Transferable photovoltaic devices

Abstract

The present invention provides a transferable photovoltaic device configuration for transferring a thin film photovoltaic device to a lower photovoltaic subcell to fabricate a tandem photovoltaic cell, the transferable photovoltaic device configuration comprising a flexible release substrate and a thin film photovoltaic device comprising (i) a first transparent conductive layer positioned above the flexible release substrate and (ii) a photoactive layer positioned above the first transparent conductive layer, wherein the first transparent conductive layer is a solution processed layer comprising at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder, and the flexible release substrate is separable from the thin film photovoltaic device after the thin film photovoltaic device is adhered to the lower photovoltaic subcell with the transparent conductive adhesive, thereby exposing the first transparent conductive layer at an outer conductive surface of the thin film photovoltaic device.

Description

[Technical Field]

[0001] Priority Cross-Reference This application claims priority from Australian Provisional Patent Application No. 2022900663, filed on 17 March 2022, the contents of which are to be understood as incorporated herein by this reference.

[0002] The present invention relates to a transferable photovoltaic device configuration for transferring a thin film photovoltaic device to a lower photovoltaic sub-cell to fabricate a tandem photovoltaic cell. The transferable photovoltaic device configuration includes a flexible release substrate and a thin film photovoltaic device on the flexible release substrate, where the thin film photovoltaic device is separable from the thin film photovoltaic device after being adhered to the lower photovoltaic sub-cell with a transparent conductive adhesive. The present invention also relates to methods for fabricating the transferable photovoltaic device configuration and methods for fabricating tandem photovoltaic cells using the transferable photovoltaic device configuration. [Background technology]

[0003] Silicon-based photovoltaic (PV) cells are by far the dominant photovoltaic technology currently used in commercial solar panels. The cost of electricity from such solar panels has dropped significantly in recent decades thanks to improved power conversion efficiencies and the benefits of large-scale manufacturing of both the input and final solar panel modules. However, single-junction PV cells, which comprise a single photoactive layer sandwiched between a pair of charge-collection electrodes, have inherent efficiency limitations due to various mechanisms, including low-bandgap spectral losses and energy loss due to band-edge relaxation (thermalization). The power conversion efficiency of state-of-the-art solar panels utilizing single-junction Si-PV cells (<27%) is currently approaching the maximum that can be expected, and therefore, a fundamental change in photovoltaic cell technology is required to provide substantial further improvements.

[0004] One approach to improving the power conversion efficiency of solar panels is to utilize multijunction PV cells, in which two (or more) PV subcells are physically stacked on top of each other. The photoactive layer of the top subcell is designed to efficiently absorb photons from the high-energy region of the solar radiation spectrum, while the photoactive layer of the bottom subcell captures lower-energy photons that pass through the semitransparent top subcell. Thus, tandem PV cells, in which two subcells are electrically connected in series via a transparent conductive interlayer (recombination layer or tunnel junction), can achieve power conversion efficiencies of up to 47% under concentrated sunlight. Therefore, in principle, tandem PV cells with a Si-PV bottom subcell could offer substantially improved power conversion efficiencies over single-junction Si-PV cells while still benefiting from modern Si-PV manufacturing techniques.

[0005] Despite these theoretical advantages, the development of commercially competitive multijunction PV cells, especially those with Si-PV bottom subcells, remains challenging due to technical and cost challenges associated with fabricating the top subcell and integrating it with the bottom subcell. Vapor deposition methods for fabricating the top subcell on the Si-PV bottom subcell have not been readily adaptable to industrial scale. Thin-film top subcells containing solution-processable photoactive layers, such as metal halide perovskite semiconductors, are particularly attractive due to their ability to adequately match bandgaps. However, layer-by-layer fabrication of the top subcell directly on the Si-PV bottom subcell using solution-processing methods remains technically and economically challenging due to the difficulty of forming uniform thin layers on large-area rigid substrates and the high material waste rate of batch deposition methods.

[0006] Previous proposals have addressed this issue by fabricating a thin-film top subcell separately and then laminating this multilayer device directly to the bottom subcell to fabricate tandem PV cells in a single processing step. Transparent conductive adhesives are used to physically bond the two devices and to provide the necessary electrical interconnection (as a recombination layer or tunnel junction) between the two subcells in the final tandem PV cell. However, thin-film devices are not self-supporting, and the thin-film top subcell must be fabricated on a film support substrate. This substrate therefore forms a relatively thick outer layer of the tandem PV cell after laminating the top and bottom subcells together, which could potentially absorb a significant portion of solar radiation and thus negatively impact power conversion efficiency. Furthermore, the film support substrate on which the thin-film electronic devices are fabricated is typically electrically insulating. While an insulating outer layer can be accommodated in small-scale devices, it is undesirable for larger surface area photovoltaic cells, where current is typically drawn through the outer surface to minimize internal resistance in solar panels.

[0007] Other reported approaches to fabricating a top subcell for subsequent lamination to a bottom subcell rely on vapor or plasma deposition techniques and / or high-temperature processing steps to create a thin-film device architecture. Therefore, the top subcell must be fabricated on a supporting substrate that can withstand these processing conditions, including transferability of the subcell to the bottom subcell when fabricating a tandem PV cell. It would be desirable to fabricate a thin-film top subcell directly on a substrate suitable for transfer to the bottom subcell using low-cost, scalable techniques. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] WO2016 / 115602A1 [Patent Document 2] WO2020 / 073082A1 [Patent Document 3] WO2016 / 090179 [Non-patent literature]

[0009] [Non-Patent Document 1] J. Ouyang, Y. Yang, Adv. Mater. 2006, 18, 2141 [Non-patent document 2] Na et al., Advanced Functional Materials 2018, 6, 1805825 [Non-patent document 3] TRKlein et al., J.Phys.D:Appl.Phys. 2021, 54, 184002 [Non-patent document 4] D. Bryant et al., Adv. Mater. 2014, 26, 7499-7504 Summary of the Invention [Problem to be solved by the invention]

[0010] Thus, there is a continuing need for methods of fabricating tandem photovoltaic cells and configurations for transferring thin film photovoltaic devices onto the bottom subcell that at least partially address one or more of the aforementioned drawbacks or provide a useful alternative.

[0011] A reference herein to a patent document or other printed material offered as prior art shall not be construed as an admission that the document or printed material was known or that the information it contained was part of the general knowledge at the priority date of any of the claims. [Means for solving the problem]

[0012] The inventors have therefore developed a method for fabricating tandem photovoltaic cells by transferring a thin-film photovoltaic top subcell initially present on a supporting release substrate to a matching bottom subcell, and then removing the release substrate to provide the tandem PV cell with a transparent conductive layer at its outer surface. The transparent conductive layer is fabricated on the release substrate by solution processing and is organic, typically containing polymeric conductive and / or binder components, but the interface between the release substrate and the transparent conductive layer allows selective delamination of the release substrate from the top subcell after the top and bottom subcells are bonded, leaving the transparent conductive layer exposed at the conductive outer surface of the tandem PV cell. This can be achieved in several ways, for example, by engineering the release substrate for intrinsically weak adhesion to the transparent conductive layer or by formulating the transparent conductive layer to be released from the release substrate in response to an external stimulus.

[0013] The presence of organic or polymeric components in the first transparent conductive layer is expected to improve the flexibility and elasticity of the first transparent conductive layer, ensuring that the physical integrity and conductivity of the first transparent conductive layer persists despite bending and compression applied to the layer during transfer from the thin film photovoltaic device to the lower photovoltaic subcell.

[0014] The thin-film top subcell is adhered to the bottom subcell via a transparent conductive adhesive layer that ultimately forms the recombination layer (also known as the tunnel junction) in the tandem photovoltaic device. In some embodiments, the transparent conductive adhesive layer initially present as the outer layer of the transferable top subcell can be activated and optionally cured to facilitate adhesion of the two subcells. Importantly, the resulting adhesive bond between the two subcells is stronger than the bond between the release substrate and the top subcell when the release substrate is separated from the thin-film top subcell.

[0015] Advantageously, the methods and devices disclosed herein can be used to fabricate tandem photovoltaic cells with silicon-based photovoltaic (Si-PV) cells, and are compatible with both the manufacturing processes for Si-PV cells and the common use of Si-PV cells in solar panel modules. Thus, a thin-film top subcell can be pressed and bonded onto a bottom subcell (e.g., a Si-PV subcell) in a roll-lamination process that involves near-simultaneous separation and removal of the release substrate. Furthermore, the resulting tandem photovoltaic cell has a conductive outer surface that can be further functionalized with a current collection network, such as a printed metal grid, as typically applied to Si-PV cells.

[0016] In a first aspect, the present invention provides a transferable photovoltaic device configuration for transferring a thin film photovoltaic device to a lower photovoltaic sub-cell to fabricate a tandem photovoltaic cell, the transferable photovoltaic device configuration comprising: a flexible release substrate; and a thin film photovoltaic device comprising: (i) a first transparent conductive layer positioned above the flexible release substrate; and (ii) a photoactive layer positioned above the first transparent conductive layer, wherein the first transparent conductive layer is a solution-processed layer comprising at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder, and the flexible release substrate is separable from the thin film photovoltaic device after the thin film photovoltaic device is adhered to the lower photovoltaic sub-cell with the transparent conductive adhesive, thereby exposing the first transparent conductive layer at an outer conductive surface of the thin film photovoltaic device.

[0017] In some embodiments, the first transparent conductive layer comprises a first activatable adhesive, and the flexible release substrate is separable from the thin-film photovoltaic device when the first activatable adhesive is activated. The first activatable adhesive can be a heat-activatable adhesive polymer. The heat-activatable adhesive polymer can be, for example, a thermoplastic polymer selected from the group consisting of ethylene-vinyl acetate (EVA) copolymer, polyethylene, polyethylene oxide (PEO), and polystyrene (PS).

[0018] In some embodiments, the flexible release substrate comprises a non-stick surface, and the flexible release substrate is separable from the thin-film photovoltaic device by delaminating the first transparent conductive layer from the non-stick surface. The non-stick surface may be provided by a low surface energy polymer. The non-stick surface may be the surface of a non-stick coating on the flexible release substrate. The non-stick coating may comprise a low surface energy polymer, for example, selected from the group consisting of fluorinated polymers and silicone polymers.

[0019] In some embodiments, the transferable photovoltaic device configuration comprises a low-adhesion sacrificial layer interposed between the flexible release substrate and the first transparent conductive layer, the low-adhesion sacrificial layer having an intrinsic low adhesion or having a low adhesion when activated such that rupturing the low-adhesion sacrificial layer allows the flexible release substrate to be separated from the thin-film photovoltaic device.

[0020] In some embodiments, the low-adhesion sacrificial layer comprises a low-adhesion organic non-polymeric solid. In some embodiments, the low-adhesion sacrificial layer comprises a third activatable adhesive, and the flexible release substrate is separable from the thin-film photovoltaic device by (i) activating the third activatable adhesive with heat or radiation and (ii) rupturing the low-adhesion sacrificial layer. The third activatable adhesive can be, for example, a thermoplastic polymer selected from the group consisting of ethylene-vinyl acetate (EVA) copolymer, polyethylene, polyethylene oxide (PEO), and polystyrene (PS), or a photodepolymerizable polymer composition selected from the group consisting of poly(phthalaldehyde) (PPHA) in combination with a photoacid generator (PAG), poly(acetal) in combination with a PAG, and polylactic acid (PLA) in combination with TiO.

[0021] In some embodiments, the low-adhesion sacrificial layer is conductive. In some embodiments, the low-adhesion sacrificial layer has a thickness of less than 100 nm, less than 50 nm, or less than 20 nm.

[0022] In some embodiments, the first transparent conductive layer comprises a first conductive component selected from the group consisting of a metal, a metal oxide, a conductive polymer or polymer composite, a fullerene or a functional derivative thereof, a carbon nanomaterial (such as graphene), a non-polymeric organic semiconductor, an organic aromatic compound, and a non-polymeric conjugated organic compound.

[0023] In some embodiments, the thin film photovoltaic device further comprises (iii) a second transparent conductive layer positioned above the photoactive layer, the second transparent conductive layer comprising a transparent conductive adhesive for adhering the thin film photovoltaic device to the lower photovoltaic subcell.

[0024] In some embodiments, the transparent conductive adhesive comprises a second conductive component comprising at least one selected from the group consisting of a metal, a metal oxide, a conductive polymer or polymer composite, a fullerene or a functional derivative thereof, a carbon nanomaterial (such as graphene), a non-polymeric organic semiconductor, an organic aromatic compound, and a non-polymeric conjugated organic compound.

[0025] In some embodiments, the transparent conductive adhesive includes a second activatable adhesive, and the second transparent conductive layer adheres to the lower photovoltaic subcell when the second activatable adhesive is activated. The second activatable adhesive can be activatable by heat, radiation, or chemical treatment. In some embodiments, the second activatable adhesive irreversibly cures when activated by a covalent bond-forming reaction.

[0026] In some embodiments, the transparent conductive adhesive is heat activatable at a temperature sufficient to release the flexible release substrate from the thin film photovoltaic device.

