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Effects of different buffer layers on the electro-luminescence performances in white organic light-emitting diodes
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 71 (2010) 922–925
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of different buffer layers on the electro-luminescence performances in white organic light-emitting diodes Peng Yu Chen a, Herng Yih Ueng a,n, Meiso Yokoyama b a b
Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, Republic of China Department of Electronic Engineering, I-Shou University, Kaohsiung County 840, Taiwan, Republic of China
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 September 2009 Received in revised form 8 February 2010 Accepted 30 March 2010
The effects of different hole injection materials as the buffer layer on the electro-luminescence (EL) performances of white organic light-emitting diodes (WOLEDs) are investigated in detail. It is found that the EL performances and electric properties were strongly dependent on the structure of the used hole injection materials with different thicknesses, which directly affected the injection and transport properties in devices, and thus the EL efficiency and lifetime. It can be seen that a hybrid buffer layer of 5 nm aluminum fluoride (AlF3)/15 nm 4,40 ,400 -tris(3-methylphenylphenylamino) (m-MTDATA) as the hole injection buffer layer shows the best EL performances in efficiency and lifetime, showing a promising hole injection material in WOLEDs. The mechanisms behind the enhanced performance of the hybrid buffer layer in WOLEDs are discussed based on X-ray photoelectron spectroscopy (XPS) measurement. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Luminescence D. Transport properties
1. Introduction and background Organic light-emitting diodes (OLEDs) are very attractive displays, which are solid-state lighting sources intended for use in the next generation [1–2]. The accelerated development of OLEDs technology over the last decade has markedly improved its performance. Among them, white emission is very important for applying OLEDs to full color displays with a color filter, backlights for liquid-crystal displays, and next generation illumination light sources. However, such characteristics of white OLEDs (WOLEDs) as power consumption must still be improved for practical applications. To reduce power consumption, the driving voltage of OLEDs must be lowered, which can be achieved by enhancing the injection of carriers from the anode electrode of OLEDs [3]. In order to increase the carrier injection, a lowpressure O2 plasma treatment has been adopted for ITO, which can increase the work function [4–5]. However, it is determined that the work function obtained after O2 plasma treatment is not high enough, and the hole injection barrier remains between the ITO and the organic layer. To further lower the injection barrier, various organic materials, such as 4,40 ,400 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) [6] and copper phthalocyanine (CuPc) [7], have been used as a hole buffer layer. In addition, Chen et al. [8] reported an OLED with a double buffer layer, which facilitates hole injection through an aligned energy level. However, the high turn-on voltage and the short lifespan are considered as unsuitable for further use.
n
Corresponding author. Tel.:886 7 5252000x4181; fax: + 886 7 5252000x4199. E-mail address: [email protected] (H. Yih Ueng).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.03.045
Subsequently, inorganic oxides, such as MoO3 [9,10], Al2O3 [11], V2O5 [12], ZnO [13], WO3 [14], Pr2O3 [15], NiO [16], SiOxNy [17], TiO2 [18], SiO2 [19], and Ta2O5 [20], and fluorides such as Teflon [21] and MgF2 [22] have been used as buffer layers between the organic and ITO layers. The use of an inorganic material can reduce the turn-on voltage and increase the lifespan; however, the efficiency using inorganic buffers usually degrades rapidly with increase in drive voltage [14–21]. Aluminum fluoride (AlF3) is an important material usually used as an optical coating; therefore, it has a very high transparency of over 90% under the visible spectrum [23]. In addition, it is appropriate for thermal vapor deposition, which makes it a candidate for a buffer layer. This study investigated the behaviors of a WOLED using a combination of AlF3/m-MTDATA as a hybrid buffer layer. Compared with a device with single layer AlF3 or single layer m-MTDATA as a buffer layer, using a hybrid buffer layer, there are significant improvements in both the luminous efficiency and turn-on voltage. Furthermore, the lifespan can be improved. The most important is that the luminous efficiency of a hybrid buffer layer degrades slower than the single layer AlF3 buffer layer. X-ray photoelectron spectroscopy (XPS) is introduced to analyze the mechanisms behind the superior performance.
