1.1 Photovoltaic effect
“Photovoltaic effect” is abbreviated as “photovoltaic effect.” Photovoltaic effect refers to the phenomenon that light produces a potential difference between different parts of a non-uniform semiconductor or a combination of semiconductor and metal. It is firstly converted from photons (light waves) to electrons and light energy conversion It is the process of electric energy; secondly, it is the process of forming voltage. With voltage, it is like building a high dam. If the two are connected, a circuit of current will be formed.
If light shines on the solar cell and the light is absorbed in the interface layer, photons with sufficient energy can excite electrons from covalent bonds in P-type silicon and N-type silicon, resulting in electron-hole pairs. The photo-generated holes generated in the P-type region and the photo-generated electrons generated in the N-type region are multiple electrons, which are blocked by the barrier and cannot pass through the PN junction. Only when the photogenerated electrons in the P-type region, the photogenerated holes in the N-type region and the electron-hole pairs (minority carriers) in the PN junction region diffuse to the vicinity of the junction electric field can they drift through the junction under the action of the built-in electric field. The photo-generated electrons are drawn to the N-type region, and the photo-generated holes are drawn to the P-type region, that is, the electron-hole pairs are separated by the built-in electric field. This results in the accumulation of photogenerated electrons near the boundary of the N-type region and the accumulation of photogenerated holes near the boundary of the P-type region. They generate a light-generated electric field opposite to the built-in electric field of the thermally balanced PN junction, and its direction is from the P-type area to the N-type area. This electric field lowers the potential barrier, and the amount of reduction is the photo-generated potential difference. The P terminal is positive and the N terminal is negative. As a result, an outward testable voltage is generated, and the junction current flows from the P-type area to the N-type area, and its direction is opposite to the photocurrent. The more electron-hole pairs produced by light on the interface layer, the greater the current. The more light energy the interface layer absorbs, the larger the interface layer, that is, the larger the cell area, and the greater the current formed in the solar cell. This is the photovoltaic effect (Figure 1-1).
Figure 1-1 Photovoltaic effect
(1) The speed of light has an important experiment in modern physics, that is, the Michelson Morley experiment was done in 1887 using the Michelson interferometer. Experiments show that in all inertial systems, the speed of light propagation in all directions in vacuum is the same, that is, they are equal to 2.998×108m/s.
(2) The nature of light In the 1860s, the British physicist Maxwell established a systematic electromagnetic field theory-Max’s equations, and predicted the existence of electromagnetic waves. After that, Hertz experimentally proved the correctness of Max’s electromagnetic field theory. Theory and practice have further proved that light is electromagnetic waves. However, the electromagnetic wave theory cannot explain the experimental rules of phenomena such as the photoelectric effect. In 1905, Einstein thought: A beam of light is a stream of particles moving at the speed of light, and these particles are called photons. Of course, many subsequent experiments have shown that light is still a particle. It can be seen that light has wave-particle duality.
(3) Planck’s constant and Einstein’s theory that photons are not only the same as ordinary particles of matter, but also have a certain amount of energy. Its energy ε is proportional to its frequency υ, namely:
In the formula, h is the Planck constant, and its value is: 6.62×10-34W.
Einstein not only thought that light is a particle, but also pointed out that in order for electrons to break away from the surface of a metal, they must overcome gravity and do a certain amount of work. For different metals, when electrons leave its surface, the work done is different. . The photon with energy hυ, when it hits the metal surface, transfers its energy to the electrons in the metal, and part of the energy is used as the work of extraction required when leaving the metal surface; the other part of the energy is converted into the metal surface. The kinetic energy of electrons, that is:
According to Einstein’s equation, it can be seen that for the same metal, the number of p is certain, and if the frequency of the irradiated light is less than its lowest frequency υ0 (also called the cut-off frequency), that is, hυ≤p, it will not happen. Photoelectric effect. If a metal is to have a photoelectric effect, the energy of the irradiated photon must be greater than the work of the metal. Therefore, the frequency must be greater than a certain value υ0. It can also be seen from Einstein’s formula that the greater the frequency of the irradiated light, the greater the energy of the photon, and the greater the velocity and kinetic energy of the electrons released from the metal, regardless of the intensity of the light. The greater the intensity of light, it only means that there are more photons, so the stronger the irradiated light, the more electrons that absorb photons and release from the metal surface, and the stronger the photocurrent. It can be seen that the size of the electricity generated by the solar cell or whether it can be generated is not only related to the frequency, but also related to the intensity of the light.