How Does This Stuff Work?
Our basic understanding of photovoltaic cells is that they harness thermal energy from the sun and magically convert it into usable power for our homes. Yet, it all seems like such a mystery. What exactly are photovoltaic cells and how do they work?
Photovoltaic cells are made of two silicon semi-conductor layers (one that is negatively charged and one that is positively charged). When light strikes the cell, some of it gets absorbed within the semiconductive material. This energy allows electrons to move more freely. These loose electrons flow in a current to metal contacts on the top and bottom of the cell, where they can flow through a circuit and create electricity.
This is going to be a throwback to high school chemistry, so bear with me, please. As you recall, atoms are full of concentric circles of electrons. The innermost shell can only hold two electrons before it is filled. The second shell can hold 8, as can the third, and so on. Atoms that hold eight electrons in their outermost shells have what is called “a stable octet,” meaning that the atom is stable and not likely to react with other atoms surrounding it. On the other hand, outermost rings that have less than 8 electrons tend to react with nearby atoms to form more stable compounds. So when we look at the silicon used in the manufacture of photovoltaic cells, we find a single atom of silicon possesses 14 electrons, arranged in an 2-8-4 pattern. A silicon atom is an opportunity-seeker, always looking to pick up four more electrons to complete its outermost circuit. It likes to share electrons with four nearby atoms, joining in a crystalline structure. This structure is very important, as the light shining on these structures causes the electrons to move about, thus creating the energy for the photovoltaic cells.
The bad news is that crystalline silicon, in its purest form, is a poor conductor of electricity because the atoms tend to move around, find their stable balance with four conjoining atoms and stay put. So manufacturers alter the silicon with impurities mixed in with the silicon atoms (for example, throwing a bit of phosphorous here and there). Since phosphorous isn’t locked up in a bond with its neighboring atoms, more electrons are able to break free and carry energy.
An electric field forms when negative and positive silicon interacts. The electric field acts as a diode that pushes electrons along a current. The cell’s electric field creates a voltage. Current and voltage combined yields power. See how this is all coming together? Since the silicon photovoltaic cells are so shiny, they tend to reflect photons away before they can be absorbed, so a special anti-reflective coating must be added to the material to create usable photovoltaic cells. To protect the cell from the elements, a glass plate is usually put over the top of it.
When light strikes our photovoltaic cells, it’s separated into all different wavelengths and energies. Some light photons have enough energy to alter electron movement. Other weaker light photons will just pass through the cell as if it weren’t even there. Some photons have too much energy to have the desired outcome. Only a select amount of light photons have the ideal amount of energy to power our photovoltaic cells, so some energy loss is inevitable.
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