User:Heiko Kempa/Printed Electronics

Printed electronics is the term for a relatively new technology that defines the printing of electronics on common media such as paper, plastic, and textile using standard printing processes. This printing preferably utilizes common press equipment in the graphics arts industry, such as screen printing, flexography, gravure, and offset lithography. Instead of printing graphic arts inks, families of electrically functional electronic inks are used to print active devices, such as thin film transistors. Printed electronics is expected to facilitate widespread and very low-cost electronics useful for applications not typically associated with conventional (i.e., silicon-based) electronics, such as flexible displays, smart labels, animated posters, and active clothing.

The term printed electronics is often used in association with organic electronics or plastic electronics, where one or more functional inks are composed of carbon-based compounds. While these other terms refer to the material system, the process used to deposit them can be either 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.

Standards development and activities
Several printed electronics industry leaders have established standards and roadmapping initiatives, which are intended to facilitate value chain development (for sharing of product specifications, characterization standards, etc.) This strategy of standards development mirrors the historical development, market introduction, and wide spread acceptance of silicon-based electronics over the past 50 years. As an example of this development of standards for printed electronics, the Institute of Electrical and Electronics Engineers (IEEE) launched an initiative to develop standards to assist in the development of the technology. To date, the IEEE Standards Association has published IEEE 1620-2004 and IEEE 1620.1-2006, which will enable the continued maturity of printed electronics. In addition, similar to the well-established International Technology Roadmap for Semiconductors (ITRS), the International Electronics Manufacturing Initiative (iNEMI) has published a roadmap for printed and organic electronics.

Basics
Printed electronics integrates knowledge and developments from printing technology and electronics as well as from chemistry and materials science, especially from organic and polymer chemistry. An important basis is the development of organic electronics, which itself is based on the development of organic electronically functional materials. Beside the electronic functionalities (conductor, semiconductor, electroluminescence etc.) the processability in liquid form (as solution, dispersion or suspension) of such materials led to the development of printed electronics. However, beside this also inorganic materials, which can be processed in liquid form, are used.

As far as printed electronics is concered with devices from organic electronics, these in part differ from conventional electronics in terms of structure, operation and functionality. Therfore design and optimization of devices and circuits under consideration of the actual fabrication method play an important role in the development of printed electronics.

For the preparation of printed electronics nearly all industrial printing methods are employed, mostly in adopted and modified form. Similar to conventional printing, where several ink layers are printed on top of each other, electronic thin-film devices are prepared in printed electronics by printing several functional layers on top of each other. However, the used materials as well as the required properties of printed layers considerably differ, so that the coherent development of printing methods and printed materials is the essential task in the development of printed electronics.

For example, the maximum resolution of printed structures in conventional printing is determined by the resolution of the human eye. Feature sizes below ca. 20 µm can not be recognized by the human eye and consequently can not be prepared with conventional printing processes. In contrast, higher resolutions are desired in electronics, because they directly influence the integration density as well as the functionality of devices (especially transistors). A similar argument holds for the precision with which layers are printed on top of each other.

Deviations in thickness and other layer properties as well as the occurence of holes are relevant in conventional printing only as far as they can be detected by the human eye. In contrast, they are important quality features for the functionality of printed devices in printed electronics. Conversly, here the visual impression is irrelevant. In addition, a broader range of materials must be processed in printed electronics, which leads to new requirements for the compatibility of printed layers in terms of wetting, adhesion and dissolving.

In comparison to conventional microelectronics printed electronics is characterized by a more simple, flexible and first of all cost-efficient fabrication. It is supposed to allow for an enhanced distribution, cross-linking and pervasion of electronics also in every day life. An example is the equipment of products with printed RFID-systems, which enable contactless identification in trade and transport. Furthermore, printed electronics allows for simple realization of specific properties and functionalities (e.g. flexible displays and solar cells).

Technologies
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. 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. Among the traditional industrial printing methods mainly inkjet and screen printing as well as the so-called mass-printing methods gravure, offset and flexographic printing are used in printed electronics. While the mass-printing methods are commonly employed as roll-to-roll methods ("web-fed"), inkjet and screen printing are mostly used as sheet-fed methods. However, also the converse variations exist.

