Prototypes of the solar battery weigh as
little as two grams and are less than
one millimeter thick. "The device is
meant to ensure that the battery is
always charged with optimum voltage,
independently of the light intensity
seen by the solar cell," according to
the paper. Dennler says that a single
cell delivers about 0.6 volts. By
shaping a module with strips connected
in series, "one can add on voltages to
fit the requirements of the device."
The organic solar cell used in the
prototype is the same technology being
developed by Konarka. (See "Solar-Cell
Rollout.") It's based on a mix of
electrically conducting polymers and
fullerenes. The cells can be cut or
produced in special shapes and can be
printed on a roll-to-roll machine at low
temperature, offering the potential of
low-cost, high-volume production.
To preserve the life of the cells,
which are vulnerable to photodegradation
after only a few hours of air exposure,
the researchers encapsulated them inside
a flexible gas barrier. This extended
their life for about 3,000 hours.
Project coordinator Denis Fichou, head
of the Laboratory of Organic
Nanostructures and Semiconductors, near
Paris, says that the second important
achievement of the European project was
the incorporation into the device of an
extremely thin and highly flexible
lithium-polymer battery developed by
German company
VARTA-Microbattery, a partner in the
research consortium. VARTA's batteries
can be as thin as 0.1 millimeter and
recharged more than 1,000 times, and
they have a relatively high energy
density. Already on the market, the
battery is being used in Apple's new
iPod nano.
Dennler says that the maturity of the
battery and the imminent commercial
release of Konarka-style organic solar
cells mean that the kind of
solar-battery device designed in the
project could be available as early as
next year, although achieving higher
performance would be an ongoing pursuit.
The paper's coauthor Toby Meyer,
cofounder of Swiss-based
Solaronix, says that the prototypes
worked well enough under low-light
conditions, such as indoor window light,
to be considered as a power source for
some mobile phones. Artificial light, on
the other hand, may impose limitations.
"Office light is probably too weak to
generate enough power for the given
solar-cell surface available on the
phone," he says.
Watches, toys, RFID tags, smart
cards, remote controls, and a variety of
sensors are among the more likely
applications, although the opportunity
in the area of digital cameras, PDAs,
and mobile phones will likely continue
to drive research. "The feasibility of a
polymer solar battery has been proven,"
the paper concludes.
Rights to the technology are held by
Konarka, though the solar company says
it has no plans itself to commercial the
battery.
