Light-emitting Layer Research Articles

High-performance full phosphorescent warm white organic light-emitting diodes with external quantum efficiency of 34.5%

In this work, we have reported full phosphorescent warm white organic light-emitting diodes (WOLEDs) consisting of three light-emitting layers not only achieving high external quantum efficiency (EQE) but also demonstrating significant color stability and low efficiency roll-off. The WOLED exhibits slight shift in Commission Internationale de l’Eclairage coordinates (CIEx,y) from (0.409, 0.405) at 1000 cd m−2 to (0.372, 0.398) at 10000 cd m−2. Moreover, the maximum external quantum efficiency is as high as 34.5% and remains >28% over wide brightness range of 100 cd m−2 to 5000 cd m−2. Our work not only provides effective design strategy for physiologically friendly warm WOLEDs but also paves the way for achieving high EQE and low roll-off all phosphorescent WOLEDs.

Efficient Stable Green Hybrid Light-Emitting Devices Using Cathode Interlayer

Hybrid photoconductive materials of SnO2 doped organic materials (SnO2:PBA-PA and SnO2:PFN; PBA-PA, 2,9-bis(1-benzylpiperidinyl)-5,6,12,13-tetrachloroanthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-(2H,9H)-tetraone; and PFN, 9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) have been employed as the photoconductive interlayer in efficient hybrid organic light-emitting diodes (OLEDs). The absorption of the SnO2:PBA-PA interlayer reduced the emission intensity at 562 nm due to spectral overlap of the absorption of PBA-PA with the electroluminance (EL) of iridium(III) bis(4,5-dimethyl-1-(naphthalene-1-carbonitrile)-1H-imidazolato-N,C2′)(acetyl acetonate) (Ir(DNC)2(acac)). The fabricated OLEDs having the configuration of indium tin oxide (ITO)/cathode interlayers SnO2/SnO2:PBA-PA/SnO2:PFN/Ir(DNC)2(acac)/MoO3/Al show luminance (L) values of 4289/18056/23135 cd/m2, current efficiency (CE) values of 2.5/15.3/16.8 cd/A, power efficiency (PE) values of 1.1/7.4/8.6 lm/W, and external quantum efficiency (EQE) values of 7.3/14.3/17.0% at 4.6/3.0/2.9 V. The SnO2:PBA-PA based device exhibit increased luminance (ZL) of 22.0%, ZCE of 8.9%, ZPE of 14.0%, and ZEQE of 15.9% compared with SnO2:PFN based device. Compared with the SnO2 device, the SnO2:PBA-PA based device exhibited increased luminance (ZL) of 81.5%, ZCE of 85.1%, ZPE of 87.2%, and ZEQE of 57.1% with reduction in turn-on voltage from 4.6 to 2.9 V. These interlayers show increased conductivity due to effective electron mobility which enhances the electron–hole recombination efficiency in the emissive layer and leads to improvement of the performances of devices. The novel photoconductive interlayers (SnO2:PBA-PA and SnO2:PFN) decreased the energy barrier (cathode interlayer:light-emitting layer) results in the decrease of the turn-on voltage. Our research findings demonstrate that the insertion of a cathode interlayer opens up a new way to enhance the device efficiency.

Analysis of luminescence decay dynamics in metal-dielectric photonic structures with organic layers

Metal-dielectric photonic structures with organic materials 4,4-bis(N-carbazolyl)-1,1-biphenyl and 4,4'-bis[4-(di-ptolylamino)styryl]biphenyl) as light emitting layers were investigated. The formation of the polaritonic modes in the investigated structures was experimentally demonstrated and a correlation between theoretical and experimental dispersion was shown. Was shown, that increase in interaction between organic exciton and optical mode leads to a significant decrease in the lower polariton emission bandwidth. Keywords: light-emitting layers, exciton, luminescence.

