Enhancing light outcoupling of organic light-emitting devices by locating emittersaround the second antinode of the reflective metal electrodeChun-Liang Lin, Ting-Yi Cho, Chih-Hao Chang, and Chung-Chih Wu Citation: Applied Physics Letters 88, 081114 (2006); doi: 10.1063/1.2178485 View online: http://dx.doi.org/10.1063/1.2178485 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Impact of preparation condition of ZnO electron transport layer on performance of hybrid organic-inorganic light-emitting diodes J. Appl. Phys. 115, 083109 (2014); 10.1063/1.4866993 Enhanced light extraction in organic light-emitting devices: Using conductive low-index layers and micropatternedindium tin oxide electrodes with optimal taper angle Appl. Phys. Lett. 100, 233303 (2012); 10.1063/1.4724306 High power efficiency tandem organic light-emitting diodes based on bulk heterojunction organic bipolar chargegeneration layer Appl. Phys. Lett. 98, 243309 (2011); 10.1063/1.3599557 Inhomogeneous luminance in organic light emitting diodes related to electrode resistivity J. Appl. Phys. 100, 114513 (2006); 10.1063/1.2390552 Carbon nanotube sheets as electrodes in organic light-emitting diodes Appl. Phys. Lett. 88, 183104 (2006); 10.1063/1.2199461

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Enhancing light outcoupling of organic light-emitting devices by locatingemitters around the second antinode of the reflective metal electrode

Chun-Liang Lin, Ting-Yi Cho, Chih-Hao Chang, and Chung-Chih Wua�

Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering, and GraduateInstitute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617,Republic of China

�Received 1 November 2005; accepted 13 January 2006; published online 24 February 2006�

Due to generally low conductivity and low carrier mobilities of organic materials, organiclight-emitting devices �OLEDs� are typically optimized for light outcoupling by locating emittersaround the first antinode of the metal electrode. In this letter, by utilizing device structurescontaining conductive doping, we investigate theoretically and experimentally the influences of thelocation of emitters relative to the metal electrode on OLED emission, and show that substantialenhancement in light outcoupling �1.2 times� or forward luminance �1.6 times� could be obtained byplacing emitters around the second antinode instead of the first antinode. Depending on the detailedcondition, the second-antinode device may also give more directed emission as often observed instrong-microcavity devices yet without suffering a color shift with viewing angles. © 2006American Institute of Physics. �DOI: 10.1063/1.2178485�

Organic light-emitting devices �OLEDs� have been thesubject of intensive investigation in recent years due to theirapplications in displays and lighting.1,2 The typical OLEDstructure usually consists of a transparent substrate �e.g. glassand plastics�, a transparent indium-tin-oxide �ITO� bottomelectrode, a highly reflective top metal electrode, and organiclayers sandwiched between electrodes. In such structures,due to the strong reflection of the metal electrode, directlyoutgoing beams of the emission interfere with the beamsreflected from the metal electrode, influencing outcoupledemission intensity.3,4 To obtain constructive interference andto optimize light extraction from the device, it roughly re-quires the locations of emitters to the metal electrode beconsistent with the antinode condition of major emissionwavelengths �i.e., the emitter-to-metal round-trip phasechange equals multiple integers of 2��.3,4 Due to generallylow conductivity and low carrier mobilities of organic mate-rials, OLEDs are typically optimized by locating emittersaround the first antinode of the metal electrode to minimizethe layer thickness and device voltage. Furthermore, placingemitters at a farther antinode by using a thicker carrier-transport layer may significantly disturb and complicate thescenario of carrier recombination �e.g., the location and dis-tribution, etc.�. Recent advances in conductive doping of or-ganic semiconductors and high-mobility materials,5–8 how-ever, may remove such constraints. In this letter, by utilizingdevice structures containing conductive doping, we investi-gate theoretically and experimentally the influences of thelocation of emitters relative to the metal electrode on OLEDemission, and show that substantial enhancement in lightoutcoupling or forward luminance of OLEDs could be ob-tained by placing emitters around the second antinode in-stead of the first antinode.

The OLED structure investigated is: glass/ITO �120 nm�/Bphen:Cs �5 wt %, 20 nm�/Bphen �20 nm�/ Alq3:C545T �1wt %, 20 nm�/�-NPD �40 nm�/m-MTDATA:F4-TCNQ �1.5

wt %, x nm�/Ag �100 nm�, which adopts the inverted struc-ture with conductive doping in carrier-transport layers forcurrent conduction and carrier injection. ITO and Ag serve asthe bottom cathode and the top anode, respectively. Otherlayers in sequence consist of 4,7-diphenyl-1,10-phenanthroline �Bphen� doped with 5 wt % Cs as then-doped electron-injection layer,6 undoped Bphen as theelectron-transport layer, tris-�8-hydroxyquinoline� aluminum�Alq3� doped with the fluorescent dye C545T as the emittinglayer,2,3 �-naphthylphenylbiphenyl diamine ��-NPD� as thehole-transport layer,9 4 ,4� ,4�-tris�3-methylphenylpheny-lamino�triphenylamine �m-MTDATA� doped with 1.5 wt %of tetrafluorotetracyano-quinodimethane �F4-TCNQ� as thep-doped hole-injection layer.5 The thickness ofm-MTDATA:F4-TCNQ is varied to adjust the distance be-tween emitters and the metal electrode. The inverted struc-ture is adopted in this work purely because of the difficultyfor our deposition system to deposit thick electron-transportlayers with n-type conductive doping using the dispenser-type Cs or Li evaporation source. Nevertheless, according toour theoretical analysis, the results for both normal and in-verted OLED structures are similar.

