APPLIED PHYSICS LETTERS 96, 153305 共2010兲
Improved light extraction efficiency of white organic light-emitting devices by biomimetic antireflective surfaces Yunfeng Li, Feng Li, Junhu Zhang, Chunlei Wang, Shoujun Zhu, Huijun Yu, Zhanhua Wang, and Bai Yanga兲 State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012, People’s Republic of China
共Received 10 February 2010; accepted 24 March 2010; published online 16 April 2010兲 One simple method to increase the light extraction from white organic light-emitting devices by using biomimetic silica antireflective surfaces is demonstrated. A silica cone array was directly etched on the opposite side of the indium–tin–oxide coated fused silica substrate. The antireflective surfaces can dramatically suppress the reflection loss and increase the transmission of light over a large range of wavelength and a large field of view. Using such surfaces, the luminance efficiency of the device in the normal direction is increased by a factor of 1.4 compared to that of the device using flat silica substrate. © 2010 American Institute of Physics. 关doi:10.1063/1.3396980兴 White organic light-emitting devices 共OLEDs兲 have attracted considerable interest owing to their potential in applications such as flat panel displays and interior lighting source.1,2 The OLEDs possess many advantages such as ultrathin, light weight, and environmentally protective. As a result, The OLEDs are considered as one of the candidates for next generation planar lighting source.3 Though electrophosphorescent OLEDs with an internal quantum efficiency of near 100% already approach the efficiency of fluorescent lamps,4,5 only about 20% of the generated light can escape from the OLEDs owing to total internal reflection 共TIR兲 in the glass substrate and waveguiding. Therefore, there is considerable potential for improvement in the external efficiency of OLEDs used for flat panel displays and interior lighting source. To date, several methods6 have been reported to increase the light extraction that include introducing textured surfaces,7 mesa shaped substrate,8 scattering mediums,9,10 microcavities,11,12 surface plasmons,13–15 ordered structures or photonic crystals,16–20 and a low refractive index silica aerogel layer21 to the devices. However, these methods often possess problems such as undesirable angle dependent emission spectra, complex and expensive fabrication processes. Madigan et al.22 designed OLEDs applying millimeter-sized hemispherical lens to the back side of the glass substrate, which avoids many shortcomings above. Introducing microlens arrays23–26 to the back side of the devices does not need alignment with the OLEDs and avoids the problem of angledependent emission. Sun and Forrest1 proposed OLEDs embedding a low refractive index grid in the organic layers and combining with microlens arrays. The resulting efficiency of the OLEDs was about 2.3 times that of a conventional OLED. Recently, Leo and co-workers2 reported an improved OLED which reached fluorescent tube efficiency. By combining high refractive index glass substrates with ordered outcoupling structures, they achieved a device with power efficiency of 90 lm W−1 at 1000 cd m−2. In this letter, we demonstrate a simple method to improve the light extraction of white OLEDs by silica biomimetic antireflective surfaces. A non-close-packed hexagonal a兲
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silica cone array was directly etched on the opposite side of the indium–tin–oxide 共ITO兲-coated fused silica substrate. The antireflective surfaces27,28 have similar dimensions with the nipple arrays on cornea of moth, which dramatically suppress the reflection loss and increase the transmission of light over a large range of wavelength and a large field of view. As a result, such surfaces are very proper as substrates of white OLEDs to improve the light extraction efficiency.6 The luminance efficiencies in the normal direction are increased by a factor of about 1.4. This method is simple, time-efficient, and reproducible. Moreover, such modified substrates exhibit good mechanical stabilities compared with other modified substrates, because there are no external materials involved. The conventional device without antireflective surfaces 关the right one of Fig. 1共a兲兴 was fabricated as a reference. The fused silica antireflective surfaces on the OLEDs were fabricated by colloidal lithography which was described in previous publication.28 The ITO film was deposited on the opposite side of the antireflective surfaces by sputtering. The ITO-coated silica substrates with and without antireflective surfaces were loaded into the multiple source organic mo-
FIG. 1. 共Color online兲 共a兲 The white OLEDs structures with 共left兲 and without 共right兲 antireflective surfaces. 关共b兲 and 共c兲兴 Top-view and cross-sectional SEM images of biomimetic silica antireflective surfaces, respectively.
