The outstanding electrical, mechanical and chemical properties of graphene make it attractive for applications in flexible electronics. However, efforts to make transparent conducting films from graphene have been hampered by the lack of efficient methods for the synthesis, transfer and doping of graphene at the scale and quality required for applications. Here, we report the roll-to-roll production and wet-chemical doping of predominantly monolayer 30-inch graphene films grown by chemical vapour deposition onto flexible copper substrates. The films have sheet resistances as low as approximately 125 ohms square(-1) with 97.4% optical transmittance, and exhibit the half-integer quantum Hall effect, indicating their high quality. We further use layer-by-layer stacking to fabricate a doped four-layer film and measure its sheet resistance at values as low as approximately 30 ohms square(-1) at approximately 90% transparency, which is superior to commercial transparent electrodes such as indium tin oxides. Graphene electrodes were incorporated into a fully functional touch-screen panel device capable of withstanding high strain.

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Roll-to-roll production of 30-inch graphene films for transparent electrodes

Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

Transparent conductors as solar energy materials: A panoramic review

Organic solar cells are promising for technological applications, as they are lightweight and mechanically robust. This study presents flexible organic solar cells that are less than 2 μm thick, have very low specific weight and maintain their photovoltaic performance under repeated mechanical deformation.

/pdf/ultrathin-and-lightweight-organic-solar-cells-with-high-4rp3u0tfpt.pdf

Ultrathin and lightweight organic solar cells with high flexibility

The photopolymerization of mixtures of multifunctional thiols and enes is an efficient method for the rapid production of films and thermoset plastics with unprecedented physical and mechanical properties. One of the major obstacles in traditional free-radical photopolymerization is essentially eliminated in thiol–ene polymerizations because the polymerization occurs in air almost as rapidly as in an inert atmosphere. Virtually any type of ene will participate in a free-radical polymerization process with a multifunctional thiol. Hence, it is possible to tailor materials with virtually any combination of properties required for a particular application. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5301–5338, 2004

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/pola.20366

Thiol–enes: Chemistry of the past with promise for the future

The increase in sheet resistance of indium–tin–oxide (ITO) films on polyethylene terephthalate with increasing tensile strain is reported. The increase in resistance is related to the number of cracks in the conducting layer which depends upon applied strain and film thickness. We propose a simple model that describes the finite but increasing resistance in the cracked ITO layer in terms of a small volume of conducting material within each crack.

Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates

A wiring harness and a touch sensor incorporating same are disclosed. The touch sensor includes a sensing electrode disposed in a touch sensitive area. The touch sensor further includes a plurality of auxiliary electrodes disposed on a self-supporting dielectric substrate in a border area. The auxiliary electrodes transmit a touch signal to electronics configured to use the touch signal to determine the touch location.

Wiring harness and touch sensor incorporating same

As opto-electronic devices become flexible, the designing of reliable devices becomes more challenging. In this paper, we focus on the degradation of flexible transparent anodes by mechanical and thermal stresses. Indium tin oxide (ITO)-coated polyethyleneterephthalate (PET) is susceptible to cracking at low strains (<2%), but has excellent electrical and optical characteristics. Conducting polymers, such as polyethylene dioxythiophene doped with polystyrene sulfonate (PEDOT:PSS), have good mechanical properties, but are severely degraded by temperature, and have inferior optical and electronic properties. We have developed a mandrel-bending automated test system to evaluate the degradation of flexible anodes in service, and find that even when the strain is below the virgin cracking threshold, there are measurable changes in ITO resistance. Cyclic loading of ITO-coated PET shows three regimes of resistance increase: 1) an increase in resistance, due to changes in sample dimension until equilibrium width is obtained (50-100 cycles); 2) a gradual linear increase in resistance, possibly due to cracking of ITO; and 3) a catastrophic failure after 50 000 cycles due to severe cracking. For PEDOT:PSS-coated PET, the resistance does not increase significantly with increasing tensile strain, and it is also less susceptible to damage from repeated bending.

Electromechanical Properties of Transparent Conducting Substrates for Flexible Electronic Displays

Stress–corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices

A touch sensor and a method of sensing are disclosed. The touch sensor includes a self-supporting flexible glass layer disposed on a conductive film. The touch sensor further includes electrical circuitry configured to detect a signal induced by capacitive coupling between the conductive film and a touch input applied to the flexible glass layer.

Darran R. Cairns

Papers

Strain-dependent electrical resistance of tin-doped indium oxide on polymer substrates

Wiring harness and touch sensor incorporating same

Electromechanical Properties of Transparent Conducting Substrates for Flexible Electronic Displays

Stress–corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices

Touch input sensing device