COMMUNICATION © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 4) 1600095 wileyonlinelibrary.com 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: kempa@bc.edu; gaojw@scnu.edu.cn 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 net- work resistance after electroplating, with only minimal reduc- tion 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 con- centration 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 transmis- sion 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 pro- cessing 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 pro- cess 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 www.MaterialsViews.com www.advmattechnol.de Adv. Mater. Technol. 2016, 1600095