Acrylic UV-curable adhesives for flexible thin film electronics encapsulation

Abstract

Cheap and efficient encapsulation is an often overlooked critical condition for commercialization of organic electronics. While there are many works, devoted to the development of flexible barrier films, the topic of adhesives for flexible organic electronics encapsulation is still not studied well enough and this thesis aims to fill this gap. It is focused on UV-curable adhesives for flexible organic photovoltaic (OPV) devices, however, a few tests on organic light-emitting diodes (OLED) were also done. The first chapter of the thesis contains an introduction to two topics: adhesives and flexible organic electronics with a focus on UV-curable adhesives and OPV devices. Test methods for devices and adhesives, used in the studies, are described, and the context of the studies is revealed. In the chapter 2, a general approach to adhesive formulation is given and a screening of commercial monomers and other additives is described. Different monomers were tested on OPV devices to find out which have the best compatibility with them. A hydrophobic commercial monomer 2-phenyl ethyl acrylate was chosen among the other monomers for its good compatibility with OPV devices and a great adhesion to the used barrier foil (PET/SiOX composite). Temperature stability of the studied adhesives and relaxation processes in them are discussed in chapter 3. Bubble formation in thin layers of cured acrylic adhesives was studied by methods of thermal analysis. Chemical stability of the cured adhesives up to temperatures above 180 °C was confirmed and a possible link between residual cure stress relaxation and bubbles formation was revealed. In chapter 4, questions of interaction between the studied adhesives and OPV devices and OLEDs are considered and factors affecting this influence are analyzed. For OPV devices, the most important factors are OPV’s active materials, back electrode materials, cure process type (cationic vs radical), hydrophilicity/hydrophobicity of the adhesive monomers and oxygen presence. The effects of so-called “unswitching during cure” and “light driven unswitching” – reversible performance loss of OPV devices during encapsulation or exploitation – are described, and a possible way of their prevention by making the cure in inert atmosphere is shown. A hypothetical explanation of these effects is given and several experiments, confirming this hypothesis are described. OLEDs degradation under the influence of acrylic adhesives is touched briefly and its patterns are described. Generally, the studied adhesives were found suitable for encapsulation of OPV devices, provided that the adhesive cure is made in absence of oxygen, and not good for encapsulation of OLEDs, for which epoxy adhesives are probably the best choice. The last chapter describes attempts to improve the performance of adhesives by a synthesis of novel monomers and viscosity modifying polymer additives. The monomers were designed to be easily synthetized from abundant reagents. The properties of the monomers and the adhesives based on them are discussed. Series of bulky hydrophobic monomers were synthetized, and adhesives with various mechanical properties were formulated on their basis, including adhesives with a low cure shrinkage and high adhesion to the barrier foil. Another successful attempt to improve processability and peel strength of the adhesives was made via prepolymerization of 2-phenyl ethyl acrylate. The adhesives using this additive featured an exceptionally high peel strength combined with a relatively low viscosity. That gives an opportunity to reduce material consumption without sacrificing encapsulation quality.