WO2020152453A1

WO2020152453A1 - Organic thin film transistor with protic group on source and drain electrodes for determining presence and/or concentration of ester

Info

Publication number: WO2020152453A1
Application number: PCT/GB2020/050126
Authority: WO
Inventors: Pascal CACHELIN
Original assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Current assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Priority date: 2019-01-21
Filing date: 2020-01-21
Publication date: 2020-07-30
Anticipated expiration: 2021-07-21
Also published as: GB2580717A; GB201900836D0; GB201902297D0

Abstract

A method of using an organic thin film transistor (OTFT) gas sensor to measure an ester, e.g. butyl acetate, in a gas environment. The response of the OTFT gas sensor to the environment is used to determine the presence and/or the concentration of the ester in the environment. The OTFT gas sensor has source and drain electrodes (107, 109) in electrical contact with an organic semiconductor layer (111), a gate electrode (103), and a gate dielectric (105) between the organic semiconductor layer and the gate electrode. A monolayer (113) containing a protic group, e.g. an amino group, is bound to surfaces of the source and drain electrodes. The OTFT gas sensor allows for ester in the environment to interact with the protic group, which causes an output/change in output from the OTFT gas sensor. Measurement of the ester can be used to determine ripeness of fruit.

Description

ORGANIC THIN FILM TRANSISTOR WITH PROTIC GROUP ON SOURCE

AND DRAIN ELECTRODES FOR DETERMINING PRESENCE AND/OR CONCENTRATION OF ESTER

BACKGROUND

Embodiments of the present disclosure relate to organic thin film transistor gas sensors. More particularly, but not by way of limitation, some embodiments of the present disclosure relate to organic thin film gas sensors configured to detect esters.

Thin film transistors (TFTs) have been previously used as gas sensors. For example, such use of thin film transistors as gas sensors is described in Feng et al, Unencapsulated Air- stable Organic Field Effect Transistor by All Solution Processes for Low Power Vapor Sensing, SCIENTIFIC REPORTS 6:20671 DOI: 10.1038/srep20671 and Besar et al, Printable ammonia sensor based on organic field effect transistor, ORGANIC ELECTRONICS, Vol. 15, Issue 11, Pages 3221-3230 (November 2014). In thin film transistor gas sensors, a semiconducting layer is in electrical contact with source and drain electrodes and a gate dielectric is disposed between the semiconducting layer and a gate electrode. Interaction of a target gas with the TFT gas sensor may alter the drain current of the TFT gas sensor. Zhang et al, Organic Field-Effect Transistors-Based Gas Sensors, CHEM. SOC. REV., 2015, 44, 2087-2107 discloses gas discrimination systems having sensory arrays of organic field effect transistors.

U.S. Patent No. 8,686,404 discloses an electronic circuit containing first and second organic thin film transistors in which each organic thin film transistor includes bottom source/drain electrodes having a self-assembled monolayer formed thereon.

Volatile organic compounds may be released during decay of food, for example as described in Phan, NT., Kim, KH., Jeon, EC. et al. ENVIRON MONIT ASSESS (2012) 184: 1683. https://doi.org/10.1007/sl0661-011-2070-2, Analysis of volatile organic compounds released during food decaying processes. J Dixon and E Hewett, NEW ZEALAND JOURNAL OF CROP AND HORTICULTURAL SCIENCE, Vol. 28: 155-173, Factors affecting apple aroma/flavour volatile concentration: a review (2000) discloses that apple flavour develops during ripening and maximum endogenous volatile concentration occurs at the climacteric peak. M. Espino-Diaz el al, FOOD TECHNOL BIOTECHNOL.; 54(4): 375-397, Biochemistry of Apple Aroma: A Review (2016 Dec.) discloses flavour of apples defined by volatile aroma compounds.

SUMMARY Ester compounds can be present in various gaseous environments. For example, butyl acetate is produced by certain fruits, for example apples.

The present inventors have found that organic thin film transistors (OTFTs) may be used to detect the presence or concentration of ester compounds in an environment. For example, the concentration of volatile ester compounds produced by fruit may be useful in determining ripeness of the fruit. Further, the present inventors have found that sensitivity of an OTFT gas sensor to ester compounds may be increased by treatment of the source and drain electrodes of the OTFT gas sensor with certain compounds.

