Light-emitting diodes are budget-friendly solid-state light sources that find use in the very diverse areas: from lighting to data display devices. The history of the discovery of the electroluminescence phenomenon and development of the technology of inorganic (LED) and organic light-emitting diodes (OLED) is presented. Today, there has been a widespread transition to light-emitting diode sources based on inorganic semiconductors, ensuring significant energy savings. However, OLED technologies continue to undergo intensive development. Advantages of OLED over other light sources and areas of their practical use are considered. We discussed the fundamental principles of manufacturing organic light sources with a white emission spectrum, based on low-molecular-weight and polymeric compounds, and considered various approaches for developing polymeric materials with a white emission spectrum. Prospects of the development of the market of OLED light sources are evaluated.

The article represents a translation of Section 6.2 of the monograph authored by A.S. Burlov, V.G. Vlasenko, D.A. Garnovskiy, A.I. Uraev, E.I. Maltsev, D.A. Lypenko, and A.V. Vannikov, Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information), dedicated to reviewing the photoluminescence and electroluminescence properties of metal complexes based on (Zn, Cd, Al, Sc, Be, B) with azoles (2-(2-hydroxyphenyl)benzothiazoles, 2-(2-hydroxyphenyl)benzoxazoles, 2-(2-hydroxyphenyl)benzimidazoles, azaindoles, oxadiazoles, and their derivatives) as ligands. Modifying such ligands by introducing electron-donating substituents into various positions of the phenyl fragments leads to significant changes in their electroluminescence characteristics. Metal complex compounds with ligands of this type are widely used for creating OLED devices emitting white light.

This review article explores and analyzes published data on the photoluminescence and electroluminescence characteristics of rare earth element (REE) complex compounds, which are promising materials for creating highly efficient OLED devices. Distinguishing features of such compounds include their narrow emission bands, Stokes shift, long lifetime, and high quantum yields, enabling high resolution and color purity in constructed OLED devices. Using REE complexes as emissive layers in light-emitting diodes ensures significant color saturation and high device efficiency.

Basic processes of the electroluminescence phenomenon in OLED structures are described. The conventional theoretical models for injection, transport and recombination of charge carriers and their relationship with the obtained experimental data are discussed. The mobility of charge carriers is a key characteristic of the electron and hole transport. Methods for measuring mobility that adequately reflect the transport of charge carriers in OLEDs are considered.

The article represents a translated, revised, and updated Section 6.3 of the monograph by A.S. Burlov, V.G. Vlasenko, D.A. Garnovskiy, A.I. Uraev, E.I. Maltsev, D.A. Lypenko, and A.V. Vannikov, Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). This section is dedicated to reviewing the photoluminescence and electroluminescence properties of metal complexes based on bi- and polydentate ligands of the azomethine type (Schiff bases). Many metal complexes of Schiff bases find wide use as luminescent materials. One of the applications of these compounds is the use as emissive layers or dopants for manufacturing OLED. The photo- and electroluminescence properties of a series of the metal complexes with the ligands based on the derivatives of hydroxybenzaldehyde, 2-(N-tosylamino)benzaldehyde, 1‑phenyl-3-methyl-4-formylpyrazol-5-one, and some others are reviewed. The systematic categorization of these compounds extends beyond their photophysical properties to consider the electroluminescence characteristics observed in OLED devices employing them as the basis. By modifying known azomethine-type ligand systems, one can obtain metal complex compounds with exceptional thermal stability, solubility, and quantum efficiency. These characteristics enable their use as luminescent materials in highly efficient OLED devices.

The article represents a translated, revised, and updated excerpt from the book Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). The main technological stages for manufacturing of laboratory samples for electroluminescence are described in general terms. The most popular laboratory techniques for the formation of thin semiconducting organic films, such as solution spin coating and vacuum thermal deposition, are considered in more detail. Some methodological approaches used in our laboratory are outlined. Measurements of the polymer layer thickness by the interference method and by atomic force microscopy are considered in detail. For the interference method, the principal sources of systematic errors are examined. Concerning atomic force microscopy (AFM), two techniques for measuring thickness are considered: the express technique (macro-needle scratching) and scratching with an AFM probe in contact mode. Systematic errors associated with the first technique are determined, followed by recommendations regarding its potential application. The last section highlights the necessary adjustment for calibrating thickness sensors during film deposition if the calibration is conducted based on macro-needle scratching results.

The review is devoted to analysis of photo- and electroluminescent properties of metal complexes (Al, Be, Mg, Ga, In, Zn, and B) based on substituted quinoline derivatives. Literature data shows that changing the substituents in quinoline ligands and the metal complexing agent can affect the color and intensity of photoluminescence in the corresponding metal complexes. OLED devices fabricated based on these complexes have exhibited high brightness and efficient electroluminescence across a broad spectral range from red to blue. Furthermore, these compounds possess excellent electron-transport properties, enabling their utilization as conductive layers in OLEDs.

