How Solar Cells Work

The advantages of solar lighting are obvious. But how does it work? How does a solar cell gather sunlight and convert it into useable electricity?

Photons to Electrons - The sun emits photons. Electricity runs off electrons. The basic job of a solar cell is to take photons from the sun and use them to produce electrons. To do this requires a photovoltaic cell. ("Photo" means light, and "voltaic" means electricity.) Photovoltaic cells are made from semiconductors, usually silicon. When light strikes the cell, the semiconductor absorbs a certain amount of it. This energy releases some electrons which can now flow freely. Metal contacts at the top of the cell draw that current off to store it in a battery.

That's the basic plan. How does it happen?

Silicon - Semiconductors such as silicon have special chemical properties. In crystalline form, pure silicon is a very poor conductor. Its electrons are arranged in shells in such a way that they're tightly bound to each other. None of the electrons are free to move about. To work as a solar cell, you have to introduce impurities.

Impurities sound bad, but they're just other atoms thrown into the mix. Introducing a phosphorous atom for every million silicon atoms can do the trick. Phosphorous atoms can bind to silicon quite easily, but with one electron left over. That unbonded electron is just waiting to be nudged away. That's the N-type ("n" for negative) of silicon.

There's also the P-type ("p" for positive) that's created by introducing boron atoms into the mix. Boron atoms also form bonds with silicon, but with one space or "hole" left over where an electron should go. These holes carry a positive charge and are free to move around, just like the electrons.

Mixing It Up - Putting the N-type and the P-type together creates an obvious opportunity for all the free electrons on the N side. They move to fill the holes that exist on the P side. But they're not completely successful. Right at the junction between the two types, the electrons and the holes mix, forming a barrier. It becomes harder and harder for electrons to cross that barrier to get to the P side. When equilibrium is reached, the result is an electric field separating the two sides. This electric field acts as a diode, a one-way street for electrons. They can easily flow from the P side to the N side, but it's hard to get back.

Let the Sun Shine In - When a photon hits the cell, it will free an electron and result in a free hole. The electron, if it's close to the electrical field on the P side, will travel down the one way street to the N side. Electron flow produces current. The electric field causes voltage. This only works if the cell absorbs the photon rather than reflecting it. So silicon, which is normally shiny, has to be given an anti-reflective coating to make all this work. But once it works, the metal contacts provide a path of least resistance for the created electricity to flow out of the cell and into storage (the battery) until it's needed.

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