Modulation of Domain Size in Polycrystalline n-Type Dicyanoperylene Mono- and Bilayer Transistors

A single molecular layer (monolayer) of organic semiconductors is proven to be sufficient to create a conducting channel for charge carriers in field-effect transistors, which is an ideal platform to investigate the correlation between molecular self-assembly and device performance. Herein, ultrathin films including mono- and bilayers of an n-type dicyanoperylene (PDI8-CN2) are solution-processed by dip-coating. The domain size of the polycrystalline layers is modulated via the surface roughness of the dielectric within an extremely narrow window from 0.15 to 0.39 nm. When the surface roughness is varied from smooth to rough, the domain size and molecular order in the monolayer are significantly decreased, leading to the reduction in electron mobility by 3 orders of magnitude. On the contrary, a lower roughness dependence is observed in the case of the bilayers, with only a slight difference in domain size and charge carrier transport. On the smooth surface, the bilayers exhibit a transistor performance identical to that of the bulk film, confirming that the first few layers near the dielectric dominate the charge carrier transport. Additionally, these results provide insights into the intrinsic role of the interfacial microstructure of small molecular organic semiconductors.