Article Enhanced Extraction from Bottom Emission by High Refractive Index Nanoparticle Scattering Layer

Chan Young Park and Byoungdeog Choi *

Department of Electrical and Computer Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeonggi-do 16419, Korea * Correspondence: [email protected]

 Received: 31 July 2019; Accepted: 26 August 2019; Published: 31 August 2019 

Abstract: High refractive index nanoparticle material was applied as a scattering layer on the inner side of a glass substrate of a bottom emission organic light emitting (OLED) device to enhance light extraction and to improve angular color shift. TiO2 and YSZ (Yttria Stabilized Zirconia; Y2O3-ZrO2) were examined as the high refractive index nanoparticles. The nanoparticle material was formed as a scattering layer on a glass substrate by a coating method, which is generally used in the commercial display manufacturing process. Additionally, a planarization layer was coated on the scattering layer with the same method. The implemented nanoparticle material and planarization material endured, without deformation, the subsequent thermal annealing process, which was carried out at temperature ranged to 580 ◦C. We demonstrated a practical and highly efficient OLED device using the conventional display manufacturing process by implementing the YSZ nanoparticle. We obtained a 38% enhanced luminance of the OLED device and a decreased angular color change compared to a conventional OLED device.

Keywords: OLED; light extraction; outcoupling; nanoparticle; high refractive index; scatter; efficiency; display; YSZ

1. Introduction In flat-panel display devices, organic light emitting (OLEDs) are widely used, from mobile phones to large size TVs because OLED has excellent electro-optical properties for display products compared to widely used liquid crystal displays (LCDs). Among the properties, the efficiency of OLED devices is an important factor for practical OLED display products. There are many studies on enhancing the efficiency of OLEDs in various technology fields, such as materials, device structures, fabrication processes, circuit design, and circuit driving. The efficiency of OLEDs has been enhanced by improving the characteristics of organic materials comprised in OLED devices and by optimizing the structures of OLED devices [1–4]. However, other approaches for finding solutions for the improvement of the outer structures of OLED devices have been studied in order to enhance the efficiency of OLED devices [5,6]. As shown in Figure1, 20% of emitted light is extracted outside of OLED devices; however, 80% of emitted light is trapped inside OLED devices and dissipated, which is caused by the difference of the refractive index of each layer comprised in OLED devices [5,7]. Various ways were tried to extract the trapped light from the OLED devices [5–7]. Several methods have been proposed to enhance the light extraction from OLEDs, including mesh structures on the glass substrate [8], silica micro spheres [9], scattering layers [10], embedded low-index grids [11], photonic crystals [12,13], micro pyramids [14], and micro lens arrays [15–18]. However, these methods could be used for limited applications such

Nanomaterials 2019, 9, 1241; doi:10.3390/nano9091241 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1241 2 of 9

Nanomaterials 2019, 9, 1241 2 of 9 as illumination products without . Most methods for enhancing the efficiency of OLED are are implemented on the outer side of the glass substrate, where the distance between the light source implemented on the outer side of the glass substrate, where the distance between the light source and and the emitting structures is relatively long, due to the thickness of the glass substrate. In display the emitting structures is relatively long, due to the thickness of the glass substrate. In display products products with many pixels, these methods cause image blurring, which is a mixture of light from with many pixels, these methods cause image blurring, which is a mixture of light from adjacent pixels adjacent pixels and poor image quality by a diffusion effect. Other approaches have been studied to and poor image quality by a diffusion effect. Other approaches have been studied to enhance the light enhance the light extraction of OLEDs, adopting nano sized structures to OLEDs. By the scattering extraction of OLEDs, adopting nano sized structures to OLEDs. By the scattering effect of the nano effect of the nano structures, such as gratings [19,20], metallic nanoparticles [21–25], and buckled structures, such as gratings [19,20], metallic nanoparticles [21–25], and buckled substrates [24,26,27], substrates [24,26,27], the light trapped in glass substrate can escape. However, nano structures have the light trapped in glass substrate can escape. However, nano structures have limited application in limited application in OLEDs due to the complicated process, requiring high cost. Additionally, their OLEDs due to the complicated process, requiring high cost. Additionally, their size limitation impedes size limitation impedes their implementation in practical OLED products. Hence, the nanoparticles their implementation in practical OLED products. Hence, the nanoparticles fabricated by easy methods fabricated by easy methods or by methods compatible with conventional manufacturing processes or by methods compatible with conventional manufacturing processes suitable for large areas are more suitable for large areas are more beneficial for practical OLEDs devices. beneficial for practical OLEDs devices. In this study, we applied high refractive index nanoparticle material as a scattering layer on the In this study, we applied high refractive index nanoparticle material as a scattering layer on the inner side of the glass substrate of the bottom emission OLED device to enhance light extraction and inner side of the glass substrate of the bottom emission OLED device to enhance light extraction and to to improve angular color shift. The nanoparticle scattering layer is located near the emitting layer of improve angular color shift. The nanoparticle scattering layer is located near the emitting layer of the the OLED device, compared to previous approaches mentioned above, so it rarely affects the image OLED device, compared to previous approaches mentioned above, so it rarely affects the image quality quality of the OLED display. We demonstrated a practical highly efficient OLED device using the of the OLED display. We demonstrated a practical highly efficient OLED device using the conventional conventional display manufacturing process by implementing the YSZ nanoparticle as the high display manufacturing process by implementing the YSZ nanoparticle as the high refractive index refractive index scattering layer. We obtained a 38% enhancement of luminance of the OLED device scattering layer. We obtained a 38% enhancement of luminance of the OLED device and a white and a white angular dependency (WAD) of 0.005 at 60 degrees from normal direction. angular dependency (WAD) of 0.005 at 60 degrees from normal direction.

