WO2019083951A1 - Thermoelectric devices, methods for making thermoelectric devices, and wearable articles of manufacture that incorporate thermoelectric devices - Google Patents

Abstract

Organic TE devices that are designed and manufactured to efficiently recover energy are provided. Each organic TE device can be made to have a relatively large number of TE legs that are positioned in a close-packed layout and wired according to a space filling curve to achieve a high fill factor, thereby increasing the output voltage of the organic TE device to make it more efficient at energy recovery, even for cases where the temperature difference across the TE active material is relatively small. The output voltage can be further increased by increasing leg length within constraints, which further improves energy recovery efficiency. The close-packed layout of the TE legs and the wiring according to the space filling curve allows the total interconnect length to be kept sufficiently short such that the resistance associated with the interconnect length does not degrade performance. Methods of printing TE devices are also provided.

Description

THERMOELECTRIC DEVICES, METHODS

FOR MAKING THERMOELECTRIC DEVICES, AND WEARABLE ARTICLES OF MANUFACTURE THAT INCORPORATE THERMOELECTRIC DEVICES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a nonprovisionai PCT international application that claims the benefit of and priority to the filing date of U.S. provisional application serial number

62/575,711, filed on October 23, 2017 and entitled "Thermoelectric Interconnects," which is hereby incorporated by reference herein in its entirety.

BACKGROUND

[0002] Thermoelectric generators (TEG) are solid-state devices that directly convert thermal energy into electrical energy. The operating principle for a TEG is based on the Seebeck effect, where a thermoelectric (TE) material generates a voltage under the application of a temperature difference. The magnitude of the generated voltage is proportional to the applied temperature difference and the constant of proportionality is the Seebeck coefficient (S).

[0003] Even for the best and most widely used TE materials based on inorganic semiconductors, the voltage from one TE leg is small (<200 μΥ/Κ). Therefore, to augment the generated voltage, TEGs are often fabricated by connecting many p-type and n-type legs electrically in series and thermally in parallel. TEGs are suitable for energy harvesting and power generation with a conversion efficiency that is proportional to the dimensionless material figure-of-merit (z7) given by zT = S²aT/k, where σ is the electrical conductivity, k isthe thermal conductivity, and Jis the absolute temperature. Due to the inherent correlation between S, σ, and k, obtaining a high zT material is challenging, and most research efforts are focused here.

[0004] TE power generation is generally based on inorganic semiconductors due to their superior TE properties at high temperatures. However, for low grade (< 200 °C) energy recovery, inorganic TE materials are unsuitable because their higher thermal conductivity results in prohibitively high system costs, predominantly due to the heat exchangers. An alternate class of materials for these lower temperature applications is organic TE materials based on electrically-conducting polymers. Organic, polymer, TE materials have several advantages: (/^') they intrinsically have low thermal conductivities, which helps maintain the temperature difference across the device, {if) the material costs are potentially lower as fewer to no heavy or rare-earth elements are used, and {iif) they can be processed from solution, which allows for scalable, cost-effective and high-throughput module fabrication.

[0005] Example applications for low grade energy recovery include self-powered sensors or internet-of-things (IoT) devices powered by ubiquitous thermal sources {e.g., hot water pipes). Furthermore, since polymers can be fabricated as light weight and flexible modules, they can be used in wearable body -heat harvesting devices as well. All of the aforementioned applications require power sources on the order of μ\Υ - mW, which requires well-designed modules to ensure that device resistance does not dominate and decrease the power output. Particularly for wearable devices, thin-film TE modules are suitable power sources. However, with this thin-film limitation, the electrical contact and interconnect resistances can become significant and can lower the overall module performance, necessitating careful consideration of module design.

[0006] While increasing zT for polymer TE materials has been an active area of research, there have been only a few studies on device architectures and system-level optimization for improving module performance for devices that use electrically-conducting polymers. In fact, polymer TEGs are currently fabricated using techniques developed for inorganic semiconductors, which do not leverage the inherent advantages of these materials.

[0007] A need exists for polymer TE devices that efficiently recover energy in cases where the temperature differences across the TE active material are relatively small. A need also exists for cost-effective methods and apparatuses for making such polymer TE devices. A need also exists for polymer TE devices that are suitable for low-grade energy recovery and that can be employed in wearable body-heat harvesting devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.

[0009] Fig. 1 is a schematic diagram of a TE device in accordance with a representative embodiment.

[0010] Fig. 2 is a plot of leg length in millimeters (mm) as a function of output power in milliwatt per square centimeter (mW/cm²) for the TE device shown in Fig. 1.

[0011] Fig. 3 is a plot of output voltage, J⁷, of an organic TE device as a function of the number of TE legs that the organic TE device has in accordance with a representative embodiment.

[0012] Fig. 4 illustrates a substrate having 3600 circular dots printed thereon by a desktop inkjet printer (EPSON XP-860) in an area of 31.2 mm by 36.0 mm, with adjacent dots representing n- type and p-type TE legs.

[0013] Fig. 5 is a top schematic view of an array of rectangular p-type legs and n-type legs for a typical flat-plate Bi₂Te₃ thermoelectric module having a traditional layout.

[0014] Fig. 6 is a side view of a known commercial inorganic TE device having rectangular p- and n-type TE legs connected electrically in series using the traditional serpentine layout architecture and wiring pattern shown in Fig. 5.

[0015] Fig. 7 shows a two-dimensional (2-D) hexagonal close-packed layout of p-type legs and n-type legs having circular, or cylindrical, cross-sections in accordance with a representative embodiment.

[0016] Fig. 8 shows an array of p-type legs and n-type legs having cylindrical cross-sections and arranged in a close-packed layout in accordance with a representative embodiment.

[0017] Fig. 9 shows an array of p-type legs and n-type legs having cylindrical cross-sections and arranged in a hexagonal close-packed layout via printing in accordance with a representative embodiment.

[0018] Figs. 10A, 10B, IOC and 10D show first order (n=l), second order (n=2), third order (n=3) and fourth order (n=4) Hilbert space filling curves, respectively.

[0019] Figs. 11 A - 1 ID show the positioning of n-type and p-type legs and interconnect wiring based on first order through fourth order Hilbert spacing curves, respectively, in accordance with a representative embodiment.

[0020] Figs. 12A - 12D show the positioning of the n-type and p-type legs and interconnect wiring based on first order through fourth order Hilbert spacing curves, respectively, for a hexagonal layout in accordance with a representative embodiment.

[0021] Figs. 13 A - 13D show top plan views of an organic TE module having a hexagonal layout according to a fifth order Hilbert spacing filling curve (i.e., n= 5 for a total of 1,024 TE legs) grouped into one, two, four and sixteen organic TE sub-modules, respectively.

[0022] Fig. 14 shows a four-leg basis cell having two n-type legs and two p-type legs in accordance with a representative embodiment.

[0023] Figs. 15A - 15D show the four possible unique arrangements of the close-packed legs having the four-leg cell basis shown in Fig. 14.

[0024] Fig. 16 shows an organic TE module made up of sixteen of the cells shown in Fig. 14 having a second order Hilbert space filling curve layout in accordance with a representative embodiment.

[0025] Fig. 17 shows a plot of output impedance, R, in ohms, output current, I, in milliamps (mA) and output voltage, V, in volts as functions of the value of of an xN configuration of TE legs that is preselected for an organic TE module having the four-leg cell basis shown in Figs. 14 - 16. [0026] Figs 18 and 19 illustrate close-packed arrangements of two TE modules that have equal FFs and close-packed positioning of p-n leg pairs, but have interconnect patterns based on the Hilbert space filling curve and on the Peano space filling curve, respectively.

[0027] Fig. 20 is a schematic diagram of a textile-based TE device having a substrate that is a commercial-grade polyester knitted fabric (e.g., t-shirt material) with sixteen pairs of p-type and n-type legs arranged thereon in a hexagonal close-packed layout of the type described above with reference to, for example, Fig. 12B.

[0028] Fig. 21 depicts steps of a process for fabricating and assembling a textile-integrated TEG device in accordance with a representative embodiment.

[0029] Fig. 22 is a photo of a prototype of a wearable TEG integrated into knitted fabric using the process shown in Fig. 21 and comprising sixteen p-n leg pairs arranged in a hexagonal close- packed layout and connected according to a Hilbert space filling curve.

[0030] Fig. 23 shows a photograph of a test setup for testing the TEG prototype shown in Fig. 22 and for recording the output voltage from the TEG prototype.

