Emission layers, OLED

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... 

Figure 7.5 shows a schematic example of the electroluminescent process in a typical two-layer OLED device architecture. When a voltage is applied to the device, five key processes must take place for light emission to occur from the device. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

Important electrical informations about OLEDs, such as charge transport, charge injection, carrier mobility, etc., can be obtained from bias-dependent impedance spectroscopy, which in turn provides insight into the operating mechanisms of the OLED [14,15,73,75 78]. Campbell et al. reported electrical measurements of a PLED with a 50-nm-thick emissive layer [75], Marai et al. studied electrical measurement of capacitance-voltage and impedance frequency of ITO/l,4-Mv-(9-anthrylvinyl)-benzene/Al OLED under different bias voltage conditions [76], They found that the current is space-charge limited with traps and the conductivity exhibits power-law frequency dependence. 

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.

Doping of ionic electroluminescent films of [Ru(bpy)3]2+ with [Os(phen)3]2+ produced single layer OLEDs with luminescence emerging predominantly from the Os MLCT excited state, but the emission energy can be tuned to some extent by varying the concentration of the dopant. The devices prove to have better stability than devices prepared from either of the pure complexes [132],... 

Yam and co-workers reported the first luminescent study of 4-coordinate gold(I) phosphine complexes, [Au(PAP)2]X (PAP = l,8-bis(diphenylphos-phino)naphthalene, 4-methyl-l,8-bis(diphenylphosphino)naphthalene X = Cl, PF6) in 2000 [28]. The complexes were shown to exhibit orange-red photoluminescence both in the solid state and in dichloromethane, and the emission origin was attributed to the triplet state derived from a [a -> 77- (naphthyl)] IL transition. The PF6 salt of the complexes were also employed as an emissive layer in the fabrication of OLEDs. 

Doped PVK thin films display intense electroluminescence from the Ndm ion and OLED devices fabricated with this active material have a maximum irradiance of 8.5 nW mm-2 and an external quantum yield of 0.007%. Further refinement of the processing will hopefully lead to a still better optimization of the performance of these Ndm-doped polymeric emissive layers (O Riordan et al., 2006). 

A benzo[ /]thioxanthene unit has been incorporated in the emission layer of an OLED display <2004USP2004140761> as part of a polycyclic pigment 614 employed for coloring polymers, inks and toners <2000USP6046335> and also as the key component 615 of a red fluorescence-conversion film and associated electroluminescent devices <1998JPP10316964>. 

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.

Table 5.2 Configuration and typical electro-optical performance of an early bilayer OLED using the low-molar-mass diamine (2) as HTL and Alqj (3) as the ETL and emission layer ...

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