Highly Efficient White Top-Emitting Organic Light-Emitting Diodes

Letter pubs.acs.org/NanoLett

Highly Efficient White Top-Emitting Organic Light-Emitting Diodes Comprising Laminated Microlens Films Michael Thomschke,§ Sebastian Reineke,‡,§ Björn Lüssem, and Karl Leo* Institut für Angewandte Photophysik, TU Dresden, George-Bähr-Strasse 1, D-01069 Dresden, Germany S Supporting Information *

ABSTRACT: White top-emitting organic light-emitting diodes (OLEDs) attract much attention, as they are optically independent from the substrate used. While monochrome topemitting OLEDs can be designed easily to have high-emission efficiency, white light emission faces obstacles. The commonly used thin metal layers as top electrodes turn the device into a microresonator having detrimental narrow and angular dependent emission characteristics. Here we report on a novel concept to improve the color quality and efficiency of white top-emitting OLEDs. We laminate a refractive index-matched microlens film on the top-emitting device. The microlens film acts both as outcoupling-enhancing film and an integrating element, mixing the optical modes to a broadband spectrum. KEYWORDS: Organic semiconductors, outcoupling, organic light-emitting diodes, optoelectronic devices, white OLEDs, top-emission

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hite organic light-emitting diodes (OLEDs)1 with their soft high-quality light have the potential to revolutionize the lighting industry. Top-emitting OLEDs, emitting light through a transparent top electrode, have attracted much attention in the past,2 as they can be processed on a wide variety of substrates, including opaque ones. This will ultimately enable the use of low-cost, flexible, and roll-to-roll processable metal substrates. However, the realization of efficient white topemitting OLEDs faces many challenges: sputtering of transparent conductive oxides causes damage to the organic layers, which triggered substantial effort on finding improved processing methods (including suffering layers for sputtering) and alternative electrodes.3 To date, thin metal layers are the cathodes of choice.3 These metal electrodes lead to strong cavity effects in the OLED, which narrow the spectral emission and increase the viewing angle dependency of the emission, thus severely limiting the use of top-emitting OLEDs as white light source. In this Letter, we show that it is possible to realize highly efficient white top-emitting OLEDs (up to 30 lm W−1) with yet unmet broadband spectra through manipulating the emission of a dual mode cavity with a microlens foil that is laminated on the device. This concept differs from the common approach to broaden the narrow emission of the cavity using a capping layer.4 In our concept, we use the microlens foil as an integrating element to mix the photons emitted under various propagation directions to a color stable and white spectrum. In addition, the foil enables an improvement of the outcoupling efficiency (up to 50%), which is beyond the enhancement possible by a capping layer.4,5 Reaching very high color rendering indices (CRI) of up to 93, our approach makes it possible to surpass the color quality of the corresponding bottom-emitting device.6 © 2011 American Chemical Society

The optical design of white top-emitting OLEDs is far more challenging than for conventional bottom-emitting devices. The semitransparent metal contact, which still proves to be the best choice to simultaneously realize high lateral conductivity and reasonable transmittance,3 turns the device into a microresonator with completely different optical properties. In particular, the interference conditions for specific wavelengths lead to strong and spectrally selective resonances.7 Typical full width at half maximum (FWHM) values of the cavity emission, that is, the emission of a wavelength-independent “white” emitter placed into a cavity, are below 100 nm. Therefore, the design of white top-emitting OLEDs with a broad spectrum and high CRI is challenging. To some extent, the cavity emission of top-emitting OLEDs can be broadened by introducing a dielectric capping layer deposited on top of the semitransparent metal contact,4 leading to reasonable CRIs and good efficiencies.8 In addition to these obstacles, top-emitting OLEDs suffer the general problem that commonly used external outcoupling for conventional devices, for example, modification of the glass/air interface of the substrates,1,9 cannot be applied, because these techniques usually damage the layers made of soft and sensitive organic layers. In the following, we report about a novel concept to realize highly efficient white top-emitting OLEDs by combining a dual mode, thick optical cavity with a refractive index-matched microlens film (C-imide series S10-2, purchased from Optmate Corp.) that is laminated on the device. Organic materials used in OLEDs have refractive indices in the range of n = 1.7−1.8.10 Therefore, we choose a microlens array with a refractive index Received: October 24, 2011 Revised: November 23, 2011 Published: December 1, 2011 424

