|Composition and related materials of solar cells|
Solar power generation devices, commonly known as solar cells, can directly convert solar energy into electricity. In solar panels, photons released by the sun release electrons in the outer layer of semiconductor materials from the bondage of atomic bonds. When the electrons are forced to move in the same direction, they can generate electric current to supply power to electronic equipment or power grid.
Since 1839, photovoltaic power generation technology has been one of the research topics in France. At present, with the major research teams in the United States, Japan and Europe accelerating the industrialization of their respective solar systems, the international market of photovoltaic industry continues to expand. Although the composition of photovoltaic power generation system materials are different, but all modules include a number of layers of materials from the light surface to the backlight surface. Sunlight first passes through the protective layer (usually glass) and then enters the battery through the transparent contact layer. In the center of the module is the adsorption material, which absorbs photons and completes the "photo generated current". The semiconductor materials depend on the specific requirements of photovoltaic system.
Below the adsorption layer material is a back metal layer for circuit conduction. The composite film layer under the back metal layer is used to make the photovoltaic module waterproof and heat insulation. Usually, an additional protective layer is added to the back of the PV module. The protective layer is made of glass, aluminum alloy or plastic.
The semiconductor materials in photovoltaic power generation system can be silicon, polycrystalline film or single crystal film. Silicon materials include monocrystalline silicon, polycrystalline silicon and amorphous silicon. Monocrystalline silicon has regular structure, and its photoelectric conversion rate is higher than that of polycrystalline silicon.
The silicon atoms in amorphous silicon are randomly distributed, and its photoelectric conversion rate is also lower than that of monocrystalline silicon. However, compared with crystalline silicon, it can capture more photons. At the same time, alloying amorphous silicon with germanium or carbon can enhance this characteristic. Copper indium selenide (CIS), cadmium telluride (CdTe) and thin film silicon are commonly used polycrystalline thin film materials, while materials with high photoelectric conversion rate, such as gallium arsenide (GaAs), usually contain monocrystalline silicon thin films. These materials are used in specific photovoltaic power generation field because of their unique properties. These characteristics include crystallinity, band gap size, absorption properties and processing difficulty. Influence of external factors on semiconductor
The order of atoms in the crystal structure determines the crystallinity of semiconductor materials, while the charge transfer, current density and energy conversion efficiency of solar cells are affected by the crystallinity. The band gap of semiconductor materials is a small amount of energy required to make the electron transition from the bound state to the free state. The band gap size is usually expressed as eg, which describes the energy difference between the valence band and the conduction band. The valence band of semiconductor materials is low energy level, and the conduction band is high energy level. The absorption coefficient is used to characterize the distance of a specific wavelength of photons penetrating the medium, which determines the ability of photons to be absorbed by the medium. The absorption coefficient is determined by the cell material and the wavelength of the absorbed photon.
The cost and processing difficulty of various semiconductor materials and devices depend on many factors, including the type and scale of materials, the production cycle and the migration characteristics of batteries in the deposition chamber. In the specific demand of photovoltaic power generation, each factor will play an important role.
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