The Dipole Moment Inversion Effects in Self-Assembled Nanodielectrics for Organic Transistors

We compare and contrast the properties of hybrid organic-inorganic self-assembled nanodielectrics (SANDs) based on alternating layers of solution-processed ZrO_x and either of two phosphonic acid-functionalized azastilbazolium π-units having opposite dipolar orientations. Conventional Zr-SAND and new inverted IZr-SAND are characterized by Kevin probe, optical spectroscopy, capacitance-voltage measurements, AFM, X-ray reflectivity, and electronic structure computation. The molecular dipolar orientation affects thin-film transistor (TFT) threshold and turn-on voltages for devices based on either p-channel pentacene or n-channel copper perfluorophthalocyanine. Specifically, Zr-SAND shifts the threshold and turn-on voltages to more positive values, whereas IZr-SAND shifts them in the opposite direction. Capping these SANDs with -SiMe₃ groups enhances the effect, affording a 1.3 V difference in turn-on voltage for IZr-SAND vs Zr-SAND-gated organic TFTs. Such tunability should facilitate the engineering of more complex circuits.