Microcavity-Free Broadband Light Outcoupling ... - ACS Publications

Dec 21, 2015 - Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials ... ABSTRACT: Flexible organic light-emitting diodes (OLEDs) h...
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Microcavity-Free Broadband Light Outcoupling Enhancement in Flexible Organic Light-Emitting Diodes with Nanostructured Transparent Metal−Dielectric Composite Electrodes Lu-Hai Xu,† Qing-Dong Ou,† Yan-Qing Li,* Yi-Bo Zhang, Xin-Dong Zhao, Heng-Yang Xiang, Jing-De Chen, Lei Zhou, Shuit-Tong Lee, and Jian-Xin Tang* Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes with superior mechanical, electrical, and optical properties. Herein, an effective nanostructured metal/dielectric composite electrode on a plastic substrate is reported by combining a quasi-random outcoupling structure for broadband and angle-independent light outcoupling of white emission with an ultrathin metal alloy film for optimum optical transparency, electrical conduction, and mechanical flexibility. The microcavity effect and surface plasmonic loss can be remarkably reduced in white flexible OLEDs, resulting in a substantial increase in the external quantum efficiency and power efficiency to 47.2% and 112.4 lm W−1. KEYWORDS: transparent composite electrode, photonic structure, light outcoupling, flexible OLED, white OLED

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enabled by phosphorescent and thermally activated delay fluoresecent materials.1,12 Instead, the external quantum efficiency (EQE), that is, the combination of IQE and light outcoupling efficiency, is limited to be ∼20% in a standard device architecture. Most of the emitted light is trapped into the ITO/substrate waveguide mode, which is a result of large mismatched refractive indices between organic layers (n ≈ 1.6− 1.8), ITO (n ≈ 1.9−2.1), and plastic substrate (n ≈ 1.5).13−15 It is therefore desirable to develop alternative TCEs for highly efficient flexible OLEDs. Various materials and structures have been proposed to function as flexible TCEs, such as carbon-based materials (e.g., nanotube,16−18 graphene,7,9 and conductive polymers19), metallic nanostructures (e.g., conducting metal oxides,20 metal nanowires,21−24 metal mesh,25,26 topological insulators,27 metal/dielectric multilayer8,28−30), and hybrid composite electrodes.31,32 Among these alternatives, a metal−dielectric

lexible organic light-emitting diodes (OLEDs) using plastic substrates have many attractive features in terms of mechanical flexibility, light weight, color gamut, and power consumption and are therefore a promising candidate for future bendable, foldable display and curved lighting applications.1−8 To realize high-performance flexible OLEDs, the key issue is how to find a way to replace the conventional indium−tin−oxide (ITO) electrode with a novel flexible transparent conductive electrode (TCE) with superior mechanical, electrical, and optical properties. ITO is the most widely used TCE in optoelectronic devices due to its excellent electrical conductivity and light transmission, but the hightemperature fabrication and the brittle nature under repeated bending condition hinders its application on flexible plastic substrates.9−11 An additional drawback of using ITO in flexible OLEDs on plastic substrates is the limited light outcoupling efficiency due to the severe trapping loss of the internally emitted photons. For highly efficient OLEDs with a carefully chosen organic emitter, the internal quantum efficiency (IQE) is now close to 100% because of the full use of singlet and triplet excitons © XXXX American Chemical Society

Received: November 19, 2015 Accepted: December 21, 2015

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substrate, which effectively eliminates the microcavity effect and thus enables the broadband and angle-independent outcoupling enhancement of the waveguide light with minimized surface plasmonic loss. As a result, the light outcoupling efficiency for white flexible OLEDs is over 2.4 times that of a conventional ITO-based device used as a comparison, resulting in a substantial increase in EQE and power efficiency to 47.2% and 112.4 lm W−1, respectively.

composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity.8,28,33 While MDCE may be an ideal candidate to replace ITO, particularly in flexible devices on plastic substrates, two key challenges should be overcome. First, forming an ultrathin metal film (≤10 nm) in a MDCE for improving the optical transmission is extremely difficult due to the dewetting problem with an isolated granular morphology, causing the adverse degradation in optical transmittance and electrical conductance with particle plasmon absorption and severe disconnection.34−36 Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics.37 To get an ultrathin metal (e.g., Ag) film with homogeneous morphology, many efforts have been made with the incorporation of a nucleation inducer (e.g., metal seeds,35 metal oxides,34 organic molecules,36 polyelectrolytes3), the construction of metallic alloy films,38 or thicker film thickness at a compromise with optical transparency. Clearly, a new strategy to minimize the microcavity effect in OLEDs using MDCE is required to make full use of this TCE for flexible applications. A potential solution is to directly integrate the photonic structures into MDCE for reducing the optical loss and keeping the color quality in flexible OLEDs. In the present work, we report an effective nanostructured MDCE (referred as NMDCE) for high-performance flexible OLEDs on low-refractive-index plastic substrates, as shown in Figure 1. The key feature of this NMDCE is that combines a

RESULTS AND DISCUSSION Design and Properties of NMDCE. Figure 1a schematically illustrates the NMDCE fabrication process on a plastic substrate and flexible OLED structure. The NMDCE was constructed with biomimetic quasi-random nanostructures through perfluoropolyether (PFPE) mold-assisted soft nanoimprint lithography (SNIL) technique (see the detailed processing description in Methods section). The PFPE molds were fabricated with assistance of an anodized aluminum oxide (AAO) template transfer and multiple mold duplication processes.39,40 As shown in Figure 1a, a UV-resin layer was drop-casted uniformly on the polyethylene terephthalate (PET) plastic substrate and embossed with the PFPE mold under UV irradiation. Then, the NMDCE was deposited on the nanostructured UV-resin/PET substrate, in which two dielectric layers of molybdenum oxide (MoO3), serving as a wetting layer and a hole-injection layer, respectively, were used to sandwich an ultrathin metallic film (as discussed below). Subsequently, the organic emitter stack and top metal electrode were deposited onto NMDCE to complete the whole device (Figure 1b). Figure 1c presents the topographic surface images of a NMDCE on a plastic substrate taken by atomic force microscopy (AFM). The surface micrographs and profiles of the PFPE mold, patterned UV-resin layer, as well as the sequentially deposited organic emitter and top metal electrode are shown in Supporting Information Figure S1. These surface morphologies indicate the successful transfer of highly compact quasi-random nanostructures from the PFPE mold to the UVresin layer through SNIL, which are followed by a subsequently deposited organic emitter stack and top electrode. The quasirandom nanostructures integrated into the NMDCE show a three-dimensional (3D) tapered morphology with a subwavelength periodicity of ∼250 nm and an average depth of 50 nm. Additionally, the fast Fourier transform (FFT) pattern of the nanostructured UV-resin layer on the PET substrate (Figure S2) exhibits the “grating vectors” in all azimuthal angles, implying the broadband and quasi-omnidirectional light diffraction capability.41,42 To deposit the NMDCE, a sophisticated strategy for the ultrathin metal film growth was employed by using a nucleation-inducing seed layer and metal co-deposition effect. Specifically, an 8 nm thick calcium-doped silver (Ca/Ag, 1:1 in weight) alloy film was fabricated on a 1 nm thick aluminum (Al) seed layer and then covered by a 1 nm thick pure Ag layer to further optimize the homogeneity and microstructure of the Ca/Ag alloy film.43 Figure 2 shows the scanning electron microscopy (SEM) images of pure Ag and Ca/Ag alloy films with a nominal thickness of 8 nm on the MoO3/substrate. It is evident that the growth of pure Ag film starts with isolated granular morphology with discrete three-dimensional (3D) islands in the Volmer−Weber growth style.29 The isolated metallic grains are known to cause the severe degradation in optical transmittance and electrical conductance due to particle

