Preparation of Single Crystals of Small Molecule Organic Semiconductor via Solvent Vapor Annealing

Abstract

Solvent vapor annealing (SVA) is a solution-assisted method which is economical and efficient to improve crystallinity in organic molecular thin films. By exposing thin films of organic semiconductors (OSCs) to an environment saturated with solvent vapors, the OSC molecules will reorganize in a higher degree of order. Here, single crystals of anthracene, TIPS-PEN (6,13-bis(triisopropylsilylethynyl) pentacene) and rubrene are grown using this method from their spun coated films. After obtaining the OSC single crystals, polarized optical microscopy (POM), X-ray diffraction (XRD), selected area electron diffraction (SAED) and atomic force microscopy (AFM) are used to characterize the morphologies and crystalline structures of the crystals. The influence factors such as solvent, temperature and the polymer medium are discussed. The solubility of the annealing solvent plays a significant role in the growth of OSC crystals during SVA. The higher the solubility of the solvent, the easier to acquire large size single crystals. Among the three solvents we used, chloroform is the most suitable solvent for anthracene film to generate single crystals as anthracene is more soluble in chloroform than in other two solutions. Moreover, with the increasing of the temperature, the crystals grow faster, but when approaching the boiling point of the solvent, the crystals will get more defects. We find that the yielded crystals in 45 °C are large in size and with fewer defects. That temperature is the optimum temperature in this system for SVA. When mixed with polymer, the SVA process will be improved greatly, and we can get large-size and high-quality OSC single crystals. First, we consider the substrate is modified by the polymer. This modifying of the interface makes OSC molecules easier to migrate, thus will enhance the growth of crystals. At the same time, with the solvent vapor absorbed, the added polymer can also act as a solvent with lower solubility during SVA, which provides a mild environment for OSC molecules to nucleate and grow.