Figure 1. Schematic diagram of multi-layer OLED structure and optical ray diagram of light propagation Figure 1. Schematic diagram of multi-layer OLED structure and optical ray diagram of light via various modes, i.e., substrate escape, substrate wave guided mode, and ITO/Organic wave propagation via various modes, i.e., substrate escape, substrate wave guided mode, and ITO/Organic guided mode. wave guided mode. 2. Materials and Methods 2. Materials and Methods TiO2 and YSZ (Yttria Stabilized Zirconia; Y2O3-ZrO2) were investigated as high refractive index nanoparticleTiO2 and materials YSZ (Yttria for Stabilized scattering layer.Zirconia; The Y refractive2O3-ZrO2) indexwere investigated of TiO2 nanoparticles as high refractive is 2.5 and index that ofnanoparticle YSZ is 2.13, materials respectively. for scattering Both TiO layer.2 and The YSZ refractive nanoparticles index are of suitableTiO2 nanoparticles for high refractive is 2.5 and index that materialsof YSZ is compared2.13, respectively. to that of Both glass TiO substrate.2 and YSZ The nanoparticles size of the TiO are2 nanoparticle suitable for was high 250–300 refractive nm, whileindex thematerials size of compared the YSZ nanoparticleto that of glass was substrate. 100–150 The nm. size The of the nanoparticles TiO2 nanoparticle were dispersed was 250–300 in propylene nm, while glycolthe size methyl of theether YSZ acetatenanoparticle (PGMA) was solvent 100–150 for nm applying. The nanoparticles to the slit coating were dispersed method, which in propylene is used inglycol the conventionalmethyl ether acetate display (PGMA) manufacturing solvent for process applying for largeto the areas.slit coating The nanoparticlemethod, which suspension is used in wasthe conventional coated on the display glass substrate, manufacturing forming process the high for large refractive areas. index The scatteringnanoparticle layer, suspension followed was by acoated thermal on the curing glass process substrate, at 300–500forming the◦C high in a furnacerefractive for index 30 min. scattering The curinglayer, followed temperature by a thermal can be controlledcuring process according at 300–500 to the ° stateC in a of furnace the coating for 30 layer, min. without The curing any deformation.temperature can A SiObe 2controlledsolution, whichaccording was to normally the state used of the at -oncoating glasslayer, (SOG), without was any used deformation. as a planarization A SiO2 material,solution, which waswas coatednormally on used the nanoparticle at spin-on glass scattering (SOG), layer. was used The as refractive a planarization index of material, the SiO2 whichplanarization was coated layer on was the 1.45,nanoparticle which was scattering similar layer. to that The of refractive the glass index substrate. of the TheSiO2 planarizationplanarization layer was was 1.45, formed which on was the scatteringsimilar to that layer of with the glass the same substrate. method The as planarization the scattering layer layer. was It formed was cured on the at scattering 500 ◦C in layer a furnace with the same method as the scattering layer. It was cured at 500 °C in a furnace for 30 min. The planarization layer had a flat surface covering the roughness of the scattering layer caused by the distribution of nanoparticles. Therefore, subsequent manufacturing process can be performed easily,

