a light scattering layer for internal light extraction of...

A Light Scattering Layer for Internal Light Extraction of OrganicLight-Emitting Diodes Based on Silver NanowiresKeunsoo Lee,†,‡ Jin-Wook Shin,†,§ Jun-Hwan Park,∥ Jonghee Lee,† Chul Woong Joo,† Jeong-Ik Lee,†

Doo-Hee Cho,† Jong Tae Lim,† Min-Cheol Oh,∥ Byeong-Kwon Ju,*,‡ and Jaehyun Moon*,†

†Soft I/O Interface Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Republic of Korea‡Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 136-713, Republic of Korea§Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577, Japan∥School of Electrical Engineering, Pusan National University, Pusan (Busan) 609-735, Republic of Korea

ABSTRACT: We propose and fabricate a random light scattering layer for light extraction in organic light-emitting diodes(OLEDs) with silver nanodots, which were obtained by melting silver nanowires. The OLED with the light scattering layer as aninternal light extraction structure was enhanced by 49.1% for the integrated external quantum efficiency (EQE). When a wrinklestructure is simultaneously used for an external light extraction structure, the total enhancement of the integrated EQE was65.3%. The EQE is maximized to 65.3% at a current level of 2.0 mA/cm2. By applying an internal light scattering layer andwrinkle structure to an OLED, the variance in the emission spectra was negligible over a broad viewing angle. Power modeanalyses with finite difference time domain (FDTD) simulations revealed that the use of a scattering layer effectively reduced thewaveguiding mode while introducing non-negligible absorption. Our method offers an effective yet simple approach to achieveboth efficiency enhancement and spectral stability for a wide range of OLED applications.

KEYWORDS: organic light-emitting diodes, light extraction, silver nanowires, wrinkle structures, finite difference time domain

1. INTRODUCTION

Organic light-emitting diodes (OLEDs) can provide a human-friendly spectrum and wide color gamut. For this reason,OLEDs have been widely used in displays and lightingequipment. Because of advancements in organic materials,device stack design, and electrode optimization, the internalquantum efficiencies of OLEDs have reached nearly 100%.Recently, the external quantum efficiency (EQE) of a

conventional bottom emission type OLED has been reportedto reach as high as 30%.1−3 However, because of the presenceof various intrinsic light confinements, the EQE of conventionalOLEDs still has low values.4−6 In practical terms, lowefficiencies are detrimental to the lifetime and energyconsumption of the device. Various methods have beensuggested to improve the efficiencies of OLEDs. Broadly,these methods can be classified as external and internal lightextraction methods.7−10 Other methods, such as internal cavitydesign,11−13 molecular orientation, and application of low SPPelectrodes,14−16 have also been suggested. External lightextraction methods can extract the confined light at the

glass/air interface. However, the light in an organic layer/highrefractive index electrode is still confined. Thus, to fully out-couple the confined light, it is technically necessary to developinternal light extraction methods. To this end, we focused ontechnologically accessible methods to form internal lightextraction structures. Existing methods include photolithog-raphy patterning and vacuum depositions, which may havecomplex processes and high costs.Recently, as a method to extract internally confined light, we

developed a light scattering layer using a fabrication methodbased on dewetting of thin metal films.17 Although the internallight extraction structure formed by the aforementionedmethod can offer good light extraction and spectral stability,the process itself requires a high vacuum-based process andhigh-precision thickness control of the metal films. In thispaper, for a low cost and simple process, we propose the

Received: March 12, 2016Accepted: June 17, 2016Published: June 17, 2016

Research Article

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© 2016 American Chemical Society 17409 DOI: 10.1021/acsami.6b02924ACS Appl. Mater. Interfaces 2016, 8, 17409−17415

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http://dx.doi.org/10.1021/acsami.6b02924

fabrication of a light scattering layer fabricated with silvernanowire (AgNW). AgNWs have been widely investigated asan alternative transparent electrode dominantly used withindium tin oxide (ITO).18−22 Unlike the deposition of thinmetal films by vacuum-based thermal evaporation, AgNWs canbe readily coated onto a substrate by various low-costprocesses.23−26 Here, we used AgNWs to form a hard maskto fabricate an internal light scattering layer. Because novacuum deposition process is required in the deposition of theAgNWs, the process time and cost are greatly reduced. Wethermally melted the AgNWs to form spatially discrete Agnanodots, which act as a hard mask. The exposed area betweenthe Ag nanodots was etched away to fabricate an internalscattering layer. On this scattering layer, we fabricated OLEDsand investigated the light extraction of the OLEDs. In additionto the extraction of the internally confined light, we usedspontaneously formed organic wrinkles to extract the lightconfined at the glass/air interface. In this work, we present theprocess for using AgNWs as a starting material and the

performance of OLEDs equipped with an internal lightscattering layer fabricated with the AgNWs.

