Crystallization and Polymorphism of Organic Semiconductor in Thin Film Induced by Surface Segregated Monolayers

Preparation of highly crystalline organic semiconductor films is vital to achieving high performance in electronic devices. Here we report that surface segregated monolayers (SSMs) on top of phenyl-C₆₁-butyric acid methyl ester (PCBM) thin films induce crystal growth in the bulk, resulting in a dramatic change in the structure to form a new crystal phase. Highly ordered crystalline films with large domain sizes of several hundreds of nanometers are formed with uniaxial orientation of the crystal structure perpendicular to the substrate. The molecular rearrangements in SSMs trigger the nucleation at a lower temperature than that for the spontaneous nucleation in PCBM. The vertical charge mobility in the SSM-induced crystal domains of PCBM is five times higher than in the ordinary polycrystalline domains. Using surface monolayers may be a new strategy for controlling crystal structures and obtaining high-quality organic thin films by post-deposition crystallization.