Invisible Cloaking, Singlet Fission Set Solar on Fire in 2015 For decades researchers have been enchanted by the vast potential of photovoltaic (PV) technology as an alternative to fossil fuels. The rub: Solar-cell efficiencies have remained well below thermo-dynamic limits, hampering widespread adoption. That may soon change, due to pivotal advances in PV technology from European researchers unveiled over the last 12 months, as chronicled on Photonics.com. In June, scientists from Finland and Spain set a new efficiency record for “black silicon” cells, prized for their ability to capture solar radiation from low angles. Because of this, they generate more electricity as compared to traditional solar cells. And in October, an international team led by researchers at Cambridge used ultrafast 2D electronic spectroscopy to reveal the dynamics of singlet fission, an electrical process triggered by light in some materials. The findings could lead to the development of new material for solar cells twice as efficient as those in use today! Among other stories covered, a team from Karlsruhe Institute of Germany developed a technique, “invisible cloaking,” that eliminates shadows cast by energy-harvesting components onto the active surfaces of solar cells — an advance that could significantly boost solar cell performance in the future. With four out of the top 10 countries by PV installation in GW in Europe (according to IHS’ analysis of the global solar market in 2015), all signs point to Europe continuing to play a pivotal role in PV advances in 2016. Read on for a full account of the top developments this year. Cloaking principle could boost solar cell performance (October 2015) Invisibility cloaking may not be reality just yet, but the principle of it could help improve the performance of solar cells in the near term. In a series of simulations, researchers at the Karlsruhe Institute of Technology (KIT) in Germany have demonstrated how cloaks made of metamaterials or freeform surfaces could eliminate shadows cast by energy-harvesting components onto the active surfaces of solar cells. Contact fingers, which extract electric current, cover up to one-tenth of the surface area of a solar cell. By guiding light around these features, more of the sun’s energy could be captured by the solar cell. An invisibility cloak (right) guides sunlight past the contacts for current removal to the active surface area of a solar cell. Courtesy of Martin Schumann/Karlsruhe Institute of Technology. The scientists pursued two approaches, both based on polymer coatings, to achieve the cloaking effect. One approach uses graded-index metamaterials designed using 2D Schwarz-Christoffel conformal maps, and the second employs freeform surfaces that are designed using 1D coordinate transformations. In the freeform surface approach, the cloak layer is grooved along the contact fingers. In this way, incident light is refracted away from the contact fingers and toward the active surface of the solar cell. Freeform surfaces are particularly promising because they could be integrated into mass production processes and can cancel shadows at virtually any angle, the researchers said. Now, Dr. Martin Wegener, a professor at KIT, said the researchers are “working on the technology to imprint the cloaking structures onto large areas of actual solar cells.” He noted that fellow KIT professor Carsten Rockstuhl is now assembling a team of researchers in Germany, including several top solar cell groups, “to further work on these matters.” Reflective roof cooler than ambient air temperature (June 2015) A potential new roofing material stays cooler than the ambient air temperature, even under the midsummer sun. Researchers, led by the University of Technology Sydney, found that the surface, comprised of a coated polymer stack on a silver thin film, exhibited nearly 100 percent solar reflectance and thermal emittance at IR wavelengths from 7.9 to 13 μm. In the study, the new surface stayed 11 degrees (or more) cooler than an existing state-of-the-art white roof nearby because it absorbed just 3 percent of incident sunlight “while simultaneously strongly radiating heat at IR wavelengths that are not absorbed by the atmosphere,” said emeritus professor Geoff Smith. An IR image shows the temperature difference between the new surface (center) and an existing cool roof used in testing. Courtesy of the University of Technology Sydney. Data was collected for an unprotected new surface and one that had aged over several days in a polluted outdoor summer environment for assessment of the impact of the buildup of dust and grime. The surface maintained its high performance in all conditions. Now, the team has set up “novel, cheap outdoor systems” that can “achieve very cold temperatures at night and well below ambient in the day in full sun,” according to Smith and fellow researcher Angus Gentle. “Our insights from the super cool [roofing material] work are leading to development of new environmental monitoring approaches to better test the beneficial urban and building impact of super cool roofs,” Smith said, adding that his team is also