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Blue light emitting diodes with bathocuproine layer
Synthetic Metals 127 (2002) 165±168
Blue light emitting diodes with bathocuproine layer D. Troadeca, G. Veriotb, A. Molitona,* a
UMOP, EA 1072, Faculty of Sciences, University of Limoges, 123 av. Albert Thomas, 87060 Limoges Cedex, France b LCR-THALES, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract Several hole transport molecules (N,N0 -diphenyl-N,N0 -(3-methylphenyl)-1,10 -biphenyl-4,40 -diamine; 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl; 4,40 ,400 -tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (TNATA); MTDATA) are blue light emitters [SPIE 3797 (1999) 120]. Realisation of monolayer structures is very easy but their performances are too weak. To improve them, we have built some multilayer structures with electron transport layer (tris(8-hydroxyquinolinate) aluminium (Alq3); bis(10-hydroxybenzo(h)quinolinate) beryllium (Bebq2)) and a bathocuproine (BCP) layer to con®ne the radiative recombinations in the hole transport layer. To improve hole injection, we inserted poly(3,4-ethylenedioxythiophene) (PEDOT) layer between the indium±tin-oxide anode and the hole transporting layer. The best results are obtained with the four-layer structure PEDOT/TNATA/BCP/Bebq2 l 508 nm, luminance 5500 cd/m2 at 10.2 V. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hole blocking layer; Bathocuproine (BCP); Blue light emitting diode
1. Introduction
2. Experimental
Flat panel displays have been capturing the attention of the electronics industry for several years. Liquid crystal displays and new displays such as plasma, plasma assisted liquid crystal and ®eld emission displays are now commercially available. On the other hand, in recent years, organic electroluminescent materials and their applications to organic light emitting diodes (OLEDs) have become increasingly important because of their potentials for low cost manufacturing, mechanical ¯exibility, wide viewing angle, and high brightness. Furthermore, these materials have low operating voltages and a wide range of emission wavelengths which allows one to realise full-colour and selfluminous ¯at panel displays [1,2]. For green light, tris(8-hydroxyquinolinate) aluminium (Alq3) is widely used as electron-transporting green-emissive material [3]. Considerable research has been undertaken for developing red OLEDs using ¯uorescent dopants such as DCM, coumarin [4], Nile Red [5], perylene or europium complexes [6]. For blue emissions, zinc complexes or ¯uorene-oxadiazole compounds [7] are reported as electron-transporting blue emitting materials. But their performances are weak and they show poor colour purity. In this paper, we studied blue light emission of the hole transporting layer in monolayer and multilayer structures.
Some of the used organic materials are synthesised at THALES LCR:
*
Corresponding author. Tel.: 33-5-55-45-74-32; fax: 33-5-55-45-72-88. E-mail address: [email protected] (A. Moliton).
tris(8-hydroxyquinolinate) aluminium (Alq3), bis(10-hydroxybenzo(h)quinolinate) beryllium (Bebq2). The others are commercial: N,N0 -diphenyl-N,N0 -(3-methylphenyl)-1,10 -biphenyl4,40 -diamine (TPD), 4,40 ,400 -tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (TNATA), 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline (bathocuproine: BCP), poly(3,4-ethylenedioxythiophene)-PSS (PEDOT-PSS). The indium±tin-oxide (ITO) anode is deposited on a 1.1 mm thick glass substrate. The ITO is commercially prepared by Merck Display Technologies and has a sheet resistance of about 20 O/sq. that corresponds to typically 100 nm thick ®lm. Prior to the organic deposition, the ITO coated glass plate is cleaned in an ultrasonic bath. The conducting polymer (PEDOT-PSS) and its solvent are ®ltered (0.45 mm) and deposited by spin coating; the ®nal thickness is 40 nm. The TPD, NPB, TNATA, BCP, Bebq2 and Alq3 are prepared in a vacuum chamber (about 10 6 Torr) by vapour deposition from heated molybdenum Ê /s and the distance boats. Typically, the deposition rate is 2 A substrate-boat is about 12 cm. Generally, the thickness of the
