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We demonstrate an efficient strategy to characterise weathering-induced photooxidative defects in microplastics. The central 13C cross polarisation NMR spectra offer high resolution and are quantitative when combined with multiple excitation.

The fragmentation of macro- into microplastics (MP) is the main source of MP in the environment. Nevertheless, knowledge about degradation mechanisms, changes in chemical composition, morphology, and residence times is still limited. Here, we present a long-term accelerated weathering study on polystyrene (PS) tensile bars and MP particles using simulated solar radiation and mechanical stress. The degradation process was monitored by gel permeation chromatography (GPC), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), 13C magic-angle spinning (MAS) NMR spectroscopy, tensile testing, and Monte Carlo simulations. We verified that degradation proceeds in two main stages. Stage I is dominated by photooxidation in a near-surface layer. During stage II, microcrack formation and particle rupturing accelerate the degradation. Depending on the ratio and intensity of the applied stress factors, MP degradation kinetics and lifetimes vary dramatically and an increasing amount of small MP fragments with high proportions of carboxyl, peroxide, and keto groups is continuously released into the environment. The enhanced surface area for adsorbing pollutants and forming biofilms modifies the uptake behavior and interaction with organisms together with potential ecological risks. We expect the proposed two-stage model to be valid for predicting the abiotic degradation of other commodity plastics with a carbon-carbon backbone.

The development of efficient blue emitter–host combinations is one of the biggest challenges in organic light-emitting diode (OLED) research. Host materials play a crucial role when it comes to enhancing the efficiency, improving the lifetime and decreasing the efficiency roll-off of the device. The need for new hosts is of prime importance, especially for blue phosphorescence and thermally activated delayed fluorescence (TADF) emitters, due to their high exciton energies. The hosts are less investigated than the emitters and require further progress. This work provides a new molecular strategy that combines an acridine derivative (donor) and pyrimidine moieties (acceptors) to obtain three novel host materials. This approach demonstrates that via careful selection of donor and acceptor units, it is possible to manage the properties of the host materials, obtaining at the same time superior thermal and morphological properties and high triplet energies up to 3.07 eV. The decrease of the conjugation in the acceptor unit was found to play a crucial role in increasing the triplet energy. The most promising host 1MPA was used to fabricate blue TADF OLEDs. Using a sky-blue emitter, we achieved electroluminescence at 491 nm and a maximum external quantum efficiency (EQE) of 13.6%, combined with a low efficiency roll-off, even beyond the practical brightness of 1000 cd m−1. The host 1MPA was also combined with a deep blue emitter to deliver a blue OLED with color coordinates of x = 0.16 and y = 0.18.

In this work, we present a versatile approach to achieve well-defined conjugated polymers and oligomers from a single standard synthesis route via preparative size-exclusion chromatography. Using a...

Microplastic particles (MP), arising from the gradual decomposition of plastics in the environment, have been identified as a global problem. Most investigations of MP cytotoxicity use pristine spherical particles available from commercial sources when evaluating their impact on mammalian cells, while only limited data is available for the more relevant "weathered microplastic". In this study, we exposed murine macrophages to polystyrene MP either after up to 130 days of accelerated ageing or in pristine condition. Weathered and pristine MP were physicochemically characterized, and their cytotoxicity was investigated using biological assays, transcriptome analysis, and metabolic pathways prediction. Whereas the response to pristine MP is mainly dominated by a TNF-α release, sharp-edged weathered MP induce broader adverse cellular reactions. This study stresses the importance of including more realistic test particles (e.g., weathered particles) in combination with a broad range of biological assays when evaluating the potential risk of microplastic exposure.

When plastics enter the environment, they are exposed to abiotic and biotic impacts, resulting in degradation and the formation of micro- and nanoplastic. Microplastic is ubiquitous in every environmental compartment. Nevertheless, the underlying degradation processes are not yet fully understood. Here, we studied the abiotic degradation of commonly used semi-crystalline, low-density polyethylene (LDPE) in a long-term accelerated weathering experiment combining several macro- and microscopic methods. Based on our observations, the degradation of LDPE proceeds in three stages. Initially, LDPE objects are prone to abrasion, followed by a period of surface cracking. A large number of secondary particles with a high degree of crystallinity are formed, with sizes down to the nanometer scale. These particles consist of highly polar oligomers leading to agglomeration in the final stage. We therefore suppose that weathered microplastic and nanoplastic particles will attach to colloidal environmental matter. This offers an explanation for the absence of free nanoplastic particles in natural samples.

