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Ag thin film on an organic silane monolayer applied as anode of organic light emitting diode
Thin Solid Films 532 (2013) 7–10
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ag thin film on an organic silane monolayer applied as anode of organic light emitting diode M. Kawamura ⁎, Y. Ishizuka, S. Yoshida, Y. Abe, K.H. Kim Department of Materials Science and Engineering, Kitami Institute of Technology, Kitami, Hokkaido, Japan
a r t i c l e
i n f o
Available online 14 November 2012 Keywords: Silver Thin films 3-Mercaptopropyl trimethoxysilane Sheet resistance Transparent electrode Anode Organic light emitting diodes Indium tin oxide
a b s t r a c t We prepared Ag thin films on glass substrates with an interlayer of 3-mercaptopropyl trimethoxysilane (MPTMS) for use as transparent anodes in organic light emitting diodes (OLEDs) as substitutes for conventionally used tin-doped indium oxide (ITO) films. The measured OLED properties reveal that a Ag (8 nm)/ MPTMS film can be used as the anode, despite having inferior optical properties to ITO films. Experiments also revealed that employing a thicker Ag film anode improved the emission properties of OLEDs. These results demonstrate that Ag/MPTMS anodes are promising for producing high-performance OLEDs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Tin-doped indium oxide (ITO) films are the most commonly used transparent conductive films and they are employed in optical devices such as organic light emitting diodes (OLEDs). With the aim of reducing the use of rare metals such as indium, alternative materials such as aluminum-doped zinc oxide and gallium-doped zinc oxide have been developed [1,2]. In addition, structures with thinner ITO layers and a thin metal interlayer, such as ITO/Ag/ITO films, have been developed [3]. On the other hand, very thin, low-resistivity metal (e.g., Au, Ag, and Cu) films are expected to be used as transparent electrodes in display devices. These films require both a high transparency and a low resistivity. Thinner films generally have higher transparency. However, very thin metal films deposited on glass or SiO2 substrates are discontinuous because they grow in Volmer–Weber mode. Film growth is influenced by various parameters including the surface condition of the substrate. The presence of a 3-mercaptopropyl trimethoxysilane (MPTMS) monolayer modified the nuclear density of very thin Cu sputtered films [4]. We found that a thinner continuous Ag film can be prepared on a thermal oxide of Si or a glass substrate with an MPTMS interlayer due to the strong interaction between Ag atoms and the mercapto moiety of MPTMS [5,6]. We also found that Ag thin films grown on MPTMS interlayer are flatter and have a lower resistivity than films grown without an interlayer. In addition, measurements using a micro scratch tester revealed that Ag films with an interlayer have a higher adhesion
⁎ Corresponding author. E-mail address: [email protected] (M. Kawamura). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.108
strength, which is preferable for realizing a high film stability. Comparison of 6-nm- and 8-nm-thick Ag films on an MPTMS interlayer revealed that the latter has a lower sheet resistance and thus seems to be more suitable as an OLED anode [6]. In the present study, we prepared Ag thin films of various thicknesses up to 20 nm on glass substrates with an MPTMS interlayer. Their electrical and optical properties were investigated and they were used as anodes in OLEDs that had the same structure. The device properties obtained with Ag/MPTMS anodes were compared with those obtained using an ITO anode. 2. Experimental details A Corning #1737 glass substrate was ultrasonically cleaned in acetone, 2-propanol, and deionized water and then cleaned in a UV/ozone cleaner. The substrate was transferred to a nitrogen-filled glove box, immersed in a 5 mM MPTMS toluene solution for 30 min, and then rinsed in toluene and dried overnight to deposit an MPTMS layer on the substrate. Ag films were deposited on glass substrates with and without an MPTMS interlayer by vacuum evaporation below 3.0×10−4 Pa using a Ag wire (99.99% purity) as the evaporation source. The thickness and deposition rate (0.40 nm/s) of the films were controlled by a quartz crystal microbalance. The electrical resistance was measured at room temperature by the four-point probe method. The optical transmittance was measured using a UV/vis spectrophotometer. The OLED devices in this study were fabricated as follows. High purity source materials of MoO3 (Mitsuwa Chemicals Co., Ltd.), α-NPD (Nippon Steel Chemical Co., Ltd.), tris(8-hydroxyquinoline) aluminum (Alq3) (Nippon Steel Chemical Co., Ltd.) and LiF (Wako Pure Chemical Industries, Ltd.) were successively evaporated on Ag/MPTMS anode
8
M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
15
prepared above. The structure of the OLED was Al (100 nm)/LiF (0.5 nm)/Alq3 (65 nm)/α-NPD (60 nm)/MoO3 (5.0 nm)/Ag/MPTMS/ glass as shown in Fig. 1a. For comparison, a commercial glass substrate (Tokyo Sanyo Shinku Co.) coated with a 150-nm-thick ITO film was also used to prepare a basic OLED. The structure with ITO anode is shown in Fig. 1b. The OLEDs had active areas of 2 × 5 mm2. The current density (J)–voltage (V)–luminance (L) characteristics were measured using a DC power supply, a digital multimeter, and a chroma meter (CS-200, Konica Minolta Sensing, Inc.).
