Researchers at HHI have now managed to double the efficiency of black silicon solar cells. They have created cells that can produce more electricity from the infrared spectrum. “We achieved that by modifying the shape of the laser pulse we use to irradiate the silicon,” says 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 into the atomic lattice, researchers can change the positions that these atoms adopt in the lattice and change the height of their ‘levels’, in other words their energy level. “We used the laser pulses to alter the embedded sulfur in order to maximize the number of electrons that can climb up while minimizing the number that can go back down,” Kontermann sums up.

Image: Black silicon is irradiated with a laser. Small image: Black silicon, magnified. © Fraunhofer HHI

In the first stage of the project, the scientists modified the laser pulses and investigated how this changed the properties of black silicon and the efficiency of solar cells made from this material. Now they are working on using different shapes of laser pulses and analyzing how this changes the energy level of the sulfur. In the future, they 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 successfully built prototypes of black silicon solar cells and their next step will be to try and merge these cells with commercial technology.

Dr. Stefan Kontermann, Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute HHI: http://www.hhi.fraunhofer.de/en/start-page/