ORGANIC FIELD-EFFECT TRANSISTOR COMPRISING A DIELECTRIC LAYER EXHIBITING HIGH DIELECTRIC PERMITTIVITY AND BEING STABLE WITH TEMPERATURE

Abstract

The invention relates to a composition comprising a blend of fluorinated electroactive polymers and having a dielectric permittivity that exhibits greater stability over the operating temperature range with respect to each polymer employed on its own. The invention also relates to formulations and films produced on the basis of said composition. The invention also relates to a field-effect transistor, at least part of the dielectric layer of which is composed of a blend of fluorinated electroactive polymers.

Claims

1. A composition comprising a blend of fluorinated electroactive polymers, said blend being composed of: a) at least one fluorinated terpolymer of formula P(VDF-X-Y) comprising units derived from vinylidene fluoride (VDF), units derived from a monomer X chosen from trifluoroethylene (TrFE), tetrafluoroethylene, chlorotrifluoroethylene (CTFE), vinyl fluoride, 1,1-chlorofluoroethylene, hexafluoropropene, 3,3,3-trifluoropropene, 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 2-chloro-3,3,3-trifluoropropene, and a third monomer Y, b) and of at least one copolymer of formula P(VDF-TrFE) comprising units derived from vinylidene fluoride and units derived from trifluoroethylene, the proportion of units derived from trifluoroethylene being greater than 45 mol % relative to the sum of the units derived from vinylidene fluoride and trifluoroethylene.

2. The composition as claimed in claim 1, wherein the monomer X is trifluoroethylene.

3. The composition as claimed in claim 1, wherein the third monomer Y is chlorotrifluoroethylene or 1,1-chlorofluoroethylene, and the proportion of units derived from the monomer Y is from 1 to 15 mol %, relative to the entirety of the units of said terpolymer.

4. The composition as claimed in claim 1, wherein the proportion by weight of component b) is between 0.1% and 50% of the total weight of the composition.

5. The composition as claimed in claim 1, wherein the proportion of units derived from trifluoroethylene in the copolymer is greater than 50 mol % relative to the sum of the units derived from vinylidene fluoride and from trifluoroethylene.

6. The composition as claimed in claim 1, further comprising up to 2% by weight of an additive, said additive being a (meth)acrylic polymer, in particular poly(methyl methacrylate).

7. The composition as claimed in claim 1, said composition exhibiting a relative permittivity εr of higher than 30 measured at 1 kHz over a temperature range from 20° C. to 80° C.

8. The composition as claimed in claim 1, wherein the fluorinated electroactive terpolymer is a relaxor ferroelectric terpolymer.

9. A formulation based on fluorinated electroactive polymers comprising the composition as claimed in claim 1 in solution in a solvent.

10. The formulation as claimed in claim 9, wherein said solvent is chosen from dimethylformamide, dimethylacetamide, dimethyl sulfoxide, ketones, ifurans, esters, carbonates, phosphates, and mixtures thereof.

11. A polymer film consisting of the formulation as claimed in claim 9.

12. The film as claimed in claim 11, exhibiting a relative dielectric permittivity that varies by +/−10 over a temperature range from 0° C. to 100° C.

13. An (opto)electronic device comprising a substrate and a film as claimed in claim 11 deposited on the substrate.

14. The device as claimed in claim 13, further comprising electrodes on either side of the film.

15. A field-effect transistor comprising a semiconductor element (3), electrodes (1) and a dielectric layer (2), wherein the dielectric layer is at least partly composed of a blend of fluorinated electroactive polymers.

16. The field-effect transistor as claimed in claim 15, wherein said blend of fluorinated electroactive polymers is composed of: at least one fluorinated electroactive terpolymer of formula P(VDF-X-Y) comprising units derived from vinylidene fluoride (VDF), units derived from a monomer X chosen from trifluoroethylene (TrFE), tetrafluoroethylene, chlorotrifluoroethylene (CTFE), vinyl fluoride, 1,1-chlorofluoroethylene, hexafluoropropene, 3,3,3-trifluoropropene, 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 2-chloro-3,3,3-trifluoropropene, and a third monomer Y, and of at least one copolymer of formula P(VDF-TrFE) comprising units derived from vinylidene fluoride and units derived from trifluoroethylene.

17. The field-effect transistor as claimed in claim 15, wherein the dielectric layer is at least partly composed of the composition as claimed in claim 1.

Description

FIGURES

[0028] FIG. 1 shows a schematic diagram of a field-effect transistor. The elements 1 represent the three electrodes: the source, gate and drain. The element 2 represents the dielectric layer which, in the invention, is composed of at least one layer produced on the basis of a blend of fluorinated electroactive polymers exhibiting high dielectric permittivity. The element 3 is the semiconductor.

