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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Solar cell turns windows into generators

For the past four years a team of researchers from Flinders University has been working to make this dream a reality – and now the notion of solar-powered windows could be coming to a not too distant future near you.

As part of his just-completed PhD, Dr. Mark Bissett from the School of Chemical and Physical Sciences has developed a revolutionary solar cell using carbon nanotubes.

A promising alternative to traditional silicon-based solar cells, carbon nanotubes are cheaper to make and more efficient to use than their energy-sapping, silicon counterparts.

“Solar power is actually the most expensive type of renewable energy – in fact the silicon solar cells we see on peoples’ roofs are very expensive to produce and they also use a lot of electricity to purify,” Dr. Bissett said.

“The overall efficiency of silicon solar cells are about 10 per cent and even when they’re operating at optimal efficiency it could take eight to 15 years to make back the energy that it took to produce them in the first place because they’re produced using fossil fuels,” he said.

Dr. Bissett said the new, low-cost carbon nanotubes are transparent, meaning they can be “sprayed” onto windows without blocking light, and they are also flexible so they can be weaved into a range of materials including fabric – a concept that is already being explored by advertising companies.

While the amount of power generated by solar windows would not be enough to completely offset the energy consumption of a standard office building, Dr. Bissett said they still had many financial and environmental advantages.

“In a new building, or one where the windows are being replaced anyway, adding transparent solar cells to the glass would be a relatively small cost since the cost of the glass, frames and installation would be the same with or without the solar component,” Dr. Bissett said.

“It’s basically like tinting the windows except they’re able to produce electricity, and considering office buildings don’t have a lot of roof space for solar panels it makes sense to utilise the many windows they do have instead.”

Dr. Bissett said the technology mimics photosynthesis, the process whereby plants obtain energy from the sun.

“A solar cell is created by taking two sheets of electrically conductive glass and sandwiching a layer of functionalized single-walled carbon nanotubes between the glass sheets,” he said.

“When light shines on the cell, electrons are generated within the carbon nanotubes and these can be used to power electrical devices.”

Although small prototypes have been developed in the lab, he said the next step would be to test the carbon cells on an “industrial stage”.

If all goes to plan, the material could be on the market within 10 years.

“When we first started the research we had no idea if it would work because we were the first in the world to try it so it’s pretty exciting that we’ve proved the concept, and hopefully it will be commercially available in a few year’s time,” Dr. Bissett said.

Dr. Bissett is a winner of Flinders inaugural Best Student Paper Award, a now annual program which aims to recognise excellence in student research across the University.

Provided by Flinders University

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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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Nine Exoplanets Discovered in Solar System's 'Twin'

In 2010, a star 127 light-years away stunned the world -- it had become the largest star system beyond our own, playing host to five, possibly seven, alien worlds. Now, the star (called HD 10180) is back in the headlines; it may actually have nine exoplanets orbiting it.

Interestingly, HD 10180 is a yellow dwarf star very much like the sun, so this discovery has drawn many parallels with our own Solar System. It is a multi-planetary system surrounding a sun-like star. But it is also a very alien place with an assortment of worlds spread over wildly different orbits.

It is believed that one of HD 10180's exoplanets is small -- although astronomers only know the planets' masses, not their physical size or composition. The smallest world weighs-in at 1.4 times the mass of Earth, making it a "super-Earth."

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When it was first revealed that HD 10180 was a multi-planetary system, astronomers of the European Southern Observatory (ESO) detected six exoplanets gravitationally "tugging" on their host star. Using the "radial velocity" exoplanet detection method, the astronomers watched the star's wobble to decipher up to seven worlds measuring between 1.4 to 65 times the mass of Earth.

Five exoplanets were found to be 12-to-25 times the mass of Earth -- "Neptune-like" masses -- while another was detected orbiting in the outermost reaches of the system with a mass of 65 Earth masses (a "Saturn-like" world), taking around 2,200 days to complete one orbit.

But now, in addition to verifying the signal of the small 1.4 Earth-mass world, there appears to be another two small alien worlds.

"In addition to these seven signals, we report two additional periodic signals that are, according to our model probabilities ... statistically significant and unlikely to be caused by noise or data sampling or poor phasecoverage of the observations," Mikko Tuomi, of the University of Hertfordshire, reports in a new research paper (PDF) accepted for publication in the journal Astronomy & Astrophysics.

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This basically means that Tuomi has reanalyzed the data from previous observations made by the HARPS spectrograph (attached to the ESO's 3.6-meter telescope at La Silla, Chile), confirmed signals relating to the seven exoplanets discovered in 2010 and uncovered two new worlds in the process.

