Alain Siove

The role of carbazole in organic light-emitting devices

Abstract New organic oligomers and polymers based on the carbazole molecule are explored for possible applications in light-emitting devices. In one case, (butyl- or octyl-) carbazole dimers and poly (N-butyl-3,6-carbazolylene) polymer were used as the hole-transporting and light-emitting layer in multilayer light-emitting diodes (LEDs). These devices yielded bright blue light (as much as about 6000 cd m−2) with high external quantum (about 10%) and luminance efficiencies (about 2 1m W−1). The other case involved ([3-octylthiophene]-[bis-(N-ethyl or octyl carbazolylene)]) multiblock copolymers as the active emitting layer in single-layer LEDs. Color tuning was achieved in these devices by changing the number of monomer units contained in the thiophene chain. We also observed an increase of the external quantum efficiency in diodes based on the copolymers with short thiophene segments that we attributed to a more balanced charge injection.

White organic light-emitting diodes with fine chromaticity tuning via ultrathin layer position shifting

Nondoped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface between a hole-transporting 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPB) layer and a blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via bright white with CIE coordinates (x=0.33, y=0.32), an ηext of 1.9%, and a color rendering index of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and 4.9nm, respectively).

Chain length effect on the electroactivity of poly(N-alkyl-3,6-carbazolediyl) thin films

Abstract Several poly(N-alkyl-3,6-carbazolediyl)s (PCZS) have been obtained by electroreduction of the corresponding dibromomonomers in the presence of a catalytic Ni(0)-based system. Depending on the nature of the alkyl group linked to the nitrogen atom, PCZS are soluble in organic solvents such as nitrobenzene, chloroform and tetrahydrofuran and insoluble in acetonitrile and alcohols. Taking these properties into account thin films have been prepared (in the neutral state) either by casting from solutions or directly by electrodeposition onto conducting substrates in the CH3CN/NBu4BF4 medium. Electrochemical behavior of these materials in the range 0–1.4 V versus saturated calomel electrode (SCE) is strongly dependent on the macromolecular chain length, but is slightly affected by the nature of the alkyl chain. Depending on the potential applied, the films are either colorless, green or blue. The redox processes involved in electrochemical oxidation of the polymer as well as those of the oligomers are discussed.

Synthesis and blue luminescence of a soluble newly designed carbazole main-chain polymer

A new electroactive polymer with alternating conjugated–nonconjugated repeating units in the main chain was synthesized by step growth polymerization of α,ω-bis(N-carbazolyl)octane in chloroform solution with excess of iron trichloride, and in the presence of N-ethylcarbazole as a terminating agent. The resulting α,ω-N-ethylcarbazole terminated poly(3,3′-bicarbazyl-N,N′-octylene)s are readily soluble in common organic solvents and have good film-forming capabilities, partly because of the special design of the polymer backbone constituted by stiff bicarbazyl chromophores linked by flexible eight-carbon segments. Soluble materials with molecular weights up to 105 g/mol and polydispersity indices around 3.3 were obtained. Purified samples exhibiting quite low polydispersity indices (in the range 1.5–1.7) can be prepared by a selective fractionation of the crude polymer from benzene/methanol mixtures. Structural analysis of these new ‘stairs-like’ polymers disclosed their well-defined character with an aromatic linkage exclusively at position 3 on the carbazole moieties. Electrochemical studies of polymer films exhibited two reversible redox processes between 0 and 1.5 V vs saturated calomel electrode. The photoluminescence (PL) of the polymer in solution and as cast films revealed an intense blue emission and the same intensity level than that of the N,N′-diethyl-3,3′-bicarbazyl molecule, taken as a model of the aromatic segment of the repeating unit. Such a processable and purifiable polymer with bicarbazyl-isolated fluorophores is quite promising for the fabrication of efficient blue light-emitting devices.