Figure 1. (a) Schematic illustrations of fabrication steps for NMDCE on a plastic substrate. (b) Flexible OLED structure with NMDCE on a plastic substrate. (c) AFM image and surface profile of the NMDCE with a pseudo period of ∼250 nm, fill factor of 0.6, and average groove depth of ∼50 nm.

novel light outcoupling structure for wavelength/angleindependent white emission with improved optical transparency, electrical conduction, and mechanical flexibility. In this NMDCE, an ultrathin calcium-doped silver alloy film (∼8 nm) was used instead of a pure metal layer for improving the film wettability and growth homogeneity, which shows high electrical conductance and low optical loss. The quasi-random nanostructures are directly integrated into the NMDCE by imprinting an index-matched UV-resin layer on a plastic B

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ITO-PET counterpart (35 Ω sq−1). Accordingly, an ultrathin metal film with reduced sheet resistance can be obtained to improve the optical transmittance and minimize the plasmon absorption effect. Optical Properties of NMDCE. To optimize the optical properties, the impact of the MoO3 thickness on the transmittance was simulated in the case of a planar MoO3/ Ca/Ag/MoO3 MDCE, in which the Ca/Ag alloy film had a thickness of 8 nm. As shown in Figure S3, the optimized MoO3 film thicknesses of the hole-injection and wetting layers can be around 10 and 25 nm, respectively, for obtaining the broadband optical transmission for white emission. It should be mentioned that using a 10 nm thickness for the MoO3 hole-injection layer allows the fine-tuning of optical transmission without altering the hole-injection property and thus the electric characteristics of OLEDs. The optical transmittance characteristics of PET substrates with various TCEs was measured and is displayed in Figure 3a. As a comparison, the corresponding transmittance spectra of bare electrodes are shown in Figure S4. It is noted that the integration of quasi-random nanostructures in NMDCE can significantly enhance the transmittance as compared to that of a planar MDCE, and the optical transmittance of the NMDCEPET substrate is even higher than that of ITO-PET substrate. For example, the NMDCE-PET substrate exhibits a comparatively uniform transmittance with an average value of ∼86% over the entire visible wavelength range. For bare NMDCE film, a high transmittance of ∼95% at 550 nm can be obtained (Figure S4), which is far beyond that of a conventional metallic film electrode. The optical properties of NMDCE match well with the spectral window of white emission in full-color display and general lighting applications. To gain further insight into the transmittance enhancement in NMDCE, energy flow (Poynting vector S) distributions were calculated for PET substrates with ITO, MDCE, and NMDCE with the excitation of an incident plane wave from air to the semi-infinite substrate.40,45 As presented in Figure 3b, ITO-

Figure 2. Comparison of surface morphology and sheet resistance of pure Ag and Ca/Ag alloy films. SEM images of (a) pure Ag film and (b) Ca/Ag alloy film with a nominal thickness of 8 nm. (c) Sheet resistance as a function of nominal film thickness of pure Ag and Ca/Ag alloy deposited on the MoO3/substrate.

plasmon absorption effect and layer disconnection.34−36,44 In contrast, a compact ultrathin Ca/Ag alloy film is formed, exhibiting a smooth and continuous morphology. The different growth behavior of the Ca/Ag alloy film is due to the prohibition of random migration and aggregation of deposited Ag atoms in the co-deposition process. The change in film morphology of the Ca/Ag alloy film is expected to enhance its electrical conductivity. Figure 2c compares the sheet resistance of pure Ag and Ca/Ag alloy films. Note that the Ca/Ag alloy film is more conductive than pure Ag film with the same thickness. The sheet resistance of an 8 nm thick Ca/Ag alloy film is only 27.1 Ω sq−1, which is even superior to that of the

Figure 3. (a) Measured optical transmittance of PET substrates with different electrodes. (b−d) Simulated energy flow diagrams of PET substrates with (b) ITO, (c) MDCE, and (d) NMDCE, where the calculated Poynting vector distributions are excited by a normally incident plane wave at 520 nm from air to various PET substrates. The small arrows depict the energy flow direction and are colored according to their relative intensity. C

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Figure 4. Normalized cross-section intensity field distributions (at 520 nm) with propagation of OLEDs on (a) ITO, (b) MDCE, and (c) NMDCE using FDTD method (Rsoft FullWave). (d) Time dependence of calculated outcoupling efficiency of OLEDs on MDCE and NMDCE relative to that of an ITO-based device.

similar behaviors can also be observed for the emissions at 480 and 620 nm (Figure S5), implying the broadband response for white emission. To elucidate the subtle interplay between internal angular power distribution and the overall light extraction enhancement, Figure 4d compares the time-integrated photon energy extracted into the air (leaky mode) for flexible OLEDs with various TCEs, which are normalized relative to the saturated extracted energy flow of ITO-based device. As shown in Figure 4d, the time-integrated photon energy extracted into air increases asymptotically with time. The calculated light outcoupling efficiency of MDCE-based OLED shows an enhancement factor of ∼1.32 compared to the ITO counterpart, while an enhancement factor of ∼2.55 is obtained for the NMDCE device. According to the literature,6 this enhanced light outcoupling is primarily ascribed to the imprinted quasirandom nanostructures, which enable additional wave vectors for the momentum-matched conservation of some confined waves (guided waves and surface plasmonic waves) into the leaky (escapable) waves. This can be further illustrated by Poynting vector simulaton as shown in Figure S6. The close interactions between waveguide and surface plasmonic modes can promote the photon flux to propagate as an extraordinary optical vortex from the device to leaky modes eventually. Particularly, both MoO3 layers can function as an optical coupling layer together with the quasi-random nanostructures, which synergistically eliminate the need for a high-refractiveindex substrate in traditional ITO-based OLEDs.2 As a result, the ITO/substrate waveguide mode in the standard planar architecture is suppressed by the gradual transition of quasirandom nanostructures in NMDCE on a plastic substrate, and thus the plasmonic loss at the organic/metal interface is reduced by the diffraction grating of the corrugated organic and electrode layers. White Flexible OLEDs Using NMDCE. To verify the charge injection and light outcoupling capability of the NMDCE, white flexible OLEDs were fabricated on PET

induced optical loss can be easily observed from the continuous decrease of light intensity inside the ITO layer (that is, arrow color variation from red to blue). Figure 3c shows that an abrupt change in optical impedance at the metal surface of planar MDCE would cause the light loss when photons travel through it. On the contrary, the energy flow traveling normally through NMDCE to the PET substrate is tuned into metallic apertures, as shown in Figure 3d, resulting in more efficient light transmission than that in the planar MDCE-PET substrate. These results match well with the measurements of optical transmittance in Figure 3a. Hence, the Poynting vector distributions provide a visualized image to understand the enhanced optical transmittance for the NMDCE-PET substrate. Optical Modeling of Flexible OLEDs. To analyze the light emission behaviors of flexible OLEDs on various TCEs, the optical modeling calculations of near-field light propagation inside the devices were conducted using 3D finite-difference time-domain (FDTD) method (see the Methods section for the detailed procedure of optical modeling). The device structures and refractive indices of the materials used for optical modeling are based on the experimentally determined parameters obtained from the AFM and ellipsometer measurements, while hexagonal closely packed nanostructures with continuously tapered profile were used for simplicity instead of randomly distributed geometry. Figure 4 presents the simulated cross-section views of the energy flux density (i.e., the Poynting vector magnitude) induced by an emitting dipole at 520 nm centered in the organic emitter, which qualitatively reveals the NMDCE influence on light manipulation in flexible OLEDs. As obtained from the theoretical calculation of intensity field distribution in Figure 4a,b, an amount of internally emitted light in planar ITO and MDCE devices will propagate mainly in the forward azimuthal angles and be confined in ITO/substrate or MDCE/substrate layers. As a comparison, more fraction of emitted light is coupled into the substrate through the grooves of quasi-random nanostructures in a NMDCE (Figure 4c), resulting in a remarkable outcoupling enhancement. The D

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Figure 5. Performance characteristics of white flexible OLEDs. (a) Current density and luminance as a function of driving voltage. (b) EQE as a function of luminance. (c) CE and PE as a function of luminance. (d) Normalized EL spectra at 1000 cd m−2. Inset is the photograph of white emission flexible OLED. (e) Normalized angle dependence of the electroluminescence intensity. Lambertian emission pattern is displayed as a dashed line. (f) CIE 1931 color coordinates of white flexible OLEDs for viewing angles between 0 and 80°.

