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Abstract
We demonstrate enhanced light out-coupling efficiency of organic light-emitting diodes by applying a multilayer stacked electrode structure consisting of fast and cost-effective sol-gel processed tantalum pentoxide (Ta2O5), thin layer of Au and molybdenum trioxide (MoO3). The application of the Ta2O5/Au/MoO3 electrode can modulate the optical characteristics of the device due to the optical microcavity effect. The refractive index of the sol-gel processed Ta2O5 thin film varied depending on the annealing temperature and reached a maximum at 400 °C (n = 2.2 at 512 nm). The influence of the refractive index of the Ta2O5 layer and the thickness of the multilayer electrode stack on the optical microcavity effect was systematically investigated. The device with the Ta2O5/Au/MoO3 electrode, fabricated at an optimum condition based on the simulation result by calculating the photon flux, exhibited 52% enhancement in light out-coupling efficiency at 1000 cd/m2 and improved color stability with the viewing angle, having near-Lambertian emission.
© 2017 Optical Society of America
1. Introduction
Organic light-emitting diodes (OLEDs) have become a key technology for commercial displays and solid-state lighting as a result of tremendous research efforts since the first discovery [1]. The internal quantum efficiency (IQE) of the OLED can reach almost 100% by harvesting both singlet and triplet exciton states by employing electro-phosphorescent materials or thermally activated delayed fluorescence materials [2–4]. The external quantum efficiency (EQE), which is defined as the product of the IQE and the light out-coupling efficiency, however, has been limited to only ~20% for typical OLEDs fabricated on the ITO-coated galss substrates [5, 6]. Approximately 80% of the light generated inside the device is trapped in the transparent conducting oxide layer (wave-guided mode) or the substrate (substrate mode) because of the total internal reflection (TIR) caused by the refractive index (n) mismatch between the organic layers (n~1.6-1.8), ITO (n~1.8-2.2), glass (n~1.5) and air (n~1.0). Thus, the enhancement of the out-coupling efficiency is one of the key issues that must be addressed in improving the device performance of OLEDs.
Many research groups have studied the light out-coupling of the substrate mode or the wave-guided mode [7]. The substrate mode can be easily out-coupled by attaching a patterned structure onto the back of a glass substrate such as micro-lens arrays [8], truncated square-pyramid luminaire [9], high-index glass substrate with a half-sphere structure [10] and light extraction film [11]. These methods enable the enhancement of the out-coupling efficiency without influencing the original device structure, but they are not suitable for highly pixelated OLED displays and some cause an optical blurring effect. The extraction of the wave-guided mode is much more challenging because it requires a modification of the internal structures inside the device. Such methods include the use of optical gratings [12], photonic crystals [13, 14], low-index grids [15], nano-sized stochastic texture surface [16] and corrugated structures [17]. These methods have proven effective, but the patterning on the internal structure requires a complex, high-cost fabrication process and can cause an unwanted modification of the device structure that can affect the electrical properties of the OLEDs.
Another way to enhance the out-coupling efficiency is to make use of the optical microcavity effect between semitransparent and highly reflective electrodes [18, 19]. For the optimization of the optical microcavity effect, the organic layer thickness is generally changed, which leads to a higher possibility of an alteration in the electrical properties of the device. In order to resolve this issue, Wang et al. reported a high out-coupling efficiency enhancement using an electrode stack consisting of a semitransparent metal thin film sandwiched between two transition metal oxides, with the bottom dielectric layer deposited by radio frequency magnetron sputtering [20]. However, in spite of its high enhancement, the use of sputtering is a time-consuming process with high material loss, which leads to a high fabrication cost that is inappropriate for commercial applications. Thus, new strategies to improve the light out-coupling efficiency using a simple and low-cost fabrication process are required.
