CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore patent application No. 10201801297S filed on 15 February 2018, the content of which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various embodiments relate to a conductive polymer composition and a method of preparing the conductive polymer composition, as well as a substrate coated with said conductive polymer composition.

BACKGROUND

[0003] Thermoelectric behavior involves the conversion of thermal gradient to electrical energy or vice versa. The ability to generate a clean source of electrical energy results in the thermoelectric (TE) field gaining popularity in the research community. This would have the potential to harness heat waste and then convert it to electricity, making a system potentially self-sustaining. TE materials could also help in cooling by application of energy to the circuit. With such advantages in TE materials, efforts have been taken to improve the thermoelectric performance of existing polymers.

[0004] Despite the inorganic materials such as Bi ₂ Te ₃ , Sb ₂ Te ₃ , PbSeTe, SiGe having good thermoelectric performance, it is accompanied by disadvantages such as high cost, toxicity, scarcity and inflexibility. More importantly, these materials are not suitable for ambient energy harvesting. These disadvantages have resulted in an increasing research interest to shift to organic conductive polymer compositions. For example, poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) is one of the most widely investigated conductive polymer compositions, which has been reported to have the highest figure of merit (ZT) value.

[0005] Despite the positive results from PEDOT:PSS doping, the current values are still insufficient for efficient heat waste conversion to useful electrical energy. Nevertheless these results do signal a potential for PEDOT in improving the thermoelectric properties.

[0006] PEDOT:dispersant-based conductive polymer compositions have been extensively studied and have produced numerous applications such as in organic light-emitting diodes, flexible organic solar cells, touchscreens, and electronic paper in industry. Another emerging application of these polymer compositions is in thermoelectric applications. However, the efficiency of these PEDOT-based materials is still insufficient for waste heat conversion applications. Furthermore, structure modifications of PEDOT are less well studied. As such, there is a need to develop high performance materials to strive towards a more efficient conversion rate. In addition, during the fabrication process, PEDOT:dispersant compositions tend to precipitate out after a few days of either doping such as pre- or post-treatment with a variety of chemicals or polar solvents, or storing at room temperature. Solving this problem would allow PEDOT:dispersant compositions to disperse better in solutions for a longer shelf life. In view of the above, there exists a need for an improved conductive polymer composition and a method of making the same that overcomes or at least alleviates one or more of the above problems.

SUMMARY

[0007] In a first aspect, there is provided a conductive polymer composition comprising a poly(3 ,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being a compound of Formula (I)

wherein:

Ri is a substituted, linear or branched Ci-C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ ; and

R ₂ is selected from the group consisting of hydrogen, an unsubstituted, linear or branched C _l -C2o-alkyl and Ri.

[0008] In a second aspect, there is provided a method for making a conductive polymer composition comprising a poly(3 ,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being defined as above, the method comprising

(i) providing an aqueous solution of the dispersant;

(ii) adding a compound of the Formula (II) to the aqueous solution

(P),

wherein Ri and R ₂ are defined as above; and

(iii) adding a peroxydisulfate to the aqueous solution.

[0009] In a third aspect, there is provided a substrate coated with a conductive polymer composition comprising a poly(3,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being defined as above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0011] FIG. 1 is a graph showing the electrical conductivity of poly(2,3-dihydrothieno[3,4-b]- l,4-dioxin-2-methanol) (PEDOT-OH) : polystyrene sulfonate (PSS) dispersant at various ratios.

[0012] FIG. 2 is a graph showing the Seebeck coefficient of PEDOT-OH: PSS dispersant at various ratios.

[0013] FIG. 3 is a graph showing the Power Factor of PEDOT-OH: PSS dispersant at various ratios.

[0014] FIG. 4 A is a graphical illustration of the setup of time-dependent open-circuit voltage (VOC) measurement and detailed geometry of PEDOT-OH:PSS film and gold (Au) electrodes.

[0015] FIG. 4B is a graph showing the time-dependent VOC generated by PEDOT-OH:PSS film and corresponding temperature profile.