We use bilayer devices with a series of three fullerene acceptors differing in order and intermolecular coupling to systematically explore the influence of electron delocalization in the acceptor phase on the dissociation efficiency of charge transfer states. Structural information from GIWAXS measurements is combined with the results of optical and electrical characterization as well as theoretical modeling. Our results indicate that an increase in CT-dissociation efficiency is directly coupled to an enhancement in electron delocalization that is particularly prominent for C60 which forms crystalline domains. Therefore, our results substantiate the concept of delocalization of electrons taking a positive role in the charge separation process, and of acceptor crystallinity being crucial in this respect.

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C60 molecules from a C60 layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C60 molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C60 can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C60 diffusion is thermally activated and not driven by WFL-type collective motion above Tg but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and t...

Carrier mobility is a key parameter for the application of conjugated polymers. In this work, a series of polyfluorenes (PF2/6) with different fractions of crosslinkable acrylate groups is investigated. Mobility measurements are carried out to assess the influence of crosslinking with different photoinitiators on the performance of the material. For the regime of low to medium charge carrier density, relevant for OLEDs and OPVs, we used a novel technique based on the injection of charge carriers from the electrodes of an optoelectronic device: MIS-CELIV (MIS: metal-insulator-semiconductor). For large charge carrier densities we performed OFET measurements. We find that using optimized conditions crosslinking does not influence the hole mobility in the investigated system. Furthermore, we demonstrate that the crosslinking process may be triggered solely by thermal activation and UV-illumination without the need of any initiator. Thus, densely crosslinked networks are obtained without the formation of undesired decomposition products from added photoinitiator. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016

In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by ...

A critical issue of bulk heterojunction (BHJ) solar cells is the instability of the morphology of the polymer:fullerene blend over long operation times. We report the synthesis of crosslinkable derivatives of the low bandgap polymer PFDTBT, poly(2,7-(9,9-dialkylfluorene)-alt-(5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))), and the stabilization of BHJ solar cells by crosslinking. Oxetane units are attached to the polymer side chains as crosslinkable functional groups. We study the crosslinking of the polymers via cationic ring opening polymerization of the oxetanes and show that our materials rapidly form insoluble networks. Our materials also crosslink in the presence of fullerenes. We report for the first time that crosslinking takes place upon prolonged heating to 100 °C without any added initiator. The best efficiency and thermal stability are found in thermally crosslinked BHJ solar cells. After 30 hours at 100 °C, 65% of the initial efficiency are retained and no further decay is observed up to 100 hours.

We show that the performance of an organic solar cell can be increased by the introduction of an additional polymeric exciton blocking layer. In order to realize this, the novel polymer PFTPDAc with pendant acrylate groups is developed. Thin films are coated from a PFTPDAc solution and subsequently crosslinked by irradiation. Thereby, the film becomes completely insoluble and allows spincoating of a second polymer layer on top. We realize a three layer solar cell which contains a crosslinked PFTPDAc interlayer on top of the molybdenum oxide anode and layers of the low-bandgap polymer PCDTBT and C60. In comparison with a reference cell without the interlayer, the EQE is significantly increased in the spectral region between 400 and 650 nm. From current–voltage measurements a power conversion efficiency of 1.8% is determined. PL measurements show that the increase of solar cell performance is attributed to exciton blocking by the PFTPDAc interlayer.

Two novel bipolar host materials BPTRZ and MBPTRZ were synthesized, in which the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic, but nonconjugated meta-linked biphenyl unit. The additional twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2′-position of the biphenyl leads to a higher triplet energy of 2.81 eV compared to 2.70 eV for BPTRZ. Both materials possess high thermal stabilities and good glass forming properties. An organic light emitting diode with MBPTRZ as host for the blue phosphorescence emitter FIrpic shows a maximum luminance of 30600 cd/m2 and a maximum external quantum efficiency of 7.0%.