[0027] In some embodiments, the first transparent conductive layer comprises a first heat-activatable adhesive polymer and the second transparent conductive layer comprises a second heat-activatable adhesive polymer, the second heat-activatable adhesive polymer having a higher melting point than the first heat-activatable adhesive polymer.

[0028] In some embodiments, the thin film photovoltaic device further comprises (iv) a first charge transport layer interposed between the first transparent conductive layer and the photoactive layer, and / or (v) a second charge transport layer positioned above the photoactive layer.

[0029] In some embodiments, the photoactive layer is a photoactive perovskite layer.

[0030] In some embodiments, the transferable photovoltaic device configuration is configured as a roll for roll lamination processing in which the thin film photovoltaic device is pressed onto the lower photovoltaic subcell and the flexible release substrate is separated from the thin film photovoltaic device.

[0031] In some embodiments, each functional layer of a thin film photovoltaic device selected from (i) a first transparent conductive layer, (ii) a photoactive layer, (iv) a first charge transport layer interposed between the first transparent conductive layer and the photoactive layer, and (v) a second charge transport layer positioned above the photoactive layer is a solution-processed layer formed on a flexible release substrate. In some embodiments, each functional layer of a thin film photovoltaic device is a solution-processed layer formed on a flexible release substrate.

[0032] In a second aspect, the present invention provides a tandem photovoltaic cell configuration comprising a bottom photovoltaic sub-cell and a transferable photovoltaic device configuration according to any embodiment of the first aspect, wherein the thin film photovoltaic device is adhered to the bottom photovoltaic sub-cell via a second transparent conductive layer comprising a transparent conductive adhesive.

[0033] In some embodiments, the bottom photovoltaic subcell is a silicon photovoltaic cell.

[0034] In some embodiments, the flexible release substrate is partially separated from the thin film photovoltaic device, thereby exposing the first transparent conductive layer at the outer conductive surface.

[0035] In a third aspect, the present invention provides a method of fabricating a transferable photovoltaic device configuration, the method comprising the steps of providing a flexible release substrate and fabricating a thin film photovoltaic device on the flexible release substrate by successively forming (i) a first transparent conductive layer over the flexible release substrate and (ii) a photoactive layer over the first transparent conductive layer, wherein the first transparent conductive layer is formed by solution processing and comprises at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder, and the flexible release substrate is separable from the thin film photovoltaic device to expose the first transparent conductive layer at an outer conductive surface of the thin film photovoltaic device.

[0036] In some embodiments, forming the first transparent conductive layer over the flexible release substrate comprises applying a first fluid composition comprising a first conductive component or a reactive precursor thereof to form a wet film over the flexible release substrate, and solidifying the wet film to form the first transparent conductive layer over the flexible release substrate.

[0037] In some embodiments, the first fluid composition further comprises a solvent, and solidifying the wet film comprises removing the solvent.

[0038] In some embodiments, the first fluid composition includes a first activatable adhesive, the first activatable adhesive being a heat-activatable adhesive polymer, and the flexible release substrate is separable from the thin-film photovoltaic device when the first transparent conductive layer is heated.

[0039] In some embodiments, the flexible release substrate comprises a non-stick surface and the first transparent conductive layer is formed directly on the non-stick surface.

[0040] In some embodiments, the flexible release substrate includes a low-adhesion sacrificial layer on a surface thereof, and the first transparent conductive layer is formed directly on the low-adhesion sacrificial layer.

[0041] In some embodiments, forming the photoactive layer over the first transparent conductive layer comprises applying a flowable composition comprising one or more photoactive layer components or precursors thereof dispersed in a solvent to form a wet film over the first transparent conductive layer, and removing the solvent from the wet film to form the photoactive layer. Optionally, the flowable composition is a perovskite precursor solution.

[0042] In some embodiments, fabricating the thin film photovoltaic device on a flexible release substrate further includes (iii) forming a second transparent conductive layer over the photoactive layer, the second transparent conductive layer comprising a transparent conductive adhesive for adhering the thin film photovoltaic device to the lower photovoltaic subcell. Forming the second transparent conductive layer may include applying a second fluid composition comprising a second activatable adhesive and a second conductive component or a reactive precursor thereof to form a wet film over the photoactive layer, and solidifying the wet film to form the second transparent conductive layer.

[0043] In some embodiments, fabricating the thin film photovoltaic device on a flexible release substrate further includes forming (iv) a first charge transport layer interposed between the first transparent conductive layer and the photoactive layer, and / or (v) a second charge transport layer above the photoactive layer.

[0044] In some embodiments, the flexible release substrate is configured as a roll, and the thin film photovoltaic device is fabricated on the flexible release substrate by coating and / or printing steps in a roll-to-roll process.

[0045] In a fourth aspect, the present invention provides a method of fabricating a tandem photovoltaic cell, the method comprising the steps of: providing a transferable photovoltaic device configuration according to any embodiment of the first aspect, or as fabricated by a method according to any embodiment of the third aspect; adhering a thin film photovoltaic device of the transferable photovoltaic device configuration to a bottom photovoltaic sub-cell by a transparent conductive adhesive, thereby electrically coupling the thin film photovoltaic device and the bottom photovoltaic sub-cell via the second transparent conductive layer; and separating a flexible release substrate of the transferable photovoltaic device configuration from the thin film photovoltaic device, thereby fabricating a tandem photovoltaic cell having the first transparent conductive layer of the thin film photovoltaic device at an outer conductive surface.

[0046] In some embodiments, the bottom photovoltaic subcell is a silicon photovoltaic cell.

[0047] In some embodiments, the thin film photovoltaic device is pressed or compressed onto the bottom photovoltaic subcell to facilitate adhesion before separating the flexible release substrate from the thin film photovoltaic device.

[0048] In some embodiments, the transferable photovoltaic device configuration is configured as a roll, with the thin film photovoltaic device adhered to the bottom photovoltaic subcell, and the flexible release substrate separated from the thin film photovoltaic device in a roll lamination process.

[0049] In some embodiments, adhering the thin film photovoltaic device to the bottom photovoltaic subcell comprises activating a transparent conductive adhesive with heat, radiation, or chemical treatment.

[0050] In some embodiments, adhering the thin film photovoltaic device to the lower photovoltaic subcell comprises activating the transparent conductive adhesive by heat at a temperature sufficient to release the flexible release substrate from the thin film photovoltaic device.

[0051] In some embodiments, the method further comprises fabricating a metallic current collecting network on the outer conductive surface of the tandem photovoltaic cell.

[0052] According to a fifth aspect, the present invention provides a tandem photovoltaic cell fabricated by a method according to any embodiment of the fourth aspect.

[0053] Further aspects of the present invention will become apparent from the detailed description of the invention that follows.

[0054] Embodiments of the present invention will now be illustrated, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0055] [Figure 1] 1 is a schematic diagram of a transferable photovoltaic device configuration 100 according to the present invention and a method 150 of fabricating a tandem photovoltaic cell 130 using the transferable photovoltaic device configuration by transferring a photovoltaic device 104 to a lower photovoltaic subcell 112. [Figure 2] 2 is a schematic diagram of a transferable photovoltaic device configuration 200 according to an embodiment of the present invention and a method 250 of fabricating a tandem photovoltaic cell 230 using the transferable photovoltaic device configuration by transferring a photovoltaic device 204 to a bottom photovoltaic subcell 212. [Figure 3] 3 is a schematic diagram of a transferable photovoltaic device configuration 300 according to an embodiment of the present invention and a method 350 of fabricating a tandem photovoltaic cell 330 using the transferable photovoltaic device configuration by transferring a photovoltaic device 304 to a bottom photovoltaic sub-cell 312. [Figure 4] 4 is a schematic diagram of a transferable photovoltaic device configuration 400 according to an embodiment of the present invention and a method 450 of fabricating a tandem photovoltaic cell 430 using the transferable photovoltaic device configuration by transferring a photovoltaic device 404 to a bottom photovoltaic sub-cell 412. [Figure 5] 1 is a block flow diagram illustrating steps of a method for fabricating a transferable photovoltaic device configuration in accordance with the present invention. [Figure 6] 1 is a schematic diagram of a method for fabricating tandem photovoltaic cells in a roll-stacking process according to an embodiment of the present invention. [Figure 7] FIG. 1 is a schematic diagram of a method for transferring a solution-processed thin film photovoltaic device from a flexible release substrate to a conductive substrate, as implemented in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0056] The present invention relates to a transferable photovoltaic device configuration for transferring a thin-film photovoltaic device to a lower photovoltaic subcell to fabricate a tandem photovoltaic cell. The transferable photovoltaic device configuration includes a flexible release substrate and a thin-film photovoltaic device on the flexible release substrate. The thin-film photovoltaic device includes a first transparent conductive layer positioned above the flexible release substrate and a photoactive layer positioned above the first transparent conductive layer. The first transparent conductive layer is a solution-processed layer and includes at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder. During use, the flexible release substrate is separable from the thin-film photovoltaic device after the thin-film photovoltaic device is bonded to the lower photovoltaic subcell with the transparent conductive adhesive. Thus, the resulting tandem photovoltaic cell has the first transparent conductive layer of the thin-film photovoltaic device exposed at its outer conductive surface.

[0057] As seen in FIG. 1 , the transferable photovoltaic device configuration 100 includes a flexible release substrate 102, which may comprise a flexible polymer film. A thin-film photovoltaic device 104 is positioned on the flexible release substrate 102 with an interface 106 between the flexible release substrate and the photovoltaic device. As will be explained in more detail below, the interface 106 is designed to release the photovoltaic device 104 once it is attached to the bottom photovoltaic subcell. The thin-film photovoltaic device 104 includes at least a solution-processed transparent conductive layer 108 positioned above the flexible release substrate and a photoactive layer 110 positioned above the layer 108. The transparent conductive layer 108 will ultimately form the top electrical contact of the tandem photovoltaic solar cell, and the photoactive layer 110 is a light-absorbing semiconductor layer with a bandgap suitable for the top subcell photoactive layer of the tandem photovoltaic solar cell, such as a perovskite photoactive layer. The thin-film photovoltaic device 104 may optionally include other layers (not shown), such as a charge transport layer (i.e., an electron-selective layer and / or a hole-selective layer) interposed between the transparent conductive layer 108 and the photoactive layer 110 and / or positioned above the photoactive layer 110.

[0058] The transferable photovoltaic device configuration 100 is configured to transfer a thin film photovoltaic device 104 to a bottom photovoltaic subcell 112 to fabricate a tandem photovoltaic cell 130, as also shown in FIG.

[0059] Accordingly, the present invention further relates to a method for fabricating a tandem photovoltaic cell. The method includes providing a transferable photovoltaic device configuration as disclosed herein and adhering a thin-film photovoltaic device of the transferable photovoltaic device configuration to a lower photovoltaic subcell with a transparent conductive adhesive. The thin-film photovoltaic device is thereby operatively coupled to the lower photovoltaic subcell via the second transparent conductive layer. The flexible release substrate of the transferable photovoltaic device configuration is then separated from the thin-film photovoltaic device. The resulting tandem photovoltaic cell has the first transparent conductive layer of the thin-film photovoltaic device at the outer conductive surface and a second transparent conductive layer between the upper and lower subcells as a recombination layer or tunnel junction.

[0060] Continuing to refer to FIG. 1 , the transferable photovoltaic device configuration 100 is used in such a transfer method (depicted by arrow 150) to transfer a thin-film photovoltaic device 104 to a bottom photovoltaic subcell 112. The bottom photovoltaic subcell 112 comprises at least a top conductive layer 114, a bottom electrical contact 116, and a photoactive semiconductor layer 118, and in some embodiments is a conventional silicon photovoltaic solar cell. Thus, the thin-film photovoltaic device 104 is adhered to the top conductive layer 114 of the bottom subcell by a transparent conductive adhesive 120 such that the thin-film photovoltaic device 104 and the bottom photovoltaic subcell 112 are operatively coupled via the transparent conductive layer 122. The transparent conductive layer 122 formed from the transparent conductive adhesive 120 thus forms a recombination layer or tunnel junction between the top and bottom subcells in the tandem photovoltaic cell 130.

[0061] In some embodiments, the transparent conductive layer 122 is initially present as a pre-formed outer layer of the thin film photovoltaic device 104, positioned above the photoactive layer 110. In other embodiments, the transparent conductive layer 122 is initially present as a pre-formed outer layer of the bottom photovoltaic subcell 112, positioned above or as the top conductive layer 114. In still further embodiments, the transparent conductive adhesive 120 may be applied as a coating to the surface of either the thin film photovoltaic device 104 or the bottom photovoltaic subcell 112 as part of a transfer method.

