October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS
5901
Enhanced light extraction efficiency in organic light emitting diodes using a tetragonal photonic crystal with hydrogen silsesquioxane Yang Doo Kim,1,† Kyeong-Hoon Han,2,† Sang-Jun Park,1 Jung-Bum Kim,2 Ju-Hyeon Shin,1 Jang Joo Kim,2,4 and Heon Lee1,3 1
Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-713, South Korea 2
Department of Material Science and Engineering and the Center for Organic Light Emitting Diodes, Seoul National University, 599, Gwanangno, Gwanak-gu, Seoul 151-744, South Korea 3
e-mail: [email protected] 4
e-mail: [email protected]
Received April 17, 2014; revised September 10, 2014; accepted September 10, 2014; posted September 11, 2014 (Doc. ID 210128); published October 10, 2014 We report an organic light emitting diode (OLED) with a hydrogen silsesquioxane as a scattering material, for enhancing light extraction efficiency. A tetragonal photonic crystal was used as pattern type, and fabricated using a direct printing technique. Planarization was accomplished using TiO2 solgel solution, having a refractive index identical to that of the indium zinc oxide transparent electrode. The current efficiency and power efficiency of the OLED increased by 17.3% and 43.4% at 10 mA∕cm2 , respectively, without electric degradation. © 2014 Optical Society of America OCIS codes: (160.4890) Organic materials; (230.3670) Light-emitting diodes; (250.0250) Optoelectronics; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/OL.39.005901
Organic light emitting diodes (OLEDs) are in use for displays, and extending their application to solid state lighting. OLEDs are potentially high quality planar light sources, with a high color rendering index and warm color. One drawback of OLEDs at this stage is lower efficiency than inorganic light emitting diodes. The efficiency of OLEDs has been improved steadily during the last couple of decades to reach an external quantum efficiency (EQE) of about 30%, without extra light extraction layers, in recent years corresponding to almost 100% internal quantum efficiency [1–3]. Still, 70% of the emitted light is confined in OLEDs as waveguide and substrate mode, or lost as the surface plasmon polariton (SPP) mode of metal electrodes or absorption. Therefore, it is important to extract the light confined in OLEDs to improve the efficiency of OLEDs. Various methods have been reported to extract light in OLEDs, which can be categorized as the external light extraction methods represented by a micro-lens array [4,5] attached on glass substrates, and internal light extraction methods such as photonic or phononic crystals [6–8], buckled structures [9], low index grids [10], and scattering nanopillars or spheres [11–15]. The external light extraction method has a limitation of extracting substrate confined light only. Internal light extraction layers (ILELs) are required to extract the waveguide or SPP mode through scattering of emitted light, using various nano and micropatterns between the substrate and device. Considering the efficiency of OLEDs, only the geometry of the scattering pattern is the most important; however, in order to achieve reliable devices, materials forming a light extracting structure should be under consideration. Various materials have been reported for scattering materials, such as SiO2 , [6,8,10,14] UVcurable resin [7,9,12], inorganic nanoparticles [13,15], and so on. SiO2 having the same refractive index as the 0146-9592/14/205901-04$15.00/0
substrate and high durability is hard to fabricate, with respect to sophisticated patterns. UV-curable resin has a similar refractive index as a glass substrate, and it is easy to make complicated patterns by using UV/nanoimprint lithography (NIL). However, polymer resins have a high gas permeability and thermal expansion coefficient, which impart difficulties to applications in a commercialized device. Inorganic nanoparticles have a high refractive index, but a matrix layer must be included, which is not effective with respect to light extraction. In this Letter, a tetragonal photonic crystal (PC) pattern was fabricated on glass, by using a direct printing technique and a simple patterning process with hydrogen silsesquioxane (HSQ) [16,17]. Surface roughness was reduced by planarization for stability of the device. An Indoped ZnO (IZO) layer and organic layers were deposited on a patterned substrate, using a sputtering system and a thermal evaporator, respectively. The optical and electrical properties of the OLED were then measured. In order to fabricate a nanopattern on glass for light scattering, HSQ (FOX-16, Dow Corning) was selected as a pattern material, owing to its superior formability than SiO2 , high