Photovoltaic devices with semiconductor contact layers

Abstract

The embodiments provided herein relate to thin-film photovoltaic devices and, more specifically, to the structure of layers, methods of forming, and use of particular combinations of materials to improve the efficiency of photovoltaic devices. Disclosed embodiments include photovoltaic devices having layer compositions of selected ll-VI semiconductor materials. The absorber layer (160) is CdSeTe based, and the adjacent back contact layer (180) is ZnSeTe based.

Description

PHOTOVOLTAIC DEVICES WITH SEMICONDUCTOR CONTACT LAYERSBACKGROUND

[0001] The present specification generally relates to thin-film photovoltaic devices and, more specifically, to the structure of layers, methods of forming, and use of particular combinations of materials to improve the efficiency of photovoltaic devices. Disclosed embodiments include photovoltaic devices having layer compositions of selected II-VI semiconductor materials.

[0002] A photovoltaic device generates electrical power by converting light into electricity using semiconductor materials that exhibit the photovoltaic effect. The efficient operation of a photovoltaic device may be negatively affected by resistance losses, scattering, short carrier lifetimes, and recombination of mobile charge carriers. To improve the flow of electrical current in a photovoltaic device, it is desirable to improve carrier lifetimes for p-type charge carriers. For a type II-VI absorber, such as an absorber comprising cadmium, selenium, and tellurium (CdSeTe), increasing the selenium content has the potential to increase charge carrier lifetimes. However the addition of Se to a CdSeTe absorber poses the problem of an increased carrier energy offset, or valence band offset (VBO), between the absorber layer and an adjacent contact layer. This can lead to high interface recombination, resulting in reduced device efficiency.

[0003] Accordingly, there is a need for alternative layer structures, compositions, and methods to improve photovoltaic device efficiency.SUMMARY

[0004] The embodiments provided herein relate to photovoltaic devices and semiconductor contact layers with selenium, as well as methods for forming the same. These and additional features provided by the embodiments described herein will be more fully understood in view of the following detailed description, in conjunction with the drawings.BRIEF DESCRIPTION OF THE 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. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals.

[0006] FIG. 1 schematically depicts layers in a photovoltaic device according to one or more embodiments shown and described herein.

[0007] FIG. 2 schematically depicts an absorber layer according to one or more embodiments described herein.

[0008] FIG. 3 schematically depicts a back contact layer according to one or more embodiments described herein.

[0009] FIG. 4 depicts a chart of valence band offset (VBO) of band gap voltages for different material fractions for use in photovoltaic devices according to one or more embodiments shown and described herein.

[0010] FIG. 5 shows experimental data for open circuit voltage (Voc) for devices having a CdSeTe absorber and a ZnSeTe back contact according to one or more embodiments shown and described herein.

[0011] FIG. 6 shows experimental data for open circuit voltage (Voc) for tested devices having a CdSeTe absorber and a ZnSeTe back contact with a graded composition according to one or more embodiments shown and described herein.DETAILED DESCRIPTION

[0012] Embodiments of a photovoltaic device for generating electrical power from light are described herein. The photovoltaic device generally includes an absorber layer and a p-type contact layer formed from type II- VI semiconductor materials. Disclosed embodiments include photovoltaic devices having layer compositions of selected combinations of II- VI semiconductor materials and dopants. The compositions, structures, and methods provide improved carrier lifetimes, band gap alignment and lattice matching between adjacent layers, and facilitate device efficiency improvements.

[0013] Higher substitution levels of Se for Te in CdSeTe-based absorber layers can increase charge carrier lifetimes and facilitate absorption of a broad spectral range. But, as Se substitution levels increase in the CdSeTe-based absorber, there is an increase in valence band offset (VBO) with zinc telluride (ZnTe) back contacts, corresponding to a CdSeTe valence band energy lower than the ZnTe valence band energy; increased substitution can also lead to shifting a p-type absorber to n-type, reduced stability, poor dopant incorporation, and reduced Voc after annealing. Pairing a p-type CdSeTe absorber with a doped ZnSeTe p-type contact layer canproduce a device with improvements in charge carrier lifetimes, reduced VBO, improved stability, good dopant incorporation, and improved Voc.

[0014] The efficient operation of a photovoltaic device may be negatively affected by resistance losses, scattering, short carrier lifetimes, and recombination of mobile charge carriers. To improve the flow of electrical current in a photovoltaic device, it is desirable to improve carrier lifetimes for p-type charge carriers. For a type II- VI absorber, such as an absorber layer comprising cadmium, selenium, and tellurium (CdSeTe), increasing the selenium content has the potential to increase charge carrier lifetimes. However, a high proportion of Se can cause undesirable changes in the crystal lattice, and can shift an absorber layer from p-type to n-type, also, the addition of Se to a CdSeTe absorber poses the problem of an increased valence band offset (VBO), or a carrier energy offset, between the absorber and an adjacent contact layer. This can lead to reduced device efficiency.

[0015] To improve overall efficiency, modification to absorber Se content can be combined with the addition of Se to an adjacent semiconductor back contact layer, or to an absorber-contacting region of a back contact layer. Additional improvements can be produced by incorporating group V dopants into the absorber layer and back contact layer. Improved photovoltaic devices can include a cadmium selenide telluride (CdSeTe) p-type absorber layer adjacent to a zinc selenide telluride (ZnSeTe) p-type contact layer.

[0016] A method of making a semiconductor device structure can include providing a CdSeTe absorber layer contacted by a ZnSeTe layer as a carrier selective hole-transport layer. In some embodiments, a doped CdSeTe thin-film p-type polycrystalline layer of an absorber stack may optionally be treated by methods including, for example, annealing, passivation, and / or chloride-heat treatment. A contiguous thin-film p-type ZnSeTe contact layer may be deposited onto the absorber layer of the absorber stack by a sputtering process. Sputtering of the ZnSeTe can be done by co-sputtering of ZnSe and ZnTe targets, or by sputtering of a ZnSeTe alloyed target. A co-sputtering process provides for compositional control by selectively alloying the ZnTe with Se, for tuning of the bandgap and valence band offset of the ZnSeTe contact layer to align features relating to Se content of the CdSeTe layer for improved photovoltaic device performance. Doping, for example, nitrogen doping during the sputtering deposition of the ZnSeTe film can be used to produce increased conductivity of the back contact layer or a region thereof.

[0017] Various embodiments of the absorber layer, contact layer, methods for forming layers of the photovoltaic device, layer structures, compositions, and methods to improve photovoltaic device efficiency are described in more detail herein.

[0018] The layers described in the following embodiments may be composed of more than one layer or film. As used herein, the term “layer” refers to a thickness of material provided upon a surface. Additionally, each layer can cover all or a portion of the device and / or all or a portion of the layer or material underlying the layer. For example, a “layer” can mean an amount of material that contacts all or a portion of a surface. In some embodiments, a layer can be a discontinuous layer, contiguous with a portion of a surface of an adjacent layer. In some embodiments, the discontinuous layer contacts, and is contiguous with, by area, at least 1%, 2%, 5%, 10%, 15% or 20% of a surface of the adjacent layer. In some embodiments, the discontinuous layer contacts less than 75%, 60%, 50%, 40%, 30%, 25%, or 20% of the surface of the adjacent layer. When a discontinuous layer is present, a first surface of an overlying layer can be adjacent to, and in direct contact with, both the discontinuous layer and a portion of the second surface of the layer below the discontinuous layer.

[0019] 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 an adjacent second layer are in direct contact with one another.

[0020] During a process to form one of the layers, the created layer forms on an outer surface, typically a top surface, of a substrate, layer stack, or substrate structure. A substrate structure may include a base layer introduced into a deposition process and any other or additional layers that may have been deposited onto the base layer in a prior deposition process or processes. A layer may include sublayers and can have compositional gradients within a layer. A layer can include one or more functional layers of material. Layers may be deposited over the entirety of a substrate with certain portions of the material later removed. Manufacturing of photovoltaic devices can include the selective removal of portions of certain layers of the stack of layers, such as by scribing or laser ablation, to divide the photovoltaic device into a plurality of photovoltaic cells.

