Highly Enhanced Light-Outcoupling Efficiency in ITO-Free Organic

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Highly Enhanced Light Outcoupling Efficiency in ITOFree Organic Light-Emitting Diodes Using Surface Nanostructure Embedded High Refractive Index Polymers Dong Woo Kim, Joo Won Han, Kwon Taek Lim, and Yong Hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15345 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Applied Materials & Interfaces

Highly Enhanced Light Outcoupling Efficiency in ITO-Free Organic Light-Emitting Diodes Using Surface Nanostructure Embedded High Refractive Index Polymers Dong Woo Kim†, Joo Won Han†, Kwon Taek Lim*, and Yong Hyun Kim* Department of Display Engineering, Pukyong National University, Busan 48513, Republic of Korea

†

These authors contributed equally to this work

*

Corresponding authors.

E-mail: [email protected], [email protected] Tel. : +82-51-629-6418; Fax : +82-51-629-6408

KEYWORDS : organic light-emitting diodes, high refractive index polymers, light extraction, silver nanowires, transparent electrodes

Abstract We develop the internal light outcoupling system (HRLOC) based on the high refractive index polyimide (PI) and metal oxide nanoparticles for organic light-emitting diodes (OLEDs) with silver nanowires (AgNWs). The spontaneously formed nano-bump structures, high refractive index, and light scattering properties of HRLOC significantly enhance the light extraction efficiency of OLEDs. Not only do the outcoupling structures improve the light

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extraction efficiency, but remarkably enhance the electrical properties of OLEDs. HRLOC leads to the regular and smooth formation of AgNWs, resulting in the improvement of the electrical properties of devices by preventing electrical shorts and leakage currents. The power efficiency of the AgNW-based OLEDs with PI is improved by a factor of 1.31 compared to the reference device with indium tin oxide (ITO) transparent electrode at a luminance of 20,000 cd/m2. The efficiency is further improved by incorporating TiO2 NPs into the PI matrix by a factor of 1.69. To our knowledge, the optically and electrically enhanced OLEDs show one of the highest enhancement factors reported for ITO-free OLEDs with internal outcoupling structures. In addition, the outcoupling structures are solution processable, thermally stable, and can be scaled up to 200×200 mm2 for large-area applications. We believe that the light outcoupling structures developed here have great potential for efficient, low-cost, and flexible ITO-free OLEDs.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted much interest in recent years owing to their promising applications in large-area flat panel displays and solid-state lighting.1,2 Significant efforts for improvement of the efficiency of OLEDs realize almost 100% of internal quantum efficiency (IQE) of devices by using phosphorescent materials.3,4 However, the mismatch of refractive indices between substrate, transparent electrodes, and organic layers causes a total internal reflection at interfaces, which limits the external quantum efficiency (EQE) of conventional OLEDs to only 20-30 %.1,5 Accordingly, numerous studies have been reported to improve the light outcoupling efficiency of OLEDs. To extract light confined in substrate (substrate mode), microlens arrays and textured substrates are widely reported.3,6,7 For the extraction of light trapped at transparent electrodes and organic layers (waveguide mode), light scattering layers,8–10 corrugated structures,11,12 and high refractive index substrate3,13 have been studied. However, the light outcoupling structures to reduce 2 Environment ACS Paragon Plus

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waveguide mode have a large difficulty because they readily damage soft organic layers and suffer from expensive and complex fabrication processes. Another issue raised in the practical applications of OLEDs is the development of alternative transparent electrodes. Indium tin oxide (ITO) is the most widely used transparent electrode material for OLEDs. However, the drawbakcs of ITO such as intrinsic brittleness, the high material cost, and the need for elevated processing temperature boost to search alternative transparent electrodes.14,15 Conductive polymers,9,16,17 metal nanowires,18–20 graphene,21,22 metal grid,23,24 thin metals,25–27 and carbon nanotubes28,29 have been extensively investigated to replace ITO as transparent electrodes for optoelectronic devices. Among them, silver nanowire (AgNW) has received much attention because of its high transmittance, low resistance, excellent mechanical flexibility, and low-cost solution processibility.30 However, the high surface roughness and irregular distribution of AgNWs can lead to significant leakage currents and electrical shorts of commonly-used organic devices. In order to solve such problems of AgNWs, various techniques such as cross-point welding,31 introduction of planarization layers,32,33 and transfer methods34,35 have been studied. Here we report the high performance internal light outcoupling system (HRLOC) which simultaneously improves the light outcoupling efficiency and electrical properties of ITO-free OLEDs based on AgNWs. HRLOC consists of light scattering TiO2 nanoparticles (TiO2 NPs) dispersed in the high refractive index polyimide (PI) matrix. The spontaneously formed nanobump structures, high refractive index, and light scattering properties of HRLOC greatly suppress the waveguide mode of device, resulting in the remarkably enhanced outcoupling efficiency of OLEDs. Moreover, HRLOC effectively assists the regular and smooth formation of AgNWs in long-range order, which improves the electrical properties of devices by preventing electrical shorts and leakage currents. Thus, the performance of AgNW-based OLEDs employing HRLOC is significantly improved compared to that of control devices 3 Environment ACS Paragon Plus