[0031] Fig. 24 is a plot of a curve corresponding to the output voltage Voc values as a function of junction temperature difference (ΔΤ) under natural convection for the recordings obtained by the test setup shown in Fig. 23.

[0032] Fig. 25 is a photograph showing the morphologies of the p- and n-type thermoelectric inks stencil printed on a knitted fabric substrate using the process depicted in Fig. 21.

[0033] Fig. 26 shows an 864-leg TEG device in accordance with a representative embodiment fabricated using the process depicted in Fig. 21.

[0034] Fig. 27 shows the 864-leg TEG device shown in Fig. 26 implemented as a wearable TEG body heat harvesting device, which is shown secured to a torso of a mannequin for illustrative purposes. DETAILED DESCRIPTION

[0035] Representative embodiments described herein are directed to organic TE devices that are designed and manufactured to efficiently recover energy. Each organic TE device can be made to have a relatively large number of TE legs that are positioned in a close-packed layout and wired according to a space filling curve to achieve a high fill factor, thereby increasing the output voltage of the organic TE device to make it more efficient at energy recovery, even for cases where the temperature difference across the TE active material is relatively small. The output voltage can be further increased by increasing leg length within constraints, which further improves energy recovery efficiency. The close-packed layout of the TE legs and the wiring according to the space filling curve allows the total interconnect length to be kept sufficiently short such that the resistance associated with the interconnect length does not degrade performance.

[0036] In accordance with a representative embodiment, the space filling curve is a Hilbert space filling curve. Using the Hilbert space filling curve for the layout wiring enables larger fill factors (FF) to be obtained when positioning the TE legs, which leads to reduced interconnect resistances. In addition, the close-packed layout provides better accommodation of non-uniform temperature distribution across the TE device as well as facile load matching conditions. These latter two attributes are important for improving performance of organic TE modules used for low grade energy harvesting applications, such as in wearable body-heat harvesting devices.

[0037] Moreover, the fractal nature of the space filling curve used for the layout allows a given organic TE device to be subdivided into organic TE sub-modules that have an impedance that is preselected to match an input impedance of a load, thereby obviating the need to use additional devices to impedance match the TE module with the load.

[0038] In the following representative embodiments, it will be shown that by positioning the TE legs based on a hexagonal close-packed layout and wiring them according to the Hilbert space filling curve, larger fill factors (FF), decreased interconnect resistances, better accommodation of non-uniform temperature distribution across the module, and facile load matching conditions are obtained. These features improve performance and make the organic TE devices well suited for low grade energy harvesting applications.

[0039] A few representative embodiments of TE devices and of methods for making TE devices will now be described with reference to Figs. 1 - 27, in which like reference numerals represent like components, elements or features. It should be noted that features, elements or components in the figures are not intended to be drawn to scale, emphasis being placed instead on

demonstrating inventive principles and concepts. It should be noted that the inventive principles and concepts are not limited to the representative embodiments described herein, as will be understood by those of skill in the art in view of the description provided herein.

[0040] In the following detailed description, for purposes of explanation and not limitation, exemplary, or representative, embodiments disclosing specific details are set forth in order to provide a thorough understanding of inventive principles and concepts. However, it will be apparent to one of ordinary skill in the art having the benefit of the present disclosure that other embodiments according to the present teachings that are not explicitly described or shown herein are within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as not to obscure the description of the exemplary embodiments. Such methods and apparatuses are clearly within the scope of the present teachings, as will be understood by those of skill in the art. It should also be understood that the word "example," as used herein, is intended to be non-exclusionary and non-limiting in nature.

[0041] The terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical, scientific, or ordinary meanings of the defined terms as commonly understood and accepted in the relevant context.

[0042] The terms "a," "an" and "the" include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, "a device" includes one device and plural devices. The terms "substantial" or "substantially" mean to within acceptable limits or degrees acceptable to those of skill in the art. For example, the term "substantially parallel to" means that a structure or device may not be made perfectly parallel to some other structure or device due to tolerances or imperfections in the process by which the structures or devices are made. The term "approximately" means to within an acceptable limit or amount to one of ordinary skill in the art. Relative terms, such as "over," "above," "below," "top," "bottom," "upper" and "lower" may be used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as "above" another element, for example, would now be below that element.

[0043] Relative terms may be used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings.

[0044] In accordance with a representative embodiment, organic TE devices or modules are fabricated using one or more known printing techniques, such as, for example, screen printing and inkjet printing. The printing methods described herein may also be used to fabricate inorganic TE devices. The terms "organic TE device" and "organic TE module," as those terms are used herein, denote a device having at least one component that comprises organic matter, namely matter constituted from carbon, oxygen, hydrogen, or nitrogen atoms. The term "organic material," as that term is used herein, denotes material that comprises organic matter, namely matter constituted from carbon, oxygen, hydrogen, or nitrogen atoms.

[0045] In screen printing, the material is directly cast onto a substrate that is covered by a pre- patterned mask. This technique has the potential to be used in continuous roll-to-roll (R2R) fabrication of square-meter organic TE devices. Higher precision fabrication with minimal material loss can be achieved by inkjet printing, which allows for non-contact deposition of small solution droplets (~pL) on a substrate.

[0046] Despite differences in these printing techniques, the printed layer thickness is generally limited to tens of microns. This makes energy harvesting from a through-plane temperature difference challenging, because only a small portion of the imposed temperature difference occurs across the active TE material, and the majority occurs across the substrate. This drastically reduces the generated voltage from the TE device, resulting in low performance for organic prototypes. For this reason, most known proposed designs for organic TEGs have used a lateral (in-plane) temperature gradient. However, as will be described below in more detail, energy harvesting based on a through-plane temperature difference is preferred in accordance with representative embodiments in which TE devices are incorporated into flexible TEGs and wearable devices.

[0047] Given that the voltage output depends on the temperature difference across the TE legs and the number of legs, two approaches can be envisioned to improve the performance of organic TEGs. The first approach is to make the TE legs longer (i.e., the active material thickness should be equal to or larger than substrate thickness), so that a higher portion of the imposed temperature difference is across the active material. The second approach is to increase the number of p-n leg pairs, so that even if the generated voltage per leg is small, many legs can provide the required output voltage.

[0048] While it is known that longer leg lengths in organic TE materials can be fabricated using molding and lithography, these techniques require modifications to the substrate, which consequently increases manufacturing costs. While it has been proposed that longer leg lengths can also be achieved using three-dimensional (3-D) printing, this technology for printing organic TEs has not been fully demonstrated, although inorganic TEs and graphene have recently been 3- D printed. Dispenser printing is another technique than can be utilized to fabricate longer TE leg lengths, but this technique requires large amounts of material, and also requires modifications to the material properties to ensure they are in the correct range for dispense printing. For these reasons, representative embodiments of the fabrication method described herein are directed to using screen printing and inkjet printing techniques to make the organic TE devices. In the following, reference to the terms "printing," "to print" and "printed" refer to inkjet printing or screen printing.

[0049] Existing inkjet printers and screen printers can be used to print the organic TE devices described herein on a substrate. Therefore, the discussion of the method for making the organic TE devices will be limited to using inkjet or screen printing for this purpose. However, it should be noted that although the preferred methods for making the organic TE modules are inkjet printing and screen printing, the organic TE devices described herein having the close-packed layout with interconnect wiring according to a preselected space filling curve are not limited with respect to the method by which they are made, as will be understood by those of skill in the art in view of the description provided herein. Methods other than printing may be used to make the TE devices described herein, as will be understood by those of skill in the art in view of the description provided herein.

[0050] In some embodiments, in order to make longer leg lengths and circumvent losses through the substrate, TE legs are printed on a porous substrate. This allows the TE material to diffuse into the substrate and to make electrical connections from both the top and the bottom surfaces of the substrate. This way, the thickness of the active material, i.e., the leg length, is at least equal to the substrate thickness, which ensures that most, if not all, of the temperature drop occurs across the TE material. It has been shown by the inventors in other literature that there is an optimum leg length that maximizes the power output from a TEG device. For a typical polymer, this is approximately 250 μπι, as will now be described with reference to Fig. 2. This makes paper a candidate substrate for printing low cost organic TE devices.

[0051] To assess the increase in power density that can be obtained using the close-packed layout of p-type and n-type legs disclosed herein, a numerical model has been developed that includes material properties, device geometry and thermoelectric effects for calculating the generated voltage from a TE device, or module, as a function of the number of legs. As an example, properties of dedoped PEDOT:PSS^sl (S = 72 μΥ/Κ, σ = 890 S/cm, and k = 0.33 W/m-K) were used for the p-type material, and properties of a fictitious n-type material with identical properties except for the sign of the Seebeck coefficient were used for the n-type material. Fig. 1 is a schematic diagram of a TE device 1 having the properties mentioned above for the p-type and n-type materials. For fixed hot- and cold-side temperatures of TH= 400 K and Tc = 300 K, a system of coupled non-linear equations that includes the Peltier effect and Joule heating terms can be solved numerically. The leg length is the design variable to optimize device geometry and performance. All input parameters used in this model are listed below in Table 1 and shown in Fig. 1. The obtained results for the optimum leg length are presented in Fig. 2, which is a plot 20 of leg length in millimeters (mm) as a function of output power in milliwatt per square centimeter (mW/cm²). For this example, it was determined, as shown in Fig. 2, that the optimum leg length for the printed organic TE device 1 having the properties described above is between about 200 and about 250 micrometers (microns) for a Kapton (300 MTB) substrate with t2 = 75 microns and = 0.45 W/m-K.

Table 1. Input parameters used for calculating the power density and optimum leg length

Parameter Symbol Value

Seebeck coefficient (p-type) Sp 72 _μν/Κ

Electrical conductivity (p-type) σ_ρ 890 S/cm

Thermal conductivity (p-type) k_p 0.33 W/m-K

Seebeck coefficient (n-type) S_n - 72 _μν/Κ

Electrical conductivity (n-type) On 890 S/cm

Thermal conductivity (n-type) k„ 0.33 W/m-K

Hot side temperature TH 400 K

Cold side temperature Tc 300 K

TE Leg diameter D 20 μηι

TE leg cross section A_c πΏ²/4 Fill Factor FF

Electrical contact resistance Rc lxl 0 ¹⁰ Ω-m²

Thickness of interconnects (copper) tl 25.4 μηι

Thermal conductivity of interconnects (copper) k, 400 W/m-K

Thickness of Kapton film (polyimide) t₂ 75 μηι

Thermal conductivity of Kapton (polyimide) k₂ 0.45 W/m-K

Electrical conductivity of interconnects (copper) Pi 1.7 x 10^"8 Ω-ηι

Overall heat transfer coefficient for the hot side U_H 5000 W/m²-K

Overall heat transfer coefficient for the cold side Uc 5000 W/m²-K

Air thermal conductivity k_a 0.03 W/m-K

[0052] Although the organic TE devices described herein can be fabricated to have longer leg lengths by the methods described herein and by other methods not specifically described herein, due to the lower Seebeck coefficient of organic materials, the output voltage per p-n leg pair will still be lower than that of its inorganic counterpart. In accordance with representative

embodiments described herein, a practical approach for augmenting the output voltage from an organic TE device or module is to increase the number of legs per unit area by using the aforementioned close-packed layout design.

[0053] Fig. 3 is a plot 30 of output voltage, V, of an organic TE device as a function of the number of TE legs that the organic TE device has. The output voltage, J⁷, from the organic TE device is directly proportional to the number of TE legs as: V = N.S_pn.AT, where S_pn is the Seebeck coefficient of one p-n leg pair (S_pn = S_P - Sn), Nis the number of p-n leg pairs, and AT is the temperature difference across the TE legs. For this example, it was assumed that AT = 5 K (representing a human touch for wearable electronics). The properties of dedoped PEDOT:PSS^2: (S = 72 μν/Κ) were used for the p-type material and the properties of a fictitious material with identical properties except for the sign of the Seebeck coefficient were used for the n-type material. As shown in the plot 30, a 2 volt (V) output voltage J⁷ can be produced with 3600 legs by an organic TE device having these properties. [0054] It should be noted that increasing the number of legs of the organic TE device does not necessarily lead to larger device sizes, as printing techniques (such as high resolution inkjet printing) allow for positioning many TE legs in a small area. Fig. 4 illustrates a substrate 40 having 3600 circular dots printed thereon by a desktop inkjet printer (EPSON XP-860) in an area of 31.2 mm by 36.0 mm, with adjacent dots representing n-type and p-type TE legs. The printed array of dots is shown beside a quarter to provide an indication of size, although the inventive principles and concepts are not limited with respect to the number of legs that can be printed in a given area. It should be noted that such high packing densities for TE legs are impossible to achieve using existing fabrication methods that are used to fabricate inorganic TEGs that include dicing and pick-and-place techniques.

[0055] Increasing the number of TE legs has one potential drawback: the total interconnect length can increase due to the p- and n-type materials being electrically wired in series, and the larger number of legs can potentially lead to the total interconnect length being too long. This, along with the thin-film limitation imposed by the through-plane temperature difference configuration, ultimately makes interconnects the dominant resistance in the system. If additional precautions are not taken, this resistance can result in unacceptably low power outputs for organic TE devices. However, as will be described below in more detail, the close-packed layout in combination with basing interconnect wiring on a preselected space filling curve ensures that the resistance associated with the interconnect wiring is not so large that

performance is degraded below acceptable levels.

[0056] Fig. 5 is a top schematic view of an array 50 of rectangular p-type legs 51 and n-type legs 52 for a typical flat-plate Bi₂Te₃ thermoelectric module (not shown). The p-type legs 51 and the n-type legs 52 are positioned in accordance with a traditional layout and wired using a traditional wiring pattern. The legs 51 and 52 are diced to have rectangular cross-sections, positioned in a square lattice using a pick-and-place machine, and wired in series using solder interconnects. Fig. 6 is a side view of a known commercial inorganic TE device 60 (Custom Thermoelectric, part # 1271 1-5L30- 25CQ) having rectangular p- and n-type TE legs connected electrically in series using the traditional layout architecture and wiring pattern shown in Fig. 5. The inorganic TE device 60 has 127 p-n leg pairs (254 TE legs) sandwiched between ceramic plates. Device dimensions are provided in Fig. 6 for FF calculations given below.

[0057] Typical leg lengths for inorganic TE modules of the type shown in Figs. 5 and 6 are 2 to 5 mm, and these legs are sandwiched between ceramic plates that serve as the hot-side and cold- side heat exchangers. The FF of the inorganic TE device is defined as the ratio of the area covered by the active TE material to the hot-side heat exchanger area (this is often the area of the ceramic plates). Based on the dimensions given in Fig. 6, the FF for a flat-plate device can be calculated to be -25%, as will now be described. Using the dimensions provided in Fig. 6, the FF for a commercial Bi₂Te3 module having 254 legs can be calculated as follows:

[0058] Fig. 7 shows a two-dimensional (2-D) hexagonal close-packed layout 70 of p-type legs 71 and n-type legs 72 having circular, or cylindrical, cross-sections in accordance with inventive principles and concepts described herein. The FF for such a layout can be calculated using the parallelogram shown in the Fig. 7. All of the circles (i.e. , TE legs) shown in Fig. 7 have a diameter equal to D. The calculation can be carried out as follows,

[0059] For organic TE devices, printing-based fabrication techniques allow for a broader range of geometries and interconnect patterns. Particularly, using high resolution inkjet printing, legs with different cross-sections such as circular or cylindrical legs with smaller diameters, for example, can be printed much closer together. This results in a higher FF for the organic TE device or module, as shown in Fig. 8, which in turn results in a higher output power density for energy harvesting applications. Fig. 8 is an array 80 of p-type legs 81 and n-type legs 82 having cylindrical cross-sections and arranged in a close-packed layout in accordance with a

representative embodiment. Although Fig. 8 has the same traditional layout shown in Fig. 5, a comparison of the layouts of Figs. 5 and 8 shows that the higher FF of the circular or cylindrical shapes of the legs 81, 82 and the close-packed layout of Fig. 8 obtained via printing enable more legs to be located in a smaller area.

[0060] Fig. 9 shows an array 90 of p-type legs 91 and n-type legs 92 having cylindrical cross- sections and arranged in a hexagonal close-packed layout via printing. The hexagonal close- packed layout helps achieve a maximum theoretical FF of -91% for circular junctions on a two- dimensional plane. Thus, the FF of the hexagonal close-packed layout shown in Fig. 9 is even higher than the FF of the layout shown in Fig. 8.

[0061] In addition to the benefit of having higher FFs that allow TE devices to have large numbers of TE legs, placing the TE legs closer together in the close-packed layout decreases the interconnect length, which in turn lowers the total resistance and results in a higher power output. This is particularly important in cases in which the organic TE device or module includes a relatively large number of legs (>2000 compared to the typical -250 legs for inorganic TE modules).

