Good Hole-transporting Ability Research Articles

Acetylene-extended triarylamines for solution-processable p-channel OFETs

New acetylene-extended molecules were developed as a semiconducting layer for application in solution-processable organic field-effect transistors through coupling reactions using substituted triphenylamines as wings and diacetylene as spacer units. The influence of donor ability and spacer strength on the electrical properties was explored by developing different acetylene-extended triarylamines with D–π and D–π–π–D architectures and their structure–properties analysis. Their optical studies revealed narrow Stoke's shift and optical bandgap values, especially for highly electron-rich systems. Encouraging results supported by their electrochemical studies that revealed high HOMO energy levels ranging from –5.1 to –5.3 eV suggested a low energy barrier and good hole-transporting ability, hence, their applicability as p-channel OFETs. Furthermore, the extended conjugation in D–π–π–D based molecules relative to their corresponding D–π system caused enhanced molecular orbital overlap along the triphenylamine and facilitated good charge transport characteristics. Upon fabrication as the semiconducting channel in top-gate/bottom-contact OFET devices, the acetylene-extended D–π–π–D system showed good p-channel properties with hole mobilities up to 1.2 cm2/Vs and the highest on/off ratio of 107.

Solution-processable orange-red thermally activated delayed fluorescence hyper-structured molecular emitters containing diphenylamine substituted carbazole host units

Realizing solution-processable orange-red thermally activated delayed fluorescence (TADF) emitters is still challenging owing to the severe aggregation-caused quenching. In this study, N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine (DDPACz) with high triplet energy and good hole transport ability was chosen as host unit. And 1,8-naphthimide-9,9-dimethyl-9,10-dihydroacridine (NDMAC) was used as red TADF unit. Then, these two functional units were linked to the inactive calix[4]resorcinarene (CRA) core to construct a series of hyper-structured molecules (HSMs), CRA-NDMAC(x)-DDPACz(8-x) (x = 0, 1, 2, 4, 6, 7 or 8). 1H NMR, MALDI-TOF MS and FT-IR spectra indicated that HSMs were successfully synthesized. UV–Vis, PL and CV results demonstrated that these HSMs inherited the photophysical and electrochemical properties of functional units. Spin-coating CRA-NDMAC(x)-DDPACz(8-x) (x = 1, 2, 4, 6, 7 or 8) as light-emitting layer, all the non-doped solution processed organic light-emitting diodes (OLEDs) showed orange-red emission. Among them, CRA-NDMAC(1)-DDPACz(7)-based device showed the highest maximum external quantum efficiency (EQEmax) of 0.94% with the emission peak at 613 nm. Furthermore, bright orange emission at a central wavelength of 592 nm with EQEmax of 9.73% and luminance of 5428 cd/m2 was achieved in the doped OLED based on CRA-NDMAC(7)-DDPACz(1) using CBP as host. These results indicate that hyper-structured molecules could be a potential strategy for the construction of orange-red TADF emitters.

Rational Design of Chrysene-Based Hybridized Local and Charge-Transfer Molecules as Efficient Non-Doped Deep-Blue Emitters for Simple-Structured Electroluminescent Devices.

Herein, we present a molecular design of chrysene-based deep-blue emissive materials (TC, TpPC, TpXC, and TmPC), in which chrysene as a core is functionalized with different triphenylamine moieties to realize a fine-tuning deep-blue fluorescence with superior electroluminescent (EL) performance. The photophysical analyses and density functional theory (DFT) calculations disclose that TC, TpPC, and TpXC possess HLCT characteristics with intense deep-blue emission in the solid-state, good hole-transporting ability, and high thermal and electrochemical stabilities. They are successfully employed as non-doped emitters in simple structured OLEDs (ITO/PEDOT : PSS : NF/emitter/TPBi/LiF : Al). In particular, TC-based device emits a deep-blue light with an emission peak at 446 nm and CIE color coordinates of (0.148, 0.096), a maximum external quantum efficiency (EQEmax ) of 4.31%, and a low turn-on voltage of 2.8 V.

