Development of Extremely High Efficacy White OLED with over 100 lm/w

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1 Journal of Photopolymer Science and Technology Volume 27, Number 3 (2014) SPST Development of Extremely High Efficacy White OLED with over 100 lm/w Nobuhiro Ide, Kazuyuki Yamae, Varutt Kittichungchit, Hiroya Tsuji, Masuyuki Ota and Takuya Komoda Eco Solutions Company, Panasonic Corporation, 1048 Kadoma,Kadoma City, Osaka Japan Non-radiation mode of OLED device was reduced by optimizing the distance between emissive layers to the metal cathode. Light distribution of OLED and optical properties of a light outcoupling substrate based on high refractive index microstructures were adjusted to achieve the better combination of the OLED device with the substrate. Those advanced optical design decreased non-radiative evanescent mode and waveguide mode, and realized a white OLED device with quite high light outcoupling efficiency of at least 56 % and outstandingly high efficacy of 133 lm/w at 1,000 cd/m 2. Keywords: OLED; light outcoupling structure, evanescent mode 1. Introduction Recently, various strategic programs are internationally executed for the global environmental protection. In order to save energy consumption by lighting, legal restrictions or phase-outs of less efficient incandescent bulbs had been decided in many countries and they will be effective before 2017 [1]. Additionally, the Minamata Convention on Mercury was agreed in October 2013, which prohibits manufacture, import and export of mercury-added products such as fluorescent lamps by Based on these backgrounds, urgent development and penetration of next generation lighting sources without mercury that realize high efficiency are expected. In these days, the performance of light emitting diode (LED) is dramatically improved and LED is rapidly spreading into the market as various types of lighting sources. LED is replacing the conventional lighting sources (incandescent bulbs and fluorescent lamps), and in 2020, LED will occupy 52 percent of worldwide lamp and luminaire shipment in volume [2]. On the other hand, organic light emitting diode (OLED) becomes another candidate of an energy efficient lighting source. OLED is an ultrathin and lightweight surface-emitting lighting source, and has some great potentialities as, for example, flexible and/or transparent lighting devices. However, in order to apply OLED to the considerable part of lighting market, further Received April 20, 2014 Accepted May 23, 2014 improvement of efficacy and reduction of cost should be required. In this paper, highly efficient white OLED with an optically optimized high efficacy all phosphorescent device on the specially designed light outcoupling substrate is described. 2. Efficiency of OLED and light outcoupling technology Luminous efficacy of OLED device is determined by the quantitative three factors, luminous efficacy of radiation (LER), electrical efficiency (EE), and external quantum efficiency (EQE). LER is dependent on the spectrum and the maximum value in white color region is in the range of lm/w [6]. EE is a ratio determined by energy band gap of an emitter and driving voltage of OLED. EQE is a ratio of photon generation to injected carrier and it is the product of internal quantum efficiency (IQE) and light extraction efficiency (LEE). Figure 1 shows the typical behavior of light generated in the OLED device. LEE is a ratio of external mode to the total emission mode, and LEE of the typical OLED structure is limited to only 20~30 %. It is because of the high refractive indices of organic layers and ITO (n ~ 1.8) and glass substrate (n ~ 1.5), and most of generated light are confined within these high refractive-index layers as waveguide and substrate modes due to total internal reflection (TIR) at interfaces. 357