looking at differences between standard roofing and existing cool roofs. “Large area roofs or urban spaces are known to thermally behave differently to small roofs and spaces. Available techniques cannot provide the required details on important key urban environmental responses relevant to energy savings, indoor and outdoor thermal comfort, and human, plant and animal health. Ours will.” The materials used in the demonstration are commercially available and potentially suitable for use on basic roofing worldwide. Black silicon solar cells see 4 percent efficiency increase (June 2015) With a newly certified 22.1 percent efficiency rating and wider angular acceptance than other materials, “black silicon” solar cells may now be ready for prime time. Researchers from Aalto University in Finland and the Polytechnic University of Catalonia in Spain achieved the new record by applying a thin, passivating film of conformal alumina to nanostructured silicon via atomic layer deposition. The new cells also integrate all metal contacts on the back side of the cell. The new efficiency record, which represents a nearly 4 percent increase over earlier black silicon solar cells, was certified by the Fraunhofer Institute for Solar Energy Systems’ CalLab. Surface recombination has long been a problem in black silicon solar cells and has limited efficiency to less than 20 percent, the researchers said. The new cells consists of a thick, back-contacted structure known to be highly sensitive to front-surface recombination. Certified external quantum efficiency of 96 percent at 300 nm demonstrates that the increased surface recombination problem no longer exists and that, for the first time, black silicon is not limiting the final energy conversion efficiency. “The energy conversion efficiency is not the only parameter that we should look at,” said professor Hele Savin of Aalto University, who coordinated the study. “Due to the ability of black cells to capture solar radiation from low angles, they generate more electricity already over the duration of one day as compared to the traditional cells.” “This is an advantage particularly in the north, where the sun shines from a low angle for a large part of the year,” she said. “We have demonstrated that in winter Helsinki, black cells generate considerably more electricity than traditional cells, even though both cells have identical efficiency values.” In the near future, the goal of the team is to apply the technology to other cell structures — in particular, thin and multicrystalline cells. “Our record cells were fabricated using p-type silicon, which is known to suffer from impurity-related degradation,” Savin said. “There is no reason why even higher efficiencies could not be reached using n-type silicon or more advanced cell structures.” The development of the cells fabricated last year will continue in an upcoming project supported by the European Union, in which Savin and her team will develop the technology further in cooperation with industry. “The surface area of the best cells in the study was already 9 cm²,” she said. “This is a good starting point for upscaling the results to full wafers and all the way to the industrial scale.” Spectroscopy elucidates solar singlet fission (October 2015) Better understanding of singlet fission, an electrical process triggered by light in some materials, could enable solar cells twice as efficient as those used today. An international team led by researchers at the University of Cambridge in England used ultrafast laser pulses to observe how single photons can be converted into two spin-triplet excitons in the organic material pentacene. The researchers confirmed that this “two-for-one” transformation involves an elusive intermediate state in which the two triplet excitons are entangled, a feature of quantum mechanics that causes the properties of each exciton to be intrinsically linked to that of its partner. By shining ultrafast laser pulses on a sample of pentacene, the researchers were able to directly observe this entangled state for the first time, and showed how molecular vibrations make it both detectable and drive its creation. The key challenge for observing real-time singlet fission is that the entangled spin-triplet excitons are essentially dark to almost all optical probes, meaning they cannot be directly created or destroyed by light. In materials like pentacene, the first stage — which can be seen — is the absorption of light that creates a single, high-energy, spin-singlet exciton. The subsequent fission of the singlet exciton into two less-energetic triplet excitons gives the process its name. But the ability to see what is going on vanishes as the process occurs. To get around this, the team used ultrafast 2D electronic spectroscopy, which involves illuminating the material with a coordinated sequence of ultrashort laser pulses and then measuring the light emitted by the excited sample. Pentacene molecules convert a single photon into two molecular excitations via the quantum mechanics of singlet fission. Courtesy of Alex W. Chin, Lawrence W. Chin and David Turban. By varying the time