dx.doi.org/10.1021/nl203743p | Nano Lett. 2012, 12, 424−428

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of nμ = 1.71 (see Experimental Methods). Thus, light created in the organic layers can enter the microlens film without passing an optical interface.1 Yang et al. already reported on single emitter top-emitting OLEDs with applied microlenses.11 However, their foil was made of PDMS having a refractive index of nPDMS = 1.43, which is even lower than nglass = 1.51 of standard glass,1 so that the coupling into the lens medium is strongly suppressed. Furthermore, they deposited micrometerthick perylene layers for passivation between the devices and lenses. In our case, we selected a simplified and improved process to merge the device with the outcoupling structure. We coat both the OLED and the microlens film with 100 nm of NPB each (Supporting Information), which are laminated with a yield above 80% after coating according to the procedure shown in Figure 1a (Methods), to guarantee optimal optical contact between OLED and microlens film. Light being coupled into the microlens film can then efficiently be mixed with respect to different directions of incoming propagation, which enables one to readdress the overall OLED design. Generally, because the interference conditions for different wavelength diverge with increasing cavity length (the distance between both metal mirrors),1 the best white top-emitting OLEDs reported so far have very small cavity length (300 nm. The blue and red emitter are located almost at the same position within the cavity (Supporting Information), where the red and blue emitter fulfill the interference condition for a λ and (3/2)λ mode, respectively. The position of the yellow second unit is optimized to also meet the field antinode of a λ mode. In total, the presented device layout forms a dual mode cavity. We discuss the following four different devices in this Letter (Experimental Methods, Supporting Information): no. 1 is designed to have the highest contribution from the blue emitter, which is best suitable to discuss the dual mode character of these devices. Device nos. 2−4 with small variation in their layer structure (Table 1), are optimized for highest efficiency and best color quality. All reference devices, that is, without applied microlens film, already make use of the capping layer concept (100 nm NPB capping layer).4,5,8 Figure 2a shows the emission of device no. 1 without foil for different viewing angles. Strong spectral changes are observed with both modes (indicated as λ and (3/2)λ mode in Figure 2a) sweeping from long to short wavelengths with increasing viewing angle. In particular for the λ mode, the device shows highest radiance at a viewing angle of ∼50−60° (Figure S5 Supporting Information). Our optical device design allows the highest possible integrated efficiencies for this kind of OLEDs,13 however it is detrimental for obtaining white emission at all viewing angles. Actually, all reference device nos. 1−4 (without foil) do not emit a high quality, white spectrum at any viewing angle (Figure S2 Supporting Information). The far-field emission of the device completely changes with application of the index-matched microlens film. Here, it is important that the lamination fully merges the two NPB layers to achieve optical contact. This is realized by heating the NPB film covering the microlens foil to 55 °C, which is close to the glass transition of NPB.14 Figure 1b shows an OLED pixel with

Figure 1. (a) Schematics of the lamination process (Methods). (b) Image of a device pixel that is only partially covered with a microlens film. Arrows point to different regions with and without optical contact of the NPB layers. Top and bottom contacts are indicated. Right: zoom in of the edge of the active area as defined by overlapping metal contacts. (c) CIE 1931 color coordinates of devices nos. 1 to 4 obtained at 0° viewing angle.

the foil covering half of the active area (actually the adjacent pixel, not shown in Figure 1b, was subject to lamination). Optical contact is only made in the bright areas. Without contact, the foil still works as a diffuser or integrating element, however with much lower efficiency. Figure 2b shows the angular emission of device no. 1 with attached foil. In contrast to Figure 2a, the spectrum improves drastically. The emission is only slightly dependent on viewing angle (Figure S3, Supporting Information) and, more importantly, is changed to a broadband, high quality white light. The angular pattern 425


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Table 1. Device Stack Information and Key Performance Figures (with Foil).a device no. no. no. no.

1 2 3 4

cavity length (nm)

2nd HTL (nm)

2nd ETL (nm)

4P-NPD layer (nm)

312 320 330 342

30 100 100 100

130 70 80 90

7 5 5 7

CIE (x,y) (0.442, (0.542, (0.516, (0.472,

0.418) 0.416) 0.420) 0.430)

CRI

EQE (%)

LE (lm W−1)

72 75 86 93

21.3 [1.51] 26.8 [1.13] 22.8 [1.38] n.a.

21.3 30.1 25.2 n.a.

