High Efficiency White OLEDs for Lighting

Transcription

1 CIE-y Journal of Photopolymer Science and Technology Volume 25, Number 3 (2012) CPST High Efficiency White OLEDs for Lighting Takuya Komoda, Kazuyuki Yamae, Varutt Kittichungchit, Hiroya Tsuji and Nobuhiro Ide Panasonic Corporation, Eco Solutions Company, Kadoma , Osaka, Japan komoda.takuya@jp.panasonic.com A light outcoupling substrate and high performance white OLEDs were investigated. The light outcoupilng substrate achieved high light coupling efficiency of 42.8 % in a white OLED. A fluorescent / phosphorescent hybrid two-unit device achieved high luminous efficacy of 56 lm/w, high CRI of 91 and long lifetime of 150,000 h at 1,000 cd/m 2. An optically and electrically optimized two-unit of all phosphorescent white OLED realized quite high luminous efficacy of 83 lm/w and was estimated to be half decay lifetime of over 100,000 h. Keywords: white OLED; phosphorescent emitter; light outcoupling; high index; color rendering index (CRI) 1. Introduction Recently, energy saving becomes one of the greatest concerns to conserve the global environment. Highly efficient next-generation lighting sources will considerably contribute to the reduction of energy consumption. Light emitting diode (LED) has been already widespread and gradually replacing the conventional lighting sources. White OLED (organic light emitting diode) is another candidate of next generation solid-state lighting source. It has various functional characteristics such as very thin appearance, no UV/IR gentle surface emission and high color rendering index (CRI), as well as environmental friendliness (for example, no mercury and, in the future, lower energy consumption than conventional lighting sources). Thanks to those features, OLED lighting is expected to be much more versatile than conventional lighting sources, and various application and prototypes are actively discussed and demonstrated. We have already achieved a high performance white OLED device of luminous efficacy of 42 lm/w, a half decay lifetime over 100,000 h and CRI of 90 at 1,000 cd/m 2 [1]. This device had a two-unit structure composed of a red/green phosphorescent unit and blue fluorescent unit on a glass substrate with a light outcoupling film attached onto the surface of the substrate. Typical structure of this system is shown in Figure 1. Key technologies in this system are a high efficiency and long lifetime phosphorescent red/green emissive unit, a deep blue fluorescent unit for high CRI, an optically and electrically designed two-unit white device structure and a wet coated hole injection layer for the better reliability [1-2] green red Black Body 0.1 deep blue CIE-x red/green phosphorescent unit transparent connecting layer deep blue fluorescent unit hole injection layer fabricated by slit-die coating light outcoupling enhancement structure Fig. 1. Typical structure of the two-unit fluorescent/phosphorescent hybrid OLED. These technologies were already applied to our commercialized OLED lighting panel. It realized the excellent and balanced performance of the luminous efficacy of 30 lm/w, LT70 (lifetime to the 70% of the initial luminance) of over 10,000 h, uniformity in luminance of over 70% in the emissive area of 8 cm x 8 cm at 3,000 cd/m 2, and high CRI of over 90 at the same time. However, further improvement in efficiency, lifetime and quality of light are strongly Received April 6, 2012 Accepted May 10,

2 optical enhancement factor 322 expected for OLED in order to proceed to the advanced stage as the next generation lighting device. As for the quality of light, a lot of required characteristics of such as the correlated color temperature (CCT), CRI, color maintenance and spatial uniformity in chromaticity are already determined by ENERGY STAR Program Requirements for Solid State Lighting Luminaires, Eligibility Criteria [3]. Optical simulation and design of white OLED device structures are indispensable approaches for the achievement of those requirements. Light outcoupling enhancement and phosphorescent emitter are quite important technologies in order to improve the performance [4-9]. In this paper, developments of a light outcoupling substrate, high efficiency fluorescent / phosphorescent hybrid white OLED and all phosphorescent white OLED are discussed. 2. Light outcoupling substrate Almost 80 % of the generated 1.2 light in the OLED device is disappeared due to the total internal 1.15 reflection and the extinction, and only about 20 % can be extracted to the air. Various 1.1 optical structures fabricated on the surface of the substrate can extract 1.05 a certain amount of light confined in the glass substrate and improve the outcoupling 1 efficiency up to about 30%. This kind of technologies were 0.95 already applied to many of commercialized OLED panels, however, remaining light 0.9was still confined in the thin transparent electrode and organic layers and finally disappeared. Some internal light outcoupling technologies such as scattering layers and diffraction gratings fabricated at the interface of glass substrate and transparent electrode can reduce the total reflection, and have already realized better light outcoupling efficiency [4-6]. High refractive index (n) substrates also improved the transmission from thin layers to the substrate, thus luminous efficacy [7]. However, these technologies are still under development and are not commercially available because of remaining issues of, for example, reliability and fabrication cost for large area OLED lighting application. Figure 2 shows the newly developed schematic structure of the light outcoupling substrate. Some semi-spherical micro lens array (MLA) structures were fabricated with high refractive index resins on various transparent plastic films [10]. In order to reduce the light absorption in this structure, extremely transparent high refractive index plastic films and materials were selected. Dimensional configuration of MLA was designed by various optical simulations. A SEM image of typical MLA is shown in Figure 3. A transparent electrode as an anode was fabricated on the other surface of the film. This film was then settled to a glass substrate, thus the light outcoupling layer composed of the plastic film as a high refractive index layer and the high refractive index microstructures with voids were fabricated between the transparent electrode and the glass substrate. light outcoupling layer Fig refractive index (n) Fig. 3. glass substrate transparent electrode high refractive index layer high refractive index microstructure with voids Schematic of light outcoupling substrate. SEM image of typical MLA structure. In this light outcoupling substrate, the high refractive index layer and microstructures decrease the total internal reflection at the interfaces around the transparent electrode. Figure 4 shows the refractive index dependence of optical enhancement factor of the light outcoupling substrates in a standard monochrome OLED device. With the higher refractive index structure, better enhancement factor was obtained. Additionally, anti-reflection treatment was done for the surfaces of the glass substrate in order to reduce the Fresnel s reflection. 3. Two-unit fluorescent/phosphorescent hybrid System On the light outcoupling substrate, the white OLED device with the two-unit structure composed

3 optical enhancement factor y of a red/green phosphorescent unit and blue fluorescent unit (mentioned in section 1) was fabricated [11-12]. Figure 5 shows a brief structure of this device; outcoupling substrate / wet coated HIL / blue fluorescent unit / connecting layer / red/green phosphorescent unit / cathode CIE 1931 x,y Chromaticity Diagram refractive index (n) Fig. 4 Refractive index dependence of optical enhancement factor in a standard monochrome OLED device. 2 black body radiation curve Fig. 6. Example of chromaticity change; : OLED on the glass substrate with film : OLED on the light outcoupling substrate : optimized OLED for the light outcoupling substrate cathode bus electrode x substrate light outcoupling substrate red/green phosphorescent unit transparent inter-unit layer deep blue fluorescent unit wet coated hole injection layer high refractive index layer high refractive index microstructure with voids Fig. 5. Schematic of the high efficacy white OLED with the light outcoupling substrate In order to achieve the better optical matching to the light outcoupling substrate (for example, improvement in the outcoupling efficiency and adjustment of the emission color to the white region), some modifications of the device structure, such as thicknesses and doping concentrations of emissive layers, were conducted. The examples of the variation in emission color are shown in Figure 6. By these optimization, luminous efficiency improved from 42 lm/w to 56 lm/w. A half decay lifetime of over 150,000 h, high CRI of 91 and color coordinates of (2, 1) within the tolerance quadrangle of correlated color temperature defined by Energy Star was obtained. Figure 7 shows the photograph of the prototype of this OLED panel with the emission area of 25 cm 2. emission area light outcoupling area wavelength (nm) anode Fig. 7. A high performance white OLED (25 cm 2 ) and the emission spectrum. 4. All phosphorescent, single unit white device In all phosphorescent white device, a blue phosphorescent emissive layer composed of a high T1 host and a blue phosphorescent emitter was required. A light blue phosphorescent emitter was selected to obtain high efficiency and enough lifetime, and the red and green phosphorescent emitters were chosen from the view of emission spectra in order to achieve white emission with high CRI when combined with the light blue emission. Schematic of the color coordinates of three primary colors (light blue, green and red) and of white are depicted in Figure 8. The position of blue emissive layer in OLED device was optically, electrically and experimentally investigated and determined to be between a green emissive layer and an electron transport layer [13]. Thus, the device structure was anode / HIL / HTL / red emissive layer / green emissive layer / blue emissive layer / ETL / EIL / cathode. 323