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 1 5 - 4
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D. Troadec et al. / Synthetic Metals 127 (2002) 165±168
hole transport layer (HTL) is around 60 nm, and for each studied structure we will specify the thickness of each layer. After the organic layer sublimations, the sample is transferred into a vacuum chamber (10 5 Torr) inside a glove box. The cathode, about 150 nm of calcium covered with 150 nm of aluminium is then deposited on top of the organic layers using two different tungsten evaporation boats. The active area of the emissive layer (EL) device, de®ned by the overlap of the ITO anode and the Ca/Al cathode, is 7.11 mm2. The EL luminance is measured with a photodiode and a Keithley 175 multimeter, the intensity with a Keithley 617 electrometer and the EL spectrum with a Zeiss spectrophotometer. All the measurements are realised at room temperature and under nitrogen controlled atmosphere. 3. Results and discussion At ®rst, we prepared monolayer structures using TPD, TNATA or NPB as ELs. These materials have blue emission but their performances are very weak: low luminance and high voltage (Table 1). Insertion of PEDOT-PSS layer between the ITO and the EL allowed us to reduce threshold voltage and to increase the maximum luminance (Table 1). This additional layer protects the EL from oxygen or indium diffusion ITO [8] and improves the hole injection by lowering the energy barrier (0.5 versus 0.67 eV) between the highest occupied molecular orbital (HOMO) and ITO. The Table 1 Performances of monolayer and bi-layer structures Structures (nm)
Vseuil (V)
Lmax (cd/m2)
VLmax V
60 60 40 40 40
6.4 9.5 3.3 3.4 4.7
82.0 61.2 240.0 30.8 7.2
16.8 16.8 8.4 5.6 10.2
TNATA NPB PEDOT 60 TNATA PEDOT 60 NPB PEDOT 60 TPD
best results are obtained with the structure ITO/PEDOTPSS/TNATA/Ca/Al with luminance 240 cd/m2 and threshold voltage 3.3 V. Tang and VanSlyke [9] inserted an HTL before the EL (Alq3) which is an electron transport layer (ETL) to improve the hole transport. In our case, the EL is an HTL, and so we insert an ETL between EL and the cathode to improve the electron injection. However, experiment showed ETL electroluminescence instead of HTL electroluminescence, as we could forecast on energy diagram (Figs. 1 and 2a). To solve this problem, we inserted a BCP layer between ETL and HTL. On the one hand, the high barrier in the HOMO levels located between HTL and BCP (0.7 eV) allows us to block the holes in the HTL; on the other hand, the low barrier in the lowest unoccupied molecular orbital (LUMO) levels between BCP and HTL (0.1 eV) facilitates the crossing of electrons from the cathode to the HTL (Fig. 2b and c). In this way, the BCP layer (hole blocking layer: HBL) allows to con®ne the radiative recombinations in the HTL. We used Alq3 and Bebq2 for the electron transport layer. In the case of HTL/ETL system, we observed green emission from the ETL, while the incorporation of BCP leads to blue emission from the HTL (Fig. 2). At ®rst, we studied TPD/BCP/Alq3 structure because Alq3 is the most popular ETL and we optimised the thickness of each layer. The best result (Fig. 3) was obtained with the structure: TPD(60 nm)/BCP(15 nm)/Alq3(30 nm), the turn on voltage was 5.9 V and the maximum luminance was 464 cd/m2 at 14.2 V. Furthermore, on the energy diagram of this structure (Fig. 2b), we noticed the large barrier in LUMO levels between Alq3 and BCP (0.7 eV). So, we used Bebq2 instead of Alq3 because its LUMO energy is more suitable; only a low energy barrier between Ca and Bebq2 (0.2 eV) and between Bebq2 and BCP (0.3 eV), for electron crossing from the cathode to the HTL (Fig. 2c), the characteristic L f V for TPD(60 nm)/BCP(15 nm)/Bebq2(20 nm)
Fig. 1. Molecular structures of the used organic materials.
D. Troadec et al. / Synthetic Metals 127 (2002) 165±168
Fig. 2. Radiative recombination in: (a) TPD/Alq3, (b) TPD/BCP/Alq3 and (c) TPD/BCP/Bebq2 structures.
Fig. 3. L f V characteristics of TPD/BCP/Alq3 and TPD/BCP/Bebq2 structures.
Fig. 4. L f V characteristics of different NPB structures.
