@article{KietzkeStillerLandfesteretal.2005, author = {Kietzke, Thomas and Stiller, Burkhard and Landfester, Katharina and Montenegro, Rivelino V. D. and Neher, Dieter}, title = {Probing the local optical properties of layers prepared from polymer nanoparticles}, issn = {0379-6779}, year = {2005}, abstract = {It is well known that the performance of solar cells based on a blend of hole-accepting and electron-accepting conjugated polymers as the active material depend crucially on the length scale of the resulting phase separated morphology. However, a direct control of this morphology is difficult if the layer is prepared from an organic solvent. To circumvent this difficulty, recently a universal method to fabricate defined nano-structured blend layer using nanoparticles dispersed in water was demonstrated. These nanoparticles were prepared with the miniemulsion method, which allows for the preparation of semiconducting polymer nanospheres (SPNs) with diameters in the range of 30 to 300 nanometres. Since the process starts from the active material dissolved in a common solvent, it can be applied to the fabrication of nanoparticles of blends of polymers with oligomers or even with inorganic materials. We present here for the first time scanning near field optical microscopy (SNOM) investigations on these novel nanostructured polymer layers. We show that by spin-coating a mixture of two different dispersions a nanoparticle monolayer with a statistically distribution of the nanoparticles can be obtained. Mixing conjugated polymer nanoparticles with some inert particles like polystyrene beads may allow for the preparation of nano-sized light emitters}, language = {en} } @article{KarageorgievNeherSchulzetal.2005, author = {Karageorgiev, Peter and Neher, Dieter and Schulz, Burkhard and Stiller, Burkhard and Pietsch, Ullrich and Giersig, Michael and Brehmer, Ludwig}, title = {From anisotropic photo-fluidity towards nanomanipulation in the optical near-field}, issn = {1476-1122}, year = {2005}, abstract = {An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated}, language = {en} } @article{MechauSaphiannikovaNeher2005, author = {Mechau, Norman and Saphiannikova, Marina and Neher, Dieter}, title = {Dielectric and mechanical properties of azobenzene polymer layers under visible and ultraviolet irradiation}, issn = {0024-9297}, year = {2005}, abstract = {Photoinduced changes in the mechanical and dielectric properties of azobenzene polymer films were measured utilizing the method of electromechanical spectroscopy. The measurements revealed a strong correlation between the time- dependent behavior of the plate compliance and the dielectric constant under irradiation. Actinic light causes a light softening of the film that also manifests itself in the increase of the dielectric constant, whereas ultraviolet irradiation results in an initial plasticization of the film followed by its hardening. The latter is accompanied by decrease of the dielectric constant. A semiquantitative model based on the kinetics of the photoisomerization process in azobenzene polymers is proposed. We assume that both visible and ultraviolet irradiation increase the free volume in the layer due to photoisomerization. Additionally, ultraviolet light increases the modulus of the polymer matrix due to the presence of a high density of azobenzene moieties in the cis state. These assumptions allowed us to reproduce the time- dependent behavior of the bulk compliance as well as the dielectric constant at different irradiation intensities, for both visible and ultraviolet light, with only two adjustable parameters}, language = {en} } @article{EgbeKietzkeCarbonnieretal.2004, author = {Egbe, D. A. M. and Kietzke, Thomas and Carbonnier, B. and Muhlbacher, D. and Horhold, H. H. and Neher, Dieter and Pakula, T.}, title = {Synthesis, characterization, and photophysical, electrochemical, electroluminescent, and photovoltaic properties of yne-containing CN-PPVs}, year = {2004}, abstract = {Alkoxy-substituted CN-containing phenylene-vinylene-alt-phenylene-ethynylene hybrid polymers (CN-PPV-PPE), 3a, 3b, and 7a, were obtained from luminophoric dialdehydes 1 by step growth polymerization via Knoevenagel reaction as high molecular-weight materials. Corresponding CN-free polymers 3c and 7b and an ethynylene-free polymer 5 with similar side chains were synthesized for the purpose of comparison. The chemical structures of the polymers were confirmed by IR, H-1 and C-13 NMR, and elemental analysis. Thermal characterization was conducted by means of thermogravimetric analysis and differential scanning calorimetry. Morphology was investigated by means of optical microscopy and small-angle light scattering. The final morphologies are determined by the molecular characteristics (side chains volume fraction, backbone stiffness) of the studied polymers. All the CN-containing polymers 3b, 5, and 7a exhibit higher fluorescence quantum yield in solid state (50 to 60\%), but lower quantum yields (12-40\%) in dilute chloroform solution, in total contrast to CN-free polymers 3c, 3d, and 7b. Identical optical, E-g(opt), and electrochemical band gap energies, E- g(ec), were obtained for 3b, 3c and 3d with intrinsic self-assembly ability, whereas a discrepancy, DeltaE(g), was observed in the cases of the fully substituted polymers 5, 7a, and 7b, whose values are dependent on the level of backbone stiffness and length of the side groups combined with the presence or absence of CN units. The incorporation