Layer In Organic Light-emitting Devices Research Articles

1,2,4-triazine derived binuclear lead(ii) complexes: synthesis, spectroscopic and structural investigations and application in organic light-emitting diodes (OLEDs).

Two novel binuclear complexes of Pb(ii) were synthesized by reacting a 3-(2-pyridyl)-5-(4-methoxyphenyl)-1,2,4-triazine (PMPT) ligand with different anionic co-ligands (1: bromide, 2: acetate and isothiocyanate) in a 1 : 1 molar ratio of PMPT ligands to lead(ii) salts. The complexes, [Pb2(μ-PMPT)2Br4] (1) and [Pb2(μ-PMPT)2((μ-CH3COO)2(NCS)2] (2), were characterized using various physicochemical techniques such as CHN analysis, FT-IR spectroscopy, and 1H NMR spectroscopy. Additionally, their structures were determined using single-crystal X-ray diffraction. Based on the obtained structural parameters, complex 1 exhibited a PbN3Br2 environment, while complex 2 displayed a PbN4O3 environment, with holodirected and hemidirected coordination spheres, respectively. Within the crystal network of the complexes, there were interactions involving C-H⋯X (X: O, S, N) as well as π-π stacking. The Pb(ii) complexes were further investigated for their potential use as the emitting layer in organic light-emitting devices (OLEDs). The current-voltage and luminescence-voltage characteristics, as well as the electroluminescence (EL) properties of the complexes, were studied.

Thermally activated delayed fluorescence tetradentate ligand-containing gold(III) complexes with preferential molecular orientation and their application in organic light-emitting devices.

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (kr) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE(S1-T1), of these complexes were estimated to be in the range of 0.16-0.18 eV. Through strategic modification of the position of the electron-donating -tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (ηout), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

Hole Injection Role of p-Type Conjugated Polymer Nanolayers in Phosphorescent Organic Light-Emitting Devices

Here, we report the hole injection role of p-type conjugated polymer layer in phosphorescent organic light-emitting devices (OLEDs). Poly(3-hexylthiophene) (P3HT) nanolayers (thickness = ~1 nm thick), which were subjected to thermal annealing at 140 °C by varying annealing time, were inserted between indium tin oxide (ITO) anodes and hole transport layers (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine, NPB). The 1 nm-thick P3HT layers showed very weak absorption in the visible light range of 500~650 nm. The device results disclosed that the presence of P3HT layers were just able to improve the charge injection of OLEDs leading to an enhanced luminance irrespective of thermal annealing condition. The highest luminance and efficiency were achieved for the OLEDs with the P3HT layers annealed at 140 °C for 10 min. Further annealing for 30 min resulted in turn-down of device performances. The emission color was almost unchanged by the presence of P3HT layers even though the color coordinates were marginally fluctuated according to the annealing time. The present result delivers the possibility to use p-type conjugated polymers (i.e., P3HT) as a hole injection layer in OLEDs.

Thermally Activated Delayed Fluorescence beyond Through‐Bond Charge Transfer for High‐Performance OLEDs

AbstractCharge transfer plays an important role in ruling the exciton dynamics of organic emitters. Most likely, intermolecular/intramolecular charge transfers occur when a donor meets a matched acceptor, which shapes the radiative decays of excitons. The generation and utilization of excitons are vital for organic light‐emitting devices (OLEDs). Thermally activated delayed fluorescence (TADF) emitters, which harvest the triplets via reverse intersystem crossing and thus have attracted intensive attention in the last decade, are very promising to address the issue of cost‐effectiveness in the commercialized OLEDs. Conventionally, intramolecular charge transfer via through‐bond interaction is ubiquitous in the state‐of‐the‐art TADF materials. Alternatively, through‐space charge transfer via either intermolecular or intramolecular interaction is found to be unique and effective in modulating the luminescent properties, such as the emissive colors, exciton lifetimes, and quantum yields. The emerging approaches to evoke the TADF mechanisms beyond through‐bond charge transfer in a single molecule and the related applications of the emitters in OLEDs are highlighted here. The timely and fashionable molecular design and device engineering regarding through‐space charge transfer to the emitting layers in OLEDs are compared and discussed. Finally, a conclusion and the perspectives of the electroluminescence of novel through‐space systems are proposed.

