light extraction from oleds for lighting applications through light scattering

Organic Electronics 8 (2007) 293–299

www.elsevier.com/locate/orgel

Light extraction from OLEDs for lighting applicationsthrough light scattering

R. Bathelt a, D. Buchhauser b,c, C. Garditz a,b, R. Paetzold b,*, P. Wellmann a

a Department of Material Science VI, University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germanyb Siemens AG, CT MM1, Guenther Scharowsky Street 1, 91052 Erlangen, Germany

c Department of Experimental Physics, University of Freiberg, Leipzigerstrasse 23, 09599 Freiberg, Germany

Received 18 July 2006; received in revised form 11 October 2006; accepted 28 November 2006Available online 22 December 2006

Abstract

OLEDs are gaining increasing interest for lighting applications as large-area light sources. Their lifetime and overallefficiency can be increased by the optimization of light outcoupling.

In this article, scattering films on the viewer’s side of the substrate to increase the outcoupling efficiency of OLEDs forlighting applications are examined. Experimental results show that the increase of outcoupling efficiency is dependent fromthe absolute number of scattering particles in the matrix.

Theoretical considerations expect an increase of outcoupling efficiency of up to 70%. Experimental results yield anincrease of outcoupling efficiency of about 22%. A software model based on raytracing to simulate light outcoupling fromOLEDs through a scattering layer is introduced to gain a deeper understanding of the light extraction mechanism. Wefound that the scattering anisotropy factor g determined using the Henyey–Greenstein phase function and the effectiveabsorbance of the OLED device have strong impact on outcoupling efficiency.� 2006 Elsevier B.V. All rights reserved.

PACS: 78.20.Bh; 85.60.Jb; 78.35.+c; 72.80.Le

Keywords: OLED; Lighting; Outcoupling enhancement; Light scattering; Simulation; Raytracing

1. Introduction

Organic light emitting diodes (OLEDs) are poten-tial candidates for next-generation flatpanel-displayapplications. With increasing efficiency and lifetime,lighting applications are becoming another focus of

1566-1199/$ - see front matter � 2006 Elsevier B.V. All rights reserved

doi:10.1016/j.orgel.2006.11.003

* Corresponding author. Tel.: +49 9131 7 33611.E-mail address: [email protected] (R. Paetzold).

interest [1]. For the latter, optimised light extractionefficiency is of particular interest. The typical designof a polymer-based OLED is made up of two layersof active polymer between a metallic cathode and atransparent anode on glass substrate. These activelayers consist of a hole-transporting layer and thelight emitting polymer (LEP). The light from theLEP must go through the device layers to be emitted.Assuming isotropic emission from the LEP andusing Snell’s Law with typical values for indices of

.

mailto:[email protected]

294 R. Bathelt et al. / Organic Electronics 8 (2007) 293–299

refraction of the device layers one can calculate that,due to reflections at the interfaces within the OLED,only 30% of the light produced is emitted.

In the past years an extensive discussion tookplace concerning surface modification [2–4] toimprove light outcoupling from organic and inor-ganic LEDs using ordered structures at the interfaceto air like e.g. micro spheres [5] or lenses [6]. At theinterface to the active layers e.g. mesa structures [7]or distributed bragg-reflectors [8] were examined.Concerning lighting applications these approachesoften have a serious effect on the emitted spectrumor radiation distribution. Another approach is theuse of random structures to achieve light scatteringlike sanding of the surface to air [3,9] or applicationof an aerogel [10]. These approaches have a merelycorrective effect on the emission concerning lightingpurposes. In [14] light extraction through a scatter-ing film has been examined using the radiative trans-port theory.

In this article, we introduce a computer simula-tion program based on a statistical approach andray-tracing to gain a deeper understanding of theoutcoupling mechanism of light from an OLEDthrough a scattering layer.

Therefore freestanding scattering films were fabri-cated and attached to white emitting OLEDs in orderto address the issue of light extraction efficiency.Light reflected at the internal interfaces is lost dueto absorption in the OLED stack, respective emis-sion from the substrate edges [11]. By changing pho-ton trajectory randomly, all substrate light modes arerecycled by scattering films. Hence, photons reflectedat the interface to air may be redirected to the OLEDsurface, increasing the total light extraction proba-bility and OLED efficiency. Using scattering filmson the viewer’s side of the substrate, solely glass orpolymer foil substrate modes may be extracted tothe viewer. Layer modes (modes confined withinthe emitting material), however, are not accessiblein this approach. From a theoretical point of view,an increase of light outcoupling efficiency of up to70% may be achievable in this approach.

