Genesis Morocco: Plasmonic Cells, How Nanoparticles can Change the Game

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Genesis Morocco

Project Genesis is a strategic sustainable development framework for Morocco to translate from being a net importer of energy and a country facing water shortage issues, into the number one producer both of clean renewable energy and water in the region.

Wednesday, February 16, 2011    <<Home

Plasmonic Cells, How Nanoparticles can Change the Game


 Overall light-gathering efficiency for solar cells using metallic nanoparticles can be improved by 30 percent a University of New South Wales study shows.

Personal Notes : Ok, its not the scoop I have hoped it would be, the study dates back to 2008 and nanoparticles have been mentioned for their role in solar since 2002. However that is the time for a technology to mature and we should expect new exciting developments very soon. 

After getting interested in the role nanoparticles can play in water desalination and purification, I decided to research their possible role for solar energy, it turns out that the potential is tremendous, an increase in efficiency and lower costs for the panels have the potential to make solar energy cost competitive very quickly and can be a real game changer. 

This is very good news for Project Genesis as any increase in efficiency as well as lower costs translate into direct dividends and also accelerate the tendency towards Equilibrium. 

What I call Equilibrium point refers to the point where renewables provide for a 100% of our energy consumption, getting to and past Equilibrium point qualify a nation as self sufficient in energy and as a possible exporter. A 30% increase in the efficiency of solar cells is a very important development.

The best candidate is silver, and it is also outstanding that this technology which smack of the future was actually used by artisans as far back as the 9th century in Mesopotamia for generating a glittering effect on the surface of pots. 

Nano is beautiful. 

Below is a compound of articles relating on this ongoing technological development. Studies are being conducted in Sweden, England, The USA and in Australia and very likely in Japan. 

Nanoparticles inspire plasmonic solar cells Mar 27, 2009

Combining the properties of plasmonics with thin-film solar cell technology could disrupt the future of grid electricity. Caryl Richards talks to Kylie Catchpole at the Australian National University to find out more about advancements in plasmonic solar cells.

As demand grows for greener power generation and energy conservation, how can renewable technologies take on the might of goliaths of the fossil fuel industry? In the case of thin-film solar cells, the weapon of choice comes in the diminutive form of metallic nanoparticles. Thanks to a combination of the resonant plasmonic properties of metallic nanoparticles with thin-film photovoltaic technology, a new generation of plasmonic solar cell has evolved with similar performance to silicon cells but at potentially a fraction of the cost

Today, plasmonic solar cells are emerging as promising candidates amongst many solar energy technologies spurring continuing research to improve device performance. One leading research group in this area is based at the Centre for Sustainable Energy Systems at the Australian National University (ANU) who are working alongside other principal groups led by Harry Atwater and Albert Polman at Caltech, California, US and the FOM-Institute, AMOLF, the Netherlands, respectively.

The group at ANU measured an enhanced photocurrent attributed to the increased trapping of light scattered into a thin-film silicon cell by silver metal nanoparticles excited at their surface plasmon resonance. Now, leading scientists in the field are looking to drive plasmonic solar cells out of the science of the small into the next big thing in the photovoltaics industry (Optics Express 16 21793).

A thin slice of the solar industry

The global photovoltaic market as a whole looks set to ride out the economic downturn with a predicted growth hitting $2.4 bn in 2011 and $7.5 bn by 2015, according to a recent report by NanoMarkets. In spite of this fact, photovoltaics will only outshine existing methods of generating electricity if they can genuinely compete with current fossil fuel technologies in terms of cost and performance. This requires at the least halving the price of current solar cells.

Thin-film cells are made from a thin semiconducting layer – usually of amorphous or polycrystalline silicon, cadmium telluride or copper indium diselenide – deposited on a cheap glass, plastic or stainless steel substrate. Now, researchers believe that thin films will succeed as alternative energy sources by eliminating the need for thick and expensive silicon wafers.

Increased light intensity

"The thickness of the thin-film silicon solar cell is only 1 or 2  µm compared with the 200 µm for the wafer cells," Kylie Catchpole, research fellow at the ANU, told OLE. "That can dramatically reduce your materials cost as it reduces the amount of high purity semiconductor that you need."

