Enhanced light emission from top-emitting organic light-emitting diodes byoptimizing surface plasmon polariton losses

Cornelius Fuchs,∗ Paul-Anton Will, Martin Wieczorek, Malte C. Gather,†

Simone Hofmann, Sebastian Reineke, Karl Leo,‡ and Reinhard ScholzTechnische Universitat Dresden, Institut fur Angewandte Photophysik,

George-Bahr-Straße 1, Dresden, Germany, 01069

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emittingdiodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is causedby the use of a hole transport layer (HTL) material possessing a low refractive index (∼ 1.52).The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP)excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. Theshift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitationsand thus increases the outcoupling efficiency, although the SPP remains constant in intensity. Theproposed method is suitable for emitter materials owning isotropic orientation of the transitiondipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparableenhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, theSPP can be modeled as a propagating plane wave within other organic materials in the opticalmicrocavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings,it would become feasible to obtain even higher enhancements of the light extraction.

PACS numbers: 78.60.Fi, 78.66.Qn, 85.60.Bt, 85.60.Jb, 73.20.Mf

I. INTRODUCTION

Organic light-emitting diodes (OLEDs) are already ap-plied as lighting or display technology today.1,2 However,the research for more efficient devices is widespread.1,3–5

Especially top-emitting OLEDs are of great interest, asthese do not radiate through a transparent substrateand therefore allow for a more flexible choice of sub-strate materials.3,6,7 The radiometric efficiency of anOLED is given as the external quantum efficiency (EQE)ηEQE, relating the number of extracted photons to thenumber of injected charges.8 Increasing the EQE re-mains an important goal in OLED related research. TheEQE can be divided into a product of three efficien-cies ηEQE = γ ηIQE ηout.

9 In this relation, γ representsthe electrical efficiency, accounts for electrical losses andcharge carrier balance. The internal quantum efficiencydenoted by ηIQE characterizes the efficiency of charge tophoton conversion. Both quantities are close to unityfor common pin-OLEDs10,11 incorporating phosphores-cent emitters.12–15 However, due to the high refractiveindices of the organic semiconductor materials causingtotal internal reflection (TIR),9,16 the outcoupling ef-ficiency ηout is typically limited to values below 30 %.Therefore, the focus in OLED related research is increas-ingly on ηout. The 30 % limit takes into account the en-hanced power dissipation into resonances of the thin op-tical microcavity.16,17 For top-emitting OLEDs, the op-

∗ cornelius.fuchs@iapp.de† Now at: SUPA, School of Physics and Astronomy, University ofSt Andrews, North Haugh, St Andrews, Scotland, UK, KY169SS‡ http://www.iapp.de

tical losses can be divided into contributions caused bythe finite extent of the optical microcavity, i.e. opticalwave guide (WG) modes, losses from absorption, and ex-citations of surface plasmon polariton (SPP) modes atmetal interfaces.18 Either the OLED suffers from largelosses due to the SPP mode, which is excited at themetallic opaque back reflector, or, if the distance be-tween organic emitter and the metal is increased, an-other WG mode as source of loss is introduced, whilethe SPP excitation is strongly reduced.18 Typically, eventhough the experimentalist is free to ponder between dif-ferent types of loss channels, known OLED layouts can-not exceed the limit mentioned above. For top-emittingOLEDs without any further outcoupling techniques, thefirst order optical microcavity incorporating a large SPPcontribution was found to be more efficient than opti-cal microcavities exhibiting higher order WG modes.18,19

Lately, anisotropic emitter materials showing a preferredspatial orientation of their transition dipole moment re-ceived great attention.15,20,21 Using such materials, theefficiency limit can be increased compared to isotropicemitters.15,21 For spatial orientations of common phos-phorescent emitter materials the enhancement was foundto be about 15 % for optimized devices, as the con-tribution of plasmonic losses for the first order deviceare reduced.15 Recently, for nearly completely horizon-tal alignment, the enhancement was found to be up to50 %22, in accordance with simulations23. To extractthe remaining power lost into SPP excitations, scatter-ing approaches have been proposed20,24,25, or methodsinvolving complex microlens structures of high refractiveindex.20,25,26

Here, we propose a method to increase the outcou-pling efficiency ηout of OLEDs by modifying the prop-erties of the SPP excitations of the simple planar op-

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tical cavity. We avoid power dissipation into evanes-cent modes by using Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as doped organiccharge transport materials with a low refractive index.The reduction of the average refractive index by about14 % results in an average increase of the outcoupling effi-ciency of about 20 % regardless of whether the emitter ex-hibits anisotropic or isotropic orientation of the transitiondipole moments, as the SPP excitation itself is retained.These results, initially obtained from optical simulations,are experimentally verified for optimized first-order greenphosphorescent top-emitting OLEDs with emitters hav-ing isotropic or preferential horizontal orientation, re-spectively.

Several studies have successfully implemented lowrefractive index materials such as PEDOT:PSS intoOLEDs. These works focused on the use of low refrac-tive index materials or PEDOT:PSS as electrode27–29,hole transport layers30 or low refractive index spacersoutside the electrical active microcavity31 in bottom-emitting OLEDs, or low refractive index photonic crys-tal layers.32 For these stratified planar bottom-emittingOLEDs, the enhancements of the EQE compared to com-mon materials (i.e. indium tin oxide electrodes) resultedfrom the drastic reduction of the microcavity size or me-diation of the properties of the wave guide. So in theseworks OLEDs of different optical orders were compared,instead of OLEDs with similar thickness of the microcavity. Furthermore, in a purely theoretical study, Smithand Barnes33 investigated the effect of reducing the re-fractive index of the entire organic layers within the opti-cal microcavity for bottom and top-emitting geometries.They found great potential for improvement, becausewith decreasing refractive index the resonant enhance-ment of non-radiative modes is reduced.