Sheet resistance (Ω Ω/sq.)
13
3. Results and discussion
11 9 7 5
3.1. Electrical and optical properties of Ag/MPTMS films
3
Fig. 2 shows the sheet resistances (Rs) of Ag/MPTMS films of various thicknesses. The sheet resistance generally decreases with metal film thickness; being discontinuous, a very thin film has extremely high sheet resistance [7]. In the present results, a 6-nm-thick Ag/MPTMS film (the thinnest film) has a sheet resistance as low as about 9 Ω/sq, indicating that it has a smooth and continuous morphology. In fact, Rs is lower at each thickness than that in our previous study [6]; this is thought to be due to the optimized angle and distance of the evaporation source relative to the substrate in the present study. The highest Rs of the Ag films in this study (~9 Ω/sq) is much lower than that (15 Ω/sq) of the ITO film. Fig. 3 shows optical transmission spectra of a glass substrate, an ITO-coated glass substrate, and 8-, 13-, and 20-nm-thick Ag films with MPTMS interlayers on glass substrates. The ITO-coated glass substrate is thinner (0.7 mm) than the other glass substrates (1.1 mm) and it is thus expected to have a higher transmittance. Consequently, the transmittances of these samples cannot be directly compared. However, the transmittance at about 520 nm, which is the peak emission wavelength of Alq3, varies by about 30% between the Ag (8 nm)/MPTMS/glass and ITO/glass samples. In addition, when the Ag thickness is increased to 20 nm, the transmittance decreases to about 35%. Thus, Ag/MPTMS films have lower transmittances than ITO films, which seems to be disadvantageous for anode of OLEDs.
1
The OLED devices shown in Fig. 1 were fabricated. First, 8-nm and 13-nm-thick Ag films on an MPTMS interlayer and ITO film were used as anodes and the device properties were measured. Fig. 4 shows the current density (J) and luminance (L) as a function of the applied voltage (V). This figure shows that the device with the Ag (8 nm)/ MPTMS anode has a slightly higher driving voltage than devices with the Ag (13 nm)/MPTMS anode and the ITO anode. Table 1 lists the voltage, luminance, current efficiency, and power efficiency of the OLED devices at a current density of 50 mA/cm 2. Here, the table includes data
a)
Al LiF Alq3 α-NPD MoO3 Ag MPTMS glass
b) Al LiF Alq3 α-NPD MoO3 ITO glass
Fig. 1. OLED structures with (a) Ag and (b) ITO anodes.
10
15
20
25
Thickness (nm) Fig. 2. Sheet resistance of Ag/MPTMS films on glass substrates as a function of Ag film thickness.