Role of new synthesized diethylenetriamine-based protic ionic liquids in organic semiconductor devices

The charge carrier transport and related transport layers are extremely essential issue for obtaining high-efficient Organic Light Emitting Devices (OLEDs) and high-performance Organic Field Effect Transistor (OFETs). In an attempt to light on the effects of diethylenetriamine (DETA) based protic ionic liquids (PILs) on OLEDs and OFET, PILs were used as a hole transport layer and dielectric layer in OLEDs and OFETs, respectively. Charge transfer properties of synthesized two novel DETA based PILs (PIL1 and PIL2) on OLED device performance were investigated by spinning on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) due to their ionic characteristics. The PIL2 based OLED device showed threefold luminance (1038 cd/m2) compared to reference device (360 cd/m2) without effecting its light output emission wavelength. When analyzed OFET performances, device with PIL1 possessed the highest performance in terms of low threshold voltage (VTh∼1.5 V), operating voltage (V∼5 V) and reproducibility in the ambient atmosphere in comparison to device without PILs. Moreover, Density Functional Theory (DFT) calculations and impedance spectroscopic analysis of ionic liquids (ILs) were investigated. The carrier transport properties and mobility measurement has been identified with impedance spectroscopy of the devices. PIL1 had p-type transport property with mobility up to 0.198 cm2/Vs while the device without PILs had mobility 0.0237 cm2/Vs.

Thermally activated delayed fluorescent polymer- assisted morphological control on perfluorinated ionomer enriched surface and exciton harvesting for phosphorescent organic light-emitting devices

Poly(3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) is one of the most successful conductive polymers, usually used as hole injecting layer in organic light-emitting devices. Due to the enhanced conductivity and tunable work function (WF), the modification of PEDOT:PSS with some additives has been proved to be cost-effective. Perfluorinated ionomer (PFI) has been reported as an effective additive to improve the WF of PEDOT:PSS, increasing from 5.2 eV to 5.95 eV with a recipe of PEDOT(1):PSS(6):PFI(25). However, the strong hydrophobicity of PFI has limited the choice of the organic solvents used to fabricate the uniform emitting layer on the modified PEDOT:PSS. To address this issue, the polymers were introduced as the additives of the emitter to overcome the non-uniform morphology on the hydrophobic surface of PEDOT:PSS:PFI. A conventional fluorescent polymer poly(9-vinylcarbazole) (PVK) and a thermally activated delayed fluorescent polymer (4-(2-(9-(heptadecan-9-yl)-6-phenyl-9H-carbazol-3-yl)-9,9-dihexyl-7-phenylacridin-10(9H)-yl)phenyl) (phenyl)methanone (PABPC2.5) were compared in parallel to reveal the influence of the polymer additive of the emitters on the overall performances of the devices. The control device based on the PFI-modified PEDOT:PSS achieved a maximum current efficiency (CE) of 0.14 cd A−1, without any polymer additives in the emitting layer. In contrast, the CE of the devices with PVK and PABPC2.5 were dramatically elevated to 7.1 cd A-1 and 4.5 cd A−1 respectively, when a small molecular thermally activated delayed fluorescence compound was employed as the emitter. Enabled by an easily soluble phosphorescent emitter acetylacetonatobis(4-(4-tert-butylphenyl)-thieno[3,2-c]pyridinato-C2,N)iridium (PO-01-TB). The device with PABPC2.5 as the additive obtained a maximum CE of 42.0 cd A−1, which is significantly higher than 15.7 cd A−1 of the one with the additive PVK, attributed to the energy transfer in the emitting layer. The polymer additives not only overcome the issue of the hydrophobicity of the modified PEDOT:PSS but endow the exciton harvesting from the polymer to the small molecular emitters.

Controlling the Electrical Characteristics of Charge Carrier Transporting Layers in Organic Light-Emitting Devices by the Host–Guest Structure: A Master Equation Approach

Controlling the electron and hole current is a crucial requirement in organic light-emitting devices (OLEDs) as it has a significant impact on the charge balance factor and the location of exciton generation zone. In this paper, via a self-consistent solution of the 3-D master equation approach coupled to Poisson’s equation, it is shown that the charge carrier and current density can be controlled in a wide range in OLEDs by utilizing a host–guest structure in the charge carrier transporting layers of the device. To this end, first, the impact of the injection energy barrier height, trap depth, and blending ratio on the charge carrier and current density is investigated. Second, the strategy of partially blending the hole or electron transporting material with an appropriate guest material is presented, and it is shown that by employing this strategy, current density can be enhanced over two orders of magnitude for high injection energy barriers, where the charge injection is most difficult. Moreover, this strategy guarantees that the transport of charge carriers to the emissive layer is conducted by the hole or electron transporting material itself rather than the guest material which is important as hole and electron transporting materials must fulfill other requirements such as high charge carrier mobility or specific excitonic energy levels.

Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices.

Electron injection layers (EILs) based on a simple polymer blend of polyethyleneimine ethoxylated (PEIE) and poly[(9,9-bis(3'-(( N, N-dimethyl)- N-ethylammonium)-propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) can suppress the dependence of organic light-emitting device (OLED) performance on thickness variation compared with single PEIE or PFN-Br EILs. PEIE and PFN-Br were compatible with each other and PFN-Br uniformly mixed in the PEIE matrix. PFN-Br in PEIE formed more fluorene-fluorene pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the work function (WF) substantially compared with single PEIE or PFN-Br polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower driving voltages and smaller dependences of device performance on EIL thickness than single PEIE or PFN-Br-based devices. These improvements were attributed to electron-transporting fluorene moieties, increased fluorene-fluorene pairs working as channels of electron transport, and the large WF reduction effect of PEIE:PFN-Br blends.

Post-Treatment-Free Solution-Processed Reduced Phosphomolybdic Acid Containing Molybdenum Oxide Units for Efficient Hole-Injection Layers in Organic Light-Emitting Devices.

We report the development of solution-processed reduced phosphomolybdic acid (rPMA) containing molybdenum oxide units for post-treatment-free hole-injection layers (HILs) in organic light-emitting devices (OLEDs). The physical and chemical properties of rPMA, including its structure, solubility in several solvents, film surface roughness, work function, and valence states, were investigated. The formation of gap states just below the Fermi level of rPMA was observed. Without any post-treatment after the formation of rPMA films, OLEDs employing rPMA as an HIL exhibited a very low driving voltage and a high luminous efficiency. The low driving voltage was attributed to the energy level alignment between the gap states formed by reduction and the HOMO level of the hole-transport layer material N,N'-bis(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine.

Isolating Degradation Mechanisms in Mixed Emissive Layer Organic Light-Emitting Devices.

Degradation in organic light-emitting devices (OLEDs) is generally driven by reactions involving excitons and polarons. Accordingly, a common design strategy to improve OLED lifetime is to reduce the density of these species by engineering an emissive layer architecture to achieve a broad exciton recombination zone. Here, the effect of exciton density on device degradation is analyzed in a mixed host emissive layer (M-EML) architecture which exhibits a broad recombination zone. To gain further insight into the dominant degradation mechanism, losses in the exciton formation efficiency and photoluminescence (PL) efficiency are decoupled by tracking the emissive layer PL during device degradation. By varying the starting luminance and M-EML thickness, the rate of PL degradation is found to depend strongly on recombination zone width and hence exciton density. In contrast, losses in the exciton formation depend only weakly on the recombination zone, and thus may originate outside of the emissive layer. These results suggest that the lifetime enhancement observed in the M-EML architectures reflects a reduction in the rate of PL degradation. Moreover, the varying roles of excitons and polarons in degrading the PL and exciton formation efficiencies suggest that kinetically distinct pathways drive OLED degradation and that a single degradation mechanism cannot be assumed when attempting to model the device lifetime. This work highlights the potential to extract fundamental insight into OLED degradation by tracking the emissive layer PL during lifetime testing, while also enabling diagnostic tests on the root causes of device instability.

A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for Electron-Injection Layers in Organic Light-Emitting Devices.