[0062] The transfer method also includes separating the flexible release substrate 102 from the thin-film photovoltaic device 104. The separation step, which typically occurs by peeling the flexible substrate to gradually break the interface 106, may occur substantially simultaneously with the bonding of the thin-film photovoltaic device 104 to the bottom photovoltaic subcell 112, or may occur as a separate operation at a later time. In either case, separation of the flexible substrate relies on (i) the bonding of the thin-film photovoltaic device 104 to the bottom photovoltaic subcell 112 via the transparent conductive layer 122, and (b) the adhesion or cohesion at the interface 106, which is weaker at least at the time of separation, rather than the adhesion and cohesion between and within other functional layers in the tandem photovoltaic cell 130. Following the separation step, the tandem photovoltaic cell 130 has a transparent conductive layer 108 at its outer conductive surface 124 from which current can be drawn when the tandem cell is in operation.

[0063] definition When the terms "comprises" and "comprising" are used in this specification (including the claims), they specify stated features, integers, steps or components, but are not to be interpreted as excluding the presence of one or more other features, integers, steps or components, or groups thereof.

[0064] As used herein, terms such as "first," "second," and "third," with respect to various features of the disclosed compositions, devices, and methods, are arbitrarily assigned labels and are intended merely to distinguish between two or more such features that may be incorporated into various embodiments. These terms do not, by themselves, indicate any particular orientation or sequence. Furthermore, it is understood that the presence of a "first" feature does not imply the presence of a "second" feature, the presence of a "second" feature does not imply the presence of a "first" feature, etc.

[0065] As used herein, the terms "top" and "bottom" refer to the top light-receiving surface in a tandem photovoltaic cell, which is the first transparent conductive layer.

[0066] As used herein, particularly in reference to functional layers of transferable photovoltaic device configurations, the term "above" refers to relative position in a multilayer device structure, including both "in direct contact with" and "separated by one or more intervening functional layers." Thus, if layer B is above layer A and layer C is above layer B, the multilayer device structure can include the following layer sequence: ABC, AXBC, ABXC, and AXBXC, where X is one or more intervening functional layers.

[0067] As used herein, the term "conductive layer" means a thin film with sufficient conductivity to transport photon-generated charge through the layer. A conductive layer can be an electrical conductor or a semiconductor. A "conductive layer" can have multiple functions, including charge selectivity.

[0068] As used herein, an "activatable" layer, adhesive, or adhesive polymer is adapted to be functionally activated for adhesion or delamination as needed by an external stimulus such as heat, radiation (e.g., actinic light), or chemical treatment.

[0069] Transferable photovoltaic device configurations Flexible release substrate The transferable photovoltaic device configuration comprises a flexible release substrate. The flexible release substrate is typically a flexible film that can be peeled away from the thin-film photovoltaic device. In some embodiments, the flexible substrate is in the form of a roll of film or tape to facilitate (i) fabrication of the thin-film photovoltaic device in a roll-to-roll manufacturing process and / or (ii) transfer of the thin-film photovoltaic device to the lower photovoltaic subcell in a roll lamination process. Alternatively, the release substrate may be formed as a flat sheet of flexible film. The flexible release substrate may comprise a flexible polymer film, for example, a polyester such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), a polyolefin such as polypropylene, or a fluorinated polymer such as ethylene tetrafluoroethylene (ETFE), as is typical for substrates in roll manufacturing processes. However, it is not excluded that the flexible release substrate can be made from other materials, such as metal foil or paper.

[0070] In some embodiments, the release substrate comprises a non-stick surface such that the flexible release substrate can be easily separated from the thin-film photovoltaic device by delaminating the adjacent transparent conductive layer from the non-stick surface. As will be understood by those skilled in the art, the non-stick surface is fundamentally weakly adhesive due to its low surface energy composition, and therefore does not require heat activation to achieve non-stick properties. A suitable non-stick surface may have a surface energy of less than 36 Dyne / cm, preferably less than 30 Dyne / cm, as measured, for example, according to ASTM D7490-13. The non-stick coating may be provided by a low surface energy polymer, such as a polymer selected from the group consisting of fluorinated polymers, such as polytetrafluoroethylene (PTFE), and silicone polymers, such as polydimethylsiloxane (PDMS). The non-stick surface may be the surface of a non-stick coating on the flexible release substrate. Suitable non-stick coatings are generally low surface energy polymer coatings, as described above. Alternatively, the flexible release substrate may comprise a self-supporting film of a suitable low surface energy polymer, the non-stick surface being the surface of the film.

[0071] Low-adhesion sacrificial layer In some embodiments, the transferable photovoltaic device configuration includes a low-adhesion sacrificial layer interposed between a flexible release substrate and the first transparent conductive layer of the thin-film photovoltaic device. As used herein, a low-adhesion sacrificial layer is a layer having intentionally weaker adhesion than the adhesion between and within other layers in the multilayer structure obtained when the thin-film photovoltaic device is attached to the lower subcell via a layer of transparent conductive adhesive. The low-adhesion sacrificial layer is either an inherently low-adhesion layer, i.e., at room temperature, or has appropriately low adhesion when activated. In either case, the flexible release substrate can be separated from the thin-film photovoltaic device by selectively breaking the low-adhesion sacrificial layer. However, it is understood that the low-adhesion sacrificial layer still requires sufficient adhesive integrity and properties so that the flexible release substrate can adhere to and support the thin-film photovoltaic device during fabrication and during transfer of the thin-film photovoltaic device to the lower subcell.

[0072] The transparent, low-adhesion sacrificial layer is preferably a very thin layer, e.g., having a thickness of less than 100 nm, less than 50 nm, or less than 20 nm. In some embodiments, the low-adhesion sacrificial layer is conductive due to the incorporation of conductive components such as metals, metal oxides, and conductive polymers or polymer composites (e.g., poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, i.e., PEDOT:PSS). The inclusion of conductive components can advantageously reduce loss of conductivity through the surface of the first transparent conductive layer if residues of the low-adhesion sacrificial layer remain on the first transparent conductive layer after separation and removal of the flexible release substrate.

[0073] In some embodiments, the low-adhesion sacrificial layer comprises a low-adhesion organic non-polymeric solid, such as a wax. In some embodiments, the low-adhesion sacrificial layer comprises an activatable adhesive. The activatable adhesive can be activated by any suitable external stimulus applied on command, such as heat or radiation, to sufficiently reduce the adhesion of the sacrificial layer. Thus, the flexible release substrate can be separated from the thin-film photovoltaic device by activating the activatable adhesive with heat or radiation and rupturing the low-adhesion sacrificial layer.

[0074] In some embodiments, the heat-activatable adhesive in the sacrificial layer is a heat-activatable adhesive, such as a thermoplastic polymer selected from the group consisting of ethylene-vinyl acetate (EVA) copolymer, polyethylene, polyethylene oxide (PEO), and polystyrene (PS). The thermoplastic polymer responds to the application of heat by melting or softening, thereby optionally reducing the adhesion of the sacrificial layer. Preferably, activation occurs at a temperature that can be achieved when pressing the transferable photovoltaic device configuration onto the lower subcell without damaging other layers in the device. In some embodiments, the sacrificial layer is heat-activatable to sufficiently reduce its adhesion at a temperature in the range of 50°C to 170°C, such as in the range of 90°C to 140°C.

[0075] In another embodiment, the activatable adhesive in the sacrificial layer is a photodepolymerizable polymer composition, optionally containing a suitable photoinitiator or photocatalyst. Examples include polymer compositions selected from the group consisting of poly(phthalaldehyde) (PPHA) combined with a photoacid generator (PAG), poly(acetal) combined with a PAG, and polylactic acid (PLA) combined with TiO. The photodepolymerizable polymer composition responds to irradiation with light of an appropriate wavelength by depolymerizing or decrosslinking, thereby reducing the adhesion of the sacrificial layer as needed.

[0076] Thin-film photovoltaic devices The transferable photovoltaic device configuration comprises a thin-film photovoltaic device positioned above a flexible release substrate at an interface configured to allow separation of the flexible substrate. The thin-film photovoltaic device generally has a multilayer architecture formed by a succession of thin-film functional layers. In some embodiments, each functional layer has a dry layer thickness of less than 50 μm. In embodiments, each functional layer has a dry layer thickness of less than 10 μm, less than 5 μm, or less than 2 μm.

[0077] Because of this architecture, thin-film photovoltaic devices are generally not self-supporting (i.e., physically unstable) and must therefore be fabricated and handled on a support substrate, here the flexible release substrate itself, by methods described hereinafter. Once transferred to the bottom subcell, the thin-film photovoltaic device becomes the top subcell in the resulting tandem photovoltaic cell, as disclosed herein.

[0078] First transparent conductive layer The thin film photovoltaic device comprises a first transparent conductive layer, which is a solution-processed layer and includes at least one organic, typically polymeric, conductive and / or binder component selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder. Advantageously, the organic or polymer component can also improve the flexibility and resilience of the first transparent conductive layer, ensuring that the physical integrity and conductivity of the first transparent conductive layer persists despite flexing and compression applied to the layer during transfer from the thin film photovoltaic device to the lower photovoltaic subcell.

[0079] The first transparent conductive layer is designed to be the outer conductive layer of the top subcell in a tandem photovoltaic cell. However, in a transferable photovoltaic device configuration, the first transparent conductive layer may be positioned above a flexible release substrate and in direct contact with the substrate, or may be separated from the substrate by an intervening sacrificial layer. The interface between the flexible release substrate and the solution-processed first transparent conductive layer is designed to facilitate release and separation of the flexible release layer once the thin-film photovoltaic device is attached to the bottom subcell. Consistent with the preferred method of fabrication disclosed hereinafter, the first transparent conductive layer can be fabricated on the release substrate by solution processing techniques such as coating or printing.

[0080] To provide the necessary conductivity, the first transparent conductive layer includes at least one conductive component, optionally held together by a binder (e.g., if the conductive component is a non-polymeric material that requires reinforcement). In some embodiments, the first transparent conductive layer includes a conductive component selected from the group consisting of a metal, a metal oxide, a conductive polymer or polymer composite, a fullerene or functional derivative thereof, a carbon nanomaterial (e.g., graphene), a non-polymeric organic semiconductor, an organic aromatic compound, and a non-polymeric conjugated organic compound. In some embodiments, the first transparent conductive layer includes a conductive component selected from the group consisting of a metal, a metal oxide, and a conductive polymer or polymer composite. Suitable metals and metal oxides may include particulate nanomaterials, including nanowires of metals (e.g., silver), nanoparticles and nanoparticulate transparent conductive oxides of other metals, such as indium tin oxide, zinc oxide, tin oxide, and nickel oxide. In some embodiments, the first transparent conductive layer includes or consists of a conductive polymer or polymer composite. A suitable conductive polymer composite is PEDOT:PSS.

[0081] In some embodiments, the first transparent conductive layer is a conductive layer (i.e., with metal-like conductivity). Suitable conductive layers can include conductive metal compositions such as Au, Ag, Al, Mg, Cu, or suitable alloys thereof. For example, such metal compositions can be present as nanoparticles in a transparent conductive matrix (i.e., in a conductive polymer) or can be reinforced by an organic binder.

[0082] In other embodiments, the first transparent conductive layer is a semiconducting charge transport layer, such as an electron-selective transport layer or a hole-selective transport layer, depending on the intended orientation of the upper subcell. Thus, during use in a tandem photovoltaic cell, the first transparent conductive layer can cooperate with the photoactive layer of the upper subcell to selectively extract either photovoltaically excited electrons or corresponding holes to the outer conductive surface of the upper subcell.

[0083] When the first transparent conductive layer is an electron selective transport layer, the first transparent conductive layer may include at least one transparent conductive oxide such as tin oxide, nickel oxide, zinc oxide, titanium dioxide, or tungsten trioxide, a fullerene derivative such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM), a pure fullerene such as C60 or C70, or a functional polymer such as polyethyleneimine ethoxylate (PEIE).

[0084] When the first transparent conductive layer is a hole selective transport layer, the first transparent conductive layer may be made of 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD), poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), poly(3,4-ethylenedioxythiophene) (PEDOT), a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT:PSS), poly(4,4-dioctylcyclopentadithiophene); The transparent conductive polymer may include at least one of P3HT, doped P3HT (poly(3-hexylthiophene-2,5-diyl)), poly(triarylamine) (PTAA), poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), or poly(N-alkyldiketopyrrolopyrroledithienylthieno[3,2-b]thiophene) (DPP-DTT), and the like.

[0085] The first transparent conductive layer may comprise a transparent conductive adhesive containing an activatable adhesive component, as described in more detail below. However, this is not required, as a non-activatable or non-tacky transparent conductive layer can be suitably laminated to the non-stick surface of a release substrate or to a low-adhesion sacrificial layer in the release substrate by solution processing methods. Therefore, separation of the release substrate need not rely on delamination caused by activation of the first transparent conductive layer itself. For example, the inventors have found that thin-film photovoltaic devices comprising a PEDOT:PSS transparent conductive layer can be delaminated from either (i) a flexible release substrate with a non-stick silicone coating or (ii) a flexible release substrate with a heat-activated low-adhesion sacrificial layer, despite the absence of an activatable adhesive component in the transparent conductive layer.

