Organic Electronics 11 (2010) 137–145

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Deep blue, efficient, moderate microcavity organic light-emitting diodes

Hyoung Kun Kim a,1, Sang-Hwan Cho b,1, Jeong Rok Oh a, Yong-Hee Lee b, Jun-Ho Lee c,Jae-Gab Lee c, Soo-Kang Kim d, Young-Il Park d, Jong-Wook Park d, Young Rag Do a,*

a Department of Chemistry, Kookmin University, Seoul 136-702, Republic of Koreab Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 350-710, Republic of Koreac School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Koread Department of Chemistry/Display Research Center, The Catholic University of Korea, Bucheon 420-743, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 August 2009Received in revised form 1 October 2009Accepted 17 October 2009Available online 23 October 2009

Keywords:Organic light-emitting diodesMicrocavityBragg mirrorBlue

1566-1199/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.orgel.2009.10.011

* Corresponding author.E-mail address: yrdo@kookmin.ac.kr (Y.R. Do).

1 These authors contributed equally to this work.

Two types of microcavity blue organic light-emitting diodes (OLEDs) with a dielectric Braggmirror (two different center wavelengths, type-A � 465, type-B � 470 nm) were designedto achieve the color coordinates of NTSC blue standard and enhance the quantum efficiencyin the normal direction. A moderate microcavity OLED was defined as a microcavity OLEDwith a single pair of TiO2/SiO2 high/low dielectric layers inserted between an indium tinoxide (ITO) layer and a glass substrate. The moderate microcavity blue OLED dopedwith 9,10-bis(30 ,50-diphenylphenyl)-10-(300 0,500 0-diphenylbiphenyl-400-yl)anthracene (TAT)exhibited excellent color coordinates (type-A; x = 0.143, y = 0.068, type-B; x = 0.139, y =0.081), which were better than the color coordinates of the NTSC standard (0.140, 0.080)and the TAT-doped conventional noncavity OLED (0.156, 0.094). There were approximately60% and 54% improvement in the relative quantum efficiency of the type-B TAT-dopedmoderate microcavity OLED, respectively, compared to those of the conventional noncavityreference OLED (ITO = 150 nm) and other reference type-II with an identical ITO layerthickness (ITO = 85 nm). These improvements in color coordinates and the relative quan-tum efficiency were attributed to the optimization of narrowed spectrum bandwidth andenhanced integrated spectrum intensity in the TAT-doped blue OLED, resulting from theeffective microcavity effect.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Organic light-emitting diodes (OLEDs) are used widelyas ideal emissive devices in lighting and flat panel displayson account of their low driving voltage, low powerconsumption, high color gamut, high contrast and rapidresponse [1–4]. For full color display applications, it isessential to develop high performance red-, green-, andblue-emitting OLEDs with high EL efficiency, good thermalproperties, and long device lifetime as well as pure

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color coordinates (1931 Commission Internationale del’Eclairage (CIE) x, y coordinates). To develop stable, effi-cient and saturated red to green and blue OLEDs, the gen-eral approach is to design and synthesize new organicsmall molecules as emitters in OLEDs. Many organic smallmolecules for red [5–7], green [8,9] and blue emitters [10–19] have been synthesized. Recently, high efficient andpure red fluorescent and green OLEDs have been developedwith CIE x, y coordinates and electrical efficiencies of (0.67,0.23) and 11 cd/A and (0.29, 0.64) and 21 cd/A, respec-tively. However, the color purity and efficiency of theblue-light emitters are still lower than the requirementsfor balancing with red and green emitters for full colordisplays. Therefore, improvements in the blue emittingperformance are necessary, particularly for large display