In contrast, the focus of this work is the enhancedlight harvesting of the already optimized first order top-emitting OLEDs and experimental verification of the the-oretical predictions. For these devices, the optical thick-ness of the microcavity layers can not be reduced to ob-tain higher optical outcoupling efficiency18, neither by re-ducing the layer thickness, nor by reducing the refractiveindex. Instead, we mitigate the losses from surface plas-mon contributions, increasing the boundary of the theo-retical limit of outcoupling efficiency for planar OLEDs.In addition, a detailed discussion of the optical effects isgiven. The theoretical discussion indicates that in thebest case the modified SPP can be modeled by a non-evanescent plane wave within the organic layers. Thus, itbecomes an accessible target for further macroscopic3,34

or microscopic24,35 outcoupling techniques.

II. DEVICES AND MATERIALS

This report investigates plain colored top-emittingOLEDs. All these devices follow the pin-conceptusing doped charge transport layers enclosing intrinsic

charge blocking and emission layers.10,11 The thick-nesses of the transport and organic capping layer wereobtained by optimization of the optical outcouplingefficiency from optical simulations. Hereby the fixeddimensions of the intrinsic layers of the OLED weretaken into account. The thicknesses of the intrinsiclayers were obtained in order to achieve the high-est internal quantum efficiencies.17,36 To guide thereader, the layer sequences of devices A to D andtheir distinctive features are illustrated in Fig. 1.The OLEDs were manufactured onto glass substrates,upon which the opaque bottom aluminum electrode(anode, 100 nm) was deposited. Next, the hole trans-port layer (HTL) was fabricated. For devices A andC, both serving as reference devices, the HTL wasevaporated using 2,2’,7,7’-tetra(N,N-ditolyl)amino-9,9-spiro-bifluorene (Spiro-TTB) doped with 4 wt.% of 2,2-(perfluoro-naphthalene-2,6-diylidene)dimalononitrile(F6-TCNNQ), where the thickness was 49 nm for deviceA and 46 nm for device C. The HTLs of devices B andD were prepared from PEDOT:PSS (Clevios P AI4083)with a thickness of 55 nm for Device B and 51 nmfor device D. Among the two different HTL materialsused in this study, Spiro-TTB:F6-TCNNQ has a highrefractive index (average value of nhigh = 1.77 over theemission spectrum of the emitter molecules), whereasPEDOT:PSS possesses a low average refractive indexnlow = 1.52. The intrinsic emission units were depositedonto the HTLs for each device. They consisted of anelectron blocking layer (EBL), a double emission layer(EML), and a hole blocking layer (HBL). The EBL wasfabricated from 2,2’,7,7’-Tetrakis-(N,N-diphenylamino)-9,9’-spirobifluorene (Spiro-TAD, 10 nm), for devices Aand B. For devices C and D the EBL was preparedfrom N,N’-Di(naphthalen-1-yl)-N,N’-diphenyl-benzidine(α-NPD, 10 nm). The EML of device A and C wereproduced from 4,4’,4”-tris(carbazol-9-yl)-triphenylamine(TCTA, 6 nm) as one host material, and 2,2’,2”-(1,3,5-Phenylen)tris(1-phenyl-1H-benzimidazole) (TPBi,12 nm) as a second EML. As emission material fordevices A and C the phosphorescent green emitterTris(2-phenylpyridine) iridium(III) (Ir(ppy)3) was dopedwith a concentration of 8 wt% into these hosts. Thedouble EML host materials for devices C and D arethe same as for devices A and B, but the optimizedthicknesses were adjusted to 8 nm of TCTA and 12 nm ofTPBi. In the devices C and D the phosphorescent greenemitter dopant bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)2(acac)) doped with 8 wt% into thematrix was used. In all devices the HBL was producedfrom 4,7-diphenyl-1,10-phenanthroline (BPhen, 10 nm).

The choice of the different green emitting moleculeswas motivated by their different alignment of their tran-sition dipole moments. Ir(ppy)3 is known to havean isotropic dipole distribution37, where in contrastIr(ppy)2(acac) shows preferential horizontal (preferentialparallel to the x-, resp. y-axis) alignment.37 To com-plete the OLED stack, an electron transport layer (ETL)

3

FIG. 1. Scheme of the fabricated devices A to D. The devices are distinguished by their phosphorescent emitter molecules,which show either isotropic (a = 0.33, Ir(ppy)3, devices A and B) orientation of their transition dipole moment, or anisotropicorientation (i.e. a = 0.23, Ir(ppy)2(acac), devices C and D). Devices A and C are reference devices with high refractive indexHTL, i.e. Spiro-TTB:F6-TCNNQ, devices B and D have a low refractive index PEDOT:PSS HTL.

of BPhen doped with cesium, a transparent metal elec-trode (cathode) consisting of 2 nm gold and 7 nm silver,and an organic capping layer (CL) of α-NPD were de-posited. For device A and B the thicknesses of the ETLand CL were 43 nm and 89 nm, and for device C and Dthe ETL thickness was 44 nm and 46 nm respectively, anda CL had a thickness of 91 nm.

III. THEORETICAL CONCEPT

A. Shift of the SPP excited at thedielectric/opaque metal interface

Every metal/dielectric interface supports SPPs, aris-ing from the coupling between surface charge oscilla-tions and the electromagnetic field. In OLEDs, SPPscan be excited due to the close proximity between themetal/dielectric interface and the radiation source. Com-monly at least the opaque bottom electrode in top-emitting OLEDs is produced from metal, because of theconvenient large broadband reflectivity and simplicity infabrication7,19,38 although there have been attempts tofabricate semi-transparent top contacts from sputteredtransparent conductive oxides (TCOs) or polymers.38,39

For the theoretical discussion, the definition of the co-ordinate system in reference to the OLED is shown inthe inset of Fig. 2. To outline the properties of theSPP within OLED microcavities at first a brief discussionfor a simplified interface of two adjacent media is given.The electromagnetic field of an SPP mode, modeled bya p-polarized time-harmonic electromagnetic plane wavewith its magnetic field Hy = H0, y eı(ux+w z), is local-ized by an exponentially decaying envelope around the

metal/dielectric interface. The coordinate system is laidout such that the plane of incidence, defined by the wavevector ~ν of the electromagnetic plane wave and the z-direction is given by y = 0. Thus, the y-component ofthe wave vector is zero. Furthermore, the interfaces be-tween the stratified media are located at planes denotedby z = const. . The localization of the electromagneticfield of the simple SPP can be seen from the dispersionrelation excited at an interface at z = 0 between a metaland a dielectric medium with dielectric functions εm andεd

40

E = ~ c u√εm + εdεm εd

. (1)

Here, E denotes the energy, u the in-plane component ofthe wave vector, c identifies the speed of light and ~ thePlanck constant. The localization of the SPP is under-stood from the definition of the out-of-plane wavenumber

w =√εd/m ν

20 − u2 and the fact that the squared in-

plane wavenumbers u2 exceed εd/m ν20 in each respective

medium, with ν0 = E/ (~ c) being the vacuum wavenum-ber. Even though the case of infinite media remains amerely theoretical example, it is useful to get a quali-tative insight of how the dispersion relation of the SPPat an interface between an opaque metal and a dielec-tric material (oSPP) can be modified. In Fig. 2 thedispersion relation according to Eq. (1) for a high re-fractive index dielectric medium (average refractive in-dex nhigh = 1.77) and a low refractive index medium(average refractive index nlow = 1.52) is shown (dashedline). Hereby, the dielectric function of the metal ismodeled by a free electron gas model dielectric func-tion εm = 1 − E2

p/E2. The plasma energy is fitted to

4

FIG. 2. Calculated dispersion relations of oSPP modes. Thecalculations are carried out either in the approximation ofEq. (1), or by taking into account the complete OLED opticalmicrocavity by using the transfer-matrix approach. For bothmethods a shift of the oSPP mode towards smaller in-planewavenumbers u is observed for a decreased refractive index ofthe dielectric medium adjacent to the opaque metal.

Ep = 12.26 eV representing aluminum for the relevantrange of photon energies. The reduced refractive indexleads to a shift ∆u = ulow − uhigh of the oSPP dis-persion relation towards smaller in-plane wavenumbersu for all relevant photon energies E. For a photon en-ergy of E = 2.6 eV, representing the upper bound ofinterest, the shift is quantified to ∆u = −3.59µm−1,whereas for the lower energy of E = 1.9 eV a smallershift of ∆u = −2.28µm−1 is achieved. From the in-planewavenumbers, the decay lengths40,41 z = |1/wdiel| withinthe dielectric can be calculated. It is found to be in therange of the extension of the optical microcavity.40 Toquantify the shift of the oSPP mode, along with the pre-dictions from the simplified example, Fig. 2 visualizes thecalculated resonance positions of the oSPPs of device Aand B. These involve all the interactions of the completeOLED optical microcavity, calculated from the transfer-matrix.41–43 Although the positions and absolute valuesof the shift of the resonance originating from the oSPP,∆u2.6 eV = −2.42µm−1 and ∆u1.9 eV = −1.02µm−1 dif-fer from the predictions derived from Eq. (1), the generaltrend for the classical case holds, and the oSPP resonanceis shifted towards smaller in-plane wavenumbers as therefractive index of the adjacent dielectric is decreased.The change in the absolute positions of the oSPP andin the slopes of the oSPP dispersions calculated from thetransfer-matrix approach compared to the classical modelof Eq. (1) arises due to the use of the dielectric functionsof all media involved, rather than average refractive in-dices of the dielectrics.

B. Implications of the oSPP shift on the powerdissipation of OLEDs

The electromagnetic radiation of OLEDs is com-monly modeled by the resonant coupling of electricdipole sources to the radiation within the opticalmicrocavity.16,17,44,45 Following this approach, the den-sity of dissipated power by the emitter K for a givenphoton energy E and in-plane wavenumber u can becalculated.17,45 The outcoupled part of the dissipatedpower

Fout = 2/ν2active

uTIR∫0

duuKout(u) , (2)

is limited by the in-plane wavenumber uTIR =sin (θTIR) νactive. This in-plane wavenumber correspondsto the emission angle θTIR = arcsin (nair/nactive) repre-senting trapped light within the optical microcavity dueto TIR with respect to the outcoupling medium, i.e. air.Here νactive = ν0 nactive describes the wavenumber withinthe active medium of the OLED with refractive indexnactive. The outcoupling efficiency ηout of an optical mi-crocavity is given by the outcoupled power Fout dividedby the total dissipated power

F = 2/ν2active

∞∫0

duuK(u) . (3)

Thus, the total power contains (i) the outcoupled ra-diation, i.e. u < uTIR, (ii) resonance contributionsfrom modes which are propagating in the organic lay-ers, but which are trapped in the device due to TIR, i.e.uTIR < u < νactive, and (iii) shares of the resonances fromradiation represented by plane waves with imaginary out-of-plane wavenumbers wactive, i.e. u > νactive such asSPPs. Keeping in mind that the dissipated power is cal-culated from the resonant coupling of the electromagneticfield to the sources, it becomes clear that for evanescentradiation the power dissipation asymptotically vanishesfor u→∞, since in this limit the electromagnetic field atthe emitter position decreases exponentially, and the res-onant coupling of the sources to the field decreases like-wise. Furthermore, enhanced dissipation of power occursinto resonances originating from WG and SPP modes, asfor these modes resonances of the transmission exist.

