The present invention provides a heterocyclic compound and an organic light-emitting device including the heterocyclic compound. The organic light-emitting devices using the heterocyclic compounds have high-efficiency, low driving voltage, high luminance and long lifespan.

Organic light-emitting device

During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) or tungsten tri-oxide (WO3) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications

The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.

Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films

P-type doping of organic wide band gap materials by transition metal oxides: A case-study on Molybdenum trioxide

Thin-film metal oxides are among the key materials used in organic semiconductor devices. As there are no intrinsic charge carriers in a typical organic semiconductor, all charges in the device must be injected from electrode/organic interfaces, whose energetic structure consequentially dictates the performance of devices. The energy barrier at the interface depends critically on the work function of the electrode. For this reason, various types of thin-film metal oxides can be used as a buffer layer to modify the electrode work function. This paper provides a review on recent progress in metal oxide/organic interface energetics, oxide valence structure and work function, as well as the impact of defects and interfacial reactions on oxide work functions. This review provides a rational guide to process engineers in selecting the best suitable electrode/oxide structures for a targeted applications. Organic semiconductors offer an attractive alternative to the traditional, silicon-based components of electronic devices. Cheaper to produce and more sustainable, they can also introduce different attributes, such as flexibility, to these devices. However, as organic materials do not typically possess intrinsic charge carriers — electrons or holes — all charges in the device must originate from the electrode and pass through the electrode-organic material interface, a process hindered by an energy barrier. Mark Greiner and Zheng-Hong Lu review recent achievements in a versatile class of buffer layer — thin films of transition metal oxides — that can be positioned between the two materials to reduce the energy barrier that limits charge injection. The researchers discuss how to select the most suitable metal oxide for a specific purpose, and then tune the thin film's properties by adjusting the thickness of the metal oxide layer, the oxidation state of its cations and the concentration of its defects. Over the last decade, metal oxides have proven to be important materials for organic electronics. Oxides are often used as charge-injection and charge-selective interlayers to engineer the electrical resistance at electrode/organic interfaces in organic devices. An oxide’s behavior as an interlayer depends strongly on the oxide’s electronic properties—such as its band structure and work function. The numerous degrees of freedom in an oxide’s electronic properties allow these characteristics to be easily modified. The present review outlines the use of metal oxides in organic electronics, and discusses the factors that affect the oxide’s properties that are relevant to oxide/organic interfaces.

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Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

The role of molybdenum oxide as anode interfacial modification in the improvement of efficiency and stability in organic light-emitting diodes

A soluble nonionic surfactant, polyethylenimine 80% ethoxylated (PEIE) solution, was used as the electron injection material in inverted bottom-emission organic light emitting diodes (OLEDs). The transparent PEIE film was formed on indium-tin-oxide cathode by simple spin-coating method and it was found that the electron injection was greatly enhanced. The devices with PEIE electron injection layer had achieved significant enhancement in luminance and efficiency. The maximum luminance reached 47 000 cd/m2, and the maximum luminance efficiency and power efficiency arrived at 19.7 cd/A and 10.6 lm/W, respectively. Our results indicate that PEIE is a promising electron injection material to realize high performance OLEDs.

A soluble nonionic surfactant as electron injection material for high-efficiency inverted bottom-emission organic light emitting diodes

We demonstrate highly efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs) by using cesium hydroxide (CsOH) doped tris-(8-hydroxyquinoline) aluminum (Alq3) as the electron injection layer on indium tin oxide cathode, which could significantly enhance the electron injection, resulting in a large increase in luminance and efficiency. The maximum luminance, current efficiency, and power efficiency reach 21000cd∕cm2, 6.5cd∕A, and 3.5lm∕W, respectively, which are 40%–50% higher in efficiency than that of IBOLEDs with cesium carbonate (Cs2CO3) doped Alq3 as the electron injection layer, where the efficiencies are only 4.5cd∕A and 2.2lm∕W. Our results indicate that CsOH doped Alq3 should be an effective electron injection layer on a wide range of electrodes to fabricate high performance OLEDs.

Cesium hydroxide doped tris-(8-hydroxyquinoline) aluminum as an effective electron injection layer in inverted bottom-emission organic light emitting diodes

Origin of improvement in device performance via the modification role of cesium hydroxide doped tris(8-hydroxyquinoline) aluminum interfacial layer on ITO cathode in inverted bottom-emission organic light-emitting diodes

By introducing tungsten oxide (WO3) doped N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) hole injection layer, the great improvement in device efficiency and the organic film morphology stability at high temperature were realized for organic light-emitting diodes (OLEDs). The detailed investigations on the improvement mechanism by optical, electric, and film morphology properties were presented. The experimental results clearly demonstrated that using WO3 doped NPB as the hole injection layer in OLEDs not only reduced the hole injection barrier and enhanced the transport property, leading to low operational voltage and high efficiency, but also improved organic film morphology stability, which should be related to the device stability. It could be seen that due to the utilization of WO3 doped NPB hole injection layer in NPB/tris (8-quinolinolato) aluminum (Alq3)-based device, the maximum efficiency reached 6.1 cd A−1 and 4.8 lm W−1, which were much higher than 4.5 cd A−1 and 1.1 lm W−1 of NPB/Alq3...

Xianfeng Qiao

Papers

The role of molybdenum oxide as anode interfacial modification in the improvement of efficiency and stability in organic light-emitting diodes

A soluble nonionic surfactant as electron injection material for high-efficiency inverted bottom-emission organic light emitting diodes

Cesium hydroxide doped tris-(8-hydroxyquinoline) aluminum as an effective electron injection layer in inverted bottom-emission organic light emitting diodes

Origin of improvement in device performance via the modification role of cesium hydroxide doped tris(8-hydroxyquinoline) aluminum interfacial layer on ITO cathode in inverted bottom-emission organic light-emitting diodes

Tungsten oxide doped N,N-'-di(naphthalen-1-yl)-N,N-'-diphenyl-benzidine as hole injection layer for high performance organic light-emitting diodes