D.A.M. Egbe, T. Kietzke, B. Carbonnier, D. Mühlbacher, H.H. Hörhold, D. Neher, T. Pakula, “Synthesis, characterization, and photophysical, electrochemical, electroluminescent, and photovoltaic properties of yne-containing CN-PPVs” Macromolecules37 (2004) 8863 DOI
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, 1H and 13C 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, Egopt, and electrochemical band gap energies, Egec, were obtained for 3b, 3c and 3d with intrinsic self-assembly ability, whereas a discrepancy, Eg, 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.
S.A. Bagnich, H. Bässler, D. Neher, “Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)”, Journal of Chemical Physics121 (2004) 9178 DOI
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.
J.Q. Qu, J.Y. Zhang, A.C. Grimsdale, K. Müllen, F. Jaiser, X.H. Yang, D. Neher, “Dendronized perylene diimide emitters: Synthesis, luminescence, and electron and energy transfer studies”, Macromolecules37 (2004) 8297 DOI
Aggregation of chromophores in the solid state commonly causes undesirable red shifts in the emission spectra and/or emission quenching. To overcome this problem, we have prepared soluble perylenetetracarboxidiimide dyes in which the chromophores are effectively shielded by polyphenylene dendrimers attached in the bay positions. Models show that attachment of the shielding units in the bay position should provide more efficient shielding than attaching them via the imide moieties. The dendrimers possess excellent film-forming properties due to alkyl substituents on their peripheries. The lack of a red shift in emission upon going from solution to the solid state indicates the dendrons suppress interaction of the emissive cores, leading to pure red-orange emission. Single-layer LEDs produce red-orange emission with relatively low efficiency especially for the higher generation dendrons, which is attributed to poor charge conduction. LEDs using blends of the dendrimers and the undendronized dye as a model compound in PVK have been investigated, and a model to extract relative charge injection rates through the dendritic scaffold from the spectral contributions in the EL spectra is developed.
X.H. Yang, F. Jaiser, D. Neher, P. V. Lawson, J.L. Brédas, E. Zojer, R. Güntner, P.S. de Freitas, M. Forster, U. Scherf, “Suppression of the keto-emission in polyfluorene light-emitting diodes: Experiments and models”, Advanced Functional Materials14 (2004) 1097 DOI
The spectral characteristics of polyfluorene (PF)-based light-emitting diodes (LEDs) containing a defined low concentration of either keto-defects or of the polymer poly(9.9-octylfuorene-co-benzothiadiazole) (F8BT) are preseneted. Both types of blend layers were tested in different device configurations with respect to the relative and absolute intensities of green blue emission components. It is shown that blending hole-transporting molecules into the emission layer at low concentration or incorporation of a suitable hole-transport layer reduces the green emission contribution in the electroluminescence (EL) spectrum of the PF:F8BT blend, which is similar to what is observed for the keto-containing PF layer. We conclude that the keto-defects in PF homopolymer layers mainly constitute weakly emissive electron traps, in agreement with the results of quantum-mechanical calculations.
B. Stiller, P. Karageirgiev, T. Geue, K. Morawetz, M. Saphiannikova, N. Mechau, D. Neher, “Optically induced mass transport studied by scanning near-field optical- and atomic force microscopy”, Physics of Low-Dimensional Structures1-2 (2004) 129
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.
A. Zen, J. Pflaum, S. Hirschmann, W. Zhuang, F. Jaiser, U. Asawapirom, J.P. Rabe, U. Scherf, D. Neher, “Effect of Molecular Weight and Annealing of Poly(3-hexylthiophene)s on the Performance of Organic Field Effect Transistors”, Advanced Functional Materials14 (2004) 757 DOI
The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 1.50 degreesC . However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers.
T. Kietzke, D. Neher, M. Kumke, R. Montenegro, K. Landfester, U. Scherf, “A nanoparticle approach to control the phase separation in polyfluorene photovoltaic devices”, Macromolecules37 (2004) 4882 DOI
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.
X.H. Yang, D. Neher, “Polymer electrophosphorescence devices with high power conversion efficiencies”, Applied Physics Letters84 (2004) 2476 DOI
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.
U. Asawapirom, F. Bulut, T. Farrell, C. Gadermaier, S. Gamerith, R. Güntner, T. Kietzke, S. Patil, T. Piek, R. Montenegro, B. Stiller, B. Tiersch, K. Landfester, E.J.W. List, D. Neher, C. Sotomayor-Torres, U. Scherf, “Materials for polymer electronics applications – semiconducting polymer thin films and nanoparticles”, Macromolecular Symposia212 (2004) 83 DOI
The paper presents two different approaches to nanostructured semiconducting polymer materials: (i) the generation of aqueous semiconducting polymer dispersions (semiconducting polymer nanospheres SPNs) and their processing into dense films and layers, and (ii) the synthesis of novel semiconducting polyfluorene-block-polyaniline (PF-b-PANI) block copolymers composed of conjugated blocks of different redox potentials which form nanosized morphologies in the solid state.
