A multifunctional dipyrimidylpyridine electron-transporter combined with Ag for robust and efficient organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have been successfully applied as displays and also recognized as a next-generation lighting technology because their several advantages such as self-emission, high luminous efficiency, full-color capability, wide viewing angle, high contrast, low power consumption, low weight, large area manufacture, transparence and flexibility. Now, the red and green phosphorescent OLEDs are qualified for the commercial products. The bottleneck still is the blue OLED. Currently, the high efficiency (100% internal quantum efficiency, and over 20% external quantum efficiency, EQE) blue OLED can be achieved using phosphorescent and thermal activated delay fluorescent (TADF) emitters, which were the crucial factor to determine color purity, efficiency, and lifetime of device. Beside the emitters, the other important factor is the host material. In this talk, we synthesized several novel carbazole-based, benzimidazole-based, and their combined derivers to be the universal hosts with wide energy bandgap for red, green, blue phosphorescent emitters and also applied for green and blue TADF emitters. These novel host materials could be classified as electron transport, hole transport, and bipolar. The carrier dynamics in OLEDs with these host materials were investigated by probing the position of main recombination zone and optimizing the efficiency performance. With increasing electron mobility of host material, the main carrier recombination zone in emitting layer was moved from the ETL side to the HTL side. Finally, the high EQE of over 30% was achieved in green and blue phosphorescent and TADF OLEDs with our host system. In addition, the operational lifetime was also improved by using our host system, comparing that of OLED with commercial mCP host.

The recombination dynamic of hole and electron and carrier transport mechanism were studied. We demonstrated a efficient green phosphorescent organic light-emitting diodes (OLEDs) as the doping concentration are varied without hole blocking layer (HBL) and electron transport layer (ETL). In spite of device without HBL and ETL, it is possible to inject the electron from the cathode into the emission layer (EML). It is practicable under the high doping ratio of the dopant in host. The high concentrated doping leads to the direct injection of hole and electron from both electrodes into iridium(III) tris(2-phenylpyridine) [Ir(ppy) 3 ] used as dopant because the workfunction of Al and the highest occupied molecular orbital (HOMO) state of 4,4'-bis[N-(naphthyl)-N-phenyl-amino]biphenyl 4,4'-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (NPB) are nearly aligned with the lowest unoccupied molecular orbital (LUMO) and HOMO energy state of Ir(ppy) 3 , respectively.

We demonstrated highly efficient green organic light-emitting diodes (OLEDs) by inserting a mixed hole blocking layer (HBL) between the emitting layer (EML) and electron transporting layer (ETL). We compared 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), N-arylbenzimidazoles trimer (TPBi), and 4,7-diphenyl-1,10-phenanthroline (Bphen) as the hole blocking layer. Thereafter, we mixed BCP with TPBi and Bphen. We obtained improved efficiency from the mixed HBL devices and the TPBi mixed with BCP device showed the best efficiency.

Organic and polymeric materials have generated interest in recent year for their potential applications in flat panel displays, such as TVs, monitors, and mobile displays, due to their wide viewing angle, fast response time, and low operating voltage with high external quantum efficiency. A well-balanced charge injection leads to an increase in the external quantum efficiency in polymer-based light-emitting diodes (PLEDs). This can be achieved by forming a multilayer structure in device fabrication and considering the charge mobility, the ionization potentials and the electron affinities of the organic materials, and the work functions of the metal electrodes. When taking the hole favor property in most conjugated polymers into consideration, it is important to introduce electron injection and transport layers for more balanced charge injection into the emission zone. The electron injection can be enhanced by using a metal cathode with a low work function, such as Ca or ionic compounds like LiF or CsF. However, active metals like Ca are known to diffuse into the organic layer due to the strong electric field inside the device, which forms quenching sites in the emission layer near the cathode. Brown et al. announced that ionic compounds could also be partially ionized and migrate into the emission layer. In fact, the multilayer structure with selected electron injection and transport layers can enhance the device performance by not only reducing the electron injection barrier and controlling the electron mobility but also preventing the migration of metals or ionic compounds into the emission layer. In general, most conjugated polymers are soluble incommon organic solvents, which cause difficulty for fabrication of multilayer polymer films. During the last decade, considerable research efforts have been carried out on multilayer PLEDs to enhance the device performance to make them suitable for practical use. Lee et al. introduced an ionomer layer next to the emitting polymer for electron injection and a hole blocking layer, which lowered the operating voltage by up to 60%. Niu et al. reported that the use of neutral surfactants blended with poly(ethylene glycol) as an electron injection layer with an Al cathode could improve the current efficiency to almost twice that of Ca/Al cathode. Recently, Tseng et al. showed a general method to solution-process multilayer PLEDs by using an intermediate liquid buffer between polymer layers. Several papers have also reported the enhancement of electron injection by using ion-conducting polymer, cross-linkable inter-layers, and water/ methanolsoluble copolymers. In spite of those improvements in multilayer PLEDs fabrication, however, it is true that we still need more materials for effective hole blocking and electron injection with an easy process. In this study, we synthesized new types of water soluble conjugated polymer and showed enhanced performance on the multi-layered polymer light-emitting diodes (PLEDs) by introducing the water soluble conjugated polymer FPQ as an electron transport layer between the emitting polymer and the cathode metal. The device performances with and without water soluble polymer layer will be compared in terms of the current-voltage characteristics, the luminance, and the external quantum efficiencies.

