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Double-layer electroluminescent devices with Langmuir–Blodgett films of amphiphilic 8-hydroxyquinoline lanthanum as emitter
Materials Science and Engineering B85 (2001) 247– 250 www.elsevier.com/locate/mseb
Double-layer electroluminescent devices with Langmuir –Blodgett films of amphiphilic 8-hydroxyquinoline lanthanum as emitter Jian-Ming Ouyang a,*, Wei-Han Ling a, Chunhe Yang b, Yongfang Li b, Gui Yu b b
a Department of Chemistry, Jinan Uni6ersity, Guangzhou 510632, People’s Republic of China Laboratory of Organic Solids, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Abstract A double-layer organic thin film electroluminescent (EL) device has been prepared by using the LB film technique. Its basic structure consists of a hole-transport layer of material, poly (N-vinylcarbazole) (PVK) and an electron-transport layer of material, the Langmuir–Blodgett films of bis [N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl). Comparing with the luminance of the single-layer EL device (ITO/LB film/Al), the double-layer EL device (ITO/PVK/LB film/Al) has a higher luminance (873 cd m − 2) and operates at a lower driving voltage (8 V). The deposited surface pressure of the LB film had strong influence on the EL intensity of the devices. The luminancent mechanism of the EL devices was discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroluminescent devices; Langmuir–Blodgett; 8-Hydroxyquinoline
1. Introduction 8-Hydroquinoline and its derivatives are known as good chelating reagents that coordinate many primary and transition metal ions [1,2]. Very interesting is that some complexes of 8-hydroxyquinoline (Hq) and its derivatives can be used as the emitting elements in electroluminescent (EL) devices [3,4]. The EL devices with the complexes Alq3, Znq2, and Beq2 etc. as emitter have a high external quantum efficiency and brightness at a driving low voltage below 10 V [5]. Since organic functional ultrathin films with a controlled thickness at a molecular size and with defined molecular orientation can be prepared by Langmuir – Blodgett (LB) technique, if some amphiphilic complexes with 8-hydroxyquinoline can be incorporated in LB films, these LB films may be used as the emitting layer of EL devices. The highly ordered arrangement of emitting molecules may be able to increase the * Corresponding author. Tel.: + 86-20-85220915; fax: +86-2085221941. E-mail address: [email protected] (J.-M. Ouyang).
luminous efficiency of EL devices. In the previous papers [6,7], we have already reported that the LB films of the amphiphilic complexes, bis[N-hexadecyl-8hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl) and bis[N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] cadmium (CdL2), can be used as emitting layer in single-layer EL devices. Green –yellow emission (the EL peak wavelengths were 515 and 490 nm) with a luminance of about 330 and 1200 cd m − 2, respectively, were achieved. Organic EL devices composed of multi layer structures of organic thin films have high brightness and operate at low driving voltages. Excellent performance of the EL devices originates from the insertion of a hole transport layer and/or an electron-transport layer between the electrode and the emitting layer [8]. With this in mind, we introduce a hole transport layer (HTL), poly(N-vinylcarbazole) (PVK), in the singlelayer EL devices we reported previously [6,7]. PVK possesses a high hole mobility between an ITO electrode and an organic emitting layer. It is expected that an improvement in emission efficiency can be obtained in this double-layer EL devices.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 9 6 - 7
J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
248
2. Experimental details
2.2. Formation of air–water monolayers and deposition of the LB films
2.1. Materials The amphiphilic complex, bis[N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl) was synthesized by the previously reported method [9]. The hole-transport layer, poly (N-vinylcarbazole) (PVK), was prepared by ourselves according to conventional methods. Fig. 1 shows the molecular structures of the materials used in this study.
LaL2Cl solution (5.0×l0 − 4 mol l − 1) in 2:1 chloroform–dimethylformanide was spread on the subphase. After 20 min equilibration period, the monolayers of LaL2Cl could easily be transferred onto ITO (indium/ tinoxide) electrode by a conventional vertical dipping LB technique at a certain surface pressure. The dipping rate was 3.0 mm min − 1.All the work was carried out in a dust-free box at a temperature of 259 1°C.
2.3. Apparatus Measurement of surface pressure– area (y-A) isotherms was carried out with a WM-2 Langmuir trough system, which is a fully computerized and programmable apparatus fitted with two movable teflon barriers. Fluorescence and the EL spectra were measured with a Hitachi F-4000 spectrophotometer. The luminance was measured by a handy model ST-86LA luminance meter. Fig. 1. The molecular structures of the materials used in the study: (a) LaL2Cl; (b) PVK.
