(581d) Controlling Self-Assembly for Enhanced Interconnection in Conjugated Polymer Networks

The properties of semiconducting electronic conjugated polymer systems are highly dependent on the thin film morphology. While numerous structural motifs have been suggested as key determinants of high charge carrier mobility, the requirement of interconnected assemblies has been deemed most influential for long range percolative charge transport. However, entanglement effects in conjugated polymers severely limits the π stacking of polymer chains into interconnected crystalline domains. The entanglement of individual chains can be reduced through solution processing methods that promote the nucleation and growth of tightly packed, ordered structures. Further advances in processing methods to form ‘tie chains’ to interconnect these ordered assemblies are still required.

Herein, we demonstrate facile solution processing methods to target the formation of interconnected assemblies. Poly(3-hexylthiophene) (P3HT) is the canonical semicrystalline conjugated polymer, and here was used to investigate the mechanism of self-assembly in solution. The polymer molecular weight distribution, solute-solvent interactions via solution environment, and quantity of seed nuclei are shown to be tunable parameters impacting the degree of interconnectivity during self-assembly. Both the generality and limitations of these approaches were investigated using a wide array of strategies to induce nucleation, including exposure to low dose UV, microfluidic flow processing, and poor solvent addition. A particularly promising approach involves the selective mixing of a nucleated polymer solution with a non-nucleated sample. Mechanistically, highly crystalline domains can be formed during primary nucleation and then later connected via secondary nucleation. These processing approaches have improved the charge carrier mobility from a base of ~10^-3 to values exceeding 0.2 cm²/V-s.

While tie chains between assembled domains are currently infeasible to characterize experimentally, examination of UV-vis spectral features of the solutions and thin film and image processing of atomic force microscopy (AFM) images can help elucidate their existence and impact. Process-structure-property relationships were developed to quantitatively describe the tradeoffs between polymer network formation and grain boundaries on charge transport. All examined cases indicate an optimal processing window for long range interconnectivity.