Quantifying tie-chain fraction and its impact on charge transport in model conjugated polymers

The presence of tie chains that connect between crystallites can critically impact the electrical properties of conjugated polymers. Yet, the community has not yet been able to directly visualize them, let alone quantify their content in conjugated polymers. We applied the Huang-Brown model, a framework commonly used to elucidate the structural origins of mechanical properties in polyolefins, to quantify the tie-chain contribution to charge transport in a series of model poly(3-hexylthiophene), P3HT, and its blends. Plotting field-effect mobility as a function of tie-chain fraction, as extracted from the Huang-Brown model, collapses the data on a single curve not previously seen when the charge transport property is plotted against molar mass. We find a threshold tie-chain fraction of 10^-3, below which intercrystallite connectivity limits macroscopic charge transport. Structural characterization via x-ray paracrystallinity analysis of these P3HT films suggests intracrystallite disorder to be the bottleneck that limits charge transport when crystallites are connected. Our study affirms the importance of connectivity between crystalline domains, with the Huang-Brown model implicating long polymer chains with rigid backbone to facilitate macroscopic charge transport.