[0029] FIG. 2 is a graph illustrating curves of relative permittivity (Y axis), measured at 1 kHz, with temperature (X axis) (rising and then falling) for three fluorinated electroactive polymers: P(VDF-TrFE-CTFE) (dashed-and-dotted curve), P(VDF-TrFE-CTFE) (solid curve) and P(VDF-TrFE) (dashed curve). The VDF/TrFEN molar compositions of the polymers are: 60/30/10, 61/35/4 and 70/30, respectively.

[0030] FIG. 3 is a graph illustrating curves of relative permittivity (Y axis), measured at 1 kHz, with temperature (X axis) (rising and then falling) for a P(VDF-TrFE) fluorinated electroactive polymer with a molar composition of 43/57.

[0031] FIG. 4 is a graph illustrating curves of relative permittivity (Y axis), measured at 1 kHz, with temperature (X axis) (rising only shown) for four compositions: (i) broken black line, P(VDF-TrFE-CFE) terpolymer containing 6.9 mol % CFE; (ii) broken gray line, P(VDF-TrFE-CFE) terpolymer with 8.2 mol % CFE; (iii) solid gray line, 70-30 blend by weight of terpolymer (ii) with a P(VDF-TrFE) copolymer with a molar composition of 54/46; and (iv) solid black line, 70-30 blend by weight of terpolymer (i) with a P(VDF-TrFE) copolymer with a molar composition of 54/46.

[0032] FIG. 5A is a graph illustrating curves of relative permittivity (Y axis) with temperature (X axis) (rising and falling) and with electric field frequency (0.1-1-10-100 kHz) for a 90-10 blend by weight of a P(VDF-TrFE-CFE) terpolymer containing 6.9 mol % CFE and a P(VDF-TrFE) copolymer with a molar composition of 54/46. In FIG. 5B: curves of relative permittivity (Y axis) with temperature (X axis) and with electric field frequency (0.1-1-10-100 kHz) for the same blend further comprising PMMA.

[0033] FIG. 6 is a graph illustrating the curve of relative permittivity (Y axis), measured at 1 kHz, with temperature (X axis) (rising and then falling) for a 50-50 blend by weight of a P(VDF-TrFE) copolymer with a molar composition of 43/57 with a P(VDF-TrFE-CFE) terpolymer containing 8.2 mol % CFE.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0034] The invention is now described in greater detail and in a non-limiting manner in the description that follows.

[0035] The invention is based first on the use of a fluorinated terpolymer (component a). The term “fluorinated” is understood to mean a terpolymer comprising -F groups.

[0036] Preferably, the fluorinated terpolymer is a relaxor ferroelectric polymer. Such a material has a weak coercive field (typically weaker than 10 V/μm), little remanent polarization (typically less than 10 mC/m.sup.2), or even none, and a maximum dielectric permittivity with temperature dependent on the frequency of the electric field.

[0037] The terpolymer, of formula P(VDF-X-Y), comprises units derived from vinylidene fluoride (VDF), units derived from a monomer X chosen from trifluoroethylene (TrFE), tetrafluoroethylene, chlorotrifluoroethylene (CTFE), vinyl fluoride, 1,1-chlorofluoroethylene (CFE), hexafluoropropene, 3,3,3-trifluoropropene, 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 2-chloro-3,3,3-trifluoropropene, and a third monomer Y.

[0038] Preferably, the monomer X is TrFE.

[0039] Preferably, Y represents units derived from CFE (1-chloro-1-fluoroethylene) or from CTFE (chlorotrifluoroethylene).

[0040] Alternatively, the third monomer may in particular be chosen from halogenated alkenes, in particular halogenated propenes or ethylenes, and for example from tetrafluoropropenes (in particular 2,3,3,3-tetrafluoropropene), chlorotrifluoropropenes (in particular 2-chloro-3,3,3-trifluoropropene), 1-chloro-2-fluoroethylene, trifluoropropenes (in particular 3,3,3-trifluoropropene), pentafluoropropenes (in particular 1,1,3,3,3-pentafluoropropene or 1,2,3,3,3-pentafluoropropene), 1-chloro-2,2-difluoroethylene, 1-chloro-2-fluoroethylene, 1-bromo-2,2-difluoroethylene, bromotrifluoroethylene, fluoroethylene (or vinyl fluoride), tetrafluoroethylene and hexafluoropropene. The third monomer may also be a perfluoroalkyl vinyl ether, of general formula Rf—O—CF═CF.sub.2, R.sub.f being a preferably C.sub.1 to C.sub.4 alkyl group. Preferred examples are PPVE (perfluoropropyl vinyl ether) and PMVE (perfluoromethyl vinyl ether).

