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Title:
FREE-STANDING AND TRANSFERABLE HIGHLY CONDUCTIVE POLYMER FILMS, AND METHOD OF MAKING
Document Type and Number:
WIPO Patent Application WO/2019/177538
Kind Code:
A1
Abstract:
A method of synthesizing a conductive poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate composite film is provided comprising contacting an annealed poly (3, 4- ethylenedioxythiophene) :polystyrene sulfonate (PEDOT:PSS) film disposed on a heated substrate with an acidic solution such as sulfuric acid, soaking the annealed poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate film and the heated substrate with Dl water to form a free-standing poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate film which is separated from the heated substrate, and contacting the free-standing poly (3, 4- ethylenedioxythiophene) :polystyrene sulfonate film with a flexible substrate such as PET or paper in water to form the conductive poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate composite film. The present disclosure also relates to a thermoelectric device comprising a conductive poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate composite film, and its method of forming. The present disclosure further relates to a free-standing conductive poly (3, 4-ethylenedioxythiophene) :polystyrene sulfonate film, and its method of synthesis.

Inventors:
KYAW, Aung Ko Ko (2 Fusionopolis Way #08-03, Innovis, Singapore 4, 138634, SG)
WONG, Dao Hwee Grayson (2 Fusionopolis Way #08-03, Innovis, Singapore 4, 138634, SG)
XU, Jianwei (2 Fusionopolis Way #08-03, Innovis, Singapore 4, 138634, SG)
Application Number:
SG2019/050137
Publication Date:
September 19, 2019
Filing Date:
March 13, 2019
Export Citation:
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Assignee:
AGENCY FOR SCIENCE, TECHNOLOGY AND RESEARCH (1 Fusionopolis Way, #20-10 Connexis North Tower, Singapore 2, 138632, SG)
International Classes:
C09D125/18; C09D145/00; H01B1/12; H01L35/12; H01L35/24
Foreign References:
CN104934140B2016-09-21
US20160064672A12016-03-03
Other References:
SARATH KUMAR, S. R. ET AL.: "Enhanced high temperature thermoelectric response of sulphuric acid treated conducting polymer thin films", J. MATER. CHEM. C, vol. 4, no. 1, 24 November 2015 (2015-11-24), pages 215 - 221, XP055640895
MENGISTIE, D. A. ET AL.: "Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants", APPLIED MATERIALS AND INTERFACES, vol. 7, no. 1, 5 December 2014 (2014-12-05), pages 94 - 100, XP055640896
MCCARTHY, J. E. ET AL.: "Fabrication of highly transparent and conducting PEDOT:PSS films using a formic acid treatment", JOURNAL OF MATERIALS CHEMISTRY C, vol. 2, no. 4, 18 November 2013 (2013-11-18), pages 764 - 770, XP055640900
Attorney, Agent or Firm:
VIERING, JENTSCHURA & PARTNER LLP (P.O. Box 1088, Rochor Post Office,,Rochor Road, Singapore 3, 911833, SG)
Download PDF:
Claims:
CLAIMS

1. A method of synthesizing a conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film, the method comprising: contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate; and

contacting the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film.

2. The method according to claim 1, wherein the heated substrate has a temperature of at least l00°C.

3. The method according to claim 1 or 2, wherein the acidic solution has a concentration of more than 75 vol%.

4. The method according to any one of claims 1 to 3, wherein the acidic solution has a concentration of 75 vol% or less.

5. The method according to any one of claims 1 to 4, wherein contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution comprises contacting the annealed poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the acidic solution for at least 60 minutes.

6. The method according to any one of claims 1 to 5, wherein the acidic solution comprises an organic acid or a sulfur-based acid.

7. A conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite film obtained by the method according to any one of claims 1 to 6.

8. A method of forming a thermoelectric device comprising a conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film, the method comprising:

contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate;

contacting the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film;

forming a plurality of thermoelectric elements from the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film onto a flexible and foldable support substrate; and

connecting the plurality of thermoelectric elements via a conductive material to form the thermoelectric device.

9. The method according to claim 8, wherein the heated substrate has a temperature of at least l00°C.

10. The method according to claim 8 or 9, wherein the acidic solution has a concentration of more than 75 vol%.

11. The method according to any one of claims 8 to 10, wherein contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution comprises contacting the annealed poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the acidic solution for at least 60 minutes.

12. The method according to any one of claims 8 to 11, wherein the acidic solution comprises an organic acid or a sulfur-based acid.

13. The method according to any one of claims 8 to 12, wherein forming the plurality of thermoelectric elements comprises adhering the plurality of thermoelectric elements to the flexible and foldable support substrate.

14. A thermoelectric device obtained by the method according to any one of claims 8 to 13.

15. A method of synthesizing a free-standing conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film, the method comprising:

contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate;

transferring the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film into an anhydrous alcohol; and

removing the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film from the alcohol with an anhydrous substrate to form the free-standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film.

16. The method according to claim 15, wherein the heated substrate has a temperature of at least l00°C.

17. The method according to claim 15 or 16, wherein the acidic solution has a concentration of more than 75 vol%.

18. The method according to any one of claims 15 to 17, wherein contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution comprises contacting the annealed poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the acidic solution for at least 60 minutes.

19. The method according to any one of claims 15 to 18, wherein the acidic solution comprises an organic acid or a sulfur-based acid. 20. A free-standing conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film obtained by the method according to any one of claims 15 to 19.