Table 1. Efficiency Comparison for White Flexible OLEDs with Different Electrodes at a Luminance of 1000 cd m−2 and at the Maximum Valuesa

a

device structure

EQE (1000 cd m−2) [%]

ΔEQE

CE (1000 cd m−2) [cd A−1]

PE (1000 cd m−2) [lm W−1]

EQE (max) [%]

CE (max) [cd A−1]

PE (max) [lm W−1]

ITO MDCE NMDCE

18.4 26.5 45.6

1 1.44 2.48

52.1 73.4 122.3

35.4 51.7 95.1

20.2 26.8 47.2

57.2 74.5 126.4

39.5 53.3 112.4

ΔEQE refers to the EQE enhancement ratio relative to that of the ITO-based device.

substrates. The white organic emitter with complementary blue and orange phosphorescent materials consists of a blueemitting layer of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2carboxypyridyl)iridium(III) (FIrpic)-doped N,N′-dicarbazolyl3,5-benzene (mCP) and an orange-emitting layer of iridium(III)bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01)-doped mCP (see the Methods section for the detailed material information and device structure). As a comparison, white flexible OLEDs with the identical organic emitters were also fabricated on planar MDCE and ITO-PET substrates in the same batch. Figure 5a plots the current density−voltage−luminance (J− V−L) characteristics of white flexible OLEDs with different TCEs. It is obvious that the use of NMDCE and planar MDCE in flexible OLEDs can provide the better electrical properties as

compared to that on an ITO electrode. It further indicates the comparably low sheet resistance of the MDCE and NMDCE and the reduction of ohmic losses at the contact to the organic emitter in flexible OLEDs.8 On the other hand, the enhanced electrical conductance of flexible OLEDs using NMDCE is partially due to the underestimated device area with the incorporation of quasi-random nanostructures as compared to the case of planar MDCE.42 More remarkably, the luminance intensity of the NMDCE device is enhanced in comparison with its counterparts using ITO and MDCE (Figure 5a). The EQE, current efficiency (CE), and power efficiency (PE) characteristics of these flexible devices are summarized in Figure 5b,c and Table 1. Note that flexible OLED using NMDCE exhibits a substantial increase in the efficiency. For instance, the EQE, CE, and PE of white flexible OLEDs using E

