Materials For Organic Light-emitting Devices Research Articles

Low-temperature solvent-free synthesis route of blue and green luminescent phases of tris(8-hydroxyquinoline)aluminum (AlQ3)

Recent research studies have shifted towards simple, efficient, and more eco-friendly synthesis methods. The current study focuses on using a free-solvent reaction to synthesize trichelate 8-hydroxyquinoline (8HQ) octahedral Al(III) coordination complex (AlQ3), which still represents the most widely used electroluminescent material in organic light-emitting devices (OLEDs). We have developed a new safe and easy route for synthesizing AlQ3 via a pure solid-state reaction between 8HQ and AlCl3 with sodium carbonate as an inorganic base. The formation of the free-solvent prepared AlQ3 was confirmed using FT-IR, UV–Vis spectroscopy, SEM/EDS analysis, powder, and single crystal. Our investigation overturns the belief that fac-AlQ3 is only formed at high temperature; the developed synthesis method enabled the production of blue-fluorescent AlQ3 at room temperature. Furthermore, this study presents a low-temperature synthesis pathway for obtaining pure AlQ3 phases for the first time. We controlled the washing step of the isolated AlQ3 to produce large amounts of blue and green luminescent AlQ3. The optical properties of the samples were determined using photoluminescence (PL) spectroscopy.

Application of carbazole derivatives as a multifunctional material for organic light-emitting devices

The object of research is newly synthesized carbazole-derived compounds and organic light-emitting structures based on them. The problem lies in the complex solution of scientific and technical problems of improving the characteristics and stability of organic light-emitting diodes (OLEDs), namely improving the brightness and energy-efficient parameters. Organic light-emitting structures of blue, blue, and green radiation with color coordinates were formed by the thermovacuum sputtering method and the solution deposition method. The turn-on voltage of the white OLED is 6 V, the maximum brightness of the light-emitting structures was 10,000 cd/m2. The devices demonstrated a sufficiently high external quantum efficiency of 5 % to 7 %. This paper reports the multifunctional application of a simple donor-acceptor organic compound, as active and host material in the emission layer of organic light emitting devices. Em1 has been used as active components in OLEDs, where Em1 is the guest emitter (Device A), the acceptor part of the excited emitter (Device B) and the host matrix of the CdSeS/ZnS alloy quantum dot. At least four different OLEDs have been designed and characterized where Em1 plays the role of the guest emitter (Device C). The external quantum efficiencies of devices A–C are characterized by values common to pure fluorescent OLEDs (up to 5 % of the theoretical limit), but these devices sustain low-efficiency roll-off of electroluminescence over a wide range of current densities. Organic light-emitting diodes based on carbazole-derived compounds, due to their color characteristics, are promising candidates for use in the latest lighting systems. A separate advantage of the data light-emitting structures is a multifunctional application of one compound for different types of light-emitting structures. In addition, organic LEDs on based on carbazole-derived compounds have low energy consumption and are environmentally friendly due to the absence of toxic substances in their architecture, which creates prerequisites for saving energy resources and reducing the industrial burden on the environment.

Aggregation-Induced Emission Metallocuboctahedra for White Light Devices.

Materials for organic light-emitting devices which exhibit superior emission properties in both the solution and solid states with a high fluorescence quantum yield have been extensively sought after. Herein, two metallocages, S1 and S2, were constructed, and both showed typical aggregation-induced emission (AIE) features with intense yellow fluorescence. By adding blue-emissive 9,10-dimethylanthracene, pure white light emission can be produced in the solution of S1 and S2. Furthermore, due to the remarkable AIE feature and good fluorescence quantum yield in the solid state, metallocages are highly emissive in the solid state and can be utilized to coat blue LED bulbs or integrate with blue-emitting chips to obtain white light. This study advances the usage of metallocages as practical solid-state fluorescent materials and provides a fresh perspective on highly emissive AIE materials.

