Design strategies of n-type conjugated polymers for organic thin-film transistors

Significant progress in n-type conjugated polymers (CPs) for organic thin-film transistors (OTFTs), which are required in making complementary metal oxide semiconductor (CMOS)-like logic circuits together with p-type CPs, has been made in the last two decades by developing novel building blocks and optimizing device structures as well as processing conditions. However, the device performance, in terms of mobility and air stability, of n-type CPs is still much lower than that of their p-type counterparts. CPs play a key role in determining the device performance. The properties of CPs, such as molecular packing structures and frontier molecular orbital energy levels, can be appropriately adjusted by molecular design. In the current review, we summarize the progress of n-type CPs from the aspect of molecular design, and four molecular design strategies, i.e., donor-acceptor (D-A) CPs with strong A units, D-A CPs with weak D units, D-A CPs with dual-A and A-A type CPs are discussed. It has been demonstrated that D-A CPs with weak D units and A-A type CPs are highly desirable in the construction of unipolar n-type CPs because they generally have low-lying highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels.