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News Article | August 30, 2016
Site: www.spie.org

Plastic foil substrates and chromium oxide interlayers are used in a novel technology that combines high efficiency, low weight, and extreme flexibility in a single platform. In the pursuit to solve the fossil fuel energy crisis, the use and development of photovoltaic technologies is thriving. In this technology, nature's most abundant source of energy—sunlight—is directly tapped. To achieve this, light-absorbing materials that are highly efficient, lightweight, low-cost, and stable during operation are required. Organolead halide perovskites1 are one such class of materials that show promise for photovoltaic applications, and they have recently become a strong focus of solar cell research. Indeed, their cell efficiency has been increased to about 20%2 within only a few years. Organolead halide perovskites are popular because their raw materials are plentiful and cheap, and because they can be fabricated in a simple manner (i.e., optimal prerequisites for cheap solar power). The limited stability of many perovskite absorbers under ambient conditions, however, may ultimately limit the widespread adoption of these materials in solar cells (especially if heavy and costly packaging is to be avoided). The main issue that gives rise to the poor stability of perovskites is water ingress3 and the subsequent liberation of highly corrosive species that rapidly damage metal contacts. In a previous attempt to overcome this problem,4 thick carbon electrodes were used to enable solar cell operation under ambient conditions. Nonetheless, if power output per solar cell weight (a critical metric for all mobile applications)—as well as power conversion efficiency—is to be optimized, alternative strategies are required. In such approaches, it is necessary to maintain the thin and light form factor of a direct band gap absorber material. In our approach,5 we thus demonstrate ultrathin (3μm), highly flexible perovskite solar cells that have stabilized 12% efficiency and a power-per-weight value as high as 23W/g. To realize these devices, we use 1μm-thick plastic foils as substrates and we process (from solution, at low temperature) pinhole-free perovskite films at high yield. We achieve perfect growth of tightly packed perovskite crystallites by treating the transparent polymer electrode with dimethyl sulfoxide. In addition, we introduce a chromium oxide–chromium interlayer, which effectively protects the metal top contacts from reacting with the perovskite, to facilitate air-stable operation. The detailed structure of our solar foils is depicted in Figure 1, together with a photograph of the freestanding ultrathin solar cell. The transparent conducting electrode and the absorber layer are processed from solution, and the electron-selective metal top contacts are thermally evaporated. Figure 1. (a) Schematic illustration of the cell stack. Polyethylene terephthalate (PET) foils (1.4μm thick) serve as the substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is the transparent hole-selective electrode. Using dimethyl sulfoxide as an additive promotes the formation of pinhole-free perovskite layers. A one-step solution precursor deposition method is used to form the methylammonium lead iodide absorber. Perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) constitute the electron-transport layers. The chromium (Cr) oxide (Cr O ) stabilizes the metal top contact so that the device can be operated in ambient air. Low-resistivity metals, e.g., gold (Au), copper (Cu), and aluminum (Al), complete the device, and a 1μm-thick capping layer of polyurethane is used for mechanical protection. (b) Photograph of freestanding 3μm-thick solar cells (with copper top-metal contacts). (a) Schematic illustration of the cell stack. Polyethylene terephthalate (PET) foils (1.4μm thick) serve as the substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is the transparent hole-selective electrode. Using dimethyl sulfoxide as an additive promotes the formation of pinhole-free perovskite layers. A one-step solution precursor deposition method is used to form the methylammonium lead iodide absorber. Perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) or [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) constitute the electron-transport layers. The chromium (Cr) oxide (Cr) stabilizes the metal top contact so that the device can be operated in ambient air. Low-resistivity metals, e.g., gold (Au), copper (Cu), and aluminum (Al), complete the device, and a 1μm-thick capping layer of polyurethane is used for mechanical protection. (b) Photograph of freestanding 3μm-thick solar cells (with copper top-metal contacts). 