Existing stretchable, transparent conductors are mostly electronic conductors. They limit the performance of interconnects, sensors, and actuators as components of stretchable electronics and soft machines. We describe a class of devices enabled by ionic conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts. We demonstrate a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reaction. The ionic conductors have higher resistivity than many electronic conductors; however, when large stretchability and high transmittance are required, the ionic conductors have lower sheet resistance than all existing electronic conductors.

https://dash.harvard.edu/bitstream/handle/1/11084802/Keplinger_StretchableTransparent.pdf?sequence=1

Stretchable, Transparent, Ionic Conductors

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand-alone electrodes for organic solar cells have been optimized using a solvent post-treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm−1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO-free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C60 bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre-heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post-treatment can be a very promising electrode material for highly efficient flexible ITO-free organic solar cells.

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post-Treatment for ITO-Free Organic Solar Cells

IO N Materials that are both conductive and stretchable could enable a spectrum of applications such as stretchable displays, [ 1 ] stretchable radiofrequency antennas, [ 2 ] artifi cial muscles [ 3 ] and conformal skin sensors. [ 4–7 ] A variety of such materials have been recently developed, such as wavy thin metals, [ 8 , 9 ] metal-coated net-shaped plastic fi lm, [ 10 ] graphene fi lms [ 11 ] and carbon nanotube (CNT)-based composites. [ 12–19 ] But several limitations typically exist in these materials including low conductivity, [ 13 , 15 , 16 , 19 ]

Highly Conductive and Stretchable Silver Nanowire Conductors

Due to its unprecedented physical properties, graphene has generated huge interest over the last 7 years. Graphene is generally fabricated in one of two ways: as very high quality sheets produced in limited quantities by micromechanical cleavage or vapor growth or as a rather defective, graphene-like material, graphene oxide, produced in large quantities. However, a growing number of applications would profit from the availability of a method to produce high-quality graphene in large quantities.This Account describes recent work to develop such a processing route inspired by previous theoretical and experimental studies on the solvent dispersion of carbon nanotubes. That work had shown that nanotubes could be effectively dispersed in solvents whose surface energy matched that of the nanotubes. We describe the application of the same approach to the exfoliation of graphite to give graphene in a range of solvents. When graphite powder is exposed to ultrasonication in the presence of a suitable solvent, the ...

/pdf/liquid-exfoliation-of-defect-free-graphene-32hzrm5jfb.pdf

Liquid Exfoliation of Defect-Free Graphene

Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low infrared transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low conductivity. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω □(-1) at 90% transmission because of the high conductivity of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the number of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ~2Ω □(-1) at 90% transmission) and remarkable mechanical flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.

/pdf/a-transparent-electrode-based-on-a-metal-nanotrough-network-3bf3ixxfu6.pdf

A transparent electrode based on a metal nanotrough network

A method to produce scalable, low-resistance, high-transparency, percolating networks of silver nanowires by spray coating is presented. By optimizing the spraying parameters, networks with a sheet resistance of R(s) ≈ 50 Ω □(-1) at a transparency of T = 90% can be produced. The critical processing parameter is shown to be the spraying pressure. Optimizing the pressure reduces the droplet size resulting in more uniform networks. High uniformity leads to a low percolation exponent, which is essential for low-resistance, high-transparency films.

Spray deposition of highly transparent, low-resistance networks of silver nanowires over large areas.

We investigate the morphological, electrical, and optical properties of carbon nanotube thin films, focusing on films with transmittance, T>90%. For films with T≈90% we measure sheet resistance of Rs 90%. Thus, while reducing t can give T>99%, the corresponding Rs increases to >40 kΩ/◻. Acid treatment improves the conductivity by doping, giving properties such as T≈98% for Rs≈10 kΩ/◻.

/pdf/very-thin-transparent-conductive-carbon-nanotube-films-on-3lixlqn0nf.pdf

Very thin transparent, conductive carbon nanotube films on flexible substrates

A droplet evaporation/stamping method was employed to fabricate closely packed arrays of Au nanorods aligned parallel or perpendicular to a support. The potential as SERS substrates was investigated using model molecule 4-aminobenzenethiol (4-ABT), for which enhanced signals where obtained compared to the signals of the bulk molecule. Enhancement factors of the order of 104 and 105 were calculated for parallel and perpendicular arrays, respectively. Quantitative Raman detection of 4-ABT was obtained with detection limits in the nM concentration range. Fabricated arrays displayed good stability and uniformity, showing their potential as sensing platforms for plasmon-induced optical molecular detection.

Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol on Au Nanorod Ordered Arrays

Devices with nanosized particles deposited on shaped surface geometries include a substrate with an active material of nanosized particles deposited on a surface of the substrate. The active material has an edge formed at a position determined with a shaped geometry of the surface.

Nanosized particles deposited on shaped surface geometries

We investigate Silver Nanowire (AgNW) networks, deposited by spray coating, on a flexible plastic substrate, for application as transparent electrodes for Indium Tin Oxide (ITO) replacement. We demonstrate that the network performance is controlled by the size distribution of the droplets impinging the substrate. Droplet size distribution can be controlled by tuning the spray pressure. Under the most favorable conditions, we achieve networks with typical transmittance T∼90% and sheet resistance Rs∼50Ω/□, and Rs∼1kΩ/□ at T∼94%.

Richard Coull

Papers

Spray deposition of highly transparent, low-resistance networks of silver nanowires over large areas.

Very thin transparent, conductive carbon nanotube films on flexible substrates

Surface-Enhanced Raman Scattering of 4-Aminobenzenethiol on Au Nanorod Ordered Arrays

Nanosized particles deposited on shaped surface geometries

Spray deposition of Silver Nanowire transparent conductive networks