T. Kietzke, D. Neher, R. Montenegro, K. Landfester, U. Scherf, H.H. Hörhold, “Nanostructured solar cells based on semiconducting polymer nanospheres”, Proc. SPIE5215 (2004) 206
The paper presents two different approaches to nanostructured semiconducting polymer materials: (i) the generation of aqueous semiconducting polymer dispersions (semiconducting polymer nanospheres SPNs) and their processing into dense films and layers, and (ii) the synthesis of novel semiconducting polyfluorene-block-polyaniline (PF-b-PANI) block copolymers composed of conjugated blocks of different redox potentials which form nanosized morphologies in the solid state.
S.A Bagnich, C. Im, H. Bässler, D. Neher, U. Scherf, “Energy transfer in a ladder-type methyl-poly(para-phenylene) doped by Pt(II)octaethylporphyrin”, Chemical Physics299 (2004) 11-16 DOI
The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped by platinum-porphyrin dye PtOEP covering the concentration 10(-3) to 5% by weight has been measured employing cw and transient techniques. Upon excitating into the range of absorption of the host strong phosphorescence of the dopant is observed. Possible ways of populating of the dopant triplet state are considered. It is shown that the main channel is singlet-singlet energy transfer among chromophor groups of the polymer followed by Forster-type transfer to the guest and subsequent intersystem crossing.
R. Zentel, M. Behl, D. Neher, A. Zen, S. Lucht, “Nanostructured polytriarylamines: Orientation layers for polyfluorene”, Abstracts of Papers of the American Chemical Society227 (2004) U517
L. Kulikovsky, D. Neher, E. Mecher, K. Meerholz, H.H. Hörhold, O. Ostroverkhova, “Photocurrent dynamics in a poly(phenylene vinylene)-based photorefractive composite”, Physical Review B69 (2004) 125216 DOI
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.
X.H. Yang, D. Neher, D. Hertel, T.K. Däubler, “Highly-efficient single layer polymer electrophosphorescent devices”, Advanced Materials16 (2004) 161 DOI
A commercially available Ir complex has been employed for the preparation of highly efficient 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.
X.H. Yang, D. Neher, C. Spitz, E. Zojer, J.L. Brédas, R. Güntner, U. Scherf, “On the polarization of the green emission of polyfluorenes”, Journal of Chemical Physics119 (2003) 6832 DOI
An experimental and theoretical study of the anisotropic optical properties of polyfluorenes (PFs) bearing ketonic defects is presented. Polarized emission experiments performed on photooxidized aligned PF layers indicate that the transition dipole of the "green" CT π-π* transition of the keto-defect is oriented parallel to the chain direction. It is further observed that the polarization ratio of the green emission is slightly smaller than that of the blue emission component originating from undisturbed chains. Quantum mechanical calculations have been performed to support these observations. It is shown that the transition dipole moment of the CT π-π* transition of the defect is slightly misaligned with respect to the π-π* transition of the undisturbed PF chain, and that the angle between both depends on the chain conformation. For the most probably 5/2 helical conformation, this angle is, however, smaller than 5°. Further, polarized PL spectroscopy with polarized excitation has been performed to determine the extent of energy migration prior to emission from the keto-defect. For excitation at 380 nm, the polarization ratio of the green emission is essentially independent of the excitation polarization, indicating almost complete depolarization of the excitation before it is captured at a defect site. In contrast to this, energy migration after direct excitation of the keto-defect is inefficient or even absent.
S.J. Tans, R.G. Miedema, L.J. Geerligs, C. Dekker, J. Wu, D. Neher, G. Wegner, “Electronic transport in monolayers of phthalocyanine polymers”, Nanotechnology14 (2003) 1043 DOI
We report on a study of the electrical-transport properties of monolayers of phthalocyaninepolysiloxane (PcPS) polymers, and evaluate their potential for use as molecular wires. Monolayers have been deposited with the Laurnuir-Blodgett technique on top of Si/SiO2 substrates with interdigitated electrodes. Current-voltage curves have been measured as a function of temperature for samples with varying electrode distance and number of monolayers. In the undoped state, the conduction is well described by the space-charge-limited-current model. From the data we obtain material characteristics such as the density of trap states within the gap and an estimate of the charge-carrier mobility. It appears that the conductivity is too low to yield a measurable current through a single PcPS polymer. Chemical doping and a field effect have been investigated. Oxygen is effective in doping the PcPS layers, resulting in a two orders of magnitude increase of the conductivity. Iodine is not effective as a dopant. By application of a voltage on a back-gate, we observe a field-effect-induced increase of the conductivity by three orders of magnitude. The effect however decays rapidly in time.
Th. Kietzke, D. Neher, K. Landfester, R. Montenegro, U. Scherf, R. Güntner, “Novel approaches to polymer blends based on polymer nanoparticles”, Nature Materials2 (2003) 408 DOI
Polymer layers can exhibit significantly improved performances if they possess a multicomponent phase-separated morphology. We present two approaches to control the dimensions of phase separation in thin polymer-blend layers; both rely on polymer nanospheres prepared by the miniemulsion process. In the first approach, heterophase solid layers are prepared from an aqueous dispersion containing nanoparticles of two polymers, whereas in the second approach, both polymers are already contained in each individual nanoparticle. In both cases, the upper limit for the dimension of phase separation is determined by the size of the individual nanoparticles, which can be adjusted down to a few tens of nanometres. We also show that the efficiencies of solar cells using two-component particles are comparable to those of devices prepared from solution at comparable illumination conditions, and that they are not affected by the choice of solvent used in the miniemulsion process.
