TABLE OF CONTENTS
CHAPTER 1
·
INTRODUCTION
CHAPTER 2
·
ORGANIC SEMICONDUCTOR
CHAPTER 3
·
COMPARISON WITH SILICON -BASED
INDUSTRY
CHAPTER 4
·
PLASTIC ELECTRONICS MATERIALS
·
CONDUCTORS
·
SEMICONDUCTORS
·
DIELECTRICS
·
SUBSTRATES
CHAPTER 5
·
PRINTING PROCESS
·
MICRO CONTACT PRINTING
·
SCREEN PRINTING
CHAPTER 6
·
PRINTING MACHINE
CHAPTER 7
·
PAPERLIKE DISPLAY SYSTEM
CHAPTER 8
·
APPLICATION
CHAPTER 9
·
BENEFITS AND ABSTACLES
CHAPTER 10
·
CONCLUSION
·
REFERENCES
CHAPTER 1
1.
INTRODUCTION
Strong, flexible, lightweight and
cheap, plastics have acquired an additional attribute in recent years: the
ability to function as semiconductors,
forming diodes and transistors in plastic integrated circuits. Now, as the
first plastic electronics products are hitting.
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.).
The market in displays that use organic
light-emitting diodes, the stage is set for a new era of pervasive computing
with polymers. Plastics may never match the sheer processing speed and
miniaturization of silicon, but they will be able to go places that silicon
cannot reach: ultracheap radio-frequency identification tags; lowend,
high-volume data storage; displays that are inexpensive, even disposable, or
that can be wrapped around a wall column; and wearable computing.
Other uses for conductive
plastics include photocells, chemical sensors and pressure-sensitive materials.
A key advantage of organic transistors over silicon is their ease of
fabrication. Building a state-of-the-art silicon chip takes weeks of work using
complex and expensive processes such as photolithography and vacuum deposition,
carried out under high temperatures in ultraclean rooms. In comparison, organic
transistors can be made using faster, cheaper processes under less carefully
controlled conditions. Finally, there is the promise of “roll-to-roll”
fabrication similar to the continuous printing presses that revolutionized
publishing.
CHAPTER 2
2. Organic Semiconductors
THE CONDUCTIVE PLASTICS in
electronics come in two broad types. One is made out of small organic
molecules, the other out of long, conjugated polymer molecules. An example of
the small-molecule variant is pentacene, which consists of five benzene rings
joined in a line . The long polymers consist of chains of hundreds or thousands
of carbon atoms. “Conjugated” means that the carbon atoms in the chains are
joined by alternating double and single bonds.
A benzene ring can also be
thought of as a short chain of six such carbon atoms, with alternating bonds,
biting its tail to form a closed loop. But that picture of alternating single
and double bonds is not the most accurate way of viewing any of these
molecules. Instead some of the double-bond electrons become delocalized, shared
among several atoms rather than localized in a specific bond between two atoms.
Such delocalization is similar to what happens in metals and semiconductors.
The delocalized electrons can exist only in states that have specific energy
levels. The permitted energies form bands that can hold only so many electrons
.
The highest-energy band
containing electrons is called the valence band, and the next higher one is the
conduction band. The small molecules, such as pentacene, are conductive in
their pure state, and they can be made directly into crystals or thin films for
use in devices. The long polymers, in contrast, are generally poor conductors
in their pure state. The reason is that their valence band is full of electrons,
which obstructs current flow. Each electron in the band has nowhere to go—it
has no empty states available where it can move. The empty spaces in the
conduction band are at too high an energy level to be of use. To change that
situation, researchers introduce special impurity atoms (called doping). The
dopant atoms either add extra electrons, which go into the conduction band, or
they remove some electrons from the valence band, creating holes, which behave
like positive particles. In either case, current can flow easily, either by
conduction electrons traveling along in the almost empty conduction band or by
holes traveling through the valence band. (From the perspective of a hole, the
valence band is almost empty: every electron there is akin to a location to
which the hole can move.)
