Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and inkjet. Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors or resistors. Printed electronics is expected to facilitate widespread, very low-cost, low-performance electronics for applications such as flexible displays, smart labels, decorative and animated posters, and active clothing that do not require high performance.[1]
The term printed electronics is related to organic electronics or plastic electronics, in which one or more inks are composed of carbon-based compounds. These other terms refer to the ink material, which can be deposited by solution-based, vacuum-based or some other method. Printed electronics, in contrast, specifies the process, and can utilize any solution-based material, including organic semiconductors, inorganic semiconductors, metallic conductors, nanoparticles, nanotubes, etc.
For the preparation of printed electronics nearly all industrial printing methods are employed. Similar to conventional printing, printed electronics applies ink layers one atop another.[2] so that the coherent development of printing methods and ink materials are the field's essential tasks.
The most important benefit of printing is low-cost volume fabrication. The lower cost enables use in more applications.[3] An example is RFID-systems, which enable contactless identification in trade and transport. In some domains, such as light-emitting diodes printing does not impact performance.[2] Printing on flexible substrates allows electronics to be placed on curved surfaces, for example, putting solar cells on vehicle roofs. More typically, conventional semiconductors justify their much higher costs by providing much higher performance.
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The maximum required resolution of structures in conventional printing is determined by the human eye. Feature sizes smaller than approximately 20 µm cannot be distinguished by the human eye and consequently exceed the capabilities of conventional printing processes.[4] In contrast, higher resolution and smaller structures are necessary in electronics printing, because they directly affect circuit density and functionality (especially transistors). A similar requirement holds for the precision with which layers are printed on top of each other (layer to layer registration).
Control of thickness, holes, and material compatibility (wetting, adhesion, solvation) are essential, but matter in conventional printing only if the eye can detect them. Conversely, the visual impression is irrelevant.[5]
The attraction of printing technology for the fabrication of electronics mainly results from the possibility to prepare stacks of micro-structured layers (and thereby thin-film devices) in a much more simple and cost-effective way compared to conventional electronics.[6] Beside this, also the possibility to implement new or improved functionalities (e.g. mechanical flexibility) plays a role. The selection of used printing methods is determined by requirements concerning printed layers, by properties of printed materials as well as economic and technical considerations in terms of printed products.
Printing technologies divide between sheet-based and roll-to-roll-based approaches. Sheet-based techniques, such as inkjet and screen printing are best for low-volume, high-precision work. 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).[4][6] While offset and flexographic printing are mainly used for inorganic[7][8] and organic [9][10] conductors (the latter also for dielectrics),[11] gravure printing is especially suitable for quality-sensitive layers like organic semiconductors and semiconductor/dielectric-interfaces in transistors, due to high layer quality.[11] In connection with high resolution, is also suitable for inorganic[12] and organic [13] conductors. Organic field-effect transistors and integrated circuits can be prepared completely by means of mass-printing methods.[11]
Inkjets are flexible and versatile, and can be set up with relatively low effort. Inkjets are probably the most commonly used method.[14] However, inkjets offer lower througput of around 100 m2/h and lower resolution (ca. 50 µm).[4] It is well suited for low-viscosity, soluble materials like organic semiconductors. With high-viscosity materials, like organic dielectrics, and dispersed particles, like inorganic metal inks, difficulties due to nozzle clogging occur. Because ink is deposited via droplets, thickness and dispersion homogeneity is reduced. Simultaneously using many nozzles and pre-structuring the substrate allows improvements in productivity and resolution, respectively. However, in the latter case non-printing methods must be employed for the actual patterning step.[15] Inkjet printing is preferable for organic semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs), but also OFETs completely prepared by this method have been demonstrated.[16] Frontplanes[17] and backplanes[18] of OLED-displays, integrated circuits,[19] organic photovoltaic cells (OPVCs) [20] and other devices can be prepared with inkjets.
Screen printing is appropriate for fabricating electrics and electronics on industrial scales due to its ability to produce thick layers from paste-like materials. This method can produce conducting lines from inorganic materials (e.g. for circuit boards and antennas), but also insulating and passivating layers, whereby layer thickness is more important than high resolution. Its 50 m²/h throughput and 100 µm resolution are similar to inkjets.[4] This versatile and comparatively simple method is used mainly for conductive and dielectric layers,[21][22] but also organic semiconductors, e.g. for OPVCs,[23] and even complete OFETs [17] can be printed.
