PECVD (Plasma Enhanced Chemical Vapor Deposition) plasma improves chemical vapor deposition. The plasma causes an intensification of the thermal movement of the material molecules. Collisions with each other cause the gas molecules to ionize, and the material moves freely and forms. positive interactions. A mixture of ions, electrons and neutral particles.
According to calculations, the rate of loss of light by reflection from the silicon surface reaches approximately 35%. The anti-reflective film can greatly improve the utilization rate of sunlight by cells and help increase the density of photogenerated current, thereby improving the conversion efficiency. At the same time, passivation of the cell surface by hydrogen in the film reduces the surface recombination rate of the emitter junction, reduces the dark current, and increases the open circuit voltage. , and improves the efficiency of thea photoelectric conversion; In the combustion process, high-temperature flash annealing breaks some Si-H and N-H bonds, and the free H further strengthens the passivation of the battery.
Since photovoltaic-grade silicon materials inevitably contain a large number of impurities and defects, the lifetime and diffusion time of minority carriers in silicon are reduced, resulting in a decrease in battery conversion efficiency H. interact with defects or impurities in the silicon. A reaction is carried out to transfer the energy band from the bandgap to the valence band or conduction band.
1. PECVD Principle
The PECVD system is a series of generators that use parallel plate coating boats and high-frequency plasma exciters. Under low pressure and high temperature conditions, the plasma generatoris directly installed in the middle of the cladding plate to react. The active gases used are silane SiH4 and ammonia NH3. These gases act on the silicon nitride stored on the silicon wafer. Different refractive indices can be obtained by changing the silane/ammonia ratio. During the deposition process, a large number of hydrogen atoms and hydrogen ions are produced, which makes the hydrogen passivation of the wafer very good.
Under vacuum and at an ambient temperature of 480 degrees Celsius, the surface of the silicon wafer is coated with a layer of SixNy by conducting electricity on the graphite boat.
3SiH4+4NH3 → Si3N4+12H2
2. Si3N4
The color of the Si3N4 film changes with its thickness. Generally, the ideal thickness is between 75. -80 nm, it appears dark blue. The refractive index of Si3N4 film is between 2.0 and 2.5. Alcohol is generally used to measure itsrefractive index.
Excellent surface passivation effect, effective optical anti-reflection performance (adapting the refractive index of the thickness), low temperature process (effective cost reduction), and the generated H ions passivate the surface of the silicon wafer.
3. Common questions in coating shops
Film thickness. The thickness of the film varies depending on the deposition time. The deposition time should be increased or decreased appropriately depending on the color of the coating. If the film turns white, the deposition time must be reduced. If the film is reddish, the deposition time must be reduced. increased appropriately.
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