Thin-film back contacts for photovoltaic devices and methods for forming the same

Abstract

Photovoltaic devices having metal telluride back contact layers and methods for forming group-V doped metal telluride back contact layers for use in photovoltaic devices are described, wherein the doped metal telluride (170) is sputter-deposited directly on the absorber (160) of the solar cell, and consists in a doped seed layer deposited with a lower sputtering energy and a bulk layer deposited with a higher sputtering energy, and / or using sputtering targets presenting a metal-rich stoichiometry in order to improve the group V dopant incorporation and activation.

Description

THIN-FILM BACK CONTACTS FOR PHOTOVOLTAIC DEVICES AND METHODS FOR FORMING THE SAMEBACKGROUND

[0001] The present specification generally relates to photovoltaic devices with thin film back contacts and, more specifically, to photovoltaic devices having metal telluride back contacts and methods for forming the same.

[0002] A photovoltaic device generates electrical power by converting light into electricity using semiconductor materials that exhibit the photovoltaic effect. Photovoltaic devices include a number of functional layers that cooperate to generate electric current. For example, an absorber layer can absorb photons and in response generate charge carriers. The back contact layer can be configured to selectively transport charge carriers generated by the absorber layer. The materials of the absorber layer and back contact layer can be selected to have complimentary material properties, e.g., band gap and morphology. In addition to material properties of the back contact, the interface between the absorber layer and the back contact layer can impact the performance of the photovoltaic device.

[0003] Accordingly, a need exists for alternative back contacts and methods for forming the same.SUMMARY

[0004] The embodiments provided herein relate to photovoltaic devices with conducting layer interconnects. 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 and in which:

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

[0007] FIG. 2 schematically depicts a cross-sectional view along 2-2 of the photovoltaic device of FIG. 1 according to one or more embodiments shown and described herein;

[0008] FIG. 3 schematically depicts a substrate according to one or more embodiments shown and described herein; and

[0009] FIG. 4 schematically depicts a process for forming a back contact layer according to one or more embodiments shown and described herein.DETAILED DESCRIPTION

[0010] Embodiments of photovoltaic devices having thin film back contacts are provided herein. Various embodiments of the photovoltaic device, as well as, compositions of and methods for forming a thin film back contact of the photovoltaic device will be described in more detail herein.

[0011] 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 a first side 102 configured to be exposed to a light source such as, for example, the sun. The photovoltaic device 100 can also define an opposing side 104 offset from the first side 102 such as, for example, by a plurality of material layers. 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. “Sunlight,” as used herein, refers to light emitted by the sun.

[0012] The photovoltaic device 100 can include a plurality of layers disposed between the first side 102 and the opposing side 104. As used herein, the term “layer” refers to a thickness of material provided upon a surface. Each layer can cover all or a portion of the surface. In some embodiments, a layer can be a substantially continuous layer, contiguous with a majority of a surface of an adjacent layer. In some embodiments, the substantially continuous layer contacts, and is contiguous with, by area, at least 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% of a surface of the adjacent layer. In some embodiments, a layer can be adiscontinuous 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.

[0013] As shown in FIG. 1, in some embodiments, the layers of the photovoltaic device 100 can be divided into an array of photovoltaic cells 200. For example, the photovoltaic device 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 photovoltaic device 100 and demarcate the photovoltaic cells 200 along the length Y of the photovoltaic device 100. The serial scribes 202 can be configured to connect neighboring cells of the photovoltaic cells 200 serially along a width X of the photovoltaic device 100. Serial scribes 202 can form a monolithic interconnect of the neighboring cells, adjacent to the serial scribe 202. The parallel scribes 204 can extend along the width X of the photovoltaic device 100 and demarcate the photovoltaic cells 200 along the width X of the photovoltaic device 100. Under operations, current 205 can predominantly flow along the width X through the photovoltaic cells 200 serially connected by the serial scribes 202. Under operations, 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. Accordingly, the serial scribes 202 and the parallel scribes 204 can demarcate the array of the photovoltaic cells 200.

[0014] Referring still to FIG. 1, the parallel scribes 204 can electrically isolate the groups 206 of photovoltaic cells 200 that are connected serially. 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 206. 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.