Utilization of Nanoporous Nickel Oxide as the Hole Injection Layer for Quantum Dot Light-Emitting Diodes.

Nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its excellent hole mobility, high environmental stability, and low-cost fabrication. In this research, a NiOx thin film and nanoporous layers (NPLs) have been utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiOx NPLs have spongelike nanostructures that possess a larger surface area to enhance carrier injection and to lower the turn-on voltage as compared with the NiOx thin film. The energy levels of NiOx were slightly downshifted by incorporating the nanoporous structure. The amount of Ni2O3 species is higher than that of NiO in the NiOx NPL, confirming its good hole transport ability. The best QLED was achieved with a 30 nm thick NiOx NPL, exhibiting a maximum brightness of 68 646 cd m–2, a current efficiency of 7.60 cd A–1, and a low turn-on voltage of 3.4 V. More balanced carrier transport from the NiOx NPL and ZnO NPs/polyethylenimine ethoxylated (PEIE) is responsible for the improved device performance.

Peripheral carbazole or cyano substitutents in poly (aryl ether) based bipolar polymeric hosts: Increase the hole or electron injection properties for blue phosphorescent polymer light emitting diodes

AbstractA series of poly (aryl ether) based bipolar polymer hosts, PCzOPOC8, PCzOPOCz, and PCzOPOCN with a peripheral carbazole or cyano unit which joins the main chain by a nonconjugated linker of a hexyl group, were synthesized and characterized. They depicted high triplet level (ET) (>2.80 eV), which was probably attributed to oxygen‐interrupted nonconjugated main chains of polymers as well as carbazole and phosphine oxide groups with high ET. While doped with a 10 wt% typical blue phosphor {iridium(III)[bis(4,6‐difluoro phenyl)pyridinato‐N, C2]picolinate, FIrpic)}, compared with those of nonsubstituted PCzOPOC8 and peripheral cyano‐substituted PCzOPOCN, PCzOPOCz with carbazole moiety gave better performances due to balance of carrier flux from carbazole moiety with good hole transport ability.

High efficiency green perovskite light-emitting diodes based on exciton blocking layer

In recent years, metal halide perovskite materials, owing to their excellent photoelectric properties including high photoluminescence quantum yield, high color purity, tunable band gap, etc., have been regarded as new-generation lighting sources and are widely used to fabricate perovskite light-emitting diodes (PeLEDs). Though great progresses have been made in recent years, neither the efficiency nor stability has not yet reached the requirements of commercialization. Thus, further improvement is needed. In this work, a small organic molecule material, namely 4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) with a wide bandgap and a good hole transport ability, is used as an exciton blocking layer by utilizing the spin-coating method to improve the stability and efficiency of PeLEDs. Highly efficient and stable CsPbBr<sub>3</sub> PeLEDs are finally realized. The physical mechanism related to the improved electroluminescence performance is investigated thoroughly. Firstly, the stepped energy level alignment is formed, since the highest occupied molecular orbital energy level (HOMO) of TAPC is located between the HOMO of (3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT: PSS) and the valence band of CsPbBr<sub>3</sub>, which is beneficial to hole injection and transport. Meanwhile, the lowest unoccupied molecular orbital level of TAPC is high enough to prevent electrons from leaking into the anode effectively and confine electrons and excitons well in the emitting layer. Secondly, the introduction of the TAPC layer can avoid the direct contact between the perovskite light emitting layer and the strong acidic layer of PEDOT:PSS, thereby eliminating the related excitons quenching, which can further increase the radiative recombination.

Solution-Processable Unsymmetrical Triarylamines: Towards High Mobility and ON/OFF Ratio in Bottom-Gated OFETs.

