Inorganic semiconductor technologies with conventional patterning offer immediate solutions, according to Kodak, which offers Chemical Vapor Deposition CVD to make it happen. This is an ambient, atmospheric process producing films of high uniformity and flexibility. At this week's conference Printed Electronics USA in San Francisco, Dr Janos Veres made these points and referred to the "huge attention" now being paid to inorganic compound semiconductors. These are typically based on zinc oxide, with indium gallium zinc oxide being particularly promising. Most of them are transparent, enabling new display concepts, for example:
The Kodak process has created transistors with mobility of 14-16 cm2/vs and on off ratios in excess of 100 million, these parameters being five to ten times better than the best organic equivalents and leading to lower power, higher frequency devices that can be fully transparent, something not yet available with organic transistors. Indeed, alumina dielectrics, aluminum electrodes without the need for contact doping and other layers have been successfully deposited and annealed at only 120 - 200 centigrade. The uniform microstructure of these coatings was investigated in collaboration with Penn State University. Deposition rate is typically 100 - 200 angstroms/ minute and large area patterning is possible by laser imaging at 3-5 micron accuracy. Indeed, he assessed that inorganic compound semiconductor transistors met the requirements of flexibility, deposition on plastic, large area, transparency, uniformity, and current driving capability that are essential to the next generation of OLEDs, electroluminescent and LCD displays.
Work on inorganic compound transistors in Portugal, the UK, the US and Japan has already been reported in Printed Electronics World - there being several relevant organisations in each of these countries. Beyond this, Dr Veres pointed to the superb mobility of 95 cm2/vs and on off ratio of 10 million achieved by LG Electronics in Korea this year, ETRI Korea producing a working OLED with ZnO drive transistors and Toppan Printing producing working electrophoretic displays driven by IGZO. Most of these employed low temperature RF sputtering.
In another lecture, Dr. Vishal Shrotriya of Solarmer Energy showed how printed and thin film organic photovoltaics needs to capture light to create energy yet Solarmer's versions, developed in collaboration with the University of California Los Angeles, obtain 65% translucence with a near-commercial 4% efficiency. However, 20% degradation occurred in only 1000 hours in the tests. Commercialisation is targeted for 2010.
Many of the new versions of non-silicon photovoltaics promise price advantages as shown below:
In addition, for its flexible organics in 2010, Solarmer targets
- Efficiency - 8%
- Lifetime - 5+ years
- Cost - 60-80 cents per Watt
This is to be achieved using solution-based processing to make fabrication of solar cells cheaper. A morphology control process of the polymer film enhances properties. This is easy to manufacture on large area substrates and production cost is claimed to be significantly lower than silicon solar cells.