WO3 multilayer electrode

Organic Electronics 31 (2016) 240e246

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

ITO-free, efficient, and inverted phosphorescent organic light-emitting diodes using a WO3/Ag/WO3 multilayer electrode Shun-Wei Liu*, Tsung-Hao Su, Po-Chien Chang, Tzu-Hung Yeh, Ya-Ze Li, Ling-Jie Huang, Yu-Hui Chen, Chun-Feng Lin Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 October 2015 Received in revised form 6 January 2016 Accepted 22 January 2016 Available online 5 February 2016

We present an indium tin oxide (ITO)-free, bottom-emission inverted phosphorescent organic lightemitting diode (PHOLED) with a maximum luminance of 280,000 cd/m2 at 8 V, total maximum current efficiency of 81.4 cd/A, and external quantum efficiency of 22.4%. The inverted OLED structure is composed of glass/WO3 (30 nm)/Ag (15 nm)/WO3 (5 nm)/BPhen:15wt% CS2CO3 (5 nm)/BPhen (30 nm)/ CBP: 8wt% Ir(ppy)3 (10 nm)/TAPC (50 nm)/WO3 (5 nm)/Ag (150 nm) multilayers. In this device structure, the WO3/Ag/WO3 (WAW) multilayer serving as a transparent cathode demonstrates a low sheet resistance (3.5 U/sq) and high optical transmittance (approximately 80%) in a visible light range of 400 e600 nm; this multilayer was prepared by thermal evaporation to form a relatively smooth morphology of the conductive thin film on the glass substrate. In addition, an electron-only WAW device was subjected to electrical characterization, and the results revealed that this device exhibited a more efficient electron injection property at the WAW/BPhen:CS2CO3 interface than the contact electrode of a standard ITO-based device. © 2016 Elsevier B.V. All rights reserved.

Keywords: Organic light-emitting diodes Transparent multilayer electrode WO3 Ag WAW Electron injection barrier Sheet resistance Atomic force microscopy Transmittance

1. Introduction High-performance organic light-emitting diodes (OLEDs) with conductive multilayer electrodes (CMEs) have received considerable attention because of their high transparency [1], low sheet resistance [2], simplified fabrication process [3], and mechanical flexibility which makes them suitable [4] for optoelectronic devices [5e7]. Regarding CMEs, several dielectric/metal/dielectric (DMD) structures, such as ZnS/Ag/Zns (ZAZ) [8], ZnO/Ag/WO3 (ZAW) [9], ZnO/Ag/MoO3 [10], ZnO/Ag/ZnO/Ag/WO3 [11], WO3/Ag/MoO3 [12], WO3/Ag/WO3 (WAW) [13], and indium tin oxide (ITO)/Ag/ITO [14], have been used in anode electrode applications to provide more efficient hole injection properties than those of standard ITO electrodes. However, additional studies must be conducted on the suitability of multilayer electrodes for various applications for realizing high-performance OLEDs. For example, Cho et al. presented a highly flexible OLED device that was based on a transparent ZnS/Ag/WO3 electrode, and they reported that the

* Corresponding author. E-mail address: [email protected] (S.-W. Liu). http://dx.doi.org/10.1016/j.orgel.2016.01.035 1566-1199/© 2016 Elsevier B.V. All rights reserved.

performance and operational stability of this device were superior to those of ITO-based devices [4]. A transparent OLED device was produced using thermally evaporable WAW [15,16] and MoO3/Ag/ MoO3 [17] as the multilayer cathode, and this device exhibited a total transmission of more than 75% in the visible range. In addition, Liu et al. reported that a thermally deposited ZAZ electrode demonstrated a relatively low sheet resistance of approximately 3 U/sq [18], which was considerably higher than that of an ITO electrode prepared by means of sputter [19] and pulsed laser deposition [20]. Therefore, thermally deposited multilayer electrodes have high potential as alternatives to ITO electrodes in organic optoelectronic devices because ITO electrodes are marred by indium scarcity and a complex fabrication process [21]. Lee et al. suggested that a high-performance inverted OLED with maximum external quantum efficiency of approximately 20% can be fabricated on an ITO electrode [22]. This study may provide an effective solution for n-channel transistors to drive the development of OLEDs because ITO can be used as a bottom cathode for OLEDs and source-drain electrode for organic thin-film transistors. Therefore, developing high-performance inverted OLEDs and efficient TCO is imperative in the field of active-matrix OLEDs that are