We present a number of polyfluorene based conjugated polymers with crosslinkable acrylate and oxetane units. These polymers can be crosslinked by free radical polymerization in the case of acrylates and by cationic ring opening polymerization for oxetanes. Upon polymerization densely crosslinked networks are formed which are completely insoluble. We show that the diffusion coefficient of C60 in polyfluorene is reduced by a factor of 1000 by crosslinking. MIS-CELIV measurements are used to monitor changes in the charge carrier mobility upon crosslinking. It shows that using appropriate conditions, e.g. low initiator concentrations or thermal crosslinking, the charge carrier mobility is not reduced by crosslinking. Solution processed three layer organic solar cells were realized with a crosslinkable fluorene based copolymer containing acrylate groups. The efficiency is increased from 1.4% for the reference to 1.8% in the three layer cell with a crosslinked exciton blocking layer. A critical issue of BHJ cells is the instability of the morphology of the polymer:fullerene blend over long operation times at elevated temperature. We present a crosslinkable derivative of the low bandgap polymer PFDTBT which contains oxetane units. BHJ cells with the crosslinked PFDTBT derivative and PCBM were tested in accelerated aging experiments at 100 °C for times up to 100 h. Stabilization was clearly observed in crosslinked BHJ cells compared to the non-crosslinked reference. We show for the first time that oxetane containing polymers can be thermally crosslinked without any added initiator. Initiator free crosslinking is particularly attractive as it avoids the formation of decomposition products, and thus potential electron traps and quenching sites from the initiator.

We present a combined detailed spectroscopic and quantum chemical study on the bipolar host materials BPTRZ and MBPTRZ in solution and in neat film. In the two compounds, the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic but non-conjugated meta-linked biphenyl unit. The two materials differ by an additional steric twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2′-position of the biphenyl. We find that while the twist shifts the triplet state in MBPTRZ to higher energies (3.0 eV in solution) compared to BPTRZ (2.8 eV in solution), this also localizes electron density on the carbazole moiety, leading to excimer formation in neat films.

In the presented work solvent-free film preparation from molecular glass resists, the evaluation of the patterning performance using thermal scanning probe lithography (tSPL) and an efficient etch transfer process are demonstrated. As the presented materials have a high tendency to crystallize and thus form crystalline films of bad quality when processed by solution casting, two component mixtures prepared by coevaporation were investigated. Stable amorphous films were obtained by selecting compatible material pairs for the coevaporation. One optimized material pair is based on trissubstituted, twisted resist materials with a distinct difference in molecular design. Here a high resolution tSPL prepared pattern of 18 nm half pitch in a 10 nm thick film is demonstrated. A further optimization is reported for “small” cubic silsequioxane molecules. Again single component films show independent to applied film preparation techniques bad film forming properties due to the high crystallinity of the symmetric cubic silsequioxane molecules. But coevaporation of the phenyl substituted octaphenylsilsequioxane combined with the fully aromatic 2,2',7,7'-tetraphenyl-9,9'-spirobi[fluorene] results in stable amorphous thin films. tSPL investigations demonstrate the patternability by writing high resolution line features of 20 nm half pitch. An important advantage of such a silicon rich resist material is that it can be directly converted to SiO2, yielding to a patterned hardmask of SiO2. This proof of principle is demonstrated and an efficient pattern transfer of 60 nm half pitch line into the underlying HM8006 is reported.