[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, whenan 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] Unless specified otherwise, values for material properties and testing conditions correspond to conditions at normal temperature and pressure, 20 degrees C and 1 atmosphere pressure. Unless specified otherwise, values for provided ranges are inclusive of endpoints and include all distinct values and further divided ranges within the entire range.

[0024] Referring now to FIG. 1, an embodiment of a photovoltaic device 100 is schematically depicted. The photovoltaic device 100 can be configured to receive light and transform light into electrical signals, e.g., photons can be absorbed from the light and transformed into electrical signals via the photovoltaic effect. Accordingly, the photovoltaic device 100 can define an energy side 102, or front side, configured to be exposed to a primary light source such as, for example, the sun. The photovoltaic device 100 can also define an opposing side 104, or back side, offset from the energy side 102. 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 (UV), infrared (IR), and visible portions of the electromagnetic spectrum. The photovoltaic device 100 can include a plurality of layers disposed between the energy side 102 and the opposing side 104.

[0025] The photovoltaic device 100 has an absorber layer 160 and a contact layer 180 between a plurality of layers making up a front layer stack 115 and a back layer stack 195. Light entering on the energy side 102 passes through the front layer stack 115. In the embodiment shown in FIG. 1, the front layer stack 115 comprises a substrate 110, a barrier layer 130, atransparent conductive layer 140, and a buffer layer 150. Light passes through the front layer stack 115 to the absorber layer 160.

[0026] In bifacial embodiments, light can enter the device and reach the absorber layer 160 through both the front layer stack 115 and the back layer stack 195.

[0027] In some embodiments, the photovoltaic device 100 is combined with a second device or submodule to form a multi -junction or tandem embodiment. In some tandem embodiments, light can enter the device and reach the absorber layer 160 through the front layer stack 115 and a portion of the light may pass through the absorber layer 160 and through the back layer stack 195 to a second submodule comprising a second absorber layer. In some tandem embodiments, light can reach the absorber layer 160 through the front layer stack 115, after passing through a second submodule comprising a second absorber layer.

[0028] Referring again to FIG. 1, the photovoltaic device 100 can include a substrate 110 configured to facilitate the transmission of light into the photovoltaic device 100. The substrate 110 can be disposed at the energy side 102 of the photovoltaic device 100. Referring to FIG. 1, the substrate 110 can have a first surface 112 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 114 substantially facing the opposing side 104 of the photovoltaic device 100. One or more layers of material can be disposed between the first surface 112 and the second surface 114 of the substrate 110.

[0029] The substrate 110 can include a transparent support layer. The transparent support layer can be formed from a substantially transparent material such as, for example, glass, such as soda-lime glass or a glass with reduced iron content. The substrate 110 can have a wavelength transmittance range, for example, 350 nm to 850 nm, 400 nm to 750 nm, or about 450 nm to about 800 nm in some embodiments. The substrate 110 may also have a transmission percentage, including, for example, more than about 50% in one embodiment, more than about 60% in another embodiment, more than about 70% in yet another embodiment, more than about 80% in a further embodiment, or more than about 85% in still a further embodiment. In one embodiment, substrate 110 can comprise a glass with about 90% transmittance. Optionally, the substrate 110 can include one or more coatings applied to the first surface 112 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 antisoiling coating, or a combination thereof.

[0030] Referring to FIG. 1, the photovoltaic device 100 can include a barrier layer 130 configured to mitigate diffusion of contaminants from the substrate 110, which could result indegradation or delamination. The barrier layer 130 can have a first surface 132 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 134 substantially facing the opposing side 104 of the photovoltaic device 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. The phrase "adjacent to," as used herein, means that two layers are disposed contiguously and without any intervening materials between at least a portion of the layers.

[0031] Generally, the barrier layer 130 can be substantially transparent, thermally stable, with a reduced number of pin holes and having high sodium-blocking capability, and good adhesive properties. Alternatively or additionally, the barrier layer 130 can be configured to apply color suppression to light. 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 can have 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, more than about 50 nm, more than 75 nm in another embodiment; and less than 200 nm, including, for example, in some embodiments, less than 120 nm, less than 60 nm, or less than about 20 nm in other embodiments.

[0032] Referring still to FIG. 1, the photovoltaic device 100 can include a transparent conductive layer 140 configured to provide electrical contact to transport charge carriers generated by the photovoltaic device 100. The transparent conductive layer 140 can have a first surface 142 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 144 substantially facing the opposing side 104 of the photovoltaic device 100. In some embodiments, the transparent conductive layer 140 can be provided adjacent to the barrier layer 130. For example, the first surface 142 of the transparent conductive layer 140 can be provided upon the second surface 134 of the barrier layer 130.

[0033] The transparent conductive layer 140 can be n-type. Generally, the transparent conductive layer 140 can be formed from one or more layers of n-type semiconductor material that is substantially transparent and has a wide band gap. It may also be referred to as a front contact layer, a transparent conductive oxide (TCO) layer, or an electron transport layer (ETL). The transparent conductive 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 canhave 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 (Cd2SnO4).

[0034] Semiconductors doped to be p-type or n-type are sometimes further characterized based on the density of respective majority charge carriers. Although the boundaries are not rigid, a material is generally considered p-type if electron acceptor carriers (i.e. “holes”) are present in the range of about 1 x 1011cm’3to about 1 x 1017cm’3, and p+type if acceptor carrier density is greater than about I x lO17cm’3. Similarly, a material is considered n-type if electron donor carriers are present in the range of about 1 x 1011cm’3to about 1 x 1017cm’3, and n+type if donor carrier density is greater than about I x lO17cm’3. The boundaries are not rigid and may overlap because a layer may 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 2 orders of magnitude (i.e. 100-fold) higher, regardless of the absolute carrier density.

[0035] The photovoltaic device 100 can include a buffer layer 150 configured to provide an insulating layer between the transparent conductive layer 140 and any adjacent semiconductor layers. The buffer layer 150 can have a first surface 152 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 154 substantially facing the opposing side 104 of the photovoltaic device 100. In some embodiments, the buffer layer 150 can be provided adjacent to the transparent conductive layer 140. For example, the first surface 152 of the buffer layer 150 can be provided upon the second surface 144 of the transparent conductive layer 140.

[0036] The buffer layer 150 may include material having higher resistivity than the transparent conductive layer 140, including, but not limited to, intrinsic tin dioxide, zinc magnesium oxide (e.g., Zni-xMgxO), silicon dioxide (SnCh), aluminum oxide (AI2O3), aluminum nitride (AIN), zinc tin oxide, zinc oxide, tin silicon oxide, or a combination 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 layer. The buffer layer 150 may have a thickness between the first surface 152 and the second surface 154, including, for example, more than about 10 nm in one embodiment, between 10 nm and about 80 nm in another embodiment, or between 15 nm and 60 nm in a further embodiment.

[0037] Referring again to FIG. 1, the photovoltaic device 100 can include an absorber layer 160 configured to cooperate with another layer and form a p-n junction within thephotovoltaic device 100. As used herein, the term “p-n junction” can encompass p-i-n, p-n, n-p, or n-i-p configurations. Accordingly, absorbed photons of the light can free electron-hole pairs and generate carrier flow, which can yield electrical power. The absorber layer 160 can have a first surface 162 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 164 substantially facing the opposing side 104 of the photovoltaic device 100. A thickness of the absorber layer 160 can be defined between 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.

[0038] Referring still to FIG. 1, the absorber layer 160 can be a poly crystalline p-type semiconductor material having an excess of positive charge carriers, i.e., holes or acceptors. The absorber layer 160 can include p-type semiconductor materials such as group II- VI semiconductors. Specific examples include, but are not limited to, semiconductor materials including cadmium, tellurium, selenium, or combinations thereof. Example compositions include, but are not limited to, cadmium telluride, ternaries of cadmium, selenium and tellurium (e.g., CdSexTei-x,), or quaternaries comprising cadmium, selenium and tellurium. The absorber layer 160 can comprise a cadmium selenide telluride (CdSeTe) p-type polycrystalline thin film.

