Black Silicon for Solar Cells: What does it mean?

Silicon is the “material of choice” for solar voltaic cells because it takes relatively little energy to excite the electrons in a silicon crystal into the conduction band, where they are free to conduct electricity that can be used to generate electric power. Scientists and engineers have spent many years trying to modify silicon crystals by doping them with other materials that would make the photovoltaic process even more efficient. In the last few years, scientists at Harvard University found that if they blasted the surface of a silicon wafer (in the presence of sulfur hexafluoride gas) with femtosecond laser pulses, the silicon received a heavy doping of sulfur and the surface of the silicon developed deep, microscopic cones. The result was that this newly formed, thin silicon layer, called “black silicon” is 200-500 times more sensitive to light than untreated silicon. How is this possible?

With its new structure, the band gap in the thin silicon layer - the difference between the valence band and the conduction band - is smaller. This means that longer wavelengths of sunlight (infrared) are also able to excite electrons into the conduction band - contributing energy to the solar-electric conversion. Furthermore, by applying a small voltage (a bias) to black silicon creates conditions in which each incoming photon can excite still more electrons. So, not only is the material responsive to wavelengths that silicon-based devices couldn’t detect in the past - it also produces a much stronger signal in response to a weak stimulus. The increased sensitivity makes black silicon good for detection applications; and the increase in absorption wavelengths improves the efficiency of silicon for solar cells.

The Harvard Scientists (Stephen Saylor and James Carey) formed a spinoff company, SiOnyx, to commercialize the process for solar energy and highly-sensitive, imaging applications, such as night vision, surveillance, digital cameras and medical imaging. For over three years, they’ve been pretty quiet about this, but in the last few months, they are starting to talk. Maybe that’s a sign that some of the applications of black silicon are about to “come to market”.

This is another example of photonics (femtosecond lasers) as an enabling technology - this time to enhance the efficiencies of solar voltaic cells.

To read the complete article about black silicon, go to:

www.xconomy.com/boston/2008/10/12/sionyx

What do you think about the potential of “black silicon” for solar cell improvement? What other breakthroughs do you see in solar electric energy?