Inorganic printed electronics employs new compounds

There are huge opportunities for companies providing inorganic chemicals to printed and potentially printed electronics. Here, Dr Peter Harrop, Chairman, IDTechEx, summarises some of the findings from the new IDTechEx report Inorganic and Composite Printed Electronics 2009-2019.

 

There are huge opportunities for companies specialising in inorganic chemicals to do business in the bold new world of printed and potentially printed, thin film electronics. Their skills will be tested to the limit in making nanoparticle, particle free and other printable ink often with low temperature annealing capability.

 

Premium pricing awaits but there are the challenges 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. For example, agglomeration of the nano particles is a problem with the doped nano silicon inks being prepared for market by several suppliers. There is the basis of a huge new industry of inorganic chemicals for printed electronics, many of them highly patented.

 

Most printed electronics relies on inorganic materials and that will continue to be the case. Well rehearsed are forms of flexible photovoltaics based on silicon ink, copper indium gallium diselenide CIGS, dye sensitised solar cells based on titanium dioxide, iodine and ruthenium. Then there are those using cadmium telluride and cadmium selenide and those with multiple thin films employing GaAs, Ge and so on in one device. However, there is even more coming along, including Fe3O4 , Au/TiO2, MgB2, LaAlO3, and other compounds. These promise printed photovoltaics that harvests infrared, light and ultraviolet in one device.

 

 

In an alternative approach, Ohio State University dopes oligothiophene with molybdenum and tungsten to generate power in response to light of wavelengths from ultraviolet to near infrared.

 

IDTechEx find that of all of printed, organic and flexible electronics in 2009, the majority are based on inorganic materials.

 

Now, the U. S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) is seeking project proposals as part of recently announced DOE funding to accelerate commercialization of solar energy technologies. NREL also announced partnerships with 13 U.S. small solar businesses, which have the capability to enter the market by 2012.

 

Within this, Ascent Solar Technologies, Inc. will develop zinc magnesium oxide window layers enabling high performance mid-bandgap CIGS photovoltaics on polyimide modules. This offers the potential to increase the performance of the devices through an increase in the absorber band gap.

 

Phosphorescent OLEDs, such as those employing iridium based dyes, have been developed by Pacific Northwest National Laboratory in the USA and others.

 

 

 

Silver is the favourite printed conductor by far but new printable copper has received considerable attention recently. Less well known is the use of a wide choice of other inorganic elements and their compounds to make organic electronics a practicable proposition as with inorganic oxides and nitrides in barrier layers and a considerable number of other inorganic elements and compounds performing different functions in these devices.

 

Indium tin oxide is proving difficult to displace as the favoured transparent electrode despite the fact that printed versions have never really caught on and it has problems with cracking and the volatile price of indium. Inorganic compounds with less indium are being developed that have low temperature curing.

 

 

In the laboratory, Organic Light Emitting Diode displays OLEDs variously employ such materials as B, Al and Ti oxides and nitrides as barrier layers against water and oxygen, Al, Cu, Ag and indium tin oxide as conductors, Ca or Mg cathodes and CoFe nanodots, phosphine sulphide, Ir and Eu in light emitting layers.

 

 

For instance, the OLLA project concluded that printed organic "conductors" are too resistive to distribute potential evenly over wide area OLEDs and copper should be used. Jian Shen of the Oak Ridge National Laboratory in the USA and his colleagues used Co and Fe magnetic nano particles to dope the structure of a polymer-based OLED in research aimed at printing of these devices. The technique not only opens up a way to get more light out of an OLED, but also allows the OLED intensity to be controlled by an external magnetic field. More information can be found in the IDTechEx report Displays and Lighting: OLED, e-paper, electroluminescent and beyond.

 

 

Barrier layers for flexible OLEDs and for flexible organic photovoltaics in the open air need to be better than those used for any other device other than possibly. We mean 10-6 grams per square meter per day of water and 10-5 cc per square meter per day of oxygen.

 

Few currently believe that the requirements of life, flexibility, large area, low cost and volume manufacture have been met, to the extent that wide area, long life, flexible OLED lighting, signage and displays can be produced.

 

 

Developers such as Vitex, Appliflex, Alcan and 3M in the USA and IMRE in Singapore use alternating metal oxide or nitride and polymer layers, examples of the inorganic layers being the oxides or nitrides of boron, aluminium and titanium and there is interest in binding up the undesirables, not just preventing them from getting through. However, not one of the developers of barrier layers is able to use printing as yet. For more information, read the IDTechEx report Barrier Films for Flexible Electronics for details.

