Let the solar shine in: Canadian researchers are hard at work on the next generation of solar cells.
In any bulk material, electrons can only exist at certain energy levels, called bands. In semiconductors, there is a gap between the low-energy valence band, where electrons are bonded to individual atoms, and the high-energy conducting band, where electrons can flow freely through the material. By absorbing energy from photons, electrons can be promoted across this gap. The band gap of silicon is about 1.1 electron-volts, which means it can be bridged by medium to high-energy photons--anything with a wavelength shorter than 1,100 nanometres (nm). This includes visible and ultraviolet light, where most solar radiation lies, but leaves out the infrared region that comprises about 19 per cent of the solar spectrum.
In order to make current flow, the excited electron must be separated from the 'hole' it left behind. Crystalline silicon cells achieve this through a p-n junction, a sandwich of two silicon wafers that have been chemically altered, or 'doped,' to have slightly different numbers of electrons and holes. The junction creates an electric field that causes the electrons to flow one way and the holes another. In order to re-unite with its hole, each electron must flow through a circuit, creating electricity.
Due to its low band gap, good charge conduction and durability, silicon can be used to make cells with overall efficiencies above 20 per cent. The main drawback is that in order to have these desirable properties, solar-grade silicon must be extremely pure and free of defects in its crystalline structure. Manufacturing this material is an extremely energy-intensive process and must be carried out in ultra-clean facilities with a high capital cost. The main driver for alternative solar technologies is the potential to use cheaper processing techniques that could drastically reduce the cost of solar power.
DYE-SENSITIZED SOLAR CELLS
Curtis Berlinguette is the director of the Centre for Advanced Solar Materials at the University of Calgary. Berlinguette's research focuses on dye-sensitized solar cells (DSSCs) that use a different mechanism to perform the two different roles played by silicon: light absorption and charge separation. In DSSCs, the semiconductor is titanium dioxide (Ti[O.sub.2], also called titania) that, unlike silicon, does not have to be ultra-pure in order to function. "It is the disorder of titania that makes this type of cell so promising. On top of that, titania is stable, environmentally benign, easy to form into films and dirt cheap," says Berlinguette. Unfortunately, Ti[O.sub.2] has too large a band gap to get excited by most sunlight, so it must be sensitized by coating it with dyes, molecules specifically designed to absorb lots of light. Typically, cells consist of a sponge-like layer of Ti[O.sub.2] nanoparticles with a high surface area. This is coated with a dye carefully designed to absorb light and inject its excited electrons into the Ti[O.sub.2] semiconductor. The electrons then flow through a circuit and are returned to the dye by a liquid electrolyte.
The design allows for potentially cheaper cells than silicon, but the dyes have room for improvement. One problem is that the dyes, which are often metal-organic complexes, tend to decompose or detach from their Ti[O.sub.2] substrate after a couple of years of exposure to sunlight. Berlinguette's group is focused on determining why this happens and how to prevent it. Last fall, they reported a new ruthenium-based dye that does not contain the components that are most likely to degrade, while maintaining a leading-edge cell efficiency of more than nine per cent.
Berlinguette doesn't see his technology as being in direct competition with silicon. "Crystalline silicon is great for solar farms because they are optimized for intense sunlight and stable for decades," he says. "DSSCs require less energy to fabricate and their power conversion efficiency is not as sensitive to light intensity. They work at all light conditions and are consequently better suited for urban environments, including indoor applications." This could include translucent solar cells coated on the inside of south-facing windows or cells built into the protective cases of smart phones and tablet personal computers.
ORGANIC PHOTOVOLTAIC CELL
Another technology that will likely see its first applications in tablets and smart phones is the organic photovoltaic (OPV) cell. These were made possible by the discovery of semiconducting polymers more than 30 years ago, but it wasn't until the 1990s that the focus turned from using them to produce light from electricity to doing the reverse. OPVs are made of two semiconducting organic compounds in which the high-energy conducting band of compound A (the electron donor) is slightly higher than that of compound B (electron acceptor). When an electron near the interface of the two compounds gets excited by a photon, it naturally flows from A to B and must run through the circuit to return to where it started. In order to maximize the interface between the two polymers, they are mixed into a non-homogeneous layer, like the light and dark regions of marble cheese. This arrangement is called a bulk heterojunction.
