Improving Miscibility of a Naphthalene Diimide‐Bithiophene Copolymer with n‐Type Dopants through the Incorporation of “Kinked” Monomers

Abstract

Abstract N‐type doping of polymer semiconductors is necessary to enable printable and efficient organic thermoelectric generators. A recently reported method relies on blending air‐stable benzimidazole derivative dopant molecules with good electron transporting materials, such as the well‐known poly{[ N , N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)}, also known as PNDIT2. One of the main limitations to doping efficiency is miscibility of the dopant with the polymer. In order to overcome such limitation, controlled amounts of the covalently incorporated, meta ‐substituted monomer 1,3‐bis(2‐thienyl)benzene (TPT) (“kinked monomer”) are introduced into the otherwise straight backbone of PNDIT2. Differential scanning calorimetry shows that crystallinity of P(NDI‐ alt ‐[T2‐ co ‐TPT]) first decreases with increasing TPT content up to 5 mol%, but then increases again for higher TPT contents. Miscibility of P(NDI‐ alt ‐[T2‐ co ‐TPT]) with the dopant 4‐(1,3‐dimethyl‐2,3‐dihydro‐1 H ‐benzoimidazol‐2‐yl)‐ N , N ‐diphenylaniline increases with increasing TPT content up to 30 mol%. The electrical conductivity of doped P(NDI‐ alt ‐[T2‐ co ‐TPT]) films is reduced with respect to PNDIT2, owing to a lower charge mobility caused by TPT units which break conjugation. Nevertheless, the doping efficiency at high doping concentration is substantially improved, with an estimated ≈20‐fold increase with respect to PNDIT2, as a result of the improved miscibility of dopant and copolymer.