Juozas V. Grazulevicius

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.

Hole-transporting hydrazones

This tutorial review covers recent contributions in the area of hole-transporting hydrazones, which are widely used in optoelectronic devices. It is addressed to students and researchers interested in the synthesis and properties of organic electroactive materials. The thermal, charge transport and other properties of electroactive hydrazones are compared and the relationships between the molecular structures and properties are emphasized. The first part discusses the low-molar-mass hydrazones and presents examples of their synthetic routes and chemical structures. In the second part, polymeric arylaldehyde hydrazones containing hydrazone moieties as the side substituents and in the main-chain are described.

High hole mobilities in carbazole-based glass-forming hydrazones

The properties of a series of carbazole-based dihydrazones are reported. The dependence of their thermal and glass-forming properties on their chemical structure is discussed. The hydrazones having phenyl substituents at the N atom of the hydrazine moiety form glasses and their amorphous films on the glass or polyester substrates can be prepared by casting from solutions. The ionization potentials of the synthesized hydrazones measured by the electron photoemission technique range from 5.24 to 5.50 eV. Hole drift mobilities of some newly synthesized carbazole-based dihydrazones appproach 10−2 cm2 V−1 s−1 at an electric field of 6.4×105 V cm−1, at 22 °C.

Air stable electron-transporting and ambipolar bay substituted perylene bisimides

Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the bay region have been designed, synthesized and characterized. The materials possess high thermal stability and form uniform films. They display a wide absorption window extending to the near infrared region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer. Ionization potential values of these perylene bisimide derivatives measured by photoelectron spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino substituted material exhibited competent electron drift mobility (>10−3 cm2 V−1 cm−1) under ambient conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay substituted perylene bisimides in order to understand the complementary ambipolar charge transport as well as the difference in the optical properties.

Influence of the hole blocking layer on blue phosphorescent organic light-emitting devices using 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole as host material

Organic blue light-emitting devices are essential for the development of future light sources and display technology. Here, we present a highly efficient host-guest system suitable for blue light emission, consisting of the wide gap host material 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) and the phosphorescent blue emitter iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). We investigate charge carrier balance as a function of hole blocking layer thickness. For optimized structures, devices with a quantum efficiency as high as 14.3% and a luminous efficacy of 21 lm/W at a luminance of 1000 cd/m2 are realized.

Highly efficient blue organic light-emitting diode with an oligomeric host having high triplet-energy and high electron mobility

We report a high-efficiency blue organic light-emitting diode (OLED) with a solution-processed emissive layer composed of an oligomeric host of poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (PCMO) that possesses high triplet-energy and high electron mobility. The device exhibited a current efficiency of 40.4 cd A−1 with an external quantum efficiency (EQE) of 21.6% and power efficiency of 28.2 lm W−1 at 230 cd m−2 or 24.7 cd A−1, 10.3%, and 15.5 lm W−1 at 1 000 cd m−2. The high efficiency may be attributed to the host possessing a high electron mobility and lower electron injection barrier, resulting in a more balanced carrier-injection. Moreover, the high electron-mobility favors the transport of electrons, resulting in a more balanced carrier-injection in the emissive layer. The device efficiency has been further enhanced to 42.6 cd A−1 (22.9%, 29.7 lm W−1) at 124 cd m−2 or 28.8 cd A−1 (15.4%, 17.8 lm W−1) at 1 000 cd m−2 by pre-heating the emissive solution at an elevated temperature before spin-coating.

A single emitting layer white OLED based on exciplex interface emission

A new triaryl molecule based on a benzene–benzothiadiazole–benzene core has been applied in a WOLED device. This very simple molecule emits from a combination of emissive states (exciton/electromer/exciplex/electroplex) to give white light with CIE coordinates of (0.38, 0.45) and a colour temperature of 4500 K.

Synthesis and Properties of Methoxyphenyl-Substituted Derivatives of Indolo[3,2-b]carbazole

The synthesis and full characterization of new derivatives of indolo[3,2-b]carbazole with differently substituted phenyl groups at nitrogen atoms is reported. Comparative study on their thermal, optical electrochemical, and photoelectrical properties is presented. The synthesized compounds are electrochemically stable. Their highest occupied molecular orbital energy values range from -5.14 to -5.07 eV. The electron photoemission spectra of the films of synthesized materials revealed the ionization potentials of 5.31-5.47 eV. Hole drift mobility of the amorphous film of 5,11-bis(3-methoxyphenyl)-6-pentyl-5,11-dihydroindolo[3,2-b]carbazole exceed 10(-3) cm(2)/V·s at high electric fields, as it was established by xerographic time-of-flight technique. In contrast to diphenylamino substituted derivatives of carbazole, no effect of the position of methoxy groups on the photoelectrical properties was observed for the synthesized methoxyphenyl-substituted derivatives of indolo[3,2-b]carbazole. The indolo[3,2-b]carbazole core has a larger resonance structure that includes 3 phenyl rings, and thus the energy gap of the HOMO and LUMO π orbitals is lower as compared to that of carbazoles. With a larger energy difference between the phenyl substituents and the core moiety, the indolo[3,2-b]carbazole derivatives studied all have a weaker coupling between the phenyl group and a much weaker dependence of the molecular properties on the position of substituents on the phenyl groups as compared to those observed in substituted carbazoles.

Hole-Transporting Glass-Forming 3,3′-Dicarbazyl-Based Hydrazones

ABSTRACT 3,3′-Di(9-(4-butylphenyl)carbazyl) and 3,3′-di(9-ethylcarbazyl)-based hydrazones were synthesized by the multi-step synthetic route including oxidative dimerization of 9-(4-butylphenyl)carbazole and 9-ethylcarbazole, formylation of the dimmers obtained and the reactions of the formyl derivatives with different hydrazines. The chemical structure of the compounds was confirmed by 1HNMR, IR and mass spectroscopy. The hydrazones synthesized form stable glasses with the glass transition temperatures exceeding 140°C. The values of ionisation potentials measured by electron photoemission technique are in the range of 5,21–5,4 eV. The hole drift mobilities in the films of the 50% solid solutions of the hydrazones in bisphenol Z polycarbonate established by the xerographic time-of-flight technique exceed 10−5 cm2/Vs at high electric fields.