IEEE Spectrum January, 2014 - 20

texas, 2050
Texas is peculiar in that it has its own electricity grid.
It's not so peculiar in that it recently suffered through a
devastating drought. That combination attracted the
attention of engineers at MIT. They were looking to
model the mix of electricity generation you wind up
with in 2050 when you consider different constraints.

may have to do with different energy characteristics in different areas of the molecule-
prevents the extra energy from being released
as heat before the spin can flip. "Because of
their shape, you can localize the electron density and therefore the spin density in specific
areas in the molecules," Lupton says, adding
that the result was surprising to the researchers, who were actually studying other facets
of OLEDs. "If you look at textbooks of physical chemistry, this should not have actually
worked," he says.
Last year another group, led by Chihaya
Adachi at Kyushu University, in Japan, demonstrated its own method for making OLEDs
without the use of metal. The researchers
designed a mole cule that relies on fluorescence for light emission, instead of the
phosphorescence used in most metal-reliant
OLEDs. Their mechanism, called thermally

It's a bit of a no-brainer that you'd get a
different mix depending on whether you
want to cut carbon dioxide or not. But
the more interesting finding is how dissimilar this low-emissions mix is from
what you get if you also want to limit
water use. The difference in the role
of nuclear energy-notoriously thirsty
for cooling water-is particularly stark.
-SaMuel k. Moore

200

*
*
**

180
160

Generation capacity (GW)

140
120
100

*
**
*

80
60
40
20
0
No limits

75% CO 2 reduction 75% CO 2 reduction
50% H 2 0 reduction

Nuclear
(water cooled)
Nuclear
(hybrid water/
air cooled)
Coal
Natural gas
combined cycle
(water cooled)
Natural gas
combustion turbine
Wind
Natural gas
combined cycle
(hybrid water/
air cooled)
Natural gas
combined cycle
(air cooled)

activated delayed fluorescence, takes the
heat released by some of the charge carriers and uses it to bump up the energy level
of others to the point that they emit light instead of heat when they relax. The Kyushu
scientists say their material has an internal efficiency of nearly 100 percent, which
translates into about 14 percent of the electricity pumped into the OLED reemerging
as light, compared with about 20 percent in
commercial OLEDs that use metal.
Alán Aspuru-Guzik, a chemist at Harvard,
sees the work by Adachi and Lupton as developments "that could lead to a breakthrough in the field of OLEDs." The work
both groups are doing could lead to better
emission in the blue end of the spectrum in
particular. Blue organic light emitters have
been the most difficult to develop.
At the moment, the price of rare metals
needed in OLED manufacturing is so low that
manufacturers are unlikely to need either
trick soon. And this new method doesn't
actually improve OLED efficiency over what
existing devices can deliver, says Lupton. But
it could prove appealing to OLED manufacturers if the price of iridium goes up.
Far more interesting may be another aspect
of Lupton's discovery: The spins of the charge
carriers in this type of OLED control the wavelength of the light emitted. "You can actually
measure the spin of the electron within your
OLED by using the color," Lupton says. And
because spin on the quantum level translates
into magnetism in the macro world, "if you design this in the right way you could make it incredibly sensitive to magnetic fields," Lupton
says. "That will actually allow us to use the
color of the OLED as a compass." He says such
a device could be more sensitive than Hall
effect devices, which are common in smartphone navigation systems and in automobiles,
where they measure the rotation of the wheels.
The research might even provide a new clue
to how birds navigate using Earth's magnetic
field. While the exact mechanism is still debated, some scientists suspect that their eyes
let them see shifts in the field as subtle variations of color. Lupton says they may be sensitive to color changes caused by alterations
in the spins of electrons. -neil savage

nEwS

jan 2014

nORTh aMERICan

SPECTRUM.IEEE.ORG

source: "Water-co 2 trade-oFFs in eLectricity generation pLanning," By mort WeBster et aL. naTurE clImaTE changE, puBLished onLine 27 octoBer 2013

state it'll emit the extra energy as a photon
rather than as heat, Lupton explains. "Long
enough" is measured in milliseconds, compared with the nanoseconds it usually takes
for emission to occur, a difference of six orders of magnitude.
The trick, which the researchers from
Regensburg, the University of Bonn, the University of Utah, and MIT explain in a paper
published online by Angewandte Chemie, lies
in the shape of the organic molecules used in
the LED. The researchers developed two kinds
of polycyclic aromatic hydrocarbons (carbonbased molecules having multiple ring-shaped
components): one called phenazine and the
other triphenylene. The atomic structures of
the molecules were such that they trapped
the charge carriers long enough for the spontaneous change to occur. A peculiarity of the
molecule-the researchers speculate that it

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Table of Contents for the Digital Edition of IEEE Spectrum January, 2014