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with ITO. The power efficiency of OLEDs with HRLOC/AgNW is improved by a factor of 1.69 compared to the ITO reference device. To our knowledge, the optically and electrically enhanced OLEDs show one of the highest enhancement factors reported for ITO-free OLEDs with internal outcoupling structures. We believe that HRLOCs combined with AgNWs developed here can be greatly promising outcoupling structures for high performance, lowcost ITO-free OLEDs.

2. EXPERIMENTAL SECTION Preparation of HRLOCs: 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and bis(3-aminophenyl) sulfone (3DDS) were purchased from Tokyo Chemical Industry. Titanium isopropoxide (TTIP, >97%), acetic anhydride (AA, >99%), chlorobenzene (CB, 99.8%) and methanol (MeOH) were purchased from Sigma Aldrich, and other reagents were used without any purification. For synthesis of PI, 7.0 g (15.7 mmol) of 6FDA and 3.9 g (15.7 mmol) of 3DDS were dissolved in 43 ml of imethylacetamide (DMAc) placed in an ice bath at 0 °C under N2 atmosphere. After 1 hr, the ice bath was removed and the solution was kept stirring at room temperature for 23 hrs. Afterwards, 15.4 g (15 mmol) of AA and 11.93 g (15 mmol) of pyridine were added into the solution at 40 °C and the mixture was stirred for 24 hrs. After the chemical imidization reaction, the solution was diluted with acetone and precipitated into excess amount of methanol. The filtered solid was washed several times with methanol and vacuum dried overnight at 50 °C. For preparation of HRLOC, 1 g of PI was dissolved in 4 ml of N-methyl-2-pyrrolidone (NMP) and the solution was further diluted with 3.4 ml of tetrahydrofuran (THF). The TiO2 NPs were synthesized according to the previous report.36 5 g (20 mmol) of TTIP and 1 g (16 mmol) of acetic acid (99.5%) were mixed for 10 min and then the mixture was injected into 100 ml of water with vigorous stirring for 30 min. After stirring at room temperature for 12 hrs, 1.44 ml of nitric acid (65%) was added to the aqueous solution. Subsequently, the solution was heated at 80-85 °C for 2 hrs to produce a stable 4 Environment ACS Paragon Plus

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bluish titania sol. 5 ml of as-prepared titania sol was poured into 40 ml of acetone to precipitate TiO2 NPs (5-10 nm), which were isolated as a white wet cake by centrifugation. 1g of TiO2 NPs was added into 2 ml of NMP and the dispersion was ultrasonicated until it became transparent dispersion. The TiO2 NP dispersion was mixed with the PI solution, and the mixture was ultrasonicated for 1 hr to obtain a transparent pale yellow high refractive index PI solution. 10 wt% of HRLOC or PI solutions were spin-coated on glass substrates (1.5×1.5 cm2) for 30 sec at 1500 rpm, and then the samples were dried on a hot plate at 80 °C. The drying temperature was gradually increased from 80 °C to 130 °C at intervals of 10 min, and the samples were allowed to stand at 130 °C for 10 min to form polymer films on the glass substrate.

Preparation of AgNW electrodes: The aqueous 0.3 wt% AgNW aqueous dispersion (Nano pyxis, average diameter~35 nm, average length~20 µm) was spin-coated on the substrates pretreated with UV / ozone at 3000 rpm for 30 sec. The coated AgNWs were dried on a hot plate at 100 °C for 10 min. The same procedure was repeated one more time to form the highly conductive AgNW networks. The sheet resistance of the AgNW electrode was measured by the van der Pauw method. The transmittance of films was examined by UV-vis spectrophotometer (Optizen pop, MECASYS). The refractive index of films was characterized by ellipsometry (M-2000D, J.A.Woollam). The morphologies of films were examined by using field emission scanning electron microscope (FESEM, JEM-2100F, JEOL) and atomic force microscopy (AFM, Icon-PT-PLUS, BRUKER) in the tapping mode.

