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New transport layers for highly efficient organic electroluminescence devices
Synthetic Metals 102 (1999) 1083-1084
New Transport Layers for Highly Efficient Organic Electroluminescence Devices B.Winkler2, F.Meghdadi 1, S.Taschl , B.Evers3, ISchneider3, W.Fischer2, F.Stelzer2, and G.Leisingl 1 Institut fiir Festkorperphysik, Technische Universitat Graz, Petersgasse 16, A-80 10 Graz, Austria 2 ICTOS, Technische Universitiit Graz, Stremayrgasse 16, A-80 10 Graz, Austria JSFB, Inst. fir Chemische Techenologie,TU-Graz, Stremayrgasse 16, A-80 10 Graz, Austria Abstract Multiheterostructure electroluminescence (EL) devices with blue emission color based on parahexaphenyl (PHP) as the emitting layer and various new organic materials as hole and/or electron transport layers (HTL, ETL) were fabricated to improve the EL efficiency. We present new ETLs, which were prepared via different metal-catalyzed coupling reactions of brominated educts in high yields. Utilizing these new organic materials enables us to obtain high performance of blue PHP EL devices with low threshold fields (0.3 MV/cm) and high external EL quantum efficiencies up to 3.4%. Keywords:
parahexaphenyl,
transport layer, electroluminescence,
multi-heterostructure
Introduction Due to its high luminescent properties, Parahexaphenyl (PHP) oligomer, has been used as an attractive emissive layer in blue organic light emitting devices [l-3]. We have already presented the realization of bright red, green and blue (RGB) EL devices based on blue PHP light emitting in combination with tailored dye layers and with appropriate filters by color conversion technique [4]. One of the most important applications of blue light emitting devices is in full color flat panel display technology. Therefore it is required that high efficiencies are achieved in these devices. Using multilayer instead of single layer structures in organic light emitting diodes (OLED) lead to improved device efficiencies by providing flexibility in the choice of materials [5].
ETL2
Fig.2: Schematic of the energy diagram of PHP oligomer and transport layers. In this work we improved the EL quantum efficiency and stability of blue PHP LEDs with new organic hole and electron transport layers. The electronic structure of the transport layers and I-V characteristics of EL devices are reported. Experimental
ETLI: ETLZ: ETW: ETL4:
4,4’-bis (2,3,4,5,6-pentafluorostyryl)biphenyl 4,4’-diaminooctafluorobiphenyl 4-bromo-2,2’3,3’4”,5,5’6,6’nonafluoroI : 1’,4’: 1“-terphenyl 2,2’3,3”,4’,4”‘,5,5”,6,6”-decafluoro -1:1’,4’:1”,4”:1”’ quaterphenyl HTLl: 3,5,3’,5’-tetramethylbenzidin + terephthaldehyde HTL2: 3,5-diamino-1,2,4-tazol + terephthaldehyde) Fig. 1: Molecular structure of PHP and used organic transport layers in multi-heterostructure LEDs.
Figure 1 shows the chemical structure of new organic transport layers in multiheterostructure PHP EL devices The synthesis of PHP is reported elswhere [6,7]. Details of the synthesis of four type of ETLs which are prepared via different metal-catalysed coupling reactions of brominated educts and two type of HTL are described in [8]. For fabrication of multilayer EL devices, with PHP and transport layers, conventional evaporation technique were used. The thickness of evaporated layers is varied between 120-220 nm. Indium-tin oxide (ITO) coated glass and evaporated Al electrodes were used as anode and cathode, respectively. The current-voltage characteristics and EL quantum efficiency were recorded with a Keithley 236 and 180 simultaneously. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all organic materials were determined by cyclic voltametric measurement.
0379-6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01374-5
F. Meghdadi
1084
. c . c I 7 n 0
PHP HTLlIPHPIETL2 PHPIETL2 PHPIETLl PHPiETL3 PHPIETL4 HTLliPHP HTLPiPHP
-1
0 0 c3 0
0
1
0
Field
A
et al. I Synthetic
2
(Mvlcm)
Fig.3:Current-electric field characteristics for multilayer device with PHP and organic transport layers.