2. EXPERIMENT SECTIONFigure 1 shows the fabrication process of an OLED device with ascattering layer as an internal light extraction structure. A SiOx layer of400 nm was deposited onto the glass substrate by plasma-enhancedchemical vapor deposition. Silver nanowires (Cambrios TechnologiesCorporation) were spin-coated onto the SiOx layer. The diameter andlength of the AgNWs are 40 nm and 30 μm, respectively. To form theetching mask, the spin-coated AgNWs film was annealed at 400 °C for2 h in air atmosphere. During the annealing process, AgNWs undergoa melting process producing Ag nanodots on the SiOx layer. Variousstudies and analyses of the melting behavior of thin metal nanowireshave been done.27−30 The diameters of the Ag nanodots varied from200 to 600 nm, which covers most of the visible light wavelengthrange. The exposed SiOx layer was etched with induced coupledplasma reactive ion etching in CF4 and Ar atmosphere. After theetching, the Ag nanodots were removed with a diluted nitric acidsolution. This process yields a light scattering layer on the glasssubstrate. A planarization layer is coated onto the light scattering layer.

Figure 1. Processing flow of substrate equipped with an internal light extraction layer.

Figure 2. SEM images. (a) Ag NW spin-coated on SiOx layer. (b) Ag nanodots formed by Ag NW melting. (c) Internal scattering SiOx layer (45° tiltview). (d) Internal scattering SiOx layer (cross sectional view). (e) Internal scattering layer with planarization layer. (f) Wrinkles for external lightextraction.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b02924ACS Appl. Mater. Interfaces 2016, 8, 17409−17415

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From the perspective of device operation, it is important to electricallystabilize the device. Rough surfaces are prone to induce electricalfailure. Additionally, to facilitate light traveling, it is desirable to have aplanarization layer with a refractive index comparable to that of atransparent anode. If the refractive index of the planarization layer ismuch lower than that of the anode, the light emitted from the organicemission layer could be confined in the anode/organic layer. An ITOwas used that is used widely as an anode for OLEDs. The refractiveindex of the ITO is ∼1.9. To planarize the scattering layer, we used anultraviolet (UV) curable resin that contains dispersed zirconiananocrystals (purchased from Pixelligent). The cured film has arefractive index of 1.81 at a wavelength of 550 nm. The resin was spincoated onto the light scattering layer and subsequently dried briefly ina nitrogen atmosphere. After that, the film was exposed to UV tofinalize the planarization layer fabrication process.The OLED device has the following stack sequence. Our OLED

had the following stack structure: ITO (150 nm)/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile [HAT-CN] (10 nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] [TAPC](45 nm)/ HAT-CN (10 nm)/TAPC (45 nm)/HAT-CN (10 nm)/TAPC (45 nm)/2,6-bis-[3-(carbazol-9-yl)phenyl]pyridine [DCzPPy]doped with 7% of fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] (20nm)/1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene [BmPyPB] (60 nm)/LiF (1 nm)/Al (120 nm). All organics were deposited with a thermalevaporation method. To achieve a high hole injection rate andelectrical stability, we used a hole transport layer (HTL), in which theHAT-CN and TAPC are alternated. The HAT-CN is a high mobilitystrong n-type organic semiconductor, which can extract electrons fromadjacent organics. Alternately deposited HAT-CN extracts theelectrons from highest occupied molecular orbital level to lowestoccupied molecular orbital level, which induces hole generation effect.Thus, the devices can be operated at low voltage with electricalstability. The OLED characteristics with HTL have been reportedelsewhere.31,32 To protect the organics from atmospheric degradation,the fabricated OLEDs were glass encapsulated in a glovebox. Theemitting area of the devices was 70 mm2 (10 mm × 7 mm). Tomaximize the efficiency, we used a wrinkle structure that has a role inan external light extraction structure of OLED. The wrinkle structurewas fabricated on an adhesive plastic film and attached to the glasssurface of a bottom emission OLED. Additional information anddetails on the fabrication process of the wrinkle structure is available inour previous report.33 The current density (J)−voltage (V) andvoltage (V)−luminance (L) characteristics of the devices weremeasured using a current/voltage source/measure unit (Keithley238) and a spectro-radiometer (CS-2000, Minolta), respectively. Theirangular spectra and luminance distributions were measured with aspectro-radiometer (Minolta CS-2000) and a goniometer-equippedsample stage. The efficiencies were measured with an integratingsphere (HM series, Dae Sung Hi-Tech).

