Using two different types of chemical etching to create features at both the micron and nanometer size scales, researchers at the Georgia Institute of Technology have developed a surface treatment that boosts the light absorption of silicon photovoltaic cells in two complementary ways.
The surface treatment increases absorption both by trapping light in three-dimensional structures and by making the surfaces self-cleaning – allowing rain or dew to wash away the dust and dirt that can accumulate on photovoltaic arrays. Because of its ability to make water bead up and roll off, the surface is classified as superhydrophobic."The more sunlight that goes into the photovoltaic cells and the less that reflects back, the higher the efficiency can be," said C.P. Wong, Regents' professor in Georgia Tech's School of Materials Science and Engineering. "Our simulations show that we can potentially increase the final efficiency of the cells by as much as two percent with this surface structure."
The silicon etching treatment 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.
In the silicon surface treatment, the two-tier roughness – created with both micron- and nano-scale structures – works in the same way as the lotus leaf, minimizing contact between the water or dust and the surface, Wong noted.
"When a water droplet reaches the surface, it sits on top of this two-tier roughness and only about three percent of it is in contact with the silicon," he explained.
Preparation of the superhydrophobic surface begins with 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 textured surface and the self-cleaning ability both help boost absorption of sunlight hitting the silicon surface.
"A normal silicon surface reflects a lot of the light that comes in, but by doing this texturing, the reflection is reduced to less than five percent," said Dennis Hess, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering. "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."
Even in desert areas where constant sunlight provides ideal conditions for photovoltaic arrays, nighttime dew should provide enough moisture to keep the cells clean, Wong said.
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.
Technical Contacts: C.P. Wong (404-894-8391); E-mail: (firstname.lastname@example.org) or Dennis Hess (404-894-5922); E-mail: (email@example.com)
John Toon | EurekAlert!
Further reports about: > Ferchau Engineering > Materials Science > Science TV > Superhydrophobic > crystalline planes > hydrogen fluoride > hydrogen peroxide > low-reflectivity treatment > nanometer size scales > photovoltaic cells > potassium hydroxide > self-cleaning > silicon photovoltaic cells > silicon surface > superhydrophobic surface > surface roughness
Multi-institutional collaboration uncovers how molecular machines assemble
02.12.2016 | Salk Institute
Fertilized egg cells trigger and monitor loss of sperm’s epigenetic memory
02.12.2016 | IMBA - Institut für Molekulare Biotechnologie der Österreichischen Akademie der Wissenschaften GmbH
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water
In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...
The efficiency of power electronic systems is not solely dependent on electrical efficiency but also on weight, for example, in mobile systems. When the weight of relevant components and devices in airplanes, for instance, is reduced, fuel savings can be achieved and correspondingly greenhouse gas emissions decreased. New materials and components based on gallium nitride (GaN) can help to reduce weight and increase the efficiency. With these new materials, power electronic switches can be operated at higher switching frequency, resulting in higher power density and lower material costs.
Researchers at the Fraunhofer Institute for Solar Energy Systems ISE together with partners have investigated how these materials can be used to make power...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
02.12.2016 | Medical Engineering
02.12.2016 | Agricultural and Forestry Science
02.12.2016 | Physics and Astronomy