Organic Electronics 11 (2010) 247–254

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Organic Electronics

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

Enhancing color gamut of white OLED displays byusing microcavity green pixels

Chih-Hao Chang, Hsing-Chieh Cheng, Yin-Jui Lu, Kun-Cheng Tien, Hao-Wu Lin,Chun-Liang Lin, Chih-Jen Yang, Chung-Chih Wu *

Department of Electrical Engineering, Graduate Institute of Photonics and Optoelectronics, and Graduate Institute of Electronics Engineering,National Taiwan University, Taipei 10617, Taiwan, ROC

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

Article history:Received 31 August 2009Received in revised form 25 October 2009Accepted 1 November 2009Available online 6 November 2009

Keywords:White OLEDsMicrocavityColor gamut

1566-1199/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.orgel.2009.11.002

* Corresponding author. Tel.: +886 2 33663636; faE-mail address: chungwu@cc.ee.ntu.edu.tw (C.-C

The combination of white organic light-emitting devices (OLED) and color filters is one ofthe most promising technologies for full-color OLED displays. Yet, the transmission spectraof most available color filters are broad and poorly match the electroluminescence spectraof white OLEDs, yielding less saturated filtered colors and small color gamut for displays. Inthis work, we report that by employing the microcavity structure only in green pixels forsuch OLED display technology, one can simultaneously achieve simplification the manufac-turing process, enhancement of color performances, and enhancement in efficiencies forOLED displays.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

White organic light-emitting devices (WOLEDs) haveattracted wide attention due to their potential applicationsin displays and solid-state lighting (SSL) [1,2]. For full-colordisplay applications, the combination of WOLEDs and colorfilters is one of the most promising technologies, since itavoids the precise shadow-masking often used in the sep-arate red–green–blue (RGB) pixilation, and thus is benefi-cial to realizations of higher display resolutions andlarge-area scale-up. However, for this technique challengesexist in generation of white electroluminescence (EL) withwide visible spectra and their perfect match with thetransmittance of color filters (CFs) [3–5]. Typically, broadEL spectra of white emission can be achieved either bymixing two complementary colors or three primary colors.Yet, the transmission spectra of most available CFs arebroad and poorly match the EL spectra of WOLEDs. Hence,the filtered EL usually exhibit poorly saturated colors andyield small color gamut than the NTSC standard.

. All rights reserved.

x: +886 2 23689178.. Wu).

In 2004, Sony reported that the white emitter with pix-ilated microcavity structures combined with CFs canachieve high brightness and good color saturation in full-color active-matrix OLED displays [6]. In general, micro-cavity OLEDs consist of a highly reflective back mirrorelectrode and a semi-transparent output mirror electrode.When appropriately tailored, microcavity OLEDs can pro-duce narrowed/more saturated EL and brightnessenhancement [7–9], useful for constructing energy-effi-cient displays with color saturation comparable to theNTSC standard. In Sony’s configuration, WOLEDs wereembedded in the microcavity structure and individualRGB emissions from RGB sub-pixels were created byadjusting the microcavity resonant wavelength of eachsub-pixel. It was accomplished through depositing andpatterning the indium–tin-oxide (ITO) layers of differentthicknesses for the respective RGB sub-pixels. The sche-matic configuration is shown in Fig. 1a. Depositing andpatterning ITO into different thicknesses, however, compli-cates the fabrication and thus is not convenient for massproduction [10].

In the WOLED + CF display technology, the most signif-icant issue resulting from residual emission with theimperfect filtration is unsaturated green emission. With

Fig. 1. (a) The schematic diagram of the WOLED + CF display with pixilated microcavity developed by Sony in 2004 and (b) the schematic diagram of theWOLED + CF display by using microcavity green pixels. The buffer layer in (b) could be an index-matching adhesive.

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the band-pass nature of green color filters, modifyingoptical properties of CF or OLED materials would give onlylimited improvement and shall not solve the problem com-pletely. Nevertheless, by employing the microcavity struc-ture only in green pixels may compensate this weaknessand also substantially simplify the manufacturing process(versus Sony’s configuration). For instance, one possibleWOLED + CF configuration with microcavity green pixelsis schematically shown in Fig. 1b. In such an approach, athin metal layer of one thickness is deposited and pat-terned only on the ITO anode of green pixels, and thenthe fabrication of WOLEDs of a same structure for all RGBpixels can be carried out simultaneously.

