Photovoltaic devices and method of manufacturing

Abstract

A photovoltaic device includes a substrate structure and at least one Se-containing layer, such as a CdSeTe layer. A process for manufacturing the photovoltaic device includes forming the CdSeTe layer over a substrate by at least one of sputtering, evaporation deposition, CVD, chemical bath deposition process, and vapor transport deposition process. The process can also include controlling a thickness range of the Se-containing layer.

Claims

1. A method of forming a photovoltaic device comprising the steps of: depositing a material comprising CdSe over a TCO layer; depositing a material comprising CdTe over the material comprising CdSe to form a precursor; annealing the precursor to form an absorber layer, whereby the material comprising CdSe and at least a portion of the material comprising CdTe interdiffuse to form a CdSeTe alloy; depositing a material comprising tellurium and zinc, over the absorber layer, to form a back contact over the absorber layer; wherein: the absorber layer is p-type; a ratio of Te atoms to a sum of Se atoms and the Te atoms throughout the absorber layer is between about 99 to 100 and about 60 to 100; and a Se concentration declines across a thickness of the absorber layer toward the back contact.

2. The method of claim 1, wherein: a ratio of Se atoms to a sum of Se atoms and Te atoms in the absorber layer is between about 5 to 100 and about 10 to 100.

3. The method of claim 1, further comprising: forming a buffer layer between the TCO layer and the absorber layer, wherein the buffer layer comprises Mg, and wherein a peak concentration of Se is located at an interface between the buffer layer and the absorber layer.

4. The method of claim 1, further comprising: activating the absorber layer by contacting a surface of the absorber layer with a material containing chlorine and heating the absorber layer to a temperature in a range of 350° C. to 475° C. for a duration of 90 minutes or less, wherein the material containing chlorine includes at least one of CdCl.sub.2, MnCl.sub.2, MgCl.sub.2, NH.sub.4Cl, ZnCl.sub.2, or TeCl.sub.4.

5. The method of claim 1, wherein the material comprising CdSe is a CdSeTe alloy.

6. The method of claim 1, wherein the step of depositing the material comprising CdTe, comprises forming a CdTe layer by vapor transport deposition of the material comprising CdTe, wherein the material comprising CdTe consists essentially of CdTe.

7. The method of claim 1, further comprising: depositing a second layer of CdTe between the material comprising CdTe and the back contact prior to the annealing step.

8. The method of claim 1, wherein, prior to annealing, a CdSeTe layer is disposed between a layer of CdSe disposed adjacent the TCO layer, and a CdTe layer disposed over the CdSeTe layer; and whereby the CdTe layer is substantially consumed during the annealing to form the CdSeTe alloy.

9. The method of claim 1, wherein, prior to annealing, a first layer of CdSe is deposited over the TCO layer, a first layer of CdTe is deposited over the first layer of CdSe, and a second layer of CdTe is deposited over the first layer of CdTe, wherein annealing forms a CdSeTe alloy and the annealing consumes substantially all of the first layer of CdSe.

10. The method of claim 1, wherein the back contact comprises one or more layers of ZnTe, CdZnTe, or ZnTe:Cu.

11. The method of claim 1, further comprising depositing an interfacial layer comprising copper between the absorber layer and the back contact.

12. The method of claim 1, further comprising: depositing a back metal electrode over the back contact, the back metal electrode comprising a MoN.sub.x layer adjacent the back contact, an aluminum layer over the MoN.sub.x layer, and a chromium layer over the aluminum layer.

13. The method of claim 1, further comprising: forming a window layer over the TCO layer; wherein: the window layer comprises a n-type semiconductor, the absorber layer is formed over the window layer, the absorber layer is p-type, and forms a p-n junction with the window layer.

14. The method of claim 1, wherein the photovoltaic device comprises a layer comprising at least one of: zinc sulfide, cadmium sulfide, cadmium selenide, zinc magnesium oxide, cadmium magnesium sulfide, cadmium tin oxide, indium tin oxide, indium-doped cadmium oxide, aluminum-doped zinc oxide, indium zinc oxide, zinc tin oxide, cadmium oxide, zinc aluminum oxide, zinc silicon oxide, zinc zirconium oxide, tin aluminum oxide, tin silicon oxide, or tin zirconium oxide.

15. The method of claim 1, wherein the material comprising CdTe includes a dopant.

16. A method of forming a photovoltaic device comprising the steps of: depositing a TCO layer; depositing a material comprising CdSe over the TCO layer; depositing a material comprising CdTe to form at least one precursor layer; annealing the at least one precursor layer to form an absorber layer, whereby the material comprising CdSe and at least a portion of the material comprising CdTe interdiffuse to form a CdSeTe alloy; and forming a back contact over the absorber layer; wherein forming the back contact comprises: depositing a layer of ZnTe over the absorber layer; depositing a layer of CdTe over the layer of ZnTe; depositing a layer of Cu; and annealing to form a copper-doped CdZnTe layer, wherein a copper dopant concentration in the back contact is a range of 0.01-1.0% Cu by atomic weight.

17. The method of claim 16, further comprising cleaning the absorber layer with an acid prior to forming the back contact.

18. The method of claim 17, wherein the acid is a mixture of hydrochloric acid and copper and the absorber layer is doped with copper during the cleaning step.

19. The method of claim 17, further comprising doping the absorber layer with copper after the cleaning step.

Description

DESCRIPTION OF THE DRAWINGS

(1) The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:

(2) FIG. 1 depicts a schematic of functional layers in a photovoltaic device according to an embodiment of the invention;

(3) FIG. 2 depicts a process for manufacturing a photovoltaic device according to an embodiment of the invention;

(4) FIG. 3 depicts a schematic of functional layers according to an embodiment of a photovoltaic device;

(5) FIG. 4 depicts a schematic of functional layers according to an embodiment of a photovoltaic device;

(6) FIG. 5 depicts a schematic of functional layers of the photovoltaic device of FIG. 3 according to another embodiment;

(7) FIG. 6 depicts a schematic of functional layers according to an embodiment of a photovoltaic device;

(8) FIG. 7 depicts a schematic of functional layers according to an embodiment of a photovoltaic device;

(9) FIG. 8 depicts a schematic of functional according to an embodiment of a photovoltaic device;

(10) FIG. 8a depicts a schematic of functional layers in the photovoltaic device of FIG. 7 prior to an annealing step;

(11) FIG. 8b depicts a schematic of functional layers in the photovoltaic device of FIG. 7 prior to an annealing step according to another embodiment of the invention;

(12) FIG. 9 depicts a schematic of functional according to an embodiment of a photovoltaic device;

(13) FIG. 10 depicts a schematic of functional layers in a thirteenth embodiment of a photovoltaic device;

(14) FIG. 11 depicts a schematic of functional layers according to an embodiment of a photovoltaic device;

(15) FIG. 12 shows the selenium to tellurium ratio in atomic percentage according to some embodiments of the invention; and

(16) FIG. 13 shows the measured quantum efficiency (QE) for photovoltaic devices according to some embodiments of the invention.