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based on n-channel transistors. However, a few studies have suggested that multilayer electrodes prepared through thermal evaporation can be used for developing efficient inverted phosphorescent OLEDs. In this study, we present the use of a WAW transparent cathode with low sheet resistance (approximately 3.5 U/sq) and high optical transmittance (nearly 80%) in a visible light range of 400e550 nm in an inverted, bottom-emission, green phosphorescent OLED. The inverted OLED comprising a WAW cathode exhibited a maximum current efficiency of 81.4 cd/A and external quantum efficiency (EQE) of 22.4%, 1.8 times higher than those of the conventional ITO-based devices. In addition, temperature-dependent electrical characterization and atomic force microscopy (AFM) were used to determine the intrinsic property of the WAW/organic layer interface. 2. Experiments Materials comprising WO3 (99.9%), Ag (99.9%), cesium carbonate (CS2CO3; 99%), 4,7-diphenyl-l,10-phenanthroline (BPhen), 4,40 N,N’-dicarbazole-biphenyl (CBP), 1,1’-bis-(4-bis(4-methyl-phenyl)amino-phenyl)-cyclohexane (TAPC), and phosphorescent green dye of fac-tris(2-phenyl-pyridine) iridium [Ir(ppy)3] were purchased from SigmaeAldrich, whereas all organic materials were purified twice in a home-made vacuum system according to standard operating procedures documented in the literature [23]. The ITO was used as reference substrates and purchased from Luminescence Technology Corp. We evaluated the electrical and electroluminescence characteristics of the device with the structure glass/ WO3 (30 nm)/Ag (15 nm)/WO3 (5, 10, 20, and 30 nm)/BPhen:15wt% CS2CO3 (5 nm)/BPhen (30 nm)/CBP: 8wt% Ir(ppy)3 (10 nm)/TAPC (50 nm)/WO3 (5 nm)/Ag (150 nm). An ITO anode (10 U/sq) was also fabricated as a reference device with the structure glass/ ITO(150 nm)/TAPC (50 nm)/CBP: 8wt% Ir(ppy)3 (10 nm)/ BPhen(30 nm)/BPhen: 15wt% CS2CO3 (5 nm)/Ag(100 nm)] for comparing the performance of the WAW device. The thicknesses of each layer were calibrated using a surface profiler (Dektak XT) before device fabrication. The substrate was cleaned in an ultrasonic bath in successive solutions of acetone and isoproponal, etched in a dilute solution of sulphuric acid, and finally dried blowing with 5N nitrogen. Subsequently, organic and metal layers were continuously deposited with a shadow mask on the bare glass substrate. The active area of the fabricated device was 4 mm2, and the devices were encapsulated in a glove box. Fig. 1 shows the configuration, energy level, and chemical structure of the fabricated

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device. The current density-voltage-luminance characteristics of the OLED devices were measured using a dc current/voltage source meter (Keithley 2400), whereas the brightness was monitored with a spectrophotometer (Photo Research PR655). To evaluate the electron injection barrier, temperature-dependent electrical measurement was performed in a cooled cryostat (Janis VPF-100) connected to a cryogenic temperature controller (Lake Shore 335) from 270 to 340 K to measure the current density of electron-only devices. Our previous study presented a more detailed description of the setup [24,25]. The AFM images of transparent conductive oxide electrodes on an n-type Si wafer were analyzed by a Park System XE-70 with a non-contact mode by using a NCHR tip cantilever. The workfunction of WAW was measured using a photoelectron spectrometer (Riken Keiki AC-2). The transmission and reflection spectra were measured with a UVevisible spectrophotometer (Thermo Scientific Evolution 220). The optical constants for neat WO3 and Ag were measured by ellipsometry (Raditech SE-950). All measurements in this study were conducted in an air environment. 3. Results and discussion We doped the electron transporting layer of BPhen with 15 wt% Cs2CO3, an alkali metal compound, enabling the reduced Cs atom to form a natural trap in the organic layer. Therefore, even if the fabricated WAW multilayer inherently features a workfunction of 5.65 eV, it can still guide electrons into the organic layer through the Cs trap in BPhen. Hence, to verify that the BPhen: Cs2CO3 organic layer is crucial for the application of WAW in inverted OLEDs, we fabricated WAW on the electrode on one side of the electron-only device and subsequently compared the effects of the BPhen organic layer doped with or without (w and w/o) Cs2CO3 on the electron injection process. Note that the electron-only device configuration is WO3 (30 nm)/Ag (15 nm)/WO3 (5 nm)/BPhen with and without 15 wt% CS2CO3 doping (5 nm)/BPhen (100 nm)/LiF (1 nm)/Al (120 nm), which determined the interfacial barrier at the WAW/organic interface. Fig. 2a indicates that the current in the Cs2CO3-doped device is substantially greater than that in the device w/o Cs2CO3, confirming that Cs2CO3 doping is imperative for the effective application of WAW in inverted OLEDs. To address the injection barrier issue, Fig. 2b and c illustrate a comparative analysis of the temperature dependences and injection barriers in the electron-only device, while the interfacial barrier was estimated from the relationship between Schottky emission current density, J,