[0041] The terpolymers of the invention may be produced by using any known process, such as emulsion polymerization, microemulsion polymerization, suspension polymerization and solution polymerization. The use of the process described in the document WO 2010/116105 is particularly preferred. This process makes it possible to obtain polymers of high molecular weight and of suitable structuring.

[0042] According to one embodiment, the molar proportion of Y units in the terpolymer has a value ranging from 1% to 15%, preferably from 1% to 12%.

[0043] According to one embodiment, the molar ratio of the VDF units to the TrFE units in the terpolymer has a value of 85/15 to 30/70 and preferably of 75/25 to 40/60.

[0044] According to one embodiment, the weight-average molar mass, which in the context of this patent application is also referred to as the “molecular weight” (Mw), of the terpolymer has a value ranging from 200 000 g/mol to 1 500 000 g/mol, preferably from 250 000 g/mol to 1 000 000 g/mol and more particularly from 300 000 g/mol to 700 000 g/mol.

[0045] The latter can be adjusted by modifying certain parameters of the process, such as the temperature in the reactor, or by adding a transfer agent.

[0046] The molecular weight distribution can be estimated by SEC (size exclusion chromatography) with dimethylformamide (DMF) as eluent, with a set of three columns of increasing porosity. The stationary phase is a styrene-DVB gel. The detection process is based on measurement of the refractive index, and calibration is performed with polystyrene standards. The sample is dissolved at 0.5 g/I in DMF and filtered through a 0.45 μm nylon filter.

[0047] The molecular weight can also be evaluated by measurement of the melt flow index at 230° C. under a load of 5 kg according to ASTM D1238 (ISO 1133).

[0048] Moreover, the molecular weight can also be characterized by a measurement of the viscosity in solution according to the standard ISO 1628. Methyl ethyl ketone (MEK) is a preferred solvent for the terpolymers for determining the viscosity index.

[0049] More generally, the molar composition of the terpolymers of the invention can be determined by various means. The conventional methods for elemental analysis of carbon, fluorine and chlorine or bromine elements result in a system of two or three independent equations having two independent unknowns (for example % VDF and % TrFE, with % Y=100−(% VDF+% TrFE)), which makes it possible to unambiguously calculate the composition by weight of the polymers, from which the molar composition is deduced.

[0050] Use may also be made of multinuclear, in this instance proton (.sup.1H) and fluorine (.sup.19F), NMR techniques, by analysis of a solution of the polymer in an appropriate deuterated solvent. The NMR spectrum is recorded on an FT-NMR spectrometer equipped with a multinuclear probe. The specific signals given by the various monomers in the spectra produced according to one or other nucleus are then identified. Thus, for example, the TrFE (CFH═CF.sub.2) unit gives, in proton NMR, a specific signal characteristic of the CFH group (at approximately 5 ppm). The same is true for the CH.sub.2 groups of VDF (broad unresolved peak centred at 3 ppm). The relative integration of the two signals gives the relative abundance of the two monomers, i.e. the VDF/TrFE mole ratio.

[0051] The combination of the relative integrations of the various signals obtained in proton NMR and in fluorine NMR results in a system of equations, the resolution of which results in the molar concentrations of the various monomer units being obtained.

[0052] Finally, it is possible to combine the elemental analysis, for example for the heteroatoms, such as chlorine or bromine, and the NMR analysis. Thus, the content of CTFE or of CFE can be determined by a measurement of the chlorine content by elemental analysis.

[0053] A person skilled in the art thus has available a range of methods or combination of methods allowing them to determine, without ambiguity and with the necessary accuracy, the composition of the terpolymers of the invention.

[0054] The invention is next based in the use of a copolymer (component b) of formula P(VDF-TrFE) comprising units derived from vinylidene fluoride and units derived from trifluoroethylene, compatible with the terpolymer and exhibiting a Curie temperature differing from that of the terpolymer.

[0055] The term “compatible” is understood to mean that the blending of the two polymers forms a homogeneous phase with a single glass transition temperature.

[0056] Characteristically, the proportion of units derived from trifluoroethylene in the copolymer is greater than 45 mol % relative to the sum of the units derived from vinylidene fluoride and from trifluoroethylene, and preferably greater than 50 mol %.

[0057] The Curie temperature of the copolymer is between 20° C. and 80° C. The Curie temperature of the polymers of the invention may be measured by differential scanning calorimetry or by dielectric spectroscopy.

