Using a specially constructed instrument system, researchers at the National Institute of Standards and Technology and the University of Colorado at Boulder have made images of particles only a few billionths of a meter wide that may be reducing the efficiency of certain light-sensitive films.
Under a contract with the Department of Energy's National Renewable Energy Laboratory, the researchers are studying the processes used to grow photovoltaic films over large surfaces. Such films are used in making solar energy panels and large flat-panel displays. The efficiency of the film in converting light into electrical current is best with homogenous, very thin films about 500 nanometers (500 billionths of a meter) thick.
In a project conducted at JILA, a joint institute of NIST and CU at Boulder, the researchers custom built a system for both growing thin films and examining them with an ultra-sensitive scanning tunneling microscope. The researchers used the new apparatus to grow amorphous (non-crystalline) films of silicon and hydrogen atoms with a technique called plasma-enhanced chemical vapor deposition, or PECVD. Images of the film were then taken at various stages throughout the growth process. The images show particles 3 to 5 nanometers in size, which form in the vapor and bond to the film surface during growth. As a new layer of silicon and hydrogen atoms deposits on the surface, these clumps cause voids within the film in the same way that a ball under an otherwise smooth blanket creates air pockets.
``Many people have studied the production of larger particles during PECVD, but these particles are suspended in the plasma and do not reach the growing film. As far as we know, no one has seen nanometer-sized particles before now, so they didn't realize that these can reach the growing film,'' says Alan Gallagher, principal investigator for the project. ``If we can figure out how to keep these particles from forming or prevent them from reaching the film surface, we should be able to improve the film's ability to convert light into electrical current.''
Current commercial solar films of this material convert about 10 percent of incoming solar energy into electrical energy. Improved efficiency is needed to lower the cost per watt to the level of more traditional methods of generating electricity.
Solar panel manufacturers also working on the NREL project will use the new JILA results to try to improve their processing methods. The JILA researchers also are developing a laser scattering system to detect the silicon/hydrogen clumps as they are forming in the plasma. Laser scattering detects larger particles but provides a method for real-time monitoring of particulate behavior in the plasma.
For further technical details, contact Alan Gallagher, Div. 848.03, JILA, Boulder, Colo. 80309-0440, (303) 497-3936, fax: (303) 492-5235, e-mail: firstname.lastname@example.org (via Internet).
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