Printed electronics

Printed electronics (English printed electronics ) refers to electronic components, modules and applications that are made ​​entirely or partly by means of printing processes. Instead of the inks, electronic functional materials, which are present in liquid or paste form, printed. Frequently, these are organic materials, inasmuch as the printed electronics is a branch of organic electronics and is regarded as a key technology for their production. Due to a significant reduction in manufacturing costs through the ability to print large-area, flexible substrates, as well as new functionalities fields of application are to be developed for the electronics that were conventional ( inorganic ) electronics previously not or only partially accessible. New developments through the printed electronics are emerging, inter alia, in applications such as RFID, displays and solar cells.

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Basics

The printed electronics combines insights and developments in printing technology, electronics and chemistry and material science, particularly organic and polymer chemistry. Pioneering the development of organic electronics, which in turn is based on the development of organic electronic functional materials. In addition to the electronic functionalities ( conductors, semiconductors, electroluminescent, etc.) the processability resulted in liquid form ( as a solution, emulsion or suspension) of such materials for the development of printed electronics. Addition, however, are also inorganic materials that can be litigating in liquid form, is used.

In this respect, it is components made of organic electronics in printed electronics, they differ in structure, functioning and functionality in part by conventional electronics. Therefore, the design and optimization of the components and circuits, taking into account of the special manufacturing process, also plays an important role in the development of printed electronics.

For the production of printed electronics, almost all industrial printing method, usually in an adapted or modified form, for use. Similar to the conventional photo printing, in which multiple layers of color are applied over one another, thin-film electronic devices are manufactured by the overprinting of several functional layers in the printed electronics. However, be different, both the materials used and the required properties of the printed layers substantially from each other, so that the coordinated adaptation and further development of the printing processes used, and of the printed materials is the central task in the development of printed electronics.

For example, the maximum resolution of the printed structures in conventional printing images is determined by the resolution of the human eye. Structure sizes below about 20 microns can not be perceived by the human eye and usually not produced in conventional printing processes. In contrast, in the electronics higher resolutions are desirable because they directly affect the integration density, but also the functionality of components (particularly of transistors). The same applies to the fit above the other printed layers.

Variations in the thickness and other film properties and the occurrence of holes are the conventional printing only insofar relevant as they can be perceived by the human eye. In contrast, put them in the printed electronics essential quality characteristics for the function of the printed components dar. Conversely, here is the visual impression irrelevant. In addition, in the printed electronics a greater variety of materials to be processed, resulting in new requirements arise to the compatibility successive layers of printed regarding wetting, adhesion and mutual partial dissolution.

Compared to conventional microelectronics, the printed electronics is characterized by a simpler, more flexible, and above all cost-effective production. It should enable electronic applications a much greater dissemination, networking and penetration in everyday life. An example of this is the equipment of packaging for everyday goods with printed RFID systems that enables contactless identification in trade and transport. In addition, the printed electronics enables easy implementation and integration of special features and functionalities (such as flexible displays and solar cells).

In general, the performance of printed electronics in terms of their respective function, apart from some exceptions, for example in the field of light emitting diodes behind the conventional electronics remains. Electronic applications with high switching frequencies and high integration density (so-called " high-end electronics") will be dominated for the foreseeable future of the traditional electronics, but also requires relatively high investment and production costs. In contrast, the printed electronics is designed as a complementary technology to the establishment of a " low- cost electronics " for applications where the high performance of conventional electronics is not required.

Method

The attractiveness of the use of printing method for manufacturing electronic primarily results from the possibility of stack of microstructured layers (and thus thin-film devices ) to a much simpler and less costly way than in conventional electronics manufacture. In addition, the possibility of a role to create new or improved functionalities (for example, mechanical flexibility). The choice of printing process used depends on the requirements of the printed layers on the properties of the printed materials as well as the economic and technical considerations of manufactured products. Of the conventional industrial printing method, inkjet printing and screen printing as well as the so-called mass-printing methods gravure, offset and flexographic printing are mainly used in printed electronics. While the mass printing processes usually as roll-to -roll process (web -fed ) are used, the screen and inkjet printing usually come as arc process ( sheet -fed ) were used. However, there are also the other variants.

