Breakthroughs in electronic inks
11 February 2008
Science fiction writer Arthur C Clarke famously said that any sufficiently advanced new science is indistinguishable from magic. The dream of printing electronics onto and into anything sounds pretty magical, particularly when it is said to involve creating bionic man, having edible electronics, smart clothing, skin patches that deliver drugs according to the need that they sense, writes Dr. Peter Harrop of IDTechEx.
However, there is now a clear road map taking us to these things. When we print electronics directly onto things, in the way that 85 per cent of barcodes are printed today, there will be little market for substrates, cases, keyboards, mounted displays and the paraphernalia (and profit makers) of electronics today. The added value will be almost entirely in the ink. Some of these clever inks will cost ten times their weight in gold but, paradoxically, because they will be printed in layers only a few atoms thick, they will create very low cost devices.
To make transistors, solar cells, batteries and all the kit of parts of the electronic designer, we need semi-conducting, conducting, insulating, light emitting, protecting and other inks. Indeed, the solvent in an ink must not destroy the delicate layer put down just before. The inks must cure at low enough temperatures to use the lowest cost plastic films, and even print on human skin.
To make edible electronics, the ink must be FDA approved. Edible electronics goes beyond the recent patents of Eastman Kodak, which refer to monitoring smart pills to prove they were taken and see when they are absorbed by the body (the circuit can no longer be sensed). Even children’s electronic toys must be edible if they are likely to be chewed. The new bio-degradable electronics printed on paper certainly comes in that category.
To make the new invisible electronics, all the layers must be transparent. In the UK, 3T Technologies Ltd calls itself the transparent electronics company and works with researchers at Cambridge University. Around the world, fully transparent photovoltaics, batteries, transistor arrays and other printed components have already been demonstrated. A watch will soon be launched where the cover glass has an invisible layer that charges the battery from heat as well as light; meaning that the battery then lasts longer than you do.
In the last few months we have seen demonstrations of smart contact lenses and even replacement eyes that provide superhuman capabilities. Bionic man is certainly an objective of many new implants and organ replacements employing bio-compatible electronics and bio-compatible inks.
Organic chemists have been particularly strident in claiming that printed electronics will end up being based on organic inks. The authorities have tended to agree and the considerable funding of research by the European Commission has usually had titles like Polyapply and Poly this and that. Companies such as Plastic Logic and Polymer Vision have been created and there is an Organic Electronics Association based in Germany. There are conferences called Plastic Electronics and Organic Electronics. So far, so misleading. What matters is what works, that it is safe and affordable and, although there are many exciting organic semi-conducting and dielectric inks being put into production, most printed electronic devices employ both organic and inorganic inks. An increasing percentage have at least one layer that combines organic and inorganic materials in one ink or at least elemental carbon with organic compounds. With partially printed thin film fuel cells, actuators, microphones, loudspeakers, lasers and more being evolved, it never was realistic to think that organic or inorganic chemistry alone could conquer all. Indeed, transparent electronics is usually based on inorganic compounds in the ink and most research on photovoltaics concerns inorganic alternatives to silicon.
Printed semiconductors are used in a special form in photovoltaics where the electroluminescent function is suppressed. Printed transistors alone have many requirements, from high frequency of operation to low power wastage, transparency to high power handling, light emission, and so on.
Put at its simplest, the higher the maximum frequency of operation of the transistor, the more gadgets you can make and sell. One of the primary properties of the semi-conducting inks put down to make transistors is the charge carrier mobility in the resulting layer, because that affects the maximum frequency of operation. Organic inks are being improved slowly in this respect but their mobility is very poor. People have realised that certain inorganic compounds with one hundred times the mobility could be put down in thin layers, but printing and curing them at high-speed reel to reel seemed impossible.
However, that is no longer the case. Tokyo Institute of Technology working with Toppan Printing is now doing that with InGaZnO semi-conductors. The organic carrier is destroyed during curing. Then along came Kovio with nanosilicon ink and one thousand times the mobility of organic semiconductors with the ability to print much smaller transistors, so that thousands can be deposited on the area taken by the silicon chip.
By contrast, organic transistor arrays tend to look like a train ticket. In the near term, Kovio alone will meet the favourite specification of RFID, replacing the silicon chip in the label with something printed that is initially replacing the chip with something 80 per cent cheaper, and later 90 per cent cheaper.
Silicon solar cells are heavy, brittle, inefficient and expensive, and most of them only sell because the supplier or user gets subsidised by government. Thin films of cadmium telluride on low cost flexible substrates are now found to be cheaper to buy, install and own than silicon, and over $1.5 billion of orders have recently been placed with First Solar to prove it.
No-one has yet worked out how to print these, but next in line are dye sensitised solar cells using ruthenium-based organic dye on titanium dioxide nanoparticles and copper indium gallium diselenide CIGS. Factories now exist ink jet printing these on low cost polymer subtrates reel to reel; notably G24 Innovations in the UK and Nanosolar in the US and Germany. DSSC makes electricity from heat and light (thermophotovoltaics) and is unusually tolerant of light at narrow angles of incidence and even polarised light from reflections. CIGS can be very efficient in converting light into electricity, but there is still a place for printed organic photovoltaics where area is not a problem, but price is critical.
Everyone wants to say they are in nano technology but in printed electronics (particularly conductors), most of these projects involve particles or film thicknesses of tens of nanometers. That often means that too much expensive material such as silver is employed, given the need to produce huge volumes of labels and huge areas of billboards to give them such things as moving colour pictures, solar panels and the like.
NanoMas Technology makes consistent three micron particles of silver in ink, thus reducing the melting point tenfold. It is possible to dip a pen in their black ink, write on acetate film and melt the particles together without distorting the film, thus creating a mirror-like layer with electrical conduction near to bulk silver by using very little material.
The most common printing technology for research and pre-production of printed electronics is screen printing, and the most common technology in full production is ink jet printing, including plastic logic flexible displays and their backplane transistor arrays. However, there are many projects employing flexo, gravure, litho and other options or a combination for different layers, often with some spin coating, chemical deposition or even sputtering as interim stages for difficult layers.
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