Chlorinated Isoindigo-Based Conjugated Polymers: Effect of Rotational Freedom of Conjugated Segment on Crystallinity and Charge-Transport Characteristics

Abstract

The chlorinated isoindigo (CI) is a promising building block for organic semiconducting materials because the favorable properties, such as ready availability, lower price, and higher capability to hold the electron density than fluorine atoms, make them advantageous for use in semiconducting materials. It was reported that CI can be more readily synthesized than fluorinated isoindigo (FI), and the CI-based conjugated polymer exhibited comparable device performance with the FI-based conjugated polymer. Chorine-substituted conjugated molecules, however, have been less investigated than that of the fluorine atom, presumably, due to the large size of the chlorine atom, which induces steric hindrance effects in the conjugated backbone. In this study, we systematically investigate the effect of the structural property, flexibility vs rigidity, of the donor unit on the crystallinity and charge transport characteristics of chlorinated isoindigo acceptor-based D-A-type conjugated polymers to understanding the structure-property relationships of chlorinated isoindigo-based conjugated polymers. Interestingly, in an X-ray diffraction analysis, although a TV unit is a more planar structure than a BT unit, the PCIBT film exhibited a much stronger peak intensity and highly extended the lamellar peak up to 400 compared to the PCITV film after the thermal annealing process. This indicated that PCIBT had a more improved molecular ordering and crystalline structure than PCITV. This may be associated with the BT unit having higher rotational freedom compared to that of the TV unit. This property of the BT unit can facilitate thermal-assisted polymer chain packing in the film state that results in an enhanced molecular ordering of PCIBT after the thermal annealing process. Therefore, PCIBT exhibited an electron mobility (1.7 cm2V–1s–1) higher than that of PCITV (0.39 cm2V–1s–1) in organic field-effect transistors due to enhanced intermolecular charge transport between polymer chains.