Thermoelectric devices and methods for their production

Abstract

The present invention relates to an n-type element for use in a thermoelectric device, wherein the n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the crystalline n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge). The invention further relates to flexible thermoelectric devices comprising the n-type elements and to methods for their preparation.

Description

THERMOELECTRIC DEVICES AND METHODS FOR THEIR PRODUCTIONFIELD OF THE INVENTION

[0001] The present invention relates to the field of thermoelectric devices, in particular, flexible thermoelectric devices and methods for their preparation. However, it will be appreciated that the invention is not limited to this particular field of use.BACKGROUND OF THE INVENTION

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Thermoelectric devices have emerged as a promising new technology for sustainable charging and cooling technologies and have attracted increased attention. There are several potential applications for thermoelectric devices including as thermoelectric generators and as cooling devices. Wearable flexible thermoelectric devices (F-TEDs) are envisaged in which the device produces electric energy by utilizing the temperature difference between the human body and the ambient environment.

[0004] Thermoelectric devices also have applications in cooling integrated circuits (ICs). For advanced ICs fabricated at 7 nm, 5 nm, or even 3 nm, thermal management is a critical challenge as the heat generated in confined areas leads to performance bottlenecks and circuit malfunctions. Efficient heat dissipation is essential to ensure performance, reliability, and longevity. The need for innovative cooling techniques has brought flexible and printable thermoelectric thin films into focus as a promising solution. By converting ambient heat into electricity, thermoelectric materials can simultaneously provide cooling while harvesting energy, making them appealing for integration with ICs in advanced semiconductor manufacturing processes. Flexible and printable thermoelectric films can be applied directly to the surface of the IC in thin layers. Screen-printable thermoelectric materials are compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication processes. Their deposition can be incorporated as a post-fabrication step without the need for substantial alterations to the IC manufacturing process. Such flexibility and thin characteristics make them suitable for application in the densely packed architectures typical of 7 nm and 3 nm nodes.

[0005] To date, although many organic and inorganic / organic hybrid materials demonstrate excellent flexibility, their thermoelectric performance is significantly inferior to that of inorganic materials. Accordingly, there is a need to improve the flexibility of inorganic flexible thermoelectric materials while maintaining high thermoelectric performance comparable to bulk materials.

[0006] To enhance the practicality of flexible thermoelectric devices, the thermoelectric material from which they are made must have a high-powerfactor (S2o) or high figure-of-merit ZT near room temperature, as well as good flexibility, low toxicity and stability. ZT =S2OTIK is used to determine the overall thermoelectric performance, where S is the Seebeck coefficient, o is the electrical conductivity, K is total thermal conductivity comprising the electronic thermal conductivity xeand the lattice thermal conductivity K\, and T is the absolute temperature.

[0007] Bismuth telluride (Bi2Tes) is a promising thermoelectric material for producing F- TEDs due to its excellent a and S at room temperature. Bi2Tes has excellent thermoelectric performance at near room temperature compared to other materials. Therefore, Bi2Tes is a promising inorganic material for F-TEDs, or screen-printable film, with recent observations of a ZT of 1.2 at room temperature in Ag-doped, highly (00 / )-oriented Bi2Tes films.

[0008] However, the process used to prepare this material is complex and energyconsuming, limiting its potential for large-scale application. Several methods for fabricating flexible Bi2Te3-based films have been reported, including vacuum filtration, screen-printing, and magnetron-sputtering. Among these methods, screen-printing is a relatively inexpensive and scalable technique suitable for both laboratory-scale research and industrial production, with low energy consumption and minimal material requirements. However, screen-printed Bi2Tes films often suffer from poor densification, leading to low o and thus impacting overall performance. Additionally, flexibility of the film was suboptimal and in need of further improvement for commercial applications. Accordingly, optimizing performance of Bi2Te3 films remains an ongoing goal.

[0009] A primary challenge for optimisation is the design of the powder for the ink. The crystal structure of Bi2Te3 is rhombohedral with the space group R3m and can be described using a hexagonal unit cell. The unit cell consists of five layers of Bi-Te (Te1-Bi-Te2-Bi- Te1). Van der Waals forces bind these layers together and cause them to stack in the direction perpendicular to the plane. The ZT of Bi2Te3 is higher in-plane than out-of-plane, which is why high (00 / ) orientation is an important characteristic for Bi2Te3-based films. In addition to carefully controlling the powder size to achieve high (00 / ) orientation in the films, the composition of the powder also needs to be optimized to ensure high initial thermoelectricperformance, which has previously been lacking. However, larger particle sizes of Bi2Tes powders tend to make the films more prone to cracking, thus affecting flexibility and stability. This presents a challenging bottleneck for producing high-performance Bi2Tes films.

[0010] Ag2Se is a narrow band-gap n-type semiconductor that exists as an orthorhombic phase (P-Ag2Se) at room temperature and transforms into a cubic phase (a-Ag2Se) at high temperatures (=130 °C). p-Ag2Se has thermoelectric properties comparable to Bi2Tes due to its high carrier mobility ( ) and low K\, making it a cost-effective alternative to Bi2Tes. However, challenges such as high porosity (=50%), excessive thickness (=0.8 mm), and limited flexibility hinder its use in wearable and microelectronic devices.

[0011] As with Bi2Tes, several methods have been reported to fabricate Ag2Se films including magnetron sputtering, vacuum filtration, co-evaporation, physical vapor deposition (PVD), and screen-printing. However, achieving a ZT exceeding 1 remains challenging across these techniques. Screen-printing is the most straight forward method although, most screen- printed Ag2Se films show poor thermoelectric performance. This is mainly due to their high porosity, which reduces o and lowers the S2o. Addressing this issue while maintaining flexibility remains a key challenge in optimizing screen-printed Ag2Se films.

[0012] It is an object of the present invention to overcome or ameliorate one or more the disadvantages of the prior art, or at least to provide a useful alternative.SUMMARY OF THE INVENTION

[0013] The present inventors have found that adding a binder to n-type thermoelectric material in the preparation of n-type elements enables cost-effective production of thermoelectric devices with improved performance and flexibility.

[0014] Accordingly, in a first aspect the present invention provides an n-type element for use in a thermoelectric device, wherein the n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the crystalline n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

[0015] According to a second aspect, the present invention provides a thermoelectric device comprising a substrate, the substrate comprising one or more n-type elements and one or more p-type elements, each n-type element being in electrical communication with a respective p-type element,wherein each n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

[0016] In a third aspect, the present invention provides a method for preparing an n-type thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge) in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material; and(ii) combining the n-type thermoelectric material with 0.05 to 20 wt% of a binder.

[0017] According to a fourth aspect, the present invention provides a method for preparing an n-type thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge) in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material;(ii) dissolving a binder comprising Te, Se, Ag, Bi, or combinations thereof, in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline binder material;(iii) combining the n-type thermoelectric material with 0.05 to 20 wt% of the binder.

[0018] In a fifth aspect, the present invention provides an n-type composition prepared by the method according to the third or the fourth aspect of the invention.

[0019] According to a sixth aspect, the present invention provides a method for preparing an n-type element for use in a thermoelectric device comprising the steps of:(a) admixing the n-type thermoelectric composition according to the fifth aspect with a solvent and an ink binder;(b) applying the n-type thermoelectric composition to a flexible substrate; and(c) annealing the flexible substrate via spark plasma sintering to form an n-type element.

[0020] In a sixth aspect, the present invention provides a method for preparing a flexible thermoelectric device comprising the steps of:(a) admixing the n-type thermoelectric composition according to the fifth aspect with a solvent and an ink binder;(b) admixing a p-type thermoelectric composition with a solvent and an ink binder;(c) applying the n-type thermoelectric composition and the p-type thermoelectric composition to a flexible substrate to form one or more n-type elements and one or more p- type elements;(d) annealing the flexible substrate via spark plasma sintering; and(e) applying flexible electrodes so that each n-type element is in electrical communication with a respective p-type element.

[0021] These and other aspects of the present invention will be more apparent to the skilled addressee upon reading the following detailed description in connection with the accompanying examples and claims.DEFINITIONS

[0022] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting.

[0023] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[0024] Unless the context clearly requires otherwise, throughout the description and the claims, the terms “comprise”, “'comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. For example, a composition, mixture, process or method that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process or method.

[0025] The transitional phrase "consisting of’ excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

[0026] The transitional phrase "consisting essentially of' is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting essentially of" occupies a middle ground between "comprising" and "consisting of".

[0027] Where the applicant has defined an invention or a portion thereof with an open- ended term such as "comprising", it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms "consisting essentially of' or "consisting of." In other words, with respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of”.

[0028] While reference may be made in this disclosure to the invention comprising a combination of a plurality of elements, it is also understood that this invention is regarded to comprise combinations which omit or exclude one or more of such elements, even if this omission or exclusion of an element or elements is not expressly stated herein, unless it is expressly stated herein that an element is essential to the applicant' s combination and cannot be omitted. It is further understood that the related prior art may include elements from which this invention may be distinguished by negative claim limitations, even without any express statement of such negative limitations herein. It is to be understood, between the positive statements of applicant's invention expressly stated herein, and the prior art and knowledge of the prior art by those of ordinary skill which is incorporated herein even if not expressly reproduced here for reasons of economy, that any and all such negative claim limitations supported by the prior art are also considered to be within the scope of this disclosure and its associated claims, even absent any express statement herein about any particular negative claim limitations.

[0029] As used herein, with reference to numbers in a range of numerals, the terms "about," "approximately" and "substantially" are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to + 1 % of the referenced number, most preferably -0 .1 % to +0 .1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[0030] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[0031] The complete disclosures of the patents, patent documents and publications cited herein are incorporated by reference in their entirety as if each were individually incorporated.

