luminous power efficiency optimization of a white organic

Luminous power efficiency optimization of a whiteorganic light-emitting diode by tuning itsspectrum and its extraction efficiency

Peter Vandersteegen,1,* Gregor Schwartz,2 Peter Bienstman,1 and Roel Baets1

1Department of Information Technology, University of Ghent/IMEC, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium2Institut für Angewendte Photophysik, George-Bähr-Strasse 1, 01069 Dresden, Germany

*Corresponding author: [email protected]

Received 8 October 2007; revised 11 January 2008; accepted 11 January 2008;posted 11 January 2008 (Doc. ID 88384); published 0 MONTH 0000

We show an increase of the luminous power efficiency of a white organic light-emitting diode (LED) withthree emitters by optimizing its spectrum and its extraction efficiency. To calculate this efficiency we usea model with four parameters: the spectra, extraction efficiencies, internal quantum efficiencies of threeemitters, and the driving voltage. This luminous power efficiency increases by 30% by use of a spectrumclose to the spectrum of the MacAdam limit. This limit gives the highest luminous efficacy for a givenchromaticity. We also show that a white organic LED with an inefficient deep blue emitter can givethe same luminous power efficiency as a white organic LED with a more efficient light blue emitter,because of their different fractions in the radiant flux. Tuning the extraction efficiency with a microcavityto the spectrum also increases the luminous power efficiency by 10%. © 2008 Optical Society of America

OCIS codes: 330.1715, 250.3680.

1. Maximizing the Luminous Power Efficiency

Any future light source should have a high efficiencywith sufficiently high color reproducibility. Theseparameters are usually given by the luminous powerefficiency [1] and the color rendering index (CRI). [2]One promising technology for future lighting and dis-play applications is the white organic light-emittingdiode (WOLED), because of its ease of fabricationand (potentially) high efficiency. Most WOLEDsare fabricated by the deposition of a stack of thin or-ganic layers on a glass substrate. The total thicknessof these layers is approximately 100nm, which pro-vides a large area that generates diffuse light.Although the first WOLED by Kido et al. [3] had aluminous power efficiency of only 1:1m=W at most,present day WOLEDs already exceed the luminouspower efficiency of the classical incandescent bulb(15 lm=W) by a factor of 2–3 [4–7]. The WOLEDs

discussed in these papers usually have three emit-ters in the stack organic layers. These papers also in-dicate several routes to increase the overall luminouspower efficiency. On the one hand one can improvethe electrical behavior to gain efficient generationof photons. On the other hand one can increase lightextraction, which should ensure that most of thegenerated photons can escape through the substrateto air. Here, we focus on an optimization of the spec-tral behavior of optical and electrical properties.

We put forward three statements with regard tohow the spectral behavior of electrical and opticalproperties of the emitters in a WOLED can increasethe overall luminous power efficiency. This luminouspower efficiency gives the ratio of the luminous fluxto the electrical power. We also use the luminousefficacy, which gives the ratio of luminous flux to ra-diant flux. Note that both properties are in units oflumens per watt (lm=W). Moreover, these two proper-ties are related by the wall plug efficiency that givesthe efficiency at which electrical power is convertedto radiant flux (W=W).

0003-6935/08/130001-01$15.00/0© 2008 Optical Society of America

1 May 2008 / Vol. 47, No. 13 / APPLIED OPTICS 1

Our methodology to illustrate these three state-ments uses a model of a three-color WOLED withfour parameters. To calculate the luminous efficacyand the luminous power efficiency, these four para-meters are used: the emission spectra of the threeemitters, the internal quantum efficiencies of thethree emitters, a wavelength-dependent extractionefficiency of three emitters, and the driving voltage.We first give a short overview of the three state-ments. We then discuss the relation of these fourparameters to our three statements. Moreover, thisoverview gives typical values for these parameters.The first statement is about optimization of the

