GB2554448A - Thermoelectric generators comprising n-doped materials with improved conductivity and seebeck coefficient - Google Patents

Abstract

Thermoelectric element comprising a layer of fullerene and/or fullerene derivative deposited with an n-type dopant. This layer may form the n-type leg 3 of a thermoelectric element. The fullerene based element may be used as the thermoelectric conversion element which is connected to a pair of electrodes. The layer may be deposited from solution, or the fullerene based layer may deposited as a first sub-layer followed by the deposition of a second sub-layer containing the n-type dopant, so as to effect doping of the fullerene based layer. The n-type dopant may be an imidazole derivative. The fullerene derivative may be of a PCBM type. The n-type dopant may constitute between 5 and 50 mol% of the layer.

Description

(71) Applicant(s):

Sumitomo Chemical Company Limited Floor 18, Sumitomo Twin Buildings,

27-1 Shinkawa 2-chome, Chuo-Ku, Tokyo 104-8260, Japan

Cambridge Display Technology Limited

Unit 12 Cardinal Park, Cardinal Way, Godmanchester,

Cambridgeshire, PE29 2XG, United Kingdom (72) Inventor(s):

Thomas Fletcher Graham Anderson (74) Agent and/or Address for Service:

Cambridge Display Technology Ltd IP Department, Unit 12 Cardinal Park, Cardinal Way, Godmanchester, Cambridgeshire, PE29 2XG,

United Kingdom (51) INT CL:

H01L 35/24 (2006.01) (56) Documents Cited:

WO 2014/152570 A2 US 20060181854 A1 Menke et al., 27 October 2014, Determining doping efficiency and mobility from conductivity and Seebeck data of n-doped C60 layers, Arxiv.org, https://arxiv.Org/abs/1410.7119?context=condmat.mtrl-sci.

Journal of Materials Science, Vol. 48,12 May 2012, Barbot et al., N-type doping and thermoelectric properties of co-sublimed cesium-carbonate-doped fullerene, pages 2785-2789.

Applied Physics letter, Vol. 99, 2 September 2011, Sumino et al., Thermoelectric properties of n-type C60 thin films and their application in organic thermovoltaic devices, Pages 93308-93308-3.

AU 2015227519

Polymer Composites, Vol. 34, Issue 10,10 July 3013, Xu et al., Enhancement in thermoelectric properties using a P-type and N-type thin-film device structure, Pages 1728-1734.

(58) Field of Search:

INT CL B82Y, H01L

Other: WPK EPODOC, TXTE and INSPEC (54) Title of the Invention: Thermoelectric generators comprising n-doped materials with improved conductivity and seebeck coefficient

Abstract Title: Fullerene N-doped materials for thermoelectric elements (57) Thermoelectric element comprising a layer of fullerene and/or fullerene derivative deposited with an n-type dopant. This layer may form the n-type leg 3 of a thermoelectric element. The fullerene based element may be used as the thermoelectric conversion element which is connected to a pair of electrodes. The layer may be deposited from solution, or the fullerene based layer may deposited as a first sub-layer followed by the deposition of a second sublayer containing the n-type dopant, so as to effect doping of the fullerene based layer. The n-type dopant may be an imidazole derivative. The fullerene derivative may be of a PCBM type. The n-type dopant may constitute between 5 and 50 mol% of the layer.

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FIG. 2a

FIG. 2b

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Electrical Conductivity (S/cm

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Power Factor PF (pW/mK²}

N-Type Dopant Content

FIG. 5

Intellectual

Property

Office

Application No. GB1616483.2

RTM

Date :27 February 2017

The following terms are registered trade marks and should be read as such wherever they occur in this document:

Sourcemeter (Page 10)

Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

THERMOELECTRIC GENERATORS COMPRISING N-DOPED MATERIALS WITH IMPROVED CONDUCTIVITY AND SEEBECK COEFFICIENT

FIELD OF INVENTION [0001] This invention relates to a thermoelectric elements and modules comprising a layer of fullerene and/or fullerene derivative doped with an n-type dopant. In addition the present invention relates to the use of fullerene and/or fullerene derivative doped with an n-type dopant in the manufacture of a thermoelectric element.

BACKGROUND OF THE INVENTION [0002] Organic thermoelectrics have attracted considerable research interest since they enable realization of flexible, large-area modules which may be manufactured and processed at low costs by using solution processing techniques.

[0003] In general, the fabrication of a thermoelectric module involves the formation of pand n-type semiconducting legs that are usually connected electrically in series and parallel to the heat gradient applied over the generator module.

[0004] In the recent years, many efforts have been made to develop new n-type materials which are both amenable to solution processing and exhibit a favourable thermoelectric performance, i.e. optimized electrical conductivity, Seebeck coefficient (which represents a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across the material), power factor and a low heat conductivity. In thermoelectric organic materials there is a well-known trade-off between doping level (meaning electrical conductivity) and Seebeck coefficient which severely limits the development of organic thermoelectric generators. Nevertheless, many applications require semiconductive legs which exhibit both high conductivity (preferably at least 0.1 S/cm) and a high absolute Seebeck coefficient (preferably at least 100 pV/K) for effective operation.

[0005] Functionalized fullerenes such as PCBM derivatives are a well known class of ntype semiconductor materials. Doping has been considered previously with a variety of dopants (e.g. imidazoles, tetrabutylammonium salts, P3HT), however, mainly focusing on the use in photovoltaics (P3HT) or organic transistors (see e.g. P. Wei et al., Journal of the American Chemical Society 2010, 132(26), 8852-8853; and C. Z. Li, et al., Advanced Materials 2013, 25(32), 4425-4430). In these applications the degree of doping is typically limited to match the device architecture.

[0006] Recently, it has been shown that doping PCBM with P3HT at levels of up to 30% may increase both the electrical conductivity and the Seebeck coefficient of thermoelectric devices (see L. Xu et al., Polymer Composites 2013, 34, 1728-1734). However, this effect is only observed upon device illumination, so that the applications for such material layers are highly limited. Moreover, even under illumination conditions the electrical conductivity of PCBM:P3HT layers is insufficient for effective use.

[0007] In view of the above, there still exists a need to provide n-type materials which exhibit both high conductivity and a favourable Seebeck coefficient and may be easily processed by solution deposition methods in the manufacture of thermoelectric elements.

SUMMARY OF THE INVENTION [0008] The present invention solves these objects with the subject matter of the claims as defined herein. The advantages of the present invention will be further explained in detail in the section below and further advantages will become apparent to the skilled artisan upon consideration of the invention disclosure.

[0009] The present inventors found that a layer comprising a fullerene and/or a fullerene derivative doped with an n-type dopant - which may be processed easily by conventional solution deposition methods - exhibits excellent electrical conductivity and at the same time provides for surprisingly high Seebeck coefficients without requiring illumination techniques and thus enables production of thermoelectric devices with excellent thermoelectric conversion efficiency.

[0010] Generally speaking, the present invention relates to a thermoelectric element comprising a layer comprising a fullerene doped with an n-type dopant and/or a fullerene derivative doped with an n-type dopant.

[0011] In a second aspect, the present invention relates to a thermoelectric module comprising a plurality of said thermoelectric elements.

[0012] In a third aspect, the present invention relates to a method of manufacturing a thermoelectric element comprising a) a step of depositing a layer comprising fullerene doped with an n-type dopant and/or fullerene derivative doped with an n-type dopant from solution; or b) a step of depositing a first sub-layer comprising a fullerene and/or fullerene derivative from solution, followed by depositing a second sub-layer comprising an n-type dopant in contact with the first sub-layer so as to effect n-doping of the fullerene and/or fullerene derivative.

[0013] In a further aspect, the present invention relates to the use of a fullerene and/or fullerene derivative doped with an n-type dopant in the manufacture of a thermoelectric element.

[0014] Preferred embodiments of the formulation according to the present invention and other aspects of the present invention are described in the following description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1a schematically illustrates the general architecture of an exemplary thermoelectric element comprising n-type and p-type legs.

[0016] FIG. 1b illustrates a thermoelectric device comprising a plurality of thermoelectric elements.

