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21 Mar 2011 | United States
New transparent conductive films from Cambrios
IDTechEx recently attended the UV.EB West conference in San Jose, which explored opportunities for this industry in printed electronics.
One of the presenters was Cambrios, based in Sunnyvale, CA, that has developed a wet processable alternative to transparent conductive oxide materials, mainly to tackle the varying costs and supply of Indium Tin Oxide (ITO).
ITO is used as a transparent conductor in the display and photovoltaic industry, but over the past decade the cost of indium has swung between lows of $100/kg to $1000/kg. It is around $400/kg now. Much of it is mined in China, where supply is restricted ensuring a higher price.
However, beyond price there are other problems with this material. It is brittle, cracking under a few percent of repeated strain, making it a challenge to create reliable flexible displays and photovoltaics. It is also not very conductive, so thick layers are used which adds to the cost. It is made usually by sputtering which is expensive.
Teresa Ramos of Cambrios spoke about their "ClearOhm" product. The company has 40 people, and produces nano materials for the display, touchpanel and photovoltaics industry. Cambrios use silver, which can be printed and in its bulk is 100 times more conductive than ITO. It is deposited by wet coating onto a surface at high speed, with 1% of the final surface being silver.
The company takes silver salt and grows crystals carefully controlling the morphology - creating long strands of silver. These are grown into large areas then formulated into inks which can be printed rapidly onto large areas.
The control can be adjusted to create high or low haze transparent materials, suitable for different applications (PV or LCD displays respectively). The product can be "tuned" for different conductivity versus transmission aspects (silver is not transparent so the more conductivity achieved the less transparent the film is). For examples, at 50ohms/sq there is 96.6% transmission or for higher haze at 15 ohms/sq there is 93.% transmission. The former has lower density - more gaps between the silver nanowires.
The transparent film is made in a two stage process - first the metallic layer is deposited, dried (at less than 100 degrees Celsius) and then to make it mechanically strong an overcoat is printed which is UV cured. The UV overcoat needs to have high transmission, be neutral in color, non-yellowing with time. Mechanically, it must be highly durable, and have good adhesion to PET and clear hard coatings on PET. The layers are coated using a roll to roll slot die or gravure, on a web moving at 50 feet per minute. The inks are made in Sunnyvale and the material is made in Tokyo. The company has also inkjet printed the materials and has developed photo-patternable transfer films.
Compared to ITO, at a given wavelength the Cambrios material has better transmission. On flex tests, for ITO sheet resistance increases quickly due to cracking but Cambrios saw no change in conductivity of their materials with 50 wraps of the material around a spindle.
The company report that sheet resistance variation is 4.21%, transmission variation is 0.11%. Cambrios is focussing on touch screen devices first, then LCDs, OLEDs and PV. Their first product is a touchscreen for a projective capacitance phone, qualified now by two customers. For AM LCD, target sheet resistance is 20 ohms/sq. Cambrios anticipate the cost to be half that of typical ITO coatings.
For more attend: Printed Electronics & Photovoltaics Europe 2011 .
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