Michael McBride

Nucleation, Growth, and Alignment of Poly(3-hexylthiophene) Nanofibers for High-Performance OFETs.

Conjugated semiconducting polymers have been the subject of intense study for over two decades with promising advances toward a printable electronics manufacturing ecosystem. These materials will deliver functional electronic devices that are lightweight, flexible, large-area, and cost-effective, with applications ranging from biomedical sensors to solar cells. Synthesis of novel molecules has led to significant improvements in charge carrier mobility, a defining electrical performance metric for many applications. However, the solution processing and thin film deposition of conjugated polymers must also be properly controlled to obtain reproducible device performance. This has led to an abundance of research on the process-structure-property relationships governing the microstructural evolution of the model semicrystalline poly(3-hexylthiophene) (P3HT) as applied to organic field effect transistor (OFET) fabrication. What followed was the production of an expansive body of work on the crystallization, self-assembly, and charge transport behavior of this semiflexible polymer whose strong π-π stacking interactions allow for highly creative methods of structural control, including the modulation of solvent and solution properties, flow-induced crystallization and alignment techniques, structural templating, and solid-state thermal and mechanical processing. This Account relates recent progress in the microstructural control of P3HT thin films through the nucleation, growth, and alignment of P3HT nanofibers. Solution-based nanofiber formation allows one to develop structural order prior to thin film deposition, mitigating the need for intricate deposition processes and enabling the use of batch and continuous chemical processing steps. Fiber growth is framed as a traditional crystallization problem, with the balance between nucleation and growth rates determining the fiber size and ultimately the distribution of grain boundaries in the solid state. Control of nucleation can be accomplished through a sonication-based seeding procedure, while growth can be modulated through supersaturation control via the tuning of solvent quality, the use of UV irradiation or through aging. These principles carry over to the flow-induced growth of P3HT nanofibers in a continuous microfluidic processing system, leading to thin films with significantly enhanced mobility. Further gains can be made by promoting long-range polymer chain alignment, achieved by depositing nanofibers through shear-based coating methods that promote high fiber packing density and alignment. All of these developments in processing were carried out on a standard OFET platform, enabling us to generalize quantitative structure-property relationships from structural data sources such as UV-vis, AFM, and GIWAXS. It is shown that a linear correlation exists between mobility and the in-plane orientational order of nanofibers, as extracted from AFM images using advanced computer vision software developed by our group. Herein, we discuss data-driven approaches to the determination of process-structure-property relationships, as well as the transferability of structural control strategies for P3HT to other conjugated polymer systems and applications.

High-Throughput Image Analysis of Fibrillar Materials: A Case Study on Polymer Nanofiber Packing, Alignment, and Defects in Organic Field Effect Transistors

High-throughput discovery of process-structure-property relationships in materials through an informatics-enabled empirical approach is an increasingly utilized technique in materials research due to the rapidly expanding availability of data. Here, process-structure-property relationships are extracted for the nucleation, growth, and deposition of semiconducting poly(3-hexylthiophene) (P3HT) nanofibers used in organic field effect transistors, via high-throughput image analysis. This study is performed using an automated image analysis pipeline combining existing open-source software and new algorithms, enabling the rapid evaluation of structural metrics for images of fibrillar materials, including local orientational order, fiber length density, and fiber length distributions. We observe that microfluidic processing leads to fibers that pack with unusually high density, while sonication yields fibers that pack sparsely with low alignment. This is attributed to differences in their crystallization mechanisms. P3HT nanofiber packing during thin film deposition exhibits behavior suggesting that fibers are confined to packing in two-dimensional layers. We find that fiber alignment, a feature correlated with charge carrier mobility, is driven by increasing fiber length, and that shorter fibers tend to segregate to the buried dielectric interface during deposition, creating potentially performance-limiting defects in alignment. Another barrier to perfect alignment is the curvature of P3HT fibers; we propose a mechanistic simulation of fiber growth that reconciles both this curvature and the log-normal distribution of fiber lengths inherent to the fiber populations under consideration.

A Polymer Blend Approach for Creation of Effective Conjugated Polymer Charge Transport Pathways

Understanding the role of the distribution of polymer chain lengths on process-structure-property relationships in semiconducting organic electronics has remained elusive due to challenges in synthesizing targeted molecular weights ( Mw) and polydispersity indices. Here, a facile blending approach of various poly(3-hexylthiophene) (P3HT) molecular weights is used to investigate the impact of the distribution of polymer chain lengths on self-assembly into aggregates and associated charge transport properties. Low and high Mw samples were blended to form a highly polydisperse sample which was compared to a similar, medium Mw control. Self-assembly was induced by preprocessing the polymer solution with UV irradiation and subsequent solution aging before deposition via blade-coating. Superior charge carrier (hole) mobilities were observed for the blend and control samples. Furthermore, their solution lifetimes exceeded 14 days. UV-vis spectral analysis suggests that low Mw P3HT lacks the mesoscale crystallinity required for percolative charge transport. In contrast, when the Mw is too high, the polymer rapidly aggregates, leading to paracrystalline disorder and structural inhomogeneity that interrupts charge-transfer pathways. The role of grain boundaries, fibrillar order, and macroscale alignment is characterized via grazing-incidence wide-angle X-ray scattering, atomic force microscopic, and optical microscopic techniques. The results presented here provide additional guidance on the interplay between polymer solubility, self-assembly, network interconnectivity, and charge transport to enable robust polymer ink formulations with reliable and reproducible performance attributes.