[0017] FIG. 2a is a diagram of a first exemplary thermoelectric element comprising a thermoelectric conversion layer.

[0018] FIG. 2b is a diagram of a second exemplary thermoelectric element comprising a thermoelectric conversion layer.

[0019] FIG. 3 is a graph which shows the lateral conductivity of C6o-PCBM:N-DMBI layer in dependence of varying dopant concentrations.

[0020] FIG. 4 depicts the Seebeck coefficient of an thermoelectric element comprising a Cso-PCBM:N-DMBI layer in dependence of varying dopant concentrations.

[0021] FIG. 5 is a graphic representation of the calculated power factor of an thermoelectric element comprising a C6o-PCBM:N-DMBI layer in dependence of varying dopant concentrations.

DETAILED DESCRIPTION OF THE INVENTION [0022] For a more complete understanding of the present invention, reference is now made to the following description of the illustrative embodiments thereof:

Thermoelectric Elements & Modules [0023] In a first embodiment, the present invention relates to a thermoelectric element comprising a layer comprising fullerene doped with an n-type dopant and/or a fullerene derivative doped with an n-type dopant.

[0024] The thermoelectric element may comprise an n-type and a p-type semiconducting leg connected via an electrically conductive layer or a pair of electrodes connected via a thermoelectric conversion layer.

[0025] Most commercially available thermoelectric modules can be found in two different configurations, one being a vertical configuration, which consists of a large number of thermocouples connected in series and sandwiched between a thermally conductive hot plate and cold plate. Thermocouples consist of a p-type leg and an n-type leg connected in series, each of which may be segmented or cascaded, i.e. consist of a series configuration of multiple materials in order to maximize the efficiency. Another type of thermoelectric module is the lateral configuration, which follows the same principle of the vertical type with the main difference that the heat source is located in the right or left side of the module.

[0026] A general schematic representation of an exemplary thermoelectric element having a vertical geometry is shown in FIG. 1a. The depicted thermoelectric element comprises a thermoelectric junction between an n-type material and p-type material having different Seebeck coefficients, which generally take the shape of n-type (3) and ptype (4) legs arranged between electrically-insulating substrates (1a and 1b), optionally with conductor layers/electrical shunt layers (2a, 2b and 2c) or interface material layers (not depicted) provided inbetween. Such junctions particularly enable to generate electric power when they are submitted to a temperature gradient (as that between the upper high temperature side and the lower low temperature side in FIG. 1a, for example), or to generate a temperature gradient when they are crossed by an electric current.

[0027] In a configuration according to FIG. 1a, it is preferred that the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant forms the n-type semiconducting leg of the thermoelectric element.

[0028] The material for the p-type legs used in this configuration is not particularly limited and may be selected from known organic and inorganic p-type semiconducting thermoelectric materials, including for example p-doped conductive polymers,.

[0029] Further examples of thermoelectric element configurations are shown in FIG. 2a and FIG. 2b, wherein the arrows indicate the direction of temperature difference to be imparted during operation of the element. Herein, the elements comprise a first electrode (12a/22a) and a second electrode (12b/22b) that are connected to each other via a thermoelectric conversion layer (13/23). In FIG. 2a, the element has a lateral geometry and both electrodes are arranged on the same substrate layer (11). The materials used for the electrode(s)/conductive layers may be suitably selected by the skilled artisan and typically include conductive metals (e.g. Ag) and metal oxides, while glass or plastics are conventionally used as materials for the substrate layers of lateral devices and ceramics are most commonly used as substrate material in vertical device configurations.

[0030] In the configurations according to FIG. 2a and FIG. 2b, it is preferred that the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant is comprised as a sub-layer of the thermoelectric conversion layer (the sub-layer being preferably in direct contact with the electrode) or forms the thermoelectric conversion layer.

[0031] It is to be understood that FIG. 1a, FIG. 2b, and FIG. 2c serve illustrative purposes only and one or more further layers may be present in the elements, such as additional substrate layers, additional electrically conductive layers and/or encapsulation layers, for example.

[0032] While it is preferred that the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant is a uniform single layer, it may be also composed of a first sub-layer comprising the fullerene and/or fullerene derivative and a second sublayer comprising the n-type dopant in direct contact with the first sub-layer as long as ndoping of the fullerene (or its derivative) is effected. Multiple sub-layers may also be provided as an alternative (i.e. fullerene/dopant/fullerene/dopant, etc.).

[0033] While the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant may comprise additional materials (including additional polymers, conductive particles, antioxidants, light or resistance enhancing agents, plasticizers etc.) beside of the n-doped PCPM-type fullerene derivative, the content of said additional materials is preferably less than 50 wt.-%, more preferably less than 30 wt.-%, further preferably less than 20 wt.-%, in embodiments less than 10 wt.-% or less than 5 wt.-% based on the total weight of the layer and it is particularly preferred that the layer consists of the fullerene doped with the n-type dopant and/or fullerene derivative doped with the ntype dopant, with the exception of residual solvent remaining in the layer after solution processing and evaporation/drying. In another preferred embodiment, the layer comprises the fullerene and/or fullerene derivative and the n-type dopant in a total content of at least 50 wt.-%, more preferably at least 60 wt.-%, still preferably at least 70 wt.-%, for example at least 80 wt.-% based on the total weight of the layer.

[0034] When using conventional organic n-type materials, the increase in charge density induced by doping typically leads to a decrease of the Seebeck coefficient. Advantageously, the materials used in the present invention enable to provide favourable electrical conductivity and simultaneously excellent Seebeck coefficients. In a preferred embodiment the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant comprises the n-type dopant in a content of 2 to 50 mol%, more preferably in a content of 5 mol% to 45 mol%, based on the total content of the functionalized/nonfunctionalized fullerene and n-type dopant. From the viewpoint of an excellent balance of electrical conductivity and Seebeck coefficient, it is preferable that the content of n-type dopant is 7 to 40 mol%, still preferably 10 to 30 mol%. Further preferably, the layer comprises the fullerene and/or fullerene derivative in a content of from 50 mol% to 98 mol%, in embodiments from 55 to 95 mol% or 60 to 90 mol% based on the total molar content of the fullerene and/or fullerene derivative and the n-type dopant.

[0035] The doped fullerene-based compound is preferably a functionalized fullerene, more preferably a PCBM-type fullerene derivative. Such derivatives include [6,6]-phenylcei-butyric acid methyl ester (CeoPCBM), [6,6]-phenyl-C71-butyric acid methyl ester (C70PCBM), [6,6]-phenyl-C85-butyric acid methyl ester (C84PCBM), and mixtures and adducts thereof, for example. Further preferably, the PCBM-type fullerene derivative is [6,6]-phenyl-C61-butyric acid methyl ester (C60PCBM).

[0036] The term “n-type dopant”, as used herein, denotes compounds which intrinsically function as electron donors or reducing agents, i.e. in their ground state. Preferably, the ntype dopant is selected from the group of non-polymeric electron donors and/or reducing agents. As preferred examples thereof in terms of stability under ambient conditions, imidazol derivatives and tetraalkylammonium salts (e.g. tetrabutylammonium fluoride) may be mentioned. Imidazole derivatives are further preferred n-type dopants, and benzoimidazole derivatives are especially preferable in view of en excellent solution processability. In a still preferred embodiment, the imidazole derivative is a compound according to General Formula (1):

(1) [0037] Herein, Ri to R4 and R6 to R10 are independently selected from any of hydrogen, a halogen, a hydroxy group, a C1-C12 alkyl group, a C1-C12 alkoxy group, a C1-C12 haloalkyl group, a substituted or unsubstituted C6-C12 aryl group, or an amino group -NR₂, wherein R is a C1-C12 alkyl group, preferably a C1-C3 alkyl group; and wherein R5, Rn and R12 are independently selected from any of hydrogen or a C1-C6 alkyl group, preferably a C1-C3 alkyl group. Specific examples of compounds falling into the definition of General Formula (1) include DMBI derivatives, such as e.g. (4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazole-2-yl)-phenyl)-dimethyl-amine (N-DMBI), 2-(2,4-dichlorophenyl)-1,3Ί dimethyl-2,3-dihydro-1 H-benzoimidazole (CI-DMBI), 2-(1,3-dimethyl-2,3-dihydro-1 Hbenzoimidazol-2-yl)-phenol (OH-DMBI), and 1,2,3-trimethyl-2-phenyl-2,3-dihydro-1Hbenzoimidazole (TMBI). In an especially preferred embodiment, the n-type dopant is NDMBI.

