Heinrich-Hertz-Institut doubles the efficiency of black silicon solar cells

October 10, 2012 // By Paul Buckley
Solar cells convert three-quarters of the energy contained in the Sun‘s spectrum into electricity – yet the infrared spectrum is entirely lost in standard solar cells. In contrast, black silicon solar cells are specifi cally designed to absorb this part of the Sun‘s spectrum – and researchers have recently succeeded in doubling their overall efficiency.

Researchers at Heinrich-Hertz-Institut, HHI have doubled the efficiency of black silicon solar cells.  If manufacturers were to equip their solar cells with this black silicon, it would boost the cells’ efficiency by enabling them to utilize the full Sun spectrum.

Around a quarter of the Sun’s spectrum is made up of infrared radiation which cannot be converted by standard solar cells – so this heat radiation is lost. One way to overcome this is to use black silicon, a material that absorbs nearly all of the sunlight that hits it, including infrared radiation, and converts it into electricity. But how is this material produced?

“Black silicon is produced by irradiating standard silicon with femtosecond laser pulses under a sulfur containing atmosphere,” explained Dr. Stefan Kontermann, who heads the Research group “Nanomaterials for Energy Conversion“ within the Fraunhofer Project Group for Fiber Optical Sensor Systems at the Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, HHI. “This structures the surface and integrates sulfur atoms into the silicon lattice, making the treated material appear black.”

“We achieved that by modifying the shape of the laser pulse we use to irradiate the silicon,” said Kontermann. This enabled the scientists to solve a key problem of black silicon: In normal silicon, infrared light does not have enough energy to excite the electrons into the conduction band and convert them into electricity, but the sulfur incorporated in black silicon forms a kind of intermediate level. You can compare this to climbing a wall: The first time you fail because the wall is too high, but the second time you succeed in two steps by using an intermediate level. However, in sulfur this intermediate level not only enables electrons to climb the ‘wall’, it also works in reverse, enabling electrons from the conduction band to jump back via this intermediate level, which causes electricity to be lost once again. By modifying the laser pulse that drives the sulfur atoms