Self-assembly of organic small molecules into an ordered thin film has been the key strategy towards efficient charge transport for organic field-effect transistors (OFETs). Solution processing is a feasible and economic way to enhance pi-pi interaction. Herein, nitrile-substituted unsymmetrical triarylamines for OFET applications with high mobility are reported. The compounds were constructed by Suzuki cross-coupling reactions under inert conditions. The HOMO level of about 5.3 eV indicates good hole-transporting ability. OFETs were assembled in bottom-gate, top-contact architecture. Devices fabricated from a binary solvent system exhibited excellent p-channel characteristics, with impressively high charge-carrier mobility of up to 2.58 cm2 V-1 s-1 and ION/OFF current ratios of 106 -107 . SEM and AFM analysis showed the efficient molecular self-assembly attained by the simple and effective solvent-engineering method. Theoretical insights obtained by DFT calculations supported by single-crystal structures showed that the crystalline nature and packing modes of these compounds ensure high mobility. The results prove that these compounds have great potential for use in numerous electronic applications, such as sensors and logic switches.

A new dibenzo[g.p]chrysene derivative as an efficient anode buffer for inverted polymer solar cells

Abstract A new dibenzo[g,p]chrysene derivative, 3,6,11,14-tetramethoxyphenylamine- dibenzo[g,p]chrysene (MeOPhN-DBC), has been designed and synthesized. This material shows good hole-transport ability, high thermal stability, and relatively high HOMO/LUMO, suitable for anode buffer in electronic devices. When 4 nm MeOPhN-DBC was inserted between the active layer and MoO3 to form double interfacial layers in the inverted polymer solar cells based on poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blended active layer, the power conversion efficiency was improved 24.7% compared to device with only MoO3 buffer (2.95% vs 3.68%). As demonstrated by photoluminescence, electrochemical impedance spectroscopy and the atom force microscopy, as well as the energy level diagram, the insertion of MeOPhN-DBC inter layer can block exciton quenching, improve electrical conductivity of the device, smooth interfacial contact, block electron flow toward anode and thus suppress carrier recombination there, leading to improved hole extraction and OPV device performance. This work demonstrates for the first time that dibenzo[g,p]chrysene derivatives can be promising materials for anode buffer in electronic devices.

Molecular engineering to enhance perovskite solar cell performance: Incorporation of benzothiadiazole as core unit for low cost hole transport materials

Through molecular engineering, a benzothiadiazole unit is incorporated as the core structure to generate a new hole transport material JY5. Cyclic voltammetry measurement suggests matched energy level of JY5 with the perovskite materials. TGA and DSC show excellent thermal stability of JY5. Hole mobility and PL decay experiments indicate good hole transport ability in JY5. As a result, perovskite solar cells utilizing JY5 as HTM outperforms the biphenyl centered analogue, and a champion efficiency of 16.87% was achieved among the HTMs studied in this work, with short-circuit current (Jsc) of 21.06 mA cm−2, open-circuit voltage (Voc) of 1.06 V, and fill factor (FF) of 0.76. The high efficiency as well as low costs make it potential alternative to the highly cost Spiro-OMeTAD.

Synthesis and properties study of a X-type dendrimer based on triphenylamine

A X-type dendrimer DTPA based on triphenylamine was designed, synthesized, and characterized by NMR and mass spectrometry. The obtained dendrimer possesses good thermal stability. Utilizing DTPA as the hole-transporting layer, the organic light emitting devices (OLEDs) were fabricated and the performances were investigated. The devices test showed that DTPA-based devices exhibited higher efficiency and higher external quantum efficiency (EQE), which indicated DTPA possesses good hole-transporting ability. Moreover, the DTPA-based devices also realized an operation lifetime of 265h at 20mA/cm2. The good hole-transporting ability and stability indicated the obtained DTPA is a promising candidate for hole-transporting materials, and has potential application in OLED devices.

