Plastic electronics ,Organic electronics, or polymer electronics, is a branch of electronics that deals with conductive polymers, plastics, or small molecules. It is called 'organic' electronics because the polymers and small molecules are carbon-based, like the molecules of living things. This is as opposed to traditional electronics (or metal electronics) which relies on inorganic conductors such as copper or silicon
Plastic Electronics allows circuits to be produced at relatively low cost by printing electronic materials onto any surface, whether rigid or flexible. it is very different from the assembly of conventional silicon-based electronics. it will lead to the creation of a whole new range of products such as conformable and rollable electronic displays, ultra-efficient lighting and low-cost, long-life solar cells. its market value is forecast to rise from $2 billion today to $120 billion in 2020.
Plastic electronic materials and high-resolution printing methods may be important technologies for new classes of consumer electronic devices that are lightweight, mechanically flexible and bendable, and that can cover large areas at low cost.
This area will be important (at least initially) not because of its potential for achieving high speed, density, and so forth but because the circuits can be rugged and bendable, and they can be printed rapidly over large areas at low cost. These features can be difficult to achieve with the brittle inorganic materials and sophisticated processing techniques that are used for conventional electronics. Bendable plastic circuits will enable new devices—electronic paper, wearable computers or sensors, disposable wireless identification tags, and so forth—that complement the types of systems that existing silicon-based electronics supports well (e.g., microprocessors, high-density RAM, etc.).
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The silicon-based electronics world is, of course, a very well entrenched, multi-billion dollar industry that offers increasingly impressive levels of processing power. But it also has the characteristics of very high capital needs (multi-billion dollars for silicon chip manufacture), potential over-specification for a number of applications, and design limitations in respect of flexible or conformable devices.
Another advantage is its processing at low-temperatures. The substrate is a solution which is printable and coatable enabling also flexible products. The additive processes might prove to be more environmentally friendly
Despite its many benefits, to date the performance of plastic electronics in terms of the actual function and performance is reduced compared to that of conventional electronics.
Fig 1[1]
It is therefore believed that Plastic electronics will, on the whole, become a winning technology platform not by ‘beating’ silicon but by complementing silicon technologies or by facilitating the development of new products (like rollable displays) where silicon just cannot be used.
The components of Plastic electronics are organic molecules and polymers that give semiconducting or light-emitting properties.
For active organic electronics, materials ranging from conductors (electrodes), semiconductors, to insulators (dielectric materials) are required.
3.1 CONDUCTORS:-
The materials used for conductors fall mainly into three categories – those based on :-
Metallic features can be printed in a number of different ways. The most common technique is to use inks that contain metal particles.
For conductors, conducting polymers are most desirable because of their mechanical flexibility and processability. However, the conductivities of conducting polymers are still lower than required.
Even though certain polymers can conduct electricity, they are still 1000 times less conductive than metals. The compounds that are most used for conductive polymers in printed are heteroaromatic polymers, based upon aniline, thiophene, and pyrole and their derivatives. Of all of the conducting polymers, the one that has been used the most as a conductor is probably PEDOT:PSS (also known as PEDT:PSS, Figure 2), which is commercially available
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Fig 2[1]. Chemical structure of PEDOT:PSS
Dispersions of PEDOT:PSS have good film forming properties, high conductivity (< 400 S/cm), high visible light transmission, and excellent stability. Films of PEDOT:PSS can be heated in air at > 100 ˚C for > 1000 hours with only minimal change in conductivity.
Another class of conductive materials that is often used for electrodes are metal oxides, particularly Indium Tin Oxide (ITO). These materials are used primarily because of their transparency. They are used where transparent electrodes are needed, particularly for light emitting or optoelectronic devices.
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3.2 SEMICONDUCTORS
Organic semiconductors can be soluble and solution processable, hence they lend them-selves to printing. The charge transport in organic semiconductors is highly dependent upon the deposition conditions, and can be influenced by many factors, including solvent, concentration, deposition technique, deposition temperature, surface treatment, surface roughness, etc.
Fig 3[1]. Chemical structures of typical organic semiconductors: (a)–(g) are p-channel materials,
and (h)–(j) are n-channel materials.
Matching combinations of p and n-type semiconductors are required for CMOS circuits.
They are chemically synthesized and formulated as printing inks.
Fig 4[1] Electronic Inks
3.3 DIELECTRICS:-
A variety of materials can be used as dielectrics. While much work has been done using inorganic (silica, alumina, and high dielectric constant oxides) dielectrics, these are not generally printable. A variety of organic polymers including polypropylene, polyvinyl alcohol, polyvinyl phenol, poly methyl methacrylate, and polyethylene terephthalate can also be used as dielectrics. Most of these are polymers that are widely used for non electronic purposes, and available in bulk quantities quite inexpensively.