The possibility of doping
conjugated polymers in this way to create a conducting or semiconducting
material was discovered in the 1970s by Alan J. Heeger (now at the University
of California at Santa Barbara), Alan G. MacDiarmid (now at the University of
Pennsylvania), Hideki Shirakawa (now at the University of Tsukuba in Japan) and
their co-workers. Heeger, MacDiarmid and Shirakawa received the 2000 Nobel
Prize in Chemistry for this work. They doped polyacetylene by exposing it in
various experiments to chlorine, bromine or iodine. These conductive plastics
have already found a number of applications other than electronic circuitry,
including use as a corrosion inhibitor, electromagnetic shielding for electronic
circuits, an antistatic coating on photographic emulsions, and a
microwave-absorbing stealth coating to hide an object from radar.
CHAPTER 3
3. COMPARISON WITH SILICON-BASED INDUSTRY
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. No. 3.1
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.
CHAPTER 4
4. PLASTIC
ELECTRONIOC MATERIALS
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.
4.1 CONDUCTORS
The materials used for conductors fall mainly into three
categories – those based on :-
·
Metals
·
organic
compounds
·
metal
oxides.
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.
.
Fig 4.1.1 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.
4.2 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 4.2.1 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 4.2.2 Electronic Inks
4.3 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.
4.4 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).
CHAPTER 5
5. PRINTING PROCESS
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
5.1 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 5.1.1 Diagram of the microcontact printing process.
Source: VDMA
5.2 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
5.2.1 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
5.2.2 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.
CHAPTER 6
6. PRINTING MACHINE
THE SMALL-MOLECULE organic
semiconductors are best fabricated into devices by vapor deposition: the
compound is vaporized in a closed chamber, either evacuated or filled with an
inert gas, and allowed to condense in a film onto a substrate. This technique
is similar to that used in the manufacture of some very quotidian products,
such as the coating on potato chip bags that prevents oxygen from diffusing through
the plastic. Polymers offer a number of fabrication techniques. One is
spin-coating, in which a disk with a blob of a solution containing the polymer
or its precursors is spun, spreading the material evenly across the disk. The
material can then be etched by photolithographic techniques similar to those
used in making conventional inorganic semiconductors or cut or imprinted in
other ways. (Some researchers have also used spin-coating with pentacene.) One
problem with conductive polymers compared with the plastics used in other
industries is their lack of solubility in convenient organic solvents. For
example-polyethylenedioxythiophene, or PEDOT, is typically laid down in an
acidic waterbased solution whose corrosive properties cause other problems. In April,
TDA Research in Wheat Ridge, Colo., announced a new form of PEDOT dubbed
oligotron, which is soluble in noncorrosive organic solvents. Shining
ultraviolet (UV) light on the liquid precursor causes its molecules to
cross-link, curing the material into an insoluble solid. Thus, it should be
possible to spin-coat oligotron and then make a pattern by shining UV light on
it through a mask. Alternatively, it could be ink-jet-printed in a pattern and
then fixed with UV radiation.
The ink-jet process is highly
analogous to graphical ink-jet printing, but instead of colored dyes, tiny
droplets of polymer solution are propelled onto the substrate in carefully
controlled patterns. So far only a large-scale proof-of-principle pattern has
been demonstrated; no electronic devices have been made. The trick to making
oligotron soluble was to attach appropriate groups on the end of the PEDOT
monomer molecules. It should be possible to create variants of oligotron with
specific properties by modifying the end groups. For instance, oligotron with
photovoltaic end groups might be used to make solar cells. Several companies
are pursuing the ink-jet technique of circuitry printing. The Palo Alto
Research Center (PARC, formerly a part of Xerox) has demonstrated such
technology; in 2003 its researchers produced the first plastic semiconductor
transistor array built entirely by
ink-jet printing . The transistors are larger than their silicon cousins and
switch more slowly, but their mobility—0.1 square centimeter per volt per
second (cm2/Vs)—is only a factor of 10 lower than that of amorphous silicon,
which is widely used in the
backplanes of liquid-crystal computer displays. (Mobility is a measure of
how readily charge carriers such as electrons travel in a material.