Mass printing

The mass printing methods gravure, offset and flexographic printing stand out in comparison with other printing methods, especially by a far superior productivity, which is expressed in a surface flow of many 10,000 m² / h, from. They are therefore suitable in a special way, to drastically reduce the manufacturing cost when they are applied to the printing electronics. Due to its high level of development and the variety of available processes and process variants they simultaneously allow high resolutions of up to 20 microns and below, high film qualities and a wide breadth of reach layer properties and processable materials. In the field of printed electronics is it like in other printing processes also to significant developments of the conventional methods. However, not only considerable know -how, but also in comparison with the other printing processes the application and adaptation of the Mass printing of printed electronics requires more effort, but the wide is still below the in conventional electronics. During the offset and flexographic printing mainly inorganic and organic fiber ( the latter for dielectrics) are used, the gravure printing is suitable because of the high attainable layer quality particularly for quality -sensitive layers such as organic semiconductor, and the semiconductor / dielectric interface layers in the transistors, in connection with the high resolution but also for inorganic and organic conductors. It could be shown that it is possible organic field- effect transistors and integrated circuits constructed therefrom produced by means of mass-printing methods completely.

Ink-jet printing

The inkjet printing is a flexible and multipurpose digital printing process, which is carried out at relatively low cost and also in the laboratory scale. Therefore, he is the printing method for printed electronics probably most widely used, but it is inferior to the mass printing technologies both in terms of the surface flow ( typically 100 m² / h) as well as in terms of resolution (about 50 microns ). It is particularly suitable for low viscosity, dissolved materials such as organic semiconductors. Highly viscous materials, such as organic dielectrics, and dispersed particles, such as inorganic metal colors, please always problems on by the clogging of the nozzles. Because of the dropwise order of the layers whose homogeneity is limited. These problems can be mitigated by appropriate measures. Through parallelization, that is, the simultaneous use of multiple nozzles, or a pre-structuring of the substrate or the productivity improvements in resolution can be achieved with respect. However, use is made in the latter case for the actual patterning step, on non- printing process. Inkjet printing is preferred for organic semiconductors in organic field-effect transistors ( OFETs) and organic light emitting diodes used ( OLEDs), there were also fully demonstrated by this method produced OFETs. Furthermore, front - and backplanes of OLED displays, integrated circuits, organic photovoltaic cells ( OPVCs ) and other components and assemblies using the inkjet printing can be produced.

Screen printing

Because of the ability to create thick layers of soft materials, screen printing has been used for a long time on an industrial scale in the production of electronics and electrical engineering. In particular conductor tracks of inorganic metals (for example, printed circuit boards, antennas, or glucose test strips ), as well as insulating and passivation layers are formed by this method, and it is important in each case a comparatively large layer thickness, but not to a high resolution. Areal throughput (approx. 50 m² / h) and resolution (about 100 microns ) are similar to inkjet printing, limited. Also in the printed electronics, this versatile and relatively simple procedure, especially for conducting and dielectric layers is applied, but it may also include organic semiconductors, eg for OPVCs, and even complete OFETs are printed.

Other methods

In addition to the conventional methods, new methods are that have similarities to print, used, including the micro- contact printing and nano- imprint lithography. Here layers with micron or nanometer resolution in a stamping procedure similar with soft and hard forms are produced. The actual structures are often in a subtractive way, for example by the application of etching masks, or a lift-off method is produced. In this way, for example, electrodes may be manufactured for OFETs. Chance and the pad printing is used in a similar manner. Occasionally, a so-called transfer method, and the application in which solid structured layers are transferred from a carrier onto the substrate, the printed electronics counted. Electrophotography (called toner or laser printing ) is not yet in the printed electronics application.

Materials

For printed electronics, both organic and inorganic materials may be used., Is a prerequisite for the next respective electronic functionality, that the materials, i.e., as a solution, dispersion or suspension, in liquid form. This applies particularly to many organic functional materials, which are used as conductors, semiconductors or insulators, too. With few exceptions, it is the inorganic materials are dispersions of metallic micro-or nanoparticles. Starting point for the development of printable electronic functional materials was the discovery of conjugated polymers (Nobel Prize in Chemistry in 2000) and their further development into soluble materials. Today a wide variety of printable materials from this class of polymers that exhibit conducting, semiconducting, electroluminescent, photovoltaic and other functional properties exist. Other polymers are usually used as insulators or dielectrics.

