Novel Patterning Method for Silver Nanowire Electrodes for Thermal

Mar 30, 2016 - *E-mail: [email protected]. ... Here, by controlling the surface wetting properties of a polydimethylsiloxane (PDMS) release template, we ar...
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Novel Patterning Method for Silver Nanowire Electrodes for ThermalEvaporated Organic Light Emitting Diodes Shuyi Liu,†,‡ Szuheng Ho,‡ and Franky So*,† †

Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27606, United States Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States

‡

S Supporting Information *

ABSTRACT: Silver nanowires (AgNWs) mesh has been used as transparent electrodes in optoelectronic devices. However, the lack of practical patterning techniques for the random percolating nanowire network has limited its applications in devices where a well-defined pixel is required. Here, by controlling the surface wetting properties of a polydimethylsiloxane (PDMS) release template, we are able to pattern the random AgNWs network with uniform conducting property; and due to the hydrophobic recovery nature of PDMS, a multilayer patterning and transferring process can be realized, resulting in a fine-patterned, smooth, and uniform AgNWs mesh/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) composite electrode. A thermal-evaporated organic light-emitting diode (OLED) is directly fabricated onto the patterned AgNWs/PEDOT:PSS composite electrode. The device shows well-defined pixel edges and a uniformly lighted pixel area. A uniform OLED with very low leakage current is realized. The enhanced efficiency compared to the controlled device with prepatterned indium tin oxide (ITO) electrode is attributed to the scattering effects of the AgNWs electrode. KEYWORDS: silver nanowires, patterning, organic light emitting diodes, surface treatments, polymer embedding, transparent electrode, light scattering

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INTRODUCTION Transparent electrodes are imperative for large area optoelectronic devices such as organic light-emitting diodes (OLEDs) and photovoltaic cells as well as touch screens. The prevalent electrode candidate is tin-doped indium oxide (ITO), which is typically deposited by vacuum deposition. It is widely used as a transparent conductor due to its low sheet resistance (20 Ω sq−1), high optical transmittance (85−90%), and good thin film quality.1,2 However, ITO electrode has two problems: the high price due to the shortage of indium and the incompatibility with flexible/bendable applications owing to its brittleness.3 A variety of materials have been developed as ITO alternatives,4 ranging from carbon nanotubes,5−7 graphene,8−13 conducting polymers,14−17 and metal nanowires.18−29 Due to its high electrical conductivity and stability, silver nanowires (AgNWs) mesh is a promising candidate to be used for transparent electrode applications, and it has sheet resistance and transmittance comparable with those of ITO.30 In AgNWs electrodes, transport of charges is via the percolating random networks. One of the challenges to fabricate electrodes using AgNWs is the uniformity of the electrical conductivity due to the nature of the nanowires. Conducting polymers such as poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) are used to fill the void areas of the nanowire network, and this approach has been found to significantly improve the conducting uniformity by facilitating charge transport between spatially separated nanowires and junction areas in the composite electrode.20,31 Although highly © XXXX American Chemical Society

conductive PEDOT:PSS (Clevios PH 1000) alone can be used as a transparent electrode,14−17 a thicker layer of PEDOT:PSS is usually required to render a low sheet resistance of the electrode,15 which decreases its transparency and increases the risk of electrical leakage due to PEDOT:PSS agglomeration. The sheet resistance is also significantly higher compared to the ITO electrode.14 While for the AgNWs/ PEDOT:PSS composite electrode, due to the small scale of the void areas in the random AgNWs network, the conducting uniformity can be significantly enhanced with a thin (30 nm) PEDOT:PSS layer without sacrificing too much transparency.32 However, there are two problems hindering the practical use of AgNWs electrodes. The first problem is the film roughness. As the length of AgNWs is on the order of tens of micrometers, the resulting roughness of AgNWs films due to the protruding nanowires leads to serious electrical shorts in devices. Attempts such as membrane vacuum filtration have been made to address the shorting problems associated with AgNWs electrodes.33 While the surface roughness of the AgNWs electrode was reduced, the film quality is still not good enough for device fabrication. More recently, using a cross-linkable polymer to embed and transfer the AgNWs mesh deposited onto a release substrate was demonstrated to be an effective way to reduce the electrodes surface roughness and the leakage current in the Received: January 19, 2016 Accepted: March 24, 2016

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DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Procedures of fabricating autopatterned silver nanowire composite electrodes.

