Enhancing the organic thin-film transistor performance of diketopyrrolopyrrole-benzodithiophene copolymers via the modification of both conjugated backbone and side chain

We developed a synthetic strategy for enhancing organic thin-film transistor performances of polymer semiconductors through the modification of the side chain to optimize the stacking conformation and the conjugated backbone to decrease the pi-pi stacking distance of polymers. Our studies demonstrate that the role of bulky alkyl chains attached to the donor unit or the acceptor unit is especially crucial for molecular stacking and aggregation and thus the OTFT device performance of polymers. However, with a larger pi-pi stacking distance in the thin film, the polymer with a bulky alkyl chain attached at the acceptor (P2) shows almost two orders of magnitude higher mobility than that with a bulky alkyl chain attached at the donor (P1). The better performance for P2 is attributed to the bulky alkyl chain at the acceptor which allows more coplanarity of the P2 backbone in the solution state, which leads to self-assembly, and finally forms a highly ordered layer-by-layer lamellar packing for P2 during spin-coating. Further improved performances were obtained by introducing two thiophene units into the polymer backbone to give P3, due to closer pi-pi stacking and in-plane pi-stacking alignment in the thin film and a higher HOMO energy level. Therefore, an optimized device performance was realized through subtle modification of the polymer structure, including both the main chain and the side chain, which provides an insight into structure-property relationships for high-mobility polymer semiconductors.