In this work, we have demonstrated the light out-coupling efficiency enhancement of OLEDs by applying a multilayer stacked electrode with a dielectric/metal/dielectric (DMD) structure. The multilayer electrode stack consists of fast and cost-effective sol-gel processed tantalum pentoxide (Ta2O5), thin layer of Au and molybdenum trioxide (MoO3). The refractive index and thickness of the Ta2O5 layer mainly determines the optical characteristics of the OLED device, with a high refractive index effectively tuning the optical microcavity at the optimum condition, maximizing the out-coupling efficiency. To validate the effect of the multilayer electrode stack, the photon flux (PF), i.e., the number of out-coupled photons, of an OLED device with a multilayer electrode stack was simulated, and the EQE was calculated based on the simulated result. The light out-coupling efficiency of an OLED device with a sol-gel processed Ta2O5/Au/MoO3 multilayer electrode stack was enhanced by 52% at 1000 cd/m2 with improved charge injection, effectively reducing the power consumption. The device also exhibited a stable color coordinate with the viewing angle compared to that of the reference device, with near-Lambertian emission characteristics.
2. Experiment
2.1 Preparation of Ta2O5 thin film
The Ta2O5 solution was prepared using the sol-gel method by dissolving tantalum(V) ethoxide (Ta(OC2H5)5) as a precursor in ethanol with the addition of glacial acetic acid [21]. The solution was left to stir for over 4 hours, and then a Ta2O5 thin film was deposited by spin-coating on a glass substrate and annealing for an hour at high temperatures from 120 °C up to 500 °C. The refractive index and thickness of the spin-coated thin film were modulated by changing the annealing temperature and the concentration of the sol-gel precursor, respectively, and measured using spectroscopic ellipsometry (Elli-SE).
2.2 Fabrication of OLED and hole-only devices
Green phosphorescent OLED devices were fabricated using a Ta2O5/Au/MoO3 multilayer stacked electrode structure as the anode (DMD-device) and compared with devices using ITO as the anode (ITO-device). The Ta2O5 thin film was deposited by spin-coating (5000 rpm, 40 s) on a bare glass substrate that was pre-cleaned with acetone, IPA, and DI water and treated with UV-ozone for 15 minutes. Then, the thin film was annealed at 400 °C for 1 hour on a hot plate in an ambient condition. All other layers were deposited using thermal evaporation in high vacuum (<10−6 torr). 4,4′-cyclohexylidene bis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) and 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were used as hole-transporting and electron-transporting layers, respectively. 8 wt% tris[2-phenylpyridinato-C2]iridium(III) (Ir(ppy)3) doped in 4′-bis(carbazol-9-yl)biphenyl (CBP) was used as the emitting layer and LiF/Al was used as the cathode. The detailed device structure and the energy level diagram are shown in Figs. 1(a) and 1(b). To compare the charge injection property of ITO and Au anodes, we also fabricated hole-only devices (HODs) with the structure of ITO or Au (15 nm) / MoO3 (10 nm) / TAPC (50 nm) / MoO3 (10 nm) / Au (50 nm).
Fig. 1 (a) Device structure and (b) energy level diagram of green phosphorescent OLEDs with a Ta2O5/Au/MoO3 multilayer-stacked electrode as the anode (DMD-device)
The current density-voltage (J–V) characteristics of the OLED device were measured using source-measurement unit (Keithley 236) and a multimeter (Keithley 2000). The luminance-voltage (L–V) characteristics were recorded with a calibrated Si photodiode (Hamamatsu S5227-1010BQ) and a photomultiplier tube. The efficiencies were calculated from the photocurrent data, and the electroluminescence (EL) spectra data were obtained using a spectroradiometer (Minolta CS-1000A). Commercial software (SimOLED) was used for the optical simulation and analysis of the multilayer stacked electrode, and the optical constants of each layer were measured using spectroscopic ellipsometry (Elli-SE). The EQE and power efficiency (PE) were calculated by considering the angular emission characteristics of the emission
3. Results and discussion
3.1 Optical microcavity effect in OLED with multilayer electrode stack
The presence of the multilayer stacked electrode structure of a thin metal layer sandwiched between two transition metal oxides undergoes the optical microcavity effect. The microcavity effect between the semitransparent and highly reflective electrodes can be described by considering the OLED device as a Febry-Pérot resonator using the concept of effective interfaces, as shown in Fig. 1(a) [22].
