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Transparent conductive electrode deposited by Cs-incorporated RF magnetron sputtering and evaluation of the damage in OLED organic layer
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Thin Solid Films 516 (2008) 5907 – 5910 www.elsevier.com/locate/tsf
Transparent conductive electrode deposited by Cs-incorporated RF magnetron sputtering and evaluation of the damage in OLED organic layer Takayuki Uchida a,⁎, Yoshihiro Kasahara a , Toshio Otomo b , Shigeyuki Seki a,1 , Meihan Wang a , Yutaka Sawada a a
Center for Hyper Media Research, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan b Matsubo Corporation, 33 Mori Bldg., 3-8-21 Toranomon, Tokyo 105-0001, Japan Available online 13 October 2007
Abstract Top-emission organic light emitting devices (TE-OLEDs) and transparent OLEDs (TOLEDs) can be fabricated by employing a transparent cathode with a low work function instead of Mg:Ag or Al:Li films. However, most transparent conductive oxides (TCOs) have a high work function (WF). Therefore, we proposed a new sputtering method for fabricating a novel transparent conductive film. Generally, the sputtering process is a plasma process involving high-energy particles. It cannot avoid bombardment-induced damage to the organic layer. We fabricated ITO:Cs films and evaluated the damage to the organic layers. The turn-on threshold voltages of TOLEDs with such low WF (4.1 eV) ITO:Cs films decreased from 8.5 V to 3.5 V and higher efficiencies were obtained; however, large leak currents were observed. © 2008 Published by Elsevier B.V. Keywords: Indium-tin-oxide; Top emission; Organic light emitting device; Low work function
1. Introduction In the future, a display will be integrated into various types of devices or instruments. The following properties are considered to be essential for satisfying these requirements: “mechanical flexibility,” “light weight,” and “robust integration on a plastic substrate or thin metal sheet.” Top-emission organic light emitting devices (TE-OLEDs) [1] and transparent OLEDs (TOLEDs) [2] technologies should be particularly useful for more advanced types of applications that realize a transparent (not interfering with the visibility and function of the original device) and highresolution (high aperture) active-matrix display with a larger number of thin film transistors (TFTs) on the bottom side. In order to realize such displays, it is necessary to develop a new lowdamage film fabrication technique for a plastic substrate and an
⁎ Corresponding author. Tel.: +81 46 242 9618; fax: +81 46 242 3000. E-mail address: [email protected] (T. Uchida). 1 Present address: Sendai National College of Technology, 4-16-1 Ayashichuo, Aoba-ku, Sendai 989-3128, Japan. 0040-6090/$ - see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.10.042
organic layer since both are very sensitive to thermal and mechanical (e.g., ion bombardment) process conditions. In the first report on TOLED [2], the transparent top electrode was deposited by a low-power RF magnetron sputtering method to avoid potential damage. Accordingly, the crystallinity of transparent electrode films inevitably was amorphous in most of the cases. On the other hand, most of the basic studies on TCOs concentrated only on their electrical conductivity and optical transmission or on their poly-crystallization, which requires annealing (heat treatment). In order to study the applications of TE-OLEDs and TOLEDs, we must first study amorphous TCOs formed by a low-damage sputtering process. This process can be used to form a TCO film having a high electron injection characteristic that can be used as a cathode; the underlayer remains undamaged during this process. We propose a new sputtering method for fabricating a novel transparent conductive film using Cs-incorporated RF magnetron sputtering [3]. The main purpose was to form a low work function TCO with high electron injection. Several other methods have been reported for fabricating such films, e.g., a method for inducing a redox reaction between TDAE of the electron-donor molecules
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T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
and ITO [4], a method using LaB6 [5], and a method for cosputtering C12A7 and ITO [6]. Cs has been used to negatively ionize a target material by lowering the work function of the target surface and generating high-density plasma at a lower sputtering gas pressure. The negatively ionized sputtered particles are accelerated by the voltage between the target and the substrate and they improve film qualities such as adhesion, density, or surface smoothness by their energy [7]. In contrast, our aim was to incorporate Cs in the film, although it had been introduced in the sputtering process. We have shown that the electronic injection improved and the driving voltage decreased in TOLEDs. Cs was shown to exist in the film with high stability [8]. Here, we examined the damage caused by this