DETAILED DESCRIPTION

(17) The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature and, thus, the order of the steps is not necessary or critical.

(18) Photovoltaic devices generally comprise multiple layers of material. FIG. 1 depicts a photovoltaic device 100 according to the invention wherein a number of layers are shown generally and described herein. The layers that are described herein, the materials used to form the layers, and/or the methods of forming the layers of the photovoltaic device 100 may be substituted, included in addition to layers described, or be absent in the embodiments of the invention described herein below and illustrated in the figures. Several specific embodiments of a novel photovoltaic device will be described with reference to the figures. It is further understood that each of the layers may be deposited in a single layer deposition from a single material, from a multi-layer deposition from a single material, or from a multi-layer deposition from a plurality of materials, as desired.

(19) The photovoltaic device 100 may include a substrate layer 105, a transparent conductive oxide (TCO) layer 110, a window layer 115, an absorber layer 120, a back contact 125, and a back metal electrode 127. The photovoltaic device 100 may further include an interfacial layer, such as a buffer layer, for example, between the various layers of the device. Photovoltaic devices may further include electrical connections, not shown, that provide a current path to communicate generated current flow, such as from one photovoltaic cell to adjacent cells in a module or from one photovoltaic module to adjacent modules in an array. Alternatively, the electrical connections may communicate the current flow to an external load device where the photogenerated current provides power.

(20) Each of the layers described in the following embodiments may be composed of more than one layer or film. 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 any amount of material that contacts all or a portion of a surface. During a process to form one of the layers, the created layer forms on an outer surface, typically a top surface, of a substrate or substrate structure. A substrate structure may include a substrate layer introduced into a deposition process and any other or additional layers that may have been deposited onto the substrate layer in a prior deposition process. Layers may be deposited over the entirety of a substrate with certain portions of the material later removed through laser ablation, scribing, or other material-removal process.

(21) The substrate layer 105 may be formed from glass, for example, soda lime glass or float glass. Alternatively, the substrate layer 105 may be formed from polymeric, ceramic, or other materials that provide a suitable structure for forming a base of photovoltaic cell. The substrate layer 105 may have additional layers applied (not shown) that promote the transmission of photons through its thickness, which may include anti-reflective coatings or anti-soiling coatings. The substrate layer 105 has the TCO layer 110 deposited thereon. The TCO layer 110 may be formed from any suitable transparent conductive oxide, including, but not limited to, indium gallium oxide, cadmium stannate, cadmium tin oxide, silicon oxide, tin oxide, cadmium indium oxide, fluorine doped tin oxide, aluminum doped zinc oxide, indium tin oxide, or various combinations of the foregoing.

(22) The window layer 115 is formed on the TCO layer 110 and may be formed from an n-type semiconductor material such as, for example, CdS, CdSSe, CdSe, zinc sulfide (ZnS), a ZnS/CdS alloy, ZnSO, zinc magnesium oxide, cadmium magnesium sulfide, cadmium tin oxide, indium tin oxide, indium-doped cadmium oxide, aluminum-doped zinc oxide, indium zinc oxide, zinc tin oxide, cadmium oxide, zinc aluminum oxide, zinc silicon oxide, a zinc zirconium oxide, tin aluminum oxide, tin silicon oxide, tin zirconium oxide, or another suitable wide-band gap and stable material. It is understood that a buffer layer (not shown) may be formed between the window layer 115 and the TCO layer 110. It is understood that the photovoltaic device 100 may omit the window layer 115, as desired.

(23) The absorber layer 120 is formed on the window layer 115 (if present) and may be formed from cadmium telluride, cadmium zinc telluride, CdSe, cadmium selenium telluride, Cd(S, Se, Te), CdSTe, copper indium gallium selenide, amorphous silicon, combinations of the foregoing, alloys of the foregoing, or any suitable p-type semiconductor material. The absorber layer 120 may be formed by a layer of material that is deposited on the device 100, or the absorber layer 120 may be formed by a plurality of layers of material deposited on the device 100 that are layer processed (e.g., annealing) to form an alloy which is the absorber layer 120. The absorber layer 120 may also be formed from multiple layers of materials that form a gradient across the absorber layer 120 once the multiple layers are annealed, or the absorber layer 120 may be formed from a single layer of material having a gradient of a material formed thereacross.

(24) The back contact 125 is an interfacial layer between the absorber layer 120 and the back metal electrode 127. The combination of the back contact 125 and the back metal electrode 127 may collectively be referred to generally as the back contact without a distinction being drawn between the layers. The back contact 125 may be formed from any material including tellurium, selenium, gold, tungsten, tantalum, titanium, palladium, nickel, silver, calcium, lead, mercury, graphite, and the like. The back contact 125 may also include ZnTe, a CdTe—ZnTe alloy (e.g., CdZnTe), ZnTe:Cu, indium nitride, HgTe, Te, and PbTe, or any other suitable material. The back metal electrode 127 provides lateral conduction of electricity to the outside circuit. The back metal electrode 127 may be formed from aluminum, copper, nickel, gold, silver, molybdenum nitride, molybdenum, chromium, oxidized metals, nitrides metals, combinations of the foregoing, alloys of the foregoing, or any other metals known to be useful as a conductor in a photovoltaic device. A suitable back contact 125 and electrode 127 is described in the commonly-owned patent application WO2014/151610 for Photovoltaic Device Having Improved Back Electrode and Method of Formation hereby incorporated herein by reference in its entirety, the disclosure of which may be relied upon for enablement with respect to the back contact 125 and electrode 127 portion of the invention.