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D. Troadec et al. / Synthetic Metals 127 (2002) 165±168
structure is shown in Fig. 3. On the one hand, we obtained higher luminance (about 700 cd/m2 at 14.6 V), and on the other, higher threshold voltage (7.0 V). To reduce the voltage, we inserted PEDOT-PSS layer like for the ITO/TPD/Ca/Al structure. Then, for the PEDOTPSS/TPD/BCP/Alq3 structure, threshold voltage was reduced from 5.9 to 4.5 V and luminance was increased from 464 cd/m2 at 14.2 V to 652 cd/m2 at 12.0 V. Finally, we studied PEDOT-PSS/TPD/BCP/Bebq2 structure with the two other HTL organic molecules (NPB and TNATA) in place of TPD. These organic molecules are more stable than TPD with increasing temperatures, and their lower HOMO improves electron injection. The luminance of the structure with NPB (3300 cd/m2 at 10.0 V) is lower than with TNATA (5500 cd/m2 at 10.2 V) and the threshold voltage is higher (respectively, 3.0 and 2.5 V). The comparison between monolayer and multilayer structures with NPB ELs is shown in Fig. 4. 4. Conclusion To realise blue light emitting diodes, we used three different hole transport molecules; TPD, NPB and TNATA for the EL. The weak performances of monolayer structures
led us to use Alq3 or Bebq2 layer to improve electron injection and transport, BCP layer to con®ne the radiative recombinations in the HTL and PEDOT-PSS to ameliorate hole injection. These modi®cations increase the luminance from 82 cd/m2 at 16.8 V for ITO/TNATA/Ca/Al structure to 5500 cd/m2 at 10.2 V for ITO/PEDOT-PSS/TNATA/BCP/ Bebq2/Ca/Al structure, and at the same time threshold voltage is reduced from 6.4 to 2.5 V.
References [1] S. Tamura, Y. Kijima, N. Asai, M. Ichimura, T. Ishibashi, SPIE 3797 (1999) 120. [2] P. Le Barny, C.R. Acad. Sci. Paris 1 (4) (2000) 378±380. [3] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 215. [4] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1986) 9. [5] P. Jolinat, R. Clergereaux, J. Farenc, P. Destruel, Appl. Phys. 31 (1998) 1257±1262. [6] K. Okada, Y.F. Wang, T. Nakaya, Synth. Met. 97 (1998) 113±116. [7] H. Antoniadis, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 21. [8] E. Gautier, A. Lorin, J.M. Nunzi, A. Schlchdi, J.J. Benattar, D. Vital, Appl. Phys. Lett. 69 (1996) 1071. [9] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913.
Blue light emitting diodes with bathocuproine layer D. Troadeca, G. Veriotb, A. Molitona,* a
UMOP, EA 1072, Faculty of Sciences, University of Limoges, 123 av. Albert Thomas, 87060 Limoges Cedex, France b LCR-THALES, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract Several hole transport molecules (N,N0 -diphenyl-N,N0 -(3-methylphenyl)-1,10 -biphenyl-4,40 -diamine; 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl; 4,40 ,400 -tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (TNATA); MTDATA) are blue light emitters [SPIE 3797 (1999) 120]. Realisation of monolayer structures is very easy but their performances are too weak. To improve them, we have built some multilayer structures with electron transport layer (tris(8-hydroxyquinolinate) aluminium (Alq3); bis(10-hydroxybenzo(h)quinolinate) beryllium (Bebq2)) and a bathocuproine (BCP) layer to con®ne the radiative recombinations in the hole transport layer. To improve hole injection, we inserted poly(3,4-ethylenedioxythiophene) (PEDOT) layer between the indium±tin-oxide anode and the hole transporting layer. The best results are obtained with the four-layer structure PEDOT/TNATA/BCP/Bebq2 l 508 nm, luminance 5500 cd/m2 at 10.2 V. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hole blocking layer; Bathocuproine (BCP); Blue light emitting diode
1. Introduction
2. Experimental
Flat panel displays have been capturing the attention of the electronics industry for several years. Liquid crystal displays and new displays such as plasma, plasma assisted liquid crystal and ®eld emission displays are now commercially available. On the other hand, in recent years, organic electroluminescent materials and their applications to organic light emitting diodes (OLEDs) have become increasingly important because of their potentials for low cost manufacturing, mechanical ¯exibility, wide viewing angle, and high brightness. Furthermore, these materials have low operating voltages and a wide range of emission wavelengths which allows one to realise full-colour and selfluminous ¯at panel displays [1,2]. For green light, tris(8-hydroxyquinolinate) aluminium (Alq3) is widely used as electron-transporting green-emissive material [3]. Considerable research has been undertaken for developing red OLEDs using ¯uorescent dopants such as DCM, coumarin [4], Nile Red [5], perylene or europium complexes [6]. For blue emissions, zinc complexes or ¯uorene-oxadiazole compounds [7] are reported as electron-transporting blue emitting materials. But their performances are weak and they show poor colour purity. In this paper, we studied blue light emission of the hole transporting layer in monolayer and multilayer structures.