Fig. 1. (a) Device configuration and (b) chemical structure and energy level of organic material used in this study.

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Fig. 2. Electrical characterization of electron only device of (a) room temperature. Plot of In(J/A*T2) versus (V-Vbi)1/2 for electron only device with (b) and without (c) in a temperature range of 270e340 K. (d) Plot of In(J0/A*T2) as a function of (1000/T) for calculating the injection barrier DB between the WAW and organic layer.

and the applied voltage, V. Such an equation can be expressed as follows [26]:

o E D n J ¼ A* T 2 exp  q DB  ½qðV  Vbi Þ=4pεr ε0 kd1=2 KT

(1)

Here, A* is the effective Richardson constant, which depends on the effective mass; T is the temperature; q is the elementary charge; DB is the interfacial barrier; V is the applied voltage; Vbi is the builtin potential of the device; εr and ε0 are the relative dielectric constant and permittivity of free space, respectively; k is the Boltzmann constant; and d is the simple thickness of the organic layer. As the same anode or cathode electrode was used in our electron only device, the parameter of Vbi was assumed to be zero for plotting the measured figure of In(J/A*T2) versus (V-Vbi)1/2 (Fig. 2b and c). Fig. 2d shows that the DB can be obtained from the following expression:

J0 ¼ A* T 2 expð  qDB =kTÞ

(2)

where J0 is the current at the zero voltage, which was obtained by Fig. 2b and c. The term In(J0/A*T2) can be plotted as a function of (1000/T), and DB can be determined by the slope of the curve. This result shows that the current in the injection barrier of the Cs2CO3doped structure is only 0.048 eV, whereas that in the injection barrier of the undoped structured is 0.43 eV, signifying a nearly 10fold difference in the electron injection barrier (WAW/organic) between these structures. To further investigate the intrinsic property of WAW thin film in our study, we measured the surface morphology and crosssectional profile of WAW by AFM, while the standard ITO was also prepared for comparison. Fig. 3 shows root mean square (rms) and cross-section results obtained from an area measuring 2 mm  2 mm. Although the WAW and ITO revealed a similar islandlike surface texture, the rms of WAW exhibited a smooth surface with an rms value of 1.4 nm (Fig. 3a) compared with the surface

morphology of ITO showing the rms value of 2.84 nm (Fig. 3b). Because the WAW just used a thermal deposition process without any post-annealing to form conductive thin film, we extracted smaller depth profiles different from those of ITO in the crosssectional profile, where the extracted value corresponded to the highest and lowest depth profiles (Fig. 3c and d). As a result, the variation depth was 8 and 14 nm for WAW and ITO, respectively. To explain such thin film morphology, we further measured the surface morphology of the neat WO3, WO3/Ag bilayer, and WO3/Ag/ WO3 multilayer as shown in Fig. 4. More interestingly, the surface morphology of neat WO3 (rms ¼ 1.15 nm) is quite smooth. This is probably why the WO3/Ag/WO3 multilayer (rms ¼ 1.4 nm) exhibited a highly uniform surface profile. Therefore, our proposed WAW process is highly inconsistent with that obtained from the method used for forming ITO thin film [19,20]. Fig. 5a shows a transmittance of the WAW multilayer thin film when the inner WO3 thickness varied. The transmittance increased as the thickness of the inner WO3 layer increased to a level similar to that of the outer WO3 layer; in other words, when the inner and outer dielectric layers of the DMD symmetrical structure demonstrated a similar thickness, the transmittance of the multilayer thin film increased. This phenomenon is attributable to the microcavity effect, through which the density of various forms of photons is redistributed, causing light of a specific wavelength to satisfy the microcavity model [27]. However, to confirm such an assumption, we assume that the optical property of the WAW multilayer electrode may also be influenced by the reflection and refractive index results, indicating a various microcavity effect on our proposed ITOfree devices. Fig. 5b shows the reflection spectra of WO3, WO3/Ag, WO3/Ag/WO3 for comparison. We found that the value of reflection decreased with increasing thickness of the inner WO3 layer. Therefore, the optical properties of a semi-transparent multilayer metal oxide thin film, i.e. transmission, reflection, and refractive index, are completely different from those of an ITO film, and a

