How Silicone Makes a Solar Cell
Silicon has special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells -- which hold two and eight electrons respectively -- are completely full. The outer shell, however, is only half full with just four electrons. A silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.
Pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about, unlike the electrons in more optimum conductors like copper. To overcome this, the silicon in a solar cell has impurities added – which are other atoms mixed in with the silicon atoms. We usually think of impurities as something undesirable, but in this case, our cell wouldn't work without them.
If we add an atom of phosphorous (with five electrons) to the silicon which has four leftover electrons; the phosphorous will bond to the silicon atoms leaving one electron that doesn't form part of a bond. This left over electron is held in place by a positive proton in the phosphorous nucleus.When energy is added to pure silicon, in the form of heat it causes a few electrons to break free and leave their atoms. These electrons are called free carriers, they wander randomly around the crystalline lattice.
This process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon.The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type ("p" for positive) has free openings and carries the opposite (positive) charge.
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