[0038] The thickness of the n-type layer may be appropriately set by the skilled artisan depending on the purpose and element configuration and generally ranges between 20 nm to 800 pm. When used as a thermoelectric leg, the thickness of the layer is preferably in a range of from 0.5 to 800 pm, more preferably from 1 to 500 pm, and further preferably from 5 to 100 pm.

[0039] The n-type material layer described above provides for both high electrical conductivity and favourable Seebeck coefficient and may be easily processed by solution deposition methods in the manufacture of thermoelectric elements.

[0040] It will be appreciated that the preferred features of the first embodiment specified above may be combined in any combination, except for combinations where at least some of the features are mutually exclusive.

[0041] In a second embodiment, the present invention relates to a thermoelectric module comprising a plurality of the thermoelectric elements described in the first embodiment. [0042] An exemplary representation of an exemplary thermoelectric module is depicted in FIG. 1b, wherein elements in accordance with the first embodiment are connected electrically in series and thermally in parallel.

Method for Manufacturing of Thermoelectric Elements & Modules [0043] In a third embodiment, the present invention relates to a method of manufacturing a thermoelectric element.

[0044] In one embodiment, the method comprises a step of depositing a layer comprising a fullerene doped with an n-type dopant and/or fullerene derivative doped with an n-type dopant from solution.

[0045] Preferably, said method further comprises the steps of: separately dissolving the fullerene and/or fullerene derivative and the n-type dopant in one or more organic solvent(s); combining and mixing the solutions; depositing the mixed solutions and drying the deposited solutions to form the layer of fullerene and/or fullerene derivative doped with an n-type dopant. It is further preferable to combine the solutions in a ratio so that the resulting mixture comprises the n-type dopant and the fullerene and/or fullerene derivative in the contents specified above in connection with the first embodiment.

[0046] In an alternative embodiment, the method may comprise a step of depositing a first sub-layer comprising a fullerene and/or fullerene derivative from solution and a step of depositing a second sub-layer comprising an n-type dopant in contact with the first sublayer so as to effect n-doping of the fullerene and/or fullerene derivative. In this case, the second sub-layer may be deposited from solution or via vapour exposure.

[0047] The solution deposition technique includes but is not limited to coating or printing or microdispensing methods like for example spin coating, spray coating, web printing, brush coating, dip coating, slot-die printing, ink jet printing, letter-press printing, stencil printing, screen printing, doctor blade coating, roller printing, offset lithography printing, flexographic printing, or pad printing. Preferably, the solution deposition method is an inkjet printing, stencil printing, screen printing, dispense printing or drop casting method, more preferably a stencil printing, screen printing, dispense printing or inkjet printing method, which provide for a scalable rout to manufacture. Inkjet printing generally involves the ejection of a fixed quantity of a liquid phase, i.e. ink, in form of droplets from a chamber through a nozzle. The ejected drops are provided onto a substrate to form a pattern. While solidification of the liquid drops may be brought about through chemical changes or crystallization, solvent evaporation is commonly used, in some cases by exposing the deposited wet film to high temperature and/or reduced pressure, preferably immediately upon printing.

[0048] The organic solvents used for dissolving each of the fullerene and/or fullerene derivative and the n-type dopant may be identical or different and are not particularly limited and may be suitably selected by the skilled artisan in view of their compatibility, boiling point and the processing conditions. In addition, a blend of multiple solvents may be used for each of the species to be dissolved.

