Flexible and Mechanically Robust Organic Light-Emitting Diodes

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Flexible and Mechanically-Robust Organic Light Emitting Diodes Based on Photopatternable Silver Nanowire Electrodes Hyungseok Kang, Iljoong Kang, Jaehun Han, Jun Beom Kim, Dong Yun Lee, Sung Min Cho, and Jeong Ho Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06599 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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The Journal of Physical Chemistry

Flexible and Mechanically-Robust Organic Light Emitting Diodes Based on Photopatternable Silver Nanowire Electrodes Hyungseok Kang1, Iljoong Kang2, Jaehun Han1, Jun Beom Kim1, Dong Yun Lee3, Sung Min Cho 1,2*, Jeong Ho Cho1, 2* 1

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea. 2 School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea. 3 Department of Polymer Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea. Abstract We developed a simple methodology for fabricating silver nanowire (AgNW) micropatterns on a plastic substrate using a photocurable polymer. The patterning method began with the lamination of a UV– curable prepolymer film onto the AgNW–coated rigid glass substrate. Selective UV exposure of the UV– curable prepolymer film through a photomask solidified the exposed regions, and the unexposed regions were simply removed by the solvent. AgNW micropatterns of various sizes and shapes could be readily formed across the entire plastic substrate. Importantly, this photopatterning process enabled the embedding of the AgNW structures into the polymer matrix, which dramatically reduced the surface roughness and enhanced the mechanical stability of the AgNW film. The AgNW structures served as transparent anode electrodes in organic light–emitting diodes (OLEDs) that performed well compared to OLEDs fabricated using conventional indium tin oxide (ITO) or conducting polymer electrodes. This simple, inexpensive, and scalable AgNW patterning technique provides a novel approach to realizing next–generation flexible electronics. *Corresponding authors: Prof. Jeong Ho Cho: Tel.:+82 31 299 4165, E−mail: [email protected] SKKU Advanced Institute of Nanotechnology (SAINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Prof. Sung Min Cho: Tel.:+82 31 290 7251, E−mail: [email protected] School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea

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1. Introduction Transparent conductive electrodes (TCEs) have attracted attention because they are a critical component in various optoelectric devices, such as organic light–emitting diodes, organic solar cells, liquid crystal displays, and touch screen panels.1-9 Indium tin oxide (ITO) is the most widely used TCE due to its high electrical conductivity and optical transparency; however, it cannot be used in plastic–based flexible electronic devices because ITO films are brittle, offer a low infrared transmittance, and can only be produced in low–throughput high–temperature processes. Additionally, indium is scarce, which increases the costs of production. Significant efforts have been devoted toward replacing ITO. Several emerging alternative TCEs have been identified with attractive electrical and optical properties: conducting polymers (polyaniline and poly(3,4–ethylenedioxythiophene):poly(styrenesulfonate))10-15 and carbon nanomaterials (carbon nanotubes (CNTs) and graphene).16-19 Most of these materials, however, exhibited a much higher sheet resistance of 200 – 100 Ω/sq (with an optical transmittance of 80 – 90%) than the values obtained from metal oxides. Significant improvements in the electrode performances are required for applications to a variety of practical devices. Silver nanowires (AgNWs) are one of the most promising TCE materials because they offer a low resistance, good optical transparency, and high mechanical flexibility.6-7, 20-21 AgNWs dispersed in a solvent can be deposited onto large–area flexible substrates using simple solution–coating techniques, such as spin– coating, spray–coating, or Myer–rod coating.3, 22-23 Percolating networks of as–coated AgNW films exhibit a sheet resistance of 20 Ω/sq with an optical transmittance of 80%. The sheet resistance of the AgNW film depends strongly on the contact resistance of the inter–nanowire junctions. Thus, many research groups have intensively studied post–welding processes, including thermal, mechanical, light, or electrochemical treatments, in an effort to reduce the junction resistance.24-28 In addition to optimizing the electrical and optical properties of the AgNWs, several technological issues must be addressed before AgNWs may be applied in practical applications: the surface roughness must be reduced, and the interfacial adhesion between AgNWs and polymeric substrates must be improved.29-36 The extremely high surface roughness is one of the most critical sources of device failure due to electrical short generation.37 The weak adhesion between the AgNWs and the plastic substrate promotes delamination of the AgNWs under external mechanical stress.38 Another important issue involves the patterning method used to fabricate AgNW films. Despite its importance, few studies have attempted to develop simple large–area patterning methods.35, 39-42 Here, we report a AgNW micropatterning method that is suitable for forming the anode electrodes in OLEDs to obtain excellent device performances. This photopatterning process not only dramatically reduced the surface roughness, it enhanced the mechanical stability of the AgNW film by embedding the AgNWs into a UV– patternable polymer matrix. This method included four steps: i) formation of a AgNW film on a hydrophobic glass substrate, ii) lamination of the UV–curable prepolymer film onto the AgNWs substrate, 2 ACS Paragon Plus Environment

