The main structure of crystalline silicon (single-crystal, polycrystalline) solar cells is crystalline silicon material, with silver on the front surface printed with a line of silver as the negative electrode, dividing the main grid lines and fine (sub-) grid lines; Outside the electrode, the rest is aluminum, which we call the back field of aluminum. It is screen-printed with aluminum paste and sintered at a high temperature of over 800 degrees using an alloying process. For a variety of reasons, there will be powder off the battery phenomenon, this article will analyze the problem.
1.0 Slurry
The paste includes a conductive paste, a resistor paste, a dielectric paste, and an encapsulated paste. The paste is a fluid composed of a functional component, a binding component, and an organic carrier.
The paste used in the back silver, back aluminum and positive silver processes is a conductive paste. In conductive pastes, the functional components are generally noble metal or noble metal mixtures. The carrier is a solution of the polymer in an organic solvent. The functional components determine the electrical and mechanical properties after film formation. The carrier determines the process characteristics of the thick film and is a temporary binder for the printed film and the dried film. The functional component and the binding component are generally in the form of a powder, and after being sufficiently stirred and dispersed in the carrier, a paste-like thick film slurry is formed. The sintered thick film conductor consists of a metal and a bonding component.
The technical performance index of the slurry refers to the functional components in the slurry (the silver-aluminum component in the back silver paste, the silver component in the positive silver paste, and the aluminum component in the back paste) after drying and sintering and the cell sheet The ohmic characteristics, which affect the battery's electrical performance indicators such as open circuit voltage, short-circuit current, parallel resistance, series resistance, conversion rate, and other technical indicators; slurry process characteristics are to achieve the above indicators to ensure that the slurry manufacturers for 3 The printing process has recommended process parameters such as particle size, viscosity, solid content of the slurry, the mesh number of the screen; the mesh length of the mesh grid is 2.5 to 5 times the size of the slurry powder; the viscosity of the slurry Affects the speed of the squeegee bar; the solid content determines the final thickness of the wet thickness after printing after drying and sintering. Different backing aluminum and positive silver three-step slurry, which determines their screen and printing parameters are different, Table 1 is the United States FERRO company's printing process characteristics of the relevant materials.
Table 1 US FERRO recommended slurry properties
2.0 Causes Shedding Cause
In the production process of the module, the phenomenon of powder cell peeling occurs mainly due to the following reasons:
1. Improper storage.
The grid line material on the front and back of the solar cell is sintered from precious metal slurry. Silver is a very active element and is easily oxidized by oxides. After the battery is opened and packaged, the gate line is in contact with the air, and is oxidized by oxygen and other oxides into silver oxide. When it is in contact with the air for a long time, the silver oxide powder is easily detached, resulting in serious power degradation of the battery.
2. Other reasons
In addition to the above reasons, there are factors related to the production process of the cell, such as: 1. The sintering temperature is not enough. 2. The back of the silicon wafer is oily. 3, printing is too thick or too thin. 4, the slurry mixing time or its own viscosity problems, etc., may cause the gate line or back field off, affecting the use of the battery.
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