However, while thin-film silicon solar cells are a cheaper alternative to silicon wafers the poor absorption of near-bandgap light remains a severe limitation on their performance.

Silver nanoparticles

"When you decrease the thickness that much, you also decrease the absorption," said Catchpole. "So for thin-film solar cells you really need to increase the absorption. For wafer-based solar cells there are already quite good ways for increasing the absorption but not for thin-film solar cells".

In line with this, the solar cells need to be structured so that light remains trapped inside to increase the absorption. For thin-film cells, the thickness range of a few microns is too small to support surface texturing commonly used in the wafer-based silicon cells where pyramids in the range of 2–10 µm are etched into the surface. This has prompted several research groups to look to alternative methods, one of which was to use the scattered light from the surface plasmon resonance of metallic nanoparticles on the surface of the thin-film cell.

According to Catchpole a texture on the surface of the thin-film solar cell can also reduce the maximum voltage produced by the cell through increased electron-hole recombination at the surface. Metal nanoparticles remain independent of the structure of the solar cell itself and so increase the absorption while leaving the electrical performance intact.

Silver takes first place

The optical properties of metal particles have been a subject of great interest in the last few decades, especially with the potential applications of plasmonic resonances in integrated optics and biosensing.

At wavelengths near the plasmon resonance, metal nanoparticles are strong scatterers of light. A plasmon arises from the collective oscillation of the free electrons in the metal particle. For particles with diameters well below the wavelength of light, the absorption and scattering cross-sections can be described by those of a point dipole. At the surface plasmon resonance, the scattering cross-section is found to exceed the geometrical cross-section of the particle, thereby increasing the amount of light scattered into the cell.

Noble metals are ideal for this purpose as they do not have many interband transitions and do not absorb much light as a result. Significant enhancements in photocurrent measurements have been found using noble metals such as silver or gold. While the dielectric functions of silver and gold are reported to be very similar, the group at ANU believes silver to be the better choice due to its lower absorption and lower cost.

"What you want is for the light to come in, scatter from the nanoparticle and go into the solar cell. You really don't want the light to be absorbed in the metal particle itself," described Catchpole. "Silver is by far the best for that. Other metal particles tend to absorb the light just because of their atomic structure."

Between the lines

While there are many techniques and materials for plasmonic solar cell fabrication, the group at the ANU uses boron-doped silicon solar cells and evaporates a layer of hemispherical silver nanoparticles close to 100 nm in size on the surface.

Starting with the silicon cell, an oxide is grown on the surface in an oxygen furnace at high temperature. The metal nanoparticles are then deposited on the thin-film silicon cells by vacuum evaporation. This process initially involves evaporating a thin silver film onto the cell surface and then heating the sample to 200 °C. Even though this is below the melting point of the metal, the layer is thin enough so that little blobs form under surface tension. This creates roughly evenly sized, evenly distributed particles on the solar cell surface.

In this way it is possible to cover any desired area with these very tiny particles. This would otherwise be a very difficult and expensive process were each individual particle to be made via techniques such as electron beam lithography.

This process also has the advantage of having no effect on the electrical performance of the solar cell and has no influence on the fabrication process of the solar cell itself (as metal evaporation is performed after the thin-film solar cell is made).

One of the main challenges that the group found, however, was getting the nano-particles close enough to the surface of the cell. "Putting the nanoparticles extremely close to the silicon surface turned out to be very important for getting a good enhancement in the absoption," described Catchpole. "A difference of 20 nm makes a big difference in this situation. You need to have the metal nanoparticles really close to the surface and so we have had to understand how the absorption enhancement works to figure out what we really need to do with the particles."

Evaporating the metal particles to within the desired 20 nm of the silicon surface requires control over the thickness of the oxide layer grown on the cell surface. This can be achieved through controlling the temperature or duration of the oxidation process or by etching the oxide layer after it has been grown.

The future's bright? 

According to Catchpole, progress in plasmonic solar cells has recently been dramatic thanks to a fuller understanding of plasmonics. "Plasmonics has become a big field. It is now possible to make nanoscale particles and nanoscale type structures, and so a lot of people have become interested in it. There has been work done to figure out what happens at that scale," she said.

Research into plasmonic solar cells is rapidly expanding, exploiting the benefits offered by plasmonics with those of thin-film technology.