2. Experimental Fig. 1 shows the configuration of WOLEDs, which is investigated in this study. The device includes different types of buffer layers: (1) AlF3 buffer layer with different thickness of 3, 5, and 7 nm,
ARTICLE IN PRESS P. Yu Chen et al. / Journal of Physics and Chemistry of Solids 71 (2010) 922–925
300 Electron injection layer(0.5nm)
LiF
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Alq3
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Alq3:DCJTB
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BAlq:TBPe
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Fig. 1. Schematic configuration of WOLEDs.
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3. Results and discussion Fig. 2(a) shows the dependence of the current density–voltage (J–V) characteristic of WOLEDs, with various thicknesses of buffer layer. As seen, the J–V performances of the devices are strongly dependent on the thickness of the m-MTDATA and AlF3 buffer layer. Generally, AlF3 buffer layer shows a higher current density compared to m-MTDATA buffer layer. For the devices with 5 nm AlF3 and 10 nm m-MTDATA buffer layer, the maximum current densities were achieved, as compared with those of the same types of buffer layer at the same operating voltage. Figs. 2(b) and (c) show the dependence of the luminance–voltage (L–V) and the luminous efficiency characteristics of WOLEDs with various thicknesses of m-MTDATA and AlF3 as buffer layer. As shown in
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(2) m-MTDATA buffer layer with different thickness of 10, 15, and 20 nm, (3) hybrid buffer layer of 5 nm AlF3/15 nm m-MTDATA, and (4) no buffer layer. After the buffer layers, 40-nm-thick N,N0 diphenyl-N,N0 -bis(1-naphthyl-phenyl)-(1,10 -biphenyl)-4,40 -diamine (NPB) was deposited as the hole transport layer, 20-nm-thick bis(2-methyl-8-quinolinolato)(paraphenylphenolato)aluminum(III) (BAlq) doped with 3% 2,5,8,11-tetra-tert-butylperylene (TBPe) was deposited as the blue-emitting layer, 20-nm-thick tris(8-hydroxyquinoline) aluminum (Alq3) doped with 2% 4(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl9-enyl)-4 H-pyran (DCJTB) was deposited as the red-emitting layer, 20-nm-thick tris-(8-hydroxyquinoline) aluminum (Alq3) was deposited as the electron transport layer, 0.5-nm-thick lithium fluoride (LiF) layer was deposited as the electron injection layer and 200-nm-thick Al was deposited as the cathode. All of the layers were deposited by vacuum vapor deposition at a pressure of 1 10-6 Torr onto an ITO-coated glass substrate (7O/&). The ITO substrate was initially cleaned using detergent and de-ionized water. The evaporation rate was ˚ 1–2 A/s. The evaporation rate and thickness of the film were determined using an oscillating quartz thickness monitor (ULVAC CRTM-9000). The active area of the devices, defined by the overlap of the ITO and Al electrodes, was 0.6 0.4 cm2. All devices were encapsulated in a dry nitrogen glove box. The luminance– current density–voltage (L–J–V) characteristics and color coordinates (CIE) were measured and recorded simultaneously by combining a PR-650 Spectra Scan Colorimeter with a Keithley 2400 programmable voltage–current source. X-ray photoelectron spectroscopy (XPS) measurements were made in an ultrahighvacuum system. The XPS device, which comprised a high-power Mg Ka(1253.6 eV) line X-ray source and an angle-resolved electron energy analyzer, had an energy resolution of 0.2 eV. All measurements were made at room temperature.
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Fig. 2. (a) Current density–voltage, (b) luminance–voltage, and (c) luminous efficiency–voltage characteristics of WOLEDs with various m-MTDATA and AlF3 thickness.
