Polymer blends, OLEDs

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. 

Another fluorene copolymer containing the luminescent dye [4-dicyanomethylene-2-methyl-6-4//-pyran (DCM) as acceptor compound was irradiatiated with UV light in the presence of gaseous triaUcylsilanes. This reagent selectively saturates the C = C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue-green electroluminescence from the polyfluorene units is recovered. Compared with previous processes based on polymer blends, this copolymer approach avoids problems associated with phase-separation phenomena in the active layer of OLEDs [256]. 

Although PPPs and its derivatives reveal extraordinarily high thermal and oxidative stabilities, corresponding single-layer OLEDs exhibit only low electroluminescence efficiencies. Higher efficiencies have been achieved by preparing polymer blends or by virtue of two-layer OLED-constructions. External efficiencies up to 3% were determined for an ITO/PVK/poly(2-decyloxy-l,4-phenylene)/Ca — OLED [86,87]. 

Figure 9.24 The chemical formula for the two components poly(3,4-ethylene dioxythiophene) (PEDOT) and poly(styrene sulphonic acid) (PSS) making up the PEDOT PSS polymer blend used in contact with the anode and for hole transport in polymer OLEDS. Note that the two molecules bind together strongly as a double bond is broken in the PEDOT units and an electron is transferred to the PSS, resulting in release of a hydrogen atom (which may bond with the reaetive radical site of the double bond. The molecules are held together, in part by the attraction of the ionic charges.

As well as being of fundamental interest to polymer science, wetting and phase behavior is highly pertinent to the performance of conducting polymer electronic devices such as photovoltaics or OLEDs. The formation of layers, previously established for model polymer blends, has also been seen for blends of conducting polymers. Here, such self-stratified... 

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. 

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. 

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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