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applications, such as digital video displays (DVD) and lap-top computer screens, which require a larger color gamut.In particular, blue emitters require the CIE x, y coordinatesof the National Television System Committee (NTSC) bluecolor standard (0.14, 0.08). With this respect, synthesizingnew blue emitters with a narrow band and saturated colormight help in obtaining a deep blue color. Many types ofblue emitters have been studied extensively, and consider-able effort has been made to improve their blue EL perfor-mance [10–19]. Thus far, the best results of a deep blue-light emitter are anthracene-cored derivatives with excel-lent color coordinates (0.156, 0.088) and good electricalefficiency [19]. The color coordinates of anthracene-coredderivatives are still inferior to the NTSC blue color stan-dard. Therefore, further improvements in blue color andluminescence efficiency are essential for applications tolarge display devices, such as televisions and computerscreens. However, the broad emission spectra due to bothvibronic transitions and strong inhomogeneous broaden-ing of the transitions in various organic molecules makeit difficult to develop deep blue color OLEDs using a newtype of blue emitter only.

Another way of overcoming the problem of limited col-or purity of blue OLEDs is the optical modification of theOLED structure and filtering out the unwanted long wave-length emission. With this respect, a simple modificationof the OLED structure, i.e. the insertion of organic layersinto a microcavity structure has been investigated exten-sively in recent years owing to the enhanced luminescence,narrowing bandwidth, and tunable emission color [20–32].Microcavity OLEDs can be divided into two categories: oneis OLEDs embedded in a weak cavity structure [20], and theother is insertion into a strong cavity structure [21–32].The former is a conventional OLED structure, in whichthe central organic layers are sandwiched between an alu-minum (Al) metal cathode and an indium tin oxide (ITO)anode. On the other hand, the latter uses a conventionalAl electrode and semitransparent metal mirror [21–24] oran ITO anode coated onto a dielectric Bragg mirror[25–32]. As previously reported, the weak microcavityhas relatively minor effects on the electroluminescence(EL) performance [20]. However, the strong microcavitymodifies the photonic mode density within the OLED sig-nificantly, improving the EL performance and tuning thecolor purity from a homogeneously broadened emitter atthe microcavity resonance [21–32].

The use of metal mirrors enhances the EL efficiency andluminance in the top- or bottom-emitting OLEDs signifi-cantly but their narrowing capability of the emission spec-trum from the metal mirror still does not satisfy the NTSCpure blue. Recently, Mudler et al. [21] obtained a high effi-cient and deep blue color (0.116, 0.136) OLED from a skyblue phosphorescent OLED using a strong microcavitystructure with a semitransparent Ag anode. Although thecolor purity of the phosphorescent OLED was improvedsignificantly using a strong metal mirror-based microcavi-ty, it is necessary to control the detailed peak position andbandwidth of the emission spectrum in order to approachthe NTSC blue color standard.

On the other hand, dielectric mirrors reduce the band-width of the EL spectrum considerably but the improve-

ment in OELD efficiency is not as high as the metal-basedstrong microcavity. In the early stages of dielectric-basedmicrocavity OLEDs, Tokito et al. [28,29] reported that thepurity of red, green and blue obtained in microcavityOLEDs was superior to the NTSC standard. However, the to-tal enhancement of their device by integration over all an-gles and wavelengths was not reported. Recently, Zhanget al. [30] and Jung et al. [31] reported the tuning of theemission color and improvement in EL efficiency simulta-neously by simply adjusting the cavity length and carefullydesigning the device structures. They narrowed the emis-sion spectrum of blue, green and red OLEDs significantly,but improved the green and red efficiency only. Quite re-cently, Cho et al. [32] reported that moderate microcavityformed by an Al cathode and one pair of high- and low-in-dex (SiNx/SiO2) dielectric layers enhanced the efficiency ofsky blue OLEDs (0.110, 0.216) by more than 26%. Althoughthere are many papers on the enhanced output of the emis-sion peaks from microcavity OLEDs, there are few reportson the high efficient and saturated blue OLEDs, which aremore efficient than noncavity conventional OLEDs andhave a deeper color than the NTSC blue color standard(0.14, 0.08). This paper reports microcavity OLEDs contain-ing dielectric Bragg mirrors with a finely tunable peak po-sition and a bandwidth of EL emission that achievesenhanced external quantum efficiency and a deep blue col-or (y 6 0.08). The significant change in bandwidth, externalquantum efficiency and color of the microcavity OLEDswith three different pairs and two lattice constants of high-and low-index dielectric layers was examined in order togain a better understanding of the effect of the degree ofmicrocavity TiO2/SiO2 layers on the efficiency and colorpurity of 9,10-bis(30,50-diphenylphenyl)-10-(300 0,500 0-diph-enylbiphenyl-400-yl)anthracene (TAT)-doped OLEDs.Among the various microcavity OLEDs, a moderate micro-cavity OLED was defined as a microcavity OLED with only asingle pair of TiO2/SiO2 layers inserted between the indiumtin oxide (ITO) layer and a glass substrate to achieve theNTSC blue standard and enhance the normally directedquantum efficiency.

