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Laser pulse improves black silicon’s solar efficiency

A solar material that takes advantage of a spectrum totally unused by standard cells could be the key to more efficient photovoltaics, now that a team in Germany has successfully doubled its efficiency.

Solar cells convert three-quarters of the energy in the sun’s spectrum into electricity, but about a quarter of its spectrum – the infrared – is lost in standard solar cells. Black silicon cells, however, are specifically designed to absorb nearly all of the sunlight that hits them, including IR radiation.

“Black silicon is produced by irradiating standard silicon with femtosecond laser pulses under a sulfur-containing atmosphere,” said Dr. Stefan Kontermann, head of the Nanomaterials for Energy Conversion department within Fiber Optical Sensor Systems at Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute (HHI) in Berlin.

To make black silicon, standard silicon is irradiated using femtosecond laser pulses in a sulfur-containing atmosphere; the inset shows black silicon magnified. Modifying the laser pulse makes the material more efficient for use in solar cells.

In normal silicon, IR light doesn’t have enough energy to excite electrons into the conduction band and convert them into electricity, but incorporating sulfur atoms into black silicon forms a kind of intermediate level, or half-step. That level not only enables the electron to jump to a higher conduction band, gaining energy, but also works in reverse, enabling the electrons from the band to jump back, causing electricity to be lost once again.

By modifying the laser pulse that drives the sulfur into the lattice, they also found that they can change the energy level of the sulfur, altering the number of electrons that can be created by a photon.

Black silicon solar cell prototypes have been built, and the next step will be to try and merge the cells with commercial technology.

“We hope to be able to increase the efficiency of commercial solar cells – which currently stands at approximately 17 percent – by 1 percent by combining them with black silicon,” Kontermann said.

The team also is planning a spinoff to market the laser system to manufacturers.

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