Technical Background on Solar Cells
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]).
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.
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.