This work describes n-type self-assembled monolayer field-effect transistors (SAMFETs) based on a perylene derivative which is covalently fixed to an aluminum oxide dielectric via a phosphonic acid linker. N-type SAMFETs spontaneously formed by a single layer of active molecules are demonstrated for transistor channel length up to 100 μm. Highly reproducible transistors with electron mobilities of 1.5 × 10-3 cm2 V-1 s-1 and on/off current ratios up to 105 are obtained. By implementing n-type and p-type transistors in one device, a complimentary inverter based solely on SAMFETs is demonstrated for the first time. Highly reproducible n-type self-assembled monolayer field-effect transistors (SAMFETs) based on a perylene derivative are reported. Electron mobilities of 1.5 × 10-3 cm2 V-1s-1 and on/off current ratios up to 105 are obtained. By implementing n-type and p-type transistors in one device, a complimentary inverter based solely on SAMFETs is demonstrated for the first time. Copyright (c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using nanoparticle dispersions for printing of semiconductors would be the easiest way to evolve from classic printing technologies towards printed electronics. However, nanoparticular thin films are unfavorable in transistor applications due to two reasons: (i) The charge transport in the thin film or at its interfaces to the gate dielectric is disturbed by the voids between the nanoparticles. (ii) These layers are highly sensitive to surface adsorbates due to their high surface to volume ratio. Atmospheric surface adsorbates, e.g. on metal oxides are known to influence the electrical properties of the thin films. In order to overcome the disadvantages of the nanoparticulate thin film, this work targets both issues with a combined approach. By choosing a qualified surface adsorbate, the perturbing surface of the nanoparticles will be passivated. By using the surface adsorbate as a linker to an electron conducting organic molecule, the n-type organic will be eligible for filling the voids between the particles. We present the synthesis of a new pyrrolidone functionalized n-type perylene diimide and its application in hetero-layer nanoparticulate zinc oxide (ZnO) field-effect transistors.

We present four derivatives of 4,4′-bis(9-carbazolyl)biphenyl (CBP) for the use as host materials in blue phosphorescent organic light emitting diodes. By replacing the para -linkage by a meta -linkage of the carbazole substituents at the central biphenyl unit materials with improved thermal and optical properties are obtained. The triplet energy of the meta -linked host materials is significantly increased to more than 2.90 eV compared to 2.58 eV for the para -linked CBP. Moreover, selective methyl substitution of the basic meta -CBP structure leads to materials with high glass transition temperatures up to 120 °C and electrochemical stability of the oxidised species against dimerisation. The high triplet energy allows the use of the meta -CBP derivatives as host materials for the carbene emitter mer -tris( N -dibenzofuranyl- N ′-methylimidazole)iridium (III) (Ir(dbfmi)) with a pure blue emission at 450 nm.

We present a series of low-molecular-weight materials based on cyclic phosphazenes for the use as host materials in blue phosphorescent organic light-emitting diodes. Substituted phenyl rings are attached to the central phosphazene ring either via phosphorus–oxygen bonds to yield phenoxy-substituted derivatives or via direct phosphorus–carbon bonds to yield phenyl-substituted derivatives. The phenoxy substituted cyclic phosphazenes were prepared by nucleophilic substitution of the six chlorine atoms in hexachlorocyclotriphosphazene with phenoxy groups, whereas the phenyl substituted cyclic phosphazenes were formed in a cyclocondensation reaction of three equivalents of substituted phosphinic amides. The phenyl substitution leads to materials with superior thermal properties compared to the phenoxy substitution. Because of the nonconjugated linkage to the phosphazene core, the host materials have very high triplet energies of more than 3 eV. In an OLED device using one compound as host for the saturated bl...

A novel phenoxy-carbazole substituted triazine host material (PCTrz) was synthesised. PCTrz exhibits a high triplet bandgap of 2.91 eV and a high glass transition temperature of 148 °C. A non-conjugated ether bond separates the oxidation- and reduction-site in the molecule. Good charge carrier transport properties in single carrier devices prove the potential as host for blue phosphorescent emitters. An organic light emitting diode with PCTrz as host for FIrpic exhibited a high current efficiency of 13.5 cd/A a luminance of 1000 cd/m2 and a maximum luminance of 29,000 cd/m2.