transparency, and almost the same refractive index as glass. Contrary to UV-curable resin (∼230 × 10−6 m∕m∕°C), the coefficient of linear thermal expansion of HSQ (18 × 10−6 m∕m∕°C) is similar to glass (3.6 × 10−6 m∕m∕°C) [18]. Patterning of HSQ can be easily accomplished using a structural transformation, from a cage structure to a network structure at high temperature (>400°C). However, in order to fabricate a pattern, the HSQ solvent should be removed. Therefore, for direct HSQ printing techniques, a polydimethylsiloxane (PDMS) mold was selected, owing to its low surface energy and solvent permeability. Figure 1 depicts fabrication of an OLED on a nanosized pattern substrate with a flattened surface. First, in order © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 20 / October 15, 2014
to fabricate a tetragonal PC, a PDMS stamp with conical structures 400 nm in height, 450 nm in diameter, and 600 nm in pitch as a square array was replicated from a master template using nanomolding. The HSQ solution was coated onto the PDMS mold using a spin-coating process at 3000 rpm for 30 s. After spin-coating in air ambient, the PDMS mold covered by the HSQ layer was contacted with a cleaned glass substrate, and the HSQ patterned layer was replicated on a glass substrate (Corning Display Technologies, AMLCD Glass) by direct printing, with an applied pressure of 5 atm for 3 min in vacuum. In this step, the HSQ solvent was evaporated through the PDMS mold. The PDMS mold was detached from the printed glass substrate, and the patterned HSQ on glass remained in the solid state. In order to remove the residual organic materials in the HSQ pattern and cure the HSQ, the HSQ nanosized pattern was annealed at 500°C for 1 h in air. Finally, the HSQ nanopattern was fabricated using indentation with a master template. The resulting nanosized pattern with sharp structures was planarized using a TiO2 solgel solution, in order to obtain low surface roughness. The material for planarization had a similar refractive index as indium zinc oxide (IZO), in order to minimize reflection between the IZO and planarization layer. After fabricating a nanopattern with a planarized surface, an IZO (150 nm, 40 ohm/sq) electrode was deposited on substrates by face targeting sputtering using an IZO target (90 wt. % In2 O3 : 10 wt. % ZnO). After deposition, all substrates are cleaned with acetone and isopropyl alcohol (IPA). The organic layers were deposited on the IZO by thermal evaporation in the following sequence: (1) rhenium oxides (ReO3 , 1 nm); (2) 1,1bis-[4-bis(4-methyl-phenyl)-amino-phenyl]-cyclohexane (TAPC, 40 nm); (3) 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN, 5 nm); (4) TAPC (40 nm); (5) HATCN (5 nm); (6) TAPC (40 nm); (7) HATCN (5 nm); (8) TAPC (40 nm); (9) 4,40 ,400 -tris(carbazol-9-yl)triphenylamine (TCTA, 10 nm); (10) bis(2-phenylpyridyl)
iridium(III) acetylacetonate [Ir(ppy)2acac] (8 wt. %) doped TCTA and bis-4,6-(3,5-di-3-pyridylphenyl)-2methylpyrimidine (B3PYMPM) cohost as a green phosphorescent emitter (30 nm); (11) B3PYMPM (40 nm); (12) lithium fluoride (LiF, 1 nm); and (13) aluminum (Al, 100 nm). Thick multilayers were deposited for protecting leakage current at the edge of the electrode. The devices were encapsulated with epoxy resin and glass canisters. A Keithley 2400 semiconductor parameter analyzer and a Photo Research PR-650 spectrophotometer were used to measure the luminescent and electric characteristics, respectively. Figure 2 shows scanning electron microscopy (SEM) images of the nanopattern, comprising conical-shaped structures and the corresponding planarized nano pattern. The height of the conical structure of the nanopattern shown in Fig. 2(a) i 400 nm, and the diameter and pitch of the pattern were 500 and 600 nm, respectively. Figure 2(b) shows an SEM micrograph of the planarized nanopattern. The uniform planarization of the nanopattern with the TiO2 layer was confirmed, and no cracks were observed. The surface roughness of the nanopattern and the planarized nanopattern was measured using atomic force microscopy (AFM), which revealed that the surface roughness was reduced from 97 to 15 nm by the planarization process. The optical properties of the nanopattern were measured using the integrating sphere system of a UV-vis spectrometer, as shown in Fig. 3. The total transmittance of the nanopattern was decreased by 12% at 630 nm, which is the same as the period of the nanocone array. However, at the FWHM of photoluminescence of Irppy2 acac, 505–565 nm, the total transmittance of the nanopattern was almost identical to that of flat glass. As presented in Fig. 3(b), the haziness of the nanopattern
Fig. 1. Schematic fabrication process of an OLED on a planarized nanopattern.