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forNanomaterials 30 min. 2019 The, planarization9, 1241 layer had a flat surface covering the roughness of the scattering layer3 of 9 caused by the distribution of nanoparticles. Therefore, subsequent manufacturing process can be performedlike on the easily,glass substrate like on the surface. glass substrateNext, the surface.conventional Next, low the temperature conventional poly low silicon temperature (LTPS) poly thin siliconfilm (LTPS) thin (TFT) film manufacturing transistor (TFT) process manufacturing was performed process on wasthe performedplanarization on layer. the planarization In the LTPS layer.TFT manufacturing In the LTPS TFT process, manufacturing the thermal process, annealing the thermal process annealing is required process at the is required highest attemperature, the highest temperature,approaching approaching580 °C, which 580 can◦C, cause which deformation can cause deformation of the nanoparticle of the nanoparticle scattering scattering layer and layer the andplanarization the planarization layer. The layer. proposed The proposed materials materials should should endure endure the high the temperature high temperature during during LTPS LTPS TFT TFTmanufacturing manufacturing process process without without any anydeformation. deformation. AA conventionalconventional bottombottom emissionemission whitewhite OLEDOLED displaydisplay devicedevice waswas usedused toto investigateinvestigate thethe eeffectffect ofof thethe highhigh refractiverefractive indexindex nanoparticlenanoparticle scatteringscattering layer,layer, asas representedrepresented inin FigureFigure2 .2. On On the the glass glass substrate,substrate, thethe nanoparticlenanoparticle scatteringscattering layer,layer, thethe planarizationplanarization layer,layer, andand thethe TFTTFT layerlayer werewere fabricatedfabricated sequentially.sequentially. Next, a a color color filter filter and and an an over over coat coat layer layer were were formed formed to toreproduce reproduce color color images images on the on theOLED OLED display . device. On Onthe the indium indium tin tin oxide oxide (ITO) (ITO) anode elec electrode,trode, the the white white OLED device,device, comprisedcomprised of of blue, blue, green, green, and and red emittingred emitting layers stackedlayers stacked up, were up, deposited were deposited by a thermal by evaporation a thermal process.evaporation The deviceprocess. was The finalized device withwas finalized an aluminum with cathodean aluminum and a topcathode glass and substrate. a top glass The sizesubstrate. of the glassThe size substrate of the was glass 370 substrate470 mm. was Additionally, 370 × 470 an mm. OLED Additionally, device without an OLED a nanoparticle device scatteringwithout a × layernanoparticle and a planarization scattering layer layer and was a alsoplanarization fabricated layer as a referencewas also fabricated device. as a reference device.

FigureFigure 2.2. SchematicSchematic diagramdiagram ofof thethe bottombottom emissionemission whitewhite OLEDOLED devicedevice withwith thethe scatteringscattering layerlayer onon thethe innerinner sideside ofof glassglass substrate.substrate.

