S. Grigalevicius

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.

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.

Hole-transporting molecular glasses based on carbazole and diphenylamine moieties

Abstract Various molecular designs based on carbazole and diphenylamine moieties are found to constitute novel hole transporting amorphous molecular materials, as characterized by differential scanning calorimetry and time of flight method. Hole drift mobility of synthesized materials were in the range 10 −4 –10 −6 cm 2 V −1 s −1 at an applied field of 3.6×10 5 V cm −1 .

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.

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.

Triplet states and energy back transfer of carbazole derivatives

Intermolecular interactions among π conjugated semiconducting molecules often give rise to totally different optical behaviours between the solid state and dilute phases. Phosphorescence spectra observed in the solid state are often lowered compared with dilute forms resulting in the red-shift of the phosphorescence spectra. Here, we demonstrate that this red-shift can be reduced by introducing side groups. We also show that such a shift is a function of interchromophoric distance with fast exponential decay. Furthermore, we show conclusively that triplet exciton transfer between the hosts and the bis[2-(4F,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) can be described in terms of the Boltzmann factor using triplet energies obtained from the solid state. These results will have implications in molecular design that utilizes triplet excitons such as organic light emitting diodes and singlet fission solar cells.

Wet-process feasible novel carbazole-type molecular host for high efficiency phosphorescent organic light emitting diodes

Wet-process organic light-emitting diodes (OLEDs) are crucial to realize cost-effective and large-area roll-to-roll fabrication of high quality displays and lightings. In this study, a wet-process feasible carbazole based host material, 2-[4-(carbazol-9-yl)butyloxy]-9-[4-(carbazol-9-yl)butyl]carbazole (6), is synthesized, and two other carbazole hosts, 2-[5-(carbazol-9-yl)pentyloxy]-9-[5-(carbazol-9-yl)pentyl]carbazole (7) and 2-[6-(carbazol-9-yl)hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole (8) are also synthesized for comparison. All the three host materials exhibit high triplet energy, and possess high solubility in common organic solvents at room temperature. On doping a green phosphorescent emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)3) into host 6, the device shows an efficacy of 51 lm W−1 and a current efficiency of 52 cd A−1 at 100 cd m−2 or 30 lm W−1 and 40.7 cd A−1 at 1000 cd m−2. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, the ability for excitons to generate on both host and guest, and excellent film integrity.

Counter-ion induced solubility of polypyridines

Protonating the pyridine rings of poly(2,5-pyridine) and the chloro-substituted prepolymer of poly(2,5-pyridine vinylene) with dodecylbenzenesulfonic acid produces polymers which can be dissolved in chloroform. This contrasts to the unprotonated polymers, which can only be dissolved in strong acids such as formic acid. This allows mixing of these polymers with other chloroform soluble conjugated polymers for use in photodiodes/solar cells. Both the prepolymer of poly(2,5-pyridine vinylene) and its protonated form can be converted to the conjugated form by heat treatment. The absorption maximum is red-shifted about 60 nm, because of the changed electronic structure.

N,N′-Diphenyl-1,4-phenylenediamine-Based Enamines Containing Reactive Functional Groups as Building Blocks for Electro-Active Polymers

N,N′-Diphenyl-1,4-phenylenediamine-based enamines containing reactive functional groups were synthesized by a multi-step synthesis route. The materials were examined by various techniques including thermogravimetry, differential scanning calorimetry, UV and fluorescence spectrometry, electron photoemission and time-of-flight techniques. The electron photoemission spectra of the layers of the amorphous materials showed the ionization potentials of ca. 5.3 eV. Hole drift mobilities in the charge transport layers prepared using one of the synthesized compounds exceeded 10−5 cm2/Vs at high electric fields.

P-152: Efficient Blue Phosphorescent OLEDs Employing Novel Oligocarbazoles as High-Triplet-Energy Host Materials

A series of 3(6),9′-linked oligocarbazoles with large triplet energies and morphological stability are reported as host materials for electrophosphorescence. In such oligocarbazoles, the triplet excitation is confined within one carbazole unit, leading to a new molecular design strategy for large-triplet-energy and high morphological stability hosts. Using these hosts, efficient blue phosphorescent OLEDs having efficiencies up to 15%, 31 cd/A and 28 lm/W are demonstrated.