Tailoring Co-crystallization over Microphase Separation in Conjugated Block Copolymers via Rational Film Processing for Field- Effect Transistors

In sharp contrast to widely studied coil-coil block copolymers (BCPs), investigation into conjugated rod-rod BCPs is relatively few and limited in scope. Moreover, the ability to exquisitely tailor microphase separation (i.e., forming two dissimilar crystals from two blocks, respectively) and co-crystallization (i.e., co-crystals of two blocks) in conjugated rod-rod BCPs offers a robust route to scrutinize their processing-structure-property relationship. Herein, we report the tailoring of co-crystalline and microphase-separated structures in a family of poly(3-butylthiophene)-block-poly(3-hexylselenophene) (denoted P3BT-b-P3HS) with different molecular weights via three processing strategies (i.e., drop-casting, meniscus-assisted solution shearing (MASS), and post solvent annealing) and the subsequent exploration of the correlation between their different structures (i.e., microphase separation and co-crystallization) and charge transport properties for use in organic field-effect transistors (OFETs). In the case of drop-casting, a larger molecular weight of P3BT-b-P3HS and a faster rate of solvent evaporation are found to favor their co-crystallization over the microphase separation of P3BT and P3HS. Interestingly, the MASS process effectively enables a phase transition from initial microphase-separated P3BT-b-P3HS to their co-crystallization. By contrast, the co-crystalline structure of P3BT-b-P3HS in a certain molecular weight range originally attained from drop-casting could be transformed into a microphaseseparated structure upon post solvent annealing. Afterward, the correlation between the crystalline structures yielded from the three processing strategies and charge mobilities of P3BT-b-P3HS is established. This study highlights the effectiveness of the three strategies in regulating co-crystallization and microphase separation in conjugated rod-rod BCPs, which in turn strengthens the fundamental understanding of phase behavior in conjugated BCPs that underpins future developments in organic optoelectronic materials and devices.