Comparative Study of Charge-Transport Behaviour of Edge-on- and Face-on-Oriented Diketopyrrolopyrrole-Based Conjugated Copolymers Bearing Chalcogenophene Units

Herein, we present comparative device analyses that help to understand charge-transport behaviour in polymer field-effect transistors (PFETs) in which synthesised diketopyrrolopyrrole-based (DPP-based) conjugated copolymers bearing chalcogenophene units are used as semiconducting channel materials. The copolymer backbone orientations were successfully modified by tailoring the spacer units of the conjugated copolymers to be face-on-oriented for DPP-based copolymers bearing methylthiophene–furan–methylthiophene units (PDPP-MT-F-MT), and edge-on-oriented for the corresponding thiophene (PDPP-MT-T-MT) and selenophene (PDPP-MT-Se-MT) containing copolymers. Higher backbone planarity and higher field-effect mobilities were expected for the edge-on-oriented DPP-based copolymers (PDPP-MT-T-MT and PDPP-MT-Se-MT) based on density functional theory calculations; however, the face-on-oriented PDPP-MT-F-MT-based PFET exhibited superior electrical performance in terms of field-effect mobility. Charge-transport analyses based on the Gaussian disorder model, focusing on the charge-transport behaviour of the disordered (amorphous) domains in the DPP-based polymer films, revealed that the localised states in the disordered domains of the face-oriented PDPP-MT-F-MT become more delocalised than those of the edge-oriented PDPP-MT-T-MT and PDPP-MT-Se-MT polymers when a gate bias is applied to their PFETs. Therefore, we attribute the electrical performance of a DPP-based PFET to the degree of delocalisation of the disordered domains of its DPP-based polymer, which has a significant impact on charge-transport behaviour.