T. Piok, S. Gamerith, C. Gadermaier, H. Plank, F.P. Wenzl, S. Patil, R. Montenegro, T. Kietzke, D. Neher, U. Scherf, K. Landfester, E.J.W. List, “Organic light-emitting devices fabricated from semiconducting nanospheres”, Advanced Materials15 (2003) 800 DOI
The fabrication of organic light-emitting devices (OLEDs) from semiconducting polymer nanospheres (SPNs) deposited from aqueous dispersions is described. It is found that the active device layer consists of a homogeneous single layer of light-emitting SPNs. The OLEDs exhibit an electroluminescence onset at the SPN energy gap, which can be attributed to field-enhanced charge-carrier injection at the nanostructured Al cathode.
X.H. Yang, D. Neher, U. Scherf, S. Bagnich, H. Bässler, “Efficient phosphorescent light-emitting diodes utilizing a ladder-type poly(p-phenylene) polymer host”, Journal of Applied Physics93 (2003) 4413 DOI
The properties of light-emitting diodes based on the electrophorescent platinum-porphyrin dye PtOEP blended into a ladder-type poly(para-phenylene) (LPPP) polymer host are presented. Due to the small difference between the highest occupied molecular orbital (HOMO) levels of the guest and the host, the operating voltage of single layer devices is almost independent of the dopant concentration in clear contrast to what has been observed in the case of wide-band gap host polymers. However, the efficiency and the color purity of these single-layer devices is quite poor, which can be attributed to the weak trapping of carriers on the phosphorescent dye. Incorporating an electron-transporting/hole-blocking layer greatly increases the efficiency of the devices but at the same time emission from the host becomes more significant. Adding a hole-transporting/electron-blocking layer further increases the efficiency of the devices. Pure red emission is, however, only obtained if the HOMO of the hole-transporting layer is well above that of the LPPP host. This effect is interpreted in terms of a direct injection of holes from the hole-transporting layer into the HOMO level of the dopant. Further, prompt fluorescence and phosphorescence studies indicate that energy transfer from LPPP singlet excitons contributes to the emission from the dopant, but that the efficiency of singlet exciton transfer is too small to explain the almost complete absence of host emission in the electroluminescence spectra of single- and three-layer devices.
X.H. Yang, T. Kietzke, F. Jaiser, D. Neher, “Polymer light emitting diodes based on LiF/Al composite cathode”, Synthetic Metals137 (2003) 1503 DOI
We report on the performances of polymer light emitting diodes utilizing LiF/Al composite cathodes. Among the devices with the composite cathodes of different concentration of LiF, the "layer by layer" device is the most efficient, which is different from the results based on small molecule organic light diode devices. The different performances of the devices can be ascribed to the variation of the electron injection barrier heights at the polymer/cathode interface, as suggested by photovoltaic experiments.
D. Sainova, D. Neher, E. Dobruchowska, B. Luszczynska, I. Glowacki, J. Ulanski, H.-G. Nothofer, U. Scherf, “Thermoluminescence and electroluminescence of annealed polymer layers”, Chemical Physics Letters371 (2003) 15 DOI
As-prepared and annealed layers of polyfluorene were studied by thermoluminescence (TL), photo luminescence (PL) and electroluminescence (EL). Upon annealing at the transition to the liquid-crystalline phase, the green emission in PL and EL becomes more pronounced. At the same time the TL spectrum broadens towards higher temperatures, indicating the introduction of additional trapping sites. In contrast, photooxidation does not broaden the TL spectrum, even so it significantly affects the emission spectra. We conclude that annealing causes the formation of non-/weakly emitting aggregates which compete with the undisturbed chains with respect to charge carrier capture and exciton recombination.
S. Lucht, D. Neher, T. Miteva, G. Nelles, A. Yasuda, R. Hagen, S. Kostromine, “Photoaddressable polymers for liquid crystal alignment”, Liquid Crystals30 (2003) 337 DOI
We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.
N. Mechau, D. Neher, V. Börger, H. Menzel, K. Urayama, “Optically-driven diffusion and mechanical softening in azobenzene polymer layers”, Applied Physics Letters81 (2002) 4715 DOI
A study of the thermally induced and photoinduced changes in the mechanical properties of thin azobenzene polymer layers utilizing electromechanical spectroscopy is presented. Upon illumination at room temperature, weak photoinduced plasticization is observed. This is well below the degree of softening induced by heating the layer above the glass transition temperature. At the same time, the polymer layer roughens, indicating light-induced diffusion of the macromolecules over micrometer distances. We propose an active photoinduced diffusion mechanism, in which free volume generated during the photoisomerization of one chromophore is reoccupied in the course of the photoinduced isomerization of an adjacent azobenzene molecule.
