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Drastic drop of external quantum efficiency at liquid nitrogen temperature in a bilayer blue phosphorescent organic light-emitting device
Synthetic Metals 217 (2016) 244–247
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Drastic drop of external quantum efficiency at liquid nitrogen temperature in a bilayer blue phosphorescent organic light-emitting device Mi-Young Ha, Da-Young Park, Min-Jae Lee, Seung-Jung Choi, Jae-Hoon Jung, Dae-Gyu Moon* Department of Materials Engineering, Soonchunhyang University, 646, Eupnae-ri, Shinchang-myeon, Asan-si, Chungcheongnam-do 336-745, South Korea
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 10 April 2016 Accepted 13 April 2016 Available online xxx Keywords: OLED Phosphorescence Blue Bilayer Liquid nitrogen temperature
A B S T R A C T
We have investigated substantial drop of external quantum efficiency (EQE) at liquid nitrogen temperature in a bilayer blue phosphorescent organic light emitting device with a structure of ITO/TAPC: FIrpic/TAZ/LiF/Al, where TAPC, FIrpic, and TAZ represent 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane, 0 iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2 ]picolinate, and 3-(biphenyl-4-yl)-4-phenyl-5-(4tert-butyl-phenyl)-1,2,4-triazole, respectively. The device exhibits a high EQE of 17.2% at room temperature although it has a simple bilayer structure. However, the quantum efficiency drops drastically to 0.8% at liquid nitrogen temperature. We studied this drastic drop of EQE in viewpoints of carrier conduction, carrier recombination, photoluminescence quantum efficiency, and energy transfer. ã 2016 Published by Elsevier B.V.
1. Introduction Since the first report on efficient bilayer organic light-emitting device (OLED) by Tang and VanSlyke [1], numerous researches have been conducted to improve the efficiency and lifetime of devices. Particularly, phosphorescent OLEDs have been widely researched in the past two decades because they can realize highly efficient displays and lighting devices by harvesting both the electro-generated singlet and triplet excitons [2]. The organic phosphors have been typically used as the guest emitters which are doped into the organic host layers inserted between functional organic layers such as carrier injection, carrier transport, and carrier blocking layers [3,4]. Utilizing these host-guest systems and multi-layer structures, the red and green phosphorescent OLEDs approaching a theoretical external quantum efficiency (EQE) of about 20% have been reported [5,6]. However, the efficient and stable blue phosphorescent device, especially having a simple organic layer structure, is still one of the important challenging issues. Since the triplet excitons of blue phosphorescent emitters have high energies, the host materials with high triplet energies are
* Corresponding author. E-mail address: [email protected] (D.-G. Moon). http://dx.doi.org/10.1016/j.synthmet.2016.04.010 0379-6779/ ã 2016 Published by Elsevier B.V.
preferred to enhance the energy transfer and avoid the backward energy transfer from the guest to the host molecules [7–9]. In addition, the carrier transporting materials contacting with emission layer should have high triplet energies to avoid the energy transfer of triplet excitons from the emission layer to the adjacent carrier transport layers [10–12]. Therefore, the wide band gap materials with high triplet energies are typically used as host and carrier transport layers in blue phosphorescent OLEDs. These wide band gap materials may limit the carrier injection and transport so that the complicated device structures with multiorganic layers have been used to achieve a high recombination efficiency and low driving voltage. There are very few reports on simple bilayer blue phosphorescent OLEDs [13], although several red and greed bilayer devices have been demonstrated [14–17]. In this paper, we report on highly efficient bilayer blue phosphorescent OLEDs. The emission layer acts as carrier injection, transport, recombination, and exciton formation in bilayer devices. Although these properties are dependent on temperature, the low temperature characteristics of the bilayer blue phosphorescent OLEDs has not been demonstrated. We observed drastic drop of external quantum efficiency at liquid nitrogen temperature in the highly efficient bilayer blue phosphorescent OLEDs. We investigated this decrease of efficiency in viewpoints of carrier conduction, carrier recombination, photoluminescence (PL) quantum efficiency, and energy transfer.