Constructing all-solution-processed green phosphorescent organic light-emitting diodes based on zinc sulfide QDs as an electron injection layer

Theoretical model for the external quantum efficiency of organic light-emitting diodes and its experimental validation

Systematic studies on carrier injection and transport are very important for achieving high efficiency in OLEDs. We demonstrate excellent green phosphorescent organic light-emitting diodes (OLED) with lithium quinolate (Liq) doped in 1,3,5-tris(N-phenylbenzimidazole-2-yl) benzene (TPBi) as the electron transport layer (ETL). The doping concentration of Liq was varied from 0% to 10%. The optimized green phosphorescent OLED with 5% Liq in the ETL showed the best efficiencies, which were maximum luminous efficiency, power efficiency, and quantum efficiency of 65.76 cd/A, 57.39 Im/W, and 20.03%, respectively. Moreover, high triplet energy states of TCTA and TPBi as a triplet exciton-blocking layer (TEBL) played a role in efficient exciton confinement.

Recently, triplet harvesting via a thermally activated delayed fluorescence (TADF) process has been established as a realistic route for obtaining ultimate internal electroluminescence (EL) quantum efficiency in organic light-emitting diodes (OLEDs). However, the possibility that the rather long transient lifetime of the triplet excited states would reduce operational stability due to an increased chance for unwarranted chemical reactions has been a concern. Herein, we demonstrate dual enhancement of EL efficiency and operational stability in OLEDs by employing a TADF molecule as an assistant dopant and a fluorescent molecule as an end emitter. The proper combination of assistant dopant and emitter molecules realized a “one-way” rapid Förster energy transfer of singlet excitons from TADF molecules to fluorescent emitters, reducing the number of cycles of intersystem crossing (ISC) and reverse ISC in the TADF molecules and resulting in a significant enhancement of operational stability compared to OLEDs with a TADF molecule as the end emitter. In addition, we found that the presence of this rapid energy transfer significantly suppresses singlet-triplet annihilation. Using this finely-tuned rapid triplet-exciton upconversion scheme, OLED performance and lifetime was greatly improved.

In this study, we report highly efficient green phosphorescent organic light-emitting diodes (OLEDs) with ultra-thin emission layers (EMLs). We use tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3), a green phosphorescent dopant, for creating the OLEDs. Under systematic analysis, the peak external quantum efficiency (EQE) of an optimized device based on the ultra-thin EML structure is found to be approximately 24%. This result is highest EQE among ultra-thin EML OLEDs and comparable to the highest efficiency achieved by OLEDs using Ir(ppy)3 that are fabricated via conventional doping methods. Moreover, this result shows that OLEDs with ultra-thin EML structures can achieve ultra-high efficiency.

Dramatically improved power efficiency and stability of organic light-emitting diodes (OLEDs) were achieved by using buckminsterfullerene (C60) as an interlayer between indium tin oxide (ITO) anode and hole transporting layer of N, N′-diphenyl-N, N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) and electron transporting layer (ETL) at the same time. The results are ascribed to the interfacial-dipole formation of C60 on the surface of ITO anode and Ohmic cathode contact of C60. The surface dipole of C60 on the ITO anode helps to lower the hole injection energy barrier from ITO to NPB. C60 also has an Ohmic cathode contact with high electron mobility in the typical structure of C60/LiF/Al. These properties of C60 make it possible to simultaneously enhance the electron and hole injection from both cathode and anode. Lowered operating voltage by surface dipole and Ohmic cathode contact of C60 can eliminate Joule heating at both organic/cathode and organic/anode interfaces and as a result, provides the improved stability of OLEDs.