CIE coordinates and CRI are obtained at 4.4 mA cm−2 in forward direction. External quantum efficiency (EQE) and luminous efficacy (LE) values are measured at 1000 cd m−2 in a calibrated integrating sphere. The values in parentheses represent the improvement in EQE compared to the corresponding device without foil. n.a. = not available. See Supporting Information for device layout details.

a

transverse electric (TE) and transverse magnetic (TM) mode splitting at high angles.17 Taking the photoluminescence of the emitters incorporated into account (Figure S6 Supporting Information), it is possible to calculate the emitted spectrum as a function of viewing angle (Figure 4b left), which shows excellent agreement with the experimentally measured emission pattern of the reference device (Figure 4b right). The same calculation is performed for the laminated device no. 1 with the observer in the high refractive index (nμ = 1.71) medium of the foil, shown in Figure 4c. The white line indicates the critical angle for light escaping the high-index material to air (for a flat interface to air), as determined by total internal reflection (35.8°). At large angles, additional modes (TM0 and TE0) appear and sweep across the complete visible spectrum from low to high energy. The left side of Figure 4d shows the calculated light distribution inside the microlens foil, again with the white line indicating the fraction of light that could escape to air when facing a flat interface (nμ/nair). Application of a microlens foil to the OLED has two effects. First, light emitted at higher angles (larger than the critical angle for a flat device) can escape to air, increasing the number of outcoupled photons. Second, it integrates the spectrum over all viewing angles and makes the emitted spectrum almost viewing-angle independent. These effects lead to the measured angular distribution of device no. 1 with applied microlens foil as shown on the right side of Figure 4d. The comparison of both plots reveals the capability of the microlens array to function as efficient integrating element, turning a highly complex mode structure into a broadband emission that is almost constant as a function of viewing angle. The optical simulation is additionally used to calculate the spectral enhancement [Sμ/Sref]calc (Supporting Information), which is plotted in Figure 2c as dashed lines. These calculation show qualitatively good agreement with the experiment, being able to resemble the spectral shape of [Sμ/Sref]exp as well as its cavity length dependency. The overestimation especially in the region of high enhancement factors can be attributed to the imperfection of the microlenses in the film (Figure S7 Supporting Information), which are assumed to function as a perfectly hemispherical, large optical system in the calculation. In conclusion, we have presented an alternative to existing routes to realize highly efficient white-light emission with superior color quality from top-emitting OLEDs. Using a microlens array enables one to exploit emission from a highly complex optical system. We expect the lamination process to be cost-effective and roll-to-roll compatible. Furthermore, water and oxygen barrier functionality can be added to the film turning it into an encapsulation film at the same time. Experimental Methods. All devices are fabricated in a single-chamber high-vacuum (base pressure ∼10−8 mbar) system by thermal evaporation of organic materials and metals. Standard glass, cleaned in ultrasonic bath with acetone, ethanol,

changes to a sub-Lambertian angular distribution (Figure S5, Supporting Information). The Commission Internationale de l’Éclairage (CIE) 1931 color coordinates of the OLEDs are plotted in Figure 1c (and in the full CIE color space in Figure S4, Supporting Information), all being close to the Planckian locus, representing perfect blackbody radiators of different color temperature. Table 1 summarizes the key figures of the white OLEDs (with microlens). The CRIs of these devices are very high. Device no. 4 reaches a CRI of 93 (Figure S10, Supporting Information) being much higher than the values obtained for the bottom-emitting white OLEDs based on the same device design6 and is to our knowledge the highest value reported for top-emitting white OLEDs. The external quantum efficiencies (EQE) and luminous efficacies are plotted in Figure 3. Unfortunately, the lamination of device no. 4 only led to partial optical contact so that the efficiency of device no. 4 could not be determined. The highest efficiency is obtained for device no. 2, reaching 26.8% EQE and 30.1 lm W−1 at 1000 cd m−2. In Figure 3, the vertical lines indicate the enhancement of the laminated device at 1000 cd m−2. The improvement in EQE varies from a factor of 1.13 to 1.51, depending on the actual device layout. The enhancement induced by the foil is reduced for OLEDs whose corresponding reference device already shows high quantum efficiency. It has to be kept in mind that the reference devices do not emit white light. The spectral enhancement [Sμ/Sref]exp, shown in Figure 2c as solid lines, can be calculated by dividing the integrated spectrum of both reference (Sref) and laminated (Sμ) pixel as obtained at the same current density j10k. Interestingly, all devices show large spectral enhancements up to a 4-fold increase in between both cavity modes (Figure 2a). Thus, the microlens foil does not only mix/integrate the emission from the reference device but also couples out modes that cannot escape to air from a flat layer. The wavelength of highest enhancement shifts to the red with increasing cavity length. For a given device structure, [Sμ/Sref]exp is very reproducible (Figure S11 Supporting Information). This strong outcoupling enhancement in the green spectral region is observed in the microscope image (close-up of Figure 1b). At the edge, where bottom and top metal contacts stop overlapping, green emission (cf.[Sμ/Sref]exp in Figure 2c) is observed that fades away with increasing distance (only to the left, where the highly reflective mirror is underneath). This is the light that is originally trapped in the organic layers and now outcoupled by the microlenses. In order to understand the effects observed, we employed a comprehensive thin-film optical simulation (Supporting Information).15,16 Figure 4a shows the cavity emission of the reference device no. 1 that couples to the far-field, that is, the modes reaching the observer in air. The two cavity modes [λ and (3/2)λ] are clearly visible, which additionally show 426


