WO3 Multilayer As Transparent Cathode

Feb 3, 2011 - We investigated the optical properties of a dielec-tric−metal−dielectric multilayer for the transparent top cathode in top-emitting ...
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Optical Properties of WO3/Ag/WO3 Multilayer As Transparent Cathode in Top-Emitting Organic Light Emitting Diodes Kihyon Hong,† Kisoo Kim,† Sungjun Kim,† Illhwan Lee,† Hyunsu Cho,‡ Seunghyup Yoo,‡ Ho Won Choi,§ Nam-Yang Lee,§ Yoon-Heung Tak,§ and Jong-Lam Lee†,* †

Division of Advanced Materials Science and Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk, 789-784, Korea ‡ Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701,Korea § LG Display Inc., Gumi, Gyungbuk, 730-030, Korea ABSTRACT: We investigated the optical properties of a dielectric-metal-dielectric multilayer for the transparent top cathode in top-emitting organic light emitting diodes (TOLEDs). The optical transmittance of the metal layer was enhanced by depositing a dielectric material which had a high refraction index n below and above the metal (Ag) layer. Due to multiple reflections and interferences, the Ag layer sandwiched between dielectric materials with a high value of n can show improved transmittance. Because the WO3 had a high value of n (>2.0), a thin WO3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO3/Ag/WO3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO3 to obtain the best transmittance value were determined by theoretical calculation, and they agreed well with the experimental results. The best results were obtained for the thermally evaporated WO3 (300 Å)/Ag (120 Å)/WO3 (300 Å) structure, a high transmittance of ∼93.5% and a low sheet resistance about ∼7.22 ohm/sq were obtained. When the top Al cathode was replaced with the WO3/Ag/WO3 multilayer, the maximum luminance value (J = 220 mA/cm2) increased from 8400 to 11700 cd/m2, and the power efficiency increased about 26%. To improve the electron injection efficiency at the cathode region, a 20-Å thick Al layer was introduced as an electron injection interlayer between the organic materials and the WO3/Ag/WO3 cathode. Using the Al interlayer decreased the operation voltage at J = 10 mA/cm2 by 6.9 V. Thus, a WO3/Ag/WO3 with an Al interlayer could promote the transparency of the top cathode and lower the electron injection barrier, enhancing the electroluminescent properties of TOLED.

1. INTRODUCTION Organic light emitting diodes (OLEDs) have shown potential in applications such as displays and general illumination due to their steadily improved efficiency, superior color balance, and high brightness.1-5 Top-emitting organic light emitting diodes (TOLEDs), featuring a bottom reflective anode, are more preferable for device integration due to the use of more complicated pixel circuits without sacrificing the aperture ratio of the pixels.6,7 Because the light generated by the recombination of holes and electrons is coupled out via the top cathode, formation of a high transparency cathode is one of the most problematic processes for high efficiency TOLEDs. Thus, serious attempts have been made by several research groups to develop top cathode materials having high transparency, low resistivity, and low work function. The most common transparent cathode materials are thermally evaporated metal or metal oxide films.8-10 Using a semitransparent Ag top electrode could induce the microcavity effect, improving the optical properties of TOLEDs about 35%.8 However, due to the high work function (∼4.7 eV) of Ag, the device showed a relatively high operation voltage (>10 V).9 Additionally, the microcavity effect induced by the semitransparent Ag cathode resulted in a narrow electroluminescence (EL) spectrum. Thus, Ag is not appropriate for high efficiency white OLEDs. To r 2011 American Chemical Society

enhance the transparency of the cathode, aluminum (Al) doped SiO was adopted as a top cathode for TOLEDs. Due to the high transmittance of SiO, the cathode showed a high transmittance of >50%.10 However, even though the Al, which has low resistivity (∼10-8 Ωcm), was doped into SiO, the cathode showed poor resistivity (∼10-5 Ωcm) due to the high resistivity of SiO, resulting in a high turn-on voltage (∼12 V) and low current density. When the thin Ag layer was deposited on an alkali metal surface such as lithium, calcium, or barium, the metal films had a high transmittance value (>70%) and low sheet resistivity (∼10 Ω/sq). Thus, TOLEDs with a Ba/Ag top cathode can have a low turn-on voltage (∼4 V) and high luminance (>10 000 cd/m2).11,12 However, these alkali metals are always susceptible to atmospheric oxidation, inducing the formation of dark spots and degradation of the lifetime during the operation. These problems have been addressed by depositing layers of transparent conducting oxides (such as indium tin oxide (ITO)) using radio frequency sputtering to improve the cathode transparency, but the high energy ITO sputtering process degrades Received: October 17, 2010 Revised: December 19, 2010 Published: February 03, 2011 3453