Fig. 2. Top and cross sectional SEM images: (a) nanopattern and (b) planarized nanopattern.
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October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS 20
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Fig. 4. (a) Current density-voltage (J-V) and (b) luminancevoltage of OLEDs on flat glass, and PTPC pattern and spectrum at normal direction (inset).
and a planarized nanopattern. In comparison with the OLED fabricated on flat glass, the current and power efficiency of OLEDs on substrates with a nanopattern (a) 120 Current efficiency (cd/A)
dramatically increased up to 97% at a wavelength of 320 nm. At 505–565 nm, the haziness of the nanopattern was enhanced by up to 30%. In order to evaluate the electrical and optical properties of OLEDs on the flat glass (reference) and the planarized tetragonal photonic crystal (PTPC) pattern, current density-voltage-luminescence (J-V-L) characteristics were measured. Figure 4 shows current density and luminance as a function of applied voltage. As shown in Fig. 4(a), the OLEDs on a patterned substrate showed higher current density at the same voltage. The low surface roughness of 15 nm still increases current density, because of the focused electric field. The spectrum at the normal direction and luminance is shown in Fig. 4(b). Because of periodic PC, the device with a PTPC pattern had a narrow spectrum and showed higher luminance at the same voltage, in comparison with the reference device. The current efficiencies of the OLED on the flat glass and PTPC pattern showed, respectively, 77.2 and 85.3 cd/ A at 2 mA∕cm2 and 71.5 and 83.9 cd/A at 10 mA∕cm2 , an enhancement ratio of 10.5% and 17.3% [Fig. 5(a)]. Enhanced current efficiency means that our PTPC pattern extracted light, which were confined at flat device. The Lambertian corrected power efficiencies of the OLED on the PTPC pattern were, respectively, 67.4 and 87.2 lm/w at 2 mA∕cm2 , and 44.5 and 63.8 at 10 mA∕cm2 , an enhancement ratio of 29.4% and 43.4% [Fig. 5(b)]. An OLED with the PTPC had a similar roll-off as an OLED on flat glass. Despite the increased charge injection, similar roll-off means the planarized substrate didn’t deteriorate the electrical performance of the OLED. In conclusion, we fabricated a PTPC pattern on glass using a direct printing technique. The surface roughness of the nanopattern was reduced by planarization using a TiO2 solgel solution. OLEDs were fabricated on flat glass
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Fig. 5. Efficiency and power efficiency of OLEDs on two types of substrates: (a) efficiency-current density and (b) power efficiency-current density for OLEDs on flat glass and the PTPC pattern.
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increased 17.3% and 43.4%, respectively. The patterned device also showed similar roll-off as a device on flat glass. This research was supported by the R&D program for Industrial Core Technology through the Korea Evaluation Institute of Industrial Technology, supported by the Ministry of Knowledge Economy in Korea (Grant No. 10040225) and the IT R&D program of MKE/KEIT (10041062, Development of Fundamental Technology for Light Extraction of OLED). † These authors contributed equally to this work. References 1. D. Tanaka, H. Sasabe, Y.-J. Li, S.-J. Su, T. Takeda, and J. Kido, Jpn. J. Appl. Phys. 46, L10 (2007). 2. M. G. Helander, Z. B. Wang, J. Qiu, M. T. Greiner, D. P. Puzzo, Z. W. Liu, and Z. H. Lu, Science 332, 944 (2011). 3. Y. S. Park, K.-H. Kim, and J.-J. Kim, Appl. Phys. Lett. 102, 153306 (2013). 4. S. Möller and S. R. Forrest, J. Appl. Phys. 91, 3324 (2002). 5. J. Lim, S. S. Oh, D. Y. Kim, S. H. Cho, I. T. Kim, S. H. Han, H. Takezoe, E. H. Choi, G. S. Cho, Y. H. Seo, S. O. Kang, and B. Park, Opt. Express 14, 6564 (2006). 6. Y.-J. Lee, S.-H. Kim, J. Huh, G.-H. Kim, Y.-H. Lee, S.-H. Cho, Y.-C. Kim, and Y. R. Do, Appl. Phys. Lett. 82, 3779 (2003).