ITO/PHP/ETL3/Al ITOIPHPIETLAIAl ITOiHTLl /Pm/Al ITOIHTL2h’HPIAl
3.4 2.56 0.33 1.45
10 18 4 5
EL
0.5 1.5 0.3 0.3
Tablel: Summary of characteristic data for single and multistructure PHP EL devices using transport layers (Rel. ELQF: Relative EL Quantum Efficiency, Uto : turn-on voltage and E,, : threshold electric field ).
Results and discussions Electroluminescence is the emission of light as a result of the injection of charges of opposite sign. In general the unbalance of the charge carriers and existence of different barrier height between electrodes and emitting layer interfaces is one of the reason for low efficiency of light emission in single EL devices. Alignment of the HOMO and LUMO levels of the appropriate organic layers assembled between the two electrodes, with high and low work function can improve the efficiency of EL devices. The PHP and the new charge transport layers which were designed in the way that the HOMO and LUMO are close to the work function of IT0 (4.8-5.1 eV) and Al (4.2 eV), respectively (see Fig.2 ). Figure 3 shows the current density versus electric field characteristics for single and multilayer PHP EL devices. By applying hole transport/electron blocking materials HTL 1 and HTL2 in PHP EL devices the threshold electric field is decreased to 0.3 MVicm. An external quantum efficiency of about 1.45% for a device with HTL2 is observed. The lowering of the electric
Metals
102 (1999)
1083-1084
field can be related to the reduction of barrier height at the interface for hole injection at the IT0 interface. which reduces the onset of the current, since the current in the PHP devices is mainly dominated by holes as it has been also observed for other conjugated polymer devices . The energy diagram in Fig.2 shows that the HOMO level of HTLl (5.5 eV) is between that of PHP (6.2 eV) and the work function of IT0 (4.9-5.1 eV), which facilitates hole injection. The characteristic data, EL quantum efficiencies, turn-on electric field and threshold voltage for all PHP EL devices are listed in table 1. When the organic materials containing electronegative F-groups are used (see Fig 1) as ETL in PHP devices, the EL quantum efficiencies similarly are increased (except ETLI) although the electron affinity or LUMO level of these materials are lower than PHP. We assume that the interface between Al and transport layers improve charge carrier injection. The ETL2,3,4 have rather good electron mobility but low hole mobilities. For that reason holes injected into the PHP are hindered by the ETL to reach the cathode, and hence are confined to the interface of PHPIETL. The injected electrons from the cathode drift through the ETL into the PHP, where they recombine with holes, located at the interface PHP/ETL. Utilizing ETL in PHP EL devices can also assist effective carrier injection from the electrode to the emitting layer accompanied by a lowering of drive voltage and blocking the holes which pass through the emitting layer and thus control the recombination process. The ETL in multilayer EL devices also acts as a spacer and therefore reduces exciton quenching near the metal electrodes [S]. In double layer PHP EL devices with ETL the turn-on voltage are varied between IO-32 volts and respective threshold electric field between OS-I.7 MV/cm. The highest EL quantum efficiency is observed for ITOIPHPIETL3IAI with value of 3.4% at maximum operating voltage of about 10 V. Conclusion We have shown the application of new transport layers in blue PHP LEDs, significantly improved the performance of the devices. These new organic materials reduced the operating voltage and turn-on electric field and result EL quantum efficiency up to 3.4%. We thank SFB Elektroaktive
Stoffe for the financial support.
References [l] W.Graupner, G.Grem, F.Meghdadi, C.Paar, G.Leising, UScherf, K.Miiller, W.Fischer, F.Stelzer, Mol Cryst., 265 (1994) 54. [2] M.Era, T.Tsutsui, and SSaito, Appl. Phys. Lett., 67 (1995) 2436. [3] F.Meghdadi, G.Leising, W.Fischer, F.Stelzer, Synth. MeC., 85 (1997) 441. [4] STasch, C.Brandst&er, F.Meghdadi, G.Leising, L.Athouel, G.Froyer, Adv.Mat., 9 (1997) 33. [5] C.W.Tang, S.A.VanSlyke, Appl.Phys. Lett., 51 (1987) 913. [6] P. Kovacic and R.M. Lange, J.Org.Chem., 29,2416(1964). [7] G.Froyer, Y.Plous, E.Dall’arche, CChevrot, A.Sivoe, Fr.Patent No. 9008091 (I 990). [8] B.Winkler, Ph.D Thesis, Technische Universitat Graz (1997).