2. Experimental

Fig. 1 shows a schematic diagram of the microcavityOLED with one, two and four pairs of high- and low-indexpair of dielectric Bragg mirrors. Two types of reference blueOLEDs were fabricated with a commercialized emitter inthe following configurations: ITO (150 nm or 85 nm)/4,40,400-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine[2-TNATA] (49 nm)/N,N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl)benzidine [NPB] (12 nm)/9,10-bis(30,50-diphenyl-phenyl)-10-(300 0,500 0-diphenylbiphenyl-400-yl)anthracene [TAT](30 nm)/8-hydroxyquinolinealuminium [Alq3] (24 nm)/LiF(1 nm)/Al (150 nm). The optical lengths of the type-A andtype-B TAT-doped microcavity blue OLEDs were attributedto the different optimized structures summarized in Table1. In the type-A microcavity blue OLEDs, one, two and fourpairs of high-(TiO2) and low-index (SiO2) layers wereplaced between the ITO layer and glass substrate to controlthe top reflectivity of the reflection band.

Fig. 1. Schematic diagram of a microcavity OLED device and chemical structure of TAT.

H.K. Kim et al. / Organic Electronics 11 (2010) 137–145 139

Fig. 1 also shows the chemical structures of the blueemitting material (TAT) in this experiment. As reportedpreviously [19], TAT was purified employing a silica columnand recrystallization, was characterized with nuclear mag-netic resonance (NMR), Fourier transform infrared (FT-IR),and fast atom bombardment mass (FAB-MS) analysis. Bora-tion and Suzuki aryl–aryl coupling reactions were used inall syntheses. The detailed synthesis route and character-ization results of TAT are reported elsewhere [19].

To examine the effect of the dielectric Bragg mirrorsexperimentally, OLEDs were fabricated on two referencesubstrates (conventional reference (ITO = 150 nm) and ref-erence type-II (ITO = 85 nm) and two types (type-A onepair and type-B one pair) of dielectric Bragg mirrors cover-ing the substrates. Table 1 summarizes the thicknesses andsheet resistances of each sample. Mono-color OLEDsamples were fabricated with an active area of2.0 � 2.0 mm2. High-index TiO2 (n = 2.3) and low-indexSiO2 (n = 1.46) thin films were deposited alternatively onglass substrates in a single chamber using an e-beam evap-oration technique [33]. The ITO layer was then depositedby magnetron sputtering with no intentional heating, andorganic layers corresponding to 2-TNATA, NPB, TAT, andAlq3 as well as a LiF film (as EIL) and aluminum metal cath-ode were evaporated sequentially to fabricate both the ref-erence and microcavity devices. Finally the OLEDs wereencapsulated with 0.7 mm Corning cover glass.

The current–voltage (I–V) characteristics of the electro-luminescence (EL) devices fabricated were obtained usinga Keithley 2611 electrometer. The EL spectra, EL brightnessand color coordinates were obtained with a spectropho-tometer (PSI Co., Ltd.) and a Minolta CS-1000A. Theenhancement ratio of the quantum efficiency was obtainedby comparing the normally directed light output of micro-

Table 1The designed and fabricated thicknesses of the ITO, TiO2, and SiO2 films for the twosheet resistances of ITO for each sample.

