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Highly Conductive and Uniform Alginate/Silver Nanowire Composite Transparent Electrode by Room Temperature Solution Processing for Organic Light Emitting Diode Lu Lian, Dan Dong, Shuai Yang, Bingwu Wei, and Gufeng He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01159 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

Highly Conductive and Uniform Alginate/Silver Nanowire Composite Transparent Electrode by Room Temperature Solution Processing for Organic Light Emitting Diode Lu Lian, Dan Dong, Shuai Yang, Bingwu Wei and Gufeng He* National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China.

KEYWORDS: silver nanowire, mechanical pressing, alginate, renovate, OLED

ABSTRACT: A novel transparent electrode composed of Alginate/silver nanowire (AgNW) with high conductivity and low roughness is fabricated via solution process at room temperature. Solgel transition of the Alginate triggered by CaCl2 solution bonds the AgNWs to the substrate tightly. Meanwhile, Cl- in the solution can renovate the cracks on the AgNW surfaces created during the mechanical pressing, resulting in a great increase of the electrical conductivity. The Alginate/AgNW composite film can reach a sheet resistance of 2.3 Ω/sq with a transmittance of 83% at 550 nm. The conductivity of the composite film remains stable after bending and tape test, demonstrating excellent flexibility and great adhesion of AgNWs to the substrate. Moreover, the

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composite film shows better stability to resist longtime storage than conventional annealed-AgNW film. The organic light emitting diode using such Alginate/AgNW composite film as anode presents comparable current densities and luminances to those of ITO anode, and higher efficiencies are obtained due to the better charge balance.

INTRODUCTION Transparent conductive electrodes are indispensable components for optoelectronic devices such as solar cells and organic light emitting diodes (OLED). The most commonly used transparent conducting materials are composed of metallic oxides, primarily sputtered indium tin oxide (ITO).1-3 However, several crucial drawbacks, such as damage of plastic substrate during hightemperature sputtering,4-6 indium scarcity,3, 7 high cost and lack of flexibility,8-10 restrict its application on flexible devices in the future. Great efforts have been made to develop sustainable alternative transparent conductors to ITO, such as chemical vapor deposition graphene,11 reduced graphene oxide,12 carbon nanotubes (CNT),13, 14 conducting polymers15-17 and metal nanowires.3, 18

Among them, silver nanowire (AgNW) film has demonstrated excellent transparency and

sheet resistance, which are comparable to that of ITO.18-21 However, AgNW films are still unsuitable for the commercial applications due to their disadvantages addressed. First, in order to reduce contact resistance between silver wires and increase the conductivity of the whole film, a subsequent sintering at a relatively high temperature approximately 200 °C is required.22, 23 This restricts its application in various heatsensitive substrates. Second, the high peaks created by overlapping junctions between wires make the surface roughness extremely high.24, 25 In practical application, these protruding


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nanostructures will easily form current pathways through the device, which may lead to shunting and shorting. Third, the adhesion between the AgNWs and substrate is too weak to resist the detachment,24-26 so that the AgNW electrodes are easily destroyed in the application. Various instrumentalities have been proposed to solve these problems, such as light-induced heating,27 microwave or radiation sintering,28, 29 two-step polymer encapsulating30, 31 and substrate modification.32 However, these methods usually involve complex procedures or expensive equipment, which obstructs their application into the commercial applications. Recently, a simple electrolyte such as a halide has been reported to trigger the coalescence of metallic nanostructures by removing the capping agent on the surface, which improve the conductivities of metallic nanostructures.33-35 The silver nanostructures undergo an Ostwald ripening process and a spontaneous coalescence occurs among them.34-37 AgNW films treated by this sintering method can reach a relatively high conductivity because of the well connection between nanowires, but the high surface roughness is still a serious problem to be solved urgently. In this work, we develop a room temperature solution processing method to achieve highly conductive and smooth AgNW composite film with great adhesion. The thick junctions between AgNWs embed into a sodium alginate (NaAlg) layer by mechanical pressing and weld together forming a particularly smooth surface. However, some micro cracks appear on the wire-junctions owing to the hard pressure, which are finally cured by calcium chloride (CaCl2) treatment and the conductivity is further improved. Meanwhile, CaCl2 can react with sodium alginate and link the polymer chains together to form undissolved network, which bond AgNWs onto the substrate tightly. The whole process is conducted at room temperature. The OLED fabricated with