This review summarizes the results of the synthesis, photoluminescence, and electroluminescence of the main classes of cyclometallated iridium(III) complex compounds. The primary focus of the review is on heteroleptic iridium(III) complexes, which contain two equivalent C^N ligands and an additional auxiliary ligand. Numerous examples demonstrate that the selection and modification of the ligand systems in iridium(III) complexes, using various donor or acceptor groups, alter significantly their spectral characteristics, often achieving record levels of brightness and emission efficiency for OLED devices manufactured based on them.

The article represents a translated, revised, and updated excerpt from the book Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). The measurement of performance characteristics of organic light-emitting diodes (OLEDs), including current–voltage, luminance–voltage, and spectral characteristics is described in detail in relaton to the laboratory measuring facility existing at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences. A detailed description of this setup, including the essential minimum set of instruments required for characterizing OLEDs, can serve as a guide for researchers embarking on experiments in the field of electroluminescence. The second part of the article describes the principal limiting factors for OLED efficiency.

The structures of J -aggregates of two monomethine dyes are compared at the meso- and nanoscale by using fluorescence and atomic force microscopy. The J -aggregates of the two dyes are found to have solely a monolayer structure and to be polymorphic. Morphologically, monolayers represent narrow skew-symmetric strips and symmetric rhombi in the case of the first and the second dyes. The second morphological type is the tubular type in both cases. For monolayers of both dyes, atomic force microscopy imaging at the ultimate resolution reveals the presence of a quasi-one-dimensional substructure consisting of densely packed nanostrips with a width in the range of 7–10 nm. The nanostrips are interpreted as building blocks emerging at early stages of monolayer crystallization that proceeds via a nonclassical multistep mechanism.

The effect of addition of J -aggregates of polymethine dye into the hole-transport layer of an organic light-emitting diode (OLED) on its characteristics and operational stability was studied. Water-soluble PEDOT:PSS (poly(3,4-ethylenedioxythiophene)) and the interpolymer complex of polyaniline and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAn-PAMPSA) were used as the matrices of polymer nanocomposites. The highest efficiency characteristics of the OLEDs with the considered structure were achieved using PAn-PAMPSA at a particular component ratio in the interpolymer complex. Additives of J ‑aggregates did not significantly affect the efficiency characteristics of the OLEDs; however, an increase in the operational stability of devices was recorded.

Abstract—J-aggregates of a carbocyanine dye (pyridinium salt of 3,3'-di-(γ-sulfopropyl)-4,5,4',5'-di(tieno-3'',2'')-9‑ethyl-tiatrimethine cyanine betaine) (CC) were studied via the high-resolution AFM method at the mesoscale and nanoscale. They were found to exist in two structurally different polymorphic forms of single layers and fibrils. The class of single-layer J-aggregates consists of irregular micron sheets and giant submicron tubes, whose structure is identical to tubular J-aggregates of other cyanine dyes. The fibrillar J-aggregates are composed of 3-nm-high fibrils forming the networks owing to merging and branching, as well as of “elementary” fibrils with a height close to the molecular sizes of dye. Structural models of elementary fibril with molecular “staircase” and “ladder” stacking types were proposed as well.

A crystallographic analysis of the images of destruction fragments of tubular monomethine dye J -aggregates has been performed for the first time. It is shown that the formation of characteristic zigzag fragments is caused by the unwrapping of cylindrical surface on plane. Possible cleavage directions for the monolayer forming the tube walls are considered and an interrelation between the destruction products and molecular packing of this monolayer is established.

New Zn(II) and Cd(II) complexes with N-{2-[(E)-(4-tert-butylphenyl)iminomethyl]phenyl}-4-methylbenzenesulfonamide were synthesized by chemical and electrochemical methods. The ligand and metal complexes were characterized by the methods of IR, 1H NMR, electron absorption spectroscopy, and X-ray absorption spectroscopy and by quantum chemical calculations. Electroluminescent devices with zinc and cadmium complexes were fabricated as emitter layers, which showed radiation brightness of 460 cd/m2 (15 V) with a maximum at 502 nm and 140 cd/m2 (15 V) with a maximum at 535 nm, respectively.

Meso- and nanoscale structural polymorphism of monolayer J-aggregates of four cyanine dyes are investigated. Three mesoscale morphological types are observed: ribbons, rhombic leaves, and tubes. Tubes are formed during cylindrical spinning of ribbons. Lined substructure characterized by the line width of about 7 nm is found in the case of monolayers of monomethine cyanine dyes at the nanoscale.