with Ag (20 nm) anode also, which will be shown below in detail. The device with the Ag (8 nm) anode has inferior properties to the others; however, an extremely thin anode is suitable as an OLED anode. Thus, ITO-free OLED devices can be fabricated by replacing ITO thin films by Ag thin films with MPTMS interlayers. When a Ag (8 nm)/MPTMS anode is used, the total anode thickness is less than 10 nm, which represents a remarkable reduction in anode thickness since the ITO thickness is generally about 150 nm. The device with a Ag (13 nm) anode has similar J–V–L properties as the ITO anode. The current efficiency of the former (4.0 cd/A) is even higher than the latter (2.6 cd/A) as shown in Table 1. This indicates that good emission properties are obtained despite the lower optical transmittance of the Ag anode. In addition, the driving voltage of the Ag (13 nm) device (Fig. 4a) is lower than that of the Ag (8 nm) device, which is probably due to its lower sheet resistance; Rs of Ag (13 nm) film is about 3 Ω/sq and that of Ag (8 nm) film is about 7 Ω/sq, as shown in Fig. 3. Although the ITO film has a higher sheet resistance (15 Ω/sq), its driving voltage is even lower than the Ag films. This is thought to be because ITO has a higher work function (4.7–4.8 eV) than Ag (4.3–4.7 eV) [8–10], which is advantageous for hole injection; this aspect seems to be more important than the sheet resistance when comparing these anodes.
100 90 glass
80
Transmittance (%)
3.2. Application of Ag/MPTMS as an OLED anode
5
ITO/glass*
70 60 50 40 30 Ag(8nm) 20 10 0 200
Ag(13nm) Ag(20nm) 400
600
800
1000
Wavelength (nm) Fig. 3. Optical transmittance spectra of the samples. The glass* means that the glass substrate for ITO film is not the same with that for Ag films.
M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
25000
1000
a)
b)
800 600
Luminance (cd/m²)
Current density (mA/cm²)
9
Ag(8nm) Ag(13nm)
400
ITO
200
20000 15000
Ag(8nm) Ag(13nm)
10000
ITO
5000 0
0 0
5
10
0
5
10
Voltage (V)
Voltage (V)
Fig. 4. (a) J–V and (b) L–V properties of OLEDs with Ag (8 nm) and Ag (13 nm)/MPTMS anodes and ITO anode.
properties were found to be superior to those of the thinnest anode. The enhanced emission is considered to be due to the microcavity effect.
Table 1 OLED properties at a current density of 50 mA/cm2.
ITO Ag (8 nm) Ag (13 nm) Ag (20 nm)
Operation voltage (V)
Luminance (cd/m2)
Current efficiency (cd/A)
Power efficiency (lm/W)
7.5 8.6 7.5 6.9
1280 1150 2010 3100
2.6 2.3 4.0 6.2
1.1 0.84 1.7 2.8
The 20-nm-thick Ag film has a markedly lower optical transmittance than those of thinner films (see Fig. 3). We fabricated a device with Ag (20 nm)/MPTMS (Rs: 2 Ω/sq) and investigated the device properties. Fig. 5 shows the J–V–L properties of the device. Unexpectedly, the device with the Ag (20 nm) anode has an extremely high luminance, as shown in Fig. 5b. On the other hand, the highest current density of the Ag (20 nm) anode device is comparable with that of ITO device, though the driving voltage is lower for the former (Fig. 5a). This is thought be due to the microcavity effect [11,12], which is known to alter the emission properties (e.g., improve the color purity). Here, the two metallic reflective films (i.e., the Al cathode and the Ag anode) are considered to function as a resonator. Consequently, the current efficiency of the device at a current density of 50 mA/cm2 was 6.2 cd/A (see Table 1), which is the highest of all the devices in this study. This result indicates that when the emission is very strong, the disadvantage of the lower transmittance of the anode is overcome. Thus, a very thin Ag film on a MPTMS interlayer can be applied as a transparent electrode in a display device that has good device properties. Thicker Ag films can be also used as anodes for these devices; their
4. Conclusion We fabricated and investigated OLED devices with very thin Ag/ MPTMS films. We found that an extremely thin Ag/MPTMS film can be applied as an OLED anode despite its lower optical transmittance. Therefore, ITO-free OLED devices can be fabricated by replacing ITO with thin Ag/MPTMS films. The Ag film thickness for anodes was not restricted to 8 nm: a 20-nm-thick Ag film was also useful for producing an OLED with high emission. Acknowledgment The authors thank Nippon Steel Chemical Co., Ltd. for its support by offering chemicals (Alq3 and α-NPD) used in this work. This research was supported in part by a Grant-in-Aid for Scientific Research (C) (no. 22560712) from the Japan Society for the Promotion of Science. References [1] [2] [3] [4]
T. Minami, H. Sato, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 24 (1985) L781. S. Takata, T. Minami, H. Nanto, Thin Solid Films 135 (1986) 183. C. Guillen, J. Herrero, Thin Solid Films 520 (2011) 1. M. Hu, S. Noda, Y. Tsuji, T. Okubo, Y. Yamaguchi, H. Komiyama, J. Vac. Sci. Technol. A 20 (2002) 589. [5] T. Fudei, M. Kawamura, Y. Abe, K. Sasaki, J. Nanosci. Nanotechnol. 12 (2012) 1188. [6] M. Kawamura, T. Fudei, Y. Abe, J. Phys. Conf. Ser. (to be published).