The spectral radiant intensity can be calculated as a function of the wavelength and emission angle, given by
where T and R are the transmittance and reflectivity at the top and bottom electrode interfaces, n is the refractive index, z is the optical distance between the emission zone and the reflective top electrode and is the intensity of the emitted light without the cavity effect. The phase shift is given bywhere and are the phase changes occurring at the top and bottom electrode interfaces, and and are the refractive index and thickness of the i-th organic layer, respectively. The resonance condition is defined as , where m is the mode index indicating the order of the microcavity [23]. In this study, the thickness of the layers between the two electrodes and the optical characteristics of the reflective cathode are fixed, so the optical effect of the multilayer stacked electrode on the enhancement of the light out-coupling efficiency is mainly related to the refractive index and thickness of the bottom dielectric layer and the thickness of the thin metal layer. These conditions affect the resonance in the microcavity, tuning the phase shift of the reflection and transmission between the metal electrodes. The optimization of the multilayer electrode stack and the OLED device is discussed in the next section.3.2 Optimization and electrical/optical characteristics of the DMD-device
The optimization of the multilayer electrode stack is a key parameter to achieve the goal of enhancing the light out-coupling efficiency in an OLED device without affecting the device structure or the electrical properties. The multilayer stacked electrode consists of sol-gel processed Ta2O5, Au and MoO3. The thin layer of Au in the electrode stack was chosen for its high work function with high conductivity, expecting improved charge injection compared to that of the conventionally used ITO. The MoO3 layer helps achieve efficient charge injection and transport from the anode to the hole-transporting layer, effectively reducing the interfacial dipole formation at the metal/organic interfaces. The refractive indices and thicknesses of the organic layers are fixed to maintain the charge balance, so the optical properties of the reflective cathode are also fixed. Therefore, the only factor that can influence the optical characteristics of the device is the properties of the multilayer electrode stack.
The refractive index of the sol-gel processed Ta2O5 plays an important role in the multilayer electrode stack because the number of photons trapped inside the glass substrate is reduced as the refractive index of the bottom dielectric layer increases [24]. The photons with an incident angle smaller than the critical angle of TIR at the dielectric/glass interface will be transmitted, while those with an incident angle larger than the critical angle of TIR at the glass/air interface (41.8 °) will be trapped inside the glass substrate. Thus, the number of photons trapped inside the glass substrate is proportional to the difference between the critical angles of TIR at the dielectric/glass and glass/air interfaces, and the refractive index of the bottom dielectric layer determines the critical angle of TIR at the dielectric/glass interface. For the optimization of the multilayer electrode stack, the refractive index characteristics of the sol-gel processed Ta2O5 thin film was first analyzed. The refractive index of the Ta2O5 thin film changes with respect to the annealing temperature, as shown in Fig. 2(a). An increase in the refractive index was observed along with the increase in the annealing temperature up to 400 °C. The refractive index of the Ta2O5 thin film decreased after annealing the thin film at a temperature over 400 °C. The refractive index of the thin film increased from 1.9 at an annealing temperature of 120 °C to 2.2 for an annealing temperature of 400 °C at the wavelength of 512 nm (emission peak wavelength of Ir(ppy)3). The high refractive index of the Ta2O5 layer annealed at 400 °C reduced the critical angle of TIR at the Ta2O5/glass interface (42.7 °) compared to that of the Ta2O5 layer annealed at 120 °C (52.2 °). As a result, most of the photons can escape the glass substrate, effectively reducing the substrate mode loss.
