BACKGROUND

Embodiments of the present disclosure relate to a de-doped thermoelectric material comprising an inherently doped organic semiconductor and a de-doping agent. The de-doped thermoelectric material can be tuned, the conductivity and Seebeck coefficient of the de- doped thermoelectric material adjusted, by changing the amount of the de-doping agent in the de-doped thermoelectric material.

Thermoelectric devices, operating by the Peltier Effect, are known. Use of electrically conductive polymers in thermoelectric devices is known, for example as disclosed in Thermoelectrics for Power Generation - A Look at Trends in the Technology, ISBN 978-953- 51-2846-5, Chapter 3, L Stepien et al, PROGRESS IN POLYMER THERMOELECTRICS.

United States Patent Publication No 2015/0243869 discloses self-doping organic materials. Sun et al, Flexible n-Type High-Performance Thermoelectric Thin Films of Poly(nickel- ethylenetetrathiolate) Prepared by an Electrochemical Method, ADV. MATER. (2016), DOI: 10. l002/adma.201104305 discloses poly(nickel ethylenetetrathiolate) for use as an n-type thermoelectric material.

Harada et al, Improved Thermoelectric Performance of Organic Thin-Film Elements Utilizing a Bilayer Structure of Pentacene and 2,3,5,6-Tetrafluoro-7,7,8,8-Tetracyanoquinodimethane (F4-TCNQ), APPLIED PHYSICS LETTERS 96, 253304 (2010) discloses a device having a bilayer structure composed of an intrinsic pentacene layer and an acceptor 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane layer

It is desirable for a thermoelectric material to possess a high Seebeck coefficient ( i.e . the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material), however there is a known trade-off between electrical conductivity and Seebeck coefficient of a thermoelectric material, for example as disclosed in Mateeva et al, Correlation of Seebeck Coefficient and Electric Conductivity in Polyaniline and Polypyrrole , J. APPL. PHYS. Vol. 83, No. 6, p.3111-3117, which discloses that logarithm of the electrical conductivity of polyaniline and polypyrrole varies linearly with the Seebeck coefficient on doping, but with a proportionality substantially in excess of a prediction from simple theory for a single type of mobile carrier.

SUMMARY

Embodiments of the present disclosure provide a thermoelectric material that is a blend of an inherently-doped-organic-semiconducting material and a de-doping agent. In some embodiments, the inherently-doped-organic-semiconducting material may be a semiconducting polymer.

In some embodiments, the de-doping agent is used to tune both the conductivity and the Seebeck coefficient of the inherently-doped-organic-semiconducting material. The“tuned” thermoelectric material may be used in a thermoelectric device.

The thermoelectric material may be formed by adding a de-doping agent to an n-doped or p- doped organic semiconductor. The organic semiconductor formed by this method may be used to form a p-type or n-type thermoelectric leg of a thermoelectric device.

Embodiments of the present disclosure provide a method of increasing the Seebeck coefficient of a conducting organic material while maintaining a conductivity suitable for thermoelectric applications.

Embodiments of the present disclosure provide a method for controllably increasing the Seebeck coefficient of a conducting organic material.

The present inventors have found that the Seebeck coefficient of an organic semiconductor doped by an n-dopant or a p-dopant may be increased by treating it with a dedoping agent having an energy level selected to cause de-dedoping of the organic semiconductor.

Accordingly, in some embodiments, a method of increasing the Seebeck coefficient of a doped organic semiconductor is provided, the method comprising the step of adding a dedoping agent to an inherently doped organic semiconductor. In some embodiments, a thermoelectric device is provided. The thermoelectric device comprises a p-type thermoelectric leg and an n-type thermoelectric leg, which legs are both disposed between a pair of electrodes. In some embodiments of the present disclosure, the n- type or the p-type thermoelectric leg comprises an inherently doped organic semiconductor and a de-doping agent.

In some embodiments, a method of forming a thermoelectric device comprising a p-type thermoelectric leg and an n-type thermoelectric leg disposed between a first and a second electrode is provided.

