Direct Detection of Dilute Solid Chemicals with Responsive Lateral Organic Diodes

Bionic Olfactory Neuron with In-Sensor Reservoir Computing for Intelligent Gas Recognition

Gas sensing and recognition are closely related to the sustainable development of human society, current electronic noses (e-noses) typically focus on detecting specific gases, with only a few capable of recognizing complex odor mixtures. Further, these sensors often struggle to distinguish between isomers and homologs, as these compounds usually have similar physical and chemical properties, yielding nearly identical sensor responses. Even the mammalian olfactory systems consisting of a large variety of receptor cells and efficient neuron networks sometimes fail in this task. The bottleneck stems from the inability to extract the fingerprints of these compounds and the inefficiency of signal processing. To address these limitations, a material-device-algorithm co-design strategy is proposed that integrates an organic field-effect transistor (OFET) array with in-sensor reservoir computing (RC) and the k-nearest neighbors (KNN) algorithm. Organic semiconductors diversify responses to different gases, while RC efficiently extracts spatiotemporal features with lower training costs and reduced energy overhead. This synergy achieves 100% classification accuracy for eight gases and 99.04% accuracy for a library of 26 gases, including mixtures, isomers, and homologs-among the highest reported accuracies. This work provides a groundbreaking hardware solution for bionic olfactory neurons with edge artificial intelligence (AI) functions, surpassing traditional e-noses.

Space-Confined Vertical Growth of Large-Size Organic Semiconductor Single Crystals

Organic semiconductor single crystals (OSSCs) have garnered considerable attention because of their high charge mobility and atomic-scale smooth surface. However, their large-size high-quality preparation remains challenging due to the inevitable defects and limited growth speed brought by traditional epitaxial growth. Here, we demonstrate a space-confined strategy, named out-of-plane microspacing in-air sublimation (OPMAS), for growing vertically millimeter-sized OSSCs in several minutes by revolutionizing the heterogeneous epitaxial growth mode severely depending on substrates into a spontaneous homogeneous growth mode free from substrates. The intrinsic driven force of this transformation is proved to be the change of surface energy, and the maximum size of crystals is thermodynamically dependent on the distance of the microspace. OPMAS has been proven to be suitable for preparing single crystals of various typical organic semiconductors. Numerous characterizations have been conducted to prove the high quality and uniformity of the thus-produced OSSCs. In addition, the photoresponse of the prepared OSSCs is highly dependent on the illumination intensities, making it suitable for high-contrast Morse coding process, demonstrating a stable switch ratio of about 3.3. This work provides a universal platform for preparing large-sized OSSCs, advancing both fundamental (opto)electronic studies and a wide range of practical applications.

Electrophilic-Attack Doped Organic Field-Effect Transistors for Ultrasensitive and Selective Hydrogen Sulfide Detection

Doping of organic semiconductors (OSCs) has been developed as an effective means of modulating the density and transfer efficiency of charge carriers; however, realization of effective doping to tailor the chemical sensing performance of OSC-based sensors still remains not explored extensively. In addition, the application of OSCs in chemical sensors is usually limited by the poor stability and low selectivity. Herein, flexible donor-acceptor copolymer-based organic field-effect transistor (OFET) chemical sensors are designed via an electrophilic attack doping strategy. The p-dopant trityl tetrakis(pentafluorophenyl) borate (TrTPFB) can be effectively doped into the host molecular N-alkyl-diketopyrrolo-pyrroledithienylthieno[3,2-b]thiophene (DPPDTT). It is simple to alter the doping efficiency and film thickness (ca. 5.5-17.7 nm) by adjusting the proportion and concentration of guest-host molecules, which endows facile carrier mobility and enhanced sensing sensitivity modulation toward reducing gases at room temperature. Particularly, 1.0 mol% TrTPFB-doped DPPDTT achieved the highest response to H2S gas, including ultralow detection concentration (0.5 ppb), excellent selectivity, high humidity stability, and long-term storage stability. This work can provide a new strategy for the potential applications of the organic electronic sensing devices.

Research Progress of Organic Field-Effect Transistor Based Chemical Sensors

Due to their high sensitivity and selectivity, chemical sensors have gained significant attention in various fields, including drug security, environmental testing, food safety, and biological medicine. Among them, organic field-effect transistor (OFET) based chemical sensors have emerged as a promising alternative to traditional sensors, exhibiting several advantages such as multi-parameter detection, room temperature operation, miniaturization, flexibility, and portability. This review paper presents recent research progress on OFET-based chemical sensors, highlighting the enhancement of sensor performance, including sensitivity, selectivity, stability, etc. The main improvement programs are improving the internal and external structures of the device, as well as the organic semiconductor layer and dielectric structure. Finally, an outlook on the prospects and challenges of OFET-based chemical sensors is presented.