7. S. Jeon, J.-W. Kang, H.-D. Park, J.-J. Kim, J. R. Youn, J. Shim, J.-H. Jeong, D. G. Choi, K.-D. Kim, A. O. Altun, S.-H. Kim, and Y.-H. Lee, Appl. Phys. Lett. 92, 223307 (2008). 8. T. Okamoto and K. Shinotsuka, Appl. Phys. Lett. 104, 093301 (2014). 9. W. H. Koo, S. M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, and H. Takezoe, Nat. Photonics 4, 222 (2010). 10. Y. Sun and S. R. Forrest, Nat. Photonics 2, 483 (2008). 11. A. Kumar, R. Srivastava, M. N. Kamalasanan, and D. S. Mehta, Opt. Lett. 37, 575 (2012). 12. C. Lee and J.-J. Kim, Small 9, 3858 (2013). 13. Y. H. Kim, J. Lee, W. M. Kim, C. Fuchs, S. Hofmann, H.-W. Chang, M. C. Gather, L. Müller-Meskamp, and K. Leo, Adv. Funct. Mater. 10, 1002 (2013). 14. J.-W. Shin, D.-H. Cho, J. Moon, C. W. Joo, S. K. Park, J. Lee, J.-H. Han, N. S. Cho, J. Hwang, J. W. Huh, H. Y. Chu, and J.-I. Lee, Org. Electron. 15, 196 (2014). 15. H.-W. Changa, Y. H. Kima, J. Lee, S. Hofmann, B. Lüssem, L. Müller-Meskamp, M. C. Gather, K. Leo, and C.-C. Wu, Org. Electron. 15, 1028 (2014). 16. Y. Igaku, S. Matsui, H. Ishigaki, J. Fujita, M. Ishida, Y. Ochiai, H. Namatsu, M. Komuro, and H. Hiroshima, Jpn. J. Appl. Phys. 41, 4198 (2002). 17. Y. Kang, M. Okada, S. Omoto, Y. Haruyama, K. Kanda, and S. Matsui, J. Vac. Sci. Technol. B29, 06FC03 (2011). 18. H. C. Liou and J. Pretzer, Thin Solid Films 335, 186 (1998).