Fabrication and Characterization of OLEDs: We fabricated solution-processed OLEDs because of its great potential for low-cost, simple, and fast production.37–39 All steps were carried out in air. As a hole transport layer (HTL), PEDOT:PSS (AI4083, Heraeus) was spun onto the indium tin oxide (ITO) or silver nanowire deposited substrates (1500 rpm, 30 s), 5 Environment ACS Paragon Plus


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followed by annealing at 120 °C. The emission layer (EML) consisted of a mixture of poly(9vinylcarbazole) (PVK) and 4,4’-N,N’-dicarbazole-biphenyl (CBP) co-hosts and the green phosphorescent dopant tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3). The PVK:CBP:Ir(mppy)3 mixture of 0.45:0.45:0.1 by weight was dissolved in CB at a concentration of 10 mg/ml. The EML solution was spin-coated onto the PEDOT:PSS film at a spin speed of 800 rpm and the film was annealed at 100 °C. 2,2′,2"-(1,3,5-Benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) dissolved in methanol as an electron transport layer (ETL) was spin-coated on the EML. The thicknesses of HTL, EML, and ETL were 35, 61, 20 nm, respectively. Finally, LiF (1nm) and Al (100 nm) were thermally evaporated in order on TPBi. The active area of the device was about 6 mm2.

3. RESULTS AND DISCUSSION Transparent PI is prepared by a step growth polymerization of 4,4’(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and bis(3-aminophenyl) sulfone (3DDS). The structure of transparent PI which possess trifluoromethyl (-CF3) and sulfone (O=S=O) functional groups in the polymer backbone is shown in Fig. 1a. Such functional groups with the elements of the high electronegativity give not only a transparent property but also properties that can be dissolved in various organic solvents by reducing the charge transfer effect in the PI backbone.40 HRLOCs are prepared with the dispersion of TiO2 NPs in PI where the TiO2 NPs act as light scattering centers and the refractive index enhancement agent. The refractive index of HRLOCs is 1.77 by adding TiO2 NPs into the PI matrix (n~1.62). The thickness of the PI matrix is around 1 µm. The TiO2 nano-bump structures formed on top of HRLOC effectively reduces total internal reflection in devices. The combined light extraction effects of HRLOC enable for excellent outcoupling enhancement of OLEDs. It is notable that HRLOCs can be readily scaled up. The HRLOC is successfully prepared on large area glass substrate (200×200 mm2) by bar-coating process, exhibiting the 6 Environment ACS Paragon Plus

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highly transparent and homogeneous film (see Fig. 1b). In addition, the high thermal resistance of PI enables for the elevated temperature processing such as the deposition of ITO (see Fig. S2 in Supporting Information). In addition to the outcoupling enhancement of HRLOCs, we employ AgNWs on HRLOCs to realize ITO-free OLEDs. The scanning electron microscope (SEM) images of the AgNW networks formed on the glass substrate, PI layer, and HRLOC are shown in Fig. 2, respectively. The AgNW network prepared on the bare glass substrate represents huge aggregated structures. In contrast, the AgNWs formed on PI and HRLOC demonstrate wellspread homogeneous networks without aggregations because the PI chain structure effectively enhances the adhesion property of AgNWs. It should be noted that the ring structures in the PI backbone cause a strong hydrophobic property. Therefore, the strong van der Walls force induced by hydrophobic PI results in the enhanced interaction between AgNWs and PI layers. 41,42

The improved adhesion property effectively holds each single wire on the PI layer during

solvent drying, which prevents an aggregation of AgNW networks. It should be noted that, due to the nature of AgNW networks, a certain portion of flat surfaces (void regions in between nanowires) in glass/AgNW or PI/AgNW are directly contacted to organic layers of OLEDs, which can lead to a total internal reflection and increases waveguide mode in OLEDs. As can be seen in Fig. 2f and 2h, HRLOCs incorporated with TiO2 NPs provide the nano-bump surface of the PI matrix therefore the flat region in between AgNW networks can be reduced and suppress the waveguide mode in OLEDs. The characteristics of the roughened nano-bump structures in HRLOCs are confirmed more clearly in the atomic force microscopy (AFM) images as shown in Fig. 3. In HRLOC, the nano-bump structures induced by TiO2 NPs are formed in the void region of AgNW networks. The diameter of nanostructure is measured to be in the range of 30 to 80 nm. Despite of nanostructures, the root-mean-square (RMS) roughness of HRLOC is only 7 Environment ACS Paragon Plus


