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Re: [APD] Article

Jerry Baker wrote:
> Stuart Halliday wrote:
>> I like the claims of near 100% efficiently, low heat and natural light 
>> bandwidth.
> I wonder if they are capable of producing the intensities required for 
> planted tanks, and a pleasing spectral distribution.

This article seems to be saying that OLEDs only produce 25 lumens per 
watt. That is around 1/4th the efficiency of a good metal halide.



Quantum quest leads to super-efficient lights

     * 18:47 12 April 2006
     * NewScientist.com news service
     * Tom Simonite

A light that lasts 20 times longer than a conventional bulb and is 75% 
more energy efficient has been developed by researchers.

The light is based on organic light emitting diodes (OLEDs) that use a 
novel combination of photon-emitting compounds to convert more energy 
into light than existing versions. It was created by Stephen Forrest 
from Princeton University in New Jersey, and colleagues at the 
University of Southern California, both in the US.

OLEDs can easily be printed onto different surfaces and could therefore 
be used to illuminate buildings and homes in completely new and more 
energy efficient ways, experts say.

John de Mello, who works on OLEDs at Imperial College London, UK, and 
was not involved with the work, is impressed by the new work. "The 
beauty of this approach is that it harvests the energy in a near-optimal 
manner," he says.

De Mello adds that the OLEDs could make new methods of lighting 
possible. "The industrial interest in these materials stems from their 
ease of processing," he says. "They can be deposited over large areas at 
relatively low cost."
Red, green and blue

OLEDs produce light using a layer of semi-conducting material made from 
an organic compound. Passing a current through this layer raises the 
energy level of electrons inside it and as these electrons return to 
their previous energy they release photons.

The components can be made to produce different colours of light by 
adding "dopant" materials to change the wavelengths of emitted photons. 
Dopants that produce the right mix of red, green and blue light can be 
combined to fine-tune a material to a particular colour.

Phosphorescent molecules are normally used as the dopant material in 
OLEDs, as they release photons a steady rate and can be easily 
fine-tuned. But the molecules only convert a limited number of photons 
and some phosphorescent materials also tend to wear out rapidly.

Forrest and colleagues developed OLEDs that avoid both these problems, 
harnessing all the photons up for grabs, and that use only 
phosphorescent compounds that are long-lasting.
Singlets and triplets

They determined that OLEDs produce photons in one of two quantum spin 
states - referred to "triplets" and "singlets" - and that the 
phosphorescent material used in most OLEDs can only transmit triplets, 
which comprise 75% of all photons.

So Forrest's team doped their OLEDs with fluorescent molecules as well 
as phosphorescent ones. The fluorescent molecules convert photons 
through a slightly different process, and can transmit singlets 
efficiently. As the fluorescent material produces photons at a blue 
wavelength, the researchers re-tuned the rest of the dopant material to 
produce the necessary wavelengths for white light. "Using this 
combination of fluorescence and phosphorescence means there's no waste," 
Forrest says.

The OLEDs developed by Forrest's team have a longer lifespan than 
existing ones, because they use only longer lasting phosphorescent 
compounds "This first model has a lifetime of around 20,000 hours," 
Forrest says.

By contrast, regular light bulbs - which supply about 40% of lighting in 
the US - normally only last for around 1000 hours, while compact 
fluorescent lights can last for around 10,000 hours. Lighting based on 
the new OLED should also be more efficient than conventional bulbs - 
they emit about 25 lumens per watt of energy, compared to about 15 for a 
normal bulb.

Jerry Baker
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