Hole-transporting Units Research Articles

Probing Electronics as a Function of Size and Surface of Colloidal Germanium Nanocrystals

Inorganic semiconductor nanoparticles are of significant interest for applications that benefit from their size-dependent properties. The work presented here focuses on the characterization of solution-based microwave synthesized Ge nanocrystals (NCs). Three differently capped Ge NCs were investigated: oleylamine (OAM), dodecanethiol (DDT), and a functionalized N4,N4,N4′,N4′-tetraphenylbiphenyl-4,4′-diamine (TPD) ligand, which is commonly used as hole-transporting units. The optical gaps followed the expected trend for quantum confinement; however, the absolute value depended upon the ligand. We found that the DDT-capped Ge NCs feature consistently larger bandgaps than OAM-capped Ge NCs of a similar size. Cyclic voltammetry (CV) was used to determine the valence band energy for OAM-capped Ge NCs, and the conduction band energy was estimated from the optical gap. By contrast, DDT-capped Ge NCs and the OAM/DDT-capped Ge NCs did not exhibit an oxidative signal in the cyclic voltammetry. This was attributed to the removal of surface defects of OAM-capped Ge NCs through stronger Ge–S surface bonds. TPD-capped Ge NCs were investigated and showed a shift to slightly higher oxidation potential compared with the free ligand and bandgap values in between that of the OAM-capped and DDT-capped Ge NCs. The higher oxidation potential is attributed to TPD orientation, and the bandgap value reflects the lower number of Ge–S bonds on the surface due to ligand sterics.

Synthesis and photovoltaic performance of N-dioctylmethyl-2,7-carbazole-alt-5,7-bis(thiophen-2-yl)-2,3-biphenylthieno[3,4-b] pyrazine copolymeric derivatives appending various donor units in phenyl moieties

A series of N-dioctylmethyl-2,7-carbazole-alt-5,7-bis(thiophen-2-yl)-2,3-biphenylthieno[3,4-b]pyrazine (PCz-3ThPz-Ph) copolymeric derivatives appending various donor units in two phenyl rings, namely, PCz-3ThPz-PhTh, PCz-3ThPz-PhFl, PCz-3ThPz-PhCz, and PCz-3ThPz-PhTpa were synthesized and characterized. The effect of these appending donor units, e.g., thiophene (Th), fluorine (Fl), carbazole (Cz), and triarylamine (Tpa), was investigated on dispersible, optical, electrochemical, and photovoltaic properties for their polymers. The copolymers of PCz-3ThPz-PhCz and PCz-3ThPz-PhTpa containing Cz or Tpa units exhibited higher short-circuit current density (Jsc) and power conversion efficiency (PCE) in their bulk heterojunction polymeric solar cells. The highest PCE of 1.66% and Jsc of 7.16 mA cm−2 were obtained in a device with the PCz-3ThPz-PhCz/PC61BM blend under AM 1.5 G irradiation (100 mW cm−2); these values are 1.78 and 1.59 times higher than the corresponding values for the PCz-3ThPz-Ph-based device. When PC61BM was placed by PC71BM, the PCz-3ThPz-PhCz-based device displayed an enhanced PCE of 2.98% and a Jsc of 10.88 mA cm−2. This work demonstrated that appending additional hole-transporting units of Cz and Tpa into the side-chain of a polymer with a D-A backbone can significantly enhance the photovoltaic performance of their resultant polymers.

Heteroleptic copper(i) sensitizers with one versus two hole-transporting units in functionalized 2,9-dimethyl-1,10-phenanthroline ancillary ligands

Tuning of phen-based ancillary ligands in [Cu(Lanchor)(Lancillary)]+ dyes leads to DSCs with efficiencies of 33.9% relative to N719 (set at 100%).