In addition to the respective electronic functionality, the processability in the printing process essential for the application in the printed electronics. These two properties may well contradict each other, so that a careful optimization is required. For example, a higher molar mass of conductive polymers acts tend to be a positive effect on the conductivity of the printed layer, but a negative effect on the solubility in the solvent used for printing. For the processing in the printing process, the properties of the liquid formulation such as viscosity, surface tension and solid content play a role, further are also interactions with previous or subsequent layers such as wetting, adhesion and mutual solubilization as well as the drying process after the deposition of the liquid layer to be considered. The use of additives to improve the processability as with conventional printing inks is severely restricted in the printed electronics, which often affect the function.

The properties of the materials used determine in large numbers as the differences between the printed and conventional electronics. On one hand, a number of advantages that are crucial for the development of this technology, the materials of printed electronics. These include not only the processability in liquid form, the mechanical flexibility and the ability to set functional properties by chemical modifications (e.g., the color of the emitted light in the active layer of OLEDs ). On the other hand, generally consist of organic, in particular from polymeric materials not highly ordered layers and interfaces, such as those used in the inorganic electronics are manufactured. This leads among other things to the fact that the conductivity far in printed conductors and the charge carrier mobility in semiconductors, printed partly below the values ​​in the inorganic layers. A currently intensively investigated point is the fact that in the majority of organic materials the perforated line is opposite to the electron conduction is preferred. Recent studies suggest that this is a specific property of organic semiconductor / dielectric interfaces, which play a central role in OFETs is. Thus far, no components were n-type, are in contrast to p-type devices, printed, so that in the printed electronics not have a CMOS, but only PMOS technology is possible. Finally, the stability with respect to environmental effects and the durability of printed electronic functional layers is typically less than that of the conventional materials.

An essential characteristic of printed electronics is the use of flexible substrates, which has a favorable effect on the production costs and enables the fabrication of mechanically flexible electronic applications. While working in the inkjet and screen printing still partly on rigid substrates such as glass and silicon, film and paper used almost exclusively in the mass printing processes due to their rotary process principle. Due to the cost advantage often is polyethylene terephthalate (PET) film, due to the higher temperature stability sometimes polyethylene naphthalate (PEN) and polyimide film (PI ) are used. Another important criteria for the use of the substrate, a low roughness, and a suitable wettability, optionally by pre-treatment ( coating, corona treatment) can be adjusted. In contrast to conventional printing, a high absorbency usually affects unfavorably. Due to the low cost and the wide range of possible applications is paper represents an attractive substrate for printed electronics, but is preparing for the high roughness and absorbency technological difficulties. Nevertheless, such developments are underway.

Among the most commonly used in printed electronics materials include conductive polymers, poly-3 ,4- ethylenedioxythiophene, which is doped with polystyrene sulfonate ( PEDOT: PSS), and polyaniline (PANI ). Both polymers are commercially available in various formulations and have been misprinted in the inkjet, screen and offset printing or in screen, flexo and gravure printing. Alternatively, silver nanoparticles in flexographic, offset and inkjet printing, also gold particles used in the latter method. Besides the polymeric and metallic materials also moves also of carbon as a robust material for printed electronic applications in the focus of this technology. Numerous polymeric semiconductors are processed in inkjet printing, which is often around Poylthiophene such as poly (3- hexylthiophene ) ( P3HT ) and poly -9 ,9- dioctylfluorencobithiophen ( F8T2 ). The latter material was printed up already in gravure printing. Various electroluminescent polymers are processed in inkjet printing, as well as active materials for the photovoltaic (eg mixtures of P3HT Fullerene derivatives ), which can also be applied by screen printing to some extent (for example, blends of polyphenylene with fullerene - derivatives ). Printable organic and inorganic insulators or dielectrics exist in large numbers and can be processed in various printing processes.