By shifting the oSPP towards smaller in-planewavenumbers, the coupling of the transition dipoles toevanescent modes with u > uoSPP is reduced as theasymptotic behavior for u � ν0 nactive is reached ear-lier compared to a geometry with an oSPP resonanceof a high average refractive index. This effect is shownin Fig. 3 a, where the power dissipation spectra of de-vices A to D are calculated for the refractive indicesnhigh (green) and nlow (red) of the HTL materials, re-spectively. The power dissipation is shown for the pho-

5

FIG. 3. a) Power dissipation density u/νactive · F of the optimized device A and C comprising the high refractive index HTL(green lines), and B and D using the low refractive index (red lines) HTL material at 510 nm wavelength. Simulations areshown for the isotropic emitter (a = 0.33, solid lines, devices A and B), and anisotropic dipole distributions (a = 0.23, dashedlines, devices C and D). A shift of the contribution to the power dissipation spectrum originating from the oSPP causing anincrease in outcoupling efficiency by a factor of ∼ 1.2 for the low average refractive index nlow. Additionally, a variation of theETL (blue) is shown where the oSPP shift is significantly smaller compared to the HTL variation. b) Calculated enhancementfactor of the optimized outcoupling efficiencies ηout with regards to the anisotropy factor a of the emitter (red squares). Theenhancement is obtained for optimized geometries using nhigh and nlow for the HTL. Furthermore, the enhancement as afunction of the average refractive index n of the HTL (green) or ETL (blue) compared to nhigh is shown for the anisotropycoefficients used in the experiments. The enhancement factors show only a weak dependence on the anisotropy factor, but theydepend heavily on the refractive index difference between nhigh and n.

ton energy corresponding to the most prominent wave-length λ = 510 nm of the green emitter materials. Twoanisotropy coefficients23 are investigated, with a = 0.33(solid lines) corresponding to isotropically distributedtransition dipole moments, i.e. Ir(ppy)3, and a = 0.23(dashed lines) corresponding to Ir(ppy)2(acac).37 Fig-ure 3 a displays three strong resonances, of which twoare located in the organic region for all anisotropy co-efficients, representing resonances of WG modes. Theresonance located at u/νactive = 0.65 is the excitation ofthe p-polarized (TM) second order resonance, and thepeak at u/νactive = 0.79 corresponds to the s-polarized(TE) first order WG excitation. The third resonancerepresents the oSPP excitation that is shifted to a valueof u/νactive = 0.96. In the region corresponding to theoutcoupled radiation, the power dissipation spectrum isdefined by shallow resonances which will form the TM3

and TE2 within the organic WG region at higher ener-gies.

Due to the shift of the oSPP the power dissipation intothe evanescent region for the lower refractive index HTLis reduced. Therefore, also the total dissipated power Fis strongly reduced by 16.8 % for the isotropic emitterand by 15.7 % for the anisotropic emitter reducing therefractive index of the HTL. Additionally, for this micro-cavity the outcoupled power Fout is slightly increased dueto the enhanced emission into resonant modes of the mi-crocavity, as can be seen from Fig. 3 a. For a = 0.33 the

relative increase of the outcoupled power is quantified tobe 5.0 % and for a = 0.23 by 2.9 %. Hence, in total theoutcoupling efficiency is increased by 22 % for a = 0.33,resp. 18 % for a = 0.23. Thus, it follows that the major-ity of the increase is achieved by the displacement of theoSPP.

Furthermore, Fig. 3 a shows the optimized power dis-sipation of a comparable refractive index variation of theETL of devices A and C (blue). Due to the extent ofthe oSPP electromagnetic field over the complete opticalmicrocavity, the reduction of the refractive index of theETL also leads to a shift of the oSPP dispersion rela-tion. But compared to the HTL variation, in this casethe shift is much weaker due to the increased distancebetween the low refractive index medium and the surfaceof the opaque metal bottom reflector. In contrast, the in-tensity and position of the higher order WG modes (TE1,TM2, and the radiative TM and TE modes) is modified.This arises due to the fact, that the electromagnetic fieldof these modes is much more localized around the emis-sion layer and ETL. Thus, the reduced refractive index ofthe ETL has a larger influence on the WG resonance po-sitions and intensities than for a modified HTL. In sum-mary, the outcoupling efficiency is enhanced by about9.8 % for isotropic dipole distributions for a refractive in-dex change of about 12 % of the ETL, and by 8.5 % fororiented dipole distributions with an anisotropy factora = 0.23. Thus, the usage of the low refractive index

6

HTL material close to the opaque back reflector can in-fluence the oSPP two times more efficiently compared toa low refractive index ETL.

To substantiate the potential of the proposed methodFig. 3 b shows the calculated enhancement factors for ηoutin dependence of the anisotropy factor (red) and the re-fractive index of the HTL (green) and ETL (blue). Forthe variation of the anisotropy coefficient the refractiveindex of the HTL was modified from nhigh to nlow. Theoutcoupling efficiencies were obtained from the simula-tion of the optimized device geometry. For the entirerange of investigated anisotropy coefficients and enhance-ment of the optimized outcoupling efficiency is observed.Even for completely in-plane oriented dipole sources theenhancements would be about around 15 %, for a refrac-tive index nlow reduced by about 14 % to the referencevalue nhigh. This is caused by the fact that the emis-sion into SPPs for emitter materials aligned in-plane isreduced compared to isotropic emitters. However, theoSPP contribution does not vanish due to the interac-tion of p-polarized radiation with the horizontally aligneddipoles. Hence, the total dissipated power is still reducedfor the complete range of anisotropy factors. Regardlessof the alignment of the dipole distribution the increasedintensities of the higher order WG modes holds, causingan increased outcoupled power, resulting in an overallenhancement. Moreover, for a range of anisotropy coef-ficients covered by existing phosphorescent emitter ma-terials, i.e. a ∈

[0.23, 0.33

], the enhancement remains

around 20 % and arises mainly from the shift of the oSPPmode. But also for a = 0.0822, recently found in a ther-mally activated fluorescent (TADF) emitter material, theenhancement would still be about 17 %.

