Dmytro Volyniuk

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.

An Ambipolar BODIPY Derivative for a White Exciplex OLED and Cholesteric Liquid Crystal Laser toward Multifunctional Devices

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10-4 and 2 × 10-4 cm2 V-1 s-1, respectively, at electric fields higher than 5.3 × 105 V cm-1. The resulting WOLED exhibited a maximum luminance of 6579 cd m-2 with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell.

Can hydrogen bonds improve the hole-mobility in amorphous organic semiconductors? Experimental and theoretical insights

Five hole-transporting triphenylamine derivatives containing methoxy and methyl groups are synthesized and investigated. The hole-mobility increases in the presence of methyl and methoxy substituents, exceeding 10−2 cm2 V−1 s−1 in the case of methyl groups. Quantum mechanical calculations on these compounds indicate very different dipole moments and intermolecular interaction strengths, with intriguing correlations with the trend in hole-mobility. Temperature dependent hole-mobility measurements indicate disorder dominated hole transport. The values of the energetic disorder parameter (σ) decrease upon methyl and methoxy substitutions despite the increase in dipole moments. This trend is discussed as a function of the interaction energy between adjacent molecules, the dipole moment, the molecular polarizability, and the conformational degree of freedom. Our results indicate that the global decrease of σ upon methyl and methoxy substitutions is dominated by the larger decrease in the geometrical randomness component of the energetic disorder. A direct correlation is established between the decrease in geometrical randomness and the increase in intermolecular interaction energies, mainly stemming from the additional C–H⋯π, O, N hydrogen bonds induced by methyl and methoxy groups.

A wet- and dry-process feasible carbazole type host for highly efficient phosphorescent OLEDs

A wet- and dry-process feasible host material is crucial to realize, respectively, low cost roll-to-roll fabrication of large area and high performance organic light-emitting diodes (OLEDs) with precise deposition of organic layers. We demonstrate in this study high efficiency phosphorescent OLED devices by employing a newly synthesized carbazole based host material 1,6-bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 5). Moreover, two other carbazole hosts 1,6-bis[3-(6-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 4) and 3,6-di(2-methoxy-3-pyridinyl)-9-ethylcarbazole (compound 6) are also synthesized for comparison. By doping a typical green emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)3) in compound 5, for example, the resultant wet-processed device exhibits at 100 cd m−2 a current efficiency of 27 cd A−1 and a power efficiency of 16.1 lm W−1. The dry-processed device shows a current efficiency of 61 cd A−1 and a power efficiency of 62.8 lm W−1. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, effective carrier injection balance, and the device architecture enabling excitons to generate on both the host and the guest.

N-annelated perylenes as effective green emitters for OLEDs

New carbazole-substituted N-annelated perylenes with different linking topologies of the chromophores were synthesized and their physical properties were studied with various experimental techniques including thermogravimetric analysis, differential scanning calorimetry, UV-vis, fluorescence and photoelectron emission spectroscopies and cyclic voltammetry. The synthesized materials exhibit extremely high thermal stability with 5% weight loss temperatures ranging from 400 to 457 °C. Photoluminescence quantum yields of the solid films of the compounds range from 33 to 51%. The ionization potentials of the solid samples established by the electron photoemission technique were found to be in the range of 5.14–5.53 eV. Using the newly synthesized compounds as emissive materials, efficient organic light-emitting diodes (OLEDs) were fabricated with the emission pattern covering the broad visible region from 475 to 675 nm. Bright green OLEDs exhibited luminance exceeding 62 000 cd m−2 and external quantum efficiency reaching 4.2%.

Nine-ring angular fused biscarbazoloanthracene displaying a solid state based excimer emission suitable for OLED application

A new biscarbazoloanthracene consisting of nine fused aromatic rings, including two pyrrole units, has been obtained in a straightforward and convergent synthesis. Computational chemistry and conformational analysis revealed that the semiconductors molecule is not planar, the two carbazole moieties being helical twisted from the plane of the anthracene unit. Photophysical and electrochemical measurements showed that this angular fused heteroacene has a low lying HOMO energy level with a wide band gap despite its extended π-conjugated molecular framework. Based on its relatively low-lying HOMO level, the semiconductor promises a high environmental stability in comparison to other related linear fused acenes and heteroacenes. The biscarbazoloanthracene has been applied as the light emitting layer in a white light emitting diode (WOLED). It is proposed that the white OLED feature is due to dual light emission properties from the active semiconductor layer being based on both the molecular luminescence of the small molecule and a discrete excimer emission made possible by suitable aggregates in the solid state. Noteworthy, this is the first reported example of such a behavior observed in a small molecule heteroacene rather than an oligomer or a polymer.

