Plastic Logic: from innovation to impact

The story of Plastic Logic started in the mid-1980s when Professor Sir Richard Friend – then a lecturer in the Department of Physics at the University of Cambridge – began to work on organic semiconductors [see Glossary below]. ‘My interest was pure curiosity,’ says Friend, who is now the Cavendish Professor of Physics at the University of Cambridge. He was interested, he explains, in gaining a basic understanding of how electrons might be made to move in carbon-based semiconductors, rather than being driven by the prospect that his research might be commercially useful.

Semiconductors – materials that conduct electricity under some conditions but not others – are used to make the integrated circuits that run computers and other electronic devices. Silicon is the best known semiconductor but, in the 1960s, researchers discovered that some organic molecules also behave as semiconductors. Specifically, small molecules that contain carbon atoms linked by alternating single and double bonds – so-called conjugated molecules – behave as semiconductors because some of their electrons are delocalised and ‘shared’ throughout the molecule. Friend wanted to know whether polymers made from building blocks of conjugated molecules would also behave as semiconductors. ‘We were interested in this type of molecule because we thought that, if they did behave as semiconductors, we might be able to use them to make electronic devices simply by dissolving the polymers in a solvent and then painting them onto a surface,’ says Friend.

By 1988, Friend’s research group had managed to make a transistor from the conjugated polymer polyacetylene. But, notes Professor Henning Sirringhaus, Hitachi Professor at the University of Cambridge and Friend’s colleague since 1997, ‘the performance of this polymer or plastic transistor was very poor because the speed at which electrons and holes move through polyacetylene – a property called mobility – is much lower than in silicon. Plastic transistors were pretty much a scientific curiosity at that point, although they did provide a useful device for studying the electrical properties of new materials.’

A serendipitous discovery

Friend’s team now started to investigate whether better transistors could be made from other conjugated polymers. ‘We thought that a poorly studied compound called poly(p-phenylene vinylene), PPV, looked promising,’ says Friend, ‘and we began a collaboration with Andrew Holmes, a natural products scientist then working in the Department of Chemistry in Cambridge, to make PPV and to use it to make transistors.’

Unfortunately, PPV was not ideal for transistors – it was too good an insulator. But rather than giving up on PPV, the researchers decided to measure its insulating properties. ‘Instead of making a parallel electrode arrangement as we do for transistors, in February 1989 we made a stacked electrode arrangement as we do in diodes and sandwiched the PPV between the two electrodes to measure its insulating abilities,’ explains Friend.

By good fortune, Dr Jeremy Burroughes, who had made the first polyacetylene transistors while a PhD student in Friend’s laboratory, used a thin, semi-transparent layer of aluminium to make the top electrode in this PPV-containing device. When Burroughes (who is now the Chief Technology Officer at Cambridge Display Technology, CDT) applied a voltage to the device, he unexpectedly saw green light coming through the electrode. Friend immediately contacted Dr Richard Jennings (Director of Technology Transfer and Consultancy Services, Cambridge Enterprise Ltd) in what was then the University’s industrial liaison office to tell him about the strange, light-emitting piece of plastic and to ask for advice on patenting this discovery.

‘As soon as Richard explained what he had seen, we began to think about applications,’ says Jennings. ‘Plastic light-emitting displays, light-emitting clothing, plastic TV screens – it didn’t take much imagination to see how these polymer light-emitting diodes [P-LEDs] might be used and my advice was to patent the invention immediately.’ A particular appeal of light-emitting plastics, say both Friend and Jennings, was that these materials could be solution-processed or painted over a large area, a much simpler process than that needed to make liquid crystal displays (LCDs), the up-and-coming display technology in the late 1980s.

Patents for P-LEDs were filed in April 1989 and April 1990. Then, in October 1990, the researchers published a letter in the journalNatureentitled ‘Light-emitting diodes based on conjugated polymers’. ‘The rest of the world simply dived in after we published. We had scores of imitators and our patent was challenged on several occasions,’ says Friend.

But, despite the academic interest in P-LEDs, Friend failed to find a UK electronics company to license and develop the invention. ‘It wasn’t that the companies weren’t willing to license the patent,’ stresses Friend. ‘It was more that they did not see organic light-emitting diodes as a core business and I was concerned that they would simply sit on our idea and not do the work needed to develop it. The quickest single way to kill a good idea is to put it into the wrong hands,’ comments Friend.