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ACS Nano MDCE at a luminance of 1000 cd m−2 are increased to 26.5%, 73.4 cd A−1, and 51.7 lm W−1, respectively, which are over 1.4 times higher than the ITO device (that is 18.4%, 52.1 cd A−1, and 35.4 lm W−1). This enhancement is mainly ascribed to the absence of strong optical loss by ITO waveguide mode.15,21,25 When the NMDCE is integrated into a flexible OLED, the EQE, CE, and PE values can be further increased to 45.6%, 122.3 cd A−1 and 95.1 lm W−1, which are over 2.4 times higher the ITO device. Particularly, the maximum EQE, CE, and PE values of white flexible OLED on NMDCE are enhanced to 47.2%, 126.4 cd A−1, and 112.4 lm W−1, respectively. It is noteworthy that the efficiencies obtained for the device on NMDCE are record performance for white OLED on thin metal film electrode and also among the highest values in white flexible OLEDs reported to date without any external outcoupling structures such as a microlens array and a hemispherical lens.2,6,9,22,25 Given that the organic emitted light used in flexible OLEDs are identical, the efficiency enhancement for the use of NMDCE is attributed to the substantial outcoupling enhancement of internally emitted photons into the substrate modes. These experimentally obtained efficiency enhancements match well with the theoretically predicted energy flow diagrams in Figure 4. Further enhancement of the device efficiency on NMDCE is thus expected to be available by outcoupling the trapped light in substrate mode (the plastic substrate). In addition, flexible OLEDs on different electrodes exhibit almost identical electroluminescent (EL) spectra (Figure 5d), and the angular emission intensity of the NMDCE device is close to the Lambertian intensity distribution with a stronger side emission (Figure 5e). According to the angle-dependent EL spectra (Figure S7), the Commission Internationale d’Eclairage (CIE) color coordinates (CIE-x and CIE-y) of white flexible OLEDs on MDCE and NMDCE are summarized in Figure 5f. It is shown that the emission color of the NMDCE device is rather stable and almost constant across the entire range of viewing angles. As discussed in Figure 4, this superior emission feature of the NMDCE device is primarily ascribed to the suppression of the optical microcavity effect induced by quasi-random nanostructures. As shown in Figure S2, the quasirandom nanostructures integrated into the front transparent electrode are duplicated by the deposited organic emitter and rear metallic electrode. The resonance wavelength of the cavity is thus disturbed due to the irregular cavity lengths, leading to the broadband and quasi-omnidirectional response of light outcoupling.6,21,42 Besides the enhanced light outcoupling, flexible OLEDs using NMDCE possess another important benefit that is the improved mechanical stability under repeated bending stress. The luminance changes of the devices on different electrodes were measured as a function of the bending cycles. As shown in Figure 6, a flexible OLED on NMDCE shows an excellent mechanical stability, and the nearly constant luminance is virtually observed after 800 continuous bending cycles. However, the luminance of the ITO device decreases quickly under the same bending conditions due to cracking in the brittle ITO and thus the decrease in electrical conductivity. Such a difference indicates that each component in the NMDCE remains intact during the repeated bending process. Note that the obstacles in the device lifetime related to the use of plastic substrates have to be solved by the development of effective thin-film encapsulation techniques.

Figure 6. Performance stability of flexible OLEDs during the bending tests. The flexibility of OLEDs on NMDCE and ITO electrodes was tested by repeatedly bending the substrate to a radius of 5 mm at a constant operating voltage of 5 V. Insets show the photographs of corresponding electrodes after continuous bending.

CONCLUSIONS In conclusion, we demonstrate a new strategy to achieve a powerful transparent conductive electrode that combines a quasi-random nanostructured optical coupling layer and an ultrathin metal alloy conduction layer. This NMDCE shows the optimum electrical conductivity, optical manipulation capability, and high tolerance to mechanical bending, which is favorable for the realization of ITO-free flexible OLEDs with state-of-the-art performance on low-refractive-index plastic substrate. The angularly and spectrally independent boost in light outcoupling of white emission is obtained by minimizing the waveguide mode, metallic electrode-related microcavity effect, and surface plasmonic loss due to the integrated quasirandom outcoupling structure in the NMDCE. The resulting white flexible OLED exhibits the high enhancement in efficiency, for example, external quantum efficiency of 47.2% and power efficiency of 112.4 lm W−1. In addition, the NMDCE proposed here has a scalable manufacturing potential in large-area flexible electronic systems and will be beneficial to the realization of next-generation flexible displays and lighting with the demands on device efficiency and angular color stability. METHODS Electrode Preparation and Characterization. The MoO3/silver alloy/MoO3 MDCE was nanostructured using soft nanoimprinting lithography with an elastomeric perfluoropolyether mold, which was prepared by a series of chemical etching and multitransfer process as described in our previous reports.39,40 To transfer the quasi-random nanostructures into UV-resin layer (D10, PhiChem) drop-casted on the PET substrate, the PFPE mold was applied to emboss the 3 μm thick UV-resin layer under a constant pressure of 1.5 bar for 10 s with a UV illumination at light power intensity of 500 mJ cm−2 at a wavelength of 395 nm (Figure 1). After the UV-resin layer was imprinted, the patterned PET substrates were transferred into a highvacuum chamber (base pressure ∼ 2 × 10−6 Torr) for the successive deposition of the MoO3/silver alloy/MoO3 structure as well as organic emitter without breaking the vacuum. Optical transmittance spectra were recorded by a UV−vis/near-IR spectrophotometer (PerkinElmer Lambda 750) with an integrating sphere. Surface morphologies were characterized by AFM (Veeco MultiMode V) in tapping mode and SEM (FEI, Quanta 200FEG). The sheet resistances of pure metal and metal alloy films were determined using a four-point probe measurement stand. F

