Plastic lights could be ready for prime time

Researchers at Wake Forest University say they have come up with a way to make light-emitting plastic that has light qualities that are comparable to those of ordinary fluorescent bulbs. They also say their development could be commercialized by the end of this year.

David Carroll, professor of physics and director of Wake Forest's Center for Nanotechnology and Molecular Materials says the key to making practical light-emitting plastic was to introduce multi-walled carbon nanotubes (MWNTs)in the emissive layer of an alternating-current field-induced polymer electroluminescence (FIPEL)device. FIPELs are basically capacitors. The nanotubes aid in generating charges within the active layer of these devices and, in so doing, makes them more efficient light generators. an active layer and dielectric.

Specifically, Carroll and his group inserted a layer of nanotube-infused plastic between a metal electrode and a transparent conducting electrode. In such devices light gets generated through the ac-field-induced creation or injection of excitons and the subsequent recombination of these holes and electrons. The nanotubes enhance charge injection and are dispersed within the material.

However, use of carbon nanotubes this way isn't new. Carroll's group took a different approach to electron injection and charge generation. They put solubilized, cut MWNTs dispersed within the active layer and tried it using two different device structures. One used a transparent bottom ITO electrode/dielectric/emissive layer/dielectric/top electrode (symmetric device). The second was a transparent bottom ITO electrode/emissive layer/dielectric/top electrode (asymmetric device).

"We suggest that electron injection is increased by multiwalled carbon nanotubes (MWNTs) within the emissive layer of the asymmetric device under ac driven fields, with electrons and holes well separated in their conduction paths from the ITO electrode," the group says. They fabbed their FIPEL devices on a glass substrate with a pre-coated ITO film that was 100 nm thick, with a sheet resistance of approximately 10 Ω/□. They say they got their highest light output by driving the devices at a frequency of 80 kHz where they saw a maximum luminance of approximately 100 cd/m² in the FIPEL device with 0.04 wt% MWNTs in the emissive layer. This value is five times higher than that of a pure plastic emissive layer-based device under the same conditions.

All in all, they say, "The charge generation and exceptional injection efficiency of MWNTs represents a novelty for organic electronics suffering from high injection barriers and high contact resistance in general."

Carroll further says the plastic lights are at least twice as efficient as compact fluorescent (CFL) bulbs and on par with LEDs.

Wake Forest news release on Carroll's work: http://news.wfu.edu/2012/12/03/taking-the-buzz-out-of-office-lights/

Here is the paper on the subject which ran in Organic Electronics: http://www.sciencedirect.com/science/article/pii/S1566119912004831