Description:
FREE-STANDING AND TRANSFERABLE HIGHLY CONDUCTIVE POLYMER

FILMS, AND METHOD OF MAKING

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10201802065Q, filed 13 March 2018, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The present disclosure relates to a conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film, its method of production and uses.

Background

[0003] The global market size of conductive polymers was estimated to be about 236 kilotons, which is equivalent to 3.1 billion USD revenue in 2014, as shown in FIG. 1. The market size keeps growing, due to growing need of shielding sensitive electronic devices from interfering signals, replacement of conventional metals and ceramics arising from the awareness of noise pollution reduction in electronic devices, improvement in fuel economy in automobile industry, and the favourable outlook towards digital electronics including smartphones, web camera and tablets. The projected global market size may become more than 500 kilotons with a revenue of 8 billons USD in 2020.

[0004] PEDOT:PSS or poly(3,4-ethylenedioxythiophene):polystyrene sulfonate is one conducting polymer that may be used in many applications such as transparent electrode for solar cells and light-emitting diode, organic electrochemical transistor and thermoelectric generator. The electrical conductivity of pristine PEDOT:PSS, however, is very low (0.1-0.5 S/cm) and treatment is typically required either by adding a secondary solvent to PEDOT:PSS dispersion, or post treatment with chemicals. Among the various kinds of treatments, post treatment with strong acid such as sulfuric acid at high temperature (more than l50°C) tends to produce the best result in terms of electrical conductivity. Despite that, this method tends to be limited to treatment of PEDOT:PSS film on a rigid and robust substrate such as glass due to the corrosive strong acid and high temperature. Hence, such conventional methods are incompatible with flexible substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and paper.

[0005] There is thus a need to provide for a solution that ameliorates and/or resolves one or more of the issues mentioned above. The solution should at least provide for a method of treating PEDOT:PSS with a strong concentrated acid, which is compatible with a flexible substrate. Summary

[0006] In a first aspect, there is provided for a method of synthesizing a conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate composite film, the method comprising:

contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate; and

contacting the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film.

[0007] In another aspect, there is provided for a conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film obtained by the method described according to the first aspect.

[0008] In another aspect, there is provided for a method of forming a thermoelectric device comprising a conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite film, the method comprising:

contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate;

contacting the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film;

forming a plurality of thermoelectric elements from the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film onto a flexible and foldable support substrate; and

connecting the plurality of thermoelectric elements via a conductive material to form the thermoelectric device.

[0009] In another aspect, there is provided for a thermoelectric device obtained by the method described according to the above aspect.

[0010] In another aspect, there is provided for a method of synthesizing a free-standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film, the method comprising:

contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution;

soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate;

transferring the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film into an anhydrous alcohol; and

removing the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film from the alcohol with an anhydrous substrate to form the free-standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film.

[0011] In another aspect, there is provided for a free-standing conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film obtained by the method described according to the above aspect. Brief Description of the Drawings

[0012] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0013] FIG. 1A shows a global market forecast of conducting polymers.

[0014] FIG. 1B shows the chemical structure that represents PEDOT (poly(3,4- ethylenedioxy thiophene)) and PSS (polystyrene sulfonate), wherein m and n denote a value ranging from 1 to 10,000.

[0015] FIG. 2 shows a schematic process flow of free-standing film that is transferable to flexible substrates.

[0016] FIG. 3 shows a photograph of the transfer process of free-standing PEDOT:PSS film to a paper.

[0017] FIG. 4A shows a large-area highly conductive PEDOT:PSS film on a flexible paper substrate.

[0018] FIG. 4B shows a large-area highly conductive PEDOT:PSS film on a flexible paper substrate that is folded.

[0019] FIG. 4C shows a large-area highly conductive PEDOT:PSS film on a flexible polyethylene terephthalate (PET) substrate.

[0020] FIG. 4D shows a large-area highly conductive PEDOT:PSS film on a flexible PET substrate that is folded.

[0021] FIG. 5 shows a free-standing highly conductive PEDOT:PSS film.

[0022] FIG. 6A shows the contour representing the 25-point sheet resistance check on about a 100 cm 2 transferred PEDOT:PSS film.

[0023] FIG. 6B shows the corresponding statistics for the 25 -point sheet resistance check on the 100 cm 2 transferred PEDOT:PSS film.

[0024] FIG. 7 shows conductivity of pristine and transferred PEDOT:PSS film at various relative humidity (RH).

[0025] FIG. 8 shows the time-dependent Voc of pristine film and transferred film measured at 60% RH and the corresponding temperature profile.

[0026] FIG. 9 shows the time-dependent Voc of transferred PEDOT:PSS film measured at 90% RH and the corresponding temperature profile [0027] FIG. 10A shows a schematic of the thermoelectric element with p-type only legs in series connection.

[0028] FIG. 10B shows the photograph of the element generating about 270 pV when it is put on a human body.

[0029] FIG. 11A shows a schematic of the thermoelectric module in which 400 thermoelectric legs are attached to a paper and connected in series before folding the paper.

[0030] FIG. 11B shows a schematic of the thermoelectric module in which 400 thermoelectric legs are attached to a paper and connected in series after folding the paper.

[0031] FIG. 11C shows a photograph of the thermoelectric module partially folded in hand.

[0032] FIG. 11D shows a photograph of the thermoelectric module completely folded in hand.

