Wet-coating techniques

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. 

One of the most salient advantages of thermal vacuum evaporation is that it enables fabrication of multilayer devices in which the thickness of each layer can be controled easily, in contrast to spin coating (see below). In addition, 2-dimensional combinatorial arrays of OLEDs, in which two parameters (e.g., the thickness or composition of two of the layers) may be varied systematically across the array, can be relatively easily fabricated in a single deposition procedure.50,12 This combinatorial fabrication greatly enhances the efficiency of systematic device fabrication aimed at optimizing the various parameters. 

Although the thickness of spun-coated films may be controlled by the concentration of the polymer in the solution, the spinning rate, and the spin-coating temperature, it is difficult to fabricate thick films and the thickness obviously cannot be monitored during deposition. In addition, no combinatorial fabrication methods have been developed for spun- coated PLEDs (see above). 

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. 

Other important techniques currently studied in the area of wet casting are screen printing, micro-stamping, and hot microprint contact.57 

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Wet-on-wet coating Technique of painting whereby further coats are applied before the previous coats have dried, and the composite film then dries as a whole. The process requires specially formulated paints. 

Hot Dip Tin Coating of Steel and Cast Iron. Hot dipping of tin [7440-31 -5] has been largely superseded by electrolytic coating techniques, especially for sheet. However, hot dipping can be the method of choice for complex and shaped parts. Very thin layers of tin are extensively used to passivate steel used for canned goods. Tin is essentially nontoxic, is nearly insoluble in almost all foods, and easily wets and completely covers steel with a pinhole-free coating. 

Porous Si02 nanoparticles deposited by a common wet-coating sol-gel technique, for instance, form a single-layer, low-refractive-index and cost-efficient antireflective coating on glass as an alternative to common multilayer compositions (Figure 4.3).4 The coating thereby obtained exhibits porosity of up to 50% and a refractive index between 1.25 and 1.3, which corresponds to a transmission maximum up to 99% and an... 

The first major obstacle encountered in developing a successful coating seems to be the method of application. The coating must wet the liber properly, be uniform in thickness and, most preferably, be free of porosity extending to the surface of the substrate. The conventional methods of applying coatings to bulk substrates arc not suitable nor adequate for fibers of small diameter. It is important to note that the selection criteria for one coating technique over another are most often diflerent. 

Due to the inherent dry nature of CVD technique, CVD polymers can be utilized as dry resist coatings. Dry resist lithography is advantageous over conventional wet resist techniques because, the whole process can be performed in a unified vacuum chamber. The... 

The spin coating technique has attracted interest, since it maintains many aspects of technical catalysts prepared by pore volume or incipient wetness impregnation, and simultaneously allows the interpretation and analysis in a similar way as the more well-defined model systems discussed above [30]. Here, a solution of the desired catalyst precursor is dropped onto a wafer covered with an oxide film, which is spun on a rotor to create a liquid layer of uniform thickness in order to mimic traditional wet impregnation preparation of catalysts. Control of the catalyst loading and particle size is to some degree achieved by varying the rotation speed, concentration, and vapor pressure of the solute. Still the method suffers, however, from many of the drawbacks associated with wet-impregnated model catalysts, which imparts detailed mechanistic studies. 

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