3.4 SUBSTRATES:-
For organic electronics, flexible polymeric substrates are generally used. Flexible substrates pose a number of challenges, however. Flexible substrates are usually not completely dimensionally stable, and this can greatly affect the resolution and registration of features printed on them. The surfaces of flexible substrates are usually too rough for device fabrication. Flexible substrates can melt or deform when exposed to high temperatures, which limits the kinds of processing that can be applied to them.
Many types of flexible substrates are also incompatible with some solvents used for organic electronic components. When exposed to such solvents, the substrates may either dissolve or swell. The flexible polymeric substrates that have been used the most in organic electronics are the polyesters polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).
In organic electronics, Printing Techniques are chosen based upon their suitability for printing the desired materials (viscoelastic properties), as well as by their capability to print the desired feature sizes (lateral resolution, ink thickness, surface uniformity) required by the device.. Some important printing methods used today are:-
4.1 MICRO CONTACT PRINTING (µCP):-
Figure 5 illustrates how the µCP process is performed. First, a master is created using micro-fabrication processes. Second, the liquid prepolymer is applied to the surface of the master. Third, the prepolymer is cured (by heating), and removed from the master. Now, ink needs to be applied to the surface of the stamp. This can be done by either applying the ink directy to the stamp (4) or by using an ink pad (5). Most often, the inks used are molecules which form self assembled monolayers (typically thi-ols) on the surface (typically gold). Sixth, the stamp is brought into contact with the surface to be patterned. Seventh, upon removal of the stamp, a self assembled monolayer (SAM) of ink is formed on the substrate surface. Finally, this SAM is used as an etch resist to selectively etch the underlying substrate surface.
Fig 5[2]. Diagram of the microcontact printing process. Source: VDMA
4.2 SCREEN PRINTING
The screen printing process is shown in Figure 7. In screen printing, the mask (emulsion) is supported by a screen (usually made of polyester or stainless steel). The screen support allows the use of areas which are not connected, which would fall through a regular stencil or mask. In screen printing, a wide variety of different screen parameters are available.
Figure 7[2]. Screen printing process. Source: VDMA
When practiced appropriately, screen printing is a non contact printing process. The screen itself should not touch the substrate. The ink is spread out over the screen and forced through it with a squeegee.
Although screen printing is not normally considered a high volume printing process, the volume can be increased considerably by using rotary screen printing. The rotary screen printing process is shown in Figure 8.
Fig 8[2]. Rotary screen printing process. Source: VDMA
In rotary screen printing, the screen is wrapped around a cylinder, and the ink is contained inside the cylinder. The cylinder rotates continuously, and the ink is fed through it. In this way, rotary screen printing can operate continuously, and increase the throughput considerably over flat bed screen printing.
We use interconnected arrays of transistors to drive circuits in flexible paper like displays that use a type of microencapsulated electro-phoretic ink.
The backplane circuits of these prototype devices consist of square arrays of 256 suitably interconnected p-channel transistors. Figure 9 illustrates an image of one of these circuits, and Figure 10 shows the various components of the display.
Fig 9[3]. Image of a printed plastic backplane circuit de-signed Fig 10[3]. Exploded view of a paper like display. for an electronic paper like display. The circuit incorporates several hundred interconnected organic transistors
A completed display (total thickness: 1mm) consists of a transparent front-plane electrode of ITO on PET and a thin, unpatterned layer of flexible electronic ‘ink’ mounted against a sheet that supports square pixel electrode pads and pin-outs; these pixel pads attach, via a conductive adhesive, to the backplanes. Each transistor functions as a switch that locally controls the color of the ‘ink’ that consists of small spheres which are filled with smaller (white) charged spheres and a colored (black) liquid (Figure 10).
Transistors in a given column have connected gates, and those in a given row have connected source electrodes. Applying a voltage to a column (gate) and a row (source) electrode turns on the transistor located at this column and row position. Activating the transistor generates an electric field between the front-plane ITO and the corresponding pixel electrode. Upon application of an appropriate electric field, the charged (white) spheres move either toward the top or the bottom of the liquid. When the (white) spheres are toward the observer, the display looks (white). When the (white) spheres are at the other side (bottom) of the display, the color of the liquid (black) is seen. The spheres and liquid can be made to be any color. The contrast is independent of viewing angle, and significantly better than newsprint.
Many different stimuli can be sensed using organic electronics, including temperature, pressure, light, and chemical identity. temperature and pressure sensors integrated into an artificial skin (Figure 11),
Figure 11[3]. “Artificial skin” flexible integrated pressure and temperature sensors .
Actuators have also been made using organic electronics. An electronic Braille actuator was recently demonstrated (Figure 20), which provided sufficient stimulus to be read by a blind persons
Figure 12[3]. Braille actuator
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