Noncavity OLEDs

Conventional reference Reference

Thickness (nm) ITO 150 85SiO2 – –TiO2 – –

Sheet resistance (±2 X/h) 13 47

cavity OLEDs at 10 mA/cm2 to that of a conventional refer-ence OLED (ITO = 150 nm).

3. Results and discussion

As reported previously [29–32], the electrical efficiency,relative quantum efficiency and color purity of the micro-cavity OLEDs depend on the thickness of the ITO, organic,low-index, and the high-index layers, respectively. The fi-nite-difference time-domain (FDTD) simulations wereused to determine the resonance wavelength of the cavityby varying the thickness of the ITO, low-index (SiO2), andhigh-index layers (TiO2). Dielectric Bragg mirrors withtwo different lattice constants were also inserted into theblue OLEDs in an attempt to tune the position of the blueemission peaks. Here, two types of quarter-wave stackswere designed to have center wavelengths of �465(type-A) and 470 nm (type-B). The thicknesses of the ITO,low-index, and high-index layers are defined as the valueswhen a deep blue color was realized. The reflectance (R)and transmittance (T) of the two types of quarter-wavestacks consisting of alternating low-index SiO2 and high-index TiO2 were measured as a function of the number ofperiods. As shown in Fig. 2, the reflectance of the type-ABragg mirrors at the blue region increased with increasingperiodic number of stacks. The central wavelength of thereflectance band shifted toward a bluish color withincreasing number of high/low stacks, which is consistentwith those in the calculation. The inset in Fig. 2 shows aside-view scanning electron microscopy (SEM) image of areal fabricated type-A dielectric multilayer comprised ofalternate TiO2 and SiO2 quarter-wave films of the fourthperiods. As shown in Table 1 and Fig. 2, the thicknesses

types of reference OLEDs and two types of microcavity OLEDs along with the

Microcavity OLEDs

Type-A Type-B

type-II One pair Two pairs Four pairs One pair

80 8580 8250 5134 44 41 36

Fig. 2. The measured diffusive reflectance spectra of two and four pairs ofTiO2/SiO2 high/low stacks coated on glass substrate. The inset shows aside-view scanning electron microscopy (SEM) image of the fourthperiods of [TiO2/SiO2] films coated with the ITO film.

Fig. 3. (a) Normalized EL spectra and (b) relative EL spectra of theconventional reference OLED (ITO = 150 nm) and type-A microcavityOLEDs with the periodic number of high/low stacks at viewing angle of 0�.The inset in (a) shows the relationship between the full width at halfmaximum (FWHM) of the EL spectrum and the periodic number of high/low stacks, and the inset in (b) shows the relationship between the ELspectrum intensity and the periodic number of high/low stacks. (c)Calculated EL spectra of the conventional reference OLED (ITO = 150 nm)and type-A microcavity OLEDs with the periodic number of high/lowstacks at viewing angle of 0�. The inset in (c) shows the relationshipbetween the calculated and measured electrical efficiency along normaldirection and the periodic number of high/low stacks.

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of the TiO2/SiO2 films match those of the dielectric films inthe designed Bragg mirrors satisfactorily.