Some of the used organic materials are synthesised at THALES LCR:
*
Corresponding author. Tel.: 33-5-55-45-74-32; fax: 33-5-55-45-72-88. E-mail address: [email protected] (A. Moliton).
tris(8-hydroxyquinolinate) aluminium (Alq3), bis(10-hydroxybenzo(h)quinolinate) beryllium (Bebq2). The others are commercial: N,N0 -diphenyl-N,N0 -(3-methylphenyl)-1,10 -biphenyl4,40 -diamine (TPD), 4,40 ,400 -tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (TNATA), 4,4-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline (bathocuproine: BCP), poly(3,4-ethylenedioxythiophene)-PSS (PEDOT-PSS). The indium±tin-oxide (ITO) anode is deposited on a 1.1 mm thick glass substrate. The ITO is commercially prepared by Merck Display Technologies and has a sheet resistance of about 20 O/sq. that corresponds to typically 100 nm thick ®lm. Prior to the organic deposition, the ITO coated glass plate is cleaned in an ultrasonic bath. The conducting polymer (PEDOT-PSS) and its solvent are ®ltered (0.45 mm) and deposited by spin coating; the ®nal thickness is 40 nm. The TPD, NPB, TNATA, BCP, Bebq2 and Alq3 are prepared in a vacuum chamber (about 10 6 Torr) by vapour deposition from heated molybdenum Ê /s and the distance boats. Typically, the deposition rate is 2 A substrate-boat is about 12 cm. Generally, the thickness of the
0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 6 1 5 - 4
166
D. Troadec et al. / Synthetic Metals 127 (2002) 165±168
hole transport layer (HTL) is around 60 nm, and for each studied structure we will specify the thickness of each layer. After the organic layer sublimations, the sample is transferred into a vacuum chamber (10 5 Torr) inside a glove box. The cathode, about 150 nm of calcium covered with 150 nm of aluminium is then deposited on top of the organic layers using two different tungsten evaporation boats. The active area of the emissive layer (EL) device, de®ned by the overlap of the ITO anode and the Ca/Al cathode, is 7.11 mm2. The EL luminance is measured with a photodiode and a Keithley 175 multimeter, the intensity with a Keithley 617 electrometer and the EL spectrum with a Zeiss spectrophotometer. All the measurements are realised at room temperature and under nitrogen controlled atmosphere. 3. Results and discussion At ®rst, we prepared monolayer structures using TPD, TNATA or NPB as ELs. These materials have blue emission but their performances are very weak: low luminance and high voltage (Table 1). Insertion of PEDOT-PSS layer between the ITO and the EL allowed us to reduce threshold voltage and to increase the maximum luminance (Table 1). This additional layer protects the EL from oxygen or indium diffusion ITO [8] and improves the hole injection by lowering the energy barrier (0.5 versus 0.67 eV) between the highest occupied molecular orbital (HOMO) and ITO. The Table 1 Performances of monolayer and bi-layer structures Structures (nm)
Vseuil (V)
Lmax (cd/m2)
VLmax V
60 60 40 40 40
6.4 9.5 3.3 3.4 4.7
82.0 61.2 240.0 30.8 7.2
16.8 16.8 8.4 5.6 10.2
TNATA NPB PEDOT 60 TNATA PEDOT 60 NPB PEDOT 60 TPD
best results are obtained with the structure ITO/PEDOTPSS/TNATA/Ca/Al with luminance 240 cd/m2 and threshold voltage 3.3 V. Tang and VanSlyke [9] inserted an HTL before the EL (Alq3) which is an electron transport layer (ETL) to improve the hole transport. In our case, the EL is an HTL, and so we insert an ETL between EL and the cathode to improve the electron injection. However, experiment showed ETL electroluminescence instead of HTL electroluminescence, as we could forecast on energy diagram (Figs. 1 and 2a). To solve this problem, we inserted a BCP layer between ETL and HTL. On the one hand, the high barrier in the HOMO levels located between HTL and BCP (0.7 eV) allows us to block the holes in the HTL; on the other hand, the low barrier in the lowest unoccupied