From a carpet of nanorods to a thin film solar cell absorber within a few seconds

Research teams at the HZB and at the University of Limerick, Ireland, have discovered a novel solid state reaction which lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound. At the EDDI Beamline at BESSY II, the scientists could observe this process in real-time when heating the sample: in a few seconds Kesterite grains formed. The size of the grains was found to depend on the heating rate. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin film solar cells. These findings have now been published in the journal “Nature Communications”.

Grain formation during growth of kesterite solar cells observed in real-time
As starting material for the formation of the kesterite film serves a “carpet of nanorods”: With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick have fabricated films of highly ordered wurtzite nanorods, which have exactly the same composition as kesterite Cu₂ZnSnS₄. With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold could now observe how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. “It is interesting to see that the complete formation of the kesterite film is so fast”, says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form – instead of a few larger grains. Additionally, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.

Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model which can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.

Novel synthesis pathway for thin film semiconductors with controlled morphology
The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu₂ZnSnS₄-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se₂ chalcopyrite solar cells which already achieved efficiencies above 20%. Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements which are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. “But we continue to focus on kesterites, because this is a really exciting topic at the moment.”

The results have been published in Nature communications doi: 10.1038/ncomms4133

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13909;sprache=en;typoid=3228

Improving solar efficiencies by 'inverse opal' structure

Researchers have shown how to increase the efficiency of thin-film solar cells, a technology that could bring low-cost solar energy. The approach uses 3-D "photonic crystals" to absorb more sunlight than conventional thin-film cells.

The synthetic crystals possess a structure called an "inverse opal" to make use of and enhance properties found in the gemstones to reflect, diffract and bend incoming sunlight.

"Usually, in thin-film silicon solar cells much of the sunlight comes right back out, but using our approach the light comes in and it is diffracted, causing it to propagate in a parallel path within the film," said Peter Bermel, an assistant professor in Purdue University's School of Electrical and Computer Engineering and Birck Nanotechnology Center.

Compared to solar cells made of silicon wafers, cost is reduced 100 times for the thin films. However, they are less efficient.

"The question is, can we make up that lower efficiency by introducing new approaches to light trapping for thin film solar cells?" Bermel said. "Can we combine low cost and high performance?"

The researchers are the first to demonstrate incorporation of the 3-D photonic crystals to increase light trapping in crystalline silicon solar cells. Experimental findings indicate roughly a 10 percent increase in efficiency over conventional silicon thin films, with further potential for improvement.

The technology is better at absorbing and harvesting near-infrared light.

"A major reason thin-film silicon solar cells have lower efficiency is that they don't absorb near-infrared light very effectively," Bermel said. "Light in the near-infrared range is important because there is a lot of solar energy in that wavelength range and also because silicon can convert near infrared light to energy if it can absorb it, but thin films don't fully absorb it."

Findings were detailed in a research paper appearing in October in the peer-reviewed scientific journal Advanced Optical Materials.

The researchers created inverse opals using a process called meniscus-driven self-assembly.

"You could make them to custom order or design, and we decided to make them for solar cells in order to improve absorption of light," Qi said.

Silicon has for many years been the dominant material used in solar cells. However, solar cells made of thick monocrystalline silicon wafers are too expensive to be practical for widespread application. This limitation has driven recent innovation in multicrystalline and thin-film silicon solar cells.

"Our premise is to use only 1 percent as much material as a silicon wafer using these thin films of crystalline silicon," Qi said.

Applications for thin-film solar cells include generating electricity for utilities and the home, as well as smaller-scale applications such as mobile charging of electronic devices.

Natural opals create rainbow patterns caused when different wavelengths of light are diffracted at different angles. Opals are made of solid silica spheres in a matrix of some other material. The new synthetic structures are called inverse opals because they consist of hollow spheres of air surrounded by silicon.

The researchers first build a standard opal structure. The spheres are placed in a solution, which evaporates, leaving the self-assembled structure.

"As it evaporates the spheres get stacked on top of the substrate right at the meniscus, the interface between the liquid and air," Varghese said.

Manufacturers now increase light absorption by etching or depositing random textures on the thin films.

"We think it is best to combine both the textured randomness as well as ordered structure," Bermel said. "The texture helps well with some wavelengths and the ordered structure will help with others."

More information: Varghese, L. T., Xuan, Y., Niu, B., Fan, L., Bermel, P. and Qi, M. (2013), "Enhanced Photon Management of Thin-Film Silicon Solar Cells Using Inverse Opal Photonic Crystals with 3D Photonic Bandgaps." Advanced Optical Materials, 1: 692–698. DOI: 10.1002/adom.201300254


Read more at: http://phys.org/news/2014-01-inverse-opal-thin-film-solar-cells.html#jCp

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

Maximizing the power conversion efficiency of thin-film silicon solar cells

Design optimization could help maximize the power conversion efficiency of thin-film silicon solar cells

Silicon is readily available, easy to process, highly stable and non-toxic. It is also one of the best materials for making solar cells. The high quality and purity of silicon needed for fabricating the most efficient silicon-based solar cells, however, has made it difficult to lower production costs for this renewable energy technology. One approach that could reduce costs is to use a microscopically thin film of silicon with a textured surface to enhance light absorption. Navab Singh at the A*STAR Institute of Microelectronics and co-workers have now highlighted several key factors affecting the power conversion efficiency of surface-textured thin-film solar cells and come up with a ‘nanopillar’ design that maximizes light absorption and minimizes production costs.