Asymmetric Alkyl Side‐Chain Engineering of Naphthalene Diimide‐Based n‐Type Polymers for Efficient All‐Polymer Solar Cells.

Abstract: The design and synthesis of three n‐type conjugated polymers based on a naphthalene diimide–thiophene skeleton are presented. The control polymer, PNDI‐2HD, has two identical 2‐hexyldecyl side chains, and the other polymers have different alkyl side chains; PNDI‐EHDT has a 2‐ethylhexyl and a 2‐decyltetradecyl side chain, and PNDI‐BOOD has a 2‐butyloctyl and a 2‐octyldodecyl side chain. These copolymers with different alkyl side chains exhibit higher melting and crystallization temperatures, and stronger aggregation in solution, than the control copolymer PNDI‐2HD that has the same side chain. Polymer solar cells based on the electron‐donating copolymer PTB7‐Th and these novel copolymers exhibit nearly the same open‐circuit voltage of 0.77 V. Devices based on the copolymer PNDI‐BOOD with different side chains have a power‐conversion efficiency of up to 6.89%, which is much higher than the 4.30% obtained with the symmetric PNDI‐2HD. This improvement can be attributed to the improved charge‐carrier mobility and the formation of favorable film morphology. These observations suggest that the molecular design strategy of incorporating different side chains can provide a new and promising approach to developing n‐type conjugated polymers. [ABSTRACT FROM AUTHOR]