[0086] In some embodiments, the first transparent conductive layer comprises or consists of a transparent conductive adhesive. Thus, the first transparent conductive layer can comprise a first activatable adhesive such that the flexible release substrate is separable from the thin-film photovoltaic device when the first activatable adhesive is activated. As used herein, an activatable adhesive is an adhesive composition, typically an adhesive polymer composition, that can be activated by any applied stimulus, such as heat, radiation, or chemical treatment, to enable adhesive bonding and / or release. In principle, a transparent conductive layer comprising an activatable adhesive can be formed on a wide range of flexible release substrates, since interfacial release is triggered by activation of the transparent conductive adhesive. Thus, the interface between the activated first transparent conductive layer and the flexible release substrate is preferably the weakest point in the structure after activation, so that the activated transparent conductive layer separates intact. However, it is not excluded that some residue of the activated transparent conductive adhesive may remain on the flexible release substrate, provided that a suitably continuous layer of transparent conductive adhesive remains present in the thin film photovoltaic device after its transfer.

[0087] In some embodiments, the first transparent conductive layer comprises a heat-activatable adhesive, such as a heat-activatable adhesive polymer. The heat-activatable adhesive polymer can be a thermoplastic polymer selected from the group consisting of ethylene-vinyl acetate (EVA) copolymer, polyethylene, polyethylene oxide (PEO), and polystyrene (PS). It is understood that when such an insulating polymer is present as the matrix of the first transparent conductive layer, a conductive component is also required to establish the necessary conductivity. Suitable conductive components include particles containing conductive metals or metal oxides and conductive polymers such as PEDOT:PSS, polyaniline (PANI), polyacetylene, polypyrrole, polythienylene vinylene, polythiophene, polyphenylene vinylene, and polyphenylene sulfide. In one such example, the first transparent conductive layer comprises a thermoplastic EVA copolymer matrix with Ag-coated conductive microspheres as a conductive additive. In another approach, the first transparent conductive layer comprises a heat-activatable conductive polymer composition, such as PEDOT:PSS in combination with a polyhydric alcohol such as D-sorbitol (see, for example, J. Ouyang, Y. Yang, Adv. Mater. 2006, 18, 2141).

[0088] A suitable heat-activatable first transparent conductive layer is preferably adapted to activate at a temperature that may be provided when pressing the transferable photovoltaic device configuration onto the lower subcell without damaging other functional layers in the device. In some embodiments, the first transparent conductive layer is heat-activatable to cause release from the flexible release substrate at a temperature in the range of 50°C to 170°C, such as in the range of 90°C to 140°C.

[0089] photoactive layer Thin film photovoltaic devices include a photoactive layer that is intended to be the light-absorbing layer of the top subcell in a tandem photovoltaic cell. However, in transferable photovoltaic device configurations, the photoactive layer is located above a first transparent conductive layer, either in direct contact with the first transparent conductive layer or separated from the first transparent conductive layer by one or more intervening layers, such as a charge transport layer. Consistent with preferred methods of fabrication as disclosed hereinafter, the photoactive layer may be solution-processed and fabricated on a release substrate.

[0090] The photoactive layer comprises a light-absorbing semiconductor with a bandgap higher than the bandgap of the bottom subcell used in the tandem photovoltaic cell. The silicon-based bottom subcell has a bandgap of approximately 1.1 eV. The highest power conversion efficiency of a tandem cell with a Si-PV subcell is believed to be when the bandgap of the photoactive layer of the top subcell is between 1.6 eV and 1.9 eV, although it is understood that bandgaps higher than this preferred range and lower bandgaps are also acceptable. Thus, in some embodiments, the bandgap of the photoactive layer in a thin-film photovoltaic device is between 1.6 eV and 1.9 eV.

[0091] In some embodiments, the photoactive layer comprises a metal halide semiconductor. In some such embodiments, the metal halide semiconductor is a photoactive perovskite layer. Organic-inorganic hybrid perovskites are considered particularly suitable because the bandgap can be tuned to a desired target by adjusting the composition of the perovskite structure. Furthermore, perovskite layers can be fabricated in thin-film format by solution processing methods, including on flexible substrates in roll-to-roll manufacturing processes. Examples of such methods are disclosed in international patent applications published as WO 2016 / 115602 A1 and WO 2020 / 073082 A1. Perovskite electrical properties are highly resistant to defects that can result from solution processing methods. Perovskite photoactive layers typically have dry layer thicknesses in the range of 0.5 to 1 μm.

[0092] The photoactive perovskite layer comprises a light-absorbing perovskite semiconductor consisting essentially of perovskite crystallites. Perovskite materials can be represented by the formula AMX3, where A is at least one cation, M is at least one cation, and X is at least one anion. When a perovskite comprises two or more A cations, the different A cations can be distributed in an ordered or disordered manner across the A sites. When a perovskite comprises two or more M cations, the different M cations can be distributed in an ordered or disordered manner across the M sites. When a perovskite comprises two or more X anions, the different X anions can be distributed in an ordered or disordered manner across the X sites. Perovskites containing two or more A cations, two or more M cations, or two or more X anions have lower symmetry than CaTiO3. Perovskites are crystalline compounds. Thus, a layer of a perovskite conductor without open pores typically consists essentially of perovskite crystallites. In perovskite-based photoactive devices, such as photovoltaic cells, the photoactive layer may comprise an organic-inorganic perovskite-structured semiconductor. However, in some embodiments, the photoactive layer may be all-inorganic, e.g., CsPb(I n X 1-n )3, where it is understood that X is a non-iodine halide.

[0093] As used herein, the term "perovskite" refers to (a) a material with a three-dimensional crystal structure related to that of CaTiO or (b) a material comprising layers of material, wherein the layers have a structure related to that of CaTiO. While both of these classes of perovskites can be used in devices according to the present invention, in some circumstances it is desirable to use perovskites of the first class (a), i.e., perovskites with a three-dimensional (3D) crystal structure. Such perovskites typically comprise a 3D network of perovskite unit cells with no separation between the layers. On the other hand, perovskites of the second class (b) comprise perovskites with a two-dimensional (2D) layer structure. Perovskites with a 2D layer structure can comprise layers of perovskite unit cells separated by (interlayered) molecules; an example of such a 2D layer perovskite is [2-(l-cyclohexenyl)ethylammonium]2PbBr4. 2D layer perovskites tend to have high exciton binding energies that favor the generation of bound electron-hole pairs (excitons) under optical excitation rather than free charge carriers. Bound electron-hole pairs may not be mobile enough to reach p-type or n-type contacts where they can transfer (ionize) and generate free charge. As a result, to generate free charge, the exciton binding energy must be overcome, which represents a loss of energy to the charge generation process, resulting in lower voltages and lower efficiencies in photovoltaic cells. In contrast, perovskites with 3D crystal structures have much lower exciton binding energies (in terms of thermal energy), thereby enabling the generation of free carriers immediately after optical excitation. Therefore, the perovskite semiconductors used in the devices and processes of the present invention are perovskites of the first class, namely, (a) perovskites with a three-dimensional crystal structure.

[0094] In some embodiments, the perovskite is in the form AM(I n X 1-n )3, where A is methylammonium ([CH3NH3] +), formamidinium ([R2N-CH=NR2] + ), and at least one cation preferably selected from cesium, M is a metal, I is iodine, X is a non-iodine halide preferably selected from Br and Cl, n is in the range of 0 to 1, and M is preferably selected from Pb, Sn, Ge, Cs, Bi. Advantageously, the band gap of such perovskite semiconductors can be continuously tuned in the range of 1.6 eV to 2.3 eV by substituting bromine for iodine, as disclosed in international patent application published as WO2016 / 090179.

[0095] In some embodiments, the photoactive layer comprises an organic photovoltaic active layer, such as those used in polymer solar cells. Such photoactive layers are often combined with fullerene-based electron acceptor materials, such as phenyl-C61-butyric acid methyl ester (PCBM), or non-fullerene acceptors, such as 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene) (ITIC) or Y6. The organic photoactive layer typically comprises an organic polymer electron donor material such as poly(3-hexylthiophene) (P3HT) or poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T). The organic photoactive layer can also be advantageously adapted for fabrication in thin film format by solution processing methods, including on flexible substrates in a roll-to-roll manufacturing process. Examples of such methods are disclosed in Na et al., Advanced Functional Materials 2018, 6, 1805825.

[0096] charge transport layer In addition to the first transparent conductive layer and the photoactive layer, the thin film photovoltaic device may further comprise one or more transparent functional layers. Consistent with preferred methods of fabrication as disclosed hereinafter, the transparent functional layers may be solution-processed layers and may be fabricated on a release substrate.

[0097] In some embodiments, the thin-film photovoltaic device further comprises a first charge transport layer interposed between the first transparent conductive layer and the photoactive layer, e.g., sandwiched between and in direct contact with these layers. In some embodiments, the thin-film photovoltaic device further comprises a second charge transport layer positioned above the photoactive layer, e.g., in direct contact with the photoactive layer. Such charge transport layers typically have a thickness of less than 0.2 μm or less than 0.1 μm.

[0098] The first charge transport layer can be an electron-selective transport layer, and the second charge transport layer can be a hole-selective transport layer, or vice versa, depending on the intended orientation of the upper subcell. Such charge transport layers can assist in selectively extracting photovoltaically excited electrons and corresponding holes toward the outer conductive surface and the recombination layer / tunnel junction of the upper subcell, respectively, thereby providing improved photovoltaic performance. However, as those skilled in the art will appreciate, charge transport layers are not required for solar cell architecture, and therefore, either or both of the first and second charge transport layers can be absent.

[0099] Suitable electron selective transport layers may include at least one transparent conductive oxide such as tin oxide, zinc oxide, or titanium dioxide, a fullerene derivative such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM), a pure fullerene mixture such as C60 or C70, or a transparent conductive polymer such as polyethyleneimine ethoxylate (PEIE).

[0100] Suitable hole selective transport layers include 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD), poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole)] (PPDT2FBT), poly(3,4-ethylenedioxythiophene) (PEDOT), a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT:PSS), poly(4,4-dioctylcyclopentadithiophene); The transparent conductive polymer may include at least one of P3HT, doped P3HT (poly(3-hexylthiophene-2,5-diyl)), poly(triarylamine) (PTAA), poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT), or poly(N-alkyldiketopyrrolopyrroledithienylthieno[3,2-b]thiophene) (DPP-DTT), MoO, and the like.

[0101] Second transparent conductive layer In some embodiments, the thin-film photovoltaic device of the transferable photovoltaic device configuration comprises a second transparent conductive layer. The second transparent conductive layer is positioned above the photoactive layer, either in direct contact with the photoactive layer or separated from the photoactive layer by one or more intervening layers, such as a charge transport layer. The second transparent conductive layer, present as an outer layer of the thin-film photovoltaic device, comprises a transparent conductive adhesive for adhering the thin-film photovoltaic device to the lower photovoltaic subcell. Thus, advantageously, the thin-film photovoltaic device can be adhered to a lower subcell, such as a silicon-based cell, simply by contacting the two layers under conditions where the transparent conductive adhesive bonds to the lower subcell.

[0102] Consistent with the preferred method of fabrication as disclosed hereinafter, the second transparent conductive layer may be a solution-processed layer and may be fabricated on a release substrate.

[0103] Preferred transparent conductive adhesives for the second transparent conductive layer have the following properties: (i) the ability to adhesively bond to the bottom subcell with sufficient adhesive strength to allow a flexible release substrate to be peeled from the thin-film photovoltaic device, optionally after first being activated by an external stimulus; (ii) the ability to form a robust adhesive interlayer between the top and bottom subcells to form a monolithic tandem photovoltaic device; (iii) high optical transparency to photons traveling through the top subcell for absorption in the photoactive layer of the bottom subcell; and (iv) suitable electronic properties to recombine hole current from one subcell with electron current from the other subcell with low voltage loss.

[0104] The transparent conductive adhesive includes a second conductive component that may include at least one selected from the group consisting of a metal, a metal oxide, a conductive polymer or polymer composite, a fullerene or functional derivative thereof, a carbon nanomaterial (such as graphene), a non-polymeric organic semiconductor, an organic aromatic compound, and a non-polymeric conjugated organic compound. In some embodiments, the second conductive component includes at least one selected from the group consisting of a metal, a metal oxide, and a conductive polymer or polymer composite.

[0105] In some embodiments, the transparent conductive adhesive comprises an activatable adhesive such that the second transparent conductive layer is adhered to the lower photovoltaic subcell when the activatable adhesive is activated. The activatable adhesive can be made activatable by any applied stimulus, such as heat, radiation, or chemical treatment.

[0106] If heat-activatable, the second transparent conductive layer is preferably adapted to activate at a temperature that can be provided when pressing the transferable photovoltaic device configuration onto the lower subcell without damaging other functional layers in the device. In some embodiments, the second transparent conductive layer is heat-activatable to facilitate adhesion to the lower subcell at a temperature in the range of 50°C to 170°C, such as in the range of 90°C to 140°C.

