The Birth of a Solar Cell
(Page 2 of 3)
July/August 1982
By TJ Byers
The fully grown crystal now goes to a grinder, where irregularities are removed and the cylinder is turned down to exactly 4 inches in diameter. Then the ingot is clamped in a vise and sliced into wafers, each of which is only about 0.020" thick. Unfortunately, the abrasive cutting blade produces a considerable amount of waste almost three-quarters of the original cylinder is reduced to dust when the crystal is cut!
RELATED CONTENT
You don’t have to have land to have a garden full of healthy food. Container gardens can exist in j...
From sunup to sundown, enjoy outdoor fun amongst Grand Junction’s breathtaking mountains....
The U.S. Department of Energy has added two 100 Awards to their trophy case — for a total of 42 — f...
If you've been considering investing in a power cultivator, this chart may save you some legwork....
The next step in the production process is a chemical treatment that removes blade marks (produced by the slicing) from the faces of the disks. Sodium hydroxide (household lye) smooths the silicon surfaces, and then the caustic element is neutralized with a sulfuric acid rinse. Once the wafers have been ultrasonically cleaned and inspected, they're ready to have their electrical components applied.
The actual process of converting sunlight into electricity takes place in a semiconductor junction that's formed on one surface of the cell. This thin layer is created by placing N-type semiconductor silicon over the face of the cell through a process called diffusion.
The wafers are first put in a quartz tube of about 8 inches in diameter and heated to 850°C (1562°F). The system is purged with inert, dry nitrogen to remove all oxygen, and a measured amount of phosphorus is injected into the tube.
Once in place, the phosphorus molecules disassociate under the intense heat and force themselves into the silicon's crystal structure, displacing silicon atoms as they do so. The penetration is very shallow, though . . . on the order of about 20 microns. (To give you a better idea of just how small that dimension is, consider that a micron is 1/1,000,000 of a meter, or slightly less than 1/25,000 of an inch!)
During diffusion, two disks are placed back to back to avoid forming a junction on both sides of the cells. However, a junction is formed around the thin outer edges of the disks, and it must be removed to prevent the cells from shorting out. The units are placed in what's called a plasma etch chamber, which trims back the perimeter enough to remove the unwanted material.
At this point the cell has become a genuine photovoltaic generator and will produce electricity if exposed to sunlight. Now, it's necessary to attach metallic contacts to the cell's surface for removing the useful electrons. This is accomplished by printing the cell's face—using a silk-screen pattern—with a grid of metallic ink. The grid wires are then baked into the surface to complete the negative connection, and the back of the wafer is coated with the same material (without the pattern) to form the positive contact.
ASSEMBLING THE MODULE
Completed photovoltaic cells are grouped together in modules, both to boost available voltage and power (a single cell's output is relatively low) and to protect the units from the elements. Because silicon is slightly hygroscopic (that is, it absorbs water), careful hermetic sealing is just as important as is shielding the cells from impact. In fact, the life expectancy of a solar panel is directly related to how well it's sealed.