Light-emitting dendrimer:exciplex host-based solution-processed white organic light-emitting diodes

Solution-processed monochrome and white organic light-emitting diodes (OLEDs) that have a light-emitting layer composed of a 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and (5-terphenyl-1,3-phenylene)bis(diphenylphosphine oxide) (POPH) exciplex host and soluble blue, green and/or red phosphorescent dendrimers have been fabricated and characterised. The OLED performance characteristics were found to be scan dependent, with the first scans having large external quantum efficiencies (EQEs) at low luminance, with subsequent scans showing stable performance. The monochrome (blue, green and red) films were found to have high photoluminescence quantum yields, relatively balanced hole and electron mobilities, and the OLEDs had stable EQEs of between 9 and 12% at 100 cd m −2 over multiple scans. The blue, green and red emissive materials were blended with the exciplex host, with their ratio tuned to achieve white emission. The optimal blend ratio provided white OLEDs that had EQEs of 11.7% and 10.6% at 100 cd m −2 and 1000 cd m −2 , respectively. The best balance of 1931 Commission Internationale d'Eclairage (CIE) coordinates (0.40,0.40), Colour Rendering Index (70), and Delta uv (0.003) was achieved for an OLED with the light-emitting layer containing the blue, green and red dendrimers in a ratio of 20.0:0.4:0.8 wt%. A feature of the white OLEDs was the stability of the CIE coordinates, with a change of only (0.013,0.005) between a luminance of 100 cd m −2 and 4000 cd m −2 . • A common exciplex host composed for solution processed blue, green, red and white emissive OLEDs. • Monochrome light-emitting dendrimer:exciplex host blend films have relatively balanced hole and electron mobilities. • Monochrome OLEDs with external quantum efficiencies of up to 22.5%. • White emitting OLEDs external quantum efficiencies of 10.6% at 1000 cd m −2 • White OLEDs with stable CIE coordinates across a broad range of luminance.

Ionic-compound based high performance perovskite light emitting diodes

Metal halide perovskite has attracted much attention due to its adjustable color, high color purity, and excellent photoelectric properties. The quality of the perovskite film is one of the key factors that affect the performance of device. Here, PEA<sub>2</sub>Cs<sub><i>n</i>–1</sub>Pb<sub><i>n</i></sub>Br<sub>3<i>n</i>+1</sub> thin films are prepared by directly doping the ionic compound additive tetraphenylphosphine chloride (TPPCl) into the perovskite precursor of the light-emitting layer based on additive assisted technology. High-quality perovskite films with uniform, less pinholes and smaller grains are obtained. Not only is the photoluminescence (PL) performance of PeLEDs improved but the electroluminescence (EL) performance of PeLEDs with a double electron transport layer also turns better. The maximum brightness is 25285 cd/m<sup>2</sup>. The maximum current efficiency is 65.9 cd/A. And the maximum EQE is 17.3%. The method of adding ionic compounds to the perovskite precursor can not only improve the carrier transport behavior, but also make the formed small n crystal phases and large n crystal phase more balance, leading to the energy funnel effect to be enhanced. Further investigation by FTIR proves that the TPPCl can passivate the perovskite film, and thus greatly improving the EQE value of the PeLED. This researchpresents a simple and efficient method of developing high-performance quasi-two-dimensional green PeLEDs.

Effect of PEDOT:PSS on the performance of solution-processed blue phosphorescent organic light-emitting diodes with an exciplex host

The acidity of the PEDOT:PSS hole injection layer was found to effect the performance of efficient solution-processed organic light-emitting diodes incorporating a light-emitting layer composed of a blue phosphorescent dendrimer:exciplex host blend.

Production of organic light-emitting diode with fluorescence featured quinoline derivative

High-priced coating devices limit producing electronic devices and circuit applications widely in laboratories. Simply In this study spin coating technique was used to create surface thin films. Also with this method, an OLED(Organic Light Emitting Diode) device was practically produced. OLED device includes mainly HTL(hole transfer layer), fluorescent layer(light-emitting layer), and an ETL(electron transfer layer). Light-emitting layers in OLED experimental studies are frequently done with commercially produced expensive fluorescence polymers. As an example, MEH-PPV (Poly[2-methoxy-5-(2’-ethyl-hexoxy)-1,4-phenylenevinylene]), Alq3 (Tris-(8-hydroxyquinolinato) aluminum) are mostly known and used fluorescent semiconductor polymers. Alternative to these fluorescent polymers, three different produced quinoline ligand products has fluorescent feature were evaluated. After comparing the fluorescence yields of the produced three complexes, it was seen that 5,7-dibromo-8-hydroxyquinoline has the highest fluorescent response from the others. OLED device production was done with a commercial MEH-PPV(commercial) fluorescent product, and produced (5,7-dibromo-8-hydroxyquinoline). Designed OLED device illumination spectrum was found in the UV(ultraviolet) region. It was concluded that this quoniline product can use as a fluorescent material to produce an OLED device.

