Painting on Solar Cells - the nano cell solution [UC Berkeley]

source: Eric Scigliano MIT technology review 2003.2

The sun may be the only energy source big enough to wean us off fossil fuels. But harnessing its energy
depends on silicon wafers that must be produced by the same exacting process used to make computer
chips. The expense of the silicon wafers raises solar-power costs to as much as 10 times the price of
fossil fuel generation—keeping it an energy source best suited for satellites and other niche applications.

Paul Alivisatos, a chemist at the University of California, Berkeley, has a better idea: he aims to use
nanotechnology to produce a photovoltaic material that can be spread like plastic wrap or paint. Not only
could the nano solar cell be integrated with other building materials, it also offers the promise of cheap
production costs that could finally make solar power a widely used electricity alternative.

Alivisatos’s approach begins with electrically conductive polymers. Other researchers have attempted to
concoct solar cells from these plastic materials (see “Solar on the Cheap,” TR January/ February 2002), but
even the best of these devices aren’t nearly efficient enough at converting solar energy into electricity. To
improve the efficiency, Alivisatos and his coworkers are adding a new ingredient to the polymer:
nanorods, bar-shaped semiconducting inorganic crystals measuring just seven nanometers by 60
nanometers. The result is a cheap and flexible material that could provide the same kind of efficiency
achieved with silicon solar cells. Indeed, Alivisatos hopes that within three years, Nanosys—a Palo Alto,
CA, startup he cofounded—will roll out a nanorod solar cell that can produce energy with the efficiency of
silicon-based systems.

The prototype solar cells he has made so far consist of sheets of a nanorod-polymer composite just 200 nanometers thick. Thin layers
of an electrode sandwich the composite sheets. When sunlight hits the sheets, they absorb photons, exciting electrons in the polymer
and the nanorods, which make up 90 percent of the composite. The result is a useful current that is carried away by the electrodes.

Early results have been encouraging. But several tricks now in the works could further boost performance. First, Alivisatos and his
collaborators have switched to a new nanorod material, cadmium telluride, which absorbs more sunlight than cadmium selenide, the
material they used initially. The scientists are also aligning the nanorods in branching assemblages that conduct electrons more
efficiently than do randomly mixed nanorods. “It’s all a matter of processing,” Alivisatos explains, adding that he sees “no inherent
reason” why the nano solar cells couldn’t eventually match the performance of top-end, expensive silicon solar cells.

The nanorod solar cells could be rolled out, ink-jet printed, or even painted onto surfaces, so “a billboard on a bus could be a solar
collector,” says Nanosys’s director of business development, Stephen Empedocles. He predicts that cheaper materials could create a
$10 billion annual market for solar cells, dwarfing the growing market for conventional silicon cells.

Alivisatos’s nanorods aren’t the only technology entrants chasing cheaper solar power. But whether or not his approach eventually
revolutionizes solar power, he is bringing novel nanotechnology strategies to bear on the problem. And that alone could be a major
contribution to the search for a better solar cell. “There will be other research groups with clever ideas and processes—maybe
something we haven’t even thought of yet,” says Alivisatos. “New ideas and new materials have opened up a period of change. It’s a
good idea to try many approaches and see what emerges.”

Thanks to nanotechnology, those new ideas and new materials could transform the solar cell market from a boutique source to the
Wal-Mart of electricity production.

Others in
NANO SOLAR CELLS, RESEARCHER, PROJECT
Richard Friend , U. Cambridge, Fullerene-polymer composite solar cells
Michael Grätzel, Swiss Federal Institute ofTechnology, Nanocrystalline dye-sensitized solar cells
Alan Heeger, U. California,Santa Barbara, Fullerene-polymer composite solar cells
N. Serdar Sariciftci, Johannes Kepler U, Polymer and fullerene-polymer composite solarcells