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2.43 nm even in a large-area of 30×30 µm2. The roughness value is in an acceptable range not to cause electrical shorts and leakage currents of organic devices. The transmittance spectra of the AgNWs formed on various substrates are shown in Fig. 4. Table 1 summarizes the sheet resistance and transmittance of the AgNWs. The reference ITO has a low sheet resistance of 10 ohm/sq and a transmittance of 83.5 % while AgNW networks formed on glass, PI layer, and HRLOC reveal higher sheet resistances of around 45-48 ohm/sq with higher transmittances of around 86-87 % at a wavelength of 400-800 nm. To investigate the effect of outcoupling systems and transparent electrodes on the performance of OLEDs, various sublayer/transparent electrodes such as reference ITO (Device_ITO), AgNW (Device_AgNW), AgNW on PI layer (Device_PI/AgNW), and AgNW on HRLOC (Device_HRLOC/AgNW) are integrated into solution-processed green OLEDs. Fig. 5a depicts the structure of devices. The current density-voltage characteristics of the OLEDs are shown in Fig. 5b. The current densities of devices based on AgNWs are higher than that of Device_ITO. Especially, the notably high current density of Device_AgNW is observed because of the leakage current induced by the aggregation of AgNWs on the glass substrate. Device_PI/AgNW and Device_HRLOC/AgNW exhibit the reduced current level compared to Device_AgNW owing to the uniform coating of AgNW networks without aggregation of networks, which is assisted by the PI matrix. The higher current of Device_HRLOC/AgNW compared to that of Device_PI/AgNW is likely due to the TiO2 NPinduced nanostructures which result in the enhanced electric field in devices by partially reducing the thickness of the emissive layer.11,43,44 Figure 5c shows the luminance-voltage characteristics of devices. OLEDs based on AgNWs show higher luminance than that of Device_ITO owing to the higher current level of devices. It is observed that Device_HRLOC/AgNW gives the higher luminance than other devices because of the effects of light outcoupling and electrical enhancement. The light scattering effects of TiO2 NPs and 8 Environment ACS Paragon Plus

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the nano-bump structures effectively extract photons in waveguide mode. Furthermore, the well-formed AgNW networks enhance the generation of photons by preventing carrier loss from the leakage current. Figure 5d shows the current efficiency and the power efficiency of OLEDs with respect to luminance and resulting parameters are listed in Table 2. The current efficiencies of Device_ITO, Device_AgNW, Device_PI/AgNW, and Device_HRLOC/AgNW are 14.18 cd/A, 8.19 cd/A, 16.95 cd/A, and 18.37 cd/A, respectively, at a brightness of 20,000 cd/m2. As expected, Device_AgNW gives the low device performance mainly caused by the high leakage current flowing in devices. In contrast, Device_PI/AgNW exhibits the improved current efficiency of a factor of about 1.20 compared to Device_ITO. This improvement is attributed to the minimized leakage current due to the homogeneous formation of AgNWs on the PI layer and the additional light extraction effect caused by the refractive index matching. The PI layer having a modest refractive index of about 1.6 contributes to reducing the total internal reflection at the interface between the PI layer and the organic layer (n~1.7-1.8). In contrast, Device_ITO suffers from a large total internal reflection because of the refractive index mismatch at the interface of glass substrate (n~1.5) and ITO (n~1.8). The current efficiency of Device_HRLOC/AgNW shows the highest current efficiency which is increased by a factor of about 1.30 compared to Device_ITO. The TiO2 NPs introduced in HRLOC result in an increase in the refractive index of the PI matrix layer, which corresponds to about 1.77. Although the high refractive index of Device_HRLOC/AgNW may cause a total internal reflection between HRLOC/AgNW and PEDOT:PSS, the performance of OLED is improved. This result demonstrate that the nano-bump structures of HRLOC exposed in the void region of AgNW networks as well as light scattering effects by TiO2 NPs effectively change the angle of rays, resulting in the improvement of light extraction. It is expected that HRLOC can reveal a greater enhancement of outcoupling efficiency if it is employed into high



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performance OLEDs with phosphorescent hole transport layer (HTL) materials (n~1.8) where low refractive index PEDOT:PSS HTL layers are not employed. Figure 5e presents the power efficiency of OLEDs. The power efficiency of Device_HRLOC/AgNW is improved by a factor of 1.69, compared to Device_ITO, corresponding to 5.55 lm/W at a brightness of 20,000 cd/m2. The remarkably enhanced power efficiency is due to the enhanced light outcoupling effect by the nano-bump structures together with the reduced operating voltage and the improved hole injection properties resulting from the uniform formation of AgNW networks on HRLOCs, which prevents carrier loss in devices. The external quantum efficiency (EQE) of Device_HRLOC/AgNW is 6.92 %, which is increased by a factor of 1.26 compared to Device_ITO, at a brightness of 20,000 cd/m2. The normalized electroluminescent spectra for all devices are almost equal as shown in Fig 5f, meaning that PI layers and HRLOCs provide the weak wavelength dependence while they enhance the light outcoupling of OLEDs.

4. CONCLUSIONS In conclusion, we have successfully developed the high performance internal light outcoupling systems based on the high refractive index PI matrix incorporated with TiO2 NPs. In addition to the greatly improved light outcoupling efficiency, the electrical properties of OLEDs are simultaneously improved by the PI layer due to the uniform formation of AgNW networks without aggregation, spikes, or irregular distribution. The power efficiency of the Device_PI/AgNW is improved by a factor of 1.31 compared to Device_ITO by reduced waveguide mode. Overall, Device_HRLOC/AgNW with TiO2 NPs shows the dramatic improvement of power efficiency, by a factor of 1.69. Furthermore, HRLOC is solution processable and can be scaled up to 200×200 mm2, which is highly promising for large-area applications. Due to the excellent thermal resistance of PI, elevated temperature processing is also possible such as the deposition of ITO. It should be highlighted that HRLOC has a great 10 Environment ACS Paragon Plus

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potential in high performance OLEDs having vacuum-processed high refractive index HTLs by an excellent refractive index matching. The performance can be further improved by optimization processes for material contents and the size of nanoparticles for HRLOC to maximize the light outcoupling effect, which will be further studied. We believe that the combination of HRLOC and AgNW networks based on the high refractive index PI and metal oxide nanoparticles will contribute to the development of high performance, low-cost ITOfree OLEDs.

Supporting Information Schematic illustration of 6FDA-3DDS polyimide synthesis process, TGA data of polyimide, photograph and TEM image of organic capped TiO2 nanoparticle sol, ellipsometry result of PI and HRLOC, current efficiency and power efficiency versus current density, list of enhancement factors of OLEDs with internal light scattering structures (PDF)

Acknowledgements This work was supported by the Technology Innovation Program (No. 10052923, the development of light extraction complex substrate for curved-OLED lighting with high-CRI) funded by the Ministry of Trade, Industry & Energy, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016R1C1B2012490).

References (1) (2) (3)

Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes: Status and Perspective. Rev. Mod. Phys. 2013, 85 (3), 1245–1293. Gather, M. C.; Köhnen, A.; Meerholz, K. White Organic Light-Emitting Diodes. Adv. Mater. 2011, 23 (2), 233–248. Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459 (7244), 234–238. 11 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(4)

(5) (6) (7) (8)

(9)

(10)

(11)

(12)

(13)

(14) (15)

(16)

(17)

(18)

(19) (20)

