Increased crystallite size in thin films of C60 and p-terphenyls via PDMS-assisted crystallization

Abstract

Polydimethylsiloxane (PDMS)-assisted crystallization (PAC) is a scalable, solution-based method for growing crystalline organic semiconductor thin films, which can be used in a variety of electronic charge transport and device integration studies. Using this method, polycrystalline C₆₀ and highly oriented crystalline p-terphenyl thin films can be grown out of solution onto amorphous substrates. Polarized optical microscopy, AFM, and GIWAXS characterization reveal that the films (1) are typically 100–200 nm thick, (2) are made up of needle- and tendril-like crystallites extending between 1–2.5 mm, and (3) have crystalline microstructures that vary with choice of solvent, temperature, and substrate pre-treatment. Both the small molecules are found to have some preferential growth parallel to the substrate. The p-terphenyl molecules arrange themselves end-on with respect to the substrate within the films—a potentially favorable orientation for in-plane charge transport. Films grown from carbon disulfide solutions cover larger areas more uniformly with thin needles compared with those grown out of o-dichlorobenzene. Growth at temperatures around ambient result in mm-long, uniformly aligned crystallites. Substrate pretreatment also enhances the uniformity and length of needle-like crystallites. This study's optimization of the PAC method parameters can promote accessible and scalable applications of crystalline organic small molecule thin films.