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Polycrystalline silicon, ready to be manufactured into photovoltaic cells.

How a slice of silicon often thinner I than a human hair can harvest sunlight to make electricity may seem like magic. But what may appear as a bit of sorcery actually boils down to uniting science and engineering wizardry with some of Earth's most abundant resources— sunshine and silicon.

Polycrystalline silicon, ready to be manufactured into photovoltaic cells.

M (Valence) Shell: 4 electrons

L Shell: 8 electrons

L Shell: 8 electrons

K Shell: 2 electrons

Silicon Atom:

14 protons 14 neutrons 14 electrons

Photovoltaic (PV) cells are made of a special class of materials called semiconductors. Of all the semiconductor materials, silicon is most commonly used because of its availability (it's the second-most abundant element in Earth's crust) and its special chemical properties.

An atom of silicon has fourteen electrons arranged in three different levels, or shells. The first two shells, those closest to the center, are completely full. The outer shell, with four electrons, is only half full. A silicon atom will always look for ways to fill up its last shell (which would like to have eight electrons). To do this, it will share electrons with four of its neighboring silicon atoms. It's like every atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that arrangement turns out to be important to the function of a PV cell.

photovoltaic effect

Making a Better Carrier

Energy added to pure silicon can cause a few electrons to break free of their bonds and leave their atoms, leaving a "hole" (an unfilled bond) behind. These "free carrier" electrons wander randomly around the crystalline lattice structure, eventually falling into another hole. But there are so few free carriers available in pure silicon that they aren't very useful. Scientists found they could improve silicon's electron carrier ability (conductivity) by adding other atoms in a process know as "doping."

Silicon doped with an atom of phosphorous here and there (maybe one for every million silicon atoms), will still bond with its silicon neighbor atoms. But phosphorous, which has five electrons in its outer shell, has one electron that doesn't have anyone to hold hands with, so it takes a lot less energy to knock it loose. As a result, most of these electrons do break free, resulting in more free carriers. Phosphorous-doped silicon is called N-type ("n" for "negative") because of the prevalence of free electrons.

But only one part of our solar cell can be N-type. The other part is typically doped with boron, which has three electrons in its outer shell. Instead of having free electrons, P-type ("p" for "positive") has free holes.

The interesting part starts when you put N-type silicon next to P-type silicon—a silicon sandwich of sorts. When the electrons and holes mix at the junction between N-type and P-type silicon, silicon's neutrality is disrupted and the free electrons mix to form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and an electric field separates the two sides. The electric field allows (and even pushes) electrons to flow from the P side to the N side, but not the other way

Polycrystalline Wafer
Polycrystalline wafers: uncoated (left) and with the telltale blue antireflective coating (right).

around. A P-N junction is commonly known as a diode—an electrical one-way valve for electricity. The special thing about PV cells is that they are diodes designed to absorb energy from sunlight.

When a photon—the electromagnetic energy of sunlight— with enough energy hits the N-layer, it knocks an electron free. These electrons stay in the N-layer. When a photon of light hits an atom in the P-layer, it knocks an electron free that can easily cross into the N-layer. The result is that extra electrons accumulate in the N-layer. A series of metal

P/N Silicon and the Function of a PV Cell

Sunlight:

Energy (photons) knocks electrons loose to move throughout crystal structure

Extra Electrons -

N-Layer:

Phosphorus doped; extra electrons create negative charge

P/N Barrier:

Electrically neutral; allows electrons to move from P-layer to N-layer, but not back

P-Layer:

Boron doped; deficient electrons create positive charge

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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