[0016] FIG. 5 A is a graphical illustration of the setup of time-dependent voltage at the load resistor (VOUT) measurement and detailed geometry of PEDOT-OH:PSS film and gold (Au) electrodes.

[0017] FIG. 5B is a graph showing the time-dependent VOUT generated by PEDOT-OH:PSS film and corresponding temperature profile.

[0018] FIG. 6 is a graph showing the time-dependent VOUT measured at the load resistor up to four cycles of the heater being switched“on” and“off ⁵ and its corresponding DT profile. DETAILED DESCRIPTION

[0019] In order to meet the above described demand, in this disclosure, a PEDOT analogue was synthesized, the PEDOT analogue being a compound of Formula (I)

wherein:

R is a substituted, linear or branched C _l -C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ ; and

R ₂ is selected from the group consisting of hydrogen, an unsubstituted, linear or branched Ci- C ₂₀ -alkyl and Ru

[0020] This PEDOT analogue in a composition with a dispersant was used for thermoelectric applications. Surprisingly, it was found that such a composition yielded a high Seebeck coefficient and a high power factor. Furthermore, due to the hydrophilic groups on the side- chains of the PEDOT analogue, such as a hydroxyl group, an amino group or a carboxy group, a higher dispersibility in water was obtained than for PEDOT without any modification. This higher dispersibility in water allows for a decreased amount of dispersant being required in order to obtain a workable suspension. Further advantageously, the method for making the conductive polymer composition is free of any pre-treatment or post-treatment step, which may be due to the advantageous properties of the PEDOT analogue. Therefore, a high Seebeck coefficient may be harnessed, without any complicated treatment involved, making it an asset to the TE research field.

[0021] Therefore, in a first aspect, there is provided a conductive polymer composition comprising a poly(3,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being a compound of Formula (I)

wherein:

Ri may be a substituted, linear or branched C _l -C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ ; and

R ₂ may be selected from the group consisting of hydrogen, an unsubstituted, linear or branched C _l -C ₂ o-alkyl and R _t .

[0022] The term“composition” as used herein refers to a mixture of components. Comprised in this composition may be a PEDOT analogue as described herein and a dispersant. Further components which may be comprised in this composition are liquids, for example water. The PEDOT analogue together with the dispersant may thus be provided as an aqueous suspension. The composition may be such that there is no covalent bond formation between the components of the composition. Hence, there would be no covalent bond formation between the dispersant and the PEDOT analogue. However, this definition does not preclude any ionic interaction between the dispersant and the PEDOT analogue. In particular, in the composition, there may exist hydrogen bonding between the dispersant and the hydrophilic groups on the side-chains of the PEDOT analogue. In case PSS and PEDOT-OH is used in the composition, there exists hydrogen bonding between -S0 ₃ H of PSS and the -OH functional group of the PEDOT-OH as well as Van der Waals interaction. A composition comprising the PEDOT analogue and the dispersant is described herein as“PEDOT analogue: dispersant”, whereas various embodiments may refer to narrower embodiments thereof, but in the same annotation.

[0023] The term“dispersant” as used herein, refers to an ionomer, which comprises ionizable groups, or ionized groups. As used herein the term“ionizable groups,” means potentially ionic groups. The ionizable groups in general correspond to the ionic groups, except that they are in the acid (such as carboxyl— COOH) or base (such as primary, secondary or tertiary amine— NH ₂ ,— NRH, or— NR ₂ ) form. The ionizable groups are such that they are readily converted to their anionic or cationic form during the dispersion/polymer preparation method as discussed below. Specific examples of anionic groups include carboxylate and sulfonate groups. Examples of cationic groups include quaternary ammonium groups and sulfonium groups. In the case of anionic group substitution, the groups can be carboxylic acid groups, carboxylate groups, sulphonic acid groups, sulphonate groups, phosphoric acid groups and phosphonate groups. As used herein the term“ionized groups” means ionic groups. The ionic groups in general correspond to the anionic or cationic forms of the ionizable groups mentioned above. The dispersant advantageously exhibits an ionic interaction with the PEDOT analogue to increase water solubility. Hence, a dispersant is typically introduced into a conductive polymer composition for the purpose of producing a“macromolecular salt” due to its ionizable or ionized functionalities. This macromolecular salt may render the ensuing conductive polymer composition more water-soluble, thereby enhancing water dispersability.