Carbazole derivative synthesis and their electropolymerization

This review article investigates the hot topics by presenting the latest advances in carbazole and its derivative synthesis and their electropolymerization processes. The carbazole-based compounds are particularly attractive due to their important photochemical and thermal stability and good hole-transport ability. Conjugated monomers of carbazole derivatives and their polymers attract interest due to their suitability in a broad range of applications, such as biosensors, corrosion inhibition, photovoltaics, electroluminescent devices, field-effect transistors and supercapacitors, etc. This review article divides two main parts. One of them includes effects of electropolymerization parameters of carbazole and chemical polymerization of carbazole derivatives. In addition, copolymers and composites of carbazole derivatives were presented in the first part. In the second part, the application of polycarbazole and its derivatives was examined as biosensors, corrosion inhibition, supercapacitor, battery, fuel cell, solar cell, electropolymerization, light emitting diodes, and OLEDs.

Benzene-cored AIEgens for deep-blue OLEDs: high performance without hole-transporting layers, and unexpected excellent host for orange emission as a side-effect.

Great efforts have been devoted to explore efficient fluorescent materials, especially deep-blue luminogens, for organic light emitting diodes (OLEDs). In this paper, according to the design idea of creating luminogens with the characteristic of aggregation induced emission (AIE), four new benzene-cored luminogens with very simple structure have been intelligently designed, in which, without an additional hole-transporting layer (such as NPB), 3TPA-CN exhibited deep-blue emission and high performance in a simple nondoped LED device with a current efficiency (CE) of 5.21 cd A-1, external quantum efficiency (EQE) of 3.89%, and CIE coordinates of (0.15, 0.14). Excitingly, as a wonderful side-effect, 3TPA-CN can serve as an excellent host for orange emissive phosphorescent OLEDs (PhOLEDs), with a maximum current and power efficiency of 57.4 cd A-1 and 52.0 lm W-1, respectively, and a corresponding maximum EQE of 18.2%, higher than that of CBP (15.7%), one popular host for orange PhOLEDs, under the same conditions, thus broadening the utilization of AIEgens as host in PhOLEDs.

Tert-Butylated spirobifluorene derivative incorporating triphenylamine groups: A deep-blue emitter with high thermal stability and good hole transport ability for organic light emitting diode applications

A tert-butylated spirobifluorene derivative incorporating triphenylamine groups was synthesized starting from the readily available reagent biphenyl. Without any hole-transport layer, an unoptimized double-layer device exhibits excellent electroluminescent performances with a radiance of 3013 cd/m2 at 9.8 V, a maximum electroluminescent efficiency of 1.71 cd/A, a maximal external quantum efficiency of 2.58% at a brightness of 4.8 cd/m2, narrow full width at half-maximum (<54 nm), and blue emission with Commission Internationale de l'Eclairage coordinates of (0.152, 0.103), which is close to the standard for blue. Compared to the reported organic light emitting diodes materials, the molecule displays very high thermal stability. In addition, the bilayer device showed a greatly improved performance as compared to a trilayer device with NPB as hole-transporting layer, which indicated that the molecule possesses good hole transport ability and can be good candidate for hole-transporting layer in organic light emitting diodes.

Triphenylene containing host materials with high thermal stability for green phosphorescent organic light emitting diode

We report series of triphenylene derivatives with good electronic properties for the use as host materials for green phosphorescent organic light emitting diodes (PHOLEDs). We applied highly planar triphenylene moieties which could provide good electron transport ability and the carbazole or dibenzothiophene moieties which could provide good hole transport ability to obtain a bipolar host materials for green PHOLEDs. From this approach we achieved relatively good current efficiency up to 64 cd/A and external quantum efficiency up to 20.3%, respectively. This was the much more improved value (by ∼22.3%) compare to that obtained from the 4,4′-N,N′-dicarbazolebiphenyl system as a reference.

