Sunlight is Earth’s most abundant energy source and is delivered
everywhere free of charge. Yet direct use of solar energy—that
is, harnessing light’s energy content immediately rather than
indirectly in fossil fuels or wind power—makes only a small
contribution to humanity’s energy supply. In 2008, about 0.1% of
the total energy supply in the United States came from solar
sources. In theory, it could be much more. In practice, it will
require considerable scientific and engineering progress in the
two ways of converting the energy of sunlight into usable forms.
Photovoltaic systems are routinely employed to power a host of
devices—from orbiting satellites to pocket calculators—and many
companies make roof-sized units for homes and office buildings.
Photovoltaic (PV) systems exploit the photoelectric effect
discovered more than a century ago. In certain materials, the
energy of incoming light kicks electrons into motion, creating a
current. Sheets of these materials are routinely employed to
power a host of d