Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength...

Organic microcavity light-emitting diodes withmetal mirrors: dependence of the emissionwavelength on the viewing angle

Aleksandra B. Djurisic and Aleksandar D. Rakic

Organic microcavity light-emitting diodes typically exhibit a blueshift of the emitting wavelength withincreasing viewing angle. We have modeled the shift of the resonance wavelength for several metalmirrors. Eight metals �Al, Ag, Cr, Ti, Au, Ni, Pt, and Cu� have been considered as top or bottom mirrors,depending on their work functions. The model fully takes into account the dependence of the phasechange that occurs on reflection on angle and wavelength for both s and p polarization, as well as ondispersion in the organic layers. Different contributions to the emission wavelength shift are discussed.The influence of the thickness of the bottom mirror and of the choice and thickness of the organicmaterials inside the cavity has been investigated. Based on the results obtained, guidelines for a choiceof materials to reduce blueshift are given. © 2002 Optical Society of America

OCIS codes: 230.3670, 160.4890, 230.3990.

1. Introduction

Organic light-emitting materials have been attract-ing considerable attention owing to their potential forapplications in low cost and high brightness thin dis-play applications. In recent years, planar microcav-ity structures have been used to improve theperformance of organic light-emitting diodes.1–11

When light-emitting material is placed inside a mi-crocavity, the spontaneous-emission photon densityof states becomes redistributed, so only wavelengthsthat correspond to the cavity modes are emitted. Inplanar microcavities this typically results in en-hancement of spontaneous-emission intensity in theperpendicular direction as well as in narrowing of theemission spectra. In addition to beneficial effects,i.e., intensity enhancement and improved purity ofcolor, microcavity organic light-emitting diodes�MOLEDs� exhibit a highly undesirable property, i.e.,dependence of the emission wavelength on the viewing

Aleksandra B. Djurisic �e-mail: [email protected].� is with theDepartment of Electrical and Electronic Engineering and Depart-ment of Physics, The University of Hong Kong, Pokfulam Road,Hong Kong, China. Aleksandar D. Rakic is with the School ofInformation Technology and Electrical Engineering, The Univer-sity of Queensland, Brisbane Qld4072, Australia.

Received 26 June 2002; revised manuscript received 26 June2002.

0003-6935�02�367650-07$15.00�0© 2002 Optical Society of America

7650 APPLIED OPTICS � Vol. 41, No. 36 � 20 December 2002

angle. This phenomenon has been well documentedexperimentally,5–11 but studies that address this prob-lem specifically have been scarce.10,11 Until now, adetailed analysis of the influence of various factors inMOLED design on the dependence of the emissionwavelength on viewing angle has been lacking.

Here we present calculations for the blueshift of theemission wavelength with increasing viewing angle intris�8-hydroxyquinoline� aluminum– �Alq3-� basedMOLEDs with metallic mirrors. The device struc-ture is illustrated in Fig. 1. There are two configura-tions of MOLEDs, depending on the type of bottommirror. The top mirror is simply a metal cathode,whereas the bottom mirror can be a semitransparentmetal layer or a distributed Bragg reflector �DBR�. Inthe latter case a layer of indium tin oxide, which servesas an anode, is deposited on the top of insulating DBRstructure. It has been shown that the enhancementthat can be achieved for silver–silver microcavities islarger than that for silver–DBR cavities.12 Also, it isexpected that MOLEDs with metallic mirrors will ex-hibit less dependence of emission wavelength on view-ing angle because of the shorter cavity length that canbe achieved compared with those for MOLEDs withDBRs as bottom mirrors.12 For these reasons wehave focused on MOLEDs with metallic mirrors. Thecalculations have been performed for eight metals �Al,Ag, Cr, Ti, Au, Ni, Pt, and Cu�. Low work-functionmetals have been considered for the top mirror �cath-ode�, and high work-function metals have been consid-

ered for the bottom mirror �anode� to ensure efficientcarrier injection. The dependence of emission wave-length on viewing angle is a consequence of the depen-dence of the phase change that occurs on reflectionfrom the metal mirrors as well as on the change in theoptical path length inside the cavity.