technique to an organic film since it appears to be possible to cause damage by using high-density plasma and kinetically energetic particles. The ITO and ITO:Cs film of the OLED being used for comparison was deposited by the typical RF magnetron sputter method. We then evaluated the characteristics of the OLED using an ITO:Cs film and evaluated the damage to the organic layer by comparing the degradation of PL intensities of BAlq (bis(2-methyl-8-quinolinolato)(p-phenylphenolato) aluminum) with an ITO:Cs film and BAlq with a standard ITO film. The leakage currents of the devices in the forward and reverse directions were measured as well. 2. Experimental 2.1. Comparison of the photoluminescence intensities of BAlq with an ITO:Cs film and BAlq with a standard ITO film For comparing the photoluminescence intensities, 40-nm BAlq's were deposited on glass substrates (Corning 7059) and
then transparent TCO films (ITO and ITO:Cs) were fabricated. During the formation of the ITO:Cs film, a Cs dispenser (Seas Getters) was heated to eject Cs vapor in the sputtering chamber. The details are described in another paper [8]. The PL intensities were measured using an excitation wavelength of 370 nm (PL measuring instrument (Horiba Jobin Yvon, FluoroMax-P)). The schematic diagram of the PL measurement is shown in the upper right in Fig. 1. 2.2. Transparent organic light emitting devices (TOLEDs) with a standard ITO film or an ITO:Cs film as a transparent cathode TOLEDs with ITO and ITO:Cs films were fabricated according to the following procedure. First, 70 nm of poly (styrenesulfonate)/poly(2,3-dihydrothieno (3,4-b)-1,4-dioxin) PEDOT-PSS was spin-coated on both ITO-coated (anode side) glass substrates and bis[N-(1-naphthyl)-N-phenyl]benzidine (NPB) as a hole transport layer and tris-(8-hydroxyquinoline) aluminum (Alq3) (40 nm) as an electron transport and emissive layer in that order. Then, 10 nm of 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP) was deposited by evaporation as an electron injection layer. Finally, two different types of TCO films (ITO and ITO:Cs) were deposited by sputtering. 3. Experimental result and discussion 3.1. PL intensities of BAlq with an ITO film and BAlq with an ITO:Cs film Before conducting intensity measurements as a damage indicator, we examined the time dependence of the fluorescence intensities of BAlq in the atmosphere. In consecutive measurements (1 h interval) over 19 h, the intensity was so stable that
Fig. 1. Photoluminescence intensities of BAlq's with Cs-incorporated ITO and standard ITO inset figure (upper right): schematic diagram of PL measurement, (lower bottom): PL spectra before and after sputtering deposition.
T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
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Fig. 2. AFM images of ITO and ITO:Cs and commercial ITO surface. (a) Commercial ITO, (b) ITO film fabricated by low-power sputtering deposition, and (c) ITO:Cs film fabricated by low-power sputtering deposition.
the attenuation was within approximately 5%. Referring to this result, we used the fluorescence intensity of BAlq as a probe for examining the damage to the organic underlayers. Fig. 1 shows the fluorescence intensities of BAlq's with ITO:Cs and ITO films. As shown in the lower left inset diagram, the PL intensities decreased when such TCO films were deposited by sputtering. The ratio of the peak values between the values before and after sputtering (PLA/PLB) shows that the PL intensity was reduced to 45% and 25% by typical and Csincorporated RF magnetron sputtering, respectively. It is necessary to consider the degradation of the material with time and the influence of the optical film on them; however, it is believed that the result mainly reflects the sputtering damage. It has been known that the light emitting region of an actual device is specified to be a several nanometers region very close to the interface of the hole transportation layer. This region doesn't seem to have been damaged. But the balance of injections of holes and electrons has to be considered. Here, the average values of degradation of the entire layers were measured. These values may not precisely reflect the degradation of the actual device; however, it is possible to determine the extent of degradation caused by different sputtering conditions for ITO and ITO:Cs films.
It is assumed that negatively ionized sputtered particles around the surface of target by introducing Cs vapor, strongly impact the organic layer and/or higher density plasma damage the organic layer. Fig. 2 shows AFM images of ITO and ITO:Cs and a commercial ITO surface: (a) commercial ITO, (b) ITO film fabricated by low-power sputtering deposition, and (c) ITO:Cs film fabricated by low-power sputtering deposition. In the case of commercial ITO, crystallization forms a polycrystal with Ra = 2.2 nm. The ITO and ITO:Cs films are amorphous with Ra = 1.19 nm and 1.54 nm, respectively, since they are fabricated by low-power sputtering deposition.