[0033] FIG. 12 shows the transmittance spectrum of transferred PEDOT:PSS film before and after compensating the transmittance of PET film.

[0034] FIG. 13 shows the sheet resistance based on number of bending cycles.

Detailed Description

[0035] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised.

[0036] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0037] The present disclosure relates to a poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (i.e. PEDOT:PSS) composite film, its method of synthesis and uses. The present PEDOT:PSS composite film, even without a substrate (i.e. free-standing), is advantageous over PEDOT:PSS produced by conventional methods, as the present free-standing PEDOT:PSS has a higher electrical conductivity compared to the electrical conductivity of PEDOT:PSS derived by conventional methods which is typically undesirable at 0.1 S/cm to 0.5 S/cm.

[0038] The present method is advantageous over conventional methods of producing PEDOT:PSS not only because of the higher electrical conductivity, but also because the present method allows for the PEDOT:PSS to be formed on a flexible substrate. Flexible substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and paper, tend to be avoided or are not used in conventional methods that involve acids and/or high temperatures, as the flexible substrates can be easily damaged by the acids and high temperatures. For example, conventional methods tend to use sulfuric acid for enhancing electrical conductivity of PEDOT:PSS, rendering flexible substrates incompatible for such conventional methods. In other words, corrosion resistant and/or high temperature resistant substrates such as glass have to be used. The present method does not require avoiding corrosive acids and high temperatures, and yet allows PEDOT:PSS to be formed on a flexible substrate.

[0039] PEDOT:PSS composite films derived from conventional methods using glass as the substrate in order for the PEDOT:PSS to be treated with acids lack flexibility. The term“flexibility” and its grammatical variants used herein refer to an object or material that can be subjected to any contortion, including folding, bending, twisting, without being damaged, and can be restored to its original structure on its own or simply undo the contortion.

[0040] The present method also allows for an adequate amount of sulfuric acid to be uniformly spread across a PEDOT:PSS film throughout the entire process of treatment without causing any spillage or overflow, rendering a uniform improvement in the electrical conductivity of the PEDOT:PSS across a large surface area.

[0041] Generally, the present method, which provides for production of a free-standing PEDOT:PSS film includes soaking the film into deionized (DI) water immediately after removing from a hot plate (without the need for cooling down), as this allows the PEDOT:PSS to detach spontaneously from the glass substrate and form the free standing PEDOT:PSS film without additional peeling step. If the film is soaked into DI water after cooling, the film does not detach from the glass substrate. [0042] The present method involves transferring the PEDOT:PSS film from a glass substrate to a flexible substrate, wherein the transferred film can be permanently adhered to the flexible substrate without surface treatment or using surfactants, as the transfer is performed in a hydrophilic environment (i.e. entirely in a water bath).

[0043] The present PEDOT:PSS film and the present method can be used to design and develop a thermoelectric module that is compact, flexible, foldable and simply fabricated using a normal printing paper as the substrate. This method is able to produce a free-standing highly conductive PEDOT:PSS film for large area manufacturing, wherein a uniform conductivity of about 1800 S/cm can be achieved even for a size of about 100 cm 2 . The free-standing PEDOT:PSS film is also transferrable to other substrates to develop flexible electronic devices. Therefore, the present PEDOT:PSS film and present method are very useful for fabricating highly conductive PEDOT:PSS film with a flexible substrate even when corrosive and high temperature treatment are employed. Furthermore, the transferred PEDOT:PSS film is attached firmly to the substrate without surface modification or adding surfactant. The adhesion between the transferred film and the substrate is so firm that the film does not peel off even after bending or folding the substrate. The transferred film on a paper substrate can be handled easily and cut into any size and shape to make thermoelectric legs. These thermoelectric legs can be attached to another paper and be electrically connected to each other to form a thermoelectric module. The whole module remains light-weight, flexible and foldable, due to the compact module design.

[0044] Using the present method, a thin PEDOT:PSS film (e.g. a thickness of 200 nm or less) can be transferred to a flexible substrate to form a transparent conductive electrode for use in flexible optoelectronic devices.

[0045] The word“substantially” does not exclude“completely” e.g. a composition which is“substantially free” from Y may be completely free from Y. Where necessary, the word“substantially” may be omitted from the definition of the invention.

[0046] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0047] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance. [0048] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0049] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0050] Details of various embodiments of the present PEDOT:PSS film and its method of production are described below.

[0051] In the present disclosure, there is provided for a method of synthesizing a conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate composite film comprising contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution, soaking the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate, and contacting the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate composite film.

[0052] The present method allows PEDOT:PSS to be formed on a flexible substrate even when an acidic solution and a heated substrate are used. To combine the PEDOT:PSS and flexible substrate, an annealed PEDOT:PSS that has been treated with the acidic solution may be soaked into water, such as a water bath, immediately without cooling to detach the PEDOT:PSS from the heated substrate. In other words, the annealed PEDOT:PSS flim and the heated substrate may be immersed into water directly after treatment with the acidic solution, such that the annealed PEDOT:PSS that contacts the water is a heated annealed PEDOT:PSS. Once immersed in water, the PEDOT:PSS detaches spontaneously from the heated substrate. Subsequently, the PEDOT:PSS may be lifted, for example, by using a pair of forceps. If the acid-treated annealed PEDOT:PSS is cooled first before soaking in water, the PEDOT:PSS cannot detach spontaneously, or even be peeled off, from the glass substrate. The term “spontaneously” used herein means that detachment of PEDOT:PSS from the heated substrate in water occurs on its own in the absence of manual intervention. The PEDOT:PSS can then be caught onto a flexible substrate used to lift the PEDOT:PSS out of the water which the PEDOT:PSS combines with and gets transferred thereon.

