APPLIED PHYSICS LETTERS 92, 023306 共2008兲
Transparent organic light-emitting diodes consisting of a metal oxide multilayer cathode Seung Yoon Ryu AMOLED Business Team, Samsung SDI, Co., Ltd., 428-5 Gongse-Dong, Kiheung-Goo, Yongin-City, Gyeonggi-Do 449-577, Republic of Korea and Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
Joo Hyon Noh, Byoung Har Hwang, Chang Su Kim, Sung Jin Jo, Jong Tae Kim, Hyeon Seok Hwang, and Hong Koo Baika兲 Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
Hee Seong Jeong, Chang Ho Lee, Seung Yong Song, Seung Ho Choi, and Si Young Park Samsung SDI, Co., LTD, 428-5 Gongse-Dong, Kiheung-Goo, Yongin-City, Gyeonggi-Do 449-577, Republic of Korea
共Received 17 October 2007; accepted 19 December 2007; published online 16 January 2008兲 The authors have developed a semitransparent, multilayered cathode of indium tin oxide 共ITO兲/Ag/ tungsten oxide 共WO3兲 for transparent organic light-emitting diodes. The device showed a weak negative differential resistance 共NDR兲, until the operating voltage of 8 V was reached. NDR was due to the resonant tunneling by both the quantum barrier and quantum well. The silver oxide 共Ag2O兲 on the Ag metal was confirmed by x-ray photoelectron spectroscopy, and the energy levels of Ag2O were quantized due to the quantum size effect and this produced the resonant tunneling channels. The device using ITO/ Ag/ WO3 with a LiF / Al bilayer was superior to those devices which only used ITO or WO3, mainly because the out coupling was enhanced by employing a WO3 material, which is much more transparent than ITO. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2835044兴 Various displays such as those in mobile phones are based on organic light-emitting diodes 共OLEDs兲,1 and these have recently appeared on the current market. The size of the OLED displays is supposed to expand in the early future to include their incorporation into large television screens. But, it is strongly related to the active matrix’s array within the thin film transistor 共TFT兲 and it should avoid the complexities of TFTs and low aperture ratios. Therefore, transparent OLEDs 共TOLEDs兲 共Ref. 1兲 for top emission have been studied with various structures using a transparent cathode to overcome the TFTs’ interference by light from devices. Until now, Mg:Ag has been commonly used in TOLED. However, mainly because a Mg:Ag cathode should be handled carefully due to its reactivity,2 many researchers have focused on a thin multilayer coating for its low resistance and optically improved transmittance.3–6 Ryu and Baik have already created a TOLED using an indium tin oxide 共ITO兲/Ag共metal兲/ ITO 共IMI兲 cathode with low resistance.2 However, an IMI cathode showed low transmittance due to the low substrate heating through e-beam deposition,2,6,7 and an ITO/Ag/ tungsten oxide 共WO3兲 共IAW兲 for higher transmittance was used in this instance. Saitoh and Hiramoto8 reported the carrier transport mechanism of the ultrasmall silicon quantum dot in a singleelectron transistor with a double barrier structure 共DBS兲. The gate oxides and the silicon quantum dot had a role on the quantum barrier and quantum well. It is well known that the effects of discrete quantum levels affect the variation of the tunneling rates to each level for current injection, which induces the negative differential resistance 共NDR兲 共Refs. 9–12兲 due to the change in the energy levels. Resonant tunneling a兲
Author to whom correspondence should be addressed. Electronic mail:
