After it was discovered that the n-on-p cell was more resistant to
radiation, we decided to make an all-out effort to develop an n-on-p
cell that could be manufactured in quantity for our new satellite. Since
we didn’t know whether we could solve this problem in time to meet the
Telstar I launch date, we “hedged” by designing the new n-on-p cells to
be the same physical size (one by two centimeters) as conventional
p-on-n cells. Thus, if the n-on-p project hit a snag, we probably could
use regular p-on-n cells.
As you can imagine, making a solar cell to fit the very high
requirements we had set for the Telstar satellite is not an easy job—and
making these cells by the thousands is even more of a task. During
October, November, and December of 1960 we carried on a crash program in
which we made hundreds of experimental cells in our laboratories, using
a variety of materials and many different manufacturing techniques.
We perfected a phosphorus diffusion process to develop the very thin
n-layer (about one forty-thousandth of an inch thick) that we needed for
our special blue-sensitive n-on-p cells. We also had to devise an
entirely new way to attach the metallic contacts to the highly polished
surfaces of our cells, using a combination of titanium and silver.
Some tricky manufacturing problems also had to be solved once the
Western Electric Company began to make the large quantity of cells
needed for the Telstar program. For example, during the diffusion of the
n-layer of the cell, the silicon slice is surrounded by phosphorus
pentoxide vapor, which covers the entire slice with an “n-skin.” This
skin must be removed from the bottom of the cell by etching or grit
blasting before the p-contact is applied. Another difficult problem
occurred when we decided to give our cells an anti-reflection coating.
Because polished silicon has a refractive index near 4 and space has an
index of 1, silicon will reflect about 34% of visible light from the
sun. However, if we apply an anti-reflection layer onto the silicon this
percentage of reflection can be considerably decreased. We found that
the best substance for this purpose was a layer of silicon monoxide only
three-millionths of an inch thick. But it was only after quite a bit of
trouble—and scrapping several thousand cells—that we were able to get
this coating to adhere properly in the right thickness.