In order to compare the performance of the conven-tional noncavity OLED and the type-A microcavity OLEDs,the color coordinates, brightness, electrical efficiency andpower efficiency should be considered. The normalizedEL spectra of both type-A devices and the conventional ref-erence (ITO = 150 nm) device (Fig. 3a) indicates that an in-crease in the periodic number of stacks decreases the fullwidth at half maximum (FWHM) of the emission spectrumup to �9 nm (four pairs) [29,30]. Single-mode blue emis-sions with peak wavelength of 460–466 nm with reducedFWHMs were realized by simply increasing the periodicnumber of SiO2/TiO2 stacks. Compared to the conventionalnoncavity devices with a broader EL spectrum (�55 nmFWHM), controlling both the narrowness and position ofthe emissive spectrum will be of great benefit in optimiz-ing the color purity of blue OLED for applications to displaydevices [32]. Fig. 3b shows the relative EL spectra of theconventional OLED and type-A OLEDs with the periodicnumber of stacks at a viewing angle of 0�. The intensityof the EL spectra increased with increasing periodic num-ber of stacks up to two pairs and then decreased with fur-ther increases. This decrease in EL intensity in the fourpairs of TiO2/SiO2 stacks was attributed to the increasedreflectance and reduced transmittance of blue light, asshown in Fig. 2. The figure compares the normally directedemission spectra recorded at a constant current density(10 mA/cm2) for the conventional noncavity OLED andtype-A microcavity OLEDs with a periodic number ofstacks. The results show that at the main peak wavelength,the emission intensity of the type-A microcavity OLEDswith one, two, and four pair stacks was 2.3, 3.3, and 2.2higher than that of the conventional reference OLED,respectively. Moreover, the EL spectra of the microcavityOLEDs were narrowed and shifted, indicating the micro-cavity effect. Therefore, the thicknesses of the ITO, low-and high-index layer affect the spectral characteristics[30]. Our FDTD calculations were also used to determine

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the normally directed emission spectrum and electricalefficiency of type-A microcavity OLEDs with one, two andfour pair stacks, indicating an improvement in the relativeelectrical efficiency of this device of �23%. Fig. 3c showsthe calculated emission spectra and relative electrical effi-ciency of the type-A microcavity OLEDs. Fig. 3b and c con-firm that the calculated shape of the emission spectra andthe enhancement of the electrical efficiency are very simi-lar to those of the measured spectra and electrical effi-ciency in the normal direction.

Fig. 4a and b shows the current density (J)–normaldirection luminance (L)–voltage (V) characteristics of the

Fig. 4. (a) Current density–bias voltage characteristics, (b) current density–noefficiency characteristics, (d) current density–power efficiency characteristics (e)(f) CIE color coordinates of the conventional reference OLED (ITO = 150 nm) andviewing angle of 0�. (For interpretation of the references to colour in this figure

conventional noncavity reference OLEDs and type-A micro-cavity OLEDs with the periodic number of stacks. The J ofthe conventional reference OLED increased faster withincreasing voltage than those of all microcavity OLEDs.The superior J–V characteristics was attributed to the en-hanced hole injection from the thicker ITO anode of theconventional OLED with a lower resistance, as summarizedin Table 1. On the other hand, the dependence of J for alltype-A microcavity OLEDs showed similar trends due tothe similar sheet resistance of the ITO anodes, irrespectiveof the periodic number of stacks. Fig. 4b shows that the dif-ference between the L–V curves of the conventional OLED

rmal direction luminance characteristics, (c) current density–electricalenhancement ratio of the quantum efficiency in the normal direction andtype-A microcavity OLEDs with a periodic number of high/low stacks at alegend, the reader is referred to the web version of this article.)

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and the microcavity OLEDs with the periodic number ofstacks is larger than that between the J–V curves. This fig-ure also shows the different trends of the L–V curves of themicrocavity OLEDs as a function of the periodic number ofstacks. The microcavity OLEDs with increasing number ofquarter-wave stacks showed decreased brightness due tothe different dependences of the microcavity effect onthe periodic number of stacks. Although higher color puritywas achieved by increasing the microcavity effect, theluminous efficiency was reduced despite the enhancedpeak intensity due to the loss of light around the shoulderof the peak at 480 nm, which has high eye sensitivity. It ap-pears that when dielectric mirrors are used, there is sometrade-off between the color purity and luminance for blue-emitting materials.