Simultaneous Incorporation of Two Types of Azo-Groups in the Side Chains of a Conjugated D-A Polymer for Logic Control of the Semiconducting Performance by Light Irradiation

A new design strategy for photoresponsive semiconducting polymer with tri-stable semiconducting states is reported by simultaneous incorporation of tetra-ortho-methoxy-substituted azobenzene (mAzo) and arylazopyrazole (pAzo) in the side chains. The trans-to-cis transformations for mAzo and pAzo groups can sequentially occur within the polymer thin film after sequential 560 and 365 nm light irradiation. Remarkably, the trans-cis isomerization of mAzo and pAzo groups can modulate the thin film crystallinity. Accordingly, the performances of the resulting field-effect transistors (FETs) can be reversibly modulated, leading to tri-stable semiconducting states after sequential 560, 365, and 470 nm light irradiation. Therefore, the device performance can be logically controlled by light irradiation at three different wavelengths. In addition, with light irradiation and device current as the input and output signals, the three-value logic gate by using single FET device can be successfully mimicked.

Monolayer Two-dimensional Molecular Crystals for an Ultrasensitive OFET-based Chemical Sensor

The sensitivity of conventional thin-film OFET-based sensors is limited by the diffusion of analytes through bulk films and remains the central challenge in sensing technology. Now, for the first time, an ultrasensitive (sub-ppb level) sensor is reported that exploits n-type monolayer molecular crystals (MMCs) with porous two-dimensional structures. Thanks to monolayer crystal structure of NDI3HU-DTYM2 (NDI) and controlled formation of porous structure, a world-record detection limit of NH3 (0.1 ppb) was achieved. Moreover, the MMC-OFETs also enabled direct detection of solid analytes of biological amine derivatives, such as dopamine at an extremely low concentration of 500 ppb. The remarkably improved sensing performances of MMC-OFETs opens up the possibility of engineering OFETs for ultrasensitive (bio)chemical sensing.

Organic Heterojunctions Formed by Interfacing Two Single Crystals from a Mixed Solution

Organic heterojunctions are widely used in organic electronics and they are composed of semiconductors interfaced together. Good ordering in the molecular packing inside the heterojunctions is highly desired but it is still challenging to interface organic single crystals to form single-crystalline heterojunctions. Here, we describe how organic heterojunctions are formed by interfacing two single crystals from a droplet of a mixed solution containing two semiconductors. Based on crystallization of six organic semiconductors from a droplet on a substrate, two distinct crystallization mechanisms have been recognized in the sense that crystals form at either the top interface between the air and solution or the bottom interface between the substrate and solution. The preference for one interface rather than the other depends on the semiconductor-substrate pair and, for a given semiconductor, it can be switched by changing the substrate, suggesting that the preference is associated with the semiconductor-substrate molecular interaction. Furthermore, simultaneous crystallization of two semiconductors at two different interfaces to reduce their mutual disturbance results in the formation of bilayer single crystals interfaced together for organic heterojunctions. These single-crystalline heterojunctions exhibit ambipolar charge transport in field-effect transistors, with the highest electron mobility of 1.90 cm² V^-1 s^-1 and the highest hole mobility of 1.02 cm² V^-1 s^-1. Hence, by elucidating the interfacial crystallization events, this work should greatly harvest the solution-grown organic single-crystalline heterojunctions.

Cyclohexyl-Substituted Anthracene Derivatives for High Thermal Stability Organic Semiconductors

A novel p-type organic semiconductor with high thermal stability is developed by simply incorporating cyclohexyl substituted aryl groups into the 2,6-position of anthracene, namely 2,6-di(4-cyclohexylphenyl)anthracene (DcHPA), and a similar compound with linear alkyl chain, 2,6-di(4-n-hexylphenyl)anthracene (DnHPA), is also studied for comparison. DcHPA shows sublimation temperature around 360°C, and thin film field-effect transistors of DcHPA could maintain half of the original mobility value when heated up to 150°C. Corresponding DnHPA has sublimation temperature of 310°C and the performance of its thin film devices decreases by about 50% when heated to 80°C. The impressing thermal stability of the cyclohexyl substitution compounds might provide guidelines for developing organic electronic materials with high thermal stability.

Strategies for Improving the Performance of Sensors Based on Organic Field-Effect Transistors

Organic semiconductors (OSCs) have been extensively studied as sensing channel materials in field-effect transistors due to their unique charge transport properties. Stimulation caused by its environmental conditions can readily change the charge-carrier density and mobility of OSCs. Organic field-effect transistors (OFETs) can act as both signal transducers and signal amplifiers, which greatly simplifies the device structure. Over the past decades, various sensors based on OFETs have been developed, including physical sensors, chemical sensors, biosensors, and integrated sensor arrays with advanced functionalities. However, the performance of OFET-based sensors still needs to be improved to meet the requirements from various practical applications, such as high sensitivity, high selectivity, and rapid response speed. Tailoring molecular structures and micro/nanofilm structures of OSCs is a vital strategy for achieving better sensing performance. Modification of the dielectric layer and the semiconductor/dielectric interface is another approach for improving the sensor performance. Moreover, advanced sensory functionalities have been achieved by developing integrated device arrays. Here, a brief review of strategies used for improving the performance of OFET sensors is presented, which is expected to inspire and provide guidance for the design of future OFET sensors for various specific and practical applications.