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High‐Resolution Near‐Field Optical Investigation of Crystalline Domains in Oligomeric PQT‐12 Thin Films

The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near‐field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT‐12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well‐ordered dichroic areas at the ten‐micrometer‐scale, which are sub‐divided into domains with different molecular in‐plane orientation. These serve as a template for the formation of smaller needle‐like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field‐effect mobility of these layers of 10−3 cm2 V−1 s−1, whereas it is limited by the presence of domain boundaries at macroscopic distances.