Plastics might seem unlikely materials for building electrical circuits but recent breakthroughs have made this possible. We discover the advantages on offer and question whether, one day, it might rival silicon.
Plastics are electrical insulators, and good ones that that, so it might seem that their use in electronic circuits would be somewhat limited. However, in recent years chemists have learned to fine tune the composition of some plastics so they behave as conductors or semi-conductors.
This, in turn, allows circuits to be created that are tough and flexible – properties not usually associated with silicon – and this might lead to rollable displays and other innovative new types of product. There are other potential benefits too. Plastics are so much cheaper than silicon and plastic circuits are also simpler to manufacture than silicon chips.
This has caused some pundits to suggest that an age of truly ubiquitous intelligence could be just round the corner. Here we look at what’s made all this possible, at the exciting new products that might result, and at when you’ll be able to get your hands on some of this revolutionary gear.
Plastic electronics: a case of mobility
An electrical current involves the flow of electrons and this fact dictates what types of materials are good electrical conductors and which are insulators. In metals, the atoms have some electrons that are not closely bound to them so they are free to move, particularly when attracted by an electrical potential, thereby allowing the flow of an electrical current. In other substances, and plastics are classic examples, the electrons are tightly bound to a particular atoms or, perhaps, shared between a pair of atoms to form a chemical bond. Now, very few electrons will move, even when a high electrical potential is applied.
Plastics are polymers which means have very large molecules comprising a particular group of atoms repeated over and over. Polythene, for example, is short for poly-ethylene which literally means that the molecule is composed of lots of repeating ethylene groups. Ethylene has the chemical formula C2H4 and is a gas.
However, when it’s polymerised, it turns into a solid with the chemical formula (C2H4)nH2 where n is a very large number. All the electrons are closely associated with the carbon or hydrogen atoms or are in the strong chemical bonds between adjacent carbon atoms, or between the carbon and hydrogen atoms. As a result, polythene is a good electrical insulator.
Those few plastics that are electrical conductors have the unique property of mobile electrons. Typically the molecules contain long strings of alternating single and double bonds between the carbon atoms in the so-called molecular backbone. In reality, though, such alternating bonds are only one way of viewing the situation and, in reality, some of the electrons in these bonds are referred to as delocalised or, in other words, they are free to move from atom to atom along the backbone.
What’s more, because polymers contain huge molecules, those electrons are able to move a considerable distance as required for electrical conduction. A classic example is polyphenylene vinylene. As you can see in the diagram of its molecular structure, it has a long string of alternating single and double bonds which does, indeed, allow electrical conduction.
The alternating single and double bonds in allow this plastic to conduct
The availability of conducting polymers means that plastics could be used to connect components together as the copper tracks do on most electronic circuit boards but, to produce active electronic components like the transistors in micro chips, another element is needed. That is the property of semi-conduction as found in silicon and a few other semi-metallic elements.
So as not to get bogged down in the nitty gritty of polymer chemistry and semiconductor physics, suffice it to say that plastics can also be fine tuned to behave as semiconductors. So, if we also add in good old-fashioned plastics, between the various formulations we now have all the essential elements – conductors, semiconductors and insulators – that are required to build electronic circuits.
Plastic electronics: the plastic advantage
Recent advances in polymer chemistry might have made it possible to create electronic circuits purely out of plastic but, given that we already have copper and silicon, it’s pertinent to ask why anyone would want to it. There are several potential benefits.
First is price. While it would be wrong to consider silicon a rare element –after all, many rocks contain silicon as does sand – extracting the silicon from its ore and purifying it to the extent required for semiconductor manufacturing is an expensive processes. Plastics, on the other hand, and cheap to make.
Far more importantly, though, is the cost of the processes required to turn the raw material into a working circuit. Creating a silicon chip is a hugely complicated and expensive multi-stage process involving deposition, etching, ion implantation and so many other techniques that have to be carried out using high tech equipment. Because plastics can be dissolved in an organic solvent, however, a polymer circuit can be created using a technique that’s very similar to inkjet printing.
Second, and perhaps more importantly, plastics are tough and flexible. Without a doubt, today’s handheld electronic gear is far more durable than it was only a few years ago. Even so, dropping your new smartphone on a concrete floor isn’t a good idea as you could easily end up destroying its screen as with the iPhone to the right. Plastics, on the other hand are virtually indestructible, a fact which, when coupled with the super low price, will surely result in totally new applications.
Plastic electronics: Organic LEDs
One of the first applications of plastics in active electronic components, and one which is starting to appear in real world products, is the OLED or organic light-emitting diode, one variant of which relies on polymers. Most of today’s screens are based on LCD technology in which liquid crystals are made transparent or opaque, thereby allowing a back light to shine through. Some LCD displays are referred to as LED displays but the use of LEDs is purely to produce the back light.
Another option is to discard with the LCD totally and use LEDs and specifically OLEDs – one per pixel – to generate the image. These are great for use in bright light, they provide an exceptionally good contrast ratio for really black blacks, and they’re starting to appear in consumer electronics gear such as the recent spate of curved screen TVs.
OLEDs are starting to appear in curved TVs like this one from Samsung
Despite the fact that OLEDs themselves are flexible, the fact that the display material has to be paired up with a layer of thin film transistors (TFTs), to turn each of the pixels on and off, limits another potential benefit. In the main the TFT layer is based on silicon technology so the display is relatively inflexible and is not entirely immune from damage.
However, Cambridge-based Plastic Logic has developed the technology for producing an all-plastic FTF backplane that can be bonded to a display layer which can be based on either the coloured OLED or the monochrome electrophoretic (i.e. Kindle-type e-paper) technology. See: all ereader reviews.
Plastic Logic’s displays are totally flexible
To date, applications have mostly benefited from the virtually indestructible nature of these all-plastic displays. For example, the displays have been used to display timetables at bus stops in Germany and in other public transport applications. Closer to home, PopSlate and PocketBook have produced smartphone cases that feature an e-paper based secondary display. Whether there really will be a market for the rollable display, that has been touted for many years, remains to be seen. However, Plastic Logic believes all plastic displays will now set free the whole sphere of wearable electronics as we’ll see later.
One of the first applications of tough plastic displays is in public transport (Photo: Plastic Logic)
Next page: the plastic processor and the future