Controlling Thin Film Morphology and Polymorphism of Electro-Active Molecules Using Solution Process

Organic semiconductors (OSCs) promise the advent of flexible, transparent electronics through the use of low temperature, solution processing methods of fabricating transistors on diverse substrates, such as plastic, glass, paper, and cloth. However, one of the main limitations of organic electronics is that OSC based thin film transistors (TFTs) are limited by low charge carrier mobilities and non-uniformity between transistors. 2,7-dioctyl[1]benzohieno[3,2-b][1]benzothiophene (C8-BTBT) is a small molecule organic semiconductor that has recently reported to have charge transfer mobilities as high as 43 cm2 V-1 s-1. This OSC can also form high quality thin films by proper control of the crystalline texture and morphology, which can be used to form thin film transistors (TFTs) with uniform charge transport properties. In this study, highly crystalline, aligned C8-BTBT thin films were formed using a flow coating process termed solution shearing. Cross-polarized optical microscopy (POM) was used to study the crystal morphology and texture of the resulting thin films. In addition, grazing-incidence X-ray diffraction (GIXD) was used to study the thin film alignment, crystal packing structure and defect formation. The results show that the solution shearing process causes the thin film formation of C8-BTBT to occur in the evaporative region, where the coating process and the evaporation of the solvent occur at similar time scales. Previous work has demonstrated that solution processing conditions and different solvents may induce polymorphism in the crystal packing, another important factor determining the charge carrier mobility of TFTs. Solvent dependent polymorphism was studied in this work using GIXD. Factors that could impact polymorphism such as solvent molecular volume are discussed. Solution shearing C8-BTBT thin films at varying processing parameters not only demonstrates that fluid dynamics play an important role in the thin film morphology control, but also provides an insight for controlling polymorphism, enabling us to achieve the ultimate goal of a large-area, roll-to-roll coating process to produce organic electronics. Solution shearing method is also considered to control the deposition of metal organic frameworks (MOFs). Cu(TCNQ) was selected as the prototypical MOF, given the existence of two distinct polymorphs and good electrical and magnetic conductivities. Two Cu(TCNQ) polymorphs were synthesized and validated by X-ray diffraction and scanning electron microscopy. Applying solution shearing to metal organic frameworks (MOFs) provides new insights in controlling properties of emerging crystalline materials and engineering functional thin film devices.