2 Waveguide mode Substrate mode Total Internal Reflection Total Internal Reflection Evanescent mode Organic and ITO layer (n ~ 1.8) Glass Substrate (n ~ 1.5) External mode Air (n = 1.0) ext = 20~30 % Fig. 1. Typical behavior of generated light in the OLED device. We have already reported highly efficient light outcoupling technology achieved by an originally developed built-up light extraction substrate (BLES) which is composed of high refractive index materials [3-5]. Schematic of BLES is depicted in Figure 2. In this system, high refractive-index material is used as a high-n layer and light outcoupling textures attached onto the glass substrate, and there are air-gaps between the high refractive index texture and the glass substrate. air gaps organic layers transparent electrode high-n layer high-n light outcoupling texture glass substrate Fig. 2. Schematic of built-up light extraction substrate (BLES). In BLES system, most of generated light in organic layers propagates through the high-n layer and the high-n light outcoupling texture. Approximately similar refractive index of the outcoupling texture to those of organic layers and transparent electrode enables the better coupling of waveguide mode to substrate mode. Additionally, thanks to the existence of the air-gap between the light outcoupling texture and glass substrate, the extracted light from the texture is able to transmit the glass substrate without TIR. Furthermore, by anti-reflection treatment, it is possible to decrease Fresnel s reflection of both surfaces of the glass. Consequently, excellent LEE of nearly 50 % was already realized in BLES system, and further optimization of spatial distribution of luminous intensity of OLED will improve the outcoupling efficiency. 3. Optical design of organic layers and light extraction structure for highly efficient OLED 3-1. Investigation of EQE EQE of OLED is simply given by Equation 1. Here, IQE is the ratio of radiation to the total injected carrier and LEE is the ratio of external mode to the total emission mode. radiation, nonradiation and evanescent stand for the fractions of radiative, non-radiative and evanescent modes, and external, substrate and waveguide stand for the fractions of external, substrate, and waveguide modes in the plain bottom emission OLED structure, respectively, and Equation 1 can be rewritten as Equations 2 and 3. EQE = IQE LEE radiation 1 1 nonradiatiion 1 1 evanescent external substrate substrate waveguide (1) (2) (3) (4) When IQE is optimized by, for example, the best carrier balance and the use of ideal emitters, the non-radiative loss is almost attributed to the evanescent mode. With regard to LEE, when carefully selected electrode and substrate which have similar refractive indices to that of organic layers are used (as BLES system), waveguide mode is coupled to the substrate mode and Equation 3 can be converted to Equation 4. Therefore, the issues in OLED to improve EQE are focused on 1) reduction of evanescent mode and 2) extraction of substrate mode Reduction of evanescent mode A lot of methods were proposed to reduce the evanescent mode in OLED devices. For example, fabrication of nanostructures between organic layer and metal cathode [7-9], back-cavity structure with thin metal cathode [10], and horizontally oriented emissive layer [11]. In this study, we paid attention to the distance from emissive zone to the metal cathode [12]. In order to quantitatively specify each mode in OLED, a bottom emitting monochrome OLED on the high refractive index substrate was investigated by the optical simulation. In this simulation, some optical parameters were assumed: 1) wavelength: 530 nm, 2) emission center: at the interface of HTL and ETL, 3) refractive indices of organic layers, ITO and substrate: 1.8, 4) dipole orientation of emission: random, 5) extinction coefficients of organic layers: zero, 6) absorption channel: Ag (cathode) and ITO (anode). Figure 3 shows the device structure in this model, and distance between emitting layer and cathode was determined and varied by the thickness of ETL. 358

3 Ag (150 nm) of WASM though the substrate with outcoupling structures to the air is especially required. ETL (0~300nm) HTL (50nm) ITO (100nm) Ag (150 nm) ETL (0~300nm) Substrate HTL (50nm) ITO (100nm) Fig. 3. Schematic of monochrome OLED for optical simulation. Air ETL thickness [nm] (=Distance between Emission Center and Cathode) Substrate Air Figure 4 shows the result of optical simulation of ETL thickness dependence of the fraction of various modes. By increasing the ETL thickness, evanescent mode is gradually reduced and when the ETL thickness is over 100 nm, most of the evanescent mode is converted to the substrate mode. In this thickness region, fraction of absorption mode is so small and the extraction of substrate mode is the most important in order to improve the efficiency. Evanescent mode Absorption Fig. 5. Calculated angular distribution of light in high refractive index substrate (n = 1.8). Here, the angular distributions of transmission though various micro structures to the air were investigated. Results are shown in Figure 6. Carefully designed micro-texture of BLES had better transmission property of WASM compared to typical scattering layer or microlens array systems. 