Structure-function relationships of acene based organic semiconductors for use in organic field-effect transistors and organic photovoltaic devices.

Abstract

The ability to readily synthesize derivatives of semi-conducting organic molecules through iterative synthesis allows for the facial tuning of both electron and physical properties. This has led to their use in devices such as field-effect transistors, light emitting diodes, and photovoltaics, bringing organic devices from the bench top to everyday use. Acenes, molecules consisting of annulated benzenes, have garnered interest for use in both single-crystals and thin films devices. However, if organic semiconductors (OSCs) are ever to compete with inorganic semiconductors, a complete structure function relationship (SFR) is required. This will require the development of efficient synthetic strategies, evaluation of solid-state packing, and device performance. Charge transport in OSCs is greatly affected by HOMO/LUMO energies of the molecule, solid state molecular packing, and thin film morphology. Through functionalization of the acene core, HOMO/LUMO energies can be modified yielding n-¬type (electron-acceptor) or p-type (electron-donating) materials. However, the addition of functional groups to acenes can also drastically alter solid-state packing resulting in new solid-state morphologies which can profoundly affect device efficiency. To vertically advance the field of organic electronics a complete understanding of SFR of new OSC must be determined in order to allow for the rational design of the next generation of OSC molecules. Over the course of my research I have synthesized two modified acene system to study SFRs of organic semiconductors: asymmetrically substituted diarylindenotetracenes (ASIs) and functionalized rubrenes. Derivatives of ASIs containing either electron donating or electron withdrawing groups were synthesized and their electronic properties were studied using UV/Visible light spectroscopy, fluorescence spectroscopy, and cyclic voltammetry (CV). Stability of these new OSC was determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). To examine the solid-state packing, single crystals of ASIs were grown and crystal morphologies were determined via single-crystal X-¬ray diffraction experiments. Organic photovoltaic devices were prepared using four ASI derivatives as the active layer n-type material and the electronic properties were determined via UV/Visible light spectroscopy, fluorescence spectroscopy, and OPV performance was measured via I-V curves. To correlate thin film morphology and OPV performance solid-state characterization was performed utilizing atomic force microscopy (AFM), grazing incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). With both the electronic and physical properties of ASIs determined correlations between ASI molecular structure, thin film morphologies and OPV device performance could be made. Through our study of ASIs we have advanced the field of OSC research and increased our understanding of SFRs of n-type semiconductors. Two different studies were done to examine the effect of functionalization on the p-type semiconductor rubrene. First, the effect of fluorination on the solid-state packing of rubrene derivatives was studied. Several fluorinated rubrene derivatives were synthesized and single crystals were grown for single-crystal X-ray experiments to examine changes in molecular solid-state packing. By closely studying both inter and intramolecular interactions in each crystal we found that intermolecular interaction, and specifically C–FX intermolecular interactions, are a major contributor to crystal packing and molecular orientation in the solid state. Second, to study the effect of nitrogen substitution on both electronic and physical properties of rubrene, a synthetic strategy was developed to prepare 5,6,11,12-¬tetraphenylnaphtho [2,3-g] quinoline, azarubrene. The total synthesis of this rubrene derivative is still in progress.