BACKGROUND

[0001] The present specification relates generally to tandem photovoltaic devices and, more specifically, to improved front contact layer stacks for lower submodules in tandem photovoltaic devices. Embodiments include contact layer stacks for use in structures having a type I-III-VI thin-film absorber lower submodule.

[0002] A photovoltaic device generates electrical power by converting light into electrical power using semiconductor materials that exhibit the photovoltaic effect. Certain semiconductor materials are more efficient at absorbing particular ranges of the electromagnetic spectrum. To improve the overall efficiency of photovoltaic devices, photovoltaic devices may incorporate stacked submodules utilizing semiconductor materials with differing absorptive properties to form a tandem device.

[0003] Laboratory experiments measuring absorption efficiency at relevant spectral ranges for separate submodules have shown that there are promising materials that might be used together to absorb a greater proportion of incident radiation. However, with the increased complexity of a tandem architecture it is also more challenging to close the gap between actual and theoretical performance. A substantial challenge for producing tandem devices, with good efficiency and manufacturability, is in providing contact layer stacks having desired electrical, optical, physical, and thermal properties.

[0004] Accordingly, a need exists for alternative layer structures for use in photovoltaic devices and for processes and materials useful in tandem photovoltaic device architecture.

DRAWINGS

[0005] The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims.

[0006] FIG. 1 schematically depicts a cross-sectional view of a tandem photovoltaic device according to one or more embodiments shown and described herein.

[0007] FIG. 2 schematically depicts an example submodule of the photovoltaic device of FIG. 1 according to one or more embodiments shown and described herein.

[0008] FIG. 3 schematically depicts a cross-sectional view along 3-3 of the photovoltaic submodule of FIG. 2 according to one or more embodiments shown and described herein. [0009] FIG. 4 schematically depicts a cross-sectional view of an example second submodule according to one or more embodiments shown and described herein.

[0010] FIGS. 5A-5F schematically depict cross-sectional views of contact stacks according to one or more embodiments shown and described herein.

DETAILED DESCRIPTION

[0011] Photovoltaic devices can be formed from a stack of functional layers formed over a substrate. Photovoltaic devices can include an absorber layer for converting light into charge carriers, and conductive layers for collecting the charge carriers. As charge carriers are generated, an electric potential is produced by the separation of charges. The positive and negative charge carriers, holes and electrons respectively, move in opposite directions, towards p-type and n-type regions on opposite sides of the absorber, resulting in an electrical current. Tandem photovoltaic devices include more than one absorber layer and associated layers for collecting the generated charge. The embodiments provided herein relate to n-type charge collection layer stacks and tandem photovoltaic devices including the same. The disclosed contact layer stacks, providing a front contact for a lower submodule, can increase device efficiency and can promote or maintain passivation of an absorber surface in the submodule.

[0012] Semiconductor material selections and compositional modifications can be used to modulate absorptive properties. Semiconductor materials for use in photovoltaic devices can, for example, include type I-III-VI materials, type II- VI materials, type III-V materials, silicon-based materials, perovskite materials, dye- sensitized solar materials, and combinations thereof.

[0013] Tandem photovoltaic devices can theoretically achieve higher total conversion efficiency than single-junction photovoltaic devices by capturing a larger portion of incident light. It is noted that the term “light” can refer to various wavelengths of the electromagnetic spectrum such as, but not limited to, wavelengths in the ultraviolet, infrared, and visible portions of the electromagnetic spectrum. “Sunlight,” as used herein, refers to light emitted by the sun.

[0014] A tandem device can have two or more stacked sub-cells or submodules, and each submodule can include active regions formed from materials having specific absorptive properties, and may include different types of semiconductor materials. In a tandem photovoltaic device for which the primary light source is from above, a light-incident top cell, or upper submodule, can be configured to absorb predominantly shorter wavelength light, while a bottom cell, or lower submodule, can be configured to absorb predominantly longer wavelengths of incident light that has passed through the upper submodule. Tandem photovoltaic devices can include bifacial devices, configured to receive incident radiation through both front and rear surfaces, typically receiving direct solar radiation on a top or front surface and receiving radiation reflected from external surfaces, including visible and infrared light, on a back or rear surface.

[0015] The provided description of technology is exemplary in nature of the subject matter, and is not intended to limit the scope, application, or uses of any specific invention claimed in this application, or in such other applications as may be filed claiming priority to this application, or patents issuing therefrom. Regarding methods disclosed, the order of the steps presented may be exemplary in nature, and, unless stated otherwise, the order of the steps can be different in various embodiments, including where certain steps can be simultaneously performed. In some example embodiments, well-known processes, well- known device structures, and well-known technologies are not described in detail. Equivalent changes, modifications and variations of some embodiments, materials, compositions, and methods can be made within the scope of the present technology, with substantially similar results.

[0016] Unless otherwise specified or indicated by context, measurements correspond to values at equilibrium and at normal temperature and pressure of 25°C and 1 atmosphere. It is noted that the terms "substantially" and "about" may be utilized to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

[0017] “A” and “an” as used herein indicate “at least one” of the item is present; a plurality of such items may be present, when possible. Although the open-ended term “comprising,” as a synonym of non-restrictive terms such as including, containing, or having, is used herein to describe and claim embodiments of the present technology, embodiments may alternatively be described using more limiting terms such as “consisting of’ or “consisting essentially of.” Thus, for a given embodiment reciting materials, components, or process steps, the present technology also specifically includes embodiments consisting of, or consisting essentially of, such materials, components, or process steps, and excluding additional materials, components or processes, for “consisting of,” or excluding additional materials, components or processes affecting the significant properties of the embodiment, for “consisting essentially of,” even though such additional materials, components or processes are not explicitly recited in this application. For example, recitation of a composition or process reciting elements A, B and C specifically envisions embodiments consisting of, and consisting essentially of, A, B and C, excluding an element D that may be recited in the art, even though element D is not explicitly described as being excluded herein.

[0018] As referred to herein, disclosures of ranges are, unless specified otherwise, inclusive of endpoints and include all distinct values and further divided ranges within the entire range. Thus, for example, a range of “from A to B” or “from about A to about B” is inclusive of A and of B. Disclosure of values and ranges of values for specific parameters, such as amounts or weight percentages, are not exclusive of other values and ranges of values useful herein. It is envisioned that two or more specific exemplified values for a given parameter may define endpoints for a range of values that may be claimed for the parameter. For example, if Parameter X is exemplified herein to have value A and also exemplified to have value Z, it is envisioned that Parameter X may have a range of values from about A to about Z. Similarly, disclosure of two or more ranges of values for a parameter encompass combination of ranges for the value that might be claimed using endpoints of the disclosed ranges.

[0019] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer, or section. Terms such as “first,” “second,” and other numerical terms when used herein do not specify a sequence or order unless explicitly stated or indicated by context. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the example embodiments.