(25) If an interfacial layer is present in the photovoltaic device 100, the interfacial layer may be formed from any number of materials and may be disposed between any of the various layers of the photovoltaic device, as desired. The interfacial layer may be a buffer layer or a barrier layer that inhibits the diffusion of chemical ions from, into, or across the substrate 105 or another layer of the device 100. For example, one interfacial layer included in the photovoltaic device 100 may be a barrier layer formed between the substrate layer 105 and the TCO layer 110. The barrier layer may be formed from any suitable material, including, but not limited to, silica, alumina, tin oxide, or silicon aluminum oxide. Another example of an interfacial layer may be a buffer layer formed between the TCO layer 110 and the window layer 115 to reduce recombination of holes and electrons at the interface of the TCO layer 110 and the window layer 115. The buffer layer may be formed of any suitable material, including, but not limited to, tin oxide, zinc oxide, zinc tin oxide, zinc doped tin oxide, indium oxide, a mixture of tin and zinc oxides, zinc stannate, zinc magnesium oxide, zinc oxysulfide, cadmium manganese sulfide, or cadmium magnesium sulfide, or combinations of the foregoing, for example.

(26) The photovoltaic device 100 may include other components (not shown) such as, bus bars, external wiring, laser etches, etc. For example, when the device 100 forms a photovoltaic cell of a photovoltaic module, a plurality of photovoltaic cells may be connected in series in order to achieve a desired voltage, such as through an electrical wiring connection. Each end of the series connected cells may be attached to a suitable conductor such as a wire or bus bar, to direct the generated current to convenient locations for connection to a device or other system using the generated current. In some embodiments, a laser may be used to scribe the deposited layers of the photovoltaic device 100 to divide the device into a plurality of series connected cells.

(27) The layers of the photovoltaic device 100 and the devices described herein may be deposited by a sputtering process. In general, sputtering involves the ejectment of atoms from the surface of a target material via energetic bombardment of ions on the surface of the target. Alternatively, the layers may be formed by any other suitable deposition process known in the art, including, but not limited to, pulse laser deposition (PLD), chemical vapor deposition (CVD), electrochemical deposition (ECD), atomic layer deposition (ALD), evaporation, or vapor transport deposition (VTD).

(28) A method of manufacturing a photovoltaic structure, the photovoltaic device 100, for example, is depicted in FIG. 2, can include sequentially forming layers on a substrate. In a first step 802, a TCO layer can be formed on a substrate layer, such as glass. In a second step 804, a window layer can be deposited over the substrate including the previously applied TCO layer and substrate layer. The window layer may include an n-type semiconductor, for example, or the window layer may be omitted. In an optional third step 806, an interfacial layer can be deposited over the substrate including the previously applied n-type window layer, TCO layer, and substrate layer.

(29) In a fourth step 808, an absorber layer can be deposited over the substrate including the optional interfacial layer(s), the n-type window layer, the TCO layer, and the substrate. The absorber layer deposition of the fourth step 808 may include the deposition of one or more precursor layers that require an annealing step or heating step (as described below) after the deposition thereof to form the absorber layer. In embodiments where the absorber layer is formed from one or more precursor layers, a first precursor layer, for example CdSe, is deposited over a substrate structure followed by the deposition of a second precursor layer, for example CdTe, over the first precursor layer. The deposited precursor layers are then annealed (see step 810) to form desired final layer form, for example CdSeTe. The annealing step(s) causes the interdiffusion of Se throughout the CdSeTe layer.

(30) In a fifth step 810, an activation process may be performed on the deposited layers. The activation step 810 can include the introduction of a material containing chlorine to the semiconductor material layers, for example cadmium chloride (CdCl.sub.2) as a bathing solution, spray, or vapor, and an associated annealing of the absorber layer at an elevated temperature. For example, if CdCl.sub.2 is used, the CdCl.sub.2 can be applied over the absorber layer as an aqueous solution. Alternatively, the absorber layer can be annealed with CdCl.sub.2 by continuously flowing CdCl.sub.2 vapor over the surface of the absorber layer during the annealing step. Alternative chlorine-doping materials can also be used such as MnCl.sub.2, MgCl.sub.2, NH.sub.4Cl, ZnCl.sub.2, or TeCl.sub.4. A typical anneal can be performed at a temperature of about 350° C.-475° C. for a total duration of 90 minutes or less, with a soaking time equal to or less than about 60 minutes.

(31) A multi-step activation step 810 may be used for each of the embodiments described herein. With each desired activation mechanism in the multi-step activation step 810, such as semiconductor grain growth, chlorine diffusion, sulfur and/or selenium inter-diffusion into the layers, a different thermal activation energy may be required. Using a multi-step process allows each activation mechanism to be optimized. As an example of a multi-step activation process, CdCl.sub.2 can be applied in a single step followed by annealing using a multi-step temperature profile. For example, the anneal temperature may be ramped up to 425° C. first, held there for a period of time (e.g. 1-10 minutes) and then ramped up further to 450°-460° C. and held there for an additional period of time (e.g., 1-10 minutes) before ramping the anneal temperature back down. This temperature profile for the above anneal results in different crystallinity characteristics of a CdTe material than those of a device activated in a single anneal step at 425° C. or alternatively at 450°-460° C. As an extension or alternative to this approach, multiple CdCl.sub.2 applications, each paired with annealing at varied times and temperatures may also be used to achieve desired layer characteristics. In a sixth step 812, a back contact can be formed over the activated p-type absorber layer.

(32) FIG. 3 illustrates a photovoltaic device 200 according to an embodiment of the invention. The layers of the photovoltaic device 200 are similar to the photovoltaic device 100 except as described below. The photovoltaic device 200 includes an n-type window layer 230 comprising cadmium sulphoselenide (CdSSe) deposited over a substrate layer 205 and a TCO layer 210. A p-type absorber layer 220 is deposited over the CdSSe layer 230. The CdSSe layer 230 and the p-type absorber layer 220 form a p-n junction in the photovoltaic device 200. A back contact 225 is formed over the p-type absorber layer 220. The TCO layer 210 allows light to pass through to a semiconductor window layer 215 while serving as an ohmic electrode to transport photogenerated charge carriers away from the light absorbing material. The back contact 225 serves as a second ohmic electrode to transport photogenerated charge carriers.