spectrum. Different spectra can give the same chro-maticity but with different luminous efficacy, whichis also called metamerism. But one spectrum yieldschromaticity with a greater luminous efficacy thanany of the other spectra [8]. Because of the close re-lationship between luminous efficacy and luminouspower efficiency, we show that a spectrum with high-er luminous efficacy yields an OLED with higher lu-minous power efficiency. For the second statement,we examine the conversion efficiency of electrical en-ergy to photons by these emitters as a function of thespectrum of these emitters. In our example, we use adeep blue emitter or a light blue emitter in a three-color OLED while we maintain the same white chro-maticity. We show that the WOLED with a deep blueemitter requires a smaller fraction of blue than theWOLED with a light blue emitter [9]. Then theWOLED with the deep blue inefficient emitter hasthe same overall luminous power efficiency as aWOLED with a more efficient light blue emitter.Third, we show that the wavelength-dependent be-havior of the extraction efficiency needs to be tunedas a function of the internal quantum efficiencies andthe spectra of the emitters.The first parameter of the model is the spectrum of

each emitter. The spectra of the emitters illustratethe first statement. Although different spectra existfor any given chromaticity (metamerism), only onespectrum exists with the highest luminous efficacy,the MacAdam limit [8]. Thus, the MacAdam limitalso gives the highest luminous power efficiency ofany light source. For example, the MacAdam limitfor illuminant A is 512 lm=W. This MacAdam limitcan be found for all chromaticities; see Fig. 1. Be-cause the spectra of most WOLEDs do not resemblethis spectrum, the luminous efficacy of these spectrais at most 350 lm=W. Changing the spectrum of oneof the emitters in a WOLED can increase the lumi-nous efficacy. A typical spectral width of an emitter isapproximately 100nm. Nevertheless, some red emit-ters based on europium can have a wavelength band-width equal to one-fourth of this width [10].The second parameter is the internal quantum ef-

ficiency of each emitter. This internal quantum effi-ciency gives the fraction of electron– hole pairs thatradiatively decay. This parameter, together with thefirst parameter, is used to illustrate the second state-ment, which states that a WOLED with a less effi-

cient deep blue emitter can have the sameluminous power efficiency as a WOLED with a moreefficient blue emitter. Note that the focus of our ex-ample is the internal quantum efficiency of the blueemitter. Why? The most efficient emitters are phos-phorescent emitters. Theoretically, these emitterscan reach an internal quantum efficiency (IQE) ofηIQE ¼ 100% [11,12]. Although a light blue phosphor-escent emitter, FIrpic, with high IQE has beendemonstrated [13], the stability of deep blue phos-phorescent emitters is still a limiting factor of thelifetime of complete phosphorescent white OLEDs[14,15]. Note that green and red emitters are alsophosphorescent emitters, but their lifetime is notthe limiting factor with regard to the lifetime ofthe device. Thus, the reason to focus on the blue emit-ter is the lack of stability of deep blue efficient emit-ters, especially when compared with the stability ofefficient green and red emitters. However, WOLEDswith a long lifetime have been shown, but these use aless efficient deep blue fluorescent emitter [16,17].Theoretically, the upper limit of the internal quan-tum efficiency of a fluorescent emitter is ηIQE ¼ 25%.Alternatively, some stacks with a blue fluorescentemitter and green and phosphorescent emitters getaround the low internal quantum efficiency of theblue emitter [18]. A fraction of the triplet excitons,some of the electron–hole pairs, which normallywould decay nonradiatively as a result of the fluores-cent blue emitter, can transfer to the phosphorescentemitters. These excitons can then decay radiativelyon these phosphorescent emitters.