K. Urayama, S. Yamamoto, Y. Tsujii, T. Fukuda, D. Neher, “Elastic properties of well-defined, high density poly(methyl methacrylate) brushes studied by electromechanical interferometry”, Macromolecules35 (2002) 9459 DOI
We have investigated the elastic properties of dry polymer brushes comprising low-polydispersity poly(methyl methacrylate) (PMMA) chains with high graft densities over 0.4 chains/nm2 by means of electromechanical interferometry. End-grafting on a glass plate by living radical polymerization yielded high-density brushes composed of the highly stretched graft chains, and the layer thickness of the brushes is 4-5 times as large as the unperturbed dimension of the graft chains. The layer thickness change induced by an applied electric field (electrostriction) is measured by a Nomarski optical interferometer as a function of temperature. The analysis of the electromechanical and dielectric data yields the plate compressibility (κp) of the brushes in the glassy and molten states. Comparison of the results for the high-density PMMA brushes and a reference spin-coated PMMA layer reveals the differences in the elastic and dielectric properties between a brush composed of highly stretched graft chains and a layer formed by the equivalent chains in the random-coiled state. The temperature dependence of the dielectric loss suggests that the glass transition temperature of the high-density brushes is ca. 10 °C higher than that of the spin-coat layer. In the glassy state, there is no appreciable difference in κp between the brushes and the spin-coat layer, whereas in the molten state, κp of the brushes is definitely (ca. 30-40%) lower than that of the spin-coated layer. This proves that the molten high-density PMMA brushes are more resistant against compression relative to the PMMA melt. The low compressibility of the molten high-density brushes is interpreted on the basis of rubber elasticity of entanglement network in the stretched state. This model successfully explains the magnitude of the plate compressibility of the brushes, which suggests that the low compressibility is mainly attributed to a strain-hardening effect of the highly stretched entanglement chains and that there exists a considerable amount of entanglements of different graft chains contributing to elastic modulus.
X.H. Yang, D. Neher, S. Lucht, H.G. Nothofer, R. Güntner, U. Scherf, R. Hagen, S. Kostromine, “Efficient polarized light-emitting diodes utilizing ultrathin photoaddressable alignment layers”, Applied Physics Letters81 (2002) 2319 DOI
We demonstrate that an ultrathin photoaddressable polymer (PAP) layer with a thickness as small as 5 nm can be utilized for the monodomain alignment of thermotropic liquid crystalline polyfluorene. The optical anisotropies in absorption and emission are found to be independent of the PAP layer thickness within a range of 5 to 30 nm. On the other hand, decreasing the PAP layer thickness greatly improves the performance of polarized blue light-emitting diodes: With a PAP layer thickness of only 10 nm, the device turns on at 5 V and reaches a brightness of 100 cd/m2 at 8 V with an efficiency of 0.66 cd/A.
A. Meisel, T. Miteva, G. Glaser, V. Scheumann, D. Neher, “Influence of dopant concentration on the morphology of hole-transporting alignment layers based on a polyimide matrix”, Polymer43 (2002) 5235 DOI
Investigations on hole-transporting alignment layers (HTALs) consisting of a polyimide matrix doped with hole-transporting materials (HTMs) at different concentrations by means of low-voltage scanning electron microscopy and atomic force microscopy are reported. These layers were recently used as HTALs for liquid crystalline polyfluorenes in polarized light-emitting diodes. For HTM concentrations below 15 wt%, phase-separation was found to be not significant, and the layer characteristics were dominated by the stiff polyimide matrix. These layers aligned polyfluorene very well, resulting in polarization ratios in electroluminescence of more than 20. On the contrary, the morphology was substantially altered at higher dopant concentrations. Moreover, microgrooves became visible after rubbing, indicating that the degree of imidization of the polyimide matrix was reduced. As a result, increasing the concentration of the HTMs above a certain level resulted in a dramatic decrease of the aligning ability of the HTAL.
M. Oda, H.G. Nothofer, U. Scherf, V. Sunjic, D. Richter, W. Regenstein, and D. Neher, “Chiroptical properties of chiral substituted polyfluorenes”, Macromolecules35 (2002) 6792 DOI
Liquid-crystalline polyfluorene (PF) homopolymers substituted with chiral alkyl side chains were synthesized, and their chiroptical properties in the solid state were investigated by means of circular dichroism (CD), circularly polarized photoluminescence (CPPL), and circularly polarized electroluminescence (CPEL) measurements. Polarization-selective scattering of light is shown to cause artifacts in the circularly polarized absorption and emission spectra in the wavelength range near or above the absorption edge, and a measurement scheme to avoid these is presented. For all derivatives, significant chiroptical effects appeared only after the solid layers have been annealed at elevated temperatures, preferably into the liquid-crystalline state of the polymer. The largest anisotropy factors were measured for a polyfluorene substituted with chiral (R)-2-ethylhexyl side chains, yielding absolute values of up to 0.28 for CPPL and up to 0.25 for CPEL. These are among the highest ever reported for a chiral conjugated polymer. Anisotropy factors for CD, CPEL, and CPPL were consistently found to follow an "odd-even effect" concerning the position of the chiral center in the alkyl side chain. If the chiral center is placed close to the polymer backbone, the CD is dominated by one peak with its maximum close to the maximum of the pi-pi* absorption band. This indicates that the chiroptical. properties are most probably caused by intrachain effects rather than by pure interchain exciton coupling. This interpretation is supported by the results of time-dependent Hartree-Fock calculations for the isolated fluorene dimer and trimer. In both cases, the anisotropy factor depends strongly on the torsion angle between neighboring fluorene units. For the trimer, a maximum anisotropy factor of 0.25, close to the maximum values determined experimentally, is predicted for a torsion angle of ca. 105degrees. Both experimental and theoretical results indicate that the chiroptical properties of these chiral substituted polyfluorenes are mainly caused by a helical conformation of the conjugated polymer backbone.