 

 

Virtually all thin film batteries are inorganic, with printed manganese dioxide zinc and deposited thin film lithium polymer and lithium ion devices in the ascendant.

 

For batteries in general, among the most promising lithium ion chemistries are LiCoO2 (Graphite), Li(Ni1-x-yCoxAly)O2 (Graphite), Li(Ni1/3Mn1/3Co1/3)O2, LiFePO4 (Graphite) (Graphite), LiMn2O4 (Graphite) (Hard carbon) (Li4Ti5O12). Several are proving practicable in thin film and ultimately printed batteries.

 

 

 

Printed alternating current (ac) electroluminescent displays on polyester film are exhibiting better colors and longer life as the chemistry improves. Traditionally they rely on printed zinc sulfide variously doped with silver, copper and iron to get the different colors, the necessary insulating and conductive layers also being printed inorganics.

 

 

Printed electrophoretic displays are proving the most successful form of flexible electronic display, used in e-readers, wristwatches, shelf edge displays, e-posters, e-packaging and even the Esquire magazine animated front and inner pages for its 75th anniversary. These employ titanium dioxide, carbon and organics to give exceptionally low power and good viewing in sunshine.

 

 

It is too early to refer to zinc oxide as the new silicon but this simple material is now proving extremely versatile. One option for photovoltaics that harvests ultraviolet, visible light and infrared is silver nanocluster doped ZnO thin films. Zinc oxide nanorods promise to be an improvement on TiO2 nanoparticles in DSSC photovoltaics according to some researchers.

 

 

Thin films of zinc oxide nanowires prove to be potent piezoelectric energy harvesters potentially suitable for micro electroelectromagnetic systems MEMS and other applications.

 

There is now a huge development effort on printed transistors based on InGaZnO and similar semiconductors which have parameters greatly superior to those of printed organic transistors. It includes Evonik Degussa and the University of Darmstadt backed by Merck Chemical in Germany; Tokyo Institute of Technology and Toppan Printing collaborating in Japan; 3T Technologies and Cambridge University CAPE collaborating in the UK and the University of California at Berkeley and Sunchon University Korea who are working together and Universities in Portugal.

 

 

Hewlett Packard, which works with Oregon State University on the subject, has already licensed two companies to make their transparent printed versions and Samsung has demonstrated LCD backplane drivers based on these transistors.

 

Thin film lasers recently announced they use zinc oxide nanowires. 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 dioxide/ ZnO composite.

 

The INANEE in Italy has demonstrated improved light extraction in OLEDs by using polystyrene ZnO nanocomposite scattering layers. Istanbul University uses sol-gel printing to create ZnO nanodots as humidity sensors.

 

 

Now we have radical new, thin, flexible components that are either printed or potentially printed. They include supercapacitors, supercabatteries, memristors and metamaterial devices. Most rely on inorganic materials wholly or in part.

 

 

There are GaAs single layer transistors from Nano e-Print that work at terahertz frequencies and experimental laminar fuel cells and a host of new printed sensors for biological and chemical monitoring.

 

Quantum dots are giving superlative light emission, photovoltaic and other phenomena in the laboratory. These dots in a carrier material are variously made of CdSe, CdSe/ZnS, PbS, PbSe and other inorganic compounds. Copper nanorods are of interest in laminar power generation and printed metal patterning smaller than the wavelength being harvested is employed in experimental nano-antenna arrays planned as a more efficient alternative to photovoltaics. Similar printing technology is used for metamaterial patterning.

 

Transparent electronics is becoming a business proposition in its own right where companies such as 3T Technologies "The transparent electronics company" are taking a lead, in this case with support from Cambridge University centre for Applied Photonics and Electronics in the UK. Currently, in the world of flexible devices, transparent transistors, ballistic diodes and ac electroluminescent displays and lighting are inorganic whereas transparent batteries, memory, loudspeakers, OLEDs are at least partly organic. Transparent flexible resistors and capacitors can be made with organic or inorganic chemistry.

 

For more information, read the IDTechEx report Inorganic and Composite Printed Electronics 2009-2019. With over 160 tables and figures, this report critically compares the options, the trends and the emerging applications. The emphasis is on technology basics, commercialisation and the key players. This report is suitable for all companies developing or interested in the opportunity of printed or thin film electronics materials, manufacturing technologies or complete device fabrication and integration.

 

 

All of these topics will be aired at the forthcoming IDTechEx Printed Electronics events: Printed Electronics Asia 2009, Tokyo, Japan and Printed Electronics USA Dec 2-3, San Jose, CA.