[0039] In some embodiments, an atomic concentration of the elements in the absorber layer 160 can vary across a thickness of the absorber layer. The concentration may form a gradient across a layer thickness. Alternatively or additionally, the concentration profile of the elements can vary through the thickness of the absorber layer 160 between the first or front surface 162 and the back or second surface 164 of the absorber layer 160. In some embodiments, the atomic concentration can vary with distance from the second surface 164 of the absorber layer 160. For example, the atomic ratio of CdTei-xSexof the absorber layer measured in bulk can be provided such that x is less than about 0.4 such as, for example, a range of about 0.1 to about 0.4. Alternatively or additionally, the atomic ratio of CdTei-xSexmeasured along any portion of the thickness of the absorber layer can be provided such that x is less than about 0.4 throughout the thickness, such as, for example, a range of 0.005 to 0.400. Generally, as used herein, reference to an atomic ratio of a layer, e.g., “the absorber has a mole ratio,” is intended to invoke the bulk measurement of the layer. Alternatively, reference to a specific location of the layer, e.g., “second surface of the absorber layer has a mole ratio,” is intended to refer to the mole ratio of the referenced location.

[0040] In embodiments where the absorber layer 160 comprises selenium and cadmium, the atomic percent of the selenium can be greater than about 0 atomic percent and less than about 20 atomic percent with respect to the alloy as a whole, or can be in a range between 0.0% to 40% as a substitution fraction with respect to tellurium. In embodiments where the absorber layer 160 comprises tellurium and cadmium, the atomic percent of the tellurium can be greater than about 30 atomic percent and less than about 50 atomic percent with respect to the alloy as a whole. Thus, for an absorber composition of CdSe(X)Te(i-X), a value of x can be 0.00 < x < 0.40. It is noted that the atomic percent described herein is representative of the entirety of the absorber layer 160, throughout its thickness, the atomic percentage of material at a particular location within the absorber layer 160 can vary with thickness depth compared to the overall composition of the absorber layer 160. It is noted that the concentration of tellurium, selenium, or both can vary through the thickness of the absorber layer 160. For example, when the absorber layer 160 comprises a polycrystalline ternary of cadmium, selenium, and tellurium (CdSexTei-x), x can vary in the absorber layer 160 with distance from the first surface 162 of the absorber layer 160. In some embodiments, the value of x can decrease in the absorber layer 160 with distance from the first surface 162 of the absorber layer 160.

[0041] According to the embodiments provided herein, the absorber layer 160 can be doped with a group V dopant such as, for example, nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), bismuth (Bi), Moscovium (Me) or a combination thereof. The total dosage of the dopant within the absorber layer 160 can be controlled. In some embodiments, an atomic concentration of Group V dopant in the absorber layer 160 can be greater than about lxl017cm'3such as, for example, between about lxl017cm'3and about 5xl020cm'3in one embodiment, between about 3xl017cm'3and about 5xl019cm'3in another embodiment.

[0042] Alternatively or additionally, the atomic concentration profile of the group V dopant can vary through the depth of the absorber layer 160. Specifically, the atomic concentration of the group V dopant can vary with distance from the first surface 162 of the absorber layer 160. It is noted that the phrase “average atomic concentration,” as used herein, can mean the average of the atomic concentration profile over a given region or thickness.

[0043] Referring still to FIG. 1, a p-n junction can be formed by providing the absorber layer 160 sufficiently close to a portion of the photovoltaic device 100 having an excess of negative charge carriers, i.e., electrons or donors. In some embodiments, the absorber layer 160 can be provided adjacent to n-type semiconductor material. Alternatively, one or more intervening layers can be provided between the absorber layer 160 and n-type semiconductormaterial. The absorber layer 160 can be p-type throughout its thickness. The absorber layer 160 can form a p-n junction with one or more layers of the front layer stack 115. 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.

[0044] Turning now to FIG. 2, with continued reference to FIG. 1, the absorber layer 160 can include a plurality of regions. In the example illustrated in FIG. 2, the absorber laye cxr 160 consists of three substantially co-planar regions, a front absorber region 160A, a central or bulk absorber region 160B, and a back absorber region 160C. In this embodiment, the bulk absorber region 160B corresponds to a middle 70% of the thickness of the absorber layer, the front absorber region 160A corresponds to 15% of the thickness of the absorber layer nearest the front layer stack 115, and the back absorber region 160C corresponds to 15% of the thickness of the absorber layer nearest the back layer stack 195. In some embodiments, a compositional or atomic ratio of the elements alloyed to form the absorber layer 160 differs between at least two regions of the three regions 160A, 160B, 160C, constituting the absorber layer 160. In some embodiments, a dopant level differs between at least two regions of the three regions 160A, 160B, 160C, constituting the absorber layer 160.

[0045] Referring to FIGS. 1 and 3, the photovoltaic device 100 can include a back contact layer 180. The back contact layer 180 can be configured to provide electrical contact to the absorber layer 160. The back contact layer 180 can be configured to mitigate undesired alteration or migration of dopants. The back contact layer 180 can have a first surface 182 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 184 substantially facing the opposing side 104 of the photovoltaic device 100. A thickness of the back contact layer 180 can be defined between the first surface 182 and the second surface 184. The thickness of the back contact layer 180 can be between about 5 nm to about 200 nm such as, for example, between about 10 nm to about 50 nm, a thickness range of 18 nm to 25 nm, or in a thickness range of 15 nm to 40 nm in an example embodiment. The back contact layer 180 can have a substantially contiguous and uniform thickness, with at least 95% of the back contact layer having a thickness deviating by less than + / - 20% of a nominal thickness range.

[0046] The back contact layer 180 can be provided adjacent to the absorber layer 160. For example, the first surface 182 of the back contact layer 180 can be provided upon the second surface 164 of the absorber layer 160.

[0047] The back contact layer 180 can comprise a zinc selenide telluride (ZnSeTe) p-type contact layer. In some embodiments, the back contact layer 180 can include a chemical compound of materials from groups I, II, VI, such as for example, one or more layers containing zinc, copper, cadmium, and tellurium in various compositions in one embodiment, or binary or ternary combinations of materials from groups I, II, VI. Further exemplary materials include, but are not limited to, zinc telluride doped with nitrogen, zinc telluride doped with copper telluride, or zinc telluride alloyed with copper telluride. In an example, the back contact layer 180 comprises Zm. ySeyTe, wherein a value of y is in a range from 0.05 to 0.50. In some examples, a concentration of selenium is graded and higher at a region adjacent the first surface 182, relative to a region adjacent a second surface 184. In an example, a value of y at the first surface 182 is in a range of 0.1 to 0.4 (0.1 < y < 0.4), and a value of y at the second surface 184 of the back contact layer 180 is in a range of 0.0 to 0.2 (0.0 < y < 0.2). In some embodiments, the back contact layer is doped with a group V dopant including at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth.

[0048] Referring to FIG. 3, the back contact layer 180 can include a plurality of regions. In some embodiments, the back contact layer 180 can be provided as a bilayer having two regions. In the example illustrated in FIG. 3, the back contact layer 180 consists of two substantially coplanar regions, a first region 180A and a second region 180B. In an example embodiment, the first region 180A corresponds to about 50% of the thickness of the of the back contact layer 180, and the second region 180B corresponds to the remaining 50% of the thickness of the of the back contact layer 180. In some embodiments, the first region 180A corresponds to 10% to 100% of the thickness of the back contact layer 180, and the second region 180B corresponds to 0% to 90% of the thickness of the back contact layer. In some embodiments, the first region 180A thickness is at least 10%, 20%, 30%, or at least 40% of the thickness of the back contact layer 180. In some embodiments, the second region 180B thickness is at least 10%, 20%, 30%, or at least 40% of the thickness of the back contact layer 180. In some embodiments, the first region 180A corresponds to 20% to 100% of the thickness of the back contact layer 180, and the second region 180B corresponds to 0% to 80% of the thickness of the back contact layer. In an example, the first region 180A thickness is equal to or greater than the second region thickness 180B. In an example, the thickness of the second region is greater than 0 and a ratio of the thickness of the second region to the thickness of the first region is in a range of 1 : 1 to 1 :20.