In some embodiments, the n-type or the p-type thermoelectric leg is formed by deposition of a thermoelectric material comprising an inherently doped organic semiconductor and a dedoping agent or a precursor thereof.

In some embodiments, a de-doping agent is used to increase/tune the Seebeck coefficient of an inherently doped organic semiconductor.

In some embodiments, a thermoelectric device is provided comprising a p-type thermoelectric leg and an n-type thermoelectric leg between first and second electrodes, wherein at least one of the p-type and n-type thermoelectric legs comprises an inherently doped polymer and a dedoping agent.

DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to the drawings, in which: Figure 1 is a schematic illustration of a thermoelectric device according to an embodiment.

DETAILED DESCRIPTION

An inherently doped organic semiconductor as described herein for treatment with a dedoping agent may be an n-doped or p-doped semiconducting organic semiconductor. It will be understood that treatment of the inherently doped organic semiconductor with the de- doping agent reduces the charge carrier concentration of the doped organic semiconductor. In case of an inherently n-doped organic semiconductor, the dedoping agent has a LUMO deeper, in some embodiments, at least 0.5 eV deeper, than that of the n-doped organic semiconductor.

In the case of an inherently p-doped organic semiconductor, the dedoping agent has a HOMO shallower, in some embodiments, at least 0.5 eV shallower, than the HOMO of the p-doped organic semiconductor.

In some embodiments, the inherently doped organic semiconductor comprises a semiconducting polymer.

HOMO and LUMO values as described herein are as measured by square wave voltammetry. Semiconducting polymers as described herein may be conjugated or non-conjugated polymers.

In some embodiments, the inherently doped organic semiconductor treated with a de-doping agent is an inherently doped polymer. By“inherently doped polymer,” as used herein, it is meant a polymer in which doping occurs during the polymerisation of monomers to form the polymer.

In some embodiments, the inherently n-doped semiconducting polymer is poly[Mx(Ni-ETT)] in which:

Ni-ETT is a repeating unit of formula:

where:

M is a cation, x is an integer for balancing negative charges on the repeat units.

M is preferably a metal cation, more preferably an alkali cation, most preferably Na or K.

In the case of an inherently n-doped organic semiconductor, in accordance with some embodiments, the de-doping agent has a LUMO of at least 4.0 eV from vacuum level.

In the case of an inherently n-doped organic semiconductor, in accordance with some embodiments, the de-doping agent has a LUMO at least 0.5 eV deeper than that of the n- doped organic semiconductor.

De-doping agents for an inherently n-doped organic semiconductor may be selected from materials known for use as a p-dopant (although it will be understood that the de-doping agent used as described herein does not p-dope the organic semiconductor).

In accordance with some embodiments, a de-doping agent for use with inherently n-doped organic semiconductors is tetracyanoquinodimethane (TCNQ) and fluorinated derivatives thereof. In some embodiments, tetrafluoro -tetracyanoquinodimethane (F4TCNQ) is used as the de-doping agent.

Merely by way of example, inherently p-doped polymers that may be used are p-doped poly(ethylenedioxythiophene) (PEDOT) and poly[Cu _x (Cu-ett)]. In some embodiments, p- dopants of PEDOT include, without limitation, polysulfonic acids for example polystyrene sulfonic acid (PSS) and fluorinated derivatives therof. De-doping agents for an inherently p-doped organic semiconductor may be selected from materials known for use as an n-dopant (although it will be understood that the de-doping agent used as described herein does not n-dope the organic semiconductor).

Exemplary de-doping agents for a p-doped organic semiconductor may be formed by activation of precursor compounds comprising a lH-benzoimidazole including, without limitation, precursor compounds comprising 1 ,3-Dimethyl-2-phenyl-2,3-dihydro-l//- benzoimidazole (DMBI), for example 4-(l,3-dimethyl-2,3-dihydro-lH-benzoimidazol- 2yl)phenyl)dimethylamine (N-DMBI).

In some embodiments, the magnitude of the Seebeck coefficient of the inherently doped organic semiconductor may be increased by at least 10 pV/K following treatment by the method of the first aspect, i. e. , adding a de-doping agent.