[0053] In various embodiments, contacting the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the flexible substrate in water may comprise immersing the flexible substrate into the water before immersing the free-standing poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film into the water. This allows the PEDOT:PSS to be simply caught onto the flexible substrate when the flexible substrate is lifted out of the water. Due to the hydrophilic nature of the PEDOT:PSS, it firmly adheres to the flexible substrate.

[0054] The present method is versatile in that various type of flexible substrate can be used. Such a flexible substrate may comprise, for example, paper, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), polycarbonate (PC), poly(propene) (PP), or poly(ethylene) (PE).

[0055] In the present method, the PEDOT:PSS may be an annealed PEDOT:PSS. The annealed PEDOT:PSS may be obtained by disposing PEDOT:PSS, e.g. already in the form of a dried film, on a heated substrate such as a hot plate having a temperature of l30°C. In various embodiments, the heated substrate may have a temperature of at least l00°C, at least 1 l0°C, at least l20°C, or at least l30°C, etc. Advantageously, by heating the substrate having the PEDOT:PSS annealed thereon, water in the film can be removed such that it may be completely or substantially free of water. Additionally, by heating the substrate having the PEDOT:PSS annealed thereon, crystallinity of the film may be improved. The annealed PEDOT:PSS on the heated substrate may be treated with an acidic solution to increase its electrical conductivity.

[0056] Treatment with the acidic solution may comprise heating the PEDOT:PSS on the heated substrate in the presence of the acid. The heating may be carried out from 130 to 200°C, 140 to 200°C, 150 to 200°C, 160 to 200°C, 170 to 200°C, 180 to 200°C, 190 to 200°C, etc. The high boiling point of the acid allows for these temperatures to be used and such temperatures allow for a more efficient treatment reaction to occur. Treatment of the annealed PEDOT:PSS on the heated substrate with the acidic solution may be carried out for a certain duration. In various embodiments, contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution may comprise contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film with the acidic solution for at least 60 mins, at least 120 mins, etc. Advantageously, by contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film with the acidic solution for at least 60 mins, the conductivity of the film may be increased as compared to an annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film which was only contacted for less than 60 mins with the acidic solution.

[0057] In certain embodiments, the acidic solution may have a concentration of more than 75 vol%, more than 80 vol%, more than 85 vol%, more than 90 vol%, or more than 95 vol%, etc. For example, the acidic solution may have a concentration of 97 vol%. At these concentration ranges and with the heating duration and temperatures mentioned above, the acidic solution does not easily damage the PEDOT:PSS film. PSS, which may react and may be removed by the acidic solution at such concentrations, may be retained as the present PEDOT:PSS is able to preserve the PSS therein, for example, by increasing the thickness of the PEDOT:PSS.

[0058] In embodiments where the PEDOT:PSS is to be combined with a flexible substrate to form a transparent conductive PEDOT:PSS composite film, the acidic solution may have a concentration of 75 vol% or less, 70 vol% or less, 65 vol% or less, 60 vol% or less, 55 vol% or less, or 50 vol% or less, etc. This means there is a higher volume of water in the acidic solution and such concentrations are advantageous as they avoid damaging the film with concentrated acid and allows the acid -treated PEDOT:PSS to be detached easily for subsequent transfer to the flexible substrate, without substantially compromising the improved electrical conductivity of PEDOT:PSS. Hence, a higher conductivity of the PEDOT:PSS may still be obtained, compared to conventional films, while damage to the film is avoided. The term “transparent” used herein means that any form of electromagnetic radiation, such as visible light, is able pass through.

[0059] In various embodiments, the acidic solution may comprise or consist of an organic acid or a sulfur-based acid. The organic acid may comprise or may be trifluoroacetic acid or formic acid. The sulfur-based acid may comprise or may be sulfuric acid or sulfurous acid. [0060] The present method may further comprise rinsing the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with water to remove residual acidic solution before contacting the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the flexible substrate. This avoids contacting the flexible substrate with an acid that may corrode the flexible substrate. The water used for rinsing may be at the same temperature as the heated substrate, as long as the PEDOT:PSS is heated before it is soaked in water for transferring onto the flexible substrate. Otherwise, there may be difficulty peeling off the PEDOT:PSS from the heated substrate for transferring onto the flexible substrate.

[0061] The present method may further comprise removing the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film from water, and drying the conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite film. The drying may be carried out by any suitable means known to a person of ordinary skill in the art.

[0062] The present disclosure provides for a conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film obtained or obtainable by the method described according to various embodiments of the first aspect. Various embodiments and advantages associated with various embodiments of the present method of the first aspect as described above, are applicable to the present conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate composite film, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity.

[0063] The present disclosure also provides for a method of forming a thermoelectric device comprising a conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite film, the method comprising contacting an annealed poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution, soaking the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate, contacting the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with a flexible substrate in water to form the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film, forming a plurality of thermoelectric elements from the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film onto a flexible and foldable support substrate, and connecting the plurality of thermoelectric elements via a conductive material to form the thermoelectric device.

[0064] Various embodiments and advantages associated with various embodiments of the present method of the first aspect as described above, are applicable to the present method of forming the thermoelectric device, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity.

