Basics of photovoltaic cells

Principles and classification of photovoltaic cells

①Principle of photovoltaic cell
Photovoltaic cell is a kind of semiconductor device based on the photovoltaic effect of semiconductor materials, which has the function of directly converting sunlight energy into electrical energy output. Most photovoltaic cells are usually PN junction photovoltaic cells composed of P-type semiconductors and N-type semiconductors. Through the diffusion process, an N-type area is formed on the P-type silicon wafer, and a PN junction is formed at the junction of the P-area and the N-area. Due to the mutual diffusion of electrons and holes near the PN junction area, a built-in electric field directed from the N area to the P area is formed in the PN junction area. If sunlight shines on the photovoltaic cell and is absorbed by the photovoltaic cell, photons with sufficient energy excite electrons from covalent bonds in P-type silicon and N-type silicon, generating electron-hole pairs. Before the electrons and holes near the PN junction recombine, they are separated from each other by the built-in electric field, so that the electrons move to the positively charged N region, and the holes move to the negatively charged P region. Through the charge separation of the PN junction, an externally testable voltage is generated between the P area and the N area.
As shown in Figure 1, under the action of the built-in electric field of the PN junction, the holes in the N area move to the P area, and the electrons in the P area move to the N area, resulting in a large number of electrons accumulating on the light-receiving surface of the photovoltaic cell, and in the backlight There is a large accumulation of cavities on the surface. If the upper and lower surfaces of the photovoltaic cell are connected to metal electrodes, and the load is connected with wires, as long as there is sunlight, current will flow through the load. When there is no sunlight on the photovoltaic cell, its electrical characteristics appear as diode characteristics.

Figure 1 Schematic diagram of crystalline silicon photovoltaic cell principle

For single crystalline silicon photovoltaic cells, the typical value of the open circuit voltage is 0.5-0.6V. The output current is 20~25mA. And the output current increases with the increase of light intensity. However, because the output voltage of the photovoltaic cell is very low Therefore, it is necessary to connect photovoltaic cells in series to obtain high voltage, and then parallel to obtain high current. At the same time, the crystalline silicon photovoltaic cell itself is relatively brittle. Can not independently resist the harsh conditions of the outside world. Therefore, external packaging is required. Leading out the external electrodes becomes a photovoltaic cell assembly device that can independently provide direct current output, that is, photovoltaic cell components. Its output power ranges from a few tenths of a watt to hundreds of watts. Several photovoltaic cell module technologies need to be connected in series and wells to form a photovoltaic cell array.

②Classification of photovoltaic cells
The most important part of a photovoltaic cell is the layer of semiconducting material, which is the part used to generate current. At present, there are many materials that can be used as the semiconductor layer of photovoltaic cells, but there are not many photovoltaic materials that can produce high energy conversion efficiency. According to the materials used, photovoltaic cells can be divided into silicon photovoltaic cells, multi-compound photovoltaic cells and organic semiconductor photovoltaic cells. Among them, silicon photovoltaic cells mainly include monocrystalline silicon, polycrystalline silicon and amorphous silicon photovoltaic cells: multi-compound compound photovoltaic cells mainly include cadmium sulfide (CdS) and cadmium telluride (CdTe) photovoltaic cells, and gallium arsenide (GaAs) photovoltaic cells , Copper indium selenium (CuInSe2) photovoltaic cells, etc.; organic semiconductor photovoltaic cells mainly include dye-sensitized photovoltaic cells and organic thin-film photovoltaic cells.