Plastics & Electronics: The New Flexibility

Plastics & Electronics: The New Flexibility

By Jon Evans

 

Computers are everywhere: all around your home—in game consoles, televisions, DVD players, and appliances—and they’ve invaded the outside world as well. On every train or plane, in every coffee shop, you’ll see someone working on a laptop; and of course, computers are also in phones, cars, MP3 players, and any other number of electrical gadgets we carry around.

            And this is just the beginning. In a few years, computers will also be on clothes, in bags, on walls, on almost every product that we buy, and even inside us, both disseminating and collecting information. This new generation of computers will be cheap, fairly simple, and, crucially, flexible and printable, and the vast majority will probably be made from electrically conducting polymers.

            As the cornerstone of the digital revolution, silicon-based computer chips have come a long way, but they have a number of limitations. For a start, they aren’t flexible: bend one too far and it will snap. As a result, the current generation of electrical devices are tough and rigid, which is fine if you’re carrying them around but less handy if you want to incorporate them into something malleable, such as fabrics.

            Furthermore, silicon chips are usually produced by photolithography, in which electronic circuits are essentially carved into a silicon chip using various physical and chemical processes. Although photolithography has been highly successful at producing electronic components at tiny scales, it is complex, time-consuming, and relatively expensive.

 

Good Conduct

For computers to truly be everywhere, silicon chips need to be supplemented with flexible, cheaper alternatives. And that means electrically conducting polymers, which may also revolutionize the world of displays and photovoltaic cells.

            Most polymers, of course, are unable to conduct electricity. They’re insulators. That’s why electric cables are usually coated with plastic, to prevent any leakage of electric current. The reason why polymers are generally not able to conduct electricity is that their component atoms are very good at “holding onto” electrons, preventing them from moving around and producing an electric current. In contrast, metal atoms are much less adept at holding onto their electrons, which is why metals are very good at conducting electricity.

            The technical explanation for this is that in solid materials with a large number of atoms in close proximity, electrons exist at discrete energy levels, known as energy bands. Usually, electrons exist in a low-energy band, in which case they don’t have enough energy to escape from their host atoms and move around. If, however, they’re given an energetic kick, such as provided by an electrical voltage, electrons can be shoved up to a higher band, in which case they can move around and generate a current.

            The gap between these low and high bands differs between materials. In insulators, it’s quite large, meaning that a substantial voltage needs to be applied before any current will flow. In metals, the two bands may actually overlap, which means even a very small voltage will initiate a current. In semiconductors such as silicon, the gap is larger than in a metal but smaller than in an insulator.

            In inorganic materials like silicon, these low-energy and high-energy bands are known, respectively, as the valence and conduction bands. But in organic materials such as polymers, they’re known as HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital).

            In most polymers, there is a large gap between the HOMO and LUMO bands. But this is not always in the case. In so-called conjugated polymers, which have alternating single and multiple bonds, the electrons tend to become de-localized from their host atoms, producing a small gap between the bands. Thus, these polymers are able to act as semiconductors, such that a small applied voltage causes electrons to move around and generate a current.

            But it’s not just electrons that generate an electrical current in conjugated polymers, or indeed in conventional semiconductors. As electrons hop up to the high-energy band and move away from their host atom, they leave behind positively charged gaps, known as holes, which are usually quickly filled by other electrons.

            These holes also appear to move about, but in the opposite direction, as electrons filling a hole in one atom leave a hole in the atom they have just left. These holes are essentially positively charged, as they represent the lack of an electron, and so also contribute to the flow of electric current. This means that when a sufficient voltage is applied to a semiconductor, electrons start flowing to the positive electrode and holes start flowing towards the negative electrode, generating an electric current.

            In many semiconductors, the electric current is generated by roughly equal numbers of electrons and holes. But semiconductors can also be chemically modified to alter the proportion of electrons to holes; this process is known as doping.

 

Doping It Out

In inorganic semiconductors, doping is achieved by introducing impurities into the semiconducting material in the form of elements with either more or fewer electrons than the semiconductor. If the impurity element has more electrons, then the resultant material has an excess of electrons. As a result, electrons become the majority charge carrier, meaning they are responsible for generating most of the current; this is known as n-type doping.