The mass-printing methods gravure, offset and flexographic printing are marked by a largely enhanced productivity in comparison with other printing methods, as expressed by a throughput of many 10.000 m²/h. They are therefore especially suitable for a dramatic reduction of fabrication costs when they are applied to printing of electronics. Due to their high level of development and the manyfold mathods and variations they allow at the same time for high resolutions down to 20 µm and below, for high layer qualities as well as for a broad range of achievable layer properties and processable materials. In the field of printed electronics, the methods are considerably developed further, which holds for the other applied printing methods as well. However, the application and adaptation of mass-printing methods for printed electronics requires not only considerable know-how, but also more effort compared to other printing methods, which nevertheless is considerably lower compared to conventional electronics. While offset and flexographic printing are mainly used for inorganic and organic  conductors (the latter also for dielectrics), gravure printing is, due to the achievable high layer quality, especially suitable for quality-sensitive layers like organic semiconductors and semiconductor/dielectric-interfaces in transistors, but, in connection with the high resolution, also for inorganic and organic conductors. It could be shown, that organic field-effect transistors and integrated circuits consisting thereof can be prepared completely by means of mass-printing methods.

Inkjet printing is a flexible and versatile digital printing method, which can be set up with relatively low effort and also at laboratory scale. Therfore it is probably the most commonly used printing method for printed electronics. However, it is inferior to mass-printing methods in terms of throughput (typically 100 m²/h) as well as in terms of resolution (ca. 50 µm). 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, repetedly difficulties due to clogging of the nozzles occur. Due to the drop-wise deposition of layers their homogeneity is limited. These problems can be moderated by suited measures. By means of parallelization (i.e. simultaneous usage of many nozzles) and pre-structuring of the substrate improvements in terms of productivity and resolution, respectively, can be achieved. However, in the latter case non-printing methods must be employed for the actual patterning step. Inkjet printing is preferably used for organic semiconductors in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs), but also OFETs completely prepared by means of this method have been demonstrated. Furthermore, frontplanes and backplanes of OLED-displays, integrated circuits, organic photovoltaic cells (OPVCs) and other devices can be prepared by means of inkjet printing.

Due to the possibility to prepare thick layers from paste-like materials, screen printing has been using since many years for the fabrication of electrics and electronics in industrial scale. Mainly conducting lines from inorganic materials (e.g. for circuit boards and antennas), but also insulating and passivating layers are prepared by means of this method, whereby a relatively high layer thickness, but not a high resolution is important. Throughput (ca. 50 m²/h) and resolution (ca. 100 µm) are limited, similar to inkjet printing. Also in printed electronics this versatile and comparatively simple method is used mainly for conductive and dielectric layers, but also organic semiconductors, e.g. for OPVCs, and even complete OFETs can be printed.

Beside the conventional methods new methods with similarities to printing are employed, among them micro-contact printing and nano-imprint lithography. Here, µm- and nm-sized layers, respectively, are prepared by means of methods similar to stamping with soft and hard forms, respectively. Often the actual structures are prepared in a subtractive manner, e.g. by deposition of etch masks or by lift-off processes. Thereby for example electrodes for OFETs can be prepared Sporadicly pad printing is used in a similar manner. Occasionally also the application of so-called transfer methods, where solid layers from a carrier are transferred to the substrate, are rated among printed electronics. Electrophotography is currently not used in printed electronics.

Materials
For printed electronics organic as well as inorganic materials are used. A prerequisite is, beside the actual functionality, the availability of the materials in liquid form, i.e. as solution, dispersion or suspension. This is in particular valid for many organic functional materials, which can be used as conductors, semiconductors or insulators. With a few exceptions, the inorganic materials in question are dispersions of metallic micro- and nano-particles. The starting point for the development of printable organic functional materials was the discovery of conjugated polymers (Nobel prize in chemistry, 2000) and their development into soluble materials. Today a large variety of printable materials from this class of polymers exists, which posess conducting, semiconducting, electroluminescent, photovoltaic and other functional properties. Other polymers are mostly used as insulators and dielectrics.

Beside the actual electronic functionality the processability in printing methods is important for the application in printed electronics. However, these two properties can be in contradiction to each other, therefore a careful optimization is necessary. For example, a higher molecular weight of conductive polymers has an influence in positive direction on conductivity, but in negative direction on solubility in the solvent used for printing. For processing with printing methods the properties of the liquid formulation like viscosity, surface tension and solid content play a role, furthermore interactions with previous and following layers like wetting, adhesion and dissolving as well as the drying procedure after depositiom of the liquid layer are to be considered. The usage of additives for improvement of processability like in conventional printing inks is largely restricted in printed electronics, because they often interfere with the actual functionality.