[0020] The term “layer” can refer to a thickness of material provided upon a surface. The layer can cover all or a portion of an adjacent surface. The phrase "adjacent to," as used herein, means that a first layer and a second layer are disposed contiguously and without any intervening materials between at least a portion of the first layer and the second layer. Accordingly, at least a portion of the first layer and the second layer are in direct contact with one another. A layer may include sublayers and can have compositional gradients within a layer. A layer can include one or more functional layers of material. [0021] When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected, or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion, for example, “between” versus “directly between.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0022] Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used for ease of description to describe relative relationships between elements or features. Spatially relative terms may be intended to encompass different orientations of the device in use or operation, or an orientation during manufacturing, in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. The device may be otherwise oriented or rotated, and the spatially relative descriptors interpreted accordingly.

[0023] Referring now to FIG. 1, an embodiment of a tandem photovoltaic device 300 is shown. The tandem photovoltaic device 300 can be configured to receive light and transform light into electrical energy, as photons are absorbed and transformed into electrical current via the photovoltaic effect. For sake of discussion and clarity, the tandem photovoltaic device 300 can define a front side 302 configured to face a primary light source, for example, the sun. The tandem photovoltaic device 300 can also define a back side 304 offset from the front side 302 by a plurality of functional layers of material.

[0024] The tandem photovoltaic device 300 can have a first submodule 100, a second submodule 500, and an interlayer 400 therebetween. The first submodule 100 can also be termed a top cell or upper submodule. The second submodule 500 can also be termed a bottom cell or lower submodule. The interlayer 400, can also be termed an interlayer stack, a dielectric stack, or a transparent coupling layer. In tandem devices, interlayer structures may be designed to be substantially transparent to wavelengths configured to be absorbed by the lower submodule. Incident electromagnetic radiation, or light, may enter a tandem device primarily through a front or top surface and into the upper submodule. Light that is not absorbed by the upper submodule reaches the interlayer. The interlayer can be configured to reflect some light energy, or photons, back into the upper submodule, and also transmit light to the back cell or lower submodule. Photovoltaic devices may include a plurality of stacked submodules, and additional intermediate layers can be provided between each submodule.

[0025] Each of the first submodule 100, the second submodule 500, and the interlayer 400 can comprise a plurality of layers. Each of the first and second submodules 100, 500 of the tandem photovoltaic device 300 can include one or more absorber layers for converting light into charge carriers, and conductive layers for collecting the charge carriers.

[0026] The first submodule 100 can have a front surface 102 substantially facing the front side 302 of the tandem photovoltaic device 300 and a back surface 104 substantially facing the back side 304 of the photovoltaic device 300. The interlayer 400 can have a first surface 402 substantially facing the front side 302 of the photovoltaic device 300 and a second surface 404 substantially facing the back side 304 of the photovoltaic device 300. The second submodule 500 can have a front surface 502 substantially facing the front side 302 of the photovoltaic device 300 and a back surface 504 substantially facing the back side 304 of the photovoltaic device 300.

[0027] As depicted in FIG. 1, incident light (Av) 10 can enter the front side 302 of the tandem photovoltaic device 300 through the first submodule 100 and a first portion 11 of light energy can be absorbed by the first submodule 100 and a remaining portion 12 of light energy can pass through the first submodule 100 to the interlayer 400. At the interlayer 400, reflected light 13 can be directed back toward the absorptive region of the first submodule 100 and transmitted light 14 can pass to the second submodule 500. Optionally, in a bifacial tandem device, back side light energy 16 can enter the back side 304 of the tandem photovoltaic device 300 toward the second submodule 500. In many implementations, back side light energy 16 can include externally reflected visible light and near infrared light. The first submodule can absorb the first portion 11 of light energy, which can include an absorbed combination of the incident light 10 and the reflected light 13. The second submodule can absorb a second portion 15 of light energy comprising the transmitted light 14 and, optionally, the back side light energy 16.

[0028] Referring now to FIGS. 2 & 3, an example embodiment of the first submodule 100 of the tandem photovoltaic device 300 is shown. The first submodule 100 can include a plurality of layers disposed between the front side 102 and the back side 104. In some embodiments, the layers of the first submodule 100 can be divided into an array of photovoltaic cells 200. For example, the first submodule 100 can be scribed according to a plurality of serial scribes 202 and a plurality of parallel scribes 204. The serial scribes 202 can extend along a length Y of the first submodule 100 and demarcate the photovoltaic cells 200 along the length Y of the first submodule 100. Neighboring cells of the photovoltaic cells 200 can be serially connected along a width X of the first submodule 100. In other words, a monolithic interconnect of the neighboring cells 200 can be formed; e.g., adjacent to the serial scribe 202. The parallel scribes 204 can extend along the width X of the first submodule 100 and demarcate the photovoltaic cells 200 along the width X of the first submodule 100. Under operation, current 205 can predominantly flow along the width X through the photovoltaic cells 200 serially connected by the serial scribes 202. Under operation, parallel scribes 204 can limit the ability of current 205 to flow along the length Y. Parallel scribes 204 are optional and can be configured to separate the photovoltaic cells 200 that are connected serially into groups 206 arranged along length Y.

[0029] In example devices, the width X is greater than 0.2 meters, greater than 0.5 meters, greater than 1.0 m, greater than 1.5 m, or in a range of 0.5 to 2.5 m. In example devices, the length y is greater than 0.2 meters, greater than 0.5 meters, greater than 1.0 m, greater than 1.5 m, or in a range of 0.5 m to 2.5 m. In some embodiments, the width X and length Y of the first submodule 100 is the same as a width and length of the second submodule.

[0030] With particular reference to FIG. 2, the parallel scribes 204 can electrically isolate the groups 206 of photovoltaic cells 200 that are serially connected. In some embodiments, the groups 206 of the photovoltaic cells 200 can be connected in parallel such as, for example, via electrical bussing. Optionally, the number of parallel scribes 204 can be configured to limit a maximum current generated by each group 206 of the photovoltaic cells 200. In some embodiments, the maximum current generated by each group 206 can be less than or equal to about 200 milliamps (mA) such as, for example, less than or equal to about 100 mA in one embodiment, less than or equal to about 75 mA in another embodiment, or less than or equal to about 50 mA in a further embodiment. In example embodiments, the first submodule comprises between 5 and 500 groups. In example embodiments, the first submodule comprises more than 10 groups, more than 20 groups, more than 50 groups, or more than 80 groups. In example embodiments, the first submodule comprises up to 500 groups, up to 400 groups, up to 350 groups, up to 250 groups, up to 200 groups, or up to 100 groups.

[0031] With particular reference to FIG. 3, the layers of the first submodule 100 can include a thin film stack provided over a substrate 110. The substrate 110 can be configured to facilitate the transmission of light into the first submodule 100. The substrate 110 can be disposed at the front side 102 of the first submodule 100. Referring collectively to FIGS. 2 & 3, the substrate 110 can have a first surface 112 substantially facing the front side 102 of the first submodule 100 and a second surface 114 substantially facing the back side 104 of the first submodule 100. One or more layers of material can be disposed between the first surface 112 and the second surface 114 of the substrate 110.

[0032] The substrate 110 can include a transparent layer. The transparent layer can be formed from a substantially transparent material such as, for example, glass. Optionally, the substrate 110 can include one or more coatings applied to a surface of the substrate 110. The coating can be configured to interact with light or to improve durability of the substrate 110 such as, but not limited to, an antireflective coating, an antifouling coating, or a combination thereof.