[0065] As already mentioned above, contacting the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the flexible substrate in water may comprise immersing the flexible substrate into the water before immersing the free-standing poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film into the water.

[0066] The present method is versatile in that various type of flexible substrate can be used. Such a flexible substrate may comprise, for example, paper, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), polycarbonate (PC), poly(propene) (PP), or poly(ethylene) (PE).

[0067] In the present method, the PEDOT:PSS may be an annealed PEDOT:PSS. The annealed PEDOT:PSS may be obtained by disposing PEDOT:PSS, e.g. already in the form of a dried film, on a heated substrate such as a hot plate having a temperature of l30°C. In various embodiments, the heated substrate may have a temperature of at least l00°C, at least 1 l0°C, at least l20°C, or at least l30°C, etc. The annealed PEDOT:PSS on the heated substrate may be treated with an acidic solution to increase its electrical conductivity.

[0068] Treatment with the acidic solution may comprise heating the PEDOT:PSS on the heated substrate in the presence of the acid. The heating may be carried out from 130 to 200°C, 140 to 200°C, 150 to 200°C, 160 to 200°C, 170 to 200°C, 180 to 200°C, 190 to 200°C, etc. Treatment of the annealed PEDOT:PSS on the heated substrate with the acidic solution may be carried out for a duration. In various embodiments, contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution may comprise contacting the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with the acidic solution for at least 60 mins, at least 120 mins, etc.

[0069] In various embodiments, the acidic solution may have a concentration of more than 75 vol%, more than 80 vol%, more than 85 vol%, more than 90 vol%, or more than 95 vol%, etc. For example, the acidic solution may have a concentration of 97 vol%.

[0070] In various embodiments, the acidic solution may comprise or consist of an organic acid or a sulfur-based acid. The organic acid may comprise or may be trifluoroacetic acid or formic acid. The sulfur-based acid may comprise or may be sulfuric acid or sulfurous acid.

[0071] The present method may further comprise rinsing the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with water to remove residual acidic solution before contacting the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film with the flexible substrate. This avoids contacting the flexible substrate with an acid that may corrode the flexible substrate. The water used for rinsing may be at the same temperature as the heated substrate, as long as the PEDOT:PSS is heated before it is soaked in water for transferring onto the flexible substrate. Otherwise, there may be difficulty peeling off the PEDOT:PSS from the heated substrate for transferring onto the flexible substrate.

[0072] The present method may further comprise removing the conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate composite film from water, and drying the conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate composite film. The drying may be carried out by any suitable means known to a person of ordinary skill in the art.

[0073] In various embodiments, forming the plurality of thermoelectric elements may comprise adhering the plurality of thermoelectric elements to the flexible and foldable support substrate. The flexible and foldable support substrate may be comprised of paper, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), polycarbonate (PC), poly(propene) (PP), or poly(ethylene) (PE). The plurality of thermoelectric elements may then be connected by a conductive material such that electricity can be conducted through the plurality of thermoelectric elements. Connecting the plurality of thermoelectric elements via the conductive material may comprise connecting the plurality of thermoelectric elements to form a series connection. In various embodiments, the conductive material may comprise silver paint or conductive carbon paste.

[0074] The present disclosure also provides for a thermoelectric device obtained or obtainable by the method described according to various embodiments of the present method of forming the thermoelectric device. Various embodiments and advantages associated with various embodiments of the present method of the first aspect and the present method of forming the thermoelectric device as described above, are applicable to the present thermoelectric device, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity.

[0075] The present disclosure further provides for a method of synthesizing a free standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film comprising contacting an annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film disposed on a heated substrate with an acidic solution, soaking the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film and the heated substrate with water to form a free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film which is separated from the heated substrate, transferring the free-standing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film into an anhydrous alcohol, and removing the free-standing poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film from the alcohol with an anhydrous substrate to form the free-standing conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film. Advantageously, by using anhydrous alcohol, the poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film does not stick to the substrate but forms a free-standing film.

[0076] The present method provides for the PEDOT:PSS to be recovered as a free standing film without combining the film with any substrate. Various embodiments and advantages associated with various embodiments of the present method of the first aspect as described above, are applicable to the present method of synthesizing a free standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity. [0077] In various embodiments, the anhydrous alcohol may comprise ethanol, methanol, or isopropyl alcohol. Water present in such alcohols may cause the film to stick to the substrate and increases difficulty of forming a free-standing film.

[0078] Treatment with the acidic solution may comprise heating the PEDOT:PSS on the heated substrate in the presence of the acid. The heating may be carried out from 130 to 200°C, 140 to 200°C, 150 to 200°C, 160 to 200°C, 170 to 200°C, 180 to 200°C, 190 to 200°C, etc. Treatment of the annealed PEDOT:PSS on the heated substrate with the acidic solution may be carried out for a certain duration. In various embodiments, contacting the annealed poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film disposed on the heated substrate with the acidic solution may comprise contacting the annealed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film with the acidic solution for at least 60 mins, at least 120 mins, etc.

[0079] In various embodiments, the acidic solution may have a concentration of more than 75 vol%, more than 80 vol%, more than 85 vol%, more than 90 vol%, or more than 95 vol%, etc. For example, the acidic solution may have a concentration of 97 vol%.

[0080] In various embodiments, the acidic solution may comprise or consist of an organic acid or a sulfur-based acid. The organic acid may comprise or may be trifluoroacetic acid or formic acid. The sulfur-based acid may comprise or may be sulfuric acid or sulfurous acid.

