Co‐Percolating Graphene‐Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes

Abstract

Transparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, fl at panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance ( R S ) vs. transparency ( T ) relationship. However, ITO is relatively expensive (due to limited abundance of Indium), brittle, unstable, and infl exible; moreover, ITO transparency drops rapidly for wavelengths above 1000 nm. Motivated by a need for transparent conductors with comparable (or better) R S at a given T , as well as fl exible structures, several alternative material systems have been investigated. Single-layer graphene (SLG) or few-layer graphene provide suffi ciently high transparency ( ≈ 97% per layer) to be a potential replacement for ITO. However, large-area synthesis approaches, including chemical vapor deposition (CVD), typically yield fi lms with relatively high sheet resistance due to small grain sizes and high-resistance grain boundaries (HGBs). In this paper, we report a hybrid structure employing a CVD SLG fi lm and a network of silver nanowires (AgNWs): R S as low as 22 Ω /ٗ (stabilized to 13 Ω /ٗ after 4 months) have been observed at high transparency (88% at λ = 550 nm) in hybrid structures employing relatively low-cost commercial graphene with a starting R S of 770 Ω /ٗ. This sheet resistance is superior to typical reported values for ITO, comparable to the best reported TCEs employing graphene and/or random nanowire networks, and the fi lm properties exhibit impressive stability under mechanical pressure, mechanical bending and over time. The design is inspired by the theory of a co-percolating network where conduction bottlenecks of a 2D fi lm (e.g., SLG, MoS 2 ) are circumvented by a 1D network (e.g., AgNWs, CNTs) and vice versa. The development of these high-performance hybrid structures provides a route towards robust, scalable and low-cost approaches for realizing high-performance TCE.

Copercolating Networks: An Approach for Realizing High-Performance Transparent Conductors using Multicomponent Nanostructured Networks

“…In this analysis, representative publications are chosen based on (i) performance (transmittance (T) versus sheet resistance (R S ) relationship), which is representative of high-quality results within the class, (ii) availability of a complete set of data (thin-film thickness (t), nanowire density/thickness, T, R S or conductivity/resistivity), and (iii) a sufficiently large thickness range, allowing meaningful interpolation or extrapolation of T over a range of~70% to 90%. In general, a variety of results have been obtained within a given class, corresponding to different deposition techniques, width and length of nanowires, or postprocessing techniques [16][17][18][19][20][21]. Most of the studies account for the optical properties of the substrate, typically by correcting for the substrate reflectance and normalizing the measured T values to those measured for the substrate (to compensate for substrate loss).…”

“…Detailed electromagnetic simulations justify Eq. (4b) for nanonets and metallic nanowire arrays [20,31], but the justification of Eq. (4a) will be considered more carefully in Section 3.…”

“…Theoretically proposed by Jeong et al [52], percolation doping of nanowires in polycrystalline graphene (involving a "hybrid 1" structure with nanowires deposited over graphene) was observed to provide high transparency and high sheet conductance. Experimental realization and physical insight into the copercolation mechanism were later demonstrated in an inverted structure ("hybrid 2" structure, in which the graphene layer was deposited on top of the nanowire network) [20]. In such a network, the individual transport bottlenecks of nanowire-nanowire junction resistances and graphene grain-boundary resistances were circumvented by each other, leading to stabilized sheet resistance of 13 Ω/ with T = 88% at a wavelength of 550 nm.…”

“…In such a network, the individual transport bottlenecks of nanowire-nanowire junction resistances and graphene grain-boundary resistances were circumvented by each other, leading to stabilized sheet resistance of 13 Ω/ with T = 88% at a wavelength of 550 nm. As a result, a number of devices, including the electrochromic device, UV LED, red LED, polymer LED, and organic solar cell, electrodes for transistors, and biosensors have been demonstrated as an application of hybrid TCE [20,21,53,[55][56][57][58][59][60] . In addition, hybrid networks have also been used as source and drain electrodes in a transistor structure [21].…”

“…In this regard, CNT-nanonet performs better than metallic nanonets [28]. In general, copercolating networks are far more stable, even those with Ag and Cu nanowires [20,29]. Here, the graphene works as a barrier to moisture to prevent oxidation and corrosion.…”