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2. Experimental Bilayer blue phosphorescent OLEDs were prepared on the indium tin oxide (ITO) coated glass substrates. The sheet resistance of the ITO film was about 10 V/&. The ITO film was patterned to define anode electrodes using photolithography processes. The patterned substrates were loaded into the vacuum chamber after cleaning with isopropyl alcohol and deionized water, followed by exposing to the oxygen plasma at 10 W. Organic, LiF, and metal layers were sequentially deposited by using a vacuum thermal evaporation method in a base pressure of about 1 106 Torr. A 0 30 nm thick iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2 ] picolinate (FIrpic) doped 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer was deposited on the patterned ITO anodes. The concentration of blue phosphorescent FIrpic guest emitter was 2%. We varied the FIrpic concentration from 0.5% to 20% in the bilayer blue phosphorescent OLEDs using tris[2,4,6-trimethyl-3(pyridine-3-yl)phenyl]borane electron transport layer [13]. The current efficiency was highest in the 2% doped device so that we fixed the FIrpic concentration to be 2% [13]. The TAPC acts as a hole transporting host material. After depositing the FIrpic doped TAPC layer, a 65 nm thick 3-(biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) electron transport layer was deposited. After depositing the organic layers, a 0.5 nm thick LiF and a 100 nm thick Al layers were sequentially evaporated through a shadow mask. The completed device structure was ITO/TAPC: FIrpic (30 nm, 2%)/TAZ (65 nm)/LiF (0.5 nm)/Al (100 nm). Fig. 1 shows the chemical structures of the organic materials, the schematic device structure, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the organic materials. The active area of the devices was 4 4 mm2. Current density-voltage-luminance characteristics of the devices were measured using computer controlled Keithley 2400 source-measure units and a calibrated fast Si photodiode (FDS010) at room temperature and liquid nitrogen temperature. The electroluminescence (EL) spectra of the devices were measured using a Minolta CS1000 spectroradiometer at room temperature and liquid nitrogen temperature.
Fig. 2. EQE curves as a function of current density for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
3. Result and discussion Fig. 2 shows the EQE curves as a function of current density for the bilayer blue phosphorescent OLEDs measured at room and liquid nitrogen temperatures. The device structure is ITO/TAPC: FIrpic (30 nm, 2%)/TAZ (65 nm)/LiF/Al. TAPC was used as a hole transporting host for the blue phosphorescent FIrpic emitter. At room temperature, the device exhibits a maximum EQE of 17.2%, corresponding to a maximum current efficiency of 37.6 cd/A at a current density of 1.04 mA/cm2. Recently, several authors reported blue phosphorescent OLEDs exhibiting maximum external quantum efficiencies of 6–11% using three organic layers [18–20]. Other authors also reported the maximum external quantum efficiencies of 10–24% using four or five organic layers [21–23]. The device shown in Fig. 1 exhibits high external quantum efficiency, although the device has a simple bilayer structure [13]. The high efficiency in our bilayer device can be attributed to the effective confinement of triplet excitons on the guest molecules and efficient recombination of injected charges [13]. Since the triplet energy of the blue phosphorescent FIrpic molecules is known to be 2.64–2.70 eV [7–9], the host material for the FIrpic should have the triplet energy higher than 2.70 eV in order to prevent the backward
Fig. 1. Chemical structures of organic materials, device structure of the bilayer blue phosphorescent OLED, and the HOMO and LUMO energy levels of organic materials.