Highly Efficient Bipolar Host Materials with Indenocarbazole and Pyrimidine Moieties for Phosphorescent Green Light-Emitting Diodes

In this paper, we report bipolar host materials, 6-(2,6-diphenyl-pyrimidin-4-yl)-12,12-dimethyl-6,12,12′-trihydroindeno[1,2-b]carbazole (DPICz1) and 6-(2,6-diphenyl-pyrimidin-4-yl)-12,12-dimethyl-6,12,12′-trihydroindeno[1,2-b]carbazole (DPICz2), with indenocarbazole moiety as a hole-transporting unit and pyrimidine moiety as an electron-transporting unit for highly efficient green phosphorescent organic light-emitting diodes. The synthesized materials show excellent electro-optical properties. Similarly, it also exhibits outstanding thermal and morphological stability due to high glass transition temperature (Tg) of 161 °C and decomposition temperature (Td) of 327–401 °C. Fabricated green phosphorescent organic light-emitting diode shows excellent current and power efficiency of 77.8 cd/A and 62.8 lm/W as well as almost ideal external quantum efficiency of 22.3% at the brightness of 1000 cd/m2.

Effect of arylamine hole-transport units on the performance of blue polyspirobifulorene light-emitting diodes

The operation of blue light-emitting diodes based on polyspirobifluorene with a varying number of N,N,N′,N′ tetraaryldiamino biphenyl (TAD) hole-transport units (HTUs) is investigated. Assuming that the electron transport is not affected by the incorporation of TAD units, model calculations predict that a concentration of 5% HTU leads to an optimal efficiency for this blue-emitting polymer. However, experimentally an optimum performance is achieved for 10% TAD HTUs. Analysis of the transport and recombination shows that polymer light-emitting diodes with 5%, 7.5%, and 12.5% TAD units follow the predicted behavior. The enhanced performance of the polymer with 10% TAD originates from a decrease in the number of electron traps, which is typically a factor of three lower than the universal value found in many polymers. This reduced number of traps leads to a reduction of nonradiative recombination and exciton quenching at the cathode.

Synthesis and Photophysical Properties of Charged Iridium(III) Cyclometallates Based on Hole-Transporting Carbazole Unit

Four novel charged Ir (III) cyclometallates based on 9-phenylcarbazole unit have been developed. By attaching isoquinoline or 5-trifluoromethylpyridine group, the light-emitting properties of the resulting Ir (III) cyclometallates have been improved. Meanwhile, 1,10-phenanthroline or 4,4′-dimethyl-2,2′-bipyridine has been utilized to modify the thermal stability. The four Ir (III) cyclometallates emit yellow light with the highest photoluminescence quantum yields (ΦP) of 0.32 (C4), which is only less than that of the state-of-art complexfac-tris (2-phenylpyridine) iridium (III) [fac-Ir (ppy)3], which showsΦPof 0.40 under the same condition. This inspiring result reveals that the bulky character of CF3group enhances the performance of reported charged Ir (III) cyclometallates and makes them to be potential phosphors in OLED applications.

Multifunctional homoleptic iridium(III) dendrimers towards solution-processed nondoped electrophosphorescence with low efficiency roll-off

A series of new iridium dendrimers comprised of bifunctional charge transport peripheral groups have been designed and facilely synthesized. The relationship between the structures and their photophysical, electrochemical and electrophosphorescent performances is investigated. Through the incorporation of rigid electron-transporting phosphine oxide groups and/or hole-transporting arylamine units, the new complexes all have good thermal stabilities with high glass-transition temperature up to 284°C. Besides, the peripheries sufficiently shield the emissive core from the intermolecular interactions and prevent luminance quenching in neat films. Solution-processed phosphorescent organic light-emitting device (PhOLED) based on bipolar phosphor 2 as neat emitter achieves a maximum current efficiency of 12.4cdA−1 with Commission Internationale de l’Eclairage coordinates of (0.57, 0.42), and the value remains at 11.5cdA−1 at a practical luminance of 1000cdm−2. This low roll-off can be attributed to the bipolar nature of the emitter. This indicates that rational incorporation of charge-transporting moieties into the sphere of iridium(III) core is a simple and effective approach to develop efficient host-free phosphors for solution-processable nondoped PhOLEDs.