Furthermore, Fig. 3 b reports the enhancement factorsof the optimized outcoupling efficiencies as a functionof the average refractive index of the HTL (green) andETL (blue) of optimized first order OLED microcavities.The refractive indices are varied for device geometriesincorporating transition dipole orientations currently re-alized by phosphorescent emitters. For average refrac-tive indices smaller than the reference value of nhigh, theenhancement increases approximately linearly decreasingthe average refractive index for both the HTL and ETLmaterials. The enhancement factors for average refrac-tive indices below 1.3 are considered to be hypothetical,as materials with such low refractive indices and reason-able conductivities are hard to obtain. Nevertheless, thisfigure indicates that at very low refractive indices thelinearity is dropped and the enhancement factor will sat-urate. This is caused by the shift of the oSPP, whichalso saturates towards average refractive indices n = nair.Likewise, it can be seen that the oSPP shift is the pre-dominant cause for the enhanced outcoupling efficiencycomparing the enhancements of the HTL variations tothe ETL alteration. Hence, it is more desirable to mod-ify the refractive index of the HTL than the index of theETL, albeit a combination of both would lead close tothe high predicted efficiencies by Smith and Barnes.33

C. Optimizing thin film SPPs and powerdissipation of implemented devices

The full benefit of our optimization strategy arises (forthe top-emitting geometries) if the excitation of resonantmodes with u > uoSPP can be avoided entirely. For top-emitting OLED optical microcavities, these modes con-sist of SPPs excited at the thin semi-transparent top-contact as long as this contact contains any metal. TheSPPs of thin metal films are the coupled states of theSPPs excited at each metal/dielectric interface, wherethey are able to form a symmetric (sSPP) and an anti-symmetric (aSPP) state, regarding the z-component ofthe electrical field for a symmetric ε1/εmet/ε1 geometry.For a non-symmetric geometry ε1/εmet/ε3 the sSPP de-scribes a mode with a minimum of the z-component ofthe electrical field within the metal, and the aSPP a modewith a field envelope changing sign inside the thin metallayer. In this simple case the dispersion relation can becalculated numerically from40,46

e2 ı δ wmet =(wmet ε1 + w1 εmet) (w3 εmet + wmet ε3)

(wmet ε1 − w1 εmet) (−w3 εmet + wmet ε3),

(4)where δ denotes the thickness of the metal layer. Cal-culating the in-plane wavenumbers for the sSPP andthe aSPP from Eq. (4) or the transfer-matrix of a com-plete OLED geometry, for common metals one obtains,< (usSPP) < < (uoSPP) < < (uaSPP) for the real partsof the in-plane wavenumbers within the visible spectralrange. However, due to the strong in-plane localizationof the sSPP, this mode is irrelevant for the power dissi-pation in OLED geometries.41,47 In contrast, the aSPPcan be excited from the emissive dipoles, similar to theoSPP, although the excitation is weaker due to the largerin-plane wavenumber uaSPP.

In Fig. 4 a the in-plane wavenumbers for the aSPPmode at a fixed photon energy of Ephoton = 2.45 eV iscalculated for for decreasing metal thicknesses δ. The di-electric functions ε1 = 2.96 and ε3 = 3.53 correspondto the ETL material BPhen:Cs and the CL materialα-NPD at this energy. The plasma energy Ep in theDrude model dielectric function of the metal was fittedto Ep = 8.22 eV, representing the silver forming the thinfilm electrode. A nonlinear shift of the aSPP position asa function of the metal layer thickness δ is observed, withincreasing shifts for decreasing thicknesses δ.

For decreased metal film thicknesses δ, the nonlin-ear shift of the aSPPs in-plane wavenumber towardshigher values causes a pronounced localization of theaSPPs electromagnetic field in and around the metal film.Thus, the interaction with the emitter sources decreases.Hence, the power dissipation into the aSPP is reducedfor decreased metal film thicknesses. This shift of theaSPP and the reduced power dissipation into the res-onance is shown in Fig. 4 b. Here the displacement ofthe aSPP excitation of the complete OLED microcavityis in good agreement with the predictions from Eq. (4)

7

FIG. 4. a) Calculated shift of the aSPP mode, from Eq. (4), for various thicknesses of the metal layer. The dielectric functionsε1 = 2.96 and ε3 = 3.53 correspond to BPhen:Cs and α-NPD respectively. The dielectric function of the metal was parametrizedas a Drude model with Ep = 8.22 eV, corresponding to silver. A non-linear increasing shift of the aSPP position decreasing themetal thickness δ is observed. b) Simulated power dissipation spectra for a photon energy of 2.45 eV taking into account thecomplete OLED stack (device A) and varying the silver top contact thicknesses δ. Due to the aSPP shift the dissipated powerinto this mode is strongly reduced.

(cf. Fig. 4 a) taking into account the dielectric functionof the top electrode silver and the additional thin goldfilm. These simulations included all layers of the opti-cal microcavity of device A with fixed thicknesses, whilethe thickness of the semi-transparent silver layer was var-ied. Due to the decreased contribution of the aSPP andthe broadening of the spectral range of high transmis-sion, the outcoupling efficiency of the OLED optical mi-crocavity is enhanced. This holds for the high refrac-tive index HTL material, ηout, high, 7 nm/ηout, high, 20 nm =1.11, as well as for the low refractive index HTL,ηout, low, 7 nm/ηout, low, 20 nm = 1.12 using isotropic dipoledistributions. But also for the anisotropic distribu-tion a = 0.23, ηout, high, 7 nm/ηout, high, 20 nm = 1.10 andηout, low, 7 nm/ηout, low, 20 nm = 1.11 are obtained.

Within the experiment, such ultra-thin metal layersare realized by using a recently adapted wetting layerelectrode approach48,49, where a seed metal is used toform a closed conductive and highly transparent metalelectrode. With this approach, the combined thicknessof the smooth metal electrode does not exceed 10 nm,enabling very high outcoupling efficiencies.

The modification of the power dissipation spectrumand the improved outcoupling efficiency are shown inFig. 5. It reports the power dissipation spectra over thecomplete spectral range for Ir(ppy)3 (devices A and B)or Ir(ppy)2(acac) emitters (devices C and D). The airlight lines and the organic light lines, defined from therefractive index of the active emitter medium, are su-perimposed to the contour plots. These lines distinguishbetween the power outcoupled into air, power dissipatedinto trapped light propagating as WG modes within theorganic, and the power dissipated into evanescent exci-tations lost as heat. Decreasing the refractive index ofthe HTL material (Fig. 5 b and d) shifts the oSPP res-onance to in-plane wavenumbers below the light line ofthe organic medium. For devices A and B, the displace-ments correspond to the ones given in the discussion ofFig. 2. The shifts for the devices C and D are given

by ∆u2.6 eV = −2.19µm−1 and ∆u1.9 eV = −0.93µm−1.Compared to A and B, the smaller shift of the oSPPfor devices C and D is attributed to the reduced HTLthickness caused by the anisotropy allowing to reduce theoptimized distance between the emitter and the opaquemetal electrode. This is also reflected by the calculatedefficiencies which are ηout,A = 30.2 %, ηout,B = 37.1 %,ηout,C = 34.8 %, and ηout,D = 41.5 %. The relative en-hancement of the outcoupling efficiency of device B com-pared to A is about 1.23, whereas the relative enhance-ment for device D compared to C is 1.19. The reductionis attributed to the smaller shift of the oSPP mode res-onance together with an decreased impact of the oSPPon the total dissipated power for anisotropic emitter ma-terials, cf. Fig. 3 b. However, for both emitter materi-als, the oSPP resonance shifts to in-plane wavenumbersuoSPP < νactive over the complete spectral range, wherethe in-plane wavenumbers of the resonances correspondto internal emission angles of about θ ≈ 75◦. Hence,this resonance can be addressed by macroscopic extrac-tion structures3,34, as within the organic semiconductormaterials the oSPP is represented by a non-evanescentpropagating plane wave. Furthermore it is worth to notethat due to the oSPP shift, the excited modes are com-pressed into a smaller range of reciprocal space. Thisimplies higher enhancement of such top-emitting OLEDsif Bragg scattering structures24,35,50,51 are applied, be-cause for one reciprocal lattice constant more modes canbe scattered into the outcoupling cone. Moreover, asthe in-plane wavenumber of the oSPP is reduced, thelattice constants for which the plasmon contribution isaddressed can be increased compared to standard HTLmaterials. Having outlined the theoretical aspects of theoptimization concept, the experimental realization is dis-cussed in the next section.

8

FIG. 5. a-d) Calculated power dissipation spectra for the relevant spectral range for devices A-D. The air light line andthe organic light line separate the outcoupled from the trapped and the trapped radiation from the evanescent excitations,respectively (black solid lines). The contributions to the power dissipation originating from the oSPP resonances are highlightedas dashed lines. Comparing devices A and B to C and D the optical thickness of the microcavities is maintained while reducingthe refractive index of the HTL Thus, the contributions from WG resonances are only slightly modified. The oSPP contributionis reduced for devices C and D due to the anisotropic emitter orientation. Hence, the outcoupling efficiencies of C and D exceedthose of devices A and B. However, the enhancements due to the oSPP shift is comparable with ηout,B/ηout,A = 1.23 andηout,D/ηout,C = 1.19.

IV. EXPERIMENTAL REALIZATION ANDRESULTS

Devices A to D were fabricated from the materialsgiven in Sec. II. All materials, except the PEDOT:PSSwere deposited by thermal evaporation in an UHV tool(Kurt J. Lesker Co.) at a base pressure of 10−7–10−8 mbar. The PEDOT:PSS HTL was spin coated ontothe opaque bottom electrode adjusting the speed of rota-tion to obtain the desired thicknesses followed by a 15 minbaking step at 110 ◦C under ambient condition. To en-hance the wetting behavior of the PEDOT:PSS solutiononto the aluminum electrode, and thus to achieve homo-geneous transparent films, the substrates containing theelectrodes were exposed to an argon plasma at a basepressure of 10−2 mbar for ten minutes. In order to obtain

comparable electrodes for all devices, the plasma treat-ment was applied to all electrodes, including the subse-quent exposure to ambient conditions. Prior to the EBLdeposition under UHV conditions, all substrates weretransferred to vacuum and again heated out for 1 h at110◦ C. Directly after the last deposition step, the deviceswere encapsulated with a glass lid under nitrogen atmo-sphere. A source measure unit (Keithley 2400) and a cal-ibrated spectrometer (Instrument Systems CAS140) pro-viding the spectral radiant intensity were used to obtainthe current-voltage-luminance (I-V -L) characteristics ofall devices. The EQEs are calculated from the I-V -Land the angle-dependent emission spectra of the OLEDs,which were obtained for emission angles θ (0◦ ≤ θ ≤ 90◦)with a spectro-goniometer using a calibrated miniaturespectrometer (USB 4000, Ocean Optics Co.). The j-V -Lcharacteristics for devices A and B are given in Fig. 6 a

9

FIG. 6. a) Measured j-V -L characteristics for devices A and B. Due to the use of spin-coated PEDOT:PSS the leakage currentsare increased, and the slope of the j-V curve is decreased beyond the threshold voltage. b) External quantum efficiencies ηEQE

obtained from the measured angle dependent emission spectra of devices A and B. The ηEQE of the device B is increased bya factor of 1.19 compared to device A, in agreement with the value of 1.23 predicted by the optical simulations. c) Measuredj-V -L characteristics for devices C and D. Similar effects due to the PEDOT:PSS occur for device D as for device B. d) EQEsderived from the measurements on devices C and D. The maximum EQE is enhanced by about 1.18, again in good agreementwith the optical enhancement value of 1.19. The errors of the EQEs for devices B and D are increased due to the increasedsample-to-sample variations caused by spin coating PEDOT:PSS.

and for C and D in Fig. 6 c, where j denotes the cur-rent density from which the current can be calculatedimplying an active area of the OLED of 6.76 mm2. BothOLEDs using the PEDOT:PSS HTL exhibit an increasein the leakage current by about one order of magnitudecompared to their reference devices. We attribute this ef-fect to the ex situ deposition of PEDOT:PSS, as this pro-cess is likely to introduce impurities into the sensitive or-ganic layers. Furthermore, we observe a decreased slopeof the j-V curve beyond the threshold voltage for the lowrefractive index HTL devices. By changing the HTL ma-terial from Spiro-TTB:F6-TCNNQ to PEDOT:PSS, thecharge injection barriers from the anode to the HTL52

and from the HTL to the EBL53 are modified. Espe-cially due to the corrosivity of the aqueous PEDOT:PSSdispersion53 the formation of capacitive interlayers on theanode surface is enhanced52, introducing additional bar-

riers for electrical transport, and thus changing the j-Vcharacteristics.