Thermally activated delayed fluorescence polymers for high-efficiency solution-processed non-doped OLEDs: Convenient synthesis by binding TADF units and host units to the pre-synthesized polycarbazole-based backbone via click reaction

At present, TADF polymers with multiple functional components are still limited due to their complicated synthetic process and low synthetic repeatability. To overcome these problems, a post-functionalization strategy was exploited to synthesize the self-host TADF conjugated polymers. A series of yellow-green TADF polymers, PCz-mCP-PxzTrz-X, X = 10, 20, 30 or 40, were successfully synthesized by grafting stoichiometric TADF units (phenoxazinetriphenyltriazine, PxzTrz) and host units (1,3-bis(N-carbazolyl)benzene, mCP) onto the side chain of a pre-synthesized polycarbazole-based backbone via a click reaction. The carbazole-containing main chain and mCP-containing side chains served as an “encapsulant”, which can transport carriers and prevent the TADF units from concentration quenching. Using PCz-mCP-PxzTrz-30 as the light-emitting layer, the solution-processed non-doped organic light-emitting diode displayed an external quantum efficiency of 15.2% at 100 cd m−2. Our synthetic method could offer a useful strategy to construct self-host TADF polymers containing multiple functional components.

New Unsymmetrically Substituted Benzothiadiazole-Based Luminophores: Synthesis, Optical, Electrochemical Studies, Charge Transport, and Electroluminescent Characteristics.

Three new benzothiadiazole (BTD)-containing luminophores with different configurations of aryl linkers have been prepared via Pd-catalyzed cross-coupling Suzuki and Buchwald–Hartwig reactions. Photophysical and electroluminescent properties of the compounds were investigated to estimate their potential for optoelectronic applications. All synthesized structures have sufficiently high quantum yields in film. The BTD with aryl bridged carbazole unit demonstrated the highest electrons and holes mobility in a series. OLED with light-emitting layer (EML) based on this compound exhibited the highest brightness, as well as current and luminous efficiency. The synthesized compounds are not only luminophores with a high photoluminescence quantum yield, but also active transport centers for charge carriers in EML of OLED devices.

Effect of dendrimer surface groups on the properties of phosphorescent emissive films

Fac -tris[2-phenylpyridinato- C 2 , N ]iridium(III) [Ir(ppy) 3 ] is a well-known phosphorescent material for organic light-emitting diodes (OLEDs) and while uniform thin films can be formed using evaporation under high vacuum, it is not sufficiently soluble to enable it to be processed from solution. Ir(ppy) 3 -cored dendrimers with solubilizing surface groups can be processed from solution to form good quality neat or guest:host blends, even when the host has relatively poor solubility. In this manuscript, we report the effect of adding different solubilizing surface groups, namely 2-ethylhexyloxy, n -propyl or t -butyl to first generation dendrons attached to the Ir(ppy) 3 core on the optoelectronic properties of neat and blend films with tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The different dendrons were found to have minimal effect on the photoluminescence spectra and efficiency of energy transfer from the host to the guest in the blend films. The hole mobility of neat films of the solution-processed Ir(ppy) 3 -cored dendrimer films was around 10 −6 cm 2 V −1 s −1 , which was an order of magnitude less than Ir(ppy) 3 films formed by evaporation. Blending the dendrimers with TCTA at a concentration of 11 mol% (the concentration of the highest film photoluminescence quantum yield) led to an order of magnitude decrease in the hole mobility compared to the neat films. However, despite the relatively low mobilities, simple two-layer OLEDs composed of a blend light-emitting layer and an electron transporting layer were found to reach a maximum EQE of 11.1% at a luminance of 1000 cd m −2 for a film with a PLQY of 60%. • Ir(ppy)3-cored dendrimers with solubilising surface groups are solution processed. • Good quality guest:host blend films formed with poorly soluble TCTA host. • Dendron type was found to have minimal effect on the host to guest energy transfer. • Charge mobility in dendrimer:TCTA blends less than in neat film. • Simple two-layer OLEDs had maximum EQEs of 11.1% at a luminance of 1000 cd m-2.