The properties of the used materials already largely determine the differences between printed and conventional electronics. On the one hand, the materials of printed electronics provide a number of advantages, which are decisive for the development of this technology. Among them are, beside processability in liquid form, mechanical flexibility as well as the possibility to adjust functional properties by means of chemical modification (e.g. the colour of the emitted light in the active layer of OLEDs). On the other hand, the highly ordered layers and interfaces known from inorganic electronics usually can not be made from organic, especially polymer materials. Among other things, this leads to a conductivity of printed conductors and a charge carrier mobility of printed semiconductors, respectively, which is in part considerably lower compared to inorganic electronics. An intensivly studied point at present is the fact, that in most organic materials hole transport is favoured against electron transport. Recent studies indicate that this is a specific feature of organic semiconductor/dielectric-interfaces, which play a major role in OFETs. Therefore no n-type, in contrast to p-type devices, devices could be printed so far and no CMOS-, but only PMOS-technology is yet possible in printed electronics. Finally, also the stability against environmental influences and the lifetime of printed electronic functional layers is typically reduced compared to conventional materials.

An important characteristics of printed electronics is the usage of flexible substrates, which is favourable in terms of production costs and allows for the fabrication of mechanically flexible applications. While injet and screen printing in part are still carried out on rigid substrates like glass and silicon, in mass-printing methods, due to their rotational principle, nearly exclusively foil and paper are used. Due to the cost advantage, frequently poly(ethylene terephthalate)-foil (PET), due to the higher temperature stability eventually also poly(ethylene naphthalate)- (PEN) and poly(imide)-foil (PI) are used. Other important criteria for the usage of the substrate are low roughness and suitable wettability, which if necessary can be adjusted by means of pre-treatment (coating, corona). In contrast to conventional printing, a large absorbency is usually disadvantageous. Due to the low costs and the manyfold applications, paper is an attractive substrate for printed electronics, however, due to its high roughness and large absorbency technological difficulties occur. Nevertheless developments in this directions are ongoing.

Among the organic semiconductors most widely used in printed electronics are 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, screen and offset printing or screen, flexo and gravure printing, respectively. Alternatively silver nanoparticles are used with flexo, offset and inkjet printing,, with the latter method also gold particles. Many polymer semiconductors are processed using inkjet printing, frequently poly(thiopene)s like poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene co-bithiophen) (F8T2) are used. The latter material has also been gravure printed. Different electroluminescent polymers are used with inkjet printing, as well as active materials for photovoltaics (e.g. blends of P3HT with fullerene derivatives) , which in part also can be deposited using screen printing (e.g. blends of poly(phenylene vinylene) with fullerene derivatives). A large number of printable organic and inorganic insulators and dielectrics exist, which can be processed with different printing methods.

Applications
Whether utilising low-cost silicon or new organic semi-conductors, the possibilities are wide ranging :


 * Disposable radio tags that reveal the location of individual items, cases or pallets in the supply chain which result in stores that are never out of stock, and automated checkouts;
 * Clothing that can monitor our heartbeat or select the correct cycle when in the washing machine
 * Pharmaceuticals that alert the patient when to take the next dose and record and download compliance history for physicians
 * Radio frequency identification (RFID) tagged milk cartons equipped with self-adjusting sell-by dates
 * Books, newspapers, magazines and posters containing printed colour advertising with moving images
 * Mobile phones printed onto packaging.

Applications includes :
 * Product ID/RFID
 * Flexible displays
 * Intelligent packaging
 * Sensors
 * Disposable computers
 * Photovoltaics
 * Tracking tags
 * Inventory tags
 * Luggage tags
 * Electronic barcodes
 * Smart cards
 * Toys

Printed photovoltaics
Gifu University (Prof. Tsukasa Yoshida) and National Chiao Tung University (Prof. Fang-Chung Chen) are working in printed photovoltaics.

Print equipment

 * Agfa
 * Canon
 * Creo
 * IBM
 * MAN Roland
 * Spectra
 * Toppan
 * Xaar
 * Xerox

Substrates and Inks

 * 3M
 * BASF
 * Dow Chemical
 * Flint Ink/Precisia
 * Mitsubishi Chemical
 * Sumitomo
 * Sun
 * Tetra Pak
 * Toyo Ink