Fig. 4c and d shows the relationship between the cur-rent efficiency–current density and power efficiency–cur-rent density of the conventional reference OLED andtype-A microcavity OLEDs. The current efficiency at10 mA/cm2 in the normal direction for the conventionalnoncavity reference OLED was 2.8 cd/A, whereas the effi-ciencies for the type-A microcavity OLEDs with one, twoand four pairs of quarter-wave stacks were 3.2, 2.4, and1.0 cd/A, indicating that the current efficiency of conven-tional reference OLED in the normal direction is slightlylower than that in the type-A microcavity OLED containinga single pair of quarter-wave stacks. Fig. 4d also shows asimilar relationship of power efficiency–current densitywith the number of quarter-wave stacks. Although the cur-rent and power efficiencies of the moderate microcavityblue OLED with one pair of high-low stacks is slightly high-er than that of the noncavity blue OLED, the photons emit-ted from the microcavity blue OLEDs was significantlyenhanced by the microcavity effect, as illustrated by theemission spectra in Fig. 3b.

For a systematic comparison of these OLEDs, theenhancement ratio of the quantum efficiency was definedas the ratio of the normally directed light output of micro-cavity OLEDs or other reference OLEDs to that of a conven-tional reference OLED (ITO = 150 nm). This enhancementratio only shows how much the EL quantum efficiency at10 mA/cm2 of microcavity OLEDs was improved relativeto that of a conventional noncavity reference OLED.Fig. 4e shows the enhancement ratio of the quantum effi-ciencies of each OLED. In contrast to the trends in currentefficiency, the quantum efficiency of type-A microcavityOLEDs with a single pair of quarter-wave stacks was supe-rior to that of the conventional reference OLED. However,the microcavity OLEDs with a higher number of pairs ofquarter-wave stacks showed lower quantum efficiency.The normally directed quantum efficiency of the type-Amicrocavity OLEDs with a single pair of high and low-indexlayers was 1.57 higher than that of the conventional refer-ence OLED. As expected, with the resonant wavelength setto 465 nm (19 nm longer than the PL peak of TAT), thetype-A moderate microcavity device with a single pair ofquarter-wave stacks showed the significantly enhancedquantum efficiency compared to the conventional refer-ence OLED device. Even with brightness slightly higherthan the conventional OLEDs, the quantum efficiency ofthe microcavity OLEDs with a single pair of quarter-wave

stacks was significantly higher than that of the conven-tional OLED device. This was attributed to the lower eyesensitivity of more pure blue moderate microcavity OLEDs.In addition, Fig. 4f shows that the CIE color coordinates ofthe type-A microcavity OLEDs with a different number ofquarter-wave stacks were purer than those of the conven-tional noncavity OLED. The increased color purity with theincreasing number of high/low stacks was attributed to theblue shift and narrowing of the emission spectrum due tothe enhanced microcavity effect. In particular, the moder-ate microcavity OLED with a single pair of high/low stacksshowed a deeper blue and higher quantum efficiency thanthe conventional noncavity OLED.

The angular dependence of the emission spectrum wasalso examined because changes in color with changingviewing angle are undesirable for display applications[28,29]. Fig. 5a and b shows the angle dependence of theemission spectra of a conventional reference OLED(ITO = 150 nm) and a type-A microcavity OLED with a sin-gle pair of high/low stacks at room temperature, respec-tively. The figures show negligible angular dependence inthe peak shape of the conventional reference OLED but ablue shift in the emission spectrum of the microcavityOLED with a single pair of high/low stacks. The CIE coordi-nates were used to assess the color changes in the OLEDs.Fig. 6 shows the angular dependence of the CIE color coor-dinates for the conventional reference OLED and type-Amicrocavity OLED with a single pair of high/low stack ona chromaticity diagram. When viewed from the normaldirection, the CIE color coordinates for the conventionalreference OLED and moderate microcavity OLED were(0.156, 0.094) and (0.143, 0.068), respectively. This sug-gests that the light emitted by the moderate microcavityOLED has deeper color purity than the conventional refer-ence standard. The results also show that the color changeratio of the moderate microcavity OLED (Dx = 13%, Dy =15%) became slightly worse than that of the conventionalreference OLED (Dx = 1%, Dy = 31%) as the viewing anglewas changed up to 60� of the normal. In contrast to the col-or change in the conventional reference OLED(0.156, 0.094 ? 0.157, 0.123) and the angular dependenceof color purity of the moderate microcavity OLED(0.143, 0.068 ? 0.161, 0.058) showed improved color pur-ity with increased viewing angle. The improved color pur-ity and comparable angular dependence of the color purityare another advantage that the moderate microcavityOLEDs have over conventional reference OLEDs.

The far-field pattern of the integrated spectrum inten-sity was measured to determine the physical characteris-tics underlying the angular dependence of a single ELemission intensity. The far-field photon radiation fromthe OLED devices was measured directly with a photodi-ode, 1 mm2 in area, which was located 10 cm from theradiation source. Fig. 7 compares the far-field profiles ofthe conventional reference and moderate microcavityOLEDs cut along the horizontal line. It shows that the non-cavity OLED radiates into air in a manner consistent with aLambertian source. It also suggests that the moderatemicrocavity OLEDs show only small variations in theirangular radiation patterns over a large angular span.Therefore, the far-field profiles of the moderate microcav-

Fig. 5. Viewing angle dependence of the EL spectra of (a) conventional reference OLED (ITO = 150 nm) and (b) type-A moderate microcavity OLED with asingle pair of high/low stacks.

Fig. 6. Viewing angle dependence of the Commission Internationale del’Eclairage (CIE) color coordinates of the conventional reference OLED(ITO = 150 nm) and type-A moderate microcavity OLED with a single pairof high/low stacks under 10 mA/cm2 of current density.

Fig. 7. The viewing angle dependence of integrated EL intensity profilesof conventional reference OLED (ITO = 150 nm) and type-A moderatemicrocavity OLED with a single pair of high/low stacks under 10 mA/cm2

of current density.

H.K. Kim et al. / Organic Electronics 11 (2010) 137–145 143

ity OLEDs with a single pair of high/low stacks were similarto that of the conventional reference OLEDs.

To achieve the NTSC blue color from TAT-doped blueOLED, the tunable effects of the microcavity (type-A onepair and type-B one pair) on OLEDs were examined bycontrolling the central wavelength of the dielectric Bragg

144 H.K. Kim et al. / Organic Electronics 11 (2010) 137–145

mirror. OLEDs were fabricated on two reference substrates(conventional reference (ITO = 150 nm) and referencetype-II (ITO = 85 nm)) and two moderate microcavity sub-strates (type-A: kcen = 465 nm, type-B: kcen = 470 nm)deposited with a single pair of high/low stacks, side by sidefor comparison. Consistent with the simulated results,Fig. 8 suggests that the emission spectrum of the type-Bmicrocavity OLED was shifted slightly to a longer wave-length compared to that of the type-A microcavity OLEDs.This indicates that the emission spectra of two types ofmoderate microcavity OLEDs were narrower than thoseof the two reference OLEDs. Fig. 8 also shows the CIE colorcoordinates in the normal direction for the referenceOLEDs and moderate microcavity OLEDs. The color coordi-nates of a thin ITO (85 nm) coated reference type-II OLEDwas slightly higher than that of the conventional referenceOLED (ITO = 150 nm). This means that the reduced ITOthickness of the noncavity OLED have a minor effect in con-trolling the color purity and EL performance. As mentioned

Fig. 8. The normalized EL spectra and CIE color coordinates of theconventional reference OLED (ITO = 150 nm), reference type-II OLED,type-A moderate microcavity, and type-B moderate microcavity OLEDunder a current density of 10 mA/cm2.

Fig. 9. Electrical efficiency and the enhancement ratio of the normally directedreference type-II OLED, type-A moderate microcavity, and type-B moderate mic

above, the light emitted from the moderate microcavityOLEDs had deeper color purity than that emitted by bothtypes of reference OLEDs. The color coordinates(x = 0.139, y = 0.081) of the type-B moderate microcavityOLED with a single pair of high/low stacks was very closeto the color coordinates of NTSC blue (x = 0.140,y = 0.080). Therefore, a pure blue color was obtained fromTAT-doped blue OLEDs simply by introducing a single pairof TiO2/SiO2 high/low dielectric stacks on the substrate.

