Structure of crystalline silicon solar cell
Solar cells are semiconductor optoelectronic devices that directly convert solar radiant energy into electrical energy.
As early as 1939, the French physicist Alexander-Edmond Becquerel discovered the photovoltaic effect. The so-called photovoltaic effect means that when light irradiates a solid or liquid system with two electrodes, a voltage can be generated between the electrodes. Solar cells based on the photoelectric effect of crystalline silicon are called crystalline silicon solar cells. When the light quantum is absorbed by the semiconductor crystal silicon with PN junction, electron-hole pairs will be generated. When these electron-hole pairs reach the PN junction, they are separated to both sides of the PN junction by the junction electric field. When the external load is connected, a photocurrent is formed and electric energy is output.
In 1954, Daryl Chapin of Bell Labs and others developed a solar cell with a photoelectric conversion efficiency of 6% using the photovoltaic effect, and then the efficiency quickly increased to 10%, which was applied to man-made solar cells. It is used as a power source on the satellite and gradually expanded to ground applications. At present, the photoelectric conversion efficiency of crystalline silicon solar cells is 17% to 20%. The highest efficiency in the laboratory is 25.6%.
The earth has abundant silicon raw materials, stable crystal structure, mature silicon semiconductor device technology, and little impact on the environment, and it is hoped that the photoelectric efficiency will be further improved, thereby reducing production costs. At present, silicon-based solar cells still occupy the solar cell market with an absolute advantage, accounting for about 90% of the total solar cells of various forms.
The crystal structure of silicon
Silicon is the basic material of existing crystalline silicon solar cells. The abundance of silicon on the earth is 25.8%. Silicon belongs to group IVa of the third period of the periodic table. Its atomic number is 14, its atomic weight is 28.085, and its atomic valence is mainly 4-valence. In silicon crystals, atoms are bonded by covalent bonds and have the crystallographic characteristics of regular tetrahedrons. The chemical bonds in silicon crystals are covalent bonds, and each atom forms 4 equivalent covalent bonds with surrounding atoms.
- The crystal structure of silicon
The silicon unit cell is a cubic crystal system. The 8 vertices and 6 face centers of the silicon unit cell have atoms. In addition, there are 4 silicon atoms in the cube, each occupying 1/4 of the diagonal of the space from the corresponding vertex. - Surface and interface of crystalline silicon
From the point of view of electron distribution, the physical surface of silicon crystal is the surface based on the outermost atoms on the surface, extending 1.0~1.5nm into the vacuum and the body.
The silicon atoms on the crystal surface can only form covalent bonds with the surrounding 3 silicon atoms. Part of the excess covalent bonds will usually be saturated by the oxygen atoms in the SiO2 present on the silicon surface, and part of the unsaturated covalent bonds Form dangling bonds. These dangling bonds and surface defects plus foreign atoms adsorbed on the surface will form a surface state. Surface electronic states will form surface energy levels, and non-equilibrium carriers will recombine through these energy levels and reduce their lifetime.
The interface state of silicon is related to the dangling bonds, impurities and defects at the interface, and the interface state density of silicon crystal is related to the orientation of the crystal plane of the substrate, and they decrease in the order of (111)>(110)>(100). The interface state is the center of carrier generation and recombination.
The interface formed by the contact between silicon and metal, insulating medium (such as SiO2, SiN, etc.) and other semiconductors plays an important role in changing the performance of silicon solar cells.