We compare different structures of organic solar cells based on zinc phthalocyanine (ZnPc) and fullerene derivatives as electron donor and acceptor materials, respectively. Bilayer devices are fabricated and characterized by current-voltage and spectrally resolved photocurrent measurements. In a novel approach, the ZnPc was combined with soluble fullerene derivatives. With a pyrrolidinofullerene bearing chelating pyrridyl-groups we observed a complexation between donor and acceptor molecules. Due to a favorable structuring of the donor-acceptor interface this leads to a significant enhancement of the solar cell performance compared to similar devices where no complexation takes place. Coevaporated bulk heterojunction mixed-layers are introduced between the pristine layers. In these optimized structures short circuit currents up to 13 mA/cm2 are observed. We investigate the voltage dependence of the spectrally resolved photocurrent of ZnPc / Buckminsterfullerene bilayer solar cells and interpret the results in terms of the Gartner model.
Ullrich Scherf, Roland Resel, Christoph Brandstaetter, Douglas Gin, Wilhelm Graupner, Arnold Niko, U. Rohr, Egbert Zojer, Stefan Tasch, Peter Markart, Y. Geerts, Emil List, Farideh Meghdadi, Klaus Muellen, Ryan Smith, P. Schlichting, Christian Zenz, Gerald Kranzelbinder, Guenther Leising
We demonstrate the fabrication and characterization of highly efficient red-green-blue (RGB) and white light emitting devices based on poly(phenylene) type materials as the hexaphenyl and the methyl substituted laddertype poly(para phenylene) (m-LPPP). The RGB-devices are fabricated with an external color conversion technique based on PHP, whereas the white light emission is generated by an internal excitation energy transfer from the blue m-LPPP component to a red light-emitting polymer in a polymer blend, which is used as the active layer in a light-emitting diode. We present photophysical properties, like spectral line-shape site selectivity of photoluminescence (PL), and electroluminescence of bulk poly(para-phenylenevinylene) PPV films and isolated PPV chains incorporated into a self- assembled matrix material, which leads to the formation of a regular hexagonal array of channels with a diameter of about 15 angstrom, in which the conjugated polymer chains are contained. The structure of the nano-composite in organic- light-emitting-diodes. A suitably structured m-LPPP waveguide shows a spectrally very narrow high directional blue-green light output when optically pumped. The high optical gain of m-LPPP is a results of the spectral separation of stimulated emission and photoinduced absorption bands, thus spectral narrowing is even observable in below cut-off waveguides. Under resonant excitation conditions, we observe strong stimulated Raman scattering.
Klaus Muellen, Markus Moser, Ullrich Scherf, Mauro Nisoli, Thomas Jost, Sandro De Silvestri, Wilhelm Graupner, Guenther Leising, Guglielmo Lanzani, Gerard Froyer, Farideh Meghdadi, Berthold Winkler, Stefan Tasch, Gerald Kranzelbinder, Laurence Athouel, Olov Ekstroem
We present efficient blue electroluminescence devices based on pure soluble poly(paraphenylene) polymers and poly(phenylene) oligomers. Highly efficient green and red emission light is produced with a new color conversion technique by pumping fluorescent dye layers with blue organic light-emitting diodes. The high intrachain order of the laddertype poly(paraphenylene) allows us to observe stimulated emission in the blue-green spectral regions, which does not compete with dissipative processes, like in other conjugated polymers. We discuss the state of the art situation to realize an optically pumped homopolymer solid state laser.
The complete knowledge of the intrinsic electronic properties of new materials like fullerenes is essential for their technical application in optoelectronic and photonic devices, as well as in terms of the fundamental physical processes. The value of the intrinsic energy gap, the shape of the bandedge, and the nature and origin of the radiative recombination channels after photoexcitation of C60 are still unsettled. We report on photoluminescence emission and excitation spectroscopy on high-quality C60 single crystals and high-quality C60 thin films grown by molecular beam epitaxy (MBE) on mica substrates.
Guenther Leising, Stefan Tasch, Robert Grubbs, Joachim Huber, Arnold Niko, Farideh Meghdadi, Gerard Froyer, Ullrich Scherf, Laurence Athouel, Martin Wagner, Lin Pu, Gabriele Kopping-Grem, Walter Fischer
Conjugated polymers and oligomers are very interesting materials, with a number of possible electronic, optoelectronic, and photonic applications. New synthesis techniques allow the preparation of extremely pure soluble poly(paraphenylenes), showing a steepness at the bandedge comparable to conventional semiconductors. One consequence of this high purity is an improved photoluminescence quantum yield, higher than 24% for thin films. Copolymers of stepladder poly(paraphenylenes) are presented, which can be understood as the polymeric version of quantum well structures. Oligomers of poly(paraphenylene), like p-hexaphenyl can be produced as highly pure, well ordered thin films, which show a high photo-luminescence quantum yield in excess of 30%. All these materials, including an improved poly(phenylenevinylene), are applied as the active layer in light-emitting devices, giving bright light emission in the visible range and we report on their performance and efficiencies. Finally, we present a new preparation technique to produce highly ordered self assembled aromatic oligoazomethines, which are applied as transport layers in multiheterolayer electroluminescence devices.
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