Fig. 2(b), the turn-on (when the devices were at 1 cd/m2) voltages of no buffer layer, 15 nm m-MTDATA buffer layer, and 5 nm AlF3 buffer layer were 4.2, 3.9, and 3.1 V, respectively. The different thicknesses of m-MTDATA buffer layer show small influence on the turn-on voltage, it only influences the variation of the
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Fig. 3. Luminance–voltage and luminous efficiency–voltage characteristics of a WOLED device with a hybrid buffer layer. The inset shows the EL spectrum of a WOLED device.
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luminance. This phenomenon also appeals in the different thickness of AlF3 buffer layers. Moreover, it is found that the maximum luminance of m-MTDATA and AlF3 buffer layer is achieved under the thicknesses of 15 and 5 nm at a constant voltage of 10 V. The luminous efficiency is the most important factor for the WOLED. Fig. 2(c) shows that the 15 nm m-MTDATA buffer layer has a highest luminous efficiency of about 9 cd/A and a 5 nm AlF3 buffer layer has a maximum luminous efficiency of about 11.8 cd/A while the no buffer device only shows a maximum luminous efficiency of about 6.2 cd/A. It is also noteworthy that the luminous efficiencies of the devices with various thicknesses of AlF3 buffer layer degraded obviously with respect to varying drive voltages. It is suggested that the quenching effect occurred under a higher drive voltage, resulting in a lower luminous efficiency. In order to redeem this problem, a hybrid buffer layer, with a combination of a 5 nm AlF3 and a 15 nm m-MTDATA, was constructed in this investigation. The results of using a hybrid buffer layer in a WOLED are shown in Fig. 3 and illustrate that the turn-on voltage was 3.1 V as well as the maximum luminous efficiency is boosted to 14.7 cd/A; the most important is that the hybrid buffer layer shows a weak quenching effect with increase in drive voltage. Moreover, the inset in Fig. 3 illustrates the electro-luminescence (EL) spectra of this device. The CIEx,y value is at about (0.33, 0.35), which is close to the white CIEx,y value of (0.33, 0.33). The improvement of the use of a hybrid buffer layer in WOLED can be illustrated in several aspects. It has been recognized that the turn-on voltage depends on the carrier injection and transportation, and the luminous efficiency depends not only on carrier injection but also on the balance of electron–hole pairs [24]. The usage of AlF3 in OLED facilitates the hole injection and reduces the turn-on voltage, and these can be explained in terms of an increase in carrier injection. To investigate the effect of carrier injection, we measured X-ray photoelectron spectra (XPS): (A) ITO substrate, (B) ITO/AlF3 (5 nm), and (C) ITO/AlF3 (15 nm). In Fig. 4, the In 3d peaks shifted to higher energies and the Al 2p peaks shifted to lower energies in the XPS spectra of (B) when compared with those in the XPS spectra of (A) and (C). Since a negatively charged atom has a higher binding energy of electrons, the spectral shifts mean carriers transfer from ITO to AlF3, leading to the increase in hole injection at the ITO/AlF3 interface. However, higher hole injection does not inevitably evolve to higher device efficiency. It has been recognized that the luminous
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Fig. 4. XPS spectra: (a) In 3d peaks and (b) Al 2p peaks observed in (A) ITO substrate, (B) ITO/AlF3 (5 nm), and (C) ITO/AlF3 (15 nm).
