Blends OLEDs

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

ErQ3 gives both PL and EL emission at 1,500 nm.207-209 Red light (614 nm) is emitted by [Eu(TTFA)3(phen)] (Figure 2(a)) doped in a poly(A-vinylcarbazole) layer.210 The complex [Eu(TTFA)3(TPPO)2] (TPPO = triphenylphosphine oxide, see Figure 2(f)) emits in the red when incorporated in OLEDs,211,212 behavior which is enhanced when the material is heat treated.204 Near-IR EL and PL emission was observed from blends of polyfp-phenylene-vinylene) with [Yb(TPP)(ACAC)] and [Er(TPP)(ACAC)] (H2TPP is 5,10,15,20-tetraphenylporphyrin) by both EL and PL 213... 

Conjugated organic polymers such as those shown in the Tables have been used in multilayer OLEDs as the HTL or combined HTL and emission layers or as the ETL or combined ETL and emission layer. The combined polymers (75-77) shown in Table 6.15 have been used as combined ETL, HTL and emission layers in various OLED configurations. Blends of these polymers have also been used to maximise OLED efficiency, although phase separation is always a problem with mixtures (blends) of main-chain polymers. 

Thermal evaporation is one of the most efficient methods for the preparation of diketo-nate complexe layers. However, many of these compounds are thermally unstable at high temperatnre and reqnire an alternative method. Milder conditions are attained on incorporating snch componnds into polymer host materials, followed by thin film deposition by the spin coating method. In these cases, the luminance of OLED devices is comparable to those measnred for devices fabricated using small molecules and polymer blends. 

In this chapter, a review of our work regarding the chemical, electrochemical, and morphological stability of vacuum-deposited Alq3 is presented. The effect of these processes on charge transport and light emission in OLEDs is also considered, and the readers may refer to some published work for more quantitative analysis.35-37 The efficacy of simple passivation schemes as well as the fabrication of crystallization-resistant blends will also be briefly discussed. 

It is not necessary to use only dyes to take advantage of the energy transfer blends of two polymers can also be used as host-guest systems (Lee et al 2002). The guest molecules can be florescent or phosphorescent in nature. However, phosphorescent dyes based on Ir and Pt complexes have provided significantly higher efficiency of OLEDs because of their ability to emit from both singlet and triplet excitons of the host molecule (Kamata et al 2002),... 

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