Interface Modified Flexible Printed Conductive Films via Ag2

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Functional Inorganic Materials and Devices

Interface modified flexible printed conductive films via AgO nanoparticle decorated Ag flake inks 2

Yuhuan Meng, Taichong Ma, Felippe J Pavinatto, and J. Devin MacKenzie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20057 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Materials & Interfaces

Interface modified flexible printed conductive films via Ag2O nanoparticle decorated Ag flake inks Yuhuan Meng†, Taichong Ma†, Felippe Jose Pavinatto† and John Devin MacKenzie*, † ‡ † Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, USA. ‡ Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA.

KEYWORDS: printed conductive films, silver ink, interface modification, Ag2O nanoparticledecorated Ag flake inks

ABSTRACT: A new approach to stable, low resistance inexpensive printed flexible conductive inks is proposed. Silver inks have been extensively studied and commercialized for applications in printed electronics due to the inherent high conductivity and stability of silver, even in particulate-based percolation networks processed at temperatures compatible with low cost polymer films such as polyethylene terephthalate (PET). Recent interest in flexible and even stretchable circuits, however, has presented new challenges for particle-based inks as mechanical strains can result in the opening of critical particle-to-particle contacts. Here we report a facile, low cost method for the single step synthesis of stable, printable nanoscale Ag2O-decorated Ag flake inks which can be converted to highly conductive Ag films at 150°C curing temperature

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without the use of limited shelf life organometallics or low metal loading nanoparticles to modify the interface between silver flakes. Analysis indicate that decoration of Ag flakes with Ag2O nanoparticles (NPs) during ink synthesis improves the conductivity and flexibility of printed silver films by forming bridging interconnections between Ag flakes after low temperature reduction of the Ag2O NPs. In this work, printed nano-decorated silver conductors with starting oxide to metal weight ratios of 5:95 exhibited lateral resistivities lower than 1.5×10-5 Ω cm, which was 35% less than films derived from undecorated Ag flake inks of the same total Ag loading and binder system. This resistivity difference increased to 45% after cyclic bend testing showing increased resilience to repeated flexing for the nano-decorated inks. Through detailed compositional and morphological characterizations, we demonstrate that such improved conductivity and flexibility is due to a more effective bridging afforded by the in-situ synthesized Ag NPs on the surface of Ag flakes. These properties, combined with the simplified syntheses method of the nano-ink, make the material a viable, advantageous alternative to the limited number of stretchable conductors currently available.


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1. INTRODUCTION In the past few decades, printed electronics has emerged as a scalable manufacturing approach that combines engineered inks with additive printing to produce a diverse array of printed electronic devices and systems.1–8 Conductive inks are fundamental building blocks for virtually any printed electronic device and they have been studied extensively starting with the invention of printed circuit boards through to emerging flexible circuits.9, 10 Conductive printing inks are typically multi-component formulations that contain conductive materials or conductive material precursors in a liquid solvent vehicle that can also include binders and additives that enable optimal electrical performance, rheology, mechanical properties and processability.11 The broad field of conductive inks includes metal-based inks,10 as well as carbon-,12 and polymer-based inks.13 Conductive inks for the deposition of silver have their own prominence in terms of relatively stable electrical properties and ease of processing.9,14 Silver is the most conductive pure element at room temperature and the native surface passivation provides a relatively low barrier to charge transport unlike the oxides of other metals.15 With similar printable formulations and processing temperatures, other metal particle inks made from metals such as Cu or Al are relatively resistive due to blocking native surface oxides that form in air or other oxidizing environments leading to high particle to particle contact resistance. This particle-toparticle resistance can dominate the overall effective resistivity of films formed from particles composed of these materials. In the formation of a printed silver conductor from discrete metallic particles, a 3D continuous conductive percolation network must be established, typically from micron-scale metal particles mixed with a non-conductive polymer matrix or binder that promotes film cohesion and adhesion to an underlying substrate. In addition to increasing mechanical stability,