[0049] In an example embodiment, the back contact layer 180 consists of the first region 180A and the second region 180B, the back contact layer 180 comprising nitrogen-doped Zni.ySeyTe, wherein a value of y in the first region 180A is in a range of 0.1 to 0.4 (0.1 < y < 0.4), and a value of y in the in the second region 180B is in a range of 0.0 to 0.2 (0.0 < y < 0.2). In some examples, a doping concentration differs between the first region 180A and the second region 180B. In some examples, a dopant concentration of a group V dopant in the second region 180B is equal to or greater then a dopant concentration of the same group V dopant in the first region 180B. In some examples, a ratio of a group V dopant concentration, in atoms per cm'3, in the second region 180B to the first region 180A is at least 1.5 : 1, at least 2:1, at least 3:1, at least 5:1, at least 10:1, at least 15:1, or in a range of 1:1 to 20:1.

[0050] Referring to FIG. 1, the photovoltaic device 100 can include a transparent conducting layer 190 configured to provide electrical contact with the back contact layer 180, the absorber layer 160, or both. The transparent conducting layer 190 can have a first surface 192 substantially facing the energy side 102 of the photovoltaic device 100 and a second surface 194 substantially facing the opposing side 104 of the photovoltaic device 100. In some embodiments, the transparent conducting layer 190 can be provided adjacent to the back contact layer 180. For example, the first surface 192 of the transparent conducting layer 190 can be provided upon the second surface 184 of the back contact layer 180. A thickness of the transparent conducting layer 190 can be defined between the first surface 192 and the second surface 194. The thickness of the transparent conducting layer 190 can be less than 500 nm such as, for example, between 40 nm and 400 nm in one embodiment, or between 60 nm and 350 nm.

[0051] According to the embodiments provided herein, the transparent conducting layer 190 can include one or more functional layers of material. The transparent conducting layer 190 can include a diffusion barrier layer operable to limit diffusion of metal species into the absorber layer 160. Materials for use in the diffusion barrier can include metal oxynitrides, nitrides, or oxides, such as, for example, titanium oxynitrides (TiNxOy) or molybdenum oxynitrides (MoNxOy). Materials for use in the diffusion barrier can include transparent conductive oxides such as, for example, tin oxide (SnCb) zinc oxide (ZnO), indium-tin oxide (In(2-x)SnxO3), cadmium oxide (CdO), and cadmium stannate (Cd2SnO4), including amorphous cadmium stannate. These transparent conductive oxides can be doped with impurities such as F, Al, In, Ga, Ti, and others to alter their electrical and optical properties. In some embodiments, the transparent conducting layer 190 comprises an amorphous cadmium stannate CdzSnCM material where 0.5 < z < 2.

[0052] The transparent conducting layer 190 can include a high conductivity layer disposed over the diffusion barrier layer and configured to provide low device series resistance. The high conductivity layer can include a degeneratively doped transparent conductive oxide. In some embodiments, the high conductivity layer can be doped n++ intrinsically or with an oxide dopant. The oxide dopants may include, but are not limited to, In2Os, Ga2Os, TiCh, Dy?©?, SnCh, Y2O3, AI2O3, or a combination thereof. In some embodiments, high conductivity layer can include cadmium oxide (CdO) such as, for example, indium oxide doped cadmium oxide (CdOJmCh) or gallium oxide doped cadmium oxide (CdO:Ga2O3).

[0053] The transparent conducting layer 190 can further include a capping layer operable to mitigate corrosion of the high conductivity layer in hot and humid environments. Accordingly, the transparent conducting layer 190 can include a stack of layers, for example: a diffusion barrier layer / a high conductivity layer / a capping layer. The capping layer can include a transparent conductive oxide, such as, but not limited to, cadmium stannate.

[0054] In some embodiments, the high conductivity layer of the transparent conducting layer 190 can be positioned further away from the back contact layer 180 relative to the diffusion barrier layer of the transparent conducting layer 190. the capping layer can be positioned further away from the back contact layer 180 relative to the high conductivity layer. Accordingly, within the transparent conducting layer 190, the high conductivity layer can be positioned between the diffusion barrier layer and the capping layer. Specifically, in some embodiments, the capping layer can be provided adjacent to the high conductivity layer.

[0055] The photovoltaic device 100 can include a back support 196 configured to cooperate with the substrate 110 to form a housing for the photovoltaic device 100. The back support 196 can be disposed at the opposing side 104 of the photovoltaic device 100. For example, the back support 196 can be formed adjacent to the transparent conducting layer 190. The back support 196 can include a supporting material, including, for example, glass, borosilicate glass, float glass, soda lime glass, carbon fiber, polycarbonate, or a polymer-based back sheet. The back support 196 and substrate 110 can protect the various layers of the photovoltaic device 100 from exposure to moisture and environmental hazards.

[0056] In some embodiments, the photovoltaic device 100 can comprise a supplemental p-type charge transport material. In some embodiments, the supplemental charge transport material can be provided between the absorber layer 160 and the back support 196. In some embodiments, the supplemental charge transport material can be a discontinuous layer. In someembodiments, the supplemental charge transport material can be a discontinuous layer contiguous with a portion of a surface of an adjacent layer. In some embodiments, the supplemental p-type charge transport material is a discontinuous layer and contacts a portion of the absorber layer 160 second surface 164. In some embodiments, the supplemental p-type charge transport material is a discontinuous layer and contacts a portion of the first surface 182 of the back contact 180.

[0057] In some embodiments, the supplemental charge transport material can be a p-type material such as, for example, a polymer, a small molecule, or an inorganic compound. The polymeric semiconductor materials can include Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] or poly[bis(4-phenyl) (2,4-dimethylphenyl) amine (PTAA), Poly(3-hexylthiophene-2,5-diyl) (P3HT), or Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). In some embodiments, the charge transport material comprises a small molecule selected from N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)-9,9'-spirobi[9H-fluorene]-2,2',7,7'- tetramine (Spiro-OMeTAD), N2,N2,N2',N2',N7,N7,N7',N7'-octakis(4-methoxyphenyl)-10- phenyl-10H-spiro[acridine-9,9'-fluorene]-2,2',7,7'-tetraamine (SAF-OMe), OMeTPA-FA, SGT- 407, Fused-F, or tetrathiafulvalene (TTF-1). In some embodiments, the charge transport material comprises a material selected from: alpha-NPD, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), PCPDTBT, PCDTBT, graphene oxide, and quinolizino acridine. In some embodiments, the supplemental p-type charge transport material comprises an inorganic compound selected from nickel oxide (NiOx), cuprous thiocyanate (CuSCN), or copper oxide (Cu2O).

[0058] Additional components can be included in the exemplary photovoltaic device 100, such as bus bars, wiring, and encapsulation.

[0059] Referring still to FIG. 1, manufacturing of a photovoltaic device 100 generally includes sequentially disposing functional layers or layer precursors in a “stack” of layers through one or more processes, including, but not limited to: sputtering, spray coating, spin coating, inkjet printing, slot-die coating, blade coating, dip coating, roll coating, 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). Once a layer is formed it may be desirable to modify the physical characteristics of the layer through subsequent treatment processes.

[0060] In an example process, an absorber layer 160 is formed on the front layer stack 115. Forming the absorber layer may include deposition of one or more compositions comprisingcadmium, tellurium, and / or selenium. Formation of the absorber layer may include introducing one or more dopants. Absorber layer formation and processing may include annealing, passivation, and / or chloride treatment steps to produce the absorber layer 160. In some embodiments, a supplemental charge transport material may be contacted to the second surface of the absorber layer to form a discontinuous layer over a portion of the absorber layer 160. The back contact layer can be formed over the absorber layer 160. Forming the back contact layer may include deposition of one or more compositions comprising zinc, selenium, and / or tellurium. The back contact layer can be provided as a contiguous layer with substantially uniform thickness. In some embodiments, the back contact layer 180 may be formed by a sputtering process.