The paper is organized in the following way: first,we address the model, the parameters and simplifi-cations taken into account in the creation of thesimulation program. In the experimental part wedescribe the determination of the model inputparameters, foil production and the measurementof the impact of scattering films on OLED emission.In the last parts we compare the simulation resultswith the experimental results.

2. Simulation

The ray-tracing computer simulation tool uses aMonte-Carlo component to simulate light outcou-pling from an OLED by means of a scattering film.

Using the MIE-Theory [12], one can calculate thescattering anisotropy exactly using particle size andthe indices of refraction of particle and matrix.However, this process may be difficult. The Hen-yey–Greenstein

P HGðcos#Þ ¼ 1� g2

4p 1þ g2 � 2g cos#ð Þ32

phase function [13], is an approach on light scatter-ing, which is widespread because of simplificationsas to which the properties of the scattering systemare summarized to the scattering anisotropy factorg. This factor is experimentally accessible. It is de-fined in between �1 and 1. The values ‘‘�1’’, ‘‘0’’,‘‘1’’ mean mirror-like backscattering, isotropic scat-tering, and no change in photon trajectory respec-tively. Only g factors >0, which imply forwardscattering, are examined in this article, as non-reflecting particles are used.

The Henyey–Greenstein distribution of a scatter-ing event is being modeled statistically through aMonte-Carlo approach. The resulting angular dis-tribution of scattered photons depends on the g-fac-tor. The frequency of scattering depends on themean free pathway between two scattering eventsand film thickness.

The software models an OLED consisting ofactive polymer and ITO, summarized to one layerwith index of refraction nLEP of 1.7, substrate glasswith ng of 1.5 and attached scattering layer.

Fig. 1 sketches typical paths of photons in thissimulated OLED. Path 1 shows a photon confinedwithin the LEP by reflection at the interface LEP/substrate and at the mirror like cathode. It is finallyrecognized as side emission from the LEP. Path 2shows a photon that is reflected back into the deviceat the interface to air after some scattering events.Path 3 shows a photon that is finally extracted tothe viewer after some reflections at the interface toair and scattering events where it has been redi-rected back to the surface. Path 4 shows a photonthat is scattered back into the OLED stack whereit may be absorbed or reflected back to the scatter-ing layer depending on the effective absorbance ofthe OLED. Combinations of these sketched pathsare possible as well. Back-reflection at the interface

Fig. 1. Sketch of typical ways of photons in the simulated OLED. A description is given in the text.

R. Bathelt et al. / Organic Electronics 8 (2007) 293–299 295

to air is the major loss mechanism at a low concen-tration of scattering particles, while backscatteringis the major loss mechanism at high concentrations.This indicates that an optimum exists between theseextremes.

As proposed elsewhere [14], the following sixinput parameters are taken into account in our sim-ulation tool. (1) The absorption coefficient of thematrix material of the scattering film a. (2) The meanfree pathway between two scattering events Xf,which depends on particle concentration and size.(3) Thickness of the scattering film x. (4) The indexof refraction of the matrix material nm is requiredto calculate the change of propagation directionthrough refraction at the layer interfaces and thecritical angle for reflection using Snell’s law. (5)The probability of backscattered photons to beabsorbed in the active layers of the OLED or atthe metallic cathode is summarized in the parameterof effective absorbance Ae. (6) The scattering anisot-

ropy factor g of the scattering system describes the‘‘strength’’ of an average scattering event followingthe Henyey–Greenstein phase function.

Nine simplifications are applied to the model. (1)The angular distribution of the emission of the LEPis assumed to be isotropic. However, interferenceeffects can be incorporated by choosing suitableemission characteristics. (2) A truncation criterionlimits the lateral size of the OLED. Photons cros-sing this limit are always being recognized as wave-guided in the respective layer. (3) All interfaces areassumed to be even. (4) LEP and ITO are summa-rized to be one layer with nLEP ¼ 1:7. (5) TheOLED emission is assumed to be azimuthally sym-metric (2D case). (6) Dispersion and (7) interferenceeffects are not taken into account. (8) The scatteringparticles are assumed to be randomly distributed inthe matrix and (9) equal in size.