V. Cimrová, D. Neher, R. Hildbrandt, M. Hegelich, A. von der Lieth, G. Marowsky, R. Hagen, S. Kostromine, T. Bieringer, “Comparison of the birefringence in an azobenzene side chain copolymer induced by pulsed and continuous-wave irradiation”, Applied Physics Letters81 (2002) 1228 DOI
A comparative study of photoinduced birefringence in azobenzene-side-chain copolymers under continuous-wave (cw) and pulsed irradiation is presented. Absolute refractive index changes were determined by the attenuated total reflection spectroscopy. We clearly demonstrated that pulsed, similarly to cw, irradiation induces a three-dimensional reorientation of azobenzene chromophores. For the same light dose, pulsed irradiation using ns pulses of appropriate pulse energy led to higher values of birefringence, which can be attributed to a cooperative motion of azobenzene side chains and thermal effects induced by the laser pulses.
A. Zen, D. Neher, C. Bauer, U. Asawapirom, U. Scherf, R. Hagen, S. Kostromine, R. F. Mahrt, “Polarization-sensitive photoconductivity in aligned polyfluorene layers”, Applied Physics Letters80 (2002) 4699 DOI
Polarization-sensitive photodiodes have been constructed utilizing an aligned polyfluorene layer oriented on a photoaligned liquid crystalline photoaddressable polymer. The diodes exhibit a pronounced dependence of the photocurrent on the polarization of the incident light, yielding a polarization sensitivity of more than a factor of 10 at the onset of the absorption. The action spectrum is symbatic for light polarized perpendicular to the polymer alignment direction whereas it does not simply correlate with the absorption characteristics for parallel polarized light. The data can be explained utilizing Ghosh's model taking into account interference effects within the polyfluorene layer.
J. N. Wilson, W. Steffen, T.G. McKenzie, G. Lieser, M. Oda, D. Neher, U.H.F. Bunz, “Chiroptical properties of poly(p-phenyleneethynylene) copolymers in thin films: large g-values”, Journal of the American Chemical Society124 (2002) 6830 DOI
A series of chirally substituted poly (p-phenyleneethynylene) copolymers was prepared by alkyne metathesis of mixtures of two different 2,5-dialkyl-1,4-dipropynylbenzenes. One of the monomers was the chiral bis-2,5-(S)-3,7-dimethyloctyl-1,4-dipropynylbenzene, and the second one was an achiral dipropynyl monomer. If the content of chiral monomer is 25-50 mol %, unusually large chiroptical effects, that is, optical dissymmetries, result in absorption (g = -0.38) and emission (g = -0.19) of these copolymers. The large dissymmetries can be explained by a supramolecular ordering of the PPEs into stranded features that are visualized by dark-field transmission electron microscopy. The strands of chirally substituted PPEs display a striated structure that suggests that the whole feature is a single chirally twisted crystallite.
K. Landfester, R. Montenegro, U. Scherf, R. Güntner, U. Asawapirom, S. Patil, D. Neher, T. Kietzke, “Semiconducting polymer nanospheres in aqeous dispersion prepared by a miniemulsion process”, Advanced Materials14 (2002) 651 DOI
T. Srikhirin, V. Cimrová, B. Schiewe, M. Tzolov, R. Hagen, S. Kostromine, T. Bieringer, D. Neher, “An Investigation of the photoinduced changes of absorption of high-performance photoaddressable polymers”, ChemPhysChem3 (2002) 335 DOI
Over three spectral regions, photoinduced absorption changes were measured for copolymers comprising a non-mesogenic and a mesogenic azobenzene side chains for applications including optical storage. On illumination, these polymers show little out-of-plane reorientation and minor changes in transmission below the absorption band.
D. Sainova, A. Zen, H.G. Nothofer, U. Asawapirom, U. Scherf, R. Hagen, T. Bieringer, S. Kostromine, D. Neher, “Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices”, Advanced Functional Materials12 (2002) 49 DOI
Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.
D. Neher, “Polyfluorene Homopolymers: Conjugated Liquid-Crystalline Polymers for Bright Blue Emission and Polarized Electroluminescence”, Macromolecular Rapid Communications22 (2001) 1365 DOI
Since the first report on blue electroluminescence from a soluble poly(9,9-di-alkylfluorene), fluorene-based homo- and copolymers have evolved as a major class of polymeric emitters for highly efficient organic light-emitting diodes. This Review is concerned with the particular properties of soluble derivatives of polyfluorene homopolymers with respect to emission properties, control of color stability and efficiency in electroluminescence, alignment in thin layers and polarized emission. A major point of discussion is the origin of unwanted emission contributions in the photoluminescence and electroluminescence spectra of solid layers as well as concepts to avoid these contributions in order to ensure stable device performance. Further, the alignment of polyfluorenes and their use in polarized light-emitting diodes is addressed.
F.C. Grozema, T.J. Savenije, M. J. W. Vermeulen, L.D.A. Siebbeles, J.M. Warman, A. Meisel, D. Neher, H.G. Nothofer, U. Scherf, “Electrodeless Measurement of the In-Plane Anisotropy in the Photoconductivity of an Aligned Polyfluorene Film”, Advanced Materials13 (2001) 1627 DOI
Quantitative data on the in-plane anisotropy of charge transport in an aligned polyfluorene (PF) film can be obtained thanks to the technique described in this communication. Since the method revealed here does not require the deposition of electrode layers, it can be more readily applied to the characterization of a large number and variety of structured, thin-layer Samples, free of the complications that often accompany DC measurements.
