There are two ways of making a silicon solar cell. In one, the body of
the cell is what we call _n-type_ silicon—that is, pure silicon that has
been doped with a small number of impurity atoms of an element such as
phosphorus or arsenic (from group V of the periodic table). This kind of
semiconductor[4] conducts electricity by means of a supply of
free-to-move electrons (negative charges) caused by the presence of
these impurity atoms. To make a workable solar cell from n-type silicon,
a thin surface layer of p-type silicon is formed by diffusing atoms of a
material from group III of the periodic table—usually boron—into the
silicon. Metallic contacts then are made to these two regions. This kind
of cell is known as a _p-on-n cell_.
The second type of solar cell is just the reverse. It begins with a body
of p-type silicon (with impurity atoms from a group III element) and
conducts electricity by means of “holes”—vacant sites where electrons
might be but are not. These holes act as free-to-move positive charges.
We can make a solar cell from this material by diffusing a layer of
n-type impurity, such as phosphorus, into it. We call this an _n-on-p
cell_ (see the _figure below_).
[Illustration: Construction of a silicon solar cell of the n-on-p
type (thickness of n-layer greatly exaggerated).]
titanium-silver evaporated contact with solder dip finish
antireflection coating
contact gridding for lower series resistance
15 mil wafer, p-type
0.4 micron front layer, n-type
The key to the operation of either type of solar cell is the junction
between the regions of n-type and p-type material—what we call the _p-n
junction_. In an actual n-on-p cell this junction is only about twenty
millionths of an inch below the surface, since that is the thickness of
the n-layer. At this point, where the hole-rich p-region meets the
electron-rich n-region, there is a permanent, built-in electric field.
As shown in the figure below, the n-layer has many free electrons
(indicated by minus signs) and a few holes (circled pluses), while the
p-region has many holes and a few electrons. When the cell is in
equilibrium, thermal agitation causes some holes to diffuse into the
p-region. We call these stray holes and electrons _minority carriers_
(the circled pluses and minuses in the figure). Thus, the n-layer has a
slight positive charge and the p-body has a slight negative charge; this
results in a difference in potential across the junction, which in
silicon amounts to about seven-tenths of a volt.
[Illustration: _Schematic diagram of an n-on-p solar cell. In the
n-layer, minuses represent free electrons, circled pluses are
minority-carrier holes; in the p-type body, pluses represent holes,
circled minuses are minority-carrier electrons._]
Sunlight is made up of individual corpuscles of energy called _photons_.
When these photons are absorbed in or near a cell’s p-n junction, they
liberate both a free-to-move negative charge and a free-to-move positive
charge—this is called generating a _hole-electron pair_. The electric
field across the p-n junction causes the holes to flow to the p-side and
the electrons to the n-side of the barrier. This flow tends to make the
p-side positive and the n-side negative, so that, when a load is
connected between them, a useful external voltage (amounting to about
six-tenths of a volt) is produced, and electric current will flow. Thus,
we have converted light energy into electrical energy.
Only part of the energy in light can be used to generate an electrical
output, since a good deal of the light striking a cell is absorbed as
heat or is reflected from its surface. The percentage of solar energy
that can be converted into usable electric power is called the cell’s
_conversion factor_ or _efficiency_. Although this can theoretically be
as high as 22%, the best cells we have made in the laboratory have
conversion factors of only about 15%, and the better commercial cells
have efficiencies of 12% or more.
Although both p-on-n and n-on-p cells were made in early laboratory
studies, the p-on-n cells gave a somewhat higher output. As a result,
all the American commercial solar cells up to 1960 were of this type,
and they were used on all satellites before Telstar I. (Russian
satellites, we believe, have used n-on-p cells from the beginning.)
The U.S. Army Signal Corps Research and Development Laboratory, however,
decided to make both p-on-n and n-on-p cells and compare their
performance. This laboratory work led to a surprising discovery: The
n-on-p cells were several times as resistant to energetic particle
radiation as were comparable p-on-n cells. These results were announced
in 1960, and confirmed by our measurements and those of other
laboratories. The timing was very fortunate, since we had just learned
of the greatly increased radiation hazards presented by the Van Allen
belts.