As result of the simulation, angularly resolveddistributions of photons in the respective layers


are obtained and the amount of photons that areabsorbed in, or emitted from the side of the respec-tive layer.

3. Experiment

Scattering films were produced by dispersing thin-walled hollow polymer micro spheres in an acrylate-based resist, which is used as matrix material. Allsamples were prepared via doctor blading of the mix-ture on a glass substrate. After UV curing the film isremoved from the substrate. The absorption coeffi-cient of the scattering layer was determined by mea-suring the transmission of unloaded films withdifferent thickness using a Perkin-Elmer Lambda 35spectrophotometer. The absorption coefficient a isfound to be (0:35� 0:04) cm�1 at 550 nm. The indexof refraction of the matrix material was measuredusing wavelength resolved ellipsometry. The indexof refraction of the matrix material nm is determinedto vary between 1.51 at 400 nm up to 1.47 at 780 nm.Please note: the refractive index close to the one ofglass (1.5) ensures a good transfer of photons fromglass to the matrix. The average particle size of(10� 2) lm was determined via measuring the diam-eter of a number of scattering particles in the matrixusing an optical microscope. As the differences ofrefractive indices of polymers are small in most cases,it can be assumed that the main scattering processtakes place at the interface between particle shelland particle hollow core rather than at the interfaceof matrix and polymer particle sphere. The effectiveabsorption Ae of the OLED used was measured usingthe reflectivity accessory of the Lambda 35 spectro-photometer and found to be (20� 10)%. Light scat-tering was determined by transilluminating afreestanding scattering film with a collimated beamof light emitted from a white inorganic LED. Theangular distribution of the scattered light in forwarddirection was determined angularly resolved using aPR 650 spectrophotometer from Spectra ScanTM.The scattering anisotropy factor g is then calculatedvia fit of the Henyey–Greenstein Phase function onthe measurement. The g-factor of the system is foundto be (0:85� 0:05).

To investigate the effect of scattering films onoutcoupling, films with differing weight loadings ofpolymer spheres and differing thickness were pre-pared and successively applied to a white emittingpolymer-based OLED (120 nm PEDOT, 80 nmLEP, active area about 2 cm2) that exhibited lam-bertian emission characteristics.

The spectrum of the OLED without a scatteringlayer applied was measured angularly resolved usingthe PR 650 spectrophotometer. Directly afterwardsa scattering layer was applied using optical gel andthe measurement was repeated. The ratio of theintensity with and without scattering layer yields aparameter referred to as outcoupling enhancementin the following.

The average of the outcoupling enhancementover the visible spectrum and all angles leads tothe overall outcoupling enhancement. The scatter-ing foil was removed after the measurement andthe intensity of the OLED was measured again toexclude fluctuations of the OLED intensity duringthe measurement. Integrating sphere measurementsare used as references.

4. Experimental results

Scattering films with particle concentrations of 5,15, 25, 40 and 50 wt.% and thickness between 40and 300 lm were produced and their impact on out-coupling was measured as described above.

Fig. 2 shows a decreasing overall enhancementwith increasing film thickness for high particle con-centrations (50 wt.%), while the situation is viceversa for low particle concentrations (5 wt.%). Amaximum enhancement of about 22% is observed.It shifts to lower film thickness with increasing par-ticle concentration. This result indicates a corre-spondence between particle concentration and filmthickness. If a matrix system with negligible absorp-tion is used, the scattering ability gets independentfrom the scattering film thickness (Fig. 2b). It onlydepends from the number of particles per area, i.e.if a thin film with high load or a thick film withlow load is used, same results can be obtained ifthe absolute number of particles per area is identi-cal. Consequently, it does not matter if the optimumoutcoupling efficiency is achieved by adjusting theconcentration of scattering particles, or throughadjusting the pathway in the film by thickness orby observation angle.

Another result is concerning the microcavity-effect, which is common in OLEDs based on broad-band emitters. With increasing observation angle itusually exhibits a blue-shift based on destructiveinterference of the red part of the spectrum [15].