[0033] Referring again to FIG. 3, the first submodule 100 can include a barrier layer 130. The barrier layer 130 can have a first surface 132 substantially facing the front side 102 of the first submodule 100 and a second surface 134 substantially facing the back side 104 of the first submodule 100. In some embodiments, the barrier layer 130 can be provided adjacent to the substrate 110. For example, the first surface 132 of the barrier layer 130 can be provided upon the second surface 114 of the substrate 110.

[0034] The barrier layer 130 may mitigate diffusion of contaminants from the substrate 110, which could result in degradation or delamination of other layers of the photovoltaic stack. The barrier layer 130 may be substantially transparent, thermally stable, with minimal pin holes, have high sodium-blocking capability, and/or good adhesive properties. The barrier layer 130 can include one or more layers of material, including, but not limited to, tin oxide, silicon dioxide, aluminum-doped silicon oxide, silicon oxide, silicon nitride, or aluminum oxide. The barrier layer 130 has a thickness bounded by the first surface 132 and the second surface 134. The thickness of the barrier layer 130 may be in a range of 10 nanometers to 200 nanometers, including, for example, more than about 10 nanometers, more than 15 nm, or less than about 20 nm in some embodiments.

[0035] With continuing reference to FIG. 3, the first submodule 100 can include a front contact layer 140 configured to provide electrical contact to transport charge carriers generated by the first submodule 100. The front contact layer 140 can have a first surface 142 substantially facing the front side 102 of the first submodule 100 and a second surface 144 substantially facing the back side 104 of the first submodule 100. In some embodiments, the front contact layer 140 can be provided adjacent to the barrier layer 130. For example, the first surface 142 of the front contact layer 140 can be provided upon the second surface 134 of the barrier layer 130.

[0036] In some embodiments, the front contact layer 140 can be n-type and may be called an electron transport layer (ETL). The front contact layer 140 can be an n-type transparent conductive oxide (TCO) layer. An n-type contact layer can be formed from one or more layers of n-type semiconductor material that is substantially transparent and has a wide band gap. Specifically, the wide band gap can have a larger energy value compared to the energy of the photons of the light, which can mitigate undesired absorption of light. The n-type contact layer can include one or more layers of material, including, but not limited to, tin dioxide, doped tin dioxide (such as F-SnCh), indium tin oxide, or cadmium tin oxide (Cd ₂ SnO ₄ ).

[0036] The first submodule 100 can include a buffer layer 150. The buffer layer 150 may be configured to provide a high-resistivity layer between the front contact layer 140 and semiconductor layers. The buffer layer 150 can have a first surface 152 substantially facing the front side 102 of the first submodule 100 and a second surface 154 substantially facing the back side 104 of the first submodule 100. In some embodiments, the buffer layer 150 can be provided adjacent to the front contact layer 140. For example, the first surface 152 of the buffer layer 150 can be provided upon the second surface 144 of the front contact layer 140.

[0037] The buffer layer 150 can include material having higher resistivity than the front contact layer 140, including, but not limited to, intrinsic tin dioxide, zinc magnesium oxide (e.g., Zni-xMgxO), silicon dioxide (SiO ₂ ), aluminum oxide (Al ₂ Os), aluminum nitride (AIN), zinc tin oxide, zinc oxide, tin silicon oxide, or combinations thereof. In some embodiments, the material of the buffer layer 150 can be configured to substantially match the band gap of an adjacent semiconductor layer, such as an absorber. The buffer layer 150 can have a thickness between the first surface 152 and the second surface 154. Example thicknesses include, for example, more than about 10 nm in one embodiment, between about 10 nm and about 80 nm in another embodiment, or between about 15 nm and about 60 nm in a further embodiment.

[0038] Referring still to FIG. 3, the first submodule 100 can include an absorber layer 160. The absorber layer may be configured to cooperate with another layer or a sublayer and form a p-i-n, p-n, n-p, or n-i-p junction within the first submodule 100. Accordingly, absorbed photons of light can free electron-hole pairs and generate carrier flow, to yield electrical energy. The absorber layer 160 can have a first surface 162 substantially facing the front side 102 of the first submodule 100 and a second surface 164 substantially facing the back side 104 of the first submodule 100. The absorber layer 160 thickness is bounded by the first surface 162 and the second surface 164. The thickness of the absorber layer 160 can be between about 500 nm to about 10000 nm such as, for example, between about 1000 nm to about 7000 nm in one embodiment, or between about 1500 nm to about 4000 nm in another embodiment.

[0039] Semiconductor materials for use in absorber layers in photovoltaic devices can include type I-III-VI materials, such as, for example, copper indium gallium sulfide/selenide (CIGS), or copper indium diselenide CuInSe2 (CIS). Suitable semiconductor materials may include type II- VI materials, for example, cadmium selenide (CdSe), cadmium telluride selenide alloys (CdSe _x Tei- _x ), or cadmium mercury telluride selenide alloys. Further examples of Group II- VI absorber materials include, but are not limited to, semiconductor materials comprising cadmium, mercury, zinc, tellurium, selenium, and other type II- VI binary, ternary, or quaternary alloys. Suitable semiconductor materials may include type III- V materials, for example, gallium arsenide. Suitable semiconductor materials may include silicon-based materials, for example, amorphous, crystalline, polycrystalline, or thin film silicon. Suitable semiconductor materials may include perovskite materials, including metal halide organic compositions, for example, methylammonium lead halide. Suitable semiconductor materials may include dye- sensitized solar materials. Semiconductor material selections and compositional modifications can be used to modulate absorptive properties.

[0040] The absorber layer 160 can further include one or more dopants. The absorber layer 160 can be doped with a dopant configured to manipulate the charge carrier concentration. In some embodiments, the absorber layer 160 can be doped with a p-type dopant. P-type dopants may include an element from Group VA (group 15) such as, for example, arsenic, phosphorous, antimony, or a combination thereof. Alternatively, or additionally, the absorber layer 160 can be doped with a Group IB (group 11) dopant such as, for example, copper, silver, gold, or a combination thereof. The total density of the dopant within the absorber layer 160 can be controlled. The amount of the dopant can vary with distance from the first surface 162 of the absorber layer 160 to produce a graded dopant concentration.

[0041] In some embodiments the absorber layer 160 has a compositional gradient. In some embodiments, the absorber layer 160 comprises an n-type material. In some embodiments, a p-n junction is present within the absorber layer 160.

[0042] In some embodiments, the absorber layer 160 is p-type and is provided adjacent to an n-type semiconductor material. Alternatively, one or more intervening layers can be provided between the absorber layer 160 and n-type semiconductor material. In some embodiments, the absorber layer 160 can be provided adjacent to the buffer layer 150. For example, the first surface 162 of the absorber layer 160 can be provided upon the second surface 154 of the buffer layer 150.

[0043] The first submodule 100 can include a back contact layer 170. The back contact layer 170 can have an opposite charge with respect to the front contact layer 140. The back contact layer 170 can be configured to mitigate undesired alteration of a dopant and to provide electrical contact to the absorber layer 160. The back contact layer 170 can have a first surface 172 substantially facing the front side 102 of the first submodule 100 and a second surface 174 substantially facing the back side 104 of the first submodule 100. A thickness of the back contact layer 170 can be defined between the first surface 172 and the second surface 174. The thickness of the back contact layer 170 can be between about 5 nm to about 200 nm such as, for example, between about 10 nm to about 50 nm in one embodiment.

