The Birth of a Solar Cell

Image Gallery

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

If you've wondered about the making of photovoltaic panels, perhaps you'd like to attend...

Regular readers of this magazine are probably well aware that photovoltaics—the production of electricity from sunlight—is rapidly becoming a very practical alternative source of energy . . . and that solar cells are likely to play an important role in our future. Photocell installations are already popping up in great numbers, and—even today—can often beat out utility power in cost-effective applications. What's more, in some instancesthe 1,000,000-watt ARCO Solar plant that's expected to begin feeding power into Southern California Edison's lines by early 1983, for example—solar electricity has become a mainstream alternative.

To most of us, however, the workings of a solar cell remain cloaked in mystery. Which is why, in an effort to probe the "secrets" of photovoltaic construction, I recently toured the ARCO Solar facility in Chatsworth, California . . . where I was impressed by the level of technical sophistication required to produce each panel. (This is not an energy alternative that—as yet, anyway—is within the reach of the backyard researcher.) I'd like you to share my experience, so let's take a minitour through the fabrication process and witness the birth of a solar cell.

SANDSATIONAL

The photovoltaic cell is made primarily of silicon, which is the most abundant element on earth. (In fact, silicon's common form is everyday sand.) Unfortunately, in its naturally occurring state, silicon contains a number of contaminants that would seriously reduce photovoltaic performance, so the ore must be refined extensively before it can be used. In fact, by the time the processed mineral reaches the manufacturing plant, in the form of polysilicon rocks, it must be at least 99.9% pure.

However, polysilicon is amorphous (that is, it has no defined crystalline structure) and thus unsuitable for efficient cell operation. Therefore, it's necessary to convert the polysilicon into a perfectly structured crystal. And just how does one make a perfect crystal big enough to use in the preparation of a photovoltaic cell? The answer is simple . . . by growing it!

CRYSTAL CULTIVATION

First of all, a perfectly arrayed seed crystal is needed to form a basis for growth. This little crystal is attached to a support rod that's suspended above a ceramic crucible full of molten polysilicon. The shaft is turned by a small electric motor, and the spinning seed is lowered until it touches the surface of the hot liquid. As the seed rotates, silicon atoms attach themselves, using the tiny crystal's structure as a pattern for perfect alignment.

At the same time that the slowly forming "jewel" is growing larger, the seed is grad ually withdrawn from the silicon solution, thereby elongating the new crystal. Over the course of about 24 hours, a crystalline cylinder about 4 inches in diameter and 20 inches in length is produced. Amazingly, during the entire day-long period the temperature of the "raw" silicon must not vary more than 2 °C above or below silicon's melting point of 1420°C (2588°F)!

RSS | Comments | E-mail | Print