Almost everyone uses a smartphone today and the displays are getting ever bigger. Larger screens are a pleasure to watch, but difficult to put inside a pocket. Therefore, Qibing Pei, a professor of materials science, is researching highly flexible and stretchable OLED displays that could allow a small elastic OLED smartphone to fit easily into one’s pocket and the screen could be expanded when viewing. That would certainly be a great help if successful, but in the meantime, there is something else, which is better than an OLED.
OLEDs require power and are expensive. Instead, carbon nanotube field emitters powering up a lighting panel are less expensive. They stimulate a phosphor in the panel to glow, much as the cathode ray tubes of the yesteryears did. The phosphor is brighter than the current OLEDs, consumes much less power compared to LEDs and is far less expensive than both of them are. Professor Norihiro Shimoi, a lead researcher at the Tohoku University in Japan is working on this technology. He uses light through a neutral density filter to illuminate nanotube field emitters to stimulate the phosphor.
Although the prototype in Professor Shimoi’s lab has yet to achieve 60-lumens per watt, it is similar in design to the flat version of the old cathode ray tube. Not expected for a commercial release before 2019, the nanotube prototype is like a lighting lamp, but with a power consumption of 1/100th of standard LED devices.
LEDs are all the rage today, owing their advantages over fluorescent and incandescent lighting because of the very low power consumption of LED based devices. With large-scale lighting, however, several LEDs have to be used together, which complicates the engineering and thermal design. On the other hand, the nanotube design is flexible enough to be formed into flat panels of any size.
Incandescent bulbs are the least efficient at a mere 15lumens per watt. In comparison, LEDs and fluorescent bulbs both produce about 100 lumens per watt. The difference is LEDs are point sources of light, whereas fluorescent bulbs spread their light over a much larger area. Organic cousins of LEDs, the OLEDs, produce about 40 lumens per watt but have the advantage of being incorporated into panels. According to Shimoi, simulating large phosphor-covered panels with electron field emitters made of carbon nanotubes will be more efficient. With their much lower power requirements, and producing 60 lumens per watt, these phosphors will potentially be brighter than OLEDs that produce only 40 lumens per watt.
Shimoi is currently working on reducing the energy loss by heat. The device employs highly crystallized carbon nanotubes and phosphors. These are coated with ITO particles. Shimoi is attempting to increase the electrical conductivity to reduce energy loss by heat. The process involves optimization of the crystallization of the carbon nanotubes along with the design of the lighting device.
Where typically, carbon nanotubes are made using semiconductor diode junctions, Shimoi has made them into excellent field emitters of electrons so that they can stimulate phosphors. Furthermore, production of these nanotubes does not require expensive clean rooms or high-temperature ovens. The nanotubes are single-walled and are grown by arcing.