Fig. 3 shows the CIE 1931 y coordinate of thecolor of the emitted light for different observationangles. Without scattering film it oscillates. Withapplied scattering film and increasing particle load

Fig. 3. Impact of scattering films on the angle-dependentmicrocavity-effect. Through angular intermixing of differentmodes the microcavity-effect is reduced and almost vanishes ata sufficiently high particle load. The CIE 1931 x - coordinate isnot displayed as it exhibits only minor angle-dependency. Plottedlines are guidelines for the eye only.

Fig. 4. Squares (solid line) denote the experimentally determinedoverall outcoupling enhancement, as described in part III.Maximum enhancement is found at a particle load of about30 wt.% for scattering films with 50 lm thickness. Open circles(dashed line) denote simulation results for the respective inputparameters. Plotted lines are guidelines for the eye only.

Fig. 2. (a) Shows the overall outcoupling enhancement plottedover film thickness for different weight loadings. (b) Shows theseresults plotted over the normalized amount of particles per area.A description is given in the text. Plotted lines are guidelines forthe eye only.


the oscillation is reduced and it almost vanishes at aparticle load of 40 wt.% through increasing angularintermixing of modes with different wavelength.

5. Simulation results

The simulation was executed with the input val-ues stated in part III. Fig. 4 shows the simulationresults using open circles and – using squares – themeasured outcoupling enhancement for scatteringfoils with 50 lm thickness and different particle con-centrations. Measurement and simulation exhibit amaximum outcoupling efficiency of about 22% ata particle load of about 35 wt.%.

The angular resolved light output of a whiteemitting OLED with attached scattering films, one

with high, and one with a rather low amount of par-ticles is examined in Fig. 5.

Symbols denote the measured values of the nor-malized light intensity emitted in the respectiveangle. Lines denote the simulation results. At highparticle concentrations (open circles, 25 wt.%), thehighest output is found in forward direction andbackscattering increases to higher angles. As theoptimum is exceeded in forward direction, out-coupling efficiency decreases to higher angles as

Fig. 5. Symbols denote measured values of the normalized lightintensity in the respective angle. Lines denote the simulationresults for the corresponding input values of thickness andparticle load. Additionally, a lambertian emission characteristic(dashed) is shown.

Fig. 6. Impact of effective OLED absorbance on outcouplingefficiency. The plot of 20% denotes the simulation result for themeasured value of OLED absorbance. Plotted lines are guidelinesfor the eye only.

Fig. 7. Impact of the g-factor on outcoupling efficiency. The plotfor the value 0.9 denotes the simulation result for the measuredvalue. Plotted lines are guidelines for the eye only.


indicated by the weak undershoot from lambertian.At low amounts of scattering particles, the behavioris vice versa. Around relatively low amounts(5 wt.%) of particles (squares), a stronger overshootfrom lambertian behavior is observable. This is dueto more optimal numbers of scattering events athigher angles because of a more optimal way lengthin the film than in forward direction. These devia-tions from lambertian disappear towards very lowamounts of particles (<1 wt.%, 50 lm) and are gen-erally relatively weak at high amounts of particles(>25 wt.%, 50 lm).

The good agreement between simulation andexperimental results in Figs. 4 and 5 shows clearlythat our raytracing algorithm simulates outcouplingefficiency and angular distribution of the emissioncorrectly quite well.

To find optimal film compositions, simulationruns with varied input parameters were performed(Figs. 6 and 7). The most relevant input parametersare thus identified by the software model. We foundthat the effective absorbance Ae and the g factorhave the strongest impact on outcoupling efficiency.

During the variation of one parameter, all otherparameters are fixed at their measured value. Thethickness of the scattering film is set to 50 lm.

Fig. 6 shows that the effective absorbance of theOLED Ae has high impact on outcoupling. Themeasured value is 20%. Reduction of OLED absor-bance leads to a significantly increased maximumoutcoupling enhancement. At low OLED absor-bance, the maximum efficiency is found at a low

particle concentration. A possible reason is, that inthis case back-reflection is a much weaker lossmechanism than absorption in the scattering film.With constant a, film absorbance is reduced througha lower way length in it because of lower amounts ofscattering particles.

Fig. 7 shows the impact of the g factor on out-coupling. We found values between 0.6 and 0.8 tobe optimal for high outcoupling efficiency. Withincreasing g factor the maximum moves to higherparticle loads, which might be due to a weaker redi-rection effect of a single scattering event at a high g

factor.