An indole derivative as a high triplet energy hole transport material for blue phosphorescent organic light-emitting diodes

A thermally stable high triplet energy material derived from an indoloacridine core and indole hole transport units, 8,8-bis(4-(1H-indol-1-yl)phenyl)-8H-indolo[3,2,1-de]acridine (BIPIA), was synthesized as the hole transport material for deep blue phosphorescent organic light-emitting diodes. The BIPIA hole transport material showed a high triplet energy of 2.95eV and high glass transition temperature of 142°C. A high quantum efficiency of 19.3% was obtained in the deep blue device using BIPIA as the high triplet energy hole transport material.

Novel Cathode Interlayers Based on Neutral Alcohol‐Soluble Small Molecules with a Triphenylamine Core Featuring Polar Phosphonate Side Chains for High‐Performance Polymer Light‐Emitting and Photovoltaic Devices

A new family of neutral alcohol-soluble small molecular materials comprised of electron-rich triphenylamine (TPA) and fluorene featuring phosphonate side chains (FEP) is reported, namely 3TPA-FEP, 2TPA-2FEP and TPA-3FEP, which have different TPA and FEP contents. Due to their good solubility in polar solvents like alcohol, multilayer devices can be fabricated by a wet process from orthogonal solvents. Polymer light-emitting devices with these materials as a cathode interlayer and Al as the cathode show greatly enhanced efficiencies in contrast to control devices without such a cathode interlayer, and their efficiencies are comparable with or even higher than devices with the low work-function metal Ba/Al as the cathode. In addition, high-performance polymer solar cells based on the poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C71 -butyric acid methyl ester (PC71 BM) system are also achieved with power conversion efficiencies of 7.21%, 6.90% and 6.89%, by utilizing 3TPA-FEP, 2TPA-2FEP and TPA-3FEP as the cathode interlayer, respectively. These efficiencies are also much higher than those for control devices without the cathode interlayer. Although TPA is well-known as a hole-transport unit, the current findings indicate that alcohol-soluble TPA-based small molecules are also a promising cathode interlayer for both electron injection and extraction.

Synthesis and photoluminescence properties of rhenium(i) complexes based on 2,2′:6′,2′′-terpyridine derivatives with hole-transporting units

Based on 2,2′:6′,2′′-terpyridine ligands (L1), five terpyridine derivatives, namely 4′-carbazol-9-yl-2,2′:6′,2′′-terpyridine (L2), 4′-diphenylamino-2,2′:6′,2′′-terpyridine (L3), 4′-bis(4-tert-butylphenyl)amino-2,2′:6′,2′′-terpyridine (L4), 4′-[naphthalen-1-yl-(phenyl)amino]-2,2′:6′,2′′-terpyridine (L5), 4′-[naphthalen-2-yl(phenyl)amino]-2,2′:6′,2′′-terpyridine (L6) and their corresponding Re(I) complexes ReL(n)(CO)3Cl (n = 1–6) have been synthesized and characterized by elemental analysis and 1H NMR spectroscopy. The X-ray crystal structure of ReL3(CO)3Cl has also been obtained. The luminescence spectra of ReL2(CO)3Cl–ReL5(CO)3Cl, obtained in CH2Cl2 solution at room temperature, show strong dπ (Re) → π* (diimine) MLCT character (λ(max) 600 nm) and a small red shift relative to ReL1(CO)3Cl. This, confirmed by the study of the triplet energy levels of the L1–L6 ligands at low temperature (77 K rigid matrix), indicates that the introduction of electron-donating moieties on the terpyridine unit decreases the triplet levels of the ligands, leading to a reduction of the energy gap between d and π* orbitals. In the solid state, upon MLCT excitation, all the complexes show an even stronger emission and a blue spectral shift (λ(max) ∼ 550 nm) compared to those obtained in solution.

