Advance Enables Cost Savings by Allowing Use of Thinner Silicon Wafers
Red Bank, NJ - (May 5, 2011) - Scientists of Natcore Technology Inc. (TSX-V: NXT; NTCXF.PK) have shown that Natcore's liquid phase deposition (LPD) process can be used to apply an antireflective (AR) coating to textured solar cells as well as standard planar cells, thus opening the door for the industry to achieve even further wafer thickness reductions by eliminating the thermal vacuum AR coating process altogether. The work was conducted by Natcore researchers at the Ohio State University in Columbus, Ohio.
One of the primary ways solar cell manufacturers are striving to reduce the cost of manufacturing silicon solar cells is to reduce the thickness of the wafers from which the cells are made. As the wafers get thinner, however, they lose the ability to fully absorb the light that strikes their front surface; consequently, the efficiency falls. A technique commonly employed to overcome this fundamental limitation is to isotropically etch the front surface of the wafer in order to form a regular pattern of small pyramids on it. The shape and angles of the pyramidal faces are such that more light is refracted into the crystal over a range of angles that prevent the light from travelling straight through the solar cell and being incompletely absorbed. The process, known as texturizing the wafer surface, has gained widespread acceptance in the solar cell industry.
Once texturized, the cells are put through the remaining standard cell processing steps, which include adding the AR coating and the contacts. A growing problem, however, is that the conventional AR coating process requires the cells to travel through a vacuum furnace. As the wafers get thinner and thinner, the conventional AR coating process causes them to warp, reducing the yield from the production process.
Reflectance is the proportion of light striking a surface that is reflected from it. For solar cells, the optimum reflectance is zero; a typical industry reflectance is about six percent. Measurements made at NASA Glenn Research Center in Cleveland, Ohio, show that the reflectance from texturized wafers coated with Natcore's LPD antireflective coating is reduced to well below two percent over the entire absorption band of silicon solar cells. This represents a two-thirds reduction from the typical reflectance achieved by standard industry practices today, and would be accompanied by an increase in cell efficiency.
"The processing steps for achieving this outstanding result will be implemented in the intelligent processing tool we're building in Silicon Valley," says Natcore Chairman Brien Lundin. "We've had strong interest in this technology from several solar cell companies in China. We'll send a number of coated wafers for each of them to process into finished solar cells before we take orders for the industrial version of our intelligent processing tool."