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energy transfer from the FIrpic to the host molecules. The triplet energy of the TAPC is 2.87 eV [10], so that the triplet excitons generated on the FIrpic molecules can be efficiently confined on the guest molecules doped into the TAPC layer. In addition, the triplet excitons on the FIrpic molecules are also effectively confined inside the emission layer without loss of triplet excitons owing to the energy transfer to the adjacent TAZ layer because the triplet energy of TAZ is 2.70 eV [24]. Furthermore, the effective blocking of holes by TAZ layer may improve the charge balance since the HOMO energy level (5.5 eV) of TAPC is shallower than that (6.3 eV) of TAZ [22,25], as shown in the energy level diagram of Fig. 1. Although the bilayer blue phosphorescent device exhibits high efficiency at room temperature, the external quantum efficiency decreases substantially at liquid nitrogen temperature as shown in Fig. 2. The device exhibits a maximum EQE of 0.8%, corresponding to a maximum current efficiency of 1.5 cd/A at a current density of 3.8 mA/cm2 at liquid nitrogen temperature. The emission characteristics of the device were measured to investigate this substantial drop of efficiency at liquid nitrogen temperature. Fig. 3 shows the EL spectra for the bilayer blue phosphorescent OLEDs measured at room and liquid nitrogen temperatures. The devices exhibit strong emission peaks at 470 and 500 nm, attributing to radiative transitions from the triplet metal to ligand charge transfer (3MLCT) and p-p* ligand states of blue phosphorescent FIrpic molecules, respectively [26]. The emission intensity at 500 nm due to the vibronic p-p* transition of FIrpic molecules decreases at liquid nitrogen temperature [22]. The formation of excitons on the FIrpic molecules may be due to the energy transfer from TAPC host and/or due to the direct recombination of electrons and holes on the FIpric molecules. Since the the HOMO level (5.7 eV) of FIrpic is deeper than that (5.5 eV) of TAPC as shown in the energy diagram of Fig. 1, the holes are injected into the TAPC molecules rather than FIrpic molecules. On the other hand, some electrons can be directly injected into the FIrpic molecules from TAZ layer since the LUMO level (2.9 eV) of FIrpic is deeper than that (2.0 eV) of TAPC. The electrons on the FIrpic molecules attract the holes on the TAPC molecules, resulting in the formation of excitons on the FIrpic molecules. The other electrons that overcome the energy barrier of 0.7 eV between TAPC and TAZ layers generate the excitons on the TAPC molecules by recombining with holes on the host molecules, transferring their energy to the guest FIrpic molecules. The PL quantum efficiency in the FIrpic doped TAPC film may be independent of the temperature since the triplet energy level (2.87 eV) of the TAPC is higher than that (2.64–2.70 eV) of the FIrpic molecules so that the efficient exothermic energy transfer occurs from the host TAPC to the guest FIrpic molecules even at liquid nitrogen temperature [7–10]. The temperature
Fig. 3. EL spectra for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
Fig. 4. Current density-voltage-luminance curves as a function of voltage for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
independency of the PL quantum efficiency was observed in the several exothermic host-guest systems such as N,N0 -dicarbazolyl3,5-benzene:FIrpic [7], 4,40 -bis(9-carbazolyl)-2,20 -dimethyl-biphenyl:FIrpic [8,9], TAPC:tris(2-phenylpyridine)iridium(III) [Ir (ppy)3] [10], and 4,40 -N,N0 -dicarbazole-biphenyl:Ir(ppy)3 [27]. It should be noted that the EL emission at around 400 nm, which corresponds to the emission from TAZ molecules [28], increases at liquid nitrogen temperature as shown in Fig. 3. The relative intensity of TAZ emission peak is about 0.12 at liquid nitrogen temperature, which is high although the TAZ is not highly fluorescent molecule. This result suggests that the significant amount of electron-hole recombination takes place in the TAZ layer at liquid nitrogen temperature, resulting in drastic drop of external quantum efficiency. Fig. 4 shows the current density-voltage-luminance curves as a function of voltage for the bilayer blue phosphorescent OLEDs at room and liquid nitrogen temperatures. The voltage for the same current density substantially increases at liquid nitrogen temperature. For example, the voltage for a current density 10 mA/cm2 increases from 10.6 to 23.2 V as the temperature decreases from room to liquid nitrogen temperature. Similarly, the voltage for the same luminance substantially increases at liquid nitrogen temperature. For example, the voltages for a luminance of 1000 cd/m2 are 10.6 and 23.2 V at room and liquid nitrogen temperature, respectively. Since this substantial increase of the driving voltage at liquid nitrogen temperature could be attributed to the suppression of carrier transport in the TAPC:FIrpic and/or TAZ layer, the current conduction characteristics of the TAPC hole only device (HOD) and TAZ electron only device (EOD) were measured at room and liquid nitrogen temperatures. Fig. 5 shows the current
Fig. 5. Current density-voltage curves as a function of voltage for the TAPC HOD and TAZ EOD at room (RT) and liquid nitrogen temperatures (LNT).
