Black silicon solar cells

Print edition : November 02, 2012

Black silicon solar cellscan absorb nearly all of the sunlight that hits them.-

SOLAR cells convert three-quarters of the energy contained in the suns spectrum into electricity. Infrared radiation, which makes up the remaining one-fourth of the spectrum, is entirely lost in standard solar cells. In contrast, black silicon solar cells can absorb nearly all of the sunlight that hits them, including infrared radiation, and convert it into electricity. Black silicon is produced by irradiating standard silicon with femtosecond laser pulses under a sulphur-containing atmosphere, explains Stefan Kontermann, who heads the research project on Nanomaterials for Energy Conversion at the Fraunhofer Institute for Telecommunications, Heinrich-Hertz Institute (HHI), Berlin. This structures the surface and integrates sulphur atoms into the silicon lattice, making the treated material appear black. In normal silicon, infrared radiation light does not have enough energy to excite the electrons into the conduction band and convert them into electricity, but the sulphur in black silicon forms a kind of intermediate level. However, this intermediate level not only enables electrons to climb the energy wall, but also works in reverse, enabling electrons from the conduction band to jump back via this intermediate level. By modifying the laser pulse that drives the sulphur atoms into the atomic lattice, researchers can change the positions that these atoms adopt in the lattice and change the height of their energy levels. In the future, the German researchers hope that a system of algorithms will automatically identify how the laser pulse should be modified in order to achieve optimum efficiency.

The researchers have already built prototypes of black silicon solar cells. Their next step is to try and merge these cells with commercial technology. We hope to be able to increase the efficiency of commercial solar cells by combining them with black silicon, Kontermann says. Their starting point is a standard commercial solar cell and black silicon is incorporated as part of the cell by removing its back cover, thereby creating a tandem solar cell that contains both normal and black silicon.

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