Thin films optical systems

From optical point of view, an OLED structure can be considered as a multilayer thin-film system composed of absorbing and nonabsorbing materials, as shown in Figure 6.27. Therefore, the optical properties and optimal structure of such a multilayer device can be investigated by applying thin-film optical analysis techniques. Based on the theory of optical admittance analysis for analyzing the optical properties of a thin-film system [92], the optical properties of an OLED thin-film system can be simulated to reduce the ambient reflection. 

There are two ways to achieve low reflection, (A) One layer of a low refractive index film, and (B) An optical multi-layer filmprepared by alternately laminating a low refractive index film and a high refractive index fihn. Both (A) and (B) utilize the interference principles of thin film optics to produce destractive interference of the reflected light at upper and the lower interfaces of the thin film-subslrate system (Macleod, 1989). To reduce the reflectance within a wide wavelength range, it is advisable to use method (B). However, this method has a problem of costs, since at least two layers, preferably at least three layers, are required. 

The optical admittance parameter has been introduced into thin-film optics with one specific aim, namely to visualize optical phenomena occurring within such systems by means of a graphical representation of the optical events known as the admittance diagram. Although this is one of a class of diagrams known collectively as circle diagrams, it is particularly powerful and attractive and therefore it is used extensively in thin-film optics. 

Experiments on transport, injection, electroluminescence, and fluorescence probe the spatial correlation within the film, therefore we expect that their response will be sensitive to the self-affinity of the film. This approach, which we proved useful in the analysis of AFM data of conjugated molecular thin films grown in high vacuum, has never been applied to optical and electrical techniques on these systems and might be an interesting route to explore. We have started to assess the influence of different spatial correlations in thin films on the optical and the electro-optical properties, as it will be described in the next section. 

Thin films are used to modify the optical characteristics of a system and a brief review of these characteristics is in order. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Numerous ternary systems are known for II-VI structures incorporating elements from other groups of the Periodic Table. One example is the Zn-Fe-S system Zn(II) and Fe(II) may substimte each other in chalcogenide structures as both are divalent and have similar radii. The cubic polymorphs of ZnS and FeS have almost identical lattice constant a = 5.3 A) and form solid solutions in the entire range of composition. The optical band gap of these alloys varies (rather anomalously) within the limits of the ZnS (3.6 eV) and FeS (0.95 eV) values. The properties of Zn Fei-xS are well suited for thin film heterojunction-based solar cells as well as for photoluminescent and electroluminescent devices. 

Garcia, J.A. and Mandelis, A., Study of the thin-film palladium/hydrogen system by an optical transmittance method, Review Scientific Instruments, 67(11), 3981,1996. 

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