Interpretation of photovoltaic cell process flow route

Interpretation of photovoltaic cell process flow route. 1 article amazing makes it clear!

Photovoltaic cells are generally divided into monocrystalline silicon, polycrystalline silicon, and amorphous silicon. Monocrystalline silicon solar cells are the fastest-developed solar cells at present. Its structure and production process have been finalized, and the products have been widely used in space and on the ground. This type of solar cell uses high-purity monocrystalline silicon rods as raw materials.

Photovoltaic cell process flow:

Texturing (INTEX) → Diffusion (DIFF) → Post-cleaning (edging/PSG removal) → Anti-reflection film coating (PECVD) → Screen, sintering (PRINTER) → Testing, sorting (TESTER + SORTER) → Packaging (PACKING)

Texturing

The purpose of texturing is to form a velvet surface on the surface of the silicon wafer to reduce the reflectivity of the photovoltaic cell. The uneven velvet surface can increase secondary reflection and change the optical path and incident mode.

Usually, alkali treatment of single crystals can obtain a pyramid-shaped velvet surface; acid treatment of polycrystalline can obtain a wormhole-shaped irregular velvet surface. The difference in treatment methods is mainly in the properties of single and polycrystalline.

Process flow: Texturing tank → water washing → alkali washing → water washing → acid washing → water washing → blow drying.

Under normal circumstances, silicon is considered unreactive with HF and HNO3 (the silicon surface will be passivated). The reaction between silicon and the mixed solution is continuous when present in a system of two mixed acids. Silicon is the most common semiconductor material.

Diffusion

Diffusion is the heart of photovoltaic cells and the P-N junction of photovoltaic cells. POCl3 is currently the most popular choice for phosphorus diffusion.

POCl3 is a liquid phosphorus source. Liquid phosphorus source diffusion has the advantages of high production efficiency, good stability, uniform and flat PN junction, and good diffusion layer surface.

POCl3 decomposes to generate phosphorus pentachloride (PCl5) and phosphorus pentoxide (P2O5) at a temperature greater than 600°C. PCl5 has a corrosive effect on the surface of silicon wafers. When oxygen O2 is present, PCl5 will decompose into P2O5 and release chlorine, so a certain flow of oxygen is introduced while diffusing nitrogen.

P2O5 reacts with silicon at the diffusion temperature to generate silicon dioxide and phosphorus atoms. The generated P2O5 is deposited on the surface of the silicon wafer and continues to react with silicon to generate SiO2 and phosphorus atoms, and forms phosphorus-silicon glass (PSG) on the surface of the silicon wafer. The phosphorus atoms diffuse into the silicon to produce an N-type semiconductor.

The efficiency of a photovoltaic cell depends on the quality of the semiconductor materials used
The efficiency of a photovoltaic cell depends on the quality of the semiconductor materials used

Etching

In the diffusion process, a back-to-back single-sided diffusion method is adopted, and phosphorus atoms will inevitably diffuse on the side and back edges of the silicon wafer.

When the sun shines, the photogenerated electrons collected on the front of the P-N junction will flow along the edge of the phosphorus diffusion area to the back of the P-N junction, causing a short circuit path. The short circuit path is equivalent to reducing the parallel resistance.

The etching process is to remove the phosphorus part of the edge of the silicon wafer, avoiding the short circuit of the P-N junction and causing the parallel resistance to be reduced.

Wet etching process flow: wafer loading → etching tank (H2SO4 HNO3 HF) → water washing → alkaline tank (KOH) → water washing → HF tank → water washing → wafer loading

HNO3 reacts and oxidizes to generate SiO2, and HF removes SiO2. The purpose of the etching alkaline tank is to polish the unused surface and make the photovoltaic cell smooth; the main solution of the alkaline tank is KOH; H2SO4 is to make the silicon wafer float and flow on the assembly line, and does not participate in the reaction.

Dry etching is to use plasma to etch thin films.

When the gas exists in the form of plasma, on the one hand, the chemical activity of the gas in the plasma will become relatively strong. Choosing the right gas can make the silicon wafer react faster and achieve etching; on the other hand, the electric field can be used to guide and accelerate the plasma, so that the plasma has a certain energy. When bombarding the surface of the silicon wafer, the atoms of the silicon wafer material are knocked out, and the purpose of etching can be achieved by physical energy transfer. spectral properties of photovoltaic cells also affect energy conversion.

PECVD

Plasma chemical vapor deposition. The reflection loss rate of sunlight on the silicon surface is as high as about 35%.
Anti-reflection film can improve the absorption of sunlight by photovoltaic cells, which helps to increase photocurrent and thus improve conversion efficiency: on the other hand, the passivation of the surface of the cell by hydrogen in the film reduces the surface recombination rate of the emitter junction, reduces dark current, increases open circuit voltage, and improves photoelectric conversion efficiency.

H can react with defects or impurities in silicon, thereby transferring the energy band in the forbidden band into the valence band or conduction band.

In a vacuum environment and at a temperature of 480 degrees Celsius, a layer of SixNy film is coated on the surface of the silicon wafer by conducting the graphite boat.

Screen printing

In layman’s terms, it is to collect current and make electrodes for solar cells. The first is the back silver electrode, the second is the printing and drying of the back aluminum backfield; the third is the printing of the front silver electrode, mainly monitoring the wet weight after printing and the width of the secondary grid line.

If the wet weight of the second pass is too large, it will not only waste the slurry, but also may not be fully dried before entering the high-temperature zone, and even all organic matter in it cannot be driven out so that the entire aluminum paste layer cannot be converted into metallic aluminum. In addition, too much-wet weight may cause the photovoltaic cell to bow after sintering.

If the wet weight is too small, all the aluminum paste will be consumed in the subsequent sintering process to form a molten area with silicon, and the alloy area is not suitable for back metal contact in terms of lateral conductivity or weldability. In addition, there may be poor appearance such as bulging.

If the width of the third grid line is too large, the photovoltaic cell will have a smaller light-receiving area and reduced efficiency.

Printing method: physical printing, drying

Sintering

Sintering is to sinter the electrodes printed on the surface of the photovoltaic cell at high temperature, so that the electrodes and the silicon wafer itself form an ohmic contact, improve the open circuit voltage and fill factor of the photovoltaic cell, and make the contact of the electrode have resistance characteristics to achieve high conversion efficiency. The sintering process can also facilitate the diffusion of -H introduced by the PECVD process into the body, which can play a good body passivation role.

Sintering method: high-temperature rapid sintering, heating method: infrared heating

Sintering is a process that combines diffusion, flow, and physical and chemical reactions. The front Ag diffuses into silicon through SiNH but cannot reach the P-N surface. The back Ag and Al diffuse into silicon. Since a certain temperature is required to form an alloy, and the stability of Ag, Al, and Si forming alloys is different, different temperatures need to be set to achieve alloying respectively.

The performance of a photovoltaic cell can be affected by temperature fluctuations and shading
The performance of a photovoltaic cell can be affected by temperature fluctuations and shading

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