M.-Y. Ha et al. / Synthetic Metals 217 (2016) 244–247
density-voltage curves as a function of voltage for the TAPC HOD and TAZ EOD at room and liquid nitrogen temperatures. The device structures of the TAPC HOD and TAZ EOD are ITO/TAPC (100 nm)/Al and ITO/TAZ (100 nm)/LiF/Al, respectively. The driving voltage of the TAPC HOD is about 6–7 V lower than that of TAZ EOD. For example, the voltages for the current density of 10 mA/cm2 are 4.4 and 10.8 V in the TAPC HOD and TAZ EOD, respectively. The lower driving voltage in the TAPC HOD results from the lower carrier injection barrier and mobility compared to the TAZ. The hole injection from anode to TAPC layer is easier than the electron injection from cathode to TAZ layer because the HOMO and LUMO levels of TAPC and TAZ are 5.5 and 2.7 eV, respectively, as shown in the energy diagram of Fig. 1. Furthermore, the hole mobility of TAPC (103–102 cm2/Vs) is 3–4 order of magnitude higher than the electron mobility of TAZ (106 cm2/Vs) [29,30]. Therefore, the carrier transport is dominated by the TAZ layer in the TAPC:Firpic bilayer device so that the electron-hole recombination efficiency is limited by electron transport in the TAZ layer. As the temperature decreases to liquid nitrogen temperature, the driving voltage increases in both devices as shown in Fig. 5. However, the amount of voltage shift is much larger in the TAZ EOD. For example, the voltages at a current density of 10 mA/cm2 are shifted by 4.4 and 12.0 V as the temperature decreases to liquid nitrogen temperature in the TAPC HOD and TAZ EOD, respectively. This result suggests that the substantially reduced carrier transport in the TAZ layer at liquid nitrogen temperature results in carrier recombination inside the TAZ layer, leading to drastic drop of current efficiency and significant fluorescence emission from the TAZ layer. 4. Conclusion We investigated the substantial drop of EQE at liquid nitrogen temperature in the highly efficient bilayer phosphorescent OLEDs. The device structure was ITO/TAPC:FIrpic/TAZ/LiF/Al. The EQE of the device was 17.2% at room temperature. The device exhibited substantial drop of EQE to 0.8% and the simultaneous increase of fluorescent emission intensity from TAZ layer at liquid nitrogen temperature. The drastic drop of EQE was attributed to the significant carrier recombination inside the TAZ layer rather than the modification of energy transfer processes to FIrpic and PL quantum efficiency of TAPC:FIrpic emission layer at liquid nitrogen temperature. The current-voltage characteristics of the TAPC HOD and TAZ EOD devices revealed that the electron conduction in the TAZ layer was much suppressed compared to the hole conduction in the TAPC layer at liquid nitrogen temperature, so that the large amounts of carriers were recombined inside the TAZ layer.
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Acknowledgements This work was supported by the Soonchunhyang University Research Fund and by the IT R&D program of MKE/KEIT (10041062, Development of fundamental technology for light extraction of OLED). References [1] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. [2] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature 395 (1998) 151. [3] M. Ikai, S. Tokito, Y. Samamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [4] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [5] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [6] W.S. Jeon, T.J. Park, S.Y. Kim, R. Pode, J. Jang, J.H. Kwon, Org. Electron. 10 (2009) 240. [7] R.J. Holmes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon, M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422. [8] S. Tokito, T. Iijima, Y. Suzuki, H. Kita, T. Tsuzuki, F. Sato, Appl. Phys. Lett. 83 (2003) 569. [9] I. Tanaka, Y. Tabata, S. Tokito, Chem. Phys. Lett. 400 (2004) 86. [10] K. Goushi, R. Kwong, J.J. Brown, H. Sasabe, C. Adachi, J. Appl. Phys. 95 (2004) 7798. [11] J. Lee, N. Chopra, S.H. Eom, Y. Zheng, J. Xue, F. So, J. Shi, Appl. Phys. Lett. 93 (2008) 123306. [12] N. Chopra, J. Lee, Y. Zheng, S.H. Eom, J. Xue, F. So, Appl. Phys. Lett. 93 (2008) 143307. [13] T.Y. Kim, D.G. Moon, Synth. Met. 160 (2010) 675. [14] C. Adachi, R. Kwong, S.R. Forrest, Org. Electron. 