Historically, the sun’s energy has most often been used by concentrating
it with a lens or mirror and then converting it to heat. We could do
this and run a heat engine, but a more direct avenue is open.
About a decade ago it was found that the junction between _p_ and _n_
semiconductors would generate electricity if illuminated. This discovery
led to the development of the _solar cell_, a thin, lopsided sandwich of
silicon semiconductors. As shown in Figure 12, the top semiconductor
layer exposed to the sun is extremely thin, only 2.5 microns. Solar
photons can readily penetrate this layer and reach the junction
separating it from the thick main body of the solar cell.
[Illustration: Figure 12 THE SOLAR CELL
_The photograph shows the solar cell in use on a satellite. The
spherical, radioisotope, thermoelectric generator at the bottom of
the satellite is used to supplement the solar cells. In the solar
cell, hole-electron pairs are created by solar photons in the
vicinity of a p-n junction._
Courtesy U. S. Air Force and National Aeronautics and Space
Administration.]
_p_ SILICON
_n_ SILICON
ELECTRON-MOLE PAIRS
JUNCTION
PHOTONS FROM SUN OR RADIOISOTOPE
ELECTRONS
ENERGY OUT
Whenever _p-_ and _n-_type semiconductors are sandwiched together a
voltage difference is created across the junction. The separated holes
and electrons in the two semiconductor regions establish this electric
field across the junction. Unfortunately, there are usually no current
carriers in the immediate vicinity of the junction so that no power is
produced.
The absorption of solar photons in the vicinity of the junction will
create current carriers, as the photons’ energy is transformed into the
potential energy of the hole-electron pairs. These pairs would quickly
recombine and give up their newly acquired potential energy if the
electric field existing across the junction did not whisk them away to
an external load.
The solar cell produces electricity when hole-electron pairs are formed.
Any other phenomenon that creates such pairs will also generate
electricity. The source of energy is irrelevant so long as the current
carriers are formed near the junction. Thus, particles emitted by
radioactive atoms can also produce electricity from solar cells,
although too much bombardment by such particles can damage the cell’s
atomic structure and reduce its output.
The solar cell is not a heat engine. Yet it loses enough energy so that
the sun’s energy is converted at less than 15% efficiency. Losses
commonly occur because of the recombination of the hole-electron pairs
before they can produce current, the absorption of photons too far from
the junction, and the reflection of incident photons from the top
surface of the cell. Despite these losses solar cells are now the
mainstay of nonpropulsive space power.