Carbazole oligomers revisited: New additions at the carbazole 1- and 8-positions (original) (raw)
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The role of carbazole in organic light-emitting devices
Synthetic Metals, 1996
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 lightemitting 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 21m W-1). The other case involved ([3-octylthiophene J-[bis-(Nethyl 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.
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 wetprocess 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 three host materials exhibit high triplet energy, and possess high solubility in common organic solvents at room temperature. Doping a green phosphorescent emitter fac tris(2- phenylpyridine)iridium (Ir(ppy)3) into host 6 , the device shows an efficacy of 51 lmW-1 and current efficiency of 52 cdA-1 at 100 cdm-2 or 30 lmW-1 and 40.7 cdA-1 at 1,000 cdm-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.
Synthetic Metals, 2007
A new carbazole derivative with a 3,3'-bicarbazyl core 6,6'-substituted by dicyanovinylene groups (6,6'-bis(1-(2,2'-dicyano)vinyl)-N,N'dioctyl-3,3'-bicarbazyl; named (OcCz2CN) 2 , was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor-acceptor type structure, (OcCz2CN) 2 was found to emit a yellow light at λ max =590 nm (with the CIE coordinates x=0.51; y = 0.47) and was used either as a dopant or as an ultra-thin layer in a blue-emitting matrix of 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi). DPVBi (OcCz2CN) 2 -doped structure exhibited, at doping ratio of 1.5 weight %, a yellowish-green light with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence efficiency η EL =1.3 cd/A, an external quantum efficiency η ext = 0.4 % and a luminance L= 127 cd/m 2 (at 10 mA/cm 2 ) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x = 0.40; y= 0.43), η EL = 2.0 cd/A, η ext = 0.7 %, L = 197 cd/m 2 (at 10 mA/cm 2 ) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.
Organic Electronics, 2006
Three new oxadiazole substituted carbazole derivatives 9-{4-[5-(4-tert-butylphenyl)-[1,3,4] oxadiazol-2-yl]-benzyl}-9 H-carbazole (t-CmOxa), 9-[4-5-phenyl-[1,3,4]oxadiazol-2-yl-benzyl]-9H-carbazole (p-CmOxa) and 9- [4-5-biphenyl-[1,3,4] oxadiazol-2-yl-benzyl]-9H-carbazole (d-CmOxa) were successfully synthesized and characterized by spectroscopy (NMR, UV-vis, mass spectrum and photoluminescence) and cyclic voltammetry measurements. Employing t-CmOxa as a host and Ir(DBQ) 2 (acac) (DBQ = dibenzo[f,h]quinoxaline, acac = acetylacetonate) as the dopant emitter, OLEDs with structures of ITO/NPB(30 nm)/Ir(DBQ) 2 (acac): t-CmOxa (30 nm, x%)/Alq 3 (30 nm)/Mg 0.9 :Ag 0.1 were fabricated without using BCP as the hole blocking layer. Red emission was obtained with CIE coordinates (x = 0.66, and y = 0.34) at 5 V and a very high external electroluminescent (EL) quantum efficiency of 9.5 ± 0.1%, and an energy conversion efficiency of 9.9 ± 0.1 lm/W were achieved for the device when the doping concentration x is equal to 4%.
Efficient red phosphorescent OLEDs employing carbazole-based materials as the emitting host
Dyes and Pigments, 2015
We report on the synthesis and characterization of a new series of electro-active carbazole-based compounds. The derivatives are thermally stable amorphous materials with glass transition temperatures in the range of 54e93 C. Electron photoemission spectra of thin layers of the materials show ionization potential in the range of 5.4e5.5 eV. The carbazole-based derivatives are fully characterized and their spectroscopic properties are determined by absorption and photoluminescence. All developed materials and commonly-used tris(4-carbazoyl-9-ylphenyl)amine (TCTA) were used as hosts in red phosphorescent organic light-emitting diodes (OLEDs) for comparison. Results indicate that a device with 3-[bis(9-ethylcarbazol-3-yl)methyl]-9-hexylcarbazole exhibited superior performance with peak efficiencies of 8.4%, 5.3 cd/A and 5.5 lm/W.