2 (2001) 37. [15] J. Meyer, S. Hamwi, T. Bülow, H.H. Johannes, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 91 (2007) 113506. [16] T.J. Park, W.S. Jeon, J.J. Park, S.Y. Kim, Y.K. Lee, J. Jang, J.H. Kwon, R. Pode, Appl. Phys. Lett. 92 (2008) 113308. [17] W.S. Jeon, T.J. Park, J.J. Park, S.Y. Kim, J. Jang, J.H. Kwon, R. Pode, Appl. Phys. Lett. 92 (2008) 113311. [18] D.R. Whang, Y. You, S.H. Kim, W.I. Jeong, Y.S. Park, J.J. Kim, S.Y. Park, Appl. Phys. Lett. 91 (2007) 233501. [19] G.T. Lei, L.D. Wang, L. Duan, J.H. Wang, Y. Qiu, Synth. Met. 144 (2004) 249. [20] Y.S. Seo, D.G. Moon, Synth. Met. 160 (2010) 113. [21] S.J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 20 (2008) 2125. [22] M.H. Tsai, H.W. Lin, H.C. Su, T.H. Ke, C.C. Wu, F.C. Fang, Y.L. Liao, K.T. Wong, C.I. Wu, Adv. Mater. 18 (2006) 1216. [23] J. Lee, J.I. Lee, K.I. Song, S.J. Lee, H.Y. Chu, Appl. Phys. Lett. 92 (2008) 203305. [24] L. Xiao, S.J. Su, Y. Agata, H. Lan, J. Kido, Adv. Mater. 21 (2009) 1271. [25] J. Kalinowski, M. Cocchi, D. Virgili, V. Fattori, J.A. Gareth Williams, Adv. Mater. 19 (2007) 4000. [26] V. Maiorano, E. Perrone, S. Carallo, A. Biasco, P.P. Pompa, R. Cingolani, A. Croce, R.I.R. Blyth, J. Thompson, Synth. Met. 151 (2005) 147. [27] K. Goushi, Y. Kawamura, H. Sasabe, C. Adachi, Jpn. J. Appl. Phys. 43 (2004) L937. [28] C.H. Chang, C.C. Chen, C.C. Wu, C.H. Yang, Y. Chi, Org. Electron. 10 (2009) 1364. [29] P.M. Borsengerger, L. Pautmeier, R. Richert, H. Bässler, J. Chem. Phys. 94 (1991) 8276. [30] H. Sasabe, E. Gonmori, T. Chiba, Y.J. Li, D. Tanaka, S.J. Su, T. Takeda, Y.J. Pu, K. Nakayama, J. Kido, Chem. Mater. 20 (2008) 5951.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Drastic drop of external quantum efficiency at liquid nitrogen temperature in a bilayer blue phosphorescent organic light-emitting device Mi-Young Ha, Da-Young Park, Min-Jae Lee, Seung-Jung Choi, Jae-Hoon Jung, Dae-Gyu Moon* Department of Materials Engineering, Soonchunhyang University, 646, Eupnae-ri, Shinchang-myeon, Asan-si, Chungcheongnam-do 336-745, South Korea
A R T I C L E I N F O
Article history: Received 19 February 2016 Received in revised form 10 April 2016 Accepted 13 April 2016 Available online xxx Keywords: OLED Phosphorescence Blue Bilayer Liquid nitrogen temperature
A B S T R A C T
We have investigated substantial drop of external quantum efficiency (EQE) at liquid nitrogen temperature in a bilayer blue phosphorescent organic light emitting device with a structure of ITO/TAPC: FIrpic/TAZ/LiF/Al, where TAPC, FIrpic, and TAZ represent 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane, 0 iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2 ]picolinate, and 3-(biphenyl-4-yl)-4-phenyl-5-(4tert-butyl-phenyl)-1,2,4-triazole, respectively. The device exhibits a high EQE of 17.2% at room temperature although it has a simple bilayer structure. However, the quantum efficiency drops drastically to 0.8% at liquid nitrogen temperature. We studied this drastic drop of EQE in viewpoints of carrier conduction, carrier recombination, photoluminescence quantum efficiency, and energy transfer. ã 2016 Published by Elsevier B.V.
1. Introduction Since the first report on efficient bilayer organic light-emitting device (OLED) by Tang and VanSlyke [1], numerous researches have been conducted to improve the efficiency and lifetime of devices. Particularly, phosphorescent OLEDs have been widely researched in the past two decades because they can realize highly efficient displays and lighting devices by harvesting both the electro-generated singlet and triplet excitons [2]. The organic phosphors have been typically used as the guest emitters which are doped into the organic host layers inserted between functional organic layers such as carrier injection, carrier transport, and carrier blocking layers [3,4]. Utilizing these host-guest systems and multi-layer structures, the red and green phosphorescent OLEDs approaching a theoretical external quantum efficiency (EQE) of about 20% have been reported [5,6]. However, the efficient and stable blue phosphorescent device, especially having a simple organic layer structure, is still one of the important challenging issues. Since the triplet excitons of blue phosphorescent emitters have high energies, the host materials with high triplet energies are
* Corresponding author. E-mail address: [email protected] (D.-G. Moon). http://dx.doi.org/10.1016/j.synthmet.2016.04.010 0379-6779/ ã 2016 Published by Elsevier B.V.
