Elsevier

Thin Solid Films

Volume 596, 1 December 2015, Pages 167-173
Thin Solid Films

Direct writing and electro-mechanical characterization of Ag micro-patterns on polymer substrates for flexible electronics

https://doi.org/10.1016/j.tsf.2015.07.064Get rights and content

Highlights

  • Ag nanoparticle synthesis method with high yield and controlled particle size

  • Low temperature direct writing of conductive Ag patterns on large-area PEN

  • Ag printed patterns on flex exhibit reliable cyclic mechanical behavior

Abstract

There is currently a great interest in developing flexible electrodes. Such components are used in most electronic devices from displays to solar cells to flexible sensors. To date most of them are fabricated using expensive vacuum techniques, and are based on transparent conducting oxides. These oxides are not entirely compatible with flexible substrates under the application of mechanical stresses, due to their brittle nature. Therefore, there is a need to explore novel low-cost, large-area fabrication methods to deposit alternative conducting materials with enhanced electro-mechanical performance.

This work focuses on Ag patterns fabricated at low temperatures (below 150 °C) on flexible polyethylene naphthalate utilizing a robotic printing approach. Such lithography-free method minimizes material waste by printing exact amounts of inks on digitally predefined locations. Additionally, it allows a broad feature size range, from a few μm to a few mm, and a variety of ink viscosities for better pattern control.

We investigate the synthesis and direct writing of Ag particle-based inks, patterned-on-flex as lines and grids in the μm scale. We report on a high-yield ink synthesis method (~ 61.6%) with controlled particle size. It is found that the electrical resistivity (1.75  10 4 Ω cm) of the patterns is in the same range with similar particle-based conductive components. The correlation between annealing temperature, microstructural evolution, and electrical performance is established. Also, the optical transmittance of the patterns can be controlled to meet specific application requirements by regulating the substrate surface area covered. Finally, the mechanical behavior under both monotonic and cyclic conditions shows a superior performance compared to brittle counterparts and underlines the potential of such metallic micro-patterns to be utilized in a wide range of flexible electronic applications. It is believed that direct writing of Ag patterns on compliant substrates may hold the key in developing the next generation of truly flexible devices.

Introduction

There is currently a great interest to advance the development of large-area transparent conducting electrodes (TCEs). TCE components are integrated into a range of flexible optoelectronic applications spanning from displays to touch sensors to solar cells [1]. Transparent conducting oxides (TCOs), such as indium tin oxide (ITO), are the current solution for such applications due to their excellent electrical and optical properties [2]. In particular, ITO exhibits the lowest resistivity in the TCO group in the order of 1–2  10 4 Ω cm and optical transmittance, at 550 nm wavelength, above 92% [3].

However, due to the scarcity of indium, the need for vacuum processing, and the limited mechanical flexibility of TCOs, owing to their brittle nature [4], [5], [6], [7], [8], there is a need to develop alternative TCEs [9] on mechanically flexible, low temperature, polymeric substrates. Currently explored alternatives include carbon nanotube films [10], found to exhibit enhanced mechanical flexibility compared to ITO but requiring electrical conductivity improvements [11]. In addition, the candidates to replace TCOs encompass electrically conductive polymers, such as poly-(3, 4-ethylenedioxythiopene):poly (styrenesulfonic acid) (PEDOT:PSS), which exhibit reversible stretchability up to 10% uniaxial tensile strain when pre-strained on silicone substrates [12]. Nevertheless, the electrical stability of PEDOT:PSS needs to be further improved since its conductivity and environmental stability are observed to decrease upon exposure to non-inert environments [13]. Some recent efforts focus on fabricating highly flexible, optically transparent graphene TCEs and graphene-conducting polymer hybrids [14], [15] but the need to further develop and refine large-area and defect-free graphene synthesis methods remains a challenge before the widespread adoption of such electrodes [16]. Additionally, the use of metallic nanostructures such as Ag nanowires is another alternative that is currently being investigated with the challenge to improve its atmospheric corrosion resistance [17].

Capitalizing on the high electrical conductivity of Ag, and other metals, is of paramount importance in flexible electronics since the utilization of Ag grids [18] is another path for flexible electrode development [19] and, even beyond TCEs, towards other applications such as flexible antennas for RFID applications [20] and strain gauge sensors [21]. However, most of the methods for fabricating such grid nano/micro-patterns are of relatively high cost or involve multi-step lithographic methods which do not offer a large-area processing potential [22]. In order to overcome such challenges novel and low-cost direct patterning methods are needed to provide large-area substrate coverage. Ink-jet printing of Ag nanoparticle-based patterns has been utilized [23] but nozzle clogging remains an issue to be resolved. Screen printing of thick Ag paste is another currently employed direct patterning method [24] with comparatively large material waste during the process. More recent efforts focus on filamentary printing approaches using relatively high viscosity Ag inks where the concentrated ink solution is extruded through a cylindrical nozzle using a robotic X–Y–Z stage. This direct writing approach offers significant advantages compared to ink-jet printing, such as large-area and precise ink placement even on curved surfaces, less clogging-related issues, and ability to write in three-dimensions (3D) [25]. Quite recently, well-defined Ag nanoparticle-based periodic arrays, printed on glass and polyimide, with center-to-center separation distances of 400 μm and using an annealing temperatures above 200 °C were demonstrated [26]. However, the ink yield from this processing route has not been emphasized [27].

In this work we report on the synthesis of high-yield Ag inks and their robotic-based direct micro-patterning on polyethylene naphthalate (PEN) substrates. We focus on relatively low Ag annealing temperatures (up to 150 °C) suitable for heat-sensitive polymer substrates and we investigate the resulting electrical and mechanical cyclic loading behavior.

Section snippets

Ink synthesis and characterization

The Ag ink is obtained through preparation of Ag nanoparticles through a modification of the solution-based process described in the work conducted by Ahn et al. [25]. Briefly, Ag particles are prepared from silver nitrate (10.19 g), poly acrylic acid (PAA) (0.57 g) and ethanolamine (8.90 g) mixed in water (35.9 g). Our approach consists of the following steps: first, the (PAA) was dissolved in half the water while magnetically stirring and the ethanolamine was then added dropwise to the stirring

Results and discussion

During the preparation of the Ag nanoparticles ethanolamine was used instead of diethanolamine as the weak base precursor for the reduction process of silver ions for particle nucleation [25], [28]. The lower viscosity of ethanolamine allows for relatively easy extraction of the nanoparticles from the precursor solution. Also, the stronger pH of ethanolamine allows for utilization of less amounts of precursor to induce particle formation and growth.

The Ag yield was calculated from the

Conclusions

The modified synthesis of the Ag ink allows for a higher yield (~ 61.6%) without significant particle coarsening over time highlighting potential routes for enhancing processing efficiency. In particular, the incorporation of ethanolamine in the particle synthesis process allows for reduction of the amount of precursor to induce particle nucleation and growth due to its stronger pH when compared to its dimer.

The efficient direct NBRD micro-patterning of Ag on PEN at low temperatures (up to 150 

Acknowledgments

K.A.S and M. A. T. A. greatly acknowledge the partial support of NSF under award no. 1343726. A. M. C. wishes to thank WVU's SURE NanoSAFE program for undergraduate research. J. J. B. acknowledges the support of WV NASA Space Grant Consortium for undergraduate research. We acknowledge the use of the WVU Shared Research Facilities.

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