The third parameter of the model is the wave-length dependence of the extraction efficiency of aWOLED. This parameter, together with the firsttwo parameters will be used to illustrate the thirdstatement. The wavelength-dependent behavior ofthe extraction efficiency can be tuned as a functionof the IQEs and the spectra of the emitters. Light ex-traction has a straightforward effect on the luminouspower efficiency of a WOLED; increasing the radiantflux by 50% for all wavelengths increases the lumi-nous power efficiency by 50%. However, most techni-ques will show a wavelength-dependent extractionefficiency. Several techniques use a corrugation ofthe planar layers to increase light extraction. Thesetechniques are a grating between glass and air [19]and microlenses or a diffusive layer between sub-strate and air [20–22]. Adding interlayers betweenglass and air also increases light extraction [23–25]. In [24,25] the authors used microcavity effectsthat greatly influenced the wavelength-dependentbehavior of extraction efficiency. Thus, extraction ef-ficiency at one wavelength can be much higher thanthe extraction efficiency at other wavelengths. Be-cause a silver cathode has a higher reflectance thanan aluminum cathode for red light, the former givesgreater microcavity effects and higher extraction ef-ficiency than the latter [26]. Nevertheless, each tech-nique to increase extraction efficiency was examinedfor only one wavelength or a small wavelength

2 APPLIED OPTICS / Vol. 47, No. 13 / 1 May 2008

region, but it is still unclear how they would changethe luminous power efficiency of a WOLED. To showthe luminous power efficiency for different wave-length-dependent extraction efficiencies, we comparetwo WOLEDs, one with and one without strong mi-crocavity effects. Although this example is limited toone method of improving the extraction efficiency,the example clearly shows the influence of the wave-length dependence of the extraction efficiency.The last parameter of the three-color WOLED

model is the driving voltage. Although this para-meter has no direct effect on the discussion, a lowdriving voltage directly increases the luminouspower efficiency. Lowering the driving voltage canbe done by doping the transport layers in the organicstack [17,27]. The lowest voltage is limited by thethermodynamic limit [28].In Section 2 we discuss the model of the three-color

WOLED. In Sections 3 s4 s5 we discuss the threestatements. Our conclusions are given in Section 6.

2. Three-Color White Organic Light-EmittingDiode Model

Our goal is to optimize the luminous power efficiencyof a three-color WOLED by changing its optical prop-erties: the spectra and the external quantum efficien-cies of the three emitters. This luminous powerefficiency (ηP; lm=W) is defined as the ratio of lumi-nous flux (F) to electrical power (Pel). To calculatethe luminous power efficiency, we use a generic mod-el of a three-color WOLED that requires four para-meters. These four parameters are the internalquantum efficiency that gives the conversion of exci-tons in photons (ηint;i), the extraction efficiency ofthese photons (ηc;i), and the power normalized radi-ant flux of the emitters [Eel;iðλÞ]. One last relevantparameter is the driving voltage of the OLED. Indexi of each of the three emitters is either b(lue), r(ed), org(reen). This model will be used to show three waysto improve the luminous power efficiency in Sec-tions 3 s4 s5. A small adaptation of the model thatis needed for Section 4 will be discussed at the endof this section.The luminous power efficiency is calculated with

the luminous flux (Fi) and electrical power (Pel;i) ofeach of the three emitters:

ηP ¼P

i¼b;g;r FiPi¼b;g;r Pel;i

: ð1Þ

For a given chromaticity of the WOLED, the contri-bution of each emitter is determined automatically.Thus, the first step is to determine the spectrum thatcorresponds with the given chromaticity. Also, thetotal spectrum then gives the luminous flux. Theemitted spectrum in air of each emitter is given by

Eop;iðλÞ ¼ Eel;iðλÞηc;iðλÞ: ð2Þ

Each spectrum corresponds to tristimulus values inthe CIE color space of 1931 [29]:

Xi ¼Z

Eop;iðλÞ�xðλÞdλ; Yi ¼Z

Eop;iðλÞ�yðλÞdλ;

Zi ¼Z

Eop;iðλÞ�zðλÞdλ: ð3Þ

Given the chromaticity of the WOLED and its colorcoordinate, (Xw;Yw;Zw), and the three emitters andtheir color coordinates, the following conditions needto be satisfied:

Xw ¼X

i¼b;g;r

AiXi; Yw ¼X

i¼b;g;r

AiYi;

Zw ¼X

i¼b;g;r

AiZi: ð4Þ

The prefactors (Ab;Ag;Ar) determine the mutualratio of radiant flux inside the organic layers. We as-sume that we can change prefractors Ai withoutchanging any other property or parameter of theOLED. The luminous flux of one emitter is then givenby

Fi ¼ 683:0Z

AiEop;iðλÞVðλÞdλ: ð5Þ

Calculating the denominator of Eq. (1) is the nextstep. Equation (1) is a function of the IQE and thepower injected into the organic layers:

Pel;i ¼Z

AiEel;iðλÞ1

ηint;iqVλhc

dλ: ð6Þ

The parameter 1=ηint gives the amount of excitonsneeded to create one photon. The ratio hc=λqV givesthe relation between the optical power of the photonand the energy of the creating exciton [30]. As statedin Section 1, one can use doped layers to minimizePel;i by minimizing the required voltage.

A small variation of this model is the creation ofwhite light with three distinct monochrome OLEDs,which differs from the previous approach, which hasan OLEDwith the three emitters in the same organiclayer stack. Generating white light with three dis-tinct monochrome OLEDs has the advantage thateach emitter can be optimized independently. Thisprinciple is also known as a horizontally stackedWOLED [7]. The only parameters used to calculatethe luminous power efficiency are the spectra andthe wall plug efficiency (ηW=W;i) of each of the threeemitters. This wall plug efficiency of an emittercan be calculated with its spectrum and its overallluminous efficacy (ηP;i).

The overall wall plug efficiency of the completeWOLED (ηW;W) can then be calculated with

ηW=W ¼P

i¼b;g;r AiPi¼b;g;r

AiηW=W;i

: ð7Þ


The fraction of radiant flux of each emitter (Ai) is di-rectly given by Eq. (4). The luminous power efficiencyof the white OLED is finally given by

ηP ¼ 683REopðλÞVðλÞdλREopdλ

ηW=W : ð8Þ

The optical spectrum of the WOLED Eop is deter-mined by the spectrum of each emitter (Eop;i) andits relative fraction (Ai).

3. Spectrum to Increase the LuminousPower Efficiency

The luminous power efficiency can be increased withspectrally narrow emitters because of the MacAdamlimit. This limit gives the highest luminous efficacyfor a given chromaticity. To determine the luminousefficacy, one must take the ratio of luminous fluxto radiant flux (lm=W). We will show that thisMacAdam limit can be used to increase the luminouspower efficiency. For illuminant A, this limit gives aluminous efficacy of 512 lm=W. The correspondingspectrum has 15% of its radiant flux emitted at450nm and 85% at 579:5nm, see Fig. 1. On the otherhand, a spectrum of a typical WOLED with thischromaticity is shown in Fig. 2(a). This spectrumhas a ratio of luminous flux to radiant flux of only305 lm=W. If the red emitter of this last spectrumis replaced by a monochromatic emitter that emitsat 585nm [ see Fig. 2(b)], the luminous efficacy ofthis spectrum gives 454 lm=W. Also, Table 1 liststhe luminous power efficiency that is calculated withthe model discussed in Section 2.In addition to the luminous power efficiency, the

color quality is also important. Therefore, in the re-mainder of this section we discuss color quality as afunction of luminous power efficiency. To measurethe quality of a light source and to measure how goodcolors are reproduced when illuminated with thissource, the CRI is often used. Recently, visual experi-ence has shown that the current CRI based rankingof a set of light sources containing white LED lightsources contradicts the visual ranking. [31]. Spectralnarrow LEDs can have good light quality but a lowCRI. Nevertheless, we will use the CRI to determinethe color quality even for a light source with a spec-trum with narrow peaks. Because the CRI of theWOLED with the monochromatic yellow emitter iswell below 80, the rest of this section looks at thetrade-off between the CRI and the increase of overall

luminous efficacy of emitters with narrower spectra.It should be mentioned that a CRI of 80 is greaterthan that of the majority of fluorescent lamps.

