Optical and Electronic Properties of Inkjet Printed Polymer Organic Semiconductor Films

Abstract

Polymer organic semiconductors (OSCs) are an emerging technology poised to revolutionalise several aspects of consumer and business electronics, such as in opto(electronic) devices. They have gathered momentum primarily due to the significant advances in the science and technology of these materias over the last two decades, their solution-processability which allows for low-energy and low-wastage materials deposition on large and flexible substrates, and their perceived environmental friendliness (e.g., no mercury or cadmium is used). This opens up new commercial markets and also new manufacturing platforms for the electronics industry. These materials can be deposited into thin films using spin-casting (sc) which has been the workhorse method over the last two decades, drop-casting (dc) and, increasingly inkjet (ijp) printing which allows large area devices to be manufactured by droplet-on-demand placement of the materials at the desired location. Among polymer OSC, regioregular poly(3-hexylthiophene) (rrP3HT) is one of the most important model that has been widely studied, such as the dependence of film morphology and orientation of the polymer domains on processing conditions. Nevertheless, these studies have not addressed the possible variation of morphology (such as order, orientation and packing) between the top and bottom interfaces across the film thickness direction. Yet the properties of the polymer chains at the interfaces are the most important to understand field-effect transport and charge injection in these materials. Also there has been very little systematic work to understand the differences in the morphology of ijp films compared to sc and dc films, and how these correlate with device characteristics, such as the field-effect mobility.

In this thesis, several aspects of both these issues are addressed, by first developing an optical model to extract from variable-angle spectroscopic ellipsometry (VASE) differences in the dielectric (n,k) spectra between the top and bottom interfaces of the same film; and then using this together with complementary techniques, such as cross-section scanning electron microscopy (SEM), atomic-force microscopy (AFM), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and field-effect transistor (FET) characterisation, to systematically study the differences and similarities between the top and bottom interfaces of rrP3HT films prepared by sc, dc and ijp. The results reveal (i) a marked difference in the degree of interchain order between the top and bottom interfaces in sc films, which may explain the differences in mobility sometimes found between these two interfaces, and (ii) unique features of ijp films ? unusually high crystallinity and low anisotropy ? which was labelled here the ?ijp morphology? which explains why ijp films exhibit much lower charge carrier field-effect mobility (FET) than sc and dc films.