Electron-transporting Groups Research Articles

Derivatives of carbazole and quinazoline as host materials for red phosphorescent OLEDs showing high efficiency and brightness

Host materials in the emitting layer play an important role in phosphorescent organic light-emitting diodes (PhOLEDs). Two red-phosphorescent bipolar host materials, 9-phenyl-9'-(4-phenylquinazolin-2-yl)-9H,9′H-3,3′-bicarbazole (BQzB2Cz) and 10-(9-phenyl-9H-carbazol-3-yl)-7-(4-phenylquinazolin-2-yl)-7H-benzo[c]carbazole (BQzB2CzB), composing of carbazole moieties of 9-phenyl-9H,9′H-3,3′-bicarbazole and 10-(9-phenyl-9H-carbazol-3-yl)-7H-benzo[c]carbazole as hole-transporting groups and quinazoline moieties as electron-transporting groups, have been synthesized and characterized. The materials exhibit good thermal stability and suitable triplet-state energy levels, and the red PhOLED prepared from BQzB2Cz and BQzB2CzB showed good performance. The OLED prepared with 3 wt% guest material doped BQzB2CzB showed the optimized performance, with the turn-on voltage of 2.8 V, the maximum brightness of 114000 cd/m2, the maximum current efficiency of 21.60 cd/A, the maximum power efficiency of 24.24 lm/W, and the maximum external quantum efficiency (EQEmax) of 25.11 % (at 132 cd/m2). In addition, an EQEmax of 18.91 % was still achieved at 10,000 cd/m2. The lifetime and efficiency of OLED based on BQzB2CzB is improved compared to those of the OLED based on BQzB2Cz. The results indicate that the strategy of designing bipolar host materials with carbazole moieties as hole-transporting groups and quinazolines as electron-transporting units is a practical approach to realizing high-efficiency and high-brightness OLEDs.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Universal Bipolar Host Materials for Blue, Green, and Red Phosphorescent OLEDs with Excellent Efficiencies and Small-Efficiency Roll-Off.

Host materials are indispensable for the fabrication of organic light-emitting diodes (OLEDs) with phosphorescent emitters, but high-quality host materials that can efficiently and simultaneously function in blue, green, and red phosphorescent OLEDs (PHOLEDs) are much rare. In this work, four bipolar materials are developed using carbazole and 9,9-dimethyl-9,10-dihydroacridine as hole-transporting groups, pyridine as electron-transporting groups, and biphenyl and m-methylbiphenyl as π-spacers. The crystal and electronic structures indicate that these materials have highly twisted conformations, which endow them with aggregation-induced emission features, intramolecular charge transfer processes, wide energy band gaps, and high triplet energies. The carrier transport ability and energy transfer property analyses show that these materials are able to achieve balanced hole and electron transports and can serve as bipolar host materials for PHOLEDs. A series of monochromatic PHOLEDs with different phosphorescent dopants, including blue-emissive FIrpic, green-emissive Ir(ppy)2(acac), and red-emissive Ir(piq)2(acac), are fabricated by employing these four host materials. The green PHOLEDs can provide an impressive luminance of up to 230 200 cd m-2. Based on an identical host material, excellent external quantum efficiencies as high as 25.12, 24.73, and 19.71%, as well as minor efficiency roll-off, are attained for blue, green, and red PHOLEDs, respectively, clearly demonstrating the promising applications as universal bipolar host materials in PHOLEDs with monochromatic light and white light.

Two green iridium(iii) complexes containing the electron-transporting group of 4-phenyl-4H-1,2,4-triazole for highly efficient OLEDs

Two novel iridium(iii) complexes containing the electron-transporting group of 4-phenyl-4H-1,2,4-triazole exhibited good photoluminescence and electroluminescence performances with an EQE of 31.43% and a mild efficiency roll-off.