preferred to enhance the energy transfer and avoid the backward energy transfer from the guest to the host molecules [7–9]. In addition, the carrier transporting materials contacting with emission layer should have high triplet energies to avoid the energy transfer of triplet excitons from the emission layer to the adjacent carrier transport layers [10–12]. Therefore, the wide band gap materials with high triplet energies are typically used as host and carrier transport layers in blue phosphorescent OLEDs. These wide band gap materials may limit the carrier injection and transport so that the complicated device structures with multiorganic layers have been used to achieve a high recombination efficiency and low driving voltage. There are very few reports on simple bilayer blue phosphorescent OLEDs [13], although several red and greed bilayer devices have been demonstrated [14–17]. In this paper, we report on highly efficient bilayer blue phosphorescent OLEDs. The emission layer acts as carrier injection, transport, recombination, and exciton formation in bilayer devices. Although these properties are dependent on temperature, the low temperature characteristics of the bilayer blue phosphorescent OLEDs has not been demonstrated. We observed drastic drop of external quantum efficiency at liquid nitrogen temperature in the highly efficient bilayer blue phosphorescent OLEDs. We investigated this decrease of efficiency in viewpoints of carrier conduction, carrier recombination, photoluminescence (PL) quantum efficiency, and energy transfer.
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2. Experimental Bilayer blue phosphorescent OLEDs were prepared on the indium tin oxide (ITO) coated glass substrates. The sheet resistance of the ITO film was about 10 V/&. The ITO film was patterned to define anode electrodes using photolithography processes. The patterned substrates were loaded into the vacuum chamber after cleaning with isopropyl alcohol and deionized water, followed by exposing to the oxygen plasma at 10 W. Organic, LiF, and metal layers were sequentially deposited by using a vacuum thermal evaporation method in a base pressure of about 1 106 Torr. A 0 30 nm thick iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2 ] picolinate (FIrpic) doped 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer was deposited on the patterned ITO anodes. The concentration of blue phosphorescent FIrpic guest emitter was 2%. We varied the FIrpic concentration from 0.5% to 20% in the bilayer blue phosphorescent OLEDs using tris[2,4,6-trimethyl-3(pyridine-3-yl)phenyl]borane electron transport layer [13]. The current efficiency was highest in the 2% doped device so that we fixed the FIrpic concentration to be 2% [13]. The TAPC acts as a hole transporting host material. After depositing the FIrpic doped TAPC layer, a 65 nm thick 3-(biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) electron transport layer was deposited. After depositing the organic layers, a 0.5 nm thick LiF and a 100 nm thick Al layers were sequentially evaporated through a shadow mask. The completed device structure was ITO/TAPC: FIrpic (30 nm, 2%)/TAZ (65 nm)/LiF (0.5 nm)/Al (100 nm). Fig. 1 shows the chemical structures of the organic materials, the schematic device structure, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the organic materials. The active area of the devices was 4 4 mm2. Current density-voltage-luminance characteristics of the devices were measured using computer controlled Keithley 2400 source-measure units and a calibrated fast Si photodiode (FDS010) at room temperature and liquid nitrogen temperature. The electroluminescence (EL) spectra of the devices were measured using a Minolta CS1000 spectroradiometer at room temperature and liquid nitrogen temperature.
Fig. 2. EQE curves as a function of current density for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
3. Result and discussion Fig. 2 shows the EQE curves as a function of current density for the bilayer blue phosphorescent OLEDs measured at room and liquid nitrogen temperatures. The device structure is ITO/TAPC: FIrpic (30 nm, 2%)/TAZ (65 nm)/LiF/Al. TAPC was used as a hole transporting host for the blue phosphorescent FIrpic emitter. At room temperature, the device exhibits a maximum EQE of 17.2%, corresponding to a maximum current efficiency of 37.6 cd/A at a current density of 1.04 mA/cm2. Recently, several authors reported blue phosphorescent OLEDs exhibiting maximum external quantum efficiencies of 6–11% using three organic layers [18–20]. Other authors also reported the maximum external quantum efficiencies of 10–24% using four or five organic layers [21–23]. The device shown in Fig. 1 exhibits high external quantum efficiency, although the device has a simple bilayer structure [13]. The high efficiency in our bilayer device can be attributed to the effective confinement of triplet excitons on the guest molecules and efficient recombination of injected charges [13]. Since the triplet energy of the blue phosphorescent FIrpic molecules is known to be 2.64–2.70 eV [7–9], the host material for the FIrpic should have the triplet energy higher than 2.70 eV in order to prevent the backward
Fig. 1. Chemical structures of organic materials, device structure of the bilayer blue phosphorescent OLED, and the HOMO and LUMO energy levels of organic materials.
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energy transfer from the FIrpic to the host molecules. The triplet energy of the TAPC is 2.87 eV [10], so that the triplet excitons generated on the FIrpic molecules can be efficiently confined on the guest molecules doped into the TAPC layer. In addition, the triplet excitons on the FIrpic molecules are also effectively confined inside the emission layer without loss of triplet excitons owing to the energy transfer to the adjacent TAZ layer because the triplet energy of TAZ is 2.70 eV [24]. Furthermore, the effective blocking of holes by TAZ layer may improve the charge balance since the HOMO energy level (5.5 eV) of TAPC is shallower than that (6.3 eV) of TAZ [22,25], as shown in the energy level diagram of Fig. 1. Although the bilayer blue phosphorescent device exhibits high efficiency at room temperature, the external quantum efficiency decreases substantially at liquid nitrogen temperature as shown in Fig. 2. The device exhibits a maximum EQE of 0.8%, corresponding to a maximum current efficiency of 1.5 cd/A at a current density of 3.8 mA/cm2 at liquid nitrogen temperature. The emission characteristics of the device were measured to investigate this substantial drop of efficiency at liquid nitrogen temperature. Fig. 3 shows the EL spectra for the bilayer blue phosphorescent OLEDs measured at room and liquid nitrogen temperatures. The devices exhibit strong emission peaks at 470 and 500 nm, attributing to radiative transitions from the triplet metal to ligand charge transfer (3MLCT) and p-p* ligand states of blue phosphorescent FIrpic molecules, respectively [26]. The emission intensity at 500 nm due to the vibronic p-p* transition of FIrpic molecules decreases at liquid nitrogen temperature [22]. The formation of excitons on the FIrpic molecules may be due to the energy transfer from TAPC host and/or due to the direct recombination of electrons and holes on the FIpric molecules. Since the the HOMO level (5.7 eV) of FIrpic is deeper than that (5.5 eV) of TAPC as shown in the energy diagram of Fig. 1, the holes are injected into the TAPC molecules rather than FIrpic molecules. On the other hand, some electrons can be directly injected into the FIrpic molecules from TAZ layer since the LUMO level (2.9 eV) of FIrpic is deeper than that (2.0 eV) of TAPC. The electrons on the FIrpic molecules attract the holes on the TAPC molecules, resulting in the formation of excitons on the FIrpic molecules. The other electrons that overcome the energy barrier of 0.7 eV between TAPC and TAZ layers generate the excitons on the TAPC molecules by recombining with holes on the host molecules, transferring their energy to the guest FIrpic molecules. The PL quantum efficiency in the FIrpic doped TAPC film may be independent of the temperature since the triplet energy level (2.87 eV) of the TAPC is higher than that (2.64–2.70 eV) of the FIrpic molecules so that the efficient exothermic energy transfer occurs from the host TAPC to the guest FIrpic molecules even at liquid nitrogen temperature [7–10]. The temperature
Fig. 3. EL spectra for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
Fig. 4. Current density-voltage-luminance curves as a function of voltage for the bilayer blue phosphorescent OLEDs at room (RT) and liquid nitrogen temperatures (LNT).