5 Solar cells that have aluminum or gold in direct contact with the perovskite degrade immediately upon exposure to ambient atmosphere. The ingress of water into the absorber layer causes this degradation of the perovskite crystal structure, i.e., by the formation of intermediate hydrated phases. The degradation culminates in the reforming of lead iodide, after enough water has permeated the film. In contrast, chromium oxide is resistant to aggressive oxidizing conditions (even nitric acid and aqua regia). It is for this reason (i.e., the excellent stability) that chromium plating is used to form corrosion-resistant coatings on various metals, and why we use a chromium oxide interlayer to provide an excellent buffer to shield the top-contact metal from chemical etching. We packaged our air-stable, thin, light solar cells (with a micrometer-thick spray-on polyurethane coating) so that they could be operated in field tests, where we used the solar panels to power various unmanned aerial vehicles. A snapshot of our model airplane, on a solar-powered flight, is shown in Figure 2. The same airplane is shown with our solar-powered blimp and a ‘solar leaf’ in a short video clip available online.6 For these tests, we powered the airplane and blimp with a 3μm-thick, 5.2g/cm2, light solar panel (with 64 individual cells). The high power-per-weight performance of our devices (i.e., up to 23W/g) is vital for such applications. Several other solar technologies for decentralized power generation and distribution (e.g., blimps, weather balloons, robotic insects, smart buildings, and aerospace applications), environmental and industrial monitoring, rescue and emergency response, as well as tactical security applications, all have a similar requirement. With our new technology we combine high power conversion efficiency, minimal weight, flexibility, mechanical resilience, operational stability, and low cost in a single platform and thus make the realization of these future concepts possible. Figure 2. Snapshot of the model airplane powered by the perovskite solar cell. This image was captured while the airplane was flying on a sunny winter afternoon (on the campus of the Johannes Kepler University). The airplane has a total weight of about 4.8g and is powered by air-stable, 3μm-thick solar arrays (with a power-per-weight value of 23W/g). The wingspan is 58cm. Through our tests we have also clearly demonstrated the high yield at which we fabricated the solar cells (even on thin, rough plastic foils). Our solar foils are extremely flexible and can endure severe mechanical deformation. In addition, they become stretchable (as shown in the video6) when they are laminated on a pre-stretched rubber band and they conform to arbitrary surfaces. Our foils are therefore ideal power sources for applications where conformability, stretchability, and light weight are required (e.g., portables, wearables, and robotics). In summary, we have presented a novel approach for achieving ultrathin, highly flexible perovskite solar cells. Our devices exhibit stable operation in air, 12% efficiency, and a power-per-weight value of up to 23W/g. The concepts we introduce (i.e., plastic foil substrates and chromium oxide interlayers) are readily applicable to the growing family of perovskite absorbers and could be used to increase the power-per-weight of such materials even further. By merging high efficiency, low weight, and extreme flexibility in our single photovoltaic platform, there seem to be few obstacles to keeping perovskite solar cells grounded. Indeed, our aeronautic models are still fully functional more than six months after their initial flights. In our future research we plan to focus on realizing perovskites with improved efficiency and moisture resistance (by exploring electrode transport materials, alternative metals, and superhydrophobic coatings). We will also investigate further ways to unify the high efficiency of perovskite cells with the low weight and flexibility of our technology. This work was supported by a European Research Council Advanced Investigators Grant (‘Soft-Map’) to Siegfied Bauer and the Austrian Science Fund's Wittgenstein Award (Solare Energie Umwandlung Z222-N19) to Niyazi Serdar Sariciftci. Department of Soft Matter Physics Johannes Kepler University Martin Kaltenbrunner received his PhD from the Johannes Kepler University Linz and then joined the Someya-Sekitani Laboratory for Organic Electronics at the University of Tokyo, Japan. He is now an assistant professor. His research interests include soft transducers, photovoltaics, as well as thin-film, flexible, and stretchable electronics. Siegfried Bauer received his PhD from the University of Karlsruhe, Germany. After stays at the Heinrich Hertz Institute in Berlin and the University of Potsdam (both Germany), he became a professor at the Johannes Kepler University Linz. He has has been head of the department since 2002. His research is devoted to functional soft matter. Institute for Physical Chemistry Johannes Kepler University Niyazi Serdar Sariciftci received his PhD from the University of Vienna, Austria. Following time at the University of Stuttgart, Germany, and the University of California at Santa Barbara, he became a professor at the Johannes Kepler University Linz in 1996. He has been a fellow of SPIE since 2009. His research is focused on organic photovoltaics and energy conversion. 3. A. M. A. Leguy, Y. Hu, M. Campoy-Quiles, M. I. Alonso, O. J. Weber, P. Azarhoosh, M. van Schilfgaarde, et al., Reversible hydration of CH NH PbI in films, single crystals, and solar cells, Chem. Mater. 27, p. 3397-3407, 2015. 5. M. Kaltenbrunner, G. Adam, E. D. Glowacki, M. Drack, R. Schwödiauer, L. Leonat, D. H. Apaydin, et al., Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air, Nat. Mater. 14, p. 1032-1039, 2015. 6. http://spie.org/documents/newsroom/videos/6223/Kaltenbrunner-solar_plane.mp4 a ‘solar leaf’ operated outdoors on sunny winter afternoons, with about 40 kilolux solar irradiation.