Determination of layer morphology of rough layers in organic light emitting diodes by X‐ray reflectivity

AbstractX‐ray reflectivity (XRR) has been proven to be a useful tool to investigate thin layers as well as buried interfaces in stacks built of very thin layers. Nevertheless, x‐ray reflectivity measurements are limited by the roughness of the layers and interfaces as the roughness destroys the interference structure, the so‐called Kiessig fringes. As investigations of thin layers in organic light emitting devices (OLEDs) are still subject of research and development, the focus of this paper is the investigation of a layer of indium tin oxide (ITO) which serves as transparent anode material in OLEDs. Due to the fabrication process, ITO shows rough surface structures, so‐called spikes, hindering the determination of the ITO layer thickness and roughness in XRR measurements. In this paper, it is theoretically and experimentally proven that a smoothing layer on the ITO enables the determination of the buried ITO layer thickness and roughness as well as the density of the spikes. Furthermore, a sputtered aluminum layer (e.g. cathode material) showing spikes in atomic force microscopy covered with a smoothing layer reveals Kiessig fringes allowing the determination of the density of buried spikes. In general, it is shown that a smoothing layer on a rough surface enhances the sensitivity of x‐ray reflectivity measurements.

Highly Efficient Electrofluorescence Material Based on Pure Organic Phosphor Sensitization**

AbstractPure organic room‐temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal complexes to fabricate highly efficient organic light‐emitting devices (OLEDs). However, applications of the reported RTP materials in OLEDs are seriously impeded by their low photoluminescence quantum yields (PLQYs) in a thin film state. To overcome these obstacles, we established a new strategy to construct highly efficient OLEDs based on a pure organic RTP material sensitized fluorescence emitter by selecting benzimidazole‐triazine molecules (PIM‐TRZ), 2,6‐di(phenothiazinyl)naphthalene (β‐DPTZN), and 5,6,11,12‐tetraphenylnaphthacene (rubrene) as host, phosphor sensitizer, and fluorescent emitter, respectively. The perfect combination of host, phosphorescent sensitizer, and fluorescent emitter in the emitting layer ensure the outstanding performance of the devices with an external quantum efficiency (EQE) of 15.7 %.

Highly Efficient Electrofluorescence Material Based on Pure Organic Phosphor Sensitization*.

Pure organic room-temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal complexes to fabricate highly efficient organic light-emitting devices (OLEDs). However, applications of the reported RTP materials in OLEDs are seriously impeded by their low photoluminescence quantum yields (PLQYs) in a thin film state. To overcome these obstacles, we established a new strategy to construct highly efficient OLEDs based on a pure organic RTP material sensitized fluorescence emitter by selecting benzimidazole-triazine molecules (PIM-TRZ), 2,6-di(phenothiazinyl)naphthalene (β-DPTZN), and 5,6,11,12-tetraphenylnaphthacene (rubrene) as host, phosphor sensitizer, and fluorescent emitter, respectively. The perfect combination of host, phosphorescent sensitizer, and fluorescent emitter in the emitting layer ensure the outstanding performance of the devices with an external quantum efficiency (EQE) of 15.7 %.

Dibenzofuran derivatives with meta- and para-triphenylamine substituents as hole-transporting materials in organic light-emitting devices

Three novel hole-transporting materials, 3,3'-(dibenzo[b,d]furan-2,8-diyl)bis(N,N-diphenylaniline) (BF-m-TPA), 4,4'-(dibenzo[b,d]furan-2,8-diyl)bis(N,N-diphenylaniline) (BF-p-TPA) and 4,4'-(dibenzo[b,d]furan-2,6-diyl)bis(N,N-diphenylaniline) (BF-2,6-TPA), were designed and synthesized. Owing to the rigid dibenzofuran core, these BF-TPA derivatives exhibited high thermal decomposition temperatures of over 395 °C and very high LUMO energy levels. Electroluminescent (EL) devices were fabricated using these three hole-transporting materials. The best device performance was obtained for BF-m-TPA, with the maximum luminance (L) of 15,230 cd/m2, luminance efficiency (LE) of 56.5 cd/A, power efficiency (PE) of 13.3 lm/W, and external quantum efficiency (EQE) of 16.3%.

