When 1:00 PM - 3:30 PM Jun 22, 2018
Where 1180 Duderstadt Center
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“Morphological Design of Nanoscale Polymer Systems”


Jill K. Wenderott
PhD Dissertation Defense

Polymer thin films are utilized in a variety of applications including membranes, coatings, organic thin film transistors, light emitting diodes, and organic photovoltaics. As seen in countless examples from gas separation to thermal transport, polymer systems exhibit properties that are greatly dependent on their morphologies, thereby affording great challenges and opportunities for informed design. To realize improvement and implementation of semiconducting, or conjugated, polymer thin films in organic electronic applications, a better understanding of the intimate and complex connection between the morphology of a polymer and its electronic transport properties is warranted. A discussion of the connection between the morphology of the film and electronic transport properties, particularly at the polymer/electrode interface, is main goal of this presentation. 

   Whereas the hopping transport rates of an atom on a lattice are largely dictated by structural defect-mediated mechanisms (e.g.: vacancy, interstitial), the mobility of a charge carrier in a polymer is generally influenced by two factors: structural disorder and energetic disorder. The structural disorder is associated with the fact that there exists a distribution of site-to-site distances into which a carrier can hop; energetic disorder arises because the sites are associated with a distribution of energies. For this reason, carrier mobilities in polymers are influenced by the morphology of the polymer. The magnitude of this effect has never been fully understood. Experiments involving the transport of carriers within different morphological environments of the same polymer would be one way to develop a better understanding of this effect. Additional insights may be gleaned by considering charge transport across a conjugated polymer/electrode interface. The alignment of energy levels and the bending of the energy band levels at the conjugated polymer/electrode interface due to charge transport across the interface impact the performance of polymers in electronic device applications. The degree of band bending is well known to vary between conjugated polymers with different chemistries. Very recently we showed that the degree of band bending varies significantly for a single conjugated polymer with changes to the morphology of the polymer.  

To realize the foregoing challenge – understanding the connection between transport and morphology – polymer films with different morphologies were prepared using conventional spin-casting and matrix-assisted pulsed laser evaporation (MAPLE) on different substrates. One key finding is that MAPLE-deposited films exhibited stronger band-bending behavior and slower out-of-plane charge mobilities compared to the spin-cast analogs. Using the band-bending data for the MAPLE-deposited films, a broader density of states (DOS) was extracted than that of the spin-cast films. This suggests that the energetic disorder is more significant in the MAPLE-deposited films.  The transport rates and the magnitude of the band-bending phenomena were rationalized in terms of current theoretical models. 

In another series of experiments, we showed that modification of the interface between the conjugated polymer and electrode with polar self-assembled monolayers (SAMs) changed the effective work function of the substrate. However, the impact of the interfacial SAM on the morphology of the polymer during fabrication and processing, which may influence the electronic properties, is not well elucidated. In this second study, modification of the substrates by SAMs was observed to further affect band-bending behavior of conjugated polymer films. The band-bending data extracted from MAPLE-deposited films on different SAM-modified substrates were analyzed and varying values for the DOSs were obtained. The variations of the DOSs occurred because the morphologies of the thin films were strongly influenced by the surface energies of the SAM-modified substrates, with lower surface energy substrates yielding thin films that were rougher and possessed broader DOSs. 

A third method used to obtain other types of polymer morphologies was solvent annealing. Post-processing with solvent annealing yields new and oftentimes more stable morphologies upon subjection of fabricated polymer films to solvent vapor. This can lead to improvements in charge carrier transport in films. In this final study, 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 observed.   

The studies presented in this talk illustrate the varied and extrinsic nature of properties that are impacted by the morphology of polymer thin films and provide insights into designing morphology for informed implementation in nanoscale applications.