(33) The TCO layer 210 may form or may be electrically connected to a front contact. The back contact 225 may form or may be electrically connected to a back contact. The front contact forms a current path through which the electrical current generated by the active layers of the photovoltaic device may flow. The back contact forms a second current path through which generated current may flow. The front contact may connect one photovoltaic cell to an adjacent cell in one direction within a photovoltaic module or, alternatively, to a terminal of the photovoltaic module. Likewise, the back contact may connect the photovoltaic cell to a terminal of the photovoltaic module or, alternatively, to an adjacent cell in a second direction within the photovoltaic module, forming a series configuration. The front contact or back contact may connect the photovoltaic cell to an external terminal of the photovoltaic module in which it is located.

(34) The n-type CdSSe layer 230 forms the window layer, that is, the n-type region of the p-n junction within the photovoltaic device 200. The thickness of the CdSSe layer 230 can be between 10 nm to 100 nm thick or alternatively between 30 nm and 75 nm thick. The CdSSe layer 230 may be composed of cadmium, sulfur and selenium in varying amounts to form a CdS.sub.1−xSe.sub.x compound, where x is in the range of about 1 to 25 at %, or alternatively between about 5 to 10 at %. The compositional ratio or atomic percentage (at %) of a compound, for example CdS.sub.1−xSe.sub.x, is determined by comparing the number of sulfur atoms and the number of selenium atoms in a given amount of the compound with the total sum of sulfur and selenium atoms in the given amount. For example, where x=10 at %, there are 9 sulfur atoms for every 1 selenium atom in a given amount of CdS.sub.90%Se.sub.10% compound.

(35) The CdSSe layer 230 can be manufactured by a deposition process, such as vapor transport deposition, atomic layer deposition, chemical bath deposition, sputtering, closed space sublimation, or any other suitable process. In forming the CdSSe layer 230 using a process requiring the evaporation of powder, such as vapor transport deposition, the CdSSe layer 230 may be formed from the co-evaporation of a blended mixture of CdS and cadmium selenide (CdSe) powder, or the evaporation of a pre-alloyed CdSSe powder. The composition of the blended powders for co-evaporation or the composition of a pre-alloyed powder can be tailored so that the as-deposited film achieves the desired CdS.sub.1−xSe.sub.x compositional ratio. Alternatively, a CdSSe layer may be formed by sequentially depositing a CdS layer followed by depositing a CdSe layer with a subsequent annealing or heat treatment process to allow alloying of the two layers to achieve the desired CdS.sub.1−xSe.sub.x compositional ratio. The annealing or heat treatment process may be a separate step in a manufacturing process or may occur concurrently with the subsequent deposition of a further layer of the photovoltaic device, for example the deposition of the p-type absorber layer 220.

(36) Although the disclosed embodiments may describe a CdSSe layer as a single layer within the device, in each case the CdSSe layer may comprise multiple layers of CdSSe of varying composition to form a continuous or step-wise gradient of the sulfur to selenium ratio. For example, the CdSSe layer 230 may be formed as a single layer of CdS.sub.1−xSe.sub.x where x is held constant throughout the formation process. Alternatively, the CdSSe layer 230 may be formed sequentially as multiple layers of CdS.sub.1−xSe.sub.x where x varies in value for each of the sequentially formed layers. For example, a first CdS.sub.1−xSe.sub.x layer may be deposited where x=5 at %, and a second CdS.sub.1−xSe.sub.x layer may be deposited where x=10 at %. These two adjacent layers may together form the CdSSe layer 230. As a further alternative, the ratio of sulfur to selenium may be varied continuously throughout the formation process so that, for example, the composition of the formed CdS.sub.1−xSe.sub.x layer at a first end is x=5 at % and at a second end is x=10 at %, and where x varies continuously from 5 at % to 10% between the first and second ends. The whole of the formed layer having the continuous gradient may form the CdSSe layer 230.

(37) The p-type absorber layer 220 may include a p-type semiconductor material to form the p-type region of the p-n junction within the photovoltaic device 200. The absorber layer 220 preferably absorbs photons passing through from the CdSSe window layer 230 to mobilize charge carriers. The absorber layer 220 may be formed of CdTe, for example. An absorber layer 220 formed of CdTe may further include impurities or dopants in the CdTe bulk material. The absorber layer 220 may be between 200 nm to 8000 nm thick, or alternatively between 1000 nm to 3500 nm thick. The absorber layer 220 may be formed over the CdSSe window layer 230 by a deposition process, such as vapor transport deposition, atomic layer deposition, chemical bath deposition, sputtering, closed space sublimation, or any other suitable process. In the following alternative embodiments, similar layers as those described in the first embodiment are included and similarly numbered (incremented by 100).

(38) In another embodiment of the invention (not shown) similar to that shown in FIG. 3, a CdS layer (not shown) is deposited between the CdSSe layer 230 and the TCO layer 210. During an activation step, the CdSSe layer 230 may diffuse into the CdTe absorber layer, thereby forming a graded CdS.sub.xTe.sub.ySe.sub.z layer at the interface between the CdSSe layer 230 and the absorber layer 220, where 0<x<1, 0<y<1, 0<z<1 at % and x+y+z=1.

(39) For example, in an embodiment including a CdSSe layer as either a window layer or as an interfacial layer between a CdS layer and a CdTe absorber layer, during the activation step the CdSSe layer may diffuse into the CdTe absorber layer, thereby forming a graded CdS.sub.xTe.sub.ySe.sub.z layer at the interface between the interfacial layer and the absorber layer, where 0<x<1, 0<y<1, 0<z<1 at % and x+y+z=1.

(40) According to another embodiment of a photovoltaic device 1000, as depicted in FIG. 4, a TCO layer 1010 is formed on a substrate layer 1005. The layers of the photovoltaic device 1000 are similar to those of the photovoltaic device 100 except as described below. A p-type cadmium selenide telluride (CdSeTe) layer 1040 is formed over the TCO layer 1010. The TCO layer 1010 may include a material doped to be n-type in order to form a p-n junction with the p-type CdSeTe layer 1040. A back contact 1025 is formed over the p-type CdSeTe layer 1040. The photovoltaic device 1000 may also include a window layer (not shown) disposed between the TCO layer 1010 and the CdSeTe layer 1040. The window layer may be formed from ZnMgO, ZnSO, CdMgS, or other suitable wide-band gap and stable materials such as those disclosed hereinabove with respect to the photovoltaic device 100.

