starship-design: PV and laser

[TLG.van.der.Linden@tip.nl (Timothy van der Linden)]

Hi all, this may develop just in time to use around 2040. It may make
beaming much cheaper and easier.

It can also be found on the Web:
http://www.sciam.com/1296issue/1296techbus4.html
(From there you can also find another link)


Scientific American: Technology and Business

CHEMICAL ENGINEERING

PLASTIC POWER

Polymers take a step forward as photovoltaic cells and lasers

For nearly 20 years, scientists have expected great things from
semiconducting polymers--chimerical chemicals that can be as pliable
as plastic wrap and as conductive as copper wiring. Indeed, these
organic compounds have conjured dreams of novel optoelectronic
devices, ranging from transparent transistors to flexible
light-emitting diodes. Few of these ideas have made it out of the
laboratory. But in the past year, researchers have added two promising
candidates to the wish list: solar cells and solid-state lasers.

The lasting appeal of these materials--also called synthetic
metals--is that they are more durable and less expensive than their
inorganic doubles. Furthermore, they are easy to make. Like all
plastics, they are long, carbon-based chains strung from simple
repeating units called monomers. To make them conductive, they need
only be doped with atoms that donate negative or positive charges to
each unit. These charges clear a path through the chain for traveling
currents.

Scientists at Advanced Research Development in Athol, Mass., have
made plastic solar cells using two different polymers, polyvinyl
alcohol (PVA) and polyacetylene (PA). Films of this copolymer,
patented as Lumeloid, polarize light and, in theory at least, change
nearly three quarters of it into electricity--a remarkable gain over
the 20 percent maximum conversion rate predicted for present-day
photovoltaic cells. Lumeloid also promises to be cheaper and safer.
Alvin M. Marks, inventor and company president, estimates that whereas
solar cells now cost some $3 to $4 per watt of electricity produced,
Lumeloid will not exceed 50 cents.

The process by which these films work resembles photosynthesis,
Marks explains. Plants rely on diode structures in their leaves,
called diads, that act as positive and negative terminals and channel
electrons energized by sunlight. Similarly, Lumeloid contains
molecular diads. Electrodes extract current from the film's surface.
To go the next step, Marks is developing a complementary polymer
capable of storing electricity. "If photovoltaics are going to be
competitive, they must work day and night," he adds. His two-film
package, to be sold in a roll like tinfoil, would allow just that.

Plastics that swap electricity for laser light are less well
developed, but progress is coming fast. Only four years ago Daniel
Moses of the University of California at Santa Barbara announced that
semiconducting polymers in a dilute solution could produce laser
light, characterized by a coherent beam of photons emitted at a single
wavelength. This past July, at a conference in Snowbird, Utah, three
research teams presented results showing that newer polymer solids
could do the same. "I'm a physicist. I can't do anything with my
hands," says Z. Valy Vardeny of the University of Utah, who chaired
the meeting. "But the chemists who have created these new materials
are geniuses."

Earlier generations of semiconducting polymers could not lase for
two main reasons. First, when bombarded with electricity or photons,
they would convert most of that energy into heat instead of light--a
problem called poor luminescence efficiency. Second, the films usually
absorbed the photons that were produced, rather than emitting them, so
that the polymers lacked optical gain--a measure of a laser medium's
ability to snowball photons into an intense pulse.

Because the newer materials have fewer impurities, they offer much
higher luminescence efficiencies and show greater lasing potential,
Vardeny states. In the Japanese Journal of Applied Physics, his
group described a derivative of poly (p-phenylenevinylene), or
PPV, with a luminescence efficiency of 25 percent. The red light was
composed of photons having the same wavelength, but it did not travel
in a single beam. In Nature, another group from the Snowbird
meeting offered a way around this shortcoming. Richard H. Friend and
his colleagues at the University of Cambridge placed a PPV film inside
a device called a microcavity. Mirrors in the structure bounced the
emitted light back and forth, amplifying it into a focused laser beam.

The third group from Snowbird, led by Alan J. Heeger of U.C.S.B.,
tested more than a dozen polymers and blends as well. Their results,
which appeared in the September 27 issue of Science, show that
these materials can emit laserlike light across the full visible
spectrum--even in such rare laser hues as blue and green. In place of
a microcavity, Heeger set up his samples so that the surrounding air
confined the emitted photons to the polymer, where they could
stimulate further emissions. "We wanted to show that a whole class of
materials do this and that they definitely provide optical gain,"
Heeger says.

The challenge now will be finding a way to power these polymers
electrically. All three groups energized their samples using another
laser, but practical devices will need to run off current delivered
from electrodes. It is no small problem. Vardeny notes that electrical
charges generate destructive levels of heat and that electrodes can
react chemically with the film, lowering the polymer's luminescence
efficiency. "It's going to be hard," Heeger concurs, "but I'm
optimistic."

--Kristin Leutwyler