Come Back Inorganic Electronics - All Is Forgiven

By Dr Peter Harrop IDTechEx Here come organics Silicon has had a good run in the electronics industry but the lowest cost chips have not changed much in price for decades and wide area silicon, as with solar cells, is heavy, expensive and in need of huge government subsidies to be sold in any volume. Truly flexible - as opposed to bendable - silicon is non-existent in practicable form. Enter organic electronics that can be printed, unlike silicon, and is potentially of low cost and very versatile, being useful for sensors, power, memory, logic, lighting and much else besides. Here we are not just talking of organic semiconductors but of dielectrics, conductors and light emitters for example. Many conferences and companies use the terms organic or plastic electronics and there is even a trade association. The progress of this new industry is phenomenal. So that is the end of the story? Not that simple Nothing could be further from the truth. Organic electronics certainly holds the high ground in many aspects of potentially low cost and wide area electronics and electrics. Nanoident of Austria recently built the world's first fab for printed organic semiconductors, focussing on photodetector arrays initially. However, these devices employ nanoparticle metals for their ink jet printed conductors, including electrodes. Inorganic and hybrid constructions are often best Most companies developing Organic Light Emitting Diodes OLEDs employ indium tin oxide ITO semi transparent electrodes and most other "organic" devices, including transistors employ metal conductors. Indeed, given the mobility and therefore the operating frequency of organic transistor semiconductors, companies such as Motorola and Hewlett Packard in the USA are additionally researching inorganic semiconductors. Even Merck of Germany, a leader in organic semiconducting inks, has done the same, partnering with the Technical University of Darmstadt in Germany. It invested $1.4 million in the joint venture's first year of operation. In other words, it is complementary to learn how to print a transparent inorganic semiconductor, with up to 400 times the mobility, alongside trying to invent stable, low cost organic semiconductors with device mobility exceeding 10 cm2/Vs, when even improving on one tenth of that is proving tough. Further, anyone waiting for a cost-effective, completely organic laminar battery to drive these circuits may have to wait a long time, though it would be nice to co-deposit one. It becomes a matter of "Shall I make the new inorganics printable?" or "Shall I make organics work better?" Not everyone is jumping the same way. Indeed there is a spectrum of choice as shown in the figure below. Here we are simplifying in calling the right side "organic" because it almost always involves metal conductors, just as the left side often involves organic substrates. The technologies live together -and that is not just an interim stage. Fast growth in inorganic printed electronics Among the fastest growing companies in the new electronics are those that offer flexible A.C. electroluminescent displays that can cover many tens of square meters, emitting a range of colours, or be incorporated in watch faces and instrument displays. They involve six to eight printed inorganic layers, including a copper doped phosphor, the only organic material being a routine plastic film substrate. Pelikon and elumin8, both in the UK, Emirates Technical Innovation Centre in Dubai, Schreiner in Germany and others are involved. Inorganic compounds hold the high ground in photovoltaics Dye sensitised solar cells DSSC reaching the market are based on titanium dioxide. CIGS solar cells researched by IBM, commercialised successfully by Miasol? in the USA and being developed by several other companies are inorganic - based on copper, indium, gallium diselenide. They are certainly wide area and flexible. Can they be printed? It is little wonder that there are a rapidly growing number of organizations developing potentially printable inorganic and hybrid electronic materials and devices. Spectrolab in the USA recently demonstrated 40.7% efficiency with a gallium arsenide/ germanium solar cell, eight times that of the best organic versions (which are improving only slowly nowadays) and up with the very best performance of heavy silicon. Spectrolab already offers commercially flexible solar cells based various inorganic compounds. Much scope for hybrid inorganic/ organic solutions In the middle ground, there is much work on improving organics by adding inorganic materials often in nanoparticle form. The new quantum dot devices and high K printable dielectrics are usually based on inorganic materials. After all, organic materials always have low permittivity and are therefore rarely ideal for transistor dielectrics or compact high value capacitors. Inorganic compound transistors driving flexible displays Last year, Toppan Printing in Japan demonstrated a flexible electrophoretic display back plane driver employing amorphous indium gallium zinc oxide semiconductors processed at room temperature. This is based on work at Tokyo Institute of Technology in Japan. Toppan Printing claims much higher mobility and therefore potentially frequency of operation than organic alternatives. Both companies have programs to commercialise their inventions though Plastic Logic of the UK is ahead with flexible electrophoretic displays because it is already building a factory in Dresden in Germany to make them using its organic transistors. Do not wait for highly conductive organic inks Progress in improving the rather resistive so-called organic conductors is also limited, so new inorganic conductors also are being developed by many companies such as Cabot, Emerson & Cuming, Parelec, NanoDynamics, FerroCorp and NanoMas Technologies of the USA. Then there are nanotube carbon and inorganic inks coming along as both conductors and semiconductors thanks to Unidym of the USA and others, though they may not be low in cost for some time. Despite this there is some traction in the marketplace for the newer organic conductors including semi-transparent ones so the situation evolves. Lighting may go either way The US Department of Energy forecasts that the new laminar electrics will eventually achieve lighting at a very environmental 160 lumens per watt efficiency vs. only 50 lumens per watt for fluorescent lighting today. However, it is careful to say that it may be inorganic or organic technology that is successful in this respect. The race is on. How it is panning out Although the term inorganic printed electronics may not be on everyone's lips any time soon and it may never be the name of a company, or a trade association, it is now recognized as an area of tremendous commercial potential and technical progress. The cohabitation of organics and inorganics is probably panning out in the manner below. This is, literally, a very fluid situation. The table shows the likely impact of inorganic printed and potentially printed technology to 2017 by giving the dominant chemistry by device and device element. Dark green shows where inorganic technology is extremely important for the active (non-linear) components such as semiconductors. Light green shows important contributions from hybrid inorganic-organic technology. Red shows where organic technology has the greatest potential over inorganic. IDTechEx has just written the first analysis of the commercialization of inorganic printed and thin film electronics. It covers the activities of all the organizations mentioned above and many others. Read "Inorganic Printed and Thin Film Electronics" www.idtechex.com for more information and attend "Printed Electronics Europe" Cambridge UK 17-18 April