[0107] In some embodiments, the transparent conductive layer is heat-activatable at a temperature high enough to also release the flexible release substrate from the thin-film photovoltaic device. For example, either the first transparent conductive layer or the low-tack sacrificial layer described herein may comprise a first heat-activatable adhesive polymer, and the second transparent conductive layer comprises a second heat-activatable adhesive polymer. The two heat-activatable adhesive polymers are selected so that a single heat treatment step causes (i) delamination at the interface between the flexible release film and the thin-film photovoltaic device and (ii) activation and adhesion of the second transparent conductive layer to the lower photovoltaic subcell. For example, the second heat-activatable adhesive polymer can have a higher melting point than the first heat-activatable adhesive polymer. Thus, when the transferable photovoltaic device configuration is hot-pressed onto the lower photovoltaic subcell, for example, in a roll lamination process, separation of the flexible release substrate can occur substantially simultaneously with adhesion of the thin-film photovoltaic device to the lower photovoltaic subcell.

[0108] In some embodiments, the activatable adhesive of the second transparent conductive layer is a thermoplastic polymer selected from the group consisting of ethylene-vinyl acetate (EVA) copolymer, polyethylene, polyethylene oxide (PEO), and polystyrene (PS). When such an insulating polymer is present as the matrix of the second transparent conductive layer, a conductive component is required to establish the necessary conductivity. The conductive component may include particle conductive elements that establish a conductive path through the thickness of the second transparent conductive layer. In one such example, a transparent conductive adhesive suitable for the recombination layer consists of a thermoplastic EVA copolymer matrix with Ag-coated conductive microspheres as a conductive additive (see TR Klein et al., J. Phys. D: Appl. Phys. 2021, 54, 184002).

[0109] The thermoplastic polymer can form a continuous matrix of the second transparent conductive layer or can be present in a discontinuous form. For example, the second transparent conductive layer can include a conductive polymer or composite, such as PEDOT:PSS, combined with a particulate thermoplastic polymer. When heat-activated during use, the thermoplastic polymer flows and the layer becomes adhered to the lower photovoltaic subcell.

[0110] In other embodiments, the second transparent conductive layer comprises a heat-activatable conductive polymer composition, such as PEDOT:PSS in combination with a polyhydric alcohol such as D-sorbitol (see, e.g., J. Ouyang, Y. Yang, Adv. Mater. 2006, 18, 2141).

[0111] In some embodiments, the transparent conductive adhesive comprises a radiation activatable adhesive such that the second transparent conductive layer is adhered to the lower photovoltaic subcell when the activatable adhesive is irradiated with suitable actinic light.

[0112] The second transparent conductive layer may be adapted to solidify or harden when activated and bonded to the lower subcell, thereby forming a robust transparent conductive interlayer between the upper and lower subcells. This may simply require cooling the heat-activated adhesive to re-harden. However, in some embodiments, the second transparent conductive layer comprises an activatable adhesive that irreversibly cures via a covalent bond-forming reaction when activated, for example, by heat or radiation. An example of such a conductive adhesive is Panacol Elecolit 3065, available from Ulbricht.

[0113] In some embodiments, the transparent conductive adhesive comprises a pressure-sensitive adhesive such that the second transparent conductive layer adheres to the lower photovoltaic subcell when the thin-film photovoltaic device is pressed onto the upper receiving surface of the lower photovoltaic subcell. For example, the second transparent conductive layer may comprise PEDOT:PSS in combination with an acrylic microemulsion pressure-sensitive adhesive (see, e.g., D. Bryant et al., Adv. Mater. 2014, 26, 7499-7504).

[0114] composition The transferable photovoltaic device configuration may be configured as a roll suitable for roll lamination processing, in which the thin film photovoltaic device is pressed against the lower photovoltaic subcell and the flexible release substrate is separated from the thin film photovoltaic device. The flexible release substrate is thus configured as an elongated film or tape when unwound. The thin film photovoltaic device may also be configured as continuous or discontinuous strips extending along the elongated film, preferably with dimensions suitable to fit a particular lower photovoltaic subcell.

[0115] Method for fabricating transferable photovoltaic device configurations The present invention also relates to a method for fabricating a transferable photovoltaic device configuration. The method includes providing a flexible release substrate and fabricating a thin-film photovoltaic device on the flexible release substrate by sequentially forming a sequence of functional layers comprising at least a first transparent conductive layer over the flexible release substrate and a photoactive layer over the first transparent conductive layer. The first transparent conductive layer is formed by solution processing and comprises at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder. The interface between the flexible release substrate and the first transparent conductive layer fabricated on the flexible release substrate is designed so that the flexible release substrate can be separated from the thin-film photovoltaic device during use, thereby exposing the first transparent conductive layer at the outer conductive surface of the thin-film photovoltaic device.

[0116] A particular advantage of the methods disclosed herein is that low-cost, readily scalable solution processing techniques can be used to fabricate the functional layers. At least the first transparent conductive layer is formed by solution processing, and in preferred embodiments, at least one, at least two, or all of the functional layers in the thin film photovoltaic device are solution-processed functional layers.

[0117] Solution-processed functional layers are formed by fabrication methods in which the materials forming the layer are deposited while in solution (known as "wet" processing methods). This is in contrast to "dry" processing methods in which such materials are deposited while in the gas or vapor phase. Solution-processed functional layers typically comprise the layer's functional material and, optionally, an organic binder. The wet solutions used to form solution-processed functional layers typically include the functional material, or precursors thereof, and an optional binder mixed together in a solvent.

[0118] An organic binder is typically required when one or more functional materials of the solution-processed layer are particulate materials that require solidification to form a properly adhesive or cohesive layer. Any suitable organic binder can be used. Examples of suitable organic binders include one or more of ethyl cellulose, butyl cellulose, nitrocellulose, hydroxycellulose, cellulose acetate butyrate, alkyd resins, epoxy resins, phenolic resins, acrylic resins, butyl carbitol, butadiene styrene rubber, polyvinylpyrrolidone, polyacrylamide, and cellulose derivatives. Polymeric organic binders are particularly preferred.

[0119] Suitable solvents include, without limitation, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile, N-methyl-2-pyrrolidone (NMP), gamma-butyrolactone (GBL), terpineol, dibutyl phthalate, butyl carbitol, turpentine, butyl glycol ether, butyl carbitol acetate, ethylene glycol ether acetate, tributyl citrate and tributyl phosphate, propylene glycol methyl ether acetate toluene, diethylene glycol butyl ether, propanol, benzyl alcohol, isopropyl alcohol, ethanol, methanol, chloroform, diethylene glycol derivatives, toluene, xylene isopropanol, ethyl acetate, water, chlorobenzene, or combinations thereof.

[0120] The wet solution for fabricating each solution-processed functional layer can be applied as a wet film onto the previous layer using any suitable method, such as, for example, at least one of casting, doctor blading, blade coating, bar coating, screen printing, inkjet printing, pad printing, knife coating, meniscus coating, slot-die coating, gravure printing, reverse gravure printing, kiss coating, micro-roll coating, curtain coating, slide coating, spray coating, flexography, offset printing, rotary screen printing, or dip coating. In exemplary embodiments, at least one, at least two, or all of the functional layers are fabricated using at least one of blade coating, screen printing, slot-die coating, gravure printing, or reverse gravure coating methods.

[0121] Once the wet film is fabricated, the solvent is removed to fabricate the thin film functional layer. A heat treatment step (also known as an annealing step) is typically used to dry the functional layer by removing the solvent from the wet film. The heat treatment step is preferably performed after each layer is applied. However, it should be understood that two or more layers may be applied before the combined layer is heat treated. Various heat treatment methods can be used depending on the specific requirements of each functional layer. In some embodiments, the heat treatment involves heating the wet film to at least 80°C, preferably to at least 100°C, and more preferably to at least 120°C. In embodiments, the heat treatment method is performed for 0 to 10 minutes.

[0122] Solution-processed functional layers can be advantageously fabricated on flexible substrates by roll-to-roll manufacturing techniques suitable for large-scale, low-cost, and high-throughput production. Thus, in some embodiments, the first transparent conductive layer is fabricated by roll-to-roll processing. In some embodiments, at least one, at least two, or all of the functional layers are fabricated by roll-to-roll processing.

[0123] A method of fabricating a transferable photovoltaic device configuration will now be described with reference to Figure 5. The method includes the step 500 of providing a flexible release substrate as generally described herein with respect to the transferable photovoltaic device configuration. Suitable flexible release substrates may be available from commercial sources. In some embodiments, the flexible release substrate is configured as a roll, such that thin film photovoltaic devices can be fabricated on the flexible release substrate by a series of coating and / or printing steps in a roll-to-roll manufacturing process.

[0124] In some embodiments, the flexible release substrate comprises a non-stick surface as described above, such that the first transparent conductive layer is formed directly on the non-stick surface, and in some embodiments, the flexible release substrate comprises a low-adhesion sacrificial layer on its surface as described herein above.

[0125] Thus, the method may include the optional step 510 of forming a low-adhesion sacrificial layer on a flexible release substrate. This may be done by applying a fluid composition including the components of the sacrificial layer to form a wet film over the flexible release substrate and solidifying the wet film to form the low-adhesion sacrificial layer. The fluid composition may include a solvent that is removed from the wet film, typically by heating, to form the low-adhesion sacrificial layer. Despite its inherent or activatable low-adhesion properties, the sacrificial layer as fabricated has sufficient adhesive integrity and adhesion properties to allow subsequent layers to be fabricated and supported.

[0126] The method includes step 520 of forming a first transparent conductive layer, as described herein, over a flexible release substrate by solution processing. The first transparent conductive layer may be formed directly on the flexible release substrate or on an intervening low-adhesion sacrificial layer (e.g., as optionally formed in step 510). The first transparent conductive layer may be formed by applying a fluid composition to form a wet film over the flexible release substrate and solidifying the wet film to form the first transparent conductive layer over the flexible release substrate. The fluid composition may include a solvent that is removed from the wet film, typically by heating, to form the first transparent conductive layer.

[0127] The fluid composition contains the necessary conductive components, as previously described herein, or their reactive precursors. The reactive precursors can include reactive metal compounds, such as metal halides or metal alkoxides, that react with moisture to form transparent conductive metal oxides in situ. Examples of such materials include titanium chloride solutions, titanium 2-propoxide solutions, and zinc acetate solutions, with and without dopants. The fluid composition can include an organic binder and / or an activatable adhesive; if the conductive component is not itself a polymer, it will contain at least one such component. A suitable activatable adhesive, such as a heat-activatable adhesive polymer, will adhere the first transparent conductive layer to the underlying layer and can be subsequently activated to release the flexible release substrate from the thin-film photovoltaic device.

[0128] The method may include an optional step 530 of forming one or more transparent functional layers, such as a charge transport layer as described previously herein, on the first transparent conductive layer. An electron selective transport layer or a hole selective transport layer may be fabricated by applying a fluid composition including a charge transport component or a reactive precursor thereof to form a wet film on the first transparent conductive layer and solidifying the wet film to form the charge transport layer. The fluid composition may include a solvent that is removed from the wet film, typically by heating, to form the charge transport layer.

[0129] The method includes step 540 of forming a photoactive layer as described herein over the first transparent conductive layer. The photoactive layer may be formed directly on the first transparent conductive layer or on an intervening transparent functional layer, such as a charge transport layer, formed in optional step 530. The photoactive layer may be fabricated by applying a flowable composition including one or more photoactive layer components or precursors thereof dispersed in a solvent to form a wet film over the first transparent conductive layer, and removing the solvent from the wet film to form the photoactive layer.

[0130] The flowable composition can be a perovskite precursor solution, which contains dissolved ionic components that crystallize into a photoactive perovskite structure when a wet film of the perovskite precursor solution is concentrated by drying. One method for preparing a suitable perovskite precursor solution is to simply dissolve the required perovskite composition in a sufficiently volatile polar solvent, such as dimethylformamide. Optionally, the perovskite precursor solution is applied to only a portion of an underlying layer (e.g., a charge transport layer) and allowed to spread over the intended footprint in response to adhesive forces between the polar perovskite precursor solution and the hydrophilic underlying layer, as disclosed in International Patent Application WO2020 / 073082.

[0131] The photoactive layer may be formed by sequentially applying two or more flowable compositions that together form the photoactive layer. An example of this approach, in which two perovskite precursor solutions are sequentially applied to form a perovskite photoactive layer, is disclosed in international patent application published as WO2016 / 115602.

[0132] The flowable composition may be for fabricating an organic photovoltaic active layer and may therefore include photoactive layer components including an organic polymer electron donor material and a fullerene or non-fullerene acceptor.

[0133] The method may include the optional step 550 of forming one or more transparent functional layers, such as a charge transport layer as described previously herein, on the photoactive layer. An electron selective transport layer or a hole selective transport layer may be fabricated by applying a fluid composition containing a charge transport component or a reactive precursor thereof to form a wet film on the photoactive layer and solidifying the wet film to form the charge transport layer. The fluid composition may include a solvent that is removed from the wet film, typically by heating, to form the charge transport layer.

