Side Chain Engineering In Conjugated Polymer Semiconductors For Organic Field-Effect Transistors

Organic field-effect transistors(OFETs) have attached widespread attention in recent years due to the low cost, light weight, solution processibility. Thanks to the constant efforts of many scientists on the optimization of molecular structures and device structures, OFETs based on organic conjugated polymers have achieved breakthrough progress with high mobility over 10 cm2 V-1 s-1, comparable with those of amorphous silicon. So far much effort has been devoted to the design of new conjugated backbones. However, more and more scientists are interested in the modification of side chains in recent years because of their complex effects on morphology and crystallinity of thin film. Our work mainly focuses on the study of the effect of side chains on OFETs’ performance.The first part of my work is to design and synthesize two narrow bandgap copolymers(PFDPP-BT and PDPP-BT) composed of bithiophene and DPP unit.PFDPP-BT contains a semi-fluorinated chain. Non-fluorinated polymer PDPP-BT was synthesized as the reference polymer to investigate the effect of fluorination of side chain on the morphology of thin film and charge-carrier mobility. Organic thin-film transistors based on these two polymers showed decent device performance.Interestingly, the performance of device based on polymer PFDPP-BT is better compared with that of PDPP-BT. The mobility of polymer PFDPP-BT is about 3 times higher than that of PDPP-BT. Atomic force microscopy(AFM) and X-ray diffraction(XRD) measurements demonstrated that the semi-fluorinated side chain had a great effect on the thin film morphology and crystallinity, which led to the enhancement in device performance.To further study the impacts of side chains on the performance of OFETs, the second part is to design and synthesize three monomers with the side chains containing three aromatic motifs. Five conjugated polymers PDTII-DT, PDTII-TVT, PTII-DT,PTII-TVT and PNDIII-TT then have been synthesized via Pd catalyzed Stille coupling reaction. The physical and chemical properties of these polymers have also been investigated. The performances of these materials are not desirable, the highest hole mobility is only about 0.15 cm2 V-1 s-1.