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Nanopillars Boost Solar Efficiency

A new design can make more-efficient thin-film solar cells using existing manufacturing equipment.

Thin-film solar cells are less expensive than traditional photovoltaics sliced from wafers, but they’re not as efficient at converting the energy in sunlight into electricity. Now a Newton, MA-based startup is developing a nanostructured design that overcomes one of the main constraints on the performance of thin-film solar cells. Solasta fabricates on arrays of nanopillars, rather than flat areas, boosting the efficiency of amorphous silicon solar cells to about 10 percent–still less than crystalline silicon panels, but more than the thin-film amorphous silicon panels on the market today. The company says that the design won’t require new equipment or materials and that it will license its technology to amorphous-silicon manufacturers at the end of this year.

Pillar power: This microscope image shows the layers of a solar cell built on a nanopillar substrate. The core of each pillar is coated first with metal, then amorphous silicon, and then a transparent conductive oxide.

Solasta’s solar architecture eliminates the tradeoff between thick and thin in thin-film solar cell design by separating the electrons’ path from the photons’ path. Light tends to reflect from thin-film cells without being absorbed. The thicker a cell’s active layer, the more incident light it will collect, and the more free electrons it will generate. But the thicker the active layer, the fewer free electrons will make it out of the cell.

The cells designed by Solasta are built on a substrate forested with long, thin, vertically arrayed nanopillars. The pillars are coated first with metal, then with a thin layer of semiconducting material such as amorphous silicon, and then with a layer of transparent conductive oxide. Though the silicon layer is thin, a photon still has a relatively long path to travel down the length of the nanopillars, and a good chance of transferring its energy to an electron. Freed electrons then travel perpendicularly over a very short path to the metal at the core of each pillar, and shimmy down this electrical pole off the cell. “Electrons never have to travel through the photovoltaic material,” says Zhifeng Ren, professor of physics at Boston College. “As soon as they’re generated, they go into the metal.” Ren founded Solasta with professors Michael Naughton and Krzysztof Kempa.

Other groups are also attempting to increase the efficiencies of thin-film solar cells by creating nanostructures that provide separate paths for electrons and photons. But the advantage of Solasta’s nanopillar substrate is that it’s compatible with the manufacturing techniques used to make today’s thin-film solar cells, which are mostly built on glass using chemical-vapor deposition techniques. “We’ll license to existing thin-film manufacturers to get them an efficiency boost without having to switch out their equipment,” says Mike Clary, CEO of Solasta. “Other people are working on nanostructured surfaces to improve the performance in one way or another, but there’s nothing close to these efficiency levels or this close to commercialization,” adds Clary.

So far, Solasta’s prototyping has been done on small cells. In the coming months, the company will work on scaling up its cells to conventional thin-film sizes. The company is also testing different substrate materials, including polymer nanowires, to determine which material provides scalability to large areas while supporting the best efficiencies. Naughton, the company’s chief technology officer, says the concept will work with any thin-film solar materials, but the company is focusing on amorphous silicon first.

The nanopillar architecture has another advantage in addition to efficiency when applied to amorphous silicon cells. “Amorphous silicon cells degrade in prolonged sunlight, reducing their efficiency by 20 to 30 percent,” says Naughton. But this degradation is much less pronounced in cells thinner than about 100 nanometers, such as Solasta’s, which should maintain their performance better over their lifetime.

The company will also develop the nanopillar architecture for new types of solar cells that take advantage of quantum phenomena at the nanoscale. The Boston College researchers recently demonstrated that ultrathin solar cells can allow “hot” electrons with very high energy levels to exit the cell. Even in thin cells, however, these electrons tend to lose their energy before they can escape. In the hope that the dual-path architecture of its nanopillars will solve this absorption problem, Solasta will work on developing nanopillar solar cells with ultrathin layers of silicon.

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