Qibing Pei

Polymer light-emitting electrochemical cells.

A device configuration for light emission from electroactive polymers is described. In these light-emitting electrochemical cells, a p-n junction diode is created in situ through simultaneous p-type and n-type electrochemical doping on opposite sides of a thin film of conjugated polymer that contains added electrolyte to provide the necessary counterions for doping. Light-emitting devices based on conjugated polymers have been fabricated that operate by the proposed electrochemical oxidation-reduction mechanism. Blue, green, and orange emission have been obtained with turn-on voltages close to the band gap of the emissive material.

Advances in Dielectric Elastomers for Actuators and Artificial Muscles

A number of materials have been explored for their use as artificial muscles. Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation. DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied. Materials combining very high energy densities, strains, and efficiencies have been known for some time. To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames. Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications.

Semiconducting polymers : A new class of solid-state laser materials

Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium. The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.

Electrochromic and highly stable poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue

Abstract The synthesis and properties of poly(3,4-ethylenedioxythiophene) (PEDOT) are reported. The polymer shows a low band-gap, 0.5 eV lower than that of polythiophene. This leads to very pronounced electrochromic properties since the doped polymer is almost transparent, with a sky-blue tint, and the neutral polymer is blue-black. The low band-gap also leads to the possibility of n-doping by electrochemical methods. The conductivity of the doped polymer is up to 200 S cm−1, and the stability is remarkable, much better than observed in our studies of the polythiophene and polypyrrole families.

POLYMER LIGHT-EMITTING ELECTROCHEMICAL CELLS : IN SITU FORMATION OF A LIGHT-EMITTING P-N JUNCTION

Solid-state polymer light-emitting electrochemical cells have been fabricated using thin films of blends of poly(1,4-phenylenevinylene) and poly(ethylene oxide) complexed with lithium trifluoromethanesulfonate. The cells contain three layers:  the polymer film (as the emissive layer) and indium-tin oxide and aluminum films as the two contact electrodes. When externally biased, the conjugated polymers are p-doped and n-doped on opposite sides of the polymer layer, and a light-emitting p-n junction is formed in between. The admixed polymer electrolyte provides the counterions and the ionic conductivity necessary for doping. The p-n junction is dynamic and reversible, with an internal built-in potential close to the band gap of the redox-active conjugated polymer (2.4 eV for PPV). Green light emitted from the p-n junction was observed with a turn-on voltage of about 2.4 V. The devices reached 8 cd/m(2) at 3 V and 100 cd/m(2) at 4 V, with an external quantum efficiency of 0.3-0.4% photons/electron. The response speed of these cells was around 1 s, depending on the diffusion of ions. Once the light-emitting junction had been formed, the subsequent operation had fast response (microsecond scale or faster) and was no longer diffusion-controlled.

Highly Flexible Silver Nanowire Electrodes for Shape‐Memory Polymer Light‐Emitting Diodes

Shape-memory polymer light-emitting diodes (PLEDs) using a new silver nanowire/polymer electrode are reported. The electrode can be stretched by up to 16% with only a small increase in sheet resistance. Large deformation shape change and recovery of the PLEDs to various bistable curvatures result in minimal loss of electroluminescence performance.

High-field deformation of elastomeric dielectrics for actuators

Abstract This paper investigates the use of elastomeric dielectric materials with compliant electrodes as a means of actuation. When a voltage is applied to the electrodes, the elastomeric films expand in area and compresses in thickness. The strain response to applied electric fields was measured for a variety of elastomers. A nonlinear, high-strain, Mooney–Rivlin model was used to determine the expected strain response for a given applied field pressure. Comparing this analytical result to with experimentally measured strains, we determined that the electrostatic forces between the free charges on the electrodes are responsible for the observed response. Silicone polymers have produced the best combination of high strain and energy density, with thickness strains up to 41% and elastic energy densities up to 0.2 MJ/m3. Response times of 2 ms have been experimentally measured. This paper also reports recent progress in making highly compliant electrodes. We have shown, for example, that gold traces fabricated in a zig-zag pattern on silicone retain their conductivity when stretched up to 80%, compared to 1–5% when fabricated as a uniform two-dimensional electrodelayer. Optimal loading of dielectric elastomers can have a significant impact on performance: and the paper describes techniques which that can increase output up to a factor of 5 compared to neutral loading conditions. Lastly, the paper briefly discusses the performance of various actuators that use dielectric elastomer materials. The technology appears to be well-suited to a variety of small-scale actuator applications.

