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High-resolution near-field optical investigation of crystalline domains in oligomeric PQT-12 thin films

: Kuehn, S.; Pingel, P.; Breusing, M.; Fischer, T.; Stumpe, J.; Neher, D.; Elsaesser, T.


Advanced Functional Materials 21 (2011), No.5, pp.860-868
ISSN: 1616-301X
ISSN: 1616-3028
Journal Article
Fraunhofer IAP ()

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.