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In this paper, the spin-dependent photophysical properties of poly( para -phenylene)-type ladder-polymers (LPPP), a highly luminescent polymer with emission yields of ∼30% in film and ∼100% in solution, and of five- and seven-member ladder-type oligo( para -phenylenes) (LOPP5 and LOPP7, respectively) are determined via their photoluminescence (PL) emission, PL excitation, and PL-detected magnetic resonance (PLDMR) spectra. In the latter experiment, we investigated spin-sensitive properties, such as the formation of polaron pairs as a function of conjugation length and interchain interaction. The observation of a PLDMR quenching resonance upon increasing the temperature and/or the excitation wavelength is discussed in relation with the formation of polarons and polaron pairs from delocalized and localized states, respectively.

Nanostructured free-standing films were obtained by polymerisation of lyotropic liquid crystal phases based on the monomer 3,4,5-tris(11′-acryloyloxyundecyloxy)benzoate. The structure of these films was investigated by X-ray diffraction. No change of the lattice due to the polymerisation procedure was observed. The low water content (below 10 wt.-%) of the liquid crystals in combination with the observed hexagonal lattice reveal that a reversed hexagonal structure (H2 phase) is present. Different types of films were investigated, obtained by varying the preparation conditions of the used lyotropic liquid crystals, such as water content, type of metal-counterion and length of the monomer. The lattice constant was found to be independent of the water content. It seems that at low water content of 8 wt.-% the lyotropic structure is still fully hydrated. This unusual behaviour is related to the tapered structure of the monomer which is composed of by a small headgroup and three extended tails for each headgroup. The lattice constant can be changed by using differently charged metal-counterions which are associated with the headgroup, as well as by varying the length of the tails on the monomer. Low resolution electron density maps of the hexagonal lattice give deeper insight in the arrangement of the monomers within the polymerised structure.

Summary form only given. Dissociation of photogenerated excitons into charge carriers (polarons) is the working principle of organic photovoltaic cells, whereas recombination of field-injected charge carriers into luminescent singlet excitons and nonradiative triplet excitons governs the efficiency of organic light emitting devices (OLED). In this paper, we address these processes in a single layer OLED based on para-hexaphenyl (PHP) by conventional and electric field-assisted femtosecond pump-probe measurements. Due to its well defined conjugation length and its chemical structure, PHP exhibits well resolved photoinduced absorption (PA) features, that can be assigned to singlet, triplet and polaron absorption. This allows to monitor the dynamics of field-induced singlet dissociation and polaron recombination in a single layer OLED structure consisting of aluminum/PHP/ITO with 200 fs time resolution.

The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.

The application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.

The realization of a polymeric injection laser diode is an important aim in organic electronics. A crucial step towards this goal lies in the understanding and fabrication of feedback structures for lasers. Such feedback structures may be fabricated either in the substrate or directly in the active polymer layer.Soft Lithography has proven itself as a useful tool to pattern a wide variety of materials. Typically one uses elastomeric molds (stamps) to pattern materials and structures whereby dimensions as low as 30 nm can be achieved.Different types of Soft Lithographic processes are applied to produce optical feedback structures for optically pumped laser arrays.

An aqueous solution of a poly(p-phenylenevinylene) (PPV) precursor is combined with a self-assembling monomer to form an inverted hexagonal lyotropic liquid-crystalline mesophase. With this strategy we obtain hexagonally-packed tubular channels ordered on a nanometer scale filled with PPV chains. Isolated and well orientated molecules can be achieved. This nanocomposite is used as an emissive layer in light emitting diodes (LEDs) built up in a sandwich structure. We compare the I/V characteristics, electroluminescence (EL) spectra and the overall performance of the nanostructured PPV to that of pure PPV LEDs.