Accordingly, in some embodiments there is provided a method of identifying the presence and / or concentration of an ester in a gaseous environment, including measurement of a response of an organic thin film transistor gas sensor to the environment and determining from the measured response the presence and / or concentration of the ester. The OTFT gas sensor may have source and drain electrodes in electrical contact with an organic semiconductor layer; a gate electrode; and a gate dielectric between the organic semiconductor layer and the gate electrode. A protic group may be bound to a surface of the source and drain electrodes. The OTFT gas sensor may be configured to provide for gas communication of the ester in the environment with the protic group.

DESCRIPTION OF THE DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology. Figure 1 illustrates a bottom gate, bottom contact OTFT gas sensor according to some embodiments;

Figure 2 illustrates a bottom gate, top contact OTFT gas sensor according to some embodiments; Figure 3 illustrates a top gate, bottom contact OTFT gas sensor according to some embodiments;

Figure 4 illustrates a top gate, top contact OTFT gas sensor according to some embodiments; and

Figure 5 is a graph of drain current vs. time upon exposure of two OTFT gas sensors according to some embodiments and a comparative OTFT gas sensor to methyl hexanoate.

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. As used herein, by a material“over” a layer is meant that the material is in direct contact with the layer or is spaced apart therefrom by one or more intervening layers.

As used herein, by a material“on” a layer is meant that the material is in direct contact with that layer.

A layer“between” two other layers as described herein may be in direct contact with each of the two layers it is between or may be spaced apart from one or both of the two other layers by one or more intervening layers.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may likewise be embodied as a computer-readable medium claim, or in other forms, such as being embodied in a means- plus-function claim.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

The techniques introduced here can be embodied as special-purpose hardware (e.g., circuitry), as programmable circuitry appropriately programmed with software and/or firmware, or as a combination of special-purpose and programmable circuitry. Hence, embodiments may include a machine -readable medium having stored thereon instructions which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), magneto-optical disks, ROMs, random access memories (RAMs), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media / machine-readable medium suitable for storing electronic instructions. The machine-readable medium includes non-transitory medium, where non-transitory excludes propagation signals. For example, a processor can be connected to a non-transitory computer-readable medium that stores instructions for executing instructions by the processor.

Figure 1 is a schematic illustration of a bottom contact, bottom gate OTFT gas sensor according to some embodiments. The bottom contact bottom gate OTFT comprises a gate electrode 103 disposed over a substrate 101, e.g. a glass or plastic substrate; source and drain electrodes 107, 109; a dielectric layer 105 between the gate electrode and the source and drain electrodes; and an organic semiconductor layer 111 in electrical contact with the source and drain electrodes. A protic layer 113 comprising a protic group is bound to a surface of the source and drain electrodes and is disposed between the source and drain electrodes and the organic semiconducting layer. The protic layer may be in direct contact with the organic semiconducting layer.

Figure 2 is a schematic illustration of a top-contact bottom gate OTFT gas sensor according to some embodiments. The top-contact bottom gate OTFT is as described with reference to Figure 1 except that the organic semiconductor layer 111 is between the dielectric layer 105 and the source and drain electrodes 107, 109.

Figure 3 is a schematic illustration of a top gate bottom contact OTFT gas sensor according to some embodiments. The top gate bottom contact OTFT gas sensor comprises source and drain electrodes 107, 109 disposed over a substrate; an organic semiconductor layer 111 in electrical contact with the source and drain electrodes; a dielectric layer 105 between the gate electrode 103 and the organic semiconductor layer; and protic layer 113 comprising a protic group is bound to a surface of the source and drain electrodes and between the source and drain electrodes and the organic semiconducting layer. The organic semiconducting layer is disposed between the source and drain electrodes and the dielectric layer.

Figure 4 is a schematic illustration of a top gate top contact OTFT gas sensor according to some embodiments. The top gate OTFT gas sensor is as described with reference to Figure 3 except that the organic semiconducting layer is disposed between the substrate and the source and drain electrodes.

In some embodiments, the dielectric layer of a top-gate OTFT gas sensor as described herein, e.g. as described with reference to Figures 3 and 4, is a gas-permeable material, preferably an organic material.

In some embodiments, the top gate electrode of a top gate OTFT gas sensor as described herein, e.g. as described with reference to Figures 3 and 4, is patterned, e.g., comprises fingers, comb like structures and/or the like, to provide channels, openings and/or the like that allow passage of gas through the top gate electrode to the protic layer.

In some embodiments, the OTFT gas sensor comprises a second semiconducting layer adjacent to the organic semiconducting layer. Such an OTFT gas sensor may be as described in any one of Figures 1-4, the OTFT gas sensor further comprising the second semiconducting layer adjacent to the organic semiconducting layer.

In some embodiments, the second semiconducting layer is disposed between the source and drain electrodes and the organic semiconducting layer.

In some embodiments, the organic semiconducting layer is disposed between the source and drain electrodes and the second semiconducting layer. In use, the OTFT gas sensor may be exposed to a gaseous environment and a response of the OTFT gas sensor may be measured. The response may be a change in drain current. A measurement unit configured to measure the response of the OTFT gas sensor may be in wired or wireless communication with processor unit configured to determine a presence, a concentration, and / or a change in concentration, of an ester in the environment.

The response of the OTFT gas sensor to an environment may be a change which is directly proportional to a concentration of ester in the environment, or directly proportional to a derivative of the concentration of ester in the environment. The OTFT gas sensor may be calibrated by measuring response of the OTFT gas sensor to one or more known concentrations of ester in an environment.

The OTFT gas sensor may be part of a gas sensor system in which the OTFT gas sensor is a first gas sensor and in which the gas sensor system comprises one or more further OTFT gas sensors different from the first OTFT gas sensor. The, or each, further OTFT gas sensor may be configured to detect a second gas in the environment other than an ester.

In some embodiments, the first OTFT gas sensor configured to detect an ester has greater sensitivity to the ester than to a second gas.

In some embodiments, the gas sensor system comprises a second OTFT gas sensor configured to detect a second gas, the second OTFT gas sensor having greater sensitivity to the second gas than the first gas. The second gas may be an alkene, optionally an alkene comprising a conjugated alkene group, for example styrene or famesene. The alkene may be capable of binding to source and drain electrodes of the second OTFT gas sensor. Such binding to source and drain electrodes of the first OTFT gas sensor may be inhibited by the pro tic group.

By“sensitivity” of an OTFT gas sensor to a gas as used herein is meant a maximum percentage change in drain current upon exposure of the OTFT gas sensor to a given concentration of the gas.

Differences between first and second OTFT gas sensors may give rise to different sensitivities. In some embodiments, first and second OTFT gas sensors may have one or more of the following differences: In some embodiments, the second OTFT gas sensor may comprise one or more OTFT gas sensors as described with reference to Figures 1-4 in which the protic layer is not present, e.g. in which the organic semiconducting layer is in direct contact with the source and drain electrodes or a non-protic layer is disposed on a surface of the source and drain electrodes and between the organic semiconducting layer and the surface of the source and drain electrodes.

Different gate arrangements of the first and second OTFT gas sensors, e.g. top gate for one of the first and second OTFT gas sensors and bottom gate for the other OTFT gas sensor. - Different OTFT device types, e.g. top gate for one of the first and second OTFT gas sensors and bottom gate for the other OTFT gas sensor.

Different source and drain electrodes for the first and second OTFT gas sensors.

Different interfaces of the first and second OTFT gas sensors between the organic semiconducting layer and the source and drain electrodes of the first and second OTFT gas sensors.

Different organic semiconductors for the first and second OTFT gas sensors, e.g. a crystalline organic semiconductor for one of the first and second OTFT gas sensors and amorphous for the other OTFT gas sensor.

Different numbers of semiconducting layers, e.g. an organic semiconducting layer as the only semiconducting layer for one of the first and second OTFT gas sensors and at least one further semiconducting layer for the other OTFT gas sensor.

Different thicknesses of the organic semiconducting layers of the first and second OTFT gas sensors.

The gas sensor system may comprise one or more control OTFT gas sensors to provide a baseline for measurements of the first and second OTFT gas sensors to take into account variables such as one or more of humidity, temperature, pressure, variation of sensor parameter measurements over time (such as variation of OTFT sensor drain current over time), and gases other than a target gas or target gases in the atmosphere. One or more control OTFT gas sensors may be isolated from the atmosphere, for example by encapsulation of the or each OFT control sensor, to provide a baseline measurement other than gases in the atmosphere.

Each of the OTFT gas sensors of the gas sensor system may be supported on a common substrate and / or contained in a common housing.

