Random networks of single-walled
carbon nanotubes (SWCNTs) offer
new-form-factor electronics such as transparent, flexible, and intrinsically
stretchable devices. However, the long-standing trade-off between
carrier mobility and on/off ratio due to the coexistence of metallic
and semiconducting nanotubes has limited the performance of SWCNT-random-network-based
thin-film transistors (SWCNT TFTs), hindering their practical circuit-level
applications. Methods for high-purity separation between metallic
and semiconducting nanotubes have been proposed, but they require
high cost and energy and are vulnerable to contamination and nanotube
shortening, leading to performance degradation. Alternatively, additional
structures have been proposed to reduce the off-state current, but
they still compromise carrier mobility and suffer from inevitable
expansion in device dimensions. Here, we propose a density-modulated
SWCNT network using an inkjet-printing method as a facile approach
that can achieve superior carrier mobility and a high on/off ratio
simultaneously. By exploiting picoliter-scale drops on demand, we
form a low-density channel network near the source and drain junctions
and a high-density network at the middle of the channel. The modulated
density profile forms a large band gap near the source and drain junctions
that efficiently blocks electron injection under the reverse bias
and a narrow band gap at the high-density area that facilitates the
hole transport under the on-state bias. As a result, the density-modulated
SWCNT TFTs show both high carrier mobility (27.02 cm2 V–1 s–1) and a high on/off ratio (>106). We also demonstrate all-inkjet-printed flexible inverter
circuits whose gain is doubled by the density-modulated SWCNT TFTs,
highlighting the feasibility of our approach for realizing high-performance
flexible and conformable electronics.