Fig. 2. The device configuration used in this study.
2.4. Electroluminescent de6ices Fig. 2 shows the device configuration in this study. The emitting layer of the double-layer device was fabricated by the LB film technique, and the basic structure is ITO/HTL (50 nm)/LB film layer (40 nm)/Al (250 nm). First, a 16 layer LB film of LaL2Cl, which possesses greenish-yellow fluorescence in its solid state [9], was deposited onto a pre-cleaned ITO-coated glass substrate. After dipping and drying the LB film, a hole transport material (PVK) was deposited. Lastly, a 250 nm thick aluminium layer was deposited on the organic layer as the top electrode at 5× 10 − 5 torr. The deposited rates were maintained at 0.2−0.4 nm s − 1 for the HTL materials and 1− 2 nm s − 1 for aluminium, respectively. The ITO electrode was cleaned according to Tang et al.’s method [3]. The sheet resistance of ITO used in this experiment was on the order of 80 V. The emitting area of the devices was approximately 2×2 mm2. All electrical measurements were carried out at ambient temperature in the air under a direct current (d.c.) condition.
3. Results and discussion
3.1. B–I cur6e of the EL de6ices Fig. 3. Luminance-current characteristics for (a) ITO/LB film/Al device; (b) ITO/LB film/PVK/Al device.
Fig. 3 shows the B–I characteristics of the singlelayer (ITO/LB film/Al) and the double-layer (ITO/
J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
249
3.2. Effect of deposited pressure on the I–V cur6e of EL de6ices
Fig. 4. I – V characteristics of the double-layer EL devices with emitters of 16 layer LaL2Cl LB films deposited at different surface pressures as emitters.
PVK/LB film/Al) EL devices. As for typical organic EL devices, the luminances were linearly proportional to the injection current in the current range from 10 to 100 mA cm − 2 for both the devices. This dependent mechanism suggested that the prompt EL process was dominant rather than a delayed EL process [10,11]. The emission intensity (873 cd m − 2) of the EL device with a PVK layer between the ITO electrode and emitting LB film layer was about three times larger than that (300 cd m − 2) [7] of the EL device composed of a LaL2Cl LB film single-layer. This result verifies that the insertion of the PVK layer is responsible for the increase in emission efficiency of this emitter. The threshold voltages are 8 and 10 V for the devices with and without PVK layer, respectively. That is, the threshold voltages of the device using a PVK layer was lower than that of the device without the PVK. In the single-layer device, mainly holes are injected into the emitting LB film layer [7], leading to poor EL efficiency. The recombination and the emission sites must be located near the cathode since LaL2Cl possesses a hole transporting ability. The insertion of the hole transport layer PVK resulted in a high emission efficiency was ascribed to that PVK can assist effective carrier injection from the electrode into the emitting layer accompanied by a lowering of drive voltage. It blocks the carriers which pass through the emitter layer and thus controls the recombination process. Furthermore, PVK helps prevent the quenching of molecular excitons at the boundary between an emitter and an electrode [12]. All of those effects are expected to result in high emission efficiency.
The deposited pressure of LB films have markedly influenced the behaviour of the EL devices with LB films as emitter. Fig. 4 shows the effect of the deposited pressure of the LB films on the I–V characteristics of the double-layer EL devices. When the LB films were prepared at surface pressure of 20 mN m − 1, the organic device has higher EL intensity then another device prepared at lower surface pressure (5 mN m − 1) When the surface pressure is lower, there may be many point defects or pinholes in the LB film in which the high tunnel current would increase exponentially with field, leading to an increase in the current density crossing the EL device and a decrease in the recombination probability of electrons and holes at higher voltage. That is, in the case of organic device prepared at lower surface pressure (5 mN m − 1), the organic thin film is unable to transmit all the charges, so that the current becomes higher nonlinear with the voltage applied [5] and the electron-hole recombination increases at a higher voltage. This is the reason why the EL intensity and the breakdown voltage applied are raised for the EL device of organic thin film deposited at the lower deposition surface pressure. However, when the surface pressure is higher (20 mN m − 1), there may be fewer point defects or pinholes in the LB film [12]. The results suggest that the EL efficiency of LaL2Cl LB films could be enhanced by using the LB films deposited at higher surface pressures such as above 20 mN m − 1.
3.3. Mechanisms In organic EL devices, the generation of light is the consequences of recombination of holes and electrons injected from the electrodes. Such carrier recombination in the organic emitter layer exicites the emitting centers. For the observed behaviors of the EL devices, because 8-hydroxyquinoline derivative-metal complexes are capable of transporting electrons [5], the current density mainly depends on electrons of LaL2Cl and its EL intensity is determined without doubt by recombination of electrons from LaL2Cl and holes from the positive electrode ITO. The radiative recombination luminance is directly proportional to electronhole radiative recombination probability, that is, proportional to electron concentration and hole concentration. When a HTL (PVK) is introduced between ITO electrode and the LB films, the hole injection can be improved through the use of a hole-transport layer, resulting in enhancement of EL efficiency as the electron-hole radiative recombination probability increases.
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J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
Acknowledgements This research work was supported by Natural Science Foundation of Guangdong Province and the Foundation of the Laboratory of Organic Solids, Institute of Chemistry, the Chinese Academy of Sciences. References [1] S. Jirf, Anal. Chim. Acta. 28 (1963) 132. [2] P.K. Mishra, V. Chakravorthy, K.C. Dash, Radio. Chim. Acta. 47 (1989) 235. [3] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.
.
[4] C.H. Chen, J.-M. Shi, Coord. Chem. Rev. 171 (1998) 161. [5] Y. Hamada, T. Sano, M. Fujita, T. FujII, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [6] J.-M. Ouyang, L. Li, Z.-H. Tai, Z.-H. Lu, G.-M. Wang, Chem. Commun., 1997, 815. [7] J.-M. Ouyang, Z.-M. Zhang, W.-H. Ling, H.-Y. Liu, Z.-H. Lu, Appl. Surf. Sci. 151 (1999) 67. [8] C. Adachi, T. Tsuitsui, S. Saito, Appl. Phys. Lett. 55 (1989) 1489. [9] J.-M. Ouyang, Z.-H. Tai, W.-X. Tang, J. Mater. Chem. 6 (1996) 963. [10] W. Hwang, K. Kao, J. Chem. Phys. 60 (1974) 3845. [11] C. Adachi, T. Tsuitsui, S. Saito, Appl. Phys. Lett. 55 (15) (1989) 1489. [12] X.-M. Yang, Sci. Bull. Sinica. 38 (1994) 653.
Double-layer electroluminescent devices with Langmuir –Blodgett films of amphiphilic 8-hydroxyquinoline lanthanum as emitter Jian-Ming Ouyang a,*, Wei-Han Ling a, Chunhe Yang b, Yongfang Li b, Gui Yu b b
a Department of Chemistry, Jinan Uni6ersity, Guangzhou 510632, People’s Republic of China Laboratory of Organic Solids, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
Abstract A double-layer organic thin film electroluminescent (EL) device has been prepared by using the LB film technique. Its basic structure consists of a hole-transport layer of material, poly (N-vinylcarbazole) (PVK) and an electron-transport layer of material, the Langmuir–Blodgett films of bis [N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl). Comparing with the luminance of the single-layer EL device (ITO/LB film/Al), the double-layer EL device (ITO/PVK/LB film/Al) has a higher luminance (873 cd m − 2) and operates at a lower driving voltage (8 V). The deposited surface pressure of the LB film had strong influence on the EL intensity of the devices. The luminancent mechanism of the EL devices was discussed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroluminescent devices; Langmuir–Blodgett; 8-Hydroxyquinoline
1. Introduction 8-Hydroquinoline and its derivatives are known as good chelating reagents that coordinate many primary and transition metal ions [1,2]. Very interesting is that some complexes of 8-hydroxyquinoline (Hq) and its derivatives can be used as the emitting elements in electroluminescent (EL) devices [3,4]. The EL devices with the complexes Alq3, Znq2, and Beq2 etc. as emitter have a high external quantum efficiency and brightness at a driving low voltage below 10 V [5]. Since organic functional ultrathin films with a controlled thickness at a molecular size and with defined molecular orientation can be prepared by Langmuir – Blodgett (LB) technique, if some amphiphilic complexes with 8-hydroxyquinoline can be incorporated in LB films, these LB films may be used as the emitting layer of EL devices. The highly ordered arrangement of emitting molecules may be able to increase the * Corresponding author. Tel.: + 86-20-85220915; fax: +86-2085221941. E-mail address: [email protected] (J.-M. Ouyang).
