Stretchable diketopyrrolopyrrole-based conjugated polymers with asymmetric sidechain designs for field-effect transistor applications

Background: The asymmetric sidechain design has been recently proposed as a potential strategy to improve the mobility-stretchability property of conjugated polymers. In this study, the influences of the asymmetric sidechain designs on the mobility-stretchability properties of DPP-based conjugated polymers are investigated. Three asymmetric sidechain combinations based on a linear alkyl chain (C12), a carbosilane chain (Si), and a siloxane-terminated chain (SiO) are designed to synthesize three polymers, P(SiO-C12), P(Si-C12), and P(SiO-Si). Methods: All polymers are synthesized via Stille coupling in a microwave reactor. Their basic properties including size exclusion chromatography (SEC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) are analyzed. Their optical and electrochemical properties are characterized using UV-vis absorption and cyclic voltammetry (CV). The morphology of the polymer films is analyzed through optical microscopy (OM), atomic force microscopy (AFM), and grazing-incidence X-ray diffraction (GIXD). Lastly, the transistor characteristics are measured using a Keithley 4200-SCS semiconductor parameter analyzer (Keithley Instruments Inc.) inside a N₂-filled glove box. Significant findings: The asymmetric SiO/Si sidechain combination enables P(SiO-Si) possess the highest degree of bimodal packing orientation because their branched geometries impose a large steric hindrance to disrupt the chain packing. As benefitting from such bimodal packing orientation, P(SiO-Si) is shown to deliver the highest hole mobility value and the best mobility-stretchability property among these three polymers. It can persist 56.7% of the initial μ_h value after 800 cycles of 60% strain while P(SiO-C12) and P(Si-C12) can only maintain 2.4% and 4.1% of the initial value. Our results clearly highlight the advantage of the asymmetric SiO/Si sidechain combination on improving the mobility-stretchability property of the derived polymer.