Fabricating thin-film solar cells uses a lot less material and can take place on a very large scale – a big advantage for reducing the installation costs that form a significant part of the whole cost of a solar system.

One of the added advantages of using metal nanoparticles is that they are generally applicable to any thin-film solar cell irrespective of the underlying semiconductor be it a silicon or organic solar cell.

"It's essentially all about cost in the solar industry. Whatever you can do to lower the cost, that is what is going to win out in the end," added Catchpole. "There are a number of things that affect cost. It can be the efficiency of the cell or it can be the cost of the process, or how fast you can do the process. But all of these things are headed towards the reduced overall cost of the solar cells."

Research is ongoing into improving the performance, which includes looking into how differences in particle size and shape influence the photocurrent measurements. The group expects a commercial form of their solar cell to emerge in the next three years.

• Kylie Catchpole is the group leader in nanophotonics at the Australian National University.

• This article originally appeared in the March 2009 issue of Optics & Laser Europe magazine.

Energetic Nanoparticles Swing Sunlight Into Electricity

ScienceDaily (Dec. 26, 2008) — Deriving plentiful electricity from sunlight at a modest cost is a challenge with immense implications for energy, technology, and climate policy. Scientists are developing a relatively new approach to solar cells: lacing them with nanoscopic metal particles. As the authors describe in a new article, this approach has the potential to greatly improve the ability of solar cells to harvest light efficiently.

Like plants, solar cells turn light into energy. Plants do this inside vegetable matter, while solar cells do it in a semiconductor crystal doped with extra atoms. Current solar cells cannot convert all the incoming light into usable energy because some of the light can escape back out of the cell into the air. Additionally, sunlight comes in a variety of colors and the cell might be more efficient at converting bluish light while being less efficient at converting reddish light.

The nanoparticle approach seeks to remedy these problems. The key to this new research is the creation of a tiny electrical disturbance called a "surface plasmon." When light strikes a piece of metal it can set up waves in the surface of the metal. These waves of electrons then move about like ripples on the surface of a pond. If the metal is in the form of a tiny particle, the incoming light can make the particle vibrate, thus effectively scattering the light. If, furthermore, the light is at certain "resonant" colors, the scattering process is particularly strong.

In the Optics Express paper, Kylie Catchpole and Albert Polman show what happens when a thin coating of nanoscopic (a billionth of a meter in size) metal particles are placed onto a solar cell. First of all, the use of nanoparticles causes the incoming sunlight to scatter more fully, keeping more of the light inside the solar cell. Second, varying the size and material of the particles allows researchers to improve light capture at otherwise poorly-performing colors.

In their work, carried out at the FOM Institute for Atomic and Molecular Physics in The Netherlands, Catchpole and Polman showed that light capture for long-wavelength (reddish) light could be improved by a factor of more than ten. Previously Catchpole and co-workers at the University of New South Wales showed that overall light-gathering efficiency for solar cells using metallic nanoparticles can be improved by 30 percent.

"I think we are about three years from seeing plasmons in photovoltaic generation," says Catchpole, who has now started a new group studying surface plasmons at the Australian National University. "An important point about plasmonic solar cells is that they are applicable to any kind of solar cell." This includes the standard silicon or newer thin-film types

Energetic Nanoparticles Swing Sunlight Into Electricity

ScienceDaily (Feb. 22, 2008) — The electrons in nanoparticles of noble metal oscillate together apace with the frequency of the light. This phenomenon can be exploited to produce better and cheaper solar cells, scientists at Chalmers University of Technology in Sweden have shown.

Electricity-generating solar cells are one of the most attractive alternatives for creating a long-term sustainable energy system, but thus far solar cells have not been able to compete economically with fossil fuels. Researchers are now looking at how nanotechnology can contribute in bringing down the cost.
Solar cells are constructed of layers that absorb sunlight and convert it to electrical current. Thinner solar cells can yield both cheaper and more plentiful electricity than today's cells, if their capacity to absorb sunlight is optimized.

One way to enhance the absorption of the solar harvesting material in a solar cell is to make use of nanoparticles of noble metal. Carl Hägglund at Chalmers has looked at how this can be done in his recently completed doctoral dissertation.