of CN units in 3b and 7a lowers their respective LUMO level by 220 and 350 meV compared to their corresponding CN-free counterparts 3c and 7b, suggesting an improvement of the electron-accepting strength. Polymers 3b and 7a are efficient electron acceptors suitable for photovoltaic application. The experiments indicate that 3b is a better electron acceptor when used together with M3EH-PPV, but transport properties seem to be better for 7a. With 3b, high external quantum efficiencies of up to 23\%, an open circuit voltage of up to 1.52 V, and a white light energy efficiency of 0.65\% could be realized in bilayer solar cell devices. LED-devices of configuration ITO/PEDOT:PSS/polymer/Ca/Al from 3b, 3c, 7a, and 7b showed low turn-on voltages between 2 and 2.5 V. The CN-free polymers 3c and 7b exhibit far better EL parameters than their corresponding CN containing counterparts 3b and 7a}, language = {en} } @article{ZenSaphiannikovaNeheretal.2005, author = {Zen, Achmad and Saphiannikova, Marina and Neher, Dieter and Asawapirom, Udom and Scherf, Ullrich}, title = {Comparative study of the field-effect mobility of a copolymer and a binary blend based on poly(3- alkylthiophene)s}, issn = {0897-4756}, year = {2005}, abstract = {The performance of highly soluble regioregular poly[ (3-hexylthiophene)-co-(3-octylthiophetie)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 x 10(-3) cm(2) V-1 s(-1), which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter delta(p) of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a Sufficient processability of the polymers from common organic solvents, a high solid state order, and applicable OFET performances}, language = {en} } @article{KietzkeNeherKumkeetal.2004, author = {Kietzke, Thomas and Neher, Dieter and Kumke, Michael Uwe and Montenegro, Rivelino V. D. and Landfester, Katharina and Scherf, Ullrich}, title = {A nanoparticle approach to control the phase separation in polyfluorene photovoltaic devices}, year = {2004}, abstract = {Polymer solar cell devices with nanostructured blend layers have been fabricated using single- and dual- component polymer nanospheres. Starting from an electron-donating and an electron-accepting polyfluorene derivative, PFB and F8BT, dissolved in suitable organic solvents, dispersions of solid particles with mean diameters of ca. 50 nm, containing either the pure polymer components or a mixture of PFB and F8BT in each particle, were prepared with the miniemulsion process. Photovoltaic devices based on these particles have been studied with respect to the correlation between external quantum efficiency and layer composition. It is shown that the properties of devices containing a blend of single-component PFB and F8BT particles differ significantly from those of solar cells based on blend particles, even for the same layer composition. Various factors determining the quantum efficiency in both kinds of devices are identified and discussed, taking into account the spectroscopic properties of the particles. An external quantum efficiency of ca. 4\% is measured for a device made from polymer blend nanoparticles containing PFB:F8BT at a weight ratio of 1:2 in each individual nanosphere. This is among the highest values reported so far for photovoltaic cells using this material combination}, language = {en} } @article{BagnichBasslerNeher2004, author = {Bagnich, Sergey A. and Bassler, H. and Neher, Dieter}, title = {Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)}, issn = {0021-9606}, year = {2004}, abstract = {The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20\% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. (C) 2004 American Institute of Physics}, language = {en} } @article{SaphiannikovaNeher2005, author = {Saphiannikova, Marina and Neher, Dieter}, title = {Thermodynamic theory of light-induced material transport in amorphous azobenzene polymer films}, issn = {1520-6106}, year = {2005}, abstract = {It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements}, language = {en} } @article{KulikovskyNeherMecheretal.2004, author = {Kulikovsky, Lazar and Neher, Dieter and Mecher, E. and Meerholz, Klaus and Horhold, H. H. and Ostroverkhova, O.}, title = {Photocurrent dynamics in a poly(phenylene vinylene)-based photorefractive composite}, issn = {1098-0121}, year = {2004}, abstract = {All parameters describing the charge carrier dynamics in a poly(phenylene vinylene)-based photorefractive (PR) composite relevant to PR grating dynamics were determined using photoconductivity studies under various illumination conditions. In particular, the values of the coefficients for trap filling and recombination of charges with ionized sensitizer molecules could be extracted independently. It is concluded that the PR growth time without preillumination is mostly determined by the competition between deep trap filling and recombination with ionized sensitizer molecules. Further, the pronounced increase in PR speed upon homogeneous preillumination (gating) as reported recently is quantitatively explained by deep trap filling}, language = {en} } @article{BagnichBasslerNeher2005, author = {Bagnich, Sergey A. and Bassler, H. and Neher, Dieter}, title = {Exciton dynamics in ladder-type methyl-poly(para-phenylene) doped with phosphorescent dyes}, issn = {0022-2313}, year = {2005}, abstract = {The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum-porphyrin dye PtOEP covering the concentration 10(-3)-5\% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. (c) 