In use, each OTFT gas sensor of the gas sensor system may be connected to a common power source, or two or more of the OTFT gas sensors may be powered by different power sources.

In use, power to all of the OTFT gas sensors of the gas sensor system may be controlled by a single switch or power to two or more of the OTFT gas sensors may be controlled by different switches.

Protic layer

In some embodiments, the protic layer is a monolayer.

In some embodiments, the protic group is formed from a compound capable of binding to the source and drain electrodes, optionally a compound of formula (I):

X¹-R¹-Y

(I) wherein R¹ is an organic residue; X¹ is a binding group for binding to the surface of the source and drain electrodes; and Y is a protic group.

Optionally, R¹ is a C_1-30 hydrocarbon group which may be unsubstituted or substituted with one or more substituents. Exemplary C_1-30 hydrocarbon groups are: linear, branched or cyclic C_6-30 alkylene; C_6-20 arylene groups, preferably phenylene; and phenylene-C_1-20 alkylene.

A preferred substituent of the C_1-30 hydrocarbon group, if present, is fluorine, and one or more H atoms of the C_1-30 hydrocarbon group may be replaced with fluorine.

Optionally, Y is -NH₂.

Optionally, X¹ is SH. Optionally, R¹ is a hydrocarbon residue, e.g. C_6-20 aromatic groups, preferably phenyl, phenyl with one or more C_1-20 alkyl groups; and phenyl-C_1-20 alkyl which may be substituted with one or more C_1-20 alkyl groups.

An exemplary compound of formula (I) is 4-aminobenzenethiol:

Non-pro tic groups

In some embodiments, the protic layer consists of one or more protic groups.

In some embodiment, the protic layer further comprises one or more non-protic groups.

In some embodiments, the non-protic group is formed from a compound of formula (II): X²-R²

(P) wherein R 2 is an organic residue and X 2 is a binding group for binding to the surface of the source and drain electrodes.

X may be selected from groups described with reference to X . R may be, without limitation, a C_1-30 hydrocarbyl group which may be unsubstituted or substituted with one or more substituents. Exemplary C_1-30 hydrocarbyl groups are: linear, branched or cyclic C_6-30 alkyl; C_6-20 aryl, preferably phenyl; and C_1-20 alkylene-phenylene.

A preferred substituent of the C_1-30 hydrocarbyl group, if present, is fluorine, and one or more H atoms of the C_1-30 hydrocarbyl group may be replaced with fluorine. Exemplary compounds of formula (II) are:

The protic group and / or, if present, the non-protic group, may be selected according to the effect, if any, it has on the work function of the source and drain electrodes and the required charge injection requirements of the OTFT gas sensor such as the work function - organic semiconductor highest occupied molecular orbital (HOMO) gap in the case of a p-type OTFT gas sensor or the work function - organic semiconductor lowest unoccupied molecular orbital (LUMO) gap in the case of a n-type OTFT gas sensor.

A monolayer comprising the protic group and, if present, the non-protic group may be formed on the source and drain electrodes by depositing thereon a binding compound for forming the protic group and, optionally, a binding compound for forming the non-protic group, for example from a solution of the binding compound or compounds in one or more solvents.

The binding compound or compounds may be selectively deposited onto the source and drain electrodes only, or may be deposited by a non-selective process such as spin-coating or dip-coating.

A bottom-contact OTFT gas sensor (top gate or bottom gate) may be formed by depositing a binding compound or binding compounds onto the source and drain electrodes over a dielectric layer and then depositing the organic semiconducting layer. Binding compound which is not bound to the source and drain electrodes, for example binding compound on the dielectric layer or the substrate following a non-selective deposition process, may be removed by washing. Organic semiconducting layer

Organic semiconductors as described herein may be selected from conjugated non polymeric semiconductors; polymers comprising conjugated groups in a main chain or in a side group thereof; and carbon semiconductors such as graphene and carbon nanotubes.

An organic semiconductor layer of an OTFT gas sensor as described herein may comprise or consist of a semiconducting polymer and / or a non-polymeric organic semiconductor. The organic semiconductor layer may comprise a blend of a non-polymeric organic semiconductor and a polymer.

Exemplary organic semiconductors are disclosed in WO 2016/001095, the contents of which are incorporated herein by reference.

The organic semiconductor may be crystalline or amorphous.

