How Things Work: Solar Cells

From calculators to highway signs to satellites, solar power today serves a variety of purposes. Solar cells, also called photovoltaic cells, are designed to convert light into electricity.

Solar cells are made of semiconductors, most commonly silicon. Each atom holds 14 electrons in its three-electron shells, the last of which is only half full. The outer shell, containing only four electrons, has the capacity to hold eight electrons.

To fill the extra space in the outer shell, silicon atoms share electrons with neighboring atoms. In a given atom, the four outer electrons are each connected to a separate, neighboring shell. When multiple atoms share electrons, the connections are called covalent bonds.

As a result, silicon atoms form what is known as a crystalline structure, a term given to describe their three-dimensional arrangement, where each atom shares electrons with four of its neighbors.

With all of its outer electrons tied up in bonds, pure silicon turns out to be a rather ineffective conductor. Unlike the electrons in copper and other good conductors, the outer electrons in silicon are not free to travel.

Thus, solar cells utilize impure silicon. Through a process called “doping,” scientists create impurities in silicon by adding other elements to the atoms. The ratio of silicon atoms to other atoms is about one million to one.

Silicon is often doped with phosphorous, which holds an extra electron in its outer shell. Left out of the lattice of covalent bonds, this fifth electron is held in place by only a proton in the nucleus. This creates what is known as an n-type region because the extra electrons constitute a free negative charge.

Silicon doped with boron, on the other hand, contains p-type regions. Boron’s outer shell has three electrons, not four, like silicon. This lack of an electron results in an empty space, or hole, which has a positive charge.

Holes can move around just like free electrons. When a hole is created, a neighboring electron will shift to fill it, thus creating a new hole in its place.
Silicon is usually doped with both phosphorous and boron, creating both positive and negative regions within the silicon. Initially, these excess charges cancel out one another.

Furthermore, silicon absorbs sunlight in the form of photons, each of which provides an electron with a certain amount of energy.

Only a small percentage of incoming photons, though, offer enough energy to free the tightly bound electrons in pure silicon. The electrons in doped silicon, however, are much more easily freed.

Once free, electrons move to the p-type region, where they are able to recombine with holes in the atoms’ outer shells. Similarly, free holes are able to travel to the n-type region.

The area where the n- and p-type regions meet is called the p-n junction, and it permits current flows to move in one direction. Electrons and holes move only to the p- and n-type regions, respectively.

After enough charge has been transferred, a region across the p-n junction called the depletion zone is formed. Here, all the holes have been filled by electrons; there is no longer any free charge. Additionally, mobile electrons and holes on either side of the depletion zone become unable to pass through it.

A buildup of negative charge in the n-type region and positive charge in the p-type region creates an electric field. Given an external conduction path connecting the n- and p-type regions, electrons will flow through it toward the p-type region.

This migration of electrons produces current, and the electric field creates voltage, or potential difference. The power supplied is simply the product of these two quantities.

To increase their effectiveness, solar cells contain two additional layers. The first layer is an antireflective coating to reduce the amount of photons lost to reflection.

Every photon that reaches a solar cell is absorbed, at least temporarily. When a photon is reflected, it is immediately reemitted, without displacing any electrons.

Additionally, most solar cells have glass cover plates, which serve as shields against weather damage.

Though the sun might seem like the most inexpensive source of power, constructing and maintaining a solar-powered home is anything but cheap. A solar-powered home requires an inverter to convert direct current (DC) to alternating current (AC), which is the type of current all household appliances are designed to handle.

Additionally, the setup requires special batteries called deep-cycle batteries, which are able to produce large amounts of power for long amounts of time. These batteries need a charge control to prevent over-charging or over-draining.