[0062] To assess the increase in power density that can be obtained using this close-packed layout of TE legs and to obtain the optimum leg length, a numerical model is described herein that includes material properties, device geometry and thermoelectric effects. As an illustration, properties of dedoped PEDOT:PSS²³ (S = 72 μΥίΚ, σ = 890 S/cm, and k = 0.33) are used for the p-type material, and properties of a fictitious n-type material with identical properties except for the sign of the Seebeck coefficient are used for the n-type material. In accordance with this representative embodiment, flexible modules suitable for use in wearable energy harvesting applications were modeled by using Kapton films instead of ceramic plates used in traditional flat-plate devices. For fixed hot-side and cold- side temperatures of TH = 400 K and Tc = 300 K (typical for low grade heat recovery), a system of coupled non-linear equations that includes the Peltier effect and Joule heating terms can be solved numerically. The leg length is a design variable that optimizes device geometry and performance. All assumptions and input parameters used in this model are listed above in Table 1.

[0063] Based on this example, it was observed that power density (normalized to the heat exchanger area) increases with leg length, reaches a maximum value called the optimum leg length, and then decreases. This is expected as the thermal and electrical resistances dominate at large leg lengths, thereby reducing the heat through the device and consequently, the power output. As was mentioned previously, the optimum leg length is found to be around 250 μπι, and a power density of over 100 mW/cm² is obtained at this optimized leg length. This is significantly higher than existing organic TEG prototypes, which indicates that printing close- packed devices in accordance with embodiments described herein is beneficial for achieving high performance. It should be noted here that this is an upper limit on performance since the FF that is practically achievable is dictated, at least in part, by the printing method.

[0064] It has been shown herein that by using printing techniques, organic TE legs can be positioned closer to each other and in a hexagonal layout to increase power density. With printing, new print patterns can be printed for interconnects between the legs. A group of patterns that can potentially be utilized as interconnects are fractal geometries with self-similarity characteristics that are referred to as space filling curves. Mathematically, space filling curves are defined as mapping functions from points on a unit interval of [0, 1] to the entire two- dimensional unit square. Hilbert space filling curves and Peano space filling curves are two examples of well-known space filling curves. To demonstrate inventive principles and concepts, the layout of the TE legs and the interconnect wiring are described herein as being based on Hilbert space filling curves of various orders, as they not only allow for extremely high FF, but also allow for good surface conformation (which is beneficial for wearable devices) and self- localization (which is beneficial for maintaining a uniform temperature across all the TE legs). It should be noted, however, that the inventive principles and concepts are not limited to using any particular space filling curve for this purpose.

[0065] A Hilbert space filling curve of order n describes how 4" points are connected. Figs. 10A, 10B, IOC and 10D show first order (n=l), second order (n=2), third order (n=3) and fourth order (n=4) Hilbert space filling curves, respectively. Figs. 11 A - 1 ID show the positioning of the n-type and p-type legs 111 and 112, respectively, and interconnect wiring 113 based on first order through fourth order Hilbert spacing curves, respectively. Figs. 12A - 12D show the positioning of the n-type and p-type legs and interconnect wiring based on first order through fourth order Hilbert spacing curves, respectively, for a hexagonal layout.

[0066] To fabricate an organic TE device or module, legs having cylindrical cross-sections can be positioned in place of the points shown in Figs. 10A - 10D to obtain the layouts shown in Figs. 11 A - 1 ID, respectively, and wired together based on the Hilbert pattern for the corresponding order. It can be seen from a comparison of Figs. 11 A - 1 ID that the FF increases and the diameter of the legs decreases as the order n of the Hilbert space filling curve increases. Moreover, a hexagonal layout for positioning TE legs can also accompany the Hilbert curve interconnects, thus allowing for much higher FFs, as shown in Figs. 12A - 12D. A hexagonal layout, as that term and variations of that term are used herein, denotes a layout where the legs are placed at the center and vertices of a hexagon, in contrast to the conventional rectangular layout where legs are placed at the vertices of a rectangle. A hexagonal layout may also be thought of as one where the legs are placed at the vertices of a rhombus, where three identical rhomboids sharing two sides form a hexagon, in contrast to the conventional rectangular layout where legs are placed at the vertices of a rectangle.

[0067] One major benefit of using space filling curves for wiring interconnects in organic TE devices is that they allow the organic TE device to be divided into fractal geometries, or sub- modules. Although in theory all space filling curves allow for dividing into sub-modules, the geometry of the resulting sub-modules differ and are dependent on the chosen space filling curve. For instance, following the Hilbert pattern, a TE device or module can be divided into M groups of identical parts, or sub-modules, connected electrically in parallel with one another, with each part, or sub-module, having N legs that are electrically connected in series. Thus, each organic TE module has a total number of xN TE legs, as discussed below in more detail.

[0068] Figs. 13A - 13D show top plan views of an organic TE module 130 having a hexagonal layout according to a fifth order Hilbert spacing filling curve (i.e., n= 5 for a total of 1,024 TE legs) grouped into one, two, four and sixteen organic TE sub-modules, respectively. The fractal nature of Hilbert space filling curve allows for sub-dividing an organic TE module into identical sub-modules that can be readily connected together along lines of symmetry in either series, parallel, or a combination thereof. A total of 1024 TE legs can be connected in eleven different configurations. Four examples of these configurations include: one module 130 with all legs connected in series, as shown in Fig. 13 A; two parallel sub-modules 132, each having 512 TE legs connected in series, as shown in Fig. 13B; four parallel sub-modules 133, each having 256 TE legs connected in series, as shown in Fig. 13C; and sixteen parallel sub-modules 148, each having 64 TE legs in series.

[0069] The organic TE module 130 containing 1024 legs wired together according to a fifth order Hilbert space filling curve can be divided into eleven combinations of M and N (denoted by (M,N)), as (1,1024), (2,512), (4,256), (8, 128), (16,64), (32,32), (64, 16), (128,8), (256,4), (512,2), and (1024, 1). In order to tessellate an organic TE module, into sub-modules, an initial organic TE module of a preselected Hilbert space filling curve order, n, such as module 130 of order n=5 shown in Fig. 13 A, can be printed such that all TE legs are connected in series (i.e., a (1, MxN) combination), and then, based on the end application, it can be divided into a desired number of sub-modules connected in parallel. This opens up avenues to tailor the module such that it is electrically impedance matched or load matched to the application. Electrical impedance matching is essential to obtaining maximum power from the organic TE module. Thus, the ability to divide an initial organic TE module after printing into two or more sub-modules having a preselected impedance is an important advantage of the printable organic TE device or module disclosed herein. One key benefit of this is that organic TE devices are no longer reliant on power conditioning circuits (e.g., DC-DC converters or boost converters) to match the impedance of the organic TE device to the application, i.e., to the circuitry that the organic TE device will electrically power.

[0070] In accordance with a representative embodiment, the wiring rules and interconnect patterns for close-packed configurations described above with reference to Figs. 10A - 13D are extended by grouping adjacent TE legs into cells on a four-leg cell basis such that each cell has two p-type legs and two n-type legs rather than the traditional two-leg cell (one p-type leg and one n-type leg) that is widely used for known TE devices. Using this four-leg cell basis, there are four possible unique arrangements of these close-packed legs, resulting in four types of cells. Fig. 14 shows one of the four-leg basis cells 140 having two n-type legs 151 and two p-type legs 152. Figs. 15A - 15D show the four possible unique arrangements of these close-packed legs 151, 152, resulting in four different types of cells 140a - 140d. For this representative embodiment, the total number of legs per cell is four, but in other embodiments the total number of legs per cell is any positive integer greater than or equal to two. The primary benefit of the four-leg basis is the facile nature of programming a printer for printing TE devices of arbitrary geometries.

[0071] In accordance with a representative embodiment, the total number of cells 140 in an organic TE module is restricted to being a power of four, which results in there being a total number, 4", of legs in an organic TE module, where n is the order of the Hilbert space filling curve. This restriction allows adjacent cells 140 rather than individual legs 151, 152 to be electrically interconnected based on the Hilbert space filling curve. Fig. 16 shows an organic TE module 160 made up of sixteen of the cells 140 shown in Fig. 14 having a second order Hilbert space filling curve layout 161. From a macroscopic perspective, the placement of the legs 151, 152 appears more random than the placement in the traditional serpentine alternating p-n placement used widely (Fig. 5), but it does contain deeper order. For the Hilbert space filling curve 161 of order «=2, there are a total of sixteen cells 140 and a total of 64 TE legs 151, 152. The legs 151, 152 of each respective cell 140 are connected in series and the sixteen cells 140 are connected in parallel to one another. It should be noted that the cells 140 can be, but are not necessarily, of the same type. For this example, two adjacent cells 140 in Fig. 16 are shown to be of types 140a (Fig. 15A) and 140b (Fig. 15B).