[0024] As mentioned before, due to the hydrophilic groups on the side-chains of the PEDOT analogue, such as a hydroxyl group, an amino group or a carboxy group, the PEDOT analogues described herein have a higher water dispersibility, which can be up to 1-3%. The higher water dispersibility is advantageous as it reduces the necessity for a dispersant, thereby allowing to decrease the content thereof.

[0025] The PEDOT analogue as used herein refers to an oligomer or a polymer of the repeating unit shown as Formula (I). Hence, the PEDOT analogue may be substantially present as a homopolymer. Similarly, the PEDOT analogue may be substantially the only PEDOT- containing component in the conductive polymer composition, meaning that the PEDOT analogue is not provided as a mixture with, for example, PEDOT. The PEDOT analogue may therefore be present in the conductive polymer composition in higher than 90wt%, or higher than 95wt%, or higher than 99wt% of PEDOT-containing components in the conductive polymer composition.

[0026] The dispersant may be a polymeric compound. According to various embodiments, the dispersant may be selected from the group consisting of polystyrene sulfonate (PSS), sodium tosylate, potassium tosylate, and a combination thereof. Using PSS may be particularly cost- effective compared with other dispersants.

[0027] According to various embodiments, the PEDOT analogue and the dispersant are mixed in a weight ratio of about 1.0 : 0.2 to about 1.0 : 5.0. Alternatively, the PEDOT analogue and the dispersant are mixed in a weight ratio of about 1.0 : 0.4 to about 1.0 : 5.0, or in a weight ratio of about 1.0 : 0.5 to about 1.0 : 5.0, or in a weight ratio of about 1.0 : 0.2 to about 1.0 : 4.0, or in a weight ratio of about 1.0 : 0.2 to about 1.0 : 3.0, or in a weight ratio of about 1.0 : 0.2 to about 1.0 : 2.5, or in a weight ratio of about 1.0 : 0.5 to about 1.0 : 3.0, or in a weight ratio of about 1.0 : 0.2 to about 1.0 : 2.5, or in a weight ratio of about 1.0 : 0.5 to about 1.0 : 2.5. The choice of the ratio may advantageously be used to tune the TE performance of the conductive polymer composition. For example, a weight ratio of about 1.0 : 0.5 of PEDOT analogue:dispersant may provide optimal results with regard to the electrical conductivity, while a weight ratio of about 1.0 : 2.5 of PEDOT analogue:dispersant may provide optimal results with regard to the Seebeck coefficient. Hence, a ratio in the range of about 1.0 : 0.5 to about 1.0 : 2.5 may be advantageous in that it provides a balance between these two properties of the conductive polymer composition.

[0028] The PEDOT analogue is defined herein according to Formula (I). The term“alkyl” as used herein, as a group or part of a group, refers to a linear or branched aliphatic hydrocarbon group, preferably a C _l -C ₂ o alkyl, more preferably a Ci-Cio alkyl, most preferably C1-C5 alkyl unless otherwise noted. Examples of suitable linear and branched CnCio alkyl substituents include methyl, ethyl, «-propyl, 2-propyl, «-butyl, sec-butyl, /-butyl, hexyl, and the like. When used in the context of Ri, the alkyl group may be a bridging group, being substituted as stated therein. When used in the context of R ₂ , the alkyl group may be a terminal, unsubstituted group.

[0029] Rt comprises substitution with a hydrophilic group, selected from the functionalities - OH, -COOH, and -NH ₂ . Hence, Ri comprises a terminal hydroxyl group, amino group or carboxylic acid. It is understood that the definition of the hydrophilic groups includes any ionic form of the functionalities, such as -O , -NH ₃ ⁺ and -COO . The hydrophilic group as defined herein is believed to result in the advantageous effects of the conductive polymer composition outlined above.