The particles involved have special optical properties owing to the fact that their electrons oscillate back and forth together at the same rate as the frequency of the light, that is, the color of the light. The particles catch the light as tiny antennas and via the oscillations the energy is passed on as electricity. These oscillations, plasmons, are very forceful at certain so-called plasmon resonance frequencies, which in turn are influenced by the form, size, and surroundings of the particles.

"What we've done is to make use of nanotechnology to produce the particles and we've therefore been able to determine the properties and see how they can enhance the absorption of light of different colors," says Carl Hägglund.

In the context of solar cells, the great challenge is to efficiently convert the energy that is absorbed in the electron oscillation to energy in the form of electricity.

"We show that it is precisely the oscillations of the particles that yield the energy, how it is transmitted to the material and becomes electricity. It might have turned out, for example, that the oscillations simply generated heat instead," says Carl Hägglund.

The efficiency of the best solar cells today is already very high. The possibility of achieving even better solar cells therefore lies in using less material and in lowering production costs.

With solar cells of specially designed nanoparticles of gold, which is what Carl Hägglund has looked at, a layer only a few nanometers thick is required for the particles to be able to absorb light in an efficient way.
The dissertation examines the effect of nanoparticles of noble metal on two different types of solar cells, which can be said to represent two extremes. In one type of solar cell the light is absorbed in molecules on a surface, and in the other type deep inside the material.

The experimental and theoretical results show that the particles can help transmit the light's energy to useful electricity in several different ways and that it's possible to enhance the absorption of solar cells both on the surface and deep inside via different mechanisms.

This work has been carried out within the framework of a materials science research program (PhotoNano) funded by the Swedish Foundation for Strategic Research

Nanoparticles Used In Solar Energy Conversion

ScienceDaily (Aug. 9, 2002) — MANHATTAN, KAN. -- An enormous source of clean energy is available to us. We see it almost every day. It's just a matter of harnessing it.

The problem with solar energy is that it has not been inexpensive enough in the past. David Kelley, professor of chemistry at Kansas State University, developed a new type of nanoparticle -- a tiny chemical compound far too small to be seen with the naked eye -- that may reap big dividends in solar power.
Kelley's team is studying the properties and technical problems of gallium selenide nanoparticles. The properties of the nanoparticle change as the size changes. One of those properties is the part of the light spectrum it absorbs.

"You can make dramatically different colors just by changing the size of the nanoparticles," Kelley said.
Kelley is developing nanoparticles that are just the right size for solar cells -- they can absorb all visible light but nothing from the invisible light at the red end of the spectrum, which would reduce voltage.

"The correct-sized nanoparticles look dark red to black. There is an optimum size and that's what you want to shoot for," Kelley said.

Today's solar panels are made with silicon. The silicon usually has impurities, which limits its efficiency. Purifying a chemical is too expensive. For that reason, smaller is better. One can fit as many nanoparticles into a golf ball as one can fit beach balls into the earth.

Only a tiny percentage of a piece of material has impurities. If the entire chunk of material makes one crystal in a solar panel, the crystal will not work. But if that chunk is broken up into 100 tiny nanoparticles, then only the few unlucky nanoparticles with the impurities will not function. All the other nanoparticles will be pure and therefore will work.

Kelley said he is a long way from developing compounds that are comparable to today's silicon solar cells, because the physics of nanoparticles is so poorly understood. By using gallium selenide, Kelley is laying the groundwork for a similar, but more complex and potentially more effective nanoparticle called indium selenide. It is difficult to make silicon nanoparticles, but indium selenide has great potential for nanoparticle solar cells, Kelley said.

"The idea is to make large, high-output solar voltaic panels that are dirt cheap to produce. It's only then that the price starts to become competitive with burning fossil fuels," Kelley said.

He nearly had to start from scratch. His team invented gallium selenide nanoparticles. Kelley said he knew six years ago that many semiconductor materials had potential use in solar power, but were not being studied because there were no methods to make them into nanoparticles.

"All these really interesting materials were being ignored and I thought it just can't be allowed to stay that way," Kelley said.

The study on the methods to produce the nanoparticles was published in the journal "Nano Letters" this year. The project was funded by the U.S. Department of Energy's Solar Photochemistry Program in Basic Energy Sciences.