[0061] In an example, a sputtering deposition method is performed by one or more processes including: magnetron sputtering, direct current sputtering, radio frequency sputtering, or reactive sputtering. A sputtering deposition method can include a plurality of sources, for example, two, three, four, or five sources. Each source or target may comprise an element or alloy comprising one or more of: zinc, selenium, or tellurium. In an example process, the sputtering power for each target can be independently controlled. In an example process, the back contact layer is formed by a co-sputtering technique, using ZnSe and ZnTe targets or sources.

[0062] In some examples, the sputtering process is performed with a carrier gas or plasma comprising a noble gas. In some examples, the carrier gas comprises at least one of: argon, helium, or neon. In some examples, nitrogen is mixed with argon producing a composition calculated from total gas flow ratios in the range of 0.1% to 2.5%, N2 / (N2 + Ar). In an example deposition chamber, having an environment consisting essentially of argon and nitrogen, a ratio of argon to nitrogen is equal to or greater than 1 : 0.0001, and equal to or less than 1 : 0.01.

[0063] In some examples, the deposition is performed in a chamber at low pressure. In some examples a deposition time for forming a layer across a substrate width is in a range of 30 to 240 seconds. In some embodiments, the deposition is performed in a deposition chamber at low pressure. In some embodiments, a deposition chamber pressure is less than 100 millibar (mbar) and greater than 0.1 microbar (ubar). In some examples, the deposition is performed at a pressure in range from 1.0 ubar to 20.0 millibar (mbar), less than 10.0 mbar, less than 5.0 mbar, less than 2.0 mbar, less than 1.0 mbar, less than 0.5 mbar, less than 0.1 mbar, less than 0.025 mbar, less than 100.0 ubar, less than 50.0 ubar, less than 25.0 ubar, less than or equal to 10.0 ubar, equal to or greater than 0.5 ubar, greater than 1.0 ubar, greater than 1.5 ubar, greater than 1.8 ubar, or equal to or greater than 2.0 ubar. In some embodiments, a deposition chamber pressure is in a range from 1.0 ubar to 20.0 ubar, or in a range from 2.0 ubar to 10.0 ubar. In some examples, thesubstrate temperature is between 20° C to 600° C, between 50° C to 500° C, between 100° C to 400° C, between 150° C to 250° C, between 50° C to 200° C, between 200° C to 400° C, between 250° C to 350° C, or between 200° C to 300° C.

[0064] In an example, the sputtering power is in a range of 10 W to 2000 W. In some examples, the sputtering power for each target is in a range of 20 W to 400 W. In some examples having a plurality of targets, the method includes simultaneously operating a first target and a second target at different power levels. In an example, a difference between power supplied to a first target and power supplied to a second target is greater than 10 W, or in a range of 10 W to 50 W. In an example, a sputtering power ratio between the sputtering power for the first target and the sputtering power for the second target can be controlled. In an example, a sputtering power ratio can be in a range from 1 :5 to 5: 1 ; in some examples the sputtering power ratio can be greater than 1:1, greater than 2: 1 , or greater than 3:1.

[0065] In some examples, a layer-forming material is deposited simultaneously along a substrate width, substantially perpendicular to the substrate path, as the substrate is conveyed past a deposition station. The substrate may be conveyed continuously along a substrate path adjacent to a deposition station, such that a substantially even thickness of material is deposited in a continuous layer over the substrate stack. In an example, a substrate has a rectangular surface with a width in a range of 0.5 to 2.0 meters and a length is in a range of 0.5 to 2.0 meters. In some examples, the deposition system includes a plurality of deposition stations. In some embodiments, the deposition system includes a first deposition station in a first controlled environment, and a second deposition station in a second controlled environment, In an example deposition process, the substrate is conveyed sequentially past the first deposition station, and then past the second deposition station. In some examples, a gas flow ratio of carrier gasses differs between the first controlled environment, and the second controlled environment. In some embodiments, a gas flow ratio of nitrogen to nitrogen and argon in the first controlled environment is in a range of 0.0% to 0.8%, and a gas flow ratio of nitrogen to nitrogen and argon in the second controlled environment is in a range of 0.5% to 2.0%.

[0066] An example deposition system can convey individual substrates into a vacuum chamber. The individual substrates can be conveyed into a sputtering chamber and past a sputtering device, such as a planar magnetron, continuously sputtering a target by an ionized gas at the sputtering pressure such that a thin film is formed on a surface of the individual substrate. The substrates can be continuously conveyed at a substantially constant linear conveyance rate, while sputtering, such that the thin film formed on the surface of the individual substrate has anaverage thickness of about 5.0 nm to about 250 nm, or a narrower range as previously described for specific layers, with a non-uniformity of about 2% to about 20% of the average thickness. The deposition system can include a plurality of sputtering devices each depositing a layer, or a sublayer of a layer. By controlling deposition target compositions, power settings, and environmental conditions, such as gas flow rates and composition ratios, the method can be used to finely tune the properties of the produced layer or layers. The method can be used for high- throughput manufacturing, directed to sputtering thin films on individual substrates defining a surface having a surface area of in a range of 0.5 m2to 2.5 m2.

[0067] In an example, the back contact layer 180 is a contiguous thin-film ZnSeTe layer that is sputter-deposited over a contiguous thin-film CdSeTe absorber layer, and the back contact layer 180 is nitrogen doped during deposition by N2 addition to an inert carrier. In an example, N2 is provided with an argon plasma in a range of 0.5% to 1.2% gas flow ratio, N2 / (N2 + Ar), under pressures in a range of 2.0 microbar to 10.0 microbar, and combined with sputtering powers in a range of 20 W to 100 W and at elevated temperatures, to sputter deposit ZnSeTe layers with tunable Se content in a thickness range of 15 nm to 40 nm. In some examples the ZnSeTe layer has a nitrogen dopant concentration in a range of 1 x 1018cm'3to 1 x IO20cm'3.

[0068] Turning now to FIG. 4, a graph of valence band offset (VBO) values in electron volts (eV) is shown for adjacent layers comprising CdSeTe and ZnSeTe for concentrations of selenium with an atomic percent substitution fraction ranging from 0 to 30% for each of CdSeTe and ZnSeTe. The x-axis shows the Se % relative to the total amount of Se and Te, for back contact compositions ranging from 0% (ZnTe) to 30% (ZnSeo.sTeo.?)- The lines on the graph show the Se %, relative to the total amount of Se and Te, for CdSeTe absorber compositions for: 0%, 5%, 10%, 20%, 30%, or: CdTe, CdSeo.o5Teo.95, CdSeo.iTeo.9, CdSeo.2Teo.8, CdSeo.sTeo.?, from top to bottom, respectively. As illustrated in the graph, the valence band offset produced by increasing Se content in a CdSeTe absorber can be at least partially compensated by increasing Se content in a ZnSeTe back contact and modulating material fractions. Additional improvements can be gained by modulating compositional gradients and doping levels.

[0069] Turning now to FIG. 5, experimental data is shown for the open circuit voltage (Voc) for devices having a CdSeTe absorber and a ZnSeTe back contact. The Se percentage, relative to the total amount of Se and Te, of the absorber was 5%, 10% or 20%, and the Se percentage, relative to the total amount of Se and Te, of the back contact was 40%, 20%, 8%, or 0%. For the devices providing data for FIG. 5, the ternary or binary compositions were substantially consistent in composition, without compositional grading across a thickness.Nitrogen was included in the deposition of the ZnSeTe layers and was unchanged between the tested devices shown in FIG. 5. As shown, at levels of Se concentration of around 20% Se in the absorber, Voc was higher with levels of Se concentration of around 40% in the back contact.