[0134] The method includes an optional step 560 of forming a second transparent conductive layer, as described herein, over the photoactive layer. The second transparent conductive layer includes a transparent conductive adhesive for adhering the thin-film photovoltaic device to the lower photovoltaic subcell. The second transparent conductive layer, as described previously herein, may be formed directly on the photoactive layer or on an intervening transparent functional layer, such as a charge transport layer, formed in optional step 550. The second transparent conductive layer may be formed by applying a fluid composition to form a wet film over the photoactive layer and solidifying the wet film to form the second transparent conductive layer. The fluid composition may include a solvent that is removed from the wet film, typically by heating, to form the second transparent conductive layer. The fluid composition will also include components of a transparent conductive adhesive, such as an activatable adhesive, and a conductive component or a reactive precursor thereof.

[0135] Method for fabricating tandem photovoltaic cells The present invention also relates to a method of fabricating a tandem photovoltaic cell. The method includes providing a transferable photovoltaic device configuration as disclosed herein or fabricated by a method as disclosed herein. The thin-film photovoltaic device of the transferable photovoltaic device configuration is adhered to the lower photovoltaic subcell with a transparent conductive adhesive. The thin-film photovoltaic device and the lower photovoltaic subcell are thereby operatively coupled through the second transparent conductive layer, which forms a recombination layer or tunnel junction in the tandem photovoltaic cell. The flexible release substrate of the transferable photovoltaic device configuration is separated from the thin-film photovoltaic device, exposing the first transparent conductive layer. The resulting tandem photovoltaic cell thus has the first transparent conductive layer of the thin-film photovoltaic device on its outer conductive surface.

[0136] The lower photovoltaic subcell can be, in principle, any type of photovoltaic cell with a suitable bandgap for the lower subcell of a tandem photovoltaic cell, including a silicon photovoltaic cell, a copper indium gallium (di) selenide (CIGS) photovoltaic cell, a cadmium telluride (CdTe) photovoltaic cell, or a perovskite photovoltaic cell. The lower photovoltaic subcell generally has a conventional architecture, including a photoactive (light-absorbing) layer and a lower electrical contact (which may be opaque). The lower photovoltaic subcell has a conductive outer layer with a receiving surface to which a thin-film photovoltaic device is attached. The outer layer can be a highly doped Si layer (n-type or p-type) or a transparent conductive oxide layer.

[0137] In some embodiments, the lower photovoltaic subcell is a silicon photovoltaic cell. The silicon photovoltaic cell can be a conventional pn homojunction cell or a heterojunction silicon photovoltaic cell. In some embodiments, the lower photovoltaic subcell has a rigid, planar structure. In some embodiments, the lower photovoltaic subcell has a bendable, thin, planar structure. In some embodiments, the lower photovoltaic subcell is a textured crystalline silicon (c-Si) solar cell.

[0138] In some embodiments, the transparent conductive adhesive is present as a pre-formed outer layer of either the thin-film photovoltaic device or the lower photovoltaic subcell. In such cases, the thin-film photovoltaic device and the lower photovoltaic subcell are contacted to adhere them via the second transparent conductive layer. In other embodiments, the method may include the additional step of applying a coating of transparent conductive adhesive to the surface of either the thin-film photovoltaic device or the lower photovoltaic subcell before contacting the thin-film photovoltaic device and the lower photovoltaic subcell.

[0139] In some embodiments, the transparent conductive adhesive is activated to facilitate adhesion and / or curing of the second transparent conductive layer. The transparent conductive adhesive can be activated with heat, radiation, or chemical treatment either before or after contacting the thin-film photovoltaic device with the bottom photovoltaic subcell.

[0140] In some embodiments, the transparent conductive adhesive is optionally heat-activated at a temperature in the range of 50°C to 170°C, such as in the range of 90°C to 140°C. For example, the transferable photovoltaic device configuration can be hot-pressed onto the lower photovoltaic subcell at an appropriate temperature and dwell time to fluidize or soften the heat-activated adhesive polymer in the transparent conductive adhesive. The activated transparent conductive adhesive can thus become adhesively bonded to the receiving surface of the lower photovoltaic subcell. When the hot press is removed, the adhesive polymer cools and hardens, providing a permanent adhesive bond.

[0141] In some embodiments, the transparent conductive adhesive is radiation activated, for example, the transferable photovoltaic device configuration is exposed to actinic radiation, such as ultraviolet light, to facilitate initial adhesion to the bottom subcell and subsequent curing via a covalent bond forming reaction to form a permanent adhesive bond.

[0142] The thin film photovoltaic device can be pressed or compressed onto the lower photovoltaic subcell to facilitate adhesion. Any suitable compression / pressing process can be used. For example, the compression / pressing process can include a roll press, such as a calendar press / laminator, uniaxial press, or isostatic press. In embodiments, the thin film photovoltaic device is applied to the lower photovoltaic subcell by compression, preferably by a press configuration, and more preferably by calendar press.

[0143] In some embodiments, the thin film photovoltaic device is gently brought into contact with the bottom photovoltaic subcell, for example, by calendar pressing the thin film photovoltaic device onto the bottom photovoltaic subcell in a roll lamination process.

[0144] A sufficient adhesive bond between the thin-film photovoltaic device and the lower photovoltaic subcell is required to ensure that the flexible release substrate can be selectively separated from the flexible release substrate when desired. However, it is understood that the thin-film photovoltaic device and the lower photovoltaic subcell do not need to be in contact across the entire intended interface before separation of the flexible release film begins. In a roll lamination process, for example, the thin-film photovoltaic device and the lower photovoltaic subcell need only be in contact and adhered below the separation interface between the flexible release substrate and the thin-film photovoltaic device. Furthermore, the strength of the final adhesive bond of the interlayer between the thin-film photovoltaic device and the lower photovoltaic subcell does not need to be fully developed by the time the flexible release substrate is separated.

[0145] The flexible release substrate can be separated from the thin-film photovoltaic device at any time after the thin-film photovoltaic device is bonded to the lower photovoltaic subcell below the separation interface. In some embodiments, the flexible release substrate is separated from the thin-film photovoltaic device in a continuous process step that can, in principle, be carried out at different times and places. For example, the thin-film photovoltaic device configuration can be bonded to the lower photovoltaic subcell on one production line, and the flexible release substrate is separated and removed on a different production line, or even during assembly of the solar cell module. In other embodiments, the thin-film photovoltaic device is bonded to the lower photovoltaic subcell, and the flexible release substrate is separated from the thin-film photovoltaic device in a single process, such that the two steps occur nearly simultaneously (i.e., within a few minutes, preferably within just a few seconds).

[0146] As disclosed herein, the separating step involves selectively breaking the interface between the flexible release substrate and the thin-film photovoltaic device. In some embodiments, for example, when the flexible release substrate has a non-stick surface or when an inherently low-adhesion sacrificial layer is interposed between the thin-film photovoltaic device and the flexible release substrate, the flexible release substrate can be separated and removed once the thin-film photovoltaic device is adhered to the lower photovoltaic subcell simply by gently peeling the release substrate away.

[0147] In other embodiments, for example, when the first transparent conductive layer is activatable or when an activatable low-adhesion sacrificial layer is interposed between the thin-film photovoltaic device and the flexible release substrate, separating the flexible release substrate from the thin-film photovoltaic device includes an activation step, which can include activating an activatable adhesive in either the first transparent conductive layer or the low-adhesion sacrificial layer, for example, by heat or radiation.

[0148] In some embodiments, the activation step involves thermally activating the first transparent conductive layer or the low-adhesion sacrificial layer at a temperature in the range of 50°C to 170°C, such as in the range of 90°C to 140°C. For example, the transferable photovoltaic device configuration can be hot-pressed onto the lower photovoltaic subcell at an appropriate temperature and dwell time to fluidize or soften the heat-activated adhesive polymer in the first transparent conductive layer or the low-adhesion sacrificial layer. This releases the adhesive bond between the flexible release substrate and the thin-film photovoltaic device, allowing the flexible release substrate to be peeled off. In other embodiments, the flexible release substrate is released by irradiating the first transparent conductive layer or the low-adhesion sacrificial layer with actinic light to activate the activatable adhesive, for example, by a depolymerization mechanism.

[0149] In some embodiments, a single activation step is used to (i) release the flexible release layer from the thin-film photovoltaic device, thereby enabling separation of the flexible release layer, and (ii) activate the transparent conductive adhesive to adhere the thin-film photovoltaic device to the bottom photovoltaic subcell. In some embodiments, the thin-film photovoltaic device is adhered to the bottom photovoltaic subcell by thermally activating the transparent conductive adhesive at a temperature sufficient to release the flexible release substrate from the thin-film photovoltaic device. For example, the transferable photovoltaic device configuration can be hot-pressed onto the bottom photovoltaic subcell at an appropriate temperature and dwell time to (i) heat-activate the transparent conductive adhesive to adhere the thin-film photovoltaic device to the bottom photovoltaic subcell, and (ii) heat-activate the first transparent conductive layer or low-adhesion sacrificial layer to release the adhesive bond between the flexible release substrate and the thin-film photovoltaic device.

[0150] In some embodiments, the transferable photovoltaic device configuration is configured as a roll, and the thin film photovoltaic device is transferred to the bottom photovoltaic sub-cell in a roll lamination process.

[0151] A method for fabricating tandem photovoltaic cells with a roll lamination process is described with reference to Figure 6. A transferable photovoltaic device configuration 600 comprises a flexible release substrate 602 and a thin film photovoltaic device 604 (covering the entire underside of the flexible release substrate 602, not visible in Figure 6) disposed as a continuous strip on the flexible release substrate 602. The interface between the flexible release substrate 602 and the transparent conductive inner layer of the thin film photovoltaic device 604 is designed to allow separation as disclosed herein. The thin film photovoltaic device 604 includes a layer of transparent conductive adhesive on its outer surface.

[0152] A plurality of lower Si-PV subcells 608 are held on a rigid translation stage 610, each of which includes a conductive outer surface with a receiving surface 612 for receiving a thin-film photovoltaic device 604. Both the translation stage 610 and the transferable photovoltaic device configuration 600 are passed through a calender press 614 in the direction indicated by arrow 615. Thus, the configuration 600, which is initially configured as a roll (not shown), is unwound and brought into contact with the lower Si-PV subcells 608. The calender roll 616 presses the thin-film photovoltaic device 604 onto the receiving surface 612 for a sufficient dwell time to bond a layer of transparent conductive adhesive to the receiving surface 612. Optionally, the calender roll 616 is heated to apply a temperature sufficient to heat-activate the transparent conductive adhesive, thereby facilitating bonding.

[0153] After the laminated structure emerges from the other side of the calendar press 614, the flexible release substrate 602 is detached from the moving stage 610 and, optionally, rewound onto another roll. Because the interface between the flexible release substrate 602 and the thin-film photovoltaic device 604 is the weakest interface after roll lamination, the flexible release substrate 602 is selectively peeled off wherever the thin-film photovoltaic device 604 is adhered to the bottom Si-PV subcell 608, i.e., from the cut-out areas 613 of the flexible release substrate 602. Thus, the tandem photovoltaic cell 620 is formed with (i) a transparent conductive layer (i.e., a transparent conductive adhesive) that forms a recombination layer or tunnel junction between the bottom Si-PV subcell and the transferred thin-film photovoltaic device 604a (top subcell), and (ii) the transparent conductive layer 618 of the thin-film photovoltaic device 604a exposed at its outer surface. Peeling of the flexible release substrate 602 from the transparent conductive layer 618 can be facilitated by heat, optionally applied by the calender roll 616, which heat activates the transparent conductive layer 618 itself or an intervening low-tack sacrificial layer. Alternatively, the flexible release substrate 602 may have a non-stick surface such that the interface between the flexible release substrate 602 and the transparent conductive layer 618 is essentially separable (i.e., unactivated).

[0154] Separation of the flexible release substrate from the thin-film photovoltaic device exposes the first transparent conductive layer on the outer conductive surface of the tandem photovoltaic cell. Removal of the insulating release substrate advantageously allows current to flow through the outer surface of the tandem cell during operation. However, on large surface area tandem photovoltaic cells, current collection via a metallic current collecting network on the outer sunlight-receiving surface is still necessary. Therefore, in some embodiments, the method still includes fabricating a metallic current collecting network on the outer conductive surface of the tandem photovoltaic cell, such as a silver metal grid, as typically applied to the outer surface of commercial-sized silicon photovoltaic cells. Optionally, one or more additional transparent conductive layers (e.g., vacuum-processed transparent conductive oxides) may be fabricated on the first transparent conductive layer of the tandem photovoltaic cell. Thus, a metallic current collecting network may be fabricated on the newly provided conductive surface.

[0155] Tandem Photovoltaic Cell The present invention also relates to a tandem photovoltaic cell configuration comprising a bottom photovoltaic sub-cell and a transferable photovoltaic device configuration as disclosed herein, wherein the thin film photovoltaic device is adhered to the bottom photovoltaic sub-cell via a second transparent conductive layer comprising a transparent conductive adhesive. The bottom photovoltaic sub-cell may be as previously disclosed herein and in some exemplary embodiments is a silicon photovoltaic cell.