(41) The CdSeTe layer 1040, as a p-type absorber layer, preferably absorbs the photons passing through from the substrate layer 1005 and TCO layer 1010 to mobilize charge carriers. The thickness of the CdSeTe layer 1040 can be between about 200 nm to 5000 nm thick or alternatively between about 500 nm and 3500 nm thick. The CdSeTe layer 1040 may be composed of cadmium, selenium, and tellurium in varying amounts to form a CdSe.sub.xTe.sub.1−x compound, where x is in the range of about 1 to about 40 at %, or alternatively between about 10 to about 25 at %. The compositional ratio (at %) of a compound, for example CdSe.sub.xTe.sub.1−x, is determined by comparing the number of selenium atoms and the number of tellurium atoms in a given amount of the compound with the total sum of selenium and tellurium atoms in the given amount. For example, where x=10 at %, there are 9 tellurium atoms for every 1 selenium atom in a given amount of CdSe.sub.10%Te.sub.90% compound.

(42) The CdSeTe layer 1040 can be manufactured by a deposition process, such as vapor transport deposition, atomic layer deposition, chemical bath deposition, sputtering, closed space sublimation, or any other suitable process noted hereinabove. In forming the CdSeTe layer 1040 using a process requiring the evaporation of powder, such as vapor transport deposition, the CdSeTe layer 1040 may be formed from the co-evaporation of a blended mixture of CdSe and CdTe powder, or the evaporation of a pre-alloyed CdSeTe powder. The composition of the blended powders for co-evaporation or the composition of a pre-alloyed powder can be tailored so that the as-deposited film achieves the desired CdSe.sub.xTe.sub.1−x compositional ratio. Alternatively, a CdSeTe layer may be formed by sequentially depositing a CdSe layer followed by depositing a CdTe layer with a subsequent annealing or heat treatment process to allow alloying of the two layers to achieve the desired CdSe.sub.xTe.sub.1−x compositional profile (i.e., a gradient of Se across the CdSeTe layer 1040). The annealing or heat treatment process may be a separate step in a manufacturing process or may occur concurrently with the subsequent deposition or annealing of a further layer of the photovoltaic device, for example the deposition of a back contact 1025 or annealing of CdTe absorber layer.

(43) The CdSeTe layer 1040 may also be manufactured by a deposition process resulting in a gradient of Se in the CdSeTe layer 1040. The gradient may result in a concentration of Se adjacent the TCO layer 1010 and a lower concentration of Se adjacent the back contact 1025. It is understood that the concentration of Se adjacent the back contact 1025 may be zero, as desired. The concentration of Se adjacent the TCO layer 1010 may be lower than a concentration of Se adjacent the back contact 1025, as desired. FIG. 12 is an exemplary illustration of the at % of Se in the CdSeTe layer 1040 where the concentration of Se adjacent the TCO layer 1010 is higher than a concentration of Se adjacent the back contact 1025. FIG. 12 illustrates the Se at % versus the depth of the CdSeTe layer 1040. Higher concentration of Se near the TCO layer 1010 relative to the back contact 1025 may allow for a higher fraction of incident radiation to be absorbed in the CdSeTe layer 1040, as shown in FIG. 13. In a device having such a gradient, the band gap of the CdSeTe absorber layer 1040 appears to shift to energies that are lower than the absorber layer of a prior art device (e.g., CdTe absorber, CdSTe absorber, or CdSTe absorbers), as evidenced by a red shifted absorption edge as measured using wavelength dependence of the quantum efficiency (QE) of the solar cells. This is consistent with extensive intermixing of the Te and Se and/or a gradient of Se within the CdSeTe absorber layer 1040 thereby creating a lower band gap alloy. Moreover, Se may improve the passivation of grain boundaries and interfaces, which can be seen through higher bulk lifetime and reduced surface recombination.

(44) The gradient formed within the CdSeTe layer 1040 may be a continuous increase in concentration (see FIG. 12), a step-wise change in concentration, or the like. The gradient formed may be formed by either depositing a material(s) on the having a desired gradient and material profile, or the gradient formed may be formed by depositing discrete layers of material that are subsequently annealed to result in a desired concentration gradient profile. For example, positive results have been obtained in a photovoltaic device 1000 having a desired gradient in the CdSeTe layer 1040 with a concentration of Se higher adjacent a TCO layer 1010 and a lower concentration of Se adjacent the back contact 1025 by annealing a stack of layers including CdSe (adjacent the TCO layer 1010)/CdSeTe/CdSe/CdTe (adjacent the back contact 1025). Positive results have also been obtained in a photovoltaic device 1000 having a desired gradient in the CdSeTe layer 1040 by annealing a stack including CdSe/CdTe. Furthermore, positive results have been obtained in a photovoltaic device 1000 having a desired gradient in the CdSeTe layer 1040 by annealing and a stack including the following stack of layers CdSe/CdSeTe/CdTe.

(45) In some embodiments, as shown in FIG. 5, the concentration of Se in the layer 1040 may vary between a plurality of regions, such as a first region 1040a and a second region 1040b, and the regions may continuously change across the thickness of the respective regions. In some instances, the rate of change in concentration in the regions may itself vary through one or more of the regions, for example, increasing in some portions and decreasing in yet other portions. Similarly, in some embodiments, the first region 1040a may have a band gap that is lower than a band gap of the second region 1040b. In such instances, the concentration of selenium in the first region 1040a relative to the second region 1040b may be in a range such that the band gap of the first region 1040a is lower than the band gap of the second region 1040b.

