PECVD graphite boat photovoltaic sintering coating / Solar cell (coating) PECVD graphite boat principle

First of all, you need to know that PECVD (Plasma Enhanced Chemical Vapor Deposition) plasma enhances chemical vapor deposition. Plasma is the intensification of thermal motion of material molecules. The collision with each other will cause ionization of gas molecules, and the material will become free-moving and formed by positive interactions. A mixture of ions, electrons and neutral particles.

According to calculations, the reflection loss rate of light on the silicon surface is as high as about 35%. The anti-reflective film can greatly improve the utilization rate of sunlight by the cell, help to increase the photogenerated current density, and thereby improve the conversion efficiency. At the same time, the hydrogen in the film The passivation of the cell surface 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 N-H key, 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 length of minority carriers in silicon are reduced, which leads to a decrease in the conversion efficiency of the battery. H can react with defects or impurities in silicon, thus reducing the bandgap. The energy band in is transferred to the valence band or conduction band.

PECVD principle

The PECVD system is a series of generators utilizing graphite boats and high-frequency plasma exciters. Under the conditions of low pressure and elevated temperature, the plasma generator is directly installed in the middle of the coating 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 indexes can be obtained by changing the ratio of silane to ammonia. 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. In a 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

Si3N4

The color of the Si3N4 film changes with its thickness. Generally, the ideal thickness is between 75 and 80nm, which appears as dark blue. The refractive index of the Si3N4 film is best between 2.0 and 2.5. It is usually measured with alcohol. 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.

Common matters in coating workshops

Film thickness. The film thickness is also different depending on the deposition time. The deposition time should be appropriately increased or decreased according to the color of the coating. If the film is white, the deposition time should be reduced. If the film is reddish, the deposition time should be increased appropriately. Each boat of films must be fully confirmed, and defective products are not allowed to flow into the next process, such as color spots and poor watermark coating. The most common surface whitishness, color difference, and white spots in the production line should be picked out in a timely manner. Among them, surface whitishness Mainly due to the thick silicon nitride film, which can be adjusted by adjusting the film deposition time; chromatic aberration film is mainly caused by air path blockage, quartz tube leakage, microwave failure, etc.; white spots are mainly caused by small black spots in the front line. 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 it is also an important indicator that reflects the efficiency of a company's solar cells. The PECVD process is generally busy. Each batch of cells needs to be monitored, and there are many coating furnace tubes, and each tube is generally Several hundred pieces (depending on the equipment), after changing the process parameters, the verification cycle will be longer. Coating technology is a technology that the entire photovoltaic industry attaches great importance to. The efficiency improvement of solar cells can be achieved through the improvement of coating technology. Scientists in the field of solar cells are also eager to do this. In the future, solar cell surface technology may become a breakthrough in the theoretical efficiency of solar cells.

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