Tetraphenylimidazole-based luminophores for explosive chemosensors and OLEDs: experimental and theoretical investigation

Four different small molecular weight tetraphenylimidazole (TPI) derivatives were synthesized and characterized. The photophysical properties, solvatochromism, fluorescence decays and electrochemical behaviours were discussed. All luminophores showed very high selectivity towards picric acid (PA) among all other nitroaromatics. All synthesized TPI luminophores showing good selectivity and high sensitivity, not only can detect PA in solution but can also be used in the contact mode for PA in thin-layer chromatography plates. The sensing mechanism was carefully studied by using nuclear magnetic resonance and density functional theory (DFT) studies. The optimal geometries and singlettriplet levels were calculated theoretically by using DFT and time-dependent DFT methods. Furthermore, examinations were carried out to know the optical, electronic, charge transport and stability properties of TPI derivatives as emissive and charge transport materials for organic light-emitting devices (OLEDs). Furthermore, all the TPI derivatives were used as emissive component for fabrication of OLED devices (I and II types) with multilayer configuration via solution process. The device configurations are ITO/PEDOT:PSS/CBP:TPI-1, TPI-2, TPI-3 and TPA-4/TPBi/LiF/Al and ITO/PEDOT:PSS/TPI-1, TPI-2, TPI-3 and TPA-4/TPBi/LiF/Al for doped (device II) and non-doped (device I) devices, respectively. Compared with all, the luminophores containing non-fluorinated linker (CH3-phenyl) showed deep-blue electroluminescence (Commission International de L'Eclairage [CIE], x = 0.20, y = 0.17), whereas the fluorinated linker (CF3-phenyl) containing luminophores exhibited bright-blue emission (CIE, x = 0.23, y = 0.21) with 1.4% external quantum efficiency.

Synthesis of acridone-naphthylamine derivative and its thermally-activated delayed fluorescence studies for application in OLEDs

Acridone (acceptor) and naphthylamine (donor) based Donor-Acceptor-Donor (D-A-D) compound (1) was synthesised, characterised and its thermally-activated delayed fluorescence (TADF) properties were studied in detail. Compound 1 is fluorescent and emits in the green region (550 nm). The energy gap between the ground and the lowest excited singlet (S1) state is estimated to be 2.55 eV. The energy gap between the CT singlet and triplet states (∆EST) was found to be ~0.3 eV. Small ∆ES1-T1 is one of the important criteria for TADF to take place in a molecule and thus detailed photophysics has been studied. Transient lifetime measurements showed an increase in the fluorescence lifetime (τ) on purging with N2, as compared with that in air-saturated solution, indicating the involvement of the triplet state in emission. Emission at 550 nm was also observed with a delay of 100 µs which corresponded to the delayed fluorescence in 1. The lifetime of TADF was found to be 176 µs. Applications of TADF materials in organic light-emitting devices (OLEDs) has gotten attention as TADF materials utilise the triplet excitons which helps in increasing internal quantum efficiency of device. Air-saturated based on 1 were fabricated and their intensity was found to be nearly as high as 17,000 Cd/m2 at 25 mA/cm2 which was comparable to many of the known TADF emitters. Acridone-amine derivatives was synthesized and its photophysical properties were carried out to establish TADF. It was also used in fabrication of OLEDs.

Periphery Design of Macrocyclic Materials for Organic Light-Emitting Devices with a Blue Phosphorescent Emitter.

Cyclo- meta-phenylenes were modified with trifluoromethyl groups at their periphery to create host materials suitable for use in blue phosphorescent organic light-emitting devices. The periphery design resulted in molecules with high triplet-state energies, which were required to support the blue emission from Ir phosphors. As a result, an external quantum efficiency of 9.9% was achieved. The most successful host, a pentameric congener, preferred CF-π/CH-π interactions in its crystalline packings, which could be beneficial for the host performance.

Electroluminescence Stability of Organic Light-Emitting Devices Utilizing a Nondoped Pt-Based Emission Layer.

