Affected by the recent news that the conversion efficiency of heterojunction batteries has broken the world record, A-share heterojunction battery concept stocks bucked the trend and surged higher. LONGi announced that after the latest certification from the Institute of Solar Energy Hamelin (ISFH) in Germany, the conversion efficiency of the silicon heterojunction cell independently developed by the company has reached 26.81%, which is currently the highest international solar cell efficiency record in the world. The current mainstream P-type cells are close to the theoretical efficiency limit, and the trend of cost reduction has slowed down. N-type cells are generally optimistic, and the market believes that the future development space will be further increased. In this section, this issue of Millennial Solar will continue to introduce them to you.

Manufacturing of heterojunction solar cells

The manufacturing process of heterojunction solar cells involves several steps as follows:

1. Wafer processing

2. Wet chemical treatment

3. Core layer deposition

4. Total cost of ownership deposition

5. Metal

Wafer processing involves cutting c-Si cells with a diamond saw. Executing this process with extreme precision will produce a high-quality c-Si layer, which translates into higher efficiency.

During wet chemical processing, organic and metallic impurities are removed from c-Si wafers. There are generally two methods of wet chemical treatment, the RCA method involves the use of concentrated sulfuric acid and hydrogen peroxide, and the cost-effective alternative of applying ozone-based processes achieves similar results.

After wet chemical treatment, a deposition process using plasma enhanced chemical vapor deposition (PECVD) is applied to deposit a-Si layers on both sides of the wafer-based layer.

The second part of the deposition process uses physical vapor deposition (PVD) by sputtering to apply the ITO, forming the TCO layer of the heterojunction solar cell. Another process uses reactive plasma deposition (RPD) to apply the TCO layer, but this is a less popular option.

The metallization process differs from conventional manufacturing processes because the hydrogen in a-Si:H limits the temperature to a maximum of 200-220ºC. The electrodes are placed on the cells through a copper electroplating or screen printing process using specially curated silver paste at low temperatures.

Classification of heterojunction solar cells

Heterojunction solar cells can be divided into two categories based on the amount of doping: n-type or p-type.

The most popular doping uses n-type c-Si wafers. They are doped with phosphorus, which gives them extra electrons for negative charge. These solar cells are immune to boron oxygen, which reduces the purity and efficiency of the cells.

P-type solar cells are more suitable for space applications because they are more resistant to the radiation levels perceived in space. The p-type c-Si wafer is doped with boron, which provides the cell with one less electron and thus carries a positive charge.

3D Confocal Microscope

3D Confocal Microscope ME-PT3000 is a high-speed confocal scanning microscope for accurate and reliable 3-dimensional (3D) measurements. Real-time confocal microscopy images are achieved through fast optical scanning modules and signal processing algorithms.

ME-PT3000 is an optical instrument specially used in the photovoltaic industry to inspect the quality of grid lines and textures on the surface of photovoltaic cells. Based on the principle of optical technology, combined with precision Z-direction scanning module, 3D modeling algorithm, etc., the device surface is non-contactly scanned and a 3D image of the surface is established. The height and width of the grid lines on the photovoltaic cells and the texture on the surface are measured through the system software. The number of pyramids is quantitatively tested to provide feedback on the quality of photovoltaic cell cleaning, texturing and screen printing processes.

E-mail: market@millennialsolar.com

The efficiency of heterojunction cells has been making breakthroughs, and the market and scale have been expanding. Millennial Solar will continue to work hard in this direction and bring you more content.