Nanocrystals of the polymer poly(9,9-dioctylfluorenyl-co-bithiophene) (F8T2) with a molecular weight of 3.2 kg/mol are grown in a para-xylene solution. The typical morphology of the crystals is needle like with typical widths of 50 nm and lengths of about 200 nm. The crystal structure and morphology are stable up to a temperature of 353 K. The structure solution is obtained by x-ray powder diffraction (XRD) pattern with data modelling by a stochastic global optimization procedure which allows simultaneous indexing and molecular packing determination. Final Rietveld refinement was applied on the most promising crystal structure with a = 1.376 nm, b = 3.105 nm, c = 2.690 nm and s = 109.5° within the space group C2/c choosing the polymer backbone parallel to the b-axis. The structural motifs of the molecular packing could be identified: aromatic units within a single polymer chain are slightly bent relative to the chain axis, octyl side chains are aligned along the polymer backbone and aromatic units of neighbouring molecules display a strong tendency to stack parallel to each other. XRD results of F8T2 with a molecular weight of 19 kg/mol reveal the same peak positions compared to the 3.2 kg/mol material, showing that both materials crystallise similarly and can be described by the same crystallographic unit cell. The smaller peak intensities together with the broader peak widths, however, show that the ability of crystal formation for the 19 kg/mol material is reduced.

We have studied the temperature dependence of phosphorescence (Ph) and delayed fluorescence (DF) in two series of poly(p-phenylene) derivatives within a temperature range from 10 to 300 K under quasi-stationary conditions. One set of materials consists of the dimer, trimer, and polymer of ethylhexyl-substituted poly(fluorene) (PF2/6) and thus allows us to assess the effects of oligomer length. The second series addresses the influence of energetic disorder and conjugation length by being composed of the polymers alkoxy-substituted poly(p-phenylene) (DOO-PPP), poly(indenofluorene) (PIF), and ladder-type poly(p-phenylene) (MeLPPP). Under low light intensities, the DF features a maximum at a certain temperature T(max). For the dimer and trimer, the T(max) coincides with the temperature at which the phosphorescence has decayed to 1/2 of the value at 10 K, while T(max) shifts to lower temperature values along the series DOO-PPP, PIF, and MeLPPP and approaches T = 0 K for MeLPPP. By applying conventional kinetic equations we show that the occurrence of a maximum in the DF intensity is the consequence of generalized thermally activated triplet exciton transport toward quenching sites. We find the quenching rates at 0 K to be in the range of 1 s(-1) for the polymers, while they are more than an order of magnitude lower for the oligomers.

Carbazole-based materials such as 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP) and its derivatives are frequently used as matrix materials for phosphorescent emitters in organic light emitting diodes (OLED)s. An essential requirement for such matrix materials is a high energy of their first triplet excited state. Here we present a detailed spectroscopic investigation supported by density functional theory (DFT) calculations on two series of CBP derivatives, where CH(3) and CF(3) substituents on the 2- and 2'-position of the biphenyl introduce strong torsion into the molecular structure. We find that the resulting poor coupling between the two halves of the molecules leads to an electronic structure similar to that of N-phenyl-3,6-dimethylcarbazole, with a high triplet-state energy of 2.95 eV. However, we also observe a triplet excimer emission centered at about 2.5-2.6 eV in all compounds. We associate this triplet excimer with a sandwich geometry of neighboring carbazole moieties. For compounds with the more polar CF(3) substituents, the lifetime of the intermolecular triplet excited state extends into the millisecond range for neat films at room temperature. We attribute this to an increased charge-transfer character of the intermolecular excited state for the more polar substituents.

The synthesis of alternating photo-crosslinkable fluorene co-oligomers via Suzuki cross coupling is reported. The co-oligomers were characterized using NMR and GPC, the results showing effective control of molecular weight. The oligomers have low Tgs of 53-63 °C and are thermally stable up to ≈300 °C. Using TPD as co-monomer, a blue-emitting oligomer was obtained, a yellow-fluorescent material for benzothiadiazole, and a red (film) and yellow-green (solution) emitting material for bithiophene. The energy levels are very similar to those of randomly linked oligomers. The photo-crosslinking behavior was studied and under optimized conditions a lateral resolution of 600 nm was obtained.

