What is a TOPCon cell? This article will give you a comprehensive understanding!
What is a TOPCon cell
TOPCON (Tunnel Oxide Passivating Contact) is called tunnel oxide passivation contact in Chinese. It is an advanced N-type silicon wafer cell technology originated in 2013. TOPCon cells are tunnel oxide passivation contact solar cells with N-type silicon as the substrate.
Compared with PERC cells, TOPCon cells use tunnel oxide (tunnel oxide) with excellent charge transfer characteristics as the charge transfer layer on the back of the cell, and then deposit a layer of doped polycrystalline silicon thin layer of about 20nm to form a back passivation contact structure, which can effectively reduce surface recombination and metal contact recombination, increase the opening voltage, and improve the energy conversion efficiency.
TOPCon is a Solar Cell Technology with tunnel oxide passivation contact based on the principle of selective carriers, which achieves a better passivation effect.
The TOPCon cell structure is an N-type silicon substrate cell. An ultra-thin layer of silicon oxide is prepared on the back of the cell, and then a thin layer of doped silicon is deposited. The two together form a passivation contact structure, which effectively reduces surface recombination and metal contact recombination, and provides greater space for further improvement of the conversion efficiency of N-PERT cells.
TOPCon cells retain and utilize the existing traditional P-type cell equipment process to the greatest extent. Only boron expansion and thin film deposition equipment are required, and there is no need for back opening and alignment, which greatly simplifies the cell production process and has low difficulty in mass production. The process equipment production line is highly compatible and can be compatible with the high-temperature preparation process production line of PERC and N-PERT bifacial cells. TOPCon cells have the advantages of low attenuation, high bifaciality, and low temperature coefficient, and have obvious power generation gain effects in terminal power stations.
Development stages of TOPCon cells
The development history of TOPCon cells can be divided into four stages: technology prototype stage, product layout stage, commercial promotion stage, and outbreak stage.
Advantages of TOPCon cells
From the performance level, TOPCon has the following advantages:
1.High conversion efficiency. Thanks to the unique passivation contact design of TOPCon cells, its conversion efficiency limit is as high as 28.7%, and the current mass production efficiency of cells of leading TOPCon companies has reached more than 25.5%, which is significantly higher than the mainstream PERC cells (current mass production conversion efficiency is 23.5%, theoretical limit is 24.5%).
2.High bifaciality. The single-watt power generation of TOPCon bifacial cells is about 3% higher than that of bifacial PERC cells. In the same ground power station application scenario, it will produce a higher power generation gain.
3.Low temperature coefficient. The temperature coefficient of N-type Topcon Modules is as low as -0.30%/℃, which is better than -0.35%/℃ of P-type modules, and the power generation performance shows excellent stability in high temperature environments.
4.Low attenuation. The boron content in phosphorus-doped N-type crystalline silicon is extremely low, and there is basically no boron-oxygen compound, and the attenuation rate has an advantage. The first-year attenuation of the TOPCon module of Aikang is 1%, and the linear annual attenuation is 0.4%, which is a significant improvement compared to the first-year 2% and linear 0.45% of the PERC module, which can bring a gain in power generation of a single-watt module throughout the life cycle.
5.Advantages in weak light performance.The TOPCon cell takes into account the responsiveness to both short waves and length, and its power generation performance is still excellent under weak light such as morning and evening and cloudy days.
From an economic perspective, TOPCon has the following advantages:
1.It is highly compatible with the PERC cell manufacturing process, and the difficulty of technology upgrade is reduced. TOPCon can be extended on the PERC process technology, and only four processes are required, including the preparation of the boron emitter, the growth of the tunnel oxide layer, the deposition of polysilicon and doping, and the cleaning after diffusion, which reduces the difficulty of technology upgrade and accelerates the promotion of TOPCon technology.
2.Smooth conversion of production lines and low equipment investment costs. The equipment investment for a new TOPCon line requires 200-250 million yuan, and the investment for a new HJT line is 350-400 million yuan. TOPCon has good equipment compatibility with existing PERC production lines. It only needs to add LPCVD/PECVD/PVD equipment for boron diffusion and polysilicon/amorphous silicon deposition. The equipment investment is 50-70 million yuan, which avoids large-scale investment in new equipment and large-scale transformation of production lines, so it is more economical.
3.TOPCon products have a large premium space. Compared with PERC components, TOPCon modules have higher single-watt power generation, higher power generation gain and lower system cost, which brings a large premium space.
TOPCon cell process
Compared with the single-crystal PERC process, the TOPConcell production process has 2~3 more steps, namely, depositing a tunneling oxide layer (ultra-thin SiO2, 1~2nm), depositing an intrinsic polysilicon passivation layer (60~100nm), and phosphorus injection.
TOPCon main processes and functions
1.Cleaning and texturing
Purpose: After the silicon wafer is cut, its edge is damaged, the lattice structure of silicon is destroyed, and the surface is seriously compounded. The main purpose of cleaning and texturing is to remove surface damage and form a surface pyramid light trapping structure. The light is irradiated on the surface of the silicon wafer through multiple refractions to achieve the purpose of reducing reflectivity.
2.Boron diffusion process
①Purpose: The main function is to prepare PN junctions. Since the solid solubility of boron in silicon is low, high temperature and longer time are required for diffusion. At the same time, the choice of diffusion source will also have an impact on the production process. Chlorides are highly corrosive, bromides are highly viscous, and the cleaning process is cumbersome, increasing operation and maintenance costs.
Boron diffusion is usually completed at a higher temperature - exceeding 1000℃, and the cycle time of boron diffusion is 150min compared to the 102min cycle required for phosphorus diffusion.