[0015] Referring collectively to FIGS. 2 and 3, the layers of 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 first side 102 of the photovoltaic device 100. Referring now to FIGS. 2 and 3, the substrate 110 can have a first surface 112 substantially facing the first 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.

[0016] The substrate 110 can include a transparent layer 120 having a first surface 122 substantially facing the first side 102 of the photovoltaic device 100 and a second surface 124 substantially facing the opposing side 104 of the photovoltaic device 100. In some embodiments, the second surface 124 of the transparent layer 120 can form the second surface 114 of the substrate 110. The transparent layer 120 can be formed from a substantially transparent material such as, for example, glass. Suitable glass can include soda-lime glass, or a glass with reduced iron content. The transparent layer 120 can have a suitable transmittance, including about 250 nm to about 1,300 nm in some embodiments, or about 250 nm to about 950 nm in other embodiments, or about 350 nm to about 800 nm in other embodiments. The transparent layer 120 may also have a suitable 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, transparent layer 120 can be formed from a glass with about 90% transmittance, or more. Optionally, the substrate 110 can include a coating 126 applied to the first surface 122 of the transparent layer 120. The coating 126 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.

[0017] Referring again to FIG. 2, the photovoltaic device 100 can include a barrier layer 130 configured to mitigate diffusion of contaminants (e.g. sodium) from the substrate 110, which could result in degradation or delamination. The barrier layer 130 can have a first surface 132 substantially facing the first 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 ofthe substrate 100. 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.

[0018] 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 suitable 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, including, for example, more than about 100 A in one embodiment, more than about 150 A in another embodiment, or less than about 200 A in a further embodiment.

[0019] Referring still to FIG. 2, the photovoltaic device 100 can include a transparent conductive oxide (TCO) layer 140 configured to provide electrical contact to transport charge carriers generated by the photovoltaic device 100. The TCO layer 140 can have a first surface 142 substantially facing the first 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 TCO layer 140 can be provided adjacent to the barrier layer 130. For example, the first surface 142 of the TCO layer 140 can be provided upon the second surface 134 of the barrier layer 130. Generally, the TCO 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. 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 TCO layer 140 can include one or more layers of material, including, but not limited to, tin dioxide, doped tin dioxide (e.g., F-SnO?), indium tin oxide, doped or undoped zinc oxide or cadmium stannate.

[0020] The photovoltaic device 100 can include a buffer layer 150 configured to provide an insulating layer between the TCO layer 140 and an adjacent semiconductor layers. The buffer layer 150 can have a first surface 152 substantially facing the first 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 TCO layer 140. For example, the first surface 152 of the buffer layer 150 can be provided upon the second surface 144 of the TCO layer 140. The buffer layer 140 may include material having higher resistivity than the TCO layer 140, including, but not limited to, intrinsic tin dioxide, zinc magnesium oxide (e.g., Zni-xMgxO), silicon dioxide (SnO?),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 140 can be configured to substantially match the band gap of an adjacent semiconductor layer (e.g., an absorber). The buffer layer 150 may have a thickness between the first surface 152 and the second surface 154, including, for example, more than about 100 A in one embodiment, between about 100 A and about 800 A in another embodiment, or between about 150 A and about 600 A in a further embodiment.

[0021] Referring still to FIG. 2, the photovoltaic device 100 can include an absorber layer 160 configured to cooperate with another layer and form a p-n junction within the photovoltaic device 100. 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 first 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 0.5 pm to about 10 pm such as, for example, between about 1 pm to about 7 pm in one embodiment, or between about 1.5 pm to about 4 pm in another embodiment.

[0022] According to the embodiments described herein, the absorber layer 160 can be formed from a p-type semiconductor material having an excess of positive charge carriers, i.e., holes or acceptors. The absorber layer 160 can include a semiconductor material, including a perovskite. The absorber layer 160 can be provided as a thin film. The absorber layer 160 can include a p-type semiconductor material such as group II-VI semiconductors. Specific examples include, but are not limited to, semiconductor materials comprising cadmium, tellurium, selenium, or a combination thereof. Some examples include, but are not limited to, binaries of cadmium and tellurium, ternaries of cadmium, selenium and tellurium (e.g., CdSexTei-x), ternaries of cadmium, zinc, and tellurium (e.g., CdZnxTei-x), a compound comprising cadmium, selenium, tellurium, and one or more additional element, or a compound comprising cadmium, zinc, tellurium, and one or more additional element.

