The Complete Guide to N-Type TOPCon Technology

TOPCon cells are a type of PV crystalline silicon cells. In recent years, due to its obvious advantages such as high conversion efficiency, low attenuation performance, and high cost-effectiveness in mass production, it has been gradually adopted by companies in the industry. Namkoo solar panels with latest N-type Topcon are also popular among consumers.

What is N-type TOPCon solar cell?

The theoretical limit efficiency of TOPCon solar panels is as high as 28.7%, which is the technology closest to the theoretical limit efficiency of crystalline silicon solar cells (29.43%), much higher than PERC (24.5%), and has great R&D potential.

2023 is the year when TOPCon technology explodes, and the market share is expected to exceed 25%.

From a time perspective, TOPCon battery technology was first proposed by Germany's Fraunhofer Solar Energy Research Institute in 2014 as a new type of passivated contact solar cell.

From a structural point of view, TOPCon is a tunnel oxide passivated contact (Tunnel Oxide Passivated Contact) solar cell technology based on the principle of selective carriers. Its cell structure is an N-type silicon substrate cell, and a layer is prepared on the back of the cell. A layer of ultra-thin silicon oxide is deposited, and then a thin layer of doped silicon is deposited. The two together form a passivating contact structure, which effectively reduces surface recombination and metal contact recombination.

TOPCon battery technology

Compared with the single crystal PERC process, the TOPCon cell production process requires 2 to 3 more steps, which are the deposition of tunnel oxide layer (ultra-thin SiO2, 1~2nm) and the deposition of intrinsic polysilicon passivation layer (60~100nm). , phosphorus injection.

The main processes and functions of TOPCon

1. Cleaning and texturing

① Purpose

After the silicon wafer is cut, its edges are damaged, the silicon lattice structure 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 irradiates the surface of the silicon wafer through multiple refractions, achieving To reduce reflectivity.

2. Boron diffusion process

① Purpose

The main function is to prepare PN junction. Since boron has low solid solubility in silicon, it requires high temperature and longer time for diffusion. At the same time, the choice of diffusion source will also have an impact on the production process. Chloride is highly corrosive and bromide is highly viscous, making the cleaning process cumbersome and increasing operation and maintenance costs.

Boron diffusion is usually accomplished at higher temperatures - in excess of 1000°C - and has a cycle time of 150 minutes compared to the 102 minutes required for phosphorus diffusion.

② Principle

The gaseous HCl and H2O generated by the reaction in the furnace tube will be evenly distributed in the furnace tube under the carry of N2. H2O will also react with BBr3 and O2 to generate B2O3, which will react to generate gaseous HBO2. HBO2 will also decompose at high temperatures to generate B2O3. It 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 the quartz device, 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 exhaust gas, which can prevent metal impurities from diffusing into the interior of the solar cells during high temperature processes.

3. SE laser doping process

① Purpose

To form a selective emitter, high-concentration doping is mainly performed at and near the contact area between the metal gate line and the silicon wafer to reduce the contact resistance between the front metal electrode and the silicon wafer; while low-concentration doping in areas other than the electrode can reduce Composite of diffusion layer. By optimizing the emitter, the output current and voltage of the solar cell are increased, thereby increasing the photoelectric conversion efficiency.

② The laser process in the TOPCon process

PERC SE is phosphorus-doped, while TOPCon SE is boron-doped. Due to the different separation coefficients of boron and phosphorus, phosphorus is easier to diffuse from silicon dioxide to silicon, while boron is easier to diffuse from silicon to silicon dioxide, requiring a larger Only a certain amount of energy can be used to promote doping, and excessive laser energy can easily cause damage to the silicon wafer, so it is more difficult to dope boron into silicon.

Compared with traditional boron diffusion, TOPCon battery superimposed SE technology can theoretically achieve an efficiency increase of 0.5%, and in actual mass production, it can achieve an efficiency increase of 0.2~0.4%.

4. Etching process

① Purpose

The main function of etching is to remove the 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 can easily form a short circuit. The surface diffusion layer affects subsequent passivation and therefore needs to be removed. At present, etching mainly uses wet methods. The back and peripheral diffusion layers are first removed in chain equipment, and then the front is processed.

5. Preparation of tunnel oxide layer and polysilicon layer

① Purpose

A 1-2nm tunnel oxide layer is deposited on the back, followed by a 60-100nm polysilicon layer to form a passivation structure. There are many ways to prepare the TOPCon passivation layer, which are mainly divided into LPCVD, PECVD, PVD routes, etc. At present, LPCVD is the main method, but the plating is serious, and the comprehensive performance of PECVD has strong potential.

6. Preparation of back anti-reflection film

① Purpose

Preparing an anti-reflective passivation film layer on the back of the battery increases the absorption of light. At the same time, the hydrogen atoms generated during the formation of the SiNx film have a passivating effect on the silicon wafer.

7. Aluminum oxide coating on the front

① Purpose

An aluminum oxide film layer is deposited on the front side of the silicon wafer, which together with other film layers forms a front-side passivation effect.

8. Preparation of front anti-reflection film

① Purpose

The front anti-reflective 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 of battery components. The front SiNx also has a protective effect on the alumina.

9. Silk screen printing-laser transfer printing

At present, in the battery printing process, most still use screen printing.

In the future, laser pattern transfer printing technology (Pattern Transfer Printing) may have more advantages in the silver paste consumption direction of N-type batteries.

Laser transfer printing is a new type of non-contact printing technology. This technology coats the required slurry on a specific flexible light-transmitting material, and uses high-power laser beam high-speed pattern scanning to transfer the slurry from the flexible light-transmitting material. Transfer it to the battery surface to form grid lines and prepare front and rear electrodes.

10. Sintering

Good ohmic contact is formed through high temperature sintering.

11. Automatic sorting

Classify battery cells with different conversion efficiencies.

TOPCon solar cell advantages

The reason why TOPCon solar cell can be adopted by a large number of enterprises is that its advantages are obvious, as follows:

1. High conversion efficiency

TOPCon is currently the solar manufacturer with the highest mass production efficiency, with an efficiency as high as 25.2%. The current mainstream PERC is 23.2%, and TOPCon is 2 percentage points higher.

It is expected that by the second half of 2023, TOPCon can reach 26.8%, PERC's efficiency will be around 23.5%, and the efficiency difference can reach 3.3 percentage points.

2. Low attenuation rate

The N-type battery silicon wafer substrate is doped with phosphorus, and there is no loss of electron capture due to the formation of a boron-oxygen pair in the recombination center, and there is almost no light-induced attenuation.

The degradation rate of TOPCon modules in the first year is about 1% (PERC is about 2%), and the average annual degradation rate after the first year is about 0.4% (PERC is about 0.45%).

3. Low temperature coefficient

On the component side, the power temperature coefficient of PERC components is -0.34%/℃, while the power temperature coefficient of TOPCon components is as low as -0.30%/℃, making the power generation of TOPCon components particularly outstanding in high-temperature environments.

4. High double-sided ratio

The double-sided rate of TOPCon can reach 80%+, and the PERC is about 70%.

Due to the vast territory of large base projects, the ground reflectivity is high (usually up to 30%). In large base projects, the power generation gain of using N-type modules with high bifacial ratio is more obvious.

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