There are a few more components left before we can really use our cell. With both current and voltage, we have power, which is the product of the two. The electron flow provides the current, and the cell's electric field causes a voltage. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to the P side to unite with holes that the electric field sent there, doing work for us along the way. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. When light, in the form of photons, hits our solar cell, its energy breaks apart electron-hole pairs. It's like a hill - electrons can easily go down the hill (to the N side), but can't climb it (to the P side). This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. Eventually, equilibrium is reached, and we have an electric field separating the two sides. However, right at the junction, they do mix and form something of a barrier, making it harder and harder for electrons on the N side to cross over to the P side. If they did, then the whole arrangement wouldn't be very useful. Do all the free electrons fill all the free holes? No. Suddenly, the free electrons on the N side see all the openings on the P side, and there's a mad rush to fill them. That's because without an electric field, the cell wouldn't work the field forms when the N-type and P-type silicon come into contact. Next we'll take a closer look at what happens when these two substances start to interact.īefore now, our two separate pieces of silicon were electrically neutral the interesting part begins when you put them together. Instead of having free electrons, P-type ("p" for positive) has free openings and carries the opposite (positive) charge. 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. N-type doped silicon is a much better conductor than pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorus, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. It takes a lot less energy to knock loose one of our "extra" phosphorus electrons because they aren't tied up in a bond with any neighboring atoms. However, there are so few of them in pure silicon, that they aren't very useful.īut our impure silicon with phosphorus atoms mixed in is a different story. These electrons, called free carriers, then wander randomly around the crystalline lattice looking for another hole to fall into and carrying an electrical current. When energy is added to pure silicon, in the form of heat for example, it can cause a few electrons to break free of their bonds and leave their atoms. It doesn't form part of a bond, but there is a positive proton in the phosphorus nucleus holding it in place. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorus has one electron that doesn't have anyone to hold hands with. Phosphorus has five electrons in its outer shell, not four. Consider silicon with an atom of phosphorus here and there, maybe one for every million silicon atoms. We usually think of impurities as something undesirable, but in this case, our cell wouldn't work without them. To address this issue, the silicon in a solar cell has impurities - other atoms purposefully mixed in with the silicon atoms - which changes the way things work a bit. The only problem is that 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. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell. It's like each atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. 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. The outer shell, however, is only half full with just four electrons. The first two shells - which hold two and eight electrons respectively - are completely full. An atom of silicon has 14 electrons, arranged in three different shells. Silicon has some special chemical properties, especially in its crystalline form.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |