Solar Energy: How do photovoltaics work?

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How does energy from the sun get transformed into electricity? Learn how photovoltaic panels work, converting light into electrical energy.

Big Ideas: 
  • How does one form of energy get converted into another?
  • How do we harness energy from the sun to use?

Solar energy is the fastest growing among the renewable energy sources being used today. Advances in the field of photovoltaics continues to improve the efficiency and lower the cost of solar panels. But how exactly do solar cells work?

The photovoltaic effect, or the use of light to directly create voltage, is how solar cells create electricity. In a solar cell, this effect results from a combination of a P-N junction and something called the photoelectric effect. Solar cells are made out of semiconductors, materials which have their properties easily changed by mixing in small amounts of relatively positive and negative atoms. The "P" in the P-N junction refers to a slightly positively charged (or p-doped) part of semiconductor. As you can probably guess, the "N" refers to a slightly negatively charge part of the semiconductor. The interface between these two parts is called a P-N junction. At this P-N junction the electric field initially naturally points from the P side to the N side due to the charge difference but the negative electrons are quickly attracted from the N side to the P side and likewise with the positive holes from the P side going to the N side (the positive charges are called holes because the positive charge is caused by a lack of electrons, not the prescence of positive charges). This causes a sort of inversion at the junction where the N side of the junction is positively charged and the P side is negatively charged so that the electric field points from the N side to the P side. When the photoelectric effect plays its part in this set up we get the photovoltaic effect, as illustrated below.1

The photoelectric effect is when light hits an electron an excites it enough to make it jump out of it's normal orbit around a nucleus of an atom and go free. This can result in either a free electron (negative charge) or a free hole (positive charge). When an electron is freed from the P side it gets pulled over to the N side by the electric field in the junction. However, the N side already has a surplus of negative charges, making the electron want to go back to the P side from which it came. Unfortunately for the electron, the electric field in the junction prevents it from going back the way it came. However, by electrically connecting the back sides of the P and N doped regions (the sides not at the junction) the electron can go back to where it came from. This desire to go back creates a voltage and current which is the photovoltaic effect. The same happens when a hole is excited and jumps from the N side to the P side. In the other two cases, a hole excited on the P side and an electron excited on the N side, nothing happens because the electric field keeps the electrons in place creating no imbalance from which energy can be derived.



Article thumbnail photo taken by David Shankbone, CC BY 3.0

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