[0044] In some embodiments, the back contact layer 170 can be provided adjacent to the absorber layer 160. For example, the first surface 172 of the back contact layer 170 can be provided upon the second surface 164 of the absorber layer 160. In some embodiments, the back contact layer 170 can be p type or p+ type and may be called a hole transport layer (HTL). In some embodiments, the back contact layer 170 further comprises an electron reflector layer. In some embodiments, the back contact layer 170 further comprises a tunnel junction having a p+ sub-region and an n+ sub-region.

[0045] Materials such as semiconductors and transparent conductive oxides can be doped with impurities to alter their electrical and optical properties. Dopants can be incorporated into functional layers to modify n-type or p-type charge carrier concentrations. Charge densities of greater than about 1 x 10 ¹⁶ cm ^-3 can be considered to be “+” type. Although the boundaries are not rigid, a material can be considered n-type if electron donor carriers are present in a range greater than about 1 x 10 ¹¹ cm ^-3 or in the range of about 1 x 10 ¹¹ cm' ³ to about 1 x 10 ¹⁶ cm' ³ , and n+ type if donor carrier density is greater than about IxlO ¹⁶ cm' ³ . Similarly, a material is generally considered p-type if electron acceptor carriers (i.e., “holes”) are present in a range greater than about 1 x 10 ¹¹ cm' ³ or in the range of about 1 x 10 ¹¹ cm' ³ to about 1 x 10 ¹⁶ cm' ³ , and p+ type if acceptor carrier density is greater than about IxlO ¹⁶ cm' ³ . The boundaries are not rigid and can overlap because a layer can be p+ relative to a layer that is p-type (or n+ relative to a layer that is n-type) if the carrier concentration is at least two orders of magnitude (i.e., 100-fold) higher, regardless of the absolute carrier density. Additionally, charge densities of greater than about 1 x 10 ¹⁸ cm ^-3 can be considered to be “++” type; and thus a layer of either n-type or p-type can be “++” relative to a layer of the same type that is itself “+” relative to yet a third layer, if the ++ layer has a same-type carrier density more than 100 fold that of the + layer.

[0046] In some embodiments, the back contact layer 170 can include combinations of materials from Groups I, II, VI, such as for example, one or more layers containing zinc and tellurium in various compositions. Further suitable materials include, but are not limited to, a bilayer of cadmium zinc telluride and zinc telluride, or zinc telluride doped with a Group V (group 15) dopant such as, for example, nitrogen. In some devices, the back contact layer 170 can include an organic compound, for example, 2,2',7,7'-tetrakis [N,N-di (4- methoxyphenyl) amino] -9,9'-spirobifluorene (spiro-OMeTAD), poly[bis(4-phenyl)(2,4,6- trimethylphenyl) amine] (PTAA) and/or poly(3-hexylthiophene-2,5-diyl) (P3HT).

[0047] A thin film junction 176 can be defined as the thin film stack primarily contributing to the photovoltaic effect. For example, in some embodiments, the thin film junction 176 can include the front contact layer 140, the buffer layer 150, the absorber layer 160, the back contact layer 170, or combinations thereof.

[0048] Referring to FIG. 3, the first submodule 100 can include a conducting layer 180. The conducting layer 180 can be configured to provide electrical contact with the back contact layer 170, the absorber layer 160, or both. In some embodiments, the conducting layer 180 can be formed towards a back side 104 of the first submodule 100. In a tandem device structure, the conducting layer 180 may be transparent. In single junction devices, or when provided as a part of a bottom cell or lower submodule, conducting layers can comprise opaque, non-transparent, metal layers. However, non-transparent layers can be unsuitable for use as the conducting layer 180 of the first submodule 100, disposed between junctions in multi-junction photovoltaic devices or tandem photovoltaic devices. The conducting layer 180 can have a first surface 182 substantially facing the front side 102 of the first submodule 100 and a second surface 184 substantially facing the back side 104 of the first submodule 100. In some embodiments, the conducting layer 180 can be provided adjacent to the back contact layer 170 or the absorber layer 160. For example, the first surface 182 of the conducting layer 180 can be provided upon the second surface 174 of the back contact layer 170 or the second surface 162 of the absorber layer 160. The conducting layer 180 thickness is bounded by the first surface 182 and the second surface 184. The thickness of the conducting layer 180 can be less than about 500 nm such as, for example, between about 40 nm and about 400 nm in one embodiment, or between about 60 nm and about 350 nm. [0049] The first submodule 100 can have a back layer 199 at the back side 104 of the first submodule 100. The back surface of the back layer 199 defines the back surface 104 of the front submodule. In some embodiments, the back layer 199 is a region of the conducting layer 180. In some embodiments, the back layer 199 comprises a support. In some embodiments, the back layer 199 comprises bussing to electrically connect the first submodule 100 to the second submodule 300. In some embodiments, the back layer 199 comprises an encapsulation layer.

[0050] Referring now to FIG. 1 and FIG. 4, the tandem photovoltaic device 300 includes a second submodule 500. The second submodule 500 can be disposed below or under the first submodule 100, referencing the front side 302 of the tandem photovoltaic device 300 as the primary light-facing top surface. As shown in FIG. 4, the photovoltaic device submodule 500 can include a plurality of layers disposed between a front surface 502 and a second surface 504 on a back side of the second submodule 500. One or more of the plurality of layers can include a photovoltaic absorber material. In some embodiments, the photovoltaic device submodule 500 can be divided into a plurality of photovoltaic cells. Described layer structures and combinations facilitate optimizing the second submodule for improved long- wavelength performance, including infrared absorption.

[0051] Referring now to FIG. 4 and FIG. 5, the plurality of layers of the second submodule 500 can include an optional capping layer 530. The capping layer 530 can be configured to mitigate undesirable impurity diffusion from the first submodule 100, which could result in degradation or delamination of other layers of the second submodule 500. The capping layer 530 may also be configured to prevent or impede moisture ingress, thereby inhibiting degradation of the second submodule 500. The capping layer 530 can have a first surface 532 substantially facing the front surface 502 of the second submodule 500 and a second surface 534 substantially facing the back surface 504 of the second submodule 500.

[0052] The capping layer 530 can include one or more layers of material. In some embodiments, the capping layer 530 comprises cadmium, tin, zinc, magnesium, oxygen, or combinations thereof. In some embodiments the capping layer 530 comprises cadmium tin oxide, tin oxide, or zinc magnesium oxide. The capping layer 530 can have a thickness bounded by the first surface 532 and the second surface 534. The thickness of the capping layer 530 may be, for example, 5 nm or more, 25 nm or greater, 60 nm or less, 50 nm or less, or in a range of 5 nm to 50 nm in an example embodiment.