We study the effects of using an emitting material (Pt(II) bis(3-(trifluoromethyl)-5-(2-pyridyl)pyrazolate—Pt(fppz)2) characterized by a preferred horizontal dipole alignment and a nearly unitary quantum yield regardless of concentration on the lifetime of organic light-emitting devices (OLEDs). Using such a material as a dopant in increasingly higher concentrations is found to lead to an increase in device stability, a trend that is different from that commonly observed with conventional OLED guests. The results are consistent with the newly discovered exciton–polaron-induced aggregation degradation mechanism of OLED materials. When this emitter is used as a neat emission layer, the material is already in a highly aggregated state, and the device is no longer affected by exciton–polaron interactions. The results demonstrate the potential stability benefits of using such materials in OLEDs.

Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices

Pyrimidine is an electron-deficient azaaromatic compound containing two nitrogen atoms at 1, 3-positions that plays a key role as an organic semiconductor or semiconducting material. Because of the high electron-accepting property induced by C═N double bonds and due to its coordination ability, pyrimidine has been incorporated as a building block in phosphorescent emitters, fluorescent emitters, bipolar host materials, and electron transporting materials in organic light-emitting devices (OLEDs). Recently, pyrimidine-based thermally activated delayed fluorescent emitters combined with various electron donors have been developed, and their device performances were far better than those based on conventional fluorescent emitters. In this review, recent progress of pyrimidine-based OLED materials is presented and accompanied by a historical overview, current status, key issues, and outlook for the next generation of high-performance OLED materials.

Synthesis and Optical Properties of Donor-Acceptor-Type Benzo[<i>b</i>]phosphole Oxides

Benzo[b]phosphole oxides exhibit attractive optical and electronic properties for applications as materials for organic light-emitting devices, photovoltaics and field-effect transistors. Recently, benzo[b]phosphole oxides introduced an electron-donating substituent have been emerged. These molecules show strong fluorescence and solvatochromism depending on the electron-donating substituent. This review highlights recent progress in developing donor-acceptor-type benzo[b]phosphole oxides and their photophysical properties.

Efficient green phosphorescent Ir(iii) complexes with β-diketonate ancillary ligands

Three new iridium complexes with β-diketonate ancillary ligands were synthesized as emissive materials for organic light-emitting devices.

Triplet-induced degradation: An important consideration in the design of solution-processed hole injection materials for organic light-emitting devices

Solution-processed organic light-emitting devices (OLEDs) still require improvements in their operational lifetime in order for them to become commercially viable. One factor that limits the lifetime of these devices is the instability of the hole injection layer (HIL). Therefore, understanding its degradation mechanism is crucial for the development of more stable solution-processed OLEDs. In this work, we use an archetypal fluorescent OLED in conjunction with an experimental solution-processed HIL in order to elucidate the degradation mechanism in these HILs. Our studies show that degradation is caused by triplet excitons. This new triplet-induced hole injection degradation is expected to be a common phenomenon in OLEDs, and therefore should have important implications for the design of stable HILs.

Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole.

Five-membered 1,3,4-oxadiazole (OZ) and 1,3,4-thiadiazole (TZ) heterocycle-based copolymers as active layer have long been ignored in solution-processable n-channel polymer field-effect transistors (PFETs) despite the long history of using OZ or TZ derivatives as the electron-injecting materials in organic light-emitting devices and their favorable electron affinities. Herein, we first report the synthesis and PFETs performance of two n-channel conjugated polymers bearing OZ- or TZ-based acceptor moieties, i.e., PNOZ and PNTZ, where simple thiophene units are utilized as the weak donors and additional alkylated-naphthalenediimides units are used as the second acceptors. A comparative study has been performed to reveal the effect of different heterocyclic acceptors on thermal properties, electronic properties, ordering structures, and carrier transport performance of the target polymers. It is found that both polymers possess low-lying LUMO values below -4.0 eV, indicating high electron affinity for both heterocycle-based polymers. Because of strong polarizable ability of sulfur atom in TZ heterocycle, PNTZ exhibits a red shift in maximal absorption and stronger molecular aggregation even in the diluted chlorobenzene solution as compared to the OZ-containing PNOZ. Surface morphological study reveals that a nodule-like surface with a rough surface morphology is observed clearly for PNOZ films, whereas PNTZ films display highly uniform surface morphology with well interconnected fiber-like polycrystalline grains. Investigation of PFETs performance indicates that both polymers afford air-stable n-channel transport characteristics. The uniform morphological structure and compact π-π stacking endow PNTZ with a high electron mobility of 0.36 cm2 V-1 s-1, much higher than that of PNOZ (0.026 cm2 V-1 s-1). These results manifest the feasibility in improving electron-transporting property simply by tuning heteroatom substitutes in n-channel polymers; further demostrate that TZ derivatives possess much superior potential for developing high-performance n-channel polymers compared to OZ derivatives.