[0023] In embodiments where the absorber layer 160 comprises tellurium and cadmium, 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 group I or V dopant such as, for example, copper, arsenic, phosphorous, antimony, or a combination thereof. The total density of the dopant within the absorber layer 160 can becontrolled. Alternatively or additionally, the amount of the dopant can vary with distance from the first surface 162 of the absorber layer 160. In alternative embodiments, the absorber layer 160 can be formed from a suitable thin film material, silicon material, or combinations thereof.

[0024] According to the embodiments provided herein, the 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 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.

[0025] The photovoltaic device 100 can include a back contact layer 170 configured to mitigate undesired alteration of the 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 first side 102 of the photovoltaic device 100 and a second surface 174 substantially facing the opposing side 104 of the photovoltaic device 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 2 nm to about 100 nm such as, for example, between about 4 nm to about 75 nm in one embodiment, between about 5 nm to about 50 nm in one embodiment, or between about 5 nm to about 30 nm in another embodiment.

[0026] 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 include binary or ternary combinations of materials from groups I, II, VI, such as for example, one or more layers containing zinc, copper, cadmium and tellurium in various compositions. Further exemplary materials include, but are not limited to, metal telluride materials. In some embodiments, the back contact 170 can be doped with a p-type dopant such as, for example, pnictides (N, P, As, Sb, or Bi), or copper. In further embodiments, the back contact 170 can be substantially copper free. For example, the back contact 170 can be formed without any copper.

[0027] Applicants have discovered that, in embodiments where the back contact layer 170 is formed from metal telluride materials, the ratio of metal to tellurium can impact doping.Specifically, a deficiency of tellurium can improve both the amount of pnictide dopant incorporated by the back contact layer 170 and the activation efficiency of the pnictide dopant. That is, both the quantity of pnictide dopant and the percentage of the pnictide dopant that is activated can be increased. Generally, the atomic ratio of metal to tellurium is greater than about 0.01 relative to stoichiometric ZnTe, using Wavelength Dispersive X-Ray Fluorescence (WDXRF). For example, the atomic ratio of Zn:Te of the back contact layer can be measured using WDXRF and compared to the atomic ratio of Zn:Te of stoichiometric ZnTe using WDXRF. The difference in the WDXRF atomic ratios gives the differential atomic ratio, so, for example, the WDXRF atomic ratio of the tellurium deficient material minus the WDXRF atomic ratio of the stoichiometric ZnTe. In embodiments where the back contact layer 170 comprises or consists essentially of ZnTe doped with one or more pnictide dopant, the differential atomic ratio of Zn:Te relative to stoichiometric ZnTe can be greater than or equal to about 2% and less than or equal to about 14% such as, for example, greater than or equal to about 4% and less than or equal to about 12% in one embodiment.

[0028] The embodiments provided herein relate to back contact layers 170 having low resistivity related to the good pnictide dopant uptake and activation. For example, nitrogen doped zinc telluride, described herein, can have a resistivity of less than about 2,000 Q-cm such as, for example, less than about 1,000 Q-cm. Some examples of the nitrogen doped zinc telluride described herein was directly deposited upon soda lime glass.

[0029] Referring still to FIG. 2, the photovoltaic device 100 can include a conducting layer 180 configured to provide electrical contact with the back contact layer 170. The conducting layer 180 can have a first surface 182 substantially facing the front side 102 of the photovoltaic device 100 and a second surface 184 substantially facing the back side 104 of the photovoltaic device 100. In some embodiments, the conducting layer 180 can be provided adjacent to the back contact layer 170. 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. A thickness of the conducting layer 180 can be defined between 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.

