Understanding the Effects of Intracrystallite and Intercrystallite Structuring on Charge Transport in Conjugated Polymers

Tue, Sep 8, 2020, 9:00 am to 10:30 am

Solution-processable conjugated polymers are promising candidates for next-generation flexible electronics. Yet, it remains challenging to predict how conjugated polymers will perform in devices because the polymer active layers are semicrystalline and the details of the heterogeneous microstructures strongly impact charge transport. In this thesis, we systematically explored the effects of microstructure on charge transport in conjugated polymers, primarily focusing on a family of model poly(3-hexylthiophene), or P3HT.

To circumvent the challenges associated with determining the molecular weights (MW) and molecular weight distributions (MWD) of conjugated polymers via conventional chromatographic methods, we demonstrated diffusion-ordered NMR spectroscopy (DOSY) to characterize their size and size distribution based on their hydrodynamic properties. With the MW and MWD of P3HT samples fully characterized, we investigated the structural origins for the generally-acknowledged MW- and MWD-dependence of their charge-transport properties. We quantified, for the first time, the role of intercrystallite tie chains on macroscopic charge transport and determined a critical tie-chain fraction, below which domain interconnectivity, and above which intradomain order, limit charge transport. Our analysis implicates the importance of long and rigid polymer chains in forming percolated networks to support macroscopic charge transport.

The presence and extent of tie-chain connectivity necessarily affect the mechano-electrical response of polymer films upon deformation. We showed that the presence of tie chains is critical to achieving strain-induced structural alignment, and accordingly, improvements in macroscopic charge transport in the strain direction. When the tie-chain fraction is instead sub-critical, the polymer film undergoes brittle fracture and the electrical properties suffer consequently. We demonstrated the promise of leveraging tie-chain fraction as a practical tuning knob for effecting the mechano-electrical properties of conjugated polymers to meet specific application needs.

The specifics of processing conditions impact the solid-state morphology of solution-processed polymers, and therefore, their charge-transport properties. We showed that the detrimental effect on charge transport of tie-chain pull-out during crystallite thickening upon post-deposition thermal annealing can overshadow the benefits of improved crystallinity. Our findings supported the notion that high crystallinity does not guarantee efficient charge transport. Instead, the unifying requirement for macroscopic charge transport is sufficient interconnectivity between crystallites.