Polymethacrylates with pendant charge-transporting groups synthesized via RAFT polymerization

The controlled radical polymerization of functional polymers with hole- or electron-transporting groups via reversible addition–fragmentation chain transfer (RAFT) process has been studied. Carbazole and 1,3,4-oxadiazole-containing methacrylic monomers, 6-(N-carbazolyl)hexyl methacrylate (CzHMA) and 4-(5-(naphthyl-1,3,4-oxadiazol-2-yl)phenyl) methacrylate (Naph-OxaMA), have been synthesized and polymerized via RAFT polymerization with cumyl dithiobenzoate (CDB) as a chain transfer agent (CTA) and 2,2-azobisisobutyronitrile (AIBN) as an initiator in a benzene solution. Good control of the polymerization is confirmed by the low polydispersity indexes (PDI) and controlled molecular weights (MW) of the resulting polymers as revealed by GPC measurements. The resulting charge-transporting polymers have been further characterized by NMR, UV–Vis spectroscopy, and cyclic voltammetry (CV). Polymeric charge-transporting materials containing carbazole and 1,3,4-oxadiazole pendant groups, respectively, have been synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization with controlled molecular weights, narrow molecular weight distributions.

Binder-free phenyl sulfonated graphene/sulfur electrodes with excellent cyclability for lithium sulfur batteries

In this study, phenyl sulfonated graphene/sulfur (PhSO3-RG/S) composites were synthesized for the first time via an in situ redox reaction in aqueous solution. These composites were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetry (TG). The results show that sulfur was thoroughly enveloped by functionalized water-soluble phenyl sulfonated graphene, providing a conductive coating for electron transport and functional groups that interact strongly with polysulfides or sulfur to improve trapping. In our study, we found that large amounts of conductive carbon additives and binders were not required when the electrodes were prepared on carbon-coated aluminum foil substrates because phenyl sulfonated graphene has excellent electrical conductivity, and contains functional groups that interact strongly with carbon and sulfur. The electrochemical tests show that the binder-free PhSO3-RG/S electrodes have high reversible capacity, good cycling stability at the current density of 0.2 C, and excellent rate capability.

Novel conjugated polymer/graphene/platinum composite for enhancing electrocatalytic oxidation of methanol

AbstractA novel conjugated polymer containing electron transport groups with high electron affinity, denoted POXD was synthesized upon polycondensation. The polymer was characterized by Fourier transform infrared (FTIR) spectra and 1H nuclear magnetic resonance (1H NMR) spectroscopy. Furthermore, the conjugated polymer/graphene (POXD/RGO) composite film was prepared, which was subsequently used as the support for the electrodeposition of platinum. The microstructure and morphology of the prepared samples were characterized by X‐ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FE‐SEM). The electrocatalytic activity for the oxidation of methanol was investigated through cyclic voltammogram method. The POXD/RGO composite film can facilitate the electrodeposition of Pt nanoparticles compared with graphene (RGO) support. The POXD/RGO/Pt composite exhibits more excellent electrocatalytic property for the oxidation of methanol, such as lower oxidation potential and higher current intensity, which might be attributed to the high electron affinity of the polymer and the interaction of the Pt nanoparticles with polymer. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers

Simple Phenanthroimidazole/Carbazole Hybrid Bipolar Host Materials for Highly Efficient Green and Yellow Phosphorescent Organic Light-Emitting Diodes

By conjugating carbazole moiety to the different positions of the rigid skeleton 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole, a series of hybrid bipolar phosphorescent hosts was synthesized, and their photophysical properties were investigated. The introduction of a rigid phenanthroimidazole moiety greatly improves their morphological stability, with high decomposition temperatures (Td) and high glass transition temperatures (Tg) in the range of 394–417 and 113–243 °C, respectively. The highly efficient green and orange phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved by employing these compounds as the phosphorescent hosts. For the device of ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/mPhBINCP:9 wt % Ir(ppy)3 (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100 nm), a maximum luminous efficiency (ηc,max) of 77.6 cd/A, maximum power efficiency (ηp,max) of 80.3 lm/W, and maximum external quantum efficiency (ηEQE,max) of 21% were obtained. Furthermore, these hosts are also applicable for the orange phosphorescent emitter (fbi)2Ir(acac), a yellow PhOLEDs with pPhBICP as host, for which a performance of ηc,max of 57.2 cd/A, ηEQE,max of 19.3%, and ηp,max of 59.8 lm/W was achieved. These results demonstrated that the phenanthroimidazole unit is an excellent electron-transporting group for constructing the bipolar phosphorescent host.

