Development of N-Type Semiconducting Polymers for Transistor Applications

The recent development of n-type semiconducting polymers in the author's group is reviewed. Fullerene polymers are initially described. By introducing a specially designed alkylated comonomer and using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction, highly soluble but high-molecular-weight fullerene polymers with a high fullerene content are obtained. Next, a new attempt to produce n-type polymers by a postfunctionalization reaction is shown. The [2+2] cycloaddition-retroelectrocyclization (CA-RE) between electron-rich alkynes and electron-deficient olefins is employed to construct acceptor units in the polymers. The frontier energy levels of typical p-type polymers containing electron-rich alkynes are lowered by the [2+2] CA-RE with cyano acceptors, such as tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Finally, rational design of high mobility n-type semiconducting polymers based on benzobisthiadiazole (BBT) and its analogues are described. A series of polymers are designed and synthesized, and their thin film transistor (TFT) performances are investigated. Although the BBT unit is suitable for TFT applications, the comonomer must be carefully selected in order to create unipolar n-type charge-transport properties. Thus, naphthalenediimide (NDI) with a deep highest occupied molecular orbital (HOMO) level is copolymerized with the BBT monomer to produce high-performance n-type polymers.