Electron transport emission, OLEDs

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

The broad PL emission spectra of some metal chelates match the requirements for white emission. Hamada et al. investigated a series of Zn complexes and found bis(2-(2-hydroxy-phenyl)benzothiazolate)zinc (Zb(BTZ)2, 246) is the best white emission candidate. An OLED with a structure of ITO/TPD/Zn(BTZ)2/OXD-7/Mg In showed greenish-white emission with CIE (0.246, 0.363) with a broad emission spectrum (FWHM 157 nm) consisting of two emission peaks centered at 486 and 524 nm (Figure 3.14) [277], A maximum luminance of 10,190 cd/m2 at 8 V was achieved. The electronic and molecular structure of Zn(BTZ)2 have been elucidated by Liu et al. [278]. There is evidence that the dimeric structure [Zn(BTZ)2]2 in the solid state is more stable than its monomer Zn(BTZ)2. They also found that the electron transport property of Zn(BTZ)2 is better than that of Alq3. 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

An example "double heterostructure" OLED shown in Figure 7c uses an ITO coated glass substrate, upon which a hole transporting layer, typically composed of a tertiary amine (eg, IV,IV-biphenyl-A IV7-bis(3-methylphenyl)l-l biphenyl-4,4 diamine, abbreviated TPD), a thin film of an emissive material such as aluminum-8-hydroxyquinoline(Alq3) and an electron-transporting layer (often an oxidiazole derivative) are sequentially deposited in vacuum (Fig. 

Schematic representation of a bilayer OLED using low-molar-mass materials incorporating a combined hole-transport and emission layer and an electron-transport layer situated between a transparent anode and a cathode.

There are several possibilities for constructing bilayer OLEDs with a more balanced charge injection. These include an electron-transport layer (ETL) and a combined hole-transport (HTL) and emission layer. Conversely a hole-transport layer and a combined electron-transport and emission layer is also effective. 

There are several reports of trilayer OLEDs using three distinct layers for electron transport, hole transport and emission. This device configuration has the advantage that each layer can be optimised for one distinct function, i.e. hole-transport, electron-transport and light emission. The problems associated with the fabrication of OLEDs with three distinct polymer layers of controlled thickness, integrity and homogeneity by the technique of deposition from solution by spin-coating are not inconsiderable. 

Figure 6.7 Plot of the relative intensity (A.U.) of polarised electroluminescence against wavelength (nm) of a bilayer OLED consisting of an electron-transport and emission layer represented by a nematic network formed by polymerising compound (86) with isotropic UV light and a combined hole-transport and a coumarin non-contact alignment layer doped with 4.4,4-tris(naphthylyl)-N-(phenylamino) triphenylamine.

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