[0156] In some embodiments, the flexible release substrate remains unseparated from the thin film photovoltaic device, however, the interface between the flexible release substrate and the thin film photovoltaic device is designed to allow removal of the flexible release substrate when desired, optionally by first releasing the interface by applying a suitable external stimulus such as heat or radiation, and then peeling the flexible release substrate away.

[0157] In other embodiments, the flexible release substrate is partially separated from the thin film photovoltaic device, thereby exposing the first transparent conductive layer at the outer conductive surface. Such a configuration may be an intermediate structure obtained when fabricating a tandem photovoltaic cell according to the methods disclosed herein.

[0158] Once the flexible release substrate is completely separated, a tandem photovoltaic cell is fabricated.Accordingly, the present invention also relates to a tandem photovoltaic cell fabricated by the methods disclosed herein.

[0159] Embodiment An embodiment of the present invention will be described with reference to Figure 2. The transferable photovoltaic device configuration 200 comprises a flexible release substrate 202 on which a thin-film photovoltaic device 204 is positioned. The thin-film photovoltaic device 204 comprises the following functional layers, in sequence, over the flexible release substrate: a first transparent conductive layer 208, a charge transport layer 226, a photoactive layer 210, a charge transport layer 228, and a second transparent conductive layer 222. The second transparent conductive layer 222 comprises a heat-activatable transparent conductive adhesive, as disclosed herein. As needed to match the architecture of the lower subcell, the charge transport layer 226 can be an electron-selective transport layer and the charge transport layer 228 a hole-selective transport layer, or vice versa.

[0160] The transferable photovoltaic device configuration 200 includes a low-tack sacrificial layer 206 interposed between a flexible release substrate 202 and a transparent conductive layer 208. The sacrificial layer 206 includes a heat-activatable adhesive (such as PEO) and optionally includes a conductive component (such as PEDOT:PSS). When heated, the heat-activatable adhesive fluidizes or softens such that the sacrificial layer develops a suitable low tack. The sacrificial layer thus forms an interface that is selectively breached when the flexible release substrate 202 is separated from the thin-film photovoltaic device 204 in the bonding and detachment method step indicated by arrow 250.

[0161] In a method of fabricating a tandem photovoltaic cell, depicted by arrow 250, the thin-film photovoltaic device 204 is transferred to the bottom photovoltaic subcell 212, which may be a silicon photovoltaic cell. The thin-film photovoltaic device 204 is thus adhered to the bottom subcell 212 by (i) contacting the second transparent conductive layer 222 with a receiving surface of the bottom subcell 212, and (ii) heat-activating a transparent conductive adhesive in the second transparent conductive layer 222 to induce adhesion. Optionally, this is done by hot-pressing the transferable photovoltaic device configuration 200 onto the bottom photovoltaic subcell 212 at an appropriate temperature and dwell time to fluidize or soften the heat-activatable adhesive polymer in the transparent conductive adhesive. After the heat-activated bonding step, the thin film photovoltaic device 204 and the bottom photovoltaic subcell 212 are functionally coupled via the transparent conductive layer 222, which forms a recombination layer or tunnel junction between the top and bottom subcells in the tandem photovoltaic cell 230.

[0162] The transfer method 250 also includes separating the flexible release substrate 202 from the thin-film photovoltaic device 204. The separation step involves (i) heat-activating the heat-activatable adhesive in the sacrificial layer 206 to reduce the adhesion of the sacrificial layer 206, and (ii) peeling off the flexible release substrate 202 to gradually break the activated sacrificial layer. Optionally, the heat activation occurs when the transferable photovoltaic device configuration 200 is hot-pressed onto the bottom photovoltaic subcell 212, such that the flexible release substrate 202 is separated substantially simultaneously with the adhesion of the thin-film photovoltaic device 204 to the bottom photovoltaic subcell 212. This may be done in a roll lamination process, as described herein with reference to FIG. 6.

[0163] Following the separation step, the tandem photovoltaic cell 230 has a transparent conductive layer 208 at the outer conductive surface 224 from which current can be drawn when the tandem photovoltaic cell 230 is in operation. Because the sacrificial layer 206 is a very thin layer (and, optionally, conductive), any residue 206a remaining on the transparent conductive layer 208 has only an acceptably small effect on the conductivity of the outer conductive surface 224.

[0164] Another embodiment of the present invention will be described with reference to Figure 3. The transferable photovoltaic device configuration 300 comprises a flexible release substrate 302 on which a thin film photovoltaic device 304 is positioned. The flexible release substrate 302 comprises a flexible polymer film 303 (such as PET) coated with a non-stick coating 306 (a low surface energy polymer such as a siloxane polymer). The thin film photovoltaic device 304 comprises, in sequence, the following functional layers: a first transparent conductive layer 308, a charge transport layer 326, a photoactive layer 310, a charge transport layer 328, and a second transparent conductive layer 322. The second transparent conductive layer 322 comprises an activatable transparent conductive adhesive as disclosed herein.

[0165] The first transparent conductive layer 308 is in direct contact with the non-stick coating 306 and therefore forms an interface that is selectively broken when separating the flexible substrate 302 from the thin film photovoltaic device 304 in the bonding and separation method step supported by arrow 350.

[0166] In a method of fabricating a tandem photovoltaic cell, depicted by arrow 350, the thin-film photovoltaic device 304 is transferred to the bottom photovoltaic subcell 312, which may be a silicon photovoltaic cell. Thus, the thin-film photovoltaic device 304 is adhered to the bottom subcell 312 by (i) contacting the second transparent conductive layer 322 with a receiving surface of the bottom subcell 312, and (ii) activating, for example by heat or radiation, the transparent conductive adhesive in the second transparent conductive layer 322 to induce adhesion. Optionally, activation is performed while pressing the transferable photovoltaic device configuration 300 onto the bottom photovoltaic subcell 312.

[0167] The transfer method 350 also includes a step of separating the flexible release substrate 302 from the thin-film photovoltaic device 304. The separation step simply involves peeling the flexible substrate 302 to gradually break the interface between the non-stick coating 306 and the first transparent conductive layer 308. The flexible substrate 302 can be separated substantially simultaneously with the bonding of the thin-film photovoltaic device 304 to the bottom photovoltaic subcell 312, for example, in a roll lamination process, as described herein with reference to FIG.

[0168] Another embodiment of the present invention is described with reference to Figure 4. The transferable photovoltaic device configuration 400 comprises a flexible release substrate 402 on which a thin film photovoltaic device 404 is positioned. The flexible release substrate 402 can be a flexible polymer film such as PET. The thin film photovoltaic device 404 comprises, in sequence, the following functional layers: a first transparent conductive layer 408, a charge transport layer 426, a photoactive layer 410, a charge transport layer 428, and a second transparent conductive layer 422. The second transparent conductive layer 422 comprises a heat-activatable transparent conductive adhesive, as disclosed herein.

[0169] The first transparent conductive layer 408 is in direct contact with the flexible release substrate 402, thus forming an interface that is selectively broken when the flexible substrate 402 is separated from the thin film photovoltaic device 404 in the adhesion and detachment method step indicated by arrow 450. The first transparent conductive layer 408 comprises a heat-activatable adhesive (e.g., a thermoplastic polymer) and a conductive component, which may be a suitable metal, metal oxide, or conductive polymer or polymer composite (such as PEDOT:PSS).

[0170] In a method of fabricating a tandem photovoltaic cell, depicted by arrow 450, the thin-film photovoltaic device 404 is transferred to the bottom photovoltaic subcell 412, which may be a silicon photovoltaic cell. The thin-film photovoltaic device 404 is thus adhered to the bottom subcell 412 by (i) contacting the second transparent conductive layer 422 with a receiving surface of the bottom subcell 412, and (ii) heat-activating a transparent conductive adhesive in the second transparent conductive layer 422 to induce adhesion. Optionally, this is done by hot-pressing the transferable photovoltaic device configuration 400 onto the bottom photovoltaic subcell 412 at an appropriate temperature and dwell time to fluidize or soften the heat-activatable adhesive polymer in the transparent conductive adhesive.

[0171] The transfer method 450 also includes separating the flexible release substrate 402 from the thin-film photovoltaic device 404. The separation step involves (i) heat-activating the heat-activatable adhesive in the first transparent conductive layer 408 to release (peel) the flexible release substrate, and (ii) peeling the flexible release substrate 402 to gradually break the interface. Optionally, the heat activation occurs when the transferable photovoltaic device configuration 400 is hot-pressed onto the bottom photovoltaic subcell 412, such that the flexible release substrate 402 is separated approximately simultaneously with the adhesion of the thin-film photovoltaic device 404 to the bottom photovoltaic subcell 412. This may be done in a roll lamination process, as described herein with reference to FIG. 6.

[0172] (Example) The present invention will now be described with reference to the following examples, which should be understood as being illustrative of, but not limiting on, the invention described herein.

[0173] Materials and Procedures A custom-built roll-to-roll slot die coating machine with a slot die head with a 50 μm shim was used to create the solution-processed films. The machine was designed to handle narrow (25 mm wide) films with minimal material usage for the purposes of the study. The sheet resistance of the films was tested using a Jandel RM3000 four-point probe system.

[0174] Example 1 Transferable photovoltaic devices on non-stick surfaces The transparent conductive layer was fabricated by applying a commercially available aqueous 1% PEDOT:PSS solution (S315, Agfa) by roll-to-roll slot die coating onto a 25 mm wide, 50 micron thick roll of PET film coated with a non-stick silicone release layer (FRA 319, Fox River). Aqueous solutions, including aqueous PEDOT:PSS solutions, dewet onto non-stick, low-surface-energy surfaces, preventing the formation of a continuous layer. Therefore, the PEDOT:PSS solution was deposited onto the substrate on a heated stage (80 °C), which reduces the surface tension of the solution and accelerates solvent evaporation, thereby mitigating the dewetting problem. An air blade (10 L / min) was also engaged to accelerate solvent evaporation before dewetting occurred. The PEDOT:PSS solution was applied at a rate of 0.3 m / min, with a 3.8 μl / cm2 flow rate. 2 (hence a wet film thickness of about 38 microns) was deposited in a continuous strip 13 mm wide and dried at 130° C. for 30 seconds.

[0175] An electron transport layer was then fabricated on the first transparent conductive layer by applying a commercial ZnO solution (2.8% in 2-propanol, InfityPV) by roll-to-roll slot die coating on the dried PEDOT:PSS film. The ZnO solution was applied at a rate of 0.3 m / min at a flow rate of 0.5 μl / cm. 2(thus a wet film thickness of about 5 microns) was deposited in a continuous strip 13 mm wide across the PEDOT:PSS strip and dried at 130° C. for 30 seconds.

[0176] The photoactive layer was then fabricated on the electron transport layer by applying a solution consisting of 8 mg of PBF-QxF and 12 mg of Y6 in dichlorobenzene per ml by roll-to-roll slot die coating at a rate of 2 μl / cm2 at a substrate speed of 0.3 m / min. 2 (hence, a wet film thickness of about 20 microns, a dry film thickness of about 260 nm) was deposited in a 7 mm wide continuous strip across the ZnO strip while on a heated stage (130°C).

[0177] The separability of multilayer photovoltaic devices comprising a transparent conductive layer, an electron transport layer, and a photoactive layer from a release substrate was investigated by a simple tape peel test (also known as the Scotch tape test). It was observed that the devices could be selectively pulled apart at the interface between the transparent conductive layer and the non-stick coating of the release layer, allowing the multilayer photovoltaic device to be transferred intact to the tape.

[0178] Example 2 Transferable photovoltaic devices with sacrificial layers The coating composition for preparing the sacrificial layer was prepared by mixing 10 ml of a commercially available aqueous 1.3-1.7% PEDOT:PSS solution (Clevios AI 4083, Heraeus), 200 mg of polyethylene oxide (PEO), a water-soluble low-melting polymer (molecular weight 100,000 daltons, melting point 65 °C), and 10 ml of 2-propanol. The sacrificial layer was fabricated by applying this mixture by roll-to-roll slot die coating onto a roll of uncoated polyethylene terephthalate (PET) film. The mixture was applied at a concentration of 1 μl / cm at room temperature. 2(hence a wet film thickness of approximately 10 microns) was deposited in a continuous strip 25 mm wide and dried at 130°C for 30 seconds. Non-tacky at room temperature, the dried film softened and became tacky when heated above 80°C. In a Scotch tape test, the tape adhered firmly at room temperature and was easily removed when heated to 80°C. Based on visual inspection, the sacrificial layer remained present on the PET film after removal.

[0179] Next, a transparent conductive layer was fabricated by applying a commercially available aqueous 1% PEDOT:PSS solution (S315, Agfa) by roll-to-roll slot die coating onto the sacrificial layer at a rate of 0.3 m / min with a flow rate of 3.8 μl / cm. 2 (hence a wet film thickness of about 38 microns) was deposited in a continuous strip 13 mm wide and dried at 130° C. for 30 seconds. The sheet resistance of the transparent conductive layer was about 80 Ω / sq.