[0032] Unless expressly stated to the contrary, "or" refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0033] The term "and / or" used in the context of "X and / or Y" should be interpreted as "X," or "Y," or "X and Y." Similarly, "at least one of X or Y" should be interpreted as "X," or "Y," or "both X and Y."

[0034] The indefinite articles "a" and "an" preceding an element or component of the invention are intended to be non-restrictive regarding the number of instances (i.e., occurrences) of the element or component. Therefore "a" or "an" should be read to include one or at least one, and the singular word form of the element or component also includes the plural unless the number is obviously meant to be singular.

[0035] As used herein, wt.% refers to the weight of a particular component relative to total weight of the referenced composition.

[0036] It will be understood that use of the term “between” herein when referring to a range of numerical values encompasses the numerical values at each endpoint of the range. Forexample, a temperature of between 80 °C and 150 °C is inclusive of a temperature of 80 °C and a temperature of 150 °C.

[0037] Various features of the embodiments of the invention disclosed herein are, for brevity, described in the context of a single embodiment, but may also be provided separately or in any suitable sub-combination. All combinations of the embodiments are specifically embraced by the illustrative embodiments disclosed herein just as if each and every combination was individually and explicitly disclosed. In addition, all sub-combinations listed in the embodiments describing such variables are also specifically embraced by the present compositions and are disclosed herein just as if each and every such sub-combination was individually and explicitly disclosed herein.

[0038] In the foregoing paragraphs, where various ratios of components have been disclosed. It will be appreciated that these ratios of components can be combined in any disclosed combination. For example, the ratio of A:B (which may be between about 100:1 and 1 :100 or any range therein), may be combined with the ratio of C:D (which may be between about 50:1 and 1:50 or any range therein), and may be combined with the ratio of E:F (which may be between about 10:1 and about 1 :10 or any range therein).BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The aspects described above, as well as other apparent aspects, advantages, and objectives of the present invention are apparent from the detailed description below in combination with the drawings, in which:

[0040] Fig. 1 illustrates characterization data of the phases and structure of screen-printed Bi2Tes thermoelectric film. (A) X-ray diffraction (XRD) patterns of Ag-doped Bi2Tes thermoelectric films with x wt.% Te (x = 0, 2.5, 5, 7.5, and 10 wt.%) by the spark plasma sintering (SPS) annealing, and a Bi2Tes thermoelectric film with x = 7.5 wt. % by the tube furnace TF annealing. (B) Orientation factors F(oo / > of the Bi2Tes thin film. Comparison of pole figures along the (006) directions of Bi2Tes thin films annealed by (C) SPS and (D) TF processes. Scanning electron microscopy (SEM) images of Bi2Tes thermoelectric films with (E) x = 0 and (F) x = 7.5 wt.% from both top and cross-sectional views. (G) Secondary electron (SE) and inset backscattered electron (BSE) SEM images of Bi2Tes thin films with x = 7.5 wt. %. (H) Corresponding energy dispersive X-ray spectroscopy (EDS) maps for overlapped, Bi, Te, and Ag elements.

[0041] Fig. 2 illustrates characterization data of the phases and structure of screen-printed Ag2Te thermoelectric film, (a) X-ray diffraction (XRD) patterns of Ag2Se thin films with differentTe contents (x = 0, 2.5, 5, 7.5, and 10 wt.%). (b) Rietveld quantitative phase analysis of Ag2Se thin films with x = 0, 5, and 10 wt.% binder (Te). (c) Corresponding lattice parameters of Ag2Se with x = 0, 2.5, 5, 7.5, and 10 wt.% binder, (d) Full X-ray photoelectron spectroscopy (XPS) survey of Ag2Se film with x = 5 wt.%. (e) Detailed XPS survey for Te 3d of Ag2Se film with x = 5 wt.% binder. Scanning electron microscopy (SEM) images of Ag2Se films with (f) x = 0 and (g) x = 5 wt.% binder (Te) from both top and cross-sectional views, (h) Secondary electron (SE) SEM images of Ag2Se film with x = 5 wt.% binder from 565 to 585 eV. (i) Corresponding backscattered electron (BSE) SEM images and energy dispersive X-ray spectroscopy (EDS) maps for Ag, Se, and Te.

[0042] Fig. 3 illustrates structural characterization of screen-printed Bi2Tes thermoelectric films with 7.5 wt% binder. (A) Typical transmission electron microscopy (TEM) image of the lamella Bi2Tes material. A focused ion beam (FIB) technique was applied to prepare the lamella sample. (B) TEM image at the phase boundary between the Bi2Tes matrix and the binder (Te) secondary phases. (C) High-resolution TEM (HRTEM) image of a phase boundary between the Bi2Tes matrix and the Te secondary phase. The corresponding fast Fourier transform (FFT) pattern at the phase boundary is shown in the inset. (D) Enlarged HRTEM image of the Bi2Tes matrix, showing lattice contrast. (E) Enlarged HRTEM image of the Bi2Te3 matrix. The inset shows the corresponding selected area electron diffraction (SAED) pattern viewing along the

[0010] direction. (F) Strain maps along different directions. (G) Inverse Fourier transform image taken from (E) presents the potential presence of edge-like dislocations. The inset shows a magnified area with a potential edge-like dislocation. (H) Magnified HRTEM image of Te. (I) Enlarged HRTEM image of Te. The inset shows the corresponding SAED pattern viewing along the

[0357] direction.

[0043] Fig. 4 illustrates structural characterization of screen-printed Ag2Se thermoelectric films with 5 wt.% binder, (a) Low-magnification high-angle annular dark-field (HAADF) image of the specimen fabricated by focused ion beam (FIB) technique, (b) Enlarged HAADF image and its corresponding EDS maps for Ag, Se, and Te elements, (c) Magnified spherical aberration-corrected scanning TEM (Cs-STEM) HADDF image taken from the selected area of a. The inset shows the corresponding fast Fourier transform (FFT) pattern with the view zone axis of

[0010] , (d) Low-magnification TEM image derived from a. (e) High-resolution TEM (HRTEM) image taken from d. (f) Magnified HRTEM image of the Ag2Se matrix. The inset shows the corresponding selected area electron diffraction (SAED) pattern viewing along the (0 1 0) direction, (g) HRTEM image with potential lattice imperfections taken from d. (h) Corresponding strain maps along different directions, (i) Inverse Fourier transform imagetaken from g presents the potential presence of edge-like dislocations. The inset shows a magnified area with a potential edge-like dislocation.

[0044] Fig. 5 is a graphical representation of the thermoelectric performance of n-type Bi2Tes thermoelectric films with 0, 2.5, 5, 7.5 or 10 wt.% binder. Electrical conductivity (a) (A), Seebeck coefficient (S) (B), and S2o (C) as a function of temperature. (D) Electron carrier concentration (ne) and mobility (JJ) at 303 K. (E) Room-temperature deformation potential (Edet and effective mass (m . Measured thermal conductivity (K) (F), lattice thermal conductivity (Kt) (G), and electronic thermal conductivity (Ke) at room-temperature. (H) Measured figure of merit (ZT) values at 303 K. (I) Comparison of the measured and predicted ne-dependent ZT.

[0045] Fig. 6 is a graphical representation of the thermoelectric performance of n-type Ag2Se thermoelectric films with differing binder content (x = 0, 2.5, 5, 7.5, and 10 wt.%). (a) Seebeck coefficient (S), (b) electrical conductivity (q), and (c) power factor (S2o) as a function of temperature, (d) Measured carrier concentration (n) and mobility ( ) at 303 K. (e) Roomtemperature effective mass (m‘) and deformation potential (Edef) calculated by the single parabolic band (SPB) model, (f) Comparison of predicted S2o by the SPB model and measured S2o as a function of n. (g) Room-temperature thermal conductivity (K), (h) and corresponding lattice thermal conductivity (KI) and electronic thermal conductivity (xe) as a function of x. (i) Comparison of ZT values as a function of x at 303 K.

[0046] Fig. 7 illustrates flexibility and performance results for the screen-printed Bi2Tes thermoelectric films and flexible thermoelectric device. (A) Measured normalized resistance change AR / Ro of the Bi2Tes thermoelectric films with x wt.% Te (x = 0, 2.5, 5, 7.5, and 10 wt.%) at different bending cycles. A photo of the flexible film under a bending radius (r) of 5 mm is shown in the inset. (B) Measured AR / Ro of the Bi2Tes thermoelectric films at different r. A photo of the flexible film under a r of 8 mm is presented in the inset. (C) Schematic illustration of the size and structure of designed 1 -unit and 4-unit thermoelectric devices. (D) Measured output voltages (V) and output powers (P) at different temperature differences (A 7s) for the 1- unit thermoelectric device as a function of loading current (A). (E) Determined output power density (w) at different ATs. An infrared photograph for temperature distribution in the 1 -unit thermoelectric device is shown in the inset (the range of sensing temperature is from 31.1 to 53 °C). (F) Measured AR / Ro of the 1-unit flexible device at different r and bending cycles. A photo of the 1-unit flexible device with a r of 8 mm is shown in the inset. (G) Measured \ / of a 4-unit device worn on the human arm during sitting and walking as a function of time. An infrared photograph of the 4-unit thermoelectric device worn on the human arm while sitting is given in the inset (the range of sensing temperature is from 23.2 to 33.3 °C). (H) Infrared photograph of the 1-unit device as a cooler after applying an input current of 58.6 mA. (I)Maximum cooling performance (A7max) of the 1-unit thermoelectric device as a function of input current.

