How They Work
Photovoltaic cells are made of semiconducting materials (usually silicon) with impurities added to certain regions to create either a surplus of electrons (n-type doping) or a scarcity of electrons (p-type doping, also called a surplus of holes). The extra electrons and holes carry electrical charges, allowing current to flow in the semiconducting material.
When a photon hits the top surface of a photovoltaic cell, it penetrates some distance into the semiconductor until it is absorbed. If the photon's energy is at least as large as the material's energy bandgap, the energy from the photon creates an electron-hole pair. Usually, the electron and the hole stay together and recombine. In the presence of an electric field, however, the negatively charged electron and the positively charged hole are pulled in opposite directions. This occurs for the same reason that one end of a magnet is attracted to another magnet while the other end is repelled.
Junctions in semiconductors create electrical fields. A junction can be formed at the border between p-and n-doped regions, or between different semiconducting materials (a heterojunction), or between a semiconductor and certain metals (forming a Schottky barrier).
The movement of the charges in the photovoltaic cell creates a voltage (electrical potential energy) between the top and bottom of the cell. Electrical contacts attached to the cell at the p and n sides (the top and bottom) complete the cell. Wires attached to these contacts make the voltage available to other devices.
The distance into the material that a photon goes before being absorbed depends on both how efficient the material is at absorbing light and the energy of the photon-high-energy photons penetrate further than low-energy photons. This is why x rays are used to image your bones, but most visible light stops at your skin.
Efficiency of a cell depends on the losses that occur at each stage of the photovoltaic process. Many of the sun's photons get absorbed or deflected in the atmosphere before reaching the earth's surface (this is described by a term called air mass). Some photons will reflect off or pass through the cell. Some electron-hole pairs recombine before carrying charges to the contacts on the ends of the cell. Some of the charges at the ends of the cells do not enter the contacts, and some energy is lost to resistance in the metal contacts and wires.
The efficiency of the cell can be increased by shining more light onto it using a concentrator (such as a focusing lens), by adding coatings (such as a mirror to the bottom of the cell to reflect unabsorbed light back into the cell), or by creating heterojunction cells with materials that have different bandgaps, and thus are efficient at absorbing a variety of wavelengths. One of the most efficient photovoltaic cells reported was two-junction cell made of gallium arsenide and gallium antimony, coupled with a concentrator that increased the intensity of the light 100 times: it worked with 33% efficiency in a laboratory. In practice, ground-based solar cells tend to have efficiencies in the teens or less.
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