MeasurementsMeasurements ofof OLEDOLED propertiesproperties werewere performedperformed byby recordingrecording current–voltagecurrent–voltage characteristicscharacteristics asas wellwell as electro-luminescence (EL) (EL) spectra. spectra. Th Thee current–voltage current–voltage was was acquired acquired using using a Keithley a Keithley 236 236voltage voltage source source unit, unit, while while the the EL EL intensity intensity and and spectr spectralal characteristics characteristics of of the the devices devices were measuredmeasured withwith aa calibrated silicon silicon photodiode (Hamamatsu (Hamamatsu , Photonics, Hamamatsu, Hamamatsu, Japan, Japan, S5227-1010BQ), S5227-1010BQ), a aphotomultiplier , tube, and and a aspectroradiometer spectroradiometer (Minolta, (Minolta, Osaka, Osaka, Japan, Japan, CS-1000). CS-1000). 3. Results and Discussion 3. Results and Discussion 3.1. Properties of the Nanoparticle Scattering Layer 3.1. Properties of the Nanoparticle Scattering Layer The scattering layer with embedded TiO2 nanoparticles was fabricated using the conventional The scattering layer with embedded TiO2 nanoparticles was fabricated using the conventional slit coating method. However, TiO2 nanoparticles are hard to disperse in PGMA solvent, therefore slit coating method. However, TiO2 nanoparticles are hard to disperse in PGMA solvent, therefore they were not dispersed uniformly. The coating thickness of the TiO2 nanoparticle scattering layer was they were not dispersed uniformly. The coating thickness of the TiO2 nanoparticle scattering layer 1.5 µm, then 1 µm thick SiO2 was coated as the planarization layer. Table1 shows the optical properties was 1.5 um, then 1 um thick SiO2 was coated as the planarization layer. Table 1 shows the optical of the TiO2 nanoparticle scattering layer, such as transmittance and haze. We observed 70% of total properties of the TiO2 nanoparticle scattering layer, such as transmittance and haze. We observed transmittance at 550 nm wavelength and 55–82% of haze. The surface of the TiO2 nanoparticle layer 70% of total transmittance at 550 nm wavelength and 55–82% of haze. The surface of the TiO2 was rough, even though the planarization layer was applied on it. Considering its surface roughness, nanoparticle layer was rough, even though the planarization layer was applied on it. Considering its it would cause poor characteristics in the subsequent LTPS TFT process. Due to poor dispersivity of surface roughness, it would cause poor characteristics in the subsequent LTPS TFT process. Due to TiO2 nanoparticles in the PGMA solvent, the curing temperature was limited to 300 ◦C, which was poor dispersivity of TiO2 nanoparticles in the PGMA solvent, the curing temperature was limited to lower than the annealing process temperature of 580 ◦C in the subsequent LTPS TFT process. Despite 300 °C, which was lower than the annealing process temperature of 580 °C in the subsequent LTPS TFT process. Despite the high refractive index of TiO2 nanoparticles, the use of conditions such as particle size, sorts of solvent, and better dispersion of TiO2 nanoparticles is required.

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the high refractive index of TiO2 nanoparticles, the use of conditions such as particle size, sorts of solvent, and better dispersion of TiO2 nanoparticles is required. Nanomaterials 2019, 9, 1241 4 of 9 Table 1. Transmittance and haze of the TiO2 nanoparticle scattering layer.

Table 1. Transmittance and haze of the TiO2 nanoparticle scattering layer. Layers Total Transmittance (%) Scattering Transmittance (%) Haze Scattering layerLayers (1.5 µm) Total Transmittance70.18 (%) Scattering Transmittance 57.54 (%) Haze 82% Scattering layer (1.5um) 70.18 57.54 82% Scattering layer (1.5 µm) Scattering layer (1.5um) 68.28 37.55 55% /Planarization layer (1 µm) 68.28 37.55 55% /Planarization layer (1um)