[0047] Fig. 8 illustrates flexibility and performance results for the screen-printed Ag2Se thermoelectric films and flexible thermoelectric device, (a) Measured normalized resistance change (AR / F?o) of the Ag2Se thin films with different Te contents (x = 0, 2.5, 5, 7.5, and 10 wt.%) as a function of different bending cycles at 5 mm bending radius (r). The inset depicts the flexible film with r of 5 mm. (b) Schematic illustration of the size and structure of as fabricated device, (c) Experimental output voltages ( V) and output powers (P) versus loading current ( / ) with different temperature differences (A 7s). (d) Determined output power densities (w) as a function of AT. The inset displays the infrared photograph showing the temperature distribution on the as-fabricated device after applying a AT. (e) Measured AP / Po of as- fabricated device versus different r with different bending cycles. The inset depicts the as- fabricated device with a r of 8 mm. (f) Photograph of as-fabricated device worn on a human arm and can provide open-circuit voltage (\Z0C) of 10.88 mV. (g) Experimental VOc of the device worn on a human arm during sitting and walking as a function of time (0-130 s). The inset displays an infrared photograph showing the temperature distribution of the device worn on a human arm while seated, (h) The infrared photograph shows the temperature distribution of the device functioning as a cooler after an input current of 69.8 mA is applied, (i) Maximum cooling performance (A7max) of the device as a function of input current.DETAILED DESCRIPTION

[0048] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and / or permutations of the disclosed embodiments and features.

[0049] The present inventors have surprisingly found that the inclusion of a binder with particles of an n-type thermoelectric material in the preparation of n-type thermoelectric elements results in a material with improved thermoelectric performance and flexibility, with screen-printed thermoelectric elements achieving a normalized power density of >3 pW cm-2K-2, ranking among the highest in screen-printed devices. Additionally, the thermoelectric material and elements can be prepared in a cost-effective manner, making them suitable for large-scale manufacture.

[0050] The inventors synthesized crystalline n-type thermoelectric material by heating the material under pressure. The crystalline n-type thermoelectric material was combined with a binder and suitable solvents to prepare a printing ink. Screen-printing and spark plasmasintering (SPS) techniques were then used to fabricate thermoelectric elements with an improved normalized power density, a Seebeck coefficient dependent power factor (S2o) of greater than about 16 pW cm-1K"2at 303 K and excellent flexibility.

[0051] The methods according to the invention enabled the production of A4-sized screen- printed n-type thermoelectric film, which has previously been challenging to achieve with many complex fabrication processes. Utilizing the method according to the invention allowed for the production of n-type thermoelectric material with controlled dimensions in the film.

[0052] In a first aspect, the present invention provides an n-type element for use in a thermoelectric device, wherein the n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the crystalline n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

[0053] Suitable n-type thermoelectric material is known in the art and includes, but is not limited to, Bi2Tes, (Bi, Sb^Tes, Bi2Se, Ag2Se, Ag2Te, Ag2S, SnSe, PbTe, PbSnS2, CoSbs, SiGe and LasTe4. In one embodiment, the crystalline n-type particles comprise the n-type thermoelectric material Bi2Tes. In another embodiment, the crystalline n-type particles comprise the n-type thermoelectric material Ag2Se.

[0054] In one embodiment, the n-type thermoelectric material is doped with a dopant. The skilled addressee will understand that the process of doping electrically conductive material comprises the introduction of impurities into the crystal lattice of the electrically conductive material to alter its physical properties, for example, electrical, optical or structural properties. Suitable dopants are known in the art and include, but are not limited to, silver (Ag), zinc (Zn), chromium (Cr), neodymium (Nd), erbium (Er), thulium (Tm), and ytterbium (Yb). In one embodiment, the n-type thermoelectric material is doped with silver (Ag).

[0055] As used herein the term “binder” will be understood to refer to a material that acts to bind crystalline n-type thermoelectric particles in a favorable orientation once applied to a substrate. Without wishing to be bound by theory, it is believed that the addition of a binder with the n-type thermoelectric material helps to connect and orientate the crystalline particles, improving densification of the pressed films and inducing an effective energy-filtering effect. This maintains a high S while further optimizing the n and carrier mobility , resulting in an outstanding S2o of greater than about 16 pW cm-1K"2at 303 K.

[0056] As an example, the addition of a binder to Ag-doped Bi2Tes n-type thermoelectric material helped to order the platelet crystals both longitudinally, to form lamella sheets, as wellas laterally, to stack the sheets in a substantially parallel array. In other embodiments, the addition of a binder results in a less structured material. However, it still acts to improve density of the n-type thermoelectric film and provides a number of other benefits as outlined below.

[0057] As will be appreciated, the method according to the invention will produce crystalline n-type thermoelectric particles with various morphologies depending, for example, on the n-type material selected, whether a dopant is added, the crystallization conditions employed, etc. Accordingly, the morphology of crystalline n-type particles prepared according to the invention includes crystals, for example, with a cuboid, tabular, plate, sheet, blade, rod or needle morphology. Regardless of morphology, the addition of a binder in the printing and annealing steps has been demonstrated to improve thermoelectric properties of the resultant n-type thermoelectric element.

[0058] Suitable binders include, but are not limited to, binders comprising Te, Se, Ag, Bi and combinations thereof. The binder may be in any form such as a granular material or may be ordered, for example, in a rod-like structure. In one embodiment, the binder comprises Te. In a further embodiment, the binder is of a rod-like structure.

[0059] In certain embodiments, the binder is present in the n-type thermoelectric material in an amount of 0.05 to 20 wt%, for example, about 1 wt%, about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 8 wt%, about 9 wt%, about 10 wt%, about 11 wt%, about 12 wt%, about 13 wt%, about 14 wt%, about 15 wt%, about 16 wt%, about 17 wt%, about 18 wt%, about 19 wt%, or about 20 wt%. In a preferred embodiment, the binder is present in the n-type thermoelectric material in an amount of about 2.5 to 10 wt%.

[0060] Improved density of the n-type thermoelectric films is attributed to the binder filling the gaps between the crystal particles of the n-type thermoelectric material, effectively reducing porosity of the material. Most inorganic films prepared by screen-printing have low density due to high porosity, which reduces a and lowers the S2o.

[0061] Additionally, phase boundaries between the binder and the crystalline n-type particles were observed to induce an energy filtering effect, selectively blocking low-energy carriers and enhancing S.

[0062] When applied to a flexible substrate, the addition of a binder was found to improve flexibility of the n-type element. This is believed to be attributed to the soldering effect of the binder, connecting the crystalline n-type particles both longitudinally and laterally. It is also believed that the binder fills the pores within the matrix of the n-type thermoelectric material,acting to improve the overall plasticity of the film and to prevent the film from cracking when the substrate is bent.

[0063] An added benefit observed with the use of a binder in the preparation of n-type elements prepared with Ag2Se is that it induced diffusion doping during the annealing step of the method according to the invention, where Te substitution for Se introduced lattice imperfections, further increasing phonon scattering and reducing KbAs a result, Ag2Se elements prepared with 5 wt.% binder achieved an exceptional S2a of 25.7 pW cm-1K"2and a competitive ZT of 1.06 at 303 K.

[0064] The n-type elements prepared according to the invention are useful in the production of thermoelectric devices, particularly flexible thermoelectric devices.

[0065] Accordingly, in one aspect the present invention provides a thermoelectric device comprising a substrate, the substrate comprising one or more n-type elements and one or more p-type elements, each n-type element being in electrical communication with a respective p-type element, wherein each n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

[0066] Suitable p-type thermoelectric material for use in the preparation of the one or more p-type elements is known in the art and includes, but is not limited to, Bio.4Sb1.6Te3, Mg2Sb3, CsBi4Tee, TlgBiTes, LaFesCoSb^, AgCuSe, AgCu and AgCuTe. In one embodiment, the p- type thermoelectric material comprises Bio.4Sb1.6Te3.

[0067] In one or more embodiments, the p-type thermoelectric material is combined with a binder before it is applied to the substrate. Suitable binders include, but are not limited to, binders comprising Te, Se, Ag, Bi and combinations thereof. The binder may be in any form such as a granular material or may be ordered, for example, in a rod-like structure. In one embodiment, the binder comprises Te. In a further embodiment, the binder is of a rod-like structure.

[0068] In certain embodiments, the binder is present in the p-type thermoelectric material in an amount of 0.05 to 20 wt%, for example, about 1 wt%, about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 8 wt%, about 9 wt%, about 10 wt%, about 11 wt%, about 12 wt%, about 13 wt%, about 14 wt%, about 15 wt%, about 16 wt%,about 17 wt%, about 18 wt%, about 19 wt%, or about 20 wt%. In a preferred embodiment, the binder is present in the n-type thermoelectric material in an amount of about 2.5 to 10 wt%.

[0069] The thermoelectric device according to the invention comprises a substrate. The substrate is not specifically limited and includes commonly used substrates such as glass substrates and substrates commonly used for flexible integrated circuits. In one embodiment, the substrate is a flexible substrate. In one embodiment, the substrate is a flexible substrate selected from the group consisting of a polyimide substrate, a polyester substrate, a polyethylene naphthalate substrate, a polyetherimide substate, a fluoropolymer substrate, and copolymers thereof. In a particular embodiment, the substrate is a flexible polyimide substrate.

[0070] The thermoelectric device according to the invention comprises n-type elements in electrical communication with a respective p-type element. Suitable means of achieving electrical communication include, for example, electrodes made of any conductive material including semiconductors, graphite, conductive polymers, conductive tapes, and conductive solutions and pastes.

[0071] Where the substrate is a flexible substrate care should be taken to select a suitably flexible electrode material. Low flexibility and high resistance might cause cracking and performance loss while using the device. Suitable materials for flexible electrodes may include a conductive polymer such as polyaniline, polyacetylene, poly(3,4-ethylenedioxythiophene) and polyphenylene vinylene, or a conductive solution or paste such as a solution or paste comprising copper or silver. In one embodiment, the electrodes are conductive polymer electrodes. In another embodiment, the electrodes comprise a conductive solution or paste. In a particular embodiment, the electrodes comprise silver paste, copper tape, or combinations thereof.

