What is PERC Cell?
PERC (Passivated Emitter and Rear Cell) Cell, full name "Emitter and Rear Passivated Cell", is a natural derivative of the conventional aluminum back field cell (BSF) structure.
Conventional BSF cells have inherent limitations in terms of photoelectric conversion efficiency because the recombination velocity in the metal aluminum film layer on the back surface cannot be reduced to below 200cm/s, resulting in only 60%-70% of the infrared radiation reaching the aluminum back layer being reflected, resulting in more photoelectric losses; while solar panels made of PERC cells have an additional layer on the back of traditional solar cells. This additional layer can capture more sunlight and convert it into electrical energy, making PERC cells more efficient than traditional cells. PERC templates can also alleviate back recombination and prevent longer wavelength heat from becoming heat that will harm cell performance.
PERC cells add two processes, back passivation and laser start-up, to the traditional BSF cells to improve conversion efficiency, and the performance is significantly improved.
Development history of PERC cells
The development history of PERC cells can be divided into four stages: technology prototype, budding, high-speed growth, and explosive period.
Advantages and disadvantages of PERC cells
Advantages: PERC cells are the most mainstream technology route on the market, with high industrialization efficiency and low production cost. It improves the light absorption and electron collection efficiency on the back of the cell by introducing a silicon oxide film on the back of the cell.
Disadvantages: The theoretical efficiency limit of PERC cells is relatively low (24.5%), and there is a problem of light-induced degradation, especially in multicrystalline PERC cells.
Technology maturity: PERC technology is already very mature, but with the upgrade and iteration of p-type to n-type technology, PERC technology is facing a reduction in market share.
Cost-effectiveness: PERC cells have cost advantages, and their costs are close to those of conventional cells, but due to limited room for efficiency improvement, they may face the risk of asset impairment and elimination in the future.
Market demand: It used to be the main force of market shipments, but with the iteration of technology, PERC cells are gradually being replaced by new N-type cell technologies, such as TOPCon.
PERC cell process
1.Texturing
Purpose of texturing: To form a velvet surface, using the principle of light trapping to reduce light reflection, increase short-circuit current (Isc), increase PN junction area, and ultimately improve the photoelectric conversion efficiency of the cell. Remove the mechanical damage layer on the surface of the silicon wafer; remove surface oil and metal impurities; (polycrystalline forms a pit-shaped velvet surface), single crystal forms a pyramid-shaped velvet surface, and increases silicon's absorption of sunlight. Texturing reaction formula Si+2NaOH+H2O →Na2SiO3 +2H2↑.
Single crystal texturing materials: potassium hydroxide, hydrogen peroxide, hydrochloric acid, additives, hydrofluoric acid.
Process control points of texturing process: ① Thinning amount (measuring instrument electronic balance); ② Reflectivity (measuring instrument D8 reflectivity tester); ③ Surface observation (measuring instrument SEM).
2.Diffusion
Purpose of diffusion: To form a PN junction. The formation principle of p-n junction is that in one area of the crystal, P-type impurities are dominant, while in another area, N-type impurities are dominant. At the junction, holes will diffuse from the P-type area to the N-type area, and free electrons will diffuse from the N-type area to the P-type area. The result of mutual diffusion is the formation of a space charge region with the electric field direction pointing from N-type to P-type, which prevents the diffusion of majority carriers and promotes the drift of minority carriers. When the two reach a balance, the width of the space charge region stabilizes. Under the condition of light, electron-hole pairs will be generated inside the cell. The electron-hole pairs will move to both sides under the action of the electric field, and connect to the external circuit to form a current.
Diffusion method: The diffusion process is to add phosphorus to a single side of the P-type silicon wafer to form a PN junction.
Diffusion materials: phosphorus oxychloride, oxygen, nitrogen.
Diffusion chemical equation: 4POCl3+3O2 → 2P2O5+6Cl2↑; 2P2O5+5Si → 5SiO2+4P↓ Diffusion process process control point: square resistance (measuring instrument square resistance tester).
Diffusion materials: phosphorus oxychloride, oxygen, nitrogen.
3.Etching
Etching principle: Use a mixed liquid of HNO3 and HF to corrode the lower surface and edge of the silicon wafer after diffusion, remove the N-type silicon at the edge, and insulate the upper and lower surfaces of the silicon wafer from each other.
Purpose of etching: During the diffusion process, phosphorus will diffuse to the edge of the silicon wafer, causing a short circuit, and form a layer of silicon dioxide containing P atoms, namely PSG, on the surface of the silicon wafer. The purpose of etching is to remove the PN junction at the edge and remove PSG. In the PERC battery process, the back needs to be polished while etching.
Edge etching principle reaction equation: Si + HNO3+HF → H2 [SiF6] + NOx + H2O
Process control points of the etching process: ① Etching thinning amount (measuring instrument electronic balance); ② Edge leakage detection (measuring instrument edge leakage tester, test current value, <0.05A).
Etching materials: potassium hydroxide, hydrochloric acid, hydrofluoric acid, nitric acid.
4.Thermal maintenance
After long-term operation, the components of thermal oxidation will produce a kind of attenuation, called PID (Potential Induced Degradation), which makes the performance of the components lower than the design standard. In order to achieve the purpose of anti-PID, a layer of silicon dioxide needs to be grown on the surface of the silicon wafer after etching. At present, our company adopts thermal oxidation process, that is, oxygen is passed through the furnace tube at high temperature to oxidize the surface.