luminous efficiency of EL devices. In the previous papers [6,7], we have already reported that the LB films of the amphiphilic complexes, bis[N-hexadecyl-8hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl) and bis[N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] cadmium (CdL2), can be used as emitting layer in single-layer EL devices. Green –yellow emission (the EL peak wavelengths were 515 and 490 nm) with a luminance of about 330 and 1200 cd m − 2, respectively, were achieved. Organic EL devices composed of multi layer structures of organic thin films have high brightness and operate at low driving voltages. Excellent performance of the EL devices originates from the insertion of a hole transport layer and/or an electron-transport layer between the electrode and the emitting layer [8]. With this in mind, we introduce a hole transport layer (HTL), poly(N-vinylcarbazole) (PVK), in the singlelayer EL devices we reported previously [6,7]. PVK possesses a high hole mobility between an ITO electrode and an organic emitting layer. It is expected that an improvement in emission efficiency can be obtained in this double-layer EL devices.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 9 6 - 7
J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
248
2. Experimental details
2.2. Formation of air–water monolayers and deposition of the LB films
2.1. Materials The amphiphilic complex, bis[N-hexadecyl-8-hydroxy-2-quinolinecarboxamide] lanthanum (LaL2Cl) was synthesized by the previously reported method [9]. The hole-transport layer, poly (N-vinylcarbazole) (PVK), was prepared by ourselves according to conventional methods. Fig. 1 shows the molecular structures of the materials used in this study.
LaL2Cl solution (5.0×l0 − 4 mol l − 1) in 2:1 chloroform–dimethylformanide was spread on the subphase. After 20 min equilibration period, the monolayers of LaL2Cl could easily be transferred onto ITO (indium/ tinoxide) electrode by a conventional vertical dipping LB technique at a certain surface pressure. The dipping rate was 3.0 mm min − 1.All the work was carried out in a dust-free box at a temperature of 259 1°C.
2.3. Apparatus Measurement of surface pressure– area (y-A) isotherms was carried out with a WM-2 Langmuir trough system, which is a fully computerized and programmable apparatus fitted with two movable teflon barriers. Fluorescence and the EL spectra were measured with a Hitachi F-4000 spectrophotometer. The luminance was measured by a handy model ST-86LA luminance meter. Fig. 1. The molecular structures of the materials used in the study: (a) LaL2Cl; (b) PVK.
Fig. 2. The device configuration used in this study.
2.4. Electroluminescent de6ices Fig. 2 shows the device configuration in this study. The emitting layer of the double-layer device was fabricated by the LB film technique, and the basic structure is ITO/HTL (50 nm)/LB film layer (40 nm)/Al (250 nm). First, a 16 layer LB film of LaL2Cl, which possesses greenish-yellow fluorescence in its solid state [9], was deposited onto a pre-cleaned ITO-coated glass substrate. After dipping and drying the LB film, a hole transport material (PVK) was deposited. Lastly, a 250 nm thick aluminium layer was deposited on the organic layer as the top electrode at 5× 10 − 5 torr. The deposited rates were maintained at 0.2−0.4 nm s − 1 for the HTL materials and 1− 2 nm s − 1 for aluminium, respectively. The ITO electrode was cleaned according to Tang et al.’s method [3]. The sheet resistance of ITO used in this experiment was on the order of 80 V. The emitting area of the devices was approximately 2×2 mm2. All electrical measurements were carried out at ambient temperature in the air under a direct current (d.c.) condition.
3. Results and discussion
3.1. B–I cur6e of the EL de6ices Fig. 3. Luminance-current characteristics for (a) ITO/LB film/Al device; (b) ITO/LB film/PVK/Al device.
Fig. 3 shows the B–I characteristics of the singlelayer (ITO/LB film/Al) and the double-layer (ITO/
J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
249
3.2. Effect of deposited pressure on the I–V cur6e of EL de6ices
Fig. 4. I – V characteristics of the double-layer EL devices with emitters of 16 layer LaL2Cl LB films deposited at different surface pressures as emitters.