[0072] For the four-leg cell basis shown in Figs. 14 - 16, the voltage output from an organic TE module having this configuration is V=N(2S_pn) T. For a larger number of legs, the organic TE module can be tessellated into N cells connected in series and groups connected in parallel for load matching, as discussed previously. Fig. 17 shows a plot 170 of output impedance, R, in ohms, output current, I, in milliamps (mA) and output voltage, V, in volts as functions of the value of of the xN configuration that is preselected for the organic TE module having the four-leg cell basis shown in Figs. 14 - 16. The bars 171, 172 and 173 in the plot 170 represent output impedance, R, output voltage, V, and output current, I, respectively, for values of M ranging from 1 to 11 and N=1024. The plot 170 will be used to demonstrate an example of the manner in which the configuration of the organic TE module can be preselected to achieve a preselected output impedance that is electrically impedance matched to a particular application to achieve the requisite load matching.

[0073] For exemplary purposes, it will be assumed that a load for a particular application requires that the organic TE module have an output impedance, R, of about 6 kilo-Ohms (kQ). This corresponds to the bar 171 that intersects the dashed line 174 in Fig. 17. It can be seen on the horizontal axis of plot 170 that this intersection corresponds to a value for M of 2. An organic TE module having 1024 TE legs is printed and wired with the corresponding

interconnections. For exemplary purposes, it is assumed that the 1024 legs are interconnected based on a hexagonal layout and a fifth order Hilbert space filing curve, i.e., n=5, which corresponds to the layout and interconnections shown in Fig. 13 A. During the initial printing, the four-leg cell basis can be used such that the 1024 legs are electrically connected in series. After printing the legs and the interconnects to make all of the electrical connections, the organic TE module is sub-divided, or cut-in-half, to obtain the 2x512 configuration shown in Fig. 13B, with the two sub-modules being electrically connected in parallel to one another.

[0074] A similar design process as that described above for a preselected value of R could be used to design an organic TE module using a preselected value for I and/or V. In other words, if the output current I and/or output voltage V that the organic TE module needs to have for a particular application is known in advance, it can be used to select the corresponding value for from the plot 170. The corresponding organic TE module may then be printed and wired as described above, and if necessary, sub-divided into sub-modules. A lookup table (LUT) may be used to obtain the value of M based on a known value for R, I and/or V.

[0075] An advantage of using Hilbert space filling curve patterns over Peano space filling curve patterns or other space-filling curve patterns (e.g., serpentine curves) arises from wearable and flexible applications where the applied temperature difference may not be uniformly distributed across all of the TE legs (i.e., all legs may not be thermally in parallel). This can lead to a lower device performance as some legs can be inactive while others are active. In this regard, the sub- module geometries that are obtained from Hilbert space filling curves result in better utilization of heat because the mapping is localized, i.e., Hilbert space filling curves ensure that the closely- spaced points in one dimension (1-D) stay closer together in 2-D as well (i.e., they preserve the locality of points). Figs 18 and 19 illustrates this concept by graphically comparing two TE modules 180 and 190, respectively, that have equal FFs and close-packed positioning of p-n leg pairs, but have different interconnect patterns - one based on the Hilbert space filling curve in accordance with representative embodiments described herein and the other based on the Peano space filling curve.

[0076] The number of TE legs that can be connected using a Hilbert space filling curve of order n and a Peano space filling curve of order m are 4" and 3^m+1, respectively. Therefore, to have a similar number of legs for the modules, a Hilbert space filling curve of order 5 is compared to a Peano curve of order 5, which have 1024 and 729 legs, respectively. Following the cutting lines in Figs. 18 and 19, four sub-modules (each having 256 legs) are obtained using the Hilbert space filling curve and three sub-modules (each having 243 legs) are obtained using the Peano space filling curve. Although the number of legs in the Hilbert sub-module is larger, its geometry provides a better localization for the connected legs by keeping them spatially closer together. This helps to ensure that all the legs in the sub-module remain thermally in parallel, which is beneficial for device performance.

[0077] It should be noted that while the use of Hilbert space filling curves for making interconnections between legs will provide better performance than using Peano or other types of space filling curves for this purpose, Peano and other types of space filling curves may be used in various embodiments. It should also be noted that although the discussions above are mainly directed toward printable organic TE modules, the hexagonal close packing and interconnect wiring based on a space filling curve extend to printable inorganic TE modules as well.

[0078] Having described various inventive principles and concepts, an organic knitted textile- integrated TEG prototype that embodies at least some of the inventive principles and concepts and that is capable of through-plane body heat harvesting under natural convection will now be described.

[0079] Miniature devices that are capable of scavenging energy from the human body have demonstrated potential in recent years as a green alternative to rechargeable lithium ion batteries. In this context, low grade heat harvesting TEGs have attracted the attention of both academic researchers and industrial developers for powering wearable electronics. Compared to other energy harvesting technologies, TE devices offer various advantages, including no moving parts during operation and minimum maintenance requirements. Textiles in particular offer various advantages for body heat harvesting applications, such as easy integration into clothing for better conformity to skin and lightweight structures that do not restrict mobility. Despite the aforementioned advantages, research efforts towards textile-integrated thermoelectric materials for body heat harvesting has proven challenging. Given that the skin-ambient temperature gradient is in the through-plane direction, a through-plane architecture is desirable for wearable TEs. However, fabrics are thin (~ 0.5 mm) and therefore limit the thickness of p- and n-type legs. As indicated above, small leg lengths adversely affect the generated voltage and overall power output as the skin-ambient temperature difference is small (5 - 10 °C). As a result, inorganic printable pastes which have a higher Seebeck coefficient to offset the small temperature difference have been used as the active thermoelectric material in textile-based TEGs. However, these inorganic pastes require a post-processing annealing step at elevated temperatures (>150 °C), which is beyond the glass transition temperature, T_g, of commercial- grade textiles. This has limited most applications to industrial-grade woven glass or mesh fabrics, rendering these devices generally unsuitable for next-to-skin applications.

[0080] An alternate class of materials for wearable TEGs are organic or polymer-based TE materials. Due to their inherently low thermal conductivity, these materials can overcome the small thickness limitation. Polymers can be processed from solution, which enables cost- effective and scalable fabrication of flexible TEGs. Currently, the development of organic TE devices on textiles has focused on immersion coating of textiles in ink dispersions. This process requires a further step, which involves cutting the coated textile into strips to mount onto the substrate, and results in an in-plane TEG structure that is unsuitable for wearable applications. Furthermore, most of the devices demonstrated consist of only p-type legs that are connected with silver ink. This is mainly due to the poor performance of available n-type organic TE materials, where research efforts are lagging behind p-type polymers. Thus, the low power factor of conducting polymers, particularly n-type materials, has restricted their wider application in wearable devices. [0081] From a device design standpoint, and given the limited area for devices mounted on the body, organic TEGs require a larger number of p- and n-type leg pairs per unit area for obtaining power outputs of μ\Υ - mW and maintaining structural flexibility. As indicated above, however, the large number of p-n pairs can result in an increased interconnect resistance unless higher fill factors (>25%) are achieved. Embodiments described herein allow the higher FF to be achieved so that a larger number of n-type and p-type leg pairs can be achieved without increasing the interconnect resistance to an unacceptable level. Thus, the methods described herein allow a high FF to be achieved in a cost-effective, scalable, and fully integrated process to realize a functioning textile-integrated TEG device for powering wearables.

[0082] In previous work by the inventors, properties of n-type poly Na(NiETT)] were optimized by identifying the reaction conditions and processing steps that yield power factors over 10 μ\ν/πι-Κ². These films of NiETT/PVDF maintain their stability in air, thereby making them suitable for textile-integrated devices. In accordance with embodiments described herein, these material enhancements are combined with the close-packed layout design described above to fabricate a prototype of a flexible, organic, textile-integrated TEG using stencil and transfer printing techniques. Using these fabrication techniques, it is demonstrated below that a textile- integrated TEG based on p- and n-type organic conducting polymers capable of through-plane body heat harvesting can be produced with a very large number of TE legs.