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ACS Nano Device Fabrication and Measurements. Flexible OLEDs were fabricated by subsequently depositing the organic layers and a LiF (0.5 nm)/Al (100 nm) bilayer cathode onto various plastic substrates by thermal evaporation with a shadow mask in high-vacuum chamber, in which the deposition rate and film thickness were monitored by a quartz crystal oscillator. Specifically, the white emitter consists of 40 nm thick di[4-(N,N-ditolylamino)phenyl]cyclohexane (TAPC) as the hole transport layer, 19 nm thick 8 wt % bis(3,5-difluoro-2-(2pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic):N,N′-dicarbazolyl-3,5-benzene (mCP) as the blue emission layer, 1 nm thick 6 wt % iridium(III)bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01):mCP as the yellow emission layer, and 50 nm thick 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) as the electron transport/hole-blocking layer. The effective device area was 144 mm2. Each series of flexible OLEDs on ITO, MDCE, and NMDCE was simultaneously fabricated in the same batch for the deposition of organic layers and metal cathode to ensure the consistent results. Flexible PET substrates were rinsed with methanol and deionized water and subsequently dried by nitrogen. For the cleaned ITO/PET substrate, the ITO surface was treated by UV ozone for 20 min. A 5 nm thick MoO3 layer was inserted between the ITO anode and organic emitter to enhance the hole injection. The current density−voltage−luminance (J−V−L) characteristics and electroluminescence spectra of OLED devices were measured simultaneously using a programmable source meter (Keithley model 2400) and a luminance meter/spectrometer (PhotoResearch PR655). The angledependent emission intensity was characterized by placing the devices on a rotating stage with one of the grooves parallel to the rotation axis. The bending test of flexible OLEDs was conducted by repeatedly bending the substrates to a radius of curvature of about 6 mm at a constant current density of 15 mA cm−2. All the measurements were conducted in ambient air. Theoretical Simulation. The in-plane waveguide modes and intensity distributions (including near-field Poynting vector S distribution) in OLEDs were simulated based on the rigorous electromagnetic theory through a FDTD approach (Lumerical FDTD Solutions 8.7.3) together with in-house generated codes, which were conducted with one single dipole located in the center of the emission layer, using perfectly matched boundary layers in all dimensions. The dipole orientation was assumed to be isotropic. For simplification, the quasi-random nanostructures were set with hexagonally packed nanocones with sinusoidal cross-section profile, whereas the groove depth, period (250 nm), and fill factor data were constructed as determined from the AFM images. For calculating the relative extraction efficiency, the emitting excitons were modeled as Gaussian oscillating dipole pulses with a lifetime of 20 fs and a wavelength of 520 nm. The pulse dipole sources with finite and fixed photon number were distributed randomly with an equal number of mutually orthogonal x-, y-, and z-polarizations. The frequencydependent refractive indices and film thickness of all the inorganic and organic layers used in this work were experimentally determined from the measurements with an alpha-SE spectroscopic ellipsometer (J.A. Woollam Co., Inc.) with the angle of incidence at 70°. The complex refractive index (n = 0.867 + i6.49) of the nanostructured Al cathode was used for calculating the contribution of the surface plasmon mode in the FDTD simulation.46

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions †

L.-H.X. and Q.-D.O. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from the National Basic Research Program of China (Grant No. 2014CB932600), the National Natural Science Foundation of China (Grant Nos. 91433116, 61520106012, 61522505, and 11474214), Jiangsu Science and Technology Department (Grant No. BK20140053), Bureau of Science and Technology of Suzhou Municipality (Grant Nos. SYG201525 and ZXG201422), and the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. REFERENCES (1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes From Delayed Fluorescence. Nature 2012, 492, 234−238. (2) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234−238. (3) Kang, H.; Jung, S.; Jeong, S.; Kim, G.; Lee, K. Polymer-Metal Hybrid Transparent Electrodes for Flexible Electronics. Nat. Commun. 2015, 6, 6503. (4) Sandström, A.; Dam, H. F.; Krebs, F. C.; Edman, L. Ambient Fabrication of Flexible and Large-Area Organic Light-Emitting Devices Using Slot-Die Coating. Nat. Commun. 2012, 3, 1002. (5) Aizawa, N.; Pu, Y. J.; Watanabe, M.; Chiba, T.; Ideta, K.; Toyota, N.; Igarashi, M.; Suzuri, Y.; Sasabe, H.; Kido, J. Solution-Processed Multilayer Small-Molecule Light-Emitting Devices with HighEfficiency white-Light Emission. Nat. Commun. 2014, 5, 5756. (6) Ou, Q. D.; Zhou, L.; Li, Y. Q.; Shen, S.; Chen, J. D.; Li, C.; Wang, Q. K.; Lee, S. T.; Tang, J. X. Extremely Efficient White Organic LightEmitting Diodes for General Lighting. Adv. Funct. Mater. 2014, 24, 7249−7256. (7) Han, T. H.; Lee, Y.; Choi, M. R.; Woo, S. H.; Bae, S. H.; Hong, B. H.; Ahn, J. H.; Lee, T. W. Extremely Efficient Flexible Organic LightEmitting Diodes with Modified Graphene Anode. Nat. Photonics 2012, 6, 105−110. (8) Wang, Z. B.; Helander, M. G.; Qiu, J.; Puzzo, D. P.; Greiner, M. T.; Hudson, Z. M.; Wang, S.; Liu, Z. W.; Lu, Z. H. Unlocking the Full Potential of Organic Light-Emitting Diodes on Flexible Plastic. Nat. Photonics 2011, 5, 753−757. (9) Li, N.; Oida, S.; Tulevski, G. S.; Han, S. J.; Hannon, J. B.; Sadana, D. K.; Chen, T. C. Efficient and Bright Organic Light-Emitting Diodes on Single-Layer Graphene Electrodes. Nat. Commun. 2013, 4, 2294. (10) Gaynor, W.; Burkhard, G. F.; McGehee, M. D.; Peumans, P. Smooth Nanowire/Polymer Composite Transparent Electrodes. Adv. Mater. 2011, 23, 2905−2910. (11) Cai, M.; Ye, Z.; Xiao, T.; Liu, R.; Chen, Y.; Mayer, R. W.; Biswas, R.; Ho, K. M.; Shinar, R.; Shinar, J. Extremely Efficient Indium-Tin-Oxide-Free Green Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 24, 4337−4342. (12) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in An Organic Light-Emitting Device. J. Appl. Phys. 2001, 90, 5048−5051. (13) Nowy, S.; Krummacher, B. C.; Frischeisen, J.; Reinke, N. A.; Brütting, W. Light Extraction and Optical Loss Mechanisms in Organic Light-Emitting Diodes: Influence of the Emitter Quantum Efficiency. J. Appl. Phys. 2008, 104, 123109.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07302. Basic information about the topographic images of nanostructured substrate and deposited layers, simulated and measured optical properties of the electrodes, the calculated field distributions in various devices, and angular dependence of emission spectra are also presented (PDF) G

DOI: 10.1021/acsnano.5b07302 ACS Nano XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acsnano.5b07302 ACS Nano XXXX, XXX, XXX−XXX