Dye-sensitized solar cells with carbon nanotube transparent electrodes offer significant cost savings

A typical dye-sensitized solar cell comprises a porous layer of TiO2 nanoparticles immersed in an organic dye. The dye absorbs the sunlight and converts the energy into electricity, which flows into the TiO2 nanoparticles. The sun-facing side of the solar cell is usually covered with a transparent electrode that carries the charge carriers away from the TiO2 and out of the solar cell. “Unfortunately, ITO electrodes are brittle and crack easily,” says Huang. “They are also expensive and could incur up to 60% of the total cost of the dye-sensitized solar cell.”

Huang and his team therefore replaced the ITO electrode with a thin film of carbon nanotubes. Carbon nanotubes conduct electricity and are almost transparent, flexible and strong, which make them the ideal material for transparent electrodes. The only drawback is that photo-generated charge carriers in the nanotube may recombine with ions in the dye, which reduces the power conversion efficiency of the solar cell.

To overcome this problem, Huang and his team placed a TiO2 thin film in between the carbon nanotube thin film and the porous layer. They found that the performance of dye-sensitized solar cells with TiO2 thin film was significantly better than those without. However, they also found that the solar conversion efficiency of their new dye-sensitized solar cells was only 1.8%, which is lower than that of conventional solar cells using ITO electrodes. This is due to the higher electrical resistances and reduced optical transparency of the carbon nanotube films, which limits the amount of sunlight entering the cell.

“We are now studying different ways to enhance the conductivity and transparency of the films,” says Huang. “Furthermore, we are planning to replace the bottom platinum electrode with carbon nanotube thin film to reduce the cost of dye-sensitized solar cells further.”

If successful, the results could have a great impact on the cost and stability of dye-sensitized solar cells.

More information: Research article in Applied Physics Letters.

Provided by Agency for Science, Technology and Research (A*STAR)

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'Tunable' metal nanostructures for fuel cells, batteries and solar energy

But Cornell chemists have now developed a way to make porous metal films with up to 1,000 times the electrical conductivity offered by previous methods. Their technique also opens the door to creating a wide variety of metal nanostructures for engineering and biomedical applications, the researchers said.

The results of several years of experimentation are described March 18 online edition of the journal Nature Materials.

"We have reached unprecedented levels of control on composition, nanostructure and functionality -- for example, conductivity -- of the resulting materials, all with a simple 'one-pot' mix-and-heat approach," said senior author Ulrich Wiesner, the Spencer T. Olin Professor of Engineering.

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How it looks to a chemist: 3-isocyanatopropyltriethoxysilane (ICPTS), links to an amino acid which in turn grabs a metal ion ("M" represents whatever metal is chosen) from a metal acetate, leaving acetic acid behind. Having these structures in mind led to the Aha! moment for researcher Scott Warren.

The new method builds on the "sol-gel process," already familiar to chemists. Certain compounds of silicon mixed with solvents will self-assemble into a structure of silicon dioxide (i.e., glass) honeycombed with nanometer-scaled pores. The challenge facing the researchers was to add metal to create a porous structure that conducts electricity.

About 10 years ago, Wiesner's research group, collaborating with the Cornell Fuel Cell Institute, tried using the sol-gel process with the catalysts that pull protons off of fuel molecules to generate electricity. They needed materials that would pass high current, but adding more than a small amount of metal disrupted the sol-gel process, explained Scott Warren, first author of the Nature Materials paper.

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Just about any metal in the entire periodic table (shown in red and blue) can be used in the new process. Those labeled in blue can be bought off the shelf from chemical supply houses in the appropriate form. Credit: Wiesner Lab

Warren, who was then a Ph.D. student in Wiesner's group and is now a researcher at Northwestern University, hit on the idea of using an amino acid to link metal atoms to silica molecules, because he had realized that one end of the amino acid molecule has an affinity for silica and the other end for metals.

"If there was a way to directly attach the metal to the silica sol-gel precursor then we would prevent this phase separation that was disrupting the self-assembly process," he explained.