We report a new series of lithium pyridyl phenolate complexes with a pendant pyridyl group, Li2BPP, Li3BPP, and Li4BPP, in which the pendant pyridines are substituted at the 2-, 3-, and 4-positions, respectively. The most important difference between these complexes is their molecular planarity; Li3BPP and Li4BPP adopt twisted bipyridine structures, whereas Li2BPP adopts a planar structure owing to the steric hindrance and chelating effect of bipyridine on the Li core. The planar structure leads to crystallization through π-π stacking interactions, and the small differences in the molecular structures of the pendant pyridine rings cause drastic differences in the physical properties of thin solid films of these complexes. We applied these complexes as electron-injection layers (EILs) in Ir(ppy)3-based organic light-emitting devices. When thin EILs were used, Li3BPP and Li4BPP afforded lower driving voltages than Li2BPP; the order of the driving voltages followed the order of their electron affinity values. Moreover, the dependence of driving voltage on the EIL thickness was investigated for each complex. Among the three LiBPP derivatives, Li2BPP-based devices showed almost negligible EIL thickness dependence, which may be attributable to the high crystallinity of Li2BPP. All LiBPP-based devices also showed higher stability than conventional 8-quinolinolato lithium-based devices.

Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime.

Solution-processed electron injection layers (EILs) comprising lithium 8-quinolate (Liq) and polyethylenimine ethoxylated (PEIE) are highly effective for enhancing electron injection from ZnO to organic layers and improving device lifetime in organic light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces the work function of zinc oxide (ZnO) by enhancing dipole formation. The intermolecular interaction between Liq and PEIE was elucidated by UV-vis absorption measurement and quantum chemical calculation. The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving voltage than that of the device without Liq. Furthermore, as doping concentration of Liq into PEIE increased, the device lifetime and voltage stability during constant current operation was successively improved.

Effects of plasma-induced high-energy particles on the damage of organic thin films

ABSTRACTWe have investigated the effect of plasma-induced damages on the electron transport layers in organic light-emitting devices. Tris(8-hydroxyquinolinato) aluminum, tris(2,4,6-trimethyl-3-(pyridine-3yl)phenyl) borane, and 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ) layers were used for studying plasma-induced degradation of electron transport materials. The thicknesses, photoluminescence spectra and the current-voltage curves of organic electron transport materials were measured after exposing to Ar plasma. The high-energy species in plasma induce the sputtering of organic layers, quenching of emission sites, and the creation of defect sites in organic layers. The plasma-induced damage was strongly dependent on the glass transition temperature, exhibiting the most serious degradation in the TAZ with low glass transition temperature.

3,4,5-Triphenyl-1,2,4-triazole-based multifunctional n-type AIEgen

The luminogens with aggregation-induced emission (AIEgens) characteristics have been widely applied in diverse areas. However, the n-type AIEgens are to be further developed. In this paper, we designed and synthesized an n-type multifunctional AIEgen of tetraphenylethene-substituted 3,4,5-triphenyl-4 H -1,2,4-triazole (BTPE-TAZ). This AIEgen can serve as both light-emitting and electron-transporting layers in organic light-emitting devices. Moreover, it also exhibits the interesting optical waveguide and reversible mechanochromic luminescence properties, which are of great potential for practical applications.

Communication—Structural Modulation of Macrocyclic Materials for Charge Carrier Transport Layers in Organic Light-Emitting Devices

A cyclo-meta-phenylene (CMP) macrocycle with donor/acceptor substituents was designed. After optimization of the synthesis routes, the CMP congener was synthesized via [3 + 3] assembly of phenylene panels in moderate yield. The molecule showed a highly crystalline nature, and a single crystal formed upon sublimation. The molecule was transparent in the visible light region and was thermally robust with a decomposition temperature of 583°C. The effect of the donor/acceptor substituents was evaluated for charge carrier transport layers in emitter-doped phosphorescent OLEDs, which showed a dramatic improvement in its electron transporting ability as well as power efficiency in devices.

