Acceptor-Acceptor-Type Conjugated Polymers for Energy Level Modulation in Semiconducting Carbon Nanotube Transistors

Numerous methods have been developed for sorting single-walled carbon nanotubes (SWCNTs), and polymer wrapping has become one of the most widely applied strategies for obtaining high-purity semiconducting SWCNTs (s-SWCNTs). Conjugated polymers can selectively sort s-SWCNTs, yet the role of frontier orbital level tuning in this process remains underexplored. In this study, we designed and compared four naphthalene diimide (NDI)-based copolymers with distinct backbones. Among them, the NDI-copolymers with a bithiophene donor (PNDI-2T) or a bis(thienyl)imide acceptor (PNDI-BTI) can effectively wrap onto the s-SWCNTs for investigating how modulation of the frontier energy levels affects polymer-nanotube interactions and device performance. Optical characterizations revealed that PNDI-BTI achieves s-SWCNT purity above 99%, higher than that obtained with PNDI-2T. Furthermore, PNDI-BTI exhibits stronger aggregation, leading to enhanced wrapping and the formation of longer, more uniform s-SWCNT networks, whereas PNDI-2T tends to produce more bundled structures. On the other hand, photoluminescence excitation mapping confirmed that the two polymers selectively wrap distinct nanotube chiralities, leading to noticeable differences in average tube diameter: PNDI-BTI favors large-diameter s-SWCNTs, while PNDI-2T preferentially interacts with small-diameter ones. Field-effect transistors fabricated with PNDI-BTI/s-SWCNTs show superior performance, exhibiting a high hole mobility of 2.1 cm² V^-1 s^-1 and a stably I_on/I_off ratio of 9 × 10³, as well as improved endurance and bias stability, which are attributed to the lower-lying energy levels and stronger π-π interactions of PNDI-BTI. These findings demonstrate that tuning the frontier orbital levels of polymers provides an effective strategy to improve sorting selectivity and device performance, offering new insights into the design of high-performance acceptor-acceptor type conjugated polymers for s-SWCNT sorting.