CA1060281A - Photovoltaic cell containing cds layer impregnated with aluminum - Google Patents
Photovoltaic cell containing cds layer impregnated with aluminumInfo
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- CA1060281A CA1060281A CA235,472A CA235472A CA1060281A CA 1060281 A CA1060281 A CA 1060281A CA 235472 A CA235472 A CA 235472A CA 1060281 A CA1060281 A CA 1060281A
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
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Abstract
ABSTRACT OF THE DISCLOSURE
A method of making low cost photovoltaic cells on a large scale basis by means of a continuous process of coat-ing sheet glass while the sheet glass moves in and has its under surface immersed in a tank of molten material, comprising forming the film of CdS microcrystals on the glass sheet, which has previously been coated with transparent SnOx to a thickness of about 3 to 6 microns, the method including inter-mittently and slowly spraying on said glass, while its exposed surface is maintained at a constant temperature in the range 500°F to 800°F, and while irradiating the sheet with intense ultraviolet light, a water solution of CdCl2 and of thiourea and of AlCl3, so as to form a film of CdS microcrystals heavily impregnated with Al at least adjacent to the SnOx, but only optionally the exposed surface of the CdS, thereafter raising the temperature of the sheet of glass to 500°C-550°C, 525°C
being optimum to avoid interaction of the CdS and the SnOx which can occur at higher temperatures or even at 525°C if too long continued, thereafter cooling the glass to approximately room temperature, thereafter coating the glass with Cu2S, the 2 of Cu2S being as nearly as possible precise, by dipping the sheet coated with CdS in a solvent which may be water, the solu-tion inlcuding a weak acid, such as tartaric acid, citric acid or lactic acid, a quantity of CuCl, and optionally a quantity of H4Ce(SO4)4 and NaCl, or some other alkaline chloride, or by electroplating to form a film of Cu2S. The Cu2S forming process proceeds by ion interchange, i.e., S from CdS combines with Cu to form Cu2S. Cu is then applied over the Cu2S, and the cell cured at about 400°F to 500°F.
A method of making low cost photovoltaic cells on a large scale basis by means of a continuous process of coat-ing sheet glass while the sheet glass moves in and has its under surface immersed in a tank of molten material, comprising forming the film of CdS microcrystals on the glass sheet, which has previously been coated with transparent SnOx to a thickness of about 3 to 6 microns, the method including inter-mittently and slowly spraying on said glass, while its exposed surface is maintained at a constant temperature in the range 500°F to 800°F, and while irradiating the sheet with intense ultraviolet light, a water solution of CdCl2 and of thiourea and of AlCl3, so as to form a film of CdS microcrystals heavily impregnated with Al at least adjacent to the SnOx, but only optionally the exposed surface of the CdS, thereafter raising the temperature of the sheet of glass to 500°C-550°C, 525°C
being optimum to avoid interaction of the CdS and the SnOx which can occur at higher temperatures or even at 525°C if too long continued, thereafter cooling the glass to approximately room temperature, thereafter coating the glass with Cu2S, the 2 of Cu2S being as nearly as possible precise, by dipping the sheet coated with CdS in a solvent which may be water, the solu-tion inlcuding a weak acid, such as tartaric acid, citric acid or lactic acid, a quantity of CuCl, and optionally a quantity of H4Ce(SO4)4 and NaCl, or some other alkaline chloride, or by electroplating to form a film of Cu2S. The Cu2S forming process proceeds by ion interchange, i.e., S from CdS combines with Cu to form Cu2S. Cu is then applied over the Cu2S, and the cell cured at about 400°F to 500°F.
Description
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BACKGROUND OF THE INVENTION
It is known to form photovoltaic cells by coating on a hot sheet of Nesa glass, or glass previously coated with SnO , a thin film of CdS, by spraying a water solution of compounds which form a layer of CdS microcrystals on the SnO , and providing a Cu2S heterojunction as a layer on the CdS layer, and forming electrodes on the film of Cu2S. In accordance with our co-pending Canadian application S.N. 215,901, filed December 12, 1974, the Cu2S layer was formed by spraying a Cu2S forming solution on the CdS film while the substrate was hot, but in accordance with the present application, the Cu2S is formed by dipping at or near room temperature or by electro-plating; at or near room temperature. Cells so formed have heretofore utilized relatively thick films of CdS, or have re-sorted to other expedients to obviate the difficulty that CdS
films generally permit permeation by Cu2S and Cu, when Cu2S is formed by dipping or electroplating, i.e., by ion exchange, which provides short circuits between the Cu2S layer and the SnO , the latter constituting the negative electrode of the cell, rendering the cell inoperative. In order to convert solar energy to electrical enery on a large scale, square miles of solar cells may be required. Since Cd is a rare and expensive metal, it becames important to form photovoltaic cells with minimum quantities of CdS and hence with extremely thin CdS films, yet that the cell be reliably fabricated ?60Z81 and have long life. We have produced CdS photovoltaic cells in which the layer of CdS is about 2. or 4. microns thick, yet which show zero shorting permeation, the layers being highly impervious to Cu2S or Cu containing solution, in an ion exchange process. We have heretofore used the method of spraying a CdS forming solution on a glass coated with SnOx, intermittently and at a slow rate, while maintaining the surface of the glass at uniform and constant temperature in the range between 500F and 800F. According to the present invention, in one embodiment, plural sprays are re-quired to form the CdS layer. All sprays employ the known materials, i.e., a solution containing essentially CdC12 and thiourea, but to one spray is added AlC13 . 6 H20, in proportions by weight equal to between 10% and 50% of Al, adding thiourea as required to combine with the Al, and in a superposed spray op-tionally a small quantity of HCl, but no AlC13 . 6 H20, or a very much reduced quantity of AlC13 . 6 H20; or, instead of applying two discrete films, one containing a large quantity of aluminum com-pound and the other containing none or very little, the coating may be formed by gradually decreasing the concentration of aluminum in proceeding from the bottom of the layer adjacent to the SnOx to the top of the layer, so that the top of the layer contains no aluminum or very little aluminum while the bottom of the layer may have as much as 50% aluminum. The quantity of the sulphur employed must be proportioned to the quantity of aluminum, forming a compound or crystal therewith.
