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How Solar Cells Work

By now just about everyone should have seen a solar cell at some point in his or her life, whether it was on a roof, in a parking lot, on a light pole, attached to a road sign, or even in a pocket calculator. But have you ever stopped to think about how a solar cell, also referred to a photovoltaic (PV) cell, actually generates electricity? If so, you've come to the right place.

Basic Explanation

Sunlight is composed of photons, which are small packets of energy that reflect off surfaces and allow us to see objects.

Solar cells are made of semi-conductors, most typically silicon. When sunlight hits a solar cell, a portion of the photons are absorbed within the semi-conductor material and its electrons are energized. The excited electrons flow through the material and create electricity.

Detailed Explanation

Niels Bohr depicted the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus, just like the Earth circling around the Sun. However, an electric force holds the electron in its orbit instead of gravity.

Bohr introduced the idea that an electron can only occupy certain stable orbits with prescribed energies. The electron orbit closest to the nucleus has the lowest amount of energy. Energy is needed to pull the electron away from the nucleus so that orbits with larger radii have higher energies. In order for an electron to jump from a lower orbit to a higher orbit, it will require an energy source - light.

The atoms of elements, besides hydrogen, contain two or more protons and an equal number of electrons. The number of protons identifies the element. Protons are located at the center of the atom in the nucleus with neutrons. electrons fill what can be considered "shells" around the nucleus similar to the orbits of the Bohr atom. the first shell can hold up to two electrons, while the second and any additional shells can only hold 8.

Most solar panels are made from silicon atoms, which have 14 protons and 14 electrons. 2 electrons fill the first shell, 8 fill the second shell, and 4 are left in the third shell. These four outermost electrons, called valence electrons, will look for other nearby atoms to help fill up this outermost shell and form the crystalline structure.

When energy is added to pure silicon, it can cause a few electrons to break free of their chemical bonds and leave their atoms, which then leaves behind a hole. These wandering electrons, called free carriers, look for another hole to fall into and carry an electric current. However, pure silicon has so few free carriers that impurities must be added on purpose (a processes called doping) to change the way it works. These impurities consist of 2 other atoms that will get the electron movement necessary to create electricity.

The first atom is phosphorous. It has 15 protons and 15 electrons, giving it 5 valence electrons. It will bond with neighboring silicon atoms, but has an extra electron that doesn't form part of a chemical bond. As a result, there are now many more free carriers, making it a much better conductor than pure silicon. This type of silicon is referred to as N-type ("n" for negative) because of the prevalence of free electrons.

The second atom is boron. It has 5 protons and 5 electrons, giving it 3 valence electrons. When silicon is doped with boron, it is referred to as P-type ("p" for positive) silicon. Instead of having free electrons, it has free openings and carries a positive charge.

When the N-type and P-type silicon come together, called the P-N Junction, an electric field is formed. The free electrons on the negative side rush to fill the openings on the positive side and a sort of barrier is formed to make it more difficult for electrons on the negative side to cross over to the positive side. There is an eventual equilibrium reached and an electric field separates the two sides. The electric field then helps push electrons from the positive side to the negative side.

When light, or photons, from the sun hit the solar cell, the energy forces the electrons out of their holes. If this happens close enough to the electric field, the field will send the electron to the negative side and the empty hole to the positive side, causing further disruption of electrical neutrality. If an external current path is provided, electrons will flow along the path to the positive side in order to unite with the empty holes that the field sent there. This electron flow provides a current and the cell's electric field causes a voltage. This means we now have power!

But before the solar cell can be used, a few more things need to be done to its exterior. On the outside, silicon is a very shiny material. In order to prevent photons from bouncing away before they've energized the atoms, an antireflective coating is applied. In order to help protect the cells from weather and other damage, they are typically covered with a glass plate and put in a sturdy frame. Then, in order to achieve useful levels of current and voltage, multiple solar cells are connected together to form a PV (photovoltaic) module. Finally, the modules are put in a frame with positive and negative terminals to make mounting and installation easier. Once the wiring of the modules is complete, the solar electricity is able to flow to the other components of the solar energy system and power the business or home.