Effect of Vertical Annealing on the Nitrogen Dioxide Response of Organic Thin Film Transistors

Charge Transport Manipulation via Interface Doping: Achieving Ultrasensitive Organic Semiconductor Gas Sensors

Organic semiconductor (OSC) gas sensors are receiving tremendous attention with the rise of wearable devices. Due to the complicated charge transport characteristics of OSCs, it is usually difficult to optimize their gas sensitivity by directly tailoring the original signals, as in many other kinds of sensors. Instead, device engineering strategies are frequently centered on enhancing the gas-film interaction. Herein, by introducing interface doping between self-assembled monolayers and triisopropylsilylethynyl-substituted pentacene films, we report a wide tuning of OSC gas sensitivity via charge transport manipulation and achieve an ultrahigh sensitivity of nearly 2000%/ppm to NO₂, simultaneously resulting in a fast square-wave-like response feature. In addition, this sensor demonstrates good humidity stability and operates well in flexible devices. More importantly, we identify that charge transport manipulation tailors the gas sensibility of OSCs by means of electronic structure instead of original signal values: compared to shallow traps, the presence of proper deep traps is conducive to gaining high sensitivity and ultrafast response/recovery speeds. This approach is also effective for tuning the sensitivity to reductive gases, verifying its generality for promoting the performance of OSC gas sensors, as well as a promising strategy for other types of sensors or detectors.

Microstructural Control of Soluble Acene Crystals for Field-Effect Transistor Gas Sensors

Microstructural control during the solution processing of small-molecule semiconductors (namely, soluble acene) is important for enhancing the performance of field-effect transistors (FET) and sensors. This focused review introduces strategies to enhance the gas-sensing properties (sensitivity, recovery, selectivity, and stability) of soluble acene FET sensors by considering their sensing mechanism. Defects, such as grain boundaries and crystal edges, provide diffusion pathways for target gas molecules to reach the semiconductor-dielectric interface, thereby enhancing sensitivity and recovery. Representative studies on grain boundary engineering, patterning, and pore generation in the formation of soluble acene crystals are reviewed. The phase separation and microstructure of soluble acene/polymer blends for enhancing gas-sensing performance are also reviewed. Finally, flexible gas sensors using soluble acenes and soluble acene/polymer blends are introduced, and future research perspectives in this field are suggested.

Grain Boundary Control of Organic Semiconductors via Solvent Vapor Annealing for High-Sensitivity NO2 Detection

The microstructure of the organic semiconductor (OSC) active layer is one of the crucial topics to improve the sensing performance of gas sensors. Herein, we introduce a simple solvent vapor annealing (SVA) process to control 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) OSC films morphology and thus yields high-sensitivity nitrogen organic thin-film transistor (OTFT)-based nitrogen dioxide (NO₂) sensors. Compared to pristine devices, the toluene SVA-treated devices exhibit an order of magnitude responsivity enhancement to 10 ppm NO₂, further with a limit of detection of 148 ppb. Systematic studies on the microstructure of the TIPS-pentacene films reveal the large density grain boundaries formed by the SVA process, improving the capability for the adsorption of gas molecules, thus causing high-sensitivity to NO₂. This simple SVA processing strategy provides an effective and reliable access for realizing high-sensitivity OTFT NO₂ sensors.

Organic Thin-Film Transistors as Gas Sensors: A Review

Organic thin-film transistors (OTFTs) are miniaturized devices based upon the electronic responses of organic semiconductors. In comparison to their conventional inorganic counterparts, organic semiconductors are cheaper, can undergo reversible doping processes and may have electronic properties chiefly modulated by molecular engineering approaches. More recently, OTFTs have been designed as gas sensor devices, displaying remarkable performance for the detection of important target analytes, such as ammonia, nitrogen dioxide, hydrogen sulfide and volatile organic compounds (VOCs). The present manuscript provides a comprehensive review on the working principle of OTFTs for gas sensing, with concise descriptions of devices’ architectures and parameter extraction based upon a constant charge carrier mobility model. Then, it moves on with methods of device fabrication and physicochemical descriptions of the main organic semiconductors recently applied to gas sensors (i.e., since 2015 but emphasizing even more recent results). Finally, it describes the achievements of OTFTs in the detection of important gas pollutants alongside an outlook toward the future of this exciting technology.

Interpenetrating Polymer Semiconductor Nanonetwork Channel for Ultrasensitive, Selective, and Fast Recovered Chemodetection

Organic semiconductor (OSC)-based gas detection has attracted considerable attention due to the facile manufacturing process and effective contact with target chemicals at room temperature. However, OSCs intrinsically suffer from inferior sensing and recovery capability due to lack of functional sites and deep gas penetration into the film. Here, we describe an interpenetrating polymer semiconductor nanonetwork (IPSN) channel possessing unreacted silanol (Si-OH) groups on its surface to overcome bottlenecks that come from OSC-based chemodetection. On the top of the IPSN, moreover, we introduced electron-donating amine (NH₂) groups as a chemical receptor because they strongly interact with the electron-withdrawing nature of NO₂ gas. The NH₂-IPSN-based field-effect transistor exhibited high-performance chemodetection such as ultrasensitivity (990% ppm^-1 at 5 ppm) and excellent NO₂ selectivity against other toxic gases. Impressively, the gas recovery was significantly improved because the NH₂ chemical receptors anchored on the surface of the IPSN suppress deep gas penetration into the film. This work demonstrates that our NO₂ chemodetection is expected to provide inspiration and guideline for realization of practical gas sensors in various industries and daily life.

Cobweb-like, Ultrathin Porous Polymer Films for Ultrasensitive NO2 Detection

Gas sensors based on polymer field-effect transistors (FETs) have drawn much attention owing to the inherent merits of specific selectivity, low cost, and room temperature operation. Ultrathin (<10 nm) and porous polymer semiconductor films offer a golden opportunity for achieving high-performance gas sensors. However, wafer-scale fabrication of such high-quality polymer films is of great challenge and has rarely been realized before. Herein, the first demonstration of 4 in. wafer-scale, cobweb-like, and ultrathin porous polymer films is reported via a one-step phase-inversion process. This approach is extremely simple and universal for constructing various ultrathin porous polymer semiconductor films. Thanks to the abundant pores, ultrathin size, and high charge-transfer efficiency of the prepared polymer films, our gas sensors exhibit many superior advantages, including ultrahigh response (2.46 × 10⁶%), low limit of detection (LOD) (<1 ppm), and excellent selectivity. Thus, the proposed fabrication strategy is exceptionally promising for mass manufacturing of low-cost high-performance polymer FET-based gas sensors.

Detection of NO2 Down to One ppb Using Ion-in-Conjugation-Inspired Polymer

Nitrogen dioxide (NO₂ ) emission has severe impact on human health and the ecological environment and effective monitoring of NO₂ requires the detection limit (limit of detection) of several parts-per-billion (ppb). All organic semiconductor-based NO₂ sensors fail to reach such a level. In this work, using an ion-in-conjugation inspired-polymer (poly(3,3'-diaminobenzidine-squarine, noted as PDBS) as the sensory material, NO₂ can be detected as low as 1 ppb, which is the lowest among all reported organic NO₂ sensors. In addition, the sensor has high sensitivity, good reversibility, and long-time stability with a period longer than 120 d. Theoretical calculations reveal that PDBS offers unreacted amine and zwitterionic groups, which can offer both the H-bonding and ion-dipole interaction to NO₂ . The moderate binding energies (≈0.6 eV) offer high sensitivity, selectivity as well as good reversibility. The results demonstrate that the ion-in-conjugation can be employed to greatly improve sensitivity and selectivity in organic gas sensors by inducing both H-bonding and ion-dipole attraction.