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these non-conductive ink components can promote advantageous rheological properties during printing such as fluid flow when the matrix is dissolved in a solvent, as well as thixotropy and pseudoplasticity which can aid in advantageous non-Newtonian flow during printing and dimensional stabilization of printed ink features immediately after the ink is deposited on to a substrate. The latter can mitigates lumping and reflow of ink features that lead to loss of pattern fidelity.16 The contact resistance and the overall resistivity in printed conductive particle ink features can, in some instances, be reduced through high temperature curing to sinter silver particles together at temperatures approaching bulk melting points.11 For flexible electronics, however, low cost substrates are often heat sensitive. Heat-stabilized polyethylene terephthalate film, for example, has a maximum process temperature of ca. 150°C,17 well below the melting point of Ag metal. Based on these temperature constraints, there has been strong interest in developing highly-conductive silver inks that can be processed at low curing temperature, well below Ag melting temperatures and polymer substrate temperature limits, to achieve good conductivity and resilience to bending. Silver organic decomposition precursor (ODP)18–20 and silver nanoparticle (NP)21,22 inks have been developed to overcome this processing temperature/performance challenge. These inks, after the appropriate thermal processing, can achieve conductivities of 10-5 Ω cm and below. ODP and NP inks can have a number of drawbacks, however, including low ink shelf life and stability, high ink synthesis costs, and the inability to print thicker, lower absolute resistance conductors due to low metal ink loading limits and high shrinkage during ink drying that can lead to cracking in thick films. Such thick conductors, sometimes exceeding 10 microns in thickness, are needed, for example, to form solar cell and battery current collectors with low series resistance-related power losses.


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Currently, micron-scale elongated Ag flakes are often used as the main conductive filler in commercial conductive silver inks as a way to reduce the number of resistive particle to particle contacts in relatively low curing temperature inks.16,23 It has been shown that adding Ag NPs from a separate synthesis to micron-scale Ag flake inks can further improve the conductivity by reducing contact resistance.24,25 In these materials, Ag NPs help sinter Ag flakes together and provide low particle to particle resistances at the lower sintering temperatures associated with NPs. Ag NPs have been successful at enabling printing of thin (typically less than 100 nm thick without repeated overprinting) inkjet printed conductors from low viscosity inks (typically 10-15 cP26). However, the high cost of Ag NPs and the difficulty of stably suspending Ag nanocrystals in inks at high metal loading levels limits the broader use of this approach. Silver flake pastes for screen printing can have silver weight loadings in excess of 70%. This is advantageous as it allows for the deposition of thicker, higher Ag mass per unit area and low resistance printed conductors in a single printing step. NP inks, such as those typically used for inkjet printing of silver conductive features, can typically achieve solid mass loadings near 30%. Also, the inkjet process, typically involving the deposition of 1-10 pL droplets of low viscosity (e.g. 10-15 cP) fluid, typically deposits Ag films that are one to two order of magnitude thinner than those formed from screen printing. Organometallic Ag precursors, such as silver neodecanoate, have also been proposed as low temperature sintering aids in flake inks.27 In addition to similar limits on metal loading as in the case of the NPs, there are also shelf life and stability limits for reactive organometallic additives and there has been little evidence showing that they improve the resilience of conductors to flexing. In this work, a novel, stable silver inks have been formulated using Ag2O nanoparticledecorated Ag micro-flake (Ag flake/Ag2O NP) as the main conductive filler. The Ag2O NPs can


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be reduced to metallic silver at low sintering temperatures after printing of the inks onto the target substrate. These reduced Ag2O NPs decrease the contact resistance between the Ag flakes and increase the electrical resilience to flexing of the electrode, as depicted in Figure 1. The Ag2O NPs with high aspect ratio are distributed evenly on the surface of Ag flakes through deposition by a facile nitrate precipitation synthesis. This approach is in contrast to depositing metallic Ag NPs directly on to micron-scale Ag flakes. The direct metal nanoparticle deposition approach is challenging due to the large amount of continuous nucleation sites provided by the flakes which tends to result in low aspect ratio, and less conformal deposition of Ag onto the flake surfaces. With the in-situ syntheses of the nano Ag2O it was found that high mass loading inks could be formed with the nano-decorated flakes using standard, high volume flake ink mixing procedures. In optimization experiments, the weight ratios of the Ag flake/Ag2O NP were varied to assess the relationship between Ag2O content and the conductivity of printed and sintered films. Moreover, the flexibility was measured by cyclic bending and increasing bend radii to show the influence of Ag2O NPs’ decoration. The fabricated silver conductive electrode exhibits a high conductivity value of 1.5×10-5 Ω cm and good flexibility. In addition, the function of the Ag2O NPs was studied through the microanalytical characterization of the silver materials prior to deposition and after sintering.


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Figure 1. Schematics of film preparation: (a) Traditional method and (b) proposed method in this work. (Ⅰ) Ag flake, (Ⅱ) Ag flake electrode, (Ⅲ) Ag flake with Ag2O nanoparticle decoration, (Ⅳ) Ag flake/Ag NP electrode.