Enhanced device performance of quantum-dot light-emitting diodes via 2,2′-Bipyridyl ligand exchange

Quantum dots (QDs) are electroluminescent (EL) materials that have been developed as promising emitters in next-generation displays. The amount of charge carriers that travel to the light-emitting layer located at the interface between the hole- and electron-transport layers must be balanced in these displays to achieve maximum luminous efficiency. An existing amine ligand (oleylamine), which was coordinated on the surface of red-emitting InP/ZnSeS/ZnS QDs, was partially exchanged with two 2,2′-bipyridyl derivatives to enhance the EL properties; the QD light-emitting diode (QD-LED) performance was also investigated. Thermogravimetric and 1H nuclear magnetic resonance analyses were used to examine the bipyridyl-ligand-partially-substituted QDs. Photoluminescence spectroscopy and transmission electron microscopy revealed that the emission wavelength (630 nm) and size (7.3 nm) of the QDs were not altered upon ligand exchange. Significant improvements were observed in the electronic properties of QD-LED devices fabricated using the bipyridyl-ligand-substituted QDs. The bipyridyl ligands lowered the charge-injection barrier and improved the charge balance in the QDs. High-performance QD-LED devices were consequently realized with an augmentation of two times in current density, and three times in brightness, external quantum efficiency, and current efficiency.

A novel and efficient technology of depositing Al2O3 film for OLEDs thin film encapsulation

Organic light-emitting diode devices (OLEDs) industry is developing rapidly, however, OLEDs are very sensitive to oxygen and water, determining how to achieve better barrier effect is one of the main challenges of flexible OLEDs technology. Based on the filtered cathode vacuum arc (FCVA) technology, a dense aluminum oxide (Al2O3) film with a thickness of about 100 nm was grown at room temperature for OLEDs encapsulation. The composition, structure, surface roughness, refractive index, light transmittance and other physical properties of the film were analyzed. The water vapor transmission rates (WVTRs) of the films were tested at 85 °C and 85% relative humidity (R.H.). Photoluminescence spectrum (PL) test was carried out to study the luminescence ability of tris(8-hydroxyquinoline) aluminum (Alq3) before and after the deposition of Al2O3 film. The results show that the amorphous Al2O3 film with dense structure prepared by FCVA has high visible light transmittance. The Al2O3 film reduces the WVTRs of the polyethylene naphthalate (PEN) film by four orders of magnitude and maintains the light-emitting performance of the organic light-emitting layer. These results indicate that the FCVA technology is a feasible method for preparing barrier films of flexible OLEDs.

Host-to-Guest Energy Transfer and Its Role in the Lower Stability of Solution-Coated versus Vacuum-Deposited Phosphorescent OLEDs

Electroluminescence (EL) degradation mechanisms in solution-coated (SOL) host:guest (H:G) systems commonly used in phosphorescent organic light-emitting diodes (OLEDs) are investigated and compared to their vacuum-deposited (VAC) counterparts. Changes in the EL, photoluminescence (PL), and time-resolved PL (TRPL) characteristics of devices comprising SOL or VAC H:G light-emitting layers (EMLs) made of the same materials and in the same device architectures during prolonged electrical driving are compared and analyzed. Hole-only devices are also utilized to study the effects of charges and excitons, separately and combined. Moreover, devices with double EMLs comprising SOL and VAC components are tested to glean additional insights into the role of host excitons in device degradation. The results indicate that the faster degradation of SOL EML devices relative to their VAC EML counterparts under electrical stress is due–at least in part–to the less efficient host-to-guest (H → G) energy transfer in these systems, which accelerates molecular aggregation in the EML. Interactions between excitons and polarons in the EMLs induce this aggregation phenomenon which occurs more strongly in the case of SOL EMLs compared to their VAC counterparts because of the higher host exciton concentration in the former as a result of the less efficient H → G energy transfer. The findings shed light on one of the root causes of the limited stability of SOL OLEDs.

Highly efficient yellow organic light-emitting diodes with slow efficiency roll-off by mixing red and green emissions

In this work, we report the design and optimization of yellow organic light-emitting diodes (OLEDs) with tris [2-(p-tolyl)pyridine]iridium (III) (Ir (mppy)3) doped 4,4′,4′-tris(N-carbazolyl)triphenylamine (TCTA) and bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium (III) (PQ2Ir (dpm)) doped 2,6-bis(3-(9H-carbazol-9-yl)phenyl)phyridine (26DCzPPy) films as green and red light-emitting layers (EMLs). Experimental results demonstrated that the designed double EMLs system is beneficial in carriers' distribution, exciton confinement and reducing triplet exciton quenching because of the matched energy levels and triplet energies of hosts and dopants. Finally, the optimal device showed a very maximal current efficiency of 65.51 cd/A, slow roll-off of efficiency with a very high current efficiency of 63.54 cd/A at 5000 cd/m2, and excellent yellow emission with CIE coordinates of (0.468, 0.493) at 10 mA/cm2.

Efficient and Color-tunable Organic Light-emitting Diodes for Rear Light Application on the Motor Vehicle

By designing double emissive layers, orange light-emitting layer (EML) and red EML and a space layer of blending hole-transporting material (HTM) with electron-transporting material (ETM), we have achieved efficient and color-tunable organic light-emitting diodes (OLEDs). It is found that the red shift of EL spectrum was enhanced when the ratio of HTM to ETM was 5:1. And a maximum current efficiency of 41.17 cd/A was reached. It has been demonstrated that two ultra-thin emissive layers could improve the current efficiency of the OLEDs. The current efficiency of the OLEDs with a 1 nm o;6range EML and a 1 nm red EML was enhanced by 27.4 % as compared to the reference devices. This is a novel approach to realizing voltage-controlled electroluminescence (EL) over different colors and intensity in a single OLED. The color-tunable OLEDs show the potential to be utilized as rear light on motor vehicle. We have investigated the effect of the ratio of HTM to ETM on the efficiency and spectral shift of the OLEDs through the control of recommendation zone.

Role of acceptor concentration-dependent FRET behind luminescence tuning in polymer capped ZnO for application in hybrid LEDs

Surface modification of the ZnO nanoparticles using polymer can be an important way to tune their optoelectronic properties. On capping the ZnO nanoparticles (NPs) using the polymer polyvinylpyrrolidone (PVP), a synchronous change in the photoluminescence emission has been observed from different defect states with different formation energies, which raises doubts about the decade-old proposed theory of passivation of defect states upon surface capping. The doubt has been confirmed by X-ray photoelectron spectroscopy (XPS) of the samples. Blue shifting of the band edge absorption on polymer capping has been attributed to a change in exciton dynamics due to the tuning of band alignment on surface modification. Polymer concentration-dependent tunability for emission from defect states has been explored using two new photophysical process – acceptor (PVP)-concentration-dependent Förster resonance energy transfer (FRET), and generation of superoxide (O2-) charge-transfer states. Surface capping has been found to improve spectral purity at the cost of a decrease in quantum efficiency. Therefore, to design a very bright hybrid light-emitting layer using polymer capped ZnO, we must consider the emission and absorption spectral overlap between the ZnO nanoparticles and the polymer, FRET, exciton diffusion and balling efficiency of the polymer.

High performance blue and white phosphorescent organic light-emitting diodes obtained by sensitizing both light-emitting and electron transport layers

In this work, novel blue and white organic light-emitting diodes (OLEDs) were designed by utilizing iridium(Ⅲ)bis(4-(tert-butyl)-2,6-diuoro-2,3-bipyridinne)acetylacetonate (FK306) as sensitizer in both light-emitting and electron transport layers. Analysis of device configuration demonstrated that FK306 molecules work as electron trappers in view of their low-lying energy levels and triplet energy, which broaden the width of recombination zone, thus enhancing carriers' recombination probability and suppressing the annihilation of excitons. Consequently, blue and white devices with low turn-on voltages were reported with current efficiencies of 71.95 and 72.23 cd A−1, external quantum efficiencies of 32.62% and 31.30%, power efficiencies of 74.85 and 70.80 lm W−1 and brightness of 43171 and 37848 cd m−2, respectively. Within electron transport layer, the optimal doping concentration of FK306 is 2 wt%, which can be easily controlled in device manufacture processes. In addition, it is much more flexible in terms of sensitizer choice for doped electron transport layer compared with light-emitting layer.