[0053] With continuing reference to FIG. 4 and FIG. 5, the second submodule 500 can include an oxide layer 540. The oxide layer 540 may be an n-type contact layer configured to provide electrical contact to transport charge carriers generated by the second submodule 500. The oxide layer 540 can have a first surface 542 substantially facing the front surface 502 of the second submodule 500 and a second surface 544 substantially facing the back surface 504 of the second submodule 500. In some embodiments, the oxide layer 540 can be provided adjacent to the capping layer 530. For example, the first surface 542 of the oxide layer 540 can be in direct contact with the second surface 534 of the capping layer 530. The oxide layer 540 thickness is bounded by the first surface 542 and the second surface 544. The oxide layer 540 thickness may be greater than 10 nm, in a range including, for example, between 10 nm and 250 nm, between 50 nm and 200 nm, between 80 nm and 200 nm, between 50 nm and 150 nm, or between about 80 nm and about 100 nm in an example embodiment.

[0054] The oxide layer 540 comprises an n-type semiconductor material that has high carrier mobility. The oxide layer 540 may have a narrower direct band gap than conventional n-type conductive oxide layers. In some embodiments the oxide layer 540 band gap is less than 3.0 eV, between 2.0 eV and 2.5 eV, between 2.0 eV and 2.3 eV, or about 2.25 eV. The oxide layer 540 can have a carrier mobility of at least 50 cm ² V ¹ s’ ¹ in one embodiment, greater than 60 cm ² V ¹ s’ ¹ , greater than 75 cm ² V’ ¹ s’ ¹ , greater than 80 cm ² V’ ¹ s’ ¹ , greater than 90 cm ² V’ ¹ s’ ¹ , greater than 100 cm ² V’ ¹ s’ ¹ , between 75 cm ² V’ ¹ s’ ¹ and 130 cm ² V’ ¹ s’ ¹ , between 80 cm ² V’ ¹ s’ ¹ and 120 cm ² V’ ¹ s’ ¹ , between 90 cm ² V’ ¹ s’ ¹ and 120 cm ² V’ ¹ s’ ¹ , or about 110 cm ² V’ ¹ s’ ¹ or more in an example embodiment.

[0055] The oxide layer 540 may comprise a narrower band gap material which absorbs shorter wavelengths of light. For example, the oxide layer 540 may absorb at least 50% of light having a wavelength between 400nm and 425nm. In an embodiment, the oxide layer 540 may absorb 40% to 80% of light having a wavelength between 400nm and 425nm. In an embodiment, the oxide layer 540 may absorb 20% to 50% of light having a wavelength between 390nm and 440nm. More conventional structures avoid materials that do not transmit substantially all visible light though a front contact structure. While the oxide layer may form a part of a thin film junction region 576, the oxide layer is not a part of the absorber and does not significantly contribute to charge carrier generation for the module.

[0056] The oxide layer 540 can include one or more layers of material. In some embodiments the oxide layer 540 comprises a metal oxide. In some embodiments the oxide layer 540 comprises cadmium oxide (CdO). The cadmium oxide may be undoped. In some embodiments the oxide layer 540 comprises cadmium oxide doped with indium, gallium, titanium, yttrium, or combinations thereof. In some embodiments the oxide layer 540 comprises doped or undoped cadmium oxide (CdO), wherein at least 98.0 atomic percent of the oxide layer is composed of cadmium and oxygen. In some embodiments the oxide layer 540 comprises heavily-doped cadmium oxide, wherein the dopant is present at a level in a range of 3.0 to 7.0 atomic percent, and wherein between 93.0 to 97.0 atomic percent of the oxide layer is composed of cadmium and oxygen. In some embodiments the oxide layer 540 comprises doped cadmium oxide, wherein the dopant is present at a level greater than 1.0 at. %, greater than 2.0 at. %, greater than 3.0 at. %, greater than 4.0 at. %, or in a range of 1.0 to 5.0 at. %. In some embodiments the oxide layer 540 comprises cadmium oxide and at least 99.0 at. % of the oxide layer is composed of cadmium and oxygen. In some embodiments, the oxide layer 540 consists essentially of cadmium oxide (CdO) and dopants comprise less than 1.0 atomic percent of the oxide layer 540. In some embodiments, the cadmium oxide (CdO) is stoichiometric. In some embodiments, the cadmium oxide is non- stoichiometric and comprises a Cd-rich film with oxygen vacancies and/or doping impurities. In some embodiments, the oxide layer consists essentially of undoped cadmium oxide.

[0057] Advantageously, the described device structure is not limited to wide band gap materials for use in the oxide layer. The oxide layer 540 may include high mobility, narrower band gap materials as compared with established semiconductor alloys used for transparent conducting layer materials. Other materials used as n-type conductive oxides, such as indium tin oxide or fluorine-doped tin oxide typically have free carrier mobility in a range of 20 to 50 cm ² /Vs. The use of a higher mobility material in the oxide layer 540 can mitigate resistive losses and long-wavelength optical losses for the second submodule 500.

[0058] Referring still to FIG. 4 and FIG. 5, the second submodule 500 can include an optional buffer layer 550 configured to provide a high-resistance layer between the oxide layer 540 and the semiconductor layers of the absorber stack. The buffer layer 550 can have a first surface 552 substantially facing the front surface 502 of the second submodule 500 and a second surface 554 substantially facing the back surface 504 of the second submodule 500. In some embodiments, the buffer layer 550 can be provided adjacent to the oxide layer 540. For example, the first surface 552 of the buffer layer 550 can be provided upon the second surface 544 of the oxide layer 540. The buffer layer 550 can include material, including, but not limited to, cadmium, sulfur, zinc, magnesium, indium, tin, fluorine, aluminum, oxygen, cadmium sulfide, zinc magnesium oxide, zinc oxide, zinc oxysulfide, indium tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, or combinations thereof. In some embodiments, the buffer layer 550 can comprise cadmium or zinc. In some embodiments, the buffer layer 550 can comprise, or consist essentially of, at least one of: cadmium sulfide, zinc oxysulfide, zinc oxide, or zinc magnesium oxide.

[0059] The buffer layer 550 has a thickness bounded by the first surface 552 and the second surface 554. The buffer layer 550 thickness may be greater than 10 nm, in a range including, for example, between 10 nm and 600 nm, between about 50 nm and about 500 nm in another embodiment, or between about 80 nm and about 100 nm in a further embodiment.

[0060] In some embodiments, a thicker buffer layer over a CIGS absorber contributes to improved passivation at the absorber-front contact interface, improving performance and cell efficiency. In some embodiments where the buffer layer 550 includes cadmium sulfide (CdS), the thickness of the buffer layer 550 can be greater than 150 nm, or greater than 200 nm, for example, greater than 250 nm, greater than 300 nm, between 200 nm and 600 nm, or between 200 nm and 500 nm.

[0061] In some embodiments where the buffer layer 550 includes zinc magnesium oxide, zinc oxysulfide, or zinc oxide, the thickness of the buffer layer 550 can be in a range of 5 nm to 80 nm, between 5 nm and 50 nm, between 5 nm and 30 nm, between 10 nm and 50 nm, between 20 nm and 50 nm, or about 15 nm.