[0030] According to various embodiments, R ₂ may be hydrogen. Hence, the PEDOT analogue may have the following structure according to Formula (III):

wherein Ri may be a substituted, linear or branched Ci-C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ .

[0031] According to various embodiments, Ri may be a substituted, linear Ci-Cio-alkyl, substituted with -OH. Accordingly. Ri may be a methanol, ethanol, «-propanol, «-butanol, «- pentanol, «-hexanol, «-heptanol, «-octanol, «-nonanol or «-decanol substituent. In one example, Ri is -CH2-OH. [0032] According to various embodiments, the dispersant may be PSS. PSS may be dispersant, which is ionizable or ionized by virtue of its -S0 ₃ H functionality, while parts of this functionality may be provided in the ionized form, i.e. as the S0 ₃ functionality. PSS typically forms a“macromolecular salt” due to its S0 ₃ functionality. This macromolecular salt may render the ensuing composition more water-soluble, thereby enhancing water dispersibility. The before mentioned higher water dispersibility of the PEDOT analogue would therefore be advantageous to reduce the necessity for PSS, thereby allowing to decrease the content thereof.

[0033] According to various embodiments, the PSS may be present in a molecular weight of about 50000 g/mol to about 150000 g/mol. Alternatively, the PSS may be present in a molecular weight of about 60000 g/mol to about 150000 g/mol, or in a molecular weight of about 70000 g/mol to about 150000 g/mol, or in a molecular weight of about 50000 g/mol to about 130000 g/mol, or in a molecular weight of about 50000 g/mol to about 110000 g/mol, or in a molecular weight of about 50000 g/mol to about 90000 g/mol, or in a molecular weight of about 50000 g/mol to about 70000 g/mol, or in a molecular weight of about 60000 g/mol to about 80000 g/mol.

[0034] According to various embodiments, the mixture may be present as a suspension. While the liquid for the suspension may be any liquid, in one embodiment, it is an aqueous suspension.

[0035] According to various embodiments, the aqueous suspension may be provided in a concentration of about 0.01 g/mL to about 0.1 g/mL. Alternatively, the aqueous suspension may be provided in a concentration of about 0.01 g/mL to about 0.08 g/mL, or of about 0.01 g/mL to about 0.06 g/mL, or of about 0.01 g/mL to about 0.04 g/mL, or of about 0.01 g/mL to about 0.03 g/mL, or of about 0.02 g/mL to about 0.1 g/mL, or of about 0.02 g/mL to about 0.07 g/mL, or of about 0.02 g/mL to about 0.05 g/mL, or of about 0.02 g/mL to about 0.03 g/mL, or may be about 0.024 g/mL.

[0036] According to various embodiments, the dispersant may be present in the aqueous suspension at a weight concentration of at most 5wt%. Alternatively, the dispersant may be present in the aqueous suspension at a weight concentration of at most 4wt%, or of at most 3wt%. Advantageously, a low content of dispersant may help in the polymerization to attain the desired electronic and physical properties. For example, PSS may have a function of an insulation polymer while PEDOT-OH may have the function of the conducting polymer. Thus, using less dispersant, for example less PSS, in the polymerization may help increase the electrical conductivity. [0037] In a second aspect, there is provided a method for making a conductive polymer composition comprising a poly(3,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being a compound of Formula (I)

wherein:

Ri is a substituted, linear or branched Ci-C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ ; and

R ₂ is selected from the group consisting of hydrogen, an unsubstituted, linear or branched Ci- C ₂₀ -alkyl and Ri, the method comprising

(i) providing an aqueous solution of the dispersant;

(ii) adding a compound of the Formula (II) to the aqueous solution

(P), wherein Ri and R ₂ are defined as above; and

(iii) adding a peroxydisulfate to the aqueous solution.