3. RESULTS AND DISCUSSIONFigure 2a,b shows the SEM images of the spin-coated AgNWon a glass substrate and Ag nanodots produced by the meltingprocess, respectively. The reported melting point of bulk Ag is962 °C. As the size decreases to nanometric scale, the meltingpoint drops. This is due to the contribution of the surfaceatoms that are weakly bonded compared to those in the bulkpart. In the case of the materials having a large surface-to-volume ratio such as nanowires, the melting point is lowereddramatically with a size decrease.34 The size of the Ag nanodotsshown in Figure 2b is on the scale of several hundrednanometers, which is due to the agglomeration of the meltedliquid Ag NW parts. The Ag nanodots are located randomly onthe SiOx layer, which is desirable for light scattering in theterms of the wavelength dependence,35 and their sizes aresimilar to the wavelength of visible light. Figures 2c,d shows theinternal scattering SiOx layer, which is obtained after dryetching and stripping of the Ag nanodots. The height and

diameter of the scattering layer is 400 nm and several hundrednanometers (∼200−600 nm), respectively. According to ourprevious study, the higher structure yields better light extractioneffect.36 The height of the structure is controllable by varyingthe etching conditions. However, from processing viewpoint, itis difficult to planarize structures with high aspect ratio. Failureof planarization results in electrical failure or low devicereproducibility. In this work, we chose a specific height thatyields stable device characteristics. The geometric sizes of thescattering structure fit within the size range for the internal lightscattering structure. Figure 2e shows the planarized lightscattering layer. Planarization is performed with a UV curableresin. Figure 2e shows the cross-sectional SEM image of thescattering layer planarized with the resin. The thickness of theplanarization layer between the top of the scattering layer andthe surface of the planarization film is ∼150 nm. Figure 2fshows the wrinkle structure that is used as an external lightextraction structure to maximize the device efficiency. Moreinformation on the wrinkle was reported in detail in ourprevious study.33 Briefly, wrinkles were formed by a UV cross-linking liquid prepolymer. To evaluate the light extractioncapacity of the light scattering layer and wrinkle, we fabricatedthree types of devices: First, a planar OLED was fabricated as areference device; Device A contained an internal light scatteringlayer planarized with a UV curable resin, and Device B was thesame as Device A except for an additional wrinkle structure forexternal light extraction.Figure 3 shows the (a) J−V and (b) V−L characteristics of

the devices. All the J values increase as the applied voltage

increases. The rates gradually decrease, which is typicalelectrical behavior of OLEDs. Ideally, because the lightextraction structure is not an electrical component, the J−Vcharacteristic of the devices should superimpose on each other.However, the current density of the devices with the lightextraction structure is relatively higher than that of the planardevice. The planarization layer is not completely flat but slightlywavy, (Figure 2e) which causes an increase in the surface areaand partially thins the organic layer. The increased surface areaand partially thinned organic layer are thought to have inducedthe cause of the higher current density compared to that of theplanar device. The V−L characteristic shows a luminancechange as a function of the applied voltage. For a given voltage,the L of the OLEDs equipped with light extraction structuresshow a higher L level than that of the planar OLED.Figure 4 shows the luminance distribution of the devices as a

function of the viewing angle at a constant current density levelof 2.0 mA/cm2. The luminance distributions of OLEDs aredependent on the thickness of organics.37 In this work, toensure stable device operation, we used OLED devices withthick HTL (165 nm). The distribution is not Lambertian but

Figure 3. (a) The J−V characteristics. (b) The L−V characteristics.



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has stronger emission in the high angle range. In the planardevice, the luminance in the high angle side is higher than thatof the normal direction. Additionally, the luminance distribu-tions of Devices A and B also have higher luminance in the sidedirections than those of the normal directions. However, as theviewing angle increases, the luminance ratio of the normaldirection to the side direction increases gradually. Theluminance ratio of the normal direction (988 cd/m2) to anangle of 60° (1518 cd/m2) is 65%. The luminance ratios ofDevice A and B are 89 and 89.4%, respectively. These resultsreveal that the scattering layer effectively scatters lightuniformly over the whole angle range (0° to ∼70°).Considering that normal direction luminance increases butstill has a lower luminance value than that of the side direction,the scattered light is not concentrated in the normal direction.The technical implication here is that, by using our structure, itis possible to modify the luminance distribution to be uniformor Lambertian-like. We used a wrinkle structure as the externallight extraction structure.33 The refractive index of the curedwrinkle matches that of a glass substrate at 1.5. Previously, wehave shown that our wrinkle is an effective structure forextracting the light confined at the glass/air interface.33 TheOLED (Device B) equipped with an internal scattering layerand external wrinkles had not only the highest luminance levelbut also the most uniform luminance distribution.Figure 5 shows the EQE and power efficiency of the devices.