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Fig. 3. Topography images of (a) WAW and (b) ITO. (c) and (d) are the corresponding cross sectional profiles.

Fig. 4. AFM images of (a) WO3 (30 nm), (b) WO3 (30 nm)/Ag (15 nm), (c) WO3 (30 nm)/Ag (15 nm)/WO3 (5 nm), and (d) ITO.

specific WAW structure enables the achievement of high consistency in the selection of light-emitting conditions (Fig. 5a), which facilitates light emission efficiency. Therefore, we assume that the large difference in the refractive index between WO3 and Ag was probably induced by the microcavity effect in the multilayer structure (Fig. 5c) so the light can be emitted at a specific angle [28]. Notably, we used the four-probe measurement method to examine

the WAW thin film (30 nm/15 nm/5 nm) and observed that the sheet resistance of the film was only approximately 3.5 U/sq, indicating that this film exhibits a greater electron conductivity than that of the standard ITO electrode (approximately 15 U/sq). This observation shows that Ag in the intermediary layer is a determinant of the electrical properties of DMD structures. Fig. 5d depicts a photograph of a WAW thin film deposited on a glass

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Fig. 5. (a) The measured optical transmittance curves of WO3 (30 nm)/Ag (15 nm)/WO3 (x ¼ 5, 10, and 20, and 30 nm) thin films. Note that the reference thin-film of Ag, WO3 (5 and 30 nm), and ITO was also shown in here for comparison. (b) The reflection spectra of WO3 (30 nm), WO3 (30 nm)/Ag (15 nm), WO3 (30 nm)/Ag (15 nm)/WO3 (5 nm), and WO3 (30 nm)/Ag (15 nm)/WO3 (10 nm). (c) The refractive index of WO3 (30 nm and 15 nm) and Ag (15 nm). (d) The photograph of WAW (30 nm/15 nm/5 nm) thin film exhibiting a high transparency in visible light and excellent electrical characteristic for conducted the current into LED to emit the EL emission.

substrate; the WAW-deposited substrate demonstrated high transparency in the visible light range and excellent electron conduction characteristics. To verify this phenomenon, we replaced single-core cables with WAW thin film to conduct the current from the power supplier into the LED devices and subsequently induce the device emitting the EL emission. We confirmed that, compared with conventional ITO electrodes, the proposed electrode demonstrates superior current conduction characteristics and can thus be used in a wider range of applications. Fig. 6 illustrates the application of the WAW multilayer thin film in the cathode of the inverted PHOLEDs. In this process, we varied the thickness of the inner WO3 layer from 5 to 30 nm and measured the electrical and electroluminescence (EL) characteristics of the fabricated devices. When the inner WO3 layer was 5 nm thick, the turn-on voltage of the WAW device was 3.2 V, which resulted in a brightness of 100 cd/m2, as shown in Fig. 6a. Note that the device applied a driving voltage of 8 V and brightness increased to 280,000 cd/m2. However, when the thickness of the inner WO3 layer increased to 30 nm, the turn-on voltage increased to 5.6 V (defined as 100 cd/m2), suggesting that the electron failed to enter the inner part of the device because the thick WO3 layer impeded electron injection. The trend shown in Fig. 6b is similar to that observed for the emission property of the device. This result showed the device with a thick thickness of inner WO3 may have an adverse effect on the carrier injection, resulting in poor device performance. Fig. 6c and d shows the current efficiency and power efficiency results, respectively. At a brightness of 1000 cd/m2, the device containing a 5 nm-thick inner WO3 layer showed a current efficiency of 81.4 cd/A and power efficiency of 66.7 lm/W. The improvement in both the current and the power efficiencies remained inversely proportional to the increase in the film thickness. Fig. 6e illustrates the EQE measurement result, signifying that

