
Photovoltaic cells are made of special materials called semiconductors such as silicon, , when light strikes the cell, a portion of it is absorbed. This means that the energy of the light goes to the semiconductor. This energy knocks electrons loose, allowing them to flow freely. PV cells have electric fields that force electrons freed by the absorbed light to flow in a specific direction. This flow i s a current which can be used externally. Pure silicon is a poor conductor of electricity because electrons are not free to move about. A solar cell has silicon with impurities . These impurities have a purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms , the phosphorous has one electron that is left alone . It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind . These electrons then move freely around the crystalline lattice looking for another hole . These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons as a result, most of these electrons do break free, and we have a lot more free carriers as compared to pure silicon. This process of adding impurities is called doping. This process is the base for the working of a photovoltaic cell which forms a solar panel.
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