Copper electroplating technology is one of the important technical routes for heterojunction solar cells  to reduce costs and increase efficiency in recent years. Its advantage is that it has stronger conductivity than traditional silver paste and has low contact resistance. The use of copper electroplating technology can improve conversion efficiency. Under the market conditions where silver prices continue to fluctuate, copper electroplating technology has attracted the attention of many manufacturers and its development continues to improve.

    Basic principles of copper electroplating electrode efficiency improvement

Copper electroplating is a non-contact electrode metallization technology. The basic principle of the process is to deposit metallic copper on the surface of the base metal through electrolytic chemical reaction to make copper grid lines and collect carriers generated by the photovoltaic effect. The main difference between copper electroplating technology and traditional screen printing technology is after the ITO process. Traditional heterojunction production lines will perform silver paste printing and sintering after the ITO process. The copper electroplating process is mainly divided into two parts: gate line patterning and metallization. After the ITO process in the patterning part, no silver paste, instead using copper for electroplating.

    Copper electroplating process

    The copper electroplating process of heterojunction solar cells mainly includes four major process links: seed layer preparation, patterning, metallization (electroplating), and post-processing. The processes of each link are complex, there are multiple technical routes, and the equipment principles vary greatly.

    In the preparation of seed layer, physical vapor deposition (PVD) is the mainstream technical solution. When electroplating directly on TCO, the contact between the plating layer and TCO is physical contact. The adhesion is mainly van der Waals force, which can easily cause the electrode to fall off. Moreover, electroplating metal on TCO is non-selective, and seeds need to be deposited on the surface of the transparent conductive film before electroplating. layer (generally 100nm) to increase the adhesion between the electroplated metal and TCO.

    In the graphic production process, the currently existing technical routes mainly include: photolithography route, laser route and inkjet printing route. Among them, the exposure and development photolithography route using photosensitive ink as raw material is the mainstream technical solution for graphics.

    In the metallization plating process, there are a variety of mainstream technical solutions such as vertical, horizontal plating, and chip-type plating.

    In the post-processing process, the main treatment methods include heat treatment, chemical, mechanical treatment, etc., with the purpose of improving the performance and appearance of electroplated copper electrodes.

    The properties of electroplated copper electrodes include conductivity, chemical stability, mechanical strength, etc.

    Conductivity is one of the most basic properties of electroplated copper electrodes and can be optimized by controlling the composition of the electroplating solution and process conditions.

    Chemical stability is an important indicator of the long-term stability of electroplated copper electrodes in solar cells, and can be improved by adding stabilizers and other methods.

    Mechanical strength is one of the important properties of electroplated copper electrodes during preparation and use, and can be optimized by controlling the composition of the electroplating solution and process conditions.

    What are the advantages and disadvantages of copper grid wires compared to traditional silver grid wires?

    The grid line needs to form a very good contact with the base material of the cell, that is, ohmic contact. The quality of ohmic contact determines the conductivity of the solar cell and whether the power generation efficiency can be optimal. Whether the contact surface is tightly coupled and the resistance (resistivity) of the gate line material to the passage of electrons or carriers will affect the quality of the ohmic contact. The greater the resistivity, the higher the load the cell has on electrons or carriers, and the worse the electron or carrier pass rate.

    advantage:

1. The conductivity of silver paste is weaker than that of copper grid wire. Because the silver paste is a mixture and a flowing colloid, while the copper grid wire is pure copper, the resistivity of the copper grid wire body electrons is lower than that of the silver paste electrons.

2. The line width of silver paste printing is wider than that of copper electroplating, resulting in lower power generation efficiency of silver paste printing.

shortcoming:

1. The gate lines of copper electroplating are thinner, which will cause copper electroplating to face the problem of whether the gate lines will be stronger and whether the adhesion can meet the requirements of component production. Because the gate wires are thinner and the adhesion area is smaller, under the same stress conditions, the adhesion of the copper grid wires is not enough. The copper grid wires are easier to fall off than the silver grid wires. The falling off of the gate wires will cause problems with the gate line contact, which will Affect the later use of cells and components.

    Optimization direction: Using semiconductor technology for patterning, the gate line width of copper electroplating can reach 10μm.

   2. After long-term use, the silver paste will not suffer from oxidation failure, while the copper gate wire will suffer from oxidation failure. As long as copper is exposed to air, it will oxidize very quickly, resulting in copper oxide or cuprous oxide, which will affect the conductivity of copper. Copper oxide will also diffuse. Copper diffuses very slowly in certain conditions such as ITO. However, copper will continue to diffuse after oxidation, which will lead to failure of grid cells or components.

Optimization direction: protect the gate lines or perform anti-oxidation treatment.

    Copper electroplating has more obvious advantages in cost reduction and efficiency improvement

    Copper electroplating is a completely silver-free and disruptive cost-reduction technology. In the traditional screen printing process, silver powder accounts for about 90% of the low-temperature silver paste, and the remaining materials are dispersed binders and glass powder. After electroplating copper, the shading loss on the front of the solar cell is further reduced, and the resistance loss of the grid line is also reduced. Comprehensive reduction, as the contact between the electrode and TCO is comprehensively improved, low-cost electrode preparation can achieve large-scale mass production, which provides a new direction for the development of solar cell metallization technology. This new technology can not only reduce costs across the board, but also further improve efficiency.