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Figure 1. (a)-(c) Admittance diagrams of dielectric, metal, and DMD multilayers. n and k indicated the refractive index and extinction coefficient of metal layer. (d) Schematic diagram of the TOLEDs with different cathode structures (100 Å-thick-Al and WO3/Ag/WO3 multilayer).

the organic materials undernearth.13 The sputtering damage on organic materials can be reduced by using the facing target sputtering (FTS) method.14 However, even if the FTS shows good progress in reducing the level of sputtering damage, it might be not enough to solve the structural problems of TOLEDs because of the low electron injection efficiency induced by the high work function of ITO (∼4.8 eV) and the incomplete shielding of the high energy charged particles.15 It is theoretically possible to design a transparent thin metal layer if the refractive index nMetal of the metal film is zero because the photon energy absorbed by a layer is proportional to nkd/λ where λ is the wavelength of incident light, k is the extinction coefficient, and d is the thickness of the metal film.16 Because there is no such ideal metal, we can choose an Ag film with the lowest value of n (nAg = 0.05-i2.90 in the visible region).17 Also, Ag has the lowest absorption in the visible spectrum among those metal films. The optical transmittance of this Ag film can be enhanced by using matching layers of dielectric films on the substrate side and the air side of the Ag layer (dielectric/metal (Ag)/dielectric structure). We can optimize the optical transmittance of this dielectric/meta/dielectric (DMD) multilayer using an admittance diagram technique as shown in Figure 1(a)-(c).17,18 The admittance of the layer system starts from the substrate (nsub,0), which is glass, and ends at the air (1,0) to obtain zero reflection condition. Figure 1(a) shows the locus of a dielectric film with index nD which is deposited on a glass substrate of refractive index nsub. The starting point is (nsub,0) and as the thickness is increased to a quarter-wave, a semicircle is traced out clockwise which reintersects the real axis in the point (nD2/nsub). A second quarter wave completes the circle. Increasing the thickness of the dielectric layer resulted in the rotation of admittance on this circle. Because the metal has an imaginary part of the refractive index, the admittance diagram of metal film is somewhat distorted from the ideal case, with a loop bowing out along the direction of the real axis (Figure 1(b)). Because the transmittance of film increased by decreasing the distance from admittance to the air (1, 0), increasing the metal layer thickness decreased the transmittance of the metal film. Figure 1(c) shows the admittance diagram of the DMD layer

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system. This DMD structure was used to reduce the distance from the admittance to the air (1,0) and to obtain a higher optical transmittance than that of the metal layer. As shown in Figure 1(c), high transmittance requires a thinner Ag film. However, the Ag film must not be too thin to avoid a discontinuous-film problem and poor electrical conductivity. However, the Ag thickness can be increased while maintaining high transmittance if dielectric materials with high refractive indices are used.19-21 Recall that the diameter of the circle in the admittance diagrams will be larger with higher n. For this reason, it is known that a DMD multilayer with high refractive index dielectric layer has an advantage in that it can fulfill the optimum “zero-reflection” condition with relatively thick Ag film, realizing both improved transparency characteristics (>80%) and low sheet resistance.22,23 The ITO/Ag/ITO and ZnS/Ag/WO3 multilayers have been used as transparent bottom anodea for bottom-emitting OLEDs, giving comparable optical properties to those of ITO-based devices.23,24 An ITO/Ag/ITO multilayer has been adopted as a cathode for transparent OLEDs. Although it showed moderate optical and electrical properties as a transparent cathode, the ITO had to be deposited by e-beam evaporation or sputtering which negatively degraded the performances of organic materials in the device.25 To prevent the degradation of organic materials, the WO3/Ag/WO3 (WAW) multilayer, which can be deposited by thermal evaporation, has been employed as a cathode for transparent OLEDs.26 No works on applications of thermally evaporated DMD multilayers as transparent top cathodes of TOLEDs, however, have yet been conducted. In this work, we study the enhancement of optical properties of TOLEDs with a thermally evaporated WAW as a transparent top cathode based on the framework of thin film optics. The application of WAW to a top cathode for TOLEDs degraded the electrical properties of devices, due to the poor electron injection ability of the WO3.26 Because the melting point of WO3 (1473 °C) is much higher than that of organic materials, hot WO3 atoms deposited on an organic layer can diffuse through the surface, forming defect states.27,28 Thus, a buffer layer which can improve the electron injection efficiency and prevent the diffusion of WO3 atoms is required. We employed a thin Al film (20 Å) as a buffer layer between the WAW cathode and the organic layer. Because of the low work function (∼4.3 eV) of Al, it could act both as an electron injection layer and as a buffer layer for WO3 diffusion. The optimal WAW with Al interlayer electrode structure was predicted by optical simulation. The improvement of electron injection efficiency was examined by synchrotron radiation photoelectron spectroscopy (SRPES) and ultraviolet photoemission spectroscopy (UPS). From these, the optical characteristics of WAW and the effect of WAW on the improvement of the electroluminescence of TOLEDs are discussed.

2. EXPERIMENTAL SECTION The schematic structure of TOLEDs is shown in Figure 1(d). Glass was used as the starting substrate. The substrate was cleaned with acetone, iso-propyl alcohol, and deionized water and then dried with high purity nitrogen gas. After cleaning process, the substrates were loaded into a thermal evaporator and Ti/Ag (150/1000 Å) was deposited as an anode. The substrate was then transferred into the treatment chamber, and then exposed for 10 min to oxygen ambient with a partial pressure of 700 mtorr under ultraviolet. The treated samples were transferred to a thermal evaporator and a 20-Å thick CuO layer 3454

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The Journal of Physical Chemistry C was deposited as a hole injection layer on the anode. Then, 4,40 Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (R-NPD, 700 Å), tris(8-hydroxyquinoline) aluminum (Alq3, 400 Å) doped with the fluorescent dye 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j) quinolizin11one (C545T, 1%), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, 50 Å), Alq3 (200 Å), and LiF (10 Å) were deposited. To improve the electron injection efficiency, a thin Al layer (20 Å) was incorporated before the deposition of the WO3 (300 Å)/Ag/WO3 transparent top cathode (Figure 1(d)). The thickness of the Ag layer was changed from 100 to 250 Å to optimize the optical transmittance and electrical conductivity. The thickness of the outer WO3 layer was also changed from 25 to 500 Å to optimize the optical transmittance. We also fabricated TOLEDs using a thin (100 Å) Al top cathode as a control device during the same process run. During the deposition, the base pressure of the chamber was maintained as low as 10-6 Torr. The active area of the device was 3  3 mm2. The current density-voltage ( J-V) characteristics and luminance (L) of the devices were measured with an HP-4156 A semiconductor parameter analyzer in nitrogen ambient. The transmittance and sheet resistance (RS) were measured using a tungsten-halogen lamp, a monochromator, and a 4-point probe. The surface morphology of the Ag layer was analyzed by atomic force microscopy (AFM). Commercial software (The Essential Macleod, Thin Film Center, Inc.) based on what is known as the characteristic matrix method was employed for the optical analysis involving a multilayer structure. The optical constants of Ag and WO3 were measured by spectroscopic ellipsometry and borrowed from the literature. In order to investigate the chemical bonding states and work function of the WAW, SRPES, and UPS analyses were performed at the 4B1 beamline in Pohang Acceleration Laboratory (PAL). An incident photon energy of 650 eV was used to obtain W 4f, O 1s, and C 1s core level spectra. The secondary electron emission spectra and valence band spectra were obtained in the main chamber using He I excitation (21.2 eV). The onset of photoemission was measured with a negative bias (-20 V) on the sample to avoid the work function of the detector. The incident photon energy was calibrated with the core level spectrum of Au 4f.

3. RESULTS AND DISCUSSION 3.1. Optimization of WAW Structure Using Optical Calculation. Figure 2 shows the calculated contour plots of transmit-

tance (λ = 510 nm) for WO3 (W0)/Ag (A)/ WO3 (W1) and Al/ W0AW1 as a function of W1 and Ag thickness with the W0 thickness fixed at 300 Å. This indicates that the Al interlayer (20 Å) affects the magnitudes of transmittance value of electrodes, but does not substantially alter their dependence on the thicknesses of W1 and Ag layers. Figure 2(a) shows that transmittance of W0AW1 can be optimized to ∼94% with a 330-Å thick W1 layer and 120-Å thick Ag layer. Our calculations showed that using a high refractive-index dielectric layer of thermally evaporated WO3 (n > 2.0) as an index-matching (antireflection) layer could enhance the transmission of the Ag metal layer and this result agrees well with previously reported values.24,25 In the W0AW1 with 20-Å thick Al interlayer, the maximum transmittance value decreased to 86.8% for 270-Å thick W1 and 100-Å thick Ag layers. Although, the 20-Å thick Al interlayer decreased the transmittance of electrode, a high transmittance of over 80% can be expected for the W0AW1 based electrode. Thus, the W0AW1 electrode with Al interlayer should be usable as a transparent top cathode for TOLEDs.

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Figure 2. Calculated contour plots of transmittance for (a) W0AW1 and (b) Al/W0AW1 structures upon variation of the thickness of the outer (W1) and Ag layers for a 300-Å-thick inner WO3 (W0) layer. During the fitting procedure, the thickness of each layer was allowed to vary only within 10% of the nominal thickness.