between the pulses in the sequence, it is possible to follow in real time how energy absorbed by previous pulses is transferred and transformed into different states. Using this approach, the team found that when the pentacene molecules were vibrated by the laser pulses, certain changes in the molecular shapes caused the triplet pair to become briefly light-absorbing, and therefore detectable by later pulses. By carefully filtering out all but these frequencies, a weak but unmistakable signal from the triplet pair state became apparent. When the molecules are vibrating, the researchers said, they possess new quantum states that simultaneously have the properties of both the light-absorbing singlet exciton and the dark triplet pairs. These quantum superpositions not only make the triplet pairs visible, they also allow fission to occur directly from the moment light is absorbed. “With developing a new understanding of the fundamental quantum dynamics at work, we can begin to ask ourselves how this knowledge might be applied to develop new molecular materials that utilize fission in an optimal way,” said research fellow Alex W. Chin, speaking recently with EuroPhotonics. “The observation in our work that vibrations and molecular motions play an important role give an extra ‘handle’ for controlling fission that can be tweaked independently of other key, static properties, like energy levels alignment.” Simpler molecules in solution, designed for singlet fission but without added complexities of the solid state, could lend themselves to much more detailed experimental study and theoretical description. “These systems may allow real time simulation and a truly atomistic understanding of what is happening,” Chin said. “Then, with the remarkable ability of synthetic chemists to build molecules to order, it should be possible to test and then exploit these new concepts of dynamics and mechanisms in a systematic way.” By using a singlet fission material, excess light energy can be used, in principle, to create an extra electronic excitation instead of generating heat (provided fission is fast enough to outcompete cooling), doubling the photocurrent and thus allowing more power out of the high energy part of the spectrum. “Understanding the dynamics of singlet fission is clearly a very important part of turning this idea into a viable technology, but it is only part of the story,” Chin said. “We also need to develop efficient ways of extracting the triplet excitons (the products of fission) and tuning them into charge.” Some milestones in this respect have already been passed, he added, but researchers are still working toward exploiting the full potential of singlet fission. It could be possible to use inorganic nanoparticles to harvest triplets “and then exploit their fascinating material properties to further transfer triplet energy into a more conventional solar cell material, such as silicon.” Lens arrays could enable rooftop solar concentrators (February 2015) Tiny solar cells sandwiched between lens arrays could bring the promise of concentrator photovoltaics (CPVs) down to a consumer scale. An international team of researchers, led by Pennsylvania State University in State College, Pa., developed a prototype rooftop CPV that incorporates high-efficiency GaAs cells less than a millimeter in size; they need to move no more than a centimeter to keep up with the sun. Existing CPV systems are about the size of billboards, and must be very accurately pointed toward the sun to track it throughout the day. Such large systems are not feasible for most rooftops. As part of the research, the cells were embedded between a pair of 3D-printed plastic lens arrays less than a centimeter thick. Each lens in the top array acted as a small magnifying glass, and was matched to a lens in the bottom array that functions like a concave mirror. With each tiny solar cell located at the focal point of the two optics, sunlight is intensified more than 200 times. An optical oil was used to lubricate the sliding cell array and also to improve light transmission. Improving consumer solar cell efficiency from the current 20 percent for silicon toward 40 percent or better with multijunction CPVs is important because increasing the power generated by a given system reduces the overall cost of the electricity it generates, according to Penn State professor Dr. Noel C. Giebink. “The vision is that such a microtracking CPV panel could be placed on a roof in the same space as a traditional solar panel and generate a lot more power,” he said. “The simplicity of this solution is really what gives it practical value.” The researchers were recently awarded a grant from the U.S. Department of Energy’s Advanced Research Projects Agency (ARPA-E) to continue their work. “Our focus is on extending our earlier proof-of-concept demonstration toward a fully operational, small-scale prototype panel incorporating multi-junction tandem cells with automated microtracking,” Giebink said. “In addition to efficiency and tracking performance, we’ll also be evaluating reliability and cost aspects needed to assess commercialization potential.”