So, in 1992, Friend, with help from the University of Cambridge and local seed venture capital, founded CDT. Although the original intention was that CDT would be a materials manufacturing company, CDT has concentrated on developing new technologies and licensing them to other companies. For example, in association with various industrial partners, CDT has developed a method to make P-LED displays using inkjet printing, thin-film transistors to stimulate the P-LED-containing pixels in displays, and polymers that emit red or blue light when stimulated instead of green light. In 2004, CDT was floated on the NASDAQ National Market and, in 2007, it was acquired by the Sumitomo Chemical Company, which maintains substantial R&D activity in and around Cambridge.

Importantly, says Friend, a strong symbiotic relationship has developed between CDT and the scientists working in the University: ‘Over the years, we have sent a lot of ideas to CDT but in return we have had access to the materials and methods that CDT has developed and this has helped us to push our fundamental research along much faster than would have been possible if we had had to do everything in the University.’

Back to transistors

While P-LEDs were being developed, some work continued in Cambridge and elsewhere on plastic transistors. Because silicon-based transistors were so good, explains Sirringhaus, ‘there wasn’t any commercial drive to work on plastic transistors and probably fewer than ten groups worldwide were working on the problem.’ Adds Friend, ‘it was really a matter of waiting for new materials to be made, waiting for the technology and science to develop to a stage where we could take the transistors forward.’

Then, in 1997, a way was found to increase the mobility of polymer semiconductors. The problem with the original polymer semiconductors had been that the long-chain molecules within these substances were disordered – ‘like a bowl of spaghetti’, says Sirringhaus. As the charge moved through this disordered mass, it encountered configurations where it didn’t know where to go and this reduced the material’s mobility. The polymer chains were disordered because, to process polymer solutions,

flexible side chains have to be attached to the polymer chains. Unfortunately, these side chains made the polymer disordered and electrically poorly conducting. The 1997 breakthrough was the discovery of a way to deposit materials from polymer solutions that consist of alternating layers of conjugated polymers lying in a plane and insulating side chains. ‘The mobility in the conjugated plane can be very high and it doesn’t matter about the mobility elsewhere in the structure,’ explains Sirringhaus.

Although the demonstration that the mobility of polymer semiconductors could rival that of inorganic semiconductors like silicon was important, before the researchers could persuade large companies or venture capitalists to invest time and/or money in their discovery, they still had to show that their new material could be used to make transistors in a practical manner.

‘At that time, we were developing methods to use inkjet printing to deposit P-LEDs onto substrates so we started to investigate whether the same process could be used to print transistors,’ says Sirringhaus. Within a few months, Sirringhaus and PhD student Takeo Kawase, on secondment from Seiko Epson, had printed a few transistors onto small substrate chips and had shown that these simple circuits performed reasonably well. ‘We now had a credible story on the materials and a credible way to make devices from them so we began to think about commercialisation,’ says Sirringhaus. Indeed, says Friend, ‘I had a strong sense that the future seminal events in the development of organic transistors were going to be engineering events, not science events, and I believed that these were most likely to happen in a well-focused industrial environment.’

Plastic Logic is founded

With this in mind, the researchers approached the entrepreneur and venture capitalist Dr Hermann Hauser, a co-founder of Amadeus Capital Partners (Cambridge) and an early investor in CDT, to see whether he would invest money in the commercial development of organic polymer transistors.

‘I remember visiting Richard and his group in the Cavendish,’ says Hauser. ‘They only had a few transistors working at this time [1998] and when they stopped working they prodded them with toothpicks!’ Luckily, Hauser, with his background in physics and interest in electronics, instantly recognised that Friend, Sirringhaus and their colleagues had made a very fundamental breakthrough and, with his help, Plastic Logic was formed in January 2000.

What is so special about plastic transistors?

When Plastic Logic started, all the electronic displays in the world were made on glass. Displays like those attached to computers contain millions of pixels, each of which is switched on and off by an individual silicon transistor. To produce these transistors, amorphous (non-crystalline) silicon is processed at high temperatures. Consequently, silicon-based transistors can only be produced on a substrate like glass that can withstand high temperatures; a plastic substrate would melt or deform. But displays that contain glass are heavy, rigid and fragile and unsuitable for use in anything but very small mobile displays. The production of plastic transistors, by contrast, does not require high temperatures so they can be laid down on plastic substrates that are much lighter, and more flexible and robust than glass. This means that large portable displays can be made by using plastic instead of silicon transistors.