(46) Although the disclosed embodiments shown in FIGS. 4 and 5 may describe a CdSeTe layer as a single layer of the absorber layer 1040 within the device, in each case the CdSeTe layer may comprise multiple layers of Cd, Se, CdSe, Cd(S, Se, Te) and/or CdSeTe of varying composition to form the gradient (continuous or step-wise) of the selenium to tellurium ratio. For example, the CdSeTe layer 1040 may be formed as a single layer of CdSe.sub.xTe.sub.1−x where x is held constant throughout the formation process. Alternatively, the CdSeTe layer 1040 may be formed sequentially as multiple layers of CdSe.sub.xTe.sub.1−x where x varies in value for each of the sequentially formed layers. For example, a first CdSe.sub.xTe.sub.1−x layer may be deposited where x=10 at %, and a second CdSe.sub.xTe.sub.1−x layer may be deposited where x=5 at %. These two adjacent layers may together form the CdSeTe layer 1040. As a further alternative, the ratio of selenium to tellurium may be varied continuously throughout the formation process so that, for example, the composition of the formed CdSe.sub.xTe.sub.1−x layer at a first end is x=10 at % and at a second end is x=5 at %, and where x varies continuously from 10 at % to 5% between the first and second ends. The whole of the formed layer having the continuous gradient may form the CdSeTe layer 1040. Alternatively, a graded CdSeTe layer 1040 may be formed from a multi-layered deposition process where each of the multi-layers includes Se (e.g., Se, CdSe, CdSeTe, Cd(S, Se, Te), and the like) and each of the layers has a different concentration of Se. For example, a first layer of material to form the CdSeTe layer 1040 adjacent the TCO layer 1010 may have a thickness less than a thickness of subsequent layers but with a higher concentration of Se therein. Another example includes a multi-layer deposition of materials that includes S in a first layer of material deposited adjacent the TCO layer 1010. When the multi-layers are annealed to form the CdSeTe layer 1040, the material of the first layer and the S present therein may be substantially present only adjacent the TCO layer 1010 or the S may be allowed to disperse throughout the layer 1040. In yet another example, the multi-layer deposition of materials to form the CdSeTe layer 1040 has a concentration of Se that declines across the thickness thereof, and that includes S adjacent the TCO layer 1010.

(47) In processes including a multi-layer deposition, the CdSeTe layer 1040 includes a plurality of grains separated by grain boundaries. In some embodiments, an atomic concentration of selenium in the grain boundaries is higher than the atomic concentration of selenium in the grains. In some embodiments, a ratio of the average atomic concentration of selenium in the grain boundaries to the average atomic concentration of selenium in the grains is greater than about 2. In some embodiments, a ratio of the average atomic concentration of selenium in the grain boundaries to the average atomic concentration of selenium in the grains is greater than about 5. In some embodiments, a ratio of the average atomic concentration of selenium in the grain boundaries to the average atomic concentration of selenium in the grains is greater than about 10.

(48) According to another embodiment of a photovoltaic device 1100, as depicted in FIG. 6, a window layer 1115 is formed over a TCO layer 1110 over the substrate layer 1105. The layers of the photovoltaic device 1100 are similar to those of the photovoltaic device 1000 except as described hereinbelow. A p-type CdSeTe layer 1140 is formed over the window layer 1115. The window layer 1115 forms a p-n junction with the p-type CdSeTe layer 1140. A back contact 1125 is formed over the p-type CdSeTe layer 1140.

(49) According to an embodiment of the invention depicted in FIG. 7, a photovoltaic device 1200 includes a TCO layer 1210 formed over a substrate layer 1205. The layers of the photovoltaic device 1200 are similar to those of the photovoltaic device 100 except as described below. A cadmium sulfur selenide telluride (Cd(S,Se,Te)) layer 1242 is formed over the TCO layer 1210. A back contact 1225 is formed over the p-type Cd(S,Se,Te) layer 1242. The Cd(S,Se,Te) layer 1242 is formed from a compound of cadmium, sulfur, selenium, and tellurium, CdS.sub.ySe.sub.xTe.sub.1−(x+y) where 0<x<1, and 0<y<1, and 0<(x+y)<1, or alternatively where 0.02<x<0.25 and 0.02<y<0.25, or as a further alternative, where 0.05<x<0.20 and 0.02<y<0.05. The Cd(S,Se,Te) layer 1242 is between about 200 nm to about 5000 nm thick, or alternatively between about 500 nm to about 3500 nm thick.

(50) In one embodiment, the Cd(S,Se,Te) layer 1242 comprises multiple sub-layers where x and y vary to provide preferred concentrations a various points through the layer thickness. For example, in one embodiment both x and y may vary to provide a higher sulfur and selenium concentration adjacent the TCO layer 1210, and decrease throughout the thickness of the layer moving away from the TCO layer 1210. In other alternative embodiments, the value of x or y or both may remain constant throughout the Cd(S,Se,Te) layer 1242 between the TCO layer 1210 and back contact 1225.

(51) The Cd(S,Se,Te) layer 1242 can be manufactured by a deposition process, such as vapor transport deposition, atomic layer deposition, chemical bath deposition, sputtering, closed space sublimation, or any other suitable process. However, in forming the Cd(S,Se,Te) layer 1242 using a process requiring the evaporation of powder, such as vapor transport deposition, the Cd(S,Se,Te) layer 1242 may be formed from the co-evaporation of a blended mixture of CdS, CdSe, and CdTe powders, or the evaporation of a pre-alloyed Cd(S,Se,Te) powder. The composition of the blended powders for co-evaporation or the composition of a pre-alloyed powder can be tailored so that the as-deposited film achieves the desired CdS.sub.ySe.sub.xTe.sub.1−(x+y) compositional ratio. Alternatively, a Cd(S,Se,Te) layer may be formed by sequentially depositing a CdS layer followed by depositing a CdSeTe layer, or various other combinations of compounds containing cadmium, sulfur, selenium and tellurium, with a subsequent annealing or heat treatment process to allow alloying of the two layers to achieve the desired CdS.sub.ySe.sub.xTe.sub.1−(x+y) compositional ratio. The annealing or heat treatment process may be a separate step in a manufacturing process or may occur concurrently with the subsequent deposition of a further layer of the photovoltaic device, for example the deposition of the back contact 1225.

