Seok-Ho Hwang, Charles Moorefield and George Newkome from the University of Akron, US, examine the use of dendrimers in organic light-emitting diodes.

It has been projected that one billion US dollars in business will be generated during 2008 from the use of organic light-emitting diodes (OLEDs) in display devices. As a result, companies such as Samsung, Sony and LG are greatly interested in OLED technology. OLEDs possess a number of advantages over conventional, non-organic light-emitting diodes including higher luminous efficiency; faster response time; lower power consumption; lower cost; lighter weight; and higher brightness and contrast, which eliminates the necessity for backlighting. OLED displays can be built on large, rigid or flexible substrates and in a virtually unlimited choice of colours.

Branched architectures shine brightly in the quest for efficient components for OLEDs

And so government agencies, industry and academia are carrying out intense research in an attempt to refine and control the electro-optical properties of OLEDs. The OLED field can be divided into three groups of electroluminescent host materials: small organic molecules, polymers and dendritic macromolecules. Dendritic chemistry - which helps to integrate classical small molecule property control with macromolecular material design and processability - is particularly promising in the field.

Dendritic architectures consist of a core, branch junctures and connectors and termini. Each dendritic component of an OLED can be best chosen to fulfill the required function. For example, the use of a conjugated polymeric core can define the emitted colour; the branching moieties can be designed to aid charge transport to or from the core; or the surface groups can be matched to the processing properties (for example, the required solubility).

To date, much of the research involving dendrimers as OLED components has focused on their use in the electroluminescent, emissive layer of the device. For example, branched, stilbenoid materials have been shown to display blue emission and increase device lifetime and stability, while elegant polyphenylene chemistry has been used to design red-orange OLEDs. And phosphazene-cored dendrimers with amino-pyrene moieties have shown photoluminescent quantum efficiencies in the range of 67 to 83 per cent. Hyperbranched and dendronised polymer constructs are also under investigation.

Logically, metallodendrimers have been examined, in particular those made with europium(II), platinum(II), and iridium(III), due to their potential to act as highly efficient phosphorescence emitters; reports of theoretical internal quantum efficiencies of 100 per cent are known. Recent integration of hole-transporting, carbazole dendrons with iridium(III) complexes reportedly yielded soluble, easily manipulated phosphorescent materials with quantum yields of 87 and 45 per cent for solution and film measurements, respectively.

The advantages of dendritic macromolecules are straightforward when their structure is considered. The surface groups can be tuned independently of the core, allowing the processing properties, such as solubility, to be altered easily. Interfacial contact between surface groups helps avoid aggregate formation, which is often observed in non-dendritic materials. Finally, band gaps can be modulated, from the outer to the inner dendritic regions, to provide the OLED components with the best charge transport properties.

Though the field is growing rapidly and its impact is far-reaching, major challenges still remain. The lack of highly efficient, stable organic light-emitting materials, the short lifetime of OLEDs and low large-scale manufacturing yields are particular problems. These drawbacks can only be overcome by an exceptional interdisciplinary research effort bridging physics, chemistry and materials sciences. Progress towards higher efficiencies using dendritic architectures will undoubtedly generate new OLED applications, for example in mobile phones, portable electronic games, personal digital assistants and tomorrow's yet-to-be-created gadgets and toys.

Read more in 'Dendritic macromolecules for organic light-emitting diodes' in issue 11 of  Chemical Society Reviews.

 

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