Thin Films of Semiconducting Polymers and Block Copolymers by Surface-initiated Polymerization

Abstract

The ability to control nanoscale morphology and molecular organization in organic semiconducting polymer thin films is an important prerequisite for enhancing the efficiency of organic thin-film devices, including organic light-emitting and photovoltaic devices. The current “top-down” paradigm for making such devices is based on utilizing solution-based processing (e.g. spin-casting) of soluble semiconducting polymers. This approach typically provides only modest control over nanoscale molecular organization and polymer chain alignment. A promising alternative to using solutions of pre-synthesized semiconducting polymers pursues instead a “bottom-up” approach to prepare surface-grafted semiconducting polymer thin films by surface-initiated polymerization of small-molecule monomers. This dissertation mainly focuses on development of an efficient method to prepare semiconducting polymer thin films utilizing surface-initiated Kumada catalyst transfer polymerization (SI-KCTP). In chapter 2, we describe SI-KCTP with a new Ni(II) external catalytic initiator to prepare polythiophene (PT) thin films. We provided evidence that the surface-initiated polymerization occurs by the highly robust controlled (quasi-“living”) chain-growth mechanism. Extensive structural studies of the resulting thin films revealed detailed information on molecular organization and the bulk morphology of the films, and enabled further optimization of the polymerization protocol. Achieving such a complex mesoscale organization is virtually impossible with traditional methods relying on solution processing of pre-synthesized polymers. In addition to controlled bulk morphology, uniform molecular organization and stability, unique feature of SI-KCTP is that it can be used for the preparation of large-area uniformly nanopatterned polymer thin films. This was demonstrated using combination of particle lithography and surface-initiated polymerization. We expanded scope of the surface-initiated polymerization towards all-conjugated diblock copolymer (polythiophene-b-poly(para-phenylene)) thin films, which is described in chapter 3. In addition to the preparation of such films, we carried out detailed structural studies and investigated optoelectronic characteristics of the films. In chapter 4, we studied using SI-KCTP to prepare poly(3,4-ethylenedioxithophene) (PEDOT) thin films. PEDOT is a practically important highly conductive conjugated polymer. Our investigation of the properties of a surface-confined PEDOT film revealed that, after doping with iodine, the film became highly conductive, with conductivity comparable to that of inorganic semiconductors. Therefore, surface-confined PEDOT films may find applications in replacing traditional inorganic electrode for the fabrication of flexible organic electronics.