A silicon atom has 14 electrons, which are distributed in 3 electron layers. The 2 electron layers inside are filled, and only the outermost layer lacks 4 electrons, which is half full, as shown in Figure 1. In order to achieve a stable structure with a full electron layer, each silicon atom can only combine with its four adjacent atoms to form a shared electron pair. From the plane, it looks like all the atoms are hand-in-hand and cross-connected to form its unique crystal. The structure fixes each electron in a specific position and cannot move as freely as free electrons in good conductors such as copper. Therefore, it also determines that silicon is not a good conductor of electricity. The silicon that is actually used in solar cells is specially treated, that is, a doping process is adopted.
The main structure diagram of the silicon semiconductor is shown in Figure 2. In Figure 2, the positive charge represents the silicon atom, and the negative charge represents the 4 electrons surrounding the silicon atom. When other impurities (such as boron, etc.) are doped into the silicon crystal, there will be a hole in the silicon crystal, and the semiconductor at this time is called a P-type semiconductor, as shown in Figure 3.
In Figure 3, positive charges represent silicon atoms, and negative charges represent 4 electrons surrounding the silicon atoms. The gray color represents the mixed atoms, because there are only 3 electrons around the atoms. When forming a covalent bond with the silicon atom, a hole state will be formed. As long as a small amount of energy is required, an electron will be accepted from the nearby atom. , transfer the empty state to the nearby covalent bond, which is the hole, with a positive charge, and the same random motion as the free electron, so the black hole as shown in Figure 3 will be generated. The hole becomes very unstable because it has no electrons, and it is easy to absorb electrons and neutralize it, forming a P-type semiconductor.
If an element with one more valence electron (such as phosphorus) is doped into silicon, only 4 of the 5 electrons in the outermost layer can form a shared electron pair with the adjacent silicon atom, and the remaining one cannot form a shared electron pair. The valence bond is still bound by the impurity center, but it is much weaker than the covalent bond. As long as a small energy is required, it will get rid of the binding, so one electron will become very active, and the semiconductor at this time is called N type semiconductor, as shown in Figure 4.
When a P-type semiconductor and an N-type semiconductor formed by doping silicon are combined, a special thin layer is formed in the interface area of the two semiconductors. The P-type side of the interface is negatively charged, and the N-type side is positively charged. Electricity. This is because the P-type semiconductor has many holes and the N-type semiconductor has many free electrons, resulting in a concentration difference. The electrons in the N region will diffuse to the P region, and the holes in the P region will diffuse into the N region. Once diffused, an “internal electric field” directed from the N to the P is formed, thereby preventing the diffusion. After reaching equilibrium, a special thin layer is formed, which is the PN junction, as shown in Figure 5.
When the doped silicon wafer is exposed to light, in the PN junction, the holes of the N-type semiconductor move to the P-type region, while the electrons in the P-type region move to the N-type region, thereby forming a current from the N-type region to the P-type region. . A potential difference is then created in the PN junction, which creates the power supply, as shown in Figure 6.
Because the surface of silicon is very bright, it reflects a lot of sunlight and cannot be used by solar cells. To this end, a protective film with a very small reflection coefficient is coated on the surface of the solar cell to reduce the reflection loss to 5% or even less. After all, the current and voltage that a photovoltaic cell can provide is limited, so many photovoltaic cells (usually 36) are used in parallel or in series to form a solar photovoltaic cell module, which can generate a certain voltage and current and output a certain power. There are more than a dozen semiconductor materials for making solar cells, so there are many types of solar cells. At present, the most mature and commercially valuable solar cells are silicon solar cells.
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