We have derived the analytical expression for theangular dependence of wavelength shift �� and per-formed calculations for several cavity designs. Ourcalculations fully take into account the dispersion ofthe organic materials inside the cavity as well as thewavelength and angle dependence of the phasechange on reflection for both mirrors. We analyzedthe relative contributions of the mirrors and of theorganic light-emitting material inside the cavity tothe wavelength shift. We demonstrate here thatwavelength dispersion in the light-emitting layer isthe dominant mechanism responsible for the wave-length shift with viewing angle, and we suggest anapproach to minimizing this adverse effect.

The paper is organized as follows. In Section 2 wedescribe in detail the model and the iterative proce-dure for the calculation of ��. In Section 3 the re-sults obtained for various cavity structures arepresented. Possible ways to reduce the dependenceof the emission wavelength on the viewing angle areproposed. Finally, conclusions are drawn.

2. Description of the Model

The resonant modes of a microcavity have to satisfy

the condition that the phase change during one roundtrip be a multiple of 2�. In other words, for normalincidence the following equation is valid:

�i

4�dini��

�� top�0, �� bot�0, �� 2m�, (1)

where � is the emission wavelength, �top�0, �� and�bot�0, �� are the angle- and the wavelength-dependent phase changes on reflection from the topand the bottom mirrors, respectively, and m is aninteger that defines the mode number. The summa-tion in Eq. �1� is performed over all the layers insidethe cavity with thicknesses di and refractive indicesni��. For oblique incidence, the resonant wave-lengths are solutions of the following equation:

�i

4�dini�� cos �i

� � �� top��top, � � ��

� �bot��bot, � � �� 2m�, (2)

where �top and �bot are incidence angles at the top andthe bottom mirrors, respectively, and �i is the angle ofpropagation within ith organic layer. From Eqs. �1�and �2� we can obtain the expression for the wave-length shift, ��:

�� 1

� ��i

4�dini�� cos �i � ni��

� ��top � ��bot�� , (3)

where

� � 2m� � �top��top, � � ��

� �bot��bot, � � ��, (4)

��top � �top��top, � � �� top�0, ��, (5)

��bot � �bot��bot, � � �� bot�0, ��. (6)

Inasmuch as Eq. �3� contains several terms thatare wavelength dependent, we used an iterative pro-cedure to calculate wavelength shift ��. The start-ing point was the wavelength shift calculated for thecase of no dispersion, and the procedure was termi-nated when the change in �� in two consecutive it-erations was smaller than 0.1 nm. The phasechanges on reflection from the top metal mirror forperpendicular ��s� and parallel ��p� polarizations canbe conveniently expressed in the following form:

where ni�� is the index of refraction of the organiclayer adjacent to the top metal mirror, �i is the angleinside the organic layer, Nm�� nm�� ikm�� is

Fig. 1. Device structure of a MOLED with two metal mirrors.HTL, hole-transport layer; ETL, electron-transport layer.

�s � tan 1� 2ni��cos �inm�� y � km�� x�

nm�� x � km�� y�2 � ni2��cos �i

2 � nm�� y � km�� x�2� , (7)

�p � tan 1� 2ni��cos �inm�� y � km�� x�

ni2�� x2 � y2� � nm

2�� km2��2 cos �i

2� , (8)

20 December 2002 � Vol. 41, No. 36 � APPLIED OPTICS 7651

the complex index of refraction of the metal, cos �m �x � iy, and �m is the angle in the metal. The anglesin Eqs. �2�–�8� are related to the viewing angle out-side cavity, �0, by Snell’s law:

Nm sin �m � ni sin �i � sin �0.

The dependence of the index of refraction on wave-length for the metals was modeled with an oscillatormodel, with the model parameters given in Ref. 13.Metals with work function � higher than 4.5 eV wereconsidered for the bottom mirror because the bottommirror also serves as an anode and should provideefficient hole injection. Therefore Al, Ag, Cr, and Ti�� 4.28, 4.26, 4.5, 4.3 eV, respectively14� were as-signed for the top mirror, and Au, Pt, Ni, and Cu �� 5.1, 5.65, 5.15, 4.65 eV, respectively14� were used forthe bottom mirror. Because of the superior opticalquality of silver thin films, which have the lowestabsorption in the visible region of all metals consid-ered, we also considered MOLEDs with both mirrorsmade from silver.

The microcavity structure considered here is glasssubstrate–bottom mirror–HTL–ETL–top mirror,where HTL denotes a hole-transport layer and ETLdenotes an electron-transport layer. Alq3 was con-sidered for the ETL, whereas for the HTL N,N�-disphenyl-N,N�-bis�3-methylphenyl�-1,1�-disphenyl-4,4�-diamine �TPD� or copper phthalocyanine �CuPc�was considered. The refractive indices of Alq3 andTPD were modeled with the Cauchy equation n�� A � B��2 � C��4, where we determined the coeffi-cients A, B, and C by fitting the data in Ref. 15.CuPc has low absorption in the spectral region 450–550 nm.16 However, the experimental data for CuPcin that spectral range16 cannot be well described withthe Cauchy equation, so a polynomial fit was used.

For the bottom mirror we determined the phasechange on reflection by applying a matrix formalismto calculate reflectance from the thin metal film upona glass substrate. The refractive index of the glasswas described with the Cauchy equation, and theindex of refraction of the metal was described by anoscillator model, as it was for the top mirror. Thelight was incident from the HTL. The thickness ofthe semitransparent metal mirror was determinedfrom the condition for the reflectivity of the mirror toachieve the optimal extraction of light from a micro-cavity LED �Ref. 17�:

R1 � min�R1crit, R1

sp,crit, R1loss�, (9)

where

R1crit � 1 �

mn2 , (10)

where m is the mode number, n is the refractive indexof the material inside the cavity, R1

loss is determinedby the losses in the cavity �for large losses, approxi-mately equal to the reflectivity of the top mirror, R2�,and

R1sp,crit � 1 � 2mS, (11)

where S is the relative spectral width ��k�k0� ob-tained by dividing FWHM of the emission spectra ofthe material by the central frequency. Because ofthe large relative spectral width of Alq3 �S � 0.16�,the reflectance of the bottom mirror for optimum lightextraction is limited by R1

sp,crit; i.e., for m � 1 thereflectance of the bottom mirror should be R1 � 0.68,and for m � 2 it should be R1 � 0.36. Based on thisvalue, we have determined the required thickness forthe bottom mirror. It should be pointed out that forreflectances greater 60% the required mirror thick-ness is more than 50 nm, resulting in large lossesowing to absorption within the mirror even for silver,which has lowest loss in the spectral region consid-ered �the transmittance of a 50-nm-thick silver mir-ror is only �0.04 at 520 nm�. The loss that is due tometal absorption is lower and the transmittancehigher for mirrors designed for m � 2 cavities.

As can be observed from Eq. �3�, the shift of wave-length with viewing angle is inversely proportional tomode number m. However, no significant improve-ment can be obtained by increasing m because doingso would require larger thicknesses of the organiclayers di, and �� is directly proportional to the thick-ness values of organic layers. Furthermore, themaximum achievable extraction efficiency �max is in-versely proportional to mode number m. Therefore,low-order �m � 1, 2� cavities are preferred for achiev-ing high extraction efficiency. The cavity order hasno significant influence on the shift in emission wave-length typically less than 1-nm difference for neigh-boring modes �m � 1, 2; m � 2, 3 etc.��.

3. Results and Discussion

Although the dependence of the emission wavelengthon viewing angle in MOLEDs is well documentedexperimentally,5–11 the physical basis for this phe-nomenon has been addressed only for silver–poly�p-phenylene vinylene�–silver microcavities by use of asimple model.10,11 Becker et al.10 investigated theposition of the resonance wavelength for various sil-ver mirror thicknesses. Based on the phase changeas a function of angle of incidence for different mirrorthicknesses calculated for the air–metal interface,they concluded that angular dependence would beminimized if thicker metal films were used. Tessleret al.11 have used a simple equation,