We measured these values with an integrating sphere. The

measurements were under a constant current density level of2.0 mA/cm2. In the case of the planar device, the measuredEQE was 22.2%. The measured EQEs of Devices A and B were33.1 and 36.7%, respectively, corresponding to enhancementsof 49.1 and 65.3%. The power efficiency of the planar devicewas 46.5 lm/W. The power efficiency of Devices A and B was73.4 and 80.9 lm/W, respectively, corresponding to enhance-ments of 57.8 and 74%. Device A had an enhancement of33.1% and 57.8% for the EQE and power efficiency,respectively. For the planar device, roughly 35% of thegenerated light is confined at the ITO (∼1.9)/organic layer(∼1.78) interface for a second-order cavity.6 Because therefractive index of the planarization layer is comparable to that

of ITO, light can travel to the light scattering layer and then outcoupled to the substrate. The waveguided light randomlyscatters at the interfaces of the scattering/planarization layersresulting in an enhanced luminance. Because of the randomnessin the distribution of the scattering components, the scatteredlight is not concentrated in a specific direction but ratheruniform in all directions (Figure 4). Device B had anenhancement of 65.3% and 74% for the integrated EQE andpower efficiency, respectively. The wrinkle has a role inextracting light confined in a glass substrate. Unlike themechanism of the internal light scattering layer, the wrinkleextracts the light by changing the light path on the boundarybetween the glass and air. Details on wrinkles have beenreported elsewhere.33

Figure 6 shows the integrated EQEs and PEs versusluminance. The OLEDs equipped with light extraction

structures had higher efficiencies in the whole luminancerange. In all cases, the EQEs and PEs decrease as the luminanceincreases. In the planar OLED case, the rate of decrease ishigher than those observed in Devices A and B. The decrease inefficiencies, which is commonly referred as roll-off, can beattributed to resistive losses and various annihilation processestaking place in the light emitting layer.38 Because we usedidentical organic stacks in all devices, the difference in electricalcharacteristics (Figure 3a) has a role in the difference in therate. The results in Figure 6 show that our light extractionstructures are applicable to a wide range of luminance levels.Figure 7a−c shows the normalized electroluminescence (EL)

spectra of the OLEDs as a function of the viewing angle. To be

Figure 4. Luminance distributions of OLEDs.

Figure 5. Efficiencies. (a) External quantum efficiencies (%) and theirenhancements. (b) Power efficiencies (lm/W) and their enhancement.

Figure 6. Efficiencies as a function of current density. (a) Externalquantum efficiencies. (b) Power efficiencies (lm/W).

Figure 7. EL spectra as a function of viewing angle. (a) Planar OLED(no light extraction). (b) Device A (internal light extractionequipped). (c) Device B (internal and external light extractionsequipped). And (d) The CIE coordinates of Planar, Device A, andDevice B OLEDs.



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used as a luminescent device, the angular spectral distortionmust be minimized. The planar device shows a considerablydistorted angular spectrum, and the variance of the full width athalf-maximum (fwhm) is 20 nm. The main peak obtained at aviewing angle of 60° is shifted by 32 nm to a longer wavelength.The spectrum distortion problem arises due to the intrinsicmicrocavity effect of the OLEDs. Especially when a strongmicrocavity effect is present in the device, severe spectrumdistortion is observed.39−41 The variance of the fwhm for bothDevice A and B is 5 nm. By applying the scattering layer andwrinkle, the variance of the fwhm can be remarkably reduced.The main peak shifts of Devices A and B are almost negligible.The internal scattering layer stabilizes the main peak shift

dramatically by scattering the light uniformly over the wholeangle range. The wrinkle also contributes to stabilize the mainpeak shift. Particularly, by using an internal scattering layer andwrinkle, it was possible to keep the main peak positionunchanged. Our results show that the structures are very usefulin preserving the original EL spectrum without distortion.Figure 7d shows the 1931 Commission internationale del′eclairage (CIE) color coordinates, which were extracted fromthe EL spectra. The standard deviations of the x and ycoordinates are 0.022 and 0.021, 0.005 and 0.003, and 0.005and 0.003 for the planar device and Devices A and B,respectively. The large value of the coordinate deviation of theplanar device is a result of a weak microcavity effect. Because of

Figure 8. OLED device structures used in FDTD simulation (a) without internal scattering layer and (b) with internal scattering layer. E2 of OLED(c) without internal scattering layer and (d) with internal scattering layer.

