US scientists have created a bright efficient area light source to compete with fluorescent tubes and OLEDs, it is claimed.
Developed at Wake Forest University of North Carolina, the field-induced polymer electroluminescent (FIPEL) is a planar light emitting structure energised by an AC field from insulated electrodes.
There is no junction. Instead the key component is a layer of polymer loaded with an iridium compound and doped multi-wall carbon nanotubes.
"The AC stimulates displacement current in the polymer, in which we have placed charge sources and field sinks from nano-materials," project head Professor David Carroll told Electronics Weekly. "The big concern for AC devices is loss in dielectric. Nano particles make it tremendously non-lossy."
The polymer mix has three different emission centres coupled with triplet scavengers, said Carroll: "You get singlets and triplets and the triplets are non-radiative. Normally OLEDs have scavengers too. In our's, the effectiveness of triplet to singlet conversion is far higher."
Its structure is similar to well-established AC electroluminescent technology.
"In AC electroluminescent, light output is around 0.1cd/A. OLED delivers 50cd/A, and ours is 62cd/A," claimed Carroll.
This 62cd/A figure is at the very cold colour temperature of 6,800-7,200K, where the technology is likely to be emitting at its most efficient.
Exactly how efficient the material is not being revealed as the figures are pending publication in the journal Organic Electronics, said Carroll.
It is available on-line from the journal Organic Electronics.
He would say that operation is at "similar voltage to OLED", and that efficiency is "certainly comparable with compact fluorescents".
The university has described the technology as "at least twice as efficient as compact fluorescent bulbs and on par with LEDs".
Currently, high-end commercial LEDs are producing around 120lm/W, straight fluorescent tubes can approach 110 lm/W, and good compact fluorescents may approach 80 lm/W. Commercial OLEDs are extremely rare and lab figures would be un-representatively high.
The actual emissive layer is 200nm if a polymer called PVK, loaded with 10% of the light-emitting iridium compound Ir(ppy)3, and a fraction of a percent of nanotubes.
This is sandwiched between two 1.2µm layers of the ferroelectric polymer P(VDF–TrFE).
The nanotubes both inject electrons and generate charges, said Carroll, and the same structure without nanotubes produces only one fifth of the light.
The spectrum can be tuned to different colour temperatures.
"It is a continuous spectrum. We can almost make a perfect photopic curve," said Carroll.
Matching the eye's response - the photopic curve - would mean poor rendering of red and purple. Can he match the sun's spectrum instead?
"Yes, we can make the sun's spectrum," said Carroll.
The big problem for OLEDs is durability - water and oxygen from the atmosphere corrode the necessarily reactive electrodes and the organic materials involved.
There are no reactive metal electrodes in the FIPEL structure
"If a triplet hangs around, it will oxidise the material," said Carroll. "We have the shortest life of any triplets - so longer material life and much longer lifetime."
But how about moisture and oxygen?
"For our device to reach 20,000 hours, they still have to be encapsulated, but they are not so sensitive as OLEDs," said Carroll. "If you use expensive encapsulation, you will get 40,000-50,000 hours."
Wake Forest is working with a company to manufacture the technology and plans to have it ready for consumers in the next year.
Carroll sees potential uses in large display lighting in shops, busses and underground trains.
Source:
http://www.electronicsweekly.com/Articles/2012/12/04/55131/fipel-more-on-the-oled-alternative-light-source.htm
