Efficiency: Black Silicon

Scientists at the Fraunhofer Institute in Germany have just succeeded in doubling the efficiency of solar cells made from black silicon.
Black silicon is formed by irradiating regular silicon with a laser under sulfuric conditions. The silicon integrates the sulfur into its composition after being treated with the laser pulses, giving the substance its characteristic black color. By changing the shape of the laser beams, the researchers were able to overcome one of the most common problems of working with silicon.
In normal silicon, infrared does not possess enough energy to excite the electrons in the conduction band necessary to generate electricity. By incorporating sulfur, an intermediate level is formed in the atomic lattice. This level acts as a step for electrons to jump to the conduction band as well as move in reverse from the band. However, energy is still lost when electrons move backwards from the conduction band. “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,” said Dr. Stefan Kontermann, one of the researchers involved in the project.