Intrinsically Stretchable Polymer Light‐Emitting Devices Using Carbon Nanotube‐Polymer Composite Electrodes

IO N Stretchable electronics are evolving from a conceptual curiosity into an important branch of modern electronics. Such devices can be potentially useful for a wide range of applications including wearable electronics, “smart skins”, and minimally invasive biomedical devices. [ 1–10 ] Conventional inorganic electronic devices are brittle and certain fl exibility may be obtained by using ultrathin layers of the inorganic materials. Buckled device confi gurations have been reported to introduce stretchability in integrated devices consisting of rigid functional components, however they lack intrinsic stretchability. [ 3 , 7 ] Here we report the fabrication of polymer light-emitting devices using single-walled carbon nanotube (SWNT)-polymer composite electrodes as both the electronand hole-injection electrodes. The devices are metal-free and can be linearly stretched up to 45% strain. This represents a proof-of-concept, highly stretchable semiconductor device wherein every part of the device is intrinsically stretchable. Stretchable devices reported so far generally employ a soft rubbery polymer to embed or bond active electronic components that are rigid. [ 1–3 ] Buckled interconnects can be made using prestrained poly(dimethylsiloxane) (PDMS) with evaporated metallic fi lms. [ 11 ] Rogers et al. also reported buckled active devices using prestrained PDMS; [ 12 , 13 ] the buckled interconnects or devices can be elongated until the vertical displacement has all been converted into planar strain, and after that the rigid components prevent further elongation. Someya et al. reported an elastic conductor formed by coating PDMS substrates with a composite consisting of carbon nanotubes, an ionic liquid salt, and a fl uorinated copolymer. [ 14–16 ] These elastic conductors have been used to wire various rigid active devices including organic light-emitting diodes (OLEDs) and sensors. [ 15 , 16 ] The interconnected devices can be made stretchable wherein the deformation occurs essentially at the elastic interconnects. The recent advancement in polymer electronics opens up new opportunities to achieve intrinsically stretchable devices. Compared with their inorganic counterparts, conjugated polymers are much more compliant. When fabricated on fl exible substrates such as polyethylene terephthlate (PET) or metal foils, highly bendable polymer devices have been demonstrated with fl exed radii as small as several millimeters. [ 17–19 ] Nonetheless,

Efficient blue polymer light‐emitting diodes from a series of soluble poly(paraphenylene)s

Three derivatives of poly(paraphenylene) (PPP) have been synthesized, all with excellent solubility in common organic solvents. Efficient blue polymer light‐emitting diodes (LEDs) are demonstrated using these PPPs as the semiconducting and luminescent polymers. Double‐layer polymer LEDs (consisting of a hole transport layer in addition to the electroluminescent layer) emit blue light with external quantum efficiencies between 1% and 3% photons per electron, when using indium tin oxide as the anode and calcium as the cathode. Using internal field emission (Fowler–Nordheim tunneling) of single carrier devices for both electrons and holes, the energies of the top of the π band and the bottom of the π* band have been determined as, respectively, 5.7 and 2.3 eV below the vacuum. The operating voltages of these LEDs have been lowered by using a porous polyaniline anode, or by blending PPP with a hole transport material. LEDs using air stable cathodes, silver, indium, aluminum, and copper, were also demonstrated...

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.