Plastic transistors have a second advantage over silicon transistors when it comes to making large displays. Electronic circuits contain many layers that have to be accurately aligned with each other. In a large display, the dimensions of the substrate inevitably change slightly during the production process. Silicon-based displays are made using a lithographic process in which patterns are sequentially deposited onto substrates using metal masks. Unfortunately, any small changes in the dimensions of the substrate during the production process mean that the masks do not line up accurately and the resultant display is defective. With displays that contain plastic transistors, computers drive the inkjet printers that make the various layers of the device so it is possible to allow for changes in the substrate’s dimensions.

From single transistors to an electronic reader

‘When Plastic Logic was founded,’ says Jennings, ‘there wasn’t a clear business plan but Hermann Hauser was a very far-sighted investor who, knowing the track record of Richard Friend and Henning Sirringhaus, was willing to put money into their company to see where it would go.’ Over the next few years, Plastic Logic raised considerable sums of money to support its work and by 2006 it had developed its plastic transistor technology sufficiently to produce a display containing a million transistors. It had also developed an application for these displays – a plastic electronic reader. Since 2006, Plastic Logic has raised more than US$100 million to build a large manufacturing plant in Dresden (Germany); its research and development department still remains in Cambridge but its corporate headquarters is now based in Mountain View (California, USA). Trials of the electronic reader with key customers should be completed by the end of 2009 and commercial production will be rolled out in 2010.

The electronic reader, which has an A4 screen that is about as heavy and thick as a sheet of paper, uses an ‘active matrix display’, an array of pixels in which each pixel contains minute plastic capsules filled with a liquid that contains black and white particles. These particles have different charges so that when an electric current is applied to a pixel, either the white or the black particles move to the front of the capsule and the pixel appears white or black. A plastic transistor behind each pixel applies the electric charges and the whole device is printed onto a thin, flexible sheet of plastic.

Plastic Logic’s electronic reader will enable users to read their own documents anywhere and will give them access to newspapers and books and, according to Friend, Sirringhaus and Hauser, it has several advantages over existing electronic readers such as Amazon’s Kindle. Its display is lighter and more robust than the glass-based displays in other readers and, because the display is bigger than those in other readers, it is more suitable for accessing newspapers. Also, the device uses very little energy because, unlike other readers, the display in the Plastic Logic reader does not need a back light. Consequently, once a page is set, it can remain in place without consuming any energy. Thus, users should be able to take a Plastic Logic reader away on holiday, for example, without having to take a battery charger.

Other hopes for plastic electronics – the need for continuing basic research

Plastic Logic should produce several hundred thousand electronic readers in 2010 and, in later years, it could be producing millions of units. But Hauser believes that plastic electronics will have much broader applications in the future. While Plastic Logic was developing its electronic reader, he explains, basic research was continuing in the University of Cambridge, where Sirringhaus’ group recently made an important breakthrough by discovering how to make a CMOS plastic transistor.

‘CMOS’ stands for complementary metal oxide semiconductor, a type of semiconductor that can be used to produce a combined n-type and p-type transistor. This type of transistor is needed to build complex devices like computer processors but for many years it seemed that it would be impossible to build plastic transistors with the properties of CMOS transistors – polymer semiconductors were all p-type semiconductors because they all carried current in the form of holes. Then, in 2005, Sirringhaus and his colleagues showed that the reason why th

ere were no n-type polymer semiconductors was because the electrons were being trapped at the interface between the semiconductor and adjacent insulators. By studying this interface, the researchers were able to produce an n-type polymer semiconductor, which opened up the possibility of designing the CMOS circuits that are necessary for the development of a broad plastic electronics industry.

However, Friend, Sirringhaus and Hauser stress that relatively little is known about polymer semiconductors and, because these materials are so different from silicon, it is not possible to rely on established semiconductor physics to understand how they work. Thus, it is essential that fundamental research on polymer semiconductors continues to be funded within UK universities. This, together with improved governmental support for the companies involved in plastic electronics, should ensure that the UK’s current lead in the field of plastic electronics is retained and that the UK reaps the financial rewards of the groundbreaking, curiosity-driven basic research in which Friend, Sirringhaus and their colleagues excel.