2. EXPERIMENTAL SECTION 2.1 Materials. Silver nitrate (AgNO3), (di)propylene glycol methyl ether and ethyl cellulose were obtained from Sigma-Aldrich. Silver flakes (EA-3106) were purchased from Metalor. Ethanol was supplied by Decon Labs. Sodium hydroxide was gained from Fisher Scientific. All chemicals were used without further purification. 2.2 Preparation of Ag flake/Ag2O NP powders. First, 0.0849 g, 0.1698g, or 0.3396g silver nitrate (AgNO3) was dissolved in 140 ml ethanol solution. Then, 1 g Ag flakes was dispersed into the solution, followed by sonicating for 30 min to guarantee good dispersion of silver flakes. Next, the solution was removed to the stir plate with stir speed at 450 rpm. 1 ml, 2 ml or 4 ml 0.5 mol/L NaOH ethanol solution was added dropwise into the Ag dispersion. When the joint solution system was mixed uniformly, adjust the stir speed at 300 rpm. After 15 hours, the product was washed with a mixture of ethanol and water three times, and then dried at 60 ℃. Ag


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flake/ Ag2O NP composites with Ag2O/Ag weight ratios of 5:95, 10:90, or 20:80 were produced from the different AgNO3 concentrations. 2.3 Preparation of Ag flake/Ag2O NP inks. A common solvent (di)propylene glycol methyl ether and ethyl cellulose were used for the ink’s solvent and binder, respectively. The weight percent of total silver content was controlled at 70%. To formulate the particles into printable inks, ethyl cellulose was added to (di)propylene glycol methyl ether at the weight ratio of 1:4 and stirred for 6 h to make the solution. After that, Ag flakes or Ag flake/Ag2O NP powders and the vehicle were mixed at the weight ratio of 7:3 using planetary centrifugal vacuum mixing (THINKY) at 1700 rpm for 15 min to gain the final silver ink. 2.4 Preparation of conductive Ag flake/NP circuits. nScrypt direct nozzle printing technology (nScrypt-3Dn-300) was used to additively fabricate silver traces through micro-dispensing with a SmartPump pneumatic device. The primary advantages of this process are its adaptability to inks with a wide range of flow properties and controllability of deposition and positioning of features and its efficient use of ink. Except for complex situations such as the formation of high resolution patterns on a 3D surface, there are few restrictions on ink rheology.28–30 Four printing parameters were adjusted to optimize printing quality and feature dimensions including, printing speed, extrusion rate, distance between nozzle head and the substrate, and pressure, to obtain Ag electrodes with different patterns on different substrates. Ceramic tips with 125 µm nozzle aperture were employed. After printing, all samples were annealing at 150 °C in air on a hot plate for 30 min.17 The final printed Ag test electrodes lines were 6 cm long and 5 mm wide. 2.5 Characterization methods. The morphology was observed with a high-resolution scanning electron microscope (SEM, Sirion XL30, FEI). Higher magnification information was obtained


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by transmission electron microscope (TEM, Tecnai G2 F20, FEI). Crystal structure was identified using X-ray diffractometry (XRD, D8 Advance, Bruker). The electrode’s thickness was measured by stylus profilometery (DektakXT, Bruker). The resistivity of silver electrode was acquired by a digital sourcemeter (Keithley 2450, Tektronix) using a four-point probe configuration.

3. RESULTS AND DISCUSSION A precipitation method was used to synthesize Ag2O NPs on the surface of Ag flakes utilizing low-cost precursors in a simple synthesis.31 According to Figure S1a (supplementary material), the average long axis size of the Ag flakes is approximately 15 μm. Ag2O NPs were observed to be scattered evenly on the surface of Ag flakes after synthesis, verifying that the nanoparticles have been successfully loaded on to the Ag flakes, as shown in Figures S1b and 2a. No significant amounts of nanoparticles were observed unattached to the flakes, indicating that the nanoparticles were coated on Ag flakes with high yield. The formed 95:5 Ag flake/ Ag2O NP powders (ratio by weight) can then be mixed with binders in high solid loading inks using standard flake ink mixing approaches.16 To characterize the 95:5 Ag flake/Ag2O NP composites on the nanoscale, TEM was used to determine nanoparticle size, structure and composition. Figure 2b depicts a micrograph of a single Ag flake. In this image, black area represents micron scale Ag flake due to the blocking of electron transmission. In contrast, nanoparticles are imaged as grey features with sizes of about 20-40 nm that can be observed surrounding the Ag flakes. Additionally, HRTEM images have given us further insight into the microstructure and crystallinity of the as-prepared Ag2O NPs (Figure 2c), indicating that the marked interplanar distances are about 0.238nm and 0.193nm, which are asscoiated with (111) planes of Ag and


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(211) planes of Ag2O, respectively. These results demonstrate that well-distributed and uniform size crystalline Ag2O NPs have been deposited by a precipitation method on Ag flakes.

