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September 23rd, 2009

Cutting PV costs, Part 1: New busbars, ‘fingers’ to cut costs by 20%?

Published on September 23rd, 2009 | By: pwray@ceramics.org

Technology Review has a video (24 min.) of a recent interview with Emanuel Sachs, 1366 Technologies’ chief technology officer and professor of mechanical engineering at MIT. He explains the physics of solar cells and how 1366 is using the techniques described below to try to make them more efficient. (Note, there is a brief sponsor video that runs first.)

This Lexington, Mass., solar startup claims to have developed three processes that can be incorporated into existing solar cell manufacturing lines to improve cell efficiency. According to the company, these technologies can be used to produce multicrystalline solar cells that are 18 percent more efficient at converting sunlight into electricity, and about 20 percent cheaper.

The current industry standard for such solar cells is 15 percent to 16 percent, according to Joonki Song, a partner with Photon Consulting, based in Boston

An accompanying Technology Review article describes the new technologies:

In a normal silicon solar cell, electrons generated in the silicon must make their way out of the material to produce an electrical current, traveling first to the top layer of the silicon and then along this layer to narrow silver lines called “fingers.” The fingers then conduct the electrons to the busbars, two or three prominent silver bands seen on the surface of most silicon solar cells. These bands shade the silicon under them, reducing the amount of light the cells can absorb.

The first new process developed by 1366 Technologies produces grooved busbars that prevent light from being reflected out of a solar panel. Instead, the grooves cause light to be redirected along the glass on top of solar panels. That light can then be absorbed by unshaded areas of the solar cell.

The second process improves the cell’s electron-conducting fingers. Although these silver lines are much narrower than the busbars, there are many more of them on a solar cell, and together they shade a significant portion of the silicon. Sachs developed a process for making much narrower lines without sacrificing their conductivity. Instead of using conventional screen-printing technology, his process involves etching troughs into the surface of the silicon and depositing silver particles into the troughs. Metal is then added to these particles via electroplating to build up the fingers. The trough keeps the lines narrow but allows the silver to be stacked relatively high, maintaining conductivity. Typically busbars and fingers shade 9 percent of a cell surface, 1366 Technologies says, but with the company’s new processes, this shading can be reduced to 2 percent. Others have developed techniques for reducing shading, but these have been expensive.

The third process decreases the amount of light reflected off the surface of the cell’s silicon by texturing its surface. This is an approach that’s been taken by others, but the texturing is done in a very regular pattern that creates less surface area than other approaches. Surface area is a problem in solar cells, because electrons are often trapped at the surface of materials, Sachs says.

Special grooved ribbon reflects light within PV unit. Credit: 1366 Technologies

1366's special grooved busbar ribbons can reflect light within a PV unit. Credit: 1366 Technologies.

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