Devices and applications

Almost all of the necessary components for electronic applications are also produced in the printed electronics. Focus of current developments form:

  • OFETs, OLEDs and OPVCs,
  • Further, diodes, various kinds of sensors, memory elements and display systems, and antennas and batteries.
  • Common electrodes and other conductive layers in the elements to be printed. In particular, the formation of the source / drain electrodes of OFETs in inkjet printing and by means of mass-printing methods is the subject of intensive developments.
  • In OLEDs and OPVCs PEDOT: PSS is used as a coating for the anode or the anode itself, and can be applied by inkjet printing. In both of these components, the pressure of the lack of suitable cathode materials printable still a big challenge
  • Likewise, for example, to find in commercial systems for theft protection in screen printing RFID antennas made ​​of metal- paint.
  • Further also be prepared, the semiconductor layers in the devices by means of printing processes. For example, inkjet and gravure printing for the active layer of the OFET, and inkjet or screen printing to the OLEDs or OPVCs be used. Fully printed OFETs could well be produced by mass printing technology in inkjet and screen printing, in the latter case a fully printed integrated circuit has been demonstrated for several OFET.

Integrated circuits from OFET, OLED displays and solar cells based on OPVCs that are manufactured using printing processes, as well as other printed components and assemblies are to be found everywhere in applications where the specific properties of printed electronics are an advantage, ie. , where simple, cost-effective, flexible and large-area electronic components are required.

Often, in this context, the use of printed RFID tags is discussed, as printed electronics will enable the creation and integration of such systems in comparison with conventional systems considerably low cost, so that the equipment of large quantities of everyday products (so-called single -item tagging ) is possible. In this vision are to replace the bar code previously used for product identification in future printed RFID tags.

Based on organic, partly also flüssigprozessierbaren materials RFID circuits have been demonstrated, but the performance of printed circuits for this application is not sufficient. In contrast, simpler, fully printed identification systems already found in applications in the market.

In the field of OLED displays, organic electronics is the most advanced in terms of commercial products. Intensive efforts aimed at reducing the use of printing method, the manufacturing cost further. A major challenge in this, in addition to the pressure of the cathode, the integration of the control electronics ( backplane ) dar. In the visions in connection therewith is all about flexible and rollable displays and large-area, flexible and thin bulbs.

In the field of organic electronics and other display systems have similar functions, such as e-paper, developed that are nearing the launch, in part and should also be made ​​in the future with the help of printing processes.

Large and flexible, printed on inexpensive substrates Organic solar cells are another vision whose realization is promoted in the context of printed electronics. However, it is still a number of problems to overcome, in addition to the pressure of the cathode, an increase in the efficiency for economic operation is required, for example.

Generally, it is assumed that up to the realization associated with the printed electronics visions take several years before, but will increasingly establish simple applications in the meantime. Not least because of the possibility of easy integration of multiple functionalities printed electronics is regarded as one of the key technologies for the implementation of new paradigms of application of electronics, which are designed to increase networking and broader penetration in many areas of life, and with which tags such as " ubiquitous computing " and " ambient intelligence " are connected.

Development of printed electronics

The development of printed electronics is closely linked to the organic electronics. The following are some important milestones of this development are listed.

  • Before 1986: screen printing of metal-containing colors for interconnects in electrical engineering / electronics, using PEDOT: PSS as an antistatic coating, use of organic photoconductors in electrophotography
  • 1986: OFET
  • 1986: OPVC
  • 1987: OLED
  • 1990: OFET with flüssigprozessierter active layer
  • 1990: OLED with flüssigprozessierter active layer
  • 1994: OFET on flexible substrate
  • 1997: OFET fabricated with screen-printed active layer
  • 1998: OLED with manufactured using inkjet printing electrode
  • 1998: integrated OLED / OFET pixels with flüssigprozessierten active layers
  • 1998: OLED with manufactured using inkjet printing of active layer
  • 1999: OPVC on flexible substrate
  • 2000: OFET with manufactured using inkjet printing electrodes
  • 2000: OLED on flexible substrate
  • 2001: OFET fabricated with inkjet printing of active layer
  • 2001: made ​​entirely screen printed OFET
  • 2001: OPVC with flüssigprozessierter active layer
  • 2001: OPVC made ​​with screen-printed active layer
  • 2004: OPVC produced with inkjet printing electrode and active layer
  • 2005: made ​​entirely in inkjet printing OFET
  • 2005: OFET with produced in offset printing electrodes of PEDOT: PSS
  • 2007: made ​​entirely with mass printing technologies integrated circuit
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