The optical model used for performing the analysisadopts a classical approach based on the equivalence be-tween the emission of a photon due to an electrical dipoletransition and the radiation from a classical electrical dipoleantenna,10–13 which can take into account loss due to elec-trodes. With plane-wave expansion of the dipole field, thefull-vectorial electromagnetic fields generated by a radiationdipole embedded in a layered structure is calculated, fromwhich the distribution of the radiation power into differentplane-wave modes and the far-field radiation related to emis-sion characteristics of an OLED are obtained. To modelemission characteristics of an OLED, it is assumed that theemitting layer contains an ensemble of mutually incoherentdipole radiators with distributions in dipole orientations �arandom isotropic distribution�, locations �a decaying expo-nential distribution from the �-NPD/Alq3 interface into Alq3with an exciton diffusion length of 15 nm�,2 and frequencies

a�Author to whom correspondence should be addressed; electronic mail:chungwu@cc.ee.ntu.edu.tw

APPLIED PHYSICS LETTERS 88, 081114 �2006�

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�using the photoluminescence �PL� spectrum of Alq3 :C545Tas the intrinsic spectral distribution of the dipole radiators�.Radiation characteristics of OLEDs are then obtained by av-eraging contributions over these distributions.

In general, radiation in OLEDs is coupled into four dif-ferent modes:14 radiation modes that are outcoupled as usefulemission, substrate modes that are trapped and waveguidedin the substrate, waveguided modes that are trapped andwaveguided in the high-index organic/ITO layers, andsurface-plasmon �SP� modes that are guided along theorganic/metal interface. The partition of OLED radiation intothese modes is strongly dependent on the location of emittersrelative to the metal electrode. As an instance, Fig. 1�a�shows the fractions of radiation from single-frequency�520 nm, corresponding to the peak wavelength of C545Temission� and single-position �the �-NPD/Alq3 interface�emitting dipoles coupled into different modes as a functionof its distance to the Ag electrode �by varying m-MTDATAthickness�. With emitting dipoles getting close to the metal�and thus smaller thickness of the overall structure�, ratios ofsubstrate modes and waveguided modes are low, yet most ofradiation is coupled into SP modes and OLED emission issignificantly quenched. With increasing the distance of emit-ting dipoles to the reflective metal, coupling into SP modes

drops rapidly and those into other modes first rise and thenbecome somewhat periodic with the distance. The maximaand minimum occurring in outcoupling �i.e. fraction of ra-diation modes� roughly correspond to antinodes and nodes ofthe metal electrode, respectively. Interestingly, the secondantinode gives the highest outcoupling efficiency than otherantinodes, particularly the first antinode condition that istypically implemented in optimized OLEDs. One notices thataround the second antinode, the highest efficiency isachieved because the coupling into SP modes has dropped toan almost negligible level and both substrate and wavegud-ied modes happen to be around their local minima.

With taking into consideration the complete distributionsof dipole frequencies and locations, Figs. 1�b� and 1�c� showthe calculated outcoupling efficiency and the forwardluminance, respectively, as a function of the distance to Ag�by varying m-MTDATA thickness�. In Fig. 1�c�, the forwardluminance is normalized to that of a conventionalOLED optimized around the first antinode, i.e., the devicewith the structure of glass/ITO �120 nm� /Bphen:Cs�5 wt %, 20 nm�/Bphen �20 nm� /Alq3 :C545T �1 wt %,20 nm�/�-NPD �40 nm�/m-MTDATA:F4-TCNQ �1.5 wt %,20 nm�/Ag �100 nm� �device A�. In Figs. 1�b� and 1�c�, oneobserves that locating emitters around the first node �m-MTDADA�90nm, device B� gives the least outcoupling,while locating emitters around the second antinode enhancesboth outcoupling and forward luminance. The conditions forobtaining maximal outcoupling and maximal forward lumi-nance are slightly different. The maximal forward luminance��1.6 times larger than that of device A� occurs around m-MTDADA=150 nm �device C�, at which the antinode con-dition is exactly satisfied by 520 nm. The maximal outcou-pling ��1.2 times larger than that of device A� occurs aroundm-MTDATA=170 nm �device D�, at which the antinodecondition is exactly satisfied by a wavelength �560 nm�larger than the peak wavelength of C545T.