5901
Enhanced light extraction efficiency in organic light emitting diodes using a tetragonal photonic crystal with hydrogen silsesquioxane Yang Doo Kim,1,† Kyeong-Hoon Han,2,† Sang-Jun Park,1 Jung-Bum Kim,2 Ju-Hyeon Shin,1 Jang Joo Kim,2,4 and Heon Lee1,3 1
Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong, Sungbuk-gu, Seoul 136-713, South Korea 2
Department of Material Science and Engineering and the Center for Organic Light Emitting Diodes, Seoul National University, 599, Gwanangno, Gwanak-gu, Seoul 151-744, South Korea 3
e-mail: [email protected] 4
e-mail: [email protected]
Received April 17, 2014; revised September 10, 2014; accepted September 10, 2014; posted September 11, 2014 (Doc. ID 210128); published October 10, 2014 We report an organic light emitting diode (OLED) with a hydrogen silsesquioxane as a scattering material, for enhancing light extraction efficiency. A tetragonal photonic crystal was used as pattern type, and fabricated using a direct printing technique. Planarization was accomplished using TiO2 solgel solution, having a refractive index identical to that of the indium zinc oxide transparent electrode. The current efficiency and power efficiency of the OLED increased by 17.3% and 43.4% at 10 mA∕cm2 , respectively, without electric degradation. © 2014 Optical Society of America OCIS codes: (160.4890) Organic materials; (230.3670) Light-emitting diodes; (250.0250) Optoelectronics; (310.6845) Thin film devices and applications. http://dx.doi.org/10.1364/OL.39.005901
Organic light emitting diodes (OLEDs) are in use for displays, and extending their application to solid state lighting. OLEDs are potentially high quality planar light sources, with a high color rendering index and warm color. One drawback of OLEDs at this stage is lower efficiency than inorganic light emitting diodes. The efficiency of OLEDs has been improved steadily during the last couple of decades to reach an external quantum efficiency (EQE) of about 30%, without extra light extraction layers, in recent years corresponding to almost 100% internal quantum efficiency [1–3]. Still, 70% of the emitted light is confined in OLEDs as waveguide and substrate mode, or lost as the surface plasmon polariton (SPP) mode of metal electrodes or absorption. Therefore, it is important to extract the light confined in OLEDs to improve the efficiency of OLEDs. Various methods have been reported to extract light in OLEDs, which can be categorized as the external light extraction methods represented by a micro-lens array [4,5] attached on glass substrates, and internal light extraction methods such as photonic or phononic crystals [6–8], buckled structures [9], low index grids [10], and scattering nanopillars or spheres [11–15]. The external light extraction method has a limitation of extracting substrate confined light only. Internal light extraction layers (ILELs) are required to extract the waveguide or SPP mode through scattering of emitted light, using various nano and micropatterns between the substrate and device. Considering the efficiency of OLEDs, only the geometry of the scattering pattern is the most important; however, in order to achieve reliable devices, materials forming a light extracting structure should be under consideration. Various materials have been reported for scattering materials, such as SiO2 , [6,8,10,14] UVcurable resin [7,9,12], inorganic nanoparticles [13,15], and so on. SiO2 having the same refractive index as the 0146-9592/14/205901-04$15.00/0
substrate and high durability is hard to fabricate, with respect to sophisticated patterns. UV-curable resin has a similar refractive index as a glass substrate, and it is easy to make complicated patterns by using UV/nanoimprint lithography (NIL). However, polymer resins have a high gas permeability and thermal expansion coefficient, which impart difficulties to applications in a commercialized device. Inorganic nanoparticles have a high refractive index, but a matrix layer must be included, which is not effective with respect to light extraction. In this Letter, a tetragonal photonic crystal (PC) pattern was fabricated on glass, by using a direct printing technique and a simple patterning process with hydrogen silsesquioxane (HSQ) [16,17]. Surface roughness was reduced by planarization for stability of the device. An Indoped ZnO (IZO) layer and organic layers were deposited on a patterned substrate, using a sputtering system and a thermal evaporator, respectively. The optical and electrical properties of the OLED were then measured. In order to fabricate a nanopattern on glass for light scattering, HSQ (FOX-16, Dow Corning) was selected as a pattern material, owing to its superior formability than SiO2 , high transparency, and almost the same refractive index as glass. Contrary to UV-curable resin (∼230 × 10−6 m∕m∕°C), the coefficient of linear thermal expansion of HSQ (18 × 10−6 m∕m∕°C) is similar to glass (3.6 × 10−6 m∕m∕°C) [18]. Patterning of HSQ can be easily accomplished using a structural transformation, from a cage structure to a network structure at high temperature (>400°C). However, in order to fabricate a pattern, the HSQ solvent should be removed. Therefore, for direct HSQ printing techniques, a polydimethylsiloxane (PDMS) mold was selected, owing to its low surface energy and solvent permeability. Figure 1 depicts fabrication of an OLED on a nanosized pattern substrate with a flattened surface. First, in order © 2014 Optical Society of America
