Organic Thin-Film Transistors as Gas Sensors: A Review

Abstract

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.

Keywords: flexible electronics; gas sensors; organic field- effect transistors; organic semiconductors; organic thin-film transistors.

Figure 1

Timeline featuring the main events related to the development of organic electronics. The chemical structures of carbon nanotubes: reprinted from [23]; published by The Royal Society of Chemistry. The chemical structure of graphene: reprinted from reference [29] with permission from Elsevier. Photographs of A. J. Heeger, A. G. McDiarmid and H. Shirakawa 2000 Nobel Prize in Chemistry recipients: reproduced from reference [30] with permission from The Royal Society of Chemistry. Graphene as a 2D building material for carbon materials of all other dimensionalities: reprinted by permission from Springer Nature Customer Service Centre GmbH: Nature, Nature Materials [31], ©2007. The photograph of flexible microprocessors: ©2012 IEEE. Reprinted, with permission, from [25].

Figure 4

P-type FET characteristic curves: (a) $I_D$ versus $V_{DS}$ for $V_{GS}$ from 4 to $-10$ V. (b) Left axis: $I_D$ versus $V_{GS}$ for $V_{DS}=-1$ V to illustrate the linear fit for $\mathsf{\mu }$ and $V_T$ calculation in triode operation. Right axis: $g_m$ versus $V_{GS}$ for $\mathsf{\mu }$ calculation from $g_{m,max}$. (c) Left axis: $\sqrt{I_D}$ versus $V_{GS}$ for $V_{DS}=-10$ V to illustrate the linear fit for $\mathsf{\mu }$ and $V_T$ calculation in saturation. Right axis: $I_D$ versus $V_{GS}$ in logarithmic scale to extract $I_{ON}$ and $I_{OFF}$, and calculate an approximate value for $SS$. (d) Left axis: off-to-on and on-to-off $I_D$ versus $V_{GS}$ scans for $V_{DS}=-1$ V featuring hysteresis. Right axis: plot of the second derivative of $I_D$ with respect to $V_{GS}$ to extract $V_T$ and illustrate the hysteresis factor calculation. (e) $J_{leakage}$ versus the perpendicular electric field in the channel ($V_{GS}$/$x_i$) for $V_{DS}=0$ V. (f) Plot of $V_{GS}$ versus log${}_{10}\left|I_D\right|$ and its first derivative to illustrate $SS$ calculation. All data were extracted from [39].

Figure 6

Illustration of currently used and innovative techniques for thin-film deposition of organic transistors as gas sensors: (a) Ink-jet printing. Reprinted from [111], with permission, from WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, ©2019. (b) Solution shearing. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Nature, Nature Materials [114], ©2013. (c) Langmuir-based monolayers. Reprinted from [115], copyright 2017, with permission from Elsevier. (d) Off-center spin coating. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Nature, Nature Communications [116], ©2014. (e) Solvent vapor annealing; (f) Physical vapor transport. Reprinted from [117], copyright 2008, with permission from Elsevier. (g) Floating film transfer method.

Figure 9

High-sensitivity gas sensors for ammonia and formaldehyde detection: (a.1) Schematic diagram of a porous DPP2T-TT TFT-based NH${}_3$ sensor. The magnified cartoon illustrates the charge transport reaction occurring at the conductive channel with NH${}_3$. (a.2) Current response to NH${}_3$ with concentrations ranging from 1 ppb to 100 ppm and pore sizes from 0 to 700 nm. (a.3) Sensor performance of transistors using printing and spin coating. All VOCs at 1 ppm. (b.1) Schematic diagram of a porous C8-BTBT TFT-based formaldehyde sensor. The atomic force microscopy figure and cross-sectional profile of the semiconductor film are shown. Formaldehyde interaction with the TFT is illustrated. (b.2) Current response of transistor with (inverted triangle) and without (triangle) pores with a PEI film as compared to pristine transistors with (circle) and without (square) pores to formaldehyde with concentrations ranging from 1 ppb to 1000 ppm. The inset shows the magnified current responses at 1 ppb. The pore size was ca. 500 nm. Reprinted from [85], with permission, from John Wiley and Sons, © 2017.

Figure 10

High-sensitivity gas sensors for nitrogen dioxide, carbon monoxide and hydrogen sulfide detection: (a.1) Schematic diagram of the TIPS-pentacene TFT-based NO${}_2$ sensor from an o-xylene solution. Illustration of the gas sensing mechanism. (a.2) Plot for the limit of detection ($LoD$) calculation. (a.3) Sensor response in the saturation and subthreshold regions towards 10 ppm of NO${}_2$, SO${}_2$, NH${}_3$ and H${}_2$S. Republished from reference [96] with permission from The Royal Society of Chemistry. (b.1) Schematic diagrams of PDPP4T-T-based TFTs for CO and H${}_2$S sensing. (b.2) PDPP4T-T-Pd(II)-based TFTs. Left graph: Current response to CO with concentrations ranging from 10 ppb to 1 ppm. Right graph: Sensor performance towards hexane (52,000 ppm), dichloromethane (DCM) (301,000 ppm), acetone (1000 ppm), ethanol (1200 ppm), H${}_2$ (pure), CO${}_2$ (pure), NO${}_2$ (100 ppm), H${}_2$S (100 ppm) and CO (1 ppm). (b.3) PDPP4T-T-Hg(II)-based TFTs. Left graph: Current response to H${}_2$S with concentrations ranging from 1 to 100 ppb. Right graph: Sensor performance towards hexane (52,000 ppm), DCM (301,000 ppm), acetone (1000 ppm), ethanol (1200 ppm), H${}_2$ (pure), CO${}_2$ (pure), NO${}_2$ (100 ppm), H${}_2$S (1 ppm) and CO (100 ppm). Reprinted with permission from reference [93]. Copyright (2019) American Chemical Society.

Figure 2

Organic thin-film structures: bottom gate, (inverted) (a) bottom contact (coplanar) and (b) top contact (staggered); top gate, (c) bottom contact and (d) top contact. Note that the device is not fully scaled, since the substrate thickness can vary from less than a micron to more than a millimeter. Stacked films are not necessarily flat and conformability depends on the deposition techniques applied.

Figure 3

P-type field-effect-transistor (FET): (a) structural parameters and device electrodes; (b) cut-off (c) triode and (d) saturation operating modes.

Figure 5

Illustration of well-established and pioneering techniques for thin-film deposition of organic electronic devices: (a) spin coating and (b) thermal evaporation.

Figure 7

Chemical structures of organic semiconducting molecules for organic thin-film transistors in gas sensing applications.

Figure 8

Chemical structures of organic molecules used as dielectrics, conductors, surface treatments and substrates for organic field effect transistors in gas sensing applications.