Figure 2. Characterization of Ag flake/Ag2O NP powders: (a) SEM image, (b) TEM image and (c) HRTEM image of 95:5 Ag flake/Ag2O NP; (d) XRD patterns of Ag flake and 70:30 flake/Ag2O NP powders.

The crystallinity of the Ag2O NPs was also confirmed by XRD. Figure 2d includes XRD patterns obtained from un-sintered Ag flake and 70:30 Ag flake/Ag2O NP powders. Compared with the Ag flake’s diffraction pattern, the diffraction pattern from Ag flakes decorated with


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Ag2O NP include a reflection at 2θ = 32.3 degrees, corresponding to the characteristic peak of cubic Ag2O (JPCDS-43-0997). Next, we used the Ag flake/Ag2O NP composite powders at different weight ratios to formulate inks. (Di)propylene glycol methyl ether and ethyl cellulose were added as ink solvent and binder. The weight percent of solvent, binder and Ag flake/Ag2O NP powder were kept at 24%, 6%, 70%, respectively. Additive nozzle printing was used to fabricate conductive features on flat and curved surfaces with the thickness ca. 10 μm. SEM was used to understand the influence of Ag2O NPs on our printed conductors. Figures 3a and 3b are top views of the printed conductors which were formed by 95:5 Ag flake/Ag2O NP powders and Ag flake, respectively. Compared with a conductive electrode formed by undecorated Ag flakes (Figure 3b), Figure 3a indicates, at the same magnification, that the number of resolvable flakes is lower because the outlines of most silver flakes are obscured by Ag deposits from Ag2O reductive sintering between flakes.32


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Figure 3. (a)(c) SEM images of Ag printed conductors formed from Ag flake/Ag2O NP inks; (b)(d) SEM images of Ag printed conductors formed from Ag flake inks; (e) TEM image of sintered 95:5 Ag flake/Ag2O NP powder; the inset shows the interplanar distance of black marked area; (f) XRD pattern of 70:30 Ag flake/Ag2O NP after 150 °C annealing.


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To probe the nano-decorated flake contact morphology, high-resolution SEM imaging was performed, as shown in Figure 3c and 3d. Compared with Ag flake conductive printed lines where flakes were connected by fewer point-contacts, the number of Ag flake contacts in the modified samples is larger due to the bridging afforded by the reduced Ag2O nanoparticles. The lower resistance for the Ag2O functionalized samples can be attributed to the high density of electrically continuous Ag NP bridging contacts between flakes.33,34 This result was verified by TEM imaging and lattice measurements. Figure 3e depicts the microstructure of 95:5 Ag flake/Ag2O NP powder samples after 150 ℃ heat treatment for 30 min. It was observed that the bridge-like structure formed between Ag flakes had a characteristic interplanar distance of 0.2401 nm, which corresponds to the (111) planes of pure Ag. The reduction of Ag2O was also observed in XRD. As shown in Figure 3e, the characteristic 32.2° 2θ peak of cubic Ag2O in the XRD spectrum of the un-sintered sample of the same formulation (Figure 2d) disappears upon sintering, further supporting the premise that the Ag2O NPs have been substantially reduced to metallic Ag. The resistivity of conductive films from the different Ag flake/Ag2O NP powders was measured using a four-point probe technique and is plotted in Figure 4a. This data demonstrates that the lowest resistivity was observed for an Ag2O NP to silver flake weight ratio of 5:95. In this case average resistivities were 1.5×10-5 Ω cm, which represents a 35% reduction as compared with inks made of pure Ag flakes. As the weight percent of Ag2O NPs increased beyond this ratio, the resistivity rises. To understand this trend, the morphology of the printed conductors has been characterized. Cross-section images of conductive electrodes with different Ag flake/Ag2O NP weight ratios are shown in Figure S2. The progressive cross section images indicate that with higher Ag2O NP compositions, the silver flakes tend to aggregate, leading to


