Technology▼Solutions Solar cells that harness infrared light
© 2005 American Chemical Society
kilowatt-hour ($/kW-h), compared with 0.0716 $/kW-h on average for retail electricity in the United States in 2001, says Michael LoCascio, chief technology officer at Evident Technologies, Inc., a nanotechnology manufacturer in Troy, N.Y.
Individual columns of bonded lead and sulfur atoms create the checkerboard pattern in this image of a 6-nanometer-wide quantum dot made with transmission electron microscopy. Such nanoparticles were suspended in a solvent and dried like paint to make a plastic infrared solar cell.
“The high price for solar cells is largely due to the use of expensive substrate materials and costly microfabrication processing,” LoCascio says. The cost per watt can be cut by reducing manufacturing expenses and increasing the cells’ efficiency at converting light to electrical power, he says. The IR plastic cell may accomplish both of these goals. Using plastic, instead of expensive crystalline silicon, is one of the keys to slashing costs. Semiconducting plastics—polymers that can conduct an electric current—are cheap and amenable to the economies of highvolume manufacturing, says Daniel McGahn, chief marketing officer for Konarka Technologies, Inc., a manufacturer of plastic solar cells in Lowell, Mass. Konarka is already making a flexible plastic solar cell
M. A. HINES, A. PEROVIC, D. D. PEROVIC, AND G. D. SCHOLES, UNIV. OF TORONTO
No batteries? No sunshine? No problem. One day, your cell phone may be fully charged by a plastic photovoltaic cell built right into the case that can use indoor light and heat, making the technology truly wireless. This futuristic scenario just got a few years closer with the invention of a plastic infrared (IR) solar cell by Ted Sargent and his colleagues at the University of Toronto. The discovery opens the door to making cheap, efficient solar photovoltaic cells that produce more power per penny than fossil fuels. “We have made the first infrared photovoltaics based on solution-processing,” Sargent says. He and his team suspended lead sulfide semiconducting nanocrystals, measuring a mere 4 nanometers (nm) in diameter, in a semiconducting plastic (Nat. Mater. 2005, 4, 138–142). By controlling the size of the nanocrystals, or quantum dots, the scientists can tune the solar cells to absorb IR light at wavelengths of 980, 1200, and 1355 nm and turn it into electric current. The low cost of plastic, combined with the advantages of harvesting power from abundant IR light, could help transform the new device into an affordable source of energy, he says. Sargent’s group is not the first to create a plastic solar cell. “There are quite a few companies working on plastic photovoltaic applications,” says Ron Pernick, cofounder and principal of Clean Edge, a renewable energy consultancy in San Francisco. But Sargent’s solar cell is the first plastic cell active in the IR portion of the light spectrum. The new technology takes aim at one of the major drawbacks of conventional crystalline silicon solar cells—their high cost per watt of power produced. Current costs for photovoltaically generated electricity range from 0.25 to 1.00 dollars per
with nanostructured materials for military and consumer applications using a process similar to that used to make photographic fi lm (Environ. Sci. Technol. 2004, 38, 376A–376A). The Pentagon is reportedly hoping to use Konarka’s solar cells to create a tent capable of generating electricity from the sun and tools that soldiers can carry on the field to recharge the batteries in their cell phones, night vision scopes, and global positioning systems. Nanocrystal plastic solar cells operate on the same principles as conventional silicon solar cells, LoCascio says. Conventional cells sandwich two layers of silicon between a pair of electrodes. One silicon layer is doped with phosphorus to generate an excess of free, negatively charged electrons, and the other layer is doped with boron to create an excess of vacant, positively charged “electron holes”. The opposite charges create an electric field at the junction where the two layers meet. When light hits the cell, some of that energy is absorbed and transferred to the free electrons, enabling them to flow to the positive electrode while the positively charged holes are swept to the negative electrode, generating an electric current. In plastic solar cells, semiconducting nanocrystals provide the free electrons while the semiconducting plastic provides the abundance of positively charged holes, Sargent says. Sargent chose his polymer, poly[2-methoxy-5-(2´-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV), for its low ionization potential, which makes it a favorable hole acceptor. Until now, these materials have only worked in the visible light spectrum, with less than spectacular efficiency, although they are comparable to second-generation thin-fi lm solar technologies, McGahn says. Konarka announced in early 2004 that its engineers had developed materials that convert 8% of the light energy into electrical power.
