February 2, 2011 — Sun-drenched roofs, windows, deserts and even clothing will someday be integrated with inexpensive solar cells that are many times thinner and lighter than current options. It is not hard to envision a time when integrated solar technologies will be ubiquitous in our increasingly energy-hungry lives. A multidisciplinary team of Stanford engineers led by Mike McGehee, Yi Cui and Mark Brongersma, and joined by Michael Graetzel at the École Polytechnique Fédérale de Lausanne (EPFL), are focused on that goal.

The Stanford/EPFL team succeeded in harnessing plasmonics to more effectively trap light within thin solar cells to improve performance and production feasability. Results were published in Advance Energy Materials.

"Plasmonics makes it much easier to improve the efficiency of solar cells," said McGehee, an associate professor of materials science and engineering at Stanford, and director of CAMP, Center for Advanced Molecular Photovoltaics, a multidisciplinary, multi-university team tackling the challenges of thin-film solar cells.

"Using plasmonics we can absorb the light in thinner films than ever before," McGehee said. "The thinner the film, the closer the charged particles are to the electrodes. In essence, more electrons can make it to the electrode to become electricity."

Figure 1. Acting like a waffle iron, silicon nanodomes, each about 300 nanometers in diameter and 200 nanometers tall, imprint a honeycomb pattern of nanoscale dimples into a layer of metal within the solar cell. Image courtesy of Michael McGehee.

Plasmonics is the study of the interaction of light and metal. Under precise circumstances, these interactions create a flow of high-frequency, dense electrical waves rather than electron particles. The electronic pulse travels in extremely fast waves of greater and lesser density, like sound through the air.

To make the plasmonic dye-sensitized solar cell (DSSC), the team imprinted a honeycomb pattern of nanoscale dimples into a layer of metal within the solar cell, like a nanoscale waffle, with the bumps on the waffle iron as domes rather than cubes. McGehee and team members spread a thin layer of "batter" on a transparent, electrically conductive base. This batter is mostly titania, a semiporous metal that is also transparent to light. Next, they use their nano waffle iron to imprint the dimples into the batter. They then layer on a light-sensitive dye that oozes into the dimples and pores of the substrate. Lastly, the engineers add a layer of silver, which hardens almost immediately.

Figure 2. Titania within the solar cell is imprinted by the silicon nanodomes like a waffle imprinted by the iron. Image courtesy of Michael McGehee.

When all those nanodimples fill up, the result is a pattern of nanodomes on the light-ward side of the silver. This bumpy layer of silver has two primary benefits. First, it acts as a mirror, scattering unabsorbed light back into the dye for another shot at collection. Second, the light interacts with the silver nanodomes to produce plasmonic effects. Those domes of silver are crucial. Reflectors without them will not produce the desired effect. Nanodomes must be precisely the right diameter, height, and spacing to fully optimize the plasmonics. Read more about nanotechnology here.

Photons enter and pass through the transparent base and the titania, at which point some photons would be absorbed by the light-sensitive dye, creating an electric current. Most of the remaining photons would hit the silver back reflector and bounce back into the solar cell. A certain portion of the photons that reach the silver, however, will strike the nanodomes and cause plasmonic waves to course outward.

Dye solar cells typically reach 8% efficiency and 7 year life spans — significantly below photovoltaic industry standard of 25% efficiency, 20-30 year lifetime. Engineers like McGehee believe that if they can convert just 15% of the light into electricity and extend the product lifecycle to a decade, the roll-to-roll production, less toxic materials, flexibility, light weight, and other benefits of this emerging thin-film tech will outweigh the lower numbers.

Learn more at http://www.stanford.edu/

Courtesy of Andrew Myers, associate communications director for the School of Engineering, Stanford.

Also read: EU partners eye metallic nanostructures for solar cells 

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January 31, 2011 — Researchers from Boston College (BC), MIT, Clemson University and the University of Virginia have used nanotechnology to achieve a 60-90% increase in the thermoelectric figure of merit of p-type half-Heusler, a common bulk semiconductor compound, the team reported in the journal Nano Letters.

The dramatic increase in the figure of merit, used to measure a material’s relative thermoelectric performance, could pave the way for a new generation of products — from car exhaust systems and power plants to solar power technology — that that runs cleaner, according to co-author Xiao Yan, a researcher in the Department of Physics at Boston College.

The team registered improvement in half-Heusler, which has been under study for its thermal stability, mechanical sturdiness, non-toxicity, and low cost. However, the application of half-Heusler has been limited because of its poor thermoelectric performance: it previously registered a peak figure of merit of approximately 0.5 at 700 oC for bulk ingots.

Yan, working with BC Professor of Physics Zhifeng Ren and MIT’s Soderberg Professor of Power Engineering Gang Chen, have increased the figure of merit value of p-type half-Heusler to 0.8 at 700 oC. Moreover, the groups’ material preparation methods proved to save time and expense compared with conventional methods.

"This method is low cost and can be scaled for mass production," Ren said. "This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner."

The researchers obtained their results by first forming alloyed ingots using arc melting technique and then creating nanoscale powders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Transport property measurements together with microstructure studies on the nanostructured samples, in comparison with that of bulk ingots, showed that the thermoelectric performance improves largely because of low thermal conductivity produced by enhanced phonon scattering at grain boundaries and defects in the material. The material was also found to have a high Seebeck coefficient, a measure of thermoelectric power.

Researchers in the BC and MIT labs are still trying to prevent grain growth during press, which accounts for the still large thermal conductivity of half-Heusler.

"Even lower thermal conductivity and improved thermoelectric performance can be expected when average grain sizes are made smaller than 100nm," said Ren, who was joined on the team by fellow Boston College researchers Giri Joshi, Weishu Liu, Yucheng Lan and Hui Wang, MIT’s Sangyeop Lee, Virginia’s Rogers Professor of Physics Joe Poons and J.W. Simonson and Clemson Professor of Physics Terry M. Tritt.

Learn more at www.bc.edu.

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