Printed electronics
Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors, capacitors, coils, and resistors.
The most commonly used solvents include ethanol, xylene, Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), toluene and water, whereas, the most common conductive fillers include silver nanoparticles, silver flakes, carbon black, graphene, carbon nanotubes, conductive polymers (such as polyaniline and polypyrrole), and metal powders (such as copper or nickel).
Feature sizes smaller than approximately 20 μm cannot be distinguished by the human eye and consequently exceed the capabilities of conventional printing processes.
[9] In contrast, higher resolution and smaller structures are necessary in most electronics printing, because they directly affect circuit density and functionality (especially transistors).
Control of thickness, holes, and material compatibility (wetting, adhesion, solubility) are essential, but matter in conventional printing only if the eye can detect them.
Gravure, offset and flexographic printing are more common for high-volume production, such as solar cells, reaching 10,000 square meters per hour (m2/h).
With high-viscosity materials, like organic dielectrics, and dispersed particles, like inorganic metal inks, difficulties due to nozzle clogging occur.
[21] Frontplanes[22] and backplanes[23] of OLED-displays, integrated circuits,[24] organic photovoltaic cells (OPVCs)[25] and other devices can be prepared with inkjets.
Screen printing is appropriate for fabricating electrics and electronics due to its ability to produce patterned, thick layers from paste-like materials.
[9] This versatile and comparatively simple method is used mainly for conductive and dielectric layers,[26][27] but also organic semiconductors, e.g. for OPVCs,[28] and even complete OFETs[22] can be printed.
The Aerosol Jet process begins with atomization of an ink, via ultrasonic or pneumatic means, producing droplets on the order of one to two micrometers in diameter.
Here, an annular flow of clean gas is introduced around the aerosol stream to focus the droplets into a tightly collimated beam of material.
Electrical interconnects, passive and active components[30] are formed by moving the print head, equipped with a mechanical stop/start shutter, relative to the substrate.
[31] A wide nozzle print head enables efficient patterning of millimeter size electronic features and surface coating applications.
The high exit velocity of the jet enables a relatively large separation between the print head and the substrate, typically 2–5 mm.
The droplets remain tightly focused over this distance, resulting in the ability to print conformal patterns over three dimensional substrates.
Despite the high velocity, the printing process is gentle; substrate damage does not occur and there is generally minimal splatter or overspray from the droplets.
[32] Once patterning is complete, the printed ink typically requires post treatment to attain final electrical and mechanical properties.
This method uses techniques such as thermal, e-beam, sputter and other traditional production technologies to deposit materials through a high precision shadow mask (or stencil) that is registered to the substrate to better than 1 μm.
By layering different mask designs and/or adjusting materials, reliable, cost-effective circuits can be built additively, without the use of photo-lithography.
Organic materials in part differ from conventional electronics in terms of structure, operation and functionality,[48] which influences device and circuit design and optimization as well as fabrication method.
They involve six to eight printed inorganic layers, including a copper doped phosphor, on a plastic film substrate.
While inkjet and screen printing typically imprint rigid substrates like glass and silicon, mass-printing methods nearly exclusively use flexible foil and paper.
Poly(ethylene terephthalate)-foil (PET) is a common choice, due to its low cost and moderately high temperature stability.
This is an active research area,[62] however, and print-compatible metal deposition techniques have been demonstrated that adapt to the rough 3D surface geometry of paper.
[63][64] Other important substrate criteria are low roughness and suitable wet-ability, which can be tuned pre-treatment by use of coating or Corona discharge.
[66] The first printed circuit was produced in 1936 by Paul Eisler, and that process was used for large-scale production of radios by the USA during World War II.
[72] Printed electronics are in use or under consideration include wireless sensors in packaging, skin patches that communicate with the internet, and buildings that detect leaks to enable preventative maintenance.
[74] There is a particularly growing interest for flexible smart electronic systems, including photovoltaic, sensing and processing devices, driven by the desire to extend and integrate the latest advances in (opto-)electronic technologies into a broad range of low-cost (even disposable) consumer products of our everyday life, and as tools to bring together the digital and physical worlds.
[76][77][78][79] Another company, Rotimpres based in Spain, has successfully introduced applications on different markets as for instance; heaters for smart furniture or to prevent mist and capacitive switch for keyboards on white goods and industrial machines.