[0081] The present method may further comprise securing edges of the free-standing conductive poly(3,4-ethylenedioxythiophene):poly styrene sulfonate film to a substrate before drying. This advantageously prevents wrinkling of the PEDOT:PSS during drying. The drying may be carried out by any suitable means known to a person of ordinary skill in the art.

[0082] The present disclosure further provides for a free-standing conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film obtained or obtainable by the method described according to various embodiments of the present method of forming the thermoelectric device. Various embodiments and advantages associated with various embodiments of the present method of the first aspect and the present method of synthesizing a free-standing conductive poly(3,4- ethylenedioxythiophene):polystyrene sulfonate film as described above, are applicable to the present free-standing conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate film, and vice versa. As the various embodiments and advantages have already been described above, they shall not be iterated for brevity.

[0083] In summary, the methods disclosed herein allows for making of a large uniform area of a highly conductive PEDOT:PSS film on a flexible substrate such as PET and paper. A free-standing highly conductive PEDOT:PSS film, and thermoelectric legs using conductive PEDOT:PSS paper, can also be fabricated. The methods disclosed herein further provides for fabricating a light-weight, flexible, and foldable thermoelectric module, and a highly conductive, transparent and flexible electrode for application in flexible optoelectronic devices

[0084] While the methods described above are illustrated and described as a series of steps or events, it is appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases.

Examples

[0085] The present disclosure relates to a method of preparing a free-standing and transferrable polymer film with high electrical conductivity. Specifically, the present method provides for a method to treat polymer PEDOT:PSS films with an acid, for example, concentrated sulfuric acid, at high temperature, followed by washing with water to remove the excess acid, resulting in a film that is dense, flexible, highly electrically conductive and transparent, wherein the polymer film’s thickness is controllable. The highly conductive polymer film can be transferred to various hard and soft substrates with satisfactory adhesion for uses as thermoelectrics and transparent electrodes.

[0086] The present method of making a highly conductive free-standing and transferrable film may be summarized as follows.

[0087] PEDOT:PSS film may be treated with a reasonable amount of concentrated sulfuric acid for a full coverage of the film with the concentrated sulfuric acid and without overflow of the acid, at l60°C for 120 mins as an example. The PEDOT:PSS film is then immediately soaked into a deionized (DI) water bath without cooling. Soaking the PEDOT:PSS film together with glass substrate into DI water immediately allows the PEDOT:PSS film to detach spontaneously from the glass substrate and forms a free-standing PEDOT:PSS film without the need for a peeling step. To transfer the free-standing film to a flexible substrate, the substrate is first immersed into the DI water bath, then the free-standing film may be caught onto the flexible substrate in the water bath, and ultimately the whole stack (PEDOT:PSS film and flexible substrate) is taken out from the water bath. The transferred film has strong adhesion with the flexible substrate without additional adhesion processes and can be used for different applications.

[0088] With the present method, a highly conductive PEDOT:PSS film (about 1800 S/cm) can be fabricated on paper and/or PET as well as form a free-standing film of its own. The resultant film may be of a large area (e.g. about 100 cm 2 or even bigger), and through the present method, such a large film can be readily transferred to different substrates regardless of the film’s thickness, wherein the PEDOT:PSS film’s thickness ranges from sub-micrometer to several micrometers.

[0089] The transferred PEDOT:PSS film on paper can be cut into small rectangle pieces to form thermoelectric legs. The individual legs can be attached to another paper and be electrically connected to each other to form a thermoelectric module. The final module remains light-weight, flexible and foldable, and can be used as wearable energy harvesting device.

[0090] The examples below also demonstrate that a thin PEDOT:PSS film (200 nm or less) can be transferred onto a PET substrate via the present method. This thin PEDOT:PSS film on PET substrate has an optical transmittance greater than 80% with a sheet resistance of 60 W/p, and hence can be used as a transparent electrode for flexible optoelectronic devices.

[0091] The present free-standing conductive PEDOT:PSS film, its method of production and uses, are described by way of non-examples, as set forth below.

[0092] Example 1: Method of Making Free-Standing and Transferable Conductive PEDOT:PSS Film [0093] A schematic process flow of free-standing film that is transferrable to flexible substrates is shown in FIG. 2. A PEDOT:PSS aqueous solution (Clevios PH1000) with a weight concentration of 1.3% and weight ratio of PSS to PEDOT of 2.5 was obtained from Heraeus. PEDOT:PSS films were prepared by drop casting 6.4 mL of PEDOT:PSS aqueous solution on 10 cm xlO cm glass substrates and then left to dry for 10 hours. The substrates were prepared beforehand by cleaning them in a sonicator sequentially in detergent water, deionised (DI) water, acetone and 2-propanol. After the films were dried, they were annealed on a hotplate at l30°C for 15 mins and left to cool till room temperature. This condition of PEDOT:PSS film is referred to as its pristine film. The thickness of the pristine film is about 7 pm and the conductivity is about 0.3 S/cm at relative humidity (RH) of 50% at room temperature. The conductivity of pristine can be varied from 0.1 S/cm to 1 S/cm, depending on the RH value.

