Direct writing of silver microfiber with precise control on patterning for robust and flexible ultrahigh-performance transparent conductor

doi:10.1016/j.jmst.2020.01.043

Abstract

Abstract:

Ultrafine silver fiber is an alternative to commercial indium tin oxide (ITO) as a new-generation flexible transparent conductor that can be used in flexible electronics. However, its primary limitation is the unrepeatable optoelectronic properties due to the disordered distribution of silver fibers. In this work, we report the in-situ direct writing of the silver microfiber pattern with high conductivity and transparency to attain a flexible transparent conductor. The silver network is composed of silver microfibers, which can be artificially designed and regularly patterned under the precise control of the fiber position and shape; this is crucial for regulating its optoelectronic properties. Herein, a high-performance conductor is achieved in the silver network with high stability. This novel conductor has a sheet resistance of 2 Ω sq^-1 at 90% transparency, which corresponds to a high Figure of merit σ_dc/σ_opt = 1742. The in-situ direct writing technique developed here is distinct from other fabrication methods because it requires no transfer steps, templates or heating. Further, this silver network is integrated into a light-printable rewritable device, and can be used as a wearable heater; this heater when driven by a 1.5 V battery attains a temperature of up to 55.6 ℃. Therefore, in-situ direct writing is expected to offer a new platform for facile, scalable, and ultralow-cost production of high-performance metal networks for flexible transparent conductors.

Key words: Silver, Transparent conductor, Direct writing, Patterning, Transparent heater

Tao Chen, Heping Li, Jing Li, Sanyuan Hu, Pin Ye, Youwei Yan. Direct writing of silver microfiber with precise control on patterning for robust and flexible ultrahigh-performance transparent conductor[J]. J. Mater. Sci. Technol., 2020, 47: 103-112.

Figures/Tables 7

Fig. 1. (a, b) Schematic diagrams of direct writing of the silver fibers. (c-h) The fabricated silver patterns: parallel/straight, regular hexagon, diamond, square, semicircle-linear and sinusoidal. (c1-h1) Surface mapping of silver fibers in patterns.

Fig. 2. Schematic diagram for obtaining the Ag network under UV irradiation (a) and the obtained Ag network transparent conductor (b). (c) XRD pattern of Ag fibers. Raman spectra (d) and FT-IR (e) of the fiber, both before and after UV irradiation for 4 h. (f) TEM image of Ag fiber.

Fig. 3. (a) Transmittance of squared silver patterns with different spacings. (b) Transmittance of silver patterns in different forms: ①sinusoidal wave, ②semicircle-linear, ③hexagon, ④rhombus. (c) Sheet resistance versus optical transmittance at 550 nm for different silver patterns. (d) Comparison of optoelectronic performance between transparent conductor in this work and those reported in the literatures [[29], [30], [31], [32],35,36].

Fig. 4. Change in sheet resistance of silver pattern (a) in acidic and alkaline solution. (b) Calcining at temperature ranging from 100 to 300 °C, and (c) bending for 1000 times. The “tape test” performed on the silver pattern: (d) before sticking the tape, (e) sticking the tape, and (f) the type being peeled off.

Fig. 5. (a) I-V curves of silver pattern with different sheet resistances. (b) Temperature reached by silver pattern when different voltages were applied; cyclic electric-heating function of the silver pattern by repeatedly loading and unloading 2 V voltage for 2 cycles (c) and 1000 cycles (d). (e) Change in sheet resistance during heat fatigue test. (f) Picture of flexible silver pattern; infrared thermal image of the silver pattern after applying a voltage of 2 V (g) and bending the exothermic product into a cylindrical shape (h). (i) Temperature as a function of input power density for silver pattern with different sheet resistances.

Fig. 6. (a) Schematic diagram for assembling light-printable rewritable device; the application of silver pattern in thermotherapy: (b1) the silver pattern on a human hand, the infrared thermal image before (b2) and after (b3) applying 1.5 V voltage. (c1) Schematic diagram for its application in an extreme low temperature situation, the infrared thermal image before (c2) and after (c3) loading the voltage.

Fig. 7. Demonstration of 3D structure construction process. Schematic diagram of the process of direct-writing the first layer (a, b) and the second layer (c). (d) Enlarged view of the second layer structure. (e) Actual image of single layer structure (e1, e3) and double layer structure (e2, e4), (e1) and (e2) are grid pattern, (e3) and (e4) are wave pattern.

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