1.Formation of pn junction:
Silicon is a semiconductor material with four valence electrons. When an element with five valence electrons is doped into silicon as an impurity, an N-type semiconductor is formed. Doped impurities have one more valence electron than silicon, and this electron can play the role of transmitting current in silicon. This kind of impurity is called donor impurity, and it is called majority carrier because of the large number of electrons. Such as phosphorus or arsenic. When an element with three valence electrons is doped into silicon as an impurity, a P-type semiconductor is formed, which is called acceptor impurity. For example, when gallium replaces silicon or germanium atoms in the crystal structure, one bond lacks an electron. This position lacking electrons is called a hole. Adjacent electrons will fill this hole and leave new holes elsewhere, and adjacent electrons will fill new holes, and so on. In this way, the movement of electrons is equivalent to the free movement of holes. While the whole remains electrically neutral.
2 PN junction luminescence produces photovoltaic effect
Light is composed of photons of different energies. When light is incident on the surface of PN junction, photons with short wavelength and high energy produce electron-hole pairs in Dingcheng of pn junction, while photons with long wavelength and low energy produce electron-hole pairs in P region. If the generated electron-hole pair has a long enough life, it will spread to the vicinity of the space charge region and be separated by the built-in electric shovel. Due to the principle of opposites attraction in electricity, the electrons in P region, a photogenerated minority carrier, enter N region, and the holes in N region, a photogenerated unbalanced minority carrier, enter P region. If the front and rear electrodes of the solar cell are open, photogenerated electrons and photogenerated holes separated by the built-in electric field accumulate on both sides of the space charge region, respectively, forming a photoelectric field. This electric field is opposite to the original built-in electric field of pn junction. The photogenerated electric field breaks the balance of P-N junction and produces photogenerated voltage, which is the photovoltaic effect of photoelectric conversion.
3. Application of 3.pn junction in solar cells
Semiconductor material P/N structure solar cell takes the P-type side as the positive electrode and the N-type side as the negative electrode. When sunlight irradiates the P/N junction, electrons in the P-type region absorb photon energy and reach the N-type region across the contact potential, forming a photoelectric potential. When the load is connected with the positive and negative electrodes, electrons will flow to the P-type region through the load and output electric energy to the load. As long as there is light, solar cells can output electric energy.
There are many kinds of semiconductor materials, and there can also be many kinds of solar cells with P/N structure. At present, solar cells based on monocrystalline silicon are widely used. Under a certain light intensity, the open circuit voltage is 0.5 to 0.6 volts. Its photoelectric conversion efficiency has been improved from below 10% at the beginning (1954) to 15% in 1970s, and now it has reached 20%. The way to improve the efficiency is to enhance the absorption of sunlight and improve the preparation process of P/N junction. Solar cells can be combined in series and parallel like other chemical cells to meet high-power applications.
This is part of my thesis, I hope it will help you.