Since the invention of organic light-emitting diodes (OLEDs) nearly 40 years ago, significant effort has been put into realizing their full potential. OLEDs exhibit several properties that make them ideal candidates for applications in displays as well as solid-state lighting: low power consumption, high contrast ratios, mechanical flexibility, and wide viewing angles. However, the low electrical conductivity and poor stability of organic materials remain critical factors that limit device performance. The focus of this work is OLED performance improvement through the introduction of novel inorganic dopants into organic charge transport layers. The results show a significant reduction in operating voltage and increase in reliability for devices with doped charge transport layers compared to those without. Further, with sufficiently high doping concentrations, it is demonstrated that device structure can be dramatically simplified. Electron-only devices (EODs) were fabricated using three different electron-transport materials: Alq3, BPhen, and TPBi to investigate the effects of Ca doping via co-evaporation. It was demonstrated that only the characteristics of the BPhen-based EOD were improved. The improvement suggests facile electron transfer from the Ca dopant to the BPhen matrix due to the low-lying LUMO level of BPhen. Despite the formation of gap states, increasing the Ca concentration up to 11.5 wt% shows a monotonic trend of decreasing operating voltage. It was also shown that above 4.1 wt% Ca the energy barrier between the cathode and electron-transport layer was sufficiently reduced to allow for the removal of the LiF electron injection layer (EIL) without any negative effect on device performance. Blue OLEDs with and without Ca in the BPhen electron-transport layer (ETL) were fabricated. The doped OLEDs showed lower operating voltage and higher luminance compared to the undoped OLEDs. While the best electrical characteristics were observed when the entire ETL was doped, it caused significant exciton quenching and reduced current efficiency. This effect was reduced by introducing an undoped BPhen spacer between the emissive layer (EML) and doped ETL. In both cases, current stressing showed that Ca is a stable dopant in BPhen. CBP homojunction devices were fabricated, with the ambipolar CBP matrix material doped p-type with MoO3 in one side, n-type with Ca in the other side, and with a BCzVBi blue emitter in the middle EML to produce blue light emission. Both hole-only devices (HODs) and EODs showed monotonic improvement as doping concentration increased, indicating that CBP was successfully doped p-type and n-type. Above 10 wt% MoO3

Highly efficient organic bistable light-emitting diodes (OBLEDs) were developed using the tandem structure of organic light-emitting diodes and organic bistable memory. A high external quantum efficiency (> 19.0%) was obtained in the OBLED by combining a green phosphorescent organic light-emitting diode and polymer resistive memory. High quantum efficiency was observed in the OBLED at both the off and on states. The high quantum efficiency was maintained up to high luminance with little efficiency roll-off.

Organic light-emitting diodes (OLEDs) possess a number of advantages such as low power consumption, light weight, wide color gamut, high response speed, and high contrast ratio. They have received widespread attention due to their tremendous commercial applications in the fields of full-color flat panel display and solid-state lighting. Although nearly 100% internal quantum efficiency of OLED has been achieved through adopting phosphorescence or thermally activated delayed fluorescence emitters. However, the majority of light generated in an emitting layer is confined within the whole device but does not escape into air due to the induced surface plasmons at the interface between metal and dielectric layers as well as the differences in refractive index between layers of OLED structures including air, glass substrate, transparent electrode as well as organic or inorganic layers. The external quantum efficiency for an OLED with a flat glass substrate is limited to~20%. A low light out-coupling efficiency severely restricts the development and application of OLED. Therefore, enhancing the light out-coupling efficiency of OLED via light extraction technology offers the greatest potential for achieving a substantial increase in the external quantum efficiency of OLED and has been one of the most attractive projects. Up to now, lots of light out-coupling technologies such as micro-lens arrays, photonic crystal, Bragg mirrors and periodic grating have been suggested to enhance the out-coupling efficiency of OLEDs. However, the periodic light out-coupling structures have a limitation that the electroluminescence intensity and spectrum of OLED usually depend on the viewing angle. The angular dependence of the emission characteristic does not hold true for actual display applications due to its deviation from the Lambertian intensity distribution. In this review, we present recent research progress of using non-period micro/nanostructures to improve the light out-coupling efficiency of OLED. In contrast to the emission directionality for OLED using periodic light out-coupling structures, the luminance distribution and spectral stability of OLED based on non-period micro/nanostructures are insensitive to viewing angle. Various light out-coupling techniques such as random micro/nano lens structure, light scattering medium layer, polymer porous scattering films, random concave-convex corrugated structure, and random buckled structure are summarized and discussed. These techniques have the potential applications in displays and solid-state lighting. Finally, summary and prospects regarding to light-coupling techniques of OLEDs are presented.

A long lifetime and high efficiency organic light emitting diode (OLED) was developed with optimization of each organic materials. We improved the operational lifetimes of the green photphorescent OLEDs based on the balance beween hole and electron inside the emission layer (EML) affects the lifetime of OLEDs. The operational lifetimes become long with increasing the density of hole carrier in the EML. For that purpose, we fabricated with using the new electron blocking layer (EBL) and hole blocking layer (MBL) and optimized the thickness of EML. The lifetimes of the green phosphorescent OLEDs developed in this study were achieved over −2.6 times better lifetimes at room temperature than that of reference device. Finally, this green phosphorescent OLEDs are fabricated for mass production of automotive application.

Novel indolocarbazole-based bipolar host materials for fabricating green phosphorescent organic light-emitting diodes with high efficiency and low roll-off