Lattice Energy as Emerging General Parameter Describing Ionic Organic Semiconducting Materials Properties

Cyanine dyes represent a class of ionic functional materials used by a broad heterogeneous science community contain-ing biologists, chemists, physicists and materials scientists. Their diverse application opportunities are ranging from fluorescence markers, nonlinear optics, CD-R data storage, photography and organic electronics. Therefore the development of cyanine dyes and their applications in chemistry, engineering, medicine and pharmacology is growing continuously. A typical leitmotif of a cyanine dye contains two nitrogen heterocyclic rings joined by a conjugated chain of carbon atoms. Due to the synthetic approach, quaternary salt precursors are required resulting in ionic compounds. Such chromophores are positively charged and require a counter anion to maintain the overall charge neutrality. Several reports were presenting certain anions as efficiency enhancers in cyanine dye based organic electronic devices. Despite their long presence in science, there is a lack of knowledge behind the mechanisms causing these efficiency enhancements. Furthermore, in the field of organic electronics, the processability of cyanine dyes towards electronic devices was restricted to solution-based methods. In the present thesis work the three main challenges in cyanine dye research, namely: efficient ion exchange procedures, mechanisms behind organic electronics figures of merit enhancement as well as the introduction of physical vapour deposition method have been addressed. An efficient halide-for-anion exchange method with the potential for industry scale up was introduced. This allowed modifying a cationic cyanine chromophore with organic or inorganic anions yielding six salts. The following investigations of these six anions by either varying organic moieties or negative charge delocalisation paved the way towards a generalized concept describing the semiconducting properties of this cationic cyanine chromophore. The lattice energy was established as a universal parameter describing cyanine salt semiconducting properties on a universal energy scale. Such decoupling of an ionic structure from its Lewis formula allows future predictions of semiconducting properties which can be widened to all organic ionic functional materials. Furthermore weakly coordinating anion influence on the volatility of a cyanine chromophore established physical vapour deposition as thin film fabrication method. Using co-evaporation of the cyanine dye and the fullerene C60 a suitable bulk heterojunction morphology was obtained. This resulted in the fabrication of the first fully vacuum deposited cyanine dye bulk heterojunction device.