Peter Strohriegl

Carbazole-containing polymers: synthesis, properties and applications

Abstract The synthesis and properties of carbazole-containing polymers are reviewed with 451 references. After a short discussion of the basic principles of photoconductivity the following classes of organic carbazole-containing polymers are reviewed: polymers with pendant carbazolyl groups, polymers containing electronically isolated carbazole moieties in the main chain, polymers with π-conjugated main chain, and σ-conjugated polymers as well as carbazole-containing molecular glasses. The present and potential applications of these materials, in electrophotographic materials, light-emitting diodes, photorefractive materials and photovoltaic devices are briefly discussed.

Charge-Transporting Molecular Glasses

Among organic materials vitrification for many years was regarded mainly as a privilege of polymers. However, recently a lot of attention is paid to organic low molar mass compounds that readily form glasses above room temperature. Such compounds are called molecular glasses or amorphous molecular materials. Among these materials the most widely studied are charge-transporting molecular glasses used in copiers and laser printers, organic light-emitting diodes, photovoltaic devices, and as photorefractive materials. Two types of molecular glasses, i.e., p-type (hole-transporting), and n-type (electron-transporting) are discussed. Work of the laboratories of the authors is emphasized. In addition, an overview of current and potential applications for these materials is presented.

Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene)

Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem=458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2.

A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes

We report a series of CBP-derivatives with superior thermal and electronic properties for the use as host materials for blue electrophosphorescent organic light emitting diodes. We applied a systematic variation of the substitution pattern in the 2- and 2′-position of the biphenyl unit and the 3- and 6-position of the carbazole moieties. In contrast to the crystalline parent compound CBP, all methyl and trifluoromethyl substituted derivatives show amorphous behaviour. Substitution in the 2- and 2′-position of the biphenyl causes a twisting of the phenyl rings. Hence, the degree of conjugation of the molecules is limited which leads to enlarged triplet energies of approximately 2.95 eV compared to 2.58 eV for CBP. The methyl substitution at the active 3- and 6-position of the pendant carbazole units yields materials with an electrochemically stable behaviour against oxidation.

Oxadiazoles and phenylquinoxalines as electron transport materials

Abstract The synthesis of a number of star-shaped oxadiazoles and phenylquinoxalines is described. These low molar mass compounds are able to form stable glasses with glass-transition temperatures up to 248 °C. The materials have been tested as electron transport layers in light-emitting devices (LEDs) together with poly(1,4-phenylene vinylene) (PPV). The two-layer LEDs show an improved quantum yield and brightness compared to PPV-monolayer LEDs.

Electron transport in starburst phenylquinoxalines

The electron transport properties of two soluble tris-phenylquinoxalines have been investigated by the time-of-flight technique. The electron mobilities for both compounds approach 10−4 cm2/V s at electric fields of 106 V/cm at room temperature. These are high values for isotropic electron transport materials suitable for use in organic light emitting diodes.

ELECTRON TRANSPORT IN A STARBURST OXADIAZOLE

In this article we report on the electron mobility of a starburst oxadiazole, 1,3,5 -[(4-tert.-butylphenyl)-2-oxadiazolyl] benzene. For direct evidence of the electron transport capability the charge drift mobility and its field dependence have been measured by the time-of-flight technique. The electron mobility showed a square root dependence of the electric field and was somewhat larger than that for PBD. In addition, the oxadiazole compound has been used as the electron transport layer (ETL) in light-emitting diodes (LEDs) together with poly(1,4-phenylene vinylene). The characteristics of these devices are compared to analogous LEDs with Alq3 as the ETL.

Heterolayer light-emitting diodes based on new oxadiazole polymers

Abstract Monolayer light-emitting diodes (LEDs) from poly(1,4-phenylene vinylene) (PPV) usually exhibit relatively low quantum efficiencies. Thus, the external efficiency of an indium-tin oxide (ITO)/PPV/A1 LED is typically 0.001%. In order to increase the quantum yield heterolayer devices with new oxadiazole polymers have been fabricated. These polymers with the electron-withdrawing oxadiazole units facilitate electron injection and transport in bilayer LEDs with PPV as hole transport and emitting layer. Thus, LEDs with external quantum efficiencies up to 0.2% have been achieved. Compared to conventional PPV LEDs no temperature dependence of the quantum efficiency is detectable, indicating improved balanced charge carrier injection at room temperature.

A study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy

Abstract The polymer poly(9,9′-dioctyl fluorene) (F8) can be easily driven between a number of different structural phases by different thermal or solvent treatments. The phases which we have identified include (i) as-spin-coated, (ii) semi-crystalline, (iii) glassy, (iv) with an extended intrachain conformation. Atomic force microscopy has been used to investigate the surface structures associated with the different phases of the polymer and different solvents used to cast the film. Raman spectroscopy has been performed on these samples to understand how the polymer conformation changes with the phases. Variations in the relative intensity of the peaks and shifts in energy are observed. We have compared the F8 spectrum with a trimer of fluorene with butyl side chains and acrylate/hydroxy endgroups attached to the fluorene main chain via hexyl spacers. We identify a very small frequency dispersion in the vibrational modes connected to an increase of the conjugation length. Therefore, the assignment of the vibrational modes of the trimer is very important for understanding the F8 spectrum.

Novel functional materials based on triarylamines–synthesis and application in electroluminescent devices and photorefractive systems

A variety of new functional materials based on triarylamines, such as low molecular weight glasses which possess hole conducting/photoconductive properties as well as amorphous bifunctional materials which combine photoconductive and non-linear optical (NLO) properties in one compound, have been synthesized. The new hole transporting glasses belong to the class of 1,3,5-tris(triaryldiamino)benzenes (TTADB). The hyperbranched structure and the large aryl groups attached as substituents lead to high glass transition temperatures (Tg) of up to 141°C in these compounds. The TTADBs do not recrystallize upon cooling from the melt, but form stable glasses. Cyclic voltammetry studies reveal multi-oxidation stages for these compounds of which the first oxidation is reversible. The HOMO energy values determined from CV for TTADB-1 and TTADB-2 are -4.82 and -4.94 eV, respectively. Light emitting diodes with the structure ITO/TTADB-2/Alq3/Al (where ITO=indium tin oxide) show high efficiency and large current carrying capacity. Further, bifunctional compounds have been synthesized in which a photoconductive moiety such as bis(carbazolyl)triphenylamine or bis(diphenylamino)triphenylamine is covalently bound to different NLO chromophores. Some of these compounds are thermally and morphologically stable amorphous materials, possessing Tg in the range from 85 to 122°C. Cyclic voltammetry measurements reveal that the HOMO energy values are between -4.81 and -5.45 eV. In photorefractive measurements using 40 µm thick samples, a diffraction efficiency of 27%, which corresponds to a refractive index modulation (Δn) of 3.5×10-3, a maximum two beam coupling gain coefficient (Γ) of 90 cm-1 and a response time of 40 ms were obtained.