Molecular Design and Process Aspects of Pyromellitic and Oligosilane Small Molecule Organic Semiconductors
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Organic semiconductors comprise organic molecular or polymeric compounds that contain conjugated electron systems with delocalized molecular orbitals, in which holes and electrons can be transferred between adjacent molecular units under appropriate conditions. If the neighboring molecules become closer and align better, the better orbital overlap will help charge transfer and thus higher mobility, the velocity of charge movement at given electric field, can be achieved. Pyromellitic diimides (PyDIs) are π-conjugated electron-transport materials based on an unusually small aromatic core (benzene). We synthesized PyDI derivatives with a systematic series of fluoroalkyl side chains and investigated their film structures and electrical performances in thin-film transistors. The effect of the length of the fluorinated segment in fluoroalkylmethylene side chains was examined. Shorter side chains within this series induce higher electron mobilities, with a maximum of 0.026 cm2/Vs achieved with the perfluorobutylmethyl side chain. Based on what we learned from the systematic comparison of different side chains, we developed different approaches to explore the potential of the pyromellitic core and further enhance its electrical performance. Attaching the optimized side chain to 3,6-dibromo PyDI allowed nearly parallel PyDI cores and an exceptional mobility of 0.2 cm2/Vs, the highest PyDI mobility yet reported. Compared with other larger conjugated systems, the combination of good mobility and wide bandgap can be obtained from PyDI through a short and relatively benign synthetic process. We also evaluated different solution process methods for growing oligosilane ordered films and successfully controlled the crystallite alignment in areas with defined shapes. Optimized device schemes and dimensions were developed to confirm the electronic conduction phenomenon and a space-limited charge mobility of 1x10-3 cm2/Vs. The testing method offered more understanding toward the utilization of oligosilane as charge transport materials, and suggests the hexasilane core as a promising building block for derivatives with greater substitution and dimensionality. Fullerenes are an important family in organic electronics, especially in bulk heterojunction organic photovoltaics. A fluoroalkylated fullerene compound was synthesized and evaluated as a solution processable n-type material with mobility of 0.01 cm2/Vs. An iodinated fullerene compound that can be used for X-ray and neutron reflectivity analysis and for further functionalization was synthesized via two different routes and structurally identified. Polystyrene bilayer thin film stacking schemes were realized using thermally cross-linked polystyrene bottom layer and spin-coated top layer. X-ray and neutron reflectometry further confirmed the film schemes as bilayer with smooth and reflective interfaces, offering the basis to probe more complicated layered device architectures.