Hartmut Krueger

Polystyrene Backbone Polymers Consisting of Alkyl-Substituted Triazine Side Groups for Phosphorescent OLEDs

This paper describes the synthesis of new electron-transporting styrene monomers and their corresponding polystyrenes all with a 2,4,6-triphenyl-1,3,5-triazine basic structure in the side group. The monomers differ in the alkyl substitution and in the meta-/paralinkage of the triazine to the polymer backbone. The thermal and spectroscopic properties of the new electron-transporting polymers are discussed in regard to their chemical structures. Phosphorescent OLEDs were prepared using the obtained electron-transporting polymers as the emissive layer material in blend systems together with a green iridium-based emitter 13 and a small molecule as an additional cohost with wideband gap characteristics (CoH-001). The performance of the OLEDs was characterized and discussed in regard to the chemical structure of the new electron-transporting polymers.

New DEA materials by organic modification of silicone and polyurethane networks

Dielectric elastomer actuators (DEAs) can be optimized by modifying the dielectric or mechanical properties of the electroactive polymer. In this work both properties were improved simultaneously by a simple process, the one-step film formation for polyurethane and silicone films. The silicone network contains polydimethylsiloxane (PDMS) chains, as well as cross-linker and grafted molecular dipoles in varying amounts. The process leads to films, which are homogenous down to the molecular level and show higher permittivities as well as reduced stiffnesses. The dipole modification of a new silicone leads to 40 times higher sensitivities, compared to the unmodified films. This new material reaches the sensitivity of the widely used acrylate elatomer VHB4905. A similar silicone modification was obtained by using smart fillers consisting of organic dipoles and additional groups realizing a high compatibility to the silicon network. Polyurethanes are alternative elastomers for DEAs which are compared with the silicones in important properties. Polyurethanes have an intrinsically high dielectric constant (above 7), which is based on the polar nature of the polyurethane fragments. Polyurethanes can be made in roll-to-roll process giving constant mechanical and electrical properties on a high level.

Efficient green electrophosphorescence based on ambipolar nonconjugated polymers: Evaluation of transport and emission properties

New materials for polymer organic light-emitting diodes based on a polymer matrix doped with phosphorescent dyes are presented. The matrix system is based on a polystyrene backbone bearing either electron or hole transporting units at the 4-position of each repeat unit. Random copolymers and polymer blend systems of the homopolymers are prepared, both with 62 wt.% electron transporting and 38 wt.% hole transporting moieties. Adding a green electrophosphorescent dye to the polymer matrix leads to efficient electroluminescence with a maximum current efficiency of 35 cd/A and a maximum external quantum efficiency of up to 10%. The mobilities of electrons and holes in the dye-doped copolymer, as measured by transient electroluminescence, are around 5 × 10−5 and 5 × 10−6 cm2/Vs, respectively, while the blend of the two homopolymers exhibits slightly lower mobilities of both types of carriers. Despite the pronounced imbalance of charge transport, the device performance is almost entirely limited by the phosphor...

Some new electron-affine polymers for organic photovoltaics

The aim of this work is to develop some new polymer materials with typical n-type semiconducting properties and low reduction peak potentials. Therefore new synthetic routes are presented leading to organo-soluble polyquinolines and polyquinoxalines. The results of the synthesis and characterization of the obtained new organo-soluble polymers are shown. The electrochemical reduction and oxidation behavior of these polymers is studied by cyclovoltammetric measurements. All studied polymers can be reversibly oxidized and reduced. In addition the absorption and luminescence properties are investigated. Polymer/polymer solar cells are prepared from blends of the new polyquinoxalines and poly-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen] as active single layer. First results are discussed.

Polymers with covalently attached dyes - materials for OLEDs and photovoltaics?

An unusual way is presented to obtain a new class of deep red emitting polyurethanes with covalently attached fluorescent dyes. The DCM-dye seems to be a favourable candidate but we had to introduce two reactive hydroxy linker groups into the molecule to enable polymerization. By copolymerization with non-dye molecules the dye content in the main chain can he varied. Red emitting device structures were realized and some of the device properties will be given.

Silicone films with high stiffness and increasing permittivity through dipole-grafting

Dielectric elastomer actuators (DEAs) are smart materials that can be optimized by modifying the dielectric or mechanical properties of the electroactive polymer. The incorporation of inorganic particles in silicone elastomers shows a permittivity enhancement and undesired stiffening. We present another concept to obtain comparable properties by dipole grafting. Therefore, the organic dipole N-ally-N-methyl-4-nitroaniline is grafted in competition with the vinyl terminated PDMS to a hydrosilane cross-linker forming the PDMS network. With this procedure PDMS films with up to 25 wt% of the dipole were solvent casted and the chemical, mechanical, electrical, plus electromechanical properties of these novel materials were investigated.

n-type semiconducting polymers as electron acceptors for organic photovoltaics

The aim of this work is to develop some new polymer materials with typical n-type semiconducting properties and low reduction peak potentials. Therefore new organo-soluble copolyquinoxalines were used in polymer blends with poly-[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylene] or regioregular poly-(3-hexylthiophene). The photovoltaic properties of these polymer blends depend on the blend morphology. So the short circuit current density can be improved by better solvent mixtures and by temper processes. But nevertheless the photovoltaic properties are low and a stable and well ordered phase morphology is difficult to obtain with polymer blend systems. Therefore another approach to receive more effective full polymer photovoltaic cells was done. Block copolymers consisting of n-type and p-type sequences in one polymer chain were synthesized. As n-type material a quinoline monomer and as p-type material 3-hexylthiophene were used. The synthesis of these new materials is described. The spectroscopic and cyclovoltammetric investigations clearly indicate their block copolymer structure. The study of their phase morphology and photovoltaic properties is in progress.

Second harmonic generation studies on novel NLO-polymers

A crosslinkable side chain NLO-polymer of the bis-azobenzene type and an AB-type main chain NLO-polymer of the sulphonyl-tolane type have been synthesized and characterized with respect to their curing and electric poling properties. NLO-susceptibilities d33 of 11 pm/V and 7.5 pm/V, respectively, have been measured after initial relaxation and proved to be highly stable. During poling, transient SH-signal and square of poling current showed similar temporal behavior. Thus transient poling current can be considered to be a useful probe to monitor and to control the poling process. Direct current electrical conductivity was found to be strongly temperature dependent for both materials.