2. Experimental

2.1. OLED fabrication

All compounds used were subject to temperature-gradient sublimation under high vacuum before use.OLEDs were fabricated on the ITO-coated glass sub-strates with multiple organic/metal layers sandwichedbetween the transparent bottom indium–tin-oxide(ITO) anode and the top metal cathode. The organicand metal layers were deposited by vacuum evaporationin a vacuum chamber with a base pressure of<10�6 Torr. The deposition system permits the fabrica-tion of the complete device structure in a single vacuumpump-down without breaking vacuum. The depositionrate of organic layers was kept at �0.2 nm/s. The activearea of the device is 2 � 2 mm2, as defined by the sha-dow mask for cathode deposition.

2.2. Device characterizations

Current–voltage–luminance (I–V–L) characterization ofthe devices was measured using an Agilent 4155B semi-conductor parameter analyzer and a Si photodiode cali-brated with Photo Research PR650. EL spectra of deviceswere collected by a calibrated CCD spectrograph. To mea-sure the EL spectra of WOLED + CF, color filters were at-tached to the devices by employing index-matching oil.Transmission spectra of CFs were measured using aspectrophotometer.

3. Results and discussion

Two types of WOLEDs were fabricated and tested. InType-I devices (Devices A and B, Fig. 2a), the emissive do-pants of two complementary colors (red and blue) wereused to form a two-component white EL. In Type-II devices(Devices C and D, Fig. 2b), the emissive dopants of threeprimary colors (red, green, and blue) were used to form athree-component white EL. Both Type-I and II devices usedtwo different anodes, i.e. ITO for non-cavity Devices A/Cand ITO/thin Ag (30 nm) for microcavity Devices B/D,respectively. In Devices B and D, the thin silver was chosenas semi-transparent reflective anode for the microcavitystructure because of its low absorption/high reflection inthe visible range and the lowest resistance among elec-trode materials [11,12]. By controlling the thickness of an-ode and organic layers, the resonant wavelength wasadjusted to match the transmittance of the green color fil-ters. The transmission spectra of RGB color filters used inthis study (micrometer-thick color filters coated on theglass substrate) are shown in Fig. 3.

Fig. 2. (a) The structures of two-component WOLEDs (Type-I devices): non-cavity Device A and microcavity Device B and (b) the structures of three-component WOLEDs (Type-II devices): non-cavity Device C and microcavity Device D.

Fig. 3. Transmission spectra of color filters used in this study (microm-eter-thick RGB color filters coated on the glass substrate).

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3.1. Color performances of two-component whiteOLEDs + CFs

In Type-I devices, the organic layers and the cathodewere: m-MTDATA: F4-TCNQ 2.0 wt% (23 nm)/a-NPD

(15 nm)/ADN (5 nm)/ADN: Perylene 1.0 wt% (10 nm)/Alq3: DCJTB 0.3 wt% (10 nm)/Alq3 (35 nm)/Cs2CO3 (1 nm)/Ag (120 nm). In these devices, 4,40,40 0-Tris(N-3-methyl-phenyl-N-phenyl-amino) triphenylamine (m-MTDATA)doped with 2 wt% of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) was used as the p-doped hole-injection layer (HIL) [13–15]. The p-type doping (F4-TCNQ)facilitates carrier (hole) injection between anode (ITO orsilver) and organic materials. N,N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl)-benzidine (a-NPD) and Tris(8-hydroxy-quinolinato) aluminium (Alq3) were used as the hole-transport layer (HTL) and electron-transport layer (ETL),respectively. A 5 nm-thickness 9,10-Di(naphth-2-yl)-anthracene (ADN) layer was used as a buffer layer betweenHTL and emitting layers [16,17]. The blue emitting layerwas composed of 9,10-Di(naphth-2-yl)anthracene (ADN)doped with 1.0 wt% Perylene; the red emitting layer wascomposed of Alq3 doped with 0.3 wt% 4-(Dicyanomethyl-ene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vi-nyl)-4H-pyran (DCJTB) [18–20]. Cs2CO3 and Ag were usedas the electron-injection layer (EIL) and cathode, respec-tively [21,22]. Two types of anodes, ITO for non-cavity De-vice A and ITO/thin Ag (30 nm) for microcavity Device B,were used, respectively. The layer thickness and the dopingconcentration were adjusted to achieve white EL for non-

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cavity WOLEDs and the wavelength-selected green emis-sion for the microcavity structure (i.e. match the resonantwavelength with the green CF) simultaneously. All thematerials used in devices are shown in Fig. 4.

