organic heterostructure devices

SVIIIILrrlC IIILrlRs ELSEVIER

Synthetic Metals 96 (1998) 123-126

Electroluminescent behaviours of polymer/organic heterostructure devices Kang-Hoon Choi 1, Do-Hoon Hwang, Hyang-Mok Lee, Lee-Mi Do, Taehyoung Zyung * Research Department, Electronics and Telecommunications Research Institute, 161 Kajong-Dong, Yusong-Ku, Taejon 305-350, South Korea Received 26 January 1998; received in revised form 16 April 1998; accepted 21 April 1998

Abstract

Polymer/organic heterostructure electroluminescent devices were fabricated with poly(2-methoxy-5-(2'-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV) for emissive polymer layer and various organic materials such as tris ( 8-hydroxyquinoline) aluminum (Alq3), rubrene (Rub), quinacridone (Qc) and 2-(4'-biphenyl)-5-(4"-tert-butylphenyl)-l,3,4-oxadiazole (PBD) for hole blocking layer. All kinds of organic low molecular materials in this study show hole blocking property, but electroluminescence (EL) characteristics of some structures were not distinctly enhanced because of re-absorption by organic materials, their poor film quality, low electron mobility. The ITO/MEHPPV/Alq3/A1 structured device shows the highest luminescence among the organic molecules used in this study at low current less than about 3 mA. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Electroluminescence; Polymer/organic heterostructures; Hole blocking; Devices

1. Introduction

Since the discovery of electroluminescence (EL) from the "rr-conjugated polymer [ 1 ], much research on polymer EL devices has progressed during the last ten years. The EL devices based on organic small molecules and polymers are very attractive for application in large-area flat panel displays [ 2 ]. The advantages of this kind of device are easy fabrication for large-area displays, easy color tuning, high luminescence, feasibility of making the flexible devices, etc. Organic or polymeric EL devices have better performances in some aspects compared with inorganic EL devices such as lower operating voltage and faster response time. Especially, the organic or polymeric EL devices can operate in current control mode. This feature is one of the important merits in commercialization of the organic or polymeric EL device from the engineering point of view. For the commercialization of these devices, two major criteria should be considered, i.e., quantum efficiency and lifetime. Firstly, the number of injected electrons from the metal cathode and the number of injected holes from the semitransparent anode should be balanced to enhance the EL quantum efficiency [ 3 ], because the hole mobility and den* Corresponding author. Tel.: +82 42 860 5777; fax: +82 42 860 6836; e-mail: thz @ard.etri.re.kr E-mail: choi @scm.saclay.cea.fr

sity in the polymer layer are respectively faster and larger than those of electrons. Blending of various emissive polymers or molecular doping in the polymer matrix has also been proposed to increase the injection of electrons and to block the hole transport in the emissive polymer layer [4-9]. Secondly, the lifetime of the device should be extended. During the operation of the device, factors like photo-oxidation of polymer, humidity and Joule heat are thought to be degradation sources of the devices [ 10-14]. Encapsulation may be one of the best policies to protect the EL device from degradation sources and to extend the lifetime of devices. However, the Joule heat caused by large current flow irrespective of light emission cannot be eliminated even by the encapsulation of the device. In this work, we have tried to reduce the current flow in order to decrease Joule heat without sacrificing the light output and hence enhance the quantum efficiency of the EL device, by fabrication of the polymer/organic heterostructure devices with various organic layers for hole blocking. The low molecular weight organics used in this study for hole blocking are tris(8-hydroxyquinoline) aluminum (Alq3), rubrene (Rub), quinacridone (Qc) and 2-(4'-biphenyl)-5(4"-tert-butylphenyl)-l,3,4-oxadiazole (PBD) of which chemical structures are shown in Fig. 1. Utilization of the low molecular weight organics has some advantages such as the avoidance of difficulties in the selection of the solvents arising

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124

K.-H. Choi et al. / Synthetic Metals 96 (1998) 123-126

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(d) Fig. 1. Molecular structures of (a) tris(8-hydroxyquinoline)aluminum (Alq3), (b) rubrene (Rub), (c) quinacridone (Qc) and (d) 2-(4'bipbenyl)-5- (4"-tert-butylphenyl)- 1,3,4-oxadiazole(PBD).

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in the case of polymer/polymer multilayer spin coating and no interdiffusion of the layers arising in the case of organic/ organic multilayer deposition.