60000
a)
b)
1000
Luminance (cd/m²)
Current density ( mA/cm²)
1200
800 600
Ag(20nm) ITO
400 200 0 0
5
Voltage (V)
10
50000 40000 30000 Ag(20nm) 20000
ITO
10000 0 0
5
Voltage (V)
Fig. 5. (a) J–V and (b) L–V properties of OLEDs with Ag (20 nm)/MPTMS anode and ITO anode.
10
10
M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
[7] K. Uozumi, M. Kamiyama, in: Hakumaku handobukku (Handbook of thin films), 1983, p. 94, (Ohmsha, Ltd., Tokyo, in Japanese). [8] L. Chong, Y.L. Lee, T.C. Wen, T.F. Guo, Appl. Phys. Lett. 89 (2006) 23513. [9] C.Y. Jiang, X.W. Sun, D.W. Zhao, A.K.K. Kyaw, Y.N. Li, Sol. Energy Mater. Sol. Cells 94 (2010) 1618.
[10] L.Y. Yang, X.Z. Chen, H. Xu, D.Q. Ye, H. Tian, S.G. Yin, Appl. Surf. Sci. 254 (2008) 5055. [11] U. Lemmer, R. Henning, W. Guss, A. Ochse, J. Pommerehne, R. Sander, A. Greiner, R.F. Mahrt, H. Bassler, J. Feldmann, O. Gobel, Appl. Phys. Lett. 66 (1995) 1301. [12] H. Becker, S.E. Burns, N. Tessler, R.H. Friend, J. Appl. Phys. 81 (1997) 2825.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ag thin film on an organic silane monolayer applied as anode of organic light emitting diode M. Kawamura ⁎, Y. Ishizuka, S. Yoshida, Y. Abe, K.H. Kim Department of Materials Science and Engineering, Kitami Institute of Technology, Kitami, Hokkaido, Japan
a r t i c l e
i n f o
Available online 14 November 2012 Keywords: Silver Thin films 3-Mercaptopropyl trimethoxysilane Sheet resistance Transparent electrode Anode Organic light emitting diodes Indium tin oxide
a b s t r a c t We prepared Ag thin films on glass substrates with an interlayer of 3-mercaptopropyl trimethoxysilane (MPTMS) for use as transparent anodes in organic light emitting diodes (OLEDs) as substitutes for conventionally used tin-doped indium oxide (ITO) films. The measured OLED properties reveal that a Ag (8 nm)/ MPTMS film can be used as the anode, despite having inferior optical properties to ITO films. Experiments also revealed that employing a thicker Ag film anode improved the emission properties of OLEDs. These results demonstrate that Ag/MPTMS anodes are promising for producing high-performance OLEDs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Tin-doped indium oxide (ITO) films are the most commonly used transparent conductive films and they are employed in optical devices such as organic light emitting diodes (OLEDs). With the aim of reducing the use of rare metals such as indium, alternative materials such as aluminum-doped zinc oxide and gallium-doped zinc oxide have been developed [1,2]. In addition, structures with thinner ITO layers and a thin metal interlayer, such as ITO/Ag/ITO films, have been developed [3]. On the other hand, very thin, low-resistivity metal (e.g., Au, Ag, and Cu) films are expected to be used as transparent electrodes in display devices. These films require both a high transparency and a low resistivity. Thinner films generally have higher transparency. However, very thin metal films deposited on glass or SiO2 substrates are discontinuous because they grow in Volmer–Weber mode. Film growth is influenced by various parameters including the surface condition of the substrate. The presence of a 3-mercaptopropyl trimethoxysilane (MPTMS) monolayer modified the nuclear density of very thin Cu sputtered films [4]. We found that a thinner continuous Ag film can be prepared on a thermal oxide of Si or a glass substrate with an MPTMS interlayer due to the strong interaction between Ag atoms and the mercapto moiety of MPTMS [5,6]. We also found that Ag thin films grown on MPTMS interlayer are flatter and have a lower resistivity than films grown without an interlayer. In addition, measurements using a micro scratch tester revealed that Ag films with an interlayer have a higher adhesion
⁎ Corresponding author. E-mail address: [email protected] (M. Kawamura). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.108
