Molecular-structure dependent polaron formation in conjugated polymers – fundamentals and applications
Creators: name: Stewart, Katherine
Publisher: name: Imperial College London
Year: 2023

description: Organic semiconducting polymers are an exciting field of research for the development of new electronic device applications due to their versatility of fabrication and operation. For continued improvement of the active layer there must be a detailed understanding of how molecular design contributes to enhanced device performance. The fundamental study of charge formation in organic polymer devices is important for optimising electrical characteristics. Understanding how polaron formation changes in each polymer system is essential to create molecular design rules for further increasing the performance of optoelectronic devices. To probe and study the process and effects of molecular structure dependent polaron formation we utilise molecular level electrochemical doping with a semicrystalline solid state ionic liquid. This unique system not only enhances the electrical and device characteristics but also allows the study of long lived stable polarons. First, we examine the effect of molecular weight and regioregularity on electrochemical doping of a homopolymer. Poly(3-hexyl)thiophene, (P3HT). This study shows that a higher regioregularity and molecular weight is required to form the required polymer packing and blending morphology for electrochemical doping. With this high-performance blend, we demonstrate the potential of directly probing the electrochemical interaction induced structure changes with Raman spectroscopy to examine the electrochemical doping mechanism. Probing in situ under gas exposure allows for not only the introduction of a highly sensitive and selective gas sensor but also provides a key scientific understanding of its molecular level operational mechanism. Second, we demonstrate the of potential our electrochemical doping probe as a tool for understanding polaron formation. Understanding the contributing factors to reorganisation energy and the process of charge formation is important for understanding how π conjugated polymers will perform in electronic devices. We show that altering the side chain density and increasing the backbone complexity from simple homopolymer P3HT to the poly(3,3'''-didodecyl quaterthiophene) (PQT12) changes the mechanism of polaron formation. The reduction in side chain density and presence of two different environments of thiophene ring results in a multistep polaron formation where we probe the individual steps to understand the molecular level changes as lattice distortion and polaron delocalisation occurs. Here we have shown that a full understanding of charge formation mechanism and how it changes the operational properties is imperative for designing the future of high-performance optoelectronic devices. Finally, we consider a series of donor acceptor copolymers formed of a diketopyrrolopyrrole acceptor unit and thiophene based donor units. In this work, we use our in situ Raman structural probe to analyse the donor-acceptor properties in terms of charge formation. We show that increasing the length of the thiophene donor unit to elongate the backbone increases the planarity of the polymer backbone and reduces the structural changes require to accommodate a polaron. We further our work on understanding the effects of side chains on charge formation and polaron delocalisation by examining the difference between branched alkyl side chains and OEG side chains. The addition of glycol side chains results in enhanced conductivity, however, require greater structural change and form more localised polarons than with alkylated side chains.