N.G. Pschirer, T. Miteva, U. Evans, R.S. Roberts, A.R. Marshall, D. Neher, M.L. Myrick, U.H.F. Bunz, “Blue solid-state photoluminescence and electroluminescence from novel poly(para-phenyleneethynylene) copolymers”, Chemistry of Materials13 (2001) 2691 DOI
We report the synthesis of a series of random copolymers composed of phenyleneethynylene and 3,7-di-tert-butylnaphthalene units by alkyne metathesis with simple catalyst systems, formed from Mo(CO)6 and 4-chlorophenol in situ. These novel copolymers give rise to efficient (Φ = 0.16-0.76) blue solid-state emission (λmax = 422-443 nm) which can be fine-tuned according to their naphthalene content differing greatly from that of standard dialkyl-PPE (λmax 500-520 nm). The effective and blue-shifted solid-state emission is a result of disrupting the solid-state alignment thereby diminishing π-π-interactions of the copolymers in addition to the efficient interruption of planarization of the conjugated backbone. Electroluminescent multilayer devices showed low turn-on voltages (5.5 V) and a maximum brightness of 100 cd/m2.
M. Grell, M. Oda, K.S. Whitehead, A.Asimakis, D. Neher, D.D.C. Bradley, “A compact device for the efficient electrically driven generation of highly circularly polarized light”, Advanced Materials13 (2001) 577 DOI
A novel, compact, and simple-to-manufacture photon recycling device, which is driven by a polymer light emitting diode (LED), generates highly circularly polarized light. The device is based on a chiral nematic liquid crystal and it uses the metal cathode of the LED simultaneously as electrode and as recycling mirror of the recycling structure.
T. Miteva, A. Meisel, W. Knoll, H.G. Nothofer, U. Scherf, D.C. Müller, K. Meerholz, A. Yasuda, D. Neher, “Improving the performance of polyfluorene-based organic light-emitting diodes via end-capping”, Advanced Materials13 (2001) 565 DOI
Efficient deep-blue electroluminescence (EL) is obtained via end-capping polyfluorene (PF) homopolymers with hole-transporting moieties (HTMs). Strong evidence that the observed improvement in device performance is related to the end-capper moieties comes from the comparison of two different end-cappers. The HTM does not disturb the liquid-crystal properties of the PF polymer. Using doped polyimide alignment layers. polarized light-emitting diodes (LEDs) with a polarization ratio in excess of twenty at an efficiency of 0.25 cd/A could be realized.
C. Bauer, G. Urbasch, H. Giessen, A. Meisel, H.-G. Nothofer, D. Neher, U. Scherf, R. F. Mahrt, “Polarized photoluminescence and spectral narrowing in an oriented polyfluorene thin film”, ChemPhysChem1 (2000) 142 DOI
The oriented polyfluorene is a promising candidate for a blue polarized emitter in organic optoelectronic devices. By examination of the anisotropic luminescence of thin films of polyfluorene, polarization-dependent spectral narrowing was found at high excitation densities, which is of particular interest in reducing the threshold for optically pumped polymer lasers. Femtosecond pump-probe experiments show optical gain at this spectral position.
K. Urayama, M. Tsuji, D. Neher, “Layer-thinning effects on ferroelectricity and the ferroelectric-to-paraelectric phase transition of vinylidenefluoride-trifluoroethylene copolymer layers”, Macromolecules33 (2000) 8269 DOI
Dielectric and electromechanical properties of vinylidene fluoride-trifluoroethylene copolymer layers with thickness ranging from 1300 nm down to 65 nm have been investigated by dielectric spectroscopy and electromechanical interferometry. The effects of layer thickness (h) on ferroelectricity and the ferroelectric-to-paraelectric phase transition are discussed on the basis of the temperature (T) dependence of the dielectric constant (ε) and electrostrictive and inverse-piezoelectric effects. In the region of h less than a few hundred nanometers, the layer-thinning effect on ε becomes prominent, and ε decreases with a decrease in h, but the phase transition temperature (Tc) and the Curie constants are not significantly influenced by layer thinning. The dependence of the electrostriction on the square of the applied electric field for unpoled films is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. The degree of the nonlinearity decreases as the layer becomes thinner, and for a 65 nm thick film the nonlinearity almost vanishes at temperatures fairly below Tc. Remanent polarizations (Pr) achieved by poling are ca. 55 mC/m2 for the films of h ≥ 90 nm, while Pr for the 65 nm thick film (40 mC/m2) is definitely lower. Differential scanning calorimetry shows that the degree of crystallinity (fraction of ferroelectric crystalline phase) decreases with reduction in film thickness, and especially the crystallinity for the 65 nm thick film is much lower than those for the thicker ones. In comparison, between the 1300 and 65 nm thick films, the reduction in the degree of crystallinity is comparable to the decrease in Pr. The variation of the dielectric constant and the degree of crystallinity on h are reasonably well explained assuming the presence of a nonferroelectric amorphous-like surface near layer. Electron microscopic studies of the 65 nm thick layer suggest a preferred orientation of the chain axis of the crystallites parallel to the film surface. The presence of the preferential crystalline orientation might as well explain the appreciably different values of the surface near layer thickness (12 and 27 nm) evaluated from the h dependence of the dielectric constant and the degree of crystallinity. The layer thickness dependence of the dielectric and electromechanical properties is interpreted as a result of a combined effect of the reduction in degree of crystallinity and the specific crystallite orientation due to layer thinning.