efficiency strongly depends on the balance of electrons and holes. The excess of hole injection results in unbalance of electron–hole pairs, resulting in luminescence quenching. This phenomenon is amplified with increase in drive voltage. Thus, AlF3 buffer layer degrades the luminous efficiency with increase in drive voltage. For the hybrid buffer layer structure, after the holes injection through the AlF3 buffer layer they still require to cross a barrier between m-MTDATA and NPB. The barrier between m-MTDATA and NPB may trap some holes. Therefore, the m-MTDATA layer can limit the transporting speed of holes to withhold excess holes injecting into the NPB layer, resulting in a balanced recombination in the emissive layer. Thus, the hybrid buffer layer results in boosted luminous efficiency and less quenching effect with increase in drive voltage. Fig. 5 plots the lifespan of WOLED with structures of AlF3 (5 nm), m-MTDATA (15 nm), and AlF3 (5 nm)/m-MTDATA (15 nm) as a buffer layer. For comparison, the initial luminance of the three devices was measured under a constant current density of 20 mA/cm2. The initial brightness of the three devices was about
ARTICLE IN PRESS P. Yu Chen et al. / Journal of Physics and Chemistry of Solids 71 (2010) 922–925
be reduced to 3.1 V, and the luminous efficiency can be improved to 14.7 cd/A when a hybrid buffer layer was used. Since the turnon voltage decreases and the efficiency increases, the power consumption as well as lifespan are then improved. Moreover, the luminous efficiency of the hybrid buffer layer also immunes to voltage variations.
100 5nm AlF3
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[1] [2] [3] [4] [5] [6]
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Fig. 5. Lifespan of WOLEDs with m-MTDATA (15 nm) and AlF3 (5 nm)/m-MTDATA (15 nm) as the buffer layer under a current density of 20 mA/cm2.
2300, 1800, and 2950 cd/m2, respectively. After approximately 300 h, the luminance decayed to 80%, 76.5%, and 82%. It is known that the use of inorganic materials can increase the lifespan of a device due to good thermal stability. Furthermore, the use of a hybrid buffer layer can further increase efficiency, and therefore reduce the Joule heat generated by power loss at the interface, resulting in a low aggregation of molecules from heat. Hence, the degradation tests show that the device with a hybrid structure has a long lifespan.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21] [22] [23]
We have fabricated a WOLED with AlF3 and m-MTDATA as a hybrid buffer layer. Results indicate that the turn-on voltage can
[24]
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Effects of different buffer layers on the electro-luminescence performances in white organic light-emitting diodes Peng Yu Chen a, Herng Yih Ueng a,n, Meiso Yokoyama b a b
Department of Electrical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, Republic of China Department of Electronic Engineering, I-Shou University, Kaohsiung County 840, Taiwan, Republic of China
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 September 2009 Received in revised form 8 February 2010 Accepted 30 March 2010
The effects of different hole injection materials as the buffer layer on the electro-luminescence (EL) performances of white organic light-emitting diodes (WOLEDs) are investigated in detail. It is found that the EL performances and electric properties were strongly dependent on the structure of the used hole injection materials with different thicknesses, which directly affected the injection and transport properties in devices, and thus the EL efficiency and lifetime. It can be seen that a hybrid buffer layer of 5 nm aluminum fluoride (AlF3)/15 nm 4,40 ,400 -tris(3-methylphenylphenylamino) (m-MTDATA) as the hole injection buffer layer shows the best EL performances in efficiency and lifetime, showing a promising hole injection material in WOLEDs. The mechanisms behind the enhanced performance of the hybrid buffer layer in WOLEDs are discussed based on X-ray photoelectron spectroscopy (XPS) measurement. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films D. Luminescence D. Transport properties
1. Introduction and background Organic light-emitting diodes (OLEDs) are very attractive displays, which are solid-state lighting sources intended for use in the next generation [1–2]. The accelerated development of OLEDs technology over the last decade has markedly improved its performance. Among them, white emission is very important for applying OLEDs to full color displays with a color filter, backlights for liquid-crystal displays, and next generation illumination light sources. However, such characteristics of white OLEDs (WOLEDs) as power consumption must still be improved for practical applications. To reduce power consumption, the driving voltage of OLEDs must be lowered, which can be achieved by enhancing the injection of carriers from the anode electrode of OLEDs [3]. In order to increase the carrier injection, a lowpressure O2 plasma treatment has been adopted for ITO, which can increase the work function [4–5]. However, it is determined that the work function obtained after O2 plasma treatment is not high enough, and the hole injection barrier remains between the ITO and the organic layer. To further lower the injection barrier, various organic materials, such as 4,40 ,400 -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) [6] and copper phthalocyanine (CuPc) [7], have been used as a hole buffer layer. In addition, Chen et al. [8] reported an OLED with a double buffer layer, which facilitates hole injection through an aligned energy level. However, the high turn-on voltage and the short lifespan are considered as unsuitable for further use.