independency of the PL quantum efficiency was observed in the several exothermic host-guest systems such as N,N0 -dicarbazolyl3,5-benzene:FIrpic [7], 4,40 -bis(9-carbazolyl)-2,20 -dimethyl-biphenyl:FIrpic [8,9], TAPC:tris(2-phenylpyridine)iridium(III) [Ir (ppy)3] [10], and 4,40 -N,N0 -dicarbazole-biphenyl:Ir(ppy)3 [27]. It should be noted that the EL emission at around 400 nm, which corresponds to the emission from TAZ molecules [28], increases at liquid nitrogen temperature as shown in Fig. 3. The relative intensity of TAZ emission peak is about 0.12 at liquid nitrogen temperature, which is high although the TAZ is not highly fluorescent molecule. This result suggests that the significant amount of electron-hole recombination takes place in the TAZ layer at liquid nitrogen temperature, resulting in drastic drop of external quantum efficiency. Fig. 4 shows the current density-voltage-luminance curves as a function of voltage for the bilayer blue phosphorescent OLEDs at room and liquid nitrogen temperatures. The voltage for the same current density substantially increases at liquid nitrogen temperature. For example, the voltage for a current density 10 mA/cm2 increases from 10.6 to 23.2 V as the temperature decreases from room to liquid nitrogen temperature. Similarly, the voltage for the same luminance substantially increases at liquid nitrogen temperature. For example, the voltages for a luminance of 1000 cd/m2 are 10.6 and 23.2 V at room and liquid nitrogen temperature, respectively. Since this substantial increase of the driving voltage at liquid nitrogen temperature could be attributed to the suppression of carrier transport in the TAPC:FIrpic and/or TAZ layer, the current conduction characteristics of the TAPC hole only device (HOD) and TAZ electron only device (EOD) were measured at room and liquid nitrogen temperatures. Fig. 5 shows the current
Fig. 5. Current density-voltage curves as a function of voltage for the TAPC HOD and TAZ EOD at room (RT) and liquid nitrogen temperatures (LNT).
M.-Y. Ha et al. / Synthetic Metals 217 (2016) 244–247
density-voltage curves as a function of voltage for the TAPC HOD and TAZ EOD at room and liquid nitrogen temperatures. The device structures of the TAPC HOD and TAZ EOD are ITO/TAPC (100 nm)/Al and ITO/TAZ (100 nm)/LiF/Al, respectively. The driving voltage of the TAPC HOD is about 6–7 V lower than that of TAZ EOD. For example, the voltages for the current density of 10 mA/cm2 are 4.4 and 10.8 V in the TAPC HOD and TAZ EOD, respectively. The lower driving voltage in the TAPC HOD results from the lower carrier injection barrier and mobility compared to the TAZ. The hole injection from anode to TAPC layer is easier than the electron injection from cathode to TAZ layer because the HOMO and LUMO levels of TAPC and TAZ are 5.5 and 2.7 eV, respectively, as shown in the energy diagram of Fig. 1. Furthermore, the hole mobility of TAPC (103–102 cm2/Vs) is 3–4 order of magnitude higher than the electron mobility of TAZ (106 cm2/Vs) [29,30]. Therefore, the carrier transport is dominated by the TAZ layer in the TAPC:Firpic bilayer device so that the electron-hole recombination efficiency is limited by electron transport in the TAZ layer. As the temperature decreases to liquid nitrogen temperature, the driving voltage increases in both devices as shown in Fig. 5. However, the amount of voltage shift is much larger in the TAZ EOD. For example, the voltages at a current density of 10 mA/cm2 are shifted by 4.4 and 12.0 V as the temperature decreases to liquid nitrogen temperature in the TAPC HOD and TAZ EOD, respectively. This result suggests that the substantially reduced carrier transport in the TAZ layer at liquid nitrogen temperature results in carrier recombination inside the TAZ layer, leading to drastic drop of current efficiency and significant fluorescence emission from the TAZ layer. 4. Conclusion We investigated the substantial drop of EQE at liquid nitrogen temperature in the highly efficient bilayer phosphorescent OLEDs. The device structure was ITO/TAPC:FIrpic/TAZ/LiF/Al. The EQE of the device was 17.2% at room temperature. The device exhibited substantial drop of EQE to 0.8% and the simultaneous increase of fluorescent emission intensity from TAZ layer at liquid nitrogen temperature. The drastic drop of EQE was attributed to the significant carrier recombination inside the TAZ layer rather than the modification of energy transfer processes to FIrpic and PL quantum efficiency of TAPC:FIrpic emission layer at liquid nitrogen temperature. The current-voltage characteristics of the TAPC HOD and TAZ EOD devices revealed that the electron conduction in the TAZ layer was much suppressed compared to the hole conduction in the TAPC layer at liquid nitrogen temperature, so that the large amounts of carriers were recombined inside the TAZ layer.
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