Schwartz, G.; Reineke, S.; Rosenow, T. C.; Walzer, K.; Leo, K. Triplet Harvesting in Hybrid White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2009, 19 (9), 1319– 1333. Gather, M. C.; Reineke, S. Recent Advances in Light Outcoupling from White Organic Light-Emitting Diodes. J. Photonics Energy 2015, 5 (1), 57607. Sun, Y.; Forrest, S. R. Organic Light Emitting Devices with Enhanced Outcoupling via Microlenses Fabricated by Imprint Lithography. J. Appl. Phys. 2006, 100 (7), 1–7. Moeller, S.; Forrest, S. R. Improved Light out-Coupling in Organic Light Emitting Diodes Employing Ordered Microlens Arrays. J. Appl. Phys. 2002, 91 (5), 3324–3327. Chang, H.-W.; Lee, J.; Hofmann, S.; Hyun Kim, Y.; Müller-Meskamp, L.; Lüssem, B.; Wu, C.-C.; Leo, K.; Gather, M. C. Nano-Particle Based Scattering Layers for Optical Efficiency Enhancement of Organic Light-Emitting Diodes and Organic Solar Cells. J. Appl. Phys. 2013, 113 (20), 204502. Kim, Y. H.; Lee, J.; Kim, W. M.; Fuchs, C.; Hofmann, S.; Chang, H.-W.; Gather, M. C.; Müller-Meskamp, L.; Leo, K. We Want Our Photons Back: Simple Nanostructures for White Organic Light-Emitting Diode Outcoupling. Adv. Funct. Mater. 2014, 24, 2553–2559. Li, L.; Liang, J.; Chou, S.-Y.; Zhu, X.; Niu, X.; ZhibinYu; Pei, Q. A Solution Processed Flexible Nanocomposite Electrode with Efficient Light Extraction for Organic Light Emitting Diodes. Sci. Rep. 2014, 4, 4307. Koo, W. H.; Jeong, S. M.; Araoka, F.; Ishikawa, K.; Nishimura, S.; Toyooka, T.; Takezoe, H. Light Extraction from Organic Light-Emitting Diodes Enhanced by Spontaneously Formed Buckles. Nat. Photonics 2010, 4 (4), 222–226. Koo, W. H.; Youn, W.; Zhu, P.; Li, X.-H.; Tansu, N.; So, F. Light Extraction of Organic Light Emitting Diodes by Defective Hexagonal-Close-Packed Array. Adv. Funct. Mater. 2012, 22 (16), 3454–3459. Kim, E.; Cho, H.; Kim, K.; Koh, T.-W.; Chung, J.; Lee, J.; Park, Y.; Yoo, S. A Facile Route to Efficient, Low-Cost Flexible Organic Light-Emitting Diodes: Utilizing the High Refractive Index and Built-In Scattering Properties of Industrial-Grade PEN Substrates. Adv. Mater. 2015, 27 (9), 1624–1631. Ellmer, K. Past Achievements and Future Challenges in the Development of Optically Transparent Electrodes. Nat. Photonics 2012, 6 (12), 809–817. Han, T.-H.; Jeong, S.-H.; Lee, Y.; Seo, H.-K.; Kwon, S.-J.; Park, M.-H.; Lee, T.-W. Flexible Transparent Electrodes for Organic Light-Emitting Diodes. J. Inf. Disp. 2015, 316 (March 2015), 1–14. Kim, Y. H.; Lee, J.; Hofmann, S.; Gather, M. C.; Müller-Meskamp, L.; Leo, K. Achieving High Efficiency and Improved Stability in ITO-Free Transparent Organic Light-Emitting Diodes with Conductive Polymer Electrodes. Adv. Funct. Mater. 2013, 23, 3763–3769. Kim, Y. H.; Sachse, C.; Machala, M. L.; May, C.; Müller-Meskamp, L.; Leo, K. Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal PostTreatment for ITO-Free Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 1076–1081. Lian, L.; Dong, D.; Yang, S.; Wei, B.; He, G. Highly Conductive and Uniform Alginate/Silver Nanowire Composite Transparent Electrode by Room Temperature Solution Processing for Organic Light Emitting Diode. ACS Appl. Mater. Interfaces 2017, 9 (13), 11811–11818. Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solution-Processed Metal Nanowire Mesh Transparent Electrodes. Nano Lett. 2008, 8 (2), 689–692. Gaynor, W.; Hofmann, S.; Christoforo, M. G.; Sachse, C.; Mehra, S.; Salleo, A.; McGehee, M. D.; Gather, M. C.; Lüssem, B.; Müller-Meskamp, L.; Peumans, P.; Leo,


Page 12 of 20

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(21)

(22)

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(34)