Computational design and selection of optimal building blocks and linking topologies for construction of high-performance host materials

Two typical hole transport groups, carbazole and diphenylamine, and two typical electron transport groups, diphenylphosphine oxide and triphenylsilane, were linked to biphenyl at its ortho/meta/para-positions to investigate the effects of building blocks and linking topologies on the structural and electronic properties of such constructed host materials via density function theory calculation. It is found that the frontier orbital levels, energy band gap, and triplet energy of host molecules can be effectively tuned by different building blocks and linking topologies. The electron-transporting nature of π-conjugated molecules can be enhanced by connecting electron-withdrawing building blocks at the ortho or meta position, but not at the para-position. Employing asymmetric building blocks with meta-type topology would be an effective strategy for the design of high-performance bipolar host materials.

A novel heteroleptic iridium complex with multifunctional ligands used for polymeric light-emitting diodes

A novel iridium complex using 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole containing both hole-transporting group and electron-transporting group as main ligand and 2-(5-p-tolyl-2H-1,2,4-triazol-3-yl)pyridine as ancillary ligand was synthesized and investigated in this paper. The iridium complex possesses high solubility, high thermal stability with 5% weight-reduction temperature (Δ T 5%) of 390 °C and glass-transition temperature ( T g) of 218 °C. Polymeric light-emitting diodes (PLEDs) based on structure of ITO/PEDOT: PSS/PVK: PBD: iridium complex/TPBI/CsF/Al fabricated by solution-processed technology were measured. The PLED using the iridium complex as phosphorescent dopant at 4 wt.% doping concentration is optimal which shows the maximum luminance of 7746 cd·cm −2, maximum current efficiency of 14.0 cd A −1 and maximum external quantum efficiency of 5.8% with CIE coordinates of (0.48, 0.50). The EL emission originates from both monomers and exciplexes formed from the iridium complex and PVK.

Synthesis and Photophysics of Two Dipolar Copolymers Containing Hole-Transporting Unit and Alq<sub>3</sub> Unit

Two novel copolymers combining the electron-transporting group (Alq3), hole-transporting group (carbazole and phenothiazine) and chromophore had been obtained. The copolymers were characterized by 1H NMR, IR and UV-vis absorption spectrum, and exhibited good solubility in a wide range of organic solvents such as toluene, THF, chloroform, DMF and DMSO. GPC shows moderate molecular masses of the copolymers around 15000 and a narrower weight distribution (PDI≈1.4). The DSC and TGA measurements indicated that the resulting copolymers have excellent thermal stability and higher glass transition temperatures than the commonly used light-emitting conjugated polymers. The PL properties of the copolymers were investigated in solution and in the solid state. Only the Alq3 emission (520 nm) was observed at high concentration (10 g/L) or in solid state. However, with the decrease in concentration of the copolymer solution, the characteristic emission of carbazole appears. This means the energy transfer taken place in higher concentration is more efficient than in lower concentration solution.

Synthesis and luminescent properties of novel bisfunctional polymeric complexes based on carbazole and 8-hydroxyquinoline groups

A fully conjugated ligand 3,6-bis (8-hydroxyquinoline-5-propenyl)- N-butyl-carbazole (B8QPC) containing hole- and electron-transporting groups was firstly synthesized through the Wittig condensation polymerization. Corresponding polymeric complexes, B8QPC ( 1) with Cu(II) ( 2) and Zn(II) ( 3), were synthesized and characterized by FT-IR, elemental Analysis, conductivity measurement and gel permeation chromatography (GPC). GPC indicated that the number average molecular weights ( M n ) are about 3800 and the degree of polymerization ( n) are about 5 for the polymeric complexes 2 and 3. UV–vis and fluorescence spectra at room temperature revealed that both the polymeric complexes 2 and 3 emit blue luminescence at 473 and 513 nm ( λ em,max) in DMSO solution and blue/green luminescence at 522 and 531 nm ( λ em,max) in solid state, respectively. Thermal property measurements show that they have good thermal stability.