5902
OPTICS LETTERS / Vol. 39, No. 20 / October 15, 2014
to fabricate a tetragonal PC, a PDMS stamp with conical structures 400 nm in height, 450 nm in diameter, and 600 nm in pitch as a square array was replicated from a master template using nanomolding. The HSQ solution was coated onto the PDMS mold using a spin-coating process at 3000 rpm for 30 s. After spin-coating in air ambient, the PDMS mold covered by the HSQ layer was contacted with a cleaned glass substrate, and the HSQ patterned layer was replicated on a glass substrate (Corning Display Technologies, AMLCD Glass) by direct printing, with an applied pressure of 5 atm for 3 min in vacuum. In this step, the HSQ solvent was evaporated through the PDMS mold. The PDMS mold was detached from the printed glass substrate, and the patterned HSQ on glass remained in the solid state. In order to remove the residual organic materials in the HSQ pattern and cure the HSQ, the HSQ nanosized pattern was annealed at 500°C for 1 h in air. Finally, the HSQ nanopattern was fabricated using indentation with a master template. The resulting nanosized pattern with sharp structures was planarized using a TiO2 solgel solution, in order to obtain low surface roughness. The material for planarization had a similar refractive index as indium zinc oxide (IZO), in order to minimize reflection between the IZO and planarization layer. After fabricating a nanopattern with a planarized surface, an IZO (150 nm, 40 ohm/sq) electrode was deposited on substrates by face targeting sputtering using an IZO target (90 wt. % In2 O3 : 10 wt. % ZnO). After deposition, all substrates are cleaned with acetone and isopropyl alcohol (IPA). The organic layers were deposited on the IZO by thermal evaporation in the following sequence: (1) rhenium oxides (ReO3 , 1 nm); (2) 1,1bis-[4-bis(4-methyl-phenyl)-amino-phenyl]-cyclohexane (TAPC, 40 nm); (3) 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN, 5 nm); (4) TAPC (40 nm); (5) HATCN (5 nm); (6) TAPC (40 nm); (7) HATCN (5 nm); (8) TAPC (40 nm); (9) 4,40 ,400 -tris(carbazol-9-yl)triphenylamine (TCTA, 10 nm); (10) bis(2-phenylpyridyl)
iridium(III) acetylacetonate [Ir(ppy)2acac] (8 wt. %) doped TCTA and bis-4,6-(3,5-di-3-pyridylphenyl)-2methylpyrimidine (B3PYMPM) cohost as a green phosphorescent emitter (30 nm); (11) B3PYMPM (40 nm); (12) lithium fluoride (LiF, 1 nm); and (13) aluminum (Al, 100 nm). Thick multilayers were deposited for protecting leakage current at the edge of the electrode. The devices were encapsulated with epoxy resin and glass canisters. A Keithley 2400 semiconductor parameter analyzer and a Photo Research PR-650 spectrophotometer were used to measure the luminescent and electric characteristics, respectively. Figure 2 shows scanning electron microscopy (SEM) images of the nanopattern, comprising conical-shaped structures and the corresponding planarized nano pattern. The height of the conical structure of the nanopattern shown in Fig. 2(a) i 400 nm, and the diameter and pitch of the pattern were 500 and 600 nm, respectively. Figure 2(b) shows an SEM micrograph of the planarized nanopattern. The uniform planarization of the nanopattern with the TiO2 layer was confirmed, and no cracks were observed. The surface roughness of the nanopattern and the planarized nanopattern was measured using atomic force microscopy (AFM), which revealed that the surface roughness was reduced from 97 to 15 nm by the planarization process. The optical properties of the nanopattern were measured using the integrating sphere system of a UV-vis spectrometer, as shown in Fig. 3. The total transmittance of the nanopattern was decreased by 12% at 630 nm, which is the same as the period of the nanocone array. However, at the FWHM of photoluminescence of Irppy2 acac, 505–565 nm, the total transmittance of the nanopattern was almost identical to that of flat glass. As presented in Fig. 3(b), the haziness of the nanopattern
Fig. 1. Schematic fabrication process of an OLED on a planarized nanopattern.
Fig. 2. Top and cross sectional SEM images: (a) nanopattern and (b) planarized nanopattern.
