K.D. Smith Bell Solar Batteries TELSTAR
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How Do We Keep Solar Cell 
Power Plants Working in Space? 

By KENNETH D. SMITH Electronics Engineer-Member of Staff, Semiconductor Device Laboratory-Bell Telephone Laboratory.
Reprinted with Permission , (c) AT&T, This article appeared in SMEC Vintage Electrics Volume 3 #1


THE PROBLEM Before we learned about the Van Allen belts, we expected that the solar cells used to power satellites would last for many years in space. We thought they would be damaged only by cosmic rays, micro-meteorites, and occasional bursts of particles from the sun. But when the solar plants of some American satellites went out of action after only a few weeks in orbit, we realized that in the future solar cell power units would need better protection from radiation damage. We had learned that satellites - and particularly medium altitude communications satellites - must spend a lot of time in regions where they will be struck by thousands or even millions of high-speed radiation particles each second.


This fact forced us to change almost all our thinking about solar power plants for satellites. To make sure they would last for several years, we had to design new types of solar cells and devise new ways of mounting them. We also had to revise our estimates of how much power we could expect to get from our cells.


If a communications satellite is to go into regular commercial service, it must continue working for several years in space. The Telstar satellite, however, was designed as an experimental project, and we decided that two years would be a reasonable lifetime to plan for. When Project Telstar began, our problem was to develop solar cells that would operate in an environment subject to strong radiation effects - and keep on operating there for two years.


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 bad 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 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 at the bottom of the page).
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 at the top of the page, 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.


Sunlight is made up of individual corpuscles of energy called photons. When these photons are absorbed in or near a cellís 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 cellís 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.


Finding Out About Radiation Damage
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.


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 didnít 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 bit 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 bad 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 bad 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 bad 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 satelliteís 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 1, we decided to mount the 3600 solar cells in 12-cell modules like those shown in the figure on the previous page-.

Each of the cells has a top contact along one edge and a bottom contact all over its base, so we were able to assemble the 12-cell groups like shingles, with the bottom edge of one cell covering the top edge of the next, leaving only the active area of each cell exposed. But this meant that each module would be over four inches long and only 14 thousandths of an inch thick - far too weak to withstand stress and vibration. To support the cells, we decided to mount them on a metallized ceramic base. But this presented a problem: If the cells were soldered directly to the base, the different thermal expansion rates of the silicon and the ceramic would cause the structure to break during the cycles of extreme changes in temperature that Telstar would pass through. We remedied this by connecting each cell to the ceramic support by a thin U-shaped strip of silver (see below). Since silver has a much higher thermal expansion coefficient than silicon, we added tiny sandwiches of Nilvar or Invar (36% nickel, 64% iron) where the cells were attached. With this mounting method, the cell modules withstood thermal and mechanical shocks much more severe than those they would undergo in actual use. In one test, for instance, an entire cell module with its cover plates was first dipped in hot water, then plunged into liquid nitrogen at a temperature of -195 Centigrade. In orbit, the temperature range for the satellite was not expected to be more than from +80 degrees to -100 degrees C, with a rate of change of no more than three degrees a minute.


Finally, we needed to find the right kind of transparent protective cover for the Telstar solar cells, both to keep micrometeorites from damaging the sensitive and very thin diffused layer and to slow down the incoming electrons to non-destructive energy levels. For micrometeorite protection, only a thin layer of hard transparent substance was needed; for electron protection, the cover plates should have a mass of no less than 0.3 gram per square centimeter (as we explained above). And there were two other important considerations: The material we used should not be darkened or discolored by prolonged exposure to ultraviolet radiation, and it should have good thermal conductance, so that some of the heat absorbed by the solar cells could be conducted out to the cover plates and reradiated. All these requirements led us to the choice of clear, man-made sapphire.


Although sapphire is more expensive and difficult to make than the equivalent quartz or glass, it only has to be 30 mils (three hundreds of an inch) thick. Twice this thickness would be required if quartz or glass were used.


We have had space to describe only a few of the things involved in designing a solar cell power plant that would work unattended out in space. We have not mentioned a good many of the tough problems that had to be worked on. But we are glad to report that we could find answers to almost all our questions. And the most significant answer is shown in the figure below , where you can see how the Telstar 1 solar power plant slowly diminished in power almost exactly as we predicted it would. -KDS


KENNETH D. SMITH was born in Galesburg, Illinois, and received a B.A. from Pomona College in 1928 and an M.A. from Dartmouth College in 1930. He joined Bell Telephone Laboratories in 1930, and has worked on the development of proximity fuzes, radar bombing systems, broadband microwave radio systems, and various semiconductor devices, including radiation-resistant solar cells for the Telstar satellite.

(add photos to go with these captions)

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.

Construction of a silicon solar cell of the n-on-p type (thickness of n-layer greatly exaggerated).

Very gradual decay due to radiation effects of the Telstar I solar cell plant in the first months after the satellite went into orbit; it was extremely close to the predicted rate (solid line).

Lengthwise diagram of a solar cell module, showing how individual cells were fixed in place.

 

SMEC NOTE: We will not attempt to go into all the details of semiconductor physics here. If you would like to know more about how solar cells work, refer to the Library of Technology at the Southwest Museum of Electricity and Communications.
 
 
 

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