Semprius's Micrometric Solar Cells Efficient in Concentrated Photovoltaic Systems

Concentrated photovoltaic systems are usually more effective than regular, flat ones, because a lens focuses the sunlight on a smaller surface of some efficient and more expensive solar cells 500 times stronger than the cells would receive in regular operation.

Heat is mostly a problem with concentrated PV systems, since it has to be efficiently dissipated through complex and expensive cooling techniques (which recover a part of it through thermoelectric materials or just by heating water).

John Rogers, from the University of Urbana-Champaign in Illinois has discovered a novel solar cell printing technique that could bring prices down and make smaller, micrometric solar cells much more effective and heat-resistant.

Semprius, a NC-based company, used the printing technology developed by Rogers, and, through a joint agreement with Siemens, they committed themselves to developing prototype systems based on the new microprinting technology, and begin the mass-production of the models in 2013. Their micro solar cells are to be perfectly suited to concentrated solar power, allowing a light power up to 1,000 times stronger to be focused onto them, without costly cooling systems.

Semprius’s solar cells measure only 600 micrometers on each side, and are made of three gallium arsenide-based semiconducting layers, each of which absorbs a different band of the light spectrum. The cells are made by chemical etching and printing, meaning fewer raw materials are to be wasted. Their efficiency, according to Sandia National Renewable Energy Laboratories is 25 to 35  percent, and can provide electricity for about 10 cents per kilowatt. The final costs of these solar cells, including installation, is expected to be somewhere between $2 and $3 per watt – regular solar panel prices.

Last year, a study by researchers at Sandia National Laboratories in Albuquerque, NM, suggested that microscale solar cells might offer various cost and design advantages. “You reduce the amount of semiconductor you need, so there can be a big cost savings,” says Gregory Nielson, head scientist on the Sandia project. “And you can do things with the optics that you can’t do with larger cells.”

Smaller solar cells are more efficient at dissipating heat. “When the cells are below a millimeter, they reject the heat so efficiently they’ll be just as cool as a one-sun panel,” without the need for any cooling systems, says Nielson. This is because the tiny cells have a much greater percentage of total area given up to heat-diffusing edges.

Regular solar cells are made by building up active layers on the surface of a semiconductor wafer, and then sawing the water into smaller 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’s solar panels could be stacked, for example, in an array such as that developed by Ryszard Dzikowski by paralleling them and focusing a larger amount of solar power into the closed stack, which would further increase their efficiency, by reflecting the sunlight into the closed chamber. The main issue you would have to overcome with these models would be heating, which is now solved by using these solar cells.