[0038] The method may include providing a compound of Formula (II) as a polymer precursor, to be added to an aqueous solution of the dispersant. Addition of a peroxydisulfate may result in formation of a radical cation of the polymer precursor, which may subsequently attack a neutral compound of Formula (II), thereby forming a covalent bond on the 2-position of the thiophene. The peroxydisulfate may be an ionic component generally described as S ₂ 0 ₈ ² . It functions to initiate polymersiation by homolysis of the -O-O-bond contained therein. Suitable counter ions for the peroxydisulfate may be (NH ₄ ⁺ ) ₂ , (Na ⁺ ) ₂ , or (K ⁺ ) ₂ . In one example, the peroxydisulfate is potassium persulfate. [0039] Advantageously, the method proceeds with nearly 100% yield (>99%), which may be attributed to the hydrophilic groups. As the hydrophilic groups increase water solubility of compounds of Formula (II), the reaction may be accelerated, resulting in an overall increased yield.

[0040] According to various embodiments, the step (iii) further comprises adding an iron halide to the aqueous solution. In one example, the iron halide is iron trichloride.

[0041] According to various embodiments, the method is conducted at ambient temperature.

[0042] According to various embodiments, the method may further comprise a step (iv) of washing the aqueous solution with resins. The resins may be selected from basic and acidic resins, and a combination thereof. The resins may be added for removal of the inorganic ions. According to various embodiments, the resins are removed with the inorganic ions. They typically do not remain in the conductive polymer composition.

[0043] According to various embodiments, the method may be free of a post-treatment with an organic liquid or an inorganic or organic acid. Advantageously, the presence of the hydrophilic groups on the PEDOT analogues eliminates the need for a pre- or posttreatment in the method, due to its advantageous properties. This advantage may result in a higher efficiency of the method, as additional treatment steps are not needed.

[0044] According to various embodiments, the organic liquid may be an acid, an alcohol, DMSO, DMF, an ether, an ester, dichloromethane, dichloroethane, and a combination thereof.

[0045] In a third aspect, there is provided a substrate coated with a conductive polymer composition comprising a poly(3,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant, the PEDOT analogue being a compound of Formula (I)

wherein:

R is a substituted, linear or branched C _l -C ₂₀ -alkyl, substituted with at least one moiety selected from the group consisting of -OH, -COOH, and -NH ₂ ; and R ₂ is selected from the group consisting of hydrogen, an unsubstituted, linear or branched Ci- C ₂ o-alkyl and Ri.

[0046] The substrate may be selected from the group consisting of solar cells, OLEDs, touch panel displays or flexible wearables. The coating may be carried out by applying the conductive polymer composition on the substrate and drying the applied composition to form the substrate coated with a conductive polymer composition comprising the PEDOT analogue and the dispersant. The conductive polymer composition may be applied on the substrate, without any particular limitation, by drop casting, spin coating, dip coating, spray coating, flow coating, screen printing, or the like, or a combination comprising at least one of the foregoing. Subsequently, the applied conductive polymer composition may be subjected to curing at a temperature of about 50°C to about 500°C, or at a temperature of about 50°C to about 400°C, or at a temperature of about 50°C to about 300°C, or at a temperature of about 50°C to about 200°C, or at a temperature of about 50°C to about l00°C, or at a temperature of about 50°C to about 80°C, or at a temperature of about 60°C to about 500°C, or at a temperature of about 70°C to about 500°C, or at a temperature of about 70°C to about l00°C, or at a temperature of about 80°C.

[0047] Summarizing the above, in this disclosure, an analogue of PEDOT with hydrophilic groups offers many good characteristics, such as better dispensability in aqueous solutions and better stability. Accordingly, the conductive polymer compositions as disclosed herein have a high Seebeck coefficient, high power output, a better repeatability and a high electric conductivity. Most importantly, it enables to deliver good TE performance (high power factor) without any treatment.

[0048] In one example, a new PEDOT analogue, poly(2,3-dihydrothieno[3,4-b]-l,4-dioxin-2- methanol) (PEDOT-OH) was synthesized and used for thermoelectric applications (TE), yielding high Seebeck coefficient, high power factor. PEDOT-OH contains hydrophilic CH ₂ OH groups on side-chains of the PEDOT structure, giving it better dispersibility in water than PEDOT. No any pre-treatment or posttreatment is required to harness its high Seebeck. By varying PEDOT-OH:PSS ratio, the TE performance of polymer PEDOT-OH can be tuned, yielding a balanced Seebeck coefficient and electrical conductivity in comparison with the pristine PEDOT:PSS film.

