Kwing Tong

Silver Nanowire Percolation Network Soldered with Graphene Oxide at Room Temperature and Its Application for Fully Stretchable Polymer Light-Emitting Diodes

Transparent conductive electrodes with high surface conductivity, high transmittance in the visible wavelength range, and mechanical compliance are one of the major challenges in the fabrication of stretchable optoelectronic devices. We report the preparation of a transparent conductive electrode (TCE) based on a silver nanowire (AgNW) percolation network modified with graphene oxide (GO). The monatomic thickness, mechanical flexibility, and strong bonding with AgNWs enable the GO sheets to wrap around and solder the AgNW junctions and thus dramatically reduce the inter-nanowire contact resistance without heat treatment or high force pressing. The GO-soldered AgNW network has a figure-of-merit sheet resistance of 14 ohm/sq with 88% transmittance at 550 nm. Its storage stability is improved compared to a conventional high-temperature annealed AgNW network. The GO-soldered AgNW network on polyethylene terephthalate films was processed from solutions using a drawdown machine at room temperature. When bent to 4 mm radius, its sheet resistance was increased by only 2-3% after 12,000 bending cycles. GO solder can also improve the stretchability of the AgNW network. Composite TCE fabricated by inlaying a GO-soldered AgNW network in the surface layer of polyurethane acrylate films is stretchable, by greater than 100% linear strain without losing electrical conductivity. Fully stretchable white polymer light-emitting diodes (PLEDs) were fabricated for the first time, employing the stretchable TCE as both the anode and cathode. The PLED can survive after 100 stretching cycles between 0 and 40% strain and can be stretched up to 130% linear strain at room temperature.

A Water-Based Silver-Nanowire Screen-Print Ink for the Fabrication of Stretchable Conductors and Wearable Thin-Film Transistors.

A water-based silver-nanowire (AgNW) ink is formulated for screen printing. Screen-printed AgNW patterns have uniform sharp edges, ≈50 μm resolution, and electrical conductivity as high as 4.67 × 10(4) S cm(-1) . The screen-printed AgNW patterns are used to fabricate a stretchable composite conductor, and a fully printed and intrinsically stretchable thin-film transistor array is also realized.

Highly efficient electrocaloric cooling with electrostatic actuation

A flexible and lightweight device uses an electrocaloric polymer film to provide exceptional cooling power. A solid way to keep cool Refrigeration relies on vapor compression that is noisy, takes up space, and is mechanically complex. Solid-state cooling requires changing an external field to drive cooling, but devices produced so far have not been efficient enough for practical applications. Ma et al. constructed a lightweight and flexible device using a thin electrocaloric polymer film, where toggling it in an electric field between a heat source and sink drives the cooling process (see the Perspective by Zhang and Zhang). The device rapidly cools down an overheated smartphone battery and has potential application for developing compact, low-profile electronics. Science, this issue p. 1130; see also p. 1094 Solid-state refrigeration offers potential advantages over traditional cooling systems, but few devices offer high specific cooling power with a high coefficient of performance (COP) and the ability to be applied directly to surfaces. We developed a cooling device with a high intrinsic thermodynamic efficiency using a flexible electrocaloric (EC) polymer film and an electrostatic actuation mechanism. Reversible electrostatic forces reduce parasitic power consumption and allow efficient heat transfer through good thermal contacts with the heat source or heat sink. The EC device produced a specific cooling power of 2.8 watts per gram and a COP of 13. The new cooling device is more efficient and compact than existing surface-conformable solid-state cooling technologies, opening a path to using the technology for a variety of practical applications.

Highly flexible organometal halide perovskite quantum dot based light-emitting diodes on a silver nanowire–polymer composite electrode

Organometal halide perovskite light-emitting diodes (LEDs) have witnessed rapid development in a short time span due to the promising electronic and optical properties of perovskite materials, which showed great potential in flexible display devices due to their ease of integration in fabrication process flows. However, simply integrating perovskite films with flexible electrodes for highly flexible LEDs has encountered obstacles, as traditionally used perovskite films show unrecoverable micrometer-sized cracks after bending. Herein, we report on highly flexible LEDs by utilizing silver nanowire based polymer electrodes as anodes, and CH3NH3PbBr3 quantum dots as the emissive layer. The resulting devices are highly flexible and mechanically robust, capable of being bent to a 2.5 mm radius and capable of undergoing 1000 cycles of repeated bending and unbending to a radius of 4 mm without discernible performance degradation. Moreover, these flexible LEDs also exhibit high performance metrics, due in part to efficient charge balance through hole-injection enhancement, with a current efficiency of 10.4 cd A−1, a luminous efficacy of 8.1 lm W−1, and an external quantum efficiency of 2.6% at a brightness of 1000 cd m−2.

Multi-Colored Light-Emitting Electrochemical Cells Based on Thermal Activated Delayed Fluorescence Host

Light-emitting electrochemical cells (LECs) with the thermally activated delayed fluorescence(TADF) host and phosphorescent guests were fabricated using solution process. It is demonstrated for the first time that TADF, a well-known phenomenon that helps to increase electroluminescence efficiency by harvesting excitons from triplet states, is used as a host in LECs. Devices with green, yellow, red and warm white emissions were fabricated, with the best devices showing more than 7000 cd/m2 stable emission and a peak efficiency over 7 cd/A. Under high voltage stress, a burst of extremely high luminance of over 30,000 cd/m2 was observed. All these LEC devices are extremely simple with only one active layer. Thus, our results could pave way to produce low- cost light source with high luminance, using TADF molecules.

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays.

Transparent Perovskite Light-Emitting Touch-Responsive Device

A light-emitting touch-responsive device (LETD) for instantaneous visualization of pressure mapping is reported. The LETD integrates an organometal halide perovskite polymer composite emissive layer and a flexible silver nanowire polyurethane composite transparent electrode. The composite emissive layer contains methylammonium lead bromide nanocrystals uniformly dispersed in a poly(ethylene oxide) (PEO) matrix and emits an intense green luminescence that peaks at 529 nm. The PEO matrix promotes the formation of small perovskite grains (∼20 nm) and a pinhole-free composite film with surface roughness of only 2.96 nm. The composite transparent electrode is separated from the emissive layer with a 100 μm thick spacer. When a local pressure is applied, a Schottky contact is formed instantaneously between the metal and the emissive layer, and electroluminescence is produced at voltages as low as 2.5 V and reaches 1030 cd/m2 at 6 V. The transparent LETD has approximately 68% transparency. It can be bent to a 6 mm radius when polyethylene terephthalate is used as the substrate. The perovskite LETD has fast response and can be pixelated to offer potential applications in robotics, motion detection, fingerprint devices, and interactive wallpapers.