The organic semiconducting layer may be deposited by any suitable technique, including evaporation and deposition from a solution comprising or consisting of one or more organic semiconducting materials and at least one solvent. Exemplary solvents include benzenes with one or more alkyl substituents, preferably one or more C_1-10 alkyl substituents, such as toluene and xylene; tetralin; and chloroform. Solution deposition techniques include coating and printing methods, for example spin coating dip-coating, slot-die coating, ink jet printing, gravure printing, flexographic printing and screen printing.

Optionally, the organic semiconducting layer has a thickness in the range of about 10-200 nm.

Second semiconducting layer

In some embodiments the second semiconducting layer, if present, comprises or consists of a transition metal halide or a pseudohalide.

The transition metal halide or pseudohalide may be a metal complex, optionally a coordination polymer. The transition metal of the transition metal halide or pseudohalide is optionally copper (I), silver (I) or cobalt and is preferably Cu (I). Optionally, the halide of a semiconducting transition metal halide is selected from fluoride, chloride, bromide, iodide or astatide.

Optionally, the pseudohalide of a semiconducting transition metal pseudohalide is selected from thiocyanate, selenocyanate and tellurocyanate.

Preferably, the transition metal halide or pseudohalide is selected from copper thiocyanate (CuSCN); silver thiocyanate (AgSCN); cuprous iodide (Cul). copper selenocyanate (CuSeCN) and copper tellurocyanate (CuTeCN). Copper thiocyanate is particularly preferred.

Electrodes

The source and drain electrodes can be selected from a wide range of conducting materials for example a metal (e.g. gold), metal alloy, metal compound (e.g. indium tin oxide) or conductive polymer.

The source and drain electrode material may be selected according to its ability to bind to the protic group. In the case where the protic group comprises a thiol binding group, the source and drain electrodes are preferably gold.

The gate electrode may be selected from any conducting material, for example a metal (e.g. aluminium), a metal alloy, a conductive metal compound (e.g. a conductive metal oxide such as indium tin oxide) or a conductive polymer.

The length of the channel defined between the source and drain electrodes of the source and drain and gate electrodes of an OTFT gas sensor as described herein may be up to 500 microns, preferably less than 200 microns, more preferably less than 100 microns.

Dielectric layer

The dielectric layer may comprise one or more organic materials, one or more inorganic materials or a mixture thereof. Optionally, dielectric materials are selected from those disclosed in Chem. Rev., 2010, 110 (1), pp 205-239, the contents of which are incorporated herein by reference. In the case of a top-gate OTFT gas sensor, the dielectric layer preferably comprises an organic material.

Preferred inorganic materials include BaTiOs, SiTi0₃, SiCE, SiNx and spin-on-glass (SOG).

Preferred organic materials are organic polymers. Exemplary polymers are, polyvinylpyrrolidine (PVP), acrylates such as polymethylmethacrylate (PMMA) and benzocyclobutanes (BCBs), poly(vinyl cinnamate) P(VCn), and partially fluorinated or perfluorinated polymers, for example poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP), P(VDF-TrFE-CTFE), and polymers comprising or consisting of tetrafluoroethene repeat units. The polymer may or may not be crosslinked. In embodiments, the dielectric layer may consist of a polymer. In embodiments, the dielectric layer may be a polymer / inorganic composite, for example as described in Materials 2009, 2(4), 1697-1733, the contents of which are incorporated herein by reference. The inorganic material of the composite may be in the form of nanoparticles. The inorganic material of the composite may have a dielectric constant of at least 5, at least 10 or at least 20.

In embodiments, the OTFT gas sensor may comprise more than one dielectric layer, optionally a dielectric bilayer in which a first dielectric layer in direct contact with the organic semiconducting layer comprises a material having a lower dielectric constant than a material of a second dielectric layer spaced apart from the organic semiconducting layer by the first dielectric layer.

In some embodiments, the dielectric layer of a top-gate OTFT gas sensor comprises a gas- permeable material, preferably an organic material, more preferably a polymer material, which allows permeation through the dielectric layer of the gas or gases to be sensed. In some embodiments, the top-gate OTFT gas sensor has a single dielectric layer between the gate electrode and the semiconducting layer. In some embodiments, the top-gate OTFT gas sensor has more than one dielectric layer between the gate electrode and the semiconducting layer, each dielectric layer being permeable to the or each target gas.