Inhibition of solution-processed 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile crystallization by mixing additives for hole injection layers in organic light-emitting devices

We report the inhibition of solution-processed 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) crystallization by mixing additives for the hole injection layers (HILs) in organic light-emitting devices (OLEDs). Various additives were mixed with the HAT-CN layer, including acrylates, compounds resembling HAT-CN and arylamine derivatives. Among these, the HAT-CN films mixed with ditrimethylol propane tetraacrylate (AD-TMP) and pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN) showed a transparent appearance and high solvent resistances. The former was ascribed to the inhibition of the crystallization. We used these additives for the preparation of electron acceptor layers in charge generation layers (CGLs) of the CGL-only devices. The device prepared by mixing HAT-CN with PPDN showed a lower driving voltage and was applied to the production of an HIL in a solution-processed OLED. The device showed a lower driving voltage and a higher luminescent efficiency than those of the device containing a conventional HIL, poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS). The inhibition of solution-processed 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) crystallization by mixing additives was successfully achieved for hole injection layers (HILs) in organic light-emitting devices (OLEDs). Among various additives, the film mixed with pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (PPDN) showed both transparent appearance and high solvent resistances to an upper layer coating solvent. We applied the HAT-CN mixed with PPDN for an HIL in an OLED including solution-processed four-layer stack. The device showed lower driving voltage and higher luminescent efficiency than a device containing conventional HIL, poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS).

Applications of Transparent Conducting Oxides in Organic Light Emitting Devices.

Organic light emitting devices (OLEDs) have received great attention in the field of flat panel display. The transparent metal oxide semiconductor materials play crucial roles in the applications of OLEDs and have strong influence on the performance of OLEDs. In this review, we mainly pay attention to the application of transparent conducting oxides (TCOs) as anodes and buffer layers in OLEDs. Currently indium tin oxide (ITO) is the most widely used anode material in OLEDs owing to the advantage on electrical and optical properties, such as high work function, low resistivity and high transparency. TCO materials, such as ZnO et al., as the anode candidates also have been discussed and analyzed. The energy level can be controlled by semiconductor doping which improve the carrier density and Hall mobility. Interfacial engineering between the anodes and the overlying organic layers is an important process to obtain the high performance of the devices. Physical, chemical and the combined treatment methods to modify the TCO/organic interfaces are reviewed. The property of anode/organic interfaces can be modified and enhanced by introducing the buffer layers between anodes and hole transport layers.

Enhancement of out-coupling efficiency due to an organic scattering layer in organic light-emitting devices

Organic light-emitting devices (OLEDs) with an organic scattering layer (OSL) were fabricated to enhance their out-coupling efficiencies. The OSL was formed through a crystallization process at room temperature after deposition in a vacuum. The luminances of the OLEDs with an 1,3-bis(cabazol-9-yl)benzene (mCP), a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), or a 4,7-diphenyl-1,10-phenanthroline (BPhen) OSL were 2.16, 2.13, and 2.11 times higher than that of the OLED without an OSL due to an increase in the scattering effect at the interface between the glass substrate and the OSL. The current efficiencies of the OLEDs with a mCP, a BCP, or a BPhen OSL were increased by 14.8%, 15.8%, and 14.8% compared to that for the OLED without an OSL. The OSL films in the OLEDs decreased the intensity of the wave-guided light, resulting in enhanced light extraction.

Unlocking the full potential of conducting polymers for high-efficiency organic light-emitting devices.

By carefully tuning the thicknesses of low-optical index PEDOT:PSS and high-index ITO layers in organic light-emitting devices (OLEDs), very high optical coupling efficiencies can be obtained through the generation of appropriate microcavity effects. These experiments result in an external quantum efficiency (EQE) of 33.7% for green phosphorescent OLEDs and even higher EQEs of 54.3% can be obtained by adopting an external out-coupling lens.

Enhancing light out-coupling of organic light-emitting devices using indium tin oxide-free low-index transparent electrodes

With its increasing and sufficient conductivity, the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been capable of replacing the widely used but less cost-effective indium tin oxides (ITOs) as alternative transparent electrodes for organic light-emitting devices (OLEDs). Intriguingly, PEDOT:PSS also possesses an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve enhanced optical out-coupling and external quantum efficiencies in comparison with conventional OLEDs using ITO.