This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

The electroluminescence of organic materials

Electroluminescent devices based on organic materials are of considerable interest owing to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the device construction and operating principles, a review is presented on recent progress in organic electroluminescent materials and devices. Small molecular materials are described with emphasis on their material issues pertaining to charge transport, color, and luminance efficiencies. The chemical nature of electrode/organic interfaces and its impact on device performance are then discussed. Particular attention is paid to recent advances in interface engineering that is of paramount importance to modify the chemical and electronic structure of the interface. The topics in this report also include recent development on the enhancement of electron transport capability in organic materials by doping and the increase in luminance efficiency by utilizing electrophosphorescent materials. Of particular interest for the subject of this review are device reliability and its relationship with material characteristics and interface structures. Important issues relating to display fabrication and the status of display development are briefly addressed as well.

https://ir.nctu.edu.tw:443/bitstream/11536/28337/1/000179449700001.pdf

Recent progress of molecular organic electroluminescent materials and devices

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

Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.

Universal energy-level alignment of molecules on metal oxides

Spiro Compounds for Organic Optoelectronics

We demonstrate that by using thin films of metal oxides, such as vanadium oxide (VOx), molybdenum oxide (MoOx) and ruthenium oxide (RuOx), as a hole-injecting layer for an organic electroluminescent (EL) device consisting of N,N'-diphenyl-N,N-bis(3-methylphenyl1)1,1'-biphenyl-4,4' diamine (TPD) and tris-(8-quinolinolato) aluminium (Alq), the EL device performance can be significantly improved. The `operating voltage' of the device is reduced with respect to a device with a well known indium - tin-oxide (ITO) electrode for hole injection. We consider that the improvement of the operating voltage is attributable to the lower energy barrier for hole injection at the metal oxide/TPD interface.

https://iopscience.iop.org/article/10.1088/0022-3727/29/11/004/pdf

Metal oxides as a hole-injecting layer for an organic electroluminescent device

PROBLEM TO BE SOLVED: To provide an organic thin film luminescent element, which can reduce the energy barrier between a positive hole transporting layer and a light emitting layer and which can emit the light at a low voltage and which has a high light emitting efficiency, by using a metal oxide thin film having a work function larger than that of the indium tin oxide(ITO) for positive electrode. SOLUTION: This luminescent element is formed by forming a positive electrode 12 on a substrate 10, and forming an organic compound layers such as a positive hole transporting layer 18 and a light emitting layer thereon, and forming a negative electrode 14 thereon. In this case, the positive electrode 12 is made of the metal oxide thin film having a work function larger than that of ITO or the two-layer structure thin film forming the two-layer structure with ITO.

Organic thin film luminescent element

A micro-optical resonator type organic electroluminescent device utilizing an organic electroluminescent material high in emission efficiency but broad in emission spectrum width, has a narrow half bandwidth in the emission peak and a superior forward directivity and an excellent monochromaticity in light not separate into components, wherein a micro-optical resonator is formed by a multi-layered mirror and a metal mirror. The optical length of the micro-optical resonator, calculated from the total thickness and the refractive index of a transparent conductive layer and a luminous layer placed between the multi-layered mirror and the metal mirror and also from the light penetration depth into the multi-layered mirror, is 1.5 times the wavelength of a desired light emitted from the device, and the desired light is set on the short wavelength side of the emission spectrum of the electroluminescent material.

Micro-optical resonator type organic electroluminescent device

A novel trispirocyclic hydrocarbon having three 9-fluorene moieties around the core of truxene (5) was prepared readily via coupling of truxenone with 2-bromobiphenyl; 5 was a high melting (>500 °C) solid. For the application of 5 to an effective hole transport material (HTM) in the OLED, a triphenylamine derivative carrying six diphenylamino groups at the 2- and 7-positions of each 9-fluorene moiety (6) was designed in order to get high thermal stability as an improved material of the TPD type HTM. The synthesis of 6 was easily achieved using 4,4′-bis(diphenylamino)-2-bromobiphenyl (9). The trispirocyclic 6 was found to show a glass transition temperature as high as 170 °C. It effects the formation of its stable cation radical upon electrooxidation in solution, and amorphous thin films in solid. A multi-layered EL device for 6 as an HTM using Alq3 as an electron transporting emitter showed good EL characteristics such as the maximum luminance of 37 000 cd m−2 at 14 V. Thus, the hexakis(diphenylamino)substituted trispirocycle 6
				(TX-F6S) can be used as an efficient and thermally stable HTM in OLEDs.

New 9-fluorene-type trispirocyclic compounds for thermally stable hole transport materials in OLEDs

An organic electroluminescent device includes an anode, a cathode, a hole injection layer, a hole transport layer, an electron capture layer, a luminescent layer, and an electron transport layer. The five layers are sequentially stuck in the above order between the anode and the cathode in the direction toward the cathode. The luminescent layer includes a host organic material, the luminescent spectrum of which has a peak between 380 nm and 510 nm, and a guest fluorescent dye. The hole injection layer and the hole transport layer respectively include a host organic material. The lowest energy level in the conduction band of the host organic material of the electron capture layer is lower than those of the hole transport layer and the luminescent layer. Therefore, the device can emit light including blue light component while the host organic material of the hole transport layer is prevented from deteriorating.

Koji Noda

Papers

Metal oxides as a hole-injecting layer for an organic electroluminescent device

Organic thin film luminescent element

Micro-optical resonator type organic electroluminescent device

New 9-fluorene-type trispirocyclic compounds for thermally stable hole transport materials in OLEDs

Organic electroluminescent device