[0083] In accordance with a representative embodiment, a commercial-grade polyester knitted fabric (e.g., t-shirt material) is used as the substrate for fabricating the textile-integrated TEG. Knitted fabrics are typically used for next-to-skin and active sports garments. This is

advantageous for close contact of the TEG with the human body, thus maximizing the temperature difference and minimizing thermal losses. As indicated above, for body heat harvesting, the TEG device should be designed to harvest a temperature difference in the through-plane direction (i.e., through the fabric). In accordance with this representative embodiment, an optional burn-out process was used to create holes in the fabric that act as cavities for depositing the p- and n-type material using a stencil printing process. The leg length in this case is thereby limited to the thickness of the selected fabric (e.g., about 0.5 mm). At this thickness limit, and given that the output voltage is proportional to both the temperature difference and number of legs, increasing the number of legs increases the voltage output per unit area. This was accomplished by positioning the TE legs in a hexagonally close-packed layout and decreasing the cross-sectional area of a single leg and the pitch between legs. In order to maintain the structural integrity and strength of the knitted fabric without compromising on flexibility and process scalability, the device was designed with an FF = 30%, which is comparable to conventional designs. However, an FF of -90% could be achieved with a different knit.

[0084] Fig. 20 is a schematic diagram of a textile-based TE device 200 having a substrate 201 that is a commercial-grade polyester knitted fabric (e.g., t-shirt material) with sixteen pairs of p- type and n-type legs 202 arranged thereon in a hexagonal close-packed layout of the type described above with reference to, for example, Fig. 12B. Each of the p-type and n-type legs 202 has a length / and diameter d. For this representative embodiment, 7 = 0.5 mm and d = 2.2 mm. Preliminary experiments showed that a leg diameter of about 2.2 mm was suitable considering the fabrication process and the rheological behavior of the TE materials used. For this embodiment, the sixteen pairs of p-type and n-type circular legs 202 were arranged in an area that is about 14 mm x about 29 mm in width and length, respectively. The legs 202 were wired according to the Hilbert interconnect pattern described above with reference to, for example, Fig. 12B (i.e., a second order Hilbert space filling curve).

[0085] For this embodiment, all-organic p-type and n-type thermoelectric inks were formulated for making the p-type and n-type legs, respectively. For the p-type material, PEDOT:PSS (CLEVIOS PH1000, Heraeus Group) with 5 vol.% DMSO was used. In order to tune the viscosity and minimize ink spreading when deposited on the fabric substrate 202, dry pellets of PEDOT:PSS (Sigma- Aldrich) were added to the dispersion (8 wt.%), followed by mixing for 15 minutes in a micro-vibration mill. For the n-type material, Poly Na(NiETT)] was synthesized following known procedures and the ink viscosity was optimized by micro-ball milling the ETT in a 100 mg/mL PVDF/DMSO solution at a weight ratio of 4: 1 (ETT:PVDF). This resulted in a viscosity range of - 30 Pa-s for both inks, which minimizes spreading when printed onto the fabric substrate 202. Since the dispersions were specifically tailored for stencil printing, a four- point probe test set-up was used to simultaneously measure the thermoelectric properties by depositing thin films (~10 μπι thickness) of these inks on 1 cm x 1 cm pre-cleaned glass substrates. The n-type samples were dried on a hot plate at 70 °C for 15 minutes and were then annealed at 130 °C for 30 minutes, as it has been shown that annealing enhances the TE properties of NiETT/PVDF films. The same drying procedure was followed for the p-type films, with the exception of the annealing step as it had no effect on the thermoelectric properties of PEDOT:PSS.

[0086] Fig. 21 depicts steps of the process for fabricating and assembling a textile-integrated TEG device in accordance with a representative embodiment. In step 1, the process of burning out holes 211 through the knitted fabric substrate 201 is performed. Step 2 comprises the process of stencil printing p-type 202a and n-type 202b ink on both sides of the fabric substrate 201 to fill the holes 211 is performed. Step 3 comprises the process of stencil printing silver interconnects (light blue) 212 on a heat transfer membrane (light gray) 213a and 213b for both sides of the device. Step 4 comprises the process of transfer printing the interconnects 212 by heat pressing the heat transfer membrane 213 on both sides of the substrate 201. Fig. 22 is a photo of a wearable TEG integrated into knitted fabric 220 using the process shown in Fig. 21 and comprising sixteen p-n leg pairs arranged in a hexagonal close-packed layout and connected according to the Hilbert curve.

[0087] To perform step 2, clear sticky paper stencils were prepared using an electronic cutter. Separate stencils were prepared for the p-type and n-type ink deposition to achieve the hexagonal close-packed layout of the legs discussed earlier with an interconnect pattern following a second order Hilbert space filling curve. First, the n-type ink 202b was deposited on the substrate 201 and dried on a hot plate at 70° C for 15 minutes. This step was repeated on the other side of the substrate 201 to ensure ink penetration across the fabric substrate 201 through the holes 211. Once dried, the samples were annealed at 130° C for 30 minutes. Next, the p-type ink 202a was deposited following the same process, with the exception of the annealing step. In step 3, stretchable silicone base silver ink was stencil printed on the two clear heat transfer membranes 213 following the Hilbert interconnect pattern, and these were cured for 5 minutes at 120 °C before heat pressing them onto opposite sides of the fabric substrate 201. It is important to note that while these devices were produced manually, the techniques described herein can easily be scaled up and automated for industrial-scale production.

[0088] The TE properties of the p- and n-type thin films stencil printed on glass are listed below in Table 2. It should be noted that these values are expected to vary from the inks printed on the fabric substrate 201 (as shown in Table 1). As such, these are preliminary measurements to guide device design and make performance predictions using a numerical model described below.

Table 2. Thermoelectric properties of the p-type and n-type polymer thermoelectric materials (thin films on lass) at room temperature.

[0089] The values measured fall within the range of properties reported for both these polymers.

Next, a numerical model developed previously by the inventors was employed to predict the performance of a 16-1 eg pair TE device for body heat harvesting under stationary conditions.

The model inputs include applied temperature differences, thin film material properties and device geometry (see Table 2 above and Table 3 below), and the model outputs are voltage and power. Under these conditions, the device open circuit voltage, Voc was 2.9 - 3.8 mV, taking into account the TE property variations measured for the thin films. The corresponding power at matched load conditions (load resistance, RL = internal device resistance = 11 Ω) ranges from ~ 0.19 - 0.33 μ\Υ for this 16-leg pair TE device using the thin film TE properties.

Table 3. Input parameters to the numerical model used for calculating TEG voltage and

Parameter Value

Junction Temperature Difference (AT) 3 K

TE Leg diameter (mm) 2.2

TE Leg length (mm) 0 5

Fill Factor (%) 30

Electrical contact resistance (Ω-m²) lxlO^"10

Electrical resistivity of silver interconnects 1.6 x 10^"1

(Ω-m)

[0090] The 16-leg pair TEG prototype on the fabric substrate 201 described above with reference to Figs. 20 - 22 was then experimentally tested. Fig. 23 shows a photograph of the test setup, which included a hot plate with temperature control as the heat source and a multimeter for recording the output voltage from the TEG prototype. The hot plate surface is covered with a 1 mm thick silicone layer in order to maintain a uniform temperature and enhance the contact with the device during testing. The ambient temperature was maintained at 23 °C throughout. Fig. 24 is a curve 240 corresponding to the device Voc values as a function of junction

temperature difference (ΔΤ) under natural convection. The slope of this curve 240 gives a module Seebeck coefficient of 1.07 ± 0.03 mV K^"1, which agrees with the thin film p- and n-type TE properties reported in Table 2. This confirms that the Seebeck coefficient remains unchanged after printing onto the fabric substrate 201. At AT = 3 K, which corresponds to body heat harvesting, Voc reached ~ 2.4 mV. The corresponding power under matched load conditions (RL = 5 kQ, based on measured device resistance) was ~ 0.3 nW. Multiple device prototypes were fabricated and fresh samples were tested three times to confirm the reported results.

