Photonics breakthrough advances thin-film solar cell performance

http://www.electronics-eetimes.com/en/photonics-breakthrough-advances-thin-film-solar-cell-performance.html?cmp_id=7&news_id=222920627&vID=209&page=1
Paul Buckley

Researchers at the University at Buffalo have developed a multilayered waveguide taper array, which is a nanoscale microchip component that claims to improve the ability to trap and absorb light and could advance the performance of thin-film solar cell technology.

The work, published March 28 in the journal Scientific Reports, explores the use of waveguide tapers to slow and ultimately absorb each frequency of light at different places vertically to catch a 'rainbow' of wavelengths, or broadband light.

We previously predicted the multilayered waveguide tapers would more efficiently absorb light, and now weve proved it with these experiments, explained lead researcher Qiaoqiang Gan, PhD, UB assistant professor of electrical engineering. This advancement could prove invaluable for thin-film solar technology, as well as recycling waste thermal energy that is a byproduct of industry and everyday electronic devices such as smartphones and laptops.

Each multilayered waveguide taper is made of ultrathin layers of metal, semiconductors and/or insulators. The tapers absorb light in metal dielectric layer pairs, the so-called hyperbolic metamaterial. By adjusting the thickness of the layers and other geometric parameters, the tapers can be tuned to different frequencies including visible, near-infrared, mid-infrared, terahertz and microwaves.

The structure could lead to advancements in a number of applications.

The multilayered waveguide taper array could improve thin-film photovoltaic cells, which are less expensive and more flexible than traditional solar cells. 

The drawback with thin-film solar cells is that they do not absorb as much light as traditional cells. Because the multilayered waveguide taper structure array can efficiently absorb the visible spectrum, as well as the infrared spectrum, it could potentially boost the amount of energy that thin-film solar cells generate.

In the field of on-chip optical communications there is the crosstalk phenomenon, in which an optical signal transmitted on one waveguide channel creates an undesired scattering or coupling effect on another waveguide channel. The multilayered waveguide taper structure array could potentially prevent crosstalk.


The multilayered waveguide taper array could help recycle waste heat generated by power plants and other industrial processes, as well as electronic devices such as televisions, smartphones and laptop computers.

It could be useful as an ultra compact thermal-absorption, collection and liberation device in the mid-infrared spectrum, said Dengxin Ji, a PhD student in Gans lab and first author of the paper.

It could even be used as a stealth, or cloaking, material for airplanes, ships and other vehicles to avoid radar, sonar, infrared and other forms of detection. The multilayered waveguide tapers can be scaled up to tune the absorption band to a lower frequency domain and absorb microwaves efficiently, added Haomin Song, another PhD student in Gans lab and the papers second author.
Reference
Broadband absorption engineering of hyperbolic metafilm patterns 

Related articles and links:

www.buffalo.edu

Abstract-Artificial birefringent metallic planar structures for terahertz wave polarization manipulation

Lei Wang, Suhua Jiang, Haifeng Hu, Haomin Song, Wei Zeng, and Qiaoqiang Gan 
 »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-2-311

We propose an artificial birefringent terahertz (THz) device constructed by subwavelength L-shaped hole arrays on a single metallic layer. This structure is able to work as a polarizer when the incident frequency is between the cut-off frequencies of two eigenmodes. When the incident wave is beyond cut-off frequencies of these two modes, it can be designed as an efficient half- or quarter-wave plate with extraordinary transmission properties. A big effective index difference from 0.254 to 0.768 is obtained using a subwavelength-thick planar structure, which can reduce the thickness of the device to one tenth of conventional quartz birefringent crystals for THz waves.

© 2014 Optical Society of America

Abstract-Artificial Birefringent Metallic Planar Structures for Terahertz Wave Polarization Manipulation

• Lei Wang, haifeng Hu, Haomin Song, Wei Zeng, Suhua Jiang, and Qiaoqiang Gan
• http://www.opticsinfobase.org/ol/upcoming_pdf.cfm?id=197426

• Abstract: We propose an artificial birefringent terahertz (THz) device constructed by subwavelength L-shaped hole arrays on a single metallic layer. This structure is able to work as a polarizer when the incident frequency is between the cutoff frequencies of two eigenmodes. When the incident wave is beyond cutoff frequencies of these two modes, it can be designed as an efficient half- or quarter-wave plate with extraordinary transmission properties. A big effective index difference from 0.254 to 0.768 is obtained using a subwavelength-thick planar structure, which can reduce the thickness of the device to one tenth of conventional quartz birefringent crystals for THz waves.

Engineers Are Catching Rainbows: Material That Slows Light Opens New Possibilities in Solar Energy, Other Fields

An up-close look at the “hyperbolic metamaterial waveguide,” which catches and ultimately absorbs wavelengths (or color) in a vertical direction. (Credit: Image courtesy of University at Buffalo)
http://www.sciencedaily.com/releases/2013/02/130217085259.htm
Feb. 15, 2013 — University at Buffalo engineers have created a more efficient way to catch rainbows, an advancement in photonics that could lead to technological breakthroughs in solar energy, stealth technology and other areas of research.

Qiaoqiang Gan, PhD, an assistant professor of electrical engineering at UB, and a team of graduate students described their work in a paper called "Rainbow Trapping in Hyperbolic Metamaterial Waveguide," published Feb. 13 in the online journal Scientific Reports.
They developed a "hyperbolic metamaterial waveguide," which is essentially an advanced microchip made of alternate ultra-thin films of metal and semiconductors and/or insulators. The waveguide halts and ultimately absorbs each frequency of light, at slightly different places in a vertical direction, to catch a "rainbow" of wavelengths.
Gan is a researcher within UB's new Center of Excellence in Materials Informatics.
"Electromagnetic absorbers have been studied for many years, especially for military radar systems," Gan said. "Right now, researchers are developing compact light absorbers based on optically thick semiconductors or carbon nanotubes. However, it is still challenging to realize the perfect absorber in ultra-thin films with tunable absorption band.
"We are developing ultra-thin films that will slow the light and therefore allow much more efficient absorption, which will address the long existing challenge."
Light is made of photons that, because they move extremely fast (i.e., at the speed of light), are difficult to tame. In their initial attempts to slow light, researchers relied upon cryogenic gases. But because cryogenic gases are very cold -- roughly 240 degrees below zero Fahrenheit -- they are difficult to work with outside a laboratory.
Before joining UB, Gan helped pioneer a way to slow light without cryogenic gases. He and other researchers at Lehigh University made nano-scale-sized grooves in metallic surfaces at different depths, a process that altered the optical properties of the metal. While the grooves worked, they had limitations. For example, the energy of the incident light cannot be transferred onto the metal surface efficiently, which hampered its use for practical applications, Gan said.
The hyperbolic metamaterial waveguide solves that problem because it is a large area of patterned film that can collect the incident light efficiently. It is referred to as an artificial medium with subwavelength features whose frequency surface is hyperboloid, which allows it to capture a wide range of wavelengths in different frequencies including visible, near-infrared, mid-infrared, terahertz and microwaves.
It could lead to advancements in an array of fields.
For example, in electronics there is a phenomenon known as crosstalk, in which a signal transmitted on one circuit or channel creates an undesired effect in another circuit or channel. The on-chip absorber could potentially prevent this.
The on-chip absorber may also be applied to solar panels and other energy-harvesting devices. It could be especially useful in mid-infrared spectral regions as thermal absorber for devices that recycle heat after sundown, Gan said.
Technology such as the Stealth bomber involves materials that make planes, ships and other devices invisible to radar, infrared, sonar and other detection methods. Because the on-chip absorber has the potential to absorb different wavelengths at a multitude of frequencies, it could be useful as a stealth coating material.
Additional authors of the paper include Haifeng Hu, Dengxin Ji, Xie Zeng and Kai Liu, all PhD candidates in UB's Department of Electrical Engineering. The work was sponsored by the National Science Foundation and UB's electrical engineering department.