[0049] As examples thereof, linear or cyclic ketones (e.g. cyclohexanone); aromatic and/or aliphatic ethers (e.g. anisole); aromatic alcohols; optionally substituted thiophenes; benzothiophenes; alkoxylated naphthalene; substituted benzothiazoles; alkyl benzoates; aryl benzoates; chlorinated solvents (e.g. chlorobenzene, trichlorobenzene, dichlorobenzene or chloroform), alkylated aromatic hydrocarbons (e.g. dialkyl- or trialkylsubstituted aromatic hydrocarbons) and mixtures thereof may be mentioned.

[0050] It has been found that the electrical conductivity and Seebeck coefficient of the solution-deposited layers may vary to a certain degree depending on the choice of the organic solvent. Particularly favourable results in this respect have been obtained by using chlorinated solvents (e.g. chlorobenzene or dichlorobenzene, preferably chlorobenzene), dialkyl-substituted aromatic hydrocarbons, trialkyl-substituted aromatic hydrocarbons, benzocycloalkanes and mixtures thereof. From the viewpoint of environment-friendliness, however, dialkyl-substituted aromatic hydrocarbons, trialkylsubstituted aromatic hydrocarbons, benzocycloalkanes and mixtures thereof are still preferable. In embodiments, the solvents are preferably 1,3,5-trialkylbenzenes comprising alkyl groups independently selected from C1-C6 alkyl groups or compounds represented by the following General Formula (2):

wherein R¹ and R² independently represent C1-C6 alkyl groups, which may be connected to each other to form a ring, and wherein R³ represents hydrogen or a C1-C6 alkyl group. [0051] As specific examples of compounds according to general formula (2), 1,2dialkylbenzene (e.g. o-xylene), 1,2,4-trialkylbenzenes (e.g. 1,2,4-trimethylbenzene, 1,2,4triethylbenzene, 1,2-dimethyl-4-ethylbenzene), 1,2,3-trialkylbenzenes (e.g. 1,2,3trimethylbenzene, 1,2,3-triethylbenzene), indane and its alkyl-substituted derivatives, and tetralin and its alkyl-substituted derivatives may be mentioned. In a preferred embodiment the dialkyl- or trialkylsubstituted aromatic hydrocarbon is a trialkylbenzene, more preferably trimethylbenzene, further preferably 1,2,4-trimethylbenzene. A preferred example of 1,3,5-trialkylbenzenes comprising alkyl groups independently selected from C1-C6alkyl groups is 1,3,5-trimethylbenzene (mesitylene).

[0052] In preferred embodiments, the thermoelectric element manufactured by the method of the third embodiment comprises one or more of the features as set our above with respect to the first embodiment.

[0053] Depending on the architecture of the thermoelectric element (see e. g. configurations shown in FIG. 1a, FIG. 2a and FIG. 2b with respect to the first embodiment), the method may comprise steps of depositing further layers, including substrate layers, electrodes, electrically conductive layers, p-type legs/layers etc.. It is understood that the methods for provision of those layers is not particularly limited and includes any methods known to the skilled artisan.

[0054] The method according to the present invention has the advantage that it enables rapid and easy fabrication of highly doped n-type semiconductive layers, which may serve as thermoelectric conversion layers or n-type thermoelectric legs, if formed at relatively high thicknesses, for example by using multiple deposition passes. Thus, flexible, largearea modules may be provided which may be manufactured and processed at low costs by using solution processing techniques, and at the same time exhibit excellent thermoelectric conversion efficiency.

[0055] In a fourth embodiment of the present invention, the method according to the third embodiment may be used to provide a thermoelectric module comprising a multiplicity of thermoelectric elements in accordance with the first embodiment.

[0056] In a fifth embodiment, the present invention relates to the use of fullerene and/or fullerene derivative doped with an n-type dopant in the manufacture of a thermoelectric element, which enables manufacturing of thermoelectric devices with improved thermoelectric power conversion performance.

EXAMPLES

Preparation & Conductivity Testing [0057] Initially, the conductivity (o) of layers manufactured by deposition of the fullerene derivative CeoPCBM doped with N-DMBI has been studied in dependence of doping levels and solvent selection.