n
Corresponding author. Tel.:886 7 5252000x4181; fax: + 886 7 5252000x4199. E-mail address: [email protected] (H. Yih Ueng).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.03.045
Subsequently, inorganic oxides, such as MoO3 [9,10], Al2O3 [11], V2O5 [12], ZnO [13], WO3 [14], Pr2O3 [15], NiO [16], SiOxNy [17], TiO2 [18], SiO2 [19], and Ta2O5 [20], and fluorides such as Teflon [21] and MgF2 [22] have been used as buffer layers between the organic and ITO layers. The use of an inorganic material can reduce the turn-on voltage and increase the lifespan; however, the efficiency using inorganic buffers usually degrades rapidly with increase in drive voltage [14–21]. Aluminum fluoride (AlF3) is an important material usually used as an optical coating; therefore, it has a very high transparency of over 90% under the visible spectrum [23]. In addition, it is appropriate for thermal vapor deposition, which makes it a candidate for a buffer layer. This study investigated the behaviors of a WOLED using a combination of AlF3/m-MTDATA as a hybrid buffer layer. Compared with a device with single layer AlF3 or single layer m-MTDATA as a buffer layer, using a hybrid buffer layer, there are significant improvements in both the luminous efficiency and turn-on voltage. Furthermore, the lifespan can be improved. The most important is that the luminous efficiency of a hybrid buffer layer degrades slower than the single layer AlF3 buffer layer. X-ray photoelectron spectroscopy (XPS) is introduced to analyze the mechanisms behind the superior performance.
2. Experimental Fig. 1 shows the configuration of WOLEDs, which is investigated in this study. The device includes different types of buffer layers: (1) AlF3 buffer layer with different thickness of 3, 5, and 7 nm,
ARTICLE IN PRESS P. Yu Chen et al. / Journal of Physics and Chemistry of Solids 71 (2010) 922–925
300 Electron injection layer(0.5nm)
LiF
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Alq3
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Fig. 1. Schematic configuration of WOLEDs.
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3. Results and discussion Fig. 2(a) shows the dependence of the current density–voltage (J–V) characteristic of WOLEDs, with various thicknesses of buffer layer. As seen, the J–V performances of the devices are strongly dependent on the thickness of the m-MTDATA and AlF3 buffer layer. Generally, AlF3 buffer layer shows a higher current density compared to m-MTDATA buffer layer. For the devices with 5 nm AlF3 and 10 nm m-MTDATA buffer layer, the maximum current densities were achieved, as compared with those of the same types of buffer layer at the same operating voltage. Figs. 2(b) and (c) show the dependence of the luminance–voltage (L–V) and the luminous efficiency characteristics of WOLEDs with various thicknesses of m-MTDATA and AlF3 as buffer layer. As shown in
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(2) m-MTDATA buffer layer with different thickness of 10, 15, and 20 nm, (3) hybrid buffer layer of 5 nm AlF3/15 nm m-MTDATA, and (4) no buffer layer. After the buffer layers, 40-nm-thick N,N0 diphenyl-N,N0 -bis(1-naphthyl-phenyl)-(1,10 -biphenyl)-4,40 -diamine (NPB) was deposited as the hole transport layer, 20-nm-thick bis(2-methyl-8-quinolinolato)(paraphenylphenolato)aluminum(III) (BAlq) doped with 3% 2,5,8,11-tetra-tert-butylperylene (TBPe) was deposited as the blue-emitting layer, 20-nm-thick tris(8-hydroxyquinoline) aluminum (Alq3) doped with 2% 4(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl9-enyl)-4 H-pyran (DCJTB) was deposited as the red-emitting layer, 20-nm-thick tris-(8-hydroxyquinoline) aluminum (Alq3) was deposited as the electron transport