The dielectric material may be deposited by thermal evaporation, vacuum processing or lamination techniques as are known in the art. The dielectric material may be deposited from solution using, for example, spin coating or ink jet printing techniques and other solution deposition techniques discussed above with respect to the semiconductor layer. In the case of a bottom gate OTFT gas sensor, the dielectric material should not be dissolved if an organic semiconductor is deposited onto it from solution. In the case of a top-gate OTFT gas sensor, the organic semiconductor layer should not be dissolved if the dielectric is deposited from solution.

Techniques to avoid such dissolution include: use of orthogonal solvents for example use of a solvent for deposition of the organic semiconducting layer that does not dissolve the dielectric layer in the case of a bottom gate device or vice versa in the case of a top gate device; cross linking of the dielectric layer before deposition of the organic semiconductor layer in the case of a bottom gate device; or deposition from solution of a blend of the dielectric material and the organic semiconductor followed by vertical phase separation as disclosed in, for example, L. Qiu, et al., Adv. Mater. 2008, 20, 1141.

The thickness of the dielectric layer is preferably less than 2 micrometres, more preferably less than 500 nm.

OTFT gas sensors as described herein may be used in detection of esters, e.g. C_1-10 alkyl esters or C_1-10 alkanoate esters, optionally C_1-10 alkyl- C_1-10 alkanoate esters, e.g. methyl octanoate; ethyl acetate; ethyl butanoate; ethyl hexanoate; propyl acetate; butyl acetate; butyl butanoate; butyl hexanoate; pentyl acetate; hexyl acetate; hexyl butanoate; hexyl hexanoate; 2-methylpropyl acetate; 2-methylbutyl acetate; ethyl 2-methylbutanoate; butyl 2-methylbutanoate; and hexyl 2-methylbutanoate.

OTFT gas sensors and gas sensor systems as described herein may be used in an environment in which an ester, e.g. butyl acetate, is produced by a natural process, e.g. by fruit.

OTFT gas sensors and gas sensor systems as described herein may be used in an environment in which one or more gases, such as styrene and butyl acetate, are produced by a natural process, e.g. by fruit. The fruit may be apples. Fruit rot may be detected by detection of styrene.

OTFT Gas Sensor Example 1

A PEN substrate was baked in a vacuum oven and then UV-ozone treated for 30 seconds. Source and drain contacts were deposited onto the substrate by thermal evaporation of 3 nm Cr followed by 40nm Au through shadow masks with channel length of 125 mm and a channel width of 4 mm. A solution of 4-aminobenzene thiol (4ABT) was spin-coated onto the source and drain electrodes from an isopropyl alcohol solution (0.1 % v/v). After 2 minutes, the substrate and source and drain electrodes were rinsed with isopropyl alcohol. Semiconducting Polymer 1, illustrated below, was deposited over the substrate by spin coating from a 1%w/v solution in 1,2,4-trimethylbenzene to a thickness of 40nm and dried at 100°C for 1 or lOmin in air. The polymer dielectric Teflon ® AF2400 was spin coated from a 2.5 %w/v solution in a 50:50 v/v blend of fluorinated solvents FC43 and FC85 to a 300nm thickness and dried at 80°C for lOmin, after a 5 minute initial drying phase while spinning. The gate was formed by thermal evaporation of Cr (3 nm) followed by A1 (200 nm) through a shadow mask to form a gate electrode having a comb structure with comb fingers of 125 microns width and gaps of 125 microns between fingers.

OTFT Gas Sensor Example 2

OTFT Gas Sensor Example 2 was prepared as described for OTFT Gas Sensor Example 1 except that a solution of 4-aminobenzenethiol and 4-fluorobenzenethiol (1:1 w/w) was deposited by spin-coating onto the source and drain electrodes rather than a solution of 4- aminobenzenethiol alone.

Comparative OTFT Gas Sensor

Comparative OTFT Gas Sensor was prepared as described for Gas Sensor Example 1 except that a solution of 4-fluorobenzenethiol (4FBT) was used in place of 4- aminobenzenethiol . The OTFT gas sensors were pulsed for 100ms every 25 s at a drain and gate voltage of -4V to give a sensor current of >10nA and exposed to methyl hexanoate vapour by flowing a carrier gas (N2) through the headspace that formed above a reservoir of methyl hexanoate liquid. This flow was then diluted using a separate flow of N2 carrier gas to set the methyl hexanoate concentration to 3,000 ppm.