1.4 Substrate mode Extraction mode ETL thickness [nm] (=Distance between Emission Center and Cathode) Fig. 4. Fractions of various modes in monochrome OLED. Relative Transmission (normalized at 0deg) Typical scattering layer Typical microlens array Optimized micro-texture Angle (deg) 3-3. Extraction of substrate mode Additionally, dependence of angular distribution of radiation within the substrate was also simulated to investigate effective method to extract the substrate mode that was converted from the waveguide mode owing to the thick ETL. Simulated result is shown in Figure 5. It indicates that resonant angle of light shifts to wider angle when the distance between emission center and cathode becomes longer. When the thickness of ETL is over 100 nm, the light in substrate is dominated by wide angular substrate mode (WASM), that is, the guided light with propagation angles of over 45 degrees. For the better outcoupling of this system, the higher transmission Fig. 6. Transmission from high refractive index (n = 1.8) substrates to the air through various optical structures. 4. Phosphorescent white OLED In order to achieve high efficacy, improvement of IQE is also required. Based on the optical simulation result of simple monochrome OLED in Section 3-2, all-phosphorescent two-unit white OLED with the design to reduce evanescent mode was investigated. Two-unit structure is beneficial to enhance EE and lifetime due to the reduction of current density to obtain desired luminance by almost half. White emission with high CRI required the intensity ratio of red : green : blue emissions as about 2 : 1 : 1 and the two-unit 359

4 structure composed of the red/blue and red/green phosphorescent units was selected as the low driving voltage system [5]. In order to reduce the evanescent mode, thick (> 100 nm) ETL with high mobility and transparency was applied. Additionally, the reduction of driving voltage by the adjustment of the interfacial injection barriers especially around the blue phosphorescent emissive layer was also investigated. In order to achieve optimum optical combination of the substrate with the OLED device, light distributions, intensities and the optical interferences at various wavelengths from four emissive layers were taken into account, and thicknesses of organic layers were carefully optimized. 5. Performance of white OLED The optimized white OLED and non-optimized (typical micro-cavity) white OLED as reference were fabricated on various substrates and, for confirmation of device design, on a high-n hemisphere. IQE of these systems would be equivalent, thus the extraction enhancement effect was able to be evaluated by EQE, which was measured by the system composed of a luminance meter and a goniometric stage. Emissive area and driving current density of these devices were 1 cm 2 and 0.6 ma/cm 2, respectively. Results are shown in Table 1. Finally, a large area OLED panel on the substrate with designed microstructure was fabricated. Emissive area was 100 cm 2, and uniform emission without dark spots and visible defects was realized shown in Figure 7. Optical and electrical performances of this panel are shown in Table 2. This OLED panel achieved outstandingly high EQE of 112 % and luminous efficacy of 133 lm/w at 1,000 cd/m 2. Color coordinates were within the white color region defined by ENERGY STAR [13] as shown in Figure 8. Despite the specific design of angular distribution in organic layers, nearly-lambertian emission pattern was observed in this panel. Estimated half-decay lifetime (LT50) was over 150,000 h. Table 2. Performance of fabricated OLED panel (100 cm 2 ) Luminance 1,000 cd/m 2 Efficacy 133 lm/w external quantum efficiency (average EQE (per unit)) 112 % (56 %) Voltage 5.4 V estimated LT50 > 150,000 h CRI 84 color coordinates (0.48, 0.43) CCT 2,600 K emissive area 100 cm 2 Table 1. External quantum efficiency of white OLEDs (1 cm 2 ) substrates optimized device (thick ETL) reference plain glass 38 % 44 % with scattering layer 82 % 