Judging from the functions of the three slurries, the back silver has the smallest functionality, so the technical threshold is relatively low.
The previous back silver is usually called silver-aluminum paste. As the name suggests, the paste contains silver and aluminum. Everyone knows that silver and aluminum are difficult to form an alloy, and silver paste containing aluminum will affect welding, but why add aluminum? I think there are two main points. First, I hope that the aluminum in the silver-aluminum paste can form a similar BSF layer between the back electrode and the battery during the sintering process like the aluminum paste. (Indeed, the previous battery cells had two or three relatively wide back electrodes.) The second is to reduce the cost of back silver. After all, the prices of silver and aluminum are very different. At that time, the solid content of back silver was still around 78%.
Since the back silver has little impact on the performance of the battery, the battery chip factory’s requirements for the back silver are mainly unit consumption, welding performance, etc. So everyone took basically the same route, removing aluminum and reducing Solid content, whether it is DuPont, Heraeus or Flow, the current solid content of silver on the back is less than 70%, and the silver content is even lower. Therefore, the obvious development direction of back silver is to reduce the silver content, reduce unit consumption, and ultimately reduce costs while maintaining or improving welding performance. The main function of aluminum paste is to form an aluminum back field on the back of the battery during the sintering process to improve the electrical performance of the battery. There are two mechanisms of action of the aluminum back field, one is aluminum gettering; the other is the formation of the so-called P+ layer.
From the battery cell itself, there are several requirements for aluminum paste, including electrical properties, appearance, warpage, adhesion, etc. Among them, electrical properties are the key. There are many relevant patents at home and abroad that can be consulted. Combined with the current requirements of cell and component manufacturers, we can see that its future development direction must be high electrical performance, low warpage, uniform appearance, and up to standard adhesion. The function of positive silver (photovoltaic auxiliary material network) is to collect current. It seems very simple, but in fact it is the most difficult of the three slurries. So far, no domestic-funded enterprise has successfully launched a positive silver.
The most basic requirements of positive silver are good printing performance, high aspect ratio, and at the same time, it can form good ohmic contact with the silicon wafer and reduce the contact resistance, so that the cell chip has high photoelectric conversion efficiency. Printability and aspect ratio are not difficult for many developers. Of course, the current lack of available raw materials in China is also an insurmountable obstacle. In fact, it is more important to control the corrosion of the anti-reflection film and silicon wafer by the slurry to form a uniform and good ohmic contact. Information shows that high sheet resistance is one of the development trends. High sheet resistance corresponds to low concentration diffusion, and it is more difficult to control sintering to form good ohmic contact. Therefore, it can be speculated that one of the development directions of positive silver is good brushing performance, high width It has a high ratio and can heavily dope the contact and form a uniform and good ohmic contact.