T. Srikhirin, A. Laschitsch, D. Neher, D. Johannsmann, “Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators”, Applied Physics Letters77 (2000) 963 DOI
Employing quartz crystal resonators, we have measured the elastic compliance of thin polymers films doped with an azobenzene dye. From a comparison of the shifts of resonance frequency on different acoustic overtones of the resonator, the films' elastic compliance was derived. Upon irradiation with ultraviolett light at low intensity, the compliance decreases by a few percent. Irradiation with visible light at high intensity, on the contrary, softens the material. We assume that, when irradiated with high-power visible light, the azobenzene molecules undergo rapid cycling through the trans- and cis-states, thereby softening the polymer matrix.
G. Lieser, M. Oda, T. Miteva, A. Meisel, H.G. Nothofer, U. Scherf, D. Neher, “Ordering, graphoepitaxial orientation, and conformation of a polyfluorene derivative of the "hairy-rod" type on an oriented substrate of polyimide”, Macromolecules33 (2000) 4490 DOI
Spin-coated films of an ethylhexyl derivative of polyfluorene can be converted on a pretreated polyimide substrate into highly oriented films by annealing in the liquid crystalline state. Together with improving orientation segregation of the wormlike molecules with respect to chain lengths and lamella formation proceeds. End groups are preferentially assembled in interlamellar regions. This morphological feature is thought to influence all measurements of intrinsic properties of polyfluorene films with similar histories. Electron diffraction patterns of the film are identical with X-ray fiber diagrams of fibers drawn from the melt and annealed in the liquid crystalline state. The experimental data show that the polymer molecules adopt a helical (5/q) conformation, packing in a trigonal unit cell. Molecular modeling based on ab initio MO calculations have been carried out to obtain independent estimates of chain geometry and conformation. These calculations are more in favor of a 5/2 rather than a 5/1 helix, with the argument of the observed packing of the individual PF chains and a plausibly low torsion angle of adjacent fluorene building blocks only for a 5/2 helix.
M. Gross, D.C. Müller, H.G. Nothofer, U. Scherf, D. Neher, C. Bräuchle, K. Meerholz, “Improving the performance of doped π-conjugated polymers for use in organic light-emitting diodes”, Nature405 (2000) 661 DOI
Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest unoccupied molecular orbital of the adjacent molecules (simultaneously, holes are injected by the anode into the highest occupied molecular orbital). The two types of carriers migrate towards each other and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped pi-conjugated polymer layers improve the injection of holes in OLED devices; this is thought to result from the more favourable work function of these injection layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such polymers, the barrier to hole injection can be continuously reduced. The use of combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electroluminescence (such as polyfluorene-based systems), finding that it is possible to significantly reduce the operating voltage while improving the light output and efficiency.
T. Miteva, A. Meisel, H.G. Nothofer, U. Scherf, W. Knoll, D. Neher, M. Grell, D. Lupo, A. Yasuda, “Polarized electroluminescence from highly aligned liquid-crystalline polymers”, Proc. ICEL 2, Sheffield 1999, Synthetic Metals111-112 (2000) 173 DOI
Two classes of polymer materials, hexyl-dodecyl copoly(phenylene-ethynylene) and poly(2,7-(9,9-di(ethylhexyl)-fluorene)), respectively, were aligned by two different alignment techniques. For the copolymer, the crystalline and liquid crystalline state were found to differ only slightly and alignment was achieved in the crystalline state, whereas the two phases differ substantially for the polyfluorene and alignment could be obtained in the liquid crystalline state. Light emitting diodes of differing structures were developed using the two materials as emissive layers. In the case of the polyfluorene, a state-of-the-art polarization ratio of 21 was realized.
M. Oda, S.C.J. Meskers, H.G. Nothofer, U. Scherf and D. Neher, “Chiroptical Properties of chiral-substituted Polyfluorenes”, Proc. ICEL 2, Sheffield 1999, Synthetic Metals111-112 (2000) 575 DOI
Chiroptical properties and structures of chiral-substituted polyfluorenes have been characterized using UV/Vis, circular dichroism (CD), linear dichroism (LD), photoluminescence (PL), circular polarized photoluminescence (CPPL), linear polarized photoluminescence (LPPL), IR, X-ray diffraction, and polarized microscopy. Light emitting diodes (LED) of these polymers have been made and found to show strong blue-light emission, with the characteristic ratio of the circular polarized electroluminescence (g(CPEL)) up to 7.0×10-2 at 420 nm, which is well above the values found for other conjugated polymers in emission.
