Black silicon is a highly absorbent semi-conductor that is produced by manipulating the nano-scale surface of the silicon to improve the amount of light absorbed. The combination of low reflectivity and the semi-conductive properties of silicon found in black silicon makes it a prime candidate for application in photovoltaic solar cells. While black silicon has been known of for 30 years or so, recent advances in its production by groups at Harvard and in Munich have pushed forward the prospects of black silicon applications in the solar cell industry.
In general, silicon surfaces have high natural reflectivity which poses a problem for the solar cell industry. Whereas silicon works well as a semiconductor, the high natural reflectivity of the material greatly hurts the efficiency of the silicon cells. A clever way to overcome the natural reflectivity is to manipulate the surface of the silicon on the nano scale. By altering the surface of the silicon to consist of a series of peaks and valleys, one greatly increases the absorption as incident photons are reflected into the peaks and absorbed by the material. This phenomena was theorized by Richard Stephens and George Cody back in the 1970s. As Stephens and Cody noticed, the reflectivity of a material can be greatly decreased by "light trapping by multiple reflections." [1] Recent production techniques of black silicon (named such as with decreased reflectivity the material appears black) have been able to deform the surface and reduce reflectivity with great success. Current methods of Svetoslave Koynov, Martin Brand and Martin Stutzmann at the University of Munich have been able to achieve "an average reduction of surface reflection to about 8% - 15%." [2] This progress will greatly improve the efficiency of silicon used in photovoltaic cells. While the theory behind black silicon has been around for some time, various different production methods have been developed that are opening the door for the future application of black silicon to the solar cell industry.
The key to production of black silicon is developing methods of morphing the surface of the material that are economical and easily applicable for mass production. Further the surface produced must maintain its features as it is further manipulated and processed into a solar cell. Eric Mazur's research group in Harvard found ways to produce black silicon by using femtosecond laser pulses to manipulate the surface. His group used laser pulses and "created arrays of sharp conical spikes" on the surface of silicon. [3] These spikes improved the absorption by increasing the self reflection of light as put forth by Stephens and Cody. The laser process is still in development by the Mazur group and offers a potential production method for the solar cell industry. Mazur and others from his research group have founded SiOnyx, a startup that is developing production methods of black silicon. [4]
Other research groups have found alternate etching techniques to manipulate the surface. The group at the University of Munich uses a wet etching process that utilizes gold atoms to deform the surface of the silicon. The wet etching is a novel method that "can be applied to various forms of bulk silicon (single, poly-, or multicrystaline) as well as to thin Si films (amorphous or microcrystalline)." [5] Further the method creates a silicon that "survives the harsh chemical and thermal treatments during solar cell processing," and thus is very promising to the solar cell industry which would need to process bulk silicon.
Black silicon has various applications, but most importantly for our uses it has promising applications in the field of solar cells. On a basic level, the material seems to combine the best of both worlds--the high absorption of black materials and the semi-conducting properties of silicon. However, before the material can start to have an effect on the solar cell industry the production methods described above must take hold on an industrial level and prove that the production can be made economical. Further, the exact application of the silicon into solar cells must be figured out as the black silicon does not exhibit the same exact properties as normal silicon. For example black silicon makes much more current when exposed to light. As Tonio Bunassisi of MIT notes, "it is not clear if black silicon can be coaxed into doing that [create a voltage in response to light] efficiently." [4] Still though, while much work needs to be done on the integration of black silicon into the solar cell industry, it is a very unique material that has many prospects for greatly improving the efficiency of solar cells.
© Tim Haefele. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. B. Stephens and G. D. Cody, "Optical Reflectance and Transmission of a Textured Surface," Thin Solid Films 45, 19 (1977).
[2] S. Koynov, M. S. Brandt and M. Stutzmann, "Black Nonreflecting Silicon Surfaces for Solar Cells," Appl. Phys. Lett. 88, 203107 (2008).
[3] T.-H. Her et al., "Microstructing of Silicon with Femtosecond Laser Pulse," Appl. Phys. Lett., 73, 1673 (1998).
[4] P. Patel, "Black Silicon - A New Type of Silicon Promises Cheaper, More-Sensitive Light Detectors," Technology Review, 29 Oct 08.
[5] S. Koynov, M. S. Brandt and M. Stutzmann, "Black Multi-Crystalline Silicon solar Cells." Phys. Stat. Sol. (RRL) 1, R53 (2007).