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Publication Date: 08/1/2009
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Printed Electronics Using More Inorganics

Printed electronics is using more inorganics and composites in the quest for higher performance, lower costs, finer feature size, stretchability and creation of radically new components such as memristors, supercabatteries and metamaterials. Here is what some developers and suppliers have been up to this year.

Lightweight CIGS. Ascent Solar Technologies, Inc., a developer of flexible thin-film photovoltaic modules, recently announced success manufacturing monolithically interconnected 5 meter long flexible lightweight modules on a polyimide substrate using copper indium gallium diselenide (CIGS).

That means such modules can be hand-carried and installed without roof strengthening, as opposed to glass modules that have to be hoisted with a crane across rooftops. It can produce 123 watts under standard test conditions with an aperture area efficiency of 9.1 percent. CIGS promises to be more efficient, have lower cost of ownership and longer life than amorphous silicon flexible alternatives for open air use.

Titanium dioxide. Electronic memory chips may soon gain the ability to bend and twist as a result of work by engineers at the National Institute of Standards and Technology (NIST) who have found a way to build a flexible "memristor" memory component out of inexpensive, readily available materials. A memristor is a resistor that changes its resistance depending on the amount of current that is sent through it — and retains this resistance even after the power is turned off.

The new device is promising because of its potential applications in medicine and other fields. The memristor is a fundamentally new component for electronic circuits developed in 2008. Low-cost sol-gel process involves depositing a thin film of titanium dioxide on polymer sheet. By adding metal contacts, the team created a flexible memory switch that operates on less than 10 volts, maintains its memory when power is lost, and still functions after being flexed more than 4,000 times.

New silicon inks. Manufacturing costs for high efficiency 50µ crystalline solar cells will now be significantly less thanks to a high-throughput industrial silicon-ink inkjet printing system, according to developers OTB Solar in partnership with Innovalight. They use inkjet printing of silicon-ink to halve the number of costly manufacturing processes required to produce highly efficient solar cells.

NanoGram silicon inks for printed electronics are based on its proprietary laser pyrolysis process.

NanoGram has developed processes to make large area, thin layers of silicon cheaply through the additive process of printing nano-Si based inks and, like Kovio which also prints its own specially developed silicon inks and focuses on transistor circuits, it intends to make complete devices, both solar cells and transistors. Key to silicon ink manufacture is to achieve the appropriate particle sizes and size distribution, which enable low temperature processing when the ink is used to create devices. The particle sizes in a given measure need to be within a controlled distribution, and the particles need to be non-agglomerated — previously a considerable problem. In addition, particle oxidation needs to be avoided in order to achieve high performance in printed electronic devices. Stable inks must also be formulated from these particles to be compatible with standard printing techniques such as ink jet printing and wide area coating. NanoGram has also developed techniques for doping the silicon particles during the reaction in highly controllable doping levels.

Another NanoGram focus is on transistors, and one application is display backplanes. Within a few months of development, the company has passed the mobility of amorphous silicon, and believes that mobility in the range of >100cm2/Vs is achievable, although for display backplanes that is not needed.

While the sintering temperature of the deposited transistors is confidential, IDTechEx notes that Teijin, the plastic substrate supplier, is a partner of NanoGram involved in a joint development agreement (JDA) to develop silicon inks for flexible electronic applications. In addition, NanoGram investors include a wide range of companies with a number from Japan, given that one of the largest applications will be display backplanes. For more information, NanoGram will present at the IDTechEx Printed Electronics USA conference and trade expo in San Jose, December 2-3.

Indium zinc oxide, zinc oxide, hafnium oxide. Zinc oxide based semiconductors hugely outperform organic alternatives in conventional transistor electrical parameters though organics have a place by sometimes being ambipolar, light emitting and potentially lower cost. Indium zinc oxide with up to 50cm2/vs mobility is the favored formulation at developers Hewlett Packard (process licensed to two companies already), Oregon State University, Tokyo Institute of Technology, Toppan Printing, Cambridge University CAPE and various Portuguese and German institutions such as the University of Karlsruhe. The more troublesome p-types have now been produced so CMOS circuits are feasible. Now add Samsung, which exhibited a printed oxide TFT array for an LCD display this year. The display was 4 inches, 100 pixels per inch (ppi) and the mobility achieved was 0.6 cm2/Vs.

Lead zirconate titanate. Lead zirconate titanate PZT is the archetypal piezoelectric used for energy harvesting and sensors. Using sol-gel processes, it is now routinely printed to reduce cost and improve performance, work proceeding at Virginia Tech, Southampton University in the UK and elsewhere.

Printable copper. In 2009, printable copper ink has been announced by NovaCentrix, Hitachi Chemical and others for antennas, interconnects, electrodes and other parts. It is argued that, in the form of the new inks, it does not have the traditional problem of an insulating oxide forming and it is not a biocide (when in nano form) subject to the severe price hikes suffered by silver.

New silver inks. Creative Materials' Medical Grade Electrically Conductive Ink is now used in a range of medical electrode applications, including ECG and TENS electrode applications, transdermal drug delivery, defibrillation, and monitoring systems. These products feature excellent adhesion to a wide variety of surfaces, including polyester, polyimide, and glass. With a high percentage silver filler, they are often used in defibrillator electrodes and to make disposable ECG electrodes. Cima nanotech CEO Jon Brodd reports his company is sampling its flexible transparent conductive film made with silver nanoparticles in emulsion that self align into a connected netting-like pattern upon drying, rather like irregular chicken wire. The random openings allow 80 percent transparency, about the same as some ITO, while the silver lines reportedly provide sheet resistance of 5 ohms/square, as good as or better than much ITO.

Sintering metal inks on low cost substrates. Novacentrix is introducing a scalable solution to processing high temperature semiconductor materials on low temperature substrates like paper and plastic, with a new generation Pulseforge tool with 10-20 times the power of the previous production models, which are aimed at drying or sintering metallic inks.  

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