Figure 6 b and d show the EQEs measured for allfour devices. The errors, derived from the sample-to-sample variation from eight measured OLEDs for eachdevice, of the EQEs for devices B and D are increasedcompared to the reference devices A and B. This is ex-plained by the spin coating process for the PEDOT:PSSHTL, because this process induces sample to sample vari-ations of the film thickness and introduces more defectson the OLEDs, as discussed earlier. For the maximaof the ηEQE the charge carrier balance γ must also bemaximized, if the maximum occurs at luminance wherenon-radiative roll-off processes can be neglected. SincePEDOT:PSS is also a doped hole transport materialwith comparable conductivity to Spiro-TTB, it is as-sumed that at the maximum γ is comparable for both

10

HTLs. However, due to this change in the electricalcharacteristics the maximum is likely to occur at dif-ferent current densities, i.e. luminance, using the PE-DOT:PSS HTL for devices B and D. Hence, insteadof comparing EQEs at fixed luminance, the maxima ofthe ηEQE will be compared for the corresponding pairsof devices. Thus, the increase in the maximum of theEQE is attributed to the enhancement originating fromthe outcoupling efficiency. We find maximized EQEs ofηEQE,A = 15.7 % and ηEQE,B = 18.7 % for devices Aand B, and ηEQE,C = 12.8 % and ηEQE,D = 15.2 % fordevices C and D. This corresponds to enhancements ofthe maximum EQEs by factors of ηEQE,B/ηEQE,A = 1.19and ηEQE,D/ηEQE,C = 1.18, in good agreement with thepredicted enhancements of the outcoupling efficienciesηout,B/ηout,A = 1.23 and ηout,D/ηout,C = 1.19. The in-creased EQEs for devices A and C for small luminanceis thus attributed to the increased leakage currents forthe PEDOT:PSS devices and the decreased charge car-rier balance γ. Furthermore, we attribute the decreasedEQEs for devices C and D, compared to the devices Aand B, to an increased sensitivity of the Ir(ppy)2(acac)emitter molecule to the deposition conditions of the an-ode in combination with the PEDOT:PSS.

V. CONCLUSIONS

In this paper, we have proposed a method to en-hance the outcoupling efficiency, and thus the EQE, ofmonochrome green top-emitting OLEDs by modifyingthe oSPP resonance of the optical microcavity. The mod-ification of the oSPP was achieved by using an HTL ontop of the opaque electrode of the OLED, with a reducedaverage refractive index compared to a state-of.the-artHTL. The simulations have predicted a linear increaseof the relative outcoupling efficiency enhancement pro-portional to the relative change of the average refractiveindex. According to these simulations, the outcouplingefficiency increases by about 20 % when using a reduction

of the average refractive index by 14 %. This enhance-ment holds for a wide range of anisotropy parameters oforganic emitter molecules. Even for completely horizon-tally aligned transition dipole moments, the enhancementwould still be about 15 % if ultra thin top contacts areused.

We have verified our results from optical simulations byfabricating top-emitting green phosphorescent OLEDs,using the isotropic emitter Ir(ppy)3 and the anisotropicemitter Ir(ppy)2(acac). In both cases we find relative en-hancement factors for the maximum of the EQEs: 1.19for Ir(ppy)3 and 1.18 for Ir(ppy)2(acac), in good agree-ment with the predictions based on the optical simula-tions.

Furthermore, we have found that the combination ofthe opaque aluminum anode and the proposed low re-fractive index HTL produces an oSPP resonance propa-gating as a non-evanescent plane wave within the layersof the optical microcavity, excluding the HTL itself andthe metal layers. Thus, this resonance becomes suitablefor further macroscopic outcoupling techniques applica-ble to top-emitting OLEDs.3,34 Besides, coherent outcou-pling techniques, such as Bragg scattering24,35, shouldbe more efficient for these types of optical microcavity,because the phase space into which modes are excited iscompressed to a more narrow range of in-plane wavenum-bers. Furthermore, the modification of the oSPP can beobtained for all photons energies within the visible spec-trum. Hence, using low refractive index HTL materialsmay be suitable to enhance the performance of whitelight top-emitting devices as well.

ACKNOWLEDGMENTS

This work was funded by the European Social Fundand the free state of Saxony through the OrganoMechan-ics project. Support from the excellence cluster cfaed isgratefully acknowledged.

1 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,B. Lussem, and K. Leo, Nature 459, 234 (2009)

2 S. Mladenovski, S. Hofmann, S. Reineke, L. Penninck,T. Verschueren, and K. Neyts, J. Appl. Phys. 109, 083114(2011)

3 M. Thomschke, S. Reineke, B. Lussem, and K. Leo, NanoLett. 12, 424 (2012)

4 C. Xiang, W. Koo, F. So, H. Sasabe, and J. Kido, LightSci. Appl. 2, 1 (2013)

5 S. Reineke, M. Thomschke, B. Lussem, and K. Leo, Rev.Mod. Phys. 85, 1245 (2013)

6 V. Bulovic, G. Gu, P. E. Burrows, S. R. Forrest, and M. E.Thompson, Nature 380, 29 (1996)

7 S. Hofmann, M. Thomschke, B. Lussem, and K. Leo, Opt.Express 19, A1250 (2011)

8 Handbook of Optoelectronics, edited by J. P. Dakin andR. G. W. Brown, Vol. 1 (Taylor & Francis Group, 2006)

9 N. K. Patel, S. Cina, and J. H. Burroughes, IEEE J. Sel.Top. Quant. Electron. 8, 346 (2002)

10 M. Pfeiffer, K. Leo, X. Zhou, J. S. Huang, M. Hofmann,A. Werner, and J. Blochwitz-Nimoth, Org. Electron. 4, 89(2003)

11 K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, Chem.Rev. 107, 1233 (2007)

12 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sib-ley, M. E. Thompson, and S. R. Forrest, Nature 395, 151(1998)