2. Experimental

Poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) was spin coated to be 60 nm thick for the emissive layer onto the thoroughly cleaned indiumtin oxide (ITO) glass substrate. The organic hole blocking layer (HBL) was formed onto the MEH-PPV film by the vacuum sublimation technique. The vacuum pressure was maintained in the range of 10 -7 Ton" during the whole sublimation process and the deposition rate was kept within 1-2 ,~/s. The aluminum cathode was formed by the thermal evaporation for a diameter of 5 mm after the organic layer deposition and the thickness was 1000 A. The final structures of the devices constructed in this study are: (i) I T O / M E H PPV(600 ,~)/Alq3(500 ,~)/AI, (ii) ITO/MEH-PPV (600 A)/Rub(450 A)/A1, (iii) ITO/MEH-PPV(600 ,~) / 0c(250 A)/A1, (iv) ITO/MEH-PPVo(600 A ) / P B D ( 4 5 0 A)/AI, o(V) ITO/MEH-PPV(600 A)/AI~I3(500 A ) / (~c(250 A)/A1, (vi) ITO/MEH-PPV(600 A)/Alq3(500 A)/Rub(250 A)/AI, (vii) ITO/MEH-PPV( 1100 A)/AI. The organic materials were purchased from Aldrich and Tokyo Kasei and used without further purification. The EL properties of the devices were observed with EL spectra and current-voltage-luminescence (1-V-L) characteristics. Details in the EL device fabrications and measurements were described elsewhere [ 15,16].

Fig. 2. NormalizedUV-visibleabsorptionspectraof organicfilmsof PBD, Rub, Qc and Alq3. All the organic films formedby vacuum sublimation technique in a vacuum pressure of 10-7 Torr at the deposition rate of 1-2 A/s. The PBD, Alq3 and Rub did not show any distinct absorption band near the wavelength of 590 nm which is the emission wavelength of a MEH-PPV single-layered EL device [ 17]. Fig. 3 represents the EL spectra observed for various polymer/organic heterostructure devices. The emission maxima of EL spectra appear at the wavelength of 590 nm for all the polymer/organic EL devices except the device of ITO/ MEH-PPV/Alq3/Rub/AI. There is no light emission from the EL device of ITO/MEH-PPV/AIq3/Rub/A1. The results indicate that all the organic layers do not contribute to the light emission. The current-applied field (l-F) characteristics of various polymer/organic heterostructure devices are shown in Fig. 4. All the heterostructure devices limit the current flow effectively compared with the MEH-PPV single-layer device. The ITO/MEH-PPV/Alq3/A1 and ITO/MEH-PPV/Alq3/Qc/ AI structures limit very efficiently the current flow in the 1.2

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3. Results and discussion

Normalized UV-visible absorption spectra of the organic materials for HBL are shown in Fig. 2. The film of Qc has the absorption maximum at the wavelength of about 570 nm.

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600

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Wavelength [rim]

Fig. 3. NormalizedEL spectraof polymer/organicheterostructuredevices.

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K.-H. Choi et al. / Synthetic Metals 96 (1998) 123-126

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devices at the same applied field, and the other heterostructure devices show higher current flow than these two structures. These current limiting features clearly appear in luminescence-current (L-l) characteristics as shown in Fig. 5. The emissive light intensities at the same current from some of the polymer/organic EL systems are greater than that of the MEH-PPV single-layer device. The ITO/MEH-PPV/Alq3 / A1 heterostructure device shows the highest luminescence with low current flow. Turn-on voltage for light emission of the ITO/MEH-PPV/Alq3/A1 structure device is observed at 4 V while those of the other polymer/organic heterostructure devices are higher than 5 V. This feature can be observed from the semilog plot of the light output-voltage characteristics ( see Fig. 6) of various polymer/organic heterostructure devices. Based on the above results, we may expect that the organic layers assist the minor carrier (here, electron) injection by blocking the injected holes at the interface between the MEH-PPV emissive polymer layer and the organic layer.