strength, which is preferable for realizing a high film stability. Comparison of 6-nm- and 8-nm-thick Ag films on an MPTMS interlayer revealed that the latter has a lower sheet resistance and thus seems to be more suitable as an OLED anode [6]. In the present study, we prepared Ag thin films of various thicknesses up to 20 nm on glass substrates with an MPTMS interlayer. Their electrical and optical properties were investigated and they were used as anodes in OLEDs that had the same structure. The device properties obtained with Ag/MPTMS anodes were compared with those obtained using an ITO anode. 2. Experimental details A Corning #1737 glass substrate was ultrasonically cleaned in acetone, 2-propanol, and deionized water and then cleaned in a UV/ozone cleaner. The substrate was transferred to a nitrogen-filled glove box, immersed in a 5 mM MPTMS toluene solution for 30 min, and then rinsed in toluene and dried overnight to deposit an MPTMS layer on the substrate. Ag films were deposited on glass substrates with and without an MPTMS interlayer by vacuum evaporation below 3.0×10−4 Pa using a Ag wire (99.99% purity) as the evaporation source. The thickness and deposition rate (0.40 nm/s) of the films were controlled by a quartz crystal microbalance. The electrical resistance was measured at room temperature by the four-point probe method. The optical transmittance was measured using a UV/vis spectrophotometer. The OLED devices in this study were fabricated as follows. High purity source materials of MoO3 (Mitsuwa Chemicals Co., Ltd.), α-NPD (Nippon Steel Chemical Co., Ltd.), tris(8-hydroxyquinoline) aluminum (Alq3) (Nippon Steel Chemical Co., Ltd.) and LiF (Wako Pure Chemical Industries, Ltd.) were successively evaporated on Ag/MPTMS anode
8
M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
15
prepared above. The structure of the OLED was Al (100 nm)/LiF (0.5 nm)/Alq3 (65 nm)/α-NPD (60 nm)/MoO3 (5.0 nm)/Ag/MPTMS/ glass as shown in Fig. 1a. For comparison, a commercial glass substrate (Tokyo Sanyo Shinku Co.) coated with a 150-nm-thick ITO film was also used to prepare a basic OLED. The structure with ITO anode is shown in Fig. 1b. The OLEDs had active areas of 2 × 5 mm2. The current density (J)–voltage (V)–luminance (L) characteristics were measured using a DC power supply, a digital multimeter, and a chroma meter (CS-200, Konica Minolta Sensing, Inc.).
Sheet resistance (Ω Ω/sq.)
13
3. Results and discussion
11 9 7 5
3.1. Electrical and optical properties of Ag/MPTMS films
3
Fig. 2 shows the sheet resistances (Rs) of Ag/MPTMS films of various thicknesses. The sheet resistance generally decreases with metal film thickness; being discontinuous, a very thin film has extremely high sheet resistance [7]. In the present results, a 6-nm-thick Ag/MPTMS film (the thinnest film) has a sheet resistance as low as about 9 Ω/sq, indicating that it has a smooth and continuous morphology. In fact, Rs is lower at each thickness than that in our previous study [6]; this is thought to be due to the optimized angle and distance of the evaporation source relative to the substrate in the present study. The highest Rs of the Ag films in this study (~9 Ω/sq) is much lower than that (15 Ω/sq) of the ITO film. Fig. 3 shows optical transmission spectra of a glass substrate, an ITO-coated glass substrate, and 8-, 13-, and 20-nm-thick Ag films with MPTMS interlayers on glass substrates. The ITO-coated glass substrate is thinner (0.7 mm) than the other glass substrates (1.1 mm) and it is thus expected to have a higher transmittance. Consequently, the transmittances of these samples cannot be directly compared. However, the transmittance at about 520 nm, which is the peak emission wavelength of Alq3, varies by about 30% between the Ag (8 nm)/MPTMS/glass and ITO/glass samples. In addition, when the Ag thickness is increased to 20 nm, the transmittance decreases to about 35%. Thus, Ag/MPTMS films have lower transmittances than ITO films, which seems to be disadvantageous for anode of OLEDs.