[0180] Next, a photoactive layer was fabricated on the transparent conductive layer by applying a solution consisting of 8 mg of PBF-QxF and 12 mg of Y6 in dichlorobenzene per ml by roll-to-roll slot die coating at a rate of 2 μl / cm2 at a substrate speed of 0.3 m / min. 2 (thus a wet film thickness of about 20 microns, a dry film thickness of about 260 nm) was deposited in a 7 mm wide continuous strip across the PEDOT:PSS strip while on a heated stage (130°C).

[0181] The separability of multilayer photovoltaic devices comprising a transparent conductive layer and a photoactive layer from a peel substrate was investigated by a Scotch tape test at 80 °C. The devices were easily peeled off from the PET substrate via thermally activated sacrificial layer tearing. The sheet resistance of the transparent conductive layer, as exposed after removal of the PET film, was 90-100 Ω / sq.

[0182] PEDOT:PSS was included in the sacrificial layer for several reasons. First, PEDOT:PSS overcomes the dewetting problem encountered when roll-to-roll slot-die coating only a solution of coated PEO. Second, the conductive PEDOT:PSS is believed to mitigate the impact of sacrificial layer residue on the conductivity of the transparent conductive layer.

[0183] Example 3 Transfer of photovoltaic devices by bonding with transparent conductive adhesives and removing the peel substrate A coating composition to prepare the separation enabling sacrificial layer was prepared by dissolving 2 g of ethylene vinyl acetate in 40 ml of xylene with stirring for 30 minutes at 50°C. The sacrificial layer was fabricated by applying this solution by roll-to-roll slot die coating onto a roll of uncoated polyethylene terephthalate (PET) film. The mixture was applied at a concentration of 0.8 μl / cm at 90°C. 2 (hence a wet film thickness of about 8 microns) was deposited in a continuous strip 25 mm wide and dried for 30 seconds by blowing hot air at 80° C. The dry film, which was non-tacky at room temperature, softened and became tacky when heated above 90° C.

[0184] Next, a transparent conductive layer was fabricated by applying a commercially available aqueous 1% PEDOT:PSS solution (S315, Agfa) by roll-to-roll slot die coating onto the sacrificial layer at a rate of 0.3 m / min and a flow rate of 3.3 μl / cm. 2 (hence a wet film thickness of about 33 microns) was deposited in a width of 13 mm onto a 25 mm wide PET film and dried for 2 minutes at 90° C. The relatively low drying temperature was chosen to avoid dissolving the sacrificial layer before the conductive layer was solidified.

[0185] Next, the electron transport layer was coated with 0.58 μl / cm at a speed of 0.2 m / min. 2The film was fabricated by applying a commercially available ZnO nanoparticle solution (InfinityPV, 2.8% in 2-propanol) by roll-to-roll slot die coating onto a transparent conductive layer, resulting in a wet film thickness of approximately 5.8 microns. The film was then dried by hot air blowing at 90°C for 2 minutes.

[0186] The coating composition for preparing the perovskite photoactive layer was prepared by mixing 21.5 mg of formamidinium iodide, 139.1 mg of methylammonium iodide, 5.6 mg of methylammonium bromide, 507 mg of lead iodide, and 1 ml of acetonitrile. Methylamine gas was bubbled through the solution until it was empty. The perovskite precursor solution was then bubbled through the solution at a rate of 0.6 m / min, with 0.58 μl / cm. 2 (thus a wet film thickness of about 5.8 microns) was applied by roll-to-roll slot die coating onto the electron transport layer.

[0187] The resulting transferable photovoltaic device configuration 700, as depicted schematically in FIG. 7, comprises a PET flexible release substrate 702, a heat-activatable low-adhesion sacrificial layer 704, a PEDOT:PSS transparent conductive layer 706, a ZnO electron transport layer 708, and a perovskite photoactive layer 710.

[0188] The transferable photovoltaic device configuration 700 was then used to transfer the thin-film photovoltaic device to a conductive substrate (Solutia OC50 substrate) comprising a PET film 712 coated with a layer of indium tin oxide (ITO) 714 by the following method (method 701 depicted schematically in FIG. 7 ): A transparent conductive adhesive (3M, conductive adhesive transfer tape) layer 716 (25 mm wide) was applied by manual lamination to the ITO layer 714. The ITO-coated PET film was then placed on a hot stage (110° C.), and the transferable photovoltaic device configuration 700 was manually laminated to the ITO-coated PET film such that the perovskite photoactive layer 710 was adhered to the ITO layer 714 via the transparent conductive adhesive layer 716. Shortly thereafter, the PET flexible release substrate 702 is peeled away, leaving the thin film photovoltaic device comprising layers 706, 708, 710 adhered to the conductive substrate by the transparent conductive intermediate layer 716, with the PEDOT:PSS transparent conductive layer 706 exposed at its outer surface. The thin film photovoltaic device was cleanly detached from the PET flexible release substrate 702 via thermally activated rupture of the sacrificial layer 704.

[0189] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, and it is to be understood that the invention includes all such variations and modifications that fall within the spirit and scope of the invention. [Explanation of symbols]

[0190] 100 Transferable Photovoltaic Device Configurations 102 Flexible release substrate 104 Photovoltaic Devices 106 Boundary 108 Transparent conductive layer 110 Photoactive layer 112 Lower photovoltaic subcell 114 Upper Conduction Layer 116 Lower electrical contact 118 Photoactive semiconductor layer 120 Transparent conductive adhesive 122 Transparent conductive layer 130 Tandem Photovoltaic Cell 150 Transfer Method 200 Transferable Photovoltaic Device Configurations 202 Flexible release substrate 204 Thin-film photovoltaic devices 206 Low-adhesion sacrificial layer 206a Residue 208 First transparent conductive layer 210 Photoactive layer 212 Lower photovoltaic subcell 222 Second transparent conductive layer 224 outer conductive surface 226 Charge transport layer 228 Charge transport layer 230 Tandem Photovoltaic Cell 250 Method for fabricating tandem photovoltaic cells 300 Transferable Photovoltaic Device Configurations 302 Flexible release substrate 303 Flexible polymer membrane 304 Thin-film photovoltaic devices 306 Non-stick covering 308 First transparent conductive layer 310 Photoactive layer 312 Lower Subcell 322 Second transparent conductive layer 326 Charge transport layer 328 Charge transport layer 350 Method for fabricating tandem photovoltaic cells 400 Transferable Photovoltaic Device Configurations 402 Flexible release substrate 404 Thin-film photovoltaic devices 408 First transparent conductive layer 410 Photoactive layer 412 Lower Photovoltaic Subcell 426 Charge transport layer 428 Charge transport layer 422 Second transparent conductive layer 450 Method for fabricating tandem photovoltaic cells 600 Transferable Photovoltaic Device Configurations 602 Flexible release substrate 604 Thin-film photovoltaic devices 604a Transferred thin film photovoltaic device, upper subcell 608 Lower Si-PV subcell 610 Rigid Moving Stage 612 Receiving Surface 613 Clipping Area 614 Calendar Press 615 directions 616 Calendar Roll 618 Transparent Conductive Layer 700 Transferable Photovoltaic Device Configurations 701 Method 702 PET flexible release backing 704 Heat Activatable Low Adhesion Sacrificial Layer 706 PEDOT:PSS transparent conductive layer 708 ZnO electron transport layer 710 Perovskite photoactive layer 716 Transparent conductive adhesive layer

Claims

1. A transferable photovoltaic device configuration for transferring a thin-film photovoltaic device to a lower photovoltaic subcell in order to fabricate a tandem photovoltaic cell, Flexible release substrate and (i) a first transparent conductive layer positioned above the flexible release substrate, and (ii) a thin-film photovoltaic device comprising a photoactive layer positioned above the first transparent conductive layer. Equipped with, The first transparent conductive layer is a solution-treated layer comprising at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder. The flexible release substrate is separable from the thin-film photovoltaic device after the thin-film photovoltaic device is bonded to the lower photovoltaic subcell with a transparent conductive adhesive, thereby exposing the first transparent conductive layer on the outer conductive surface of the thin-film photovoltaic device, thus providing a transferable photovoltaic device configuration.

2. The transferable photovoltaic device configuration according to claim 1, wherein the first transparent conductive layer comprises a first activatable adhesive, and the flexible release substrate is separable from the thin-film photovoltaic device when the first activatable adhesive is activated.

3. The transferable photovoltaic device configuration according to claim 2, wherein the first activatable adhesive is a thermally activatable adhesive polymer.

4. The transferable photovoltaic device configuration according to claim 1, wherein the flexible release substrate has a non-adhesive surface, and the flexible release substrate can be separated from the thin-film photovoltaic device by peeling the first transparent conductive layer from the non-adhesive surface.

5. The transferable photovoltaic device configuration according to claim 4, wherein the non-adhesive surface is the surface of the non-adhesive coating on the flexible release substrate.

6. The transferable photovoltaic device configuration according to claim 1, comprising a low-tack sacrificial layer interposed between the flexible release substrate and the first transparent conductive layer, wherein the low-tack sacrificial layer has intrinsically low tackiness, or has low tackiness when activated, such that the flexible release substrate can be separated from the thin-film photovoltaic device by breaking the low-tack sacrificial layer.

7. The transferable photovoltaic device configuration according to any one of claims 1 to 6, wherein the thin-film photovoltaic device further comprises (iii) a second transparent conductive layer positioned above the photoactive layer, the second transparent conductive layer comprising the transparent conductive adhesive for bonding the thin-film photovoltaic device to the lower photovoltaic subcell.

8. The transferable photovoltaic device configuration according to claim 7, wherein the transparent conductive adhesive comprises a second activatable adhesive, and the second transparent conductive layer is bonded to the lower photovoltaic subcell when the second activatable adhesive is activated.

9. The transferable photovoltaic device configuration according to any one of claims 1 to 6, wherein the photoactive layer is a photoactive perovskite layer.

10. The transferable photovoltaic device configuration according to any one of claims 1 to 6, wherein the thin-film photovoltaic device is pressed onto the lower photovoltaic subcell and the flexible release substrate is configured as a roll for a roll lamination process in which the thin-film photovoltaic device is separated.

11. A tandem photovoltaic cell configuration comprising a lower photovoltaic subcell and a transferable photovoltaic device configuration according to any one of claims 1 to 6, wherein the thin-film photovoltaic device is bonded to the lower photovoltaic subcell via a second transparent conductive layer containing a transparent conductive adhesive.

12. A method for fabricating a portable photovoltaic device configuration, The steps include providing a flexible release substrate and The steps include: continuously forming (i) a first transparent conductive layer on the flexible release substrate and (ii) a photoactive layer on the first transparent conductive layer to fabricate a thin-film photovoltaic device on the flexible release substrate; Includes, The first transparent conductive layer is formed by solution treatment and comprises at least one selected from a conductive polymer or polymer composite, an activatable adhesive, and an organic binder. A method wherein the flexible peelable substrate is separable from the thin-film photovoltaic device in order to expose the first transparent conductive layer on the outer conductive surface of the thin-film photovoltaic device.

13. The method according to claim 12, wherein forming the first transparent conductive layer on the flexible release substrate comprises applying a first fluid composition containing a first conductive component or its reactive precursor to form a wet film on the flexible release substrate, and solidifying the wet film to form the first transparent conductive layer on the flexible release substrate.

14. The method according to claim 13, wherein the first fluid composition comprises a first activatable adhesive.

15. The method according to claim 14, wherein the first activatable adhesive is a heat-activatable adhesive polymer, and the flexible release substrate is separable from the thin-film photovoltaic device when the first transparent conductive layer is heated.

16. The method according to claim 12 or 13, wherein the flexible release substrate has a non-adhesive surface, and the first transparent conductive layer is formed directly on the non-adhesive surface.

17. The method according to claim 12 or 13, wherein the flexible release substrate includes a low-tack sacrificial layer on its surface, and the first transparent conductive layer is formed directly on the low-tack sacrificial layer.

18. The method according to claim 12 or 13, wherein the flexible release substrate is configured as a roll, and the thin-film photovoltaic device is fabricated on the flexible release substrate by a coating step and / or printing step in a roll-to-roll process.

19. A method for fabricating a tandem photovoltaic cell, The steps of providing a portable photovoltaic device configuration as described in any one of claims 1 to 6, or a portable photovoltaic device configuration as manufactured by the method described in claim 12 or 13, A step of bonding the thin-film photovoltaic device of the transferable photovoltaic device configuration to a lower photovoltaic subcell using a transparent conductive adhesive, thereby bonding the thin-film photovoltaic device and the lower photovoltaic subcell via a second transparent conductive layer, A step of separating the flexible peelable substrate of the transferable photovoltaic device configuration from the thin-film photovoltaic device, thereby fabricating a tandem photovoltaic cell having the first transparent conductive layer of the thin-film photovoltaic device on its outer conductive surface. Methods that include...

20. The method according to claim 19, wherein the lower photovolt subcell is a silicon photovolt cell.

21. The method according to claim 19, wherein the transferable photovoltaic device configuration is configured as a roll, the thin-film photovoltaic device is bonded to the lower photovoltaic subcell, and the flexible release substrate is separated from the thin-film photovoltaic device in a roll lamination process.

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