[0049] The conductive polymer compositions disclosed herein may be applied in thermoelectric devices, solar cells, OLEDs, touch panel displays or flexible wearables.

[0050] As used herein, the term“about”, in the context of molecular weight ranges, typically means +/- 10% of the stated value, more typically +/- 5% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

[0051] Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0052] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0053] Hereinafter, the present disclosure will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.

EXAMPLES

[0054] Various embodiments relate to a conductive polymer composition comprising a poly(3,4-ethylenedioxythiophene) (PEDOT) analogue and a dispersant.

[0055] Exemplary, a PEDOT-OH:PSS conductive polymer composition has been synthesized and the advantages described.

[0056] Example 1: Synthesis of PEDOT-OH in PSS dispersant

[0057] The PEDOT-OH: PSS (1 :1) was used as an illustrative example, the synthesis of which is graphically shown in the Scheme above. 15 mL deionized water was loaded into a pre- cleaned round bottom flask (RBF). Then 1.8 mL of polystyrene sulfonate (Mw: 70000 g/mol, 18% dispersion in water) was charged into the water solution. The solution was stirred at 1000 rpm for 30 mins. Then EDOT-OH (64 mg) was loaded into the solution under stirring. The resulting solution was further stirred at 1000 rpm for 30 mins. Then potassium persulfate (220 mg) was charged into the reaction mixture followed by 5 mg of FeCl ₃ . The resulting solution was stirred at ambient temperature for 24 hours until a blue-dark solution was obtained. The resulting solution was sequentially washed with basic and acidic polymer resins to remove the excess inorganic ions. The polymer resins were removed via centrifuge at 5000 rpm for 10 mins. In this way, polymer PEDOT-OFPPSS aqueous dispersion was obtained. Similarly, PEDOT-OH:PPS in other ratios, such as 1 :0.5 and 1 :2.5, were also obtained.

[0058] Example 2: Coating process for PEDOT-OH via drop-casting

[0059] A glass slide (2.5 cm by 2.5 cm) was sonicated in deionised water, methanol and acetone for 5 min respectively. The glass slide was then further dried under the nitrogen blow for 2 min. It was then placed under ozone treatment for 10 min. 0.4 mL of PEDOT-OH: PSS solution was drop-casted on the pre-treated glass slide and cured at 80 °C for 30 min. The glass slide was then cooled to room temperature and tested for electrical conductivity measurements and Seebeck coefficient testing.

[0060] Example 3: Performance Evaluation of PEDOT-OH: PSS films

[0061] As shown in FIG. 1, electrical conductivity decreased as the ratio of PEDOT-OH:PSS increased. The highest electrical conductivity recorded was for the PEDOT-OH:PSS ratio of 1 :0.5, at a conductivity of 139 S/cm. Though it was not comparable to that of PEDOT:PSS, however PEDOT-OH:PSS Seebeck coefficient is significantly larger than that of PEDOT:PSS.

[0062] As shown in FIG. 2, Seebeck coefficient was between the range of 62— 1000 uV/K, where the highest Seebeck Coefficient obtained was 1000 uV/K for PEDOT-OH:PSS 1 :2.5.