            In contrast, if the impurity element has fewer electrons, the resultant material has a dearth of electrons and an excess of holes. As a result, holes become the majority charge carrier; this is known as p-type doping.

            The same can be done with conjugated polymers, by oxidizing or reducing them. Oxidizing a conjugated polymer reduces the number of available electrons, thereby making holes the dominant carrier, while reducing a conjugated polymer increases the number of available electrons, making them the dominant carrier. It can also be done by adding specific electron-accepting materials, such as fullerenes, to the conducting polymer.

            Doping a semiconductor with an excess of electrons or holes can greatly increase its conductivity. But that’s not all, because joining a p-type semiconductor to an n-type semiconductor offers a way to finely control the flow of electric current, and is the foundation for our digital world.

            Take transistors, the tiny electrical switches that form the basis of all modern computers. Transistors essentially consist a layer of an n-type semiconductor and a layer of a p-type semiconductor, connected to which are three electrodes known as the source, drain, and gate. A transistor is set up in such a way that when it is switched off, electrons and holes cannot move between the source and drain electrodes. This is because the electrons are trapped in the n-type layer and the holes are trapped in the p-type layer.

            Only when a voltage is applied to the gate electrode can electrons and holes start to move, generating a current between the source and drain electrodes. Thus voltage at the gate electrode acts as the switch that turns the transistor off and on.

 

LEDs and OLEDs

A similar process occurs with light-emitting diodes (LEDs). Here, an emissive layer is sandwiched between an n-type layer and a p-type layer, with the positive electrode connected to the p-type layer and the negative electrode connected to the n-type layer. When a voltage is applied to the LED, the electrons in the n-type layer travel toward the positive electrode and the holes in the p-type travel toward the negative electrode. They meet in the emissive layer and combine, producing light.

            The opposite happens in a photovoltaic cell. Here, light striking the junction between an n-type layer and a p-type layer kicks an electron from the low-energy band to the high-energy band, leaving a hole behind in the low-energy band. In other words, light produces an electron-hole pair, known as an exciton.

            The junction between the positively charged n-type layer and the negatively-charged p-type layer also sets up an electric field. This splits the newly formed exciton apart, pulling the electrode into the n-type layer and the hole into the p-type layer and generating an electric current.

            Initially, silicon was used as the basis for transistors, LEDs, and solar cells, and it still dominates. But the development of an increasing number of conducting polymers has allowed scientists to produce polymer versions of transistors, LEDs, and solar cells.

            It has been known for over 100 years that some polymers can conduct electricity, but it was only in the 1970s that the first polymer semiconductor device was developed, a transistor-like switch based on polyacetylene. Since then, a range of semiconducting polymers have been developed. For instance, conducting polymers such as poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS), polyphenylene vinylene (PPV), and polyaniline (PANI) are now regularly used to produce transistors, photovoltaic cells, and LEDs.

            Organic LEDs (OLEDs) were the first polymer-based electronic devices to hit the market, in the form of flat-screen displays. In 2007, Sony launched the first OLED television, known as XEL-1, although it had only a 29-cm screen that was little more than a prototype. More recently, however, both Sony and Microsoft have launched media players with OLED screens.

            The advantage that OLED screens have over LCD screens is that as OLEDs release light directly, they don’t need a light-emitting backplane. As a result, OLED screens can be much thinner; Sony’s XEL-1 possesses a screen just 3 mm thick. In addition, OLED screens have a much larger viewing angle than LCD screens.

 

Achieving Flexibility

But the real benefit of OLEDs, as with all other polymer electronic devices, is that they can be printed onto flexible surfaces, which means they can be used to produce screens that roll up or can be incorporated into clothing. A number of companies are already developing these kinds of flexible displays, including the UK company Cambridge Display Technology.

            The ability to print these devices, as polymers can be processed in solution and laid down one layer at a time, means that they can be produced more easily and cheaply than conventional silicon-based devices. This can be done by inkjet printing and even by a roll-to-roll process similar to that used to print newspapers, offering the ability to print transistors and simple circuits on an industrial scale.

            The first such roll-to-roll process has been developed by the German company PolyIC, a joint venture between the electronics giant Siemens and the printing company Leonhard Kurz. This process is now being used to print simple integrated circuits for use as radio-frequency identification (RFID) tags, which are used to track products and livestock. PolyIC’s process is able to produce 15 circuits in parallel at speeds of around 20 m/min, each circuit costing just a few cents to make. The company sees similar circuits eventually forming fully interactive tickets, labels, and credit cards.