A series of novel carbazole-based materials, DPACz1, DPACz2 and DPACz3 having diphenylamino moieties at 1-and 8-positions of carbazole have been synthesized and characterized for the first time. The introduction of diphenylamino substituents at 1-or 1,8-positions of carbazole resulted into increase of the band-gap compared with the previously reported 3,6-or 2,7-substituted ones. The HOMO levels increased from DPACz1 to DPACz2 with the addition of one additional diphenylamino unit, and further increased in case of DPACz3, a dimer of DPACz1 having a benzidine moiety. The materials have high triplet energy levels of 2.68, 2.60 and 2.45 eV, respectively. Based on suitable HOMO levels and high triplet energies, the newly synthesized diphenylaminocabazoles were investigated for their potential as solution-processable host materials for green phosphorescent OLEDs with the device configuration, [ITO/ PEDOT:PSS/Emitting layer/TPBi/CsF/Al]. All the devices emitted typical green light with high luminance and had low turn-on voltages along with good luminous efficiencies which were further improved by adjusting charge balance using PBD, as a co-host. The basic characteristics and the preliminary OLED results showed the usefulness of our new materials, and this kind of 1-/1,8-substitution of carbazole would open a new way of materials design.
Dyes and Pigments, 2019
New efficient carbazole-based emitters containing tetra-/triphenylethene units were developed for boosting efficiency of non-doped fluorescent organic light-emitting diodes. Comparable study of the properties of derivatives containing one or two tetra-/triphenylethenyl units was performed using various experimental and theoretical techniques. Depending on the substitution pattern, emitters exhibited strong blue or green emission, which was enhanced by aggregation. Compounds with two substituents showed higher glass transition temperatures (up to 120 ºC) and lower ionization potentials (of ca. 5.15 eV) comparing to mono substituted derivatives. Time-of-flight hole drift mobility values of the studied compounds with two substituents reached 10-3 cm 2 /Vs at high electric fields. Non-doped fluorescent OLEDs based on carbazole derivative containing two tetraphenylethenyl units demonstrated extremely high external quantum efficiency as for simple fluorescent organic light-emitting devices, which reached 5.32 %.
Molecular Crystals and Liquid Crystals, 2018
Two oligoethers containing electroactive pendent pyridinyl-carbazole moieties have been synthesized by the multi-step synthetic route. Full characterization of their structures is presented. The oligomers represent derivatives of very high thermal stability with initial thermal degradation temperatures exceeding 400 C. Glass transition temperatures of the amorphous materials were also very high and reached values of 124 C and 145 C, respectively. Bipolar chemical structure having oligomer, i.e. poly{3-(2-methoxy-3-pyridinyl)-9-(3methyloxetan-3-yl)methyl-carbazole} was tested as host material for green phosphorescent organic light emitting diode using tris(2phenylpyridine)iridium(III) as a triplet emitter. A green device containing 10 wt% of the green guest demonstrated among all the devices the best performance with current efficiency of 8.8 cd/A and power efficiency of 5.1 lm/W at 100 cd/m 2. At higher brightness, such as 1000 cd/m 2 , used for illumination applications, this PhOLED showed enhanced efficiency of 11.7 cd/A (5.4 lm/W) with brightness exceeding 4000 cd/m 2 .
Molecules (Basel, Switzerland), 2018
Four low molecular weight compounds-three of them new, two of them with carbazole (Cz) as functional group and the other two with thienopyrroledione (TPD) group-were used as emitting materials in organic light emitting diodes (OLEDs). Devices were fabricated with the configuration ITO/PEDOT:PSS/emitting material/LiF/Al. The hole injector layer (HIL) and the emitting sheet were deposited by spin coating; LiF and Al were thermally evaporated. OLEDs based on carbazole derivatives show luminances up to 4130 cd/m², large current efficiencies about 20 cd/A and, cautiously, a very impressive External Quantum Efficiency (EQE) up to 9.5%, with electroluminescence peaks located around 490 nm (greenish blue region). Whereas, devices manufactured with TPD derivatives, present luminance up to 1729 cd/m², current efficiencies about 4.5 cd/A and EQE of 1.5%. These results are very competitive regarding previous reported materials/devices.