Despite what shortsighted critics say, solar power is the wave of the future . . . and—of course—it's been the basis of our energy supply all along, since most of our current power sources would not be available today if it weren't for of Sol. Unfortunately, the process of converting sunlight directly into electricity—which is the form of energy most useful to us—by means of solar cells has been prohibitively expensive . . . until now!
That's right . . . just recently, a genuine breakthrough in photovoltaic technology pioneered by a research arm of the Radio Corporation of America (RCA) has brought affordable solar power in reach. They've made it possible to reduce the cost of sun-produced electricity from an average of $10 a watt to $1.00 (or less) for that same watt . . . which, unbelievably enough, is even less expensive than some utility-supplied power!
The truth is, photoelectricity has been around for some time. It was first recognized in 1839, and was eventually developed into a useful source of energy with the advent of wafer-type solar cells in the mid-1950's. In a nutshell, a conventional photovoltaic cell works like this: A semiconductor material (in this case, silicon, which is a component of sand and is thus plentiful) is first refined to a high degree of purity in an electric furnace. Next, this 99% pure material is "cleansed" even further through various chemical processes, and is then converted under intense heat from a granular form into a single-crystal state.
During the conversion process, the silicon is "doped" with either boron or phosphorus to form P (positive) or N (negative) ingots. The impregnated crystals are then sliced very thin and joined, P to N, using a doping agent and more heat. With that done, minute nickel or aluminum strips are attached . . . to conduct current and act as terminals. When sunlight penetrates the thin P-layer, a reaction takes place in the "barrier" between the P and N layers and electricity is produced.
Obviously, then, the cost of manufacturing a wafer-type solar cell can be quite high. Besides the expense of merely producing the necessary ultra pure silicon, the substance must be further processed by physically slicing the ingots . . . then each cell must be assembled, which also increases cost. So, not only are the necessary materials at a premium, but the manufacturing steps themselves run into big dollars.
The Secret's in the Sauce
How, then, has RCA broken through the price barrier? To put it simply, the firm has attacked the problem from a whole new direction. Conventional photoelectric cells are relatively effective in terms of light-to-power conversion, achieving an impressive 10% (or higher) efficiency in the field. However, to achieve such a high rate of efficiency, the cells must be manufactured from only the purest grade silicon . . . anything less simply wouldn't yield the magic number.
RCA researchers decided to try a different approach: If manufacturing costs could be reduced drastically, they reasoned; cell efficiency would no longer be of prime importance. So, as early as 1974, RCA technicians began developing what they call hydrogenated amorphous silicon cells . . . hardware that not only uses inexpensive "impure" silicon, but requires less costly manufacturing techniques as well.
According to Dr. David Carlson, group leader of RCA's photovoltaic research team, the fabrication process is completely different from that used for conventional single-crystal silicon solar cells. "We start with a gas called silane, which consists of silicon atoms surrounded by four hydrogen atoms, and we use an electrical pre-charge—much like that in a fluorescent tube—to break up the silane molecules and deposit a film containing between 5 and 50 percent hydrogen by atomic weight. In other words, the hydrogen is chemically bonded into the silicon . . . and it's the hydrogen that makes the cells work. Prior to our discovery, it was generally accepted among scientists in photovoltaic research that you couldn't use amorphous, or noncrystal, silicon semiconductors to make solar cells because there were too many broken bonds in the 'impure' structures. But it turns out that the hydrogen 'heals' such defects and makes the material a fairly good semiconductor."
Of course, less costly silicon is only part of the story . . . the fact that the new process minimizes production costs is just as important. Since the gas and precharge technique doesn't require intense heat (necessary temperatures have been reduced from over 1,400°C to 200°C), not only is less energy consumed in production, but many types of inexpensive material (including glass, sheet metal, and even plastic) can now be used as a base or substrate . . . where only a high grade of metal was acceptable before.
In addition, the innovative method cuts corners by combining processing steps. The substrate section is first coated with a very thin steel or titanium conductive layer, then alternate P- and N-doped layers-sandwiched around a "spread" of undoped silane about half a micron thick -are deposited. (The beauty of working with a gas, by the way, is that different layers can be "painted" on . . . and the "dope"—in this case phosphine and diborane gas—can easily be premixed with the silane.) The final step involves coating the structure with a transparent conductive oxide-Indian tin-which naturally carries electricity and allows light to pass through. The entire process is so relatively quick and easy (it may take anywhere from 1 to 20 minutes) that it makes the production of "old-fashioned" single-crystal silicon cells almost impractical . . . and, even more important, cuts costs to the point where the glass substrate is the most expensive material used.
Efficiency is the Key
Before the company goes into full swing with the amorphous silicon solar cell program, its people would like to increase the efficiency of the little converters. So far, researchers have managed to reach a conversion factor of nearly 5% . . . which would put the cells in the range of $1.00 per peak watt for large quantities-right now. Carlson hopes, however, to be able to increase that percentage in the near future . . . since the material they are working with now is capable of about an 8% efficiency. (He also claims that in the next few years a 10% efficiency will likely be achieved.)
Today RCA . . . Tomorrow the World
Naturally, the effect that the mass production of inexpensive photovoltaic cells could have on society as a whole is nothing short of mind boggling. Since such devices will be well within the financial reach of the average American family, it isn't inconceivable that in the near future rooftops all across the nation will be covered with arrays of the 3" X 3" miniature generating stations. The energy provided by photovoltaic cells can, of course, be utilized in several ways . . . either totally independent of the local utility by adding battery storage (perhaps in combination with electrically heated water storage for space heating if desired), or—on a limited scale—in concert with the power company . . . by swapping that utility's energy at night for solar electricity during the day.
But that's not all . . . since the cells are capable of functioning even under adverse conditions (RCA researchers find their creations can produce between 20 and 30% of total power on an overcast day), future possibilities include using them as a source of power for electric propulsion vehicles (in conjunction with lightweight batteries for nighttime use).
What it all boils down to is that tomorrow's dream is here today . . . and it shouldn't be long—perhaps two years or less—before we all can enjoy low-cost, independently produced electricity!
EDITOR'S NOTE: New developments in photovoltaic research within the past few months may make it possible for us all to enjoy energy independence in the very near future: Keep a weather eye posted toward MOTHER EARTH NEWS' pages, because in the next few issues we should be reporting on several other pioneering groups who are on the verge of making similar momentous discoveries.