Mario Leclerc is a professor of polymer chemistry at Universite Laval who specializes in OPVs. In his cells, the electron acceptor is usually (6,6)-phenyl C61 butyric acid methyl ester (PCBM), a molecule that includes the soccer ball-shaped fullerene group. As for the electron donor, Leclerc's group has developed a plethora of polymeric compounds in their search for the ideal combination of properties, including band gap, light absorption and charge conductivity. Some of the compounds he has developed are derivatives of poly(N-alkyl-2,7-carbazole) or PCD, which is noted not just for its ability to conduct holes but also the ease with which different side groups can be substituted on the nitrogen atom, making it easy to tweak the material's properties. In 2009, one of these derivatives, called PCDTBT, was used in a solar cell that achieved 6.1 per cent efficiency, which at the time was a world record. More recently, in collaboration with the National Research Council's Steacie Institute of Molecular Sciences (NRC-SIMS), Leclerc's group reached 8.1 per cent efficiency using a different electron-donating polymer called PDTSTPD.
Leclerc says that the maximum efficiency of OPVs is likely to be around 10 to 12 per cent due to the fact that amorphous polymers can't transport electricity as well as ordered crystalline silicon. Their advantage lies in being able to use solution processing techniques that are far cheaper and more flexible than those used to make silicon. "Imagine you can make a solar-absorbing ink, which you could print at room temperature at a speed of three metres per minute," says Leclerc. "Clearly this would reduce the cost." OPVs are also interesting for their flexibility; the NRC has produced a prototype flexible OPV that could be stitched into the side of clothing or handbags.
QUANTUM DOT SOLAR CELLS
Of all the advanced solar technologies on the horizon, quantum dot solar cells may be the most unique. Quantum dots are extremely small particles of semiconducting substances, typically only a few nanometres in diameter. They are so small that their excited electrons are restricted and cannot flow freely in the way that they do in a bulk crystal, giving them unique properties that are intermediate between individual molecules and bulk materials. One advantage of this is that, unlike dyes or organic polymers, quantum dots can be tuned to absorb different wavelengths of light simply by making them slightly bigger or smaller, rather than changing their chemical composition.
Quantum dots were discovered in the 1980s but their application to solar cells only began in earnest in the early part of this century. One of the handful of labs working on quantum dot solar cells belongs to Ted Sargent at the University of Toronto, whose team of researchers includes chemists, materials scientists and electrical engineers. Their quantum dot material of choice is lead (II) sulfide (PbS), which they form into quantum dots by slowly condensing it out of a salt solution. The more time the dots are given to form, the bigger they get. By making quantum dots of different sizes, it's possible to create a multi-junction solar cell in which successive layers absorb different wavelengths of light, making better use of the solar spectrum. Last summer, Sargent's laboratory managed to do just that. Their two-junction quantum dot solar cell--a world first--had one layer tuned to absorb light below 775 nm and another below 1,240 nm, with an overall efficiency of 4.2 per cent.
There is a dilemma inherent in quantum dot solar cells. "To conduct electrons and holes, you need to have sufficient communication from one quantum dot to the next," says Illan Kramer, a PhD candidate in Sargent's group. "But if you bring them to the point where they're in physical contact, you start to lose some of that quantum confinement," Kramer says. "You need to bring them very close together without actually touching." The solution is to cover each quantum dot in a passivation layer, a coating of organic molecules that keeps the dots separate. Last fall, Sargent's lab worked out a way to replace the traditional bulky passivation agents with smaller and simpler halogen ions. This allowed the overall efficiency to climb to six per cent, currently the best available for quantum dot technology.
Quantum dots still have a way to go in order to catch up with DSSCs and OPVs, let alone silicon. Still, their ability to exploit quantum phenomena may ultimately give them the best crack at success. "There's no other system I can think of where by changing a physical parameter, you get an effectively different material," says Kramer. "Even with a single junction, by using different sizes of quantum dots, you could change the behaviour of the whole solar cell in an advantageous way. So we're inventing entirely new device architectures through this unique materials system."
If there's a theme to all this research, it's that solar energy should not necessarily be thought of as a drop-in replacement for fossil fuels, hydroelectricity, or nuclear power. Although silicon will likely continue to dominate solar farms for some time, the new materials being developed at these and other labs across Canada allow us to imagine a world where power generation is widely distributed: from self-charging tablets and laptops to buildings that can offset their need to draw from the grid. The ability to harness solar energy is a trick that the natural world figured out a long time ago; now it's time for chemical innovation to make that ability accessible to everyone.
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|Title Annotation:||CHEMISTRY: SOLAR INNOVATION|
|Publication:||Canadian Chemical News|
|Date:||May 1, 2012|
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