"Creating a Global Industry in Northeast Japan". Professor Koichi Koike, a professor at Tohoku University in Tokyo who developed new materials for solar cells, expressed with full confidence. Koike developed the technology to use the standard as the silver wiring of the solar cell light-receiving surface electrode for a cheaper copper wiring. Koike has developed copper-magnesium alloy materials used as semiconductor barrier metals in the past and has been adopted by semiconductor manufacturers. Koike's goal is to use this experience to establish a copper paste material manufacturing company and cultivate it into a world-class company.
Copper wiring printed onto silicon wafers
The silver paste used to form the silver wiring accounts for approximately 1/4 of the material cost of the solar cell. By replacing the silver paste with a cheaper copper paste, the material cost can be instantly reduced by 20%.
The data obtained by the research team showed that the wiring resistance value using copper paste is lower than that of silver paste. By combining this technology with the wiring width reduction technology, the conversion efficiency of the battery can also be improved. "Overseas research institutions believe that it can increase 3 to 4 percentage points" (Koike).
Therefore, the replacement of silver wiring with copper wiring has attracted the attention of global solar cell manufacturers. However, the diffusion of copper into silicon exists in this area, resulting in deterioration of the performance of solar cells and oxidation of copper during sintering, resulting in increased resistance values. Tohoku University solved the above problems by (1) forming a diffusion barrier between the silicon wafer and the copper wiring to prevent copper diffusion, and (2) improving the sintering conditions of the copper paste to prevent oxidation.
Simultaneous oxidation and reduction
The diffusion barrier layer in (1) is composed of an oxide having a thickness of 5 to 10 nm. Not only can it prevent copper from spreading, but it also has conductivity and high adhesion to copper paste (Figure 1). Due to its thinness, it has little effect on the transmission of sunlight. The current diffusion barrier layer is formed by immersing a silicon wafer in a solution-like material and sintering it at a temperature of about 500°C.
Figure 1: Copper paste after printing diffusion barrier
Tohoku University has developed a copper paste material for solar cells. After applying a diffusion barrier on the silicon wafer, copper paste is printed to form the wiring. (Photo courtesy of Tohoku University)
The sintering conditions of the copper paste in (2) are partially placed in a reducing gas. The silver paste is sintered in the atmosphere, and the resin in the paste becomes volatile oxide and can be easily removed. However, copper paste is placed in nitrogen to prevent oxidation, making it difficult to remove the resin. To remove the resin and increase the oxygen concentration, the copper surface will oxidize. Although the relevant process details have not been disclosed, it has been said that a process of simultaneously performing oxidation and reduction in a sintering furnace has been developed.
After sintering at 500°C for 1 minute, the copper wiring resistivity becomes about 5 μΩcm which is equal to or smaller than that of the Ag wiring. Even if the sintering time is prolonged, the resistivity does not increase, so that the oxidation can be sufficiently prevented while oxidizing copper (FIG. 2). It has also been confirmed that the resistivity can be reduced to about 3 μΩcm when adjusting the sintering gas.
Figure 2: Sintering conditions for 1 minute at 500°C
After the copper paste is printed, the resistivity decreases when sintered at a temperature of 500°C or higher. When the sintering time is extended to 1 minute or more, the resistivity does not change greatly. (Photo courtesy of Tohoku University)
New companies also have the opportunity
At present, the development of key technologies has been completed, and the characteristics of solar cells have been confirmed in 2012. We plan to complete the development of mass production technologies in 2013 (Figure 3). In the meantime, a copper paste material manufacturing company will be established, and at the same time, it will grant technical licenses to material manufacturers, manufacturing equipment manufacturers, solar cell manufacturers, etc., and promote the practical application of Note 1).
Figure 3: The goal is to achieve business operations as quickly as possible
The development of key technologies and proof of concept is scheduled to be completed in 2012 (a). In the future, while developing mass production technologies, it will provide technical licenses to various manufacturers to realize business operations as soon as possible (b). In the meantime, a new copper paste manufacturing company will be established.
Koike’s belief that it is possible to “bring a world-class company†to copper paste material manufacturing companies lies in the fact that there have been precedents for new materials to sweep the market. In the past, companies had developed new silver paste materials that could increase cell-cell conversion efficiency by 0.35 percentage points, and quickly increased their share. Therefore, the copper paste material can be increased by 3 to 4 percentage points, even if it is a newly involved company, it can also fully expand its share. (Reporter: He Jiji, "Nikkei Electronics")
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