New Developments at The University of Cambridge

Keynote speaker at Printed Electronics Europe  was Professor Sir Richard Friend, of the Cavendish Laboratories at the UK's University of Cambridge. He started the day with a presentation on the performance of organic semiconductors in electronic applications. Sir Richard was the founder of the core technology used by CDT and Plastic Logic. This covered areas such as OLEDs (Organic Light Emitting Diodes), FETs (Field Effect Transistors) and photovoltaics and the development of various processing technologies.

 

His wide-ranging talk gave an interesting overview and included commercial uses of polymer displays, for example, on the Philips Shaver there is a display which shows how much charge remains on the battery. Although, as Sir Richard pointed out jokingly, there would be a lot more battery life remaining if it didn't have to drive the display. But it is simple applications such as this that are driving forward OLED displays in the commercial world.

 

Although many of the currently used OLED displays are small - such as those on the shaver or on music players - the largest OLED screen, at the time of going to press, is the 40 inch (1 meter) display used in the flat screen television by Epson, comprising of four 20inch back panels placed together then inkjet printed over.

 

Despite the improvements that have been made over the last few years, Sir Richard said that he sees a lot more discovery to come. He says that the more that understanding of the technology grows, the more that it improves. For example, although originally quite short lived the blue in OLED displays now has a lifetime of more than 100,000 hours.

One of the challenges for printing transistor circuits is the need to have very small channel lengths.

 

Source:Dow Chemical 2003

 

The smaller the channel length the faster the switching speed of the transistor. This has been one of the reasons why RFID is a tough application for printed electronics because achieving UHF (900MHZ) printed tags would be very difficult due to components which would need to operate at high frequencies, although several companies say 13.56MHz is possible. Inkjet printing drops makes it hard to get very fine resolutions and a low channel length - you need the drops to be as close as possible but not to touch. Plastic Logic has been doing this by using lithography to pattern regions that are hydrophobic and hydrophilic (water hating and water loving respectively). The inkjet drops are accordingly "doped" and when they land on the substrate they are attracted to fill the patterned regions. This give sub 5 micron channel lengths. However, this process cannot be truly described as a fully inkjet printed process because it relies on lithography which is expensive and slows production. Sir Richard unveiled some of their new work, where the first drop is coated so that it repels the second drop based on the hydrophobic/hydrophilic principal detailed above. The second drop is jetted so that it slightly overlaps the first one, but because it is repelled it is pushed away and settles. Using this "self aligning" technique, the channel length, just using inkjet printing, is now on the order of the molecular length of the coating. Sir Richard said this is less than the channel length of transistors used in the latest Pentiums today. This very exciting development means UHF and higher frequencies could now be possible in due course, but work is of course needed to move this from a university experiment to commercial R&D and then, hardest of all, to a commercial process. In particular, work is required to create thinner dielectrics. The principle of this is outlined below. Plastic Logic are developing these ideas.

source: Plastic Logic

 

A second area of development is to rethink the structure of transistors. Transistors in silicon are horizontal because they are built up using many steps and various masks. However with printing it is easier to control the thickness of a layer than the spacing (which is also heavily affected by the substrate type), so work is going on at The University of Cambridge to develop transistors which are vertically built up.

 

For more information, details of the announcements and developments made at Printed Electronics Europe will be published here on Printed Electronics Review over the next few weeks. Refer back for details.

 

You can purchase the conference proceedings and audio of the presentations by going to www.printelec.com .