THIN-FILM, AMORPHOUS-SILICON PHOTOVOLTAICS

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The semiconductor silicon layers on the glass base are so thin that they're measured in millionths of an inch . . . they total just 20 millionths, to be exact. As a result, the entire cell formation process uses only 1/300 as much material as is required for the production of a conventional solar cell!

COST VERSUS EFFICIENCY

Not only are the material costs for thin-film cells considerably lower than for single-crystal units (with an accompanying reduction in waste), but the steps involved in producing them are fewer. Moreover, the process lends itself to automation. Therefore, the major expenses involved in fabricating thin-film cells will undoubtedly be the cost of the glass and the aluminum backing.

Amorphous cells do have one disadvantage. At present, single-crystal wafers are capable of operating at about 10% efficiency, which means that approximately one-tenth of all the sun's energy striking the face of the cell is converted into electricity. Production amorphous cells, on the other hand, function—so far—at only about 4% efficiency. In order to generate the same amount of power as a conventional cell, therefore, the area of the amorphous cell must be 2-1/2 times as large. Nevertheless, laboratory amorphous units have already exceeded 8% efficiency, and there's little doubt that this figure can eventually be duplicated on an assembly line.

Furthermore, two other features of the thin-film cell serve to offset its lower efficiency. For one, the spectral response of the amorphous silicon more closely approximates that of sunlight, which means that the material reaches its efficiency level by making use of a wider range of the energy available. And the output voltage of the individual amorphous wafer is higher than that of a single-crystal cell, thereby reducing the number of units needed in series to generate a given voltage.

In the final analysis, though, the choice between single-crystal and thin-film photovoltaics boils down to a trade-off between efficiency and reduced production cost. And all things considered, it looks as though the amorphous cells could prove to be a bargain.

THEY'RE IN PRODUCTION

If you're under the impression that the amorphous cell is still just another pie-in-the-sky dream, you'd better think again. Although you can't buy a module for your home as yet, amorphous cells are being manufactured on a production basis right now . . . in Japan!

Sanyo Electric, the same company that makes stereos and televisions, has recently opened the largest photovoltaics manufacturing plant in the world, turning outyou guessed it—thin-film, amorphous-silicon solar cells. And there are a number of calculators, radios, and clocks—on the market today—powered by such cells. Unfortunately, it's impossible to say how long it might take for panels to be available outside Japan, since the initial production is destined for domestic use.

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