Figure 9. (a) The EQE and enhancement ratio calculated as a function of planarization layer thickness. The power fraction of each mode as afunction of ETL thickness (b) without internal scattering layer and (c) with internal scattering layer.



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the uniform scattering of light, the standard deviation is greatlyreduced in the case of Devices A and B.To elucidate the light out coupling effect of the scattering

layer, finite difference time domain (FDTD) simulations wereconducted and then compared with experimental results. TheOLED device structures used in the FDTD simulation areshown in Figure 8a,b. To build the relevant OLED structure,shown in Figure 8b, nanopillars with a height of 380−420 nmand a radius of 140−260 nm were randomly distributed.Depending on the position of the source relative to thescattering layer, different results could be obtained. This effectwas minimized with a dipole source array and a random initialphase on each point. Our OLED light source is an array ofdipoles. On the light source plane xy, we generated 8 × 8 or 64positions. On each position we located three dipoles. The initialphase of each dipole was set randomly. This feature was tomimic the low coherence characteristics of OLED light. Thetechnical details of constructing random scattering layers andlight sources are described elsewhere.42 To effectively performthe FDTD simulations, we set the spatial grid as 20 nm in eachdirection and took a calculation volume of 6.0 × 6.0 × 5.6 μm3.Mirror boundary conditions and perfect matched layers wereset along the x, z directions and y direction. The wavelengthused in simulation was 515 nm with a bandwidth of 70 nm.Figure 8c,d shows the internal electric field (E2) distributions ofthe OLEDs. Compared to the E2 of the planar OLED case(Figure 9c), the E2 of the OLED with the scattering layer(Figure 8d) clearly propagates in an extended manner with amuch stronger intensity. This is interpreted as the light outcoupling of the trapped light in the ITO−organic layer due tothe scattering layer.As can be seen in Figure 2e, the surface of the planarization

layer is slightly wavy. Thus, there is a need to extract aneffective planarization layer thickness, tp, which yields anenhancement ratio in agreement with the experimental resultshown in Figure 5a. As depicted in Figure 8b, the tp is definedas the spacing between the top of the SiOx nanopillar and theterminal of the planarization layer. To obtain the tp, the EQEsand their enhancement ratios were calculated as a function ofthe planarization layer thickness shown in Figure 9a. Toestimate the effect of thickness of planarization layer on thedevice efficiency, we simulated the device efficiency as thethickness of planarization layer changes. Because of the changein the microcavity length, the enhancement ratio slightlyoscillates, as the tp changes. At tp = 140 nm, and anenhancement ratio of 1.5 was obtained. Referring to Figure5a, this value is very close to the enhancement obtained in theactual OLED device. We used tp = 140 nm to construct modepower fraction plots of the OLEDs with and without thescattering layer. In the simulation, a maximum EQE enhance-ment ratio of 1.55 times was observed at tp = 100 nm. Figure9b,c shows the power fraction of each mode as a function of theelectron transport layer (ETL) thickness. Significant changewas observed in the wave-guided mode by the introduction ofthe scattering layer. The loss due to the wave-guided modedecreased over the entire range of the ETL thicknessconsidered. The wave-guided mode decreases from 14% to4% in thickness of ETL 60 nm. In both results, there was noevident change in the fraction of the surface plasmon resonance(SPR) mode and substrate mode. Additionally, presumably dueto the presence of the planarization layer, non-negligibleabsorption occurs over the entire range of the ETL thickness.At an ETL thickness of 60 nm, the out coupled light fractions

of the planar and scattering layer equipped OLEDs were18.80% and 28.54%, respectively. The enhancement ratio wascalculated to be 1.52, which is close to that of the experimentalresult of 1.49 times. The simulations show that our scatteringlayer can effectively extract the wave-guided mode and enhancethe EQE of OLEDs.

4. SUMMARYIn summary, we fabricated an internal light scattering structurethat enhances the efficiency and stabilizes the emissionspectrum. Out scattering layer was fabricated with silvernanodots, which were obtained by melting silver nanowires.The light scattering layer equipped with a high refractive indexplanarization layer remarkably enhanced the device efficiency ofOLEDs. The integrated maximum EQE and power efficiency ofthe device with the scattering layer and wrinkle were 36.7% and80.9 lm/W, respectively, corresponding to enhancements of65.3 and 74% compared with the planar device. The internalscattering layer not only enhances the efficiency but alsostabilizes the EL spectrum. On the one hand, the planar devicehas a considerably distorted EL spectrum. On the other hand,the device with the light scattering layer and wrinkle hasreduced variance of the fwhm and no main shift as the viewingangle changes. To verify the light extraction capacity of the lightscattering layer, an optical simulation was also done withFDTD, and the simulation result showed a similar enhance-ment as the experimental result. The proposed light scatteringstructure in this paper is not limited to OLEDs. It can be readilyapplied to various photonics devices to improve their efficiencyand stabilize the spectrum.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected]. (B.K.J.)*E-mail: [email protected]. (J.M.)NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by “Technology Development of Lowcost Flexible Lighting Surface”, which is a part of the R&Dprogram of Electronics and Telecommunications ResearchInstitute and by the National Research Foundation grant(2014R1A2A1A10051994) funded by the Korean government.

■ REFERENCES(1) Kim, K. H.; Moon, C. K.; Lee, S. Y.; Kim, J. J.; et al. HighlyEfficient Organic Light-Emitting Diodes with Phosphorescent EmittersHaving High Quantum Yield and Horizontal Orientation of TransitionDipole Moments. Adv. Mater. 2014, 26, 3844−3847.(2) Sun, J. W.; Lee, J. H.; Moon, C. K.; Kim, K. H.; Shin, H.; Kim, J.J. A Fluorescent Organic Light-Emitting Diode with 30% ExternalQuantum Efficiency. Adv. Mater. 2014, 26, 5684−5688.(3) Udagawa, K.; Sasabe, H.; Cai, C.; Kido, J. Low-Driving-VoltageBlue Phosphorescent Organic Light-Emitting Devices with ExternalQuantum Efficiency of 30%. Adv. Mater. 2014, 26, 5062−5066.(4) Nowy, S.; Krummacher, C.; Frischeisen, J.; Reinke, N. A.;Brutting, W. Light Extraction and Optical Loss Mechanisms inOrganic Light-Emitting Diodes: Influence of the Emitter QuantumEficiency. J. Appl. Phys. 2008, 104, 123109−123117.(5) Koh, T. W.; Choi, J. M.; Lee, S.; Yoo, S. Optical OutcouplingEnhancement in Organic Light-Emitting Diodes: Highly ConductivePolymer as a Low-Index Layer on Microstructured ITO Electrodes.Adv. Mater. 2010, 22, 1849−1853.



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(6) Meerheim, R.; Furno, M.; Hofmann, S.; Lussem, B.; Leo, K.Quantification of Energy Loss Mechanisms in Organic Light-EmittingDiodes. Appl. Phys. Lett. 2010, 97, 253305−253307.(7) Koo, W. H.; Youn, W.; Zhu, P.; Li, X. H.; Tansu, N.; So, F. LightExtraction of Organic Light Emitting Diodes by Defective Hexagonal-Close-Packed Array. Adv. Funct. Mater. 2012, 22, 3454−3459.(8) Kim, E.; Cho, H.; Kim, K.; Koh, T. W.; Chung, J.; Lee, J.; Park,Y.; Yoo, S. A Facile Route to Efficient, Low-Cost Flexible OrganicLight-Emitting Diodes: Utilizing the High Refractive Index and Built-In Scattering Properties of Industrial-Grade PEN Substrates. Adv.Mater. 2015, 27, 1624−1631.(9) Lim, B. W.; Suh, M. C. Simple Fabrication of a Three-Dimensional Porous Polymer Film as a Diffuser for Organic LightEmitting Diodes. Nanoscale 2014, 6, 14446−14452.(10) Cho, D. H.; Shin, J. W.; Moon, J.; Park, S. K.; Joo, C. W.; Cho,N. S.; Huh, J. W.; Han, J. H.; Lee, J.; Chu, H. Y.; Lee, J. I. SurfaceControl of Planarization Layer on Embossed Glass for Light Extractionin OLEDs. ETRI J. 2014, 36, 847−855.(11) Xiang, C.; Koo, W.; So, F.; Sasabe, H.; Kido, J. A SystematicStudy on Efficiency Enhancements in Phosphorescent Green, Red andBlue Microcavity Organic Light Emitting Devices. Light: Sci. Appl.2013, 2, e74.(12) Manna, E.; Fungura, F.; Biswas, R.; Shinar, J.; Shinar, R. TunableNear UV Microcavity OLED Arrays: Characterization and AnalyticalApplications. Adv. Funct. Mater. 2015, 25, 1226−1232.(13) Mazzeo, M.; Mariano, F.; Genco, A.; Carallo, S.; Gigli, G. HighEfficiency ITO-Free Flexible White Organic Light-Emitting DiodesBased on Multi-Cavity Technology. Org. Electron. 2013, 14, 2840−2846.(14) Kim, J. B.; Lee, J. H.; Moon, C. K.; Kim, S. Y.; Kim, J. J. HighlyEnhanced Light Extraction from Surface Plasmonic Loss MinimizedOrganic Light-Emitting Diodes. Adv. Mater. 2013, 25, 3571−3577.(15) Chen, C. Y.; Lee, W. K.; Chen, Y. J.; Lu, C. Y.; Wu, C. C.; et al.Enhancing Optical Out-Coupling of Organic Light-Emitting Deviceswith Nanostructured Composite Electrodes Consisting of Indium TinOxide Nanomesh and Conducting Polymer. Adv. Mater. 2015, 27,4883−4888.(16) Xiao, Y.; Yang, J. P.; Cheng, P. P.; Zhu, J. J.; Xu, Z. Q.; Deng, Y.H.; Lee, S. T.; Li, Y. Q.; Tang, J. X. Surface Plasmon-EnhancedElectroluminescence in Organic Light-Emitting Diodes IncorporatingAu Nanoparticles. Appl. Phys. Lett. 2012, 100, 013308−013311.(17) Shin, J. W.; Cho, D. H.; Moon, J.; Joo, C. W.; Park, S. K.; Lee, J.;Han, J. H.; Cho, N. S.; Hwang, J.; Huh, J. W.; Chu, H. Y.; Lee, J. I.Random Nano-Structures as Light Extraction Functionals for OrganicLight-Emitting Diode Applications. Org. Electron. 2014, 15, 196−202.(18) Margulis, G. Y.; Christoforo, M. G.; Lam, D.; Beiley, Z. M.;Bowring, A. R.; Bailie, C. D.; Salleo, A.; Mcgehee, M. D. SprayDeposition of Silver Nanowire Electrodes for Semitransparent Solid-State Dye-Sensitized Solar Cells. Adv. Energy. Mater. 2013, 3, 1657−1663.(19) Lee, H. J.; Hwang, J. H.; Choi, K. B.; Jung, S. G.; Kim, K. N.;Shim, Y. S.; Park, C. H.; Park, Y. W.; Ju, B. K. Effective Indium-DopedZinc Oxide Buffer Layer on Silver Nanowires for Electrically HighlyStable, Flexible, Transparent, and Conductive Composite Electrodes.ACS Appl. Mater. Interfaces 2013, 5, 10397−10403.(20) Lee, J.; Lee, P.; Lee, H.; Lee, D.; Lee, S. S.; Ko, S. H. Very LongAg Nanowire Synthesis and Its Application in a Highly Transparent,Conductive and Flexible Metal Electrode Touch Panel. Nanoscale2012, 4, 6408−6414.(21) Kim, T.; Canlier, A.; Kim, G. H.; Choi, J.; Park, M.; Han, S. M.Electrostatic Spray Deposition of Highly Transparent Silver NanowireElectrode on Flexible Substrate. ACS Appl. Mater. Interfaces 2013, 5,788−794.(22) Coskun, S.; Ates, E. S.; Unalan, H. E. Optimization of SilverNanowire Networks for Polymer Light Emitting Diode Electrodes.Nanotechnology 2013, 24, 125202−125209.(23) Choi, D. Y.; Kang, H. W.; Sung, H. J.; Kim, S. S. Annealing-Free,Flexible Silver Nanowire−Polymer Composite Electrodes Via a