[0063] FIG. 3 shows the power factor of the PEDOT-OH:PSS, where the highest power factor was observed at 54 uW/(m*K ² ) for PEDOT-OH:PSS 1 :0.5. [0064] Like PEDOT:PSS, PEDOT-OH:PSS also exhibits a mixed ion-electron conductor (MIEC) characteristic and hence its thermovoltage varied over time even under consistent DT. Therefore, we carried time-dependent open-circuit voltage (VOC) measurement to determine the Seebeck voltage produced over time. FIG. 4A illustrates the setup of time-dependent VOC measurement, and the geometry of electrodes and PEDOT-OH: PSS film used in the measurement. FIG. 4B shows the VOC over time and the corresponding temperature profile measured at relative humidity (RH) 60%. As shown in FIG. 4B at AT= 0 K, the VOC was almost zero and no thermal voltage was produced. At time (t) = 500s, the heater was turned on at one side (referred to as "hot" in diagram). Shortly after turning on the heater, AT reached ~ 2K and VOC also increased to - 11 mV at the peak. A negative polarity is obtained as the positive terminal of the source meter was probed on the cold side of the sample. After reaching the peak, the VOC decreased until t = 5500s although AT is maintained around 1.5K. After t= 5500s, the VOC became stable and maintained its value at ~ lmV. Upon switching off the heater at t= 6500s, VOC decreased to zero and overshot to positive voltage. Then, VOC returned back to zero when AT approached zero.

[0065] From the time-dependent VOC measurement, it can be shown that transient huge thermovoltage is generated at the transition between heating to cooling or vice versa. This kind of characteristic is usually observed in MIEC due to the ion diffusion and relaxation upon heating and cooling. The VOC of -l lmV at the peak can be attributed to the ionic Seebeck coefficient. The VOC of ~1 mV at the plateau can be ascribed to the electronic Seebeck coefficient. From the time-dependent VOC measurement and its corresponding temperature profile, it can be concluded that transient ionic Seebeck of PEDOT-OH:PSS can be as high as 5.5 mVK ¹ and permanent electronic Seebeck is -667 pVK ¹ , which is higher than that of PEDOT:PSS (50 pVK ¹ for ionic Seebeck and 16 pVK ¹ at RH 60%).

[0066] To verify that the thermovoltage generated by the ionic Seebeck can be delivered to external load, the voltage was monitored at the load resistor (VOUT). FIG. 5 A illustrates the setup ofVOUT measurement. A load resistor (Rioad) with 22kQ resistance was connected to the two electrodes and the voltage across the Ri _oa d was measured using a Keithley meter. As shown in FIG. 5B, at AT= OK, the VOUT is almost zero because no thermal voltage is delivered to the load. At time (t) = 500s, the heater is turned on at one side and VOUT across the Rioad increased to - 3.3 mV at the peak and then decreased to - 250 pV at t= 3300s. The VOUT was consistent at 250 pV after t= 3300s. The corresponding AT between two electrodes during the heating was about 2K. After the heater is switched off at t= 4000s, the VOUT slowly decreased, overshot to positive voltage and then eventually returned back to zero. The detection of voltage across the Ri _oad suggests that thermovoltage from both ionic Seebeck and electronic Seebeck can be delivered to the load.

[0067] Despite its huge Seebeck coefficient, the ionic Seebeck is usually instantaneous and hence it is useful mainly for intermittence heat source. In order to confirm the application of PEDOT-OH:PSS film in intermittence heat source, the heat being switched“on” and“off’ was simulated for up to four cycles and the VOUT was monitored at the load resistor. FIG. 6 reveals the time-dependent VOUT measured at the load resistor for up to four cycles of heater switched “on” and“off’ and the corresponding DT profile. In the first cycle, upon heating DT to ~ 2K, the VOUT increased to ~ 3.3mV, which is similar to the peak VOUT in the previous cycle test, then decreased to 300 mn when the heater was switched off at t=4000s. During the cooling, the VOUT decreased to zero, overshot to positive voltage and finally reversed back to zero. Different from that cycle test, the heater was turned on again at t= 8000s while DT remained at -0.9K, without waiting DT to reach zero, in order to mimic the situation of intermittence heat source with 4000s of“on” time and 4000s of“off’ time. Because DT ¹ OK at the starting of the second cycle, the VOUT was lower than the first cycle, but still as high as ~l 8mV at the peak and ~ 200 pV at the plateau. Nevertheless, the VOUT did not further drop in the next cycles, maintaining the same voltage level. This simulation clearly showed that PEDOT-OH:PSS responsed very well to heating and cooling cycles and the huge ionic Seebeck of PEDOT- OH:PSS is applicable for intermittence heat source.

[0068] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.