Solar energy utilization and solar cell

What is diffusion?

admin on September 28, 2022 0 Comments

1.Diffusion principle analysis

The P-N junction is the core process in the solar cell preparation process. The cell is able to generate electricity because of the photoelectric effect produced by the P-N junction, and the purpose of diffusion is to produce the P-N junction. Nowadays, industrial production is generally done by diffusing phosphorus on P-type semiconductors to form N-type semiconductors, thus forming P-N junctions. Industry generally uses nitrogen gas carrying liquid POCI for high-temperature diffusion in tubes. The main use of POCL liquid source diffusion, is to pass nitrogen through the liquid source containing diffusion POCI, so as to carry POC. vapor into the diffusion furnace at high temperatures, impurities POC, vapor decomposition at high temperatures, and the formation of saturated vapor pressure, and then pass an excess of oxygen, the resulting phosphorus atoms through the high temperature in the surface of the silicon wafer to the internal diffusion. Its characteristics are mainly simple equipment, relatively easy to operate, better uniformity, and suitable for industrial mass production.

At present, industrial production is commonly used boron-doped P-type silicon wafers as a substrate, through the surface diffusion of phosphorus to obtain N-type heavily doped layer, the formation of P-N junction at the intersection, the use of liquid phosphorus trichloroxide as a source of phosphorus,nitrogen as a carrier gas to bring POCI, diffusion furnace quartz tube, the chemical reaction occurs as follows

When POC13 is thermally decomposed, the decomposition of POC13 is not sufficient without the participation of O2, and the products are PCI5 and P2O5.PCI5 is not easily decomposed in the tube and has a certain corrosive effect on the silicon wafer, and destroys the surface state of the wafer. But in the case of sufficient O2, PCI5 will further decompose into P2O5 and C12, and the generated P2O5 will further react with the silicon wafer to produce SiO2 and phosphorus atoms, and form a layer of phosphorus-containing SiO2on the surface of the silicon wafer, i.e., phosphorus-silica glass, and then carbon atoms will diffuse to the surface of the silicon wafer at high temperature.

2.Industrial grade diffusion process

Diffusion in industry is mainly divided into processes such as warming, oxidation, diffusion, deep diffusion and cooling, and diffusion equipment is shown in the figure.

Diffusion equipment

The general process of diffusion is shown in the picture

 The general process of diffusion

The following are the specific steps:

1.Before the quartz boat into the tube to be purged with nitrogen, the main role of purging is to remove the impurity gas in the furnace tube to prevent pollution of the diffusion process

2. put the boat: general diffusion in and out of the boat are relatively slow, mainly because the sudden cold and heat of the silicon wafer will destroy the hardness of the silicon wafer fragile.

3.warming: furnace door open, the furnace tube temperature loss leads to large temperature fluctuations, so generally give a certain amount of time for the furnace tube to warm up, constant temperature, this time will also check the gas tightness in the tube to ensure that the diffusion reaction is carried out properly.

4.Oxidation: Before diffusion, a sufficient amount of oxygen is introduced into the tube to oxidize the surface of the wafer, so that the wafer can be more uniform when the real diffusion takes place, and the tube is filled with sufficient oxygen to facilitate the next step of diffusion.

5.Diffusion: Pass nitrogen into the tube with phosphorus trichloride, and pass a sufficient amount of oxygen, the product of high-temperature decomposition of phosphorus trichloride in the oxygen-rich state further reacts with silicon, generating singlet phosphorus and allowing phosphorus atoms to enter the silicon wafer, forming a P-N junction.

6.Deep diffusion: The main function of this step is to further advance the phosphorus atoms, allowing them to diffuse into the silicon wafer to form a P-N junction, and then allowing the toxic waste gas to be reacted and discharged.

7.Cooling: After the diffusion is completed, a large amount of helium gas is introduced to cool down the process.

8.Boat pickup: After the cooling process is completed, the wafers are automatically and slowly discharged from the furnace.

In the diffusion, phosphorus trichloride gas containing a phosphorus source is first passed into the quartz tube at 810°C to form an atomic atmosphere filled with phosphorus around the high-temperature wafer, allowing phosphorus atoms to diffuse internally through the polysilicon surface for about 10 min, with a diffusion depth of about several hundred nanometers; further high-temperature treatment is then carried out so that the phosphorus atoms pre-deposited on the surface continue to diffuse deeper into the wafer, forming an n+/n layer, such that The structure is beneficial to the subsequent electrode preparation, because the n+ layer not only can form an ohmic contact with the metal electrode and reduce the contact resistance to obtain a good filling factor, but also can prevent the metal atoms from diffusing into the interior of the substrate during the electrode preparation, which can reduce the surface compound loss brought by the electrode. The control points of the diffusion process are as follows.

①The center value of silicon wafer square resistance is within 80~85Ω. Square resistance is the square resistance, also known as surface resistance, is a square of thin film conductive material edge-to-edge resistance, square resistance is only related to the thickness of the conductive film and other factors, where the square resistance reflects the phosphorus concentration and junction depth of the cell after diffusion, experimental studies show that the size of the square resistance is inversely proportional to the diffusion temperature, the higher the temperature the more intense the reaction, the greater the surface phosphorus concentration will be, so the lower the square resistance. In general, the smaller the square resistance indicates that the deeper the junction formed by phosphorus diffusion, such as junction is too deep, the dead layer is more obvious, the less sub-life is lower, which in turn affects the photoelectric conversion efficiency, the smaller the dead layer of the shallow junction, the higher the life of the minority carriers, it will increase the probability of photogenerated carriers being collected, the battery short-wave correspondingly better: but the shallow junction will also cause an increase in series resistance: the actual junction depth in industrial production is 0.3~0.5µm is better.

② Tube unevenness <5%, sheet unevenness <8%. In the sampling test of diffusion should not only look at the size of the central party resistance, but also look at the uniformity of the whole piece and the whole tube, expressed in terms of unevenness, the formula is as follows.

Calculation formula

Each wafer will be tested at 9 points, namely: top left, top right, top right, left edge, center value, right edge, bottom left, bottom right and bottom left, all measured at a distance of 1cm from the edge of the wafer, and the uniformity of diffusion of the whole wafer and the whole tube will be tested by this method.

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