In some embodiments, the magnitude of the Seebeck coefficient of the inherently doped organic semiconductor is at least 50 pV/K following treatment by the method of the first aspect, i.e., adding a de-doping agent.

In some embodiments, the conductivity of the inherently doped organic semiconductor following treatment by the method of the first aspect, i.e., adding a de-doping agent, is at least 1 S/cm. in some embodiments, an inherently n-doped or p-doped organic semiconductor treated according to the method of the first aspect, i.e., adding a de-doping agent, is used to form a p- type or n-type thermoelectric leg respectively of a thermoelectric device. Figure 1, which is not drawn to any scale, schematically illustrates a thermoelectric device according to some embodiments of the present application.

The thermoelectric device comprises at least one thermoelectric couple, each thermoelectric couple comprising a first electrode 103, a second electrode 107, and a p-type thermoelectric leg l05p and an n-type thermoelectric leg 105h between the first and second electrodes. At least one of the p-type thermoelectric legs and the n-type thermoelectric legs is formed from, respectively, a p-doped organic semiconductor and an inherently n-doped organic semiconductor treated with a de-doping agent, as described herein.

In the case where one of the n-type and p-type thermoelectric legs does not comprise a p- dopant or an n-dopant respectively, the active material of the thermoelectric leg may be selected from known thermoelectric materials as disclosed in, for example, J. Mater. CHEM. C, 3, 10362 (2015), and CHEM. SOC. REV., 45, 6147-6164 (2016), the contents of which are incorporated herein by reference.

In some embodiments, the n-type thermoelectric leg comprises an inherently n-doped organic semiconductor and a de-doping agent, as described herein, and the p-type thermoelectric leg may or may not comprise a de-doping agent.

In some embodiments, the p-type thermoelectric leg comprises an inherently p-doped organic semiconductor and a de-doping agent, as described herein, and the p-type thermoelectric leg may or may not comprise a de-doping agent.

The thermoelectric device may comprise an insulating structure 109 defining wells into which the material or materials for forming the n-type and p-type thermoelectric legs may be deposited. The insulating structure may be one or more layers patterned to define the wells, for example a patterned layer of positive or negative photoresist.

A composition comprising an inherently doped organic semiconductor and a de-doping agent or de-doping agent precursor as described herein for forming the n-type and p-type thermoelectric legs may be deposited by any suitable technique, and, in accordance with some embodiments, by deposition of an ink comprising the composition dissolved or dispersed in one or more solvents.

A thermoelectric leg formed by deposition of an ink onto an electrode may have higher contact resistance than a thermoelectric leg formed by another process, for example evaporation. Accordingly, an ability to tune the conductivity of a thermoelectric leg formed from an ink may be particularly advantageous.

Suitable techniques for depositing an ink are coating or printing methods including, without limitation, roll-coating, spray coating, doctor blade coating, slit coating, ink jet printing, screen printing, dispense printing, gravure printing and flexographic printing. In some embodiments, dispense printing is used. In dispense printing, each thermoelectric leg is formed by depositing a continuous flow of ink from a nozzle positioned above the first electrode. It will be understood that no ink is dispensed in regions between each thermoelectric leg.

In some embodiments, an ink formulation comprising an inherently n- or p-doped organic semiconductor and a de-doping agent is deposited to form the n-type, or p-type thermoelectric leg, respectively. According to these embodiments, the Seebeck coefficient of the doped organic semiconductor has been increased before the ink is deposited.

In embodiments, an ink formulation comprising an inherently n- or p-doped organic semiconductor and a precursor of the dedoping agent is deposited followed by activation of the precursor, for example by heating or irradiation of the precursor to form the de-doping agent. According to these embodiments, the Seebeck coefficient of the doped organic semiconductor is increased after the ink is deposited.

In some embodiments, the inherently n- or p-doped organic semiconductor and the de-doping agent are deposited separately. According to these embodiments, the Seebeck coefficient of the doped organic semiconductor is increased after it has been deposited and upon deposition of a de-doping agent or upon deposition and activation of a de-doping agent precursor.