13 C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. For-rest, J. Appl. Phys. 90, 5048 (2001)

14 S.-K. Kwon, K. Thangaraju, S.-O. Kim, K. Youngjin, andY.-H. Kim, in Organic Light Emitting Diode, edited by

11

M. Mazzeo (InTech, 2010)15 K.-H. Kim, C.-K. Moon, J.-H. Lee, S.-Y. Kim, and J.-J.

Kim, Adv. Mater. 26, 3844 (2014)16 W. L. Barnes, J. Mod. Opt. 45, 661 (1998)17 M. Furno, R. Meerheim, S. Hofmann, B. Lussem, and

K. Leo, Phys. Rev. B 85, 115205 (2012)18 R. Meerheim, M. Furno, S. Hofmann, B. Lussem, and

K. Leo, Appl. Phys. Lett. 97, 253305 (2010)19 S. Hofmann, M. Thomschke, P. Freitag, M. Furno,

B. Lussem, and K. Leo, Appl. Phys. Lett. 97, 253308(2010)

20 W. Brutting, J. Frischeisen, T. D. Schmidt, B. J. Scholz,and C. Mayr, Phys. Status Solidi A 210, 44 (2013)

21 J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725(2000)

22 C. Mayr, S. Y. Lee, T. D. Schmidt, T. Yasuda, C. Adachi,and W. Brutting, Adv. Funct. Mater. 24, 5232 (2014)

23 T. D. Schmidt, D. S. Setz, M. Flammich, J. Frischeisen,D. Michaelis, B. C. Krummacher, N. Danz, andW. Brutting, Appl. Phys. Lett. 99, 163302 (2011)

24 Y.-G. Bi, J. Feng, Y.-F. Li, Y. Jin, and Y.-F. Liu, Appl.Phys. Lett. 100, 053304 (2012)

25 J. Frischeisen, B. J. Scholz, B. J. Arndt, T. D. Schmidt,R. Gehlhaar, C. Adachi, and W. Brutting, J. Photon. En-ergy 1, 011004 (2011)

26 B. J. Scholz, J. Frischeisen, A. Jaeger, D. S. Setz, T. C. G.Reusch, and W. Brutting, Opt. Express 20, A205 (2012)

27 M. Cai, T. Xiao, R. Liu, Y. Chen, R. Shinar, and J. Shinar,Appl. Phys. Lett. 99, 153303 (2011)

28 J. Choi, T.-W. Koh, S. Lee, and S. Yoo, Appl. Phys. Lett.100, 233303 (2012)

29 Y.-H. Huang, C.-Y. Lu, S.-T. Tsai, Y.-T. Tsai, C.-Y. Chen,W.-L. Tsai, C.-Y. Lin, H.-W. Chang, W.-K. Lee, M. Jiao,and C.-C. Wu, Appl. Phys. Lett. 104, 183302 (2014)

30 A. Kohnen, M. C. Gather, N. Riegel, P. Zacharias, andK. Meerholz, Appl. Phys. Lett. 91, 113501 (2007)

31 T. Tsutsui, M. Yahiro, H. Yokogawa, K. Kawano, andM. Yokoyama, Adv. Mater. 13, 1149 (2001)

32 Y. Sun and S. R. Forrest, Nat. Photonics 2, 483 (2008)33 L. H. Smith and W. L. Barnes, Org. Electron. 7, 490 (2006)34 C.-J. Yang, S.-H. Liu, H.-H. Hsieh, C.-C. Liu, T.-Y. Cho,

and C.-C. Wu, Appl. Phys. Lett. 91, 253508 (2007)

35 T. Schwab, C. Fuchs, R. Scholz, A. Zakhidov, K. Leo, andM. C. Gather, Opt. Express 22, 7524 (2014)

36 C. Murawski, P. Liehm, K. Leo, and M. C. Gather, Adv.Funct. Mater. 24, 1117 (2014)

37 P. Liehm, C. Murawski, M. Furno, B. Lussem, K. Leo, andM. C. Gather, Appl. Phys. Lett. 101, 253304 (2012)

38 W. Cao, J. Li, H. Chen, and J. Xue, J. Photon. Energy 4,040990 (2014)

39 S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, andW. Huang, Adv. Mater. 22, 5227 (2010)

40 S. A. Maier, Plasmonics: Fundamentals and Applications(Springer Science+Business Media LLC, 2007)

41 T. J. Davis, Opt. Commun. 282, 135 (2009)42 J. F. Revelli, L. W. Tutt, and B. E. Kruschwitz, Appl.

Optics 44, 3224 (2005)43 R. E. Smith, S. N. Houde-Walter, and G. W. Forbes, IEEE

J. Quantum. Elect. 28, 1520 (1992)44 H. Benisty, R. Stanley, and M. Mayer, J. Opt. Soc. Am. A

15, 1192 (1998)45 M. A. Webster, J. L. Auld, S. J. Martin, and A. B. Walker,

in Proc. SPIE, Organic Light Emitting Materials and De-vices VII, Vol. 5214, edited by Z. H. Kafafi and P. A. Lane(2004) pp. 300–309

46 H. Raether, Surface Plasmons on smooth and rough sur-faces and on gratings (Springer Berlin Heidelberg, 1986)

47 M. Furno, M. C. Gather, B. Lussem, and K. Leo, Appl.Phys. Lett. 100, 253301 (2012)

48 S. Schubert, J. Meiss, L. Muller-Meskamp, and K. Leo,Adv. Energy Mater. 3, 438 (2013)

49 T. Schwab, S. Schubert, S. Hofmann, M. Frobel, C. Fuchs,M. Thomschke, L. Muller-Meskamp, K. Leo, and M. C.Gather, Advanced Optical Materials 1, 707 (2013)

50 B. J. Matterson, J. M. Lupton, A. F. Safonov, M. G. Salt,W. L. Barnes, and I. D. W. Samuel, Adv. Mater. 13, 123(2001)

51 L. H. Smith, J. A. E. Wasey, and W. L. Barnes, Appl.Phys. Lett. 84, 2986 (2004)

52 S. K. M. Jonsson, W. R. Salaneck, and M. Fahlman, J.Mater. Res. 18, 1219 (2003)

53 T. Yamanari, T. Taima, J. Sakai, J. Tsukamoto, andY. Yoshida, Jpn. J. Appl. Phys. 49, 01AC02 (2010)