With all other parameters at their measuredvalue, the simulation predicts an increase of themaximum outcoupling enhancement from 22% ata g factor of 0.9 to about 40%, which is consistentwith [14], at a g factor of 0.7. As the g factor canbe tuned more easily by choosing a suitable systemof matrix and particle, this parameter is the bestapproach on outcoupling, rather than the deviceparameter of effective absorbance Ae.

6. Summary

In this contribution we have explained the differ-ences between theoretical estimation and experi-mental result in the attempt to improve OLEDoutcoupling efficiency through scattering. We havegained a deeper understanding of the outcouplingmechanism through a computer simulation basedon raytracing. After experimental determination ofthe input parameters we found that the outcouplingenhancement depends mainly on the absolute num-ber of scattering particles per area in the respectivesystem of matrix material and particle. It could bedemonstrated that the simulation tool describes out-coupling enhancement and angular distribution ofthe emission correctly to a great extend.

Finally, light scattering offers inherent advanta-ges like a more constant color over all observationangles and encryption of small defects [11].

References

[1] A. Bergh, G. Crawford, A. Duggal, R. Haitz, The promise andchallenge of solid-state lighting, Phys. Today 54 (2001) 12.

[2] Bergh, Saul, Method of manufacturing surface texturedhigh-efficiency radiating devices and devices therefrom, US.Pat. 3,739,217, 1973.

[3] M. Scheffel, A. Hunze ,J. Birnstock, J. Blassing, W. Rogler,G. Wittmann, A. Winnacker, Enhanced light extraction bysubstrate modification of organic electroluminescent devicesin: Proceedings of Eur. Conf. Org. Elect. Related Phenom-ena’01, 158 2001.

[4] Windisch, Heremans, Knobloch, Kiesel, Dohler, Dutta,Borghs, Light emitting diodes with 31% external quantumefficiency by outcoupling of lateral waveguide modes, Appl.Phys. Lett. (1999) 74, p. 2256–2258.

[5] T. Yamasaki, K. Sumioka, T. Tsutsui, Organic light emittingdevice with an ordered monolayer of silica microspheres as ascattering medium, APL 76 (2000) 1243–1245.

[6] S. Moeller, S.R. Forrest, Improved light out-coupling inorganic light emitting diodes employing ordered microlensarrays, J. Appl. Phys. 91 (2002) 3324–3327.

[7] G. Gu, D.Z. Garbuzov, P.E. Burrows, S. Venkatesh, S.R.Forrest, M.F. Thomson, High-external-quantum-efficiencyorganic light emitting devices, Opt. Lett. 22 (1997) 396–398.

[8] J.M. Lupton, B.J. Matterson, I.D.W. Samuel, M.J. Jory,W.L. Barnes, Bragg scattering from periodically microstruc-tured light emitting diodes, APL 77 (2000) 3340–3342.

[9] M.H. Lu, C.F. Madison, J.C. Sturm, Tech. Dig. Int.Electron. Dev. Meeting 607 (2000).

[10] T. Tsutsui, M. Yahiro, H. Yokogawa, K. Kawano, M.Yokoyama, Doubling coupling-out efficiency in organic lightemitting devices using a thin silica aerogel layer, Adv. Mater.13 (2001) 1149–1152.

[11] C. Gaerditz, R. Paetzold, D. Buchhauser, R. Bathelt, G.Gieres, C. Tschamber, A. Hunze, K. Heuser, A. Winnacker,J. Amelung, D. Kunze, OLED lighting based on whitebroadband polymer emitters, Proc. SPIE 5937 (2005)59370L.

[12] Bergmann-Schaefer, Lehrbuch der Experimentalphysik, Bd.III ,,Optik‘‘, 9. Auflage, de Gruyter, Berlin, 1993.

[13] L.G. Henyey, J.L. Greenstein, Diffuse radiation in theGalaxy, Astrophys. J. 93 (1941) 70–83.

[14] J.J. Shiang, T.J. Faircloth, A.R. Duggal, ExperimentalDemonstration of increased organic light emitting deviceoutput via volumetric light scattering, J. Appl. Phys. 95(2004) 2889–2895.

[15] H. Becker, S.E. Burns, N. Tessler, R.H. Friend, Role ofoptical properties of metallic mirrors in microcavity struc-tures, J. Appl. Phys. 81 (1997) 2825–2829.

light extraction from oleds for lighting applications through light scattering

Documents