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NaAlg(CaCl2)/AgNW composite anode has comparable performance to the same structure with ITO anode. EXPERIMENTAL SECTION Electrode Fabrication: Sodium alginate (Sinopharm Group Co. ltd.) was dissolved in water with a concentration of 0.5 wt%, and then spin-coated onto clean substrates at a rate of 1500, 2000, 3000, 4000 and 6000 rpm to produce NaAlg films with various thicknesses. Silver nanowires were synthesized by reducing silver nitrate in the presence of ethylene glycol and poly (vinylpyrrolidone) (PVP).38, 39 The average diameter and length of the AgNWs were 90 nm and 15 μm respectively. The AgNWs were dispersed into ethanol, and then spin-coated onto the NaAlg films, marked as NaAlg/AgNW. As the concentration increased, both the transmittance and the sheet resistance of the films decreased. Considering both parameters, the figure of merit of the samples was calculated and reached the maximum value at the concentration of 3 mg/ml, as shown in Figure S1, which was selected as the optimized value for the following progress. The pressure loading was conducted by a self-designed compressor driven by compressed air, and the applied pressure was controlled accurately by an air pressure boosting device. The film was covered by another clean glass substrate edge to edge, and the sandwich structure was transferred into the compressor and applied with various force.40 After pressing, the films were immersed into the CaCl2 solution (5 wt%) for 5 minutes, and the NaAlg turns into CaAlg partially, marked as NaAlg(CaCl2)/AgNW. At last, the as-prepared films were rinsed by deionized water and dried by nitrogen. OLED Device Fabrication: PEDOT: PSS (Clevios AI4083, H. C. Starck) was spin-coated in air at 1500 and 1000 rpm on the NaAlg(CaCl2)/AgNW and ITO anode respectively and then


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annealed at 100 °C for 10 min. The process was repeated twice to get a 60 nm-thick PEDOT: PSS layer as hole-injecting layer (HIL). Then the films were transferred into a high-vacuum chamber (operating pressure below 5×10-6 Torr) to deposit following organic and metal materials. The OLED used 1 nm-thick molybdenum trioxide (MoO3) as a second HIL, 80 nmthick N,N-dicarbazolyl-3,5-benzene (mCP) as hole transporting layer (HTL), 0.2 nm-thick iridium(III)bis(2-(4-triﬂuoromethylphenyl)pyridine) tetraphenylimidodiphosphinate (Ir(tfmppy)2(tpip)) as emitting layer (EML),41-43 57.5 nm-thick 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl) benzene (TPBi) as electron transporting layer (ETL), 1 nm-thick lithium fluoride (LiF) as electron injection layer (EIL), and 80 nm-thick aluminum (Al) as cathode. A quartz-crystal monitor was used to monitor the deposition rate. The active area was 3 × 3 mm2, which was defined by the overlap of the anode and cathode. Characterization: The film thickness was measured by KLA-Tencor Alpha Step D-120 Stylus Profiler. The sheet resistance was evaluated by a four-point probe system and obtained from averaged value of sixteen different positions on a sample. The optical transmittance was characterized by a spectrophotometer (MAPADA UV-3100PC). Scanning electron microscopy (SEM) micrographs were taken on a field emission scanning electron microscope (FE-SEM, Sirion 200). The surface morphology and roughness were analyzed by an atomic force microscope (AFM, Nanonavi E-Sweep). The performances of OLED devices were characterized by computer-controlled source meter (Keithley 2400) and luminance colorimeter (BM-7A, Topcon). RESULTS AND DISCUSSION


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The fabrication process of NaAlg(CaCl2)/AgNW composite film is illustrated in Figure 1a. Sodium alginate solution and AgNWs suspension are spin-cast on the clean substrate successively. Subsequently, various intensities of mechanical pressures are applied on the surfaces, followed by a CaCl2 treatment. Clean AgNWs have a large surface energy, and PVP is easily absorbed on the surface of AgNWs as capping agent during the synthetic process to lower the energy and stabilize them.35, 44 Thus the pristine AgNW film shows a poor electrical conductivity because some neighboring nanowires do not connect physically. A AgNW film with the same nanowire density annealed at 180 °C for 15 min is fabricated as a reference, marked as annealed-AgNW, since high temperature annealing is the conventional way to remove organic residues on the nanowire surfaces and improve the contacts between the wires and consequently the film conductivity. Thus, the sheet resistance of the annealed-AgNW film (10.1 Ω/sq) is much lower than that of NaAlg/AgNW film (58.1 Ω/sq), shown in Figure 1b. In order to improve the conductivity and smooth the surface simultaneously, mechanical pressing is applied to weld the junctions between AgNWs. When the pressure is lower than 7 MPa, the sheet resistance of the annealed-AgNW decreases as the intensity increases; But it goes to bad contact when the pressure increases to larger than 7 MPa. As shown in the SEM image in Figure 1c, deformations and destructions occur on the AgNWs ensembles of annealed sample, and some AgNWs are detached from the substrate because of the bad adhesion to the substrate. However, the sheet resistance of NaAlg/AgNW composite film decreases dramatically from 58.1 Ω/sq to 5.9 Ω/sq when the pressure increases to 7 MPa, and does not change much as the pressure increases further, indicating that the junctions between wires are welded completely and the higher pressure does not improve the contacts anymore. Moreover, the AgNWs sinking into NaAlg film stick to the substrate tightly, and won’t be detached under hard pressure.


