Morphological Design of Nanoscale Polymer Systems: Influence of Film Fabrication, Post-Processing Conditions, and Interfaces on Properties

Abstract

Polymer thin films provide unique challenges and opportunities with properties that can be tuned by modifying the overall structure, or morphology. To realize improvement and implementation of polymer thin films in advanced applications, understanding of the intimate and complex connection between polymer morphology and properties is warranted. This dissertation focuses on the morphological design of polymer systems that exhibit unique properties and interpreting these systems in terms of a theoretical framework that exists to describe inherently-disordered polymer landscapes. The findings of this dissertation illustrate the wide tunability of properties accessible with informed design and control of (i) film fabrication, (ii) interfacial modification by self-assembled monolayers (SAMs) and interactions, and (iii) post-processing conditions. Energy level alignment, and the corresponding band bending of the energy levels of conjugated polymers due to charge transfer at the polymer/electrode interface, impacts the performance of organic electronic devices. It was shown that the degree of band bending, known to vary between conjugated polymers, differs as significantly in a single conjugated polymer with varied morphologies. The morphologies investigated were fabricated via spin-casting and matrix-assisted pulsed laser evaporation (MAPLE), which imparts a unique globular morphology to conjugated polymer films. MAPLE-deposited films possessed stronger band-bending behavior and slower out-of-plane charge transport as compared to the spin-cast analogs. From band-bending data, a wider density of states (DOS) was extracted for the MAPLE-deposited films than the spin-cast films, indicating more energetic disorder in the MAPLE-deposited films arising from the spatial disorder the films possess. Modification of the interface between the conjugated polymer and electrode with polar SAMs changes the effective work function of the substrate and results in a surface energy change. These changes to the interface further affect band-bending behavior of conjugated polymer films. In MAPLE-deposited films, band-bending data was fit to extract a variety of DOS widths for these films atop varied substrates modified by SAMs. It was found that the surface energy of the substrate modified by the SAM influenced the thin MAPLE-deposited film morphology, with lower surface energy substrates yielding thin films that were rougher and possessed wider DOSs. Post-processing with solvent annealing yields new morphologies upon subjection of fabricated films to solvent vapor. Band-bending behavior of conjugated polymer films revealed that solvent annealing led to more uniform spin-cast films, especially in the thicker films (>40 nm), with narrow DOSs. A separate study of solvent annealing on bismuth-based perovskite films noted the films had increased grain size, preferential grain orientation in the face-on direction, and reduction in film roughness upon solvent annealing. These structural changes led to improvements in both intragrain charge conductivity and power conversion efficiencies in devices containing the solvent-annealed films. Interfacial interactions at the polymer/substrate interface are responsible for several thickness-dependent polymer properties like glass transition temperature and chain mobility. Strong (e.g. hydrogen bonding) interfacial interactions were seen to influence the morphology of an insulating polymer film near the substrate, with preferential vertical orientation of the average dipole moment, as compared to weak (e.g. van der Waals) interfacial interactions. Realized with a combination of techniques to measure surface potential and ratios of in- to out-of-plane orientation of signature bonds of the polymer, these findings illustrate the influence of interfacial interactions on polymer thin film structure.