Now, having given you a very brief account of how a solar cell works,
let us return to our three-part problem. The first objective was to
study all the aspects of radiation damage. To do this, we had to find
out how much radiation the Telstar satellite would encounter; we needed
to estimate the concentration of high-energy particles—both electrons
and protons—at various altitudes and locations. Several government
agencies are now carrying on research in this important area, but at the
time of the Telstar I launch we did not know exactly how much radiation
the satellite would run into. And the high-altitude nuclear explosion of
July 9, 1962 (the day before Telstar I went into orbit) may have
increased the quantity of high-energy electrons injected into its path.
We also wanted to find out whether electrons and protons would do the
same damage to solar cells. Several kinds of cells were exposed at Bell
Laboratories and at various university research laboratories to a wide
range of radiation dosages. The experiments showed, generally, that the
damage effects of electrons and protons should be about the same.
Although protons are 1840 times as massive as electrons, there are a
great many more electrons in the Van Allen belts, so that an unprotected
solar cell would be much more likely to be injured by electrons than by
protons.
In fact, we found that the Van Allen belt protons have so much energy
that they can go through transparent shielding material as much as
several centimeters thick and still damage a solar cell. Thus, to screen
our cells from protons we would need very thick transparent cover
plates, and this added weight would be intolerable. So we decided to use
no proton shielding at all.
With electrons, the situation is different; they are much lighter and
have much less energy. Also, if their energy is reduced below a certain
level (about 180 thousand electron volts) electrons will not be able to
knock silicon atoms out of position, and thus cannot harm a solar cell.
We experimented with a number of different kinds and thicknesses of
cover plates, and found that transparent material with a mass of 0.3
gram per square centimeter would slow down electrons enough to make them
no problem.
Another radiation study helped us take advantage of the fact that solar
cells respond differently to light of different wave lengths. If the
surface layer of a cell is extremely thin, it will absorb blue, green,
and yellow light well, but may be much less sensitive to the deeply
penetrating red and infrared waves. We experimented with n-on-p cells
having very shallow p-n junctions, exposing them to an extremely strong
radiation dosage. The cells still responded very well to blue and green
light, even though most of their response to infrared and red light was
lost. These findings convinced us that we should work to make our new
cells as blue-green sensitive as possible, since they were going to be
exposed to heavy radiation.