Continuous Two-Step Spray-Coating Method. Nanoscale 2013, 5,977−983.(24) Maenosono, S.; Okubo, T.; Yamaguchi, Y. Overview ofNanoparticle Array Formation by Wet Coating. J. Nanopart. Res.2003, 5, 5−15.(25) Hauger, T. C.; Al-Rafia, S. M. I.; Buriak, J. M. Rolling SilverNanowire Electrodes: Simultaneously Addressing Adhesion, Rough-ness, and Conductivity. ACS Appl. Mater. Interfaces 2013, 5, 12663−12671.(26) Mahajan, A.; Francis, L. F.; Frisbie, C. D. Facile Method forFabricating Flexible Substrates with Embedded, Printed Silver Lines.ACS Appl. Mater. Interfaces 2014, 6, 1306−1312.(27) Wang, B.; Wang, G.; Chen, X.; Zhao, J. Melting Behavior ofUltrathin Titanium Nanowires. Phys. Rev. B: Condens. Matter Mater.Phys. 2003, 67, 193403−193406.(28) Wen, Y. H.; Zhu, Z. Z.; Zhu, R.; Shao, G. F. Size Effects on theMelting of Nickel Nanowires: A Molecular Dynamics Study. Phys. E2004, 25, 47−54.(29) Wang, J.; Chen, X.; Wang, G.; Wang, B.; Lu, W.; Zhao, J.Melting Behavior in Ultrathin Metallic Nanowires. Phys. Rev. B:Condens. Matter Mater. Phys. 2002, 66, 085408−085411.(30) Wen, Y. H.; Zhang, Y.; Zheng, J. C.; Zhu, Z. Z.; Sun, S. G.Orientation-Dependent Structural Transition and Melting of AuNanowires. J. Phys. Chem. C 2009, 113, 20611−20617.(31) Joo, C. W.; Moon, J.; Han, J. H.; Huh, J. W.; Lee, J.; Cho, N. S.;Hwang, J.; Chu, H. Y.; Lee, J. I. Color Temperature Tunable WhiteOrganic Light-Emitting Diodes. Org. Electron. 2014, 15, 189−195.(32) Joo, C. W.; Moon, J.; Han, J. H.; Huh, J. W.; Shin, J. W.; Cho,D. H.; Lee, J.; Cho, N. S.; Lee, J. I. White Transparent Organic Light-Emitting Diodes with High Top and Bottom Color Rendering Indices.J. Inf. Disp. 2015, 16, 161−168.(33) Moon, J.; Kim, E.; Park, S. K.; Lee, K.; Shin, J. W.; Cho, D. H.;Lee, J.; Joo, C. W.; Cho, N. S.; Han, J. H.; Yu, B. G.; Yoo, S.; Lee, J. I.Organic Wrinkles for Energy Efficient Organic Light Emitting Diodes.Org. Electron. 2015, 26, 273−278.(34) Qi, Y.; Cagin, T.; Johnson, W. L.; Goddard, W. A., III Meltingand Crystallization in Ni Nanoclusters: The Mesoscale Regime. J.Chem. Phys. 2001, 115, 385−394.(35) Koo, W. H.; Jeong, S. M.; Araoka, F.; Ishikawa, K.; Nishimura,S.; Toyooka, T.; Takezoe, H. Light Extraction from Organic Light-Emitting Diodes Enhanced by Spontaneously Formed Buckles. Nat.Photonics 2010, 4, 222−226.(36) Shin, J.-W.; Cho, D.-H.; Joo, C. W.; Moon, J.; Lee, J.; Park, S.K.; Han, J.-H.; Cho, N. S.; Kang, B.-K.; Chu, H. Y.; Lee, J.-I. Thestructural optimization of the random scattering layer to improve the lightextraction efficiency of white OLEDs. The 5th International ConferenceWhite LEDs Solid-State Lighting, Jeju, Korea, Nanophotonic Semi-conductors Laboratory, 2014.(37) Lee, J.; Chopra, N.; So, F. Cavity Effect on Light Extraction inOrganic Light Emitting Devices. Appl. Phys. Lett. 2008, 92, 033303−033305.(38) Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off inOrganic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801−6827.(39) Forrest, S. R.; Burrows, P. E.; Shen, Z.; Gu, G.; Bulovic, V.;Thompson, M. E. The Stacked OLED (SOLED): A New Type ofOrganic Device for Achieving High-Resolution Full-Color Displays.Synth. Met. 1997, 91, 9−13.(40) Joo, C. W.; Moon, J.; Hwang, J.; Han, J. H.; Shin, J. W.; Cho, D.H.; Huh, J. W.; Chu, H. Y.; Lee, J. I. Improved Device Performances inPhosphorescent Organic Light-Emitting Diodes by MicrocavityEffects. Jpn. J. Appl. Phys. 2012, 51, 09MH01−09MH04.(41) Choy, W. C. H.; Ho, C. Y. Improving the Viewing AngleProperties of Microcavity OLEDs by Using Dispersive Gratings. Opt.Express 2007, 15, 13288−13294.(42) Kim, J. W.; Jang, J. H.; Oh, M. C.; Shin, J. W.; Cho, D. H.;Moon, J.; Lee, J. I. FDTD Analysis of the Light Extraction Efficiency ofOLEDs with a Random Scattering Layer. Opt. Express 2014, 22, 498−507.



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