4 CIE-y light blue green 3,000K red CIE-x Fig. 8. Schematic of primary colors and white in all phosphorescent devices. By the introduction of blue emissive layer, increase of the driving voltage was observed. It would be due to the interfacial injection barrier of electrons from the electron transport layer to the blue phosphorescent emissive layer, and various electron transport materials with different energy levels were evaluated in order to reduce the driving voltage. Additionally, in order to achieve high quantum efficiency as well as an appropriate emission balance among red, green and blue, carrier transport properties of host materials and doping concentrations in three emissive layers were investigated and designed. This white OLED structure was fabricated onto the light outcoupling substrate mentioned in section 2 in this paper. For the reduction of the light extinction in the device, a highly reflective electrode was employed. The change in emission spectrum from the conventional OLED (with a glass substrate and an aluminum cathode) was observed due to the difference of the optical and electrical characteristics, and total design of the OLED device was re-optimized to adjust the carrier balance and color coordinate to the white region with high CRI. The white OLED device on the light outcoupling substrate realized a high efficiency of luminous efficacy of 87 lm/w, half decay lifetime of over 10,000 h, high CRI of 86 at the color coordinates of (63, 36) (color temperature: 2,860 K) at 1,000 cd/m 2. Emission area was 1 cm 2. External quantum efficiency was 42.8 %, thus light outcoupling efficiency of the light outcoupling substrate in this device was at least 42.8 %. Emission spectrum of the single unit white OLED device is shown in Figure Wavelength (nm) Fig. 9. Emission spectrum of the single unit white OLED device 5. All phosphorescent, multi-unit white device In order to improve the lifetime dramatically, twounit all phosphorescent white OLEDs were investigated [13]. Emissive layers of red, green and light blue were separated into two emissive units. Various combinations of two emissive units (see Figure 10, for example) were investigated by spectral simulation in order to obtain high CRI white emission with the intensity ratio of red : green : blue of about 2 : 1 : 1. After the experimental examination of various combinations, the two-unit white OLED device structure with red/blue and red/green was selected. RGB RGB GB R RG RB Fig. 10. Examples of various combinations of two emissive units for high CRI white emission. The two-unit device (emission area: 1 cm 2 ) composed of the red/blue and red/green phosphorescent units and aluminum cathode was deposited onto the light outcoupling substrate (Device A). The order of four emissive layers as well as the emission intensity ratios of red to blue in the red/blue unit and red to green in the red/green unit were carefully designed optically and electrically. This white OLED device achieved luminous efficacy of 73 lm/w at 6.6 V, external quantum efficiency of 74 % and estimated half decay lifetime of about 90,000 h at 1,000 cd/m 2, respectively. Emission spectrum is shown in Figure 11. The color coordinates of (48, 14) were very close to the black body radiation curve, and color temperature of 2,900 K and CRI of 81 were realized. 324

5 y The reduction of driving voltage and further improvement in light outcoupling efficiency were investigated. The interfacial injection barriers especially around the blue phosphorescent emissive layer and transport properties of some layers were again examined. In order to improve the light outcoupling efficiency, the highly reflective cathode and more transparent materials for some layers were applied, and optical structure of the device was redesigned to the combination of the high light outcoupling substrate and the highly reflective cathode (Device B). As a consequence, driving voltage decreased to 6.1 V and higher luminous efficacy of 83 lm/w at 1,000 cd/m 2 was obtained. Color coordinates was (67, 28) (color temperature: 2,750 K) and CRI was 86, respectively. Light outcoupling efficiency about 40 % was achieved also in the two-unit white OLED device. Estimated half decay lifetime was over 100,000 h. Table 1 summarizes the performance of the developed all phosphorescent white devices. Table 1. luminous efficacy driving voltage estimated half decay lifetime Fig. 11. devices CRI color coordinates color temperature Device A Performance of white devices Device B Device A 73 lm/w 6.6 V 90,000 h 81 (48, 14) 2,900 K Device B 83 lm/w 6.1 V 100,000 h 83 (67, 28) 2,750 K Wavelength (nm) Emission spectrum of all phosphorescent white 6. Conclusions The light outcoupling substrate as well as high performance white OLEDs were investigated. Light Fig. 12. CIE 1931 x,y Chromaticity Diagram black body radiation curve x Device A Device B Color coordinates of white devices outcoupilng substrate achieved high light coupling efficiency of 42.8 % in the white OLED. The fluorescent / phosphorescent hybrid two-unit device achieved high luminous efficacy of 56 lm/w, high CRI of 91 and long lifetime of 150,000 h at 1,000 cd/m 2 in the 25 cm 2 panel. All phosphorescent system achieved better performance. With the light outcoupling substrate, the single unit white OLED showed quite high luminous efficacy of 87 lm/w and half decay lifetime of about 10,000 h. In optically and electrically optimized two-unit system, luminous efficacy of 83 lm/w and estimated half decay lifetime of over 100,000 h were realized. Acknowledgments 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 started in March We thank to Idemitsu Kosan Co., Ltd. as a member of the project and their high performance host and electron transport materials for phosphorescent OLED systems, and Universal Display Corporation, Nippon Steel Chemical Co., Ltd., and Nissan Chemical Industries, Ltd. for their kind provisions of phosphorescent emitters and high performance materials. References [1] T. Komoda, H. Tsuji, N. Ito, T. Nishimori, N. Ide, SID 10 Digest (2010) 993. [2] T. Kawaguchi, Y. Ikagawa, M. Yamamoto, IDW 09 (2009)

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