Crystal Grain Orientation in Organic Homo- and Heteroepitaxy of Pentacene and Perfluoropentacene Studied with X-ray Spectromicroscopy

We show that the prototypical p- and n-conducting molecular semiconductors pentacene (PEN) and perfluoropentacene (PFP) exhibit correlated crystal orientation in neighboring grains within a thin film. We use scanning transmission X-ray microscopy (STXM) to measure the film topography in PEN and PFP, and importantly X-ray/optical dichroism also makes it possible to map the grain orientation. PEN exhibits an average grain size of 0.46 ± 0.05 μm2, but clusters of aligned grains are measurably larger at >1.9 μm2. This finding is rationalized through nucleation of small grains that maintain the epitaxial relation with an underlying larger grain during homoepitaxy. The orientation of PEN grains in (buried) layers of PEN/PFP heterostructures is also assessed with STXM, but no orientational in-plane alignment is found between layers of the two different materials. The findings are important to quantify the number and type of (orientational) grain boundaries for an understanding of charge carrier mobility and exciton diffusion.