Tuning the microstructure and electronic performance in organic thin-film transistors using chemical modifications at interfaces

A challenge with the study of organic thin-film transistors rests in understanding the mechanism behind microstructure formation and its effects on the charge carrier mobility. Often, increased order in the molecular packing of the organic semiconductor film results in a superior mobility. The microstructure of the organic thin-films can be tuned by chemically modifying the surfaces using treatments, such as self-assembled monolayers (SAMs). In 2,8-difluoro-5,11-bis(triethylsilylethynyl) (diF-TES ADT) devices with pentafluorobenzenethiol (PFBT) treated contacts, for example, high mobility regions are produced on the gold contacts, due to large grain growth, while low mobility regions are formed off the contacts, as a result of small grain growth. To explore the mechanism driving this effect, we selectively choose fluorinated contact treatments which are able to introduce targeted interactions at interface between organic semiconductor and SAM-treated contacts. This allows us to isolate key mechanisms behind microstructure formation. Through selecting a specific number of fluorine atoms and placing them into key locations on a benzene thiol base, we can tune the field-effect mobility from 10-3 cm2V-1s-1 to 0.1 cm2V-1s-1 in this organic semiconductor. We combine FET measurements with GIXD data to correlate thin-film microstructure with electronic properties. To investigate the mechanism responsible for these differences in device performance, we perform first-principle density functional theory calculations, which allow us to quantify the interactions at the interfaces between the organic semiconductor and the various contact treatments. We find that Fluorine-Fluorine interactions between the organic semiconductor and the Fluorinated SAM used for contact treatment induce this unique microstructure. Through this knowledge one can selectively choose processing parameters to attain desired film microstructure and improve device performance.