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iii) selective UV exposure of the AgNW film embedded in the UV–curable prepolymer, and iv) removal of the unexposed regions using a solvent. This method readily formed AgNW micropatterns with various sizes and shapes across the entire plastic substrate. These patterns served as transparent anode electrodes for the fabrication of OLEDs that performed excellently compared to OLEDs prepared using conventional ITO or conducting polymer electrodes.12,

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This simple, inexpensive, and scalable AgNW patterning technique

provides a novel approach to realizing next–generation flexible electronics.

2. Experimental section Patterning of the AgNWs film: A silver nanowire (AgNW) dispersed in isopropyl alcohol was purchased from Nanopyxis Co. The diameter and length of nanowires were 25 ~ 35 nm and 25 ~ 35 µm, respectively. The hydrophilic glass substrate was treated with hydrophobic octadecyltrichlorosilane (ODTS). AgNW solution was deposited onto the ODTS–treated glass substrate using the Meyer–rod coating method (#10, 14, 20, and 28: RD Specialist Inc.). The deposition density of the AgNWs network was modulated to systematically vary the number of Meyer rod. Separately, 80 wt% SU–8 2150 (Microchem Co.) disolved in SU-8 thinner was spin–coated onto the polyethylene terephthalate (PET) substrate. The thickness of the SU– 8 prepolymer film was around 10 µm. This SU–8 film onto the PET was laminated onto the preprepared AgNW film. The AgNWs–embedded SU–8 film was exposed by UV light (365 nm and 25 mW/cm2) for 35 s through the photomask. The area exposed by UV light was cross–linked, while the area unexposed by UV light was removed by the SU–8 developer. The surface morphologies of the AgNWs films were measured using a tapping–mode AFM (D3100 Nanoscope V, Veeco) and SEM (JSM-7600F, JEOL Ltd.). The sheet resistance of the AgNWs films was charaterized using the four–point probe technique (Keithley 2182A and 6221) and the optical transmittance was measured using a UV–visible spectrophotometer (Agilent 8453). OLEDs Fabrication: Onto the embedded AgNW electrode/PET substrate, a 100 nm thick–PEDOT:PSS layer (Heraeus-clevios) was deposited by spin–coating method. Then, 40 nm–thick N,N'–bis–(1–naphyl)– N,N′–diphenyl–1,1′–biphenyl–4,4′–diamine (NPB) and subsequent 50 nm–thick tris(8–hydroxyquinolinato) aluminium (Alq3) layers were thermally deposited onto PEDOT:PSS layer. Finally, a 1.5 nm– thick lithium quinolate (Liq) and a 100 nm–thick Al were deposited by thermal evaporation. The emission area was 1 x 1 cm2. The current–voltage–luminance characteristics were measured using a Keithley 236 source measurement unit and a Minolta CS2000 Spectroradiometer.

3. Results and discussion Figure 1 presents a schematic diagram of the procedure used to fabricate the photopatterned AgNW films. This patterning method began with the formation of a uniform large–area AgNW film on the hydrophobic octadecyltrichlorosilane (ODTS)–treated glass substrate using the Meyer–rod coating method. The water contact angle of the ODTS–treated substrate was 110°. The low surface energy of the ODTS 3 ACS Paragon Plus Environment


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facilitated delamination of the AgNW film from the substrate using a UV–curable polymer.44 The AgNW deposition density was controlled systematically by varying the Meyer–rod number (# 10, 14, 20, and 28). Separately, a UV–curable prepolymer (SU-8) was spin–coated onto the UV–ozone–treated polyethylene terephthalate (PET) substrate. The introduction of the UV–ozone treatment dramatically enhanced the interfacial adhesion between the UV–curable polymer and the PET film, which strengthened the adhesion relative to that between the AgNW film and the ODTS–treated glass substrate.45 The UV–curable prepolymer film, which had been spin–coated onto the PET, was laminated onto the prepared AgNW/glass substrate. The viscous sticky prepolymer filled the spaces between AgNWs, embedding and mechanically interlocking the nanowires to form a polymer matrix with intimate contact with the AgNWs. The AgNWs embedded in the UV–curable prepolymer films were selectively exposed to UV light for 35 s, which cross– linked the regions exposed to UV light. Finally, the AgNW film embedded in the UV–cured polymer film was detached from the glass substrate, and the unexposed regions were removed using the SU–8 developer solvent. The weak adhesion at the AgNW–OTDS interface and mechanical interlocking between the AgNWs and the SU–8 facilitated the clean delamination of the SU–8 film from the glass substrate. Figure 2a displays an optical microscopy image of the resulting AgNW micropatterns for OLED fabrication on a PET substrate. Sharp, fine pattern edges were observed without defects. No polymeric residue remained on the region unexposed to UV light. Scanning electron microscopy (SEM) images confirmed the successful AgNW pattern formation on the PET substrate (Figure 2b). The dramatic reduction in the surface roughness due to the embedding of the AgNW structure was characterized by atomic force microscopy (AFM), as shown in Figure 2c. As–coated films revealed protruding nanowires, which formed localized feature heights exceeding 75 nm. The root–mean–square (RMS) surface roughness of the as–spun film was 28 nm, significantly larger than the value obtained from commercial ITO films. The surface roughness significantly affected the electrical performances of the OLED devices, because the hills at wire–wire junctions as well as the voids between wires could give rise to electrical shunts and shorts in the OLEDs. By contrast, the embedded AgNW film formed by our patterning process formed a smooth and flat surface (the right panel of Figure 2c). Specifically, the RMS roughness and peak–to–valley height in the film decreased dramatically to 0.73 nm and 4.7 nm, respectively, after the nanowires had been embedded in the polymer matrix. Note that this patterning method completely flattened the rough surface and removed local spikes from the AgNWs surface. Figure 2d shows an optical image of the flexible transparent photopatterned AgNW film. The optical transmittance (inset of Figure 2d) and the sheet resistance of the resulting embedded AgNW film were found to be 80.9% at 550 nm and 8.2 Ω/sq, respectively. The density of the AgNW network was systematically controlled by varying the Meyer–rod number. The spacing between adjacent metallic coils in the Meyer rods increased with increasing the rod number, which yielded a higher deposition of AgNWs. Figure 2e shows the optical transmittance of the AgNW films, prepared using four different Meyer–rods, as a function of the wavelength. As the rod number increased 4 ACS Paragon Plus Environment

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from #10 to #28, the AgNW deposition density increased, as shown in the inset of Figure 2e. The greater deposition of AgNWs decreased both the optical transmittance and the sheet resistance of the AgNW film, as summarized in Figure 2f (closed symbols). For example, the rods #10 and #28 produced sheet resistances of 12.2 and 3.9 Ω/sq, respectively. Similarly, the optical transmittance was linearly related to the nanowire density; that is, an 84.2% transmittance was obtained from rod #10, and a 66.8% transmittance was obtained from rod #28. Importantly, the sheet resistance and optical transmittance values varied by less than 10% after embedding (open symbols), indicating a high AgNW transfer efficiency using this patterning method. The surface morphology of each sample was monitored by AFM, as shown in the inset of Figure 2g. All four embedded AgNW films exhibited smooth flat surfaces compared with the as–spun AgNW film. The RMS roughness values of the four films extracted from the AFM images are summarized in Figure 2g. The AgNW films with high densities (rod #20 and #28) exhibited slightly higher surface roughnesses due to the incomplete penetration of the UV–curable prepolymers into the nanowires. We therefore selected an embedded AgNW electrode, fabricated from rod #14, with an optical transmittance of 80.9% and a sheet resistance of 8.2 Ω/sq (RMS roughness = 0.73 nm) for use in subsequent OLED applications. The mechanical flexibility and robustness of the embedded AgNW films were evaluated by measuring the sheet resistance during fatigue cycles (Figure 3a and 3b). Both compressive and tensile strains of 2.75% were applied to the AgNWs films. The change of the sheet resistance was expressed as (R– R0)/R0, where R is the sheet resistance at a certain cycle and R0 is the initial sheet resistance. The values for the embedded AgNW films were compared with those obtained from the as–coated AgNWs and the commercial ITO electrodes. The lab–built bending test setup is shown in the inset of Figure 3a and 3b. Two edges of the film were fixed, and the sheet resistances were monitored over time during the bending cycles. The ITO sheet resistance increased sharply after 1 cycle. Both the as–coated AgNWs and the embedded AgNW films were much more stable than the ITO film. For the as–coated AgNWs, the gradual increase in the sheet resistance was observed over 10 cycles under tensile strain and over 400 cycles under compressive strain. The variations in the sheet resistance values of the as–coated AgNW film were attributed to the slipping and delamination of the AgNW junctions.