With reference to Figure 5 and Table 1, OTFT Gas Sensor Examples 1 and 2 showed a much greater response in terms of change in drain current than the Comparative OTFT Gas Sensor. Without wishing to be bound by any theory, this may be due to hydrogen bonding between the amino group of OTFT Gas Sensor Examples 1 and 2 and the ester. Use of both 4ABT and 4FBT in OTFT Gas Sensor Example 2 resulted in less variability between OTFTs than for OTFT Gas Sensor Example 1

Table 1

Claims

1. A method of identifying the presence and/or concentration of a ester in an gaseous environment, comprising: providing an organic thin film transistor (OTFT) gas sensor in the gaseous environment, the OTFT gas sensor comprising: source and drain electrodes in electrical contact with an organic

semiconductor layer; a gate electrode; and a gate dielectric disposed between the organic semiconductor layer and the gate electrode, wherein a protic group is bound to a surface of the source and drain electrodes and the OTFT gas sensor is configured to provide for gas communication of the ester in the environment with the protic group; measuring of a response of the OTFT gas sensor to the environment; and determining from the measured response the presence and/or concentration of the ester.

2. A method according to claim 1, wherein a monolayer comprises the protic group bound to the surface of the source and drain electrodes.

3. A method according to claim 1 or 2, wherein the protic group is formed from a compound of formula (I):

X¹-R¹-Y

(I) where: R¹ is an organic residue and X¹ is a binding group for binding to the surface of the source and drain electrodes; and

Y is a protic group.

4. A method according to claim 3, wherein Y is -NH2.

5. A method according to claim 3 or 4, wherein X¹ is SH.

6. A method according to any one of claims 3-5, wherein R¹ is a hydrocarbon residue.

7. A method according to any one of claims 3-6, wherein R¹ is phenylene.

8. A method according to any one of the preceding claims, wherein a non- protic group is bound to the surface of the source and drain electrodes.

9. A method according to claim 8, wherein the non-protic group is formed from a compound of formula (II):

X²-R²

(I) where: R 2 is an organic residue and X 2 is a binding group for binding to the surface of the source and drain electrodes.

10. A method according to any one of the preceding claims, wherein the ester is a C_1-10 alkyl ester.

11. A method according to any one of the preceding claims, wherein the ester is a C_1-10 alkanoate ester.

12. An organic thin film transistor (OTFT) gas sensor configured to measure ester in a gaseous environment, comprising: a source electrode and a drain electrodes in electrical contact with an organic semiconductor layer; a gate electrode; and a gate dielectric disposed between the organic semiconductor layer and the gate electrode, wherein: a protic group is bound to a surface of the source and drain electrodes; and the OTFT gas sensor is configured to provide for gaseous communication between the environment and the protic group.

13. The organic thin film transistor (OTFT) gas sensor of claim 12 or claim 13, wherein the protic group comprises a monolayer bound to the surface of the source and drain electrodes.

14. The organic thin film transistor (OTFT) gas sensor of claim 12, wherein the protic group is formed from a compound of formula (I):

X¹-R¹-Y

Y is a protic group.

15. The organic thin film transistor (OTFT) gas sensor of claim 14, wherein Y is -NH₂.

16. The organic thin film transistor (OTFT) gas sensor of claim 14 or 15, wherein X¹ is SH.

17. The organic thin film transistor (OTFT) gas sensor of any one of claims 14 through 16, wherein R¹ is a hydrocarbon residue.

18. The organic thin film transistor (OTFT) gas sensor of any one of claims claim 14 through 16, wherein R¹ is phenylene.

19. The organic thin film transistor (OTFT) gas sensor of any one of claims cl2 through 18, wherein a non-protic group is bound to the surface of the source and drain electrodes.

20. The organic thin film transistor (OTFT) gas sensor of claim 19, wherein the non-protic group is formed from a compound of formula (II):

X²-R² (I) where: R 2 is an organic residue and X 2 is a binding group for binding to the surface of the source and drain electrodes.

21. The organic thin film transistor (OTFT) gas sensor of any one of claims claim 12 through 20, wherein the ester is a C_1-10 alkyl ester.

22. The organic thin film transistor (OTFT) gas sensor of any one of claims claim 12 through 20, wherein the ester is a C_1-10 alkanoate ester.