T.K. Däubler, R. Bittner, K. Meerholz, D. Neher, “Charge carrier photogeneration, trapping and space-charge field formation in PVK-based photorefractive materials”, Physical Review B61 (2000) 13515 DOI
We studied the dark conductivity (jdark), the photoconductivity (jphoto), and the charge carrier photogeneration efficiency η of poly(N-vinylcarbazole)-based photorefractive (PR) materials with different glass-transition temperatures (TG) and chromophore content (ρCHR). Measurements were carried out at wavelengths similar to those used in degenerate four-wave mixing (DFWM) and two-beam coupling (2BC) experiments. Both thick (37 μm) and thin samples (approximate to 1 μm) were analyzed. Photoconductivity experiments at different temperatures show that both jdark and jphoto are thermally activated. For jdark the activation is not related to the glass-transition temperature of the blends, whereas photocurrents exhibit a universal behavior with respect to Tr = TG - T. The charge carrier photogeneration efficiency η was measured by xerographic discharge experiments. eta was found to be independent of both TG and of ρCHR. The photoconductivity gain factor G defined as the number of charge carriers measured in photoconductivity in relation to the number of carriers initially photogenerated as determined by the xerographic experiments is used to compare the results of photoconductivity and xerographic discharge experiments. G is found to be much smaller than unity even for thin samples, which indicates that the mean free path of the photogenerated charge carriers is less than 1 μm at photoelectrical equilibrium. Using Schildkraut's model for the space-charge field formation in organic PR materials, trap densities Ti of approximately 1017 cm-3 could be derived from G. The field and temperature dependence of Ti is independent of ρCHR and might account for the universal Tr dependence of jphoto. The estimated trap densities are used to calculate the first-order Fourier component of the space-charge field in the PR materials illuminated with a sinusoidal intensity pattern. Modifying Schildkraut's model so that the tilt between the applied electric field and the index of refraction grating is taken into account yields saturation fields of approximately 100 V/μm in agreement with findings from PR experiments. The dramatic decrease of the space-charge field when the temperature exceeds the glass-transition temperature of the blend is fully explained by a decrease in trap density. The fact that the trap density depends on the temperature with respect to TG and not on the absolute temperature suggests that the relevant traps are most likely of conformational nature.
M.U. Pralle, K. Urayama, G.N. Tew, D. Neher, G. Wegner, S.I. Stupp, “Piezoelectricity in polar supramolecular materials”, Angewandte Chemie - International Edition39 (2000) 1486 DOI
Units with large effective dipole moments (shown schematically) are suggested to explain the observation of spontaneous piezoelectric behavior in polar self-assembled films comprised of clusters formed by rod-coil molecules. The piezoelectric response of heated films remains upon cooling.
D. Sainova, T. Miteva, H.G. Nothofer, H. Fujikawa, I. Glowacki, J. Ulanski, U. Scherf, D. Neher, “Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules”, Applied Physics Letters76 (2000) 1810 DOI
Adding low-molecular-weight hole-transporting molecules (HTM) with different oxidation potentials to the polyfluorene emission layer of single-layer light-emitting diodes causes significant changes in the device properties. The pronounced increase in luminance efficiency combined with a decrease in current is attributed to significant hole trapping, as further suggested by thermoluminescence experiments. Using a oligo-triphenylamine HTM with an ionization potential of similar to 4.9 eV, light-emitting diodes with stable blue emission, a brightness of 800 cd/m2 and an efficiency of 0.87 cd/A were realized. Further, the red-emitting contribution to the spectra as observed in the pure polymer devices could be fully suppressed.
H.G. Nothofer, A. Meisel, T. Miteva, D. Neher, M. Forster, M. Oda, G. Lieser, D. Sainova, A. Yasuda, D. Lupo, W. Knoll, U. Scherf, “Liquid Crystalline Polyfluorenes for Blue Polarized Electroluminescence”, Macromolecular Symposia154 (2000) 139 DOI
A series of 9,9-dialkyl-poly(fluorene-2,7-diyl)s containing linear and branched alkyl substituents with a M-n of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.
M. Oda, H.G. Nothofer, G. Lieser, U. Scherf, S.C.J. Meskers. D. Neher, “Circularly polarized electroluminescence from liquid-crystalline chiral polyfluorenes”, Advanced Materials12 (2000) 362 DOI
Circularly polarized electroluminescence (CPEL) has potential application in optical data storage and processing. It was recently discovered that CPEL could be generated from chiral-substituted, conjugated polymers. These authors have synthesized a range of polyfluorenes that form liquid crystals and emit bluish light with a significant (200 times greater than in the first report) circularly polarized component.
T. Miteva, L. Kloppenburg, D. Neher, and U.H.F. Bunz, “Interplay of Thermochromicity and Liquid Crystalline Behavior in Poly(p-phenyleneethynylen)s:π-π-Interactions or Planarization of the Conjugated Backbone?”, Macromolecules33 (2000) 652 DOI
C. Bauer, R. Böhmer, S. Moreno-Flores, R. Richert, H. Sillescu, D. Neher, “Capacitive scanning dilatometry and frequency dependent thermal expansion of polymer films”, Physical Review E61 (2000) 1755 DOI
The dilatometric properties of polymer films near and above their glass-transition temperatures were explored using capacitive high-frequency detection in temperature ramping as well as in harmonic temperature cycling experiments. The broad applicability of capacitive scanning dilatometry is demonstrated by the investigation of macromolecular systems of vastly different polarity such as polystyrene, polybutadiene, and polyvinylacetate. From temperature cycling experiments the real and imaginary parts of the frequency-dependent thermal-expansion coefficient are determined in the sub-Hz regime.