March 26, 2009 Solar power from photovoltaic cells are widely recognized as an integral part of a clean green future, and any development that can make these cells more efficient, no matter how small, assists in making this future a reality. A team of researchers at Georgia Tech have developed a surface treatment that boosts the light absorption of silicon photovoltaic cells by trapping light in three-dimensional structures and by making the surfaces self cleaning.
In the silicon surface treatment uses two different types of chemical etching to create features at both the micron and nanometer size scales. The two-tier roughness – created with both micron- and nano-scale structures - mimics the superhydrophobic surface of the lotus leaf, which uses surface roughness at two different size scales to create high contact angles that encourage water from rain or condensation to bead up and run off. As the water runs off, it carries with it any surface dust or dirt – which also doesn’t adhere because of the unique surface properties.
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Preparation of the surface begins with the use of a potassium hydroxide (KOH) solution to etch the silicon surface. The solution preferentially removes silicon along crystalline planes, creating micron-scale pyramid structures in the surface. An e-beam process is then used to apply nanometer-scale gold particles to the pyramid structures. Using a solution of hydrogen fluoride (HF) and hydrogen peroxide (H2O2), a metal-assisted etching process with gold as the catalyst produces the nanometer-scale features. The feature size is controlled by the diameter of the gold particles and the length of time the silicon is exposed to the etching. Finally, the gold is removed with a potassium iodide (KI) solution and the surface coated with a fluorocarbon material, perfluorooctyl tricholosilane (PFOS).
The combination of increased light absorption from the resulting textured surface and the self-cleaning ability both help boost absorption of sunlight hitting the silicon surface. The team says simulations show that this surface structure can potentially increase the final efficiency of the cells by as much as two percent. Two percent doesn’t sound like much but as Georgia Tech School of Chemical and Biomolecular Engineering professor, Dennis Hess, points out, “As much as 10 percent of the light that hits the cells is scattered because of dust and dirt of the surface. If you can keep the cells clean, in principle you can increase the efficiency. Even if you only improve this by a few percent, that could make a big difference.” The team also said that even in desert areas where constant sunlight provides ideal conditions for photovoltaic arrays, night-time dew should provide enough moisture to keep the cells clean.
The research team, which also included Yonghao Xiu, Shu Zhang and Yan Liu, is working with Georgia Tech’s University Center of Excellence for Photovoltaics Research and Education, headed by Professor Ajeet Rohatgi of Georgia Tech's School of Electrical and Computer Engineering, to evaluate the surface treatment with real solar cells. However, adoption of the superhydrophobic surface treatment will ultimately depend on its long-term robustness and cost. “Because the structures are so small, they are fairly fragile,” Hess noted. “Mechanical abrasion to the surface can destroy the superhydrophobicity. We have tried to address that here by creating a large superhydrophobic surface area so that small amounts of damage won’t affect the overall surface.”
Large scale cost estimates haven’t yet been done, but Hess said the additional etching and vacuum deposition steps shouldn’t add dramatically to the already complex manufacturing process used for fabricating silicon PV cells. In addition to photovoltaic cells, the surface treatment could be used to create anti-bacterial coatings on medical equipment, micro-electromechanical devices that don’t stick together, and improved microfluidic devices.
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