(52) According to another embodiment of the invention shown in FIG. 8, a photovoltaic device 1400 includes a TCO layer 1410 formed over a substrate layer 1405. An absorber layer 1421 is formed adjacent the TCO layer 1410. The absorber layer 1421 may be formed via the annealing of a CdTe layer 1420 and a CdSe layer 1411, as described herein. A back contact 1425 is formed over the CdTe layer 1420, and a back metal electrode 1427 is formed over the back contact 1425. The TCO layer 1410 may have any thickness desired, such as between about 300 Å and about 4500 Å, or more specifically between about 300 Å and about 800 Å, or more specifically between about 3500 Å and about 4500 Å. In this embodiment, the TCO layer 1410 is formed from a plurality of layers of material comprising SnO.sub.2 and SnO.sub.2:F. If more than one layer of each SnO.sub.2 and SnO.sub.2:F is present, that materials may be alternating. The CdSe layer 1411 formed on the SnO.sub.2 layer has a thickness of about 900-1500 Å. The CdTe layer 1420 has a thickness of about 1.7-2.5 microns. The CdTe layer 1420 may be formed from the deposition of a single layer of material, or the CdTe layer 1420 may be formed from the deposition of two or more layers of CdTe, Cd, and/or Te. The combination of the CdSe layer 1411 and the CdTe layer 1420 layer may have a thickness of about 0.5-3.5 microns, as desired. The back contact 1425 is formed from ZnTe. Alternatively, the back contact 1425 may be formed from a layer of ZnTe and a layer of CdZnTe alloy. The back contact 1425 may also include a layer of Cu disposed thereon with a concentration of about 0.01-1% Cu by atomic weight. The back metal electrode 1427 is formed from a layer of MoN.sub.x adjacent the back contact 1425, a layer of chromium, and a layer of aluminum disposed therebetween. The chromium layer may have a thickness from about 0-200 Å, such as about 200 Å, the aluminum layer may have a thickness 700-1000 Å, such as about 750 Å, and the MoN.sub.x layer may have a thickness from about 50-170 Å, such as about 170 Å.

(53) A method of manufacturing the photovoltaic structure 1400 includes steps similar to those described above and shown in FIG. 2 except that the CdSe layer 1411 is deposited on the TCO layer 1410. The CdSe layer 1411 may be deposited using a sputtering deposition process. A first layer of CdTe 1420a (a precursor layer) is then deposited on the CdSe layer 1411. A second layer of CdTe 1420b (another precursor layer) is then deposited on the first layer of CdTe 1420a. Once the second layer of CdTe 1420b has been applied, the CdTe layer 1420 is formed, as shown in FIG. 8a. The CdSe 1411 and the CdTe layer 1420 are then annealed to form an alloy thereof, CdSeTe, as the absorber layer 1421 as shown in FIG. 8. The underlying layers, once annealed may result in an absorber layer 1421 having a thickness of about 0.5-3.5 microns, of about 1-3 microns, a thickness of about 1 micron, a thickness of about 3.0 microns, as desired. The annealing process may consume substantially all of the CdSe layer 1411, or a portion of the CdSe layer 1411 may remain between the TCO layer 1410 and the CdSeTe absorber layer 1421. Similarly, the annealing process may consume substantially all of the CdTe layer 1420, or a portion of the CdTe layer 1420 may remain between the CdSeTe absorber layer 1421 and the back contact 1425.

(54) The embodiment shown in FIG. 8b is similar to the embodiment described with respect to FIG. 8a with methods of forming each substantially the same except that a CdSeTe layer 1413 is deposited on the CdSe layer 1411 prior to deposition of the CdTe layer 1420 in either a single layer deposition of CdTe or in multiple layers of CdTe. The CdSe layer 1411, the CdSeTe layer 1413, and the CdTe layer 1420 are then annealed to form the absorber layer 1421 as shown in FIG. 8. The annealing process may consume substantially all of the CdSe layer 1411 or a portion of the CdSe layer 1411 may remain adjacent the TCO layer 1410. Similarly, the annealing process may consume substantially all of the CdTe layer 1420 or a portion of the CdTe layer 1420 may remain between the CdSeTe absorber layer 1421 and the back contact 1425.

(55) Once the absorber layer 1421 is formed (either from the structure described above and shown in FIG. 8a or from the structure described above and shown in FIG. 8b), the absorber layer 1421 is then cleaned to remove impurities, debris, oxides, and the like that have formed thereon. The absorber layer 1421 may be cleaned with oxalic acid, hydrochloric acid, or any acid or other suitable cleaning material. The absorber layer 1421 may be then be doped with Cu. For example, if hydrochloric acid is used to clean the absorber layer 1421, the absorber layer 1421 may be subsequently doped with Cu in a concentration of about 0.5-1.0 ppm. The hydrochloric acid cleaning step may be combined with the Cu doping step, as desired.

(56) The back contact 1425 is then deposited on the cleaned absorber layer 1421. The back contact 1425 may be formed via the deposition of a single ZnTe layer or from the deposition of a ZnTe layer adjacent the absorber layer 1421 and a CdZnTe layer adjacent the ZnTe layer. The CdZnTe layer may be formed by depositing a layer of CdTe on the ZnTe layer and then annealing the same to form the alloy therebetween. The back contact 1425 may also include a layer of Cu disposed thereon with a concentration of 0.01-1% Cu by atomic weight. The layer of Cu may be disposed between the absorber layer 1421 and the back contact 1425, between the layers of the back contact 1425, or between the back contact 1425 and the back metal electrode 1427, as desired. The back metal electrode 1427 is then deposited on the back contact 1425. The back metal electrode 1427 is formed from a layer MoN.sub.x formed adjacent the back contact 1425, then a layer of aluminum, and then a layer of chromium.

(57) FIG. 9 shows a photovoltaic device 1500 according to another embodiment of the invention. The photovoltaic device 1500 is similar to the photovoltaic device 1400 of FIG. 8 except that the device 1500 includes a window layer 1515 between a TCO layer 1510 and an absorber layer 1521. Like the photovoltaic device 1400, the photovoltaic device 1500 includes a substrate layer 1505 with the TCO layer 1510 formed thereon, the absorber layer 1521, and a back contact 1525 disposed between a back metal electrode 1527 and the absorber layer 1521. The window layer 1515 is formed from CdS.

(58) FIG. 10 shows a photovoltaic device 1600 similar to the embodiment of FIG. 8 except as described hereinbelow. The substrate layer 1605 has a TCO layer 1610 formed from SnO.sub.2:F. An undoped, higher resistivity buffer layer 1612 formed from SnO.sub.2 is formed over the TCO layer 1610. The buffer layer has a thickness of about 20 nm-60 nm with a carrier concentration of 1×10.sup.17 to 1×10.sup.19 cm.sup.−3. The photovoltaic device 1600 does not include a window layer. The absorber layer 1620 is formed from a graded composition of CdSe.sub.xTe.sub.1−x. A bandgap of the absorber layer 1620 is reduced by the presence of Se therein. The variable “x” ranges between 0.05<x<0.30 proximate to the TCO layer 1610; and x<0.01 at a distance from about 800 nm to about 2000 nm away from the TCO layer 1610. The crystal structure is a zinc blend having a residual hexagonal wurtzite structure <5% volume (less than about 5 grains per 100). A total thickness of the absorber layer 1620 ranges from about 1500 nm to about 4500 nm. The absorber layer 1620 can be formed by depositing CdSe/CdTe, CdSe/CdSe.sub.xTe.sub.1−x/CdTe, CdTe/CdSe/CdTe, CdS.sub.ySe.sub.1−y/CdTe or CdSe.sub.xTe.sub.1−x/CdTe film stacks followed by an annealing process similar to that described above with respect to FIG. 8. A back contact 1625 formed from ZnTe or a CdZnTe—ZnTe combination is formed thereon, with a back metal electrode 1627 formed over the back contact 1625.