A series of new donor-substituted 1,3,5-triazines (TRZ 1−7) has been prepared by nucleophilic substitution of cyanuric chloride with carbazole, 3-methylcarbazole, phenol, and 3,5-dimethylphenol. These s-triazines have been investigated as host material for blue phosphorescent light-emitting diodes (OLEDs). All triazine based hosts were characterized regarding their optical and thermal properties. Different substitution patterns resulted in high glass-transition temperatures (Tg) of up to 170 °C and triplet energies (ΔE(T1−S0)) of up to 2.96 eV. The application as host material for the blue phosphor bis(4,6-difluorophenylpyridinato-N,C2)picolinato-iridium(III) (FIrpic) yielded maximum current efficiencies up to 21 cd/A.

In this contribution we show that random fluorene cooligomers with photo reactive acrylate units can be prepared in a simple 1-step Yamamoto synthesis. The acrylate functionalities are preserved quantitatively under Yamamoto conditions. NMR and Maldi-ToF measurements point to an almost statistical incorporation of the comonomers into the oligomer chain. Maldi-ToF analyses give a further insight into the chain compositions, and we found fluorene-only oligomers to be present in low quantities. Thin films of the aromatic amine containing cooligomer show a blue fluorescence, the benzothiadiazole oligomer shows yellow photoluminescence, and the bithiophene oligomer emits orange-red light upon excitation. Compared to pure fluorene oligomers with a HOMO of 5.7 eV the HOMO levels of the TPD and bithiophene derivatives are increased to 5.25 and 5.31 eV, respectively, whereas the HOMO level of the benzothiadiazole oligomer is decrased to 5.85 eV. Photolithography experiments reveal that a careful optimization of th...

Here, a study of the electric field induced quenching on the phosphorescence intensity of a deep-blue triplet emitter dispersed in different host materials is presented. The hosts are characterized by a higher triplet excitonic level with respect to the emitter, ensuring efficient energy transfer and exciton confinement, whereas they differ in the highest occupied molecular orbital (HOMO) alignment, forming type I and type II host/guest heterostructures. While the type I structure shows negligible electric field induced quenching, a quenching up to 25% for the type II at a field of 2 MV/cm is reported. A similar quenching behaviour is also reported for thin films of the pure emitter, revealing an important luminescence loss mechanism for aggregated emitter molecules. These results are interpreted by considering Coulomb stabilized excitons in the type II heterostructure and in the pure emitter, that become very sensitive to dissociation upon application of the field. These results clarify the role of external electric field quenching on the phosphorescence of triplet emitters and provide useful insights for the design of deep-blue electrophosphorescent devices with a reduced efficiency roll-off.

Polyfluorene represents a unique model to study the influence of intramolecular conformation on the electronic properties of chromophores with an extended π-conjugation. According to the degree of planarity between the adjacent repeat units the electronic and optical properties can change substantially. This peculiar spectroscopic behavior has been described by identifying different phases, namely the glassy, the γ- and the β-phase. Here, we present low-temperature single-molecule spectroscopy of a series of oligofluorenes differing in the number of monomeric units, in order to gain information on the influence of chain length on the polymorphism. By monitoring the energy of the 0-0 transition we have classified single molecules belonging to the different phases. We demonstrate that a large number of molecules start to form the β-phase only when more than 9 repeat units constitute the molecular chain. The implications for the control of morphology in polyfluorene thin films are discussed.

In this paper we describe the use of combinatorial vapor deposition techniques for the optimization of blue organic light emitting diodes (OLEDs). In these devices the star shaped molecule 1 with a triphenylamine core and three fluorene side groups serves as hole transport and emitting layer. Compound 2 with a much lower lying HOMO and a larger bandgap is used as hole blocking layer. Using combinatorial vapor deposition 42 OLEDs with thickness gradients of both the hole transport and the hole blocking layer have been simultaneously prepared on one substrate. The physical characterization of the devices clearly shows that a hole blocking layer of the star shaped molecule 2 is necessary in order to obtain pure blue emission with CIE coordinates of x = 0.15 and y = 0.15. A thickness of only 5 nm of the blocking layer is sufficient, and with increasing layer thickness the brightness of the blue devices drops. The blue devices exhibit a brightness of 400 cd m -2 and a luminous efficiency of 2 cdA -1 . The thickness variations of both the hole transport and the hole blocking layer have been made in one combinatorial evaporation experiment on a single substrate using a set of movable masks. This demonstrates how efficient combinatorial methods can be used for the development of OLEDs.