Highly efficient, diffraction-limited laser emission from a vertical external-cavity surface-emitting organic laser

We report on a solid-state laser structure functioning as the organic counterpart of a vertical external-cavity surface-emitting laser (VECSEL) design. The gain medium is a poly(methyl methacrylate) film doped with Rhodamine 640, spin casted onto the high-reflectivity mirror of a plano-concave resonator. Upon pumping by 7 ns pulses at 532 nm, a diffraction-limited beam (M(2)=1) was obtained, with a conversion efficiency of 43%; higher peak powers (2 kW) could be attained when resorting to shorter (0.5 ns) pump pulses. The spectrum was controlled by the thickness of the active layer playing the role of an intracavity etalon; tunability is demonstrated at over and up to 20 nm.

Laser operation in nondoped thin films made of a small-molecule organic red-emitter

Stimulated emission in small-molecule organic films at a high dye concentration is generally hindered by fluorescence quenching, especially in the red region of the spectrum. Here we demonstrate the achievement of high net gains (up to 50 cm−1) around 640 nm in thermally evaporated nondoped films of 4-di(4′-tert-butylbiphenyl-4-yl)amino-4′-dicyanovinylbenzene, which makes this material suitable for green-light pumped single mode organic lasers with low threshold and superior stability. Lasing effect is demonstrated in a distributed Bragg resonator configuration, as well as under the form of random lasing at high pump intensities.

Synthesis and characterization of poly (N-butyl-3,6-carbazolediyl)

SummaryPoly (N-butyl-3,6-carbazolediyl) was synthesized by electrochemcial reduction of the NiBr2, 2,2′bipyridine/3,6 dibromo N-butylcarbazole system in N,N dimethylacetamide. The favorable interactions between solvent and butyl groups lead to the solubilization of the electroactive polymer in organic medium and allowed the determination of macromolecular and structural features by means of classical methods. From 13C NMR investigations it was concluded to a Π conjugation centered on the nitrogen atom of the carbazolic structure.

Photoluminescence of donor–acceptor carbazole chromophores

Absorption and emission features of various chromophores both in the solid amorphous state and in solution are presented. These 1-(N-ethyl-carbazolyl)-2-substituted-2-cyanovinylene molecules contain in their structure the electron-donor carbazole nucleus and cyanovinylene bearing different acceptor moieties such as either another nitrile function, a methylester, a phenyl, or a para-nitro-phenyl group. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less red-shifted. It is found that the methylester derivative displays the best relative photoluminescence efficiency among all the samples. From solvatochromic studies we showed that the remarkable luminescence of the cyanoester derivative can be attributed to a strong dipole moment in the excited state. Furthermore, cyanoester microcrystallites exhibited spectral narrowing which was attributed to an amplified spontaneous emission.

Blue organic light emitting diodes based on bicarbazyle derivates: Device stability and multilayer configuration

The device stability of blue light emitting diodes based on N,N′-diethyl-3,3′-bicarbazyle, (EtCz)2, are substantially improved by coevaporation with N,N′-diethyl-3,3′-bicarbazyle-6,6′ dicarbaboxylic acid, (OcCzCOOH)2. We attribute this effect to the formation of a network of hydrogen bonds in the alloy related to the presence of carboxyl groups. A three layer device with transport layers for both electrons and holes improves the quantum efficiency: With an Al cathode we obtain a luminance of 100 cd/m2 at current density levels of 50 mA/cm2. We find that the current voltage characteristics of (EtCz)2 is well described by our recent microscopic model for space charge limited currents in an assembly of conjugated polymer segments. The incorporation of an electron transport layer splits and shifts the electroluminescent band to lower energies and we observe a significant additional downshift after a period of three weeks which we attributed to molecular diffusion at the interface between the emitting and elec...

Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder forms

We present absorption and photoluminescence features of four samples of carbazole molecules substituted with various electron–acceptor groups. These molecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron–donor carbazole nucleus and cyanovinylene bearing either another nitrile function, an ethylester, a phenyl, or a para-nitrophenyl groups. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the samples and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowing and shift of the peak emission. We obtain further narrowing of the emission band and further redshifting of the emission when we illuminate, transversely, a glass capillary containing the crystalline sample by an ultraviolet light-emitting diode.