[0072] In one or more embodiments, the thermoelectric device according to the invention comprises a flexible substrate. In one embodiment, the flexible thermoelectric device (F-TED) according to the invention is a wearable F-TED that produces electric energy by utilizing the temperature difference between the human body and the ambient environment. In another embodiment, the F-TED is a cooling device, for example, for use in cooling integrated circuits and the like.

[0073] In one embodiment, the one or more n-type elements and one or more p-type elements extend from at least one surface of the substrate. The inventors have found that orientating the one or more n-type elements and one or more p-type elements to extend from the substrate allows for the inclusion of more elements per unit area. The extended elementsalso promote heat dissipation, allowing for a higher AT that is stable for a longer period of time. In a particular embodiment, the one or more n-type elements and one or more p-type elements extend from one surface of the substrate.

[0074] In one embodiment, the n-type elements and / or the p-type elements extend from the one surface of the substrate at an angle from. In a preferred embodiment, both the n-type elements and the p-type elements extend from one surface of the substrate at an angle. A flexible electrode forms an apex between an n-type element and a respective p-type element. Advantageously, extending the p-type elements and / or the n-type elements from the substrate at an angle increases AT compared to a device comprising a parallel structure when heat is applied to the substrate.

[0075] In one aspect the present invention provides a method for preparing an n-type thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge) in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material; and(ii) combining the n-type thermoelectric material with 0.05 to 20 wt% of a binder.

[0076] A “composition” refers to an admixture of the n-type thermoelectric material and the binder, optionally in a medium such as a solvent.

[0077] In one embodiment, the composition is in the form of a powder comprising the n- type thermoelectric material and the binder.

[0078] In another embodiment, the composition is in the form of a solution in which the n- type thermoelectric material and the binder are dissolved in a solvent. Suitable solvents include, but are not limited to, water, terpineol, polyethylene glycol, ethanol, butanol, toluene, ethylbenzene, chlorobenzene, N-methyl-2-pyrrolidone, N,N-dimethylformamide, cyclohexanol, cyclohexanone, and mixtures thereof. The solution may contain additional mediums including binders, plasticizers, dispersants, pigments, wetting agents, etc.

[0079] The term “precursor n-type thermoelectric material” as used herein will be understood to mean a compound that participates in a chemical reaction to form a new compound suitable for use as an n-type thermoelectric material. As an example, Bi2Os, Ag2O, and Na2TeC>3 are suitable precursor n-type thermoelectric materials for the preparation of Ag- doped Bi2Tes n-type thermoelectric material. In this instance, the term “precursor n-typethermoelectric material” will be understood to encompass each of these precursors. Similarly, Se and AgNCh are suitable precursor n-type thermoelectric materials for the preparation of Ag2Se n-type thermoelectric material.

[0080] The method according to the invention comprises dissolving a precursor n-type thermoelectric material in a polar solvent. The term “polar solvent” will be understood to refer to a solvent comprised of atoms with highly differing electronegativities and having a partial charge or significant dipole moment. The skilled addressee will understand that the choice of solvent may vary depending on, for example, the n-type thermoelectric material and preferred process conditions. Suitable polar solvents include, but are not limited to, ethylene glycol, glycerol, benzyl alcohol and combinations thereof. In one embodiment, the n-type thermoelectric material is dissolved in ethylene glycol.

[0081] Once dissolved in the polar solvent, the precursor n-type thermoelectric material is combined with a base. The “base” will be understood to mean a chemical compound that reacts with acids in an acid-base reaction and includes metal oxides, hydroxides and alkoxides. In one embodiment the base is a hydroxide selected from the group consisting of NaOH, LiOH, KOH, Mg(OH)2, Ca(OH)2and combinations thereof. In a particular embodiment, the base is NaOH.

[0082] The solution of a precursor n-type thermoelectric material and a base dissolved in a polar solvent is then heated under pressure to prepare crystalline n-type thermoelectric material. Methods for heating the reaction under pressure are known in the art and include, for example, performing the reaction in an autoclave, in a pressure vessel or in a pressure reactor. In one embodiment, the solution of a precursor n-type thermoelectric material and a base dissolved in a polar solvent is heated in an autoclave.

[0083] In some embodiments, the solution of a precursor n-type thermoelectric material and a base dissolved in a polar solvent is heated from about 210 °C to about 250 °C, for example, about 215 °C to about 245 °C, about 220 °C to about 240 °C, about 225 °C to about 235 °C, or about 230 °C. In one embodiment, the n-type thermoelectric material and the base dissolved in a polar solvent is heated to 230 °C.

[0084] The inventors have found that doping the thermoelectric material with a dopant such as silver (Ag) improves the carrier concentration n, and combined with optimized synthesis parameters, enhances production of lamella n-type Bi2Tes thermoelectric material with controlled dimensions. This, in turn, enables production of n-type elements with high (00 / ) orientation, high , and good flexibility. Accordingly, in one embodiment, step (i) of the abovemethod further comprises dissolving a dopant in the polar solvent, together with the precursor n-type thermoelectric material and the base.

[0085] In one embodiment, the n-type thermoelectric material is a silver-doped n-type thermoelectric material.

[0086] In one or more embodiments, the binder comprises Te, Se, Ag, Bi or combinations thereof. The binder may be in any form such as a granular material or may be ordered, for example, in a rod-like structure. In one embodiment, the binder comprises Te. In a further embodiment, the binder is of a rod-like structure.

[0087] In one embodiment, the binder is prepared by a process similar to that employed to prepare the crystalline n-type thermoelectric material.

[0088] Accordingly, in one aspect the present invention provides a method for preparing an n-type Bi2Tes thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge) in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material;(ii) dissolving a binder comprising Te, Se, Ag, Bi, or combinations thereof, in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline binder material;(iii) combining the n-type thermoelectric material with 0.05 to 20 wt% of the binder.

[0089] In another aspect, the present invention provides an n-type composition prepared by the method according to the invention.

[0090] In a further aspect, the present invention provides a method for preparing an n- type element for use in a thermoelectric device comprising the steps of:(a) admixing the n-type thermoelectric composition prepared by a method according to the invention with a solvent and an ink binder;(b) applying the n-type thermoelectric composition to a flexible substrate; and(c) annealing the flexible substrate via spark plasma sintering to form an n-type element.

[0091] In yet another aspect, the present invention provides a method for preparing a flexible thermoelectric device comprising the steps of:(a) admixing the n-type thermoelectric composition prepared by a method according to the invention with a solvent and an ink binder;(b) admixing a p-type thermoelectric composition with a solvent and an ink binder;(c) applying the n-type thermoelectric composition and the p-type thermoelectric composition to a flexible substrate to form one or more n-type elements and one or more p- type elements;(d) annealing the flexible substrate via spark plasma sintering; and(e) applying flexible electrodes so that each n-type element is in electrical communication with a respective p-type element.

[0092] Suitable solvents for admixing the n-type thermoelectric composition and the p- type thermoelectric composition are known in the art and include, but are not limited to, water, terpineol, polyethylene glycol, ethanol, butanol, toluene, ethylbenzene, chlorobenzene, N- methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, cyclohexanol, cyclohexanone, and mixtures thereof.

[0093] The admixtures of steps (a) and (b) in the above method further comprise an ink binder. The ink binder may be any suitable material known in the art including, but not limited to, ethyl cellulose, acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyvinyl chloride (PVC), polyvinylpyrrolidone (PVP) or polystyrene based resins, and combinations thereof and copolymers thereof.

[0094] The admixtures of steps (a) and (b) may further comprise a plasticizer. Suitable plasticizers are known in the art and include, but are not limited to, dibutyl phthalate, dimethyl phthalate, diisobutyl phthalate, butyl benzyl phthalate, bis(2-ethylhexyl) phthalate and combinations thereof.

[0095] The admixtures of steps (a) and (b) may comprise additional mediums including binders, plasticizers, dispersants, pigments, wetting agents, etc.

[0096] The n-type thermoelectric composition and the p-type thermoelectric composition may be applied to the flexible substrate by any method known in the art including, but notlimited to, vacuum filtration, screen printing, co-evaporation and magnetron-sputtering, to form one or more n-type elements and one or more p-type elements.

[0097] In a preferred embodiment, the n-type composition and the p-type composition are applied to the flexible substrate via screen-printing. The ability to screen-print the n-type thermoelectric composition and the p-type thermoelectric composition allows the method to be compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication processes. Deposition of the compositions can be incorporated as a post-fabrication step without requiring substantial alterations to the IC manufacturing process. Such flexibility and thin characteristics make the method suitable for application in densely packed IC architectures, typical of 7 nm and 3 nm nodes. Screen-printing is also a cost-effective means of preparing flexible thermoelectric devices.

[0098] Once the n-type thermoelectric composition and the p-type thermoelectric composition are applied to the flexible substrate the substrate is annealed via spark plasma sintering. As mentioned above, the addition of a binder with the n-type thermoelectric material helps to connect and orientate the crystalline particles when the substrate is annealed under pressure, such as by spark plasma sintering. This maintains a high S while further optimizing the n and carrier mobility , resulting in an outstanding S2o of greater than about 16 pW cm-1K"2at 303 K.