The solvent or solvents for an ink as described herein may comprise or consist of one or more polar aprotic solvents such as N-methylpyrrolidone; dimethylformamide; propylene carbonate; and dimethylsulfoxide; and one or more polar, protic solvents such as water or Ci _-6 alcohols.

Ink formulations preferably further comprise a polymeric binder. The binder may make layers formed from the formulation more resilient and less likely to crack than films in which no binder is present.

The polymeric binder may be a conjugated or non-conjugated polymer.

Exemplary non-conjugated binders include, without limitation, polyvinylpyrrolidone, PVDF, polyacrylates, preferably PMMA, and polystyrene. Exemplary conjugated polymer binders include, without limitation, P(NDI20D-T2), poly[(9,9-di-H-octylfluorenyl-2,7-diyl)-fif/i-(benzo[2,l,3]t hiadiazol-4,8-diyl)] (F8BT) and benzodifurandione-based PPV (BDPPV).

The ink formulation may contain only one polymeric binder or may contain more than one polymeric binder.

Compositions comprising or consisting of an inherently doped organic semiconductor and a de-doping agent, as described herein, may, in some embodiments, contain at least 1 weight % of the de-doping agent, at least 5wt of the de-doping agent or at least 10 wt % of the dedoping agent. The thermoelectric device illustrated in Figure 1 is supported on a substrate 101. The substrate may consist of a single layer or may comprise two or more layers of different materials. The device may be a flexible device supported on a flexible substrate 101. A flexible thermoelectric device as described herein preferably has a bend radius of 30 mm or less, optionally 20 mm or less. The bend radius may be at least 5 mm or at least 10 mm. in some embodiments, the thickness of the thermocouple(s) of the thermoelectric device is in the range of about 50-500 microns, optionally 50-200 microns.

The thermoelectric device may contain only one thermoelectric couple. In some embodiments, the thermoelectric device comprises a plurality of electrically connected thermoelectric couples as illustrated in Figure 1. For simplicity, only two thermoelectric couples are illustrated in Figure 1, however it will be appreciated that a larger number may be connected in an array.

The thermoelectric couples may be connected to one another in series, parallel or a combination thereof. The first and second electrodes may each form a pattern of a plurality of conducting pads connecting the thermoelectric couples. The first and second electrodes are each preferably in the form of a patterned layer defining a plurality of conductive pads. The first and second electrodes may each independently consist of a single conductive layer or two or more conductive layers. The or each conductive layer may consist of a single conductive material or may comprise two or more materials. Conductive materials for forming the conductive layers are preferably selected from metals and conductive metal compounds, for example conductive metal oxides. Exemplary metals are copper, aluminium and gold. The first and second electrodes optionally each independently have a thickness in the range of about 1-10 microns.

Following formation of the thermoelectric legs, in some embodiments, the second electrode is deposited thereover by a suitable deposition technique including, without limitation, evaporation and sputtering. In other embodiments, a second substrate (not shown in Figure 1) carrying patterned second electrodes on a surface thereof is brought into contact with the thermoelectric legs to complete the device.

In operation as a thermoelectric generator, the first or second substrate is brought into contact with a surface having a temperature higher than the environmental temperature. Pads of the first or second electrode are electrically connected to a load.

In operation as a thermoelectric cooler, the first or second substrate is brought into contact with a surface to be cooled and a voltage is applied to the device.

Measurements

HOMO and LUMO levels as described anywhere herein are as measured by square wave voltammetry.

Equipment:

CHI660D Electrochemical workstation with software (IJ Cambria Scientific Ltd))

CH1 104 3mm Glassy Carbon Disk Working Electrode (IJ Cambria Scientific Ltd))

Platinum wire auxiliary electrode

Reference Electrode (Ag/AgCl) (Harvard Apparatus Ltd) Chemicals

Acetonitrile (Hi-dry anhydrous grade-ROMIL) (Cell solution solvent)

Toluene (Hi-dry anhydrous grade) (Sample preparation solvent)

Ferrocene - FLUKA (Reference standard) Tetrabutylammoniumhexafluorophosphate - FLUKA (Cell solution salt)

Sample preparation

The polymers were spun as thin films (~20nm) onto the working electrode. Dopants or dopant precursors were measured as a dilute solution (0.3w%) in toluene.