[0094] The pristine film was treated with highly concentrated (97%) sulfuric acid (H2SO4) to increase the conductivity as well as to make a free-standing film. The treatment begins by dropping 6.4 mL of H2SO4 onto the pristine PEDOT:PSS film and left to heat on a hotplate at l60°C for 120 mins. The volume of H2SO4 mentioned above was used to ensure that H2SO4 uniformly spreads on the film throughout the whole process of treatment without causing any spillage or overflow. By using that amount, H2SO4 remains everywhere on the PEDOT:PSS until the end of the treatment, which makes the conductivity uniform over the entire film surface area. After heating for 120 mins, the PEDOT:PSS film is immediately soaked into a deionized (DI) water bath without cooling. Soaking into DI water immediately after removing from the hot plate allows the PEDOT:PSS to detach spontaneously from the glass substrate and forms a free-standing PEDOT:PSS film without additional peeling step. However, the PEDOT:PSS film, which is soaked into the DI water only after cooling to the room temperature, cannot peel off. The free-standing film is soaked in another fresh DI water bath to remove the residual acid. This process is repeated a few times to get rid of the acid completely.

[0095] The free-standing film soaked in the DI water can be easily transferred to various flexible substrates including paper and polyethylene terephthalate (PET). To transfer the freestanding film to a flexible substrate, the substrate is first immersed into the DI water bath, then the free-standing film was caught on the substrate in the water bath (FIG. 3) and finally the whole stack (PEDOT:PSS film and flexible substrate) is removed from the water bath. The transferred PEDOT:PSS on the flexible substrate is then dried in a laboratory oven. Due to the hydrophilic nature of PEDOT:PSS and the fact that the transfer process is done in the DI water, the freestanding PEDOT:PSS is firmly attached to the substrate without additional surface modification or surfactant. The transferred PEDOT:PSS film permanently sticks to the flexible substrate and does not peel off even after the drying process.

[0096] The free-standing film soaked in the DI water can also be recovered without transferring to a substrate. In order to do so, the free-standing film soaked in the DI water bath is first transferred into an ethanol bath. Then the free-standing film is caught on a dry (anhydrous) substrate and dried in an oven. Because the ethanol is less polar than water, the free-standing film cannot be attached to the substrate and a dry free standing PEDOT:PSS film is obtained after drying. To prevent wrinkling of the PEDOT:PSS during the drying, the edge of the PEDOT:PSS film is taped to the underlying substrate and cut out after drying.

[0097] Example 2: Electrical and Physical Properties of Free-Standing And Transferred PEDOT:PSS Film

[0098] The size of the treated film (both free-standing and transferred film) is slightly smaller than the pristine film due to the shrinkage and removal of PSS during the treatment. Nevertheless, a large-area film with more than 9 cm x 9 cm is obtained after the treatment as shown in FIG. 4 A to 4D and FIG. 5. The thickness of the film also reduces to 2.5-2.7 pm. The conductivity of the free-standing and transferred film on the flexible substrate can reach more than 1800 S/cm, which is at least comparable to known conductivity of PEDOT:PSS film on a large-area flexible substrate. Moreover, the sheet resistance of the PEDOT:PSS film is uniform over the entire large area. FIG. 6A shows the 25-point sheet resistance check over the sheet of about 100 cm 2 . The statistics (FIG. 6B) show that the sheet resistance varies only between 2.13 W/p and 2.62 W/p with a median value of 2.3 W/p and standard deviation of only 0.14 W/p. The slight variation in sheet resistance can be attributed to the variation in thickness of the film rather than uniformity of the treatment. The transferred PEDOT:PSS is also less susceptible to the exposure to moisture. As shown in FIG. 7, the conductivity of pristine can be varied up to one order of magnitude (from 0.1 S/cm to 1 S/cm by varying from 10% RH to 90% RH) whereas the transferred film exhibits relatively consistent conductivity (varying from about 1800 S/cm to about 1600 S/cm only) by varying the humidity level from 10% RH to 90% RH.

[0099] The Seebeck coefficient of transferred PEDOT:PSS film is 15 mn/K - 17 mn/K, which is comparable to that of pristine PEDOT:PSS (about 16 pV/K). The pristine PEDOT:PSS film exhibits a transient ionic Seebeck during the transition from heating to cooling or vice versa due to mixed ion-electron conductor behavior of PEDOT:PSS. In contrast to the pristine film, the transferred film does not show this transient ionic Seebeck, which can be clearly seen in time-dependent Seebeck coefficient measurement. FIG. 8 reveals the time-dependent Seebeck measurement monitoring the open-circuit voltage (VOC) generated from the pristine and transferred film during the heating and cooling at 60% RH. As shown in FIG. 8, in the pristine film, the VOC increases initially to -300 pV upon heating to AT = 5.6 K but it decays to -100 pV after about 10,000 s although the constant heating of AT = 5.6 K is applied. Upon cooling, at first the VOC swings back to +100 pV, then decays slowly back to zero when AT approaches zero. This factor of 52 pWm^K 1 is from the transferred PEDOT:PSS film. However, the pristine PEDOT:PSS film gives the power factor of only 7.68 x 10 3 pWm^K 1 . The transferred PEDOT:PSS film on flexible substrate such as paper and PET is bendable, foldable and easier to handle than free-standing PEDOT:PSS film. Bending tests of transferred PEDOT:PSS film on both paper and PET substrates up to 5000 cycles to check the bendability of transferred film were carried out. The conductivity and Seebeck coefficient barely changed even after 10,000 cycles of bending test (FIG. 13).