News Article | December 15, 2016
Site: www.eurekalert.org

For the first time, light is used to specifically design defined molecule chains; publication in Nature Communications Chemists of Karlsruhe Institute of Technology (KIT) have succeeded in specifically controlling the setup of precision polymers by light-induced chemical reactions. The new method allows for the precise, planned arrangement of the chain links, i.e. monomers, along polymer chains of standard length. The precisely structured macromolecules develop defined properties and may possibly be suited for use as storage systems of information or synthetic biomolecules. This novel synthesis reaction is now reported in open-access Nature Communications. (DOI: 10.1038/NCOMMS13672) Chemical reactions may be triggered by light at room temperature. This effect was used by KIT scientists to specifically link molecules to defined polymer chains under light. "In many conventional processes, polymer chains of variable length are produced. The building blocks are arranged randomly along the chain," says Professor Christopher Barner-Kowollik of the KIT Institute for Chemical Technology and Polymer Chemistry (ITCP). "We wanted to develop a light-induced method for polymer structuring, which reaches the precision of nature," the Holder of the Chair for Preparative Macromolecular Chemistry adds. The models in nature, e.g. proteins, have an exactly defined structure. The new, light-induced synthesis method allows for customized molecule design, with the building blocks being arranged at the positions desired similar to a string of colored pearls. "By controlling the structure of the molecule, the so-called sequence, properties of macromolecules can be controlled," Barner-Kowollik says. "Sequence-defined polymers might also be used as molecular data and information storage systems." Information might be encoded by the sequence of monomers, similar to the genetic information of the DNA. The team of Barner-Kowollik now presents the new light-induced and highly precise polymerization method in Nature Communications under the heading of "Coding and Decoding Libraries of Sequence Defined Functional Copolymers Synthesized via Photoligation." The developers expect the fundamental method to become a tool for chemists, biologists, and materials scientists and to be the key to future macromolecular chemistry. The new method was developed under the Collaborative Research Center 1176 "Molecular Structuring of Soft Matter" which is funded by the German Research Foundation (DFG) and coordinated by KIT. For the first four years, a budget of EUR 9 million is available to the Collaborative Research Center that started in January 2016. Nicolas Zydziak, Waldemar Konrad, Florian Feist, Sergii Afonin, Steffen Weidner, and Christopher Barner-Kowollik: Coding and Decoding Libraries of Sequence Defined Functional Copolymers Synthesized via Photoligation. DOI: 10.1038/NCOMMS13672 Karlsruhe Institute of Technology (KIT) pools its three core tasks of research, higher education, and innovation in a mission. With about 9,300 employees and 25,000 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe. KIT - The Research University in the Helmholtz Association Since 2010, the KIT has been certified as a family-friendly university.


