Solar cells can convert up to three-quarters of the energy contained in the Sun‘s spectrum into electricity, yet the infrared spectrum is entirely lost in standard solar cells.
Around a quarter of the Sun’s spectrum is made up of infrared radiation which cannot be converted by standard solar cells; that heat radiation is lost. One way to overcome it 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?
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 might fail because the wall is too high, but another time you can succeed by making two efforts, 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 found they can change the positions that these atoms adopt in the lattice and change the height of their ‘levels’, in other words their energy level.
“Black silicon is produced by irradiating standard silicon with femtosecond laser pulses under a sulfur containing atmosphere,” explains Dr. Stefan Kontermann, who heads the Nanomaterials for Energy Conversion section of 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.”
If manufacturers were to equip their solar cells with this black silicon, it would significantly boost the cells’ efficiency by enabling them to utilize the full Sun spectrum. The researchers at HHI have now managed to double the efficiency of black silicon solar cells.
“We achieved that by modifying the shape of the laser pulse we use to irradiate the silicon,” says Kontermann. “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."
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.
“We hope to be able to increase the efficiency of commercial solar cells – which currently stands at approximately 17 percent – by one percent by combining them with black silicon,” Kontermann says. Their starting point is a standard commercial solar cell: The experts simply remove the back cover and incorporate black silicon in part of the cell, thereby creating a tandem solar cell that contains both normal and black silicon.
The researchers are also planning a spin-off: This will be used to market the laser system that manufacturers will be able to acquire to expand their existing solar cell production lines. Manufacturers would then be able to produce the black silicon themselves and include it in the cells as standard.
The ‘Customized light pulses’ project was one of this year’s winners in the ‘365 Places in the Land of Ideas’ competition; the awards ceremony is due to be held in Goslar on October 11th, 2012.
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