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diodes 共RTDs兲 共Refs. 9–12兲 are found when the energy of the quisebound state of the quantum well matches that of an incident electron. In the IAW cathode, semiconducting WO3 with a band gap of 2.7 eV was selected as a outside quantum barrier and a transition metal oxide to increase a number of interesting optical and electrical properties.13 Silver oxide14 共Ag2O兲 of the surface on Ag can also usually show semiconducting and quantized properties because of the approximately nanometer range and the quantum size effect.15 There are also various methods to make the thin film of Ag2O, including substrate heating and oxygen flow.14 Previous works on electronic structures and the optical properties of Ag2O films showed that Ag2O had a band gap of ⬃1.3 eV and an ionization potential of ⬃5.3 eV.14 The authors have studied the characteristics of an IAW multilayer cathode with resonant tunneling using DBS on the polymer light-emitting diodes 共PLEDs兲. We adopted an OLED structure of ITO 共150 nm兲/poly共styrene sulfonate兲doped poly共3,4-ethylene dioxythiophen兲 共PEDOT:PSS兲 共40 nm兲/poly共9,9-dyoctilfluorene兲 共PFO兲 共60 nm兲 / LiF 共0.6 nm兲 / Al 共6 nm兲 / ITO 共40 nm兲 / Ag 共12 nm兲 / WO3 共40 nm兲. PEDOT:PSS and PFO were spin coated as a hole transport layer and an emitting layer, respectively. All of the metal, including LiF, were deposited by e-beam evaporation in a vacuum process. The concrete process of experiment and measurement for such devices was explained in our previous paper in detail.2,6 The transmittances at 550 nm and the sheet resistances of various multilayer cathodes are shown in Table I. The Al 20 nm showed a transmittance of 43% and a sheet resistance 共Rs兲 of 13 ⍀ / 䊐. The IMI, WO3 / Ag/ WO3 共WAW兲, SiO2 / Ag/ SiO2 共SAS兲, and IAW exhibited a transmittance of 27%, 90%, 68%, and 40%, respectively. Although the multilayer consisted of a single layer with poor transmit-
0003-6951/2008/92共2兲/023306/3/$23.00 92, 023306-1 © 2008 American Institute of Physics Downloaded 18 Mar 2010 to 216.96.184.125. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
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TABLE I. Transmittance and sheet resistance of various cathodes.
Conditions
Transmittance 共% at 550 nm兲
Sheet resistance 共⍀ / 䊐兲
Al 20 nm ITO 40 nm/ Ag 12 nm/ ITO 40 nm WO3 40 nm/ Ag 12 nm/ WO3 40 nm SiO2 40 nm/ Ag 12 nm/ SiO2 40 nm ITO 40 nm/ Ag 12 nm/ WO3 40 nm
⬃43 ⬃27 ⬃90 ⬃68 ⬃40
⬃13 ⬃6 ⬃6 ⬃6 ⬃6
tance, they showed little decrease in transmittance because there is a light pathway due to multiple reflections.3–5 However, all the Rs results of the multilayer are nearly the same. It indicates that the dominant factor of Rs in a multilayer is the Ag thin film, even though the carrier mobility of ITO is superior to those of WO3 and SiO2. Assuming that the total resistance results from the resistances of the three single layers coupled in parallel, it is possible to calculate the specific resistivity of the metal layer. This assumption is justified if film boundary effects are negligible,3–5 1 RTotal
=
1 RITO
+
1 1 + . RAg RWO3
共1兲
We investigated the x-ray photoelectron spectroscopy for the surface of the reference Ag thin film, and for Ag on ITO. The binding energy of Ag, Ag2O, and AgO in Ag 3d5/2 is 368.2, 367.7, and 367.1 eV, respectively.16,17 In Fig. 1共a兲, obviously, the plot represents that the dominant composition of the Ag reference and the Ag on ITO, is Ag2O. This is mainly not only because of the substrate being heated to 100 ° C and the rates of oxygen flow for the ITO evaporation process, but also because of the native oxide.7,14 However, the atomic concentration of Ag2O in Ag on ITO is higher than that in the Ag reference, which means there is a thicker Ag2O film in Ag on ITO than that in that of the Ag reference.
