Inorganic chemistry used more widely

In a recent meeting in London, that was presenting the new electronics to those outside the industry, one speaker was waxing eloquent about "Organic Electronics" when someone at the back said, "Wait a minute - you mean like organic vegetables - no poisons, right?" In fact the speaker was using verbal shorthand for printed and potentially printed electronics. Ironically, today this is mainly a matter of inorganic rather than organic chemistry and the next ten years are unlikely to see the inorganic part drop below 50% of the high value materials required.

For example, even so-called Organic Light Emitting Diodes OLEDs for signage and "wallpaper" lighting need copper bus bars deposited to distribute the potential evenly over wide areas. The largest market for OLEDs will be for flexible versions and these need excellent barrier layers against water and oxygen to prevent such molecules damaging the fragile organic layer that emits the light. Such barrier layers typically alternate layers of oxides of boron, aluminium or titanium with polymers but the cost, flexibility and gas permeability need further improvement.

 

Some of these inorganic barrier layers actually bind up the water and oxygen so there is some clever chemistry to do here.

Most printed displays use some inorganic material. AC electroluminescent displays traditionally use copper doped zinc sulfide but many better new inorganic alternatives are about to be announced. In ac electroluminescent displays, the insulating, conducting (bus bar and transparent layers) and passivation layers all consist of printed inorganics.

 

 

The huge animated color ac electroluminescent display by elumin8 at the new Heathrow Airport Terminal 5.

The new photovoltaics variously uses cadmium selenide, cadmium telluride, titanium dioxide and copper indium gallium diselenide CIGS with printed nano-silicon from US and Japanese developers a strong new contestant and InGaAs giving the highest efficiency of all in the space program. GaAs photovoltaics are also used.

 

Nano-silicon has also led to the best printed transistors, in the laboratory at least, and that has been on treated stainless steel foil, reel to reel. Zinc oxide based transistors such as InGaZnO recipes also give far better performance than organics and they are, uniquely so far, transparent. Indeed, yesterday's article described how Hewlett Packard has just licensed its version to both Inpria and Xtreme Energy. Other versions are being commercialised in the UK (Cambridge University CAPE with 3T Technologies "The Transparent Electronics Company") and Japan (Toppan Printing with Tokyo Institute of Technology) and institutions in Portugal and elsewhere are also active, so things are on the move. Printed transistor and capacitor dielectrics must have high permittivity and dielectric strength to be thin for low voltage and best performance, so printed barium titanate, hafnium, lanthanum, zirconium and other salts are being printed. Ionic effects in polymers can give high permittivity but there are stability issues.

The best printed conductors employ silver inks - increasingly with particles only 3nm across, because that uses less silver and reduces the annealing temperature so low cost polymer substrates can be used. However, copper alternatives are now offered and dealing with the insulating oxide on the copper is now a target of development programs, so connections become easier. Additive aluminium used for antennas has a similar challenge but it is in use in RFID labels. Printed silver is used in keyboards, battery testers on batteries, talking tablecloths, pillow radios, interactive board games, medical disposables and much more besides. Printed tin and nickel have been used for resistors, including heaters in apparel. Transparent electrodes for printed displays, photosensor arrays and lighting are almost entirely printed or sputtered with indium tin oxide, with other tin oxide formulations being researched as a lower cost alternative. There is also a body of work on antimony tin oxide transparent electrodes.

Laminar batteries are almost all inorganic, with manganese dioxide zinc and lithium in the forefront but perhaps nickel metal hydride and others becoming a possibility. The thin film lithium batteries (not yet printed -a challenge for chemists) can employ cadmium, titanium and other chemistry. Inorganic sensors are already a large business. Examples are biomedical sensors and silver patterns sensing which pill was removed when from a blisterpack and which package was opened without authority.

There is much more to this story, including laminar fuel cells and lasers recently announced that use zinc oxide nanowires and other compounds. Indeed quantum dots are giving superlative light emission, photovoltaic and other phenomena in the laboratory. The dots are variously made of CdSe, CdSe/ZnS, PbS, PbSe and other inorganic materials. Zinc oxide nanorods have been used in laminar piezo-power and copper nanorods are also of interest in laminar power generation. In one patent for a laminar laser, a film of ZnO is grown on a suitably adapted polycrystalline underlayer in which the grains are surrounded by electrically insulating boundaries. Other such lasers employ silicon dioxideZnO composite. GaAs single layer thin film transistors outperforming the silicon chip to work at terahertz frequencies have been made by Nano e-Print. It has changed its name from Plastic e-Print for obvious reasons.

 

 

Clearly there are huge opportunities for companies specialising in inorganic chemicals to do business in the bold new world of printed and potentially printed electronics. Their skills will be tested to the limit in making nanoparticle, porous, printable ink with low temperature annealing and other morphologies. Then add the challenge of preparing most of the above inks in the very different rheology, viscosity and the different annealing temperatures and times and so on needed for the different printing machines from litho to inkjet and the different substrates. You have the basis of a huge new industry of inorganic chemicals for printed electronics, many of them highly patented and premium priced.

The term organic will not be left to those talking about vegetables. Often the inorganics are made in composite form with organics and organics can be the carrier for the ink that is destroyed on annealing. On the other hand, the organic carrier for the quantum dots is retained in the final device. Many devices are entirely organic. Organic electronics has a huge future but the inorganic part of the demand for materials for printed electronics - currently the biggest - should be attracting more in the chemical industry. They should not be put off by misleading terminology and government programs that always seem to be called poly this or that and are indeed dedicated to organic options because the organic chemists put in the most effort to land the government grants. Some enthusiasts even think that it will all end up as organic electronics in the end but that is a "triumph of hope over reality".

 

 

Yesterday, IDTechEx spoken on printed electronics at both NERM2008, the 37th Northeast Regional Meeting of the American Chemical Society in the USA and at the UK Trade & Investment Chemical Sector Initiative on Printed Electronics in the UK at Haydock Park near Manchester. Both inorganic and organic specialists were present in force. At the UK event, it was pointed out that perhaps 80% of appropriate UK and European grants are going to organics despite them being only half of the opportunity, This is because the umbrella funding programs are given names like "Organic and Wide Area Electronics" (European Commisssion in Brussels) has the effect that those with inorganic solutions do not apply. It is an accident: it is not deliberate, but the organic chemists already on board are, of course, in no hurry to correct the situation. It suits them very well that the new electronics/ printed electronics is often considered to be synonymous with plastic electronics/ organic electronics. On questioning, one professor who lectured on "Organic Electronics" in the afternoon at Haydock Park admitted that he should have used the title "Soluble Electronics" because he was indeeed talking about the total opportunity. A member of the audience who sells inorganic electronic chemicals asked when someone will set up an inorganic electronics association. He got no satisfaction beyond suggestions that nomenclature should change. However, the UK Trade & Investment Chemical Sector Initiative on Printed Electronics deserves credit for addressing the full opportunity in this event and its ongoing support for the sector. It is those in government that give development grants to universities and companies that need to communicate better and stop repelling half of the potential industry. For more read Inorganic Printed and Thin Film Electronics and attend Printed Electronics Asia or Printed Electronics USA.