layer, 0.5-nm-thick lithium fluoride (LiF) layer was deposited as the electron injection layer and 200-nm-thick Al was deposited as the cathode. All of the layers were deposited by vacuum vapor deposition at a pressure of 1 10-6 Torr onto an ITO-coated glass substrate (7O/&). The ITO substrate was initially cleaned using detergent and de-ionized water. The evaporation rate was ˚ 1–2 A/s. The evaporation rate and thickness of the film were determined using an oscillating quartz thickness monitor (ULVAC CRTM-9000). The active area of the devices, defined by the overlap of the ITO and Al electrodes, was 0.6 0.4 cm2. All devices were encapsulated in a dry nitrogen glove box. The luminance– current density–voltage (L–J–V) characteristics and color coordinates (CIE) were measured and recorded simultaneously by combining a PR-650 Spectra Scan Colorimeter with a Keithley 2400 programmable voltage–current source. X-ray photoelectron spectroscopy (XPS) measurements were made in an ultrahighvacuum system. The XPS device, which comprised a high-power Mg Ka(1253.6 eV) line X-ray source and an angle-resolved electron energy analyzer, had an energy resolution of 0.2 eV. All measurements were made at room temperature.
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Fig. 2. (a) Current density–voltage, (b) luminance–voltage, and (c) luminous efficiency–voltage characteristics of WOLEDs with various m-MTDATA and AlF3 thickness.
Fig. 2(b), the turn-on (when the devices were at 1 cd/m2) voltages of no buffer layer, 15 nm m-MTDATA buffer layer, and 5 nm AlF3 buffer layer were 4.2, 3.9, and 3.1 V, respectively. The different thicknesses of m-MTDATA buffer layer show small influence on the turn-on voltage, it only influences the variation of the
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Al 2p
ITO/AlF3(15nm) Normalized Intensity(a.u.)
luminance. This phenomenon also appeals in the different thickness of AlF3 buffer layers. Moreover, it is found that the maximum luminance of m-MTDATA and AlF3 buffer layer is achieved under the thicknesses of 15 and 5 nm at a constant voltage of 10 V. The luminous efficiency is the most important factor for the WOLED. Fig. 2(c) shows that the 15 nm m-MTDATA buffer layer has a highest luminous efficiency of about 9 cd/A and a 5 nm AlF3 buffer layer has a maximum luminous efficiency of about 11.8 cd/A while the no buffer device only shows a maximum luminous efficiency of about 6.2 cd/A. It is also noteworthy that the luminous efficiencies of the devices with various thicknesses of AlF3 buffer layer degraded obviously with respect to varying drive voltages. It is suggested that the quenching effect occurred under a higher drive voltage, resulting in a lower luminous efficiency. In order to redeem this problem, a hybrid buffer layer, with a combination of a 5 nm AlF3 and a 15 nm m-MTDATA, was constructed in this investigation. The results of using a hybrid buffer layer in a WOLED are shown in Fig. 3 and illustrate that the turn-on voltage was 3.1 V as well as the maximum luminous efficiency is boosted to 14.7 cd/A; the most important is that the hybrid buffer layer shows a weak quenching effect with increase in drive voltage. Moreover, the inset in Fig. 3 illustrates the electro-luminescence (EL) spectra of this device. The CIEx,y value is at about (0.33, 0.35), which is close to the white CIEx,y value of (0.33, 0.33). The improvement of the use of a hybrid buffer layer in WOLED can be illustrated in several aspects. It has been recognized that the turn-on voltage depends on the carrier injection and transportation, and the luminous efficiency depends not only on carrier injection but also on the balance of electron–hole pairs [24]. The