FIG. 2. 共Color online兲 I-V Plot of devices with multilayer cathodes deposited by e-beam evaporation. The inset shows the current density versus a 쎻, b and 쎻 c indicate the starting region, the peak region, applied voltage. 쎻, and the valley region in I-V characteristics, respectively. 共The plots of Al and IMI cathode came from Ref. 2.兲
Moreover, when the thin film of semiconductor is formed in nanoscale, the energy level can be quantized by the quantum size effect.15 However, the tiny thickness of Ag2O in the Ag reference and Ag on ITO is thought to be similar. Saitoh and Hiramoto et al.8 have already showed that the ultrasmall silicon quantum dot can be formed with this the quantized energy level. Consequently, the ultrathin Ag2O makes the discrete quantized energy level and controls the resonant tunneling rate in a metal oxide multilayer cathode. Figure 1共b兲 shows the data of Auger electron spectroscopy 共AES兲 regarding the depth profile of various metal oxide multilayer cathodes. All of the 共a兲 IMI, 共b兲 WAW, and 共c兲 IAW cathodes show that in the region of the interface between the metal oxide and Ag, there are mixing zones where the overlapping of O and Ag peaks at the interface, even though there is some diffusion by sputtering kinetic energy.13 Nevertheless, the level of overlapping of the elements is nearly same, which indicates there are similar adequate thicknesses of Ag2O in metal oxides, whether oxygen flow and substrate heating are used or not. Subsequently, the double Ag2O can be thought to form on both sides between metal oxides and the Ag thin film, and quantized similarly.15 Lee and Dickinson18 reported that the energy level of the Ag nanocluster, including two or three layers, could be quantized, whereas different nanoclusters have different energy levels.15 However, the thin Ag film seems over the 8 nm in AES depth profile, which means that the Ag thin film should not be theoretically quantized by the quantum size effect. In Fig. 2, even though we adopt the WAW, SAS, and IAW multicathodes, there is the same tendency for the current injection of resonant tunneling, due to the quantum barrier and quantum well, because the quantized energy level of Ag2O by the quantum size effect determines the NDR.8 In the case of SiO2, almost minimal current seems to flow into the device because of the low conductivity and charge densities for the current flow. However, Kim and Shin19 reported that the conductivity enhancement was achieved by the introduction of zinc cations into the amorphous silica layer. That means that if we promptly handled the SAS cathode, we could gain a better current injection in our TOLED. Even though the WAW cathode has the higher transmittance, the current injection is poor because of the low charge density
FIG. 1. 共Color online兲 共a兲 High resolution XPS Ag 3d spectrum obtained from the Ag sample and the Ag surface on ITO with oxygen flow and the substrate heating until 100 ° C. 共b兲 Auger depth profiles for IMI, WAW, and IAW. Downloaded 18 Mar 2010 to 216.96.184.125. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
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FIG. 4. 共Color online兲 Energy band diagram of the resonant tunneling diode using IMI cathode 共a兲 at the starting region, 共b兲 at the peak region, and 共c兲 at the valley region in I-V characteristics.
FIG. 3. 共Color online兲 V-L Characteristics and efficiencies of devices with the multilayer cathode deposited by e-beam evaporation: 共a兲 L-V curves of the devices in bottom emission; 共b兲 Brightness-current density curves of the devices in bottom emission; 共c兲 L-V curves of the devices in top emission. 共d兲 Brightness-current density curves of the devices in top emission. 共The plots of Al and IMI cathode came from Ref. 2.兲
and mobility. Except for the SAS cathode, in the region over 8 V, all of them exhibited the current injection of trappedcharge-limited 共TCL兲 currents and space-charge-limited currents conventionally.20 It indicates that there are two separate mechanisms for resonant tunneling current injection; one for the low voltage region and one for TCL currents for the high voltage region, respectively. WO3 is known to be an n-type semiconductor via oxygen vacancy. WO3 is also known to be governed by the hopping conduction mechanism in which the electrons are localized and hop between metal ions in different valence states.13 Nevertheless, the WAW device shows poor current injection due to the lower mobility and charge density relative to those of the ITO. The IAW cathode also shows poor current injection