N-type Semiconducting Polymers Based on Dicyano Naphthobisthiadiazole: High Electron Mobility with Unfavorable Backbone Twist
Keitaro Iguchi, Tsubasa Mikie, Masahiko Saito, Kimihiro Komeyama, Takuji Seo, Yutaka Ie, Itaru Osaka
Abstract
Development of n-type semiconducting polymers is essential for organic electronic devices. In this work, we synthesize a new strong acceptor unit by converting the fluorine groups on the naphthobisthiadiazole (NTz) moiety into cyano groups via a nucleophilic aromatic substitution reaction. As a result, the p-type character observed for polymers based on NTz and difluorinated NTz is switched into an ambipolar and/or n-type character in polymers based on dicyano NTz because of the significantly improved electron affinity and thus lowered lowest unoccupied molecular orbital levels. Interestingly, although the cyano groups cause an unfavorable sterical twist in the backbone, the polymers show high electron mobilities in organic field-effect transistors. We highlight that the dicyano NTz-based polymers form a high crystalline structure similar to the noncyano NTz-based polymers and that there seems to be short contacts between the cyano group and the NTz core in the adjacent polymer chain. These structural features would account for the high mobilities despite the backbone twisting. We propose that the introduction of cyano groups into a large π-system with an electron-poor nature is crucial for the development of n-type semiconducting polymers.