Synthesis and Property of New Propeller Shaped Emitting Materials for Organic Light-Emitting Devices.

New propeller type emitting compound, namely 3,6-di-anthracen-9-yl-9,10-bis-(4-anthracen-9-yi-phenyl)-phenanthrene[TAnDAP] and 3,6-bis-(10-phenyl-anthracen-9-yl)-9,10-bis-[4-(10-phenyl-anthracen-9-yl)-phenyl]-phenanthrene [TAnPDAP] were synthesized through Suzuki and McMurry reactions. We investigated their physical properties such as optical, electrochemical, and electroluminescent properties. The two compounds were used as an emitting layer in OLED devices: ITO/2-TNATA (60 nm)/NPB (15 nm)/non-doped: TAnDAP or TAnPDAP (35 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (200 nm). The TAnDAP OLED device showed C.I.E. value of (0.28, 0.41) and luminance efficiency of 3.81 cd/A at 10 mA/cm2. The TAnPDAP device showed C.I.E. value of (0.20, 0.27) and high luminance efficiency of 5.40 cd/A at 10 mA/cm2. TAnPDAP was found to show better luminance efficiency and C.I.E. value than TAnDAP because it has a bulky 9-phenylanthracene.

High thermal stability fluorene-based hole-injecting material for organic light-emitting devices

Novel N1,N3,N5-tris(9,9-diphenyl-9H-fluroen-2-yl)-N1,N3,N5-triphenylbenzene-1,3,5-triamine (TFADB) was synthesized and characterized as a hole-injecting material (HIM) for organic light-emitting devices (OLEDs). By incorporating fluorene group TFADB shows a high glass-transition temperature Tg>168°C, indicative of excellent thermal stability. TFADB-based devices exhibited the highest performance in terms of the maximum current efficiency (6.0cd/A), maximum power efficiency (4.0lm/W), which is improved than that of the standard device based on 4-4′-4″Tris(N-(naphthalene-2-yl)-N-phenyl-amino)triphenylamine (2T-NATA) (5.2cd/A, 3.6lm/W). This material could be a promising hole-injecting material, especially for the high temperature applications of OLEDs and other organic electronic devices.

Highly efficient non-doped blue electroluminescent materials for organic light-emitting devices

Three new born blue light emitting materials, 1-naphthalen-1-yl-, 4-methylnaphthalen-1-yl- and 4-methoxynaphthalen-1-yl- based phenanthro[9,10]imidazole-2-yl-1,1′-biphenyl derivatives through two step procedure using inexpensive catalysts.

Fused Methoxynaphthyl Phenanthrimidazole Semiconductors as Functional Layer in High Efficient OLEDs.

Efficient hole transport materials based on novel fused methoxynaphthyl phenanthrimidazole core structure were synthesised and characterized. Their device performances in phosphorescent organic light emitting diodes were investigated. The high thermal stability in combination with the reversible oxidation process made promising candidates as hole-transporting materials for organic light-emitting devices. Highly efficient Alq3-based organic light emitting devices have been developed using phenanthrimidazoles as functional layers between NPB [4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl] and Alq3 [tris(8-hydroxyquinoline)aluminium] layers. Using the device of ITO/NPB/4/Alq3/LiF/Al, a maximum luminous efficiency of 5.99cd A(-1) was obtained with a maximum brightness of 40,623cdm(-2) and a power efficiency of 5.25lmW(-1).