Effect of polysiloxane encapsulation material compositions on emission behaviour and stabilities of perylene dyes

The replacement of inorganic conversion dyes with their organic counterparts in LED application bears a large potential for the reduction of rare earth elements in these devices. A major challenge of this substitution is the emission and stability of organic dyes, which is more sensitive to the composition of the polymer matrix they are embedded in than inorganic systems. In this study we systematically investigated the influence of the composition and structure of a low refractive index (LRI) polydimethylsiloxane (PDMS) and a high refractive index (HRI) polymethylphenysiloxane (PMPS) based encapsulation material on the optical properties of two different embedded perylene diimide dyes. Both dyes show low solubility in the PDMS matrix, which also fosters the heat- or light-induced degradation of the incorporated dyes. Contrary phenyl containing polysiloxane encapsulation materials enhance dye solubility, improve quantum yields, and promote heat and radiation resistance. Bulky -aryl substituents at the dye structure decrease the probability of dye-dye interaction and increase the absolute quantum yields additionally. Increased photostability and no leaching was observed when the dye was covalently attached to the polymer matrix. Additionally covalent bonding to and improved solubility of the organic dyes in the polysiloxanes allow for a solvent free processing of such dye-matrix combinations. In conclusion a good matching between the matrix and the dye is crucial for a substitution of inorganic conversion dyes by organic ones in LED devices.