Fig. 3. Transmittance of TOLED at the turn-off state. Glass/ITO/PEDOT-PSS/ NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
Fig. 4. Luminance–voltage characteristics of TOLED. Glass/ITO/PEDOT-PSS/ NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
3.2. Solution for the problems encountered in TCO film formation for TOLED with an ITO:Cs film Considering the sputtering damage caused to the organic layers, methods such as soft deposition with a slow rate or an additional buffer layer have been proposed and the leakage currents of devices have been studied in relation to the damage. In this research, we fabricated TOLEDs using ITO or ITO:Cs films as transparent cathodes and examined their characteristics.
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T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
Fig. 5. Current–voltage characteristics of TOLED. Glass/ITO/PEDOT-PSS/NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
Fig. 3 shows the transmittance (a basic characteristic) of TOLED in the turn-off state. In the visible region, TOLEDs with ITO and ITO:Cs cathodes had transmittances of 72% and 69%, respectively. The transmittance of an ITO and ITO:Cs film on glass was 90% and 87%, respectively. Cs-incorporated films were slightly brownish. Further, the luminance–voltage characteristics are shown in Fig. 4. The turn-on threshold voltages (luminance: 1 cd/m2) of TOLEDs with TCO films of ITO:Cs decreased from 8.5 V to 3.5 V and the maximum luminance of approximately 1000 cd/ m2 was obtained at 13 V. In this manner, the electronic electron injection characteristics were improved by the incorporation of Cs in ITO, and the drive voltage was decreased. Fig. 5 shows the current density–voltage characteristic. These devices exhibited typical OLED characteristics on the application of a forward bias. In the inset figure, a device with an ITO:Cs film was shown to have 20 times greater leakage current under the application of a reverse bias. This indicates that the leakage current was increased due to the introduction of Cs vapor during sputtering. We could enhance the electron injection by using an ITO:Cs film. However, it attenuated the PL intensity and increased the leakage current due to the damage to the organic layer. Therefore, it is essential to develop technology that avoids damage to the organic layer, for example, adopting a buffer layer. Further, optimization is required in terms of the conductivity, transparency, electron injection, and reduced damage. 4. Summary Transparent conductive films (ITO:Cs films) fabricated by Csincorporated RF magnetron sputtering were formed and analyzed. The turn-on threshold voltages (luminance: 1 cd/m2) of TOLEDs with such low WF (4.1 eV) TCO films were decreased from 8.5 V
to 3.5 Vand higher efficiencies were obtained. These films will be good candidates for transparent cathodes in TOLEDs. However the current density–voltage characteristic showed that these devices having ITO:Cs films have larger leakage current from the device with ITO:Cs under the application of reverse bias. This indicates that the leakage current is increased due to the introduction of Cs vapor during sputtering. Although we can enhance the electron injection by using an ITO:Cs film, this causes attenuation of the PL intensity and increases of leakage current due to the damage caused to the organic layer. Acknowledgment This work has been partly supported by the Grant-in-Aid for Scientific Research (C) No. 19560357 of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) from 2007 to 2008. References [1] Sasaoka, M. Sekiya, A. Yumoto, J. Yamada, T. Hirano, Y. Iwase, T. Yamada, T. Ishibashi, T. Mori, M. Asano, S. Tamura, T. Urabe, SID'01, Dig., 2001, p. 384. [2] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 68 (1996) 2606. [3] T. Uchida, T. Mimura, S. Kaneta, M. Ichihara, M. Ohtsuka, T. Otomo, Jpn. J. Appl. Phys. 44 (2005) 5939. [4] W. Osikowicz, X. Crispin, C. Tengstedt, L. Lindell, T. Kugler, W.R. Salaneck, Appl. Phys. Lett. 85 (2004) 1616. [5] C. Jian-bo, J. Quan, L. Zu-lun, C. Wen-bin, Y. Gang, R. Hai-bo, Q. Cheng, Proc. SPIE Int. Soc. Opt. Eng. 5519 (2004) 250. [6] D.-H. Yoon, International Symposium on C12A7 and Nanoporous Materials, 2007. [7] N.-W. Paik, S. Kim, Rev. Sci. Instrum. 73 (2002) 1212. [8] T. Uchida, T. Mimura, M. Ohtsuka, T. Otomo, M. Ide, A. Shida, Y. Sawada, Thin Solid Films 496 (2006) 75.
