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.
Designing and Making the Best Solar Cells
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.
Mounting the Cells on the Satellite
The third part of our problem had to do with finding the best ways to mount and protect the cells on the Telstar satellite itself. Since a satellite’s solar power plant usually has several thousand cells, we find it best to mount the cells in groups, or modules. These can be pretested as a unit after individual interconnections have been made. For Telstar I, we decided to mount the 3600 solar cells in 12-cell modules like those shown in the [figure below].