Experiments were conducted on devices A–D for com-parison with analyses. Figure 2 shows the current-voltage�I-V� characteristics �Fig. 2�a�� and the efficiency character-istics �Figs. 2�b� and 2�c�� of these devices. Electrical prop-erties of these devices �Fig. 2�a�� are almost identical despitevery large variation in thickness of the hole-injection layer�m-MTDATA:F4-TCNQ�, indicating the effectiveness of theconductive doping in enhancing conductivity. As expectedfrom the analysis, with emitters being located around thenode, device B has substantially reduced efficiency com-pared to device A, while by locating emitters around thesecond antinode of the metal, device C exhibits significantlylarger forward luminance and slightly larger outcouplingthan the conventional first-antinode device A �25 cd/A, 5.0%photon/electron versus 14.2 cd/A, 4.5%�, and device Dshows most enhanced outcoupling �20 cd/A, 5.4% versus14.2 cd/A, 4.5% of device A�. In Figs. 1�b� and 1�c�, mea-sured efficiencies of devices A–D �symbols� are comparedwith calculated ones �lines�. Fairly good agreement is ob-tained, indicating accuracy of the simulation.

The angular dependence of electroluminescence �EL�characteristics is also an important concern in applications.Along with photoluminescence �PL� of Alq3 :C545T, Figs.3�a�–3�c� show measured �symbols� and simulated �lines� ELspectra with relative intensities at viewing angles of 0° and60° off the surface normal for devices A, C, and D, respec-tively, in which fairly good agreement is again obtained.

FIG. 1. Efficiency characteristics of the device of glass/ITO�120 nm�/Bphen:Cs �5 wt %, 20 nm�/Bphen �20 nm� /Alq3 :C545T �1 wt %,20 nm�/�-NPD �40 nm�/ m-MTDATA:F4-TCNQ �1.5 wt %, x nm�/Ag�100 nm�: �a� Calculated fractions of radiation coupled into different modes,�b� calculated �line� and measured �symbol� outcoupling efficiency, and �c�calculated �line� and measured �symbol� forward luminance �normalized tothe x=20 nm case�, as a function of the distance between emitters and thereflective Ag electrode �by varying m-MTDATA thickness�. Results of �a�are calculated with single-frequency �520 nm� and single-position ��-NPD/Alq3 interface� emitting dipoles, while those of �b� and �c� are calcu-lated with the complete dipole distributions.

081114-2 Lin et al. Appl. Phys. Lett. 88, 081114 �2006�

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Figure 3�d� compares angular distributions of the measuredEL intensity �normalized to 0° intensity� for the same threedevices. Because the antinode condition is set for a largerwavelength, EL of device D at small angles is broader thanPL of C545T. Other than that, devices C and D do not exhibitan obvious color shift with viewing angles. For device C�with highest forward luminance�, due to enhanced forwardluminance, it also exhibits more directed emission �Fig. 3�d��

as often observed in strong-microcavity devices, yet withoutsuffering color shift with viewing angles.3,13 Such character-istics may be useful for display applications, since—withoutinvolving more complicated fabrication and tuning of micro-cavity OLED structures—advantages similar to those of mi-crocavity OLEDs can be obtained by simply increasing thelayer thickness.

The findings here may also have certain implications tothe recent development of tandem OLEDs that have multipleOLED units stacked vertically in series.15–17 In the mostsimple viewpoint, one would expect proportional increase inthe luminance efficiency �i.e., cd/A� with the number ofemitting units. In few reported tandem OLEDs, one, how-ever, notices that the enhancement in cd/A efficiency couldsignificantly exceed such a proportional increase.15–17 Theresults here provide a better understanding of such unusualefficiency gain, since the emitting unit farther away from thereflective metal electrode could contribute a larger cd/A ef-ficiency or total outcoupling.

In summary, we have investigated theoretically and ex-perimentally emission characteristics of OLEDs as the func-tion of the distance between emitters and the reflective metalelectrode. It is found that locating emitters around the secondantinode of the metal electrode gives enhanced outcouplingand forward luminance, compared to the conventional first-antinode device. Depending on the detailed condition, thesecond-antinode device may also gives more directed emis-sion as often observed in strong-microcavity devices, yetwithout suffering a color shift with viewing angles.

The authors acknowledge financial support from Na-tional Science Council of Republic of China. One of theauthors �C.-L.L.� is also grateful for financial support fromMediaTek Fellowship.

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FIG. 2. �a� I-V characteristics, �b� external EL quantum efficiencies, and �c�cd/A efficiencies of devices A–D.

FIG. 3. Measured �symbols� and calculated �lines� EL spectra with relativeintensities at viewing angles of 0° and 60° for �a� device A, �b� device C,and �c� device D. �d� Polar plots of measured EL intensities �normalized tothe 0° intensity� for devices A, C, and D. In �a�–�c�, PL of Alq3 :C545T isalso shown for comparison.

081114-3 Lin et al. Appl. Phys. Lett. 88, 081114 �2006�

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