Yaping Zang, Dazhen Huang, Chong-an Di,* and Daoben Zhu* electrical signal amplification. These two fundamental processes are determined by the stimuli/active-layer interaction and the performance of the device, respectively. Although high mobility is not an essential requirement for an OFET sensor, it can facilitate a superior signal amplification and high response speed. After years of tremendous progress in both functional materials and device engineering, OFETs with field-effect mobility over 1.0 cm2 V−1 s−1 can be routinely obtained.[13–16] These impressive performances are sufficient to support the requirement of signal amplification for remarkable sensors, leaving the tuning of stimuli-electrical signal transduction as a key strategy towards ultra-sensitive OFET-based sensors. The achievement of highly efficient signal transduction relies not merely on the development of organic semiconductors with both field-effect modulation and sensing properties, but also is significantly dependent on the utilization of device engineering (e.g., optimization of device geometry and interface properties) to strengthen the specific analyte/active layer interaction and to construct novel stimuli transduction approach. With this in mind, great efforts in device engineering of OFETs have been performed to construct a series of sensing devices and flexible sensing circuits. In this research news, we are intend to give an illustration of our recent efforts in terms of active-layer thickness modulation, surface functionalization, and device geometry optimization in enabling OFET based sensors. In addition to the emphasis on our works, we also refer to recent reports from other groups to offer an insight into the development of OFET-based sensors.
Device Engineered Organic Transistors for Flexible Sensing Applications
Organic thin-film transistors (OFETs) represent a promising candidate for next-generation sensing applications because of the intrinsic advantages of organic semiconductors. The development of flexible sensing devices has received particular interest in the past few years. The recent efforts of developing OFETs for sensitive and specific flexible sensors are summarized from the standpoint of device engineering. The tuning of signal transduction and signal amplification are highlighted based on an overview of active-layer thickness modulation, functional receptor implantation and device geometry optimization.
1. Introduction Organic thin-film transistor (OFET)-based sensors have been the subject of intense research, not only owing to the intrinsic features of organic semiconductors in mechanical flexibility, light-weight and solution processibility, but also because of their potential high sensitivity due to the combined functionality of signal transduction and amplification in one device.[1–3] Moreover, at the level of the basic device operating mechanism, organic semiconductors are susceptible to noncovalent interaction including hydrogen bonds, charge transfer, dipole-dipole interactions, photoexcitation and reversible transformations, enabling effective detection of various stimuli. Benefiting from these unique advantages, the focus of recent research has been devoted to the development of various chemical, physical and biological sensors. To date, many OFETs based sensing elements have been successfully demonstrated for the sensitive detection of chemical compounds, biological specials, pressure, temperature and light, illustrating the rapid development of this exciting research area.[1,3–12] For an OFET-based sensor, sensitivity and specificity are dominated by stimuli-electrical signal transduction and
Y. P. Zang, D. Z. Huang, Dr. C. A. Di, Prof. D. B. Zhu Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]
; [email protected]
Y. P. Zang, D. Z. Huang University of Chinese Academy of Sciences Beijing 100049, China
Adv. Mater. 2016, 28, 4549–4555
2. Tuning Sensitivity of OFETs Through Active-Layer-Thickness Modulation The charge transport and sensing process of a typical OFETbased sensor occur in the conductive channel located on the few molecule layers near the dielectric layer. The sensing performance of most OFET-based sensors is therefore limited by both the diffusion of analyte molecules through organic film and noncovalent van der Waals analyte/semiconductor interactions. One of the most powerful strategies towards maximum sensitivity is to use the “buried” conductive channel as the interaction “surface” to ensure efficient signal transduction, because analyte molecules can directly adsorb onto conductive channels without diffusion through a dense organic film. Utilization of
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) Schematic illustration of the film thickness modulation for OFET-based sensors. b) Schematic illustration of the ultrathin film fabrication processes. c) Molecular structure of NDI3HU-DTYM2 and NDI(2OD)(4tBuPh)-DTYM2. d) Transfer curves of NDI(2OD)(4tBuPh)-DTYM2 devices with different semiconductor thicknesses. e) Photographs of high transparency NDI(2OD)(4tBuPh)-DTYM2 thin films with different thickness and flexible NDI(2OD)(4tBuPh)-DTYM2 based ultrathin-film transistor sensors. f) Current responses of NDI(2OD)(4tBuPh)-DTYM2 based thin-film and ultrathinfilm transistors to NH3. Reproduced with permission. Copyright 2013, Wiley-VCH.