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Figure 3. External quantum efficiency versus current density j for both reference and laminated devices. The positions of the vertical lines indicate the current density j1K corresponding to 1000 cd m−2 of the laminated device (Supporting Information). The numbers displayed are the improvements observed at j1k. Additionally, the luminous efficacies of the laminated devices are displayed (solid lines).

the top-emitting OLEDs is done according to the procedure depicted in Figure 1a. Directly after the fabrication of the OLEDs, without breaking the vacuum, both the top-emitting OLED and the microlens film are coated with 100 nm NPB each and transferred to a glovebox with nitrogen atmosphere afterward. The microlens film is cut and heated on a hot plate to 55 °C for 15 min (the flat, NPB-covered side pointed away from the plate).14 Without cooling down, the microlens is transferred on the top-emitting OLED so that the NPB layers are in contact. Immediately afterward, a soft, hemispherical rubber stamp (8 mm in diameter) is used to press the microlens foil to the device with a pressure of (3.0 ± 0.5) MPa with a duration of 8 s, inducing only a slight increase in leakage currents (Figure S8 Supporting Information). Lamination yield typically is above 80% based on this method. After lamination, the devices are packaged with an additional glass (having a 250 μm cavity) and epoxy resin in nitrogen atmosphere. The active area of the OLED pixels is 6.76 mm2. For best comparability, more than one pixel is prepared on each individual substrate so that one pixel can be used as reference device and another one with identical layer layout and thickness for the lamination process. Current density−voltage−luminance characteristics for all devices are measured with an automated source-measure unit 2400 (Keithley Instruments) and a calibrated spectrometer CAS 140 AT (Instrument Systems Optische Messtechnik). At current densities corresponding to 1000 (j1k) and 10 000 (j10k) cd m−2 of the laminated devices (for details of our measurement technique, refer to the Supporting Information), efficiencies are measured in a calibrated integrating sphere (Instrument Systems Optische Messtechnik). The efficiency (EQE and LE) versus current density j characteristics shown in Figure 3 are calibrated to the efficiencies obtained at j1k in the integrating sphere (both for reference and laminated devices). Slight spectral changes with increasing current density (Figure S9, Supporting Information) are not taken into account in the EQE and LE versus j plots. Note that this two-unit stacked device theoretically allows an internal quantum efficiency of 200%, as two photons can be emitted for every electron measured6 (the LE remains comparable to single unit devices). Angular-resolved measurements are performed with a custom-

Figure 2. (a) Emitted spectrum of device no. 1 without foil as a function of viewing angle obtained at 4.4 mA cm−2. The arrows indicate the shift of emitted bands observed in the blue and yellow/red region. (b) Angular emission properties of device no. 1 with laminated microlens foil. (c) Spectral outcoupling factor [Sμ/Sref]exp calculated for device nos. 1, 2, and 3 as obtained at j10k. Dashed lines are enhancements factors derived from optical simulation ([Sμ/Sref]calc). Additionally stated are the corresponding device cavity lengths. Dashed horizontal line indicates an outcoupling enhancement factor of 1.

