Hunting for the best material from which to build organic solar cells can be like seeking the proverbial haystack needle, but now scientists at the National Institute of Standards and Technology (NIST) and the Naval Research Laboratory may have a better search tool for the nascent industry. The team's research findings in ("Hot photocarrier dynamics in organic solar cells") show it is possible to test a candidate material quickly and directly, using off-the-shelf laser technology. The method bypasses the costly, time-consuming step of constructing a prototype solar cell for each material to be evaluated. 
Organic materials are attractive to the solar industry because of their comparative low cost and physical flexibility. NIST findings reveal how to test a candidate organic material's viability at converting light to electricity both quickly and directly. (Image: Cappello/Georgia Tech, Courtesy National Renewable Energy Lab) "We'd like to give companies and manufacturers an alternative to trial and error," says NIST research chemist Ted Heilweil. "It takes a long time to develop photovoltaic materials for market. Screening them using our method would be much faster." Organic materials (e.g. plastics) hold a particular attraction for the solar industry, largely because of their comparative low cost and physical flexibility. Organics inspire the possibility of one day painting an inexpensive solar array onto most any surface, even one that bends and moves, and simply replacing it with a fresh coat when it wears out. At this point, organics are far less efficient at converting sunlight to electricity than traditional silicon-based technology, but ideas for better materials come at a fast clip. Unfortunately, sifting through these candidates and zeroing in on the most promising ones is expensive and arduous. Because it entails building a prototype cell for each prospective material, relatively few candidates get tested. The team's new method sidesteps this problem by using ultrafast lasers to probe a candidate material's abilities directly—and without electrical contacts. They found that when shining pulses of visible light onto a sample to mimic the sun, they could probe the sample's electronic behavior with a second laser pulse near the microwave range of the spectrum. When the sample absorbs these "terahertz" waves, its properties change in easily detectable ways. Just how the terahertz pulse changes is dependent on the material's viability at converting light to electricity. To test their method, the team looked at a number of mixed organic molecules and polymers whose abilities were well-understood from conventional prototyping. "We looked at small organics and polymers that people in the solar industry have been using as benchmarks, and we saw the same relative behavior with our terahertz measurements," Heilweil says. "We're pretty confident that our method can tell you what is useful to know." The team is using the method as part of its own ongoing materials search, Heilweil says.
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Organic materials are attractive to the solar industry because of their comparative low cost and physical flexibility. NIST findings reveal how to test a candidate organic material's viability at converting light to electricity both quickly and directly. (Image: Cappello/Georgia Tech, Courtesy National Renewable Energy Lab) "We'd like to give companies and manufacturers an alternative to trial and error," says NIST research chemist Ted Heilweil. "It takes a long time to develop photovoltaic materials for market. Screening them using our method would be much faster." Organic materials (e.g. plastics) hold a particular attraction for the solar industry, largely because of their comparative low cost and physical flexibility. Organics inspire the possibility of one day painting an inexpensive solar array onto most any surface, even one that bends and moves, and simply replacing it with a fresh coat when it wears out. At this point, organics are far less efficient at converting sunlight to electricity than traditional silicon-based technology, but ideas for better materials come at a fast clip. Unfortunately, sifting through these candidates and zeroing in on the most promising ones is expensive and arduous. Because it entails building a prototype cell for each prospective material, relatively few candidates get tested. The team's new method sidesteps this problem by using ultrafast lasers to probe a candidate material's abilities directly—and without electrical contacts. They found that when shining pulses of visible light onto a sample to mimic the sun, they could probe the sample's electronic behavior with a second laser pulse near the microwave range of the spectrum. When the sample absorbs these "terahertz" waves, its properties change in easily detectable ways. Just how the terahertz pulse changes is dependent on the material's viability at converting light to electricity. To test their method, the team looked at a number of mixed organic molecules and polymers whose abilities were well-understood from conventional prototyping. "We looked at small organics and polymers that people in the solar industry have been using as benchmarks, and we saw the same relative behavior with our terahertz measurements," Heilweil says. "We're pretty confident that our method can tell you what is useful to know." The team is using the method as part of its own ongoing materials search, Heilweil says.
Array of nanowires gallium phosphide made with an electron microscope. (Image: Eindhoven University of Technology) The electricity produced by a solar cell can be used to set off chemical reactions. If this generates a fuel, then one speaks of solar fuels - a hugely promising replacement for polluting fuels. One of the possibilities is to split liquid water using the electricity that is generated (electrolysis). Among oxygen, this produces hydrogen gas that can be used as a clean fuel in the chemical industry or combusted in fuel cells - in cars for example - to drive engines. Solar fuel cell To connect an existing silicon solar cell to a battery that splits the water may well be an efficient solution now but it is a very expensive one. Many researchers are therefore targeting their search at a semiconductor material that is able to both convert sunlight into an electrical charge and split the water, all in one; a kind of 'solar fuel cell'. Researchers at TU/e and FOM see their dream candidate in gallium phosphide (GaP), a compound of gallium and phosphide that also serves as the basis for specific colored leds. A tenfold boost GaP has good electrical properties but the drawback that it cannot easily absorb light when it is a large flat surface as used in GaP solar cells. The researchers have overcome this problem by making a grid of very small GaP nanowires, measuring five hundred nanometers (a millionth of a millimeter) long and ninety nanometers thick. This immediately boosted the yield of hydrogen by a factor of ten to 2.9 percent. A record for GaP cells, even though this is still some way off the fifteen percent achieved by silicon cells coupled to a battery. Ten thousand times less material According to Bakkers, it's not simply about the yield - where there is still a lot of scope for improvement he points out: "For the nanowires we needed ten thousand less precious GaP material than in cells with a flat surface. That makes these kinds of cells potentially a great deal cheaper," Bakkers says. "In addition, GaP is also able to extract oxygen from the water - so you then actually have a fuel cell in which you can temporarily store your solar energy. In short, for a solar fuels future we cannot ignore gallium phosphide any longer."
Artist’s rendering of the distribution of sphingomyelin (red spheres with yellow heads) in a lipid raft. (Image: Mikiko Sodeoka, RIKEN Synthetic Organic Chemistry Laboratory) In a previous collaboration, a team of researchers led by Mikiko Sodeoka at the RIKEN Synthetic Organic Chemistry Laboratory and Katsumasa Fujita at Osaka University developed a way to image small, mobile bioactive molecules in living cells, with the potential to greatly enhance our understanding of cellular processes. The method involves tagging bioactive molecules with small alkyne molecules and then imaging them using Raman microscopy—a technique that detects the vibrations of molecules by exciting them using a microscope-focused laser beam. Unlike the comparatively large fluorescent tags used in conventional approaches, the alkyne tags used in Sodeoka’s method do not alter the physical properties of bioactive molecules or impede their motion in cells. One of Sodeoka’s collaborators, Michio Murata at Osaka University, suggested applying the technique to lipid rafts—small domains in cell membranes that are rich in lipids such as cholesterol and sphingomyelin and that play important roles in membrane signaling and protein trafficking. “Murata told us it would be a real breakthrough if we could ‘see’ the distribution of sphingomyelin in the raft structure,” Sodeoka says. But this required overcoming two major challenges. Because lipid rafts are constantly moving in the cell membrane, it is very difficult to pin them down long enough to obtain an image. Furthermore, the low number of lipid rafts in membranes makes it hard to extract their signals from the background noise. The researchers overcame both challenges by drawing on the strengths of their respective teams. Murata’s group made long imaging times possible by preparing artificial membranes with a raft-like composition and immobilizing them on a surface, while Sodeoka’s and Fujita’s groups obtained strong signals from lipid rafts by employing a small, strongly Raman-active conjugated diyne tag and their highly sensitive Raman microscope. Using these tactics, the researchers observed a gradually varying distribution of sphingomyelin in ordered rafts (Fig. 1). “Many people assumed that ordered and disordered domains in lipid rafts were clearly separated, but our results suggest the existence of some interface area,” Sodeoka explains. “This was an unexpected finding.” The technique developed by the team opens the way for in-depth functional studies of lipid rafts. “Before our work, there was no method capable of quantifying the molecular distribution of one specific lipid in an ordered domain,” says Sodeoka. “Our next goal is to visualize one specific lipid in living cells.”