The goal of the next discussion is to find a spec-trum that still has a higher luminous power effi-ciency than the default spectrum but with asufficiently high CRI. To achieve this, we vary thespectrum of one of the emitters. Two possible meth-ods to change the spectrum of an emitter are shiftingits peak position or narrowing the spectrum. The

Fig. 1. (Color online) Maximal luminous efficacy of a color pointin CIE 1931 xy color space; for example, illuminant A can achieve512 lm=W[8]. 4/CO

Fig. 2. (Color online) Spectra of three emitters that yields illumi-nant A. (a) Spiro-DPVBi, Ir(ppy)3 in TCTA, and Ir(MDQ)(acac) inalpha-NPD. (b) The red emitter was replaced by a monochromaticemitter at 585nm. 4/CO

Table 1. Luminous Properties of the Spectra in Fig. 2(a)a

Luminous Properties Fig. 2(a) ( lm=W) Figure 2(b) ( lm=W)F

Popt305 454

ηP for ðηint;b; ηint;g; ηint;rÞ ¼ ð1:0; 1:0; 1:0Þ 39 66ηP for ðηint;b; ηint;g; ηint;rÞ ¼ ð0:25;1:0;1:0Þ 30 42

aThe driving voltage is 3:075V and the extraction efficiency is 20% for all wavelengths and all the emitters.


shift of a spectrum is shown in Fig. 3(a) and expres-sion (9); the narrowing of a spectrum is shown byFig. 3(b) and expression (10). The spectrum of anemitter is given by ϕðλÞ, a shift by λ0. A narrow factorof 0.5 means that the spectrum is half as wide inwavelength as the default spectrum:

ϕiðλÞ → ϕðλ − λ0Þ; ð9Þ

ϕiðλÞ → ϕ�λmax þ

λ − λmax

narrow factor

�: ð10Þ

Let us now define the boundary conditions that weused in Eqs. (1)–(8) to calculate the luminous powerefficiency (ηP) and the CRI for a basic OLED model.The extraction efficiency is 20% and the driving vol-tage is 3:075V, which is close to the thermodynamiclimit to emit deep blue. Moreover, the green andred emitters have an internal quantum efficiencyof ηint;g ¼ ηint;r ¼ 100%. Because no phosphorescentdeep blue emitters with a long lifetime are known,we again make a distinction between a WOLEDwith a blue fluorescent emitter (ηint;b ¼ 25%) and ablue phosphorescent emitter (ηint;b ¼ 100%). Table 1shows that replacement of the red emitter by amono-chromatic red emitter indeed improves the luminouspower efficiency by 30%.With the previous values, Fig. 4 shows the differ-

ent luminous power efficiencies of the WOLED forthe illuminant A chromaticity (CIE xy 1931: 0.4475and 0.4074) for variations of the blue and red emit-ters. Because the properties of the WOLED are inde-pendent of most variations of the green emitter,variations of the green emitter are not shown. More-over, variations of the green spectrum are significantonly when the shift is large enough to overlap withthe spectrum of either a blue or a red emitter.Varying the red emitter has the largest influence

on the luminous efficacy and the luminous powerefficiency because of the chosen warm chromaticity.We compared two WOLEDs with a CRI above 80:

a red emitter with its peak at 600nm and with anarrow factor of 0.5 and a default red emitter. Thenarrower red emitter in the WOLED still gives asatisfying CRI of 80 but increases the luminouspower efficiency (ηP ¼ 47 lm=W) for a WOLED witha blue phosphorescent emitter. The default red emit-ter in a WOLED gives a CRI of 90 and an overall lu-minous power efficiency of 39 lm=W (phosphorescentblue); see Table 1. Although this effect is slightlysmaller in a WOLED with a blue fluorescent emitter,we again see a relative improvement.

For a WOLED with a blue fluorescent emitter, alsovariations of the blue fluorescent emitter show an in-crease of the overall luminous efficacy. We see that avariation of the blue phosphorescent emitter of theWOLED with a blue phosphorescent emitter doesnot influence the overall luminous efficacy. The in-crease of the overall luminous efficacy is caused bya decrease in the amount of light emitted by the blueemitter, which is the least efficient emitter. Thiseffect is discussed in more detail in Section 4.