OFETs with an air dielectric layer represents a useful method to address this critical requirement. In this device geometry, the conductive channel is exposed to the analyte with direct analyte/semiconductor interactions. The enhanced interaction can be verified by observing an improved sensitivity for sulphur dioxide (SO2) detection using an air dielectric transistor of copper phthalocyanine (CuPc) when compared with conventional devices. The air dielectric-based device exhibits a high sensitivity of 119 % at a low detection limit of 500 ppb. However, these devices can be hardly constructed with thin films and suffer from problems in aqueous sensors, which limit their wide application in different flexible sensing elements. Downscaling the thickness of the active layer to a minimum number of molecular layers or even a monolayer, offers an important approach to manipulate the sensing capacity of
OFETs (Figure 1a). Commonly used OFETs possess a semiconductor layer with a thickness of several tens of nanometers, the relatively thick feature creates barriers to direct interaction between the conductive channel and the analyte. In comparison to conventional thin film, the exposed conductive channel of ultra-thin film devices is preferred to improve the sensitivity and response speed. Systematic studies on activelayer thickness modulation for OFET based sensors have been performed by Katz’s group. By exploiting the advantages of the vacuum-deposition technique, the thickness of the 5,5′-bis (4-hexylphenyl)-2,2′-bithiophene (6PTTP6) based semiconductor layer can be controlled precisely. A 1.3 monolayer has been determined as the minimum semiconductor thickness to form the conductive channel of a working OFET. It should be mentioned that the ultrathin OFET exhibited a significantly
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 4549–4555
Adv. Mater. 2016, 28, 4549–4555
was constructed via solution-shearing techniques. The high mobility of up to 0.59 cm2 V−1 s−1, ultrathin active layer (1−2 molecular layer) and strong interaction between HTEB and NH3 allowed ultra-sensitive detection of NH3 with a high sensitivity of 40% to a low limit-of-detection of 100 ppb. In addition to the application in gas sensors, ultra-thin film OFETs possess promising applications in sensitive aqueous sensors. With a core-cladding liquid-crystalline pentathiophene and sol–gel HfO2 thin film as semiconductor and dielectric layers, respectively, Chen et al. recently reported a monolayer field-effect transistor (MFET) with mobility of 0.08 ± 0.01 cm2 V−1 s−1. Because of the inherent high sensitivity that arises from an active monolayer being exposed to the environment, the MFET device showed impressive capability of the in situ detection of trace amounts of chemical species in aqueous solution. Using melamine as a representative example, the MFET demonstrated three orders of magnitude higher sensitivity, as low as 0.01 ppm, and a faster response time of 2 s than that of thick thin-film transistor sensors. This suggests once again that ultra-thin organic films are highly beneficial for enhanced sensing performance for both chemical and biological sensors.
higher sensitivity and faster response speed to dimethyl methylphosphonate (DMMP) than their counterpart with a thicker semiconductor layer. In contrast to the ultrathin film device prepared by the vacuum-deposited process, downscaling of the film thickness to less than 10 nm via solution-processing techniques is a challenge despite the several reports on Langmuir–Blodgett (LB), self-assembly, and dip-coating techniques. The development of facile yet efficient techniques to fabricate high performance ultrathin-film organic transistors is highly desired towards a sensitive sensor. Our group developed a simple spin-coating approach, namely, “on-the-fly-dispensing-spin-coating”, with which the solution is dispensed on a highly rotated substrate (Figure 1b). The tangential force can break the static wetting balance and enable the successful deposition of a thin film on an octadecyltrichlorosilane (OTS) modified hydrophobic substrate with a precisely controlled thickness. Two n-type naphthalene diimide derivatives (NDIs), namely, NDI3HU-DTYM2 and NDI(2OD)(4tBuPh)-DTYM2, were employed to construct ultra-thin-film devices (Figure 1c). The results revealed that, for both semiconductors, an ultrathin film with thickness of 2−10 nm could be well controlled by modulating the solution concentration and rotation speed. NDI(2OD) (4tBuPh)-DTYM2 based double-layer OFETs with an active-layer thickness of 4 nm exhibited a high mobility of 0.6 cm2 V−1 s−1 (Figure 1d). In contrast to conventional OFETs, devices with an ultra-thin active layer possess significant potential in transparent and flexible sensors for various applications. By utilizing polyacrylonitrile (PAN) and polymethylsilsesquioxane (PMSQ) as dielectric layer on the flexible polyethylene terephthalate (PET) substrate, a high transparent flexible NDI(2OD)(4tBuPh)DTYM2 based ultrathin-film transistor sensor has been successfully demonstrated (Figure 1e). The ultra-thin-film nature and high device performance enable its faster response and higher sensitivity to the analytes as compared with conventional thick film devices. As an example, the response signal of an NDI(2OD)(4tBuPh)-DTYM2 based ultra-thin device to 100 ppm ammonia (NH3) was one order of magnitude stronger than that of thick film (70 nm) devices. Benefiting from the obviously improved sensing performance, the detection of 10 ppm NH3 with a response and recovery time of less than 20 seconds has been realized (Figure 1f). The result represents the first highly sensitive gas sensor based on a high-performance n-channel organic transistor without functional receptor. With selection of the appropriate active organic semiconductor of dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT-C6), Li et al. recently reported a versatile strategy for the growth of dendritic microstripes with 4−6 molecular layers (6.5−11 nm in thickness) via a dip-coating process. The one-dimensional continuous, microstructured and ultrathin nature of the film allowed relatively high performance OFETs with a mobility range from 0.02 to 0.3 cm2 V−1 s−1. As expected, the ultra-thin devices exhibited a high sensitivity of 100 to 50 ppm NH3 and a short response/recovery time of 27–36 s/4–10 s. In contrast, the thicker film based devices exhibited a low sensitivity of approximately 10 and a long response/recovery time of 70−130 s, respectively. More recently, highly ordered crystalline ultra-thin film (