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Colossal Figure of Merit in Transparent-Conducting Metallic Ribbon Networks

Qiang Peng, Songru Li, Bing Han, Qikun Rong, Xubing Lu, Qianming Wang, Min Zeng, Guofu Zhou, Jun-Ming Liu, Krzysztof Kempa,* and Jinwei Gao*

Q. Peng, S. Li, B. Han, Q. Rong, Prof. X. Lu, Prof. M. Zeng, Prof. J.-M. Liu, Prof. K. Kempa, Prof. J. Gao

Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials

South China Normal University Guangzhou 510006, China

E-mail: [email protected]; [email protected] Prof. K. Kempa

Department of Physics Boston College

Chestnut Hill, MA 02467, USA Prof. G. Zhou

Electronic Paper Displays Institute Academy of Advanced Optoelectronics South China Normal University Guangzhou 510006, China Prof. J.-M. Liu

Laboratory of Solid State Microstructures Nanjing University

Nanjing 210093, China Prof. Q. Wang

School of Chemistry & Environment South China Normal University Guangzhou 510006, China DOI: 10.1002/admt.201600095

in the morphology of the seed-layer is needed for successful applications.

Here, we developed inexpensive metallic ribbon networks (MRNs) by employing the self-cracking technique, pioneered by members of our team,^[18] and subsequently used by other groups.^[7,32] We demonstrate, that the nanoribbon character of our network allows for an extraordinary reduction of the network resistance after electroplating, with only minimal reduction in transmission. The resulting network achieves a record high figure of merit of over 30 000. Networks with such ultralow resistance are desired for high power LED light sources.

The fabrication process includes four steps, shown schemati- cally in Figure 1a. First, is the formation of a self-cracking egg white sacrificial mask (yellow layer). Second, deposition of the seed-layer metal network (black layer) by sputtering or pos- sibly by a solution process (work in progress). The sacrificial layer removal represents the third step, and finally the fourth step is the deposition of the electroplating layer (red layer).

Figure 1b shows the schematic of the electroplating process:

simply biasing the network relative to the reference electrode in a solution containing metal ions leads to metal build-up on the metal ribbon network (pink layer).

Figure 2a shows a SEM image of an Ag seed-layer MRN on a PET substrate (surface roughness of a seed-layer MRN see Figure S1a, Supporting Information); each line is a ribbon of

≈60 nm thick and a few micrometer wide. The inter-ribbon spacing is in the range of 20–100 μm. We can roughly control the linewidth and inter-ribbon spacing by changing the concentration of sacrificial materials (e.g., egg white), spinning speed, and duration; for details see Table S1 of the Supporting Information. The inset in Figure 2a shows a magnified view of the flat ribbon junction. Note, that it is this ribbon character, and overall scale that allows for substantial metal over coating, without significantly reducing transmission. For example, a very large 5 μm overcoat by plating increases 100 times the line thickness (i.e., reduces resistance 100 times), but reduces the inter-ribbon distance by only 10%, and therefore the transmission by about 20%. This is the ribbon effect, discussed in more detail further below.

Figure 2b is a SEM image of the Ag based MRNs, with the seed layer (marked A) on the left and silver-plated section (marked B) seen on the right; this section was immersed in the plating solution (shown in Figure 1b). Figure 2c is a SEM image of a plated MRN with uniform metal network coverage (surface roughness of a plated MRN see Figure S1b, Supporting Information). The top inset shows the side-view, confirming high uniformity of the plating process (height profile of a plated MRN see Figure S1c, Supporting Information); the bottom inset Transparent conducting electrodes (TCEs) are essential com-

ponents of many applications, such as solar cells, light emit- ting diodes (LEDs), light sources based on LED, touch-screen displays, wearable electronics, etc.^[1–5] Doped-metal oxide films such as the tin-doped indium oxide (ITO) have dominated the field so far, but due to their brittleness and relatively high cost cannot be used in the new-class of flexible devices.^[2,6,7]

Recently, development of TCEs based on new materials, such as graphene, carbon nanotubes, nanowires, as well as their combinations opened the possibility for a viable ITO replace- ment.^[2,8–13] Metallic nanowires have led the way.[3,9,11,14–16]

However, many problems still remain, including uniformity, high interwire contact resistance, and shorts caused by the out of plane nanowires.^[2,17–19] Strategies have been proposed to remedy these problems,^[10,19–25] but these make the processing prohibitively expensive.^[19,20,26] Continuous metallic networks have recently emerged as an alternative.^[2,7,18]

These can be fabricated by a variety of techniques,[10,20,22,23]

but are still quite expensive. Recently a solution plating process has been proposed to fabricate metallic networks,^[9,27–31]

but the optoelectronic performance is still lower than that of the convectional ITO. Improvements in the processing, and

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in this figure shows the enlarged image of a seamless junction.