The improvement of EL efficiency may be simply explained with the energy band model. Fig. 7 represents the schematic energy band diagrams of the emissive polymer, MEH-PPV, and various organic layers for hole blocking. The improvement of EL efficiency should be determined by the combination of the effects of hole blocking and the minor carrier injection. When we vary the HOMO or LUMO levels of HBLs, we might identify that the improvement of EL efficiency is due to hole blocking or due to the electron injection. The Rub and Alq3 have the same LUMO level, but Alq3 has lower HOMO level than Rub. As we see in Fig. 5, the EL efficiency of ITO / MEH-PPV / Alq3 / AI i s improved 20 time s that of ITO/MEH-PPV/Rub/A1. This indicates that the hole blocking is more efficient in ITO/MEH-PPV/Alq3/A1, since this device has a larger energy barrier at the interface between the HOMO levels of the emissive and the HBL. Due to amorphous structure, moderate electron mobility, excellent film formability, and the low ionization potential (IP) of 5.7 eV [ 18], Alq3 is regarded as suitable ETL and/or hole blocking layer (HBL) material in an effort to enhance the quantum efficiency of LEDs. However, by keeping the HOMO level constant and varying the LUMO level, we may separate the effects of hole blocking from the effect of the minor carder injection. With Rub and Qc of which HOMO levels are the same but LUMO levels are 3.2 and 3.9 eV,

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K.-H. Choi et al. /Synthetic Metals 96 (1998) 123-126

respectively, we observe the larger current flow in ITO/ MEH-PPV/Qc/A1, at the same electric field, than in ITO/ MEH-PPV/Rub/A1 as seen in Fig. 4. This indicates the improvement of electron injection in the ITO/MEH-PPV/ Qc/Al device. Lower turn-on voltage of the Qc device than that of Rub, as seen in Fig. 6, also supports the efficient electron injection in the Qc device. Even though the HOMO levels of Qc and Rub are lower than that of MEH-PPV, the light emission is not so efficient. The low light intensity of the ITO/MEH-PPV/Qc/A1 and ITO/MEH-PPV/Alq3/ Qc/AI heterostructure devices may be attributed to the strong re-absorption by the organic layer since their UV-visible absorption band is widely overlapped with the emission spectra of MEH-PPV. Furthermore, Qc has a good hole transporting capability [ 19] so that the hole blocking may not be so efficient. Since PBD has the lowest HOMO level of 6.2 eV, it could be expected that the hole blocking is the most efficient. The high turn-on voltage and luminescence in the I T O / M E H PPV/PBD/AI device seems to indicate that the PBD layer blocks the hole transport efficiently. However, when the bias voltage increases above 15 V, the current increases abruptly upon bias application as seen in Fig. 6. The ITO/MEH-PPV/ PBD/A1 structure device shows a higher luminescence power of nearly 1 ~W (about 200 cd/m 2) than that of the ITO/ MEH-PPV/AIq3/A1 structured device, but this structured device shows more than 10 times higher current flow for light emission. This high current flow may produce higher Joule heat and that device may be easily degraded. We assume that this current flow behavior of the ITO/MEH-PPV/PBD/AI structure device may be due to the injection of the excess carriers from both electrodes. When we examined the surface morphology of the PBD layer with the optical microscope, it was rougher than any other layer because of the easy crystallization. Therefore, this excess carder injection to the PBD layer may be attributed to the rough morphology and crystallization of the PBD layer. Rough morphology due to the crystallization affects the interface between the emissive polymer and PBD leading to poor hole blocking. Furthermore, crystallization of the organic layer may increase the current flow of the device. From this result, we can see that the film quality of HBL is also an important parameter for EL efficiency.

4. Conclusions We have investigated polymer/organic heterostructure EL devices for reduction of current flow and increase in light emission. All the devices emit light at wavelength 590 nm, indicating that all the organic materials in this study do not emit their own light. The ITO/MEH-PPV/AIq3/AI heterostrueture device shows the best electrical and EL prop-

erties in this study. It shows a high luminescence at the current of about 3.5 mA and an operating voltage at 4 V. By varying the HOMO and LUMO levels of HBLs, we observe that the improvement of EL efficiency depends strongly upon the band mismatch between the HOMO levels of the emissive layer and the HBL, the morphology of HBL, and the physical properties of the HBL such as carder mobility. But the luminescence from Rub and Qc layer devices shows low luminescence due to re-absorption by superposing their absorption bands with the emission band of MEH-PPV. The ITO/ MEH-PPV/PBD/A1 structured device shows the highest luminescence, but the current flow is not limited, which is attributed to their poor morphology due to crystal formation.

Acknowledgements The authors gratefully acknowledge the support of this research by the Ministry of Information and Communication of Korea.

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