Synthesis and photoluminescent properties of a novel copolymer containing Eu-complex moieties and N-vinylcarbazole segments

Abstract A novel copolymer containing Eu-complex moieties and N-vinylcarbazole (NVK) segments was synthesized and characterized by FT-IR, elemental analysis, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), UV–vis absorption spectra and photoluminescence (PL) spectra. The copolymer exhibits a high glass transition temperature (Tg = 203 °C), high thermal decomposition temperature (Td = 345 °C) and good hole-transporting ability (HOMO energy level: −5.38 eV). Particularly, the copolymer shows an interesting photoluminescence (PL) feature: whether in solid state or chloroform solution, the copolymer always emits the characteristic emission of Eu3+ due to the 5D0 → 7FJ (J = 0–4) transitions; besides, the copolymer solution emits the characteristic emission of carbazole and the copolymer solid emits the characteristic emission of metalloquinolate. These phenomenons indicate that the NVK segments are not only used as light-harvesting groups to absorb energy and transfer energy to ligand and then to Eu3+, but also as barrier to form a local microenvironment for Eu-complex moieties. Moreover, the microenvironment was studied by PL emission spectra and Judd-Ofelt analysis, indicating that the introduction of NVK segments increases the polarity and asymmetry of the chemical environment of the europium ion.

An X-shaped solution-processible oligomer having an anthracene unit as a core: A new organic light-emitting material with high thermostability and efficiency

A new X-shaped solution-processible oligomer with a glass transition temperature of higher than 200 °C based on an anthracene derivative was prepared, and it showed good hole-transporting ability in organic light-emitting diodes (OLED). Such oligomer was also employed as the emitting layer to give the devices showing blue emission.

P-Doped p-phenylenediamine-substituted fluorenes for organic electroluminescent devices

Two novel p-phenylenediamine-substituted fluorenes have been designed and synthesized. Their applications as hole injection materials in organic electroluminescent devices were investigated. These materials show a high glass transition temperature and a good hole-transporting ability. It has been demonstrated that the 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) doped p-phenylene-diamine-substituted fluorenes, in which F4-TCNQ acts as p-type dopant, are highly conducting with a good hole-transporting property. The organic light emitting devices (OLEDs) utilizing these F4-TCNQ-doped materials as a hole injection layer were fabricated and investigated. The pure Alq 3-based OLED device shows a current efficiency of 5.2 cd/A at the current density of 20 mA/cm 2 and the operation lifetime is 1500 h with driving voltage increasing only about 0.7 mV/h. The device performance and stability of this hole injection material meet the benchmarks for the commercial requirements for OLED materials.

Dual functions of a new n-type conjugated dendrimer: light-emitting material and additive for polymer electroluminescent devices

We demonstrate a novel light-emitting diode (LED) of a graded bilayer structure that comprises poly(N-vinylcarbazole) (PVK) with good hole transport ability as the energy donor and a new distyrylanthracene-triazine-based dendrimer with enhanced electron transport ability as the light-emitting molecule. The device contains a graded bilayer structure of the PVK film covered with the dendrimer film prepared by sequential spin-casting of the dendrimer layer from a solvent that only swells the PVK layer. The bilayer device demonstrated a significantly enhanced electoluminescence quantum efficiency compared with the dendrimer single layer device or the PVK : dendrimer blend device with optimized composition. We also prepared composite LEDs with an MEH-PPV : emissive dendrimer blend. By doping the electron-deficient MEH-PPV layer with a small amount of the distyrylanthracene-triazine-based dendrimer, we could not only enhance the device performance but also depress the long-wavelength emission of MEH-PPV.

Theoretical study on electronic structures and luminescent properties of phenol–pyridyl boron complexes

Electronic properties, such as HOMO and LUMO energies, band gaps and the ionization potentials ( I P) of five boron complexes containing mixed phenol–pyridine donor set, were studied by the density functional theory with Becke–Lee–Young–Parr composite exchange correlation functional (B3LYP). The absorption spectra of these complexes were also investigated by time dependent density functional theory (TDDFT) with 6-31G ∗ basis set. The calculated results indicated that the HOMO and LUMO of the complexes are both destabilized by electron-donating substituents (CH 3 and NH 2). Seen from I Ps, they all exhibit good hole-transporting ability, and their correlations with HOMO energies is linear. The TDDFT calculations showed that different substituents on the phenol–pyridyl unit lead to nominal change on the absorptions of these boron complexes. The calculations on the structural, electronic and optical properties of the compounds are in good agreement with the experimental data.