[0058] The composition according to the invention is a blend of fluorinated relaxor ferroelectric polymers comprising at least one fluorinated terpolymer (component a) and at least one P(VDF-TrFE) copolymer with a molar composition of greater than 45 mol % TrFE (component b).

[0059] In the composition according to the invention, the proportion by weight of component b) is between 0.1% and 50%, preferably between 1% and 45% and advantageously between 5% and 40% of the total weight of the composition.

[0060] The composition of the invention comprises at least one terpolymer as described above (optionally two or more of them) and at least one copolymer as described above (optionally two or more of them).

[0061] The composition of the invention may also comprise an additive, the role of which is to increase the dielectric permittivity. According to one embodiment, the composition comprises up to 2% by weight of an additive, said additive being a (meth)acrylic polymer, in particular poly(methyl methacrylate) (PMMA).

[0062] Another subject of the invention is a formulation (or ink) based on fluorinated electroactive polymers comprising the composition described above in solution in a solvent. According to one embodiment, said solvent is chosen from dimethylformamide, dimethylacetamide, dimethyl sulfoxide, ketones, in particular acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclopentanone, furans, in particular tetrahydrofuran, esters, in particular methyl acetate, ethyl acetate, propyl acetate, butyl acetate and propylene glycol methyl ether acetate, carbonates, in particular dimethyl carbonate, phosphates, in particular triethyl phosphate, and mixtures thereof.

[0063] The solvent may be present in the formulation in a proportion by weight of at least 50%, and preferably of at least 80%.

[0064] The formulation of the invention can be produced by dissolving its various compounds in a solvent. The fluorinated electroactive polymers may be dissolved in the solvent at the same time or one after the other, or separately, with the formulations then being mixed. It is preferable to dissolve the terpolymer in the solvent before the copolymer.

[0065] The invention provides in particular films produced on the basis of formulations according to the invention and deposited on a substrate. The substrate may, for example, be a polymeric substrate, such as a poly(ethylene terephthalate) or polyethylene naphthalate substrate, or a paper, glass or silicon substrate.

[0066] Preferably, the film is deposited by the solvent or molten route, then dried (evaporation of the solvent) and annealed to increase its crystallinity (by heating at a temperature lower than the melting point of the composition and higher than the Curie temperature of the composition for a period of time of greater than or equal to one minute).

[0067] Advantageously, the film according to the invention exhibits what is considered a stable relative dielectric permittivity that varies by +/−10, preferably by +/−5 and advantageously by +/−2 over a temperature range from 0° C. to 100° C., preferably from 10° C. to 80° C. and advantageously from 15° C. to 70° C.

[0068] The dielectric permittivity may be measured by means of a Sefelec LCR 819 LCR meter, which enables measurement of a capacitance which is proportional to the permittivity.

[0069] This film is therefore suitable for manufacturing electronic devices that require a stable permittivity over a wide range of temperatures for their operation.

[0070] The invention therefore provides electronic devices comprising a substrate and at least one film according to the invention. The term “electronic device” is understood to mean either a single electronic component or a set of electronic components capable of performing one or more functions in an electronic circuit, such as transistors (notably field-effect transistors), chips, batteries, photovoltaic cells, light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), sensors, actuators, transformers, haptic devices, electromechanical microsystems, electrocaloric devices, and detectors.

[0071] According to certain variations, the electronic device is more particularly an optoelectronic device, i.e. a device that is capable of emitting, detecting or controlling an electromagnetic radiation.

[0072] According to one embodiment, the device includes at least one film of the composition of the invention and electrodes on either side, and thus forms an actuator.

[0073] Another subject of the invention relates to an organic field-effect transistor comprising (with reference to the appended FIG. 1), a semiconductor element (3), electrodes (1) and a dielectric layer (2). Characteristically, the dielectric layer is at least partly composed of a blend of fluorinated electroactive polymers.

[0074] According to one embodiment, said blend of fluorinated electroactive polymers is composed of: [0075] at least one fluorinated electroactive terpolymer of formula P(VDF-X-Y) comprising units derived from vinylidene fluoride (VDF), units derived from a monomer X chosen from trifluoroethylene (TrFE), tetrafluoroethylene, chlorotrifluoroethylene (CTFE), vinyl fluoride, 1,1-chlorofluoroethylene, hexafluoropropene, 3,3,3-trifluoropropene, 1,3,3,3-tetrafluoropropene, 2,3,3,3-tetrafluoropropene, 1-chloro-3,3,3-trifluoropropene, 2-chloro-3,3,3-trifluoropropene, and a third monomer Y, [0076] and of at least one copolymer of formula P(VDF-TrFE) comprising units derived from vinylidene fluoride and units derived from trifluoroethylene. The monomer X is preferably trifluoroethylene.

