Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Enabling Multifunctional Organic Transistors with Fine-Tuned Charge Transport Chong-an Di,*,† Hongguang Shen,†,‡ Fengjiao Zhang,†,‡ and Daoben Zhu† †
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Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China CONSPECTUS: Organic field-effect transistors (OFETs) are promising candidates for many electronic applications not only because of the intrinsic features of organic semiconductors in mechanical flexibility and solution processability but also owing to their multifunctionalities promised by combined signal switching and transduction properties. In contrast to rapid developments of high performance devices, the construction of multifunctional OFETs remains challenging. A key issue is fine-tuning the charge transport by modulating electric fields that are coupled with various external stimuli. Given that the charge transport is determined by complicated factors involving material and device engineering, the development of effective strategies to manipulate charge transport is highly desired toward state-of-the-art multifunctional OFETs. In this Account, we present our recent progress on device-engineered OFETs for sensing applications and thermoelectric studies of organic semiconductors. The interactions between organic semiconductors and the target analyte determine the performance of chemical sensors based on OFETs. We introduced gas receptors and in situ tailored molecular antenna on the surface of ultrathin active layers. The engineered interfaces enable direct and specific semiconductor−analyte interactions, as demonstrated in developed chemical sensors and biosensors with prominent sensitivity and good selectivity. In comparison with chemical stimuli, many physical stimuli such as pressure typically possess a limit effect on the charge transport properties of organic semiconductors. By utilizing the suspended-gate geometry, the carrier concentration in a conductive channel can be controlled quantitatively by the pressure dominated changes in the capacitance of an air dielectric layer, allowing for ultrasensitive pressure detection in a unique manner. More importantly, the transduced current can be further processed by a synaptic OFET, in which the proton/electron coupling interfaces contribute to the dynamic modulation of carrier concentration, thus mimicking biological synapses. The integrated pressure sensor and synaptic OFETs, namely, the dual-organic-transistor-based tactile-perception element, has exhibited promising applications in artificial intelligence elements. Aiming at revealing thermoelectric (TE) properties of organic semiconductors, we also investigated field-modulated TE performance of several high-mobility semiconductors by varying the driving electric field to the temperature gradient. This has been confirmed to offer a strategy to accelerate the search for promising TE materials from well-developed organic semiconductors. By tuning the charge transport process in the device, the functional modulation of OFETs has experienced significant progress in the preceding years. The exploration of new ways to create OFETs with more fascinating functionalities is still full of opportunities to obtain greater benefit from organic transistors.
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INTRODUCTION
efficiency over 15% and 17% in single junction and tandem structure, respectively.7,8 More importantly, organic semiconductors are considered to possess more fascinating functionalities, for example, thermoelectric and spintronic properties,9,10 by virtue of their weak intermolecular interaction, unique delocalized electronic states, and multiple aggregation structures. Despite these exciting properties, multifunctional studies of organic devices are far from satisfactory due to challenges not only in the exploration of multifunctional semiconductors but also on the fine-tuned modulation of fundamental processes via device engineering.
The discovery of conducting polymers in the 1970s and initial studies on organic optoelectronic devices such as organic fieldeffect transistors (OFETs), organic photovoltaics (OPVs), and organic light-emitting diodes (OLEDs) in the 1980s have enabled the emergence of organic electronics.1−4 Thereafter, organic semiconductors have attracted increasing attention owing to their particular advantages in chemical versatility, flexibility, and solution processability, implying their promising large-area applications in low-cost electronic devices. After expansive developments over the past five years, organic electronics have made great achievements, as indicated by the commercial application of OLEDs, remarkable field-effect mobility over 20 cm2 V−1 s−1,5,6 and high photovoltaic © XXXX American Chemical Society
Received: January 15, 2019
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DOI: 10.1021/acs.accounts.9b00031 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. Multifunctional investigation of device engineered organic transistors.