[0070] Turning now to FIG. 6, experimental data is shown for the open circuit voltage (Voc) for devices having a CdSeTe absorber and a ZnSeTe back contact with a graded composition in the absorber and with absorber doping. The absorber layer comprises an arsenic- doped CdSeTe layer with higher concentrations of Se near a first surface of the absorber layer and lower Se concentrations of Se near the second surface and the back contact layer. The back contact included nitrogen-doped ZnSeTe prepared by co-sputtering ZnTe and ZnSe at various power ratios, resulting in the Se% levels shown in the legend of Fig 5, or a layer of nitrogen-doped ZnTe as a control. Se concentration, relative to the total amount of Se and Te, in the nitrogen-doped ZnSeTe material was provided at 8%, 20%, and 40%. Prior to depositing the back contact, the absorber stacks were processed by a chloride heat treatment for a duration of about 40 minutes (left side) or about 70 minutes (right side).

[0071] Band gap (Eg) was also assessed for the tested devices, measured from the front side. Measurements indicated that for the control device having a ZnTe back contact, longer chloride heat treatment can produce higher band gap, however, the longer chloride heat treatment can also detrimentally reduce Voc. Testing indicated that Voc loss from absorber processing steps, such as long chloride heat treatment where Se diffuses more strongly to the back of the absorber, can be recovered by incorporating Se into the back contact layer, contributing to increased device efficiency. Specifically, an increase of 40mV was measured for the nitrogen doped ZnSeTe in which the Se concentration is 40%, relative to the control device with a nitrogen doped ZnTe contact layer.

[0072] It should now be understood that the embodiments provided herein, relate to the use of back contact layers comprising zinc selenide telluride (ZnSeTe) with absorber layers comprising cadmium selenide telluride (CdSeTe) to improve photovoltaic device efficiency. Specifically, the use of back contact layers comprising ZnSeTe can increase p-type charge carrier lifetimes, reduce valence band offset, improve II- VI lattice alignment, modulate bandgap for use in bifacial devices, and increase Voc to improve device efficiency. Improved layers and photovoltaic devices can be produced by controlling aspects of the deposition process and device fabrication.

[0073] According to the embodiments provided herein, a photovoltaic device can include a cadmium selenide telluride (CdSeTe) absorber layer and a zinc selenide telluride (ZnSeTe) back contact layer. The absorber layer can be doped and can have a concentration gradient of selenium. The absorber layer can be p-type throughout its thickness. The absorber layer can form a p-n junction with one or more layers of the front layer stack. The back contact layer can be adjacent to the absorber layer. The back contact layer can be doped. The back contact layer can have a concentration gradient of selenium. The back contact layer can comprise a layer of ZnSeTe and a layer of ZnTe. The back contact layer can be p-type throughout its thickness.

[0074] According to the embodiments provided herein, a method for forming a photovoltaic device can include forming a zinc selenide telluride (ZnSeTe) back contact layer over a cadmium selenide telluride (CdSeTe) absorber layer. In some embodiments the back contact layer can be deposited by sputtering. In some embodiments a dopant can be incorporated during deposition. In some embodiments the dopant is a Group V element. In some embodiments the dopant is provided as a gas during deposition.

[0075] According to the embodiments provided herein, a photovoltaic device can include a front layer stack comprising at least one n-type layer; an absorber layer comprising cadmium, tellurium, and selenium; and a back contact layer comprising zinc, tellurium, and selenium. The absorber layer can be p-type throughout. The absorber layer can have a first surface and a second surface. The back contact layer can have a first surface disposed on the second surface of the absorber layer. The back contact layer can be p-type throughout.

[0076] The absorber layer can form a p-n junction with the front layer stack. In some embodiments, the absorber layer is doped with at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth.

[0077] In some embodiments, the absorber layer has a mole ratio of CdSe(X)Te(i-x), wherein a value of x at the second surface of the absorber layer is less than or equal to 0.40 and equal to or greater than 0.02. In some embodiments, the absorber layer has a mole ratio of CdSe(X)Te(i-x), wherein a value of x at the second surface of the absorber layer is equal to or greater than 0.15. In some embodiments, the absorber layer has a mole ratio of CdSe(X)Te(i-x), wherein a value of x at the second surface of the absorber layer is equal to or greater than 0.20.

[0078] In some embodiments, the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50 throughout the layer. In some embodiments,the back contact layer has a mole ratio of ZnSe(y)Te(i-y), where an average value of y in the back contact layer is in a range from 0.15 to 0.35.

[0079] In some embodiments, the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.10 and less than or equal to 0.40. In some embodiments, the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.20. In some embodiments, the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.25. In some embodiments, the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.30.

[0080] In some embodiments, the back contact is doped with a Group V dopant. In some embodiments, the back contact layer can be doped with at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth. In some embodiments, the back contact is a ZnSeTe layer doped with a nitrogen dopant at a concentration in a range of 1 x 1018cm’3to 1 x IO20cm’3.

[0081] In some embodiments, the dopant has a graded composition with lower concentrations of the dopant near the first surface and higher concentrations of the dopant near a second surface of the back contact layer. In some embodiments a concentration of the dopant at the second surface is between 2 to 20 times greater than a concentration of the dopant at the first surface.

[0082] In some embodiments, the back contact has a resistivity in a range from 0.02 to 50.0 ohm-cm. In some embodiments, the back contact has a resistivity less than 40.0 ohm-cm, less than 25.0 ohm-cm, less than 10.0 ohm-cm less than 7.0 ohm-cm, or less than 5.0 ohm-cm.

[0083] In some embodiments, the back contact layer has a total thickness in a range of 5 nm to 150 nm. In some embodiments, the back contact layer has a total thickness less than 100 nm, less than 60 nm, less than 50 nm, less than 40 nm, less than 30 nm, or less than 25 nm. In some embodiments, the back contact layer comprises at a first region and a second region that are substantially coplanar. In some embodiments the first region has a thickness in a range of 2 nm to 100 nm, or a thickness less than 40 nm, less than 30 nm, less than 25 nm, less than 20 nm, less than 15 nm, less than 12 nm, less than 10 nm, or in a range of 5 nm to 25 nm. In some embodiments the second region has a thickness in a range of 2 nm to 100 nm, or a thickness less than 40 nm,less than 30 nm, less than 25 nm, less than 20 nm, less than 15 nm, less than 12 nm, less than 10 nm, or in a range of 5 nm to 25 nm.

[0084] In some embodiments, the back contact layer can have a first region proximate to the absorber layer and the back contact layer can have a second region between the first region and a back layer stack. The first region of the back contact layer can have a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the first region of the back contact layer is greater than 0.01 and less than or equal to 0.50; and the second region can have a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the second region of the back contact layer has a value in a range from 0 to 0.20.

[0085] In some embodiments, the absorber layer comprises a back absorber region, the back absorber region comprising 15% of the thickness of the absorber layer, wherein the back absorber region is proximate to the second surface of the absorber layer, and wherein a composition of the back absorber region comprises CdSe(X)Te(i-X), wherein x is less than or equal to 0.40 and equal to or greater than 0.02. In some embodiments, a composition of the back absorber region comprises CdSe(X)Te(i-X), wherein x is greater than 0.05, greater than 0.10, equal to or greater than 0.15, equal to or greater than 0.20, or is equal to or greater than 0.25.

[0086] In some embodiments, a band gap of the back contact layer at the first surface of the back contact layer is at least 0.65 eV greater 0.25 eV greater than a band gap of the second surface of the absorber layer. In some embodiments, the absorber layer at the second surface has a band gap between 1.35 eV and 1.75 eV.

[0087] In some embodiments, a valence band offset between the absorber layer and the back contact layer at the second surface of the absorber layer is less than 0.10 eV. In some embodiments, a valence band offset (VBO) between the absorber layer and the back contact layer at the second surface of the absorber layer is between 0.0001 eV and 0.0750 eV. In some embodiments, the VBO between the absorber layer and the back contact layer at the second surface of the absorber layer is between 0.000 eV and 0.075 eV, less than 0.065 eV, less than 0.055 eV, less than 0.050 eV, less than 0.045 eV, less than 0.035 eV, or less than 0.030 eV.