[0077] According to one embodiment, the dielectric layer is at least partly composed of the composition according to the invention, wherein the proportion of units derived from trifluoroethylene in the copolymer b is greater than 45 mol %, preferably greater than 50 mol %, relative to the sum of the units derived from vinylidene fluoride and from trifluoroethylene.

EXAMPLES

[0078] The following examples illustrate the invention without limiting it. A formulation at 7% by weight in butan-2-one (methyl-ethyl-ketone, MEK) is produced by mixing the one or more electroactive polymers in a round-bottomed flask surmounted by a vertical condenser and heated at 80° C. for 16 h. After complete dissolution, the solution is filtered through a 1 μm filter made of PTFE.

[0079] A film of about 250 nm is produced on a silicon substrate, on a spin coater, from the formulation prepared above. It is then dried at 60° C. for five minutes. The films obtained are then annealed at 115° C. for two hours. The upper electrode, made of gold, is deposited by evaporation under vacuum or by sputtering.

[0080] The dielectric properties of the films are measured by impedance spectroscopy.

[0081] The various compositions studied are presented in table 2 below.

COMPARATIVE EXAMPLE—FIG. 2

[0082] FIGS. 2 and 3 show a large change in dielectric permittivity with temperature. Additionally, for the P(VDF-TrFE) copolymer with a molar composition of 70/30, the hysteresis between rising and falling temperature is substantial. Therefore, for the same temperature, there are two possible dielectric permittivity values, which is not desirable for use in field-effect transistors. However, for the P(VDF-TrFE) copolymer with a molar composition of 43/57, the hysteresis is negligible.

[0083] The hysteresis observed for some P(VDF-TrFE) copolymers can also be measured by differential scanning calorimetry which makes it possible to measure the Curie temperatures at the transition peak on heating and on cooling. Table 1 contains the Curie temperature values at the transition peak on the second heating and third cooling, measured by DSC. For the P(VDF-TrFE) copolymer with a molar composition of 43/57, delta T is smaller than 2° C.

[0084] The article by Su R. et al.; Polymer, 2012, 53, 728-739, DOI: 10.1016/j.polymer.2012.01.001 confirms the presence of this hysteresis for a P(VDF-TrFE) copolymer with a molar composition of 51/49, with a Curie temperature measured at 64° C. on heating and 60° C. on cooling.

TABLE-US-00001 TABLE 1 T.sub.Curie (° C.) VDF/TrFE molar composition Second heating Third cooling Delta T 70/30 104.7 60.1 44.6 55/45 63.0 60.0 4.6 43/57 59.9 58.1 1.8

TABLE-US-00002 TABLE 2 Compo- % by sition Component a Component b weight FIG. A P(VDF-TrFE-CTFE) 2 dashed- 10 mol % CTFE and-dotted curve B P(VDF-TrFE-CTFE) 2 solid 4 mol % CTFE curve C P(VDF-TrFE) 2 dashed 70/30 curve D P(VDF-TrFE) 3 43/57 E P(VDF-TrFE-CFE) 4 broken 6.9 mol % CFE black curve F P(VDF-TrFE-CFE) 4 broken 8.2 mol % CFE gray curve G P(VDF-TrFE-CFE) P(VDF-TrFE) 70/30 4 solid 8.2 mol % CFE 54/46 gray curve H P(VDF-TrFE-CFE) P(VDF-TrFE) 70/30 4 solid 6.9 mol % CFE 54/46 black curve I P(VDF-TrFE-CFE) P(VDF-TrFE) 90/10 5A 6.9 mol % CFE 54/46 J P(VDF-TrFE-CFE) P(VDF-TrFE) 90/10 + 5B 6.9 mol % CFE 54/46 PMMA K P(VDF-TrFE-CFE) P(VDF-TrFE) 50/50 6 8.2 mol % CFE 43/57

EXAMPLES ACCORDING TO THE INVENTION—FIGS. 4 to 6

[0085] The terpolymers on their own exhibit a dielectric permittivity that varies greatly with temperature. However, stability in dielectric permittivity is observed in the case of the studied blends with a TrFE-rich copolymer (FIGS. 4 and 6). Adding a copolymer does not lead to the emergence of hysteresis in the blend (FIG. 5). An increase in dielectric permittivity is observed. This result is surprising and differs from the examples in patent WO 2017/093145 in which the addition of 3% or 6% PMMA decreases permittivity (comparative examples C2 and C3).