Ordering and Preferred Orientation in Films of Polymeric Semiconductors

Abstract

Various aspects related to molecular ordering and preferred orientation in films of polymeric semiconductors is presented. The polymers poly(alkylthiophene)s (PATs) and poly(3,4- ethylenedioxythiophene)s (PEDOTs), being polymers of great interest both scientifically and for applications, were studied in the form of films obtained by solution- and spin casting, and also by the Langmuir technique. Several experimental techniques have been used for characterising the films, most importantly a number of X-ray techniques, including synchrotron radiation, and electron spin resonance.

For parameterizing the uniaxial distribution function for molecular order in solution cast PAT films, obtained by X-ray diffraction (XRD), Gaussian-like distribution functions were found to be superior to Legendre expansions in terms of requiring fewer parameters. Relevant geometrical corrections were studied and applied.

In situ grazing incidence diffraction (GID) was used to study the drying process during solution casting, revealing a strong dependence on the ventilation rate of the sample cell. Low rates promote drying from the edges directly onto the substrate. At high ventilation rates, a skin of solid polymer is formed on top of the solution, whose crystalline part is of restricted correlation length and with a swollen unit cell. This cell can be observed at the polymer-air interface while large quantities of solvent still reside in the interior of the drying solution.

Ultra thin films on water were obtained by the Langmuir technique. Using GID and X-ray reflectometry, it was found that the films were homogeneous and of thickness 10-15nm. The crystalline a-axis (side chains) tend to orient perpendicularly to the water. Due to the hydrophobicity of the side chains, the order deteriorated towards the water. The films were also studied during in situ doping.

By exploiting the fact that the molecular Landé g-tensor of individual thiophene rings is anisotropic, we demonstrate by Monte Carlo simulations and analytically derived expressions that anisotropy can be studied by electron spin resonance, with results in agreement with the XRD results.

Electrochemically doped films of methyl- and decyl-substituted PEDOT films on glass substrates were studied using GID. The data indicate a high degree of π- stacked polymer chains, i.e. a formation of polymer sheets. A certain correlation was found between such sheets, interpreted as smectic ordering.