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

March 15, 2012

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Carbon nanotube electrodes. The use of carbon nanotubes has a significant cost advantage. However, in earlier designs (left), the carbon nanotubes degraded through chemical processes (e-: electrons, I3-: ions in the liquid). Using a thin protective layer of titanium oxide now stabilizes the nanotubes (right), increasing the performance of these cells. Credit: 2011 AIP

Solar energy is one of the most promising forms of renewable energy, but the high cost of conventional solar cells has so far limited its popularity. To increase the competitiveness of solar energy, scientists have turned to the development of dye-sensitized solar cells — solar cells that use low-cost organic dyes and titanium dioxide (TiO2) nanoparticles in place of expensive semiconductor and rare earth elements to absorb sunlight. Zhaohong Huang at the A*STAR Institute of Materials Research and Engineering and co-workers have now reduced the cost of dye-sensitized solar cells even further by replacing indium tin oxide (ITO) — the standard material for transparent electrodes — with carbon nanotubes.

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)

Source : http://www.physorg.com/news/2012-03-dye-sensitized-solar-cells-carbon-nanotube.html

Solar cell turns windows into generators

March 20, 2012

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(PhysOrg.com) -- Imagine a world where the windows of high-rise office buildings are powerful energy producers, offering its inhabitants much more than some fresh air, light and a view.

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

Source : http://www.physorg.com/news/2012-03-solar-cell-windows.html