Layers In Electroluminescent Devices Research Articles

On the non-bonding valence band and the electronic properties of poly(triazine imide), a graphitic carbon nitride.

Graphitic carbon nitrides are covalently-bonded, layered, and crystalline semiconductors with high thermal and oxidative stability. These properties make graphitic carbon nitrides potentially useful in overcoming the limitations of 0D molecular and 1D polymer semiconductors. In this contribution, we study structural, vibrational, electronic and transport properties of nano-crystals of poly(triazine-imide) (PTI) derivatives with intercalated Li- and Br-ions and without intercalates. Intercalation-free poly(triazine-imide) (PTI-IF) is corrugated or AB stacked and partially exfoliated. We find that the lowest energy electronic transition in PTI is forbidden due to a non-bonding uppermost valence band and that its electroluminescence from the π-π* transition is quenched which severely limits their use as emission layer in electroluminescent devices. THz conductivity in nano-crystalline PTI is up to eight orders of magnitude higher than the macroscopic conductivity of PTI films. We find that the charge carrier density of PTI nano-crystals is among the highest of all known intrinsic semiconductors, however, macroscopic charge transport in films of PTI is limited by disorder at crystal-crystal interfaces. Future device applications of PTI will benefit most from single crystal devices that make use of electron transport in the lowest, π-like conduction band.

Optical properties of multinary copper chalcogenide semiconductor nanocrystals and their applications in electroluminescent devices

In the past few decades, semiconductor nanocrystals (semiconductor quantum dots) have received widespread attention due to their high color purity and size-dependent optical properties as well as their potential applications in the next generation of lightening and displays. At present, most of the emitting layer materials in the quantum-dot electroluminescent devices are focused on cadmium-based chalcogenide quantum dots, but cadmium is harmful to the environment and human health, which faces great challenges in the future commercialization. Therefore, it is necessary to develop an environment-friendly, cadmium-free semiconductor quantum dot with excellent optoelectronic performance. In recent years, multinary copper chalcogenide semiconductor nanocrystals have attracted extensive attention due to their low toxicity, tunable luminescent properties and potential applications in optoelectronic devices. Ternary copper-based chalcogenide semiconductor nanocrystals (such as CuInS2, etc.) can be considered as the counterparts of the II-VI compounds, in which the divalent metal ions (such as Zn2+, etc.) are replaced by monovalent copper ions (Cu+) and trivalent indium ions (In3+). As a result, the crystal structure of ternary I-III-VI nanocrystals is similar to that of II-VI nanocrystals. Up to date, ternary CuInS2 nanocrystals are the most widely studied among different multinary copper chalcogenide semiconductor nanocrystals. The crystal structure of ternary CuInS2 nanocrystals mainly includes sphalerite, wurtzite and chalcopyrite phase, and different-phase CuInS2 nanocrystals often have different optical band gaps. With the development of colloidal chemical synthetic technique, most of the multinary copper chalcogenide semiconductor nanocrystals can be synthesized through a colloidal chemical method. The reaction parameters including temperature, reaction precursor activity and surface ligand in the synthesis process play an important role in the tuning of the size, morphology and composition of the resultant nanocrystals. Generally, ternary CuInS2 nanocrystals have a low photoluminescence quantum yield due to their internal and surface defects. Since most of the multinary copper chalcogenide semiconductor nanocrystals exhibit non-stoichiometric characteristics, the number of copper vacancies also affects their luminescence performance to some extent. In order to improve the luminescence performance, it is a common strategy to introduce Zn2+ into the Cu-In-S system to form a quaternary Cu-In-Zn-S alloyed structure or grow a wide-band gap semiconductor material (such as ZnS) onto its surface to form a core-shell structure. However, due to higher ion mobility of copper ions at a relatively high temperature and the similar radius of Cu+ ion and Zn2+, cation exchange reaction is likely to occur in the epitaxial growth of ZnS shells. Therefore, it is difficult to distinguish the alloyed and core-shell structure for the multinary copper chalcogenide semiconductor nanocrystals. With the in-depth understanding of the relationship between the nanostructure and luminescence properties of multinary copper chalcogenide semiconductor nanocrystals, these nanocrystals have been widely used as light-emitting layers in electroluminescent devices, and a significant progress has been made. At present, different strategies have been exploited to improve the device performance of the electroluminescent devices based on multinary copper chalcogenide semiconductor nanocrystals, such as changing the device structure, regulating the nanostructure of materials, and device interfacial engineering. This review gives a detailed summary on the relationship between the composition, surface ligands, crystal structure as well as the nanostructures and optical properties of the multinary copper chalcogenide semiconductor nanocrystals. Moreover, the progress of electroluminescence devices based on the multinary copper chalcogenide semiconductor nanocrystals is highlighted, and finally the prospect of the development of multinary copper chalcogenide semiconductor nanocrystals is concluded.

Bright and efficient red emitting electroluminescent devices fabricated from ternary europium complexes

[Eu(btfa)3DPEPO] and [Eu(nta)3DPEPO] are bright, red emitting complexes. They have been incorporated into single and double emitting layer electroluminescent devices.

Optical properties of P- type polypyrrole thin film synthesized by pulse laser deposition technique: Hole transport layer in electroluminescence devices

Defect free, high-quality P-type Polypyrrole (PPy) thin film was deposited on ITO substrate at 150°C by Pulse Laser deposition (PLD) technique. Fourier Transform Infrared spectroscopy (FTIR) confirmed the PPy formation as a thin film without any cleavage in the functional group. A lower shift in vibration mode was observed for PPy thin film when compared to PPy powder confirming higher conductivity in both polarons and bipolarons region. Morphology of the thin film was uniform and smooth which was revealed by Scanning electron microscopy (SEM). Optical properties were studied systematically. 59.7% transmittance and lower band gap of 1.73 eV was observed for the thin film. The optical conductivity of the PPy thin film was 0.27 S/cm. Dielectric properties of the PPy thin film were investigated in detail. Room temperature photoluminescence spectra for PPy thin film exhibited three peaks at 373 nm, 425 nm, and 486 nm which was due to π→π* electronic transitions, polarons and bipolarons respectively. Higher luminescence efficiencies of PPy thin film can be used as good hole transport layer [HTL] in electroluminescence devices.

Solution processed and low-temperature annealed Eu3+-doped YVO4 thin films

Semiconducting and luminescent thin films could be served as the emitting layer in electroluminescent devices. In this work, Eu3+-doped yttrium vanadium oxide (YVO4) luminescent thin films were directly deposited by a solution processing method, and they were annealed at a low temperature of 400 °C. Such low annealed temperature is compatible with commonly used transparent conductive electrodes, such as tin-doped indium oxide and fluorine-doped tin oxide. In addition, the luminescent thin films have a band gap of 3.94 eV and an absolute photoluminescence quantum yield of 23%. These results revealed that Eu3+-doped YVO4 luminescent thin films have a high potential in electroluminescent devices.

Semi-fluorinated p-terphenyl liquid crystals

p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray diffraction analyses it was found that the p-terphenyls carrying two fluorine atoms develop nematic and/or smectic (A and C) phases. On the contrary, the p-terphenyls substituted with four fluorine atoms are not liquid crystals. Differences found in the thermal behavior are explained in terms π – πF interactions arising from the electron withdrawing ability of the fluorine atom. Such π – πF interactions let the p-terphenyls stack in a staggered configuration that explains the stability of the smectic C phase. On the other hand, the semi-fluorinated p-terphenyls absorb and emit in the blue region of the electromagnetic spectrum; a measured large Stokes shift indicates that these compounds are nearly transparent to their own emitted light. This optical characteristic, combined with liquid crystals properties of low order, makes these new p-terphenyls potential candidates for linearly polarized blue-emitting layers in electroluminescent devices.

Solution processed blue-emitting and hole-transporting materials from truxene-carbazole-pyrene triads

Abstract A new series of truxene derivatives containing different numbers of dipyrenylcarbazole substituents are prepared and evaluated as emissive and hole-transporting layers in electroluminescent devices. The compounds are synthesized from Cu-catalyzed C-N coupling between the iodotruxene core and 3,6-dipyrenylcarbazole and characterized by 1H NMR, 13C NMR and MALDI-TOF mass spectroscopy. These compounds exhibit strong fluorescence in the blue region and show excellent thermal stabilities with Td10% above 440 °C. They are tested as non-doped blue emitters in the OLEDs with the structure of ITO/PEDOT:PSS/EML/BCP/LiF:Al. The derivative containing two dipyrenylcarbazole moieties exhibits the highest maximum brightness of 8.0 × 103 cd/m2 at 9.6 V with a low turn-on voltage (Von) of 3.4 V. In the devices of structure ITO/PEDOT:PSS/HTL/Alq3/LiF:Al, the compound also shows superior hole-transporting properties in comparison with the standard compound (N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine). The device fabricated from this compound as the HTL shows the highest maximum brightness of 4.5 × 104 cd/m2 at 10.8 V with a low turn-on voltage (Von) of only 2.8 V.

A Host Material for Deep-Blue Electrophosphorescence Based on a Cuprous Metal–Organic Framework

Although metal–organic frameworks (MOFs) have promising applications in various fields, efforts to utilize MOFs as emissive layers in electroluminescent devices have not progressed well due to their intrinsic nature. Fortunately, by using fluorene derivatives and cuprous ions, a novel MOF (Cu-P6) has been synthesized for blue electrophosphorescence. The topological structure of Cu-P6 displays a 3-connected uninodal net with a cross-stacking pattern. Such a structure allowed phosphorescent Cu-P6 to exhibit a good heat resistance and luminous efficiency (Φ = 0.62) at crystalline state. By containing both donor and acceptor in the structure, Cu-P6 possesses a balanced electron and hole mobility. Especially, electrical-driven deep-blue emission of a MOF was reported for the first time. Overall, this work may open new perspectives on developing highly efficient electrophosphorescent devices based on MOFs. The utilization of earth-abundant cuprous ions is another attractive feature of Cu-P6.

Shortening Response Time of Light-Emitting Electrochemical Cell By Addition of Ionic Liquid into Iridium Complex Film

Light-emitting electrochemical cells (LECs) are single layer electroluminescent devices consisting of a luminescent material. A large number of LECs using Iridium(Ⅲ) as the metal core to emit various color variations have been evaluated recently. However, there are many problems about performance or stability. We have aimed the improvement of the performance and stability of LECs using Ir complex. So, we have synthesized yellow emission ionic transition-metal complex, Ir(ppy)2(dtbbpy)NTf2. The resulting product is confirmed by 1H NMR. The absorption and emission spectra of Ir(ppy)2(dtbbpy)NTf2 in acetonitrile solution at room temperature were measured. The absorption bands were observed in the range of 230-350 nm assigned to spin-allowed π-π* and in the range of 350-500 nm assigned to spin-forbidden MLCT, LLCT, LC transitions. The emission peak was observed at 558 nm using an excitation wavelength of 420 nm. The redox behavior was characterized by cyclic voltammetry. We fabricated light-emitting electrochemical cell using Ir(ppy)2(dtbbpy)NTf2. Electrogenerated chemiluminescence peak was observed at 556nm indicating yellow emission. Addition of ionic liquid, (1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide) into Ir(ppy)2(dtbbpy)NTf2 film in the mixing weight ratio of 4:1 makes response time decreased dramatically. And electroluminescence layer thickness was controlled by adjusting the concentration of Ir(ppy)2(dtbbpy)NTf2 in acetonitrile solution to shorten response time.

Colloidal nanocrystals for quality lighting and displays: milestones and recent developments

AbstractRecent advances in colloidal synthesis of nanocrystals have enabled high-quality high-efficiency light-emitting diodes, displays with significantly broader color gamut, and optically-pumped lasers spanning the whole visible regime. Here we review these colloidal platforms covering the milestone studies together with recent developments. In the review, we focus on the devices made of colloidal quantum dots (nanocrystals), colloidal quantum rods (nanorods), and colloidal quantum wells (nanoplatelets) as well as those of solution processed perovskites and phosphor nanocrystals. The review starts with an introduction to colloidal nanocrystal photonics emphasizing the importance of colloidal materials for light-emitting devices. Subsequently,we continue with the summary of important reports on light-emitting diodes, in which colloids are used as the color converters and then as the emissive layers in electroluminescent devices. Also,we review the developments in color enrichment and electroluminescent displays. Next, we present a summary of important reports on the lasing of colloidal semiconductors. Finally, we summarize and conclude the review presenting a future outlook.

Influences on the white emission and stability of single layer electroluminescent devices

A detailed survey about the influences on the white emission color of polyfluorene based polymer light emitting diodes (PLEDs) is reported. First, the effect of active layer thickness was studied. Subsequently keeping the polymer thickness at optimum level, PLEDs were fabricated varying polymer concentrations. All fabricated devices were fully characterized in terms of luminance, current–voltage characteristics, efficiencies, electroluminescent spectra, and CIE color coordinates. It was found that at higher polymer concentrations, electroluminescence spectra shifted to the bathochromic region so that the resulting color becomes warm white. Furthermore, the accelerated lifetimes of the PLEDs were measured and the results are discussed in terms of polymer inter-chain interactions. Consequently, the 8mg/ml was found to be the optimum level not only for the device performances but also for the device lifetime.

Preparation and Properties of Poly(phenazasiline) with Various Substituents

Poly(phenazasiline) (PPhenaz) with various substituents was prepared, and the substituent effect was discussed. PPhenaz was prepared by dehalogenative polycondensation of the corresponding dibromophenazasiline (Phenaz). The conductivity of an electrochemically doped PPhenaz film was 10-1–101 S cm-1. Using PPhenaz as a hole-transporting layer, a double layer electroluminescent (EL) device was fabricated. The properties of the EL device improved, when the substituent of PPhenaz was small. We also fabricated organic thin-film transistors (OTFTs) using PPhenaz films. The transistors showed p-type properties. The conductivity and the transistor properties were not dependent on the density of phenazasiline in PPhenaz, whereas the properties of PPhenaz-containing EL devices depended on the size of the substituent in PPhenaz.

Luminescent Properties of ZnS:Mn2+/ZnS Core/Shell Nanocrystals

High-quality ZnS:Mn2+/ZnS core/shell nanocrystals (NCs) with a core crystal diameter of 6.1 nm and 1.15 nm thick shells were synthesized via a high-boiling solvent process. The energy levels of the conduction band and valance band are estimated to be -3.2 eV and -6.8 eV by cyclic voltammetry and ultraviolet-visible (UV-vis) absorption spectra. The ZnS:Mn2+/ZnS NC emission peak is primarily located at 580 nm under 310 nm light excitation, originating from the charge transition from 4T1 to 6A1 within the 3d5 configuration of the Mn2+ ion. Based on ZnS:Mn2+/ZnS NCs as the active layer electroluminescent devices, the emission peak mainly locates at 460 nm with one shoulder emission peaking at 580 nm. The photoluminescence and electroluminescence properties of ZnS:Mn2+/ZnS NCs are investigated in the view of charge carrier injection and energy level alignment.

Synthesis of bifluorene-based molecular materials: effect of C-9 spirocyclohexane functionalization and end-group tailoring

The preparation and properties of the first spirocyclohexane-functionalized bifluorene-based monodispersed molecular materials is described. The obtained compounds were characterized by 1H NMR, 13C{ 1H} NMR, IR, DSC, UV–vis and photoluminescence both in solution and in the solid state. The molecules show emissions ranging from the blue to the blue-green region, and higher glass transition temperatures and spectral stability with respect to the analogous compounds containing 9,9,9′,9′-tetrahexyl-[2,2′]-bifluorene core. The materials were used as active layers in electroluminescent devices with ITO/PEDOT-PSS/ SB1– 4/Ca/Al and ITO/PEDOT-PSS/ SB1– 4/BCP/Ca/Al configurations.

Synthesis, Photophysical, and Electroluminescent Properties of Arylenevinylenes-co-pyrrolenevinylenes Derived from Divinylaryl Bridged Bispyrroles

Synthesis, characterization, and photophysical properties of four different π-conjugated oligomers containing alternate arylenevinylene and pyrrolenevinylene repeat units are reported. The optoelectronic properties are traced from the molecules to oligomers and to thin films and single layer electroluminescent (EL) devices. These oligomers exhibited strong absorption and emission corresponding to π−π* transitions which vary with electronic nature of the aryl moieties. The band gaps of these oligomers are in the range 1.84−2.04 eV and they emit in the orange to red region. The photoluminescence (PL) spectra of the oligomers P1−P4 in solution and film state are significantly different from each other. The time-resolved fluorescence decay in dichloromethane solution showed a fast decay profile for the anthracene-bridged oligomer and a relatively slow decay for benzene-bridged oligomer. Polymer light emitting diodes (PLEDs) with single active layer of oligomers with the configuration ITO/PEDOT:PSS/oligomer/Ca...

