Structure-function Relationships of Organic Semiconductors in Electronic Devices

Abstract

Organic field-effect transistors (OFETs) are essential components of organic semiconductor-based electronics. In order to engineer the best performance from OFETs, it is desirable to understand the processing-structure-function relationships present in the organic active layer of these devices. To elucidate the processing-structure-function relationships present in thin films of contorted hexabenzocoronene (HBC), I examined the effect of changing the active layer film thickness of HBC OFETs on the device’s performance. Fabricating OFETs from thin films with thicknesses ranging from 15 – 250 nm, I first determined device mobilities, then determined the orientational and growth kinetics characteristics of these films using grazing incidence x-ray diffraction, near edge x-ray absorption fine structure spectroscopy, and in situ microscopy during isothermal crystallization experiments. Although mobility magnitudes were not reproducible across multiple data sets, HBC gradient thickness OFETs showed an increase in mobility with increasing film thickness between 30 – 70 nm within the same data set, corresponding with an increase in the proportion of crystals with “edge-on” oriented pi planes perpendicular to the substrate. Crystal growth and nucleation modes do not appear to be primary factors in determining the mobility increase, although differences in spherulite characteristics could be responsible for the variation across data sets.