A factor of 10 is a relatively
small difference; amorphous silicon lags behind crystalline silicon by a factor
of 1,000.) Dow, Motorola and Xerox have formed an alliance to develop
polymer inks and printing methods, as have DuPont and Lucent Technologies,
as well as Universal Display Corporation and Sarnoff.
Fig.
6.1.1 Plastic electronics circuit
Fig.
6.1.2 PLASTIC TRANSISTOR ARRAYS
were
patterned using ink-jet printing by the Palo Alto Research Center (PARC). The
lower image shows 12 transistors of an array. The dots are about 40 microns in
diameter. The researchers used a semiconducting polymer ink developed by Beng
Ong of the Xerox Research Center of Canada. The transistors would be suitable
for use in active-matrix displays, electronic paper and other applications.
Fig 6.1.
ELECTRONIC PAPER produced by Philips has a resolution of 85 pixels per inch and
can be rolled up into a cylinder four centimeters in diameter. The paper uses
pentacene thin-film transistors that are manufactured at room temperature from
a liquid solution. The transistors operate fast enough to display video.
CHAPTER
7
7. PAPERLIKE
DISPLAY SYSTEM
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. An image of one of these circuits, and
the various components of the display.
Fig.7.1 Image of a printed plastic backplane circuit
de-signed display
Fig.7.2 Exploded view of a paper like 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.
CHAPTER 8
8. APPLICATION
·
SENSORS
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 ,
FIG. 8.1 “Artificial skin” flexible integrated pressure and temperature
sensors
·
ACTUATORS
Actuators have also been made using organic
electronics. An electronic Braille actuator was recently demonstrated which
provided sufficient stimulus to be read by a blind persons
Fig. 8.2 Braille actuator
CHAPTER 9
9. BENIFITS AND ABSTACLES
·
Organic
electronics are lighter, more flexible, and less expensive than their inorganic
counterparts.
·
They
are also biodegradable (being made from carbon).
·
This
opens the door to many exciting and advanced new applications that would be
impossible using copper or silicon.
·
However,
conductive polymers have high resistance and therefore are not good conductors
of electricity.
In many cases they also have shorter
lifetimes and are much more dependant on stable environment conditions than
inorganic electronics would be.
CHAPTER 10
10. CONCLUSION
The overall objective of the project was
the development of a technology for organic electronic complementary circuits
on flexible substrates operating at frequencies far above 10 kHz. The dramatic
increase of the switching speed of representative organic circuits is achieved
by reducing the critical device dimensions and the parasitic contributions
whilst strictly controlling the mobility. The technology is based on NIL hot
embossing, which is an innovative and seminal reel-to-reel compatible, fast and
parallel high-resolution patterning technique in the field of nanoimprint
lithography. Parasitic capacitance contributions are minimized via
self-alignment of source and drain electrodes with respect to the gate
structures. A combination of short-channel, fast organic complementary circuits
with a self-aligned nanoimprinting process has to be classified as radically
innovative in the field of complex electronic applications opening a series of
new potentials and fields of applications.
The use of a complementary technology
comparable to the CMOS approach of the standard semiconductor industry leads to
lower power dissipation, higher noise margin, better robustness and easier
design of the circuits, but involves challenges as for example optimum crystalline
growth of two organic semiconductors on the same substrate and the output
current matching of p- and n-type devices.
Parasitic
capacitances in organic thin film transistors are predominantly formed by a
non-vanishing overlap of electrodes and conduction lines that are outside the
active transistor region but only separated by the gate dielectric layer. In
order to minimize this contribution the overlap between the gate electrode with
the source and drain electrodes should be as small as possible
11. REFERENCES
·
For
the topic of seminar:
·
For
the figure:
·
For
the content:
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