[0062] Advantageously, in the embodiments where the buffer layer 550 includes cadmium sulfide (CdS), zinc oxysulfide (ZnOS), zinc oxide, or zinc magnesium oxide, and is formed in contact with the front surface 562 of the absorber layer 560, the buffer layer 550 can provide improved absorber/front contact interface passivation performance and cell efficiency without sacrificing optical performance of the second submodule 500. The metric of passivation improvement is a reduction of surface recombination velocity (SRV). An SRV value for a surface of a CIGS absorber may range from about 10 ⁷ cm/s, for little or no passivation, to about 1-10 cm/s, indicating good passivation. In some embodiments, an SRV for a CIGS absorber with the buffer layer 550 is in a range of 0.01 cm/s to 10 cm/s, in a range of 0.1 cm/s to 1.0 cm/s, or in a range of 0.1 cm/s to 10 cm/s.

[0063] Referring to FIG. 5, the second submodule 500 can optionally include a diffusion barrier layer 520. The diffusion barrier layer 520 can limit diffusion of metal species into active areas of the second submodule 500, for example, into an absorber layer 560 of the second submodule 500. Diffusion of metal species into an absorber layer can degrade conversion efficiency of the second submodule 500. Such degradation and decreased performance can be particularly associated with hot and/or humid environments. Thus, the use of a suitable diffusion barrier layer 520 can improve performance of the tandem photovoltaic device 300. [0064] The diffusion barrier layer 520 can have a first surface 522 substantially facing the front side 502 of the second submodule 500 and a second surface 524 substantially facing the back side 504 of the second submodule 500. A thickness of the diffusion barrier layer 520 can be defined between the first surface 522 and the second surface 524. The thickness of the diffusion barrier layer 520 can be less than about 125 nm such as, for example, between about 2 nm and about 100 nm, or between 5 nm and 50 nm.

[0065] The diffusion barrier layer 520 may be provided between the absorber layer 560 and the oxide layer 540 of a contact stack 545. In some embodiments, as shown in FIG. 5A, the diffusion barrier layer 520 can be provided between the oxide layer 540 and the buffer layer 550 of the contact stack 545A. Thus, in these embodiments, the first surface 522 of the diffusion barrier layer 520 can be directly adjacent to the second surface 544 of the oxide layer 540 and the second surface 524 of the diffusion barrier layer 520 can be directly adjacent to the first surface 552 of the buffer layer 550. In some embodiments, as shown in FIG. 5B, the diffusion barrier layer 520 is omitted, and the contact stack 545B may comprise the oxide layer 540, the buffer layer 550, and the capping layer 530. In another embodiment as illustrated in FIG. 5C, the buffer layer 550 may be omitted from the contact stack 545C, and the diffusion barrier layer 520 positioned between, and directly in contact with, both the absorber layer 560 and the oxide layer 540. In some embodiments, as shown in FIGS. 5D- 5F, the capping layer 530 may be omitted from the contact stack 545D, 545E, 545F.

[0066] The diffusion barrier layer 520 may include an oxide. The diffusion barrier layer 520 may include an oxynitride, for example, titanium oxy-nitride (TiN _x O _y ) or molybdenum oxy-nitride (MoN _x O _y ). The diffusion barrier layer 520 may include, for example, tin oxide (SnO2) zinc oxide (ZnO), indium-tin oxide (In(2- _x )Sn _x O3), or cadmium-tin oxide (Cd2SnO4). These materials can be doped with impurities, such as F, Al, In, Ga, Ti, to alter their electrical and optical properties.

[0067] A contact stack 545 of the second submodule 500 can include a plurality of layers between the absorber 560 and the first submodule 100. The contact stack 545 may be n-type. The contact stack includes the oxide layer 540 and at least one additional layer between the oxide layer 540 and a first surface 562 of the absorber layer 560. The contact stack 545 may include the oxide layer 540 and the diffusion barrier layer 520. The contact stack 545 may include the oxide layer 540 and the buffer layer 550. The contact stack may further include the capping layer 530 and/or the diffusion barrier layer 520.

[0068] As provided herein, the contact stack 545 can be configured to improve electron transport and charge carrier collection for the second submodule. In embodiments of the invention, the oxide layer 540 is the primary conductive layer of the contact stack 545. In some embodiments, the front contact stack has an average absorption of at least 50% to light having a wavelength in a range of 400 nm to 425 nm.

[0069] Referring still to FIG. 4, the second submodule 500 can include an absorber layer 560 configured to cooperate with another layer and form a p-n junction within the second submodule 500. Accordingly, absorbed photons of the light can free electron-hole pairs and generate carrier flow, which can yield electrical energy. The absorber layer 560 can have a first surface 562 substantially facing the first surface side 502 of the second submodule 500 and a second surface 564 substantially facing the back surface 504 of the second submodule 500. The absorber layer 560 thickness is defined between the first surface 562 and the second surface 564. The thickness of the absorber layer 560 can be between about 500 nm to about 10000 nm such as, for example, between about 1000 nm to about 7000 nm in one embodiment, or between about 1500 nm to about 4000 nm in another embodiment.

[0070] The absorber layer 560 can be formed from a p-type semiconductor material. The absorber layer 560 can include a p-type semiconductor material such as type I-III-VI materials, type II- VI materials, type III-V materials, silicon-based materials, perovskite materials, or dye- sensitized solar materials. The absorber layer 560 can further include one or more dopants.

[0071] The absorber layer 560 of the second submodule 500 can comprise a type I-III-VI semiconductor material, such as, for example, copper indium gallium sulfide/selenide (CIGS) material. As used herein, the term “CIGS” encompasses a CuInSe2 (CIS) material, and encompasses a silver-alloyed (ACIGS) material, unless otherwise indicated. The absorber layer 560 can be provided as a thin film. The absorber may have a graded composition and bandgap. The type I-III-VI semiconductor material may be represented by the formula (Ag, Cu)(In, Ga)(S, Se)2 . In some embodiments, the absorber comprises at least one of: copper indium diselenide, copper gallium diselenide, copper indium gallium diselenide, silver indium gallium diselenide, or silver indium diselenide. These I-III-VI compounds may be further mixed to form more complex compositionally graded absorbers. For example, according to the general formula above, Cu can be substituted with Ag, and In can be substituted with Ga, to form a I-III-VI absorber with the desired physical properties such as with respect to bandgap or polycrystalline grain size. In some embodiments, the mole fraction of Ag in the absorber [Ag]/[Ag+Cu] is less than 25%. In another embodiment, [Ag]/[Ag+Cu] is between 25% and 50%. In yet another embodiment, [Ag]/[Ag+Cu] is between 50% and 75%. In some other embodiments, the mole fraction of Ga in the absorber [Ga]/[In+Ga] is less than 5%. In another embodiment, [Ga]/[In+Ga] is less than 10%. In yet another embodiment, [Ga]/[In+Ga] is less than 20%. While specified mole fractions reflect the average value throughout a layer thickness, it is understood that mole fractions of Ag or Ga can vary through the thickness of the absorber layer. The absorber layer 560 can further include one or more dopants.