New Transport Layers for Highly Efficient Organic Electroluminescence Devices B.Winkler2, F.Meghdadi 1, S.Taschl , B.Evers3, ISchneider3, W.Fischer2, F.Stelzer2, and G.Leisingl 1 Institut fiir Festkorperphysik, Technische Universitat Graz, Petersgasse 16, A-80 10 Graz, Austria 2 ICTOS, Technische Universitiit Graz, Stremayrgasse 16, A-80 10 Graz, Austria JSFB, Inst. fir Chemische Techenologie,TU-Graz, Stremayrgasse 16, A-80 10 Graz, Austria Abstract Multiheterostructure electroluminescence (EL) devices with blue emission color based on parahexaphenyl (PHP) as the emitting layer and various new organic materials as hole and/or electron transport layers (HTL, ETL) were fabricated to improve the EL efficiency. We present new ETLs, which were prepared via different metal-catalyzed coupling reactions of brominated educts in high yields. Utilizing these new organic materials enables us to obtain high performance of blue PHP EL devices with low threshold fields (0.3 MV/cm) and high external EL quantum efficiencies up to 3.4%. Keywords:
parahexaphenyl,
transport layer, electroluminescence,
multi-heterostructure
Introduction Due to its high luminescent properties, Parahexaphenyl (PHP) oligomer, has been used as an attractive emissive layer in blue organic light emitting devices [l-3]. We have already presented the realization of bright red, green and blue (RGB) EL devices based on blue PHP light emitting in combination with tailored dye layers and with appropriate filters by color conversion technique [4]. One of the most important applications of blue light emitting devices is in full color flat panel display technology. Therefore it is required that high efficiencies are achieved in these devices. Using multilayer instead of single layer structures in organic light emitting diodes (OLED) lead to improved device efficiencies by providing flexibility in the choice of materials [5].
ETL2
Fig.2: Schematic of the energy diagram of PHP oligomer and transport layers. In this work we improved the EL quantum efficiency and stability of blue PHP LEDs with new organic hole and electron transport layers. The electronic structure of the transport layers and I-V characteristics of EL devices are reported. Experimental
ETLI: ETLZ: ETW: ETL4:
4,4’-bis (2,3,4,5,6-pentafluorostyryl)biphenyl 4,4’-diaminooctafluorobiphenyl 4-bromo-2,2’3,3’4”,5,5’6,6’nonafluoroI : 1’,4’: 1“-terphenyl 2,2’3,3”,4’,4”‘,5,5”,6,6”-decafluoro -1:1’,4’:1”,4”:1”’ quaterphenyl HTLl: 3,5,3’,5’-tetramethylbenzidin + terephthaldehyde HTL2: 3,5-diamino-1,2,4-tazol + terephthaldehyde) Fig. 1: Molecular structure of PHP and used organic transport layers in multi-heterostructure LEDs.
Figure 1 shows the chemical structure of new organic transport layers in multiheterostructure PHP EL devices The synthesis of PHP is reported elswhere [6,7]. Details of the synthesis of four type of ETLs which are prepared via different metal-catalysed coupling reactions of brominated educts and two type of HTL are described in [8]. For fabrication of multilayer EL devices, with PHP and transport layers, conventional evaporation technique were used. The thickness of evaporated layers is varied between 120-220 nm. Indium-tin oxide (ITO) coated glass and evaporated Al electrodes were used as anode and cathode, respectively. The current-voltage characteristics and EL quantum efficiency were recorded with a Keithley 236 and 180 simultaneously. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all organic materials were determined by cyclic voltametric measurement.
0379-6779/99/$ see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01374-5
F. Meghdadi
1084
. c . c I 7 n 0
PHP HTLlIPHPIETL2 PHPIETL2 PHPIETLl PHPiETL3 PHPIETL4 HTLliPHP HTLPiPHP
-1
0 0 c3 0
0
1
0
Field
A
et al. I Synthetic
2
(Mvlcm)
Fig.3:Current-electric field characteristics for multilayer device with PHP and organic transport layers.