[0091] The large discrepancy between the model prediction and the experimental power output can be attributed to the large internal resistance of the fabricated device; the measured resistance is 5 kQ, which is significantly higher than 11 Ω predicted by the model. The device resistance comprises the p-n leg resistances, the interconnect resistance and the electrical contact resistance. The numerical model assumes negligible contact and interconnect resistances, and p- and n-type leg resistances were calculated based on values for thin films on glass, which was found to be very different from the TE material-fabric composite. A separate measurement was performed to quantify the total interconnect resistance, and it was found to account for only 0.1% of the total measured device resistance. Therefore, the higher device electrical resistance can be attributed to a combination of p-n leg resistances and contact resistance. The former is due to the lower electrical conductivity of the polymer ink after infiltrating the fabric (due to its insulating nature and different packing of particles within the holes) compared to thin films on glass. The contact resistance arises from a mismatch in chemical properties and imperfect contact geometry of the interconnects and the TE materials.

[0092] The silver ink-TE polymer chemical compatibility was first explored through testing several flexible silver inks. Among these, it was found that the silicone base silver ink selected for use in the prototype gave the best performance over time compared with the other silver inks which gave prohibitively high resistances (~ΜΩ range) over a period of a few days. With the selected silver ink, the device resistance remained consistent over a few days but an increase in device resistance (up to 20 kQ) was noticed due to repeated flexing over weeks. However, it is worth noting that the selected silver ink maintained a low resistance (~ 0.3 Ω per interconnect) after printing on the heat transfer membranes 213a, 213b and with repeated flexing prior to fusing on the fabric substrate 201. This emphasizes that the higher device resistance reported is not from the silver interconnects, and is instead arising due to poor contact with the polymer ink upon flexing.

[0093] This is also supported by analyzing the p-n leg morphology. Fig. 25 is a photograph showing the morphologies of the p- and n-type thermoelectric inks stencil printed on the knitted fabric substrate 201. After drying and annealing, concave surfaces and cracks form resulting in uneven leg surface that manifests as a large electrical contact resistance. The p-type ink forms a concave surface after drying, while the n-type ink has a tendency to shrink and form cracks. These uneven leg topographies could be the origin of the higher contact resistance due to the compromised contact area between interconnects and the leg surfaces. Further optimization of organic thermoelectric ink formulations could enhance the printing quality and overall device performance.

[0094] Despite the large device resistance, the 16-leg pair prototype demonstrates voltage and power outputs comparable to previously reported printed inorganic textile-based TEGs. This is also a notable improvement to polymer based TEGs, and demonstrates a wearable, organic, textile-integrated, TEG that is capable of through-plane body heat harvesting.

[0095] While further improvements can be made, the above discussion demonstrates an advanced level of integration of thermoelectric material into textile fabrics. In order to demonstrate the scalability of the fabrication process described above with reference to Fig. 21, an 864-leg TEG device was fabricated across a 25 x 25 cm area designed as "GT," which is the Georgia Institute of Technology logo. Fig. 26 shows the 864-leg TEG device 260 and demonstrates the high printing definition of the p- and n-type TE inks 202a, 202b of the 864-leg hexagonal close-packed layout through the fabric substrate 201 as well as the interconnect pattern (Hilbert curve) 212 employing the same device geometries and assembly process used for the 16-leg pair prototype. Fig. 27 shows the 864-leg TEG device 260 implemented as a wearable TEG body heat harvesting device, which is shown secured to a torso of a mannequin for illustrative purposes. This scaled-up prototype yielded a voltage, Voc = 47 mV representing body heat harvesting.

[0096] As discussed earlier, the limiting factor in the developed devices is the high internal electrical resistance and low power factor of the organic TE material. A device-level enhancement is possible through using thicker fabric substrates to support larger temperature differences. Knitted fabric structures are currently available in a wide range of thicknesses (up to 1 mm) and are still practical as clothing material. Further studies and optimization of the organic thermoelectric ink formulations on fabric and compatibility with the silver ink used for interconnects may lead to reductions in the contact resistance and improvements in material properties. It is believed based on numeric modeling that the 864-leg prototype described above would be capable of harvesting body heat to give a maximum power of 9 μ\Υ {Voc = 104 mV, RL = 298 Ω), assuming bulk electrical conductivity of films on glass (reported in Table2) was achieved and contact resistance was negligible. If state-of-the-art inorganic thermoelectric materials with higher zT were optimized for printing and made flexible in accordance with inventive principles and concepts described herein, then it is predicted that the inorganic TEG device will be capable of generating -300 μ\Υ, which is three times higher than the minimum 100 μ\Υ target for powering wearable electronics.

VARIOUS ASPECTS ACCORDING TO INVENTIVE PRINCIPLES AND CONCEPTS

[0097] In accordance with one aspect, an organic TE device is provided that comprises a substrate comprising at least a first surface and a second surface and an array of cells of TE legs comprising organic material and being disposed in the substrate in a preselected close-packed arrangement substantially in parallel with one another and substantially perpendicular with the first and second surfaces of the substrate. Each cell has at least one n-type TE leg and one p-type TE leg. The p-type and n-type TE legs of each cell are electrically connected in series by electrically-conductive interconnects. The cells are electrically connected in parallel to one another by electrically-conductive interconnects. The interconnects are arranged in a pattern of a preselected space filling curve.

[0098] In accordance with another aspect,_the preselected close-packed arrangement is a hexagonal close-packed arrangement.

[0099] In accordance with one or more of the above aspects, the preselected space filling curve is a Hilbert space filling curve.

[00100] In accordance with one or more of the above aspects, the Hilbert Space filling curve has an order, n, that is greater than or equal to one.

[00101] In accordance with one or more of the above aspects, the order n of the Hilbert Space filling curve is greater than or equal to two.

[00102] In accordance with one or more of the above aspects, each cell has at least two n-type TE legs and two p-type TE legs that are electrically connected in series by electrically- conductive interconnects.

[00103] In accordance with one or more of the above aspects, the n-type TE legs and p-type TE legs have a preselected length that ranges from 100 nanometers to 1 centimeter.

[00104] In accordance with one or more of the above aspects, the n-type TE legs and p-type TE legs have a preselected length that ranges from 1 micrometer to 1 millimeter.

[00105] In accordance with one or more of the above aspects,_the array of cells comprises at least 2 pairs of n-type and p-type legs.

[00106] In accordance with one or more of the above aspects, the array of cells comprises at least 256 pairs of n-type and p-type legs.

[00107] In accordance with one or more of the above aspects, the substrate comprises a fabric.

[00108] In accordance with one or more of the above aspects, the substrate a fabric comprising woven textile fibers.

[00109] In accordance with one or more of the above aspects, each TE leg is substantially cylindrical, or circular, in shape. [00110] In accordance with one or more of the above aspects, each p-type TE leg and each n- type TE leg comprises p-type ink and n-type ink, respectively.

[00111] In accordance with one or more of the above aspects, each interconnect comprises electrically-conducting ink.

[00112] In accordance with one or more of the above aspects, the organic TE device is an organic TEG comprising a wearable body heat harvesting device that can be worn on the body of a living being for harvesting body heat generated by the living being.

[00113] In accordance with one or more of the above aspects, the array has a total of MxN TE legs, where M and N are positive integers, where MxN = 4ⁿ, where n is an order of the preselected space filling curve and is greater than or equal to one.

[00114] In accordance with the aspect immediately above, the array has a total of MxN TE legs, where M and N are positive integers, where MxN = 4ⁿ, where n is an order of the preselected space filling curve and is greater than or equal to one, and the array is sub-dividable into M sub- modules, each of the sub-modules having N TE legs. The TE legs of each of the M sub-modules are electrically connected in series with the TE legs of the respective sub-module by

interconnects and the M sub-modules are electrically connected in parallel to one another by interconnects.

[00115] In accordance with the aspect immediately above, the value of M is preselected to provide the organic TE device with a preselected output impedance, voltage or current.

[00116] In accordance with one or more of the above aspects, the substrate has holes formed therein that extend in between the first and second surfaces of the substrate, and each TE leg has a first end that is adjacent or in contact with the first surface and has a second end that is adjacent or in contact with the second surface.

[00117] In accordance with one or more of the above aspects, the preselected space filling curve is a Peano space filling curve.

[00118] In accordance with another aspect, a method of making a TE device is provided comprising:

providing a substrate comprising at least a first surface and a second surface;

printing an array of TE legs on the substrate in a preselected close-packed arrangement, the array comprising cells of TE legs, each cell having at least one n-type TE leg and one p-type TE leg;

electrically connecting the p-type and n-type TE legs of each cell in series with

electrically-conductive interconnects; and

electrically connecting the cells in parallel with one another with electrically-conductive interconnects.

[00119] In accordance with another aspect, during the steps of electrically connecting the p-type and n-type TE legs of each cell in series and electrically connecting the cells in parallel with one another, the interconnects are arranged in a pattern of a preselected space filling curve.