News Article | December 15, 2016
Site: phys.org

Chemists of Karlsruhe Institute of Technology (KIT) have succeeded in specifically controlling the setup of precision polymers by light-induced chemical reactions. The new method allows for the precise, planned arrangement of the chain links, i.e. monomers, along polymer chains of standard length. The precisely structured macromolecules develop defined properties and may possibly be suited for use as storage systems of information or synthetic biomolecules. This novel synthesis reaction is now reported in open-access Nature Communications. Chemical reactions may be triggered by light at room temperature. This effect was used by KIT scientists to specifically link molecules to defined polymer chains under light. "In many conventional processes, polymer chains of variable length are produced. The building blocks are arranged randomly along the chain," says Professor Christopher Barner-Kowollik of the KIT Institute for Chemical Technology and Polymer Chemistry (ITCP). "We wanted to develop a light-induced method for polymer structuring, which reaches the precision of nature," the Holder of the Chair for Preparative Macromolecular Chemistry adds. The models in nature, e.g. proteins, have an exactly defined structure. The new, light-induced synthesis method allows for customized molecule design, with the building blocks being arranged at the positions desired similar to a string of colored pearls. "By controlling the structure of the molecule, the so-called sequence, properties of macromolecules can be controlled," Barner-Kowollik says. "Sequence-defined polymers might also be used as molecular data and information storage systems." Information might be encoded by the sequence of monomers, similar to the genetic information of the DNA. The team of Barner-Kowollik now presents the new light-induced and highly precise polymerization method in Nature Communications under the heading of "Coding and Decoding Libraries of Sequence Defined Functional Copolymers Synthesized via Photoligation." The developers expect the fundamental method to become a tool for chemists, biologists, and materials scientists and to be the key to future macromolecular chemistry. The new method was developed under the Collaborative Research Center 1176 "Molecular Structuring of Soft Matter" which is funded by the German Research Foundation (DFG) and coordinated by KIT. For the first four years, a budget of EUR 9 million is available to the Collaborative Research Center that started in January 2016. More information: Nicolas Zydziak et al. Coding and decoding libraries of sequence-defined functional copolymers synthesized via photoligation, Nature Communications (2016). DOI: 10.1038/NCOMMS13672


News Article | December 15, 2016
Site: www.cemag.us

Chemists of Karlsruhe Institute of Technology (KIT) have succeeded in specifically controlling the setup of precision polymers by light-induced chemical reactions. The new method allows for the precise, planned arrangement of the chain links, i.e. monomers, along polymer chains of standard length. The precisely structured macromolecules develop defined properties and may possibly be suited for use as storage systems of information or synthetic biomolecules. This novel synthesis reaction is now reported in Nature Communications. Chemical reactions may be triggered by light at room temperature. This effect was used by KIT scientists to specifically link molecules to defined polymer chains under light. “In many conventional processes, polymer chains of variable length are produced. The building blocks are arranged randomly along the chain,” says Professor Christopher Barner-Kowollik of the KIT Institute for Chemical Technology and Polymer Chemistry (ITCP). “We wanted to develop a light-induced method for polymer structuring, which reaches the precision of nature,” the Holder of the Chair for Preparative Macromolecular Chemistry adds. The models in nature, e.g. proteins, have an exactly defined structure. The new, light-induced synthesis method allows for customized molecule design, with the building blocks being arranged at the positions desired similar to a string of colored pearls. "By controlling the structure of the molecule, the so-called sequence, properties of macromolecules can be controlled,” Barner-Kowollik says. “Sequence-defined polymers might also be used as molecular data and information storage systems.” Information might be encoded by the sequence of monomers, similar to the genetic information of the DNA. The team of Barner-Kowollik now presents the new light-induced and highly precise polymerization method in Nature Communications under the heading of “Coding and Decoding Libraries of Sequence Defined Functional Copolymers Synthesized via Photoligation.” The developers expect the fundamental method to become a tool for chemists, biologists, and materials scientists and to be the key to future macromolecular chemistry. The new method was developed under the Collaborative Research Center 1176 “Molecular Structuring of Soft Matter” which is funded by the German Research Foundation (DFG) and coordinated by KIT. For the first four years, a budget of EUR 9 million is available to the Collaborative Research Center that started in January 2016.