1
The OLED devices shown in Fig. 1 were fabricated. First, 8-nm and 13-nm-thick Ag films on an MPTMS interlayer and ITO film were used as anodes and the device properties were measured. Fig. 4 shows the current density (J) and luminance (L) as a function of the applied voltage (V). This figure shows that the device with the Ag (8 nm)/ MPTMS anode has a slightly higher driving voltage than devices with the Ag (13 nm)/MPTMS anode and the ITO anode. Table 1 lists the voltage, luminance, current efficiency, and power efficiency of the OLED devices at a current density of 50 mA/cm 2. Here, the table includes data
a)
Al LiF Alq3 α-NPD MoO3 Ag MPTMS glass
b) Al LiF Alq3 α-NPD MoO3 ITO glass
Fig. 1. OLED structures with (a) Ag and (b) ITO anodes.
10
15
20
25
Thickness (nm) Fig. 2. Sheet resistance of Ag/MPTMS films on glass substrates as a function of Ag film thickness.
with Ag (20 nm) anode also, which will be shown below in detail. The device with the Ag (8 nm) anode has inferior properties to the others; however, an extremely thin anode is suitable as an OLED anode. Thus, ITO-free OLED devices can be fabricated by replacing ITO thin films by Ag thin films with MPTMS interlayers. When a Ag (8 nm)/MPTMS anode is used, the total anode thickness is less than 10 nm, which represents a remarkable reduction in anode thickness since the ITO thickness is generally about 150 nm. The device with a Ag (13 nm) anode has similar J–V–L properties as the ITO anode. The current efficiency of the former (4.0 cd/A) is even higher than the latter (2.6 cd/A) as shown in Table 1. This indicates that good emission properties are obtained despite the lower optical transmittance of the Ag anode. In addition, the driving voltage of the Ag (13 nm) device (Fig. 4a) is lower than that of the Ag (8 nm) device, which is probably due to its lower sheet resistance; Rs of Ag (13 nm) film is about 3 Ω/sq and that of Ag (8 nm) film is about 7 Ω/sq, as shown in Fig. 3. Although the ITO film has a higher sheet resistance (15 Ω/sq), its driving voltage is even lower than the Ag films. This is thought to be because ITO has a higher work function (4.7–4.8 eV) than Ag (4.3–4.7 eV) [8–10], which is advantageous for hole injection; this aspect seems to be more important than the sheet resistance when comparing these anodes.
100 90 glass
80
Transmittance (%)
3.2. Application of Ag/MPTMS as an OLED anode
5
ITO/glass*
70 60 50 40 30 Ag(8nm) 20 10 0 200
Ag(13nm) Ag(20nm) 400
600
800
1000
Wavelength (nm) Fig. 3. Optical transmittance spectra of the samples. The glass* means that the glass substrate for ITO film is not the same with that for Ag films.
M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
25000
1000
a)
b)
800 600
Luminance (cd/m²)
Current density (mA/cm²)
9
Ag(8nm) Ag(13nm)
400
ITO
200
20000 15000
Ag(8nm) Ag(13nm)
10000
ITO
5000 0
0 0
5
10
0
5
10
Voltage (V)
Voltage (V)
Fig. 4. (a) J–V and (b) L–V properties of OLEDs with Ag (8 nm) and Ag (13 nm)/MPTMS anodes and ITO anode.
properties were found to be superior to those of the thinnest anode. The enhanced emission is considered to be due to the microcavity effect.