Figure 2d shows a photograph of completed, strongly plated copper based MRNTCs on a PET substrate. Excellent transparency and mechanical flexibility is clearly visible. While we demonstrate here only silver and copper MRN, our method (cracking mask and electroplating) is universal, and could be applicable to different metals or metallic oxides.

Figure 3a shows compilation of optoelectronic properties of various TCs, including our MRNs, compared to other TC such as the metal nanotrough,^[19] NW networks,^[33] the graphene/

metal grid hybrid network,^[34] the self-cracking network,^[18] leaf venation and spider web networks,^[35] networks fabricated by electrospinning fibermask,^[36] electroplating metal networks,^[9]

and the conventional ITO film (150 nm thick). The lines represent parameters corresponding to a fixed figure of merit F.

For more details of figure of merit see ref.^[18]. Our plated MRNs have transmittance ranging from 60% (at sheet resistance

≈0.03 Ω sq⁻¹) to 95% (at sheet resistance

≈3 Ω sq⁻¹). Specifically, the F values for our samples cluster between dashed purple and dashed olive lines, represent colossal F = 10 000 and 30 000, respectively. Figure 3b shows the optical transmittance and sheet resistance for two, our standard Ag- and Cu-plated MRN, as well as ITO (150 nm thick). Both MRNs show large and nearly frequency-independent light transmittance over the entire visible spectrum, which is comparable or better than that of the conventional ITO. However, with almost the same transparency level the electrical conductivity <1 Ω sq⁻¹, which is much less than the conventional ITO (≈13 Ω sq⁻¹). We have also performed the X-ray diffraction studies of our MRN. In Figure 3c the characteristic peaks of all elements used are easily identifiable, and the absence of oxide peaks in the patterns demonstrates that the metal films have not oxidized during the processing in air.

Figure 4a shows improvements in F with increasing metal plating time; both Cu and Ag plating curves show a maximum.

For Ag plating, the maximum F reaches to 27 000, and for Cu plating exceeds 31 000, far surpassing other TC. The ribbon effect is confirmed by the resistance and transmission varia- tions during plating, shown in Figure 4b, where the relative reductions in R_s and T due to plating, given by (R₀–R)/R₀ and (T₀–T)/T₀, respectively, are plotted versus plating time. R₀ and T₀ are the sheet resistance and the transmittance of the network before plating. Clearly, with the exception of ultrashort

plating times (R₀–R)/R₀ >> (T0–T)/T₀. This is the ribbon effect. Note, that the networks with maximum FoM have transmittance of only ≈70%. The networks with transmittance

>90% have much smaller ribbon dimensions

<2 μm, but also much smaller FoM ≈ 1000.

A very similar effect occurs for the Cu based MRNs. This effect is further confirmed in the Figure 4c, where height and width of the line is plotted versus plating time; for a very short time, while the line thickness is very small, line width is large (ribbon), after 10 min plating, the height catches up with the width, leading to essentially semicircular lines (in cross-section) (inset in Figure 4c), also seen in the top inset in Figure 2c and Table S2 of the Supporting Information.

MRN network lines after shorter time plating remain invisible to bare eyes. This could make them applicable in touch screen and display. At longer time plating, very high conductivity is achieved at a price of network visibility and haze. Such networks are perfect for a solar cell and high-power LED light.

In fact, the large haze is an added value for such applications; the haze indicated large light trapping which is important for solar cells. In recent work^[37] it was demonstrated, that covering an LED device with an array of www.MaterialsViews.com www.advmattechnol.de

Adv. Mater. Technol. 2016, 1600095 Figure 1. Schematic diagram of a) the MRN fabrication and b) electroplating of metal layers.

Different color represents different layers, shown as rectangular bars.

Figure 2. Morphologies of MRNs. a) SEM image of a silver based seed-layer MRN (ribbon) on a PET substrate. The inset shows the ribbon character of this network. b) SEM image of an “as is” MRN (marked A), and the silver-plated section (marked B). c) SEM image of the plated MRN array. Top inset shows the side-view, and the bottom one presents the enlarged seamless junction of this network. d) Photograph of our completed, strongly plated copper TC on a PET substrate.

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in the high-current condition. This high-current condition is strongly affected by the ohmic losses in the resistive contacts, which are given by R_sI². To avoid device overheating, the series resistance of the window electrode R_s must be minimized, for example, by addition of NWs,^[37] or better yet by employing our plated MRNs. With reduced R_s, the injection current, and the corresponding LED electroluminescence can be increased, without device overheating.

