Das Projekt "Structured Templates of (6,6)-Phenyl C61-Butyric Acid Methyl Ester (PCBM) for Applications in Photovoltaics and Photonics" wird vom Umweltbundesamt gefördert und von Eidgenössische Materialprüfungs- und Forschungsanstalt durchgeführt. We present a new strategy for controlling organic thin film morphologies that can find use in photovoltaics and photonics. The ability to control the thin film morphology of organic semiconducting materials on different length scales will determine the usefulness of this class of materials in future opto-electronic devices. Structuring on the micron to sub-micron level is of interest for photonic effects. The morphology and phase separation on the 10-20 nm scale in binary materials critically determines device performance in organic photovoltaic cells. Furthermore, the control of the arrangement of organic molecules on a molecular level has proven to be crucial, both for electronic transport properties as well as for optical properties. A demixing process where the lateral size of the phase structure can be controlled over more than two orders of magnitude is liquid-liquid dewetting (LLD). In LLD, a blend solution phase separates into a transient bilayer, followed by destabilization of the layers via an interfacial disjoining pressure. Films spin coated from PCBM/cyanine dye mixtures phase separate by LLD. The mechanism results in a large variety of phase morphologies and, in particular, self-similar phase structures with dimensions ranging from 5 micron to well below 50 nm in a controllable fashion. Moreover, in this materials system, LLD induces a specific 2-dimensional arrangement of dye molecules on PCBM surfaces, most impressively demonstrated by enormous changes in the dye absorption characteristics (H-aggregation). The combination of length scale control in the superordinated phase structure with the initiation of molecular order (aggregation) gives LLD unique attributes and potential. In this project, we propose to use the liquid-liquid dewetting process for the manufacturing of stabilized, in-soluble scaffolds of PCBM by selectively removing the dye. Long range order can be induced through surface energy patterning. We want to address issues related to solvent stability, long-range order and infiltration properties of PCBM templates, knowledge which is necessary to make full use of the potential of PCBM templates. Applications of these templates are manifold, ranging from screening of donor materials for organic solar cells to photonic structures for all-optical switching elements
Das Projekt "Nanostructured polymer layers for interface-enhanced organic solar cells (InterCell)" wird vom Umweltbundesamt gefördert und von Eidgenössische Materialprüfungs- und Forschungsanstalt durchgeführt. Thin films of blends of organic semiconducting materials are increasingly used as active layers in light-emitting diodes and photovoltaic devices. The arrangement of the components at the nanometer level is the key to device perfomance, and the challenge is to optimize charge generation and transport at the same time. In this project, we used two fundamental structure formation mechanisms to control the thin-film morphology in organic photovoltaic devices. In both cases we used molecular self-assembly processes and a simple large-area compatible coating process from solution for film fabrication. Thereby, we preserve the low-cost potential that organic materials inherently offer for the fabrication of optoelectronic devices, as opposed to the various top-down printing and direct writing methods available to create and transfer structures on the sub-100 nm length scale. In one example, surface-directed spinodal demixing of an active/guest polymer mixture during spin coating was used to fabricate a vertically segregated bilayer film with a rough interface. Using a selective solvent, the guest polymer was then removed and the remaining film covered with a second active component. Bulk spinodal decomposition is the structure-determining process for large guest polymer weights and leads to a rather coarse interface structure. Only when surface segregation favours phase separation into a bilayer, submicron interface structures developed. With use of polystyrene as guest polymer, a poly(p-phenylenevinylene) derivative as electron donor and the acceptor C60, this resulted in much-improved solar cell performance, with external power efficiencies more than 3 times higher than those reported for that particular material combination so far. The second approach specifically relates to the patterning of cyanine dyes. Cyanines are charged cationic molecules and are accompanied by a negative counter ion. Cyanines intrinsically have properties which are useful for high-performing solar cells, but little is known about the nanoscale self-organization properties of molecular ionic blends. We recently found that thin films spin-coated from a cyanine dye/PCBM (a C60 derivative) mixture show small-scale phase-separated morphologies. The mechanism leading to these morphologies does not occur by phase separation alone, but by destabilization of interfaces in a transient bilayer that forms during spin coating. Both layers destabilize via a process called liquid-liquid dewetting. We believe that electrostatic forces drive the destabilization of the films. We found that liquid-liquid dewetting results in a large variety of phase morphologies, with tunable dimensions well below 50 nm. Fine tuning of the morphology can be achieved by material independent parameters such as film thickness and annealing temperature. Solar cells were fabricated and performance figures were related to the internal film structure. (abridged text)