Researchers at KIT have developed a material suited for photovoltaics. For the first time, a functioning organic solar cell consisting of a single component has been produced on the basis of metal-organic framework compounds (MOFs). The material is highly elastic and might also be used for the flexible coating of clothes and deformable components. This development success is presented on the front page of the journal ("Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?"). 
Organic solar cells made of metal-organic frameworks are highly efficient in producing charge carriers. (Figure: Wöll/KIT) “We have opened the door to a new room,” says Professor Christof Wöll, Director of KIT Institute of Functional Interfaces (IFG). “This new application of metal-organic framework compounds is the beginning only. The end of this development line is far from being reached,” the physicist emphasizes. Metal-organic frameworks, briefly called MOFs, consist of two basic elements, metal node points and organic molecules, which are assembled to form microporous, crystalline materials. For about a decade, MOFs have been attracting considerable interest of researchers, because their functionality can be adjusted by varying the components. “A number of properties of the material can be changed,” Wöll explains. So far, more than 20,000 different MOF types have been developed and used mostly for the storage or separation of gases. The team of scientists under the direction of KIT has now produced MOFs based on porphyrines. These porphyrine-based MOFs have highly interesting photophysical properties: Apart from a high efficiency in producing charge carriers, a high mobility of the latter is observed. Computations made by the group of Professor Thomas Heine from Jacobs University Bremen, which is also involved in the project, suggest that the excellent properties of the solar cell result from an additional mechanism – the formation of indirect band gaps – that plays an important role in photovoltaics. Nature uses porphyrines as universal molecules e.g. in hemoglobin and chlorophyll, where these organic dyes convert light into chemical energy. A metal-organic solar cell produced on the basis of this novel porphyrine-MOF is now presented by the researchers in the journal Angewandte Chemie (Applied Chemistry). The contribution is entitled “Photoinduzierte Erzeugung von Ladungsträgern in epitaktischen MOF-Dünnschichten: hohe Leistung aufgrund einer indirekten elektronischen Bandlücke?“ (photo-induced generation of charge carriers in epitactic MOF-thin layers: high efficiency resulting from an indirect electronic band gap?). “The clou is that we just need a single organic molecule in the solar cell,” Wöll says. The researchers expect that the photovoltaic capacity of the material may be increased considerably in the future by filling the pores in the crystalline lattice structure with molecules that can release and take up electric charges. By means of a process developed at KIT, the crystalline frameworks grow in layers on a transparent, conductive carrier surface and form a homogeneous thin film, so-called SURMOFs. “The SURMOF process is suited in principle for a continuous manufacturing process and also allows for the coating of larger plastic carrier surfaces,” Wöll says. Thanks to their mechanical properties, MOF thin films of a few hundred nanometers in thickness can be used for flexible solar cells or for the coating of clothing material or deformable components. While the demand for technical systems converting sunlight into electricity is increasing, organic materials represent a highly interesting alternative to silicon that has to be processed at high costs before it can be used for the photoactive layer of a solar cell.
