ABSTRACT
As a technique for the production of organic electronics, printing has two main advantages, efficient use of the material and high production speed. For these to be truly useful, the quality of the printed layers-their thickness, homogeneity, etc.—must conform to the requirements of the produced devices. Unfortunately, this is not easy to reach. Getting close to the performance of a spin-coated or evaporated device is only possible if the printing process is closely monitored and optimized. This, in turn, necessitates a precise measurement of the properties of the involved materials and the resulting device’s structural and electronic characteristics. In this chapter parameters and measurement techniques are presented that are suitable for this. More precisely, the characterization of substrates and printing fluids, typical structural defects in printed layers, and methods for
their detection, as well as techniques for proving the functionality of printed organic semiconductors, are covered. It is important to note that while this chapter is devoted to printed layers, the characterization techniques and many of the typical defects are the same for other kinds of wet deposition, for example, slot-die coating. Additionally, some literature on coating is listed in the “Suggested Readings” section. 4.1 Viscosities and Surface TensionsAs described in Chapter 3, the printing process can be split into several substeps. For example, the acquisition of printing fluid, its transfer to the substrate, its relaxation on it-these are completely different processes described by quite dissimilar equations. However, two parameters always crop up in wet deposition, the fluid’s and adjacent solids’ surface tensions as well as the viscosity of the former. These can be complex, changing with strain, time, and the fluid’s solid content, and hence need to be thoroughly analyzed. In this section methods for the necessary measurements are presented. 4.1.1 Measuring Viscosity
The viscosity of a fluid moderates any flow within it. To be more precise, dynamic viscosity is the ratio of shear stress and shear rate. Dividing this by the fluid’s density yields the kinematic viscosity. Fluids are categorized by their viscosity’s dependence on the shear rate; some examples are shown in Fig. 4.1.