[0058] For this purpose, solutions of CeoPCBM and N-DMBI were prepared at a concentration of 10 mg/ml in different organic solvents (i.e. chlorobenzene (CB) and 1,2,4trimethylbenzene (TMB)), by weighing the solids in air and adding the solvent in a nitrogen-filled glove-box. If required, heat was applied to ensure full dissolution. The solutions were then combined in a ratio of volumes to achieve different concentrations of N-DMBI to CeoPCBM in mol% (10 mol%, 20 mol% and 30 mol% specifically) based on the total content of the fullerene derivative and the n-type dopant. For example, for a N-DMBI concentration of 20 mol%, 0.932 ml of CeoPCBM solution was combined and mixed with 0.068 ml of N-DMBI solution, giving a total volume of 1 ml.

[0059] Thereafter, each of the mixed solutions were deposited onto substrates with prepatterned Ag electrodes of silver and dried on a hotplate at a temperature of approximately 60°C to provide elements in accordance with the configuration shown in FIG. 2a. Once dried, the samples were then baked on a hotplate for 1 hour at approximately 150°C. Upon cooling the samples were subjected to conductivity tests. [0060] Initially, the resistance of the samples was measured in a glove-box by connecting the pre-patterned electrodes to a Keithley 2400 Sourcemeter and making 4-point measurements. In particular, the resistance was measured for electrode gaps of 1, 2, 3 and 4 mm in a TLM (Transmission Line Model) type test. Thickness of the samples was measured by scratching a channel in the material and using a surface profiler to measure the step height. The lateral conductivity was then calculated from the regression of resistance measurements and the measured thickness in accordance with the following equation (I):

with R denoting the resistance, L the electrode spacing, T the layer thickness, W the sample width and athe conductivity of the sample.

[0061] The results of the conductivity calculations are shown in FIG. 3 and demonstrate that excellent conductivities of between about 0.7 and 2.4 S/cm have been achieved, with the maximum value being attained by using trimethylbenzene solvent and a doping level of 10 mol%.

Seebeck Coefficient Measurement & Thermoelectic Power Factor [0062] Thereafter, the Seebeck coefficient of each sample was measured by placing the sample on two peltier elements to generate a temperature gradient across the sample. Contacts were made to the pre-patterned electrodes to measure the generated voltage and the resistance across pre-patterned thermistors. This data was logged as the temperature gradient (217) is driven between 0°C and 10°C. The temperature was measured when ΔΤ = 0°C using thermocouples placed on the substrate surface. Using the logged resistance data, a measurement of substrate temperature, and thus ΔΤ, was made, which was referenced to the thermocouple temperature. The generated voltage was then plotted against the temperature gradient to derive the Seebeck coefficient S from the slope according to the following equation (II):

= 5ΔΤ (II) [0063] The Seebeck coefficients determined for each sample are plotted in FIG. 4. While FIG. 4 shows the expected tendency that the increase in charge density by increasing doping levels leads to a decrease of the Seebeck coefficient, it is noted that even at a dopant concentration of 30 mol%, absolute Seebeck coefficients of well above 200 pV/K are observed.

[0064] It is to be noted that these values of conductivity and Seebeck coefficient (in combination) are among the highest known for organic n-type materials. The respective values were combined into a thermoelectric power factor PF as a figure of merit according to equation (III):

BF = -S² (^N|) [0065] A graphic representation of the results is shown in FIG. 5. It is demonstrated that thermoelectric power factors in the range of about 5 to 30 pV/mK² are achieved.

[0066] Once given the above disclosure, many other features, modifications, and improvements will become apparent to the skilled artisan.

REFERENCE NUMERALS

1a: substrate layer (high-temperature side) 1b: substrate layer (low-temperature side) 2a/2b/2c: conductor layers/electrode 3: n-type leg

4: p-type leg

11/21 a/21 b: substrate layer

12a/22a: first electrode

12b/22b: second electrode

13/23: thermoelectric conversion layer

Claims (16)

1. A thermoelectric element comprising a layer comprising a fullerene doped with an ntype dopant and/or fullerene derivative doped with an n-type dopant, the layer being preferably comprised in or forming the n-type semiconducting leg of the thermoelectric element.

