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Enschede, Netherlands

News Article
Site: http://www.spie.org/x2358.xml

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.


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. Source


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. Source

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