Synthesis and Emission Behavior of Liquid-Crystalline Main-Chain Polyesters Containing Carbazole and Oxadiazole Moieties

Organic light-emitting diodes (OLEDs) based on chromophore-containing liquid crystals (LCs) can emit polarized light due to self-assembly of mesogenic chromophores. The stability of LC phases in LC polymers with a mesogenic group in the main chain is higher than that in the polymers containing a mesogenic group in the side chain. In this study, a novel family of polyesters containing a carbazole moiety as a hole-transporting unit and an oxadiazole (OXD) moiety as an electron-transporting unit in the main chain, P10-DOXDCz and P16-DOXDCz, were designed and synthesized. These polyesters were prepared by transesterification of N-hexyl-3,6-di[2-([1,3,4]oxadiazol-5-yl)-4-methylbenzoate]carbazole and 1,10-decanediol or 1,16-hexadecanediol at 220 °C for 20 h. They showed a nematic phase. Both P10-DOXDCz and P16-DOXDCz emitted intense blue fluorescence with a maximum at 439 nm and 450 nm in thin films, respectively, and their fluorescence quantum yields were 0.74 and 0.81 in solution, respectively. Both compounds exhibited polarized emission in a LC phases, and the order parameters estimated from the polarized emission spectra were 0.10 for P10-DOXDCz and 0.14 for P16-DOXDCz. Fabricated devices, ITO/PEDOT:PSS/P10-DOXDCz and P16-DOXDCz/MgAg, emitted blue light when a positive voltage was applied to the ITO electrode.

New tetraphenylethylene-containing conjugated polymers: Facile synthesis, aggregation-induced emission enhanced characteristics and application as explosive chemsensors and PLEDs

In this paper, tetraphenylethylene (TPE) units, one of the typical aggregation-induced emission (AIE) moieties, are utilized to construct a new functional polyfluorene (PF) P1, which exhibited the exciting property of the aggregation-induced emission enhancement (AIEE), instead of the aggregation-caused quenching (ACQ) of normal PFs, and could probe the explosive with high sensitivity both in the nanoparticles and solid state. Three other TPE-containing polymers, P2–P4, were also successfully prepared, and demonstrated good performance as explosive chemosensors and light-emitting materials. P3, bearing carbazole as hole-transporting units showed the best performance with a maximum luminance efficiency of 1.17 cd/A and a maximum brightness of 3609 cd/m2 at 12.9 V in its light-emitting diode device.

Mesogen-jacketed liquid crystalline polymers: from molecular design to polymer light-emitting diode applications

This critical review covers the recent progress in the chemical structure design and light-emitting performance evaluation of electroluminescent mesogen-jacketed liquid crystalline polymers (MJLCPs). By applying the concept of MJLCPs, two main chromophoric building units, electron-transporting and hole-transporting units with large steric hindrance, are attached to the backbones solely or together. We highlight the structure–properties–performance relationships of electroluminescent MJLCPs. Device performances can be enhanced by the “jacketing effect”, which inhibits the aggregation of conjugated structures, fluorescence quenching, and excimer formation.

Quantitative analysis of the guest-concentration dependence of the mobility in a disordered fluorene-arylamine host-guest system in the guest-to-guest regime

The charge transport in a polyspirobifluorene derivative with copolymerized N,N,N′,N′-tetraaryldiamino biphenyl (TAD) hole transport units is investigated as a function of the TAD content. For TAD concentrations larger than 5%, guest-to-guest transport is observed. It is demonstrated that in this regime the charge carrier density dependent mobility can be described consistently with the extended Gaussian disorder model, with a density of hopping sites which is proportional to the TAD concentration and comparable to the molecular density.

Improved OPV Efficiency of Fluorene-Thiophene-Based Copolymers with Hole- and Electron-Transporting Units in the Main Chain

In this paper, we have synthesized a series of fluorene-thiophene-based copolymers by Suzuki coupling polymerization for the use of donor materials in organic photovoltaic cells (OPVs). New conjugated polymers have an electron-deficient moiety such as benzothiadiazole (BT), oxadiazole (OX) and triazole (TZ) or an electron-rich triphenylamine (TPA) moiety or both in the main chains as third or fourth comonomers. The photovoltaic properties of the polymers were varied by introduced electron- or hole-transporting units. The order in the efficiency of the photovoltaic cells was P1 (BT) < P2 (TPA) < P3 (TPA, BT) < P4 (TPA, OX) < P5 (TPA, TZ). The highest power conversion efficiency (PCE) of 0.33% (P5) with an open-circuit voltage (V OC) of 0.76 V and a short circuit current (J SC) of 1.43 mA/cm2 was achieved by device annealing at 80°C. The relationship between molecular structures and photovoltaic properties was systematically established herein.

