Beauty in Simplicity
At its core, solar panel technology is very simple. The sun beams energy down to earth in tiny packets of sunlight called photons. When a photon hits a semiconductor such as silicon–the most common solar panel material—the packet of sunlight transfers its energy to an electron inside the silicon panel. This energized electron is suddenly free to move.
The electron is attracted to a metal plate attached to the back of the silicon panel. When the electron touches this metal contact it flows into a circuit. This circuit, filled with flowing electrons, carries electrical current, and flows into your building, providing electricity. Most solar cells are very small, so they’re linked together into modules, which in turn are linked together to form panels. The more solar panels you link together, the more current is created, the more energy you generate. It’s that easy.
P-doped and N-doped and Electric Fields, Oh My!
Of course, if it were too simple solar cells would have been invented long ago. A key discovery that made the modern solar cell possible was the addition of dopants to pure semiconductor materials. This did not happen until World War II.
Doping a semiconductor is not as cruel and unusual as it at first may sound. In fact, doping a semiconductor actually makes it more electrically useful. That’s largely because the dopant is simply another basic element: usually phosphorous or boron.
Phosphorous has one more free electron than silicon, while boron has one fewer free electron. When you sprinkle some phosphorous atoms onto one half a silicon panel, and sprinkle some boron atoms onto the other half of the silicon panel, suddenly one half of the semiconductor has an extra batch of free electrons, while the other half has a relative deficit of free electrons, called holes. As you might guess, neither half is happy.
The side with too many free electrons, the phosphorous side, is called N-doped. ‘N’ is for negative, since electrons carry negative charge. The side with too few free electrons, the boron side, is called P-doped, for positive.
Where the two halves of doped silicon meet, the free electrons will leave the N-doped side and move to P-doped side where there are holes waiting to be filled. The electrons keep moving to the P-doped side until equilibrium is reached and there are an equal number of free electrons on both halves.
The funny thing about this movement of free electrons from N-doped to P-doped is that before the electrons moved, both halves of the doped silicon had perfectly neutral charges. Though there were more free electrons on the phosphorous side than on the boron side, there were also more protons (positively charged cores of atoms). In fact, the number of protons on the phosphorous side exactly balanced the number of electrons. But unlike protons, free electrons can move. So they did. By leaving, the electrons made the phosphorous side positively charged. Similarly, by moving to the P-doped half of the silicon, the electrons made the boron side negative.
The different net charges on these two halves of the silicon semiconductor creates an electric field. From the electron’s perspective, this field is like a hill. It is easy to run down the hill towards the N-doped side where protons eager to be neutralized by the electrons are waiting. On the other hand, the P-doped side is a steep climb because it has many more electrons than protons. Though the electrons are happy where they are, if and when they are excited, the electric field will push them towards the N-doped section of the semiconductor.
Enter the Sun
What happens then, when a packet of light from the sun hits the doped semiconductor and excites an electron? Well, the electric field pushes the excited electron towards the protons on the N-doped half. Once it arrives, however, it wants to return to the P-doped half where there are fewer free electrons (recall why the electrons moved from the N-doped to the P-doped region in the first place). The problem is, the electron no longer can. It isn’t possible for it to climb back up the steep hill to get to the P-doped side.
This is where the metal plate on each side of the semiconductor become important. The metal plate on the N-doped side is connected to a thin wire that attaches to the P-doped side. The electrons know that if they run through the wire, completing the circuit they can get back to the P-doped side successfully. So they jump into the wire and as they flow through the wire, they’re directed to the main circuit of your building (technically, to the DC/AC inverter), where they deposit the energy transferred to them by the packet of sunlight.