APPLIED PHYSICS LETTERS 97, 253308 共2010兲
Top-emitting organic light-emitting diodes: Influence of cavity design Simone Hofmann,a兲 Michael Thomschke, Patricia Freitag, Mauro Furno, Björn Lüssem, and Karl Leob兲 Institut für Angewandte Photophysik, Technische Universtät Dresden, George-Bähr-Strasse 1, 01069 Dresden, Germany
共Received 1 September 2010; accepted 3 December 2010; published online 23 December 2010兲 We report on red top-emitting organic light-emitting diode structures with higher order cavities. The emission zone is placed in the first, second, and third antinodes of the electric field in the cavity by increasing the hole transport layer thickness. Furthermore, the thicknesses of the cathode and the capping layer are varied to achieve high efficiencies. Using doped charge transport layers and a phosphorescent emitter, we reach up to 29%, 17%, and 12% external quantum efficiencies for first, second, and third order devices, respectively. An optical model is further used to analyze the angular dependent emission. © 2010 American Institute of Physics. 关doi:10.1063/1.3530447兴 Top-emitting organic light-emitting diodes 共OLEDs兲 are a promising approach to fabricate high brightness and high resolution active matrix OLED displays.1 Top-emitting OLEDs generally consist of a highly reflecting bottom anode, organic layers, and a semitransparent cathode for outcoupling.2,3 Due to the reflecting metal mirrors the device exhibits strong microcavity effects, which lead to a strong dependence of outcoupled light, color, and brightness on the device structure and viewing angles.4,5 Many improvements of top-emitting OLED performance and outcoupling have been reported recently.6–10 Different metal structures and materials have been used to achieve better reflectivity and charge injection properties.6–8 An organic capping layer has been introduced to improve light extraction.2,9 Less work has been done investigating internal light loss channels such as waveguided and surface plasmon modes.3 As the strength of coupling to surface plasmons depends on the distance of the emitter molecules to the metal contact, a promising approach is a shift of the emission zone away from the anode by increasing the hole transport layer 共HTL兲 thickness. For OLEDs in bottom-emitting configuration, the influence of cavity design has been intensely investigated using similar models as the authors propose here.11,12 Furthermore, the enhancement of light outcoupling by placing the emission zone into the second antinode of the electric field has been demonstrated.13,14 In this letter, we compare the spectral angular dependent emission characteristics and the external quantum efficiencies of red top-emitting OLEDs in the first, second, and third antinodes, which are optimized by optical simulations in respect of the maximum number of outcoupled photons. We use a monochrome OLED stack employing doped charge transport layers and an organic capping layer.15,16 The organic layers, shown in Fig. 1, are layered between the two metal contacts. For morphology reasons we apply a combination of 40 nm aluminum and 40 nm silver as anode. The cathode is a semitransparent silver layer. All devices are deposited onto glass substrates by thermal evaporation in an UHV chamber 共Kurt J. Lesker Co., Pittsburgh兲 at a base pressure of about 10−8 mbar. The a兲
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samples are encapsulated immediately after preparation under nitrogen atmosphere. We use N , N , N⬘ , N⬘-tetrakis共4-methoxyphenyl兲-benzidine 共MeOTPD兲 doped with 4 wt % Novaled dopant p-type 2 共NDP-2, Novaled AG, Dresden兲 as p-type hole injection and transport layer. This dopant has similar electric properties to 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane 共F4-TCNQ兲 and exhibits a conductivity of approximately 10−5 S / cm. The n-type electron injection and transport layer 共ETL兲 is 64 nm of 4,7-diphenyl1,10-phenanthroline 共BPhen兲 doped with Cs, which shows a conductivity comparable to the HTL. 10 nm of N , N⬘di共naphthalen-1-yl兲-N , N⬘-diphenyl-benzidine 共␣-NPD兲 and 10 nm of BPhen are implemented as electron and hole blocking layers 共EBL/HBL兲 to confine charge carriers in the emitting layer. The 20 nm thick emission layer consists of ␣-NPD doped with 10 wt % of the phosphorescent emitter iridium共III兲bis共2-methyldibenzo-关f,h兴chinoxalin兲 共acetylacetonat兲 关Ir共MDQ兲2共acac兲兴. The current-voltage-luminance characteristics of the devices are measured with a Keithley 2400 source measure unit, and the spectral radiant intensity is provided by a calibrated Instrument Systems GmbH CAS140 spectrometer. Angular-dependent electroluminescence spectra are measured with a spectrogoniometer including an optical fiber and a calibrated Ocean Optics USB4000 miniature fiber optic spectrometer.
FIG. 1. 共Color online兲 Layer design of the devices in first, second, and third cavity orders. To avoid losses to surface plasmon modes at the bottom electrode, the emission zone is shifted away by increasing the hole transport layer thickness.
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FIG. 2. 共Color online兲 Simulation of photon flux in dependence on ETL, HTL 共a兲, and top contact thickness 共b兲.
The optimization of layer thicknesses is realized using optical simulations based on classical electrodynamics.17 Quantum physical effects such as the Purcell effect are not taken into account. As ␣-NPD is a hole transport material,18 the emission profile is assumed to be a delta distribution, located at the interface between BPhen and ␣-NPD: Ir共MDQ兲2共acac兲. As illustrated in Fig. 2, the thicknesses of the HTL, ETL, and the cathode have a strong influence on the photon flux 共PF兲, i.e., the number of outcoupled photons, given by PF ⬀
2 hc
冕冕
Ie⬘共, 兲sin dd ,
共1兲
where denotes the wavelength, denotes the viewing angle, Ie⬘ denotes the simulated spectral radiant intensity, h denotes the Planck constant, and c denotes the speed of light in vacuum. The photon flux decreases with increasing order of the cavity m, given by the resonance condition m = 2nl / res, where n is the refractive index, l is the length of the cavity, and res is the resonance wavelength. Due to the redistribution of light in the device and the increase in mode density with increasing m, less photons are contributing to outcoupled modes with larger m. Thus, the number of waveguided modes is increased. From the simulated results in Fig. 2共b兲, one can readily see that the optimal cathode thickness has to be reduced to obtain a maximum photon flux for second and third order cavities. To achieve the maximum external quantum efficiency 共EQE兲 and to circumvent experimental errors, devices with a small thickness variation 共5%–10%兲 of the HTL and the cathode for first, second, and third cavity orders have been fabricated. The layer structures of the best performing devices are shown in Fig. 1 and the corresponding spectral angular dependent radiation patterns in Fig. 3. In Fig. 3, the 0° spectrum shows a peak 40–50 nm redshifted compared to the maximum of the photoluminescence spectrum of the red emitter. At larger viewing angles 共40°– 50°兲 most photons are extracted, which leads to a super-
FIG. 3. 共Color online兲 Measured angular distribution 共lines兲 of spectral radiant intensity and simulation 共dots兲 for top-emitting organic light-emitting diodes with highest external quantum efficiency at the same current density for 共a兲 first, 共b兲 second, and 共c兲 third cavity orders. The spectra are normalized to the peak of the 0° spectrum.
Lambertian emission characteristic. All spectra are fitted by adjusting layer thicknesses in careful consideration of experimental uncertainties 共⬍10%兲. The experiments 共lines兲 are in good accordance with the simulation 共dots兲. Thus, a delta distributed emission profile located at the EML/HBL interface is a reasonable assumption in our optical simulation. The narrowing of the spectra with increasing cavity order is overcome using a thinner silver cathode. Taking the measured angular dependent radiation into account, the EQE is further calculated by EQE =
2e Ihc
冕冕
Ie共, 兲sin dd ,
共2兲
where e is the elementary charge, I is the current through the OLED, and Ie is the measured spectral radiant intensity. The current-voltage characteristic is similar for all three devices 共Fig. 4, left兲. The small differences at large voltage might be due to the high resistivity of the HTL and different fabrication conditions. Figure 4 共right兲 shows the EQE in
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most efficient device with 29% external quantum efficiency has been obtained in a first order cavity. The authors thank Novaled AG, Dresden for providing their materials. This work has been funded by the BMBF under Contract No. 13N 8855, project acronym “R2FLEX.” 1
FIG. 4. 共Color online兲 Current-voltage characteristics of the devices for first, second, and third cavity orders 共left兲 and external quantum efficiency as a function of luminance 共right兲. The highest efficiency of 29% is obtained in a first cavity order.
dependence of the forward luminance. Maximum EQEs of 29%, 17%, and 12% are achieved for the first, second, and third order cavities, respectively. Compared to reported literature values for OLEDs with a red phosphorescent emitter 关12% for top-emitting 共Ref. 19兲 and 20% for bottom-emitting OLEDs 共Ref. 20兲兴, the highest EQE in this work is a considerable enhancement. Furthermore, the quantum efficiency values clearly demonstrate the potential of top-emitting OLEDs compared to the conventional bottom emission architecture. However, one major drawback is the strong variation of the emission characteristics with the viewing angle. To overcome the spectral shift, an approach comprising a diffuser, such as scattering particles in the capping layer or microlens foils, is promising.21 The strong decrease in EQE with increasing cavity order is in good agreement to the simulated photon flux 共Fig. 2兲, meaning that a large number of photons is trapped in waveguided modes and surface plasmon modes when increasing the cavity length. In summary, the number of outcoupled photons of topemitting OLEDs with a higher order cavity has been optimized through optical simulation by adjusting transport layers, as well as the cathode thickness. It is found that the optimized devices have a similar angular radiation pattern exhibiting a strong color shift with the viewing angle. The
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