Electrochemical cell

The measurement cell contains the electrolyte, a glassy carbon working electrode onto which the sample is coated as a thin film, a platinum counter electrode, and an Ag/AgCl reference glass electrode. Ferrocene is added into the cell at the end of the experiment as reference material (LUMO (ferrocene) = -4.8eV).

Examples Formulation ofNiETT with F4TCNQ

NiETT was formulated as a printable dispersion in DMSO, using PVDF as a binder (4:1 w / w Ni:ETT:binder, 50mg/ml total solids).

A bulk solution of PVDF in DMSO (lOmg/ml) was made first. NiETT (60mg) was weighed into a clean glass vial and glass beads (1mm, Sigma) added to create two layers of beads. The PVDF solution (l.5ml) was added and the vial sealed under nitrogen. The mixture was vortexed at lOOOrpm for lh to form the dispersion. F4TCNQ was prepared as a bulk solution (50mg/ml) in DMSO just before use. Aliquots of Ni:ETT dispersion and F ₄ TCNQ solution were mixed together to form the final formulations, as set out in Table 1.

Table 1

Preparation of films

Films were prepared by drop casting 20 mT. of the formulation onto glass substrates with pre- pattemed electrodes and photoresist bank to control the deposition area. The films were dried for 30 minutes at 50°C under vacuum. After drying the films were immediately transferred to a nitrogen filled glovebox. Substrates

Glass substrates were prepared by depositing metal and patterning via photolithography to form electrodes with leadouts and resistive elements. An ink containing bank was formed over the electrodes using photoresist, typically SU-8, where open areas are bounded by the bank to contain the ink and providing a defined sample size. In this case a square of approximately 9mm x 9mm was formed between the electrodes.

Conductivity

The sample resistance was measured by the two probe method via the pre-patterned electrodes and leadouts using a handheld multimeter. As the sample is defined as a square this gives a direct measurement of sheet resistance. Contact resistance was negligible. The sample thickness was determined using a Dektak surface profilometer, allowing sample conductivity to be derived.

Seebeck Coefficient

The Seebeck coefficient was measured using the differential technique on a custom made test fixture. The glass substrate was placed to bridge two pettier devices, one in cooling mode and the other in heating mode. Electrical contact was made to the leadouts and resistive elements via pogo pins.

A small current was supplied to the resistive elements via a battery. The voltage across the sample and the resistive elements, and temperature of the peltier devices, were measured via a datalogger (Pico Technology TC-08).

Current was supplied to the peltier devices to generate a temperature gradient across the sample; the current was then removed allowing the temperature gradient to decay to zero whilst logging the change in voltage generated by the sample and the change in resistance of the resistive elements. The substrate temperature was calculated using the coefficient of resistivity of the resistive elements (rearrangement of equation 1, where n is the ID of the resistive element), which then allowed the temperature gradient across the sample to be determined and thus the Seebeck coefficient (Equation 2).

The effect of de-doping agent F ₄ TCNQ on n-doped Ni(ETT) is set out in Table 2. In all compositions, Ni(ETT) was mixed with PVDF in a 4:1 weight ratio. Table 2

The weight percentage of F4TCNQ given in Table 2 is that of the whole composition, i.e. F4TCNQ, Ni(ETT) and PVDF. As shown in Table 1, the magnitude of the Seebeck coefficient increases with increasing p- dopant concentration. Although there is a trade-off between the Seebeck coefficient and electrical conductivity, the change in electrical conductivity and Seebeck coefficient can be controlled by selecting the amount of dopant added to the inherently doped polymer, allowing the Seebeck coefficient and / or the electrical conductivity to be tuned according to the requirements of the application of the conducting polymer.

Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.