Study on Intrinsic Stretchability of Diketopyrrolopyrrole-Based π-Conjugated Copolymers with Poly(acryl amide) Side Chains for Organic Field-Effect Transistors

The development of a π-conjugated polymer with hydrogen-bonding moieties has aroused great attention because of the improved molecular stacking and the hydrogen-bonding network. In this study, PDPPTVT (diketopyrrolopyrrole-thiophenevinylenethiophene) and PDPPSe (diketopyrrolopyrrole-selenophene) alkylated with a carbosilane (SiC8) side chain and poly(acryl amide) (PAM)-incorporated alkyl side chain were prepared, and their structure–performance and structure–stretchability correlation were evaluated. By incorporating the DPPTVT backbone and 0, 5, 10, or 20% PAM-incorporated alkyl side chain, the μh value could reach 2.0, 0.97, 0.74, and 0.42 cm2 V–1 s–1, respectively (P1 to P4). The polymer with the PDPPSe backbone and 5% PAM-incorporated alkyl side-chain (P5) exhibited the maximum μh value of 0.96 cm2 V–1 s–1. By extending the PAM moiety from the backbone with alkyl spacers, the solid-state packing and edge-on orientation can be properly maintained. Surprisingly, the PAM-incorporated alkyl side-chain can provide a hydrogen-bonding network serving as sacrificial bonding to mechanical deformation. Therefore, the relevant changes in the crystallographic parameters including the crystalline size and the in-plane π–π stacking distance with a 100% external strain were less than 4 and 0.8%, respectively, from P1 to P3. Therefore, P3 achieved an excellent stretchability while maintaining its molecular orientation and charge-transporting performance. Even with 100% external strain, P3 still provided an orthogonal μh over 0.1 cm2 V–1 s–1. Moreover, by substituting the TVT moiety with the Se moiety, the ductility of the backbone can be further increased when the elastic modulus decreases from 0.80 to 0.36 GPa for P2 to P5. The achieved high μh retention is over 20% after 500 stretching–releasing cycles with a 60% external strain perpendicular to the channel direction for the polymer composed of PDPPSe and 5% PAM content. The results manifest that our newly designed DPP with the PAM-incorporated alkyl side chain provides a promising approach to promote the intrinsic stretchability of the π-conjugated polymers.