2004 Elsevier B.V. All rights reserved}, language = {en} } @article{GalbrechtYangNehlsetal.2005, author = {Galbrecht, Frank and Yang, X. H. and Nehls, B. S. and Neher, Dieter and Farrell, Tony and Scherf, Ullrich}, title = {Semiconducting polyfluorenes with electrophosphorescent on-chain platinum-salen chromophores}, issn = {1359-7345}, year = {2005}, abstract = {The synthesis of statistical fluorene-type copolymers with on-chain Pt-salen phosphorescent units and their use in electrophosphorescent OLEDs is reported}, language = {en} } @article{StillerKarageorgievGeueetal.2004, author = {Stiller, Burkhard and Karageorgiev, Peter and Geue, Thomas and Morawetz, Knut and Saphiannikova, Marina and Mechau, Norman and Neher, Dieter}, title = {Optically induced mass transport studied by scanning near-field optical- and atomic force microscopy}, issn = {0204-3467}, year = {2004}, abstract = {Some functionalised thin organic films show a very unusual property, namely the light induced material transport. This effect enables to generate three-dimensional structures on surfaces of azobenzene containing films only caused by special optical excitation. The physical mechanisms underlying this phenomenon have not yet been fully understood, and in addition, the dimensions of structures created in that way are macroscopic because of the optical techniques and the wavelength of the used light. In order to gain deeper insight into the physical fundamentals of this phenomenon and to open possibilities for applications it is necessary to create and study structures not only in a macroscopic but also in nanometer range. We first report about experiments to generate optically induced nano structures even down to 100 nm size. The optical stimulation was therefore made by a Scanning Near-field Optical Microscope (SNOM). Secondly, physical conditions inside optically generated surface relief gratings were studied by measuring mechanical properties with high lateral resolution via pulse force mode and force distance curves of an AFM}, language = {en} } @article{YangNeher2004, author = {Yang, X. H. and Neher, Dieter}, title = {Polymer electrophosphorescence devices with high power conversion efficiencies}, issn = {0003-6951}, year = {2004}, abstract = {We demonstrate efficient single-layer polymer phosphorescent light-emitting devices based on a green-emitting iridium complex and a polymer host co-doped with electron-transporting and hole-transporting molecules. These devices can be operated at relatively low voltages, resulting in a power conversion efficiency of up to 24 lm/W at luminous efficiencies exceeding 30 cd/A. The overall performances of these devices suggest that efficient electrophosphorescent devices with acceptable operating voltages can be achieved in very simple device structures fabricated by spin coating. (C) 2004 American Institute of Physics}, language = {en} } @article{YangNeherHerteletal.2004, author = {Yang, X. H. and Neher, Dieter and Hertel, D. and Daubler, T. K.}, title = {Highly efficient single-layer polymer electrophosphorescent devices}, issn = {0935-9648}, year = {2004}, abstract = {A commercially available Ir complex has been employed for the preparation of highly efficient (see Figure) single-layer phosphorescent polymer light,emitting diodes by use of appropriate thermal treatment and proper adjustment of the layer composition. These devices exhibit essentially no dependence of the driving field on the concentration of the Ir complex, suggesting that the build-up of space-charge in the layer is insignificant}, language = {en} } @article{SrikhirinCimrovaSchieweetal.2002, author = {Srikhirin, T. and Cimrova, V. and Schiewe, B. and Tzolov, M. and Hagen, R. and Kostromine, S. and Bieringer, Thomas and Neher, Dieter}, title = {An Investigation of the photoinduced changes of absoprtion of high-performance photoaddressable Polymers}, year = {2002}, language = {en} } @article{SianovaZenNothoferetal.2002, author = {Sianova, D. and Zen, Achmad and Nothofer, Heinz-Georg and Asawapirom, Udom and Scherf, Ullrich and Hagen, R. and Bieringer, Thomas and Kostromine, S. and Neher, Dieter}, title = {Photoaddressable alignment layers for fluorescent polymers in polarized electroluminescence devices}, year = {2002}, language = {en} } @article{WilsonSteffenMcKenzieetal.2002, author = {Wilson, J. N. and Steffen, W. and McKenzie, T. G. and Lieser, G. and Oda, Masao and Neher, Dieter and Bunz, Uwe H. F.}, title = {Chiroptcial properties of poly(p-phenyleneethynylene) copolymers in thin films : large g-values}, year = {2002}, language = {en} } @article{ZenNeherBaueretal.2002, author = {Zen, Achmad and Neher, Dieter and Bauer, C. and Asawapirom, Udom and Scherf, Ullrich and Hagen, R. and Kostromine, S. and Mahrt, R. F.}, title = {Polarization-sensitive photoconductivity in aligned polyfluorene layers}, year = {2002}, language = {en} } @article{SrikhirinLaschitschNeheretal.2000, author = {Srikhirin, Toemsak and Laschitsch, Alexander and Neher, Dieter and Johannsmann, Diethelm}, title = {Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators}, year = {2000}, language = {en} } @article{NothoferMeiselMitevaetal.2000, author = {Nothofer, Heinz-Georg and Meisel, A. and Miteva, T. and Neher, Dieter and Forster, M. and Oda, Masao and Lieser, G. and Sainova, Dessislava and Yasuda, A. and Lupo, D. and Knoll, W. and Scherf, Ullrich}, title = {Liquid crystalline polyfluorenes for blue polarized electroluminescence}, year = {2000}, language = {en} }