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Figure 1. a) Schematic procedure of NaAlg(CaCl2)/AgNW composite transparent electrode fabrication. b) Sheet resistance versus intensity of pressure for annealed-AgNW and NaAlg/AgNW films on glass. c) SEM image of Annealed-AgNW film after 10 MPa pressing.


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Figure 2. Sheet resistance versus thickness of Alginate film for NaAlg(CaCl2)/AgNW samples.

Figure 2 reveals the sheet resistances of the composite films with various Alginate film thicknesses: 0 nm, 50 nm, 100 nm, 200 nm, 350 nm, applied with an identical pressure of 10 MPa. For pristine NaAlg/AgNW film without mechanical pressing, the sheet resistance increases with the thickness increasing. As alginate is an insulating material, the electrical conduction occurs only within the AgNWs network. For pristine NaAlg/AgNW films, AgNWs are easier to sink into the NaAlg film as the NaAlg becomes thicker, which leads to loose contacts between nanowires. However, after pressing, the sheet resistances of samples drop dramatically to lower than 5 Ω/sq, and have little changes with the NaAlg thickness. As the pressure loaded, AgNWs weld together to form a uniform path for electrical current and that is independent of NaAlg thickness. After CaCl2 treatment, the sheet resistances decrease further. As a reference, the sheet


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resistance of the sample without alginate film drops to 6.5 Ω/sq from 201.3 Ω/sq after CaCl2 immersion, which is a little lower than NaAlg/AgNW samples after CaCl2 treatment. It has been reported that Cl- in the CaCl2 solution can trigger the spontaneous coalescence of the AgNW junctions.33, 34 AgNWs of NaAlg/AgNW samples are partly embedded in the NaAlg films, and only the top part can contact with the Cl- when immersed in the CaCl2 solution. However, without NaAlg film, AgNWs on the substrate have a full contact with the Cl-, resulting in better electrical conductivity than NaAlg/AgNW samples. SEM is used to investigate the morphology of the AgNWs films during the fabrication process. Without alginate film, AgNWs stack loosely on the substrate, as shown in Figure 3a, 3e. In contrast, nanowires in NaAlg/AgNW composite film (Figure 3b, 3f) coalesce together and stick to the substrate by the surface tension. But the wire junctions are much higher than a single wire. When mechanical pressure loaded (Figure 3c, 3g), the junctions are pressed to the same height as the single ones, which significantly lowers the film roughness. However, a few cracks can be seen clearly at the connection points of wires, which have negative effects on the electrical conduction of the whole film. Because of the large surface energy of fresh crack surfaces, organics including PVP are easily attached on the surfaces as capping agent. After CaCl2 treatment (Figure 3d, 3h), the cracks on the junctions disappear and the junctions between wires become smooth and complete. The Cl- in the CaCl2 solution can detach the capping agents and an Ostward ripening is undergone to renovate the cracks by dissolution of small crystal and redeposition of the dissolved species on the surfaces of larger crystals.34-36 Cracks on the AgNWs depart the wire-junction into inhomogeneous fragments. With the CaCl2 treatment, small fragments redeposit onto larger parts, resulting in a completely intact junction. The schematic illustrations of the renovation are shown in Figure 3i.


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Figure 3. SEM images of a) pristine AgNWs, b) NaAlg/AgNW composite film, c) NaAlg/AgNW composite film after mechanical pressing, d) NaAlg/AgNW composite film after mechanical pressing and CaCl2 treatment. e, f, g, h) Schematic diagrams of junction between AgNWs corresponding to (a, b, c, d). i) Schematic illustrations of a possible mechanism of crack renovation.


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Figure 4. AFM images and height line profiles of AgNWs. a, b) Pristine AgNWs. c, d) NaAlg/AgNW composite film after mechanical pressing. e, f) NaAlg/AgNW composite film after pressing and CaCl2 treatment.

