Organizing the Work
Our work on the solar cells for Telstar I began in October, 1960. With just a little more than a year to go before the satellite had to be ready, there was no time to lose. So we decided to break down the over-all problem into three parts:
- Finding out how radiation would affect various kinds of solar cells;
- Making experimental cells and, when the best had been picked, determining the best ways to make them in the large quantities we would need; and
- Developing ways to mount the cells on the Telstar satellite so that they would withstand the stresses of being launched, the effects of radiation particles, and extreme changes in temperature.
A different group of people began work simultaneously on each of these three parts of the problem, with each of them going ahead under the assumption that the others would be successful. Each group had to find the answers to many very interesting questions, but since our space is limited we can only discuss some of them here. Before doing so, however, we must say something about what a solar cell is and how it works.
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