[0099] Flexible electrodes are applied after the substrate is annealed so that each n-type element is in electrical communication with a respective p-type element. As mentioned above, care should be taken to select a suitably flexible electrode material. Low flexibility and high resistance might cause cracking and performance loss while using the device. Suitable materials for flexible electrodes include, but are not limited to, a conductive polymer such as polyaniline, polyacetylene, poly(3,4-ethylenedioxythiophene) and polyphenylene vinylene, or a conductive solution or paste such as a solution or paste comprising copper or silver. In one embodiment, the electrodes are conductive polymer electrodes. In another embodiment, the electrodes comprise a conductive solution or paste. In a particular embodiment, the electrodes comprise silver paste applied to the substrate. In one embodiment, the silver paste is screen- printed on the flexible substrate.[000100] The above method may further comprise the step of disposing the one or more n- type element(s) and the one or more p-type element(s) to extend from one surface of the flexible substrate. This may be achieved by applying the n-type thermoelectric composition and the p-type thermoelectric composition to a flexible substrate, annealing the substrate and then cutting around the perimeter of each n-type element and each p-type element except fora hinge section, and folding each n-type element and p-type element along the hinge section such that it extends from one surface of the flexible substrate.[000101] Alternatively, extension of the one or more n-type element(s) and the one or more p-type element(s) from the substrate may be achieved by applying the n-type thermoelectric composition and the p-type thermoelectric composition to a flexible substrate, annealing the substrate and then cutting around the perimeter of each n-type element and each p-type element. Each n-type element and each p-type element may then be applied to a flexible substrate, for example, with silver paste or copper tape.[000102] The inventors have found that orientating the one or more n-type elements and one or more p-type elements to extend from the substrate allows for the inclusion of more elements per unit area. The extended elements also promote heat dissipation, allowing for a higher AT that is stable for a longer period of time. In a particular embodiment, the one or more n-type elements and one or more p-type elements are disposed to extend from one surface of the substrate.[000103] In one embodiment, each n-type element and / or each p-type element is disposed to extend at an angle from one surface of the flexible substrate, with a flexible electrode forming an apex between an n-type element and a respective p-type element.[000104] In a preferred embodiment, each n-type element and each p-type element are disposed to extend at an angle from one surface of the flexible substrate, with a flexible electrode forming an apex between an n-type element and a respective p-type element. In a particular embodiment, each n-type element and p-type element extend from the substrate at a 30° angle relative to the substrate, and with an angle at the p-n junction of 120°.EXAMPLES[000105] The present invention will now be described with reference to the following examples, which should be considered in all respects as illustrative and non-restrictive.Materials[000106] Bismuth oxide (Bi2Os, 99.99 %, Sigma-Aldrich), sodium tellurite (Na2TeOs, 99.9 %, Sigma-Aldrich), silver oxide (Ag2O, 99.9 %, Sigma-Aldrich), tellurium dioxide (TeC>2, 99.9 %, Sigma-Aldrich), ethylene glycol (EG, 99 %, Sigma-Aldrich), and sodium hydroxide (NaOH, 96 %, Sigma-Aldrich) were used for solvothermal synthesis of n-type Bi2Tes thermoelectric material without further purification. Selenium (Se, 99.99 %, Sigma-Aldrich), silver nitrate (AgNOs, 99 %, Sigma-Aldrich), ethylene glycol (EG, 99 %, Sigma-Aldrich), tellurium dioxide(TeC>2, 99.99 %, Sigma-Aldrich) and sodium hydroxide (NaOH, 96 %, Sigma-Aldrich) were used for solvothermal synthesis of n-type Ag2Se thermoelectric material without further purification. Ethyl cellulose (Sigma-Aldrich), terpineol (Sigma-Aldrich), dibutyl phthalate (Sigma-Aldrich), and dispersant (Disperbyk-110, BYK USA Inc.) were used for fabricating the printing ink. Commercial Bio.4Sb1.6Te3 powders (Sigma-Aldrich) were used for fabricating p- type elements.EXAMPLE 1 : Synthesis of Ag-doped lamellar n-type Bi2Tes thermoelectric material[000107] Bi2C>3, Ag2O, and Na2TeOs were dissolved into EG (36 ml) with stirring by a magnetic stirrer to form a clear solution. After that, 4 ml NaOH (5 mol L-1) was dropped into the solution and formed the precursor solution. The solution was then sealed in a 125 ml polytetrafluoroethylene-lined stainless-steel autoclave. The autoclave was heated in an oven at 230 °C for 19 h, followed by natural cooling to room temperature. After synthesis, the product was cooled to room temperature naturally, and then collected by centrifugation and washed with deionized water and ethanol several times. Finally, the washed product was dried in the oven at 60 °C for 24 h.EXAMPLE 2: Synthesis of n-type Ag2Se thermoelectric material[000108] Se and Ag2NO were dissolved into EG (36 ml) with stirring by a magnetic stirrer to form a clear solution. After that, 4 ml NaOH (5 mol L-1) was dropped into the solution and formed the precursor solution. The solution was then sealed in a 125 ml polytetrafluoroethylene-lined stainless-steel autoclave. The autoclave was heated in an oven at 230 °C for 19 h, followed by natural cooling to room temperature. After synthesis, the product was cooled to room temperature naturally, and then collected by centrifugation and washed with deionized water and ethanol several times. Finally, the washed product was dried in the oven at 60 °C for 24 h.EXAMPLE 3: Synthesis of Te binder material[000109] TeO2 was dissolved in EG, and NaOH was added to the solution. The solution was then sealed in a 125 ml polytetrafluoroethylene-lined stainless-steel autoclave. The autoclave was heated in an oven at 230 °C for 19 h, followed by natural cooling to room temperature. After synthesis, the product allowed to cool to room temperature and then collected by centrifugation and washed with deionized water and ethanol several times. Finally, the washed product was dried in the oven at 60 °C for 24 h.EXAMPLE 4: Fabrication of n- and p-type thermoelectric films[000110] An ink binder solution was prepared by mixing ethyl cellulose (10 wt.%), terpineol (80 wt.%), and dibutyl phthalate (10 wt.%). The solution was stirred magnetically for 2 h at 80 °C to produce a clear solution. The n-type thermoelectric material prepared in Example 1 or Example 2 in powder form or a commercially available p-type powder (80 wt.%) was then mixed with the binder solvent (7.5 wt.%), terpineol (10 wt.%), and dispersant (2.5 wt.%) to prepare a printing ink. The solution was stirred for 2 h to produce a homogeneous viscous solution. The prepared ink was printed onto the flexible polyimide (PI) substrate. The printed films were dried at 200 °C for 0.5 h and annealed by spark plasma sintering (SPS-211 Lx, Fuji Electronic Industrial CO., Ltd., Japan) with 3 MPa at 450 °C for 10 min. The fabricated films are suitable for use as n-type or p-type thermoelectric elements.EXAMPLE 5: Characterization of the n-type thermoelectric material and thermoelectric filmMorphology and structural characterization[000111] The morphology and structural characteristics of the Bi2Tes and Ag2Se powder and films were analyzed through SEM (JEOL JSM-7100F) and TEM (HITACHI HF 5000), respectively. The chemical composition and crystal lattice structure of the Bi2Tes and Ag2Se films were determined by XRD (Bruker-D8) with CuKa radiation operated at 40 kV and 40 mA over an angular range of 10° to 80° in 0.02° increments. The element distribution and the chemical composition of the products were determined through EDS mapping and spot analysis (equipped in JEOL JSM-7100F and HITACHI HF 5000).[000112] X-ray diffraction (XRD) characterizations were performed to investigate the phase information of fabricated n-type thermoelectric films with varying amounts of Te binder material added (Bi2Tes or Ag2Se + xTe, where x = 0, 2.5, 5, 7.5, and 10 wt.%) (Figs. 1A and 2a). XRD patterns were compared in the 20 range of 10 to 80°, operated at 40 kV and 40 mA with a step size of 0.02°. n-Type Bi2Te3 thermoelectric film[000113] For the n-type Bi2Tes thermoelectric films, all diffraction peaks can be indexed to the PDF standard cards for Bi2Tes (#00-015-0863) and Te (#03-065-3370). As the Te content increases, the intensity of its diffraction peaks gradually strengthens. Additionally, all films pressed and annealed via SPS exhibit strong (006) orientation. This is because synthesis under pressure produces typical two-dimensional (2D) Bi2Tes lamellae. During sintering, these lamellae stack layer by layer under pressure, resulting in a distinct (00 / ) orientation. For comparison, films with x = 7.5 % were prepared and annealed in a tube furnace (TF). Because TF annealing lacks pressure, the XRD results show that the films exhibit a strong (015)orientation instead of a (006) orientation, demonstrating the importance of pressure. The orientation factor (F(oo / >) of the films was determined (Fig. 1 B) (J. Xie et al., Chem. Eng. J. 435, 135172 (2022)), indicating that the F<poi) of films annealed by the SPS process is approximately twice that of the film annealed by the TF process, validating the XRD results. The pole figures of Bi2Tes films with 7.5 wt.% Te annealed by SPS or TF, respectively, were compared along the (006) directions (Figs. 1C, D). These results further confirm that the SPS technique achieves a large amount of (00 / ) orientation in the film. n-Type Ag2Se thermoelectric film[000114] All major diffraction peaks align with the orthorhombic Ag2Se phase (PDF #04- 002-0445) and hexagonal Te phase (PDF #04-027-7719) for the n-type Ag2Se thermoelectric films. As the Te content increases, the intensities of Te-related diffraction peaks strengthen, particularly displaying a (100) preferred orientation, likely due to the pressure applied during SPS. For a more detailed analysis, Rietveld refinement of the XRD data was performed for films with 0, 5, and 10 wt.% Te, as shown in Fig. 2b. The refinement results reveal that as the Te content increases, the lattice parameters (a, b, c) of Ag2Se expand, suggesting that Te2-ions (97 pm) replace Se2-ions (50 pm), as depicted in Fig. 2c. Fig. 2d shows the full X-ray photoelectron spectroscopy (XPS) spectrum of the Ag2Se film with 5 wt.% Te, confirming the presence of Ag, Se, and Te. Fig. 2e provides high-resolution scans of Te 3d5 / 2, revealing three distinct states of Te in the Ag2Se film: Te° (573 eV), Te2’ (576.1 eV), and Te4+(572.6 eV). These findings support both the XRD and Rietveld refinement results, further confirming the diffusion of Te into the Ag2Se structure.SEM Characterization[000115] Scanning electron microscopy (SEM) characterizations coupled with energy- dispersive X-ray spectroscopy (EDS) were used to investigate the structural and compositional information of the fabricated n-type thermoelectric films with varying amounts of Te binder material (Bi2Tes or Ag2Se + xTe). n-Type Bi2Te3 thermoelectric film[000116] Figs 1 E and F show SEM images of the n-type Bi2Tes thermoelectric film with x = 0 and x = 7.5 wt.% Te from both top and cross-sectional views. The top-view SEM images show that the addition of a Te binder substantially reduces porosity of the Bi2T e3thermoelectric film. The cross-sectional views shows that the thickness of Bi2Te3 film is more constant (approximately 4.5 pm) with increasing amounts of the Te binder. Similar results were observed for films comprising 2.5 or 5 wt% Te.[000117] Secondary electron (SE) and backscattered electron (BSE) images were taken of the film with x = 7.5 wt.% Te binder to study distribution of the binder in the film. Figs. 1G and 1 H show the corresponding EDS maps including overlapping elements and individual elements of Bi, Ag, and Te. As can be seen, the Te binder fills the voids between the grains of lamella Bi2Tes thermoelectric material and acts to bond the surrounding grains together. Hence, the Te binder significantly reduces the porosity of the film by filling the pores between the lamella Bi2Tes thermoelectric material. n-Type Ag2Se thermoelectric filmFigs. 2f-g present both top and cross-sectional SEM images of Ag2Se films with 0 and 5 wt.% Te. The top-view images reveal that adding Te significantly improves the densification of the Ag2Se film. As the Te content increases, the film thickness remains approximately constant at around 15 pm, as shown in the cross-sectional views. The film thickness and uniformity show only slight variation (12-16 pm) with different Te content since all films were fabricated using 160 mesh screens. This consistency is attributed to two factors: first, the Te content in the precursor is relatively low compared to Ag2Se; second, all films were annealed under the same SPS conditions, which involved high temperature and pressure, ensuring uniform densification and minimal influence of additional Te on film morphology. Further analysis of the Ag2Se film with 5 wt.% Te, including secondary electron (SE) and backscattered electron (BSE) images, along with the corresponding EDS maps, is shown in Figs. 2h-i. These results confirm the presence of Ag, Se, and Te elements. Te is observed filling the gaps between the Ag2Se grains, effectively reducing the porosity of the film, which aligns with the findings for n-Type Bi2Tes thermoelectric films.TEM Characterization[000118] Comprehensive transmission electron microscopy (TEM) characterizations were performed via focused ion beam (FIB) on the n-type Bi2Tes thermoelectric film with 7.5 wt.% Te binder and n-type Ag2Se thermoelectric film with 5 wt.% Te binder to assess the structural characteristics of the film. n-Type Bi2Te3 thermoelectric film[000119] Fig. 3A illustrates a typical TEM image and shows the overall structural information of the specimen. A phase boundary between the Bi2Tes matrix and Te binder can be seen in the high-magnification TEM image of Fig. 3B. Fig. 3C illustrates a high-resolution TEM (HRTEM) image of the phase boundary taken from Fig. 3B. The inset in Fig. 3B illustrates the corresponding fast Fourier transform (FFT) pattern. As can be seen, Te andBi2Tes exhibit distinct lattice information. The smooth phase boundary indicates a good bonding effect between Te and Bi2Tes.[000120] This is a key factor for good flexibility of the thin film. Fig. 3D displays a magnified HRTEM image of the Bi2Tes matrix taken from Fig. 3C in which lattice distortion can be seen, attributed to the Ag-doping induced point defects of Agsi (W.-Y. Chen et al., Chem. Eng. J. 475, 146428 (2023)). Ag+(1.15 A) has a larger ionic radius than Bi3+(1.03 A), which potentially induces the lattice strain. Fig. 3E is a magnification the labelled area in Fig. 3D and illustrates local structural disorders. Confirmation of the