[0030] According to the embodiments provided herein, the conducting layer 180 can include one or more functional layers of material. The conducting layer 180 can include one or more layers conducting material such as, for example, metal, nitrogen-containing metal,transparent oxides, or a combination thereof. Each layer can contribute structural or electrical characteristics such that the stack of layers of conductive material have desired performance characteristics. Suitable metals include, but are not limited to, silver, nickel, copper, aluminum, titanium, palladium, chrome, molybdenum, gold, or combinations thereof. Suitable examples of a nitrogen-containing metals include, but are not limited to, aluminum nitride, nickel nitride, titanium nitride, tungsten nitride, selenium nitride, tantalum nitride, or vanadium nitride. Suitable transparent oxides include, but are not limited to, tin oxide (SnCh) zinc oxide (ZnO), cadmium oxide (CdO), cadmium stannate (Cd2SnO4), and amorphous cadmium tin oxide (CdxSnCU material where 0.5<x<2). These transparent 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 conducting layer 180 can be non-transparent, for example, composed of a stack of metals. Alternatively, the conducting layer 180 can be substantially transparent, including, for example, formed from one or more transparent metal oxides and / or metal nitrides, or formed as a tunnel junction using suitable transparent oxides.

[0031] The photovoltaic device 100 can include a back support 190 configured to cooperate with the substrate 110 to form a housing for the photovoltaic device 100. The back support 190 can be disposed at the opposing side 104 of the photovoltaic device 100. For example, the back support 190 can be formed over the conducting layer 180. The back support 190 can include a suitable material, including, for example, glass (e.g., soda-lime glass). It should be noted that the term “over” can mean that an object or a first layer is attached directly or indirectly to a surface of a second layer. Accordingly, a first layer that is “over” a second layer can be attached directly to the surface of the second layer or attached to one or more intervening objects or layers at a position that is offset from the surface of the second layer. In some embodiments, one or more additional layers, interlayers, photovoltaic cells, or a combination thereof can be provided between the conducting layer 180 and the back support 190. Thus, the embodiments provided herein can be configured as bifacial or tandem photovoltaic devices.

[0032] Referring collectively to FIGS. 2 and 4, manufacturing of a photovoltaic device 100 generally includes sequentially disposing functional layers or layer precursors in a “stack” of layers through one or more thin film deposition processes, including, but not limited to, sputtering, spray, evaporation, molecular beam deposition, pyrolysis, closed space sublimation (CSS), pulse laser deposition (PLD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), atomic layer deposition (ALD), or vapor transport deposition (VTD). In some embodiments, sputtering can be utilized to deposit the back contact layer 170.

[0033] Referring now to FIG. 4, a method 210 for forming a back contact layer over an absorber layer 160 is schematically depicted. The method 210 can include a process 212 for surrounding the absorber layer 160 a vacuum ambient. For example, the second surface 164 of the absorber layer 160 can be exposed within a vacuum processing chamber 214. As provided herein, the absorber layer 160 can comprise cadmium and tellurium or cadmium, tellurium and selenium. Additionally, the absorber layer 160 can be doped with a group V dopant, such as, but not limited to arsenic. Accordingly, the second surface 164 of the absorber layer 160 can include cadmium, tellurium, and selenium. Optionally, the second surface 164 of the absorber layer 160 can include more tellurium than selenium, or be substantially free of selenium in one embodiment.

[0034] The vacuum processing chamber 214 can be configured to provide a pressure and temperature controlled environment to facilitate deposition of the back contact 170 using one or more sputter targets 216. In some embodiments, the vacuum processing chamber 214 can be configured to maintain a vacuum ambient having a vacuum pressure of less than about 1 Torr such as, for example, less than 500 mTorr, or a range of about 1 mTorr to about 200 mTorr in one example. Alternatively or additionally, the vacuum ambient can include a partial pressure of pnictide dopant. For example, the pnictide dopant can be provided at a partial pressure of less than about 20% of the vacuum ambient, such as for example, in a range of 0.5% to 15.0%, less than about 10% of the vacuum ambient in one embodiment, or less than about 5% of the vacuum ambient in another embodiment.

