The use of organic semiconductors (OSCs) for electronic applications is of interest due to their myriad desirable properties, including being lightweight, flexible and low-cost. This work focuses primarily on the synthesis and characterisation of novel molecular OSCs for field-effect transistor (OFET) applications. While the majority of high-performing OFETs are comprised of p-type OSCs, the performance of n-type OSCs still lags behind. New n-type OSCs are eagerly sought for use in complementary circuits and organic photovoltaics (OPVs), where balanced charge transport between the n-type and p-type components is crucial.
In Chapter 2, core fluorination is used as a tool to modify the n-type characteristics and device performance of an indacenodithiophene (IDT)-based molecular OSC – 2,7-dihexyl-IDT-di(C(CN)2). Fluorination is shown to effectively depress the lowest-unoccupied molecular orbital (LUMO) energy level as well as cause a surprising improvement in solubility. OFET devices fabricated via solution-processing of the OSC thin-film display high mobility in the absence of non-idealities, in addition to good retention of performance in ambient conditions.
The following chapters (Chapters 3 – 5) then seek to modify this fluorinated core and investigate the effects on electronic, physical and device performance. In Chapter 3 we perform alkyl side-chain engineering, including modifications to the side-chain length, branching and regiochemistry. The nature and position of the alkyl side-chains are shown to have a significant impact on the physical properties – particularly processability and charge transfer characteristics – as well as device performance. In Chapter 4, the use of an alternative electron-withdrawing group (EWG) – N-cyanoimine – is explored in an attempt to avoid several issues associated with dicyanomethylene. Chapter 5 focuses on further modifications to the fluorinated core, including i) end-functionalisation via direct heteroarylation coupling and ii) substitution of the fluorine groups via nucleophilic aromatic substitution.
Finally, in Chapter 6 a route to a novel class of nonacyclic IDT-based ladder-type molecular is outlined. Electronic and physical properties are explored to examine the applicability of these materials as OSCs for OFETs.