Due to its outstanding carrier transport capabilities pentacene is a prominent candidate for the active semiconducting layer in organic thin film transistors. This compound crystallizes in a layered structure with herringbone arrangement within each layer. Pentacene appears in several polymorphic structures, which differ basically by their c-axis lengths, meaning that the angle at which the molecules adsorb relative to the substrate changes from phase to phase. Obviously the interaction of the π-electron systems between adjacent molecules depends strongly on the stacking nature of the molecules. It has been argued, that a smaller angle between the molecular axis and the surface normal results in a larger orbital overlap which is expected to give better carrier transport properties. Therefore it is of major interest to clarify and control the growth conditions for the different phases. We have synthesized high-quality pentacene epitaxial thin films under different growth conditions and investigated them by atomic force, scanning electron and polarization microscopy. The aim was to identify the critical growth parameters with respect to surface quality, thickness and crystallinity of the thin films and with respect to homogeneity, size and shape of grains, ordering, substrate dependence, morphology and phase formation of the polycrystalline thin films. The polymorphic phase identification and the macrostructure of the films were determined by Xray diffraction, whereas micro-structural differences, small impurity concentrations and the temperature dependence of the structure were

Electric field-induced charge photogeneration in ladder-type poly(para-phenylene) is investigated by field-assisted femtosecond pump-probe experiments carried out on light emitting diodes. The characteristic photoinduced absorption band at 1.9 eV allows one to directly monitor the polaron population. We find that polarons are formed by exciton fission without intermediate states on a time scale of 10 ps. The buildup kinetics of the polaron population suggests a dissociation driven by exciton diffusion during interchain thermalization.

We determined the internal electric field distribution in light emitting electro-chemical cells (LECs) and light emitting diodes (LEDs) based on methyl substituted poly(paraphenylen) (mLPPP) by performing electroabsorption measurements as a function of an external bias voltage. Based on these results we outline the working principle of both types of devices. In the case of the LEC we observed an abrupt increase in the EA signal above a threshold voltage of about 2V which we attributed to junction formation due to in-situ electrochemical doping. For single layer LEDs we found the behavior expected for metal/insulator/metal/structures, i.e. the electric field drops linearly over the bulk polymer and no space-charge regions were observed.These findings are important for improving the performance of organic electroluminescent devices.

A new class of precursor polymers, as first described by Louwet et al. [1], was used to make organic LEDs. Device properties are relatively independent from the side chain to be eliminated. However introduction of two methyl groups onto the phenyl ring introduces significant changes in absorption, photoluminescence (PL) emission, PL quantum efficiency and electroluminescence spectra. From XRD there is an indication that this effect might be related to a different packing of the polymer chains.

The emission color of organic-light-emitting diodes (OLEDs) can be tuned by doping the active layer with fluorescent polymers, dyes or macromolecules. We built white light emitting OLEDs with an external electroluminescence (EL) quantum efficiency of up to 1.8% by doping a blue-light-emitting laddertype poly(paraphenylene) with an orange emitting fully π-conjugated macro-molecule. For the EL spectra we found a much stronger contribution from the guest emission than in photoluminescence, which is attributed to charge carrier trapping at the guest and subsequent EL in addition to excitation energy migration of singlet excitons from the host to the guest.

We show that gain relaxation dynamics in thin films of methyl-substituted poly-(para-phenylene)-type ladder polymer, investigated by femtosecond pump–probe spectroscopy under high excitation fluence, is affected by amplified spontaneous emission (ASE), which provides for an efficient excited state population depletion mechanism. The ASE influence on the photoluminescence spectral narrowing is also investigated. A general model which accounts for both excited state population dynamics and photoluminescence spectral evolution is presented.

We have developed a method that allows us to measure the emission profiles of millimetre-sized high-power light emitting diodes with micron resolution. The optical cross-sections recorded provide detailed information on the light intensity distribution of LEDs within the substrate till a distance of several tens of millimetres, which can be used to map out strategies for an optimization of the power efficiency or the tailoring of the light distribution of high power LEDs.

We present experiments performed under high-excitation conditions in films of methyl-substituted poly(para-phenylene) ladder-type polymer (m-LPPP): an electroluminescent material characterized by a high intrachain order, a narrow conjugation length distribution, and an emission in the blue spectral region. Photoluminescence (PL) and pump-probe experiments (with tunable probe) were performed with the second harmonic of a CPA Ti:sapphire laser as the excitation source.

The transport properties of polyacetylene anti polyaniline films have been investigated by electrically detected magnetic resonance (EDMR) at room temperature. In polyacetylene, a spin-dependent interchain tunneling involving a polaron and a soliton is detected. In polyaniline, a similar process is found but involving a polaron-polaron transition. The EDMR signal in polyaniline is found to be dependent on the dc electric field. For low electric fields (F < 1000 V/cm); one resonance line is observed, for higher fields, two resonance lines are observed, having similar g-factors but different linewiths.

The goal of this combined experimental and computational study is to investigate the structural conformation of oligo( para -phenylenes) in the crystalline phase, in particular the planarity of this type of molecules. To this end we have performed Raman experiments on para -terphenyl and para -quaterphenyl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C–C interring stretch mode at 1280 cm −1 and the C–H in-plane bend mode at 1220 cm −1 have been tracked. We find that upon increasing temperature at ambient pressure the intensity ratio I 1280 / I 1220 drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increasing pressure up to about 15 kbar. In the computational part, the Raman frequencies and activities of isolated 3P and 4P molecules were calculated within restricted Hartree–Fock formalism with the interring tilt angles varying from 0 to 90°. These calculations confirm that the I 1280 / I 1220 intensity ratio can be related to the planarity of the molecules. Three-dimensional bandstructure calculations within density functional theory were applied to determine phonon frequencies and estimate Raman activities for the polymer poly( para -phenylene). These simulations show that the same conclusions hold for crystalline environment.

Femtosecond pump–probe spectroscopy was applied to a guest–host system of conjugated polymers, the blue-emitting poly(para-phenylene) (m-LPPP) and the red-emitting poly(perylene-co-diethynylbenzene) (PPDB). Excitation energy transfer between m-LPPP and PPDB, monitored by the formation of the PPDB photobleaching band at 2.15 eV, takes place on a picosecond timescale. The non-exponential kinetics of the excitation energy transfer and its dependence on the acceptor (PPDB) concentration provide clear evidence for a Forster-type energy transfer.

Hexaphenyl thin films (HTF) are widely used as an electro-active organic medium in blue light emitting diodes. The optical parameters of the HTF-based devices significantly depend on their microstructural properties.HTF of different types are produced by physical vapor deposition on glass substrates applying specific sample preparation conditions. The microstructural properties of HTF are characterized using X-ray diffraction line profile analysis and atomic force microscopy (AFM). Diffraction peaks representing three different types of preferred growth in HTF are analyzed, namely textures with (00λ), (223) and (203) net planes oriented parallel to the substrate. No additional line-broadening (compared to silicon powder used as standard) is observed in the case of a film prepared at high substrate temperature of 170 °C. On the other hand, considerable broadening is detected in a film with the substrate kept at room temperature. Multiple line analysis documents that the crystallite size and lattice strain for the sample is 150 nm and 3×10−4, respectively. Single line analysis performed on the other reflections reveal size-induced broadening for a crystallite size in the range 40 to 50 nm. From AFM data we obtained that the maximum roughness of the surface is about 40 nm. The results indicate that the deposition temperature significantly influences the microstructural properties and that higher substrate temperature promotes a higher mobility of the molecules on the substrate enabling growth of larger crystallites with lower strain.

The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications is probed by doping the blue light emitting methyl-substituted ladder-type poly(para-phenylene) with small concentrations of a highly fluorescent (pi) -conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence are modeled and discussed by means of a two step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Forster type mechanism alone cannot account for the experimental facts in such a guest host system. We have used the same materials to tune the emission color of organic- light-emitting diodes (OLEDs). Electroluminescence (EL)- current characteristics of the fabricated devices are presented. We found that the EEM process in OLEDs is more efficient for the EL process due to charge carrier trapping on the guest material compared to PL. For dopand concentrations as low as 1 w% one observes an increase of the photoluminescence as well as the electroluminescence quantum efficiency ((eta) EL) by approximately 50% with respect to the pure host polymer efficiencies. We realized a white light emitting diode with an external (eta) EL of up to 1.8% with the emission color being independent of the applied bias.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have investigated momentum dependent excitations in conjugated organic oligomer materials. Theoretical simulations based on the intermediate neglect of differential overlap (INDO) approach coupled to a single configuration interaction (SCI) technique considering the proper transition matrix elements for inelastic electron scattering are compared to experimental electron energy-loss spectroscopy (EELS) investigations. In this work we concentrate on sexiphenyl (6P), sexithienyl (6T) and beta-carotene. 6P and 6T are widely used in a number of electronic and optoelectronic devices and together with beta-carotene they can be regarded as model substances for the three big classes of conjugated polymer materials, namely phenylenes, thiophenes and acetylenes. Both experimental and theoretical results underline the validity of the so called quasi-band-structure model for the description of periodic molecules with only a finite number of repeat units. We also highlight the influence of chain orientation on EELS spectra and discuss the reasons for certain deviations between molecule-based simulations and the excitation processes occurring in the actual molecular crystals upon inelastic electron scattering.

We investigate, both experimentally and theoretically, a series of novel molecules consisting of conjugated segments (such as stilbene, naphthylene, and anthrylene) that are separated from each other by nonconjugated bridges. Excited state localization effects are studied theoretically by post-Hartree–Fock calculations—taking into account electron correlation effects. In this context, we compute the electron-hole two-particle wave functions for the prominent excited states and discuss the nature of the molecular orbitals involved in their description. We also investigate geometry relaxation effects following the electronic excitations in order to locate the regions where the strongest rearrangement of the electron density occurs. These conceptionally different approaches (relying also on different semiempirical Hamilton operators and configuration interaction techniques) yield consistent results regarding the localization of the excitations and thus prove helpful to determine the nature of the lowest exci...

The electronic states in conjugated organic materials are usually characterized either by quantum-chemical approaches relying on a molecular picture, or by solid-state band structure models requiring infinite periodicity of the chains. In this contribution we present calculations showing that a band structure like character is inherent also in molecular systems and discuss the gradual evolution of a full band structure upon increasing the number of repeat units in oligomer molecules. The main feature of a band structure approach is the description of the electronic eigenstates in momentum space. We have therefore developed a method to describe momentum dependent excitation processes in conjugated organic materials based on a post-Hartree–Fock approach including electron–electron correlation effects. The results of the simulations performed for oligo(phenylene)s of different chain lengths agree very well with experimental electron energy loss spectroscopy (EELS) data.

We describe the synthesis of soluble liquid crystalline poly( p -phenylenevinylene)s with cyanobiphenyl as the mesogenic group linked to the vinylene group via a flexible alkyl chain. The polymers were well characterized by spectroscopic methods (NMR, IR, UV—Vis, photoluminescence (PL)) and showed PL emission of green light with a maximum around 540 nm. Polarized UV—Vis spectroscopy of polymer films prepared on oriented substrates indicated a high orientation of the backbone and mesogenic units.

The optical properties of highly oriented films of para-hexaphenyl, a conjugated material which is suitable for blue light emission applications, were investigated. Wave-guiding calculations, with anisotropic optical coefficients were combined with angledependent photoluminescence measurements, and single-layer device electroluminescence. The microstructure of the vacuumdeposited films is controlled by the deposition method, allowing tuning of the optical properties. We propose optimal configurations for light output with LED's and optically pumped device applications.

The diamagnetic shielding and the Meisner expulsion of the 90 K superconductor YBa 2 Cu 3 O x are presented. Ca substitution on the Ba site is carried out to study the influence on the superconducting properties. The substituted material appeared to be a two phase mixture as determined by x-ray diffraction. With increasing substitution we observe a decrease of the orthorhombic distortion, which is investigated by Raman spectroscopy. Raman results and resistivity studies of the new BiSrCaCuO-multiphase system are also presented.