Mixing‐Induced Orientational Ordering in Liquid‐Crystalline Organic Semiconductors

Organic semiconductors (OSCs) often exhibit thermotropic liquid-crystalline (LC) phases in which molecular orientational order is partially lost. The formation of a temporary LC state via a solution process is expected to promote printing-based device production, although a method of controlling molecular ordering over LC states has not yet been developed. Here, it is demonstrated that long-axis molecular ordering can be controlled under ambient conditions using unsymmetrically alkylated OSCs with high layered crystallinity. Introducing a small amount of OSC molecules with longer alkyl-chain lengths leads to the conversion of the long-axis disordered phase into a bilayer-type ordered phase with improved hole mobility in the transistor characteristics. Analyses of the entropies of the transitions to the smectic LC phases at elevated temperatures demonstrate that the long-axis molecular ordering can be driven by a balance between intralayer flip-flop motion and core-core interactions through LC phases. The proposed LC-mediated solution processing paves the way toward high-end printed electronics.