2
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Current Density (mA/cm )
October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS 20
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Fig. 4. (a) Current density-voltage (J-V) and (b) luminancevoltage of OLEDs on flat glass, and PTPC pattern and spectrum at normal direction (inset).
and a planarized nanopattern. In comparison with the OLED fabricated on flat glass, the current and power efficiency of OLEDs on substrates with a nanopattern (a) 120 Current efficiency (cd/A)
dramatically increased up to 97% at a wavelength of 320 nm. At 505–565 nm, the haziness of the nanopattern was enhanced by up to 30%. In order to evaluate the electrical and optical properties of OLEDs on the flat glass (reference) and the planarized tetragonal photonic crystal (PTPC) pattern, current density-voltage-luminescence (J-V-L) characteristics were measured. Figure 4 shows current density and luminance as a function of applied voltage. As shown in Fig. 4(a), the OLEDs on a patterned substrate showed higher current density at the same voltage. The low surface roughness of 15 nm still increases current density, because of the focused electric field. The spectrum at the normal direction and luminance is shown in Fig. 4(b). Because of periodic PC, the device with a PTPC pattern had a narrow spectrum and showed higher luminance at the same voltage, in comparison with the reference device. The current efficiencies of the OLED on the flat glass and PTPC pattern showed, respectively, 77.2 and 85.3 cd/ A at 2 mA∕cm2 and 71.5 and 83.9 cd/A at 10 mA∕cm2 , an enhancement ratio of 10.5% and 17.3% [Fig. 5(a)]. Enhanced current efficiency means that our PTPC pattern extracted light, which were confined at flat device. The Lambertian corrected power efficiencies of the OLED on the PTPC pattern were, respectively, 67.4 and 87.2 lm/w at 2 mA∕cm2 , and 44.5 and 63.8 at 10 mA∕cm2 , an enhancement ratio of 29.4% and 43.4% [Fig. 5(b)]. An OLED with the PTPC had a similar roll-off as an OLED on flat glass. Despite the increased charge injection, similar roll-off means the planarized substrate didn’t deteriorate the electrical performance of the OLED. In conclusion, we fabricated a PTPC pattern on glass using a direct printing technique. The surface roughness of the nanopattern was reduced by planarization using a TiO2 solgel solution. OLEDs were fabricated on flat glass
0.8
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100 80 60 40 20
Reference device Device with PTPC
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Fig. 3. Optical properties of flat glass and the photonic crystal pattern: (a) total transmittance and (b) haziness.
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100 90 80 70 60 50 40 30 20 10 0
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10 2 Current Density (mA/cm )
Fig. 5. Efficiency and power efficiency of OLEDs on two types of substrates: (a) efficiency-current density and (b) power efficiency-current density for OLEDs on flat glass and the PTPC pattern.
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increased 17.3% and 43.4%, respectively. The patterned device also showed similar roll-off as a device on flat glass. This research was supported by the R&D program for Industrial Core Technology through the Korea Evaluation Institute of Industrial Technology, supported by the Ministry of Knowledge Economy in Korea (Grant No. 10040225) and the IT R&D program of MKE/KEIT (10041062, Development of Fundamental Technology for Light Extraction of OLED). † These authors contributed equally to this work. References 1. D. Tanaka, H. Sasabe, Y.-J. Li, S.-J. Su, T. Takeda, and J. Kido, Jpn. J. Appl. Phys. 46, L10 (2007). 2. M. G. Helander, Z. B. Wang, J. Qiu, M. T. Greiner, D. P. Puzzo, Z. W. Liu, and Z. H. Lu, Science 332, 944 (2011). 3. Y. S. Park, K.-H. Kim, and J.-J. Kim, Appl. Phys. Lett. 102, 153306 (2013). 4. S. Möller and S. R. Forrest, J. Appl. Phys. 91, 3324 (2002). 5. J. Lim, S. S. Oh, D. Y. Kim, S. H. Cho, I. T. Kim, S. H. Han, H. Takezoe, E. H. Choi, G. S. Cho, Y. H. Seo, S. O. Kang, and B. Park, Opt. Express 14, 6564 (2006). 6. Y.-J. Lee, S.-H. Kim, J. Huh, G.-H. Kim, Y.-H. Lee, S.-H. Cho, Y.-C. Kim, and Y. R. Do, Appl. Phys. Lett. 82, 3779 (2003).
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