Synthesis, photoluminescent and electroluminescent properties of a novel europium(III) complex involving both hole- and electron-transporting functional groups

A novel europium(III) complex involving a carbazole fragment as hole-transporting group and an oxadiazole fragment as electron-transporting group was synthesized and used as red light-emitting material in organic light-emitting diodes (OLEDs). The complex is amorphous, and exhibits high glass transition temperature ( T g = 157 °C) and high thermal stability with a 5% weight loss temperature of 367 °C. Two devices, device 1: ITO/NPB (40 nm)/Eu(III) complex (30 nm)/Alq 3 (30 nm)/LiF (1 nm)/Al (100 nm) and device 2: ITO/NPB (40 nm)/3% Eu(III) complex: CBP (30 nm)/BCP (10 nm)/Alq 3 (30 nm)/LiF (1 nm)/Al (100 nm), were fabricated, where NPB is N, N′-di(naphthalene-1-yl)- N, N′-diphenyl-benzidine, Alq 3 is tris(8-hydroxyquinoline) Al(III), CBP is 4,4′-bis(carbazole-9-yl)-biphenyl, and BCP is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, respectively. In contrast with device 1, owing to less self-quenching and better charge confinement, device 2 shows improved performances: the maximum luminance of device 2 was dramatically increased from 199 to 1845 cd/m 2, the maximum current efficiency was increased from 0.69 to 2.62 cd/A, the turn-on voltage was decreased from 9.5 to 5.5 V, and higher color purity was attained.

Synthesis and luminescent properties of Cu(II), Zn(II) polymeric complexes with electron- and hole-transporting groups

A novel ligand 3,6- bis(1,10-phenathroline-[5,6- d] imidazole-2- yl)carbazole (Bpic) containing hole- and electron-transporting groups was firstly designed. Its polymeric complexes of Bpic with Cu(∥), Zn(∥) were successfully synthesized. The UV-vis absorption, fluorescence spectra and thermal properties of these complexes were investigated. At room temperature, complexes 2, 3 emit blue luminescence from 445 to 485 nm in DMSO solution, and emit green/yellow and orange luminescence from 523 to 585 nm in solid state. In comparison with the free ligand, the polymeric complexes exhibit a bathochromic shift. It can be assigned to the extended π-conjugation of the complexes.

A multicomponent rhenium-based triplet emitter for organic electroluminescence

A novel Re complex functionalized by a hole-transport and an electron-transport groups was utilized to fabricate organic light-emitting devices (OLEDs). A current efficiency up to 8.2 cd A −1 corresponding to a power efficiency of 4.6 ml W −1 and a peak brightness as high as 5500 cd m −2 were obtained. These results represent the best values reported for OLEDs based on rhenium complexes. Short lifetime, enhanced carrier injection capability of Re complex, and efficient charge-trapping followed by exciton confinement in the emissive layer should be responsible for the outstanding electrophosphorescent performances.

Structural control of photoluminescence of four poly(urethane-urea- co-1,3,5-triazine)s: Synthesis and characterization

A series of segmented poly(urethane-urea)s containing 1,3,5 triazine in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers in which the chromophore concentration varied from 4.2% to 18.1%. Although triazine emission is located in the UV region, the films with higher content of the chromophore emitted a visible blue light (425 nm) when excited at the very red-edge of the absorption band. The photophysical properties of the materials were strongly dependent on the relative amount of triazine moieties along the main chain. Isolated moieties emit in copolymers with small amount of triazine groups, indicating that even though in solid state, these moieties tend to be apart. Two photophysical consequences were observed when the amount of triazine increases: there is some energy transfer process involving isolated moieties with consequent decrease of the lifetime and an additional red-edge emission attributed to aggregated lumophores. The mono-exponential decay observed for the isolated form is substituted by a bi-exponential decay of the aggregated species. The materials were not strong emitters, but since the N-containing triazine moieties are good electron transport groups, the polymers have potential application as electron transport enhancers in various applications.