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Although the efficiency of Sargent’s first IR system has an abysmal-sounding power-conversion efficiency of 0.001%, he emphasizes that his device is simply a prototype of how to capture IR energy. He is optimistic that adjustments to the system, such as using long, thin nanorods instead of spherical crystals, can boost conduction of electrons. IR photovoltaics have greater potential because half of the energy in sunlight occurs in the IR, at wavelengths ranging from 700 nm to 1 millimeter, Sargent says. Expanding the reach of plastic photovoltaics to harvest the IR spectrum as well as visible light is another way to boost efficiency and lower the cost of solar power, he says. “Our calculations show that, with further improvements in efficiency, combining infrared and visible photovoltaics could allow up to 30% of the sun’s radiant energy to be harnessed,” adds Peter Peumans, an electrical engineer at Stanford University. The discovery of a nanocrystal active in the IR was pure serendipity. “We initially set about creating a paintable infrared photodetector useful in optical communications,” Sargent says. One day, researchers noted that the device produced a current and developed a voltage even though there was no external source of electrical power. The IR-driven plastic photovoltaic was born. The key to the solar cell’s IR sensitivity is in the lead sulfide nanocrystals. The 4-nm spheres are smaller than the radius of an excited electron’s orbit. The effect of this socalled quantum confinement is that the light wavelengths at which the quantum dots begin to absorb energy are directly related to the crystals’ size, Sargent says. This means that by changing the size of the nanocrystals, he can tune his plastic solar cell to any wavelengths desired, from the IR to the visible spectrum. Sargent’s solar cell is currently being held back because, for each photon of light absorbed, it is not producing many pairs of the electrons and holes needed to generate an electric current, Peumans says. Because the nanocrystal is not very good at absorbing light, the experimental cell must be thick to absorb enough light, but that slows transport of the charges to the electrodes, he notes. The lead sulfide nanocrys-
tals currently provide the best bandgap, 0.5 electronvolts, for capturing energy from the IR, Peumans says. “We are reducing the thickness of the layer of molecules on the surface of our nanoparticles that impede the escape of the energy-containing charges we wish to harvest,” Sargent says. He and his colleagues are also improving the choice of polymer in which the nanoparticles are embedded, in order to improve conduction of the charges. “The concept we have shown could see commercial implementation within 3–5 years,” he predicts. “The plastic solar cell with infrared capabilities may be a breakthrough technically, but the real question is: What will it take to move this development from the lab to the marketplace?” Pernick asks. The biggest issue with nontraditional solar-cell technologies is the potential for degradation over the product’s lifetime, he adds. Silicon photovoltaics will work at close to their rated efficiency for 25 years, but that kind of longevity has not been seen in the new thinfilm plastic technologies. “The other issue is efficiency—if you’re at less than 10% efficiency, you’re going to have a hard time competing in the marketplace for many applications,” Pernick says. “The 10% efficiency threshold assumes the traditional photovoltaic markets. Our mission is to expand the use of photovoltaics to include devices, systems, and structures that cannot currently be powered by photovoltaics due to the limitations in form factor,” McGahn says. Efficiency may become less important if the device can be mass-manufactured at low cost, he adds. Despite the challenges, innovative solar technologies such as Sargent’s are receiving lots of attention—and funding, Pernick says. Photovoltaics are at a point where they are escaping the rigid-box form they have been trapped in for years and are being implanted into plastic, glass, and roofing material, he says. “What we’re seeing here is a shift to an integrated paradigm where solar will be embedded into products,” Pernick continues. Sargent agrees and says he hopes one day soon to see IR plastic solar cells powering a range of applications from consumer electronic devices to homes and businesses. —JANET PELLEY
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