THIN-FILM, AMORPHOUS-SILICON PHOTOVOLTAICS

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Imagine what would be possible if solar cells were available for 70¢ per watt! At that price, a fully equipped home would receive its electricity at a cost that's equivalent to utilityproduced energy billed at 10¢ per kilowatt-hour (KWH). Power company rates now average 7¢ per KWH in the U.S., and the dime-per figure promises to be upon us all too soon. (In other parts of the world—Japan, for example—even that price is already a fond but fading memory.)

But is it really possible that solar cells will sell for 70¢ per watt in the next few years? The answer is yes, if current trends in thinfilm cell research are any indication of what can be done.

And one of the hotbeds of such investigation is Japan, which began to organize a national effort to study photoelectricity back in 1974. You see, that country recognized—nearly ten years ago—that fuel prices were going nowhere but up! Since then, photovoltaics researchers have taken great strides toward the goal of making solar electricity a practical, economical alternative to that generated with imported oil.

The Japanese investigated numerous alternatives, and—in 1979—settled on thin-film, amorphous-silicon technology as the best bet. In fact, many of that country's scientists anticipate that once the technology has matured, the cost of such cells could drop to 70¢ per watt by 1985 . . . and to as low as 25¢ per watt by 1990.

AMORPHOUS SILICON

Unlike the popular single-crystal wafer I described a couple of issues back (see issue 76, page 178), amorphous silicon has physical properties that can best be compared with those of glass. Instead of a rigid crystalline framework, the material has a more fluidlike form that lacks orderly structure. (In fact, the word amorphous means "without definite form".)

It's the shapeless nature of this kind of silicon that holds so much promise for photovoltaic production. In contrast to the timeconsuming, energy-intensive chore of growing single-crystal wafers, producing amorphous cells is actually quite simple.

Fabrication of the cell begins with a sheet of ordinary glass, over which a very thin layer of conductive metal—actually a mixture of indium oxide and tin oxide, referred to as the ITO layer—is deposited. Its purpose is to provide the positive contact for the cell.

A layer of P-type semiconductor silicon is then applied on top of the ITO by a plasma process. (The material is released from its gaseous carrier by means of radio-frequency bombardment.) Then a layer of undoped (nonconducting) silicon, which acts as an insulating barrier, is formed over the two previous coatings. A third and final silicon deposit consists of N-type semiconductor material. And backing up the whole assembly—providing the negative contact—is a covering of aluminum.