First of all, we need to know PECVD (Plasma Enhanced Chemical Vapor Deposition). Plasma is the intensification of the thermal motion of material molecules. The collision between them will cause the gas molecules to be ionized, and the material will become a mixture of freely moving positive ions, electrons and neutral particles that interact with each other.
It is estimated that the reflection loss rate of light on the silicon surface is as high as about 35%. The anti-reflection film can greatly improve the utilization rate of solar light by the battery cell, which helps to increase the photogenerated current density and thus improve the conversion efficiency. At the same time, the hydrogen in the film passivates the surface of the battery cell, reduces the surface recombination rate of the emitter junction, reduces the dark current, increases the open circuit voltage, and improves the photoelectric conversion efficiency. The high-temperature instantaneous annealing in the burn-through process breaks some Si-H and NH bonds, and the freed H further strengthens the passivation of the battery.
Since photovoltaic-grade silicon materials inevitably contain a large amount of impurities and defects, the minority carrier lifetime and diffusion length in silicon are reduced, resulting in a decrease in the conversion efficiency of the battery. H can react with defects or impurities in silicon, thereby transferring the energy band in the bandgap into the valence band or conduction band.
1. PECVD Principle
The PECVD system is a series of generators using PECVD graphite boat and high-frequency plasma exciters. The plasma generator is directly installed in the middle of the coating plate to react under low pressure and elevated temperature. 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 ratio of silane to ammonia. During the deposition process, a large amount of hydrogen atoms and hydrogen ions are generated, making the hydrogen passivation of the wafer very good. In a vacuum and an ambient temperature of 480 degrees Celsius, a layer of SixNy is coated on the surface of the silicon wafer by conducting the PECVD graphite boat.
3SiH4+4NH3 → Si3N4+12H2
2. Si3N4
The color of Si3N4 film changes with its thickness. Generally, the ideal thickness is between 75 and 80 nm, which appears dark blue. The refractive index of Si3N4 film is best between 2.0 and 2.5. Alcohol is usually used to measure its refractive index.
Excellent surface passivation effect, efficient optical anti-reflection performance (thickness refractive index matching), low temperature process (effectively reducing costs), and the generated H ions passivate the silicon wafer surface.
3. Common matters in coating workshop
Film thickness :
The deposition time is different for different film thicknesses. The deposition time should be appropriately increased or decreased according to the color of the coating. If the film is whitish, the deposition time should be reduced. If it is reddish, it should be appropriately increased. Each boat of films should be fully confirmed, and defective products are not allowed to flow into the next process. For example, if the coating is poor, such as color spots and watermarks, the most common surface whitening, color difference, and white spots on the production line should be picked out in time. The surface whitening is mainly caused by the thick silicon nitride film, which can be adjusted by adjusting the film deposition time; the color difference film is mainly caused by gas path blockage, quartz tube leakage, microwave failure, etc.; white spots are mainly caused by small black spots in the previous process. Monitoring of reflectivity, refractive index, etc., safety of special gases, etc.
White spots on the surface :
PECVD is a relatively important process in solar cells and an important indicator of the efficiency of a company's solar cells. The PECVD process is generally busy, and each batch of cells needs to be monitored. There are many coating furnace tubes, and each tube generally has hundreds of cells (depending on the equipment). After changing the process parameters, the verification cycle is long. Coating technology is a technology that the entire photovoltaic industry attaches great importance to. The efficiency of solar cells can be improved by improving coating technology . In the future, solar cell surface technology may become a breakthrough in the theoretical efficiency of solar cells.
Post time: Dec-23-2024