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Organic solar cells made of metal-organic frameworks are highly efficient in producing charge carriers. (Figure: Wöll/KIT) “We have opened the door to a new room,” says Professor Christof Wöll, Director of KIT Institute of Functional Interfaces (IFG). “This new application of metal-organic framework compounds is the beginning only. The end of this development line is far from being reached,” the physicist emphasizes. Metal-organic frameworks, briefly called MOFs, consist of two basic elements, metal node points and organic molecules, which are assembled to form microporous, crystalline materials. For about a decade, MOFs have been attracting considerable interest of researchers, because their functionality can be adjusted by varying the components. “A number of properties of the material can be changed,” Wöll explains. So far, more than 20,000 different MOF types have been developed and used mostly for the storage or separation of gases. The team of scientists under the direction of KIT has now produced MOFs based on porphyrines. These porphyrine-based MOFs have highly interesting photophysical properties: Apart from a high efficiency in producing charge carriers, a high mobility of the latter is observed. Computations made by the group of Professor Thomas Heine from Jacobs University Bremen, which is also involved in the project, suggest that the excellent properties of the solar cell result from an additional mechanism – the formation of indirect band gaps – that plays an important role in photovoltaics. Nature uses porphyrines as universal molecules e.g. in hemoglobin and chlorophyll, where these organic dyes convert light into chemical energy. A metal-organic solar cell produced on the basis of this novel porphyrine-MOF is now presented by the researchers in the journal Angewandte Chemie (Applied Chemistry). The contribution is entitled “Photoinduzierte Erzeugung von Ladungsträgern in epitaktischen MOF-Dünnschichten: hohe Leistung aufgrund einer indirekten elektronischen Bandlücke?“ (photo-induced generation of charge carriers in epitactic MOF-thin layers: high efficiency resulting from an indirect electronic band gap?). “The clou is that we just need a single organic molecule in the solar cell,” Wöll says. The researchers expect that the photovoltaic capacity of the material may be increased considerably in the future by filling the pores in the crystalline lattice structure with molecules that can release and take up electric charges. By means of a process developed at KIT, the crystalline frameworks grow in layers on a transparent, conductive carrier surface and form a homogeneous thin film, so-called SURMOFs. “The SURMOF process is suited in principle for a continuous manufacturing process and also allows for the coating of larger plastic carrier surfaces,” Wöll says. Thanks to their mechanical properties, MOF thin films of a few hundred nanometers in thickness can be used for flexible solar cells or for the coating of clothing material or deformable components. While the demand for technical systems converting sunlight into electricity is increasing, organic materials represent a highly interesting alternative to silicon that has to be processed at high costs before it can be used for the photoactive layer of a solar cell.
Under a magnetic field, 1D arrays of protein-encased superparamagnetic nanoparticles reversibly assemble into bundles. (© American Chemical Society) The lateral assembly of superparamagnetic nanoparticles (SNPs) into 1D structures has been predicted theoretically, but scientists have found it difficult to synthesize assemblies that are both thermodynamically stable and sufficiently rigid to inhibit entanglement. Yet such 1D SNP arrays continue to attract broad scientific attention due to their potentially useful properties such as a large magnetic susceptibility in one dimension, which could allow them to self-assemble under a moderate magnetic field, and a thermally fluctuating magnetic spin under ambient conditions that makes them magnetically isotropic—features that are not found in more conventional ferromagnetic nanoparticles. Taking an unusual approach, Daigo Miyajima, Takuzo Aida and colleagues from the RIKEN Center for Emergent Matter Science and other institutions in Japan have now succeeded where others have not by encasing the nanoparticles in protein nanotubes. The researchers formed their protein nanotubes by a process called supramolecular polymerization using the barrel-shaped protein, GroEL. This polymerization results in the formation of micrometer-long protein nanotubes with hydrophobic internal cavities housing the SNPs. These protein ‘jackets’ both protect the superparamagnetic nanoparticles, which are formed from iron oxide, and permit the self-assembly of ordered 1D nanoparticle arrays. In the 1D structure, the magnetic moments of the individual SNPs arrange into a tip-to-tip configuration that induces a large magnetic moment along the long axis of the 1D SNP array. Under a magnetic field, this causes the 1D structures to assemble into thick bundles (Fig.). The protein jackets provide the physical separation needed to allow the bundles to completely dissociate when the magnetic field is turned off to give the original, isolated 1D arrays. The bundles formed are also quite selective, with arrays of similar lengths tending to bundle together. These characteristics give the system excellent homogeneity and high stability under ambient conditions. “The most interesting aspect of our system is that the nanoparticle arrays reside inside the protein nanotubes,” says Miyajima. “We envisage broadening this concept to functional materials, dual-responsive materials and even a magneto-induced gelation system. Also, because the magnetic field is a non-invasive directing force, we are interested in making biomaterials, such as smart magnetic resonance imaging contrast agents controlled by chemical signals.”
