APPLIED PHYSICS LETTERS 89, 173502 共2006兲
Planarized SiNx/spin-on-glass photonic crystal organic light-emitting diodes Yoon-Chang Kim, Sang-Hwan Cho, and Young-Woo Song Corporate Research and Development Center, Samsung SDI Co., Ltd., Kyeonggi-Do 449-902, Korea
Yong-Jae Lee and Yong-Hee Lee Department of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
Young Rag Doa兲 Department of Chemistry, Kookmin University, Seoul 136-702, Korea
共Received 3 June 2006; accepted 4 September 2006; published online 23 October 2006兲 The light extraction characteristics of low-index spin-on-glass 共SOG兲-assisted, planarized photonic crystal organic light-emitting diodes 共PC OLEDs兲 are reported. The light extraction efficiencies of planarized two-dimensional 共2D兲 SiNx / SOG PC OLEDs 共type II兲 and 2D SiNx / SOG PC OLEDs with an additional high-index SiNx layer 共type III兲 are significantly better under typical operating conditions than those of the first generation of 2D SiO2 / SiNx PC OLEDs 共type I兲. The enhancements in the extraction efficiencies of type-II and type-III PC OLEDs are about 63% and 85%, respectively, with respect to those of conventional OLEDs with indium tin oxide layers of identical thicknesses. These improvements in extraction efficiencies are attributed not only to the liberation of the photons trapped in the high-index guiding layer but also to a reduction in the surface plasmon contribution. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2364160兴 The light extraction efficiencies of the organic lightemitting diodes 共OLEDs兲 fabricated to date are typically only small fractions of their internal efficiencies.1–4 Several out-coupling schemes have been implemented with the aim of improving light extraction from OLEDs.1–21 Recently, two-dimensional 共2D兲 slab photonic crystals 共PCs兲 were introduced into semiconductor LED structures.22–25 These results have prompted us and other groups to apply a similar approach to OLEDs while ensuring that the spatial resolution of the display devices is not degraded.12–21 Very recently, we showed that the introduction of 2D SiO2 / SiNx PC layers into OLEDs is an effective way to solve the light-trapping problem.15–17 Previously, we reported that incorporation of the 2D SiO2 / SiNx PC layer improved the light extraction efficiency by over 50% compared to that of the equivalent conventional OLED. However, in the above studies, the improvement in light extraction efficiency achieved by incorporating PC layers into OLEDs was smaller than expected.17 As reported previously,15 2D SiO2 / SiNx PC structures were fabricated by two-step laser hologram patterning, etching of 2D SiO2 nanorods on a glass substrate, SiNx overcoating using plasma-enhanced chemical vapor deposition 共PECVD兲, and planarization. Details of the fabrication of the 2D SiO2 / SiNx PC slab have been presented previously.15–17 Figures 1共a兲 and 1共b兲 show scanning electron microscope 共SEM兲 images of a tilted cross-sectional view of two type-I PC OLEDs containing a 2D SiO2 / SiNx PC slab. As can be seen in these images, the fabricated 2D SiO2 / SiNx PC slabs are not perfectly flat, indicating that the PECVD and subsequent polishing processes did not yield acceptably smooth surfaces. In fact, both type-I PC OLEDs have corrugated metallic top layers. The emission spectra recorded at constant current density 共20 mA/ cm2兲 for the conventional a兲
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OLED and type-I 2D SiO2 / SiNx PC OLEDs are shown in Fig. 1共d兲. The extraction efficiency of the type-I 2D PC OLED with a flatter surface is much greater than that of the less flat type-I 2D PC OLED. This difference can be attributed in part to current leakage at the uneven interface and also to the formation of surface plasmons at the interface between the corrugated Al cathode and the organic layer. Therefore, further improvement should be possible if structures can be fabricated that reduce the undesirable effects arising from nonplanar PC layers. In this letter, we investigate two types of efficient spin on glass 共SOG兲-assisted 2D PC OLED structures and the fabrication processes for planarization.
FIG. 1. Focused ion beam SEM images of PC OLEDs 共type I兲 on a 2D SiO2 / SiNx PC layer 共⫻65 000兲: 共a兲 less flat 2D PC slab and 共b兲 flatter 2D PC slab. 共c兲 Schematic diagram of a modified OLED employing a 2D PC slab 共type I兲: 2D SiO2 / SiNx PC layer 共200 nm兲 / ITO 共150 nm兲/organics 共145 nm兲 / Al cathode 共200 nm兲. 共d兲 Emission spectra of OLEDs with and without a 2D PC layer 共two shaped PC OLEDs兲 at the interface between the glass layer and the ITO electrode.
