Solar concentrate: This solar lens array concentrates light onto microscale solar cells inside.
The key to making Semprius’s cells is a printing process developed by researchers led by John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign.
Solar cells are typically made by building up active layers on the surface of a semiconductor wafer, then sawing the wafer into pieces. Semprius’s printing process begins by treating wafers in much the same way. But instead of sawing, the company uses chemical etching to score the surface of a wafer into microscale cells, leaving them attached to the wafer’s surface by a small tab. The key to the etching step is adding a sacrificial layer when the wafers are treated. The chemical etchant eats away at just this layer, cleaving the cells from the surface. A robot bearing a polymer stamp then moves over the wafer, picking up the cells and placing them on top of an array of ceramic backings printed with electrical contacts. The process uses only a thin layer of the surface of the wafer, which can be sent back to the foundry to be reused. Each four-inch wafer can be used to produce 36,000 cells.
Each cell is then topped with a tiny spherical ball lens. “Normally there’s a huge hot spot at the center of the cell, but the ball lens uniformly distributes the light,” says Joseph Carr, Semprius’s CEO. These lenses capture sunlight from a wide angle. Finally, the lens-topped cells are grouped into 14-inch arrays, which are topped with silicone lenses that direct sunlight onto the smaller ball lenses. Together, the optical system concentrates the sun’s light 1,000 times. These arrays are stacked on a light tracker to make an 18-by-8-foot solar module.
Semprius plans to license its printing technology to enable volume production of the modules by 2013. The company plans to develop sun-tracking control systems with Siemens, and to further develop its microprinting technology, which is compatible with a range of semiconducting materials, including silicon.