AFM images are used to further characterize the surface morphologies of the samples. The root mean square (RMS) of pristine AgNWs is measured at 27.9 nm, and the top-to-bottom height of the surface is at the range of 70-170 nm (shown in Figure 4a, 4b), depending on the number of wire-stacks on the junctions. In comparison, after mechanical pressing, the RMS of the NaAlg/AgNW film decreases to 4.1 nm, with the AgNWs rising above the surface lower than 55


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nm, shown in Figure 4c, 4d. The height of the wire-junctions is almost the same as individual wires, demonstrating a full welding of the wire-wire junctions. After CaCl2 treatment, the height of the AgNWs is almost unchanged, but the RMS decreases to 3.8 nm, shown in Figure 4e, 4f. This can be attributed to the renovation of the AgNWs cracks and the sol-gel transition of the alginate. The sodium alginate reacts with the calcium ion (Ca2+) to form water resist alginate calcium, and the AgNWs ensembles are fixed to the substrate tightly.18 In addition, as a biocompatible material, alginate can be used in various bioelectronic devices.

Figure 5. Transmittance of ITO, annealed-AgNW, NaAlg/AgNW film, NaAlg/AgNW film after pressing, NaAlg/AgNW film after pressing and CaCl2 treatment. All the samples are prepared on glass, and their sheet resistances are noted.


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Figure 5 shows the optical transmittance and sheet resistance of ITO, the annealed-AgNW, NaAlg/AgNW film, NaAlg/AgNW film after pressing, NaAlg/AgNW film after pressing and CaCl2 treatment. At a wavelength of 550 nm, the transmittance of ITO is 90.0%, which is higher than the other four samples. The NaAlg/AgNW composite film without pressing is 86.3%, quite close to that of the annealed-AgNW (86.5%). After mechanical pressing, the transmittance of the composite film decreases slightly to 82.9%, attributed to the reduced height and increased width caused by pressing. After CaCl2 treatment, the transmittance of the composite film is almost invariant, while the sheet resistance decreases from 5.3 Ω/sq to 2.3 Ω/sq, demonstrating the well welding and renovation of the AgNWs assembles. The figures of merit φTC45 is used to characterize the electrical performance and optical of the samples simultaneously, which is defined as following:

TC

Tav10  Rsh

The Tav is the transmittance, while Rsh is the sheet resistance. At a wavelength of 550 nm, comparing the φTC of ITO, annealed-AgNW and NaAlg(CaCl2)/AgNW composite film, φTC(ITO)=5.62×10-2 Ω-1, φTC(annealedAgNW)=2.32×10-2 Ω-1, φTC(NaAlg(CaCl2)/AgNW)=6.67×10-2 Ω-1. The φTC of the NaAlg(CaCl2)/AgNW composite film is almost triple that of the annealed-AgNW film, and higher than ITO. Thus, the NaAlg(CaCl2)/AgNW composite film performs the best.


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Figure 6. a) The bending test of ITO, annealed-AgNW and NaAlg(CaCl2)/AgNW composite film on PEN substrate. Photographs before and after tape test and the SEM images after tape test of b, c) annealed-AgNW film and d, e) NaAlg(CaCl2)/AgNW composite film.

In order to investigate the flexibility of the NaAlg(CaCl2)/AgNW composite film, bending test is conducted on sample fabricated on polyethylene naphthalate (PEN) substrates, taking ITO and annealed-AgNW films on PEN as references. A concave and a convex compression with a bending radius of 6 mm is included in a bending cycle, and the sheet resistances of the samples are mesured every 50 bending cycles. Shown in Figure 6, the ITO film goes to bad contact


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quickly becauese of its brittleness. The sheet resistance of annealed-AgNW increases 670% from 10.6 Ω/sq to 81.6 Ω/sq after 1000 bending cycles, while that of NaAlg(CaCl2)/AgNW composite film does not alter significantly. Mechanical tape tests25 are used to further study the adhesion of AgNWs to the substrates. A clear boundry can be seen on the annealed-AgNW sample after tape test, shown in Figure 6b, indicating part of AgNWs are detached from the substrate by the tape. And the SEM image in Figure 6c comfirms that only a few AgNWs are left on the substrate. In contrast, there is no obvious change on the NaAlg(CaCl2)/AgNW sample after the tape test (Figure 6d). Since the AgNWs are fixed to the substrate tightly by the gel alginate calcium, almost all the AgNWs are in original positions and are not detached from the substrate (Figure 6e).