Thin Solid Films 516 (2008) 5907 – 5910 www.elsevier.com/locate/tsf
Transparent conductive electrode deposited by Cs-incorporated RF magnetron sputtering and evaluation of the damage in OLED organic layer Takayuki Uchida a,⁎, Yoshihiro Kasahara a , Toshio Otomo b , Shigeyuki Seki a,1 , Meihan Wang a , Yutaka Sawada a a
Center for Hyper Media Research, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan b Matsubo Corporation, 33 Mori Bldg., 3-8-21 Toranomon, Tokyo 105-0001, Japan Available online 13 October 2007
Abstract Top-emission organic light emitting devices (TE-OLEDs) and transparent OLEDs (TOLEDs) can be fabricated by employing a transparent cathode with a low work function instead of Mg:Ag or Al:Li films. However, most transparent conductive oxides (TCOs) have a high work function (WF). Therefore, we proposed a new sputtering method for fabricating a novel transparent conductive film. Generally, the sputtering process is a plasma process involving high-energy particles. It cannot avoid bombardment-induced damage to the organic layer. We fabricated ITO:Cs films and evaluated the damage to the organic layers. The turn-on threshold voltages of TOLEDs with such low WF (4.1 eV) ITO:Cs films decreased from 8.5 V to 3.5 V and higher efficiencies were obtained; however, large leak currents were observed. © 2008 Published by Elsevier B.V. Keywords: Indium-tin-oxide; Top emission; Organic light emitting device; Low work function
1. Introduction In the future, a display will be integrated into various types of devices or instruments. The following properties are considered to be essential for satisfying these requirements: “mechanical flexibility,” “light weight,” and “robust integration on a plastic substrate or thin metal sheet.” Top-emission organic light emitting devices (TE-OLEDs) [1] and transparent OLEDs (TOLEDs) [2] technologies should be particularly useful for more advanced types of applications that realize a transparent (not interfering with the visibility and function of the original device) and highresolution (high aperture) active-matrix display with a larger number of thin film transistors (TFTs) on the bottom side. In order to realize such displays, it is necessary to develop a new lowdamage film fabrication technique for a plastic substrate and an
⁎ Corresponding author. Tel.: +81 46 242 9618; fax: +81 46 242 3000. E-mail address: [email protected] (T. Uchida). 1 Present address: Sendai National College of Technology, 4-16-1 Ayashichuo, Aoba-ku, Sendai 989-3128, Japan. 0040-6090/$ - see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.10.042
organic layer since both are very sensitive to thermal and mechanical (e.g., ion bombardment) process conditions. In the first report on TOLED [2], the transparent top electrode was deposited by a low-power RF magnetron sputtering method to avoid potential damage. Accordingly, the crystallinity of transparent electrode films inevitably was amorphous in most of the cases. On the other hand, most of the basic studies on TCOs concentrated only on their electrical conductivity and optical transmission or on their poly-crystallization, which requires annealing (heat treatment). In order to study the applications of TE-OLEDs and TOLEDs, we must first study amorphous TCOs formed by a low-damage sputtering process. This process can be used to form a TCO film having a high electron injection characteristic that can be used as a cathode; the underlayer remains undamaged during this process. We propose a new sputtering method for fabricating a novel transparent conductive film using Cs-incorporated RF magnetron sputtering [3]. The main purpose was to form a low work function TCO with high electron injection. Several other methods have been reported for fabricating such films, e.g., a method for inducing a redox reaction between TDAE of the electron-donor molecules
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T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
and ITO [4], a method using LaB6 [5], and a method for cosputtering C12A7 and ITO [6]. Cs has been used to negatively ionize a target material by lowering the work function of the target surface and generating high-density plasma at a lower sputtering gas pressure. The negatively ionized sputtered particles are accelerated by the voltage between the target and the substrate and they improve film qualities such as adhesion, density, or surface smoothness by their energy [7]. In contrast, our aim was to incorporate Cs in the film, although it had been introduced in the sputtering process. We have shown that the electronic injection improved and the driving voltage decreased in TOLEDs. Cs was shown to exist in the film with high stability [8]. Here, we examined the damage caused by this technique to an organic film since it appears to be possible to cause damage by using high-density plasma and kinetically energetic particles. The ITO and ITO:Cs film of the OLED being used for comparison was deposited by the typical RF magnetron sputter method. We then evaluated the characteristics of the OLED using an ITO:Cs film and evaluated the damage to the organic layer by comparing the degradation of PL intensities of BAlq (bis(2-methyl-8-quinolinolato)(p-phenylphenolato) aluminum) with an ITO:Cs film and BAlq with a standard ITO film. The leakage currents of the devices in the forward and reverse directions were measured as well. 2. Experimental 2.1. Comparison of the photoluminescence intensities of BAlq with an ITO:Cs film and BAlq with a standard ITO film For comparing the photoluminescence intensities, 40-nm BAlq's were deposited on glass substrates (Corning 7059) and
then transparent TCO films (ITO and ITO:Cs) were fabricated. During the formation of the ITO:Cs film, a Cs dispenser (Seas Getters) was heated to eject Cs vapor in the sputtering chamber. The details are described in another paper [8]. The PL intensities were measured using an excitation wavelength of 370 nm (PL measuring instrument (Horiba Jobin Yvon, FluoroMax-P)). The schematic diagram of the PL measurement is shown in the upper right in Fig. 1. 2.2. Transparent organic light emitting devices (TOLEDs) with a standard ITO film or an ITO:Cs film as a transparent cathode TOLEDs with ITO and ITO:Cs films were fabricated according to the following procedure. First, 70 nm of poly (styrenesulfonate)/poly(2,3-dihydrothieno (3,4-b)-1,4-dioxin) PEDOT-PSS was spin-coated on both ITO-coated (anode side) glass substrates and bis[N-(1-naphthyl)-N-phenyl]benzidine (NPB) as a hole transport layer and tris-(8-hydroxyquinoline) aluminum (Alq3) (40 nm) as an electron transport and emissive layer in that order. Then, 10 nm of 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP) was deposited by evaporation as an electron injection layer. Finally, two different types of TCO films (ITO and ITO:Cs) were deposited by sputtering. 3. Experimental result and discussion 3.1. PL intensities of BAlq with an ITO film and BAlq with an ITO:Cs film Before conducting intensity measurements as a damage indicator, we examined the time dependence of the fluorescence intensities of BAlq in the atmosphere. In consecutive measurements (1 h interval) over 19 h, the intensity was so stable that
Fig. 1. Photoluminescence intensities of BAlq's with Cs-incorporated ITO and standard ITO inset figure (upper right): schematic diagram of PL measurement, (lower bottom): PL spectra before and after sputtering deposition.
T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
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Fig. 2. AFM images of ITO and ITO:Cs and commercial ITO surface. (a) Commercial ITO, (b) ITO film fabricated by low-power sputtering deposition, and (c) ITO:Cs film fabricated by low-power sputtering deposition.
the attenuation was within approximately 5%. Referring to this result, we used the fluorescence intensity of BAlq as a probe for examining the damage to the organic underlayers. Fig. 1 shows the fluorescence intensities of BAlq's with ITO:Cs and ITO films. As shown in the lower left inset diagram, the PL intensities decreased when such TCO films were deposited by sputtering. The ratio of the peak values between the values before and after sputtering (PLA/PLB) shows that the PL intensity was reduced to 45% and 25% by typical and Csincorporated RF magnetron sputtering, respectively. It is necessary to consider the degradation of the material with time and the influence of the optical film on them; however, it is believed that the result mainly reflects the sputtering damage. It has been known that the light emitting region of an actual device is specified to be a several nanometers region very close to the interface of the hole transportation layer. This region doesn't seem to have been damaged. But the balance of injections of holes and electrons has to be considered. Here, the average values of degradation of the entire layers were measured. These values may not precisely reflect the degradation of the actual device; however, it is possible to determine the extent of degradation caused by different sputtering conditions for ITO and ITO:Cs films.
It is assumed that negatively ionized sputtered particles around the surface of target by introducing Cs vapor, strongly impact the organic layer and/or higher density plasma damage the organic layer. Fig. 2 shows AFM images of ITO and ITO:Cs and a commercial ITO surface: (a) commercial ITO, (b) ITO film fabricated by low-power sputtering deposition, and (c) ITO:Cs film fabricated by low-power sputtering deposition. In the case of commercial ITO, crystallization forms a polycrystal with Ra = 2.2 nm. The ITO and ITO:Cs films are amorphous with Ra = 1.19 nm and 1.54 nm, respectively, since they are fabricated by low-power sputtering deposition.
Fig. 3. Transmittance of TOLED at the turn-off state. Glass/ITO/PEDOT-PSS/ NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
Fig. 4. Luminance–voltage characteristics of TOLED. Glass/ITO/PEDOT-PSS/ NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
3.2. Solution for the problems encountered in TCO film formation for TOLED with an ITO:Cs film Considering the sputtering damage caused to the organic layers, methods such as soft deposition with a slow rate or an additional buffer layer have been proposed and the leakage currents of devices have been studied in relation to the damage. In this research, we fabricated TOLEDs using ITO or ITO:Cs films as transparent cathodes and examined their characteristics.
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T. Uchida et al. / Thin Solid Films 516 (2008) 5907–5910
Fig. 5. Current–voltage characteristics of TOLED. Glass/ITO/PEDOT-PSS/NPB/Alq3/BCP/Cathode. (a) Cathode = ITO:Cs, (b) Cathode = ITO.
Fig. 3 shows the transmittance (a basic characteristic) of TOLED in the turn-off state. In the visible region, TOLEDs with ITO and ITO:Cs cathodes had transmittances of 72% and 69%, respectively. The transmittance of an ITO and ITO:Cs film on glass was 90% and 87%, respectively. Cs-incorporated films were slightly brownish. Further, the luminance–voltage characteristics are shown in Fig. 4. The turn-on threshold voltages (luminance: 1 cd/m2) of TOLEDs with TCO films of ITO:Cs decreased from 8.5 V to 3.5 V and the maximum luminance of approximately 1000 cd/ m2 was obtained at 13 V. In this manner, the electronic electron injection characteristics were improved by the incorporation of Cs in ITO, and the drive voltage was decreased. Fig. 5 shows the current density–voltage characteristic. These devices exhibited typical OLED characteristics on the application of a forward bias. In the inset figure, a device with an ITO:Cs film was shown to have 20 times greater leakage current under the application of a reverse bias. This indicates that the leakage current was increased due to the introduction of Cs vapor during sputtering. We could enhance the electron injection by using an ITO:Cs film. However, it attenuated the PL intensity and increased the leakage current due to the damage to the organic layer. Therefore, it is essential to develop technology that avoids damage to the organic layer, for example, adopting a buffer layer. Further, optimization is required in terms of the conductivity, transparency, electron injection, and reduced damage. 4. Summary Transparent conductive films (ITO:Cs films) fabricated by Csincorporated RF magnetron sputtering were formed and analyzed. The turn-on threshold voltages (luminance: 1 cd/m2) of TOLEDs with such low WF (4.1 eV) TCO films were decreased from 8.5 V
to 3.5 Vand higher efficiencies were obtained. These films will be good candidates for transparent cathodes in TOLEDs. However the current density–voltage characteristic showed that these devices having ITO:Cs films have larger leakage current from the device with ITO:Cs under the application of reverse bias. This indicates that the leakage current is increased due to the introduction of Cs vapor during sputtering. Although we can enhance the electron injection by using an ITO:Cs film, this causes attenuation of the PL intensity and increases of leakage current due to the damage caused to the organic layer. Acknowledgment This work has been partly supported by the Grant-in-Aid for Scientific Research (C) No. 19560357 of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) from 2007 to 2008. References [1] Sasaoka, M. Sekiya, A. Yumoto, J. Yamada, T. Hirano, Y. Iwase, T. Yamada, T. Ishibashi, T. Mori, M. Asano, S. Tamura, T. Urabe, SID'01, Dig., 2001, p. 384. [2] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 68 (1996) 2606. [3] T. Uchida, T. Mimura, S. Kaneta, M. Ichihara, M. Ohtsuka, T. Otomo, Jpn. J. Appl. Phys. 44 (2005) 5939. [4] W. Osikowicz, X. Crispin, C. Tengstedt, L. Lindell, T. Kugler, W.R. Salaneck, Appl. Phys. Lett. 85 (2004) 1616. [5] C. Jian-bo, J. Quan, L. Zu-lun, C. Wen-bin, Y. Gang, R. Hai-bo, Q. Cheng, Proc. SPIE Int. Soc. Opt. Eng. 5519 (2004) 250. [6] D.-H. Yoon, International Symposium on C12A7 and Nanoporous Materials, 2007. [7] N.-W. Paik, S. Kim, Rev. Sci. Instrum. 73 (2002) 1212. [8] T. Uchida, T. Mimura, M. Ohtsuka, T. Otomo, M. Ide, A. Shida, Y. Sawada, Thin Solid Films 496 (2006) 75.