� � 2�mLeffn cos��, (12)

where m is the mode number, n is the material insidethe cavity, � is an internal angle, and Leff is theeffective cavity length, which includes any additionallength introduced by the mirrors. Based on Eq. �12�,they conclude that, to decrease the wavelength shiftas cos�� decreases, one should increase Leff, n, orboth. This simplified analysis, however, does notprovide any detailed insight into the dependence ofthe wavelength on the viewing angle. Here we pro-vide a detailed analysis of the dependence of theemission wavelength on the viewing angle in Alq3-based MOLEDs. We have chosen Alq3 because this


material has been widely used for applications inorganic light-emitting devices. Unlike devicesbased on poly�p-phenylene�, Alq3-based MOLEDs arefabricated by thermal evaporation, which permitseasy deposition of multilayer structures and hencehigher device efficiency because separate hole-transport and electron-transport materials can beused. Because significant emission wavelengthshifts with viewing angle in Alq3-based MOLEDshave been reported 580 to �530 nm at 50°, 560 to�530 nm at 40°,6 610 to �500 nm at 80°, and 540 to�500 nm at 50° �Ref. 7��, it is important to analyzethis phenomenon and investigate ways to reduce thedependence of wavelength on viewing angle.

Figure 2 shows the phase change on reflection fromthe top metal mirror as a function of the viewingangle outside the cavity, �0. Incidence angle �top isrelated to the viewing angle by Snell’s law. It can beobserved that the phase change on reflection for thelight that can be extracted from the cavity is smallerbecause the refractive index of the material inside thecavity is larger than 1. Figure 3 shows the depen-

dence of the phase change on reflection from a silvermirror as a function of viewing angle and wavelength.It is obvious that the actual phase change at a givenviewing angle will be modified by the resonancewavelength shift, so an iterative calculation of Eq. �3�is a must. Figure 4 shows the relationship of phasechange to viewing angle for the bottom mirror. Weevaluated the emission wavelength shift versus view-ing angle for several combinations of top and bottommirrors. The wavelength shift for the s polarizationis not influenced notably by the choice of mirrors,whereas for p polarization a marked difference can beobserved for different mirror combinations �as muchas a 10-nm difference in wavelength shift�. Thesmallest emission wavelength shift is obtained for thecavities with bottom Au or Ag mirrors and a top Agmirror, whereas the largest shift can be observed incavities with a top Al mirror and a bottom Pt or Nimirror.

We have also simulated the wavelength shift forseveral emission wavelengths at �0 � 0°. Resultsobtained for a MOLED with a bottom Au and a top Agmirror are shown in Fig. 5. We have assumed thatthe thickness of Alq3 layer is 20 nm �because of lowdispersion of the refractive index of Alq3 in the regionof interest, the layer should be thin�, and then wecalculated the required thickness of the HTL �TPDwas used as the HTL for the results shown� for thedesired emission wavelength. It can be observedthat the wavelength shift at �0 � 75° in a cavity thatis resonant at 480 nm is almost two times smallerthan the shift in the cavity that is resonant at 600nm. Obviously, the dispersion in the layers insidethe cavity plays a significant role in reducing the shiftin emission wavelength. The shift of the emissionwavelength with increasing viewing angle originatesmainly in the change of the optical path within or-ganic layers �almost entirely for s polarization, andpartly compensated for p polarization�. Thereforethe choice of the materials inside the cavity is evenmore important than the appropriate choice of themirrors.

TPD and Alq3, which are commonly used in organic

Fig. 2. Phase change on reflection from the top mirror for s and ppolarization with light incident from Alq3 as a function of viewingangle outside the cavity. The phase change on reflection from aAg mirror in air is shown for comparison.

Fig. 3. Dependence of the phase change on reflection from a Agmirror for s and p polarization as a function of viewing angleoutside the cavity.

Fig. 4. Phase change on reflection from the bottom mirror for sand p polarization with light incident from TPD as a function ofviewing angle outside the cavity.


light-emitting diodes, have low dispersion in the spec-tral region of interest, as shown in Fig. 6�a�, and thusthe emission wavelength shift in MOLEDs based onthese two materials is large. Phthalocyanine mate-rials are sometimes used as hole-transport or hole-injection layers in Alq3-based OLEDs. With CuPcas a hole-transport layer, the shift in emission wave-length with increasing viewing angle can be reduced,as shown in Fig. 6�b�. The calculated wavelengthshift corresponds to a cavity that is resonant at 500nm with a 20-nm thick Alq3 layer HTL thicknessdetermined from the resonance condition, Eq. �1��with Ni bottom and Cr top mirrors. However, thereduction achieved is still insufficient. Sufficient re-duction may be obtained for devices operating nearthe short-wavelength edge of the transmission win-dow of CuPc, so the blueshifted light would be ab-sorbed, which would eliminate excessive wavelengthshift but would result in the reduction of viewingangles at which emission is possible.

