Role of Sub-Nanometer Dielectric Roughness on Microstructure and Charge Carrier Transport in α,ω-Dihexylsexithiophene Field-Effect Transistors

The effect of dielectric roughness on the microstructure evolution of thermally evaporated α,ω-dihexylsexithiophene (α,ω-DH6T) thin films from a single molecular layer to tens of monolayers (ML) is studied. Thereby, the surface roughness of dielectrics is controlled within a sub-nanometer range. It is found that the grain size of an α,ω-DH6T ML is affected by dielectric roughness, especially for 1.5 ML, whereby the transistor performance is barely influenced. This can be attributed to a domain interconnection in the second layer over a long-range formed on the rough surface. With deposition of more layers, both microstructure and charge carrier transport exhibit a roughness-independent behavior. The structural characterization of α,ω-DH6T 10 ML by grazing-incidence wide-angle X-ray scattering reveals that the interlayer distance is slightly decreased from 3.30 to 3.15 nm due to a higher roughness, while an unchanged π-stacking distance is in excellent agreement with the roughness-independent hole mobility. This study excludes the influence of molecular-solvent interaction and preaggregation taking place during solution deposition, and provides further evidence that the microstructure of the interfacial layer of organic semiconductors has only minor impact on the bulk charge carrier transport in thicker films.