molecular orbital (LUMO) levels between BCP and HTL (0.1 eV) facilitates the crossing of electrons from the cathode to the HTL (Fig. 2b and c). In this way, the BCP layer (hole blocking layer: HBL) allows to con®ne the radiative recombinations in the HTL. We used Alq3 and Bebq2 for the electron transport layer. In the case of HTL/ETL system, we observed green emission from the ETL, while the incorporation of BCP leads to blue emission from the HTL (Fig. 2). At ®rst, we studied TPD/BCP/Alq3 structure because Alq3 is the most popular ETL and we optimised the thickness of each layer. The best result (Fig. 3) was obtained with the structure: TPD(60 nm)/BCP(15 nm)/Alq3(30 nm), the turn on voltage was 5.9 V and the maximum luminance was 464 cd/m2 at 14.2 V. Furthermore, on the energy diagram of this structure (Fig. 2b), we noticed the large barrier in LUMO levels between Alq3 and BCP (0.7 eV). So, we used Bebq2 instead of Alq3 because its LUMO energy is more suitable; only a low energy barrier between Ca and Bebq2 (0.2 eV) and between Bebq2 and BCP (0.3 eV), for electron crossing from the cathode to the HTL (Fig. 2c), the characteristic L f V for TPD(60 nm)/BCP(15 nm)/Bebq2(20 nm)
Fig. 1. Molecular structures of the used organic materials.
D. Troadec et al. / Synthetic Metals 127 (2002) 165±168
Fig. 2. Radiative recombination in: (a) TPD/Alq3, (b) TPD/BCP/Alq3 and (c) TPD/BCP/Bebq2 structures.
Fig. 3. L f V characteristics of TPD/BCP/Alq3 and TPD/BCP/Bebq2 structures.
Fig. 4. L f V characteristics of different NPB structures.
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structure is shown in Fig. 3. On the one hand, we obtained higher luminance (about 700 cd/m2 at 14.6 V), and on the other, higher threshold voltage (7.0 V). To reduce the voltage, we inserted PEDOT-PSS layer like for the ITO/TPD/Ca/Al structure. Then, for the PEDOTPSS/TPD/BCP/Alq3 structure, threshold voltage was reduced from 5.9 to 4.5 V and luminance was increased from 464 cd/m2 at 14.2 V to 652 cd/m2 at 12.0 V. Finally, we studied PEDOT-PSS/TPD/BCP/Bebq2 structure with the two other HTL organic molecules (NPB and TNATA) in place of TPD. These organic molecules are more stable than TPD with increasing temperatures, and their lower HOMO improves electron injection. The luminance of the structure with NPB (3300 cd/m2 at 10.0 V) is lower than with TNATA (5500 cd/m2 at 10.2 V) and the threshold voltage is higher (respectively, 3.0 and 2.5 V). The comparison between monolayer and multilayer structures with NPB ELs is shown in Fig. 4. 4. Conclusion To realise blue light emitting diodes, we used three different hole transport molecules; TPD, NPB and TNATA for the EL. The weak performances of monolayer structures
led us to use Alq3 or Bebq2 layer to improve electron injection and transport, BCP layer to con®ne the radiative recombinations in the HTL and PEDOT-PSS to ameliorate hole injection. These modi®cations increase the luminance from 82 cd/m2 at 16.8 V for ITO/TNATA/Ca/Al structure to 5500 cd/m2 at 10.2 V for ITO/PEDOT-PSS/TNATA/BCP/ Bebq2/Ca/Al structure, and at the same time threshold voltage is reduced from 6.4 to 2.5 V.
References [1] S. Tamura, Y. Kijima, N. Asai, M. Ichimura, T. Ishibashi, SPIE 3797 (1999) 120. [2] P. Le Barny, C.R. Acad. Sci. Paris 1 (4) (2000) 378±380. [3] S.A. VanSlyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 215. [4] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1986) 9. [5] P. Jolinat, R. Clergereaux, J. Farenc, P. Destruel, Appl. Phys. 31 (1998) 1257±1262. [6] K. Okada, Y.F. Wang, T. Nakaya, Synth. Met. 97 (1998) 113±116. [7] H. Antoniadis, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 21. [8] E. Gautier, A. Lorin, J.M. Nunzi, A. Schlchdi, J.J. Benattar, D. Vital, Appl. Phys. Lett. 69 (1996) 1071. [9] C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913.