Table 1 OLED properties at a current density of 50 mA/cm2.
ITO Ag (8 nm) Ag (13 nm) Ag (20 nm)
Operation voltage (V)
Luminance (cd/m2)
Current efficiency (cd/A)
Power efficiency (lm/W)
7.5 8.6 7.5 6.9
1280 1150 2010 3100
2.6 2.3 4.0 6.2
1.1 0.84 1.7 2.8
The 20-nm-thick Ag film has a markedly lower optical transmittance than those of thinner films (see Fig. 3). We fabricated a device with Ag (20 nm)/MPTMS (Rs: 2 Ω/sq) and investigated the device properties. Fig. 5 shows the J–V–L properties of the device. Unexpectedly, the device with the Ag (20 nm) anode has an extremely high luminance, as shown in Fig. 5b. On the other hand, the highest current density of the Ag (20 nm) anode device is comparable with that of ITO device, though the driving voltage is lower for the former (Fig. 5a). This is thought be due to the microcavity effect [11,12], which is known to alter the emission properties (e.g., improve the color purity). Here, the two metallic reflective films (i.e., the Al cathode and the Ag anode) are considered to function as a resonator. Consequently, the current efficiency of the device at a current density of 50 mA/cm2 was 6.2 cd/A (see Table 1), which is the highest of all the devices in this study. This result indicates that when the emission is very strong, the disadvantage of the lower transmittance of the anode is overcome. Thus, a very thin Ag film on a MPTMS interlayer can be applied as a transparent electrode in a display device that has good device properties. Thicker Ag films can be also used as anodes for these devices; their
4. Conclusion We fabricated and investigated OLED devices with very thin Ag/ MPTMS films. We found that an extremely thin Ag/MPTMS film can be applied as an OLED anode despite its lower optical transmittance. Therefore, ITO-free OLED devices can be fabricated by replacing ITO with thin Ag/MPTMS films. The Ag film thickness for anodes was not restricted to 8 nm: a 20-nm-thick Ag film was also useful for producing an OLED with high emission. Acknowledgment The authors thank Nippon Steel Chemical Co., Ltd. for its support by offering chemicals (Alq3 and α-NPD) used in this work. This research was supported in part by a Grant-in-Aid for Scientific Research (C) (no. 22560712) from the Japan Society for the Promotion of Science. References [1] [2] [3] [4]
T. Minami, H. Sato, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 24 (1985) L781. S. Takata, T. Minami, H. Nanto, Thin Solid Films 135 (1986) 183. C. Guillen, J. Herrero, Thin Solid Films 520 (2011) 1. M. Hu, S. Noda, Y. Tsuji, T. Okubo, Y. Yamaguchi, H. Komiyama, J. Vac. Sci. Technol. A 20 (2002) 589. [5] T. Fudei, M. Kawamura, Y. Abe, K. Sasaki, J. Nanosci. Nanotechnol. 12 (2012) 1188. [6] M. Kawamura, T. Fudei, Y. Abe, J. Phys. Conf. Ser. (to be published).
60000
a)
b)
1000
Luminance (cd/m²)
Current density ( mA/cm²)
1200
800 600
Ag(20nm) ITO
400 200 0 0
5
Voltage (V)
10
50000 40000 30000 Ag(20nm) 20000
ITO
10000 0 0
5
Voltage (V)
Fig. 5. (a) J–V and (b) L–V properties of OLEDs with Ag (20 nm)/MPTMS anode and ITO anode.
10
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M. Kawamura et al. / Thin Solid Films 532 (2013) 7–10
[7] K. Uozumi, M. Kamiyama, in: Hakumaku handobukku (Handbook of thin films), 1983, p. 94, (Ohmsha, Ltd., Tokyo, in Japanese). [8] L. Chong, Y.L. Lee, T.C. Wen, T.F. Guo, Appl. Phys. Lett. 89 (2006) 23513. [9] C.Y. Jiang, X.W. Sun, D.W. Zhao, A.K.K. Kyaw, Y.N. Li, Sol. Energy Mater. Sol. Cells 94 (2010) 1618.
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