A silicon atom has 14 electrons, which are distributed on three electron layers. The two electron layers inside have been filled. Only the outermost layer lacks four electrons, which is half full, as shown in Figure 1. In order to achieve a stable structure in the full-electron layer, each silicon atom can only combine with its four adjacent atoms to form a shared electron pair. From a plane view, it looks like all the atoms are holding hands, staggered and connected to form its unique crystal structure. , Each electron is fixed in a specific position and cannot move freely like free electrons in good conductors such as copper. Therefore, this also determines that silicon is not a good conductor of electricity. The silicon that is actually used in solar cells is specially processed, that is, a doping process is adopted.
The main structure of a general semiconductor is shown in Figure 2. In Figure 2, the positive charge represents the silicon atom, and the negative charge represents the four electrons surrounding the silicon atom. When other impurities, such as boron, phosphorous, etc. are added to the silicon crystal, when boron is added, there will be a hole in the silicon crystal. At this time, the semiconductor is called a P-type semiconductor, as shown in Figure 3.
In Figure 3, the positive charge represents the silicon atom, and the negative charge represents the four electrons surrounding the silicon atom. The light gray indicates the doped boron atom. Because there are only 3 electrons around the boron atom, it will form a hole state while forming a covalent bond with the silicon atom. As long as the energy is small, it will accept an electron from a nearby atom. , The hole state is transferred to the nearby covalent bond. This is the hole. It has a positive charge. The hole and the free electron do the same irregular movement, so the black color shown in Figure 3 will be produced. Hole, this hole becomes very unstable because there are no electrons, and it is easy to absorb electrons and neutralize.
When 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 adjacent silicon atoms, and the remaining one electron cannot be formed. Covalent bond, but still bound by the impurity center, but much weaker than the constraint of covalent bond, as long as a small amount of energy will get rid of the bondage, so an electron will become very active. At this time, the semiconductor is called N-type semiconductor, as shown in Figure 4. The light gray ones are phosphorous atoms, and the black ones are extra electrons.
When the P-type and N-type semiconductors formed by silicon doping are combined, a special thin layer is formed in the area of the interface between the two semiconductors. The P-type side of the interface is negatively charged, and the N-type side is positive. Electricity. This is because P-type semiconductors have many holes and N-type semiconductors have many free electrons, and there is a difference in concentration. The electrons in the N area will diffuse to the P area, and the holes in the P area will diffuse to the N area. Once diffused, an “internal electric field” from N to P is formed, thereby preventing the diffusion from proceeding. After reaching the equilibrium, such 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, and the electrons in the P-type region move to the N-type region, thereby forming a transition from the N-type region to the P-type region. Current. Then a potential difference is formed in the PN junction, which constitutes a power supply, as shown in Figure 6.
After all, the current and voltage that a solar cell can provide is limited, so many solar cells (usually 36) are used in parallel or series to form a solar cell module, which can generate a certain voltage and current and output a certain power. There are currently more than a dozen types of semiconductor materials for manufacturing solar cells, so there are many types of solar cells. At present, the solar cell with the most mature technology and commercial value is the silicon solar cell.