Thiophene backbone-based polymers with electron-withdrawing pendant groups for application in organic thin-film transistors

Two thiophene backbone-based hole-transporting polymers, namely, poly[(2,2′-bithiophene-5,5′-diyl)- alt -(5,7-bis(2-butyloctyl)-4 H ,8 H -benzo[1,2- c :4,5- c ′]dithiophene-4,8-dione-1,3-diyl)] ( PT2-BDD ) and poly[(3,3′-difluoro-[2,2′-bithiophene]-5,5′-diyl)- alt -(5,7-bis(2-butyloctyl)-4 H ,8 H -benzo[1,2- c :4,5- c ′]dithiophene-4,8-dione-1,3-diyl)] ( PF2-BDD ), were prepared by using electron-withdrawing pendant groups such as ketone and fluorine moieties. They both exhibited a planar backbone with efficient π conjugation, which is suitable for hole transport in organic thin-film transistors (OTFTs). However, the fluorinated one ( i.e. , PF2-BDD ), despite its perfect backbone planarity and strong intra- and intermolecular interactions, could not enhance the OTFT performance; due to its solvent resistance, electron negativity, and random orientation, PF2-BDD showed 10 times lower hole mobility than the non-fluorinated polymer ( i.e. , PT2-BDD ). Nonetheless, the two-dimensional grazing incidence X-ray diffraction and temperature-dependent absorption spectra of the synthesized polymers provided crucial information to understand the relationship between their hole transport behavior and molecular structure. The suboptimal molecular ordering of by PF2-BDD quick freezing during hot-solution spin-coating hindered an efficient hole transport, whereas the more crystalline structure of PT2-BDD resulted in higher hole mobility in the corresponding OTFT.