In conclusion, the MacAdam limit plays an impor-tant role in this hypothetical WOLED. The maximalluminous power efficiency has been found by repla-cing the red emitter with a monochrome red emitter.Although the WOLED with the monochrome redemitter has a low CRI, the CRI can be improvedby using slightly broader emitters, while maintain-ing an increase in the overall luminous efficacy.

Fig. 3. (Color online) (a) Shifting and (b) stretching of the spectraof the blue emitterwere obtained, respectively, with expressions (9)and (10).4/CO

Fig. 4. (Color online) Variation of the spectral intensity of eachof three emitters, as defined in Fig. 3. These variations affectthe luminous efficacy, the CRI, and the luminous power efficiency. 4/CO


4. Fraction Radiant Flux of the Emitter

The second statement relates to the conversion effi-ciency of electron–hole pairs to photons by use ofthese emitters, which is the IQE. We can use twoemitters with roughly the same color to create aWOLED of a given chromaticity. For example, wecan use a deep blue emitter or a light blue emitterin a three-color OLED to create the same white chro-maticity. But a deep blue emitter requires a smallerfraction of the total light than does a light blue emit-ter [9]. Therefore, the luminous power efficiency ofWOLEDs with either emitter can be equal.Here we show a comparison of a less efficient deep

blue fluorescent emitter versus a more efficient lightblue phosphorescent blue emitter. For this compari-son a hypothetical WOLED is created by combiningthe three distinct monochrome OLEDs in Fig. 5.These OLEDs are described in the litarature; seeFig. 5. To explain the second statement, we thereforemake use of the wall-plug efficiencies of monochromeOLEDs instead of their IQEs. Nevertheless, the con-clusions based on the wall plug efficiency can be ex-tended to the IQE. Figure 5 and Table 2 give theproperties of these emitters.Equations (7) and (8) give, respectively, the wall

plug efficiency and the luminous power efficiencyof a hypothetical three-color WOLED. The OLEDsin Fig. 5 have a driving voltages larger than 6V.Thus lowering the driving voltage in some of theseOLEDs could increase the wall plug efficiency by afactor of 2.Table 3 lists the results for the WOLEDs with

either bI, bII or bIII. Though the wall plug efficiencyof the phosphorescent bIII is almost twice that offluorescent bII, the luminous power efficiencies ofthe WOLEDs are equal. This is caused mainly bythe lower fraction of the required blue when the blueis more deep. It should be noted that a WOLED witha more efficient blue phosphorescent, such as re-cently presented in [13], would outperform both

the WOLEDs described in this section. Nevertheless,a factor of almost 1.66 in wall plug efficiency, 3% ver-sus 5%, is compensated because of the lower fractionof the blue emitter in the radiant flux.

5. Tuning the Extraction Efficiency

The third statement relates to the wavelength de-pendence of the extraction efficiency and its rela-tion to the electrical properties of a WOLED.Although the different strategies mentioned inSection 1 increase the extraction efficiency, most ofthe cited papers discuss this increase only for onewavelength or a small wavelength range. Our focushere is to match a strong wavelength-dependentextraction efficiency with other properties of aWOLED. To achieve a strong wavelength-dependentextraction efficiency, we use microcavity effects in

Fig. 5. (Color online) Monochrome emitters found in the litera-ture for the CIE xy 1931 chromaticity diagram. The luminouspower efficiencies of these emitters are also shown. 4/CO

Table 2. Properties of the Emitters shown in Fig. 5

bI bII bIII g r

Undoped MADN Doped MADN with BD1 PhOLEDColor Coordinates in the CIE 1931 Color Space (0.15,0.66) (0.14,0.13) (0.17,0.35) (0.25,0.66) (0.62,0.38)Luminous Power Efficiency ηP;i½lmW � 1.2 3.9 14 63.5 15.9Wall Plug Efficiency ηW=W;i 0.01 0.03 0.05 0.13 0.06