82 % with designed microstructure 105 % 97 % For the confirmation high-n hemisphere 132 % 120 % The optimized device on the substrate with designed microstructure showed quite high EQE of 105 % and 132 lm/w, and these values were better than other systems. WASM-rich light distribution was appropriate for the better light outcoupling (improved to 105 % from 97 %), and the designed micro-structure was more effective to extract WASM compared to a substrate with a scattering layer (105 % from 82 %). Additionally, the optimized device on the high-n hemisphere lens showed EQE of 132 % and efficacy of 160 lm/w. Compared to the reference device, improvement of 12 % (132 % from 120 %) was achieved and it would be due to the elimination of evanescent mode. 10 cm Fig. 7. Photograph of OLED panel (100 cm 2 ) 6. Conclusion High efficacy white OLED panel with luminous efficacy of 133 lm/w and emissive area of 100 cm 2 was achieved by the advanced optical design focused on the elimination of evanescent mode and the substrate for the better transmission of WASM, and low-voltage all-phosphorescent OLED device. This performance would be the highest value in white OLEDs ever reported. 360

5 y CIE 1931 x,y Chromaticity Diagram black body radiation curve wavelength (nm) x Fig. 8. Color coordinates and spectrum of OLED panel (100 cm 2 ). 7. Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as Fundamental Technology Development of Next Generation Lighting of High-efficiency and High-quality project from March 2010 to February We thank to Idemitsu Kosan Co., Ltd. as a member of the project, Universal Display Corporation for their kind provisions of their high performance materials, and all of related companies who kindly provided us various materials and tools for this study. We also thank to Dr. Taku Hirasawa, Dr. Yasuhisa Inada, and Mr. Akira Hashiya, Device Solutions Center, R&D Division, Panasonic Corporation, for fruitful discussions and kind provisions of optical simulation data. References 1. Solid-State Lighting Research and Development Multi-Year Program Plan 2013, U. S. Department of Energy (2013). 2. T. Baumgartner, F. Wunderlich, D. Wee and A. Jaunich, "Lighting the way: Perspectives on the global lighting market, Second edition," McKinsey & Company, Inc. (2012). 3. T. Komoda, K. Yamae, V. Kittichungchit, H. Tsuji, N. Ide, Extremely High Performance White OLEDs for Lighting, SID 2012 Digest, pp. 610 (2012). 4. K. Yamae, H. Tsuji, V. Kittichungchit, Y. Matsuhisa, S. Hayashi, N. Ide, T. Komoda, High-Efficiency OLEDs with Built-up Outcoupling Substrate, SID 2012 Digest, pp. 694 (2012). 5. K. Yamae, H. Tsuji, V. Kittichungchit, N. Ide, T. Komoda, Highly Efficient White OLEDs with over 100 lm/w for General Lighting, SID 2013 Digest, pp. 916 (2013). 6. T. W. Murphy, Jr., Maximum spectral luminous efficacy of white light, J. Appl. Phys., 111 (2012) J. Frischeisen, Q. neu, A. Abdellah, J. B. Kinzel, R. Gehlhaar, G. Scarpa, C. Adachi, P. Lugli, W. Brütting, Light extraction from surface plasmons and waveguide modes in an organic light-emitting layer by nanoimprinted gratings, Optics Express, 19 (2011), Issue S1, pp. A7-A P. A. Hobson, S. Wedge, J. A. E. Wasey, I. Sage, W.L. Barnes, Surface plasmon mediated emission from organic light-emitting diodes, Adv. Mater., 14 (2002) S. Murano, D. Pavicic, M. Furno, C. Rothe, T. W. Canzler, A. Haldi, F. Löser, O. Fadhel, F. Cardinali, O. Langguth, Outcoupling Enhancement Mechanism Investigation on Highly Efficient PIN OLEDs using Crystallizing Evaporation Processed Organic Outcoupling Layers, SID 2012 Digest, pp. 687 (2012). 10. A. Mikami, T. Goto, Optical Design of Enhanced Light Extraction Efficiency in Multi-Stacked OLEDs Coupled with High Refractive-Index Medium and Back-Cavity Structure, SID 2012 Digest, pp. 683 (2012). 11. J. Frischeisen, D. Yokoyama, A. Endo, C. Adachi, W. Brütting, Increased light outcoupling efficiency in dye-doped small molecule organic light-emitting diodes with horizontally oriented emitters, Org. Electron., 12(5) (2011) (2011). 12. S. Nowy, B. C. Krummacher, J. Frischeisen, N. A. Reinke, and W. Brütting, Light extraction and optical loss mechanisms in organic light-emitting diodes: Influence of the emitter quantum efficiency, J. Appl. Phys., 104(12) (2008) ENERGY STAR Program Requirements for Solid State Lighting Luminaires, Eligibility Criteria - Version 1.1 (2008). 361