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less-uniform conductive network formation due to the increasement of the specific surface area of Ag flake/Ag2O NP powders.32 Although aggregation can be avoided by increasing the solvent content, higher solvent loading in the inks leads to less compact printed and sintered conductive films.35 This explanation is further supported by mass density measurements which showed that the printed conductor density is increased due to the addition of the Ag2O NPs (Figure S3) with the samples derived from 95:5 Ag flake/Ag2O NP weight ratios having the highest density. These density trends corresponded to the relationship between composition and conductivity of silver electrode.36 At the highest Ag2O compositions, the density of the conductive electrode decreases, which we associate with the irregular aggregation of high Ag2O NP compositions which prevents the film from fully densifying. In addition to the direct influence on printed film conductivity, Ag2O NPs decoration also impacts the resilience to mechanical flexing. To investigate the mechanical flexibility and stability of the printed conductive networks, several resistance measurements were conducted. For the bending-resistance tests, four bending states with curvatures of 0, 0.72, 1.19, and 1.92 cm-1 were used as shown in Figure S4. Figure 4b shows the change of electrical resistance with different conductor formulations as a function of static curvature. Bare Ag flakes ink showed lower flexibility with resistivity increasing more rapidly with increasing curvature as compared with Ag flake/Ag2O NP-derived conductors. In the process of bending, the tensile mechanical strain leads to separation of flakes and the reduction in flake to flake contact, which degrades the continuous conductive network.32 As discussed above, reduced Ag2O NPs serve as bridges between flakes, increase the density of the conductive network and increase the number of contact points between adjacent Ag flakes, thereby increasing the relative conductivity37 under mechanical strain.33 As shown in Figure 4b, the resistivity changes of conductive lines formed


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from optimal Ag flake/Ag2O NP formulations are significantly smaller than that observed for bare Ag flakes. The physical configuration of the two printed conductor lines at different curvatures is also shown in Figure S5. Resilience to flexing is another important consideration for flexible electronic products. Hence, we investigated electrical resistance change of different conductive electrodes as a function of the number of bend cycles with the curvature of 1.92 cm-1. The results shown in Figure 4c shows that non-decorated silver flake inks have lower flexibility and lower bending durability. The printed conductive lines printed form inks without Ag2O NPs showed considerable changes in resistivity after 50 cycles which are associated with unbridged and fewer contacts between flakes. Conductive films printed from optimal Ag flake/Ag2O NP formulation demonstrated better durability as indicated by better bend resistance.38


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Figure 4. (a) Electrical resistance of different Ag electrodes formed by different inks. (b) Change of electrical resistance in different silver conductive electrode as a function of curvature. (c) Change of electrical resistance in different silver conductive electrode as a function of bend cycles. (Bar chart refers to left axis, while line chart refers to right axis in (b) and (c)).


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4. CONCLUSIONS In summary, a novel syntheses method for Ag2O NP-decorated Ag flake inks has been developed and used to fabricate low resistivity printed conductors with excellent flexibility. In this approach, we used a simple and controlled method to load Ag2O NPs on the surface of Ag flakes and then mixed these decorated flakes as a single, room temperature-stable conductive filler into printable inks. After printing and sintering at PET-compatible temperatures, the Ag flakes formed lower resistance continuous conductive networks assisted by the reduction of Ag2O NPs. The reduced Ag2O NPs decreased the contact resistance and increased the resilience of printed conductors to cyclic flexing. By controlling the weight ratio of Ag flakes to Ag2O NPs, the conductivity was optimized for fabricating high-performance printed conductors resulting in an average resistivity 1.5×10-5 Ω·cm with improved bend resistance and durability as compared printed conductors without Ag2O decoration.


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ASSOCIATED CONTENT Supporting Information Available: It is available free of charge.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Yuhuan Meng: 0000-0001-5821-2872 Felippe Jose Pavinatto: 0000-0002-6223-973 John Devin MacKenzie: 0000-0002-3884-5127 Taichong Ma:0000-0002-7001-9909

Notes The authors declare no competing financial interest.

Acknowledgement. This work was supported by Huawei Technologies Co., Ltd. (China) under Huawei Innovation Research Program (HIRPO20160904) and has been conducted in part at the Washington Clean Energy Testbeds, Clean Energy Institute, University of Washington (UW) and at the Molecular Analysis Facility (MAF), a National Nanotechnology Coordinated Infrastructure site at the University of Washington which is supported in part by the National Science Foundation (grant NNCI-1542101).


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Interface Modified Flexible Printed Conductive Films via Ag2

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