Table 3. Luminous Power Efficiency of the Composed WOLEDs`

bI Very Deep Blue bII Deep Blue bIII Light Blue

Wall plug efficiency ηW=W of the WOLED 0.050 0.060 0.066Luminous Power Efficiency (ηP) of the WOLED 22:3 lm=W 26:6 lm=W 26:7 lm=WRelative Fraction of the Emitters to Create Illuminant A (Ab;Ag;Ar) (0.11, 0.27, 0.62) (0.125, 0.255, 0.62) (0.26, 0.13, 0.61)aAWOLED with color point illuminant A can be created by combining blue, green, and red emitters. The WOLEDs have the same green

and red emitters as in Table 2, however, each WOLED used only one of the three blue emitters listed in Table 2.


the realistic OLED stack listed in Table 4. The designof the additional interference layers is based on [25].To calculate the extraction efficiency of this struc-ture, we use a plane wave expansion method [32].To calculate the luminous power efficiency we useEqs. (1)–(8).To show the influence of the extraction efficiency of

WOLEDs on the luminous power efficiency, we com-pare four different WOLEDs. Two basic WOLEDshave no interference layers between indium tin oxide(ITO) and glass, the other two WOLEDs have addi-tional interlayers. The difference between the twobasic WOLEDs is their blue emitter, which iseither a phosphorescent or a fluorescent emitter.The green and red emitters in both devices are phos-phorescent emitters. To maximize the luminouspower efficiency, two parameters (tNET5; tNHT5) arevaried. The other two WOLEDs have three addi-tional layers. Again, there is a distinction betweena complete phosphorescent device and one with ablue fluorescent emitter. These structures havefour parameters (tNET5; tNHT5; tlow; thigh). The optionallayers have parameter tlow for the layers of low re-fractive index and the parameter thigh for the layerof high refractive index. In addition to the IQEof the emitter and the layer stack, the model inSection 2 uses the emitters spectra [Fig. 2(a)] anda driving voltage of 3:1V.The global optimization of the two basic WOLEDs

is a brute force method in which we look for the high-est luminous power efficiency in a two dimensionalparameter space. The calculation of the extractionefficiency of one OLED for one wavelength takesa few seconds on a system with a 2GHz OpteronProcessor (AMD, Sunnyvale, California). Moreover,a complete scan of the parameter space takes atleast one day. Therefore, a global optimization of fourparameters would take too long, and we optimize theparameters of the interference layers (tlow; thigh)

only locally for the previous organic layer stackoptimized by brute force. The organic layer stackis the optimized basic stack. The results of the opti-mization of all four WOLEDs is given in Table 5and Fig. 6.

These optimizations show that adding interfer-ence layers on a basic WOLED increases the overallluminous efficacy by at most 10%; see Table 5. Thisis true for both WOLEDs with a fluorescent blueemitter and the WOLEDs with a phosphorescentblue emitter. For example, Figs. 6(a) and 6(c) showthe extraction efficiency of the basic WOLEDwith a phosphorescent blue emitter and the basicWOLED with interlayers. Although the differencein the luminous power efficiency is only 10%, therelative change of the extraction efficiency is ap-proximately 50% at some wavelengths. Although

Fig. 6. (Color online)Wavelength-dependent extraction efficiencythat corresponds with the values listed in Tables 4 and 5 for dif-ferent stack configurations. Basic WOLED with (a) a blue phos-phorescent emitter and (b) a blue fluorescent emitter. WOLEDswith additional interference layers with (c) a blue phosphosrescentand (d) a blue fluorescent emitter. The green and red emitters arealways phosphorescent. 4/CO

Table 4. Typical stack of a three color White OLEDa

Material Refractive Index at 550nm Thickness

Al 0:96–6:69 j 100nmNET5 1.76 tNET5ETL 1:75–0:0092 j 10nmBlue emitter 1.80 10nmInterlayer 1.78 5nmGreen emitter 1.78 3nmRed emitter 1.80 10nmHTL 1:75–0:0092 j 10nmNHT5 1:75–0:0092 j tNHT5ITO 1:82–0:0113 j 90nm

Begin optional interference layersSiO2 1.46 tlowNb2O5 2.38 thighSiO2 1.46 tlow

End optional interference layersGlass 1.52 mmAir 1.0

aEach color is generated by one layer. The optional layers provide a stronger wavelength-dependent extraction efficiency than the de-fault stack.