On the other hand, the scattering layer with embedded YSZ nanoparticlesnanoparticles was successfully fabricated using the conventional slitslit coatingcoating method.method. ZrO2, comprisingcomprising YSZ has a monoclinic monoclinic crystal structure at room temperature. It changes to a a tetr tetragonalagonal crystal structure an andd a cubic cubic crystal crystal structure structure as temperature increases. YSZ YSZ comprised comprised of of Yttria doped ZrO 2 hashas a a stable crystal structure even when temperature increases. YSZ YSZ has has characteristic characteristicss of of chemical chemical stability, stability, thermal resistance, low thermal conductivity, and highhigh strength.strength. It has similar optical properties to ZrOZrO2,, including a high refractive index.index. TheThe YSZYSZ nanoparticle nanoparticle scattering scattering layer layer was was coated coated with with various various thicknesses thicknesses of 1.5 ofµ 1.5m, µ µ µ µ um,2.5 2.5m, 4.0um, 4.0m, andum, 7.0and m,7.0 respectively,um, respectively, and then and was then cured was atcured 500 ◦atC. 500 Next, °C.1 Next,m of 1 SiOum2 ofwas SiO coated2 was coatedas the planarizationas the planarization layer. Figure layer.3 Figureillustrates 3 illustrates the morphology the morphology of the YSZ of nanoparticle the YSZ nanoparticle scattering scatteringlayer and thelayer planarization and the planarization layer including layer includ a crossing sectional a cross sectional view, measured view, measured by scanning by scanning electronmicroscopy microscopy (SEM). The (SEM). surface The surface of the planarization of the planarization layer was layer flat was with flat small with defects,small defects, like the like glass the glasssubstrate. substrate. Moreover, Moreover, the density the density of the of YSZ the YSZ nanoparticle nanoparticle scattering scattering layer layer was was lower lower than than that that of the of planarizationthe planarization layer. layer. The changeThe change of thickness of thickness of the of planarization the planarization layer fromlayer1 fromµmat 1 theum coatingat the coating step to step0.6 µ tom 0.6 after um the after curing the curing step means step means that the that coated the coated SiO2 solution SiO2 solution permeated permeated into the into vacancies the vacancies of the ofnanoparticle the nanoparticle scattering scattering layer. The layer. large The diff erencelarge difference between refractive between index refractive of the index YSZ nanoparticles of the YSZ (2.13)nanoparticles and the (2.13) SiO2 material and the (1.45)SiO2 withinmaterial the (1.45) interpenetrating within the interpenetrating areas enhanced theareas scattering enhanced eff theect. Tablescattering2 shows effect. optical Table properties 2 shows of optical the YSZ properties nanoparticle of scatteringthe YSZ layer.nanoparticle We obtained scattering 58–78% layer. of total We obtainedtransmittance 58–78% of of the total YSZ transmittance nanoparticle of scattering the YSZ na layernoparticle at 550 scattering nm wavelength layer at and 550 24–55%nm wavelength of haze. andIt was 24–55% observed of haze. that It transmittance was observed decreased that transmitta and hazence decreased increased and as the haze thickness increased of scatteringas the thickness layer wasof scattering increased. layer This was means increased. that the This thick means scattering that th layere thick enhances scattering the layer scatter enhances effect due the toscatter increasing effect duelight ditoff usionincreasing during light its propagation. diffusion during Therefore, its thepropagation. scatter effect Ther andefore, transmittance the scatter can be effect controlled and transmittanceby modulating can the be thickness controlled ofthe by scatteringmodulating layer. the Moreover,thickness of the the YSZ scattering nanoparticle layer. scattering Moreover, layer the YSZand thenanoparticle planarization scattering layer havelayer a and uniform the planarization surface and a layer high curinghave a temperatureuniform surface of 500 and◦C, a so high the curingsubsequent temperature LTPS TFT of process500 °C, so can the be subsequent performed easily.LTPS TFT process can be performed easily.

(a) (b)

Figure 3. SEMSEM images images of of the YSZ nanoparticle scattering layer (LSM) and the planarization layer (LSPM) after the curing process: ( (aa)) Top Top view of planarization layer; ( b) Cross sectional view of the scattering layer and the planarization layer.

Table 2. Transmittance and haze of the YSZ nanoparticle scattering layer.

Thickness of Thickness of Total Scattering Haze Scattering Layer (um) Planarization Layer (um) Transmittance (%) Transmittance (%)

1.5 1.0 78.43 19.13 24%

2.5 1.0 74.63 20.06 27%

4.0 1.0 65.84 25.47 39%

7.0 1.0 58.43 32.15 55%

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Table 2. Transmittance and haze of the YSZ nanoparticle scattering layer.

Thickness of Scattering Thickness of Planarization Total Scattering Haze Layer (µm) Layer (µm) Transmittance (%) Transmittance (%) 1.5 1.0 78.43 19.13 24% 2.5 1.0 74.63 20.06 27% 4.0 1.0 65.84 25.47 39% 7.0 1.0 58.43 32.15 55% Nanomaterials 2019, 9, 1241 5 of 9

3.2. Performance Performance of of the OLED Device with High Refractive Index Nanoparticle Scattering Layer A conventionalconventional bottom bottom emission emission white white OLED OLED device device for flat-panel for flat-panel display productsdisplay wasproducts fabricated. was fabricated.The characteristics The characteristics of the OLED of devices the OLED with devices and without with and the YSZwithout scattering the YSZ layer scattering are illustrated layer are as illustratedFigure4. The as Figure OLED 4. devices The OLED had three devices peaks had at three red, peaks green, at and red, blue green, color and wavelengths. blue color wavelengths. The OLED Thedevices OLED were devices used aswere the used light sourcesas the light to investigate sources tothe investigate enhancement the enhancement of light efficiency of light of theefficiency OLED withof the an OLED embedded with an YSZ embedded scattering YSZ layer. scattering layer.