Culfaz P.Z.,Soft Matter | Culfaz P.Z.,University of Twente | Buetehorn S.,Chemical Process Engineering AVT.CVT | Utiu L.,RWTH Aachen | And 5 more authors.
Langmuir | Year: 2011

The fouling behavior of microstructured hollow fibers was investigated in constant flux filtrations of colloidal silica and sodium alginate. It was observed that the fouling resistance increases faster with structured fibers than with round fibers. Reversibility of structured fibers- fouling was similar during silica filtrations and better in sodium alginate filtrations when compared with round fibers. The deposition of two different silica sols on the membranes was observed by NMR imaging. The sols had different particle size and solution ionic strength and showed different deposition behaviors. For the smaller particle-sized sol in deionized solution (Ludox-TMA), there was more deposition within the grooves of the structured fibers and much less on the fins. For the alkali-stabilized sol Bindzil 9950, which had larger particles, the deposition was homogeneous across the surface of the structured fiber, and the thickness of the deposit was similar to that on the round fiber. This difference between the deposition behavior of the two sols is explained by differences in the back diffusion, which creates concentration polarization layers with different resistances. The Ludox sol formed a thick polarization layer with very low resistance. The Bindzil sol formed a slightly thinner polarization layer; however, its resistance was much higher, of similar magnitude as the intrinsic membrane resistance. This high resistance of the polarization layer during the Bindzil sol filtration is considered to lead to quick flow regulation toward equalizing the resistance along the fiber surface. The Ludox particles were trapped at the bottom of the grooves as a result of reduced back diffusion. The fouling behavior in sodium alginate filtrations was explained by considering the size-dependent deposition within the broad alginate size distribution. The better reversibility of fouling in the structured fibers is thought to be the result of a looser deposit within the grooves, which is more easily removed than a compressed deposit on the round fibers. © 2011 American Chemical Society.


Van Puyvelde P.,Soft Matter | Vananroye A.,Soft Matter | Hanot A.-S.,Soft Matter | Dees M.,Dow Chemical Company | And 2 more authors.
International Polymer Processing | Year: 2013

The effect of pressure on the viscosity of polymer melts is an often forgotten parameter due to the inherent difficulty to measure this quantity. Different experimental approaches have already been undertaken in literature in the past. A popular methodology to measure the pressure dependence of the viscosity is to use a capillary rheometer equipped with a counter pressure chamber in which the exit pressure can be controlled. In order to process the data, one of the key elements is the Bagley correction that is required to determine the correct entrance pressure at a specific shear rate. In all analysis approaches presented in literature on data at controlled exit pressure, the Bagley correction was always determined at atmospheric exit pressure, disgarding possible effects of an enhanced exit pressure. In this paper, a new analytical approach is presented that for the first time allows for a direct assessment of the entrance pressures obtained when capillary measurements are performed with controlled counter pressures. It is demonstrated, using polycarbonate, that the entrance pressure correction needed to obtain correct viscosity values under pressure is significantly different than the one needed to correct measurements performed at atmospheric exit pressure. © (2013) Trans Tech Publications, Switzerland. © Carl Hanser Verlag GmbH & Co. KG.