The immediate result is a nanostructure of metal, silica and carbon, with much more metal than had been possible before, greatly increasing conductivity. The silica and carbon can be removed, leaving porous metal. But a silica-metal structure would hold its shape at the high temperatures found in some fuel cells, Warren noted, and removing just the silica to leave a carbon-metal complex offers other possibilities, including larger pores.

The researchers report a wide range of experiments showing that their process can be used to make "a library of materials with a high degree of control over composition and structure." They have built structures of almost every metal in the periodic table, and with additional chemistry can "tune" the dimensions of the pores in a range from 10 to 500 nanometers. They have also made metal-filled silica nanoparticles small enough to be ingested and secreted by humans, with possible biomedical applications. Wiesner's group is also known for creating "Cornell dots," which encapsulate dyes in silica nanoparticles, so a possible future application of the sol-gel process might be to build Graetzel solar cells, which contain light-sensitive dyes. Michael Graetzel of the École Polytechnique Fédérale de Lausanne and innovator of the Graetzel cell is a co-author of the new paper. The measurement of the record-setting electrical conductivity was performed in his laboratory.

The research has been supported by the Department of Energy and, through several channels, the National Science Foundation.

Provided by Cornell University (news : web)

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iPhones Powered by Hydrogen Fuel Cells?

Apple has applied for two hydrogen fuel cell patents. Citing consumer awareness about fossil fuel's environmental and political impact, the move indicates the company has been looking into a new system to recharge their portable device batteries for over a year.

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"As a consequence of this increased consumer awareness," one application stated, "electronics manufacturers have become very interested in developing renewable energy sources for their products, and they have been exploring a number of promising renewable energy sources such as hydrogen fuel cells."

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The application goes on to say: "Hydrogen fuel cells have a number of advantages. Such fuel cells and associated fuels can potentially achieve high volumetric and gravimetric energy densities, which can potentially enable continued operation of portable electronic devices for days or even weeks without refueling."

Using hydrogen fuel cells to power mobile devices is nothing new. Horizon's MINIPAK and Toshiba's Dynario have been on the market for years, yet they aren't exactly small enough to integrate with mobile phones unless you're going for the Zack Morris look, 80's brick cell phone included.

However, Apple says their sleek fuel cell design would be able to eliminate the need for a bulky battery pack.

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The patent applications were published by the US Patent & Trademark Office last week. The first patent, "Fuel Cell System to Power a Portable Computing Device", was filed in August 2010, while the second patent, "Fuel Cell System Coupled to a Portable Computing Device" was filed in April 2011, suggesting Apple has had their eye on this technology for a while.

[Via GizMag]

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Vroom! World's Fastest Cells Drag Race

Ladies and gentlemen, start your... Petri dishes.

It may not have been the Indy 500, but a line of fetal mesenchymal bone marrow cells from Singapore recently out-dashed dozens of contenders to take the checkered flag at the World Cell Race. Claiming their title as the world's fastest cells, the microscopic racers zoomed across a Petri dish at the whiplash-inducing speed of 5.2 microns per minute, or 0.000000194 miles per hour.

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Results were announced December 3 at the American Society for Cell Biology's annual meeting in Denver, Colo. Fifty participating labs from around the world used 70 cell lines to not only race, but examine cell movement during the development of embryos, organs and cancer.

Teams shipped the cells frozen to designated laboratories in Boston, London, Heidelberg, Paris, San Francisco and Singapore. Once thawed, the cells were placed in "race tracks" that were 400 microns long (0.015748 inches) and coated with a substance that gave the little guys some tire-like traction. Digital cameras recorded the cells for 24 hours to determine, out of the 200 cells, which one was the fastest to reach the end of the track.

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A line of unaltered breast epithelial cells took second place and third place went to the the same cell type only altered to reflect patterns observed in cancerous cells. Researchers responsible for the winning cells received Nikon digital cameras and World Cell Race medals.

[Via Nature]

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