Semiconducting polymers based on electron-deficient π-building units

The development of new semiconducting polymers is one of the most important issues to address in improving the performance of organic electronic devices. Donor-acceptor semiconducting polymers, in which the backbone consists of alternating electronrich (donor) and electron-deficient (acceptor) heterocyclic pi-building units, are emerging materials that promote intermolecular interactions and offer desirable electronic structures, in turn leading to high charge carrier transport. This review focuses on the development of donor-acceptor semiconducting polymers using electron-deficient pi-building units, such as thiazolo[5,4-d] thiazole, benzo[1,2-d: 4,5-d'] bisthiazole, naphtho[1,2-c: 5,6-c'] bis[1,2,5] thiadiazole and thieno[3,2-b] thiophene-2,5-dione. All of the polymers form crystalline structures in thin films and possess deep highest occupied molecular orbital energy levels; consequently, the polymers demonstrate high charge carrier mobilities with high air stability. Based on these studies, several key parameters were identified that must be taken into account when designing high-performance polymers: the symmetry of the building units, backbone shape and delocalization of the pi-electrons along the backbone. Notably, many of the polymers synthesized in the studies exhibit high photovoltaic performance, indicating that such crystalline polymers are also useful for solar cell applications. Thus, this review affords an important guideline for the development of high-performance semiconducting polymers.