Electroluminescence

December 14, 2012

Robert Duvall, who is best known to science fiction fans for his role in THX 1138 (1971, George Lucas, Director), had a somewhat earlier science fiction role. In The Invaders, episode 20, season 1, of Voyage to the Bottom of the Sea (1965, Sobey Martin, Director; Irwin Allen, Creator), he played Zar, a survivor from the Earth of millions of years ago.

Earthmen of that time were quite superior to us, since Zar is able to learn English in just a few hours. Before Zar shows his sinister side, he's taken on a tour of the Seaview, the futuristic research submarine that discovered him in suspended animation. Noticing the ship's incandescent lighting, Zar comments how the present-day Earthmen had not yet learned how to create light without heat.

Even in those early days, before widespread manufacture of light-emitting diodes, there was a decade's old solid state technology available for production of light without a heated filament. This was electroluminescence, which was discovered in silicon carbide in 1907, and in copper-doped zinc sulfide (ZnS:Cu) in 1923.[1] By the 1960s, to paraphrase Henry Ford, you could have any inefficient solid state light you wanted, as long as it was red (LED), or green (electroluminescent). We needed to wait until the 1990s for the first white LEDs.

Despite its solid state advantage, electroluminescence wasn't used that much as a light source because it was inefficient, as the table shows. Electroluminescent devices also require a high voltage supply to operate. Nowadays, local generation of high voltages is not a problem with our miniature switching power supplies, but it was a problem in the sixties. Nevertheless, electroluminescent panels were a popular backlighting option, and they functioned well for some signage. Alas, they were not intense enough for room lighting, even if white light had been an option.

Technology	Lumens/Watt (Commercial max.)	Efficiency (approx. %)
Combustion (candle)	0.3	0.04
Electroluminescent	5	0.75
Incandescent	20	3.0
Fluorescent	100	15
LED	150	22

As in all electrically-driven light sources, electroluminescence arises from the recombination of electrons and holes, producing photons. These electron-hole pairs are produced by the intense electric field placed across the material. Unlike diode light sources, the voltage supplied to electroluminescent devices is of the alternating current type, typically about a thousand volts per millimeter.

This is quite a high voltage, since the dielectric strength of most inorganic materials is of the order of 10,000 volts per millimeter. Mica is an exception, since its strength is more than a 100,000 volts per millimeter, and that's why it's useful in many radio frequency circuits.

It's possible to make a white electroluminescent light through the use of red, green and blue materials. Green can be produced from copper-doped zinc sulfide, as mentioned above; zinc sulfide doped with manganese (ZnS:Mn) can produce an orange-red color in thin film form; and zinc sulfide doped with silver (ZnS:Ag) will produce blue. All of these are low efficiency materials.

A team of physicists from the Center for Nanotechnology and Molecular Materials, Wake Forest University (Winston-Salem, NC) and Trinity College (Dublin, Ireland) have developed an electroluminescent light with more than twice the efficiency of compact fluorescent lamps, and about the same efficiency as LEDs.[2-4] These devices, which were under development for about two years, are so easy in their manufacture that there are plans for a first production run in 2013.[2,4]

Wake Forest University physics professor David Carroll (right), and Greg Smith, working on their new field-induced polymer electroluminescence lighting technology. (Wake Forest University photograph by Ken Bennett, used with permission).

The device, based on a field-induced polymer electroluminescence, is formed from nanoparticles in a polymer matrix.[2,4] The emissive layer has multi-walled carbon nanotubes (MWCNTs) dispersed in poly-(N-vinylcarbazole) doped with fac-tris(2-phenylpyridine)iridium(III). The MWCNTs are present at 0.04 weight-%. The polymer without the nanotubes is also electro-emissive, but at a fifth the emissivity.[3] The device structure is shown in the figure.

Wake Forest University electroluminescent device. ITO is the transparent indium-tin-oxide electrode, and PVF-trFE is polyvinylidene fluoride-trifluoroethylene. The emissive layer is described in the text. (Image by the author, rendered with Inkscape.)

This device is easily made into a large are emitter, and similar devices have a achieved a lifetime of at least a decade.[2,4] David Carroll, leader of the research group, thinks that this device will supersede organic light-emitting diodes for room lighting application.[4]

The only practical problem I see is the use of indium, a rare and expensive metal, for the transparent electrode. When we're considering many square feet for a lighting unit, this would be a problem. As I discussed in a previous article (Transparent and Conductive, June 10, 2011), research is ongoing for alternative transparent conductors.

References:

1. Jeffrey A. Hart, Stefanie Ann Lenway and Thomas Murtha, "A History of Electroluminescent Displays," Indiana University Web Site, September 1999.
2. Katie Neal, "Taking the buzz out of office lights," Wake Forest University Press Release, December 3, 2012.
3. Yonghua Chen, Gregory M. Smith, Eamon Loughman, Yuan Li, Wanyi Nie and David L. Carroll, "Effect of multi-walled carbon nanotubes on electron injection and charge generation in AC field-induced polymer electroluminescence," Organic Electronics, vol. 14, no. 1 (January, 2013), pp. 8-18.
4. Matt McGrath, "Plastic bulb development promises better quality light," BBC News, December 3, 2012.