②Principle
The gaseous HCl and H2O generated by the reaction in the furnace tube will be evenly distributed in the furnace tube under the carrier of N2. H2O will also react with BBr3 and O2 to generate B2O3 and react to generate gaseous HBO2. HBO2 will also decompose at high temperature to generate B2O3, which can achieve uniform distribution of B2O3 on the surface of solar cells. On the other hand, H2O will also react with B2O3 deposited in the furnace tube, thus avoiding the deposition of B2O3 on the wall of the diffusion furnace tube, extending the service life of quartz devices, and increasing the effective boron source. HCl can also react with metal impurities on the surface of solar cells and in the furnace tube to generate gaseous metal chlorides, which are discharged with the tail gas, and can avoid metal impurities from diffusing into the interior of solar cells during high temperature.3.
3.SE laser doping process
①Purpose: To form a selective emitter, high concentration doping is mainly performed at the contact point between the metal grid line and the silicon wafer and its vicinity to reduce the contact resistance between the front metal electrode and the silicon wafer; low concentration doping is performed in the area outside the electrode to reduce the recombination of the diffusion layer. By optimizing the emitter, the output current and voltage of The Solar Cell are increased, thereby increasing the photoelectric conversion efficiency.
②The process where the laser is located in the TOPCon process
PERC SE is doped with phosphorus, while TOPCon SE is doped with boron. Due to the different separation coefficients of boron and phosphorus, phosphorus is more likely to diffuse from silicon dioxide to silicon, while boron is more likely to diffuse from silicon to silicon dioxide. More energy is required to promote doping, and excessive laser energy is easy to cause damage to the silicon wafer, so it is more difficult to dope boron into silicon. Compared with traditional boron diffusion, TOPCon cell superimposed SE technology can theoretically achieve an efficiency improvement of 0.5%, and in actual mass production, it can achieve an efficiency improvement of 0.2~0.4%.
4.Etching process
Purpose: The main function of etching is to remove BSG and back junction. The diffusion process will form a diffusion layer on the surface and periphery of the silicon wafer. The peripheral diffusion layer is prone to short circuit, and the surface diffusion layer affects the subsequent passivation, so it needs to be removed. At present, etching mainly adopts wet method, first removing the back and peripheral diffusion layers in the chain equipment, and then processing the front.
5.Preparation of tunnel oxide layer and polysilicon layer
Purpose: Deposit a 1-2nm tunnel oxide layer on the back, and then deposit a 60-100nm polysilicon layer to form a passivation structure. There are many ways to prepare the TOPCon passivation layer, mainly divided into LPCVD, PECVD, PVD routes, etc. LPCVD is currently the main method, but the plating is serious, and the comprehensive performance of PECVD has strong potential.
6.Preparation of back anti-reflection film
Purpose: Prepare an anti-reflection passivation film on the back of thecell to increase the absorption of light. At the same time, the hydrogen atoms generated during the formation of the SiNx film have a passivation effect on the silicon wafer.
7.Front aluminum oxide plating
Purpose: Deposit an aluminum oxide film layer on the front of the silicon wafer, and form a front passivation effect together with other film layers.
8.Preparation of front anti-reflection film
Purpose: The front anti-reflection film has basically the same effect as the back. In addition, the aluminum oxide film deposited on the front is very thin and is easily damaged in the subsequent production ofcell components. The front SiNx also has a protective effect on aluminum oxide.
9.Screen printing-laser transfer
Currently, screen printing is still mostly used in thecell printing process. In the future, laser pattern transfer printing technology may have more advantages in terms of silver paste consumption of N-type batteries. Laser transfer printing is a new type of non-contact printing technology. This technology is to coat the required paste on a specific flexible light-transmitting material, and use a high-power laser beam to scan the paste at high speed to transfer the paste from the flexible light-transmitting material to thecell surface to form a grid line and prepare the front and rear electrodes.
10.Sintering
Good ohmic contact is formed through high-temperature sintering.
11.Automatic sorting
Grade cells with different conversion efficiencies.
According to statistics, among thecell projects invested and expanded in 2023, there are 80 TOPConcell projects with a total capacity of 787.2GW, accounting for 41%; 51 HJTcell projects with a scale of 302.2GW, accounting for 16%; 6 BCcell projects represented by Longi Green Energy and Aixu, with a total scale of 139.5GW, accounting for 7%; 13 perovskitecell projects with a capacity of 30.31GW, accounting for 1%. Although the remaining projects have not clarified the technical route, most of them are N-type technology.
From the technical category of the signed projects, in 2023, there are 38 TOPConcell projects with a total scale of 431GW, accounting for 33%; 27 HJTcell projects with a total capacity of 179.2GW, accounting for 14%; the scale ofcell projects using BC technology is 110.5GW, accounting for 9%; the scale of perovskitecell projects is 21.31GW, accounting for 2%; the remaining projects have not clarified the technical route.
In 2023, the average conversion efficiency of n-type TOPCon cells reached 25.0%, and the average conversion efficiency of heterojunction cells reached 25.2%, both of which were significantly improved compared with 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%. The total share of n-type cells reached about 26.5%, of which n-type TOPCon cells accounted for about 23.0% of the market, heterojunction cells accounted for about 2.6%, and XBC cells accounted for about 0.9%, all of which were significantly improved compared with 2022.
According to data released by InfoLink Consulting in August, the total shipments of the five majorcell cell suppliers, Zhongrun Solar, Jietai Technology, Tongwei Co., Ltd., Aixu Co., Ltd. and Yingfa Ruineng, were 77.96GW in the first half of 2024, of which PERCcell cells had a cumulative shipment of about 37.3GW; TOPConcell cells accelerated their overtaking, with a cumulative shipment of about 40.6GW, accounting for nearly 52% of the total shipments.
Trends in market share of differentcell technology routes from 2023 to 2030.