we see a decrease of the extraction efficiency in theblue emitter, the luminous power efficiency is in-creased because of the increase of the extraction ef-ficiency in the red emitter. This improvement isexplained by the larger radiant flux needed for thered emitter than for the blue emitter. Increasingthe efficiency of the red emitter compensates forthe decrease in efficiency of the blue emitter. More-over, if the peak extraction efficiency had been placedat 500nm, the luminous power efficiency would havebeen approximately 33 lm=W, a decrease of 30%. TheWOLED with a blue fluorescent emitter, Figs. 6(b)and 6(d), show an increase of 10% in the luminouspower efficiency. However, because the blue emitteris much less efficient, the luminous power efficiencyalso requires an increase in extraction efficiency ofthe blue emitter.Although the increase in luminous power effi-

ciency by use of these microcavities is limited, wehave shown the importance of tuning the extractionefficiency to some parameters of aWOLED. Althoughthe relative improvement of the luminous power ef-ficiency is limited to 10%, the increase of the extrac-tion efficiency at some wavelengths is greater than50%. Placing this peak of extraction efficiency at an-other wavelength can decrease the luminous powerefficiency by 30%.

6. Conclusion

We have shown an increase in luminous power effi-ciency of a three-color white OLED by optimizingthe spectra and the extraction efficiencies of threeemitters. This optimization considers optical andelectrical parameters. The optical parameters arethe spectra and extraction efficiencies of the emit-ters; the electrical parameters are the driving vol-tage and the internal quantum efficiencies of theemitters. The model has been used to validate threestatements.First, we have examined variations of the spec-

trum of the MacAdam limit that have the highestluminous efficacy for a given chromaticity. The spec-trum of the MacAdam limit has only two infinitelysharp peaks. By changing the spectrum to be closerto the spectrum of the MacAdam limit, we can in-crease the luminous efficacy and also the luminouspower efficiency by 30%while retaining a sufficientlyhigh CRI of 80. A spectrum with narrower peaks

would not have a sufficiently high CRI. On the otherhand, recent work has shown that the CRI mightunderestimate the color quality of spectra withnarrow peaks.

The second statement was demonstrated by a com-parison of two WOLEDs for which we use emittervalues found in the literature. Using a WOLED witha deep blue fluorescent emitter instead of a WOLEDwith a light blue phosphorescent emitter lowers thefraction of radiant flux emitted by the blue emitter.This lower fraction compensates for the lower effi-ciency of the deep blue emitter. Thus, the overallluminous power efficiency of the two WOLEDs isalmost equal.

Third, tuning the extraction efficiency with thespectrum of a WOLED can also increase the lumi-nous power efficiency. To demonstrate this, we com-pared two WOLEDs: a basic three-color WOLED andthe WOLED with additional interference layers withstrong microcavity effects. Although the relative in-crease of the overall luminous power efficiency is lim-ited to 10%, the increase in the extraction efficiencyat some wavelengths is greater than 50%. Placingthis peak of the extraction efficiency at the wrongwavelength can decrease the overall luminous effi-cacy by 35%. Although this example is limited toone method to increase extraction efficiency, itclearly shows the importance of tuning the extractionefficiency to other parameters of the WOLED.

The research presented in this paper has beenfunded by the European Comission under contractIST- 004607 (OLLA). Peter Vandersteegen thanksHelmut Bechtel (Philips Research Aachen, Ger-many) and Eric Bretschneider for their help withthe colorimetry.

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Table 5. Basic White OLEDs with and without Interference Layersa

Basic White OLEDsηint;b; ηint;g; ηint;r ¼ 1:0; 1:0; 1:0 Fig. 6(a) tNET5, tNHT5 ¼ 48nm, 60nm ηP ¼ 48 lm=Wηint;b; ηint;g; ηint;r ¼ 0:25; 1:0; 1:0 Fig. 6(b) tNET5, tNHT5 ¼ 45nm, 30nm ηP ¼ 32 lm=W

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