Fig. 2 (a) Refractive index change of Ta2O5 thin film with respect to the annealing temperature, (b) simulated normalized photon flux of DMD-device with respect to annealing temperature compared with that of the ITO-device (Ref)
To validate the effect of the annealing temperature on the out-coupling efficiency of OLEDs, the PFs of the DMD-devices with different annealing temperatures of the Ta2O5 thin film were simulated, given by
where is the simulated spectral radiant intensity, is the wavelength, is the viewing angle, is the Planck constant, and is the speed of light in vacuum. Figure 2(b) shows the normalized PF of the DMD-device with respect to the annealing temperature of the Ta2O5 layer compared with that of the ITO-device (ref). The results indicate that the sol-gel processed Ta2O5 layer annealed at 400 °C exhibits the highest refractive index compared to the thin films annealed at different temperatures, which effectively reduces the substrate mode loss, thereby improving the out-coupling efficiency. Although the optical property of the Ta2O5 thin film was optimized at high annealing temperature of 400 °C, it is noted that the PF for devices with the Ta2O5 thin film annealed at different temperatures even at low temperatures (≤200 °C) is still higher than the reference device.The thickness of the multilayer electrode stack was optimized by calculating the PF with respect to the Ta2O5 and Au layer thicknesses, as shown in Fig. 3(a). The thickness of the MoO3 layer was fixed at 10 nm to maintain the charge balance and the electrical properties of the OLED device. The optimum thicknesses of the Ta2O5 and Au layers with the 1st order microcavity to maximize the PF were 65 nm and 15 nm, respectively. Figure 3(b) indicates the calculated and measured EQEs of the OLED device using a multilayer electrode stack at the optimum thickness. According to the simulation results, the DMD-device exhibits superior optical performance compared to that of the ITO-device. The simulated results show good agreement with the experimental results, indicating that the microcavity effect effectively modulated the out-coupling efficiency.
Fig. 3 (a) Normalized photon flux with respect to the thickness of the Ta2O5 and Au layers. (b) Calculated (solid line) and measured (open marker) EQEs of the DMD-device with an Au layer thickness of 15 nm.
Another simulation was conducted based on the optical properties of the materials in use, to estimate the optical microcavity effect and the resonance condition. The spectral radiant intensity of the optical microcavity is related with the transmittance and reflectance of the anode as described in Eq. (1). Figures 4(a) and 4(b) show the transmittance and reflectance of the ITO, Au and the DMD multilayer electrode stacks with different Ta2O5 layer thickness. The calculated spectral radiant intensity of the ITO-device, Au-device and DMD-devices is shown in Fig. 4(c). Changing the anode from ITO to DMD multilayer electrode red-shifts the spectral radiant intensity of the device due to different transmittance and reflectance properties. When the thickness of the Au layer is fixed, the resonance peak is determined by the thickness of the Ta2O5 layer. As the thickness of the Ta2O5 layer increases, the resonance peak red-shifts and the PF is optimized with a maximum value at the Ta2O5 thickness around 65 nm as shown in Fig. 4(d).
Fig. 4 The simulated (a) transmittance and (b) reflectance of ITO, Au and DMD electrode stacks with different Ta2O5 layer thickness. (c) Normalized spectral radiant intensity of ITO-device, Au-device and DMD-devices with different Ta2O5 layer thickness. (d) Normalized photon flux with respect to the thickness of Ta2O5 layer.
Based on the optimized device architecture, green phosphorescent OLED devices were fabricated, and their electrical/optical characteristics are shown in Fig. 5. The current densities of the ITO-device and the DMD-device were very similar, with the application of the multilayer electrode structure causing no significant change in the electrical characteristics [see Fig. 5 (a)]. The turn-on voltage of the ITO-device and DMD-device is 3.3 V and 3.2 V, respectively, and the operating voltage at 1000 cd/m2 is 6.2 V and 5.7 V, respectively [see Fig. 5 (b)]. The reduction in the operating voltage of the device comes from the higher work function of Au (5.2 eV) compared to ITO (4.8 eV), improving the charge injection from the anode through the HIL to the HTL. The inset of Fig. 5 (a) compares the J–V characteristics of the HODs and the device with the Au anode shows higher current density compared to that of the ITO anode, indicating that the charge injection from the anode is improved by applying the Au layer instead of ITO. Figure 5(c) shows the normalized EL intensity of each device, with a shoulder peak at 542 nm observed in the EL spectrum of the DMD-device. The increased shoulder peak intensity indicates the validity of the optical microcavity effect between the multilayer electrode stack and the Al cathode, resulting in the enhancement of the light out-coupling efficiency. The EQE of the ITO-device and the DMD-device with respect to the current density is shown in Fig. 5(d). The EQE of the DMD-device (21.1%) has been enhanced by a factor of 1.52 compared to that of the ITO-device (13.9%) at 1000 cd/m2.