0003-6951/2006/89共17兲/173502/3/$23.00 89, 173502-1 © 2006 American Institute of Physics Downloaded 23 Oct 2006 to 210.123.45.229. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 3. Dependence of calculated enhancement of extraction efficiency on the thickness of the SOG overcoating the 2D SiNx / SOG layer of 2D PC OLEDs.
FIG. 2. Schematic diagram of a modified OLED employing 2D PC slab: 共a兲 2D SiNx / SOG PC layer 共300 nm兲 / SOG-overcoated layer 共20 nm兲 / ITO 共150 nm兲/organics 共145 nm兲 共type II兲, and 共b兲 2D SiNx / SOG PC layer 共300 nm兲 / SOG-overcoated layer 共20 nm兲 / SiNx high-index layer 共260 nm兲 / ITO 共80 nm兲/organics 共145 nm兲 共type III兲. 共c兲 Field-emissiontype SEM images of the top view of the 2D high-index material nanohole pattern 共⫻50,000兲: period of 350 nm, hole diameter of 250 nm, and height of 300 nm. Focused ion beam SEM images of the cross-sectional view of PC OLEDs inserted with 共d兲 a flatter 2D SiNx / SOG PC layer 共⫻40 000兲 and 共e兲 a high-index 共SiNx兲-coated 2D SiNx / SOG PC layer 共⫻55 000兲.
As shown schematically in Figs. 2共a兲 and 2共b兲, the 2D SiNx / SOG PC layer was inserted at the interface between the glass substrate and the indium tin oxide 共ITO兲 electrode to improve the degree of flatness. To fabricate a flatter PC slab for PC OLEDs 共type II兲, we selected a 2D nanohole array of high-index SiNx 共n = 1.93兲 as the base structure, which was filled with a low-index SOG 共n = 1.28兲. First, a SiNx film was deposited onto the glass substrate using PECVD. Two-step irradiated laser-interference lithography and reactive-ion etching 共RIE兲 were then used to generate a square-lattice 2D pattern of nanoholes in the SiNx film.18,26 Figure 2共c兲 shows a 2D nanohole array with lattice constant of ⬃350 nm, nanohole diameter of ⬃250 nm, and height of ⬃300 nm. The nanoholes in the array are regular and uniform, with a fill factor of about ⬃40%. Spin coating was employed to fill the nanoholes with the SOG material 共Nanoglass-E, Honeywell Electronic Materials兲. The resultant high-index-contrast SiNx / SOG 共1.93/ 1.28兲 PC is expected to give stronger diffraction than the low-index-contrast SiNx / SiO2 共1.93/ 1.48兲 PC. However, this process leaves an unwanted SOG layer over the 2D PC pattern, which degrades the extraction efficiency if this layer is too thick. In fact, additional total internal reflection occurs at the interface between ITO 共n = 1.8兲 and SOG 共n = 1.28兲. Therefore, the SOG layer needs to be adjusted by subsequent etch-back processes under an Ar + CHF3 gas flow. After etching, the surface of the 2D PC slab was found to be smooth and flat, and thus to be an excellent substrate for 2D PC OLEDs 关Figs. 2共d兲 and 2共e兲兴. An additional high-index SiNx layer was deposited over the 2D PC slab by PECVD. We fabricated OLEDs on a conventional
substrate and two other substrates covered with PC layers, side by side for comparison. In order to find the optimal thickness of the overcoated SOG layer, the finite-difference time-domain 共FDTD兲 method was employed.15–17 The relative enhancement of the extraction efficiency is obtained by comparing the light output of the 2D PC OLED to that of the equivalent conventional OLED. These results indicate that when the thickness of the overcoated SOG layer is equal to a critical thickness of 28 nm, the extraction efficiency is higher than when the thickness of this layer is zero; the critical thickness in Fig. 3 is defined as the thickness at which the additional total internal reflection effect is almost negligible. RIE processes 共socalled etch-back processes兲 were employed to remove the overcoated SOG layer and to planarize the 2D SiNx / SOG PC layer until the thickness of the SOG overcoated layer was less than the critical thickness. Figure 2共d兲 shows a SEM image taken from the side of PC OLED inserted with 2D SiNx nanohole arrays 共2D SiNx / SOG PC slab兲 coated with a 20-nm-thick SOG film, resulting in a PC OLED with a flatter 2D PC slab. This indicates that the series of fabrication processes produce a flatter PC OLED 共type II兲 that coincides with the designed structure in Fig. 2共a兲. Figure 2共b兲 shows another type of 2D PC OLED with improved extraction efficiency and a planarized 2D PC slab covered with an additional high-index film 共type III兲. We speculated that the high-index layer would either collect photons in the adjacent organic layer through the interface between the PC slab and the organic layer or redirect the light emitted from the organic layer to the front side through a microcavity effect. Our FDTD calculations were also used to determine the extraction efficiency of a 2D PC OLED with a high-index layer between the organic layer and the 2D SiNx / SOG PC slab, and suggest a improvement in the extraction efficiency for this device of ⬃20%. Figure 2共e兲 shows a SEM image taken from the side of a PC OLED based on a 2D SiNx / SOG PC slab with a high-index coating. This also demonstrates that our series of fabrication processes produce a flatter PC OLED with a high-index coating 共type III兲, and provides a good match with the designed structure in Fig. 2共b兲. To determine the effects of the planarized 共types II and III兲 2D SiNx / SOG PC layers experimentally, we fabricated OLEDs on both a conventional substrate and a substrate patterned with a PC layer, side by side for comparison. After fabricating a 2D SiNx / SOG PC layer onto the glass substrate, the ITO layer and the SiNx / ITO double layers were deposited to produce types II and III, respectively. The SiNx
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FIG. 4. Current density-efficiency characteristics of the two conventional OLEDs 共ITO of 80 and 150 nm兲 and of the two types of 2D SiNx / SOG PC OLEDs 共with and without a high-index layer兲.
