Fabrication techniques, OLEDs

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. 

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. 

Although ITO is still one of the most widely used anode materials for OLEDs, other alternatives suited for OLEDs may also be used for making optical destructive anode for high contrast OLED displays using this technique. For example, a multilayer optical destructive anode may be fabricated using other oxide materials, including Sn02, FTO, AZO, IZO,... 

OLEDs grown and encapsulated using these techniques are beginning to show significant promise. Recently, Chwang et al. demonstrated the effects of flexing a 64x64 (180 dpi) passive-matrix flexible OLED (FOLED) display fabricated on a PET substrate with thin film encapsulation [166], In addition, lifetimes of thin-film-encapsulated OLED test pixels on flexible substrates have now been demonstrated to be thousands of hours [162,167],... 

The OLEDs were fabricated on the pre-patterned, pre-cleaned indium tin oxide (ITO) coated glass substrates. The substrates were patterned using standard photolithography technique and then cleaned with soap solution followed by boiling in trichloroethylene and isopropyl alcohol. The films were finally dried under vacuum. After cleaning the... 

Although several methods may be used to fabricate the organic layers in OLEDs, such as thermal evaporation, Langmuir-Blodget deposition or spin coating from solutions, RE + /3-diketonates usually are deposited by thermal evaporation techniques, under high vacuum ca 10 Pa) thus, volatility and thermal stability of the diketonates are required. ... 

The existing OLED fabrication procedures fall into two major categories (1) thermal vacuum evaporation of the organic layers in small molecular OLEDs, and (2) wet coating techniques of the polymer layers in PLEDs. 

In this technique, a film of the solution containing the soluble polymer is spread with uniform thickness over the substrate using a precision doctor blade. 53 In contrast to spin-coating, the doctor-blade technique is very useful for fabricating relatively thick films, but does not enable the fabrication of films < 100 nm thick, which are commonly used in OLEDs. 

Chapters 7-9 report new strategies to improve the characteristics of organic display, such as the contrast and the mechanical flexibility. Indeed high contrast and mechanical flexibility are the real factors which make these devices strongly competitive with those based on liquid crystals. In conclusion, chapter 10 shows the technology to fabricate efficient white light OLEDs for lighting applications. In particular, the various techniques to improve the... 

The first example of using the [2+2] cycloaddition for crosslinking of organic semiconductor layers was reported in 1997 by Remmers et al. [10]. It was a derivative of poly-p-phenylene (PP) deposited by the Langmiur-Blodgett (LB) technique (Fig. 9.5(a)). Polarized absorption and fluorescence of the films was reported, but no OLED devices were fabricated. 

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