Technical Field

[0001] The present disclosure relates to a method of producing a monograin membrane and a monograin membrane produced by said method.

Background

[0002] A monograin membrane is a membrane comprising grains in a single (monograin) layer and connected by a thin layer of binder or filler.

[0003] For electrical uses, the monograin membranes are formed of semiconductor grains connected by an electrically insulating binder or filler. The semiconductor grains are electrically exposed from the membrane binder/filler and may be connected by an electrode layer or any function layers on one or both sides of the membrane.

[0004] Such monograin membranes may be used in a variety of technologies. In one example, a monograin membrane comprising a layer of photosensitive semiconductor grains embedded in a non-conductive binder layer which holds the grains in place may be formed for use in a solar cell. The photosensitive semiconductor grains may be, for example, an n-type conductive material optionally further including a p-type conductive material

[0005] The use of monograin membranes can provide advantages over thicker membranes, for example improved efficiency and reduced weight.

[0006] A variety of methods of making conductive monograin membranes have been reported. The reported methods to provide a monograin membrane in which the grain surfaces arc exposed on both sides of the membrane can be generally classified as etching, or grinding and polishing methods. [0007] Among those methods, etching is the most commonly used for monograin membrane production. The etching processes typically includes sprinkling the grains onto a temporary adhesive layer supported by a substrate. Then, a binder is poured over the grains to fill space between grains as well as covering the top surface of the grain. Once the binder hardens, etching is undertaken to remove the upper layer of binder and expose the top surface of the grains. The etching may be a chemical etching or a physical etching or combination of both. The etching necessarily damages the surface of the grain from which the binder is being removed.

[0008] In the grinding and polishing process, the binder is layered on top of a first adhesive layer prior to adding the grains onto the binder surface. The grains are left to settle and embed into the binder whereby the lower surface of the grains are coated in the binder. A second adhesive layer and a substrate is then applied to cover the exposed portions of the grains. The reverse side of the membrane is subsequently ground away to expose the surface of the grains on this side. The ground surface may further be polished to smoothen the exposed grains. As with the etching method, the grinding and polishing method necessarily damages the surface of the grain from which the binder is being removed.

[0009] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims.

Summary

[0010] According to one aspect, the present disclosure provides a method of producing a monograin membrane comprising a plurality of grains connected by a binder layer such that surfaces of the grains are electrically exposed on both sides of the membrane, the method comprising: providing a membrane support structure; depositing a layer of a negative photoresist onto the membrane support structure; forming a monolayer of the grains on an upper surface of the negative photoresist layer. embedding the grains into the negative photoresist layer. exposing the upper surface of the negative photoresist layer to radiation to cross-link exposed photoresist thereby to form the binder layer, wherein the embedded grains act as a photomask preventing exposure of portions of the negative photoresist to said radiation; removing the membrane support structure and unexposed negative photoresist.

[0011] According to another aspect, there is provided a monograin membrane produced by a method disclosed herein.

[0012] According to yet another aspect, there is provided use of a monograin membrane produced by a method disclosed herein in a solar cell, a lithium-ion battery, electric-to-radiation conversion, or a radiation detector.

Definitions

[0013] With regards to the definitions provided herein, unless staled otherwise, or implicit from context, the defined terms and phrases include the provided meanings. Unless explicitly stated otherwise, or apparent from context, the terms and phrases below do not exclude the meaning that the term or phrase has acquired by a person skilled in the relevant art. The definitions are provided to aid in describing particular embodiments, and are not intended to limit the claimed invention, because the scope of the invention is limited only by the claims. Furthermore, unless otherwise required by context, singular tenns shall include pluralities and plural tenns shall include the singular.

[0014] Throughout the disclosure, reference is made to the membrane having a monograin thickness, or to the membrane being a monograin membrane. It will be understood that monograin thickness or monograin membrane refers to the membrane as having a thickness defined by the thickness of the single layer of grains forming the membrane.

[0015] Throughout this disclosure, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e., one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms “a", “an" and “the" include plural aspects unless the context clearly dictates otherwise. For example, reference to “a" includes a single as well as two or more; reference to “an" includes a single as well as two or more; reference to “the" includes a single as well as two or more and so forth.

[0016] The term “and/or”, e.g., “X and/or Y" shall be understood to mean either “X and Y" or “X or Y" and shall be taken to provide explicit support for both meanings or for either meaning.

[0017] Unless otherwise indicated, the terms “first" “second" etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these tenns refer. Moreover, reference to a “second" item does not require or preclude the existence of lower-numbered item (e.g., a “first” item) and/or a higher-numbered item (e.g., a “third" item).

