Flexible Carbon Nanotube-Based Optical Sensor Sheet Integrated with Ultra-Thin Organic Circuits

Flexible optical sensors have attracted considerable attention in the development of wearable and shape-adaptable devices for health monitoring, non-invasive inspection, and biochemical analysis [1–5]. This study develops an ultra-flexible optical sensor using carbon nanotube (CNT) films and organic circuits to detect wavelengths from near-infrared to THz. Owing to their formation on ultra-thin polymer substrates, the CNT films demonstrate a 21-fold increase in sensitivity when compared to those fabricated on rigid thick substrates; further, they exhibit durability against bending and crumpling. The CNT films are connected to organic circuits comprising organic thin-film transistors (OTFTs), with which CNT signals are selected and amplified. The functional CNT films are integrated into a flexible optical sensor as an active matrix, facilitating internal imaging from any surface using a broadband light, including THz.<br/>Flexible photodetectors have been intensively developed in recent years; for example, thin-film inorganic semiconductors, metal halide perovskites, and organic semiconductors have been employed as photodetector materials, demonstrating excellent sensitivity and flexibility [2–4]. While these flexible photodetectors are limited to detecting ultra-violet to near-infrared light, the detection of mid-infrared to THz for non-invasive inspection and chemical analysis is crucial. To address this issue, CNTs have garnered significant attention as flexible photodetectors; they can detect a broad range of electromagnetic waves (from visible light to millimeter waves) using the photothermoelectric effect at room temperature [5,6]. However, CNT optical sensors have been developed predominantly as single-element or small-scale sensors with low integration density. These types of sensors require signal processing via external circuits and spatial scanning for highly sensitive imaging, thereby limiting their portability, scalability, and scanning speed for imaging devices.<br/>In this study, CNT films are fabricated on ultrathin Parylene substrates and exhibit broadband optical response from near-infrared to THz. The CNT films exhibit a 21-fold increase in sensitivity when compared with those on thick rigid substrates owing to the suppression of heat diffusion by an ultra-thin film. Furthermore, the photoresponse of CNT films is stable after bending and crumpling. The CNT films are connected to an organic amplifier and a matrix circuit that is constructed using OTFTs with the channel layer of dinaphtho [2,3-b:2′,3′-f]thieno [3,2-b]thiophene. The organic amplifier circuit exhibits a 10-fold gain and successfully amplifies the THz detection signals. Finally, the active matrix is designed for a flexible broadband imager for non-invasive internal inspection from the surface of an object.<br/><br/>[1] Y. Luo, et al., ACS Nano 17 5211–95 (2023)<br/>[2] Z. Rao, et al., Nat. Electron. 4 513–21 (2021)<br/>[3] W. Wu, et al., Adv. Mater. 33 2006006 (2021)<br/>[4] G. Simone, et al., Adv. Funct. Mater. 30 1904205 (2020)<br/>[5] K. Li, et al., Sci. Adv. 8 eabm4349 (2022)<br/>[6] M. Zhang, et al., ACS Nano. 13 13285-92 (2019)