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K. Color in the Corners: ITO-Free White OLEDs with Angular Color Stability. Adv. Mater. 2013, 25 (29), 4006–4013. Wu, J.; Becerril, H. a.; Bao, Z.; Liu, Z.; Chen, Y.; Peumans, P. Organic Solar Cells with Solution-Processed Graphene Transparent Electrodes. Appl. Phys. Lett. 2008, 92 (26), 263302. Lee, B. H.; Lee, J.-H.; Kahng, Y. H.; Kim, N.; Kim, Y. J.; Lee, J.; Lee, T.; Lee, K. Graphene-Conducting Polymer Hybrid Transparent Electrodes for Efficient Organic Optoelectronic Devices. Adv. Funct. Mater. 2014, 24 (13), 1847–1856. Kim, Y. H.; Müller-Meskamp, L.; Leo, K. Ultratransparent Polymer/Semitransparent Silver Grid Hybrid Electrodes for Small-Molecule Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1401822. Kim, Y. H.; Schubert, S.; Timmreck, R.; Müller-Meskamp, L.; Leo, K. Collecting the Electrons on N-Doped Fullerene C 60 Transparent Conductors for All-VacuumDeposited Small-Molecule Organic Solar Cells. Adv. Energy Mater. 2013, 3 (12), 1551–1556. Meiss, J.; Riede, M. K.; Leo, K. Optimizing the Morphology of Metal Multilayer Films for Indium Tin Oxide (ITO)-Free Inverted Organic Solar Cells. J. Appl. Phys. 2009, 105 (6), 63108. Schubert, S.; Müller-Meskamp, L.; Leo, K. Unusually High Optical Transmission in Ca:Ag Blend Films: High-Performance Top Electrodes for Efficient Organic Solar Cells. Adv. Funct. Mater. 2014, 24 (42), 6668–6676. Schubert, S.; Hermenau, M.; Meiss, J.; Müller-Meskamp, L.; Leo, K. Oxide Sandwiched Metal Thin-Film Electrodes for Long-Term Stable Organic Solar Cells. Adv. Funct. Mater. 2012, 22 (23), 4993–4999. Williams, C. D.; Robles, R. O.; Zhang, M.; Li, S.; Baughman, R. H.; Zakhidov, A. a. Multiwalled Carbon Nanotube Sheets as Transparent Electrodes in High Brightness Organic Light-Emitting Diodes. Appl. Phys. Lett. 2008, 93 (18), 183506. Rowell, M. W.; Topinka, M. a.; McGehee, M. D.; Prall, H.-J.; Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Organic Solar Cells with Carbon Nanotube Network Electrodes. Appl. Phys. Lett. 2006, 88 (23), 233506. Sannicolo, T.; Lagrange, M.; Cabos, A.; Celle, C.; Simonato, J. P.; Bellet, D. Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: A Review. Small 2016, 12 (44), 6052–6075. Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Greyson Christoforo, M.; Cui, Y.; McGehee, M. D.; Brongersma, M. L. Self-Limited Plasmonic Welding of Silver Nanowire Junctions. Nat. Mater. 2012, 11 (3), 241–249. Gaynor, W.; Burkhard, G. F.; McGehee, M. D.; Peumans, P. Smooth Nanowire/Polymer Composite Transparent Electrodes. Adv. Mater. 2011, 23 (26), 2905–2910. Bormann, L.; Selzer, F.; Weiß, N.; Kneppe, D.; Leo, K.; Müller-Meskamp, L. Doped Hole Transport Layers Processed from Solution: Planarization and Bridging the Voids in Noncontinuous Silver Nanowire Electrodes. Org. Electron. physics, Mater. Appl. 2016, 28, 163–171. Nam, S.; Song, M.; Kim, D.-H.; Cho, B.; Lee, H. M.; Kwon, J.-D.; Park, S.-G.; Nam, K.-S.; Jeong, Y.; Kwon, S.-H.; Park, Y. C.; Jin, S.-H.; Kang, J.-W.; Jo, S.; Kim, C. S. Ultrasmooth, Extremely Deformable and Shape Recoverable Ag Nanowire Embedded Transparent Electrode. Sci. Rep. 2014, 4, 4788. Zeng, X.-Y.; Zhang, Q.-K.; Yu, R.-M.; Lu, C.-Z. A New Transparent Conductor: Silver Nanowire Film Buried at the Surface of a Transparent Polymer. Adv. Mater. 2010, 22 (40), 4484–4488.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(36)

(37)

(38)

(39)

(40)

(41)

(42)

(43)

(44)