usage of AlF3 in OLED facilitates the hole injection and reduces the turn-on voltage, and these can be explained in terms of an increase in carrier injection. To investigate the effect of carrier injection, we measured X-ray photoelectron spectra (XPS): (A) ITO substrate, (B) ITO/AlF3 (5 nm), and (C) ITO/AlF3 (15 nm). In Fig. 4, the In 3d peaks shifted to higher energies and the Al 2p peaks shifted to lower energies in the XPS spectra of (B) when compared with those in the XPS spectra of (A) and (C). Since a negatively charged atom has a higher binding energy of electrons, the spectral shifts mean carriers transfer from ITO to AlF3, leading to the increase in hole injection at the ITO/AlF3 interface. However, higher hole injection does not inevitably evolve to higher device efficiency. It has been recognized that the luminous
456
72
75
78 81 Binding Energy (eV)
84
Fig. 4. XPS spectra: (a) In 3d peaks and (b) Al 2p peaks observed in (A) ITO substrate, (B) ITO/AlF3 (5 nm), and (C) ITO/AlF3 (15 nm).
efficiency strongly depends on the balance of electrons and holes. The excess of hole injection results in unbalance of electron–hole pairs, resulting in luminescence quenching. This phenomenon is amplified with increase in drive voltage. Thus, AlF3 buffer layer degrades the luminous efficiency with increase in drive voltage. For the hybrid buffer layer structure, after the holes injection through the AlF3 buffer layer they still require to cross a barrier between m-MTDATA and NPB. The barrier between m-MTDATA and NPB may trap some holes. Therefore, the m-MTDATA layer can limit the transporting speed of holes to withhold excess holes injecting into the NPB layer, resulting in a balanced recombination in the emissive layer. Thus, the hybrid buffer layer results in boosted luminous efficiency and less quenching effect with increase in drive voltage. Fig. 5 plots the lifespan of WOLED with structures of AlF3 (5 nm), m-MTDATA (15 nm), and AlF3 (5 nm)/m-MTDATA (15 nm) as a buffer layer. For comparison, the initial luminance of the three devices was measured under a constant current density of 20 mA/cm2. The initial brightness of the three devices was about
ARTICLE IN PRESS P. Yu Chen et al. / Journal of Physics and Chemistry of Solids 71 (2010) 922–925
be reduced to 3.1 V, and the luminous efficiency can be improved to 14.7 cd/A when a hybrid buffer layer was used. Since the turnon voltage decreases and the efficiency increases, the power consumption as well as lifespan are then improved. Moreover, the luminous efficiency of the hybrid buffer layer also immunes to voltage variations.
100 5nm AlF3
Luminance (%)
95
925
15nm m-MTDATA 5nm AlF3 /15nm m-MTDATA
90
85
References
80
[1] [2] [3] [4] [5] [6]
75
70 0
50
100
150 200 Lifetime (h)
250
300
Fig. 5. Lifespan of WOLEDs with m-MTDATA (15 nm) and AlF3 (5 nm)/m-MTDATA (15 nm) as the buffer layer under a current density of 20 mA/cm2.
2300, 1800, and 2950 cd/m2, respectively. After approximately 300 h, the luminance decayed to 80%, 76.5%, and 82%. It is known that the use of inorganic materials can increase the lifespan of a device due to good thermal stability. Furthermore, the use of a hybrid buffer layer can further increase efficiency, and therefore reduce the Joule heat generated by power loss at the interface, resulting in a low aggregation of molecules from heat. Hence, the degradation tests show that the device with a hybrid structure has a long lifespan.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
4. Conclusions
[20] [21] [22] [23]
We have fabricated a WOLED with AlF3 and m-MTDATA as a hybrid buffer layer. Results indicate that the turn-on voltage can
[24]
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