2. The thermoelectric element according to claim 1, wherein the layer comprises 5 to 50 mol% of the n-type dopant based on the total content of the of the fullerene and/or fullerene derivative and n-type dopant.

3. The thermoelectric element according to claim 2, wherein the layer comprises the fullerene and/or fullerene derivative in a content of 50 mol% to 98 mol%, and the n-type dopant in a content of 2 to 50 mol%, based on the total content of the fullerene and/or fullerene derivative and the n-type dopant.

4. The thermoelectric element according to any of claims 1 to 3, wherein the content of components other than the fullerene and/or fullerene derivative doped and the n-type dopant in the layer is less than 50 wt.-% relative to the total weight of the layer.

5. The thermoelectric element according to any of claims 1 to 4, wherein the fullerene derivative is a PCBM-type fullerene derivative, preferably [

6,6]-phenyl-C6i-butyric acid methyl ester.

7. The thermoelectric element according to claim 6, wherein the n-type dopant is an imidazole derivative, preferably a benzoimidazole derivative.

8. The thermoelectric element according to claim 7, wherein the imidazole derivative is a compound according to General Formula (1):

(1) wherein Ri to R4 and R6 to Rw are independently selected from any of hydrogen, a halogen, a hydroxy group, a C1-C12 alkyl group, a C1-C12 alkoxy group, a C1-C12 haloalkyl group, a substituted or unsubstituted C6-C12 aryl group, or an amino group -NR2, wherein R is a C1-C12 alkyl group, preferably a C1-C3 alkyl group; and wherein R5, Rn and R12 are independently selected from any of hydrogen or a C1Cs alkyl group, preferably a C1-C3 alkyl group.

9. The thermoelectric element according to claim 8, wherein the n-type dopant is selected from any of N-DMBI, CI-DMBI, OH-DMBI, TMBI or DMBI, the n-type dopant being preferably N-DMBI.

10. The thermoelectric element according to any of claims 1 to 9, wherein the thermoelectric element comprises an n-type and a p-type semiconducting leg connected via a electrically conductive layer and the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant forms the n-type semiconducting leg of the thermoelectric element; or wherein the thermoelectric element comprises a pair of electrodes connected via a thermoelectric conversion layer, the thermoelectric conversion layer comprising the layer comprising the fullerene and/or fullerene derivative doped with the n-type dopant.

11. The thermoelectric element as claimed in any preceding claim in which the fullerene layer is deposited from solution.

12. A thermoelectric module comprising a plurality of thermoelectric elements according to any of claims 1 to 11.

13. A method of manufacturing a thermoelectric element comprising:

a) a step of depositing a layer comprising fullerene doped with an n-type dopant and/or fullerene derivative doped with an n-type dopant from solution; or

b) a step of depositing a first sub-layer comprising a fullerene and/or fullerene derivative from solution, followed by depositing a second sub-layer comprising an n-type dopant in contact with the first sub-layer so as to effect n-doping of the fullerene and/or fullerene derivative.

14. The method according to claim 13, wherein the method further comprises a step of separately dissolving the fullerene and/or fullerene derivative and the n-type dopant in one or more organic solvent(s); and/or wherein step a) comprises the steps of combining and mixing the solutions, preferably so that the resulting mixture comprises 5 to 50 mol% of the n-type dopant based on the molar content of the fullerene and/or fullerene derivative; and depositing the mixed solutions and drying the deposited solutions to form the layer of fullerene and/or fullerene derivative doped with an n-type dopant.

15. The method according to claim 14, wherein the organic solvent is selected from a chlorinated solvent, a dialkyl-substituted aromatic hydrocarbon, a trialkyl-substituted aromatic hydrocarbon, a benzocycloalkane, and mixtures thereof, wherein the dialkyl- or trialkylsubstituted aromatic hydrocarbon is preferably a trimethylbenzene, the chlorinated solvent is preferably chlorobenzene, and the benzocycloalkane is preferably indane.

16. Use of fullerene doped with an n-type dopant and/or a fullerene derivative doped with an n-type dopant in the manufacture of a thermoelectric element.

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