Scanning Probe Microscopy Study of Electronic Properties in Alkyl-substituted Oligothiopene-based Field-Effect Transitors

AbstractIt appeared in the past decades that semi-conducting organic liquid crystals could easily replace the inorganic semi-conductors to manufacture field-effect transistors (FET). They can be easily processed by simple methods such as inkjet printing. These simple and cheap manufacturing methods pave the way to new applications for plastic electronics: electronic tags, biosensors, flexible screens, … The performance of these liquid crystal nanomaterials is due to their specific nanoscale structure. However, one limitation to the improvement of organic electronic devices is an incomplete understanding of their optoelectronic properties at the nanoscale. The organic semiconductor films often contain a combination of many ordered and disordered regions, grain boundaries and localized traps. These features impact charge transport and trapping at the sub-100nm length scales [1]. Electrical SPM techniques such as STM, KPFM, EFM and CS-AFM have the potential to provide the correlation between the electronic properties directly and local film structure and have already made important contributions to the field of organic electronics. Here we report on the investigation of the structural and electronic properties of p-conductive organic field-effect transistors based on alkyl-substituted oligothiophenes with bottom-contact structure. For this purpose we use atomic force microscopy (AFM) and Kelvin-probe force microscopy (KPFM) in dual frequency mode under ambient conditions. This study helps to determine the local potential in the channel of active OFETs. On the other hand the molecular arrangements of these molecules on the HOPG surface have been studied using scanning tunnelling microscopy (STM) at the liquid-solid interface