[0018] As used herein, the phrase “at least one of", when used with a list of items, means different combinations of one or more of the listed items may be used and only one of the items in the list may be needed. The item may be a particular object, thing, or category. In other words, “at least one of" means any combination of items or number of items may be used from the list, but not all of the items in the list may be required. For example, “at least one of item A, item B, and item C” may mean item A; item A and item B; item B; item A, item B, and item C; or item B and item C. In some cases, “at least one of item A, item B, and item C” may mean, for example and without limitation, two of item A, one of item B, and ten of item C; four of item B and seven of item C; or some other suitable combination.

[0019] As used herein, the term “about”, unless stated to the contrary, typically refers to +/- 10%, for example +/- 5%, of the designated value.

[0020] Throughout the present specification, various aspects and components of the invention can be presented in a range format. The range format is included for convenience and should not be interpreted as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range, unless specifically indicated. For example, description of a range such as from 1 to 5 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 5, from 3 to 5 etc., as well as individual and partial numbers within the recited range, for example, 1, 2, 3, 4, 5, 5.5 and 6, unless where integers are required or implicit from context. This applies regardless of the breadth of the disclosed range. Where specific values are required, these will be indicated in the specification.

[0021] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The phrase “consisting of" means the enumerated elements and no others. Brief Description of Drawings

[0022] Embodiments of the present disclosure are further described and illustrated as follows, by way of example only, with reference to the accompanying drawings in which:

[0023] Figure 1 shows a flowchart of an embodiment of a method for producing an embodiment of a monograin membrane; and

[0024] figures 2A-2D show cross-sectional side views of various stages of the preparation of a monograin membrane in accordance with an embodiment of the present disclosure.

Detailed Description

[0025] In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, an embodiment of the present disclosure. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure.

[0026] With reference to Figures 1-2D, the present disclosure provides a method 100 (Figure 1) for producing a monograin membrane 10 comprising a plurality of grains 12 connected by a binder layer 14 such that surfaces 16 of the grains 12 are electrically exposed on both sides 18, 20 of the membrane 10 (figures 2A-2D).

[0027] Monograin membranes 10 of the present disclosure may be used, for example, in Si-tandem solar cells (ix. cells comprising layers of membranes on Si-bottom cell, wherein the different layers absorb different wavelengths of light). The monograin membrane 10 may be used for the topmost layers to capture and convert the visible light, and allow the infrared portion to pass through to the lower layer(s). In some embodiments, the method 100 produces a membrane 10 that is thinner than achievable with conventional manufacturing processes such as etching or grinding and polishing.

[0028] Although the method 100 will be described below primarily with respect to forming monograin membranes 10 for use in a solar cell (not shown), it will be appreciated that monograin membranes 10 produced in accordance with methods described herein could also be used in other fields and technologies. For example, monograin membranes formed of solid conductor lithium crystals in a polymer binder may be prepared for use in solid-state lithium-ion batteries. Additional non-limiting examples of uses for monograin membranes produced in accordance with the disclosed methods may also include use in electric-to-radiation energy conversions or in radiation detecton.

[0029] The type and size of the grains 12 is not particularly limited and will vary depending on the intended final use of the monograin membrane 10.

[0030] It is noted that in the accompanying figures, the grains 12 are shown as uniform in size and substantially spherical. However, it will be appreciated that the shape and size of the grains 12 forming the monograin membrane 10 may be varied. For example, the grains 12 forming the monograin membrane 10 may be substantially homogenous of similar shape and diameter. Alternatively, the grains 12 forming the monograin membrane 10 may vary in shape and/or diameter.

[0031] The shape of the grains 12 may be a regular, well-defined shape (e.g. spherical) or irregular.

[0032] The size of the grains 12 forming the monograin membrane 10 may be defined by an average diameter D of the grains 12.

[0033] In some embodiments, it may be desirable to have grains 12 of maximum and minimum diameters within a certain range of the average diameter D of the grains 12. For example, too large a variation in diameter D size may lead to a less effective membrane 10, for example due to the surfaces 16 of a portion of the grains 12 not being electrically exposed on both sides 18, 20 of the membrane 10, or difficulties in producing the membrane 10.

[0034] In an example, the minimum and maximum diameters of the grains 12 forming the monograin membrane 10 are within ±50% of the average grain diameter D, within ±45% of the average grain diameter D, ±40% of the average grain diameter D, within ±35% of the average grain diameter D, ±30% of the average grain diameter D, within ±25% of the average grain diameter D, ±20% of the average grain diameter D, within ±15% of the average grain diameter D, ±10% of the average grain diameter D, or less.

[0035] The average grain diameter D will also vary depending on the grain type and intended use of the monograin membrane 10. In an embodiment, the average grain diameter D is 1 micron, 5 microns, 10 microns, 15 microns, 20 microns, 25 microns, 30 microns, 35 microns, 40 microns, 45 microns, 50 microns, 60 microns, 70 microns, 80 microns, 90 microns, 100 microns, 110 microns, 120 microns, 130 microns, 140 microns, 150 microns, 160 microns, 170 microns, 180 microns, 190 microns or 200 microns. The average grain diameter D may be in a range from one of the described lower values to any one of the upper values.