The β Phase Formation Limit in Two Poly(9,9-di-n -octylfluorene) based Copolymers

Random copolymers of poly(9,9-di-n-octylfluorene) (PF8) incorporating 0, 8, 12, 15, and 20% dibenzothiophene (DBT), and copolymers with 2, 5, 8, 12, and 15% dibenzothiophene-S,S-dioxide (S-unit) were synthesised. Absorption and emission spectra of thin films indicate that the DBT system shows a linear decrease of toluene vapour induced β phase with increasing DBT content to a 20% cutoff, whilst in the S-unit copolymers the β phase is present up to 12% co-monomer content, and at 15% the characteristic absorption peak is absent or masked. These results demonstrate the limits, in thin films, at which the β phase can be formed in widely used PF8 copolymer systems for device applications and clearly show that it is practical to use copolymers having electron or hole transport units in the polyfluorene backbone and still be able to form efficient β phase emission sites.

Electroluminescent Properties of an Electrochemically Cross‐Linkable Carbazole Peripheral Poly(Benzyl Ether) Dendrimer

The electroluminescent (EL) properties of a cross-linkable carbazole-terminated poly(benzyl ether) dendrimer, G(3)-cbz DN, doped into a PVK:PBD host matrix with a double-layer device configuration are investigated. Different concentrations of the guest material can control device efficiency, related to chromaticity of white emission and the origin of excited-state complexes occurring between hole-transporting carbazole units (PVK or G(3)-cbz DN) and electron-transporting oxadiazole (PBD). Two excited states (exciplex and electroplex) generated at the interfaces of PVK/G(3)-cbz DN and PBD result in competitive emission, exhibiting a broad band in the EL spectra.

Synthesis, Characterization and Photovoltaic Properties of Di-Anchoring Organic Dyes

Three new organic dyes comprising carbazole, iminodibenzyl and phenothiazine moieties, as electron donors and di-anchoring rhodanine rings as the electron acceptors, were synthesized and evaluated for use in dye-sensitized solar cells. A solar cell employing dye-containing phenothiazine as a hole-transporting unit and di-anchoring rhodanine rings as the electron acceptors exhibits a short circuit photocurrent density of 10.6 mA cm-2, an open-circuit voltage of 0.658 V and a fill factor of 0.7, corresponding to an overall conversion efficiency of 4.91% at standard AM 1.5 sunlight.

Synthesis and characterization of a novel bipolar Alq3-based copolymer containing carbazole and phenothiazine groups

A novel bipolar copolymer combining hole-transporting units, electron-transporting units and light-emitting units (Alq3) had been synthesized via free radical copolymerization of N-vinylcarbazole and Alq3 monomer containing phenothiazine group. The chemical structure and composition of the copolymer were characterized by 1H NMR, 13C NMR, FT-IR spectroscopy, and elemental analysis as well as gel permeation chromatography (GPC). The number-average molecular weight (Mn) and polydispersity of these copolymers with different Alq3 contents were around 20000 and 1.4, respectively. The DSC and TGA measurements indicated that the copolymer has excellent thermal stability and high glass transition temperatures (Tg). The optical properties of the copolymer in solution and solid state were investigated by UV-vis and photoluminescence (PL) spectra. Additionally, the effects of Alq3 content, concentration, excitation wavelength and solvent on PL spectra were studied and the results indicated that there exists efficient energy transfer in the process of photoluminescence. The introduction of carbazole and phenothiazine, as hole-transporting groups and energy transfer stages, provides an effective way to improve carries transporting property and energy transfer efficiency, consequently improving luminescence properties of Alq3-based copolymer.

Dipyrenylcarbazole Derivatives for Blue Organic Light‐Emitting Diodes

Blue moon: Three new light-emitting materials, comprising highly fluorescent pyrenes with hole-transporting carbazole units, have high thermal stabilities and optoelectronic properties that are suitable for application in electroluminescent devices. A device fabricated from 1 as an emissive material exhibits the most-promising I-V-L characteristics as a blue organic light-emitting diode.