Fig. 5 (a) J–V characteristics, inset: J–V characteristics of hole-only devices, (b) L–V characteristics, (c) normalized EL intensity and (d) EQE of the ITO-device and DMD-device.
3.3 Angular emission characteristics of DMD-device
The change of the anode from ITO to the multilayer electrode stack must maintain the color stability of the OLED device. An improved efficiency and Lambertian-like emission with less spectral dependence is also required for display applications of OLEDs. To verify the color stability and the angular emission characteristics, the CIE color coordinates were first calculated based the OLED device spectra at different viewing angles, as shown in Fig. 6(a). The CIE coordinates of the DMD-device at an incident angle of 0° were (0.30, 0.64), which is comparable to those of the ITO-device (0.29, 0.63). Little change in the color coordinates was observed with the increased viewing angle, as the device maintained its color stability. The angular emission characteristics are shown in Fig. 6(b), with improved out-coupling efficiency observed throughout the viewing angle range and exhibiting near-Lambertian emission characteristics.
Fig. 6 (a) Measured CIE-x and CIE-y coordinates as functions of viewing angle. (b) Angular emission characteristics of OLED device with multilayer electrode stack compared with that of ITO. (c) Normalized EL spectra of OLED device with ITO at different viewing angles. (d) Normalized EL spectra of OLED device with multilayer electrode stack at different viewing angles.
Figures 6(c) and 6(d) show the normalized EL spectra of the ITO-device and the DMD-device, respectively. No perceptible changes in the EL spectra were observed, but for the DMD-device, the shoulder peak at 542 nm decreases as the viewing angle increases, slightly blue-shifting the overall emission spectra. This is due to the blue-shift of the resonance wavelength (), which is calculated from Eq. (2) as
Thus, of the DMD-device at the optimum condition with the 1st order microcavity becomes a cosine function of the incident angle and blue-shifts with the increasing viewing angle.3.4 Operation stability of the DMD-device
The operation lifetime and operating voltage of the ITO-device and DMD-device as a function of operation time at an initial luminance of 1000 cd/m2 were compared in Fig. 7. The DMD-device showed comparable lifetime compared to the ITO-device with lower operating voltage as expected from the J–V characterisitics shown in Fig. 5. We assume that the little difference in the operation lifetime is related to the metal/oxide/organic interface interactions [25], and further study is required to clarify the degradation mechanism of the device with the multilayer stacked electrode structure. Overall, the DMD-device effectively enhances the device’s out-coupling efficiency and reduces the operating voltage, with sustaining operational stability, compared to the ITO-device.
Fig. 7 (a) Operation lifetime and (b) operating voltage of the ITO-device and DMD-device as a function of operation time at an initial luminance of 1000 cd/m2.
4. Conclusions
The optical and electrical properties of OLEDs with sol-gel processed Ta2O5/Au/MoO3 electrode (DMD-device) were investigated. They exhibit an enhancement in the light out-coupling efficiency compared to that of the conventional ITO electrode (ITO-device). The sol-gel processed Ta2O5 as a bottom dielectric layer in the multilayer electrode stack facilitates a simpler fabrication with a lower cost compared to that of the sputtering-based fabrication. The utilization of a semitransparent thin Au layer causes an optical microcavity effect with a highly reflective Al cathode and improves the charge injection property of the OLED. The thickness of the sol-gel processed Ta2O5 and Au layers were optimized through optical simulation by calculating the PF. The high refractive index (n~2.2) of the Ta2O5 thin film annealed at 400 °C results in enhanced light out-coupling efficiency with near-Lambertian and color-stable properties. The DMD-device reduced the driving voltage by 0.5 V and improved the EQE by a factor of 1.52, at 1000 cd/m2, compared to that of the ITO-device. The work presented here suggests a simple and low-cost strategy for enhancing the light out-coupling efficiency in OLED display applications with superior electrical characteristics and stable angular emission properties without deteriorating the device stability.
Funding
Industrial Strategic Technology Development Program funded by the Ministry of Trade, Industry and Energy of Korea, a part of the Korea Evaluation Institute of Industrial Technology (no. 10042412); Samsung Display Co., Ltd.
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