Electronic properties of dioctylterthiophene-based organic thin-film transistors: A Kelvin probe force microscopy study

It appeared in the past decades that semi-conducting organic liquid crystals could be used as the active layer in organic thin film transistors (OTFTs). They can be processed by simple methods such as inkjet printing, which paves the way to applications for cheap plastic electronics such as electronic tags, biosensors, and flexible screens. However, the measured field-effect mobility in these OTFTs is relatively low compared to inorganic devices. Generally, such low field-effect mobility values result from extrinsic effects such as grain boundaries or imperfect interfaces with source and drain electrodes. It has been shown that reducing the number of grain boundaries between the source and drain electrodes improves the field effect mobility. Therefore, it is important to understand the transport mechanisms by studying the local structure and electronic properties of organic thin films within the channel and at the interfaces with source and drain electrodes in order to improve the field-effect mobility in OTFTs. Kelvin probe force microscopy (KPFM) is an ideal tool for that purpose since it allows to simultaneously investigate the local structure and the electrical potential distribution in electronic devices. In this work, the structure and the electrical properties of OTFTs based on dioctylterthiophene (DOTT) were studied. The transistors were fabricated by spin-coating DOTT on the transistor structures with untreated and treated (silanized) channel silicon oxide. The potential profiles across the channel and at the metal-electrode interfaces were measured by KPFM. The effect of surface treatment on the electrical properties, charge trapping phenomenon and hysteresis effects is demonstrated and analyzed.