A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V.

Organic Electroluminescent Diodes

In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illuminati...

Small molecular weight organic thin-film photodetectors and solar cells

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (approximately 15 lm W(-1)) incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron-hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 +/- 0.5 per cent and 37.6 +/- 0.6 lm W(-1), respectively, decreasing to 18.4 +/- 0.5 per cent and 23.8 +/- 0.5 lm W(-1) at a high luminance of 500 cd m(-2).

/pdf/management-of-singlet-and-triplet-excitons-for-efficient-4jwg5omlrj.pdf

Management of singlet and triplet excitons for efficient white organic light-emitting devices

During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

Light-emitting devices based on organometallic platinum complexes as emitters

Magnetic field effects on emission and current in Alq3-based electroluminescent diodes

We examine various electronic processes that underlie the quenching of the emission from highly efficient phosphorescent and electrophosphorescent organic solid-state molecular systems. As an example, we study the luminescent efficiencies from the phosphorescent iridium (III) complex, fac tris (2-phenylpyridine) iridium $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$ doped into a diamine derivative doped polycarbonate hole-transporting matrix and in the form of vacuum-evaporated films, as a function of electric field. We demonstrate that the observed decrease in electrophosphorescence efficiencies at high electric fields, and electric-field-induced quenching of phosphorescence from neat $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$ solid films is due to the field-assisted dissociation of Coulombically correlated electron-hole (e-h) pairs. They are formed in a bimolecular recombination process prior to the formation of emissive triplet excitons, or are charge-transfer (CT) states originating from the localized electronic excited states as a result of the initial charge separation upon photoexcitation, respectively. It is found that the high-field dependence of the quenching efficiency in both cases follows the three-dimensional Onsager theory of geminate recombination, the fit yielding the initial intercarrier distance ${(r}_{0})$ of the carrier pairs. We find ${r}_{e\ensuremath{-}h}g~3.5\mathrm{nm}$ for the triplet exciton precursor pairs in the bimolecular recombination, and ${r}_{\mathrm{CT}}=1.8\ifmmode\pm\else\textpm\fi{}0.1\mathrm{nm}$ for the initial carrier separation from the photo-excited electronic states. Triplet-triplet and triplet-charge carrier annihilation processes are shown to play major roles in the decrease of the electrophosphorescence efficiency within the lower-field regime at lower current densities. Summarizing the results allows us to point out some emitter features important for identifying phosphors useful for practical electroluminescent devices.

Quenching effects in organic electrophosphorescence

Mixing of Excimer and Exciplex Emission: A New Way to Improve White Light Emitting Organic Electrophosphorescent Diodes

There is growing interest in organic electroluminescence (EL). A great deal of progress has been made recently in improving the performance of various classes of organic EL devices. Some of these are now adequate for many applications. However, specialists focusing on selected aspects of organic EL devices have often lost contact with the general subject of EL. Therefore, a review covering all aspects of EL mechanisms and their experimental manifestation seemed necessary. This article is concerned with the new EL device physics that can be realized using crystals, or films made of organic materials, as electrically and optically active components, in devices ranging from simple single-component light emitting diodes (LEDs), through double- and multi-layer LEDs to light emitting electrochemical cells (LECs) and organic LED-based light transducers. The investigation of the properties of these devices has provided in turn a very effective method for studying the basic EL phenomena in these materials. Since the subject of the present review has generated a huge amount of literature, and it is impossible to mention here all that has been done, we have attempted to provide an outline of the background of the field of organic EL, and discussed in some detail those aspects most relevant to the EL device physics. Because of the diversity of the types of material and EL structure, there is no single, simple description of EL in organics. Therefore, the initial sections of the article are devoted to a discussion of the types of EL and related phenomena, such as carrier injection and recombination or nature of emitting states. Then, the fundamentals of the fabrication of various types of EL devices are discussed along with the most representative examples. In general, the reader will find in the article a brief historical review of the subject as well as a description of the latest trends in organic EL research covering all the new concepts and most important data which have become available before the time of publication.

https://iopscience.iop.org/article/10.1088/0022-3727/32/24/201/pdf

Jan Kalinowski

Papers

Light-emitting devices based on organometallic platinum complexes as emitters

Magnetic field effects on emission and current in Alq3-based electroluminescent diodes

Quenching effects in organic electrophosphorescence

Mixing of Excimer and Exciplex Emission: A New Way to Improve White Light Emitting Organic Electrophosphorescent Diodes

Electroluminescence in organics