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BASIC CONCEPTS FOR MULTIFUNCTIONAL OFETs A conventional OFET is composed of an organic active layer, a dielectric layer, source−drain electrodes, and a gate electrode.11,15,16 When a gate voltage is biased, charge carriers accumulate near the dielectric/semiconductor interface to induce a conductive channel. Carriers then inject from the source electrode into the organic layer and migrate through the conductive channel to the drain electrode. This operation is analogous to an electronic switch in which the carrier concentration is modulated over several orders of magnitude. In contrast to significant modulation of charge transport behavior with electric field, the carrier concentration and mobility can be fine-tuned by other external fields and stimulus to endow the OFETs with diverse functionalities beyond electronic switching properties. The so-called multifunctional OFETs typically exhibit a well-modulated current output upon combined fields or exhibit multiple signal outputs upon a typical field modulation. Most multifunctional OFETs, including sensors, memories, and phototransistors, operate with the former mechanism, whereas light-emitting OFETs (LEFETs) follow the latter mechanism with the simultaneous output of light and electrical signals.21 Although different OFETs exhibit various characteristics, they share many commonalities in terms of operating mechanism, interface properties, and device geometries. There are three approaches to achieve the combined and fine-tuned modulation of charge transport in an OFET (not include LEFETs). The first one is creating direct and specific semiconductor−stimuli interactions (Figure 1). The device characteristics, including mobility or carrier concentration, are determined quantitatively by the combined fields of the gatingfield and exposed stimuli. Stimuli detection, especially in the concentration of chemical species and biospecies, can be realized via decoupling analysis of the current. In addition, many stimuli possess negligible effects on charge transport of organic semiconductors, rendering the aforementioned model ineffective. The second method to fabricate multifunctional OFETs thus relies on manipulating carrier concentrations with external-stimuli-dominated gating fields. Because the carrier concentration in an OFET is determined by the capacitance of the dielectric layer, the gate voltage, and interfacial trap density, development of unique devices with designed
OFETs are three terminal devices with electric properties dominated by two physical processes, carrier injection and charge transport.11,12 Field-modulated charge transport enabled switching capability and rendered them attractive candidates for logic circuits and active-matrix displays, such as radio frequency identification (RFIDs) cards and the driving backplanes of OLEDs. Till now, studies on OFETs have focused on the development of semiconductors with strong intermolecular interactions and the construction of devices with perfect conductive channels. These investigations have contributed to the achievement of unprecedented mobilities, insight understanding of structure−property relationships of organic semiconductors, and construction of various integrated circuits.13,14 Moreover, benefiting from an exposed active layer with field-modulated carrier concentration, OFETs offer a platform for manipulating charge transport toward multifunctional devices and to reveal novel properties of organic semiconductors. However, the charge transport in OFETs is determined by combined effects of molecular-scale packing and nano- and microscale structure ordering of organic semiconductors, interfacial properties of the functional layers, and device geometry. It therefore makes studies on multifunctional OFETs particularly challenging but highly desired. In this Account, we highlight our recent progress on multifunctional OFETs. Initially, chemical species and biospecies manipulated charge transport has been realized to construct sensitive, specific sensors, by the introduction of ultrathin OFETs with specific receptors. Moreover, benefiting from well-designed pressure-electric-field coupling geometry and proton−electron coupling interfaces, the quantitative and dynamic modulation of carrier concentration in the conductive channel have been achieved to enable suspended-gate-OFETbased pressure sensors and synaptic OFETs, respectively, thus allowing the construction of a tactile-perception system. The final part details our studies on the field-modulated thermoelectric (TE) investigations of organic semiconductors. Notably, we do not intend to simply bring sensing OFETs and TE-OFETs together, but rather to indicate that the electric-field modulated charge transport studies of organic semiconductors can open a door for screening promising organic TE materials with state-of-the-art performances. B
DOI: 10.1021/acs.accounts.9b00031 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 2. (a) Illustration of an ultrathin-film OFET for chemical sensors. (b) Responses of NDI(2OD)(4tBuPh)-DTYM2 based devices to 100 ppm of NH3 with semiconductor thickness of 4 and 70 nm. (c) Responses of the ultrathin-film device to 10 ppm of NH3. (d) Illustration of an OFET-based sensor with absorbed gas receptor. (e) Real-time response of a gas-receptor implanted OFET to 10 ppm of HCl and HBr. (f) Selectivity of the gas receptor implanted OFETs. Reproduced with permission from refs 33 and 34. Copyright 2013 and 2014 by John Wiley and Sons.
determined by signal transduction and amplification, which are further affected by the stimuli-semiconductor interaction and the charge transport properties of the devices. Considering the charge transport in an OFET occurs in the conductive channel near the dielectric/semiconductor interface, crucial to tuning the sensing performance of the OFETs relies on the interface-engineering-assisted modulation of charge transport by the chemical/biospecies. Creating a direct stimuli−semiconductor interaction interface represents a powerful strategy to manipulate charge transport by external stimuli. Typical OFETs comprise an active layer of several tens of nanometers. The thicker feature than the conductive channel introduces a barrier to the analyte. Therefore, the sensing performance is limited by the diffusion of analyte molecules through nonconductive organic layers. An effective method to overcome this open issue is to make the “buried” conductive channel serve as the exposed “surface” to minimize analyte diffusion.15 Till now, various approaches, including downscaling the thickness of the active layer,26,27 utilization of OFETs with an air dielectric layer28 and introduction of porous-structured active layer,29 have been developed to endow the devices with prominent sensitivity and low limit-of-detection. Notably, the thickness modulation for OFET-based sensors has received particular attention. As an example, Katz et al. demonstrated ultrathin OFETs based on vacuum-deposited 5,5′-bis(4-hexylphenyl)-2,2′-bithiophene.27 Particularly, the 1.3-monolayer-based device operate well with significantly improved sensitivity to dimethyl methylphosphonate than its counterpart with a thicker active layer. Despite this achievement, an issue of ultrathin-OFET-based sensors is the downscaling of the active-layer thickness to