(a) (b)

(c) (d)

Figure 4. Characteristics of OLED devices with and withoutwithout the YSZ scattering layer. The thickness of the the YSZ YSZ scattering scattering layer layer is is2.5 2.5 um:µm: (a) (Electroluminescencea) spectrum spectrum of the of OLED the OLED devices. devices. The spectrumThe spectrum data dataare normalized are normalized at 1mA/cm at 1mA2/;cm (b2) ;(Characteristicsb) Characteristics of luminance of luminance for different for different voltages; voltages; (c) Characteristics(c) Characteristics of current of current density density for for different different voltages; voltages; and and (d (d) )Characteristics Characteristics of of current current efficiency efficiency for difference difference luminance.

In the the previous previous study, study, employing employing a a scattering scattering laye layerr to to OLED OLED devices devices as asa layer a layer or film or film coated coated on outeron outer surface surface of the of theglass glass substrate substrate reported reported enha enhancednced light light efficiency efficiency of OLEDs of OLEDs [10]. [ 10However,]. However, the distancethe distance from from the thelight light emitting emitting layer layer to the to thescattering scattering layer layer was was very very long, long, including including the the thickness thickness of theof the glass glass substrate substrate 0.5 mm 0.5 mmcompared compared to the to the pixelpitch pitch80 um. 80 Therefore,µm. Therefore, the light the scatter light effect scatter caused effect mixingcaused mixingcolors among colors amongpixels pixelsand led and to led image to image blur blur on onthe the display display screen. screen. In In this this work, work, we implemented the scattering layer on the inner surface of the glass substrate. The distance from the light emitting layer to the scattering layer was short, about half of the pixel pitch, so a good image quality without image blur could be achieved by the proposed method. The emitted light at the OLED device between the anode electrode and the electrode propagated to various directions while it passed the scattering layer. The scattered light changed the direction of propagation through the glass substrate, which limited the total internal reflection. Therefore, the light from the OLED substrate wave guided mode could be extracted, which enhanced the light efficiency of the OLED device.

Nanomaterials 2019, 9, 1241 6 of 9 implemented the scattering layer on the inner surface of the glass substrate. The distance from the light emitting layer to the scattering layer was short, about half of the pixel pitch, so a good image quality without image blur could be achieved by the proposed method. The emitted light at the OLED device between the anode electrode and the cathode electrode propagated to various directions while it passed the scattering layer. The scattered light changed the direction of propagation through the glass substrate, which limited the total internal reflection. Therefore, the light from the OLED substrate Nanomaterials 2019, 9, 1241 6 of 9 wave guided mode could be extracted, which enhanced the light efficiency of the OLED device. Table3 shows the relative luminance of OLED devices with the implemented YSZ nanoparticle Table 3 shows the relative luminance of OLED devices with the implemented YSZ nanoparticle scattering layer compared to the control device without the scattering layer. In the presence of the scattering layer compared to the control device without the scattering layer. In the presence of the scattering layer, the total luminance and luminance in normal direction of the OLED device enhanced, scattering layer, the total luminance and luminance in normal direction of the OLED device enhanced, compared to the reference device without the scattering layer. At the thickness of scattering layer, compared to the reference device without the scattering layer. At the thickness of scattering layer, 2.5 2.5 µm, a 29% enhancement of total luminance and a 38% enhancement of luminance in normal um, a 29% enhancement of total luminance and a 38% enhancement of luminance in normal direction direction were achieved. As the result, the optimal thickness of the scattering layer was observed were achieved. As the result, the optimal thickness of the scattering layer was observed at 2.5 um. at 2.5 µm. Moreover, the luminance enhancement is not proportional to the increase of the scatter Moreover, the luminance enhancement is not proportional to the increase of the scatter effect. As the effect. As the thickness of scattering layer was increased, the luminance decreased. This means that thickness of scattering layer was increased, the luminance decreased. This means that the the transmittance of the thick scattering layer lowers as the thickness increases. To analyze optimal transmittance of the thick scattering layer lowers as the thickness increases. To analyze optimal thickness of the scattering layer implemented in the OLED device, the luminance change was simulated thickness of the scattering layer implemented in the OLED device, the luminance change was usingsimulated a geometrical using a geometrical optics tracing optics method. tracing The meth LightToolsod. The LightTools program (Synopsys, program (Synopsys, Mountain View, Mountain CA, USA)View, was CA, used USA) for was the used simulation. for the Assimulation. Figure5a shows,As Figure the 5a simplified shows, the device simplified structure device used structure for the simulationused for the also simulation approximated also approximated constant values constant of parameters values of parameters such as the such refractive as the refractive index and index the thicknessand the ofthickness each layer. of each The maximumlayer. The luminance maximum enhancement luminance enhancement was calculated was as 32%calculated enhancement as 32% µ atenhancement a thickness ofat 2.5a thicknessm, in good of 2.5 agreement um, in good with agre theement experimental with the experiment result. Moreover,al result. the Moreover, maximum the luminancemaximum enhancement luminance enhancement in normal direction in normal was calculateddirection was as 42% calculated at the center as 42% of theat the device. center Figure of the5 showsdevice. the Figure simulation 5 shows result the ofsimulation luminance result change of luminance in the OLED change device. in the OLED device.