the optimal WO3 layer thickness is 5 nm. At a brightness of 1000 cd/ m2, the EQE of approximately 22.4% was obtained. In our processing system, the reference OLED reference device demonstrated the following EL performance at 1000 cd/m2 to obtain current efficiency of 42.5 cd/A, power efficiency of 25 lm/W, and EQE of 15.5%. This efficiency performance is inferior to that of the inverted WAW OLED device developed in this study. Fig. 6f presents the EL spectra of four device types with dissimilar thickness levels of inner WO3. As shown in this figure, when the thickness of the inner WO3 layer increased, the spectrum demonstrated a clear shift in a wavelength range of 550e650 nm. This is attributable to the inverted device structure with the WAW electrode designed to induce the microcavity effect in the device. For example, the photon in the device must pass through the conductive transparent WAW thin film for it to be emitted. However, the changes in the inner WO3 layer thickness influenced the microcavity in the multilayer thin film, affecting the photon that was selected to pass the thin film. Consequently, a shift in the EL spectrum was observed. On the other hand, we conducted another experiment to verify the phenomenon. We measured the light emitted from ITO-based devices with 5- and 10-nm thick inner WO3 layers at varying angles (Fig. 7a). The light emitted from the ITO-based devices at various angles conformed to a Lambertian pattern, whereas that emitted from WAW-based devices with 5- and 10-nm thick inner WO3 layers exhibited significant differences at varying angles because of the microcavity effect [28,29]. Therefore, we determined that when the thickness of the inner WO3 layer decreased, the ability of the side of the device to illuminate light weakened. By contrast, when the layer thickness increased, the ability of the side to illuminate light elevated. Finally, we adopted large OLED devices (pixel area: 36 mm2) to observe the quality of the illuminated light (Fig. 7b). The quality of the light illuminated from the WAW-based devices was just as high as that emitted from the ITO-based devices.

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Fig. 6. (a) Luminance vs. voltage, (b) current density vs. voltage, and EL performances (c) current efficiency, (d) power efficiency, (e) external quantum efficiency vs. current density, and (f) normalized EL spectra for WAW OLEDs.

However, in this work we used the novel fabricated method to prepare the proposed WAW-based device with thermal deposition at room temperature exhibiting a highly conductive and ultrasmooth transparent electrode. In addition, this WAW electrode could be integrated with an inverted device configuration to produce an ITO-free OLED device featuring excellent luminous efficiency. 4. Conclusion

Fig. 7. (a) EL viewing profile and (b) emission quality for optimal WAW and reference ITO devices.

Our experimental results revealed that when the inner WO3 layer of the optimal WAW film was 5 nm thick, the inverted PHOLED device demonstrated the highest performance. In addition, this device showed a brightness of 280,000 cd/m2 when the operating voltage was 8 V. At a brightness of 1000 cd/m2, the WAW device exhibited a current efficiency of 81.4 cd/A and an EQE of 22.4%, demonstrating a performance superior to that of the ITO-free inverted PHOLED device. In our electrical characterization experiment, we determined that applying the WAW electrode in an inverted device reduced the injection barrier to 0.048 eV through a BPhen:Cs2CO3 structure; using the WAW electrode thus provided an excellent interface for electron injection. We fabricated a WAWbased inverted OLED device featuring excellent luminous efficiency

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by using the proposed processing approach. Acknowledgements The authors acknowledge financial support from the Ministry of Science and Technology (Grant Nos. MOST 104-2623-E-011-001-D, 104-2221-E-011-126, and 104-2119-M-131-001). In addition, the corresponding author (S.-W. Liu) is grateful to Mr. H.-H. Wu, Syskey Technology Corporation (Taiwan), for his assistance in designing the fabrication system. References [1] K. Hong, K. Kim, S. Kim, I. Lee, H. Cho, S. Yoo, H.W. Choi, N.Y. Lee, Y.H. Tak, J.L. Lee, J. Phys. Chem. C 115 (2011) 3453. [2] K.S. Yook, S.O. Jeon, C.W. Joo, J.Y. Lee, Appl. Phys. Lett. 93 (2008) 013301. [3] X. Liu, X. Cai, J. Qiao, J. Mao, N. Jiang, Thin Solid Films 441 (2003) 200. [4] H. Cho, C. Yun, J.W. Park, S. Yoo, Org. Electron 10 (2009) 1163. [5] Y.C. Han, M.S. Lim, J.H. Park, K.C. Choi, Org. Electron 14 (2013) 3437.
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