An efficient one‐step synthesis of 9,9‐di(2‐ethylhexyl)‐2,7‐fluorene oligomers via an endcapping reaction is reported. Controlled endcapping demands a full conversion of functional groups and thus ...

The influence of temperature treatment on three types of conjugated polymer thin films, named poly(3-hexylthiophene) (P3HT), polyarylamine (PAA) and octylfluorene–bithiophene copolymer (F8T2) is studied. A detailed knowledge of the film morphology and crystalline structure is important since the performance of organic thin film transistors is extremely sensitive to small changes of morphology and structure. Samples are prepared via a spin-casting process on thermal oxidized silicon wafers. The influence of heat treatment in the range from ambient temperatures to 600 K is studied with specular X-ray reflectivity, X-ray diffraction and differential scanning calorimetry. The morphological parameters like layer thicknesses, electron densities and roughnesses of the interface and of the surface are calculated from the XRR measurements. The maximum change of layer thickness due to heat treatment is 15% of the initial layer thicknesses. The maximum variation of the mean electron densities are about 20% and the rms surface roughness vary from 2 A up to 20 A as a result of annealing. Interface roughness show variation of about 1 A. Strong variations of the morphological parameters next to phase transition temperatures are observed as well.

In this paper we describe the synthesis of four new bisindenocarbazole derivatives, prepared by selective bromination of bisindenocarbazole in the 7‐ and 7′‐positions, followed by Suzuki crosscoupling with alkyl‐substituted phenyl, biphenyl and fluorene units. From this new class of fused aromatics a liquid crystalline derivative is reported for the first time. The bisindenocarbazole with two 4‐hexylphenyl side groups exhibits a broad nematic phase between 180 and 250°C, whereas the other derivatives are crystalline or form molecular glasses. All bisindenocarbazoles exhibit high thermal stabilities above 300°C and show excellent electrochemical stability. HOMO and LUMO levels of −5.4 eV and −2.3 eV, respectively, were determined by cyclic voltammetry and optical spectroscopy. The bisindenocarbazoles display a strong blue fluorescence with up to 56% quantum yield in solution.

Aligned thin films of the liquid-crystalline polymer poly((9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene) were prepared, and the correlation between the optical an- isotropy and the structural properties was shown. A series of samples with different thicknesses were prepared via a spin-casting process on rubbed polyimide surfaces. The alignment of the polymer chains was obtained by a tem- perature treatment just below the clearing temperature. The degree of alignment was investigated with ultraviolet- visible absorption spectroscopy and in-plane X-ray diffrac- tion. Independently, each technique revealed Hermans ori- entation functions in the range of 0.75-0.8. Surprisingly, a layer-thickness dependence was not observed. In addition, the X-ray diffraction pole figure technique revealed that the polymer chains were uniaxially aligned along the rubbing direction. The aligned films were in the nematic state, with the director elongated along the rubbing direction. 2007 Wiley Periodicals, Inc. J Appl Polym Sci 107: 1817-1821, 2008

In the presented work solvent-free film preparation from tailored molecular glass resists, their thermal analysis, the characterization of etch resistance for plasma etching transfer processes, and the evaluation of the patterning performance using scanning probe lithography (SPL) tools, in particular electric field and thermal based SPL, are demonstrated. Therefore a series of fully aromatic spiro-based and tris-substituted twisted resist materials were systematically investigated. The materials feature very high glass transition temperatures of up to 173 °C, which allows solvent-free thin film preparation by physical vapor deposition (PVD) due to their high thermal stability. The PVD prepared films offer distinct advantages compared to spin coated films such as no pinholes, defects, or residual solvent domains, which can locally affect the film properties. In addition, PVD prepared films do not need a post apply bake (PAB) and can be precisely prepared in the nanometer range layer thickness. An observed sufficient plasma etching resistance is promising for an efficient pattern transfer even by utilizing only 10 nm thin resist films. Their lithographic resolution potential is demonstrated by a positive and a negative tone patterning using electric field, current controlled scanning probe lithography (EF-CC-SPL) at the Technical University of Ilmenau or thermal scanning probe lithography (tSPL) investigations at the IBM Research - Zurich. High resolution tSPL prepared patterns of 11 nm half pitch and at 4 nm patterning depth are demonstrated.