[0035] As provided herein, the back contact layer 170 can be formed from a metal telluride material. Accordingly, the one or more sputter targets 216 can include metal and tellurium. Optionally, the purity of the one or more sputter targets 216 can be controlled such that the one or more sputter targets 216 consist essentially of metal and tellurium. In some embodiments, the one or more sputter targets 216 can be tellurium deficient. Specifically, the one or more sputter targets 216 can have an excess by atomic ratio of metal compared to the stoichiometric amount of the metal telluride. For example, in some embodiments, the metal telluride can be ZnTe, and the one or more sputter targets 216 can have an excess by atomic ratio of Zn of between about 2% and about 16% relative to ZnTe such as, for example, an excess by atomic ratio of Zn of between about 2% and about 10% relative to ZnTe in one embodiment, or an excess by atomic ratio of Zn of between about 4% and about 8% relative toZnTe in another embodiment. For clarity is noted that the term “excess” can mean the amount of additional metal in the target mixed with a remainder of the metal telluride. That is, when the one or more sputter targets 216 has an excess by atomic ratio of Zn of about 2%, the one or more sputter targets 216 is composed of about 98% ZnTe and about 2% Zn. As noted above, the deficiency of tellurium can improve pnictide doping. For example, it was observed that nitrogen doping of a ZnTe back contact can be improved using tellurium deficient sputter targets with the same nitrogen partial pressure in the vacuum ambient.

[0036] Referring still to FIG. 4, the method 210 can include a process 220 for sputtering a seed layer 176. For example, the one or more sputter targets 216 can be energized with a first sputter source 222 in the vacuum ambient, and a voltage can be applied between the first sputter source 222 and the partially formed photovoltaic device. Suitable sputter sources include, but are not limited to, planar magnetrons or rotary magnetrons. Accordingly, atoms / molecules can be removed from the from the one or more sputter targets 216, and emitted towards the second surface 164 of the absorber layer 160 at a first kinetic energy. The first kinetic energy can be selected to mitigate atomic disordering at the second surface 164 of the absorber layer 160 and to the protect the second surface 164 of the absorber layer 160. Accordingly, the first sputter source 222 can be configured as an RF sputtering source, e.g., the power can be alternating current (AC), with a frequency of greater than 1 MHz and a peak to peak voltage of between about 500 V and 1.5 kV. Moreover, applicants have discovered that group V dopants at the second surface 164 of the absorber layer 160 is susceptible to deactivation due to excessive ion bombardment. Accordingly, it is desirable to form the seed layer 176 directly on the second surface 164 of the absorber layer 160. The seed layer 176 can have a thickness 178 of less than about 15 nm such as, for example, between about 2 nm to about 10 nm in one embodiment, or between about 2 nm to about 5 nm in one embodiment. Additionally, it is noted that the seed layer 176 can be doped during deposition with the pnictide dopant present in the vacuum ambient.

[0037] The method 210 can include a process 230 for sputtering a bulk layer 232. For example, the one or more sputter targets 216 can be energized with a second sputter source 234 in the vacuum ambient, and a voltage can be applied between the second sputter source 234 and the partially formed photovoltaic device. Suitable sputter sources include, but are not limited to, planar magnetrons or rotary magnetrons. Accordingly, atoms / molecules can be removed from the from the one or more sputter targets 216, and emitted towards the seed layer 232 at a second kinetic energy. According to the embodiments provided herein, the secondkinetic energy can be greater than the first kinetic energy. The second sputter source 234 can be configured as a pulsed DC sputtering source, e.g., the power can be a switched direct current (DC) pulsed at less than 500 kHz with a voltage of greater than 2 kV. Additionally, the bulk layer 232 can be doped during deposition with the pnictide dopant present in the vacuum ambient. It is noted that multiple vacuum processing chambers 214 can be utilized according to the embodiments provided herein. For example, the seed layer 176 and bulk layer 232 can be sputtered in separate vacuum processing chambers 214, each with their own sputter targets 216 and respective first sputter source 222 and second sputter source 234. Alternatively, the seed layer 176 and bulk layer 232 can be sputtered in the vacuum processing chamber 214.