38, 46

The better stability under compression than under

tensile strain could be explained by the fact that the tensile strain more strongly influenced delamination than the compressive strain. For the embedded AgNW film, however, the sheet resistances remained constant even after 1000 cycles because the embedded AgNW structure minimized delamination or slipping at the AgNW junctions. The 3M Scotch–tape detachment tests were also performed on the as–coated AgNW and embedded AgNW films (Figure 3c). The as–coated AgNW film exhibited the gradual increase in the sheet resistance with the detachment cycle, whereas negligible changes in the sheet resistance were obtained from the embedded AgNWs over 1000 detachment cycles. The as–coated AgNWs within the tape contact region were fully detached from the PET substrate after 100 times detachment cycles, as shown in Figure



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3d. The AgNW film embedded in the UV–curable polymer matrix exhibited a significantly enhanced stability, consistent with the above fatigue test results. The transparent and flexible embedded AgNWs fabricated using our patterning method were successfully used in OLEDs, as shown in Figure 4a. The basic structure of our OLED consisted of six layers: a patterned AgNW anode, a PEDOT:PSS hole–injecting layer, an N,N'–bis–(1–naphyl)–N,N′– diphenyl–1,1′–biphenyl–4,4′–diamine (NPB) hole transport layer, a tri(8–hydroxyquinolinato)aluminum (Alq3) emitting layer, a lithium quinolate (Liq) electron–injecting layer, and an Al cathode layer. Figures 4b–4h plot the performances of the OLEDs based on the patterned and embedded AgNWs in comparison with the performances obtained using commercial ITO or conducting PEDOT:PSS electrodes. Figures 4b and 4c show the current density–luminance–voltage characteristics of the OLEDs prepared using the embedded AgNWs, ITO, or PEDOT:PSS. The device fabricated with PEDOT:PSS alone displayed a higher operating voltage, a lower current density, and a lower luminance (12.8 mA/cm2 and 1710.6 Cd/m2 at 6.7 V) than the device fabricated with ITO (35.2 mA/cm2 and 323.5 Cd/m2 at 6.7 V). The higher sheet resistance of PEDOT:PSS (275 Ω/sq at 92% at 550 nm) than of ITO (12 Ω/sq at 85% at 550 nm) impeded the spread of a uniform current across the entire anode area and hampered the balanced recombination of electrons and holes in the device. On the other hand, the OLEDs prepared with the embedded AgNWs exhibited current densities and luminance that were indistinguishable from those of the devices prepared with commercial ITO. This result was attributed to the low sheet resistance (8.2 Ω/sq) of the embedded AgNWs, which compensated for the low transmittance (80.9% at 550 nm) that blocked some portion of the luminance compared to the ITO layer. The enhanced light extraction efficiency induced by light scattering from the AgNW surface improved the luminance. The embedded AgNW devices displayed the lowest leakage current below the light emission threshold of 3 V. The as–coated AgNWs were used as the anode electrodes in OLEDs that otherwise performed poorly due to their extremely high surface roughness, as shown in Figure S1. The luminous efficiency, power efficiency, and external quantum efficiency (EQE) of the OLED prepared with the embedded AgNWs were comparable (4.6 CdA–1, 2.1 lm W–1, and 1.5% at 6.7 V) to the corresponding values obtained from devices prepared with ITO (4.9 CdA–1, 2.2 lm W–1, and 1.5% at 6.7 V), as shown in Figures 4d, 4e, and 4f. Figure 4g shows the electroluminescence (EL) spectra, collected under different viewing angles, of the OLEDs prepared with the embedded AgNWs, ITO, or PEDOT:PSS. The spectra collected at all angles exhibited nearly identical EL peaks at 534 nm. The light intensities of both the ITO and PEDOT:PSS devices depended strongly on the viewing angle: the spectra remained constant up to 60° viewing angles but decreased significantly above 60°. By contrast, the OLEDs prepared with the embedded AgNWs exhibited almost no change in the emission spectra across the viewing angles tested. Figure 4h shows the angular dependence of the normalized luminances of three types of OLEDs. The luminances were normalized by the luminance measured along the normal direction (0°). The relative luminances at wide angles were slightly higher for the AgNW devices than for the ITO devices due to the 6 ACS Paragon Plus Environment

