Electroluminescence (EL) of organic materials has been known since the 1960s. Resulting devices are commonly called organic light-emitting diodes (OLED). Intense research activity, however, started in the late 1980s after the development of the multilayered cell structure by Tang and Vanslyke, who demonstrated the bright green emission of aluminum tris(8-hydroxyquinolinate) (Alq3) in thin film organic layers. Alq3 and phenylamine contribute to high-performance EL devices because they can form uniform thin films under 1000 Å and have good carrier transport properties and strong fluorescence. Coordination complexes are a class of main electroluminescent materials, which have both the high fluorescence quantum efficiency of organics and the good stability of inorganics. Therefore, complexes are regarded as one of the most promising luminescent materials for application. The coordination complexes used for electroluminescence require saturated metal coordination numbers, electrical neutrality, excellent vacuum sublimation properties and good solubility in organic solvents.
Application
Coordination complexes have been widely used as electroluminescent materials in EL devices, and a few simple examples are given below.
- 8-hydroxyquinoline derivative metal complexes
Alq3 is generally considered the best electroluminescent material for organic EL devices. It has excellent properties as an electroluminescent material, namely, highly stable film formation characteristics, good carrier transport properties, and good heat resistance. It is widely believed that other 8-hydroxyquinoline derivative metal complexes can be used as electroluminescent materials for EL devices, because studies have shown that other 8-hydroxyquinoline derivative metal complexes also exhibit fluorescence.
Europium complexes are known as excellent red phosphors which exhibit intense fluorescence at around 610 nm with a narrow spectral bandwidth. Kido and his co-workers [1] reported the fabrication of an EL device with Eu (TTA) as an electroluminescent material, in which Eu (TTA) was introduced into a spin-coated film by mixing it with a hole transporting polymer. They reported a red emission from the Eu complex with a maximum luminance of 0.3 cd/m2.
The most promising candidates are iridium (III) phenylpyridine complexes, which are reported to be widely used as electroluminescent materials in EL devices. For example, Thompson and co-workers doped three Ir complexes into the emissive region of multilayer, vapor-deposited OLEDs. The Ir-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current-voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%.
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Reference
- Skotheim, T.; et al. Electroluminescence from polysilane fifilm doped with europium complex. Chem. Lett. 1991, 1267-1270.
