Organic electronics are beginning to make significant inroads into the commercial world, and if the field continues to progress at its current, rapid pace, electronics based on organic thin-film materials will soon become a mainstay of our technological existence. Already products based on active thin-film organic devices are in the market place, most notably the displays of several mobile electronic appliances. Yet the future holds even greater promise for this technology, with an entirely new generation of ultralow-cost, lightweight and even flexible electronic devices in the offing, which will perform functions traditionally accomplished using much more expensive components based on conventional semiconductor materials such as silicon.

The path to ubiquitous and low-cost organic electronic appliances on plastic

Luminescent Functional Metal–Organic Frameworks

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Principles Of Fluorescence Spectroscopy

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

White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

White Organic Light‐Emitting Devices for Solid‐State Lighting

Introduction: Branched macromolecules or dendrimers have provided a rich seam of research in terms of both innovative chemistry and applications.1-14 For example, dendrimers have been studied for use as low-dielectric materials,15 as templates for the growth of single-wall carbon nanotubes,16 as catalysts,17-19 and in biological applications,20-23 including biosensors,24 magnetic resonance imaging,25-28 and drug delivery.29-33 However, it has only been more recently that such macromolecular structures have been explored in terms of their electronic and optoelectronic properties, which is the focus of this series of reviews. For example, charge-transporting dendrimers have become an important class of organic semiconducting material34 and significant effort has focused on light harvesting and energy transfer from a peripheral dye or chromophore to an emissive dye at the center or focus of the dendrimer.35-39 Organic semiconductors have become increasingly important as the active component in applications including organic light-emitting diodes (OLEDs),40-42 transistors,43,44 photovoltaic (PV) cells,45,46 optical amplifiers,47,48 and lasers.49-51 Traditionally, organic semiconductors have fallen into two main classes, small molecules and polymers, and these materials and their applications will be covered in detail by other authors. Small molecules are generally processed by evaporation techniques and have the advantages that the structure−property relationships are relatively simple to understand, the materials are mono(disperse), and they are deposited in a pure form. On the other hand, conjugated polymers are soluble and can be deposited from solution by processes such as spin-coating and ink-jet printing, which opens up the exciting prospect of simple, fast, large-area, low-temperature device manufacturing. An additional advantage for conjugated polymers is that solution processing is potentially less wasteful of material than evaporation for devices that require patterning. However, it is often difficult to control the polydispersity, molecular weight, backbone defects, and end groups of conjugated polymers reproducibly. Branched macromolecules, known as dendrimers, also have the advantage of being solution processable but by careful design can incorporate the control over the optoelectronic properties that is reminiscent of small molecules. In addition, the dendritic architecture provides a number of other attractive properties, including the ability to independently control the processing and optoelectronic properties; providing the processing power to enable simple chromophores to be deposited as stable amorphous films; dendrimer generation as a tool for controlling the intermolecular interactions that govern device performance; and the ability in well-defined dendrimers to have high chemical purity. In this review, we will focus on synthetic strategies that have been investigated for the preparation of optoelectronically active solution-processable dendritic materials and concentrate on two different applications, namely OLEDs and solar cells, in which they have been used. In the context of OLEDs, we limit the discussion to light emission, as branched macromolecules for charge transport will be discussed in the review by Shirota. We will also briefly comment on other recent light-emitting and -absorbing branched molecular materials that have been used in OLEDs and solar cells.

Development of Dendrimers: Macromolecules for Use in Organic Light-Emitting Diodes and Solar Cells

Dendrimers are now an important class of light-emitting material for use in organic light-emitting diodes (OLEDs). Dendrimers are branched macromolecules that consist of a core, one or more dendrons, and surface groups. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. The first light-emitting dendrimers were fluorescent but more recently highly efficient phosphorescent dendrimers have been developed. OLEDs containing light-emitting dendrimers have been reported to have external quantum efficiencies of up to 16 %. The solubility of the dendrimers opens the way for simple processing and a new class of flat-panel displays. In this Review we show how the structure of the light-emitting dendrimers controls key features such as intermolecular interactions and charge transport, which are important for all OLED materials. The advantages of the dendrimer architecture for phosphorescent emitters and the way the structure can be varied to enhance materials performance and device design are illustrated.

The Development of Light-Emitting Dendrimers for Displays†

We report a new family of homoleptic iridium(III) complexes that emit blue phosphorescence at room temperature. The iridium( III) complexes are comprised of phenyltriazole ligands and were easily prepared via short synthetic routes. The parent fac-tris(1-methyl-5-phenyl-3-propyl-[1,2,4]triazolyl)iridium(III) complex exhibits blue photoluminescence (PL) with emission peaks at 449 and 479 nm and has a solution PL quantum yield of 66%. The emission was sequentially blue-shifted by the attachment of one and two fluorine atoms to the ligand phenyl ring with the fac-tris{1-methyl-5-(4,6-difluorophenyl)-3-propyl-[1,2,4]triazolyl} iridium( III) complex having the 1931 Commission Internationale de l'Eclairage coordinates of (0.16, 0.12) at room temperature. In contrast, when the phenyl ring of the ligands was substituted by trifluoromethyl, the PL spectrum was red-shifted when compared to the parent compound whereas if the trifluoromethyl group was attached to the triazole ring, the emission was blue-shifted. The radiative rates of these new blue iridium( III) complexes were found to be in the range of 2-6 x 105 s(-1), indicating that the emission had varying amounts of metal-to-ligand charge-transfer character. Molecular orbital calculations showed that for the fluorinated complexes the contribution of the ligand triplet character to the emissive energy state increased with the hypsochromic shift in emission. This was confirmed by time-resolved PL measurements, which showed that the complex with the deepest blue emission had the slowest radiative decay rate.

Blue Phosphorescence from Iridium(III) Complexes at Room Temperature

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

Green Phosphorescent Dendrimer for Light‐Emitting Diodes

We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

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Shih-Chun Lo

Papers

Development of Dendrimers: Macromolecules for Use in Organic Light-Emitting Diodes and Solar Cells

The Development of Light-Emitting Dendrimers for Displays†

Blue Phosphorescence from Iridium(III) Complexes at Room Temperature

Green Phosphorescent Dendrimer for Light‐Emitting Diodes

High-efficiency green phosphorescence from spin-coated single-layer dendrimer light-emitting diodes