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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.

In the past two decades a significant research effort has focused on the photophysical properties of advanced organic materials for optoelectronic devices. Examples include the development of organic white light emitting diodes (WLEDs) since they are at the heart of display technology [1] and offer potential applications as novel lighting sources that are less expensive and more efficient than conventional incandescent and fluorescent illumination sources. [2][3][4] The most impressive characteristics of organic WLEDs are those based on single emissive dopant, where molecular excitons are harnessed to form triplet excimers. [5][6][7][8] They combine molecular phosphorescence with the red-shifted excimer phosphorescence that yields the emission approaching white light. Here, we use electron donor-electron acceptor emitter layers, in which broad emission band of an exciplex mixes with excimer emission, enabling us to form an efficient white device with particularly high color rendering index of CRI = 90. One of the most stormily developing classes of organic photonic devices nowadays are organic light-emitting-diodes (LEDs).[1] Among them, white-light-emitting-diodes (WLEDs) are of particular interest because they offer low-cost alternatives for back-lights in flat panel displays and are considered as future illumination sources which are able to operate at low voltages with high luminance efficiency. [2][3][4] By definition, the emission spectrum of WLEDs must cover possibly uniformly the whole visible spectrum of electromagnetic radiation. Several routes have been employed to realize this goal, the fabrication of stacked [3] or multilayer [9] LED structures with separated molecular emitters, that is blue, green and red, was proposed initially. Recently, a high-performance organic WLED has been fabricated that exploits three different emitters mixed together in one emissive layer to get stable color balance at a high external electroluminescence (EL) quantum efficiency (QE) of 11 % photons/electron (ph/e) and color rendering index CRI = 85.[10] The blue fluorescence of a dye dopant was mixed with green and red emissions of two phosphorescent dopants. However, this approach requires a very careful adjustment of the concentration of each dye because energy transfers from the higher energy blue dye to the green dye and from the green dye to the red dye. A simplification of the device structure can be achieved by combining molecular (monomer) and excimer phosphorescence from one emitter doped in a single emissive layer. [5,7,8] Single dopant WLEDs give voltage independent white emission with external EL QE as high as 16 % ph/e [7,8] but, due to the excessive distinctiveness of individual components in the structured emission spectra, their CRIs do not exceed 75. [5,7,8] In efforts to improve the WLEDs based on a combination of monomer and excimer spectra, we now report the achievement of well balanced efficient white emission from a single emissive layer comprised of an electron donor (D) and an electron phosphorescen...

Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature

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