[0072] In some embodiments, the absorber layer 560 has a graded bandgap, with a higher bandgap in a front region proximate to the first surface 562, relative to the bandgap in a back region proximate to the second surface 564, wherein the back region is further from the first surface 162 than the front region. In some embodiments, the absorber layer of the second submodule comprises type I- III- VI semiconductor material wherein all bandgap values for the absorber layer are within a range of 0.9 eV to 1.2 eV, or within a range of 1.0 to l.leV. In some examples, the absorber layer has a graded bandgap, wherein a first substantially planar region of the absorber layer has a band gap that differs from a second substantially planar region of the absorber layer, wherein the difference is at least 0.05 eV, at least 0.10 eV, at least 1.50 eV, or in a range of 0.05 eV to 0.30 eV. In some embodiments, the absorber layer 560 has a double-graded bandgap, with a narrower bandgap within a bulk region of the absorber layer 560, and relatively broader bandgaps both in a front region proximate to the first surface 562 and in a back region proximate to the second surface 564. In an example having a double-graded band gap, a minimum value for the narrower bandgap within the bulk region of the absorber layer 560 is in a range of 1.0 to l.leV, and the broader bandgap in at least one of the front region proximate to the first surface 562 or the back region proximate to the second surface 564, is broader than the minimum value by an amount in a range of 0.05 eV to 0.1 eV.

[0073] According to the embodiments provided herein, the p-n junction can be formed by providing the absorber layer 560, having at least one p-type layer, sufficiently close to a portion of the second submodule 500 having an excess of negative charge carriers; e.g., electrons or donors. In some embodiments, the absorber layer 560 can be provided adjacent to n-type semiconductor material. Alternatively, one or more intervening layers can be provided between the absorber layer 560 and n-type semiconductor material. In some embodiments, the absorber layer 560 can be provided adjacent to the buffer layer 550. For example, the first surface 562 of the absorber layer 560 can be provided upon the second surface 554 of the buffer layer 550 with no intervening layers.

[0074] The contact stack 545 forms part of the junction with the absorber layer 560. Without wishing to be bound by theory, it has been found that absorber interface passivation, absorber free carrier lifetime, and the band gap of charge transport layers influences the device Voc and device performance. In a tandem device, performance can be enhanced by reducing the Voc deficit of the second submodule. Device composition can be graded so that absorber bandgap is not constant throughout the absorber stack of the submodule. The Voc deficit can be evaluated as Eg/q - Voc, measured where the Eg is the smallest absorber bandgap energy and q is the electron charge, also referred to as the elementary charge. This quantifies the Voc deficit relative to submodule bandgap. By reducing Voc deficit, device performance can be improved. In some embodiments, the Voc deficit, measured in millivolts, is between 350-450 mV. For a device with a minimum absorber bandgap energy of leV, this results in a device Voc between of 550-650mV. For a device with a minimum absorber bandgap energy of l.leV, this results in a device Voc between 650-750 mV. In some embodiments, the open circuit voltage of the junction multiplied by the electron charge is within 450 millielectronvolts (meV) of a smallest bandgap energy of the absorber layer. In some embodiments, the Voc deficit is less than 450 mV, less than 425 mV, less than 420 mV, less than 400 mV, or less than 390 mV. In some embodiments, the Voc deficit is in a range of 350-450 mV, 350-425 mV, 375-425 mV, 350-400 mV, or 370-420 mV. Device Voc and Voc deficit are determined for the device operating at standard test conditions under 1000 W/m ² illumination and at 25C.

[0075] The second submodule 500 can include a back contact layer 570 to provide electrical contact to the absorber layer 560. The back contact layer 570 may be configured to mitigate undesired alteration of the dopant. The back contact layer 570 can have a first surface 572 substantially facing the front surface 502 of the second submodule 500 and a second surface 574 substantially facing the back side 504 of the second submodule 500. The back contact layer 570 thickness can be defined between the first surface 572 and the second surface 574. In some embodiments, the thickness of the back contact layer 570 can be between about 5 nm and about 200 nm such as, for example, between about 10 nm and about 50 nm in one embodiment.

[0076] In some embodiments, the back contact layer 570 can be provided adjacent to the absorber layer 560. For example, the first surface 572 of the back contact layer 570 can be provided upon the second surface 564 of the absorber layer 560. The back contact layer 570 may be p-type.

[0077] In some embodiments, the back contact layer 570 can include combinations of materials from Groups I, II, VI, such as for example, one or more layers containing zinc and tellurium in various compositions. Further suitable materials include, but are not limited to, a bilayer of cadmium zinc telluride and zinc telluride, or zinc telluride doped with a Group VA (group 15) dopant such as, for example, nitrogen.

[0078] The thin film junction 576 can be defined as the layer stack primarily contributing to the photovoltaic effect. For example, in some embodiments, the thin film junction 576 can include the transparent conductive oxide layer 540, the buffer layer 550, the absorber layer 560, the back contact layer 570, or combinations thereof.

[0079] Referring to FIG. 4, the second submodule 500 can include a conducting layer 580. The conducting layer 580 can be configured to provide electrical contact with the back contact layer 570, the absorber layer 560, or both. The conducting layer 580 can have a first surface 582 substantially facing the first side 502 of the second submodule 500 and a second surface 584 substantially facing the second side 504 of the second submodule 500. In some embodiments, the conducting layer 580 can be provided adjacent to the back contact layer 570 or the absorber layer 560. For example, the first surface 582 of the conducting layer 580 can be provided upon the second surface 574 of the back contact layer 570 or the second surface 562 of the absorber layer 560. The conducting layer 580 thickness can be defined between the first surface 582 and the second surface 584. The thickness of the conducting layer 580 can be less than about 500 nm such as, for example, between about 40 nm and about 400 nm in one embodiment, or between about 60 nm and about 350 nm.

[0080] The conducting layer 580 can be formed towards a back side of the second submodule 500 with respect to the absorber layer 560. The conducting layer 580 can be disposed at the back side of the second submodule 500 and can use opaque, non-transparent metal layers as constituents. Alternately, in a bifacial tandem device, the conducting layer 580 can be transparent to at least some wavelengths.

[0081] The second submodule 500 can have a back layer 599 at the back side 504 of the second submodule 500. The back surface of the back layer 599 defines the back surface 504 of the second submodule 500. In some embodiments, the back layer 599 comprises an encapsulation material. In some embodiments, the back layer 599 comprises a back support. In some embodiments, the back layer 599 is a region of the conducting layer 580. In some embodiments, the back layer 599 can be transparent to at least some wavelengths.

[0082] The tandem photovoltaic device 300 can include a plurality of layers and, where layers are not specified as adjacent, the photovoltaic device may include intervening layers not depicted. Layers can include one or more functional layers of material and a single layer may have a compositional gradient therein. A deployment-ready device may further include electrical connections, encapsulation, structural support, mounting support, and other components.

[0083] Photovoltaic devices may contain several material layers deposited sequentially over a substrate. Steps for manufacturing a photovoltaic device may include sequentially disposing functional layers or layer precursors in a “stack” of layers through one or more deposition processes, including, but not limited to, spin coating, spray coating, slot coating, blade coating, dip coating, sputtering, evaporation, molecular beam deposition, pyrolysis, closed space sublimation (CSS), pulse laser deposition (PLD), chemical vapor deposition (CVD), electrochemical deposition (ECD), atomic layer deposition (ALD), or vapor transport deposition (VTD). The as-deposited layer or layer stack may be treated or processed to modify characteristics using one or more methods, for example, by annealing, passivating, heating, vapor contact, or chemical treatment. Manufacturing of photovoltaic devices can further include the selective removal of portions of certain layers of the stack of layers, such as by scribing, to divide the photovoltaic device into a plurality of photovoltaic cells.