[0038] The bulk layer 232 can have a thickness 236 that is larger than the thickness 178 of the seed layer 176. For example, the thickness 236 of the bulk layer 232 can be at least twice as thick as the thickness 178 of the seed layer 176 such as, for example, at least about 5 times thicker in one embodiment. In some embodiments, a ratio of the thickness of the seed layer to the thickness of the bulk layer is in a range from 1 :2 to 1 :20, in a range from 1 :3 to 1 : 10, or in a range from 1 :4 to 1 : 15. Although two different sputtering processes are utilized to from the seed layer 176 and the bulk layer 232, the processes can be controlled to yield layers having substantially similar epitaxial quality. Moreover, it is believed that the ordering of the processes helps to achieve substantially uniformity. An interface 238 between the seed layer 176 and the bulk layer 232 can be formed within the back contact layer 170 without introducing a significant change in crystal structure. Accordingly, the seed layer 176 and the bulk layer 232 can be stacked and combined to form a substantially consistent back contact layer 170. Specifically, the seed layer 176 can form the first surface 172 of the back contact layer 170 and the bulk layer 232 can form the second surface 174 of the back contact layer 170.

[0039] It should now be understood that low kinetic energy sputtering techniques can be utilized to create a seed layer upon a group V doped CdTe absorber to prevent dopant deactivation at the interface surface of the absorber. Accordingly, low temperature roll-over effects associated with loss of low temperature coefficient and reductions in device efficiency can be mitigated. Additionally, telluride deficiencies can be introduced into the back contact to improve doping - in both quantity and activation, and reduce resistivity.

[0040] According to the embodiments provided herein, a method for forming a back contact layer of a photovoltaic device can include surrounding a surface of an absorber layer of a photovoltaic device with a vacuum ambient. The surface of the absorber layer can include cadmium and tellurium. The vacuum ambient can include a pnictide dopant. The method canfurther include sputtering a seed layer directly on the surface of the absorber layer. The one or more metal telluride sputter targets can be energized with a first sputter source in the vacuum ambient. Metal and tellurium can be emitted from the one or more metal telluride sputter targets at a first kinetic energy. The seed layer can be doped with the pnictide dopant. The method can further include sputtering a bulk layer on the seed layer. The one or more metal telluride sputter targets can be energized with a second sputter source in the vacuum ambient. Additional metal and tellurium can be emitted from the one or more metal telluride sputter targets at a second kinetic energy. The bulk layer can be doped with the pnictide dopant. The second kinetic energy can be greater than the first kinetic energy. The seed layer and the bulk layer can combine to form the back contact layer of the photovoltaic device.

[0041] In another embodiment, a photovoltaic device can include an absorber layer in direct contact with a back contact layer. The absorber layer can include cadmium and tellurium, and can be doped with a group V dopant. The back contact layer can consist essentially of zinc and tellurium doped with a pnictide dopant. A differential atomic ratio of Zn:Te of the back contact layer can be greater than or equal to 2% and less than or equal to 14% relative to stoichiometric ZnTe using Wavelength Dispersive X-Ray Fluorescence.

[0042] In some example methods for forming the back contact layer, 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 meters and a length is in a range of 0.5 to 2 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 embodiments, a sputter target comprises zinc and tellurium wherein the sputter target has more zinc than tellurium. In an example process, a seed layer of the back contact layer is formed at the first deposition station and a bulk layer is formed at the second deposition station. In some embodiments, the deposition rate of the bulk layer is greater than the deposition rate of the seed layer.

[0043] In some embodiments, the back contact layer comprises a type II-VI semiconductor material. In an example, the type II-VI semiconductor material includes zinc and tellurium. In an example, the back contact comprises nitrogen-doped zinc telluride and the back contact layer is not stoichiometric.