[00100] Example 3: Method of Making Flexible and Foldable Thermoelectric Legs and Module

[00101] As the transferred PEDOT:PSS film on paper is easy to handle, it can be cut into small rectangular pieces (10 mm x 4 mm) to make thermoelectric legs. The thermoelectric legs were glued to another paper and connected to each other by silver paint to form thermoelectric element and module. FIG. 10A and 10B show a schematic of thermoelectric element with p-type only legs which are connected in series connection. [00102] Since the conductivity of the transferred PEDOT:PSS film is high, the resistance of each leg is only 20 W-25 W and the resistance of the element with five legs and silver paste connection is only about 135 W. At AT = 26 K, the output voltage of 2.2 mV and no-load current of 15 mA are produced from the thermoelectric element. Due to the series connection of the element, the output voltage is very similar to the submission of the thermal voltage of individual legs. When the thermoelectric element is put on the human body, a voltage of about 270 pV is produced.

[00103] Based on this thermoelectric element design, a thermoelectric module in which 400 legs are attached on a paper (31 cm x 10 cm) and connected in series by silver paste, can be made as shown in FIG. 11 A to 11D. Then, the paper is folded along the folding lines. The dimension of the whole thermoelectric module is only 10 cm x 1.5 cm x 0.5 cm at the completely folded position. The module is still light-weight, flexible and foldable and hence it can be carried around easily and kept even in the pocket. The resistance of the whole module with 400 legs is only 4.5 kW, including the resistance of the silver wires. The module is able to generate the thermal voltage of about 75 mV and no-load current of about 16 pA at AT = 10 K. Hence the power output of 1.2 pW is generated at no-load condition and AT = 10 K.

[00104] Example 4: Method of Making Transparent Conductive PEDOT:PSS Film on Flexible Substrate

[00105] Using the same transfer method, a transparent conductive PEDOT:PSS film on flexible substrate for the application of transparent conductive electrodes for optoelectronic devices was fabricated.

[00106] In order to make transparent conductive PEDOT:PSS film on flexible substrate, first of all, the PEDOT:PSS aqueous solution was spin coated (instead of drop cast used for opaque film) on a 2.54 cm x 2.54 cm glass substrate, which was prepared beforehand by cleaning them in a sonicator sequentially in detergent water, deionised water, acetone and 2-propanol. The spin coating process was repeated another three times to obtain a film thickness of about 200 nm. The spin coated PEDOT:PSS was annealed at l30°C for 15 mins to completely remove the solvent. Different from a drop-cast film, diluted H 2 S0 4 was employed to carry out treatment of the film because concentrated H2SO4 is so strong that it damages the film. Diluting H2SO4 in water (2: 1 volume ratio) gives the optimum result, in the sense that PEDOT:PSS film peels off easily for the subsequent transfer process without damaging the film. The higher volume of water also results in a similar conductivity of the film but the film is not easily peeling off. The other treatment parameters for the spin-coated film are the same as that of drop- cast film. After the treatment, the spin-coated film can be successfully transferred to a flexible substrate such as PET, using the same procedure used for the drop-cast film.

[00107] The resultant transferred film has a thickness of about 100 pm and sheet resistance of 56-62 W/p, which is comparable to that of indium tin oxide film, resulting in the conductivity of about 1600-1800 S/cm. The transmittance of transferred PEDOT:PSS film with PET substrate is 75% at 500 nm. The transmittance of the transferred film alone is as high as 85% after compensating the transmittance of PET film, as shown in FIG. 12. Hence the transferred film on PET is suitable for transparent conductive electrode in flexible optoelectronic devices.

[00108] Example 5: Commercial and Potential Applications

[00109] The present method provides for a free-standing large-area film with a uniformly high electrical conductivity. This method is suitable for large scale manufacturing of a free-standing highly conductive PEDOT:PSS film. The resultant PEDOT:PSS film has a uniform conductivity of about 1800 S/cm and has a size of about 100 cm 2 .

[00110] The present method provides for the capability of transferring the PEDOT :PSS to flexible substrates, such as paper and PET for fabricating flexible electronic devices. Therefore, the present method is very useful for fabricating highly conductive PEDOT:PSS film on a flexible substrate that is not conventionally carried out as such flexible substrates are prone to damage by corrosion from use of acids or high temperature.

[00111] In the present method, the transferring of a PEOT:PSS film is easily achieved and the film can be attached firmly to the transferred substrate without surface modification or adding surfactant. The transferred film has extremely good adhesion with various substrates, wherein the adhesion between the transferred film and the substrate can be so firm that the film does not peel off even after bending or folding the substrate. Thus, the resultant film has a long-term anti-bending property and yet with stable electrical conductivity that barely changes even after 10,000 cycles of bending test. [00112] The resultant film thickness can also be manipulated to control optical transparency. For example, the resultant transferred film may have a thickness of about 100 pm and a sheet resistance of 56-62 W/p, which is at least comparable to that of indium tin oxide film, and the conductivity of the resultant film can be at least about 1600 to 1800 S/cm.

[00113] Based on the present method, a thin PEDOT:PSS film (less than 200 nm) can be made and transferred to a flexible substrate to form a transparent conductive electrode for use as a flexible optoelectronic devices. The transmittance of such a transferred film alone can reach at least 85% after compensating the transmittance of a PET substrate.

[00114] Moreover, where paper is used as the substrate, the transferred film onto a paper can be handled easily and cut into any size and shape to make thermoelectric legs. The thermoelectric legs are attached to another paper and electrically connected to each other to form a thermoelectric module. The whole module is light-weight, flexible, foldable, and yet has a compact module design.

[00115] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.