layer was deposited by PECVD, and the ITO layer was deposited by rf-magnetron sputtering with no intentional heating. Subsequent application of photolithographic patterning and the etching processes to the ITO produced the ITO stripe patterns. OLEDs consisting of organic layers of N , N⬘-di共naphthalene-1-yl兲-N , N⬘-diphenyl-benzidine, tris共8hydroxyquinoline兲 aluminum 共Alq3兲-coumarin 6 共C6兲, and Alq3 were prepared by a previously reported series fabrication processes.15–17 Electroluminescence measurements showed the currentdensity-luminance characteristics of the four types of OLEDs with and without the 2D SiNx / SOG PC layer. The luminance values of the two types of 2D PC OLEDs and a conventional OLED measured in the normal direction 共 = 0 ° 兲 under dc excitation at 20 mA/ cm2 were found to be 4494 共type III, ITO thickness of 80 nm兲, 3477 共type II, ITO thickness of 150 nm兲, 2380 cd/ m2 共ITO thickness of 80 nm兲, and 2143 共ITO thickness of 150 nm兲. Figure 4 shows the relationships between current efficiency and current density for the conventional OLEDs as well as the 2D PC OLEDs 共Types II and III兲, for a fixed lattice constant of 350 nm.17 In fact, the critical PC parameters were chosen to be similar to those used in our previous experiments.16,17 For the conventional OLEDs, the current efficiencies, at 20 mA/ cm2, in the normal direction were found to be 10.7 共ITO of 150 nm兲 and 11.7 cd/ A 共ITO of 80 nm兲. In comparison, the efficiencies of the type-II and -III 2D PC OLEDs were measured to be 17.4 共ITO of 150 nm兲 and 21.6 cd/ A 共ITO of 80 nm兲, respectively, indicating that the efficiencies in the normal direction are 63% and 85% higher than those of the corresponding conventional OLEDs. These results indicate that the extraction efficiency of the 2D SiNx / SOG PC OLED is larger than that of our previous 2D SiO2 / SiNx PC OLED. The flatter interfaces in the PC structure introduced here are believed to be partly responsible for reducing the optical and electrical losses. For the type-III OLED, the additional SiNx layer enhances the light extraction through the microcavity effect. In this case, the total optical thickness of the high-index layers is close twice the wavelength of the emitted light. As shown in Figs. 3 and 4, the calculated enhancement of the total amount of emitted light is slightly different from that for the measured extraction in the normal direction, because the integrated value includes contributions from the anisotropic diffraction fringes in the far-field radiation patterns of the PC OLEDs.18 Figure 4 also shows the Commission Internationale de’Eclairage color coordinates in the normal direction for the conventional OLEDs and the 2D PC OLEDs. Although the emission colors of the 2D PC OLEDs do not exactly
coincide with those of the conventional OLEDs, the light emitted by the 2D PC OLEDs is of slightly higher color purity than that emitted by the conventional OLEDs. In addition, our measurements of the angular dependence of the emission spectrum confirmed that the angular dependence of the color change of the 2D PC OLEDs 共types II and III兲 is similar to that of the conventional OLEDs, consistent with our previous work on PC OLEDs.15–18 In summary, we report the successful introduction of SOG-planarized 2D PC layers into OLED structures. We showed experimentally that the incorporation of a flatter PC layer or both a high-index overcoated and flatter 2D PC layer improved the light extraction efficiency by over 63% and 85% compared to the conventional OLED in the normal direction. And the flatter 2D PC layer is found to be more favorable for light extraction. This work was supported by Grant No. 2005-02522 of the Nano R&D program and Grant No. R11-2005-04800000-0 of the ERC program from the Ministry of Science and Technology in Korea. 1
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