Conductive Polymers: Plastic Electronics

courtesy of the Office of Science and Technology

Introduction to Conductive Polymers

Traditional Inorganic Semiconductors

Traditional Electronics

New Organic Semiconducting Plastics

Discovery and History

Printing Polymers

Applications of Conductive Plastics

Solar Cells

Medicine Delivery

Solar Windows

Current Areas of Research

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This is a technique for mass producing conductive polymers that is very fast and inexpensive.  Take for example transistors.  Plastic semiconductors can be used in to make transistors in the same way that traditional semiconductors are used. The printing technique can be compared to the process of words being printed on paper by an ink jet printer.  Polymers are mixed with a liquid to form  the “ink,” which can be printed onto a sheet of plastic in the correct pattern. Hundreds of identical transistors could be printed with ease and at a low cost.  The low cost of processing organic semiconductors makes them a better option than inorganic semiconductors. The prices of many electronic devices will decrease if they  switch to organic plastic semiconductors.

Courtesy of Novelia

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Tiny plastic solar cells can be incorporated into glass windowpanes, allowing them to collect electricity throughout the day. The windows would absorb solar energy all day long, while at the same time functioning as normal windows. While this application sounds futuristic, it is not far beyond our reach.  If these transparent electronics were incorporated into regular glass windows, buildings could receive some of their electricity from hidden solar panels in the windows rather than bulky solar panels on the roof.

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Conductive polymers are also changing medical technology. One idea is a battery-operated patch worn by a patient over a long period of time that administers a daily dosage of medicine directly into the body.  Rather than taking pills everyday, the patient need only replace the patch every month or so.  The polymers can be designed to release ions when a small current is applied, and this ability can be adapted to the medical needs of a patient.  Again the polymers would make this product cost effective, because they can be processed so easily.

Other research aims to “explore the use of the plastics in biomedical sensors that could, for example, display a certain color if a person has a particular infection,” writes an anonymous source for the Chemical Engineering Progress magazine . This application is still in development, but if it can become a reality it could make inexpensive medicine accessible to areas in the world that are lacking in health care.  Organizations such as Doctors Without Borders would be able to treat more people for the same amount of money.

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Solar panels are made from photovoltaic cells which essentially take energy from sunlight and convert it to electrical energy.   Traditionally, semiconductors such as Indium Tin Oxide (ITO) are used because their electrons can move when sunlight excites them, which creates the electricity.  One of the reasons for the high cost of solar panels is the price of ITO.  Several types of polymers including polyethlene, dioxythiophene, and polyaniline, can effectively replace the ITO in the solar cells.  While they are slightly less conductive, their cost is significantly lower and makes them a good alternative.

Polymers could replace other expensive metals in devices such as liquid crystal and plasma displays, and touch screens.

Transparent plastic solar cells, courtesy of greenlaunches.com

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Conductive polymers are plastics that can conduct electricity just as metals and semiconductors can.  However unlike the latter two, which are both inorganic materials, polymers are organic carbon based molecules.  Inorganic materials can be expensive, so many companies are interesting in the opossiblilty of replacing the inorganic semiconductors with more affordable organic semiconducting polymers.   These conductive polymers work similarly to the conductors already used in most of the electronic devices of the modern world, but they are light weight, tunable, inexpensive, and can be processed very easily.  They can be mass producted through a process similar to the way an ink jet printer works.  The polymers can literally be printed onto a peice of plastic or other surface, saving time, energy and money. Semiconducting polymers are beginning to replace traditional semiconductors in devices such as transistors and solar cells, where they can significantly lower the cost of production.  According to the article Electroplastics for Plastic Electronics which was published in the American Chemical Society magazine, “reducing the cost of electronic devices is the driving forces behind plastic electronics.”  These little polymers have big potential and many applications.

Flexible organic transistor, courtesy of Beckman Institute, University of Illinois

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Conductive plastics are not exactly a new technology, but only recently has their full potential been realized. Before now, a problem stood in the way of the practical applications of conductive polymers.   They could easily be made conductive, but they would lose their conductivity whenever they were molded into a form that would be useful. However, if treated with an acid, they can be shaped while still retaining their conductive abilities.  This discovery is attributed to scientists working at Princeton University.  Although conductive polymers have been around for over thirty years, the accomplishments of the Princeton University team have made conductive polymers into the novel material that is attracting so much attention today.