What's more, these two new signals represent another two super-Earths, says Tuomi. One is 1.9 times more massive than Earth and the other is 5.1 Earth-masses.

Although these may be "super-Earths," the only similarity to Earth is their mass, so don't go getting excited that we may have spotted the much sought-after Earth analogs.

The 1.4 Earth-mass exoplanet has an orbital period of only 1.2 days. The two new super-Earths also have very tight orbits, where their "years" last only 10 and 68 days. Therefore, any question of life (as we know it) existing on these worlds is moot -- they will likely be hellishly hot, with no chance of liquid water existing on their surfaces. It's debatable whether these worlds could hold onto any kind of atmosphere as they would be constantly sandblasted by intense stellar winds.

ANALYSIS: Billions of Habitable Worlds in Our Galaxy?

As we continue hunting for exoplanets, it's only a matter of time until we make the groundbreaking discovery of an Earth-mass world orbiting its sun-like star within the habitable zone -- the distance from a star where water may exist in a liquid state. Unfortunately, even if planet-hunting projects -- like NASA's Kepler space telescope -- detect such an "Earth-like" world, we'd need an even more powerful means of detecting whether or not such a world even has an atmosphere, let alone whether it has a solid surface with oceans of liquid water.

And as for detecting any kind of life, we may actually have to physically go there. But in the case of HD 10180, 127 light-years is one long trek.

Source: "Evidence for 9 planets in the HD 10180 system," Tuomi, 2012. arXiv:1204.1254v1 [astro-ph.EP]

Image credit: ESO

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Christmas Solar Eruption to Hit Earth and Mars

It's coming right at us! But don't worry, it shouldn't hurt too much when it hits.

On Christmas Day, the sun decided to get into the festive mood by laying on some decorations. Lacking the tinsel and tacky glow-in-the-dark reindeer on its front lawn, our nearest star decided to create a humongous coronal mass ejection (CME) in the shape of an interplanetary bauble, firing it right at us.

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One of NASA's twin Solar-Terrestrial Relations Observatory (STEREO) spacecraft managed to get a side-on view of the CME racing toward Earth and Mars on Dec. 26. Also, the veteran NASA/ESA Solar and Heliospheric Observatory (SOHO) nabbed a picture of the CME at around the same time (pictured above).

All predictions suggested the CME would likely hit us on Dec. 28 (Wednesday) and it appears that is going to happen.

Depending on the orientation of the magnetic field wrapped around the CME bubble of highly charged solar particles when it hits the Earth's geomagnetic field, there will be a chance of some auroral activity.

"There is a 20-40% chance of geomagnetic storms! If you live at a high latitude, look out for #auroras today," tweeted NASA's Solar Dynamics Observatory team on Wednesday.

ANALYSIS: Epic Geomagnetic Storm Erupts

Although being hit by a CME is a very well-known phenomenon for Earth, a CME impact on Mars will have a very different effect.

"Different world, different space weather," writes NASA's Tony Phillips of Spaceweather.com.

When a CME hits Earth, our planet's global geomagnetic field deflects the energy, interacts with the CME's magnetic field and funnels the solar energetic particles toward high latitude regions. The more fierce the geomagnetic storm, the deeper the particles penetrate.

As these particles rain down on high-latitude regions -- typically forming an "oval" around the polar caps when viewed from space -- interactions between the solar particles and molecules in our atmosphere generate light. The light is known as aurorae.

On Mars, it's a different story.

The Red Planet does not have a global magnetic field. Whereas Earth's magnetic field and thick atmosphere protects us from the worst ionizing effects of the sun's high-energy particles, Mars' thin atmosphere and insignificant magnetic field allow these CME particles to hit the surface.

Mars is therefore often bathed in the sun's high-energy particles -- a factor that could seriously hamper future human colonization efforts.

However, Mars does have pockets of magnetic "umbrellas." They are the ancient remnants of a once global magnetic field. It is thought that early in Martian history a huge asteroid smashed deep into the planet, interrupting the inner "dynamo" that generated its global magnetic field.

This effectively "switched off" Mars' geomagnetic field, leaving its atmosphere open to the ravages of the solar wind. That may be one of the reasons why the Martian atmosphere is 100 times thinner than Earth's -- it has quite literally been "blown away."

NEWS: Martian Air Blown Away by Solar Super Wave

So if you are lucky enough to witness the beautiful, dynamic aurora that may occur tonight or tomorrow, spare a thought for Mars where the majority of that radiation will rain down onto its surface.

Image: The sun as seen through SOHO's LASCO C3 instrument. The glare of the sun is obscured by an artificial "occulting disk" so that faint CME features can be seen in interplanetary space. The Earth-directed CME can be seen emerging from the left of the disk. Credit: NASA/ESA

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