            Despite these developments, conducting polymers still suffer from a number of important limitations, primarily that they are not as effective at conducting electric charge as silicon semiconductors. For simple applications, such as RFID tags, this isn’t really a problem, especially as they are fairly cheap to produce.

            It’s also not too much of a problem for organic photovoltaic cells (OPCs). The best OPCs only have about a third of the efficiency of crystalline silicon photovoltaic cells, converting 5% of the light hitting them to electricity, as opposed to around 15%. But this is offset by the fact that they can be produced much more cheaply and applied directly to surfaces. So efficiency is replaced by sheer numbers.

            “OPCs can be realized much easier and more cost efficiently,” says Klaus Hecker, managing director of the Organic Electronics Association, a European trade association. “Solar cells can now be directly integrated in their intended applications. They become a functional part as well as a design component of a product.”

            It is, however, a problem for more advanced applications. So scientists are busy developing new polymers with enhanced properties. Making this process more difficult is the fact that the precise suite of desired properties is not always the same.

 

Application-Specific

“It depends completely on the application,” explains Iain McCulloch, deputy director of Imperial College London’s Centre for Plastic Electronics. “In general, semiconducting polymers should be soluble in printing-friendly solvent. An OLED semiconducting polymer requires bandgaps to be designed for the color you would like to emit. In a semiconducting polymer for a transistor, the charge carrier mobility needs to be high, which requires highly crystalline, highly ordered, highly oriented polymers.”

            One way to produce better polymers for use in OPCs is to increase the area of the junction between the n-type and p-type polymers, which can be done by mixing the polymers together. This is because excitons don’t last for too long before the electron and hole recombine, so they need to be split apart before this happens. This splitting takes place only at junctions between the n-type and p-type layers, so you want these junctions to be as widespread as possible.

            So rather than having a single layer of n-type polymer and a single layer of p-type polymer, with a single junction between them, you develop a blend of the two polymers with numerous layers and a complex network of junctions. But you must make sure that each layer has an uninterrupted path to the appropriate electrode.

            Such “bulk heterojunction polymers,” as they are known, produce much more efficient OPCs than conventional conducting polymers. A team of scientists from the University of California (USA), Santa Barbara, led by Guillermo Bazan, recently developed a new process for producing bulk heterojunction polymers, which involves using microwaves to induce polymerization. Using this process, they developed two polymers that were able to increase the current density in OPCs by a factor of four.

            They now intend to use this process to produce many more enhanced polymers. “We plan to take advantage of this approach both to generate new materials that will increase solar cell efficiencies and operational lifetimes, and to re-evaluate previously considered polymer structures that should exhibit much higher performance than they showed initially,” says Bazan.

 

Flex Time

But for flexible applications, it’s no good having “bendy” conducting polymers if other aspects of the organic devices, such as the batteries and electrical connections, are made from conventional, rigid materials. Scientists are now making advances on both these fronts.

            Last year, for instance, chemists and material scientists at the University of Illinois (USA) at Urbana-Champaign developed a way to produce flexible metal connections, which could bend without breaking. This involved using inkjet printing to deposit thin slivers of silver nanoparticles between polymer-based electrical components. This thin sliver formed a kind of freestanding bridge between the components, which could move and bend with the underlying flexible substrate.

            Also last year, chemists at Uppsala University in Sweden developed a flexible, organic rechargeable battery based on the conducting polymer polypyrrole. They coated polypyrrole onto cellulose fibers, greatly increasing the available surface area and thus the charging capacity. Using these polypyrrole-coated cellulose sheets as electrodes and sandwiching them between simple sheets of filter paper soaked in salt water as the electrolyte, the chemists were able to produce a flexible battery that could hold and discharge electricity very effectively.

            With all the different polymer components required for flexible electronics now starting to come together, scientists are already exploring some entirely novel applications. For example, scientists at the University of Tokyo recently developed a flexible array of 676 organic transistors and showed that it can be used as an artificial, pressure-sensitive skin. (See photo on pages 00-00.)

 

Jon Evans is a freelance science writer based in Chichester, UK.