Electrical transport properties characterization of PVK (poly N-vinylcarbazole) for electroluminescent devices applications

Electrical transport properties of PVK (poly N-vinyl carbazole), a polymer exhibiting an intrinsic electroluminescence (EL) in blue region, have been investigated. In particular, resistivity and charge carrier mobility have been assessed by means of I–V measurements and by Field Effect (FET) doping, respectively. Dielectric constant has been also evaluated by means of C–V measurements performed on an ITO/PVK/Al single layer electroluminescent (EL) device. All electrical characterizations, carried out in different environmental conditions to deeply investigate PVK intrinsic conduction mechanisms, allowed to assess the good quality of spin coated PVK thin films. Electroluminescent measurements performed on ITO/PVK/Al and on ITO/PVK/Alq3/Al have been also performed. Finally, charge injection mechanisms from the electrodes to PVK have been investigated also as a function of the temperature. Experimental results have been compared with some well known theoretical models.

Synthesis and polymerization of a new nitrogen-containing bis-ortho-diynylarene monomer

This study focuses on the preparation, characterization, and optical properties of new bis(3,4-diphenylethynylphenyl)phenylamine. This is the first nitrogen-containing bis-ortho-diynylarene (BODA) monomer having a nitrogen atom as the spacer group. BODA monomers are usually prepared from common bisphenols, thereby providing great synthetic versatility and the opportunity to develop a wide array of novel polyarylene thermosets by varying the aromatic spacer group. The new bis(3,4-bisphenylethynylphenyl)phenylamine was synthesized in five steps. This compound emits an intense blue color (λ = 438 nm) upon irradiation by UV light and may be suitable for use as an emitting layer in electroluminescent devices. Bis-(3,4-bisphenylethynylphenyl)phenylamine and its polymer have photoluminescence quantum yields 34 and 38%, respectively, and long excited-state lifetimes of 3.2 and 3.6 ns, respectively. The structure of the monomer and its polymer were characterized using spectroscopic techniques including Ultraviolet–visible Spectrophotometer, Photoluminescence Spectrophotometer, Fourier Transform infrared spectroscopy, and Gel Permeation Chromatography. The polymerizations were studied by Differential Scanning Calorimeter. The amount of weight loss and the thermostability of the nitrogen-containing polymer were determined from thermogravimetric analysis. The electrical conductivity of neat HCl-doped BODA-derived polymer film was measured according to the standard four-point probe technique. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6988–6996, 2006

Synthesis, structure and properties of a novel quinoid compound

A novel quinoid compound α,α,α′,α′-tetra(4- tert-butylphenyl)-1,3,4-oxadiazole-quinodimethane (TPOQ) was synthesized through oxidative dehydrogenation in basic condition. The structure was characterized by elemental analysis, 1H NMR, and mass spectrum. UV–vis spectra, photoluminescence (PL) spectra, and electron spin resonance (ESR) measurements showed that the compound had a small bandgap and π–π * transition process produced an increase of polarity. Pure red emission at near 670 nm was observed from a single layer electroluminescence (EL) device, the emission color was unchanged with increase of applied voltage, which showed that TPOQ could be used as red emission material.

Charge mobilities and luminescence characteristics of blue-light emitting bent carbazole trimers connected through vinylene linkers—effect of nitrile substituents

Two new blue-light emitters, 3,6-bis(4-carbazolylstyryl)- N-4-methylphenylcarbazole (Cz 3) and 3,6-bis[4-(3,6-dicyanocarbazolyl)styryl]- N-4-methylphenylcarbazole (Cz 3(CN) 4), were found to form high glass-transition temperature ( T g), amorphous organic glasses, when vacuum deposited. Cz 3(CN) 4 exhibited a much higher T g value (242 °C) than Cz 3 (163 °C). It was found that Cz 3 is an excellent hole transporter ( μ h + = 1.3 × 10 −4 cm 2/V s, μ e − = 2.8 × 10 −8 cm 2/V s) while Cz 3(CN) 4 shows a faster electron mobility ( μ e − = 3.1 × 10 −6 cm 2/V s) than hole mobility ( μ h + = 2.0 × 10 −8 cm 2/V s), which is perceived to tell us that the presence of the electron-attracting nitrile (CN) groups facilitates the electron mobility. The single layer electroluminescence (EL) device (ITO/EML/Li:Al) of Cz 3(CN) 4 performed much better (external quantum efficiency ∼3.0 × 10 −2%) than that of Cz 3 (external quantum efficiency ∼1.2 × 10 −3%).

Light Emitting Devices from Transition Metal Complexes

Transition metal complexes have emerged as promising candidates for applications in solid state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. An efficiency of 10 lm/W –among the highest efficiencies in a single layer electroluminescent device– was recently demonstrated in an iridium complex. We present recent experimental results and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.