Schlüsselwörter
(Englisch)
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organic solar cell; polymer demixing; liquid-liquid dewetting; polymer blend
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Forschungsprogramme
(Englisch)
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COST-Action P12 - Structurin of Polymers
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Kurzbeschreibung
(Englisch)
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We propose a versatile method to produce nanostructured layers of conjugated p-type semiconducting polymers. The concept is based on the phenomenon of polymer demixing in thin solid films. In a first step, an immiscible conjugated polymer/guest polymer mixture will be spin-coated from a common solvent. The influence of the solvent, rotation speed and rate of evaporation on the phase-separation process and the developing three-dimensional morphology will be studied. Different guest polymers with variable molecular weights will be used to induce a surface-induced phase separation with the preferential aggregation of the conjugated polymer at the surface. Annealing of the as-prepared films will be used to drive the system towards equilibrium and to tune the domain sizes. Processing conditions will be optimized that after removal of the guest polymer with a selective solvent, the semiconducting polymer forms a covering layer with islands emerging from the surface. The hickness of the layer and the heights and widths of the islands will be tuned to less than 100 nm. In a second step, the rough polymer surface will be covered with a n-type semiconducting organic material, again via spincoating in a selective solvent. We propose to use this bilayer structure for the production of novel high-performance photovoltaic devices. The cell architecture consists of a large, bulk-like, interface area between the electron donor and acceptor molecules with connecting paths to the respective electrodes for both materials at the same time. This concepts combines the advantages of the well-known planar bilayer (enhanced charge transport) and electron donor-acceptor blend configuration (high efficiency of charge generation) in a single device. The novel device configuration might contribute to the benchmark 10% power conversion efficiency for organic solar cells.
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Partner und Internationale Organisationen
(Englisch)
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BE, BG, CH, CZ, DE, EE, ES, FR, IT, LT, MK, NL, NO, PL, RO, SE, UK
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Abstract
(Englisch)
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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. This will allow deriving more quantitative aspects of the dependence of the cell efficiency on the heterojunction film morphology.
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Datenbankreferenzen
(Englisch)
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Swiss Database: COST-DB of the State Secretariat for Education and Research Hallwylstrasse 4 CH-3003 Berne, Switzerland Tel. +41 31 322 74 82 Swiss Project-Number: C05.0112
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