Figure 3(a) shows the comparison of the measured transmittance and the simulation result for W0AW1 (W0 = 300 Å, Ag = 120 Å) as a function of various outer WO3 (W1) thicknesses. The calculated result indicates that transmittance of W0AW1 increased as outer WO3 (W1) layer thickness increased from 0 to 300 Å. Our calculation results verified this. For the 300-Å thick WO3, the electrode showed a higher measured transmittance (93.5%) than any other thickness. The transparency mechanism and origin of the enhanced optical property of DMD have been dealt with by several research groups from the viewpoint of surface plasmon resonance (SPR) and thin-film optic theory. It was reported that the high index of refraction contrast between Ag and the dielectric layer results in plasmon resonance coupling such that enhancement of visible transparency more than 80% can be achieved.29,30 However, the surface plasmon emission, although often termed “nonradiative” since it is not directly observable, is actually radiative in character and can be coupled out into air by using a Bragg scattering. For this reason, most of the enhanced optical property induced by SPR can occur only at a localized surface plasmon state, including an Ag nanopaticle, a nanodot, and a nanohole.31,32 Because our WAW multilayer is composed of continuous WO3 and Ag films, the increased optical transmittance at a specific WO3 thickness can be explained by an optical out coupling effect based on thin-film optic theory. The optical phase thickness (δ) of thin film can be described as follows:18 2π ndcos θ δ¼ λ 3455

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Figure 4. Tapping-mode AFM height images of Ag films (1  1 μm2) deposited on 300-Å thick WO3 layer for different Ag thickness: (a) 80-Å Ag, (b) 100-Å Ag, (c) 120-Å Ag, and (d) 150-Å Ag. Figure 3. (a) Calculated value (line) of transmittance (λ = 510 nm) for W0AW1 electrode as a function of W1 thickness. A transfer matrix method is used for the calculation. Corresponding results obtained from experiments (symbols) are also shown for comparison. Inset: the optical constants (refractive index, extinction coefficient) of WO3 and Ag used for optical analysis. (b) Calculated (lines) and measured (dots) transmittance of W0AW1 and 20-Å thick Al/W0AW1 electrodes.

where λ is the wavelength of incident light, n is the refractive index of the thin film, d is the thickness of the thin film, and θ is the angle of incident light. When the physical thickness (d) of the film increased, the optical thickness (nd) and optical phase thickness (δ) also increased. When the optical thickness is a multiple of λ/2, δ can be simplified to δ = mπ (m = 0, 1, 2, 3 3 3 ), showing maximum transmittance. Thus, optical transmittance of W0AW1 showed its highest value at a specific W1 thickness. Figure 3(b) shows the calculated and measured transmittance of W0AW1 and Al/W0AW1 as a function of wavelength. The calculation shows high transmittance of over 80% can be expected for the W0AW1 electrode. The measured spectrum is in good agreement with the calculated one, confirming the accuracy of our simulation. For the Al/W0AW1, the calculation showed a decrease of transmittance. The transmittance value (λ = 510 nm) decreased from 92.9% to 82.9% when the 20-Å thick Al interlayer was inserted. The optical constants of WO3 and Ag were measured as a function of wavelength by spectroscopic ellipsometry (inset of Figure 3(a)). Considering the ideal value of WO3 (n = 2.0), the relatively low refractive index (n = 1.9) of WO3 resulted from the low density of film due to the insufficient adatom mobility during the low temperature deposition.33 3.2. Surface Morphology of Ag Film in WO3/Ag/WO3 Electrode. Figure 4 shows AFM topography images of the Ag films deposited on 300-Å thick WO3 as a function of Ag thickness. The root-mean-square (rms) roughness changes from 19.5 to 11.0 Å with increasing Ag thickness. This clearly shows that increasing the thickness of the Ag layer resulted in a transition of the surface shape from islands to a continuous film. The AFM surface analysis showed that the transition thickness of the Ag layer was 120 Å. It has been

Figure 5. (a) Transmittance at a 510-nm wavelength and sheet resistance of WAW multilayer cathode as a function of Ag thickness. (b) Transmittance of WAW, Al (10 Å)/WAW, 100 Å-thick-Al, and 120 Å-thick-Ag electrodes as a function of wavelength.

reported that the surface roughness and morphology of a thin film are important in determining the optical properties of multilayer films. Thus, we can expect that 120 Å is the optimal Ag thickness in the WAW electrode. Our transmittance values verified this. 3.3. Measurement of Transmittance of DMD Multilayer. Figure 5(a) shows the change in measured transmittance (λ = 510 nm) and RS of the WAW multilayer as a function of Ag 3456

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The Journal of Physical Chemistry C thickness. The WAW had a high transmittance of ∼90% in the visible region. The 120-Å thick Ag sample was more transparent (93.5%) than any other samples. The lower transmittance at the thickness less than 120 Å originates from the discontinuous island growth of Ag, leading to scattering losses at the interface of Ag with WO3 (Figure 4).34 However, the transmittance decreases at Ag thicknesses above 120 Å because of the high reflectance of Ag layer. In the case of electrical conductivity, the WAW multilayer showed low RS value (