Improved Black Silicon Lets Conventional Solar Cells Harness Infrared Light

black silicon tandem© Fraunhofer HHI

Researchers at the Fraunhofer Institute have been busy making improvements to an already exciting technology, black silicon. Black silicon solar cells are specifically designed to absorb nearly all of the sun's spectrum, including infrared radiation, and convert it into electricity. Conventional solar cells, on the other hand, are able to convert the three-quarters of the spectrum not including infrared into electricity. If these two technologies could work together, conventional solar cells could be far more efficient.

The Fraunhofer researchers believe that we're close to seeing that happen. The team has found a way to double the efficiency of black silicon cells, which are made by irradiating standard silicon "with femtosecond laser pulses under a sulfur containing atmosphere." That integrates the sulfur into the silicon and makes the silicon appear black. The researchers were able to double their efficiency by changing the shape of the laser pulses.

The Fraunhofer Institute explains:

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,” said Dr. Stefan Kontermann, one of the researchers.

They've already built prototypes of these improved black silicon cells and now they will start trying to merge the technology with conventional silicon solar cells. By simply removing the back cover of a standard solar cell and applying a black silicon cell, which creates a tandem solar cell, the efficiency could be improved by one percent.

The other thing the researchers are working on is making the laser system ready for mass production so that solar cell manufacturers could just add it to their production lines, make the black silicon components themselves and add it to their existing solar cells.

Improved Black Silicon Lets Conventional Solar Cells Harness Infrared Light
Conventional solar cells only harness three-fourths of the spectrum, but improvements made to black silicon could change that.

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