Synthesis and properties of new poly(terfluorene) derivatives containing spirobifluorene and electron transport groups for stable blue electroluminescence

AbstractWe have synthesized three new blue‐emitting poly(terfluorene) derivatives containing spirobifluorene and electron transport groups (quinoline or oxadiazole). The strategy is to connect the monomers via positions 2 and 2′ of spirobifluorene, which significantly restricts the interchain interaction and effectively adjusts the conjugation length. The incorporation of electron‐deficient units (quinoline or oxadiazole) into the positions 7 and 7′ of spirobifluorene tunes the emission band and reduces the lowest unoccupied molecular orbital energy level. Blue electroluminescence with narrow emission was achieved in the devices of ITO/PEDOT/polymer/Ca/Al. The maximum luminances are in the range of 102–235 cd/m2 and the maximum photometric efficiencies are in the range of 0.17–0.21 cd/A. All the polymers show good spectral stability, and are promising for use as stable blue‐emitting or electron‐transport/injection materials in polymeric light‐emitting diodes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4517–4529, 2005

Organic negative-resistance devices using PPV containing electron-transporting groups on the main chain

A kind of novel structure polymer, a poly(phenylenevinylene) (PPV) derivative containing electron-transporting groups on the main chain (OPPV), was developed according to the polycondensation mechanism. The I–V characteristics at the low-voltage region with negative differential resistance were observed in the same model of organic light-emitting diodes with the OPPV polymer. The different external parameters, including the device structure, layer thickness, morphology, and homogeneity of indium tin oxide and the polymer, were also investigated. The performance improvement in the organic negative differential resistance and a possible mechanism are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 589–593, 2001

Synthesis, photo-, and electroluminescent properties of the soluble poly[(2,5-diphenylene-1,3,4-oxadiazole)- 4,4?-vinylene

A novel soluble conjugated polymer, poly[(2,5-diphenylene-1,3,4-oxadiazole)-4,4′-vinylene] (O-PPV), containing an electron-transporting group on the main chain of PPV, was synthesized according to HORNER mechanism. The oligo-polymer with Mw = 1000 and Td = 270°C is soluble in chloroform and tetrahydrofuran. The photoluminescent (PL) properties were investigated using different concentrations of solid-state O-PPV/PEO blends absorption and selective excitation measurements. The results show that PL arises from interchain charge-transfer states in solid-state O-PPV. Compared with the analogous single-layer device constructed with PPV (ITO/PPV/Al), which emits two peaks at λ = 520 nm and 550 nm (shoulder), the electroluminescence (EL) spectrum of the device [ITO/O-PPV (80 nm)/Al] is a broad peak at λmax = 509 nm. The quantum efficiency (0.13%) of the device ITO/O-PPV/Al is much higher than that of the device ITO/PPV/Al, due to the introduction of the electron-transporting group–oxadiazole units in the main chain of PPV. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3535–3540, 1999

Doped Organic Light-Emitting Diodes Based on Random Copolymers Containing Both Hole and Electron Transport Groups

ABSTRACTDoped organic light-emitting diodes (OLEDs), such as PVK containing the electron transport molecule PBD and molecular dyes, have demonstrated three-color capabilities and efficiencies over 1%, but there are concerns about reliability due to recrystallization of the small molecule PBD. In this work we describe the incorporation of both hole and electron transport groups into a single copolymer to avoid recrystallization and phase segregation (which could occur if two separate polymers were used) as well as the application of such copolymers in dye-doped OLEDs. The polymers were synthesized through free radical copolymerization of the electron- donating monomer N-vinylcarbazole (NVK) with the electron-withdrawing monomer 2-(4-tert-butylphenyl)-5-{4-[(4'-vinyl)phenylmethoxy] phenyl}-1,3,4- oxadiazole (BVO). The radical reactivity ratios of the two monomers were estimated to be rNVK=0.052 and rBvo=12. The copolymers are transparent in the visible region, homogeneous as characterized by both GPC and DSC, and have good thermal stability. External quantum efficiencies of 0.07%, 0.3% and 0.4% were achieved in LEDs with device structures of ITO/COP:C47/Mg:Ag, ITO/COP:C6/Mg:Ag and ITO/COP:NR/Mg:Ag, respectively, where COP stands for copolymer, C47 for Coumarin 47, C6 for Coumarin 6 and NR for Nile Red. The introduction of the oxadiazole group balances the injection of holes and electrons by decreasing the hole injection and transport ability and enhancing the electron injection and transport ability of the copolymers relative to PVK.