Sky-blue aggregation-induced emission molecules for non-doped organic light-emitting diodes

Triphenylethene moieties decorated with different carbazole-based substitutions were synthesized to realize molecules harvesting strong blue aggregation-induced emission (AIE). Among these compounds, 1,1,2-triphenyl-2-[9-(2-ethylhexyl)carbazol-2-yl]ethylene (3) and 1,1,2-triphenyl-2-[9-((3-methyloxetan-3-yl)methyl)carbazol-2-yl]ethylene (4) were found to possess suitable carrier transport capabilities, high thermal stability, and strong blue aggregation-induced emissions. In addition, bipolar charge transport for carbazole derivatives substituted with a triphenylethylene moiety was found. Both compounds 3 and 4 were selected as emitters to fabricate non-doped blue organic light-emitting diodes (OLEDs). The respective maximum efficiencies of compounds 3- and 4-based AIE OLEDs were recorded at 3.5% (8.8 cd A−1 and 8.5 lm W−1) and 3.8% (9.0 cd A−1 and 8.4 lm W−1), respectively. Furthermore, a red phosphorescent emitter was used to combine compounds 3 and 4 for white OLEDs, giving respective peak efficiencies of 2.8% and 2.7%. These results demonstrate the effective design concepts of the triphenylethene moiety decorated with different carbazole-based substitutions.

Green and red phosphorescent organic light-emitting diodes with ambipolar hosts based on phenothiazine and carbazole moieties: photoelectrical properties, morphology and efficiency

The two low-molar mass compounds 10,10′-(9-ethyl-9H-carbazole-3,6-diyl)bis(10H-phenothiazine) and 10-(9-ethyl-9H-carbazol-3-yl)-10H-phenothiazine were synthesized as ambipolar hosts for phosphorescent organic light emitting diodes. The structure and properties of these compounds were studied by X-ray crystallographic analysis, extensive UV-vis and fluorescence spectrometry, cyclic voltammetry and theoretical calculations. These compounds exhibited triplet energies of 2.6 eV; the shallow ionization potentials in the interval of 5.10–5.25 eV. The ambipolar semiconductor properties of the amorphous layers for both compounds were proved by the experimental and Marcus hooping theory methods. Importantly, the compounds appeared to be ideally suited for the green and red phosphorescent organic light-emitting diodes as the hosts for Ir(III) dyes. The maximum power and external quantum efficiencies up to 47.5/40.6 lm W−1 and 20.0/10.5% were observed for the green and red devices, respectively. The quenching processes such as a triplet–triplet annihilation and triplet-polaron quenching contributing to the roll-off of the quantum efficiency were investigated. The considerable widths of the exciton formation zone of 13 and 17 nm were estimated for the best performance devices. The morphology of the physical vapor deposited layers of the different composition mixtures of the hosts and Ir(III) emitters was examined using atomic force microscopy. Based on the obtained morphological information the preliminary correlation between the changes in a surface morphology and the widths of the exciton formation zone were established.

Deep-Blue High-Efficiency TTA OLED Using Para- and Meta-Conjugated Cyanotriphenylbenzene and Carbazole Derivatives as Emitter and Host

Elaboration of the appropriate host materials proved to be not less important for the fabrication of a highly efficient OLED than the design of emitters. In the present work, we show how by simple variation of molecular structure both blue emitters exhibiting delayed fluorescence and ambipolar high triplet energy hosts can be obtained. The compounds with a para-junction revealed higher thermal stability (TID up to 480 °C), lower ionization potentials (5.51-5.60 eV), exclusively hole transport, and higher photoluminescence quantum efficiencies (0.90-0.97). Meta-linkage leads to ambipolar charge transport and higher triplet energies (2.82 eV). Introduction of the accepting nitrile groups in the para-position induces intensive delayed fluorescence via a triplet-triplet annihilation up-conversion mechanism. By utilization of the para-substituted derivative as an emitter and the meta-substituted isomer as the host, a deep-blue OLED with the external quantum efficiency of 14.1% was fabricated.

Synthesis and characterisation of a carbazole-based bipolar exciplex-forming compound for efficient and color-tunable OLEDs

A new ambipolar fluorophore, 3,6-di(4,4′-dimethoxydiphenylaminyl)-9-(1-naphthyl)carbazole, was synthesized and its physical properties were studied by differential scanning calorimetry, thermogravimetric analysis, UV-vis absorption, luminescence and photoelectron emission spectroscopy, cyclic voltammetry and a time of flight method. The latter technique indicates that the compound demonstrates bipolar semiconducting properties. Using the synthesized compound as an emissive material, a single-layer OLED with an electroluminescence spectrum containing a voltage-dependent electroplex emission band in the region of 550–650 nm was fabricated. Another OLED was fabricated with an additional electron transporting bathophenanthroline layer that forms a direct interface with the layer of 3,6-di(4,4′-dimethoxydiphenylaminyl)-9-(1-naphthyl)carbazole. A strong exciplex-type band in the electroluminescence spectrum of this OLED with an emission maximum at ca. 540 nm was observed. The electroluminescence spectra of both devices were found to be clearly dependent on the applied bias. This effect can be useful for the development of efficient and colour-tunable OLEDs.