December 6, 2012, Printed Electronics USA, Santa Clara, CA—Erkki Soininen Canatu from Canatu described the work in carbon nanotubes and fullerenes for flexible and formable touch screens. The materials are positioned as replacements for ITO-based touch sensors.
Carbon nanotubes with a fullerene attached are conductive, transparent, non-reflective, and flexible as conductors. This work is addressing the issue of display fade-out in bright areas and in sunlight. The problem is worse on displays with touch screens, because the ITO is a key contributor to the fade-out phenomenon. Ambient light in a dark room is about 0 lux, in a bright room this increases to about 1000 lux, and most displays start to fade in this level of light. Daylight is 40,000 lux and washes out almost every display.
Comparing and contrasting the main conductor materials for touch screens shows that ITO has a contrast ratio of 4:1, and a reflectivity of 1:7. Carbon nanotubes have a contrast ratio of 8:1 and lower reflectivity than ITO. This increase in contrast allows the backlight intensity to be reduced, reducing total power consumption by up to 30 percent.
Other reflections in the film stack for LCDs further reduce the readability of the LCDs. The physics have to adhere to the Fresnel equations relating to indices of refraction and reflections. It turns out that the carbon nanotubes can be placed in a medium to achieve the same refractive index as the adjacent layer, resulting in about 1 percent reflections. In comparison, ITO reflects about 12 percent of the light, and can be coated with an anti-reflective coating to get down to 5 percent. The elimination of the reflections means simpler layers and fewer films are needed to get good displays.
The basic flow for a projected capacitive touch detector start with a low reflectance display. The display color filters are added over an air gap and anti-reflective coatings or direct contact. The touch sensor is added on top of the stack, and anti reflective coatings are placed on the front surface.
The move to flexible or formed surfaces creates new problems, as ITO is a very brittle material. The industry is moving from flat glass to curved or rigid 3-d shapes, and to semi-flexible displays. In the longer term, the industry would like to see rollable and foldable displays to enable new form factors in portable devices.
To date, the foldable substrates with carbon nanobulbs have been robust and stable. Over 1000 folds of 0-180 degrees and back have not resulted in any conductor breaks nor in any significant change in resistance, both indicators of fracturing. The nano-materials are amenable to direct, dry printing and can be set up for roll-to-roll processing. The generation of the nano-materials is an aerosol process that forms the nano-bulbs on any films.
