Surprise breakthrough on cheap solar cells to make electricity
The quest for a solar cell that can produce electricity at a competitive price had begun to seem more like a hopeless quest than a practical possibility.
But recently, researchers at the Boeing Aerospace Company here have engineered an unexpected breakthrough that makes potentially inepensive "thin film" solar cells considerably more credible.
There are two schools of thought among those attempting to develop economical solar cells, also known as photovoltaics.
One school is working with cyrstals of silicon of the sort used on space satellites. These cells can be made with high efficiencies: converting as much as 18 percent of the incident sunlight into electricity. Their problem is cost. Despite a more than 200-fold reduction from the astronomical price for space applications, the power they produce remains quite expensive. And each additional cost reduction has been a little harder to achieve.
The alternative approach is to perfect so-called "thin film" devices. These consist of a film of light-sensitive material five-thousandths of a millimeter thick. Because these films are extremely thin, the use of even exotic and expensive materials becomes feasible. And there is a general consensus among the experts that, should a suitable material be developed, these cells can be mass-produced by processes similar to those used to make photographic film. Here, the problem has been efficiency.
For several years the conversion efficiency of thin film cells has hovered around 6 percent. Government-spon- sored analyses have concluded that at least a 10 percent efficiency is needed to achieve the 50 cent-per-peak-watt goal set by the Department of Energy for 1990.
Bow it appears that this efficiency barrier has been broken. Boeing's Reed Mickelsen and Wen Chen, working with a "dark horse" photovoltaic material, have demonstrated a 9.4 percent eficiency with a true thin film cell.
The material -- a compound of copper, indium, and selenium on cadmium sulfide -- was not one favored by the experts. Although it looked theoretically promising, preliminary efforts at increasing its efficiency were unsuccessful. "We were at a stage where we didn't have much hope," admits Dick Burke of the Solar Energy Research Institute (SERI), the project manager.
Dr. Burke lists several reasons why this advance is significant besides the high efficiency. The Boeing scientists have built the cells on an inexpensive backing, or substrate, of aluminum ceramic. Also, the material appears to be very stable. Some photovoltaic compounds require airtight encapsulation to obtain long life.
In less than a year, the Boeing researchers were able to increase the cell efficiency from 6.7 to 9.4 percent. Best of all, "They have done this quite simply, without any high cost compoenents" says Dr. Burke.
With "optimization" Dr. Mickelson believes that as much as an 11 percent efficiency may be obtained.
So far, the researchers have produced 16 cells, each a square centimeter in size. They have all had efficiencies in the 9 percent, range, the researcher says.
Sigurd Wagoner, co-inventor of this material and a photovoltaics expert at SERI, says he is pleasantly surprised at the strides Drs. Mickelson and Chen have taken.
But He says he believes it is still too soon to determine the final configuration that thin film solar cells with take.
Still, he thinks this will give a big emotional boost to workers in the field. Breaking the 7 percent barrier should spur other researchers.
"This makes the thin film cell, which had become almost proverbial, considerably more credible," Dr. Wagoner says.
This advance has come at an opportune time for the backers of the federal photovoltaic program. The cost sutting fervor on Capitol Hill has led to increasing scrutiny of the Department of Energy's photovoltaic budget. The pressure has been toward paring the smaller, higher risk projects such as that of the Boeing researchers.
"I hope Congress gets the message," says one program manager.