This video covers the research being conducted at Princeton University and outlines some exciting possible applications of plastic electronics:

Plastic Electronics at Princton University

The original discovery of conductive plastics was made by three research scientists : Alan J. Heeger, Alan G, MacDiarmid, and Hideki Shirakawa, who won the Nobel Prize in chemistry in 2000 for their work in this area.  Although their work was only officially recognized recently, they have been studying conductive polymers for over thirty years.  The field of conductive polymers has been in development for a long time, and is finally picking up speed as more interest in the many applications of plastic electronics rises.

Like many of the important discoveries in the world of science, this one was somewhat of an accident. Back in the 1970′s, Shirakawa was working with polymers called polyacetylene when he unintentionally gave one of his samples too much of the iodine vapor -the catalyst he was using for his experiment- and created a strange metallic-looking polymer that could conduct electricity.  By adding the iodine vapor, he was doping the polymer, in the same way that silicon is doped with other elements to make it conductive.  He began to investigate further and collaborated with other scientists interested in the potential of these plastics, and thus began the study of conductive polymers.

When the field was young, there was a lot of excitement about the potential of  conductive plastics for use in batteries.  Developing cars that could run off of electricity stored in light weight plastic batteries was the ultimate goal.  Ambitions and hopes were high, but the plastic batteries did not quite work out and there was not enough interest in this research for any real development to take place.  That is not to say that the conductive polymers were forgotten, rather they took off in directions that no one had expected.  The current research focuses on their use in solar panels as photo-receptors.

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Where can plastic electronics be found?

Courtesy of Science and Technology Archives, OLED, Instructables, Hack n Mod, and Lenovo

Plastic electronics have applications in many different areas: They are currently beginning to replace the old technology in solar cells,  fuel cells, field- effect transistors, TV and computer displays, LEDs, transistors, photo diodes, rechargeable batteries, and even lasers.  The advantages of plastic electronics over traditional electronics is namely that using organic rather than inorganic materials simplifies the production process and reduces the cost.  Three specific applications are explored in more detail below:

Solar cells:

Semiconducting plastics can replace the traditional semiconductor, Induim Tin Oxide and lower the cost of solar panels.

Medicine Delivery:

Polymers are incoorporated into patches that delevery medicine directly into the body.

Solar windows:

Windows with hidden solar cells that harvest solar energy.

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Polymers can behave as semiconductors when they are processed a certain way.   Polymers become conductive in a similar way to inorganic semiconductors such as silicon and germanium.  Dopants can be added to change the conductivity of the plastics.  However, there are several types of plastic semiconductors, such as filled polymers, ionically conducting polymers which are two commonly used varieties.

Filled polymers are polymers that have been doped with various conductive materials.  These “fillings,” including graphite fibers and metal particles, give the polymer its conductivity.  They are the most commonly used and the oldest form of semiconducting plastics.  They are easily processed and can be used in many different types of electronics.  Despite their versatility, they are several disadvantages to using filled polymers: They are made from an inconsistent blend of materials, so they are difficult to reproduce exactly.  The filler materials are not evenly dispersed throughout the polymer, so their conductivity is not constant.

Ionic Polymerscan be considered a branch of filled polymers because they are filled with ionic materials.  They are distinct however, because the ions are only conductive when they are in an aqueous solution with water.  Ionic polymers are sensitive to changing humidity; If they dry out, they become insulators rather than conductors.  They are very easy to produce in great abundance, making them accessible and inexpensive.  Ionic polymers are most commonly used in rechargeable batteries, fuel cells, and polymer LEDs.

Rechargeable Batteries: Courtesy of Maricopa

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What can you make with semiconductors?

Diodes, transistors, and silicon chips are common electronics that use semiconductors.

A diode is a small device that uses both N-type and P-type semiconductors.  Electricity can only flow in one direction through the diode.

When the battery is set up like this, the electrons in the diode are attracted to the positive end of the battery and the positive holes are attracted to the negative end.  Nothing is moving across the diode. However if it is flipped around, the negative end of the battery repells the electrons in the diode and forces them to move. They fill in the positive holes in the in the P-type side of the diode and the flow continues through the battery.

Batteries only work when they are oriented correctly in the device because diodes are used to only allow electrical flow in one direction.  If the battery could damage the device if the diodes were not used to protect it.

A transistor is made from a sandwich of two N-types and one P-type semiconductors or two P-types and one N-type semiconductors.  Transistors work as switches, once a small amount of current is applied, a much larger current is able to run through.  Essentially a little bit of electricity turns the whole thing on.

A silicon chip is a compilation of many transistors arranged on a piece of silicon.  They can carry lots of electrical information despite their small size.  Silicon chips can be used to make microprocessor chips, which are used in computers and other electronics.

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