Liu, B. T.; Tang, S. J.; Yu, Y. Y.; Lin, S. H. High-Refractive-Index Polymer/inorganic Hybrid Films Containing High TiO2 Contents. Colloids Surfaces A Physicochem. Eng. Asp. 2011, 377 (1–3), 138–143. Xie, G.; Luo, J.; Huang, M.; Chen, T.; Wu, K.; Gong, S.; Yang, C. Inheriting the Characteristics of TADF Small Molecule by Side-Chain Engineering Strategy to Enable Bluish-Green Polymers with High PLQYs up to 74% and External Quantum Efficiency over 16% in Light-Emitting Diodes. Adv. Mater. 2017, 29 (11). Lee, S. Y.; Yasuda, T.; Komiyama, H.; Lee, J.; Adachi, C. Thermally Activated Delayed Fluorescence Polymers for Efficient Solution-Processed Organic LightEmitting Diodes. Adv. Mater. 2016, 28 (21), 4019–4024. Tang, M. C.; Lee, C. H.; Lai, S. L.; Ng, M.; Chan, M. Y.; Yam, V. W. W. Versatile Design Strategy for Highly Luminescent Vacuum-Evaporable and SolutionProcessable Tridentate Gold(III) Complexes with Monoaryl Auxiliary Ligands and Their Applications for Phosphorescent Organic Light Emitting Devices. J. Am. Chem. Soc. 2017, 139 (27), 9341–9349. Liaw, D. J.; Wang, K. L.; Huang, Y. C.; Lee, K. R.; Lai, J. Y.; Ha, C. S. Advanced Polyimide Materials: Syntheses, Physical Properties and Applications. Prog. Polym. Sci. 2012, 37 (7), 907–974. Moreno, I.; Navascues, N.; Arruebo, M.; Irusta, S.; Santamaria, J. Facile Preparation of Transparent and Conductive Polymer Films Based on Silver Nanowire/polycarbonate Nanocomposites. Nanotechnology 2013, 24 (27), 275603.1-275603.11. Selzer, F.; Weiß, N.; Bormann, L.; Sachse, C.; Gaponik, N.; Müller-Meskamp, L.; Eychmüller, A.; Leo, K. Highly Conductive Silver Nanowire Networks by Organic Matrix Assisted Low-Temperature Fusing. Org. Electron. 2014, 15 (12), 3818–3824. Fujita, M.; Ueno, T.; Ishihara, K.; Asano, T.; Noda, S.; Ohata, H.; Tsuji, T.; Nakada, H.; Shimoji, N. Reduction of Operating Voltage in Organic Light-Emitting Diode by Corrugated Photonic Crystal Structure. Appl. Phys. Lett. 2004, 85 (23), 5769–5771. Lee, C.; Kang, D. J.; Kang, H.; Kim, T.; Park, J.; Lee, J.; Yoo, S.; Kim, B. J. Simultaneously Enhancing Light Extraction and Device Stability of Organic LightEmitting Diodes Using a Corrugated Polymer Nanosphere Templated PEDOT:PSS Layer. Adv. Energy Mater. 2014, 4 (8), 1301345.


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Figures

Figure 1. (a) The chemical structure of transparent polyimide. (b) Photograph of HRLOC prepared on a large-area glass substrate (200× 200 mm2).



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Figure 2. SEM images of (a, d) AgNW, (b, e) PI/AgNW, and (c, f) HRLOC/AgNW. Schematic illustration of (g) AgNW, (h) PI/AgNW, and (i) HRLOC/AgNW.


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Figure 3. AFM topography images of (a) AgNW, (b) PI/AgNW, and (c) HRLOC/AgNW.



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Figure 4. Transmittance curves of glass, ITO, AgNW, PI/AgNW, and HRLOC/AgNW. The transmittance values of films include the glass substrate.


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Figure 5. (a) Device structure, (b) current density-voltage curves, (c) luminance-voltage curves, (d) current efficiency, (e) power efficiency, and (f) electroluminescence spectra of Device_ITO, Device_AgNW, Device_PI/AgNW, and Device_HRLOC/AgNW.



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Table 1. Sheet resistance and transmittance for ITO and AgNW transparent electrodes.

ITO AgNW PI/AgNW HRLOC/AgNW

Sheet resistance [ Ω/sq-1 ] 10.0 45.2 48.0 46.9

Transmittance Avg. [%] 83.5 86.1 87.0 85.9

Transmittance @550 nm [ % ] 90.5 86.3 87.3 88.3

Table 2. A summary of the performance of the devices. Current efficiency [cd/A] (1000 cd/m2) ITO 4.04 AgNW 1.08 PI/AgNW 3.97 HRLOC/AgNW 4.71

Current efficiency [cd/A] (20000 cd/m2) 14.18 8.19 16.95 18.37

Power efficiency [lm/W] (1000 cd/m2) 1.44 0.45 1.64 2.18

Power efficiency [lm/W] (20000 cd/m2) 3.28 2.02 4.29 5.55

Table Of Contents (TOC)


EQE [%] (1000 cd/m2) 1.58 0.41 1.56 1.81

EQE [%] (20000 cd/m2) 5.50 3.12 6.49 6.92

Highly Enhanced Light-Outcoupling Efficiency in ITO-Free Organic

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