[0088] In some embodiments, the back contact layer comprises zinc selenium telluride doped with nitrogen, wherein the nitrogen dopant is present in the back contact layer at a level in a range of 0.006 to 0.020 atomic percent.

[0089] In some embodiments, the photovoltaic device includes a transparent conducting layer over the back contact layer. The transparent conducting layer can be a n+ layer.

[0090] According to the embodiments of the present disclosure, a method for forming a photovoltaic device can include providing a front layer stack; forming an absorber layer over the front layer stack; and depositing a back contact layer over the absorber layer. The absorber layer can include cadmium, tellurium, selenium. The absorber layer can be p-type throughout. The absorber layer can have a first surface disposed on the front layer stack and a second surface. The back contact layer can be formed over the second surface of the absorber layer. The back contact layer can be p-type throughout.

[0091] In some embodiments, the method can include treating the absorber layer with a cadmium chloride heat treatment prior to forming the back contact layer. In some embodiments, the cadmium chloride heat treatment can include contacting the second surface of the absorber layer with cadmium chloride and maintaining the absorber layer stack at a temperature in a range of 300°C to 500°C for a duration. In some embodiments, the method can include treating the absorber layer with a cadmium chloride heat treatment for a duration of between 45 minutes and 120 minutes. In some embodiments, the duration can be greater than 45 minutes. In some embodiments, the duration can be greater than 60 minutes. In some embodiments, the duration can be less than 90 minutes. In some embodiments, the duration can be in a range of 50 to 80 minutes.

[0092] In some embodiments, the step of depositing the back contact layer can include simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, wherein a sputtering power for the first target differs from a sputtering power for the second target by at least 10 W. In some embodiments, a sputtering power for the first target differs from a sputtering power for the second target by between 10 W to 50 W. In some embodiments, a sputtering power for the first target differs from a sputtering power for the second target by between 15 W to 40 W. The method can include depositing the back contact layer by sputtering ZnSe and ZnTe in an environment comprising nitrogen, thereby depositing the back contact as a nitrogen-doped alloy of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50.

[0093] In some embodiments, the step of depositing the back contact layer is performed in a deposition chamber having a controlled environment. The deposition chamber may have an atmosphere comprising an inert carrier gas, or a combination an inert gas with nitrogen. The carrier gas may comprise one or more of helium, neon, or argon. In some embodiments, the deposition chamber atmosphere may comprise less than 5.0% nitrogen, less than 4.0% nitrogen, less than 3.0% nitrogen, or less than 2.0% nitrogen by mole. The deposition chamber atmosphere may comprise a trace amount of air and / or water vapor. In some embodiments, the depositionchamber atmosphere may comprise oxygen at a level less than 0.01%, or less than 0.001%. In some embodiments, the deposition chamber atmosphere may comprise oxygen at a level between 10.0 ppm and 500 ppm. In some embodiments, the deposition is performed in a deposition chamber at low pressure. In some embodiments, a deposition chamber environment has a ratio of inert gas to nitrogen equal to or greater than 1 : 0.0001 and equal to or less than 1 : 0.01. In some embodiments, a concentration of nitrogen differs between first deposition chamber environment and a second deposition chamber environment. In an example deposition chamber, having a total pressure in in a range of 1.0 microbar to 50.0 microbar, a partial pressure of nitrogen can be in a range of 0.0001 microbar to 0.1000 microbar.

[0094] In some embodiments, the step of depositing the back contact layer is performed in a controlled environment comprising argon and nitrogen, wherein a gas flow ratio of nitrogen, relative to a combined flow of nitrogen and argon, is in a range of 0.2% to 2.5%. In some examples, nitrogen is mixed with argon producing a composition calculated from total gas flow ratios in the range of 0.1% to 2.5% , N2 / (N2 + Ar). In some embodiments, a gas flow ratio, N2 / (N2 + Ar), is 0.5% to 2.2%. In some embodiments, a gas flow ratio, N2 / (N2 + Ar), is less than 2.0%. In some embodiments, a gas flow ratio, N2 / (N2 + Ar), is less than or equal to 1.8%.

[0095] In some embodiments, the absorber layer at the second surface has a band gap between 1.3 and 1.9, and the absorber layer comprises Se; and a back contact is provided over the absorber layer, the back contact comprising ZnSeyTe(i-y) wherein: y is greater than 0.005 and less than 0.5, the back contact is doped with a Group V dopant, and the Group V dopant is activated such that the back contact has a lower resistivity than a comparative undoped layer. In some embodiments, the back contact layer has a mole ratio of ZnSe^Teq.y), wherein y is greater than 0.10 and less than 0.50. In some embodiments, y is greater than 0.15 and less than 0.40.

[0096] According to embodiments of the present disclosure, a method for forming a back contact layer of a photovoltaic device, can include depositing a back contact layer over an absorber layer; wherein: the absorber layer comprises cadmium, tellurium, and selenium; the absorber layer is p-type throughout; the back contact layer comprises zinc, tellurium, and selenium; and the back contact layer is p-type throughout. In some embodiments, the step of depositing the back contact layer comprises: depositing a first region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a first sputtering power ratio of ZnSe to ZnTe, thereby depositing the first region of the back contact as an alloy of ZnSe(y)Te(i-y), wherein y is greater than 0.01 and less than 0.50; and depositing a second region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a secondsputtering power ratio of ZnSe to ZnTe, thereby depositing the second region of the back contact as an alloy of ZnSe(y)Te(i-y), wherein y is equal to or greater than 0 and less than 0.20. In some embodiments, the first sputtering power ratio is greater than the second sputtering power ratio.

[0097] It is noted that the terms "substantially" and "about" may be utilized herein 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.

[0098] 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.

Claims

CLAIMSWhat is claimed is:

1. A photovoltaic device, comprising: a front layer stack comprising at least one n-type layer; an absorber layer comprising cadmium, tellurium, and selenium; and a back contact layer comprising zinc, tellurium, and selenium, wherein: the absorber layer is p-type throughout; the absorber layer has a first surface disposed on the front layer stack and a second surface; the back contact layer is p-type throughout, and the back contact layer has a first surface disposed on the second surface of the absorber layer.

2. The photovoltaic device of claim 1, wherein: the absorber layer has a mole ratio of Cd Se(x)Te(i-X), wherein a value of x at the second surface of the absorber layer is less than or equal to 0.40 and equal to or greater than 0.02.

3. The photovoltaic device of claim 1, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50.

4. The photovoltaic device of claim 1, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.10 and less than or equal to 0.40.

5. The photovoltaic device of claim 1, wherein: the back contact is doped with a Group V dopant.

6. The photovoltaic device of claim 1, comprising a supplemental p-type charge transport material between the absorber layer and the back contact layer, wherein the supplemental p-type charge transport material is a discontinuous layer, whereby at least a portion of the second surface of the absorber layer is in direct contact with at least a portion of the first surface of the back contact layer.

7. The photovoltaic device of claim 1, wherein:the back contact layer has a first region proximate to the absorber layer; the back contact layer has a second region between the first region and a back layer stack; the first region has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the first region of the back contact layer is greater than 0.01 and less than or equal to 0.50; and the second region has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the second region of the back contact layer has a value in a range from 0 to 0.20.

8. The photovoltaic device of claim 1, wherein: the absorber layer forms a p-n junction with the front layer stack; and the absorber layer is doped with at least one of nitrogen, phosphorus, arsenic, antimony, or bismuth.

9. The photovoltaic device of claim 1, wherein: the back contact is doped with at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth.

10. The photovoltaic device of claim 1, wherein: the absorber layer comprises a back absorber region, the back absorber region comprising 15% of the thickness of the absorber layer, wherein the back absorber region is proximate to the second surface of the absorber layer, and wherein a composition of the back absorber region comprises Cd Se(x)Te(i-X), wherein x is less than or equal to 0.40 and equal to or greater than 0.02.