and iso-propanol, is used as substrates for these top-emitting OLEDs. The OLED is a two-unit stacked device based on a layout we have published recently.6 Details for the materials used and the device layer stack are given in the Supporting Information (Figure S1). The microlens film (C-imide series S10-2, Optmate Corp.) has a refractive index of nμ = 1.71 and comprises 9 μm wide (pitch 10 μm), 5 μm high lenses in a hexagonal array (supplier information, Figure S7 Supporting Information). The lamination process of the microlens film on 427


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Figure 4. Mode analysis for device no. 1. (a) Calculated cavity modes that couple to the far-field without microlens foil. (b) Emission spectra in air as a function of viewing angle (left, simulation; right, measurement). (c) Cavity modes inside the microlens film. (d) Simulated spectral intensity distributions in the film (left) and measured emission in the far-field (right). White lines in (c) and (d) indicate the critical angle of total internal reflection between microlens film and air for the case of a flat interface.

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made goniometer setup at a current density of 4.4 mA cm−2. Microscope images are taken with a light-microscope (Jenaval, Carl Zeiss Jena) with backlight illumination (SXH200) and a digital camera (Canon Powershot G9) attached to the microscope objective. Optical model calculations are based on treating the emitting molecules as radiating electrical dipoles with isotropic orientation embedded in a thin film environment (Supporting Information).

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REFERENCES

(1) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. Nature 2009, 459, 234−239. (2) Bulovic, V.; Gu, G.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Nature 1996, 380, 29. (3) Chen, S. F.; Deng, L. L.; Xie, J.; Peng, L.; Xie, L. H.; Fan, Q. L.; Huang, W. Adv. Mater. 2010, 22, 5227−5239. (4) Hung, L. S.; Tang, C. W.; Mason, M. G.; Raychaudhuri, P.; Madathil, J. Appl. Phys. Lett. 2001, 78, 544−546. (5) Thomschke, M.; Nitsche, R.; Furno, M.; Leo, K. Appl. Phys. Lett. 2009, 94, 083303. (6) Rosenow, T. C.; Furno, M.; Reineke, S.; Olthof, S.; Lüssem, B.; Leo, K. J. Appl. Phys. 2010, 108, 113113. (7) Meerheim, R.; Nitsche, R.; Leo, K. Appl. Phys. Lett. 2008, 93, 043310. (8) Freitag, P.; Reineke, S.; Olthof, S.; Furno, M.; Lüssem, B.; Leo, K. Org. Electron. 2010, 11, 1676−1682. (9) Möller, S.; Forrest, S. R. J. Appl. Phys. 2002, 91, 3324−3327. (10) Greiner, H. Jpn. J. Appl. Phys., Part 1 2007, 46, 4125−4137. (11) Yang, C. J.; Liu, S. H.; Hsieh, H. H.; Liu, C. C.; Cho, T. Y.; Wu, C. C. Appl. Phys. Lett. 2007, 91, 253508. (12) Schwartz, G.; Pfeiffer, M.; Reineke, S.; Walzer, K.; Leo, K. Adv. Mater. 2007, 19, 3672−3676. (13) Hofmann, S.; Thomschke, M.; Freitag, P.; Furno, M.; Lüssem, B.; Leo, K. Appl. Phys. Lett. 2010, 97, 253308. (14) Xu, M. S.; Xu, J. B.; An, J. Appl. Phys. A 2005, 81, 1151−1156. (15) Neyts, K. A. J. Opt. Soc. Am. 1998, A 15, 962−971. (16) Furno, M.; Meerheim, R.; Thomschke, M.; Hofmann, S.; Lüssem, B.; Leo, K. Proc. of SPIE 2010, 7617, 761716. (17) Tessler, N.; Burns, S.; Becker, H.; Friend, R. H. Appl. Phys. Lett. 1997, 70, 556−558.

ASSOCIATED CONTENT

S Supporting Information *

The contents include details about the device structure, measurement procedures, and optical modeling. Further material is provided addressing electrical and spectral properties of the devices with and without microlens film attached as well as reliability issues of the lamination process. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Address ‡ Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States. Author Contributions § These authors contributed equally to this work.

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ACKNOWLEDGMENTS The research forming the basis for this letter was funded by the German Bundesministerium für Bildung und Forschung with the support code 13N11060 (project acronym “R2FLEX”). The authors would like to thank M. Furno for the development of the optical simulation tool and T. Günther and A. Bunk for technical assistance throughout the device preparation. 428


Highly Efficient White Top-Emitting Organic Light-Emitting Diodes

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