The relation between molecular packing or morphology and chemical structure or processing conditions: The effect on electronic properties

In the past few decades, mainly two kind of organic semiconductors, namely small molecules and polymers, have been dealt with. It turns out that the difference between these two categories in terms of charge carrier transport arises from the potentially different morphologies and the molecular packing. There are many studies showing the effect of the chemical structure on the electronic properties. However, in this study, the focus is on the role of processing conditions which is found to be of at least equal importance. To study a range of morphologies and packing in as similar molecules, two systems prepared by "Click"-type chemistry are chosen, with the major difference between them being the replacement of a flat unit with one that introduces a slight twist to the aromatic skeleton. Through AFM and X-ray studies, it is shown that the molecule with the potentially flat geometry can exhibit a high degree of π-π stacking, leading to morphologies ranging from polycrystalline to single crystals while the other is always in the amorphous film state. The transport properties are compared using organic field effect transistor (OFETs) in both top and bottom contact configurations. A small molecular change using "Click" in units or a different processing procedure can change the molecular packing which in turn affects the morphology and hence the electronic properties. Using these two knobs, it is possible to obtain amorphous film to single crystal devices and thus tune the device properties. It is found that reliable comparison of different chemistries can be done only within the stable processing window of each.