The majority of organic materials have low disper-sion and low refractive indices in the spectral regionbelow the first absorption band. To explore the pos-

sibility of further reduction of the shift in emissionwavelength with viewing angle, we performed calcu-lations for a hypothetical composite material. It hasbeen demonstrated that a high refractive index andincreased dispersion can be achieved in hybridpolymer–inorganic films.18 Synthesis of small nano-particles of a variety of materials,19,20 including GaPnanoparticles with 9-nm average size,21 has been re-ported. Therefore preparation of compositepolymer–inorganic semiconductor films is feasibleand may be a useful option to explore to achievegreater variation in the refractive-index parameters

Fig. 5. �a� Comparison of the wavelength shifts for cavities withAu–Ag and Pt–Al mirrors. �b� Emission wavelength shift versusviewing angle for different starting wavelengths �i.e., differentthicknesses of organic materials inside the cavity� in a cavity witha Au bottom and a Ag top mirror.

Fig. 6. �a� Refractive indices of hole-transport materials over thespectral region of interest. The refractive index of Alq3 is alsoshown. �b� Shift in emission wavelength relative to viewing anglefor several hole-transport materials. Thick curves and closedsymbols denote s polarization; thin curves and open symbols de-note p polarization.


of layers in organic devices. We chose to simulate acomposite consisting of GaP nanoparticles inbisphenyl-A polycarbonate �PC�. GaP has been cho-sen as a semiconductor with a high refractive indexand a suitable bandgap value �which is transparentin the green spectral region�. The refractive indicesof PC �Ref. 22� and of GaP �Ref. 23� were taken fromthe literature, and the dielectric function �ε� andhence the index of refraction of the composite filmwere calculated by use of the effective-medium ap-proximation24:

εa � �ε�

εa � 2�ε�fa �

εb � �ε�

εb � 2�ε��1 � fa� � 0, (13)

where εa and εb are the dielectric function values oftwo components and fa is the volume fraction of thecomponent a. The results obtained for two differentvolume fractions of GaP are also shown in Figs. 6�a��refractive index� and 6�b� �wavelength shift�. It canbe observed that the expected emission wavelengthshift is significantly smaller than that when TPD isused as a HTL. Further reductions in the depen-dence of emission wavelength on viewing angle maybe achieved with custom-designed asymmetric Braggmirrors whose phase shifts would partly cancel thechange introduced by the material inside the cavity.Another, more realistic, possibility is to use organicmaterials with narrower emission spectra, such asrare-earth complexes. If the noncavity emissionspectrum is sufficiently narrow, the use of the cavitywill simply result in a smaller viewing angle than fornoncavity devices, but no emission wavelength shiftdetectable by the human eye over the angle rangewhere such MOLED emits can be achieved.

4. Conclusions

We have presented a model for the dependence ofemission wavelength on viewing angle in organic mi-crocavity light-emitting diodes with metal mirrors.We have found that the main contribution to thewavelength shift originates in the change of the op-tical path inside the organic materials, which can bereduced by use of more-dispersive materials �i.e., ma-terials with a larger increase of the refractive index atshorter wavelengths�. However, we found that, inAlq3-based devices, owing to the large width of theemission spectrum and the low dispersion of the re-fractive index this effect cannot be eliminated.Therefore to utilize fully the benefits of a microcavitystructure one should employ materials with narroweremission spectra, such as rare-earth complexes, andHTLs or ETLs or both with large dispersion.

This research was partly supported by a Universityof Hong Kong University Research Committee seedfunding grant.

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Organic microcavity light-emitting diodes with metal mirrors: dependence of the emission wavelength...

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