Table 3. Ratio of the luminance of OLED devices with the YSZ scattering layer to the luminance of the Table 3. Ratio of the luminance of OLED devices with the YSZ scattering layer to the luminance of reference OLED without the scattering layer, for various layer thickness. the reference OLED without the scattering layer, for various layer thickness. Changed Ratio of Total Changed Ratio of Normal Thickness of Scattering Layer (µm) Changed Ratio of Total Changed Ratio of Normal Thickness of Scattering Layer (um) Luminance Direction Luminance Luminance Direction Luminance 00 (without (without scattering scattering layer) layer) 100%100% 100% 100% 1.51.5 123%123% 131% 131% 2.52.5 129%129% 138% 138% 4.04.0 126%126% 135% 135% 7.0 119% 128% 7.0 119% 128%

(a) (b)

FigureFigure 5. 5.Simulated Simulated luminance luminance change change of of the the OLED OLED device device withwith the the implemented implem YSZ scattering layer: (a()a) Schematic Schematic diagram diagram and and parameters parameters of of the the OLED OLED device; device; ( b(b)) Simulated Simulated total total luminance luminance and and center center luminanceluminance to to normal normal direction direction as as a a function function of of the the thickness thickness of of the the scattering scattering layer. layer.

FigureFigure6 6illustrates illustrates the the relative relative angular angular luminance luminance distribution distribution of of the the OLED OLED device device with with the the implementedimplemented YSZYSZ nanoparticlenanoparticle scatteringscattering layerlayer asas aa functionfunction ofof thickness thickness ofof thethe scatteringscattering layers. layers. Compared to Lambertian distribution, for the reference device it was observed that the luminance at high angles was larger than that at normal direction. The OLED devices with implemented scattering layers showed enhanced luminance distribution at all the angles. Moreover, at the scattering layer thickness of 2.5 um, the most enhanced luminance was achieved. Consequently, the YSZ nanoparticle scattering layer enhanced the light extraction efficiency to all angular directions by the optimized scatter effect. Moreover, enhanced light extraction at high viewing angles improved the angular color change of the OLED device. As Figure 7 shows, the white angular dependency (WAD), which represents color change by angular viewing direction, decreased significantly. The color coordinate change (Δu’v’) of the reference device at 60° was 0.028, while that of the OLED device with the 2.5 Nanomaterials 2019, 9, 1241 7 of 9

Compared to Lambertian distribution, for the reference device it was observed that the luminance at high angles was larger than that at normal direction. The OLED devices with implemented scattering layers showed enhanced luminance distribution at all the angles. Moreover, at the scattering layer thickness of 2.5 µm, the most enhanced luminance was achieved. Consequently, the YSZ nanoparticle scattering layer enhanced the light extraction efficiency to all angular directions by the optimized scatter effect. Moreover, enhanced light extraction at high viewing angles improved the angular color change of the OLED device. As Figure7 shows, the white angular dependency (WAD), which representsNanomaterials color 2019 change, 9, 1241 by angular viewing direction, decreased significantly. The color coordinate7 of 9 changeNanomaterials (∆u’v’) 2019 of, the9, 1241 reference device at 60◦ was 0.028, while that of the OLED device with the7 2.5 of µ9 m thickum scatteringthick scattering layer waslayer 0.005, was 0.005, which which is an 82%is an decrease 82% decrease of color of color change. change. Hence, Hence, due todue the to scatter the effectscatterum ofthick the effect YSZscattering of nanoparticle the YSZ layer nanoparticle was scattering 0.005, scattering which layer, weis laan achievedyer, 82% we decrease achieved not only of not enhancedcolor only change. enhanced light Hence, extraction light due extraction e ffito ciencythe butefficiencyscatter also improved effect but of also the angular improved YSZ nanoparticle color angular change scatteringcolor of thechange OLED layer, of thewe device. achievedOLED device. not only enhanced light extraction efficiency but also improved angular color change of the OLED device.