[0010] view direction comes from the selected area electron diffraction (SAED) pattern (Fig. 3E, inset).[000121] Fig. 3F shows a calculation of the strain field using the geometric phase analysis algorithm, indicating the strain caused by the point defects significantly affects the y- direction of the lattice distortion. Fig. 3G provides an inverse Fourier transform image corresponding to Fig. 3E, showing a potential edge-like dislocation. An enlarged view of the edge-like dislocation can be seen in the inset image, attributed to the introduction of point defects in the Bi2Tes matrix. Figs. 3G-H illustrate low- and high-magnification TEM images taken from Fig. 3C, and the corresponding SAED pattern is provided in the inset in Fig. 3H, from which, the viewed direction of Te is

[0357] and the viewed direction is along the

[0010] for the Bi2Tes matrix. n-Type Ag2Se thermoelectric film[000122] A focused ion beam (FIB) technique was used to prepare a lamella specimen of Ag2Se with 5 wt.% Te. Fig. 4a shows a low magnification scanning transmission electron microscopy (STEM) image of the lamella sample. Fig. 4b presents an enlarged high-angle annular dark-field (HAADF) STEM image along with EDS maps for the individual Ag, Se, and Te elements, derived from a selected area in Fig. 4a. These images reveal a uniform distribution of Ag, Se, and Te, indicating homogeneous Te doping (diffusion) throughout the sample. Fig. 4c displays a high-magnification, spherical aberration-corrected scanning TEM (Cs-STEM) HAADF image, with insets showing a magnified Cs-STEM HAADF image and a fast Fourier transform (FFT) pattern. This confirms that the image was captured along the

[0010] zone axis of the Ag2Se matrix, demonstrating a well-ordered lattice structure and confirming the high crystallinity of the prepared film.[000123] Fig. 4d presents a magnified high-resolution TEM (HRTEM) image from the same region, showing lattice contrast that indicates lattice distortion. This distortion is attributed to the substitution of Se2-ions by Te2-ions within the Ag2Se lattice, which alignswith the results from the XRD Rietveld refinement. Figs. 4e-f show low- and high-magnification HRTEM images of the Ag2Se matrix, taken from an area without noticeable lattice contrast, as seen in Fig. 4d. The inset shows the corresponding selected area electron diffraction (SAED) pattern with indexed information, confirming the imaging direction is along the