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FIG. 2. EL spectra in the normal direction of OLEDs with structures 共black line兲 and without structures 共grey line兲 at the current density of 25 mA/ cm2. The inset is transmission of antireflective surfaces 共black line兲 and planar silica substrate 共grey line兲.
lecular beam deposition system, respectively, and the organic and metal materials were vapor deposited at a background pressure of 6 ⫻ 10−4 Pa. The fluorescent material of 2 , 5 , 2⬘ , 5⬘-tetrakis共2,2-diphenylvinyl兲 biphenyl 共TDPVBI兲 and phosphorescent material of iridium共III兲 bis共2-共9 , 9⬘-spirobi关fluorene兴-7-yl兲pyridine-N , C2⬘兲 acetylacetonate 关共SBFP兲2Ir共acac兲兴 were used to emit blue and yellow light, respectively.29,30 The structure of these devices was ITO/N,N-diphenyl-N,N-bis共1-naphthyl兲-共1,1-biphenyl兲4,4-diamine 共NPB兲 共40 nm兲/共TDPVBI兲 共30 nm兲/N,N⬘dicarbazolyl-3,5-benzene 共mCP兲: 8% 共SBFP兲2Ir共acac兲 共20 nm兲/1,3,5-tri共phenyl-2-benzimidazolyl兲-benzene 共TPBI兲 共40 nm兲/LiF 共0.5 nm兲/Al 共100 nm兲, where NPB is used as the hole-transporting layer and TPBI as the electron-transporting layer. The shadow masks were used to define the electrode areas and to make several 4 mm2 devices per substrate. Figures 1共b兲 and 1共c兲 show the scanning electron microscopy 共SEM兲 images of the silica biomimetic antireflective surfaces. We can see that the silica cone arrays are hexagonally non-close-packed just like the nipple arrays on the cornea of moth, and the root diameter of the cone is about
FIG. 3. 共Color online兲 Ratio of EL peak intensities 共I / Io, I, and Io are the EL peak intensity with and without antireflective surfaces兲 at the current density of 25 mA/ cm2 as a function of viewing angle.
FIG. 4. 共Color online兲 Normalized power efficiency 共a兲 and output light intensity 共b兲 of white OLEDs as a function of current density.
190 nm. We can also see that the top of the cone is smooth. The cross-sectional images of them display the tapered profile of the cone arrays. The cone arrays are vertical to substrate with 210 nm spacing and 241 nm in height 关Fig. 1共c兲兴. The cone arrays can dramatically improve the transmission of light over a large range of wavelength.28 Using such surfaces as substrates, the electroluminescent 共EL兲 intensity can be largely enhanced. Figure 2 shows the EL spectra in the normal direction of two OLEDs at the current intensity of 25 mA/ cm2. We can see that at peak wavelengths, the intensities of them are dramatically enhanced after using antireflective surfaces as substrates of OLEDs. Besides, the EL intensity enhancement of long wave is larger than that of short wave. This phenomenon is consistent with that for the antireflective surfaces used in our work,28 the enhanced transmission of long wave band is larger than that of short wave band 共inset of Fig. 2兲. Figure 3 shows the ratio of EL peak intensity 共I / Io, I, and Io are the EL peak intensity with and without antireflective surfaces.兲 at the current density of 25 mA/ cm2 as a function of viewing angle. The angular dependence of light emission was tested by putting the sample vertically on the center of a rotation stage described in our previous publication.26 The intensities of the peak emission from the OLEDs with the silica antireflective surfaces are larger than those of the device without antireflective surfaces at all viewing angles. Moreover, the enhancement of peak emission for long wave is larger than that of short wave as mentioned above. For planar silica substrate, because of TIR, the trans-
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mission of light reduces rapidly as the viewing angle increasing, especially above 40°. However, for biomimetic antireflective surface the transmission of light does not reduce rapidly like that of planar substrate.28 As a result, the ratio of the peak intensities of EL spectra increases rapidly when the viewing angles are above 40°. Figure 4共a兲 shows the normalized power efficiency of OLEDs as a function of the current density 共normalized to the maximum power efficiency of the device without the antireflective surfaces兲 measured in the normal direction. The enhancement of power efficiency is 1.4 for the OLEDs with the antireflective surfaces. The output light intensity in the normal direction of the OLEDs with and without antireflective surfaces as a function of the current density is shown in Fig. 4共b兲. We can see that the light intensities of both OLEDs increase as the current intensity increasing. Besides, a significant enhancement in light intensity is obtained for OLED with antireflective surfaces. In summary, we demonstrate that using biomimetic antireflective surfaces as the back side of white OLED substrates is promising for enhancement of the device light extraction efficiency. Owing to their broadband and large view of antireflective properties, the luminance efficiency in the normal direction for the white OLED is increased by about 1.4 times and the enhancement can be very large for the viewing angle above 40°. This method is simple, timeefficient, and reproducible. Therefore, the method mentioned here can be introduced in any OLEDs without any alteration of device structure and materials design. This work was supported by the National Science Foundation of China 共Grant Nos. 20921003, 20874039, and 60706016兲 and the National Basic Research Program of China 共Grant No. 2007CB936402兲. We are grateful for technical support from Dr. Jing Feng. Y. Sun and S. R. Forrest, Nat. Photonics 2, 483 共2008兲. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, and K. Leo, Nature 共London兲 459, 234 共2009兲. 3 B. W. D’Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater. 16, 624 共2004兲. 4 C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, J. Appl. 1 2
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