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

Abstract

The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin fi lms. Using a combination of spectroscopic methods, including scanning near-fi eld optical microscopy, we study the domain structure and packing quality of highly crystalline thin fi lms of oligomeric PQT-12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identifi cation of regions with uniform molecular orientation. We fi nd 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 fi eld-effect mobility of these layers of 10 − 3 cm 2 V − 1 s − 1 , whereas it is limited by the presence of domain boundaries at macroscopic distances.

Imaging Nanoscale Morphology of Semiconducting Polymer Films with Photoemission Electron Microscopy

“…A major drawback of this technique is that it does not deliver information about the orientation of polymer chains inside crystallites and the local degree of order. [24,25] Additional information, e.g., about the angle between transition dipole moment and polymer backbone, can be extracted by Raman spectroscopy, which can be combined with SNOM. [19][20][21] These techniques allow spatially averaged insights into the overall orientation of polymer chains and the overall degree of order in the sample.…”

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

“…[40][41][42][43] Using polarized near-field absorption probes, Kuehn and coworkers investigated local aggregated domains of thiophene oligomers by scanning a white-light NSOM probe over a semitransparent sample and collected the transmitted light in wide-field detected with an imaging spectrometer.…”

Imaging the domain structure of organic semiconductor films

“…These observations indicate a complex, hierarchical microstructure with distinctive correlated molecular orientation in relatively large domains, yet with variations in crystallinity on the submicron length scale that do not correlate with the observed dichroism. 22 …”

“…22 Figure 3 shows that polycrystalline oligo-PQT-12 films consist of domains ranging from 5 to 20 lm. The majority of the film consists of regions with high dichroic order; however, disordered defects and grain boundaries are also evident that make up roughly 5% of the area of the film.…”

“…NSOM measurements have been used relatively early on for the characterization of light-emitting conjugated polymers, 17,18 and for probing local photophysics 19,20 and device action, 21 but have only recently been used with polycrystalline thin-film organic transistors in mind. Kuehn et al 22 have recently used NSOM to characterize the domain structure of spin-coated oligomeric PQT-12 films where PQT-12 is the polythiophene poly(3,3 00 -didodecyl-[2,2 0 :5 0 ,2 00 ,2 000 ]-quaterthiophene). Transistors based on PQT-12 have achieved field-effect mobilities of order 0.2 cm 2 /Vs.…”