PVK/LB film/Al) EL devices. As for typical organic EL devices, the luminances were linearly proportional to the injection current in the current range from 10 to 100 mA cm − 2 for both the devices. This dependent mechanism suggested that the prompt EL process was dominant rather than a delayed EL process [10,11]. The emission intensity (873 cd m − 2) of the EL device with a PVK layer between the ITO electrode and emitting LB film layer was about three times larger than that (300 cd m − 2) [7] of the EL device composed of a LaL2Cl LB film single-layer. This result verifies that the insertion of the PVK layer is responsible for the increase in emission efficiency of this emitter. The threshold voltages are 8 and 10 V for the devices with and without PVK layer, respectively. That is, the threshold voltages of the device using a PVK layer was lower than that of the device without the PVK. In the single-layer device, mainly holes are injected into the emitting LB film layer [7], leading to poor EL efficiency. The recombination and the emission sites must be located near the cathode since LaL2Cl possesses a hole transporting ability. The insertion of the hole transport layer PVK resulted in a high emission efficiency was ascribed to that PVK can assist effective carrier injection from the electrode into the emitting layer accompanied by a lowering of drive voltage. It blocks the carriers which pass through the emitter layer and thus controls the recombination process. Furthermore, PVK helps prevent the quenching of molecular excitons at the boundary between an emitter and an electrode [12]. All of those effects are expected to result in high emission efficiency.
The deposited pressure of LB films have markedly influenced the behaviour of the EL devices with LB films as emitter. Fig. 4 shows the effect of the deposited pressure of the LB films on the I–V characteristics of the double-layer EL devices. When the LB films were prepared at surface pressure of 20 mN m − 1, the organic device has higher EL intensity then another device prepared at lower surface pressure (5 mN m − 1) When the surface pressure is lower, there may be many point defects or pinholes in the LB film in which the high tunnel current would increase exponentially with field, leading to an increase in the current density crossing the EL device and a decrease in the recombination probability of electrons and holes at higher voltage. That is, in the case of organic device prepared at lower surface pressure (5 mN m − 1), the organic thin film is unable to transmit all the charges, so that the current becomes higher nonlinear with the voltage applied [5] and the electron-hole recombination increases at a higher voltage. This is the reason why the EL intensity and the breakdown voltage applied are raised for the EL device of organic thin film deposited at the lower deposition surface pressure. However, when the surface pressure is higher (20 mN m − 1), there may be fewer point defects or pinholes in the LB film [12]. The results suggest that the EL efficiency of LaL2Cl LB films could be enhanced by using the LB films deposited at higher surface pressures such as above 20 mN m − 1.
3.3. Mechanisms In organic EL devices, the generation of light is the consequences of recombination of holes and electrons injected from the electrodes. Such carrier recombination in the organic emitter layer exicites the emitting centers. For the observed behaviors of the EL devices, because 8-hydroxyquinoline derivative-metal complexes are capable of transporting electrons [5], the current density mainly depends on electrons of LaL2Cl and its EL intensity is determined without doubt by recombination of electrons from LaL2Cl and holes from the positive electrode ITO. The radiative recombination luminance is directly proportional to electronhole radiative recombination probability, that is, proportional to electron concentration and hole concentration. When a HTL (PVK) is introduced between ITO electrode and the LB films, the hole injection can be improved through the use of a hole-transport layer, resulting in enhancement of EL efficiency as the electron-hole radiative recombination probability increases.
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J.-M. Ouyang et al. / Materials Science and Engineering B85 (2001) 247–250
Acknowledgements This research work was supported by Natural Science Foundation of Guangdong Province and the Foundation of the Laboratory of Organic Solids, Institute of Chemistry, the Chinese Academy of Sciences. References [1] S. Jirf, Anal. Chim. Acta. 28 (1963) 132. [2] P.K. Mishra, V. Chakravorthy, K.C. Dash, Radio. Chim. Acta. 47 (1989) 235. [3] C.W. Tang, S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913.
.
[4] C.H. Chen, J.-M. Shi, Coord. Chem. Rev. 171 (1998) 161. [5] Y. Hamada, T. Sano, M. Fujita, T. FujII, Y. Nishio, K. Shibata, Jpn. J. Appl. Phys. 32 (1993) L514. [6] J.-M. Ouyang, L. Li, Z.-H. Tai, Z.-H. Lu, G.-M. Wang, Chem. Commun., 1997, 815. [7] J.-M. Ouyang, Z.-M. Zhang, W.-H. Ling, H.-Y. Liu, Z.-H. Lu, Appl. Surf. Sci. 151 (1999) 67. [8] C. Adachi, T. Tsuitsui, S. Saito, Appl. Phys. Lett. 55 (1989) 1489. [9] J.-M. Ouyang, Z.-H. Tai, W.-X. Tang, J. Mater. Chem. 6 (1996) 963. [10] W. Hwang, K. Kao, J. Chem. Phys. 60 (1974) 3845. [11] C. Adachi, T. Tsuitsui, S. Saito, Appl. Phys. Lett. 55 (15) (1989) 1489. [12] X.-M. Yang, Sci. Bull. Sinica. 38 (1994) 653.