resulting solution processed polymer-based devices.34,35 However, as evaporated small molecule OLEDs are more vulnerable to surface roughness compared to solution-processed devices, high leakage current in evaporated devices is still a challenge even though AgNWs are embedded in a polymer.36 The second problem hindering the use of AgNWs electrodes is the difficulties in patterning AgNWs electrodes. Although several reports in the literature have addressed this issue with different printing techniques,37−39 these processes are not applicable to patterning the electrodes in optoelectronic devices which require uniform and transparent 2D nanowire mesh networks. For example, electrohydrodynamic jet printing has been demonstrated for printing fine-patterned AgNWs lines.37,40 However, due to the alignment of the nanowires, only one-dimensional conductivity can be achieved which is not suitable for electrode applications. Given the geometric nature of the AgNWs, the feature size of the AgNWs electrodes is limited to about the length of the NWs which is typically 10− 100 μm. Therefore, fine patterning of AgNWs random networks that can be used as transparent electrodes for large scale production in optoelectronic devices still remains to be a challenge. In this work, we developed a process to pattern AgNWs electrodes by manipulating the hydrophilicity and hydrophobicity on selected areas of the polydimethylsiloxane (PDMS) surface to control its surface wettability and thus adhesion to the AgNWs suspension. Oxygen plasma or UVozone treatment was used to break the chemical bonds of the PDMS surface to introduce polar functional groups, which changes its surface properties from hydrophobic to hydrophilic.41,42 By selecting the UV-ozone exposed area, the resulting PDMS can be used as a release template to define the AgNWs deposition area. As a result, a fine patterned AgNWs electrode with good precision can spontaneously be formed on the template upon spin-coating. A AgNWs electrode with a line-width less than 150 μm was demonstrated with this technique. Due to the spin-coating process, the AgNWs mesh with a randomly percolating AgNWs network can be formed and uniform conducting electrodes can be realized. Given the fact that silver nanowires used in this work (Seashell Technology) are 10−20 μm in length, our electrode with a line-width of 150 μm is close to the resolution limit of the patterned AgNWs mesh network. To acquire a highly smooth surface within the electrode area, the deposited AgNWs on the PDMS release template was covered with a layer of low-

viscosity UV-curable epoxy, which is then used to embed the rough AgNWs top surface, serving as a new substrate with patterned AgNWs electrodes for device fabrication. To minimize the damage during the delamination process, the adhesion between the cross-linked epoxy and PDMS should be weak. Here, the UV-ozone exposed area on the PDMS release templates remains to be hydrophilic during the AgNWs spincoating process and eventually undergoes a slow hydrophobic recovery at room temperature. To accelerate the hydrophobic recovery process, the sample can be annealed at elevated temperatures. Upon hydrophobic recovery, the epoxy along with the embedded AgNWs can be easily peeled off from the PDMS release template, serving as a substrate for device fabrication. As a result, we are able to fabricate high quality thermal-evaporated OLEDs on transparent AgNWs electrodes with sharp pixel definition. This autopatterning process of the AgNWs electrode by controlling the PDMS release template surface wetting circumvents the complicated developing and etching procedures in a conventional photolithography process. More importantly, it is compatible with large scale fabrication for flexible devices.

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EXPERIMENTAL SECTION

The PDMS release template was prepared by spin-coating a layer of PDMS onto a glass substrate, and then the template was cured at 100 °C for 1 h. The as-prepared PDMS surface exhibiting strong hydrophobicity is demonstrated in Figure S1. A shadow mask with the desired electrode pattern was in contact with the PDMS surface during the 1 h UV-ozone exposure. The region exposed to ozone becomes hydrophilic, while the region covered by the shadow mask remains hydrophobic. With this hydrophilic surface, aqueous-based solutions such as PEDOT:PSS can be used for the autopatterning process. We thereby incorporated PEDOT:PSS into the AgNWs electrode to increase the conductivity uniformity. The stepwise embedding and transfer process is illustrated in the schematic diagrams (Figure 1). During the electrode fabrication, a 30 nm thick PEDOT:PSS (Clevious AI 4083) was spin-coated onto a PDMS template. After drying the PEDOT:PSS film on the hot plate at 110 °C for 5 min, the AgNWs suspension was spin-coated on top of PEDOT:PSS followed by annealing at 130 °C for 40 min to remove the residue solvent and melt nanowires contacting areas into junctions. Finally, a low-viscosity epoxy (Norland Optical Product 81) was dispensed on top, and a glass substrate was gently placed on top of the epoxy layer followed by UV curing. Due to the hydrophobic recovery nature of PDMS, the top glass substrate (along with AgNWs/ PEDOT:PSS and epoxy) can be easily peeled off from the PDMS release template. The PEDOT:PSS film plays three roles here: First, B

DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces due to the well-defined patterns on the UV-ozone cured PDMS templates, the PEDOT:PSS film acts as a seeding layer to provide good adhesion to the subsequently deposited AgNWs mesh film from its aqueous-based suspension. Due to the good wettability of PEDOT:PSS to the AgNWs suspension, the spin-coated AgNWs mesh only adheres to the PEDOT:PSS layer on top of PDMS, thus forming a patterned AgNWs/PEDOT:PSS composite electrode. Second, PEDOT:PSS can fill the void areas between nanowires, improving the conducting uniformity of the AgNWs electrode. Third, as the layer structure becomes inverted after the transferring process, the PEDOT:PSS at the surface of the composite electrode will facilitate hole injection in OLEDs without using a separate hole injection layer in the device. The original coating sequence is PDMS/ PEDOT:PSS/AgNWs/epoxy, but the stacking sequence will be reversed to epoxy/AgNWs/PEDOT:PSS after it is peeled off from the PDMS template. While it is very difficult to pattern PEDOT:PSS films by conventional photolithography, this architecture is desirable for conventional OLED fabrication with autopatterned PEDOT:PSS as the hole injection layer in contact with the rest of the functional layers. To fabricate thermal-evaporated bottom-emitting OLED devices, a prepatterned commercial ITO electrode coated with a 30 nm PEDOT:PSS HIL was used as a control. Both AgNWs/PEDOT:PSS and ITO/PEDOT:PSS samples were loaded into a thermal evaporator to deposit a 45 nm-thick layer of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1phenyl-1-H-benzimidazole) (TPBi): 30 wt% tris[2-phenylpyridinatoC2,N]iridium(III) (Ir(ppy)3) as a hole transport layer (HTL), a 20 nm-thick layer of TPBi: 8 wt% Ir(ppy)3 as an emitting layer (EML), a 10 nm-thick layer of TPBi as an exciton blocking layer (EBL), a 30 nm-thick layer of tris(8-hydroxyquinoline)aluminum (Alq3) as an electron transport layer (ETL), and a 1 nm-thick layer of lithium fluoride (LiF) and a 100 nm-thick layer of aluminum (Al) as a cathode. All layers were evaporated with a rate of 0.1−1 Å s−1 at a base pressure of 5 × 10−7 Torr. The current−voltage−luminance (J-V-L) characteristics were measured by a program-controlled Keithley 2400 source meter and Keithley Series 6485 picoammeter with a calibrated Newport silicon photodiode. The luminance was measured using a Konica Minolta luminance meter (LS-100), and the EL spectrum was obtained from an Ocean Optics spectrometer.

stacking layer. For the AgNWs electrode prepared from a 5 mg mL−1 solution, the electrode shows comparable performance to ITO in terms of sheet resistance and transmittance. Optical microscopy is used to examine the pixel definition of the autopatterned AgNWs electrodes. Figure 3a shows the edge of the AgNWs electrode is sharp and clean, indicating the surface wetting control of PDMS template is of high precision. The AgNWs coating is homogeneous with no nanowires residues beyond the defined areas. Using the same method, we successfully patterned a AgNWs stripe with a width of about 150 μm simply via spin-coating onto a pretreated PDMS template (Figure 3b). It should be noted that in this approach, the metal shadow mask is in close contact with the PDMS surface using double-side adhesive tape. After the removal of the shadow masks followed by spin-coating, there are some nanowire residues at the edge of a previously covered area due to the adhesive residue on the PDMS surface. Nevertheless, the patterning quality in such a small scale can be further enhanced by replacing the thick metal shadow mask with a thin shadow mask during the PDMS surface treatment. Considering the fact that ozone is only generated in the UV exposed area, we believe that a photomask is applicable to confine the UV light and thus ozone generated in certain treated areas on PDMS, enabling a more precise control of the autopatterned AgNWs mesh networks in smaller scales. Our transfer method is used to embed the AgNWs within the epoxy layer to “planarize” the AgNWs electrode surface. As shown in Figure S2, AgNWs directly spin-coated onto PDMS template exhibits a rough surface, and the peak to valley distance within the scan region is up to 300 nm. On the contrary, our transfer technique has significantly reduced the surface roughness of the AgNWs electrode as shown in Figure 4a. However, the coarse surface (roughness over 50 nm on average) over a large area is still problematic for OLED fabrication. This coarse surface could be attributed to the sharp ends of the nanowires partially protruding into the soft PDMS template during spin-coating. When PEDOT:PSS is used as a buffer layer between PDMS and AgNWs for the autopatterning process, further planarization of the AgNWs electrode is achieved. Due to the weak adhesion between PEDOT:PSS and PDMS after annealing, the PEDOT:PSS layer can readily be peeled off from the PDMS surface, and the highly smooth PEDOT:PSS/PDMS interface in turn becomes the top surface of the epoxy/AgNWs/PEDOT:PSS stack, providing the necessary contact electrode for subsequent OLED fabrication. Figure 4b presents the atomic force microscopy (AFM) images and height profile of the transferred electrodes fabricated with PEDOT:PSS. The surface roughness is significantly improved to less than 15 nm over a large area in the AgNWs/ PEDOT:PSS composite electrode, which can be used as an anode for either solution processed or thermal-evaporated OLEDs. The thickness of the PEDOT:PSS spin-coated onto PDMS is around 30 nm, which is proved to be sufficient to prevent AgNWs from protruding into the PDMS template. The significantly lower transmittance (Support Information Figure S3) of the AgNWs electrodes peeled-off from the PEDOT:PSScoated PDMS template serves as another proof that the PEDOT:PSS layer (which contribute to the lower transmittance) was transferred together with AgNWs to form the AgNWs/PEDOT:PSS composite electrode during the peelingoff process, demonstrating the feasibility of the multilayer transferring process.

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RESULTS AND DISCUSSION The concentration of AgNWs suspension is optimized to achieve both high transparency and high conductivity spincoated AgNWs films. Figure 2 shows the transmittance and sheet resistance of the AgNWs electrodes (transferred to epoxy) and ITO. For AgNWs films, by increasing the concentration of nanowire suspensions, the sheet resistance is decreased by 1 order of magnitude with the transmittance decreasing gradually, which is attributed to the thick AgNWs

Figure 2. Sheet resistance and transmittance of silver nanowires and ITO electrodes. C

DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Optical microscopy image of (a) the AgNWs electrode pixel edge and (b) the thin AgNWs electrode.

Figure 4. AFM topography images and height profile of epoxy-embedded AgNWs electrodes after peeling-off from (a) the bare PDMS template and (b) the PEDOT:PSS coated PDMS template. The surface roughness is significantly reduced for the AgNWs/PEDOT:PSS composite electrode peeled off from the PEDOT:PSS coated PDMS template due to the smooth interface between PEDOT:PSS and PDMS templates.

Figure 5. (a) Device architecture of the thermal-evaporated OLEDs. (b) The current density−voltage−luminance (J-V-L) characteristics. (c) Current efficiency versus luminance characteristics and (d) electroluminescence spectra of OLEDs with AgNWs and ITO anodes.

To demonstrate the feasibility of our autopatterned AgNWs/ PEDOT:PSS composite electrodes for OLED applications, we fabricated both solution-processed and thermal-evaporated OLEDs using AgNWs/PEDOT:PSS composite electrodes

right after the peeling-off process and ITO electrode/ PEDOT:PSS HIL as a reference. As the solution processed films can planarize the coarse or protruding surface of the AgNWs layer, it is expected that the electrical shorts due to the D

DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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spectrum of AgNWs and ITO OLEDs. The narrower EL spectrum of the AgNWs OLEDs suggests that the charge recombination zone and microcavity effects are different in two types of devices.27,28,34 As the emission profile of the OLEDs with AgNWs/polymer composite electrodes is very close to the Lambertian distribution,27 the weaker edge emission observed in the AgNWs/polymer composite electrode implies that the substrate mode is more efficiently extracted in the AgNWs OLEDs due to the scattering effect.28 To verify this, we attached both microlens foil and half-sphere onto the pixel areas of ITO and AgNWs OLEDs and measured the device current efficiency (Figure S6). With microlens foil, the overall current efficiency enhancement is 24% for AgNWs OLEDs and 47% for ITO OLEDs; with half-sphere, the overall current efficiency enhancement is 54% for AgNWs OLEDs and 73% for ITO OLEDs. As the half-sphere more efficiently extracts the substrate mode of the device, the current efficiency is almost the same for two types of devices due to the fully extracted substrate mode. These results give a solid proof that substrate modes are already partially extracted in OLEDs with AgNWs electrodes before attaching any light out-coupling lens, leading to the higher current efficiency of the device. It is noteworthy that the smaller current efficiency enhancement in thermalevaporated AgNWs OLEDs might be associated with the small pixel area (2 mm × 2.3 mm) and thick capping glass (1.1 mm), which limits the scattering effect. The capping glass used here is merely for the convenience of the epoxy peeling-off procedure and device fabrication, and it is not mandatory during the polymer-transferring/peeling-off process. Our results indicate that the AgNWs electrode has an additional benefit for light extraction in OLEDs due to its scattering properties.