[00120] In accordance with one or more of the above aspects relating to the method, the step of printing the array of TE legs on the substrate comprises stencil printing p-type ink and n-type ink on the substrate to form the p-type and n-type TE legs, respectively.

[00121] In accordance with one or more of the above aspects relating to the method, the substrate is a knitted fabric substrate and the step of printing the array of TE legs on the substrate comprises stencil printing p-type ink and n-type ink on the knitted fabric substrate to form the p- type and n-type TE legs, respectively.

[00122] In accordance with one or more of the above aspects relating to the method, the substrate is a knitted fabric substrate and the step of printing the array of organic TE legs on the substrate comprises:

forming holes in the knitted fabric substrate; and

stencil printing p-type ink and n-type ink on the first and second sides of the knitted fabric substrate to form the p-type and n-type TE legs, respectively, to fill the holes.

[00123] In accordance with one or more of the above aspects relating to the method, the steps of electrically connecting the p-type and n-type TE legs of each cell in series and electrically connecting the cells in parallel with one another comprise:

stencil printing first electrically-conducting interconnects on a first heat transfer membrane;

stencil printing second electrically-conducting interconnects on a second heat transfer membrane; and

transfer printing the first and second electrically-conducting interconnects on the first and second surfaces of the substrate by heat pressing the first and second heat transfer membranes on the first and second sides of the substrate, respectively.

[00124] It should be noted that the inventive principles and concepts have been described with reference to representative embodiments, but that the inventive principles and concepts are not limited to the representative embodiments described herein. Although the inventive principles and concepts have been illustrated and described in detail in the drawings and in the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure, and the appended claims.

Claims

CLAIMS What is claimed is:

1. An organic thermoelectric (TE) device comprising:

a substrate comprising at least a first surface and a second surface;

an array of cells of TE legs comprising organic material and being disposed in the substrate in a preselected close-packed arrangement substantially in parallel with one another and substantially perpendicular with the first and second surfaces of the substrate, each cell having at least one n-type TE leg and one p-type TE leg, the p-type and n-type TE legs of each cell being electrically connected in series by electrically-conductive interconnects, the cells being electrically connected in parallel to one another by electrically-conductive interconnects, the interconnects being arranged in a pattern of a preselected space filling curve.

2. The organic TE device of claim 1, wherein the preselected close-packed arrangement is a hexagonal close-packed arrangement.

3. The organic TE device of one or more of claims 1 - 2, wherein the preselected space filling curve is a Hilbert space filling curve.

4. The organic TE device of one or more of claims 1 - 3, wherein the Hilbert Space filling curve has an order, n, that is greater than or equal to one.

5. The organic TE device of one or more of claims 1 - 4, wherein the order n of the Hilbert Space filling curve is greater than or equal to two.

6. The organic TE device of one or more of claims 1 - 5, wherein each cell has at least two n- type TE legs and two p-type TE legs that are electrically connected in series by electrically- conductive interconnects.

7. The organic TE device of one or more of claims 1 - 6, wherein the n-type TE legs and p-type TE legs have a preselected length that ranges from 100 nanometers to 1 centimeter.

8. The organic TE device of one or more of claims 1 - 7, wherein the n-type TE legs and p-type TE legs have a preselected length that ranges from 1 micrometer to 1 millimeter.

9. The organic TE device of one or more of claims 1 - 8, wherein the array of cells comprises at least 2 pairs of n-type and p-type legs.

10. The organic TE device of one or more of claims 1 - 9, wherein the array of cells comprises at least 256 pairs of n-type and p-type legs.

11. The organic TE device of one or more of claims 1 - 10, wherein the substrate comprises a fabric.

12. The organic TE device of one or more of claims 1 - 11, wherein the fabric comprises woven textile fibers.

13. The organic TE device of one or more of claims 1 - 12, wherein each TE leg is substantially cylindrical, or circular, in shape.

14. The organic TE device of one or more of claims 1 - 13, wherein each p-type TE leg and each n-type TE leg comprises p-type ink and n-type ink, respectively.

15. The organic TE device of one or more of claims 1 - 12, wherein each interconnect comprises electrically-conducting ink.

16. The organic TE device of one or more of claims 1 - 15, wherein the organic TE device is an organic TE generator (TEG), the organic TEG comprising a wearable body heat harvesting device that can be worn on the body of a living being for harvesting body heat generated by the living being.

17. The organic TE device of one or more of claims 1 - 16, wherein the array has a total of MxN TE legs, where M and N are positive integers, where MxN = 4ⁿ, where n is an order of the preselected space filling curve and is greater than or equal to one.

18. The organic TE device of one or more of claims 1 - 17, wherein the array is sub-dividable into M sub-modules, each of the sub-modules having N TE legs, and wherein the TE legs of each of the M sub-module are electrically connected in series with the TE legs of the respective sub- module by interconnects and wherein the M sub-modules are electrically connected in parallel to one another by interconnects.

19. The organic TE device of claim 18, wherein the value of M is preselected to provide the organic TE device with a preselected output impedance.

20. The organic TE device of claim 18, wherein the value of M is preselected to provide the organic TE device with a preselected output voltage.

21. The organic TE device of claim 18, wherein the value of M is preselected to provide the organic TE device with a preselected output current.

22. The organic TE device of one or more of claims 1 - 21, wherein the substrate has holes formed therein that extend in between the first and second surfaces of the substrate, and wherein each TE leg has a first end that is adjacent or in contact with the first surface and has a second end that is adjacent or in contact with the second surface.

23. The organic TE device of claim 1, wherein the preselected space filling curve is a Peano space filling curve.

24. A method of making a thermoelectric (TE) device comprising:

providing a substrate comprising at least a first surface and a second surface;

printing an array of TE legs on the substrate in a preselected close-packed arrangement, the array comprising cells of TE legs, each cell having at least one n-type TE leg and one p-type TE leg;

electrically connecting the p-type and n-type TE legs of each cell in series with

electrically-conductive interconnects; and

electrically connecting the cells in parallel with one another with electrically-conductive interconnects.

25. The method of claim 24, wherein the TE device is an organic TE device, and wherein the TE legs comprise organic material.

26. The method of one or more of claims 24 - 25, wherein during the steps of electrically connecting the p-type and n-type TE legs of each cell in series and electrically connecting the cells in parallel with one another, the interconnects are arranged in a pattern of a preselected space filling curve.

27. The method of one or more of claims 24 - 26, wherein the step of printing the array of TE legs on the substrate comprises stencil printing p-type ink and n-type ink on the substrate to form the p-type and n-type TE legs, respectively.

28. The method of one or more of claims 24 - 26, wherein the substrate is a knitted fabric substrate and wherein the step of printing the array of TE legs on the substrate comprises stencil printing p-type ink and n-type ink on the knitted fabric substrate to form the p-type and n-type TE legs, respectively.

29. The method of one or more of claims 24 - 26, wherein the substrate is a knitted fabric substrate and wherein the step of printing the array of organic TE legs on the substrate comprises:

forming holes in the knitted fabric substrate; and

stencil printing p-type ink and n-type ink on the first and second sides of the knitted fabric substrate to form the p-type and n-type TE legs, respectively, to fill the holes.

30. The method of any of claims 24 - 29, wherein the steps of electrically connecting the p-type and n-type TE legs of each cell in series and electrically connecting the cells in parallel with one another comprise:

stencil printing first electrically-conducting interconnects on a first heat transfer membrane;

stencil printing second electrically-conducting interconnects on a second heat transfer membrane; and

transfer printing the first and second electrically-conducting interconnects on the first and second surfaces of the substrate by heat pressing the first and second heat transfer membranes on the first and second sides of the substrate, respectively.

31. The method of one or more of claims 24 - 30, wherein the preselected close-packed arrangement is a hexagonal close-packed arrangement.

32. The method of one or more of claims 26 - 31, wherein the preselected space filling curve is a Hilbert space filling curve.

33. The method of one or more of claims 26 - 32, wherein the Hilbert Space filling curve has an order, n, that is greater than or equal to one.

34. The method of one or more of claims 26 - 33, wherein the order n of the Hilbert Space filling curve is greater than or equal to two.

35. The method of one or more of claims 25 - 34, wherein the TE device is an organic TE generator (TEG) and wherein the substrate is a knitted fabric, the organic TEG comprising a wearable body heat harvesting device that can be worn on the body of a living being for harvesting body heat generated by the living being.

36. The method of claim 24, wherein the TE device is an inorganic TE device.