[0044] An example deposition system can convey individual substrates into one or more vacuum chambers. The individual substrates can be conveyed into one or more sputtering chambers and past a sputtering device, such that a thin film is formed on a surface of the individual substrate. In some embodiments, a deposition chamber pressure is less than 100 millibar (mbar) and greater than 0.1 microbar (pbar). In some examples, the deposition is performed at a pressure in range from 1.0 pbar 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 pbar, less than 50.0 pbar, less than 25.0 pbar, less than or equal to 10.0 pbar, equal to or greater than 0.5 pbar, greater than 1.0 pbar, greater than 1.5 pbar, greater than 1.8 pbar, or equal to or greater than 2.0 pbar. In some embodiments, a deposition chamber pressure is in a range from 1.0 pbar to 20.0 pbar, or in a range from 2.0 pbar to 10.0 pbar. In some examples, the substrate 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 20° C to 150° C, between 200° C to 400° C, between 250° C to 350° C, or between 200° C to 300° C. The substrates can be continuously conveyed at a substantially constant linear conveyance rate, while sputtering, such that the thin film is formed on the surface of the individual substrate.

[0045] In an example, the pnictide dopant can be provided at a mixing ratio in the vacuum ambient in a range from 0.001% to 20%. In some examples, the pnictide is nitrogen, having a mixing ratio of at least 0.001%, 0.01%, 0.05% 0.1%, 0.5%, 1.0%, 1.5%, 1.5%, 2.0%, or 5.0%. In some examples, the pnictide is nitrogen having a mixing ratio of less than 20.0%, less than 15.0%, less than 10.0%, less than 5.0%, less than 2.0%, less than 1.5%, less than 1.0%, less than 0.5%, or less than 0.1%. In some embodiments, a mixing ratio at a first deposition station can differ from a mixing ratio at a second deposition station.

[0046] 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 properties of the produced layer or layers can be finely tuned. The method can be used forhigh-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.

[0047] 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.

[0048] 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 method for forming a back contact layer of a photovoltaic device comprising: surrounding a surface of an absorber layer of a photovoltaic device with a vacuum ambient, wherein the surface of the absorber layer comprises cadmium and tellurium, and the vacuum ambient comprises a pnictide dopant; sputtering a seed layer directly on the surface of the absorber layer, wherein the one or more metal telluride sputter targets is energized with a first sputter source in the vacuum ambient, metal and tellurium are emitted from the one or more metal telluride sputter targets at a first kinetic energy, and the seed layer is doped with the pnictide dopant; sputtering a bulk layer on the seed layer, wherein the one or more metal telluride sputter targets is energized with a second sputter source in the vacuum ambient, additional metal and tellurium are emitted from the one or more metal telluride sputter targets at a second kinetic energy, and the bulk layer is doped with the pnictide dopant, and wherein: the second kinetic energy is greater than the first kinetic energy; and the seed layer and the bulk layer combine to form the back contact layer of the photovoltaic device.

2. The method of claim 1, wherein the one or more metal telluride sputter targets consist essentially of zinc and tellurium.

3. The method of claim 2, wherein the pnictide dopant is nitrogen.

4. The method of claim 3, wherein each of the one or more metal telluride sputter targets has an excess by atomic ratio of zinc of greater than or equal to 2% and less than or equal to 16% relative to ZnTe.

5. The method of claim 1, wherein the pnictide dopant is provided at a partial pressure of less than or equal to 20% of the vacuum ambient.

6. The method of claim 1, wherein the seed layer and the bulk layer are sputtered in different vacuum processing chambers.

7. The method of claim 1, wherein the back contact layer comprises zinc and tellurium and has a resistivity of less than or equal to 2,000 Q-cm.

8. The method of claim 1, wherein the first sputter source is configured as an RF sputtering source.

9. The method of claim 1, wherein the second sputter source is configured as a pulsed DC sputtering source.

10. The method of claim 1, wherein the seed layer has a first thickness and the bulk layer has a second thickness, and the second thickness is at least twice as thick as the first thickness.

11. A photovoltaic device comprising an absorber layer in direct contact with a back contact layer, wherein: the absorber layer comprises cadmium and tellurium, and is doped with a group V dopant, the back contact layer consists essentially of zinc and tellurium doped with a pnictide dopant, and a differential atomic ratio of Zn:Te of the back contact layer is greater than or equal to 2% and less than or equal to 14% relative to stoichiometric ZnTe using Wavelength Dispersive X-Ray Fluorescence.

