Black Silicon Solar Cells to Capture More Light

In the quest to generate more power from solar cells, many scientists have their eye on infrared light.

About one quarter of the energy in sunlight is essentially lost on solar cells because they can’t convert light in the infrared spectrum into electricity. That has led to an active field of research in so-called black silicon, in which the surface of silicon solar cells is modified to absorb rather than reflect infrared light.

German research institute Fraunhofer says it has developed a way to double the efficiency of black silicon. If incorporated into existing photovoltaic manufacturing, the technique could improve silicon cell efficiency, which averages around 17 percent, by 1 percent. Depending on the cost the technology would add, that could help lower the cost of solar power.

Black silicon is made by striking solar cells with lasers in the presence of sulfur gas to change the cells’ texture. “This structures the surface and integrates sulfur atoms into the silicon lattice, making the treated material appear black,” Stefan Kontermann, who heads the Nanomaterials for Energy Conversion research group at the Fraunhofer Institute, explains in a statement.

Fraunhofer researchers improved the efficiency of black silicon by controlling the laser pulse that drives the sulfur atoms into the silicon cell. Changing the position of the sulfur reduces the amount of infrared energy needed to excite electrons so that they are converted into electricity. It effectively creates an intermediate energy level where more infrared light is able to produce electricity. “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 says.

Researchers have made a prototype device and hope to develop an algorithm to best determine how laser pulses should be modified to change the energy level of the sulfur. Fraunhofer said it intends to create a spinoff company to commercialize the research so that manufacturers can make tandem solar cells with normal and black silicon. 

The National Renewable Energy Laboratory in Colorado has developed a chemical treatment to improve black silicon performance, which has been licensed by Natcore Technology (see “Black Silicon Makes Solar Cells Cheaper”).