because of the low characteristic of the current injection. Figure 3 depicts that the luminance of the IAW is higher than for any other devices because the radiative exitons can be survived by the out coupling of WO3 through its more transparent optical properties, even though the current injection is still lower than that of the IMI and Al cathode due to the low carrier mobility. The efficiency of the device for IAW is much higher than that of others. However, in the IAW cathode, the ITO supports the current injection from the cathode into the PFO for the electrical property, and WO3 adjusts the out coupling through the multiple reflection and higher transmittance than that of the ITO. Subsequently, the IAW cathode is the most effective for the performance of the TOLED in terms of electrical and optical properties. The authors have already examined the performance of the small molecule devices with the IMI cathode, which still showed the resonant tunneling. However, due to the substrate heating, the performance was not as good as that of the PLED.2 Figure 4共a兲 shows that when the applied voltage is low, only one aligned state of the quantum well is accessible for electrons due to a large energy barrier. It is only available in the low voltage region because in the high voltage region, the TCL currents are dominant. When the voltage is increased substantially and the first aligned and next excited states are lined up with the Fermi level in the emitter, a new current channel through the next aligned excited state also
opens up, as shown in Fig. 4共b兲. At this point, the lowest unoccupied molecular orbital level of the PFO is also much more bent through the thin LiF / Al than that of the starting region of the I-V characteristics.2 Figure 4共c兲 shows that when the voltage is further increased and the first aligned state in the well goes below the conduction band edge in the emitter, the currents channel, which has contributed to electron transport, is cut off. This phenomenon, resonant tunneling through the quantized and aligned energy level in the quantum well, is the origin of the abrupt current decrease, or NDR.2,8 In order that such an NDR may appear, the energy barrier must be comparable with, or larger than, the Fermi energy in the emitter reservoir.2 Because the potential barrier is decreased with a LiF / Al layer around the valley region of the I-V characteristics, the increase of current can be explained by the charge tunneling dipole model.21–23 In summary, the TOLEDs of the IAW, WAW, and SAS cathodes were performed with resonant tunneling, with a quantum barrier of metal oxide, and a quantum well of Ag. The Ag2O on the surface of the Ag was quantized by the quantum size effect, and dominated the quantum tunneling rates. It induced the NDR in the RTDs. The authors offer special thanks to Professor Jun Yeob Lee from Dankook University for his valuable advice. This work was supported by the Brain Korea 21 共BK21兲 fellowship program at Korea’s Ministry of Education. M.-H. Lu and M. S. Weaver, Appl. Phys. Lett. 81, 3921 共2002兲. S. Y. Ryu and H. K. Baik, Appl. Phys. Lett. 91, 093515 共2007兲. 3 M. Bender and W. Seelig, Thin Solid Films 326, 67 共1998兲. 4 A. Kloppel and A. Scharmann, Thin Solid Films 365, 139 共2000兲. 5 J. Lewis and S. Grego, Appl. Phys. Lett. 85, 3450 共2004兲. 6 S. Y. Ryu and H. K. Baik, Appl. Phys. Lett. 90, 033513 共2007兲. 7 I. Hamberg and C. G. Granqvist, J. Appl. Phys. 60, R123 共1986兲. 8 M. Saitoh and T. Hiramoto, Appl. Phys. Lett. 79, 2025 共2001兲. 9 T. J. Park, J. H. Kwon, and J. Jang, Appl. Phys. Lett. 89, 151114 共2006兲. 10 V. J. Goldman and D. C. Tsui, Phys. Rev. Lett. 58, 1256 共1987兲. 11 F. W. Sheard and G. A. Toombs, Appl. Phys. Lett. 52, 1228 共1988兲. 12 T. J. Foster and G. A. Toombs, Phys. Rev. B 39, 6205 共1989兲. 13 K. H. Yoon and D. H. Kand, Appl. Phys. Lett. 68, 572 共1995兲. 14 C. W. Chen and C. C. Wu, Appl. Phys. Lett. 83, 5127 共2003兲. 15 R. Rossetti and R. Hull, J. Chem. Phys. 82, 552 共1984兲. 16 G. B. Hoflund and Z. F. Hazos, Phys. Rev. B 62, 11126 共2000兲. 17 J. B. Han and Q. Q. Wang, Mater. Lett. 60, 467 共2006兲. 18 T. H. Lee and R. M. Dickson, Proc. Natl. Acad. Sci. U.S.A. 100, 3043 共2003兲. 19 S. W. Kim and Y. W. Shin, Thin Solid Films 437, 242 共2003兲. 20 P. E. Burrows and S. R. Forrest, J. Appl. Phys. 79, 7991 共1996兲. 21 S. T. Zhang and X. Y. Hou, Appl. Phys. Lett. 84, 425 共2003兲. 22 X. J. Wang and J. M. Zhao, J. Appl. Phys. 95, 3828 共2004兲. 23 J. M. Zhao and X. Y. Hou, Appl. Phys. Lett. 84, 2913 共2004兲. 1 2
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