ITO/PHP/ETL3/Al ITOIPHPIETLAIAl ITOiHTLl /Pm/Al ITOIHTL2h’HPIAl
3.4 2.56 0.33 1.45
10 18 4 5
EL
0.5 1.5 0.3 0.3
Tablel: Summary of characteristic data for single and multistructure PHP EL devices using transport layers (Rel. ELQF: Relative EL Quantum Efficiency, Uto : turn-on voltage and E,, : threshold electric field ).
Results and discussions Electroluminescence is the emission of light as a result of the injection of charges of opposite sign. In general the unbalance of the charge carriers and existence of different barrier height between electrodes and emitting layer interfaces is one of the reason for low efficiency of light emission in single EL devices. Alignment of the HOMO and LUMO levels of the appropriate organic layers assembled between the two electrodes, with high and low work function can improve the efficiency of EL devices. The PHP and the new charge transport layers which were designed in the way that the HOMO and LUMO are close to the work function of IT0 (4.8-5.1 eV) and Al (4.2 eV), respectively (see Fig.2 ). Figure 3 shows the current density versus electric field characteristics for single and multilayer PHP EL devices. By applying hole transport/electron blocking materials HTL 1 and HTL2 in PHP EL devices the threshold electric field is decreased to 0.3 MVicm. An external quantum efficiency of about 1.45% for a device with HTL2 is observed. The lowering of the electric
Metals
102 (1999)
1083-1084
field can be related to the reduction of barrier height at the interface for hole injection at the IT0 interface. which reduces the onset of the current, since the current in the PHP devices is mainly dominated by holes as it has been also observed for other conjugated polymer devices . The energy diagram in Fig.2 shows that the HOMO level of HTLl (5.5 eV) is between that of PHP (6.2 eV) and the work function of IT0 (4.9-5.1 eV), which facilitates hole injection. The characteristic data, EL quantum efficiencies, turn-on electric field and threshold voltage for all PHP EL devices are listed in table 1. When the organic materials containing electronegative F-groups are used (see Fig 1) as ETL in PHP devices, the EL quantum efficiencies similarly are increased (except ETLI) although the electron affinity or LUMO level of these materials are lower than PHP. We assume that the interface between Al and transport layers improve charge carrier injection. The ETL2,3,4 have rather good electron mobility but low hole mobilities. For that reason holes injected into the PHP are hindered by the ETL to reach the cathode, and hence are confined to the interface of PHPIETL. The injected electrons from the cathode drift through the ETL into the PHP, where they recombine with holes, located at the interface PHP/ETL. Utilizing ETL in PHP EL devices can also assist effective carrier injection from the electrode to the emitting layer accompanied by a lowering of drive voltage and blocking the holes which pass through the emitting layer and thus control the recombination process. The ETL in multilayer EL devices also acts as a spacer and therefore reduces exciton quenching near the metal electrodes [S]. In double layer PHP EL devices with ETL the turn-on voltage are varied between IO-32 volts and respective threshold electric field between OS-I.7 MV/cm. The highest EL quantum efficiency is observed for ITOIPHPIETL3IAI with value of 3.4% at maximum operating voltage of about 10 V. Conclusion We have shown the application of new transport layers in blue PHP LEDs, significantly improved the performance of the devices. These new organic materials reduced the operating voltage and turn-on electric field and result EL quantum efficiency up to 3.4%. We thank SFB Elektroaktive
Stoffe for the financial support.
References [l] W.Graupner, G.Grem, F.Meghdadi, C.Paar, G.Leising, UScherf, K.Miiller, W.Fischer, F.Stelzer, Mol Cryst., 265 (1994) 54. [2] M.Era, T.Tsutsui, and SSaito, Appl. Phys. Lett., 67 (1995) 2436. [3] F.Meghdadi, G.Leising, W.Fischer, F.Stelzer, Synth. MeC., 85 (1997) 441. [4] STasch, C.Brandst&er, F.Meghdadi, G.Leising, L.Athouel, G.Froyer, Adv.Mat., 9 (1997) 33. [5] C.W.Tang, S.A.VanSlyke, Appl.Phys. Lett., 51 (1987) 913. [6] P. Kovacic and R.M. Lange, J.Org.Chem., 29,2416(1964). [7] G.Froyer, Y.Plous, E.Dall’arche, CChevrot, A.Sivoe, Fr.Patent No. 9008091 (I 990). [8] B.Winkler, Ph.D Thesis, Technische Universitat Graz (1997).