A Solution Processed Ultrathin Molecular Dielectric for Organic Field-Effect Transistors

Leakage-free, thin molecular dielectric layers with sizable macroscopic polarization are valuable for organic field-effect transistor applications. Molecules that are designed for large polarizability tend to be accompanied by a rigid structure which limits their dynamic field response. In this context, we report a novel molecular dielectric material based on a derivative of tetrabromophthalic anhydride (TBPA). By introduction of an intramolecular twist in the molecule design, steric hindrance between neighboring polar moieties is introduced which disrupts extended ordering and leads to the formation of homogeneous, pinhole-free, amorphous and ultrathin films down to a monolayer. A high specific capacitance of the order of microfarads is observed in solution processed monolayer films which are utilized to fabricate low-voltage polymer transistors. The molecular picture shows the formation of self-assembled nanoclusters due to noncovalent intermolecular interactions which result in a macroscopic dipole moment and subsequently a high dielectric constant. The structure–property correlation is interpreted from frequency-dependent dielectric constant measurements (ε(ω)) and density functional theory studies. Subsequently, these unique properties are utilized in polymer field-effect transistors (PFET), which exhibit low leakage and turn-on at 0.6 V.