FigureFigure 6. 6.Relative Relative angularangular luminanceluminance distribution of of OLED OLED devices devices with with the the implemented implemented YSZ YSZ nanoparticleFigure 6. Relative scattering angular layer luminance versus the distribution thickness of of scattering OLED devices layers, with compared the implemented to the reference YSZ nanoparticle scattering layer versus the thickness of scattering layers, compared to the reference device devicenanoparticle and Lambertian scattering distribution layer versus. The the Lambertian thickness ofdistribution scattering is la plottedyers, compared using scale-up to the to referencethe same and Lambertian distribution. The Lambertian distribution is plotted using scale-up to the same value valuedevice of and reference Lambertian device distribution at 0 degree. The for relativeLambertian comparision. distribution is plotted using scale-up to the same of referencevalue of reference device at devi 0 degreece at 0 fordegree relative for relative comparision. comparision.

µ FigureFigure 7. 7.Change Change of of color color coordinates coordinates ((∆Δu’v’)u’v’) of the OLED device device with with the the implemented implemented 2.5 2.5 um mthick thick YSZYSZFigure nanoparticle nanoparticle 7. Change scattering ofscattering color coordinates layer layer versus versus (Δ theu’v’)the viewingviewing of the OLED angle. device with the implemented 2.5 um thick YSZ nanoparticle scattering layer versus the viewing angle. 4. Conclusions 4. Conclusions The implemented YSZ nanoparticle material and planarization material endured, without deformation,The implemented the subsequent YSZ thermalnanoparticle annealing material proce anssd at planarization the temperature material ranged endured, to 580 °C. without Highly efficientdeformation, OLED the devices subsequent can be thermal manufactured annealing using proce thess conventional at the temperature display ranged manufacturing to 580 °C. processHighly byefficient implementing OLED devices YSZ nanoparticles.can be manufactured A 38% usingenhancement the conventional in luminance display of manufacturingthe OLED device process was achievedby implementing by the scattering YSZ nanoparticles. layer with Aa 38%thickn enhaess ncementof 2.5 um, in comparedluminance toof the the device OLED withoutdevice wasthe scatteringachieved bylayer. the Moreover,scattering thelayer decreased with a thickn angularess color of 2.5 change um, compared was obtained to the by deviceimplementation without the of thescattering nanoparticle layer. scatteringMoreover, layer, the decreased where the angular white angular color change dependency was obtained (WAD) byΔu’v’ implementation was reduced toof the nanoparticle scattering layer, where the white angular dependency (WAD) Δu’v’ was reduced to

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4. Conclusions The implemented YSZ nanoparticle material and planarization material endured, without deformation, the subsequent thermal annealing process at the temperature ranged to 580 ◦C. Highly efficient OLED devices can be manufactured using the conventional display manufacturing process by implementing YSZ nanoparticles. A 38% enhancement in luminance of the OLED device was achieved by the scattering layer with a thickness of 2.5 µm, compared to the device without the scattering layer. Moreover, the decreased angular color change was obtained by implementation of the nanoparticle scattering layer, where the white angular dependency (WAD) ∆u’v’ was reduced to 0.005 at 60 degrees from normal direction. Thus, we believe that this study can provide a simple, practical, and low-cost method for improving the performance of OLED display products.

Author Contributions: Conceptualization, C.Y.P.; B.C. methodology, C.Y.P. investigation, C.Y.P. writing, C.Y.P. supervision, B.C. funding acquisition, B.C. Funding: This research was supported by Industrial Human Resources and Skill Development Program (N0001415, Display Expert Training Project for Advanced Display equipment and components engineer) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). Conflicts of Interest: The authors declare no conflict of interest.

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