[0010] zone axis of Ag2Se.[000124] Fig. 4g displays a high-magnification HRTEM image of a selected area with lattice contrast, and Fig. 4h shows corresponding strain maps along various directions. These maps indicate that the strain is primarily along the x-direction, caused by the lattice distortion resulting from the Te2-substitution in the Ag2Se lattice. Lastly, Fig. 4i presents inverse Fourier transform images of Fig. 4g, revealing the presence of potential edge-like dislocations. The inset provides an enlarged view of these dislocations, which are likely caused by the substitution of Se2-by Te2-within the Ag2Se lattice.EXAMPLE 6: Thermoelectric properties of the n-type thermoelectric films[000125] The thermoelectric performance of n-type thermoelectric films with different amounts of Te binder (Bi2Tes or Ag2Se + xTe, x = 0, 2.5, 5, 7.5, and 10 wt.%) was evaluated from 303 K to 383 K. All data were measured along the direction perpendicular to the sintering pressure. The hall carrier concentration n values and carrier mobility values were measured by a Hall system (CH-70, CH-magnetoelectricity Technology Co., Ltd., China). The ZEM-3 was used to measure the S and o of the Bi2Tes film from 303 K to 383 K. The thermal conductivity K values were calculated by the formula K =D*Cp*p (2), where D is the thermal diffusivity and Cpis the specific heat capacity. D was measured by a laser flash method (LaserPIT, Advance Riko., Inc., Japan) along the in-plane direction. The electronic thermal conductivity xewas calculated using Ke=LoT (2), where L is the Lorenz number. For precision, the results were measured at least five times. n-Type Bi2Te3 thermoelectric film[000126] Figs. 5A-C illustrate temperature-dependent a, S, and S2o values for the n-type Bi2Tes thermoelectric films with the differing amounts of Te binder. Both o and the S are gradually enhanced when the amount of binder is increased from 0 to 7.5 wt.%, thereby achieving an S2o of 18.5 pW cm-1K"2at 303 K. A decrease is observed for both the o and the S when the amount of binder is increased to 10 wt.%, possibly due to excess Te, leading to a drop of S2o. To further understand the o, S, and S2o values with different amounts of binder in the film, the x-dependent electron carrier concentration neand the were measured at room temperature (Fig. 5D). When x is increased from 0 to 7.5 wt.%, the Te contentgradually fills the pores in the film, densifying the film, and thereby enhancing thee. When x is increased from 7.5 to 10 wt.%, excessive formation of the Te secondary phase may lead to the carrier scattering, decreasing thee. As x increases from 0 to 7.5 wt.%, the negradually decreases. This is because Te is a typical p-type material, which may release hole carriers to offset the electron carriers in the film. When x is increased from 7.5 to 10 wt.%, the increase in neis mainly derived from the drop ine.[000127] Fig. 5E shows a comparison of the calculated effective mass m* and the deformation potential Edef as a function of x at room temperature. The calculation is based on the single parabolic band (SPB) model. When the amount of binder is increased from 0 to 10 wt.% m* gradually increases and remains constant. This serves as clear evidence for the energy filtering effect, ascribed to the interface between the Bi2Tes matrix and the Te binder. Density-functional theory calculations were performed to confirm this. The results indicate that the bandgap of Te is 0.19 eV, which is approximately two times the bandgap (0.07 eV) of the Ag-doped Bi2Tes matrix. Hence, the significant difference in band structure between the Te and Bi2Tes matrix produces an effective barrier for filtering the low-energy carriers, explaining the increased S. With the increases of x, the Edef gradually decreases and remains constant. Considering that the Te binders fill the poles in the Bi2Tes matrix, Te binders can alter the deformation ability of the overall film structure. In other words, Te binders act as “buffers” to boost the overall plasticity of the film, and prevent cracking during bending. When x increases over 7.5 wt.%, the Edef remains constant since the overall lattice deformation that can be introduced has reached its limit. n-Type Ag2Se thermoelectric filmFigs. 6a-c presents the temperature-dependent S, o, and S2o of Ag2Se / Te films. As x increases from 0 to 5 wt.%, both S and o improve, resulting in an optimized S2o of 25.7 pW cm-1K-2at 303 K. However, when x increases from 7.5 to 10 wt.%, excessive formation of the secondary Te phase within the Ag2Se film leads to a decline in o, counteracting the increase in S and reducing S2o. To further understand the variations in S and a at room temperature, Hall measurements were conducted to evaluate n and p as functions of x (Fig. 6d). As x increases from 0 to 5 wt.%, p gradually improves because Te fills the pores in the Ag2Se film. However, when x increases from 7.5 to 10 wt.%, the excessive Te phase enhances phonon scattering, leading to a reduction in p. n decreases as x increases from 0 to 10 wt.% which, along with the increase in S, can be explained by the calculated bandgap changes based on first-principles DFT calculations: Ag2Se has a bandgap of 0.07 eV, while Te-doped Ag2Se exhibits a slightly larger bandgap of 0.127 eV. The widening bandgap after Te doping makesit more difficult for electrons to transition to the conduction band, resulting in lower n and higher S.To better understand the variations in and n with different Te contents, Fig. 6e compares the effective massand deformation potential (Edef) as functions of x, based on the single parabolic band (SPB) model. As x increases from 0 to 7.5 wt.%, m* gradually rises and then stabilizes. This trend may be attributed to the formation of a secondary T e phase within Ag2Se. The phase boundaries between Te and Ag2Se can induce an energy filtering effect, selectively blocking low-energy carriers, and enhancing the S. First-principles DFT calculations were performed to confirm the presence of this mechanism. The results show that the bandgap of Te is nearly 0 eV, while Ag2Se has a bandgap of 0.07 eV, and Te-doped Ag2Se exhibits an increased bandgap of 0.127 eV. The significant differences in band structures among Te, Ag2Se, and Te-doped Ag2Se create an effective energy barrier, facilitating the energy filtering effect. When x increases from 7.5 to 10 wt.%, m* decreases due to the suppression of n, as the secondary Te phase within Ag2Se becomes saturated. Regarding Edef, it initially decreases with x increase from 0 to 5 wt.% since Te atoms fill gaps within the Ag2Se matrix, altering the material’s overall deformation behavior, consistent with the findings for n-Type Bi2Tes thermoelectric film. However, as x increases from 5 to 7.5 wt.%, Edef gradually rises, indicating reduced lattice deformability. This suggests that Te doping becomes more dominant once the Te content within Ag2Se reaches saturation. Fig. 6f compares the S2o predicted by the SPB model with measured S2o as a function of n, demonstrating that Te doping effectively optimizes n to its optimal range.[000128] To evaluate thermal transport performance of the n-type thermoelectric films with different amounts Te (x = 0, 2.5, 5, 7.5, and 10 wt.%), a photothermal intensity technique (PIT) alternating current (AC) method for in-plane thermal diffusivity D measurements was performed. The K was calculated using the equation K = D x Cp* p, where p is mass density (determined via the specific gravity method) and Cpis the specific heat capacity. n-Type Bi2Te3 thermoelectric film[000129] Fig. 5F shows the K values as a function of x at room temperature. When increasing x from 0 to 7.5 wt.%, K was slightly increased due to the enhancement of densification of the film. When x is increased from 7.5 to 10 wt.%, increasing various lattice imperfections and excessive phase boundaries may decrease K. Fig. 5G shows the comparison of xeand Ki as a function of x at room temperature, where Keis calculated through the formula xe= L o T, and K can be calculated by K = K - xe, and Lorentz number L is determined by the SPB model. When x is 7.5 wt.%, a relatively low K of 0.19 W m-1K"1canbe obtained. When increasing x from 0 to 7.5 wt.%, the Keincreases with x due to the enhancement of o derived from improvede, and Ki decreases with x due to the strong phonon scattering derived from increased phase boundaries and various lattice imperfections. When x increases over 7.5 wt.%, the xeis decreased due to the excessive Te enhancing carrier scattering and suppressing the o, and the K is increased due to excessive Te second phase commencing to contribute to the lattice thermal transport of the entire film. The peak ZT of 1 .3 is obtained at 303 K for the Bi2Tes film with 7.5 wt.% Te (Fig. 5H). This ZT value is outstanding compared to other reported near-room-temperature materials fabricated through screenprinting (77, 24-40). Comparing the measured and predicted ne-dependent ZT, as calculated by the SPB model, leads to the conclusion that the addition of Te contents in the Bi2Tes film has effectively optimized the ne(Fig. 5I). n-Type Ag2Se thermoelectric film[000130] Fig. 6g shows the K values as a function of x at room temperature. As x increases from 0 to 5 wt.%, K rises due to improved densification, as Te fills the pores within the Ag2Se films. However, when x increases beyond 7.5 wt.%, K declines due to enhanced phonon scattering caused by lattice imperfections and excessive phase boundaries. To distinguish the contributions of xeand K\, Kewas calculated using the equation xe= LoT and determined K as K = K - Ke, where the Lorentz number (L) was derived from the SPB model. Fig. 6h illustrates the variation of xeand K with x at room temperature. From 0 to 5 wt.% Te, Keincreases due to a rise in o associated with enhanced film density, while K decreases as stronger phonon scattering occurs from lattice distortions and additional boundaries. Beyond 7.5 wt.% Te, Kedeclines as excessive Te phase formation intensifies phonon scattering and reduces o. Meanwhile, K increases, indicating that the excess Te phase contributes to lattice thermal transport. Fig. 6i shows the ZT values as a function of Te content, with a maximum ZT of 1.06 achieved at 303 K for the Ag2Se film containing 5 wt.% Te. This performance is competitive with previously reported results for screen-printed thermoelectric films.EXAMPLE 7: Flexibility of the n-type thermoelectric films[000131] Flexibility of the n-type thermoelectric films with differing Te contents x (x = 0, 2.5, 5, 7.5, and 10 wt.%) was tested with various bending cycles and bending radius r. Fig. 7A shows a comparison of the measured normalized resistance change A ? / ?oof Bi2Tes films with different Te contents at different bending cycles. Fig. 7B shows the measured A ? / ?o of the n- type Bi2Tes thermoelectric films as a function of bending radius r. All of the n-type Bi2Tes thermoelectric films remained functional after undergoing 1000 bending cycles with a bending radius of 5 mm. The A ? / ?owas only < 3% for a film with a Te binder content of 7.5wt.%. These results highlight the n-type Bi2Tes thermoelectric film’s flexibility and provide an insight of how the properties will degrade after wear.[000132] Fig. 8a shows the normalized resistance change (AP / Po) as a function of bending cycles of the n-type Ag2Se thermoelectric film at a radius (r) of 5 mm. The inset depicts the flexible film with r of 5 mm. After 1 ,000 bending cycles, AP / Po remains below 5% for all samples, demonstrating the excellent flexibility of the as-fabricated Ag2Se films.EXAMPLE 8: Fabrication of a flexible thermoelectric device[000133] Flexible thermoelectric devices (F-TEDs) were designed and fabricated to evaluate the practical application potential for the n-type Bi2Tes thermoelectric material and Ag2Se thermoelectric material. The n-type thermoelectric material prepared in Example 1 or Example 2 in powder form, or a commercially available p-type Bio.4Sb1.eTe3 powder, was printed on to a polyimide (PI) substrate according to Example 4 to prepare n-type elements and p-type elements. The printed device was dried at 200 °C for 0.5 h and annealed via spark plasma sintering (SPS) with 3 MPa at 450 °C for 10 min. n-Type Bi2Te3 thermoelectric film[000134] A novel F-TED was designed in which the n-type elements comprised Bi2Tes with 7.5 wt.% Te and the p-type elements comprised Bio.4Sb1.eTe3 with 5 wt.% Te to validate performance of the device in practice (Fig. 7C). After the n-type elements and p-type elements were prepared, Ag paste was printed on the annealed substrate to form the electrode and then dried at 100 °C for 15 min. Finally, n-type elements and p-type elements were cut by hand and folded to extend from the flexible substrate. P-type Bio.4Sb1.6Te3 films with different Te contents x (x = 0, 2.5, 5, 7.5, and 10 wt.%) were also prepared and evaluated.[000135] The fabricated device comprised n-type and p-type elements extending from the substrate, which can build a higher AT and include more elements per unit area (Fig. 7C). Additionally, the fin-like elements effectively promote heat dissipation and retain a stable AT for a longer period of time. A 1-unit device was prepared with two pairs of n- and p-type elements, from which a device with higher output power (P) can be easily assembled with glue or solder to connect multiple 1-unit devices (Fig. 7C). Fig. 7D shows the comparison of the measured open-circuit voltage V and P of the device with loading current ( / ) at diverse ATs, and Fig. 7E shows comparison of determined w at different ATs. The inset shows an infrared photograph of temperature distribution on the 1-unit device after applying a AT. When a AT of 20 K was applied, an open-circuit voltage Vocof 13.8 mV and a P of 2.9 pW was obtained, achieving a competitive w of 1.2 mW cm-2and an outstanding wnof up to 3 pW cm-2K"2Additionally, the data demonstrated the stability of the device while proving its ability to effectively leverage material performance.[000136] Flexibility of the 1-unit F-TED was also validated. Fig. 7F compares the measured A ? / ?o of a 1-unit device with the different bending cycles as a function of r. A photo of the 1-unit flexible device with a r of 8 mm is shown in the inset. The results show that the performance loss is approximately 5% after 1000 bending cycles with a r of 8 mm, indicating the high reliability and flexibility of the 1-unit device. Fig. 7G shows the measured Vof a 4-unit device worn on a human arm while the subject is sitting and walking as a function of time. The inset shows the infrared photograph of temperature distribution on the 4-unit device worn on the human arm while standing.[000137] The cooling performance of 1-unit device was evaluated to verify the device in practice. Fig. 7H shows an infrared image of a 1-unit device functioning as a cooling device with an input current of 58.6 mA. The device achieved a stable cooling performance with a AT of 7 K. Fig. 7I depicts the change of maximum cooling performance (ATmax) of the 1-unit device by increasing input current. With an input current of 84.2 mA, the ATmax reaches 11.7 K. Notably, there is yet a heat sink connected to the hot side of the device, which will otherwise further give raise to the ATmax. Overall, the observed high performance and good flexibility confirms that the F-TED has practical applications. n-Type Ag2Se thermoelectric film[000138] To evaluate the practical application potential of the Ag2Se thermoelectric film, an F-TED was designed featuring a triangular structure of p-n junctions. The p-type elements were fabricated using commercial Bio.4Sb1.6Te3 powders through screen printing, the same process used to produce the Ag2Se films. To enhance thermoelectric performance of Bio.4Sb1.6Te3, Te was also introduced into the p-type films, with an optimal Te content of 5 wt.%, as determined in the study of n-type Bi2Te3 thermoelectric film. Ag paste was selected for the device electrodes and Cu tape based on the band banding concept (Zavanelli N, Yeo W-H.. ACS Omega 6, 9344-9351 (2021)). Ag was employed as the contact material for the p- type elements (Bio.4Sb1.6Te3). Ag has a work function of 4.3-4.7 eV, which is close to or higher than the Fermi level of p-type Bi-Sb-Te alloys (typically 4.5-4.7 eV), enabling near-ohmic contact for hole injection. Cu was used for the n-type elements (Ag2Se), which has a work function of around 4.5 eV. This aligns well with the lower Fermi level of n-type Ag2Se (typically 4.1-4.3 eV), promoting efficient electron injection and minimizing potential Schottky barriers. Additionally, these materials offer low electrical resistance, ease of use, and costeffectiveness, making them well-suited for flexible thermoelectric applications.[000139] The p-type and n-type thermoelectric elements were prepared according to Example 4. Ten pairs of p-type and n-type elements with dimensions of 1.2 cm x 0.5 cm where then cut from the polyimide substrate using a laser cutter (TROTEC SPEEDY 300). The elements were attached to a polyimide substrate using silver paste and copper tape. The p-type and n-type elements are positioned at a 30° angle relative to the substrate, while the angle at the p-n junction is 120°.[000140] Fig. 8b illustrates the structure and dimensions of the fabricated device. Its design effectively extends the leg length without increasing vertical space, enabling a higher AT in compact environments such as electronics. ANSYS simulations of the temperature distribution were conducted to assess thermal stability. These results demonstrated that the device featuring a triangular structure can establish a larger AT compared to the conventional parallel-structured device when heat is applied to the hot side. Fig. 8c presents the experimental output voltage (V) and power (P) as functions of loading current ( / ) at different ATs. The maximum open-circuit voltage (\Z0C) and P can reach 77.6 mV and 26.2 pW, respectively, at AT of 20 K. Fig. 8d compares the w as a function of AT. The inset shows an infrared image of the temperature distribution across the device under an applied AT. A competitive w of 1.9 mW cm-2is achieved when AT is 20 K and thereby leading to an outstanding wnof 4.8 pW cm-2K"2.[000141] Bending tests were performed to assess mechanical flexibility of the device. Fig. 8e shows the measured AR / Ro as a function of r and bending cycles. The inset displays the device under a rof 8 mm. Even after 1 ,000 cycles at rof 5 mm, AR / Ro remains below 10%, demonstrating excellent flexibility and stability.[000142] Wearability of the device was further examined. When worn on a human arm, the device generated a \Z0Cof 10.88 mV, as shown in Fig. 8f. Additionally, Fig. 8g shows that the device provides a stable \Z0Cwhen worn on a human arm while walking or sitting, confirming its capability to harness body heat for energy generation. The cooling performance of the device was evaluated with and without heatsinks. Fig. 8h presents an infrared image of the temperature distribution when operating as a cooler under an input current of 69.8 mA. A temperature difference of 16.8 K was achieved under this condition. Fig. 8i further plots the ATmax as a function of input current, showing that ATmax reaches 29.8 K at 92.4 mA without external heat sinks. These results confirm that the device is well-suited for various applications.[000143] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in manyother forms, and in particular features of any one of the various described examples may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.