[0084] In some embodiments, the first submodule 100 and the second submodule 500 are mechanically stacked, rather than monolithically integrated, and joined to form the tandem device 300. Using a stacked structure facilitates separate manufacturing processes for each submodule during most layer formation steps. Advantageously, each module may be formed in parallel, and processing parameters, like temperature, may be adjusted with respect to the architecture and composition of a single module type. Thus, for example, one submodule can be formed using low temperature deposition methods, such as spraying or roll coating, while the another submodule may use higher temperature layer formation and processing steps, such as vapor transport deposition, sputtering, or high-temperature annealing, without risking damage to temperature-sensitive layers in the other submodule.

[0085] In an example method, a partly-formed second submodule layer stack comprising a layer of a semiconductor absorber material is provided. A material for forming the buffer layer 550 is deposited onto the first surface 562 of the absorber layer 560. A material for forming the diffusion barrier layer 520 may optionally be deposited onto the first surface 552 of the buffer layer 550. A material for forming the oxide layer 540 is deposited onto the first surface 522 of the diffusion barrier layer 520, or in embodiments omitting the diffusion barrier layer 520, the material for forming the oxide layer 540 may be deposited onto the first surface 552 of the buffer layer 550. A material for forming the capping layer 530 is optionally deposited onto the first surface 542 of the oxide layer 540. [0086] The absorber layer 560 of the partly-formed second submodule layer stack may comprise a CIGS material. In some embodiments, the step of depositing the buffer layer 550 onto the first surface 562 of the absorber layer 560 passivates an absorber-buffer interface of the CIGS absorber. In some embodiments the absorber layer is subjected to a passivation treatment prior to formation of a buffer layer 550 over the absorber layer 560, and the buffer layer 550 maintains passivation at the absorber-buffer interface. A passivation treatment may include an alkali-fluoride treatment. In an example passivation method, NaF, KF, or RbF is contacted with or deposited on a surface 562 of the CIGS absorber layer 560 and the partly- formed layer stack may be heated in an annealing step, whereby the surface 562 is passivated. In some embodiments, the treatment includes depositing an alkali-fluoride material layer having a thickness of 2-20 nm or about 10 nm. In some embodiments, the treated absorber layer 560 may include a passivated region at the first surface 552.

[0087] In an alternate example method, a second submodule layer stack may be formed over a front sheet or over the interlayer. A material for forming the capping layer 530 is deposited onto the front sheet or over the interlayer 400. A material for forming the oxide layer 540 is deposited onto the second surface 534 of the capping layer 530. A material for forming the buffer layer 550 is deposited onto the second surface 544 of the oxide layer 540. A material for forming the absorber layer 560 is deposited onto the second surface 554 of the buffer layer 550. The back contact layer 570 is formed over the absorber layer 560. The conductive layer 580 is formed over the back contact layer.

[0088] The capping layer 530, the oxide layer 540, the diffusion barrier layer 520, and the buffer layer 550 can be deposited using thin film deposition techniques, including, but not limited to, sputtering, including direct current (DC), pulsed DC, and radio frequency (RF) sputtering; reactive sputtering; vacuum evaporation; pulsed laser deposition; chemical vapor deposition. In some embodiments, the oxide layer 540 is formed by sputtering. In some embodiments, the buffer layer 550 is formed by sputtering, vapor transfer deposition (VTD), or chemical bath. Oxygen partial pressure may be controlled during the deposition as to influence free carrier densities and subsequently the refractive indices and extinction coefficients as well as electrical conductivities of the layers. Further control of the free carrier densities can be achieved by intentionally doping the layers. Doping may be achieved during the deposition process using either doped targets or sources, or a combination of doped and undoped targets or sources.

[0089] Scribing and most electrical connections can be formed after forming most of the layers, and before encapsulating and combining the first submodule 100 and the second submodule 500. The first submodule 100 and second submodule may be electrically connected to form the tandem photovoltaic device 300. As used herein, the phrase “electrically connected” can mean that constituent layers cooperate to form a substantially ohmic contact directly with one another or indirectly via one or more additional components. Accordingly, current can flow between the upper submodule and the lower submodule. In some embodiments, the upper submodule and the lower submodule can be electrically connected in series. Alternatively, the upper submodule and the lower submodule can be electrically connected in parallel such as, for example, via one or more additional components or conductors.

[0090] It should now be understood that described contact layer stacks can be utilized as part of a high efficiency tandem photovoltaic module. For example, the high mobility of the described oxide layer can mitigate resistive losses and long-wavelength optical losses for the second submodule. The described contact layer stacks, providing a front contact for a lower submodule, can increase device efficiency and can promote or maintain passivation of an absorber surface in the submodule. The described contact layer stacks promote efficient n- type charge collection.

[0091] According to embodiments described herein, a tandem photovoltaic device can include a contact stack. The tandem photovoltaic device can include a top, first, or upper submodule and a bottom, second, or lower submodule. The contact stack can include an oxide layer. The oxide layer may comprise cadmium oxide (CdO). The oxide layer may have an n-type carrier mobility greater than 50 cm ² V ¹ s’ ¹ . The oxide layer may have a bandgap of in a range of 2.0 eV to 3.1 eV. The contact stack can include a buffer layer. The buffer layer can be between the oxide layer and an absorber layer of the second submodule. In some embodiments, first surface of the buffer layer may directly contact a second surface of the oxide layer, and a second surface of the buffer layer may directly contact a first surface of the absorber layer of the second submodule. The absorber layer may comprise a type I- III- VI semiconductor material. The contact stack may be between the first submodule and the absorber layer of the second submodule. The contact stack forms a junction with the absorber layer. In some embodiments, a Voc deficit of the junction is within a range of 350- 450 mV with respect to a smallest bandgap energy of the absorber layer divided by the electron charge. In some embodiments, the contact stack can have an average absorption of at least 50% to light having a wavelength in a range of 400nm to 425nm. In some embodiments, the absorber layer of the second submodule comprises a type I- III- VI semiconductor material that includes Ag, the absorber layer has a graded bandgap, and all bandgap values for the absorber layer are within a range of 0.9 eV to 1.2 eV.

[0092] In some embodiments, the contact stack includes a capping layer. A second surface of the capping layer may directly contact a first surface of the oxide layer. The capping layer can include a metal oxide. The metal oxide may include one or more of: cadmium (Cd), tin (Sn), zinc (Zn), magnesium (Mg), manganese (Mn), aluminum (Al), indium (In), or fluorine (F).

[0093] According to embodiments described herein, a partly-formed tandem photovoltaic device can include a second submodule of a tandem photovoltaic device. The second submodule can include a contact stack. In some embodiments, the contact stack includes an oxide layer having high n-type charge carrier mobility, and includes a buffer layer. In some embodiments, the contact stack is provided adjacent to an absorber layer comprising a type I- III- VI semiconductor material. In some embodiments the buffer layer promotes or maintains passivation at an absorber-buffer interface.

[0094] According to embodiments described herein, methods for making a contact layer stack are provided and methods for forming a tandem photovoltaic device are described. In some embodiments, the methods for making a contact layer include sequential deposition of layers using thin film deposition techniques.

[0095] While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications may be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of the claimed subject matter.