12. The photovoltaic device of claim 11, wherein the back contact layer a resistivity of less than or equal to 2,000 Q-cm.

13. The photovoltaic device of claim 11, wherein the pnictide dopant is nitrogen.

14. The photovoltaic device of claim 11, wherein the group V dopant is arsenic.

15. A method for forming a back contact layer of a photovoltaic device comprising: surrounding a surface of an absorber layer of the photovoltaic device with a vacuum ambient, wherein the surface of the absorber layer comprises cadmium and tellurium, and the vacuum ambient comprises a pnictide dopant; providing one or more metal telluride sputter targets;sputtering a seed layer directly on the surface of the absorber layer, wherein the seed layer is doped with the pnictide dopant; sputtering a bulk layer on the seed layer, and wherein: the seed layer and the bulk layer combine to form the back contact layer of the photovoltaic device.

16. The method of claim 15, wherein the bulk layer is doped with the pnictide dopant.

17. The method of claim 15 or 16, wherein: the one or more metal telluride sputter targets is energized with a first sputter source in the vacuum ambient, metal and tellurium are emitted from the one or more metal telluride sputter targets at a first kinetic energy; the one or more metal telluride sputter targets is energized with a second sputter source in the vacuum ambient, metal and tellurium are emitted from the one or more metal telluride sputter targets at a second kinetic energy; and the second kinetic energy is greater than the first kinetic energy.

18. The method of any one of claims 1 or 15-17, wherein the pnictide dopant is nitrogen.

19. The method of any one of claims 1-2 or 15-18, wherein one of the one or more metal telluride sputter targets comprises zinc and tellurium and has an excess by atomic ratio of zinc of greater than or equal to 2% and less than or equal to 16% relative to stoichiometric ZnTe.

20. The method of any one of claims 1-2 or 15-19, wherein the pnictide dopant is provided at a partial pressure of less than or equal to 20% of the vacuum ambient.

21. The method of any one of claims 1-2 or 15-20, wherein the seed layer and the bulk layer are sputtered in different vacuum processing chambers.

22. The method of any one of claims 1-2 or 15-21, wherein the back contact layer comprises zinc and tellurium and has a resistivity of less than or equal to 2,000 Q-cm.

23. The method of any one of claims 1-2 or 17-22, wherein the first sputter source is configured as an RF sputtering source.

24. The method of any one of claims 1-2 or 17-23, wherein the second sputter source isconfigured as a pulsed DC sputtering source.

25. The method of any one of claims 1-2 or 15-24, wherein the seed layer has a first thickness and the bulk layer has a second thickness, and the second thickness is at least twice as thick as the first thickness.

26. The method of claim 25, wherein a ratio of the second thickness to the first thickness is greater than 2: 1 and less than or equal to 20: 1.

27. The method of any one of claims 1-2 or 15-26, wherein the vacuum ambient pressure is less than 200 mTorr.

28. The method of claim 27, wherein the vacuum ambient pressure is equal to or greater than 0.5 mTorr and less than or equal to 75 mTorr.

29. The method of any one of claims 1-2 or 15-28, wherein each of the one or more metal telluride sputter targets has an excess by atomic ratio of zinc of greater than or equal to 2% and less than or equal to 16% relative to ZnTe.

30. A photovoltaic device comprising an absorber layer in direct contact with a back contact layer, wherein: the absorber layer comprises cadmium and tellurium, and is doped with a group V dopant, the back contact layer consists essentially of zinc and tellurium doped with a pnictide dopant.

31. The photovoltaic device of claim 30, wherein a differential atomic ratio of Zn:Te of the back contact layer is greater than or equal to 2% and less than or equal to 14% relative to stoichiometric ZnTe using Wavelength Dispersive X-Ray Fluorescence.

32. The photovoltaic device of claims 30 or 31, wherein the pnictide dopant is nitrogen.

33. The photovoltaic device of any one of claims 30-32, wherein the group V dopant is arsenic.

34. The photovoltaic device of any one of claims 30-33, wherein the back contact layer comprises zinc and tellurium and has a resistivity of less than or equal to 2,000 Q-cm.

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