The thermal stability and molecular order in monolayers of two organic semiconductors, PBI-PA and PBI-alkyl, based on perylene derivatives with an identical molecular structure except for an anchor group for attachment to the substrate in PBI-PA, are reported. In situ X-ray reflectivity measurements are used to follow the stability of these monolayers in terms of order and thickness as temperature is increased. Films have thicknesses corresponding approximately to the length of one molecule; molecules stand upright on the substrate with a defined structure. PBI-PA monolayers have a high degree of order at room temperature and a stable film exists up to 250 °C, but decomposes rapidly above 300 °C. In contrast, stable physisorbed PBI-alkyl monolayers only exist up to 100 °C. Above the bulk melting point at 200 °C no more order exists. The results encourage using anchor groups in monolayers for various applications as it allows enhanced stability at the interface with the substrate.

We present a dynamical study on the nonlinear conduction behaviour in the commensurate charge-density-wave phase of the quasi-one-dimensional conductor DCNQI$_2$Cu below 75 K. We can accurately simulate magnitude and time-dependence of the measured conductivity in response to large voltage pulses by accounting for the energy exchange between the phononic and electronic subsystems by means of an electrothermal model. Our simulations reveal a distinct non-equilibrium population of optical phonon states with an average energy of E$_{ph}$ = 19 meV being half the activation energy of about $\Delta$E$_a$ = 39 meV observed in DC resistivity measurements. By inelastic scattering, this hot optical phonon bath generates additional charge-carrying excitations thus providing a multiplication effect while energy transferred to the acoustic phonons is dissipated out of the system via heat conduction. Therefore, in high electric fields a preferred interaction of charge-carrying excitations with optical phonons compared to acoustic phonon modes is considered to be responsible for the nonlinear conduction effects observed in DCNQI$_2$Cu.

In this letter we describe the synthesis of the first liquid crystalline bisindenocarbazole derivative. The novel bisindenocarbazole exhibits a broad nematic mesophase between 180 and 250 °C, which was characterized by polarizing microscopy and small angle X-ray scattering. The material shows an excellent electrochemical stability and a strong blue fluorescence with a quantum yield of 49% in solution.

We report the synthesis of five new 5,5′-bis(9,9′-dialkylfluorene-2-yl)-2,2′-bithiophenes 4a–e as active materials for the use in OFETs. Depending on the type of alkyl substituents crystalline or amorphous materials are obtained. Cyclovoltammetry shows that the materials are electrochemically stable and have a HOMO level at −5.3 eV. The different morphologies of thin films of 4a–c have great influence on the performance of the materials in OFETs. The field effect mobilities are in the range of 10−5 cm2/Vs in an amorphous film of 4c to 3 × 10−3 cm2/Vs in a polycrystalline film of 4a. This high mobility remains constant after 3 months at ambient conditions, which proofs the high environmental stability of this class of materials.

The synthesis of nest fluorene containing pbotocrosslinkable reactive mesogens is described, Both mono-disperse trimers or pentamers and oligomeric mixtures containing two photocrosslinkable acrylate end groups were obtained by reactions. The pentamer 12a shows an ideal phase behavior for orientation experiments with a broad nernatic phase between the glass transition at. -10 °C and 123 °C. In the oligomeric mixtures 14a-g the transition temperature from the nematic to the isotropic phase can be tailored from 100 to 310°C by oligomeric mixture 14c were oriented on rubed polyimide layers and oriemation ratios of 15:1 in photoluminescent were obtained. Experiments with different film theiknesses show that the orientation is not homogeneous thronghout the film but decreases with increasing distance from the orientation layer.