News Article | November 22, 2016
Site: www.eurekalert.org

Liquids are an important part of our everyday lives. Fluids such as water are Newtonian, and their viscous behavior is well understood. However, many common fluids are viscoelastic. These fluids, such as those commonly found in cosmetics, soaps and paints, possess a combination of viscous, liquid-like and elastic, solid-like properties and we know surprisingly little about how they flow. Despite not knowing much about their flow properties, manufactures add these fluids to many different types of everyday products. Without viscoelastic fluids, life would feel much different. We wouldn't be able to enjoy the rich lather of shampoos, nor the chewy texture of a gummy candy, nor the springy comfort of a well-built athletic shoe. To understand more about these fluids, researchers from the Micro/Bio/Nanofluidics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) study the flow properties and behavior of different viscoelastic fluids. Prof. Amy Shen, leader of the unit, and Dr. Simon Haward, the group leader of the unit, are investigating two specific types of liquids commonly used in manufactured products: polymer solutions and 'living polymer' solutions. Polymers are long molecules comprised of repeating subunits. Polymeric solutions have a wide range of applications, particularly in the formulation of foods, inks, paints and even prosthetic fluids such as eyedrops and artificial saliva. During flow, these long polymer molecules can become stretched out like rubber bands, which give the fluid its elasticity. In a collaborative study with Massachusetts Institute of Technology researcher Professor Gareth McKinley, Shen and Haward observed the flow patterns of a series of viscoelastic polymer solutions through a 4-way junction (Figure 1). Using a technique called flow-induced birefringence, they showed that as the rate of flow through the junction was increased, polymer molecules became highly stretched out at in a narrow strand passing through the center of the junction. Flow induced birefringence is caused by small measurable changes in the refraction of light passing through a liquid when it is made to flow. These changes in light refraction directly correlate to elastic stresses in the flowing fluid. The researchers found that the strong elasticity within the birefringent strand caused severe distortions of the observed flow patterns. Increasing the flow rate further led to the onset of large fluctuations or instabilities in the flow patterns. These experiments allowed the researchers to show that the mechanism for the onset of instability in this stretching flow is consistent with that for viscoelastic instabilities in other, more simple kinds of flows. In a curved pipe, for example, the onset of instability can be quite well predicted depending on the precise geometric conditions and fluid properties. However, until now it has never been shown that similar predictions can be applied to stretching flows. Many industrial processes, such as extrusion, fiber-spinning and inkjet printing, involve stretching flows of viscoelastic fluids. Flow instabilities generally have a detrimental effect on the quality of end products and so directly limit the rates at which such processes can be carried out. The ability to predict the onset of instabilities in such flows can aid in optimizing processing rates and obtaining superior end products. The results of the study are published in the open access Nature Publishing journal Scientific Reports. The Micro/Bio/Nanofluidics Unit also studies the flow of 'living polymers'. Like polymers, these materials form long chains of multiple repeating units, but unlike polymers, these units are not chemically bound together, but rely on other forces for cohesion. Wormlike micelles (WLM), a type of 'living polymer', form long, rod-like aggregates suspended in a solution. As with polymers, these materials have numerous industrial applications, including as additives in shampoos and cosmetics and as materials to enhance oil and gas recovery (EOR). WLM solutions are pumped into shale during fracking in order to extract more oil and gas from these underground rock formations. The solutions are initially thick and gel-like, which allows them to generate high pressures and fracture the shale. However, when they come into contact with the hydrocarbons, the micelles disassemble allowing the solution to behave more like water and easily flow out of the rock. These shale formations contain many obstructions that alter the flow of solutions within. Prof. Shen decided to use a simplified model to study the flow pattern of WLM solutions when a blockage is present. Dr. Ya Zhao, a former graduate student of Prof. Shen at the University of Washington, built a micro-scale channel in which she could observe the flow of WLM solutions around a cylinder acting as an obstruction in the flow path. She then compared the flow patterns of a Newtonian fluid and a WLM solution by observing the streaks formed by fluorescent tracer particles. She also measured the growth of stresses in the WLM solution using flow-induced birefringence. This research, published in Soft Matter and featured on the inside front cover, revealed some unique flow patterns with the WLM solution. "Unlike Newtonian fluids," Shen explains, "WLM solutions initiate flow instability upstream of the obstruction. Additionally, it was discovered that the degree of instability was directly related to the size of the blockage compared to the total width of the channel". Determining how these materials flow is vitally important in optimizing their applications. These materials exist in a wide variety of products and are exploited in many industrial processes, making their optimization a priority for manufacturers. Determining their flow behavior is one step closer to achieving the full potential of these products.

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