The EL spectra of Type-I devices at 1000 cd/m2 areshown in Fig. 5a. With carefully adjusting the relative ratioof the two complementary colors in Type-I devices, the ELspectrum of Device A exhibited a white emission with CIEcoordinates of (0.34, 0.30), which is close to the equal-en-ergy white at CIEx,y = (0.33, 0.33). One notices that there isabsence of the green part in EL of Type-I Devices A. Inmicrocavity Device B, to ensure sufficient light emissionthrough the thin Ag anode, the thickness of silver was setat 30 nm to avoid serious power loss. As such, the micro-cavity effect is not strong enough to remove completelyresidual emission in the blue and red ranges, but they arealready largely suppressed. The resonant wavelength ofDevice B was set at 531 nm and the device showed greenemission with CIEx,y = (0.23, 0.65).

Fig. 4. Chemical structures of m

To measure the EL spectra of WOLED + CF, color filterplates were attached to the devices by employing index-matching oil. Fig. 5b and c show the filtered normalizedEL spectra and the color gamut of Type-I Devices (A andB) when combined with CF. In Device A, the filtered ELspectra exhibited rather saturated blue and red colors atCIEx,y = (0.13, 0.11) and CIEx,y = (0.67, 0.32), respectively.On the contrary, the weak intensity in the green regionbroadened the filtered green emission and produced anunsaturated green color at CIEx,y = (0.28, 0.61). In DeviceB, the residual blue and red emission was further sup-pressed by the green CF and the filtered green EL was fur-ther narrowed. As shown in Fig. 5c, with the microcavitystructure, the filtered green emission (along the normaldirection) shifted from CIEx,y = (0.28, 0.61) (Device A) toCIEx,y = (0.25, 0.70) (Device B). By adopting the microcavitygreen pixel, the color gamut of the WOLED + CF displaycould be enlarged from 74.6% NTSC (Device A) to 91.2%NTSC (using the microcavity Device B for the green pixel)

aterials used in devices.

Fig. 5. (a) The normalized EL spectra of Type-I devices in the normaldirection, (b) the normalized EL spectra of Type-I devices coupled with CF,and (c) color gamut of Type-I devices coupled with CF at different viewingangles.

Fig. 6. (a) The normalized EL spectra of Type-II devices in the normaldirection, (b) the normalized EL spectra of Type-II devices coupled withCF, and (c) color gamut of Type-II devices coupled with CF at differentviewing angles.

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in the normal direction. Fig. 5c also shows the CIE coordi-nates of EL of microcavity Device B at different viewing an-gles, which shows slight blue shift with viewing angles. Assuch, the color gamut of the WOLED + CF display employ-ing the microcavity green pixel would vary from 91.2%NTSC at 0�, to 96.6% NTSC at 30�, and to 84.6% NTSC at

60�. Therefore, enlarged color gamuts could be obtainedover wide viewing angles. In general, the microcavity effectinduces slight blue shift of the EL spectrum/peak (color) ofthe microcavity green pixel with viewing angles [9]. Nearthe viewing angle of 30�, the EL happens to reaches the

Fig. 7. (a) Current–voltage–luminescence (I–V–L) characteristics of Type-I devices, (b) current–voltage–luminescence (I–V–L) characteristics of Type-IIdevices, and (c) luminance efficiency vs. luminance of Type-I and II devices.

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most saturated green, thus giving the largest color gamut.We also checked the viewing-angle characteristics ofmicrocavity Device B without CF. Without green CF, thecolor gamut of the WOLED + CF display would have a sig-nificant variation from 89.54% NTSC at 0� to 29.14% NTSCat 60�. Therefore, the results indicate that the CF not onlypurifies emission colors but also reduces the color shiftwith viewing angles in the current technology.

3.2. Color performances of three-component whiteOLEDs + CFs

Triple emission layers were used in Type-II devices. Theorganic layers and cathode in Type-II devices were: m-MTDATA: F4-TCNQ (23 nm)/a-NPD (15 nm)/ADN (5 nm)/ADN: Perylene 1.0 wt% (7.5 nm)/Alq3: C545T 1.0 wt%(5 nm)/Alq3: DCJTB 0.3 wt% (7.5 nm)/Alq3 (35 nm)/Cs2CO3

Table 1Luminance efficiency of WOLED + CF.

Device Luminance efficiency (cd/A) (at 103 cd/m2)

Original +CF_Blue +CF_Green +CF_Red

Type-IA 5.49 0.56 1.57 1.33B 10.23 — 5.10 —

Type-IIC 7.72 0.70 2.89 1.28D 14.96 — 8.08 —

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(1 nm)/Ag (120 nm). In Type-II devices, Alq3 doped with2,3,6,7-Tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10- (2-benzothiazolyl) quinolizino-[9,9a,1gh] coumarin (C545T)was used as the green emitting layer (Fig. 4) [23,24]. ITOwas used as the anode for the non-cavity device C andITO/thin Ag (30 nm) was used as the composite anode forthe microcavity Device D.