(59) In the embodiment of FIG. 10, a CdCl.sub.2 annealing treatment is provided. The structure is annealed in the presence of CdCl.sub.2 flux at a temperature sufficiently high and for a duration sufficiently long to achieve a mid-film grain size of >2 um and with a continuous Se concentration grading. Typical conditions for annealing would be at a temperature from about 420° C. to about 460° C. for between about 5 minutes to about 60 minutes. Chemical additions of CdCl.sub.2 to lower eutectic temperature of the CdCl.sub.2—CdTe system can be used. For example, from about 20 mg/liter to about 200 mg/liter NaCl in from about 50 gm/liter to about 300 gm/liter aqueous CdCl.sub.2 solutions can be added.

(60) Another photovoltaic device 1700 according to another embodiment of invention is shown in FIG. 11. The embodiment of FIG. 11 is similar to the embodiment of FIG. 10 except as described herein. The photovoltaic device 1700 includes a substrate layer 1705 having a TCO layer 1710 formed thereon. A buffer layer 1712 is formed on the TCO layer 1710. To eliminate intermixing of the materials forming an absorber layer 1720 that would result in a continuous Se profile with a peak Se concentration located at an interface between the device buffer layer 1712 and the absorber layer 1720, a deposition sequence is changed such that the absorber is formed with multiple layers of material including Se (e.g., Se, CdSe, and the like). A layer with the highest Se concentration is deposited after a layer a lower Se concentration. In the embodiment of FIG. 11, a layer 1720a of CdTe or low Se mole fraction CdSe.sub.xTe.sub.1−x is deposited first followed by a deposition of a CdSe or large Se mole fraction CdSe.sub.xTe.sub.1−x layer 1720b. A further layer 1720c of CdTe and/or a low Se mole fraction CdSe.sub.xTe.sub.1−x is deposited last. The further layer 1720c may be the same as the layer 1720a or different therefrom so long as the layer 1720c has a lower Se concentration than the layer 1720b. A subsequent CdCl.sub.2 treatment intermixes the layers 1720a, 1720b,1720c thereby creating a continuous Se profile in the absorber layer 1720. During the CdCl.sub.2 treatment Se diffuses to the interface between the buffer layer 1712 and the absorber layer 1720, which is favorable to interface passivation. The Se concentration at the interface, however, is still lower than in a device where a CdSe or high Se mole fraction CdSe.sub.xTe.sub.1−x, such as that of the layer 1720b, is deposited first. This deposition sequencing improves an alignment of the buffer and absorber conduction bands while not impacting current collection. Collection losses due to excessive buffer/absorber conduction band offset CBO are avoided, while the Se concentration profile is optimized for maximum light absorption and device performance.

(61) According to additional embodiments of the invention, the absorber layer of the photovoltaic devices disclosed herein, for example photovoltaic device 1400, 1500, 1600, and/or 1700 may include an absorber layer 1421, 1520, 1620, and/or 1720 that may have one of the following compositions to address concerns regarding Se diffusion into a CdTe layer thereof during an annealing process to obtain a desired Se profile: a) A first structure provides three layers including a layer of CdSe having a thickness from 0.15 μm to about 0.25 μm/from about 0.25 μm to about 0.5 μm CdSeTe/from about 2.75 μm to about 3.25 μm CdTe. The CdSeTe layer may be CdSe.sub.xTe.sub.1−x where x is from about 10 at % to about 30 at % Se. b) Another structure provides four layers including from about 0.15 μm to about 0.35 μm thickness CdSe/from about 0.75 μm to about 1.5 μm thickness CdTe/from about 0.1 μm to about 0.25 μm) thickness CdSe/about 1.5 μm to about 3 μm CdTe. c) Another structure provides four layers including from about 0.15 μm to about 0.35 μm thickness CdSe/from about 0.75 μm to about 1.5 μm thickness CdTe/from about 0.1 μm to about 0.5 μm thickness CdSeTe/from about 10% to about 30% Se)/about 1.5 μm to about 3 μm thickness CdTe. d) Another structure provides five layers including from about 0.1 μm to about 0.5 μm thickness CdSeTe and having from about 10 at % to about 30 at % Se/from about 0.15 μm to about 0.35 μm thickness CdSe/from about 0.75 μm to about 1.5 μm thickness CdTe/from about 0.1 μm to about 0.25 μm thickness CdSe/about 1.5 μm to about 3 μm thickness CdTe. e) Another structure provides five layers including from about 0.1 μm to about 0.5 μm thickness CdSeTe and having from about 10 at % to about 30 at % Se/from about 0.15 μm to about 0.35 μm thickness CdSe/from about 0.75 μm to about 1.5 μm thickness CdTe/from about 0.1 μm to about 0.5 μm thickness CdSeTe and having from about 10 at % to about 30 at % Se/about 1.5 μm to about 3 μm thickness CdTe.

(62) In each of the structures a)-e), the presence of CdSe layers, particularly the presence of CdSe as a third or fourth layer, slows down the Se fluxing from the first CdSe layer due to the small Se concentration gradient. The CdSe layer is allowed to mix with the CdTe layer to form a uniformly thick CdSeTe alloy layer.

(63) For each of the structures a)-e), a device activation process may be performed (e.g., semiconductor grain growth, chlorine diffusion, sulfur and/or selenium inter-diffusion into the layers, and the like). In some embodiments of the invention, the activation step involves a process wherein the CdTe surface is treated with a CdCl.sub.2 concentrated solution for a period from about 25 minute to about 40 minutes. The device activation process may be followed by a first recharging process for a first duration, which can be performed in either one or two steps. The recharging process may then followed by a second recharge step having a second duration less than the first duration to replenish any lost Cl.

(64) From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.