Amorphous Silicon-Based Solar Cells Come Into Their Own

April 03, 1997

San Francisco -- Three companies will mass-produce amorphous silicon-based solar cells this year, which may spell the beginning of commercially cost effective solar power, according to a Penn State researcher.

"The next four or five years could determine the future of the amorphous silicon photovoltaic industry," says Dr. Christopher R. Wronski, the Leonhard Professor of Microelectronic Materials and Devices.

The 1970s energy crisis stirred interest in commercial solar power, and the discovery that amorphous silicon, laid down as a thin film using standard depositional techniques, could be used for solar cells, gave hope to the field.

"Unfortunately for the solar industry, 20 years ago we discovered the Staebler-Wronski Effect," Wronski told attendees at the Materials Research Society conference today (Apr. 3) in San Francisco. "Ironically, the sunlight that produces electricity in these amorphous solar cells, also degrades their efficiency."

The original amorphous silicon solar cells had efficiencies of 1 to 2 percent, but in a short time, initial efficiencies reached 10 percent. By the mid-1980s, large area panels measuring a square foot could be made and by the mid-1990s, these large panels had stabilized efficiencies of 10 percent.

"But for initial efficiencies, degradation by exposure to sunlight causes 20 to 30 percent efficiency loss, lowering the available power over time," says Wronski.

"We still don't agree on the mechanism behind the degradation, but materials and manufacturing improvements have compensated to the point where companies now feel they can economically mass produce amorphous silicon solar cells," says Wronski.

The three companies that plan to market a combined annual output of 25 megawatts of capacity are Solarex Corporation, United solar Systems Corporation (U.S.S.C.) and Canon Japan. Solarex is aiming their marketing at solar farms and plans to produce 10 megawatts of capacity per year. U.S.S.C. and Canon are focusing on the rooftop market with annual output of 20 and 5 megawatts of capacity respectively. This rooftop market consists of urban rooftops with solar cells covering large areas.

Amorphous silicon-based solar cells are deposited on thin sheets of base material, usually glass but sometimes plastic or a metal. These cells consist of a conducting layer and three very thin films of amorphous silicon-based material with the middle layer electrically neutral and the bottom and top layers having opposite electrical properties. Large area solar panels are made of smaller solar cells connected in series.

While eliminating the Staebler-Wronski Effect is not currently possible, engineering approaches have been able to mitigate the effect on power production. Making the silicon layers thinner decreases the degree of degradation which is caused by sunlight-induced defects in the material. Changes in the composition of the silicon material have also helped with efficiency.

Unlike other light-induced chemical changes like those produced when light hits photographic film, the Staebler-Wronski Effect is reversible by annealing. By heating the material to 300 degrees Fahrenheit, the degradation is reversed, but, while this is a worthwhile laboratory approach, it does not help commercially, because the materials will simply degrade again when exposed to sunlight.

The most inventive approach to increasing conversion of sunlight to power has been to stack solar cells on panels. This increases both the stability and the efficiency. Recently, the stacking of three cells has achieved an increase in initial efficiency to over 14 percent.

Today, the 700 milliwatts of power from the sun that fall on a square inch of solar cell is converted by a double junction solar cell, at 8 percent efficiency, to produce 56 milliwatts per square inch and by a triple junction solar cell to produce 100 milliwatts per square inch.

"That isn't a lot of power," says Wronski, "but it is free energy and all that is needed is a large area."

EDITORS: Dr. Wronski can be reached at (814) 865-0930.

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