The CVD process begins with tanks containing an initiator material (red) and one or more monomers (purple and blue), which are the building blocks of the desired polymer coating. These are vaporized, either by heating them or reducing the pressure, and are then introduced into a vacuum chamber containing the material to be coated. The initiator helps to speed up the process in which the monomers link up in chains to form polymers on the surface of the substrate material. (Illustration courtesy of Karen Gleason) (click on image to enlarge) This vapor deposition of polymers has opened the door to a variety of materials that would be difficult, and in some cases impossible, to produce in any other way. For example, many useful polymers, such as water-shedding materials to protect industrial components or biological implants, are made from precursors that are not soluble, and thus could not be produced using conventional solution-based methods. In addition, says Gleason, the Alexander and I. Michael Kasser Professor at MIT, the CVD process itself induces chemical reactions between coatings and substrates that can strongly bond the material to the surface. Gleason’s work on polymer-based CVD began in the 1990s, when she did experiments with Teflon, a compound of chlorine and fluorine. That work led to a now-burgeoning field detailed in a new book Gleason edited, titled “CVD Polymers: Fabrication of Organic Surfaces and Devices” (Wiley, 2015). At the time, the thinking was that the only way to make CVD work with polymer materials was by using plasma — an electrically charged gas — to initiate the reaction. Gleason tried to carry out experiments to prove this, beginning by running a control experiment without the plasma in order to demonstrate how important it was for making the process work. Instead, her control experiment worked just fine with no plasma at all, proving that for many polymers this step was not necessary. But the equipment Gleason used allowed the temperature of the gas to be controlled separately from that of the substrate; having the substrate cooler turned out to be key. She went on to demonstrate the plasma-free process with more than 70 different polymers, opening up a whole new field of research. The process can require a lot of fine-tuning, but is fundamentally a simple set of steps: The material to be coated is placed inside a vacuum chamber — which dictates the maximum size of objects that can be coated. Then, the coating material is heated, or the pressure around it is reduced until the material vaporizes, either inside the vacuum chamber or in an adjacent area from which the vapor can be introduced. There, the suspended material begins to settle onto the substrate material and form a uniform coating. Adjusting the temperature and duration of the process makes it possible to control the thickness of the coating. With metals or metal compounds, such as those used in the semiconductor industry, or the silvery coatings inside snack bags, the heated metal vapor deposits on a cooler substrate. In the polymer process, it’s a bit more complex: Two or more different precursor compounds, called monomers, are introduced into the chamber, where they react to form polymers as they deposit on the surface. Even high-temperature CVD processing has evolved, with great potential for commercial applications. For example, the research group of John Hart, an associate professor of mechanical engineering, has built a roll-to-roll processing system using CVD to make sheets of graphene, a material with potential applications ranging from large-screen displays to water-filtration systems. Hart’s group and others have used CVD to produce large arrays of carbon nanotubes, materials with potential as new electrodes for batteries or fuel cells. “It’s a very versatile and widely used manufacturing process,” Hart says, “and a very general process that can be tailored to many different applications.” One great advantage of CVD processing is that it can create coatings of uniform thickness even over complex shapes. For example, CVD can be used to uniformly coat carbon nanotubes — tiny cylinders of pure carbon that are far more slender than a hair — such as to modify their mechanical properties and make them react chemically to certain substances. “By combining two CVD processes — one to grow the carbon nanotubes, and another to coat the nanotubes — we have a scalable way to manufacture nanomaterials with new properties,” Hart says. Much progress in CVD research in recent years traces back to Gleason’s unexpected discovery, back in the 1990s, that the process could work without plasma — and her follow-up on that finding. “You need to pay attention when a new thing happens,” she says. “That’s sort of the key.”