The electroluminescence measurements showed thecurrent density–luminance characteristics of the fourtypes of OLEDs with and without the microcavity high/low stack. The luminance values of the two types ofmoderate microcavity OLEDs and reference OLEDs mea-sured in the normal direction (h = 0�) under dc excitationat 10 mA/cm2 were 320 (type-A one pair, ITO thicknessof 80 nm), 340 (type-B one pair, ITO thickness of85 nm), 280 (conventional, ITO thickness of 150 nm),and 290 cd/m2 (conventional, ITO thickness of 85 nm).Fig. 9 shows the current efficiency and enhancement ra-tio of the normally directed quantum efficiency for twoconventional OLEDs and two types of moderate micro-cavity OLEDs. For the conventional OLEDs, the currentefficiencies and relative quantum efficiencies at 10 mA/cm2 in the normal direction were 2.8 cd/A and 1.00 (con-ventional reference) and 2.9 cd/A and 1.04 (referencetype-II). By comparison, the current efficiencies andquantum efficiencies of the type-A and type-B moderatemicrocavity OLEDs were 3.2 cd/A, 1.57 and 3.4 cd/A, 1.60,respectively. This indicates that the current efficiencies ofthe moderate microcavity OLEDs in the normal directionare higher than those of the corresponding conventionalOLEDs. Moreover, the relative quantum efficiencies of thetype-A and type-B moderate microcavity OLEDs in thenormal direction were 57% and 60% higher than thoseof the corresponding conventional reference OLEDs. Thisalso suggests that the type-B moderate microcavity OLEDwith color purity close to the NTSC blue standard hadhigher efficiency than both conventional reference OLEDsand reference type-II with an ITO layer of identicalthickness.

quantum efficiency of the conventional reference OLED (ITO = 150 nm),rocavity OLED under current density of 10 mA/cm2.

H.K. Kim et al. / Organic Electronics 11 (2010) 137–145 145

4. Conclusion

This study examined the effects of inserting dielectricBragg mirrors in an OLED with the aim of achieving NTSCpure blue and enhancing its relative quantum efficiency.First the FDTD method was used to calculate the resonancewavelength of the cavity while varying the thickness of theorganic and high/low dielectric layers. The introduction ofa single pair of TiO2/SiO2 high/low stacks into the OLEDstructure (denoted as a moderate microcavity OLED) is aneffective way of enhancing the quantum efficiency andachieving a NTSC pure blue color from a TAT-based blueOLED. It was demonstrated experimentally that the incor-poration of two types of moderate microcavity OLEDs witha single pair of high/low stacks achieved pure blue CIE col-or coordinates (type-A; x = 0.143, y = 0.068, type-B;x = 0.139, y = 0.081). The type-A and type-B moderatemicrocavity OLED showed more than 57% and 60%improvement in relative quantum efficiency in the normaldirection, respectively, compared to the conventional ref-erence OLED (ITO = 150 nm) in the normal direction. Theyalso improved the relative quantum efficiency by morethan 51% and 54%, respectively, compared to the referencetype-II (ITO = 85 nm). In addition, the moderate microcav-ity OLED is more favorable for the angular dependence ofthe color purity and EL intensity. This simple techniqueof introducing a dielectric Bragg mirror with a single pairof high/low dielectric stacks into a TAT-doped blue OLEDcan be generalized for the development of efficient andsaturated blue OLEDs from the standpoint of realizing adeep blue color in computer and television screens, andfor full color display applications.

Acknowledgments

This study was supported by grant number 2008-03573of the Nano R&D Program and grant number R11-2005-048-00000-0 of the ERC program through the Korea Sci-ence and Engineering Foundation, funded by the Ministryof Education, Science and Technology. This study waspartly supported by the IT R&D program of Ministry ofKnowledge Economy/Institute for Information TechnologyAdvancement (2009-F-020-01). This work was also sup-ported by the faculty research program 2009 of KookminUniversity of Korea. H.K. Kim and S.-H. Cho contributedequally to this work.

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