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The working principle of solar cells
|The working principle of solar cells is based on the photovoltaic effect of semiconductor PN junctions. The so-called photovoltaic effect is an effect of electromotive force and current when the object is illuminated, the distribution of charge in the object changes. When sunlight or other light illuminates a semiconductor PN junction, a voltage, called photovoltaic voltage, appears on both sides of the PN junction.
When light irradiates on the P n junction, an electron-hole pair is generated. The carriers generated near the p-N junction in the semiconductor are not recombined and reach the space charge region. Attracted by the internal electric field, electrons flow into the n region, and holes flow into the P region. As a result, excess electrons are stored in the n region, and there are excess holes in the P region. They form a photoelectric field near the p-n junction opposite to the direction of the barrier. In addition to partially offsetting the barrier electric field, the photovoltaic field also makes the p region charged positively and the N region charged negatively. The thin layer between the N region and the P region produces the electromotive force, which is called the photovoltaic effect.
When energy is added to pure silicon (in the form of heat, for example), it causes several electrons to break away from their covalent bonds and atoms. Every time an electron leaves, it leaves a hole. These electrons then wander around the lattice, looking for another hole to settle in. These electrons are called free carriers, and they carry currents. When pure silicon is mixed with phosphorus atoms, only a small amount of energy is needed to escape some "redundant" electrons of phosphorus atoms (the outermost five electrons). When phosphorus atoms are doped, the silicon obtained is called N-type ("n" means negative electricity). Only part of the solar cell is N-type. Another part of silicon is doped with boron. The outer electron layer of boron has only three electrons instead of four, so P-type silicon can be obtained. There are no free electrons in P-type silicon