rough surface can be mitigated in solution processed OLEDs. We fabricated multilayer solution processed green OLEDs with the following structure: AgNWs or ITO anode/PEDOT:PSS/ spin-coated 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP): 8 wt% tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy) 3)/thermalevaporated TPBi/LiF/Al (Figure S4). Both devices were made with a large pixel size of 1 cm × 0.6 cm. From the J-VL characteristics, the AgNWs device shows a sharp turn-on voltage and EL characteristics comparable with the ITO device. We have also observed an overall enhancement of current efficiency up to 40% in the AgNWs device and haze around the pixel area due to the scattering effect of AgNWs electrodes (Figure S5). We then continue to fabricate the thermal-evaporated OLEDs on the AgNWs/PEDOT:PSS composite electrodes. For thermal-evaporated OLEDs, the AgNWs/PEDOT:PSS composite electrodes were patterned with four 2 mm-width stripes (as shown in Figure 2), and both devices with AgNWs and ITO electrodes were made with a small pixel size of 0.2 cm × 0.23 cm. For our autopatterned AgNWs/PEDOT:PSS transparent electrodes, the variation for the width of patterned electrode stripes is less than 5%, indicating good control in the autopatterning process. The architecture for thermal-evaporated OLEDs is shown in the Experimental Section and Figure 5a. The current density−voltage-luminescence (J-V-L) characteristics are shown in Figure 5b. The clear turn-on voltage and low current density below turn-on in the J−V curves of AgNWs devices give a solid proof that leakage current is absent in the devices due to the smooth surface of the AgNWs composite electrode. The turn-on voltage is 2.8 V for both ITO and AgNWs devices, indicating good hole injection from both electrodes to emitting layers. The current efficiency of AgNWs and ITO OLEDs is compared in Figure 5c. As no leakage or current preferential path is observed in OLEDs with AgNWs electrodes, the AgNWs OLED yields a higher current efficiency than that of the ITO device over the entire measured luminance. The maximum current efficiency of the AgNWs device is 54.4 cd A−1 at 60 cd m−2 and remains 49.0 cd A−1 at 1000 cd m−2, whereas the ITO devices show a maximum efficiency of 47.6 cd A−1 at 37 cd m−2 and roll-off to 44.6 cd A−1 at 1000 cd m−2. The higher current efficiency of AgNWs devices as compared to ITO control devices is attributed to the scattering effect of AgNWs (Figure 6) as reported in the literature.27,28 Figure 5d shows the electroluminescence (EL)

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CONCLUSION In summary, we developed an autopatterning and transferring technique based on manipulation of the surface wetting properties of PDMS. The autopatterning method shows the potential of applying electrodes to any type of electronic devices such as OLEDs, organic thin film transistors and photovoltaic cells via the solution process, which can be used for a low cost, large scale, and high throughput manufacturing process. In addition, this method is compatible with fabricating pixel with sharp definition with a scale up to 100 microns. A fine device pattern can be achieved without using the photolithography process. Furthermore, as a buffer layer, the PEDOT:PSS layer can also be patterned and transferred using the same technique, rendering a ultrasmooth surface of the transferred AgNWs/PEDOT:PSS composite electrode. High quality solution-processed and thermal-evaporated OLEDs with a UV-curable resin/AgNWs/PEDOT:PSS composite electrode have been demonstrated. Up to 40% and 15% enhancement in efficiency compared with ITO devices was also observed in solution-processed and thermal-evaporated OLEDs, respectively. The enhanced current efficiency is due to the scattering effect from AgNWs. As a result, our work paves the way for applications of the AgNWs electrode in various types of electronic devices.

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Figure 6. Electroluminescence picture of OLEDs with (a) the commercial ITO anode and (b) the AgNWs anode. The pixel size is 2 mm × 2.3 mm. The pictures were taken at a luminescence intensity of 500 cd m−2. The scattering effect can be observed in the AgNWs electrode.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00719. E

DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Demonstration of hydrophobic/hydrophilic surface wetting control of PDMS release template, AFM image showing coarse topography of AgNWs mesh spin-coated onto bare PDMS release template, transmittance of AgNWs electrodes transferred from bare PDMS and PEDOT:PSS coated PDMS release template, device architecture and performance of solution-processed OLEDs with AgNWs and ITO electrodes, photos demonstrating scattering effect of solution-processed OLEDs with AgNWs electrodes, current efficiency of the ITO and AgNWs OLEDs with microlens foil and half-sphere (PDF)

AUTHOR INFORMATION

Corresponding Author

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

S.L. and S.H. contributed equally. Funding

All of the authors received funding from Wintek, Taiwan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Wintek for the funding support to this project.

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REFERENCES

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DOI: 10.1021/acsami.6b00719 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX