll-off was observed with UGH3 single host device. (Current density of half external quantum efficiency (J0 ) for UGH3 single EML device, and double EML Device B with interlayer were 7.9 and 49 mA/cm2 , respectively.) The external quantum efficiency (EQE = 10.0%) of Device B at high brightness of 5000 cd/m2 was two times higher than that (4.5%) of the previous single EML device. 4. Conclusion We have demonstrated that blue phosphorescent OLEDs performance at high current region was significantly improved by using D-EMLs with the hole-transport-type host (mCP) and the electro transport-type host (UGH3). Moreover, this D-EMLs exhibited highly enhanced performance and reduced roll-off at high brightness (EQE = 10.0% at 5000 cd/m2 ) compared to conventional blue PHOLEDs with a single EML. We attributed this enhancement to the distribution of exciton generation zone along with confinement of exciton within emissive layer. Acknowledgment We gratefully acknowledged Prof. Jun Yeob Lee (Department of Polymer Science and Engineering of Dankook University) for fruitful
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discussion, and Ms. K.-I. Song and Ms. S.J. Lee (ETRI) for assistance of EL device experiments. This work was supported by the future technology development program of MOCIE/ITEP [2006-10028439, OLED Lighting]. References [1] M.A. Baldo, D.F. O’Brien, Y. You, Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [2] G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, J. Salbeck, Appl. Phys. Lett. 85 (2004) 3911. [3] C. Adachi, M.A. Baldo, M.E. Thomposon, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [4] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082. [5] R.J. Homes, S.R. Forrest, Y.-J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422. [6] S. Tokito, T. Iijima, Y. Suzuri, H. Kira, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 569. [7] J. Lee, J.-I. Lee, K.-I. Song, S.J. Lee, H.Y. Chu, Appl. Phys. Lett. 92 (2008) 133304. [8] R.J. Holmes, B.W. D’Andrade, S.R. Forrest, X. Ren, J. Li, M.E. Thompson, Appl. Phys. Lett. 83 (2003) 3818. [9] Y. Zheng, S.-H. Eom, N. Chopra, J. Lee, F. So, J. Xue, Appl. Phys. Lett. 92 (2008) 223301. [10] J. Lee, J.-I. Lee, K.-I. Song, S.J. Lee, H.Y. Chu, Appl. Phys. Lett. 92 (2008) 203305. [11] J.-W. Kang, S.-H. Lee, H.-D. Park, W.-I. Jeong, K.-M. Yoo, Y.-S. Park, J.-J. Kim, Appl. Phys. Lett. 90 (2007) 223508. [12] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. [13] S. Reineke, K. Walzer, Karl Leo, Phys. Rev. B 75 (2007) 125328. [14] V.I. Adamovich, S.R. Cordero, P.I. Djurovich, A. Tamayo, M.E. Thompson, B.W. D’Andrade, S.R. Forrest, Org. Electron. 4 (2003) 77. [15] S.H. Kim, J. Jang, J.Y. Lee, Appl. Phys. Lett. 90 (2007) 223505. [16] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, Appl. Phys. Lett. 93 (2008) 113301. [17] X. Zhou, D.S. Qin, M. Pfeiffer, J. Blochwitz-Nimoth, A. Werner, J. Drechsel, B. Maennig, K. Leo, Appl. Phys. Lett. 81 (2002) 4070.