In conclusion, we have developed high performance metallic networks, by employing the self-cracking technique, combined with the metal electroplating. While the self-cracking technique produces metallic ribbon networks, the process of electroplating is used to overcoat the network ribbons, which leads to a dramatic reduction of the network resistance, with minimal reduction in transmission. The corresponding figure of merit of the resulting metallic network can exceed 30 000. Such

networks could be employed in high performance, high-power LED lighting. The network processing is simple, inexpensive, and scalable. Currently, the most expensive part is the sputtering of the seed network. This could be replaced in the future by wet chemical methods, such as the metal salt reduction or nanoparticle ink deposition.

Experimental Section

Fabrication of the Self-Cracking Template: A diluted egg white solution was spin-coated (Spin coater, Laurell, USA) on desirable substrates (glass or PET). Self-cracking process occurred in air, at temperatures of 25–70 °C. The concentration of the egg white (in water) was about 0.6 g mL⁻¹, and the spin coating speed in the range of 200–800 rpm.

The cracking character was controllable with the concentration of egg white, coating speed, and the temperature. The details have been shown in Table S1 of the Supporting Information.

Figure 3. Optoelectronic properties and X-ray characterization of plated MRNs. a) Compilation of optoelectronic properties of various TCs. The lines correspond to fixed F, as defined in the legend. b) Optical transmittance and sheet resistance of the Ag- and Cu-plated MRNs, as well as ITO (150 nm thick) in the visible wavelength. c) X-ray diffraction peaks of the Ag- and Cu-plated MRNs.

Figure 4. F value and characterization of plated MRNs. a) F versus plating time. b) Relative reductions in Rs and T versus plating time for the Ag-based MRN. c) Average height and width of the MRN lines plotted versus plating time. Inset shows the schematic diagram of the plating process from a metal ribbon to a semicircle rod.

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Metal Seed Network Film Deposition: Thermal evaporation (SKY Vacuum Technology Company, China), and sputtering (AJA International.

ATC Orion 8, USA) were used to deposit Ag seed-layers (≈60 nm). Then the cracked template was removed by rinsing in deionized water.

Fabricating of Metallic Ribbon Networks Based on Seed-Layers:

100 mL copper plating solution composed of 1.56 g CuSO4·5H2O, 6.25 g Na4P2O7·10H2O, and 1.58 g Na2HPO4·12H2O, mixed with 0.3 g NH4NO3

in distilled water, was used for the electroplating deposition. 100 mL silver electroplating solution is prepared by dissolving 4 g AgNO₃, 22.5 g Na2S2O3, and 4 g KHSO3 in distilled water. A home-made plating set-up is used in the process, with a seed-layer MRNas the cathode and a Cu (or Ag) foil as the anode. By applying potential between the two electrodes, Cu (or Ag) was deposited over the seed-layer MRN. Finally, the metallic ribbon networks were rinsed in distilled water, and subsequently dried.

Performance Measurements: The morphologies of samples were characterized with the commercial SEM system (ZEISS Ultra 55, Carl Zeiss, German.), and the commercial optical microscope (MA 2002, Chongqing Optical & Electrical Instrument Co., Ltd). The surface roughness and height profile of metallic networks were conducted by 3D confocal microscope (NanoFocus AG, Oberhausen, Germany). We used an X-ray diffraction system (PAN analytical, X’Pert-Pro MPD PW 3040/60 XRD with Cu-Kα1 radiation, Netherland) to determine the nanoparticle phase information. The sheet resistance of samples was measured by the van der Pauw method, with four contacts deposited at corners of a square sample (2 cm × 2 cm), and recorded with a Keithley 2400 Source meter. The optical transmittance was measured by using the integrated sphere system (Ocean Optics, USA). All transmittance results presented are normalized to the substrate (glass or PET).

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

Q.P. and S.L. contributed equally to this work. This work was supported by grants from the NSFC (Grant No. 51571094), The National Key Research Program of China (2016YFA0201002), and by Guangdong province funds (Grant Nos. 2014B090915005, 2014A030313447, 2013KJCX0056, HD14CXY010, and 2014A010103024). K.K. thanks the Boston College Ignite Program for additional financial support.

J.M. thanks the NSFC grant (No. 51431006). This work was partially supported by Program of Changjiang Scholar and Innovative Research Team (No. IRT13064), and Guangdong Innovative Research Team (No.

2011D039), and Guangdong Engineering Technology Research Center of Efficient Green Energy and Environmental Protection Materials.

Received: May 23, 2016 Revised: June 20, 2016 Published online:

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