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

## TL;DR

This study explores how adjusting polymer energy levels improves the sorting and performance of semiconducting carbon nanotubes in transistors.

## Contribution

The novel use of acceptor–acceptor-type conjugated polymers to modulate energy levels for enhanced carbon nanotube sorting and device performance.

## Key findings

- PNDI-BTI achieves over 99% purity of semiconducting carbon nanotubes.
- PNDI-BTI forms longer, more uniform nanotube networks compared to PNDI-2T.
- Transistors with PNDI-BTI show high hole mobility and stable performance.

## Abstract

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.