Thermal oxidation reaction equation: Si + O2→ SiO2
Thermal oxidation process control point: hydrophilicity (3~5S diffusion area is twice the original).
Thermal oxidation materials: nitrogen, oxygen.
5.Back passivation
Back passivation: Al2O3+SiNx is plated on the back surface; AlOx film is mainly used for back passivation to improve Voc, Isc; SiNx film is mainly used to protect the AlOx film to prevent the back aluminum paste from penetrating the back passivation layer to destroy the passivation effect.
Back passivation chemical formula: Al(CH3)3+N2O→Al2O3+N2+CH4+C+H2 SiH4+NH3→SiNx+H2
Purpose of back passivation: reduce the recombination rate of the back surface, improve the response of the back surface battery in the long-wave band, and repair the back surface state.
Back passivation materials: trimethyl aluminum, laughing gas, argon, ammonia, silane.
6.PECVD
PECVD: It uses microwaves or radio frequencies to ionize the gas containing the atoms that make up the film, and forms plasma locally. The plasma is very chemically active and easily reacts to deposit the desired film (SiNx:H) on the substrate.
PECVD function: form an anti-reflection film on the surface of the battery to improve the absorption of sunlight; perform surface passivation and body passivation; prevent metal ions, water, etc. from corroding the battery; improve the Uoc of the battery; reflect the long-wave light back to the silicon wafer body for reabsorption, and improve Isc.
PECVD process control points: reflectivity, reflectivity (measuring instrument ellipse deviation).
7.Back laser
Back laser aluminum oxide and silicon nitride are non-conductive materials. Before printing the back electric field, a part of aluminum oxide and silicon nitride needs to be removed to achieve electrical contact between the back field and the silicon substrate. The laser groove process is a physical process, which uses a laser beam to directly bombard the back passivation layer according to a certain pattern.
8.Screen printing
Screen printing: Print electrodes and electric fields on the surface of silicon wafers, and print back electrodes, back electric fields, and positive electrodes in sequence;
Screen printing materials: positive electrode screen, back electrode screen, back electric field screen, front silver paste, back silver paste, back aluminum paste, scraper.
9.Sintering
The purpose of sintering: to make the electrode and silicon wafer form a good ohmic contact.
The structure of the sintering furnace: loading area-drying area-sintering area-cooling area-unloading area.
Light attenuation: the phenomenon of reduced efficiency caused by light in the early stage of solar cell use.
Electric injection method: The principle of anti-light decay is to control the valence and distribution of hydrogen in solar cells by electrical means to passivate defects and impurities. It is a hydrogen passivation mechanism introduced by current.
AOI (Automatic Optical Inspection), that is, automatic optical inspection. It is mainly used for color sorting and appearance inspection of solar cells. AOI inspection is divided into two parts: optical part and image processing part. The image to be inspected is obtained through the optical part; the image processing part is used to analyze, process and judge.
The basic principle of EL testing is to pass a 1-40mA forward current into The Solar Cell, and the electric energy excites the atoms in the ground state on both sides of the action and diffusion. The atoms in the excited state are unstable and spontaneously radiate to produce a near-infrared band spectrum (900-1700nm). The intrinsic transition in spontaneous radiation is understood through the effect of the filter and the exposure degree of the film. Through the relationship between the minority carrier lifetime, density and light intensity, the exposure degree of the film is used to judge whether there are defects in the silicon wafer.
IV test tests the conversion efficiency of the solar cell under simulated sunlight. The test conditions are AM1.5, the test temperature is between 23℃-27℃, the test light intensity is between 950W/m2-1050 W/m2, and the test machine needs to be calibrated with a standard sheet before use.
Single-sided and double-sided P-PERC battery process production process
PERC cell layout
In 2024, the share of N-type batteries represented by TOPCon will fully exceed P-type PERC, and the industry expects the share to reach 70%.
According to research, at SNEC in 2024, more than 53 companies launched TOPCon products, 7 companies launched BC battery products, and P-type was hard to find. It can be seen that in just one year, the survival space of P-type components has been continuously compressed, and N-type has become the well-deserved protagonist, and its development speed is astonishing.
The life cycle of PERC batteries is entering a countdown. At the same time, how to properly dispose of the large-scale old PERC production capacity has become an important issue facing photovoltaic manufacturers.
For example, leading enterprises Tongwei and Aixu announced that they have or plan to transform PERC battery production lines into N-type TOPCon, while Junda shares chose to prioritize asset impairment.
According to data from the China Photovoltaic Industry Association, in 2023, the average conversion efficiency of p-type PERC multicrystalline black silicon cells reached 21.4%, an increase of 0.3 percentage points from 2022; the average conversion efficiency of p-type PERC ingot monocrystalline cells reached 22.7%, an increase of 0.2 percentage points from 2022. In 2023, p-type monocrystalline cells all adopted PERC technology, with an average conversion efficiency of 23.4%, an increase of 0.2 percentage points from 2022.
In 2023, the newly put into production mass production lines are mainly n-type cell production lines. As the production capacity of n-type cells is gradually released, the market share of PERC cells has been compressed to 73.0%.
In 2023, as the size of mainstream PERC cells increases, 9 main grid and above technologies will become the new market mainstream, of which 9BB technology accounts for about 12.9% of the market, 10BB technology accounts for about 38.4% of the market, and 11BB and above account for about 48.7%.
Trends in market share of various main grid technologies for PERC cells from 2023 to 2030.