11. The photovoltaic device of claim 1, wherein: a valence band offset between the absorber layer and the back contact layer at the second surface of the absorber layer is less than 0.075 eV.

12. The photovoltaic device of claim 1, wherein: a valence band offset between the absorber layer and the back contact layer at the second surface of the absorber layer is less than 0.035 eV.

13. The photovoltaic device of claim 1, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein y is greater than 0.10 and less than 0.50.

14. The photovoltaic device of claim 1, wherein: the back contact layer comprises zinc selenium telluride doped with nitrogen, wherein the nitrogen dopant is present in the back contact layer at a level in a range of 0.006 to 0.020 atomic percent.

15. The photovoltaic device of claim 1, comprising a transparent conducting layer over the back contact layer.

16. A method for forming a photovoltaic device, comprising: providing a front layer stack; forming an absorber layer over the front layer stack; and depositing a back contact layer over the absorber layer; wherein: the absorber layer comprises cadmium, tellurium, and selenium; the back contact layer comprises zinc, tellurium, and selenium; the absorber layer is p-type throughout; the absorber layer has a first surface disposed on the front layer stack and a second surface; the back contact layer is p-type throughout, and the back contact layer has a first surface disposed on the second surface of the absorber layer.

17. The method of claim 16, wherein the step of depositing the back contact layer comprises sputtering ZnSe and ZnTe in an environment comprising nitrogen, thereby depositing the back contact as a nitrogen-doped alloy of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50.

18. The method of claim 16, wherein: the step of forming the absorber layer comprises treating the absorber layer with a cadmium chloride heat treatment; and the step of depositing the back contact layer comprises: depositing a first region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a first sputtering power ratio of ZnSe to ZnTe in a first environment; and depositing a second region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a second sputtering power ratio of ZnSe to ZnTe, in a second environment; wherein: first sputtering power ratio is greater than the second sputtering power ratio; and the second environment comprises a higher concentration of nitrogen than the first environment.

19. The method of claim 16, wherein the step of depositing the back contact layer is performed in an environment comprising argon and nitrogen, wherein a gas flow ratio of nitrogen, relative to a combined flow of nitrogen and argon, is in a range of 0.2% to 2.5%.

20. A photovoltaic device comprising: an absorber layer having a band gap between 1.3 and 1.9, the absorber layer comprising Se; and a back contact layer over the absorber layer, the back contact comprising ZnSeyTe(i-y) wherein: y is greater than 0.005 and less than 0.5, the back contact is doped with a Group V dopant, and a valence band offset between the absorber layer and the back contact layer is less than 0.075 eV.

21. A photovoltaic device, comprising: a front layer stack comprising at least one n-type layer; an absorber layer comprising cadmium, tellurium, and selenium, wherein the absorber layer is p- type throughout; and a back contact layer comprising zinc, tellurium, and selenium, wherein the back contact layer is p-type throughout.

22. The photovoltaic device of claim 20 or 21, wherein the absorber layer has a first surface disposed on the front layer stack and a second surface, and the back contact layer has a first surface disposed on the second surface of the absorber layer.

23. The photovoltaic device of any one of claims 20-22, wherein: the absorber layer has a mole ratio of Cd Se(X)Te(i-X), wherein a value of x at a second surface of the absorber layer is less than or equal to 0.40 and equal to or greater than 0.02.

24. The photovoltaic device of any one of claims 20-23, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50.

25. The photovoltaic device of any one of claims 20-24, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y at the first surface of the back contact layer has a value equal to or greater than 0.10 and less than or equal to 0.40.

26. The photovoltaic device of any one of claims 20-25, wherein: the back contact is doped with a Group V dopant; and a concentration of the Group V dopant is higher at the second surface of the back contact than at the first surface of the back contact.

27. The photovoltaic device of any one of claims 20-26, comprising a supplemental p-type charge transport material between the absorber layer and the back contact layer, wherein the supplemental p-type charge transport material is a discontinuous layer, whereby at least a portion of the second surface of the absorber layer is in direct contact with at least a portion of the first surface of the back contact layer28. The photovoltaic device of any one of claims 20-27, wherein: the back contact layer has a first region proximate to the absorber layer; the back contact layer has a second region between the first region and a back layer stack; the first region has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the first region of the back contact layer is greater than 0.01 and less than or equal to 0.50; and the second region has a mole ratio of ZnSe(y)Te(i-y), wherein a value of y in the second region of the back contact layer has a value in a range from 0 to 0.20.

29. The photovoltaic device of claim 28, wherein the second region is doped with a nitrogen dopant at a concentration in a range of 1 x 1018cm'3to 1 x IO20cm'3.

30. The photovoltaic device of any one of claims 20-29, wherein: the absorber layer forms a p-n junction with the front layer stack; and the absorber layer is doped with at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth.

31. The photovoltaic device of any one of claims 20-30, wherein: the back contact is doped with at least one of: nitrogen, phosphorus, arsenic, antimony, or bismuth.

32. The photovoltaic device of any one of claims 20-31, wherein: the absorber layer comprises a back absorber region, the back absorber region comprising 15% of the thickness of the absorber layer, wherein the back absorber region is proximate to the second surface of the absorber layer, and wherein a composition of the back absorber region comprises Cd Se(x)Te(i-X), wherein x is less than or equal to 0.40 and equal to or greater than 0.02.

33. The photovoltaic device of any one of claims 20-32, wherein: a valence band offset between the absorber layer and the back contact layer at the second surface of the absorber layer is less than 0.075 eV.

34. The photovoltaic device of any one of claims 20-32, wherein: a valence band offset between the absorber layer and the back contact layer at the second surface of the absorber layer is less than 0.035 eV.

35. The photovoltaic device of any one of claims 20-34, wherein: the back contact layer has a mole ratio of ZnSe(y)Te(i-y), wherein y is greater than 0.10 and less than 0.50.

36. The photovoltaic device of any one of claims 20-35, wherein: the back contact layer comprises zinc selenium telluride doped with nitrogen, wherein the nitrogen dopant is present in the back contact layer at a level in a range of 0.006 to 0.020 atomic percent.

37. The photovoltaic device of any one of claims 20-36, comprising a transparent conducting layer over the back contact layer.

38. A method for forming a photovoltaic device, comprising: providing a front layer stack; forming an absorber layer over the front layer stack; and depositing a back contact layer over the absorber layer; wherein: the absorber layer comprises cadmium, tellurium, and selenium; the absorber layer is p-type throughout; the back contact layer comprises zinc, tellurium, and selenium; and the back contact layer is p-type throughout.

39. The method of claim 38, wherein the absorber layer has a first surface disposed on the front layer stack and a second surface; and the back contact layer has a first surface disposed on the second surface of the absorber layer40. The method of claim 38 or claim 39, wherein the step of depositing the back contact layer comprises sputtering ZnSe and ZnTe in an environment comprising nitrogen, therebydepositing the back contact as a nitrogen-doped alloy of ZnSe(y)Te(i-y), wherein y is greater than 0.005 and less than 0.50.

41. The method of any one of claims 38-40, wherein: the step of forming the absorber layer comprises treating the absorber layer with a cadmium chloride heat treatment; and the step of depositing the back contact layer comprises: depositing a first region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a first sputtering power ratio of ZnSe to ZnTe in a first environment; and depositing a second region by simultaneously sputtering with a first target comprising ZnSe and a second target comprising ZnTe, at a second sputtering power ratio of ZnSe to ZnTe, in a second environment; wherein: first sputtering power ratio is greater than the second sputtering power ratio.

42. The method of claim 41, wherein the second environment comprises a higher concentration of nitrogen than the first environment.

43. The method of any one of claims 38-42, wherein the step of depositing the back contact layer is performed in an environment comprising argon and nitrogen, wherein a gas flow ratio of nitrogen, relative to a combined flow of nitrogen and argon, is in a range of 0.2% to 2.5%.

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