Other methods with similarities to printing, among them micro-contact printing and nano-imprint lithography are of interest.[24] Here, µm- and nm-sized layers, respectively, are prepared by methods similar to stamping with soft and hard forms, respectively. Often the actual structures are prepared subtractively, e.g. by deposition of etch masks or by lift-off processes. For example electrodes for OFETs can be prepared [25][26] Sporadically pad printing is used in a similar manner.[27] Occasionally so-called transfer methods, where solid layers are transferred from a carrier to the substrate, are considered printed electronics.[28] Electrophotography is currently not used in printed electronics.
Both organic and inorganic materials are used for printed electronics. Ink materials must be available in liquid form, for solution, dispersion or suspension.[29] They must function as conductors, semiconductors, dielectrics, or insulators. Material costs must be fit the application.
Electronic functionality and printability can interfere with each other, mandating careful optimization.[5] For example, a higher molecular weight in polymers enhances conductivity, but diminishes solubility. For printing, viscosity, surface tension and solid content must be tightly controlled. Cross-layer interactions such as wetting, adhesion, and solubility as well as post-deposition drying procedures affect the outcome. Additives often used in conventional printing inks are unavailable, because they often defeat electronic functionality.
Material properties largely determine the differences between printed and conventional electronics. Printable materials provide decisive advantages beside printability, such as mechanical flexibility and functional adjustment by chemical modification (e.g. light color in OLEDs).[30]
Printed conductors offer lower conductivity and charge carrier mobility.[31]
With a few exceptions, inorganic ink materials are dispersions of metallic micro- and nano-particles.
PMOS but not CMOS is possible in printed electronics.[32]
Organic printed electronics integrates knowledge and developments from printing, electronics, chemistry, and materials science, especially from organic and polymer chemistry. Organic materials in part differ from conventional electronics in terms of structure, operation and functionality,[33] which influences device and circuit design and optimization as well as fabrication method.[34]
The discovery of conjugated polymers[31] and their development into soluble materials provided the first organic ink materials. Materials from this class of polymers variously possess conducting, semiconducting, electroluminescent, photovoltaic and other properties. Other polymers are used mostly as insulators and dielectrics.
In most organic materials, hole transport is favored over electron transport.[35] Recent studies indicate that this is a specific feature of organic semiconductor/dielectric-interfaces, which play a major role in OFETs.[36] Therefore p-type devices should dominate over n-type devices. Durability (resistance to dispersion) and lifetime is less than conventional materials.[32]
Organic semiconductors include the conductive polymers poly(3,4-ethylene dioxitiophene), doped with poly(styrene sulfonate), (PEDOT:PSS) and poly(aniline) (PANI). Both polymers are commercially available in different formulations and have been printed using inkjet,[37] screen[21] and offset printing[9] or screen,[21] flexo[10] and gravure[13] printing, respectively.
Polymer semiconductors are processed using inkjet printing, such as poly(thiopene)s like poly(3-hexylthiophene) (P3HT)[38] and poly(9,9-dioctylfluorene co-bithiophen) (F8T2).[39] The latter material has also been gravure printed.[11] Different electroluminescent polymers are used with inkjet printing,[15] as well as active materials for photovoltaics (e.g. blends of P3HT with fullerene derivatives),[40] which in part also can be deposited using screen printing (e.g. blends of poly(phenylene vinylene) with fullerene derivatives).[23]
Printable organic and inorganic insulators and dielectrics exist, which can be processed with different printing methods.[41]
Inorganic electronics provides highly ordered layers and interfaces that organic and polymer materials cannot provide.
Silver nanoparticles are used with flexo,[8] offset [42] and inkjet.[43] Gold particles are used with inkjet.[44]
A.C. electroluminescent (EL) multi-color displays can cover many tens of square meters, or be incorporated in watch faces and instrument displays. They involve six to eight printed inorganic layers, including a copper doped phosphor, on a plastic film substrate.[45]
CIGS cells can be printed directly onto molybdenum coated glass sheets.
A printed gallium arsenide germanium solar cell demonstrated 40.7% conversion efficiency, eight times that of the best organic cells, approaching the best performance of heavy silicon.[45]
Printed electronics allows the use of flexible substrates, which lowers production costs and allows fabrication of mechanically flexible circuits. 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 higher temperature stability. Poly(ethylene naphthalate)- (PEN) and poly(imide)-foil (PI) are alternatives. Paper's low costs and manifold applications make it an attractive substrate, however, its high roughness and large absorbency make it problematic for electronics.[42]
Other important substrate criteria are low roughness and suitable wettability, which can be tuned pre-treatment (coating, corona). In contrast to conventional printing, high absorbency is usually disadvantageous.
Printed electronics are in use or under consideration for:
Technical standards and roadmapping initiatives are intended to facilitate value chain development (for sharing of product specifications, characterization standards, etc.) This strategy of standards development mirrors the approach used by silicon-based electronics over the past 50 years. Initiatives include: