Carboxylic acid mediated self-assembly of small molecules for organic thin film transistors

A p-type small molecule bearing dicarboxylic acid functional group (-COOH) is synthesized and evaluated for field-effect transistor properties. We discover and report for the first time, that the -COOH groups assist in the passivation of surface traps on the dielectric layer and simultaneously facilitate the self-assembly of the molecules via inter-molecular hydrogen bonding resulting in crystalline active channels. A 9-fold decrease in the threshold voltage was observed for the transistors made using the -COOH functionalized molecule, QT-DA, compared to its ester analogue, QT-ES, providing an evidence of surface passivation. This resulted in an increase in the hole mobility of QT-DA by up to 2 orders of magnitude. It was shown that QT-DA adopts a vertical alignment with respect to the substrate due to preferential interaction between the -COOH groups and the SiO₂ surfaces.