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light scattering effects of the AgNWs. Negligible luminance was observed at an 80° viewing angle in the ITO devices. The high color stability, at different angles, of the OLEDs prepared with the embedded AgNW is advantageous for corner lighting applications. Finally, the mechanical flexibility of each OLED was investigated over 5000 fatigue cycles under a 2.75% applied tensile strain (Figure 5). The luminance of the device was monitored during the cycles and was compared with its initial value. The OLED prepared with the ITO anode lost its luminance even after 10 cycles due to mechanical failure of the ITO. By contrast, the OLEDs prepared with the patterned and embedded AgNWs showed no detectable changes in the luminance up to 5000 fatigue cycles. The fatigue tests demonstrated the stable and reliable operation of the OLEDs fabricated with the embedded AgNWs. Overall, the embedded AgNWs prepared using our patterning method offered a good strategy for fabricating flexible displays and lighting applications because the patterned and embedded AgNWs effectively replaced brittle ITO electrodes while providing comparable luminous efficiencies.

4. Conclusion In conclusion, we developed a method of photopatterning AgNW films for use in OLED anode electrodes. This method was highly reproducible and effective, dramatically reducing the surface roughness while enhancing the mechanical stability of the AgNW film. This process included four essential steps: i) AgNW film formation on a glass substrate, ii) lamination of a UV–curable prepolymer film, iii) selective UV exposure of the embedded AgNW film, and iv) removal of the unexposed regions. The resulting patterned AgNW films were successfully used as anode electrodes in OLEDs. The novel AgNW patterning method, which is easy and scalable, offers great promise for the manufacture of next–generation flexible electronics. Supporting Information Figure S1. Light emission image of the OLEDs with the as−coated AgNWs anode. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements This work was supported by the KANEKA/SKKU Incubation Center (financially supported by Kaneka Corp. in Japan) and the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (NRF–2013M3A6A5073177), Korea.



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Figure 1. (a) Schematic diagram showing the photopatterning steps applied to the AgNW films in this study.



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Figure 2. (a) Optical microscopy image of the photopatterned AgNW film. (b) SEM images of the AgNW film pattern edge. (c) AFM images of the AgNW films coated onto a rigid glass substrate, and the embedded AgNW films. (d) Optical image of the AgNW films patterned onto a plastic substrate. The inset shows the optical transmittance of the AgNW film as a function of the wavelength. (e) Optical transmittance, as a function of the wavelength, of the AgNW films coated onto a rigid glass substrate using different Myer–rod bars. The inset shows SEM images of the resulting AgNW films. (f) Plot of the optical transmittance vs. sheet resistance for the as–coated AgNW films and the embedded AgNW films. (g) RMS roughnesses of the embedded AgNW films. The inset shows the AFM images of all samples.


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Figure 3. (a) (R-R0)/R0 of ITO, as–coated AgNWs, and embedded AgNWs as a function of the fatigue cycling number during a 2.75% tensile strain. (a) (R-R0)/R0 of ITO, as–coated AgNWs, and embedded AgNWs as a function of the cycling number during a 2.75% compressive strain. The inset shows the experimental setup used in the bending tests. (c) (R-R0)/R0 of the as–coated AgNWs and embedded AgNWs as a function of the detachment time using the 3M scotch tape. (d) SEM images of the as–coated AgNW film and embedded AgNW film after one hundred detachment cycles.



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Figure 4. (a) Schematic diagram showing the device structure of the OLED based on the embedded AgNW anode electrode. The lower panel shows a light emission image of the device under bending. (b) Current density, (c) Luminance, (d) Luminous efficiency, (e) Power efficiency, (f) External quantum efficiency of the OLEDs prepared using ITO, PEDOT:PSS, or embedded AgNW anode electrodes. (g) EL spectra, measured at specific viewing angles, of the OLEDs prepared using ITO, PEDOT:PSS, or embedded AgNW anode electrodes. (h) Angular distribution of the OLED luminance.


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Figure 5. Mechanical stability of the OLEDs prepared using ITO or embedded AgNW anode electrodes.



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TOC Figure


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Flexible and Mechanically Robust Organic Light-Emitting Diodes

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