Fig. 6a shows the EL spectra of Type-II devices at1000 cd/m2. With the addition of the green emitting layerand the emphasis of green emission, the EL spectrum ofType-II Device C exhibited a greenish white color withCIEx,y = (0.34, 0.40). The larger EL intensity in green wave-lengths of Type-II devices enhances the microcavity effectin the microcavity Type-II Device D, giving a much sharpergreen EL. Comparing the EL spectra of microcavity devicesof both types (Devices B and D), one notices that the resid-ual blue and red emissions were much more suppressed inDevice D. The resonant wavelength in Device D was set at526 nm and the device showed more saturated green EL atCIEx,y = (0.22, 0.67).

Fig. 6b and c shows the normalized filtered EL spectraand the color gamut of Type-II Devices (C and D) whencombined with CF. Compared to the two-componentsWOLED Device A, the additional green emitter in DeviceC does not improve color gamut of the WOLED + CF(71.4% NTSC for Device C vs. 74.6% NTSC for Device A; par-tially due to the degradation of the filtered blue color inaddition to the broad filtered green color). In microcavityDevice D + CF, the filtered green emission produced evenmore saturated green color at CIEx,y = (0.21, 0.74), and thecolor gamut of the WOLED + CF display could be enlargedfrom 71.4% NTSC to 92.7% NTSC (using microcavity DeviceD for the green pixel) in the normal direction. As in the pre-vious case, the microcavity structure combined with CFcould mitigate the color shift with viewing angles. The col-or gamut of the WOLED + CF display employing the micro-cavity green pixel vary from 92.7% NTSC at 0�, to 95.7%NTSC at 30�, and to 83.0% NTSC at 60�, still showing en-larged color gamuts over wide viewing angles. In general,the microcavity effect induces slight blue shift of the ELspectrum/peak (color) of the microcavity green pixel withviewing angles [9]. Near the viewing angle of 30�, the ELhappens to reaches the most saturated green, thus givingthe largest color gamut.

3.3. Current–voltage–luminescence (I–V–L) characteristics ofall devices

Current–voltage–luminescence (I–V–L) characteristicsof Type-I and Type-II devices are shown in Fig. 7a and b,respectively. For both types of devices, both the non-cavityand microcavity devices show similar I–V characteristics,indicating the effectiveness of the p-type conductive dop-ing (in the hole-injection layer) for facilitating hole injec-tion from the Ag anode. The luminance efficiency vs.luminescence for all Type-I and Type-II devices are shownin Fig. 7c. The non-cavity Type-II device (Device C) showshigher luminance efficiency than the non-cavity Type-I de-vice (Device A), partly because the green dopant was usedin Type-II devices. For the same type of devices, the micro-cavity device also shows higher luminance efficiency than

the non-cavity device (Device B vs. Device A and Device Dvs. Device C), partly due to eyes being more sensitive to thegreen color and partly due to microcavity effects on bright-ness enhancement.

The luminance efficiencies of all the devices with/with-out color filters are listed in Table 1. For Type-I devices, theefficiencies of filtered green EL (at 1000 cd/m2) for DevicesA and B are 1.57 and 5.1 cd/A, respectively. For Type-II de-vices, the efficiencies of filtered green EL (at 1000 cd/m2)for Devices C and D are 2.89 and 8.08 cd/A, respectively.It is interesting to note that owing to the better match be-tween the EL spectra of the microcavity OLEDs (B and D)and the transmission of the green color filter, the efficiencyloss caused by the color filter (for green pixels) is substan-tially lower, compared to the case of the conventionalWOLED. Hence, by employing the microcavity structuremerely in green pixels of the WOLED + CF technology,one can simultaneously achieve simplification the manu-facturing process, enhancement of color performances,and enhancement in efficiencies for OLED displays.

4. Conclusions

The combination of WOLEDs and color filters is one ofthe most promising technologies for full-color OLED dis-plays. Yet, the transmission spectra of most available CFsare broad and poorly match the EL spectra of WOLEDs,yielding less saturated filtered colors and small color ga-mut. In this work, we report that by employing the micro-cavity structure only in green pixels for the WOLED + CFdisplay technology, one can simultaneously achieve sim-plification the manufacturing process, enhancement of col-or performances, and enhancement in efficiencies for OLEDdisplays.

Acknowledgements

The authors gratefully acknowledge the financial sup-port from National Science Council.

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