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[PV Panel Quality] PVG Solutions: Pioneer of Bifacial Solar Cells (2)

Improvement of reliability through unique production process

2014/11/23 16:50
Kenji Kaneko, Nikkei BP CleanTech Institute
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Continued from [PV Panel Quality] PVG Solutions: Pioneer of Bifacial Solar Cells (1)

Double-sided power generation cells are very promising in terms of high conversion efficiency. But, unlike general single-sided power generation cells, they have issues regarding durability and reliability.

"The solar cell production process involves surface treatment by high-temperature thermal diffusion, and manufacturing control is difficult because quality variation tends to occur in the diffusion process," said Shinji Goda, executive managing director of the PV Business Division at PVG Solutions. "General single-sided power generation cells go through one diffusion process while double-sided power generation cells go through two types of diffusion processes and require higher level production technologies."

If the quality varies due to improper control of the diffusion processes, the reverse voltage property of cells deteriorates, and leakage current tends to occur inside the cells, resulting in heating and increasing risks of problems such as hot spots.

Advanced technology used to overcome challenges in 'double-sided' cell production

Two types of diffusion processes are required because sunlight needs to reach both the front and back surfaces. When a solar cell, a kind of semiconductor, receives light, electricity flows through it due to potential differences when different kinds of semiconductors are used on the front and back surfaces of silicon crystal wafers and the wafers are layered. To change the property of semiconductors, phosphorus or boron is diffused at 830-980°C, which is a widely known approach.

For production of single-sided power generation cells, a method to screen-print aluminum paste on the back surface and burn it was developed, which reduced production costs and is, therefore, generally used. Following this development, the diffusion process only requires diffusion of phosphorus on the front surface.

However, for production of double-sided power generation cells, aluminum coating makes the cells unclear and cannot be incorporated. For that reason, two types of diffusion processes, "phosphorous diffusion" and "boron diffusion," are incorporated to make both sides clear.

The Saijo Plant of PVG Solutions is characterized by the unique systems that it incorporates in the diffusion process, preparation process and masking process. Silicon crystal wafers undergo etching treatment in alkali, and the treated cell wafers are put in a furnace, about 600 sheets at a time. The "phosphorous diffusion" is performed on the front surface and the "boron diffusion" is performed on the back surface at high temperatures. The layered structure of semiconductors is formed by the processes explained above. The products are completed after printing electrodes on both sides, followed by a quality check (Fig. 7 & 8 & 9).

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