Claims

CLAIMS1. An n-type element for use in a thermoelectric device, wherein the n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the crystalline n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

2. A thermoelectric device comprising a substrate, the substrate comprising one or more n- type elements and one or more p-type elements, each n-type element being in electrical communication with a respective p-type element, wherein each n-type element comprises crystalline n-type thermoelectric particles bound together by a binder, wherein the n-type thermoelectric particles comprise at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge).

3. The n-type element of claim 1 or the thermoelectric device of claim 2, wherein the n-type thermoelectric material is selected from the group consisting of Bi2Tes, (Bi, Sb^Tes, Bi2Se, Ag2Se, Ag2Te, Ag2S, SnSe, PbTe, PbSnS2, CoSb3, SiGe and La3Te4.

4. The n-type element of claim 1 or the thermoelectric device of claim 2, wherein the binder comprises Te, Se, Ag, Bi and combinations thereof.

5. The thermoelectric device of any one of claims 2 to 4, wherein the one or more p-type elements comprise a p-type thermoelectric material selected from the group consisting of Bio.4Sb1.6Te3, Mg2Sb3, CsBi4Tee, TlgBiTes, LaFesCoSb^, AgCuSe, AgCu and AgCuTe.

6. The thermoelectric device of any one of claims 2 to 5, wherein substrate is a flexible substrate.

7. The thermoelectric device of claim 6, wherein the flexible substrate is selected from the group consisting of a polyimide substrate, a polyester substrate, a polyethylene naphthalate substrate, a polyetherimide substate, a fluoropolymer substrate, and copolymers thereof.

8. The thermoelectric device of any one of claims 2 to 7, wherein the one or more n-type elements and one or more p-type elements extend from at least one surface of the substrate.

9. A method for preparing an n-type thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium(Ge), in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material; and(ii) combining the n-type thermoelectric material with 0.05 to 20 wt% of a binder.

10. The method of claim 9, wherein the polar solvent of step (i) is selected from the group consisting of ethylene glycol, glycerol, benzyl alcohol and combinations thereof.

11. The method of claim 9 or 10, wherein the base of step (i) is selected from the group consisting of NaOH, LiOH, KOH, Mg(OH)2, Ca(OH)2and combinations thereof.

12. The method of any one of claims 9 to 11, wherein the solution prepared in step (i) is heated in an autoclave.

13. The method of claim 12, wherein the solution prepared in step (i) is heated from about 210 °C to about 250 °C in an autoclave.

14. A method for preparing an n-type thermoelectric composition, the method comprising the steps of:(i) dissolving a precursor n-type thermoelectric material comprising at least one of bismuth (Bi), tellurium (Te), selenium (Se), silver (Ag), antimony (Sb), or germanium (Ge), in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline n-type thermoelectric material;(ii) dissolving a binder comprising Te, Se, Ag, Bi, or combinations thereof, in a polar solvent, adding a base to the solution and heating the solution under pressure to prepare crystalline binder material;(iii) combining the n-type thermoelectric material with 0.05 to 20 wt% of the binder.

15. An n-type composition prepared by the method of any one of claims 9 or 14.

16. A method for preparing a flexible thermoelectric device comprising the steps of:(a) admixing the n-type thermoelectric composition of claim 15 with a solvent and an ink binder;(b) admixing a p-type thermoelectric composition with a solvent and an ink binder;(c) applying the n-type thermoelectric composition and the p-type thermoelectric composition to a flexible substrate to form one or more n-type elements and one or more p- type elements;(d) annealing the flexible substrate via spark plasma sintering; and(e) applying flexible electrodes so that each n-type element is in electrical communication with a respective p-type element.

17. The method of claim 16, wherein the solvent in step (a) and step (b) is selected from the group consisting of water, terpineol, polyethylene glycol, ethanol, butanol, toluene, ethylbenzene, chlorobenzene, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, cyclohexanol, cyclohexanone, and mixtures thereof.

18. The method of claim 16 or 17, wherein the ink binder in step (a) and step (b) is selected from the group consisting of ethyl cellulose, acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyvinyl chloride (PVC), polyvinylpyrrolidone (PVP) or polystyrene based resins, and combinations thereof and copolymers thereof.

19. The method of any one of claims 16 to 18, wherein the composition in step (a) and / or step (b) further comprises a plasticizer selected from the group consisting of dibutyl phthalate, dimethyl phthalate, diisobutyl phthalate, butyl benzyl phthalate, bis(2-ethylhexyl) phthalate and combinations thereof.

20. The method of any one of claims 16 to 19, wherein the composition in step (a) and / or step (b) further comprises a dispersant.21 . The method of any one of claims 16 to 20, comprising printing the n-type composition and the p-type composition on the flexible substrate.

22. The method of any one of claims 16 to 21 , comprising annealing the flexible substrate via spark plasma sintering at 3 MPa and 450 °C for about 10 minutes.

23. The method of any one of claims 16 to 22, wherein each n-type element and each p-type element is disposed to extend from one surface of the flexible substrate.

24. The method of claim 23 wherein each n-type element and / or each p-type element is disposed to extend at an angle from one surface of the flexible substrate, with a flexible electrode forming an apex between an n-type element and a respective p-type element.

25. A flexible thermoelectric device prepared by the method of any one of claims 16 to 24.

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