Highly efficient blue phosphorescent organic light-emitting diodes with low operation voltage

In this work, we report an efficient blue organic electroluminescent device based on phosphorescent emitter iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6), which displayed excellent thermal stability. To balance carriers’ distribution and reduce operation voltage, double light-emitting layers device structure was designed by selecting 4,4′,4′-tris(N-carbazolyl)triphenylamine (TcTa) and 2,6-bis(3-(9Hcarbazol-9-yl)phenyl)phyridine (26DCzPPy) as host materials because of their stepwise energy levels. By optimizing the doping concentration of FIr6 to be 26 wt%, well balanced carriers’ distribution on emitter molecules and broadening recombination zone were realized. Finally, high performance blue device with slow efficiency roll-off and high current efficiency up to 56.83 cd/A (obtained at 700 cd/m2) was realized. At the certain brightness of 1000 (3.9 V) and 5000 cd/m2 (4.8 V), the same device retained efficiencies of 56.33 and 51.45 cd/A, respectively.

Interface charge transport of multilayer devices for exact analysis of charge behavior in organic optoelectronic devices

A unipolar charge device was utilized to gain information of the charge balance in organic optoelectronic devices to enhance efficiency and operational stability. Usually, a unipolar charge device is designed as a multilayer structure to improve charge transport and block injection of counter charges from electrodes. In this paper, the correlation between charge transport across the interlayer and corresponding bulk electrical parameters from quasi-static analysis was investigated. Although only the interlayer material was modified, the charge carrier mobility and drift charge carrier density of the light-emitting layer were totally changed, which originated from large series resistance due to charge accumulation at the interface. This result indicates that quantitative analysis of electrical properties from unipolar charge devices should be performed as background analysis to ensure efficient interface charge transport during operation.

Anomalous magnetic-field response of doped devices based on the thermally activated delayed fluorescent material of CzDBA

CzDBA is a donor-acceptor-donor (D-A-D)-type of thermally activated delayed fluorescence (TADF) material that has been recently reported in the literature; it has better luminous performance than ordinary D-A-type TADF materials. To explore the microscopic processes involved in CzDBA-based light-emitting devices, we fabricated two kinds of devices with light-emitting layers of pure CzDBA and 4CzTPN-Ph films and measured their magneto-luminescence (MEL) and magnetic conductance (MC) at room temperature. Experimental results show that although both of them contain carbazole groups, their MEL traces present two different line shapes; the former (CzDBA-based OLEDs) exhibits positive MEL curves dominated by intersystem crossing processes (ISC), whereas the latter depicts negative MEL traces governed by reverse ISC (RISC). In addition, CzDBA-based OLEDs have the same line shapes of MEL and MC traces. To investigate this anomalous magnetic-field response in CzDBA further, we prepared a doped device with a light-emitting layer of mCBP: x %CzDBA. Experimental results show that compared with devices with pure CzDBA as an emission layer, doped devices exhibit similar MEL and MC line types but have opposite temperature-dependent MEL traces. Detailed analyses related to MEL and MC traces show that the ISC process of polaron pairs dominates pure CzDBA devices, making their MEL values always positive. However, the doping of CzDBA into an mCBP host results in a strong exciton energy transfer (ET) process in the doped devices; this ET process is suppressed at low temperature, leading to a weakened RISC effect and an abnormal temperature effect different from that of pure CzDBA devices. In addition, the competition of the energy transfer process between host and guest and the trapping effect of charge carriers produce a nonmonotonic change in the MEL amplitude of the doped device by increasing the dopant concentration. This change further proves the correctness of the proposed microscopic mechanisms for these CzDBA-based OLEDs. This work is beneficial to the further understanding of the evolution mechanisms of the microscopic processes in devices based on excellent CzDBA luminescent materials recently discovered in literature.