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Figure 7. a) Sheet resistance change of Annealed-AgNW and NaAlg(CaCl2)/AgNW film at a constant humidity of 30% and room temperature condition. b) The rate of sheet resistance change as a function of days. SEM images of c) annealed-AgNW and d) NaAlg(CaCl2)/AgNW film after 30 days exposed in air.

The stability of the conducting films seriously affects their applications in practical devices. In order to investigate the conductivity change of NaAlg(CaCl2)/AgNW composite film in the ambient condition, its sheet resistance was measured everyday, with an annealed-AgNW sample as a reference. As shown in Figure 7a-b, at the 21st day, the sheet resistance of annealed-AgNW film increases significantly from 10.5 Ω/sq to 15.4 Ω/sq with a change rate of 46.3%, while the conductivity of NaAlg(CaCl2)/AgNW composite film change slightly with a low rate of 7%. The SEM images in Figure 7c-d show the morphologies of the surfaces after 30 days in air. Nanoparticles on the nanowires are observed on annealed-AgNW samples. It has been reported that a thin oxide layer would generate during the annealing progress because of the activation of O2 diffusion at high temperature.31 This oxide layer has nagative effect on the stability of the conducting film. On the contrary, the surface of AgNWs on the NaAlg(CaCl2)/AgNW composite film is quite clean. The gel alginate calcium around the AgNWs forms a hydrophobic protective layer, which protects the AgNWs from corrosion and oxidation sufficiently.


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Figure 8. Characteristics of OLED devices with NaAlg(CaCl2)/AgNW composite anode and ITO anode. a) Schematic section of the OLED device. b) Current density versus voltage. c) Luminance versus voltage. d) Current efficiency versus luminance.

OLED devices are fabricated on NaAlg(CaCl2)/AgNW composite film and ITO anodes. The structure of the devices is shown in Figure 8a, and the energy level diagram of the device is illustrated in Figure S2. For comparison, the same structure is also built on annealed-AgNW film, but the device shorts out at 3 V without lighting. This can be attributed to the thick junctions on the surface of the annealed-AgNW film, which are prone to form current pathways between the electrodes. Though the NaAlg(CaCl2)/AgNW composite electrode shows lower


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sheet resistance than that of ITO, the work function of AgNWs is 4.2 eV, lower than that of ITO (4.8 eV), which makes it more difficult to inject holes from the anode into the organic layer, leading to lower current density, as shown in Figure 8b. The OLED with NaAlg(CaCl2)/AgNW anode shows comparable luminance to the device with ITO anode, and the turn-on voltage is the same at 3.6V. At a voltage of 10 V, the luminance of the OLED with NaAlg(CaCl2)/AgNW anode is 22890 cd/m2, while that of ITO anode is 19790 cd/m2. The current efficiency of the divece with NaAlg(CaCl2)/AgNW anode is 43.1 cd/A at a luminance of 1000 cd/m2, while that of ITO anode is 30.7 cd/A. As the device structure is hole-abundant, less hole injection results in higher current efficiency. The OLED device with NaAlg(CaCl2)/AgNW anode shows better charge balance due to the slightly poorer hole injection of the composite anode than ITO.

CONCLUSION We have developed an effective method to fabricate highly conductive and smooth transparent electrode with enhanced adhesion without high-temperature annealing. Via mechanical pressing, AgNWs weld together to the same height and embedded into the sodium alginate layer, resulting in a uniform composite film which is smooth enough for thin-film devices. After CaCl2 treatment, a stable NaAlg(CaCl2)/AgNW composite electrode is obtained with a sheet resistance of 2.3 Ω/sq and a transmittance of 83%. On one hand, the chloride ions in CaCl2 solution renovate the cracks on the surface of AgNWs created by the mechanical pressing and the electrical conductivity of the film increase greatly. On the other hand, the calcium ions in the CaCl2 solution cross-link the alginate to form water resist gel, which bonds the AgNWs to the substrate tightly against bending and detaching, and enhances the film stability for longtime storage. Using the NaAlg(CaCl2)/AgNW composite film as anode, the OLED demonstrates


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comparable current density to the device with ITO anode, and the current efficiency is 1.4 times as high as that of ITO. This technique can be helpful for the development of ITO-free flexible transparent electrode for optoelectronic devices application. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: NaAlg(CaCl2)/AgNW films with various concentration of AgNWs suspension, energy level diagram of the OLED device (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +86 21 34204371; Tel: +86 21 34207045 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61675127, 61377030).


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