(631a) Invited Talk: Controlling Electronic Properties Through Block Copolymer Self-Assembly and Crystallization in Poly(3-alkylthiophenes)

The performance of organic electronic devices depends on the crystalline structure and the morphology at the nanometer length scale. Block copolymers containing a semicrystalline conjugated polymer, such as poly(3-alkylthiophene) (P3AT), can simultaneously address these design considerations by confining crystallization to well-defined nanoscale domains. However, in many conjugated block copolymer systems, crystallinity dominates the thermodynamics of self-assembly such that the melt phase morphology can be significantly distorted. In these cases, periodic domains characteristic of block copolymer self-assembly are not observed, and the morphology at the nanoscale is nearly identical to that of the semicrystalline homopolymer. We show that the rational selection of the alkyl side chain in P3ATs results in control of the thermal transitions over a range of 150C, and that the optoelectronic properties are not significantly altered in the process. Crystallization can be observed in real-time and the resulting UV-vis absorbance and field-effect transistor mobility are correlated to the structural evolution. Interestingly, while the degree of crystallinity increases monotonically over the course of an hour, the hole mobility increases by nearly two orders of magnitude once a threshold value is reached. Additionally, when these poly(3-alkylthiophenes) are incorporated into block copolymers, the decreased melting transition allows for cases in which the strength of segregation is sufficiently high during crystallization to maintain the microphase separated morphology. Phases such as hexagonally-packed cylinders and lamellae are obtained, and it is shown that the crystalline structure of the conjugated polymer is preserved even under confinement. Consequently, the optical bandgap and hole mobility, as determined by time-of-flight measurements, are identical to that of the parent homopolymer.