[0036] The grains 12 may comprise an n-type material. In some embodiments, the grains 12 may comprise a cadmium compound, for example cadmium telluride (CdTe), cadmium sulphide (CdS) or cadmium selenide (CdSe). In other embodiments, the grains may comprise a lithium compound, for example lithium aluminium titanium phosphate (LATP), lithium lanthanum titanate (LLT), lithium aluminium germanium phosphate (LAGP), or lithium aluminium titanium tantalum phosphate (LATTP).

[0037] The method 100 comprises providing 102 a membrane support structure 22. The membrane support structure 22 comprises a temporary substrate 24 that provides mechanical support for the production of the membrane 10. The temporary substrate 24 may be any suitable material for providing support during production, for example the temporary substrate 24 may be formed of metal, glass or plastic. [0038] In accordance with the method 100 disclosed herein, a layer 26 of a negative photoresist 27 is deposited 104 onto the membrane support structure 22. Depositing 104 the layer 26 of negative photoresist 27 onto the membrane support structure 22 may be achieved by any suitable method, for example depositing 104 the layer 26 of negative photoresist 27 may be achieved by slot-die coating or spin coating.

[0039] In an embodiment, a thickness <12 of the negative photoresist layer 26 is not more than half of the average diameter D of the grains 12. In this way, the monograins 12 of the monograin membrane 10 are roughly half exposed.

[0040] The thickness d2 of the negative photoresist layer 26 may be 5 microns, 10 microns, 15 microns, 20 microns, 25 microns, 30 microns, 35 microns, 40 microns, 45 microns, 50 microns, 60 microns, 70 microns, 80 microns, 90 microns, or 100 microns. The thickness d2 of the negative photoresist layer 26 may be in a range from one of the described lower values to any one of the upper values.

[0041] In an advantageous embodiment, the membrane support structure comprises a layer of thermoplastic material 28 on the temporary substrate 24. In this way, the thermoplastic material provides a deformable layer onto which the negative photoresist layer can then be applied. As will be discussed further below, the use of a combination of thermoplastic and negative photoresist layers in the method of producing a monograin membrane can allow for control of the thickness of the binder layer of the formed membrane, as well as the degree exposure of the grains.

[0042] The thermoplastic material is not particularly limited and may be any suitable material for supporting the negative photoresist layer 26 during the production of the monograin membrane 10. In an embodiment, the thermoplastic material may be selected to have a softening temperature close to the pre-baking temperature of negative photoresist. In this way, minimal heating is required in order to soften and deform the thermoplastic material. [0043] The thennoplastic material may have a softening temperature of from 20°C to 250°C. For example, the thennoplastic material may have a softening temperature of less than 200°C, less than 150°C, less than 100°C, less than 90°C, less than 80°C, less than 70°C, less than 60°C, less than 50°C, less than 40°C, or less than 30°C.

[0044] In one embodiment, the thennoplastic material is ethylene vinyl acetate (EVA).

[0045] The layer 28 of thennoplastic material may be applied to the temporary substrate 24 by any appropriate method, for example the thermoplastic material may be applied by slot-die coating or spin coating.

[0046] Where a thermoplastic forms part of the membrane support structure 22, a total thickness dl of the combined thennoplastic 28 and the photoresist 26 layers may be less than half of the average diameter D of the grains 12.

[0047] The total thickness dl of the combined thennoplastic 28 and negative photoresist 26 layers may be 5 microns, 10 microns, 15 microns, 20 microns, 25 microns, 30 microns, 35 microns, 40 microns, 45 microns, 50 microns, 60 microns, 70 microns, 80 microns, 90 microns, or 100 microns. The total thickness dl of the combined thennoplastic 28 and negative photoresist 26 layers may be in a range from one of the described lower values to any one of the upper values.

[0048] A specific thickness d3 of the thermoplastic layer 28, where used, can be varied depending on how much exposure of a backside surface 30 of the grains 12 is required. It will be appreciated that a relatively thicker thennoplastic layer 28 will result in greater degree of exposure of the backside surface 30 of the grains 12 and in a relatively thinner binder layer 14 of the formed monograin membrane 10. Conversely, a relatively thinner thennoplastic layer 28 will result in a lower degree of exposure of the backside surface 30 of the grains 12 and result in a relatively thicker binder layer 14 of the formed monograin membrane 10. [0049] It will be appreciated that monograin membranes formed in accordance with methods of the present disclosure can achieve thinner membranes 10, e.g. membranes 10 having a thickness d4 in the range of approximately 10 microns to 50 microns, as compared with the thicknesses achievable by the etching or grinding and polishing processes which have a minimum thickness of at least 100 microns.

[0050] While the method 100 of producing the monograin membrane 10 will now be described with respect to the membrane support structure 22 comprising the thermoplastic layer 28, it will be understood that the method 100 can be varied to be applied to an embodiment in which the photoresist layer 26 is formed without the thermoplastic layer 28, for example where the negative photoresist layer 26 is formed directly on the temporary substrate 24 or onto an adhesive layer (not shown) such as a double sided tape.

[0051] With reference to Figures 1 and 2B, the method 100 comprises forming 106 a monolayer of the grains 12 on an upper surface 36 of the negative photoresist layer 26. Forming the initial monolayer of the grains 12 on the upper surface 36 of the negative photoresist layer 26 may comprise depositing grains on the upper surface 36 of the negative photoresist layer 26. Excess grains 12 can be removed from the surface 36 of the negative photoresist layer 26, for example, by moving or shaking the membrane support structure 22 or blowing of gas along the surface 36 of the negative photoresist layer 26.

[0052] The method 100 further comprises, after forming 106 the monolayer of the grains 12, embedding 108 the grains 12 into the negative photoresist layer 26. The embedding 108 may be achieved without intervention (i.e. through gravity alone), or may be facilitated for example by applying pressure to the surface 16 of the grains 12 such as by flat pressing and/or by heating the thermoplastic layer 28 to soften the thermoplastic layer 28, allowing for deformation so that the grains 12 become embedded in the negative photoresist layer 26. [0053] As can be seen in Figure 2B, after embedding 108, the grains 12 are interconnected by bridging sections 38 of the negative photoresist 27 and a film 40 of negative photoresist 27 is formed on the backside surface 30 of the grains 12.

[0054] The method 100 further comprises exposing 110 the upper surface 36 of the negative photoresist layer 26 to radiation, for example UV-radiation, in order to cross- link (cure) any exposed negative photoresist 27. By applying the radiation to the upper surface 36 of the negative photoresist layer 26 followed by a photoresist post-baking procedure, the bridging sections 38 of the negative photoresist 27 are exposed to the radiation such that cross-linking (curing) occurs. The cross-linked negative photoresist forms the fixed binder layer 14 of the monograin membrane 10.

[0055] The grains 12 embedded in the negative photoresist layer 26 act as a photomask (e.g. block the transmission of the radiation) preventing exposure of portions of the negative photoresist 27 to said radiation, for example, the films 40 of negative photoresist 27 formed on the backside surfaces 30 of the grains 12 and therefore cross-linking of this shielded film 40 of negative photoresist 27 does not occur. In this way, the unexposed (ix. not cross-linked) negative photoresist 27 can be removed by photoresist development, leaving behind a monograin membrane 10 comprising a plurality of grains 12 connected by the binder (ix. cross-linked negative photoresist) layer 14 such that the surfaces 16 of the grains 12 are electrically exposed on both sides 18, 20 of the membrane 10.

[0056] The method 100 further comprises removing 112 the membrane support structure 22 and unexposed negative photoresist 40. In some embodiments, the membrane support structure 22 may be directly peeled off the monograin membrane 10 and the unexposed negative photoresist 40 on the backside surfaces 30 of the grains 12 removed (e.g. by dissolving with a developer) to form a free standing monograin membrane 10. Such a free standing membrane 10 may be used, for example, in lithium-ion battery technology. In other embodiments, after exposing the upper surface 36 of the negative photoresist layer 26 to radiation to cross-link the exposed negative photoresist 27, the monograin membrane 10 may be first transferred to a fixed substrate 42, as shown in Figure 2D, which is fixed to the monolayer of the grains 12 and any applied additional functional layers 44 with a glue 46 such as an epoxy resin.

[0057] Advantageous methods according to the present disclosure avoid restrictions in temporary substrate selection such as the requirement for high temperature resistance as the present method 100 can be undertaken at low processing temperatures and chemical inertness. In this way, the selection of the temporary substrate 24 forming the basis of the membrane support structure 22 is not particularly limited and may be rigid or flexible, glass, metal, or polymer, transparent or non-transparent.

[0058] Methods according to the present disclosure can also allow for improved control of a binder layer thickness d5 to ensure the membrane 10 is truly of one-grain thickness d4, enabling more light transmission per unit of surface area, as more of the photosensitive grains 12 in the membrane 10 are exposed and therefore capable of receiving light. Specifically, the single grain thickness d4 of the monograin membrane 10 may reduce contact resistances and sunlight screening between the grains 12. This may thereby boost the solar conversion efficiency of the membrane 10.

[0059] The relatively thin membranes 10 achievable by methods according to the present disclosure may have lower manufacturing and transport costs due to the reduced levels of material required to form the product and the provision of a simpler manufacturing process that allows for higher throughput.

[0060] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.