It is then found that that portion of the CdS layer which contains a large proportion of alumi-num is-extremely hard d ~ ~
so that it can only with difficulty be removed by application of acid or by scraping, is highly adherent to the Sn~ layer, so that it can only with difficulty be removed, and is highly im-pervious to materials involved in forming a Cu2S layer by ion exchange, or to Cu2S, and inhibits diffusion of Cu through a CdS
layer, where the Cu2S is formed by dipping or electroplating, i.e., by an ion exchange process. The inclusion of aluminum in CdS in the large quantities specified does not constitute a doping procedure, but is a new compound or alloy or a new crystalline form, having physical properties quite distinct from those of CdS
or CdS with small amounts of aluminum, in terms of hardness, toughness and imperviousness, but which remains photoconductive.
For example, it is known that A12S3 and CdS crystals are both hexagonal, and the Al may form A12S3 crystals or a composite crystal or alloy, of Al, Cd, S, or a ternary compound, though tests indicate the latter possibility to be unlikely. It has been found that if the entire film of CdS is heavily impregnated with Al and the cell remains operative, but at reduced efficiency.
SUMMARY OF THE INVFNTION
The method of making a CdS - Cu2S photovoltaic cell of the present invention comprises forming a layer of CdS containing aluminum on a transparent vitreous substrate, coated with SnO and providing a layer of Cu S over the layer of CdS with the aluminum being provided in the CdS layer in an amount to effectively in-hibit the penetration of Cu through the CdS layer to the SnO coating.
Preferably the method of fabricating a photovoltaic cell comprises spraying on a hot insulating transparent substrate coated with SnOx, a water solution of cadmium chloride, thiourea and an aluminum containing soluble compound in heavy concentration.
On the latter may be superposed a further sprayed layer of cadmium 1 .
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chloride and thirourea, having little or no included aluminum.
The aluminum concentration in the CdC12 layer may be concentrated most heavily in the underside of the CdS layer, i.e., be dispersed throughout the CdS layer with a much heavier concentration at the bottom of the layer than occurs at the top, where no Al may occur or only little, or the Al may be dispersed equally throughout, the process being conducted while the sprayed surface of the substrate is maintained at a constant temperature in the range - 4 a -' -~o~
about 500F to 800F, and thereafter heating the coated substrate to about 525C, cooling to approximately room temperature, and thereafter coating with Cu2S solution either by dipping or by electroplating to form a heterojunction, and forming an electrode over the heterojunction.
DETAILED DISCLOSURE
The process disclosed in co-pending application S.N. 215,901 is applied to a glass substrate as the glass sub-strate travels along a tank containing a molten salt or a molten metal. The sheet of glass may be supported solely by the molten liquid or partially by the molten liquid and partially by extraneous supports, so that its bottom surface is immersed in the liquid and the liquid continuously supplies heat to the substrate. In the course of spraying the substrate with solu-tions, the upper exposed surface of the substrate is cooled by the spray. It is then necessary to conduct the spraying operation intermittently and over so small a portion of the substrate surface at any instant of time, that the substrate surface can acquire from the molten material enough heat to recover the temperature of the exposed surface of the substrate between the sprays applied to any area. This process provides a uniform layer thickness, and also facilitates maintenance of constant temperature, or maintains the temperature more nearly constant than is otherwise feasible. -Subject to the considerations stated in the immediately preceding paragraph, glass is assumed to be at approximately the temperature of the heated liquid, the temperature of which is such as to maintain the exposed surface of the glass at a temperature in .
. ~ ., .
the range 500 F to 800F, and it is assumed for the purpose of the present invention that the glass has been coated with SnOx in a thin transparent layer, as by the methods taught in our parent application. Spraying is advantageously accomplished under intense ultraviolet light, which is absorbed by CdS and enhances the internal energy of the CdS crystals.
EXAMPLE I
In a first example of the invention, two solutions are prepared. The first solution may be in the proportion:
BACKGROUND OF THE INVENTION
It is known to form photovoltaic cells by coating on a hot sheet of Nesa glass, or glass previously coated with SnO , a thin film of CdS, by spraying a water solution of compounds which form a layer of CdS microcrystals on the SnO , and providing a Cu2S heterojunction as a layer on the CdS layer, and forming electrodes on the film of Cu2S. In accordance with our co-pending Canadian application S.N. 215,901, filed December 12, 1974, the Cu2S layer was formed by spraying a Cu2S forming solution on the CdS film while the substrate was hot, but in accordance with the present application, the Cu2S is formed by dipping at or near room temperature or by electro-plating; at or near room temperature. Cells so formed have heretofore utilized relatively thick films of CdS, or have re-sorted to other expedients to obviate the difficulty that CdS
films generally permit permeation by Cu2S and Cu, when Cu2S is formed by dipping or electroplating, i.e., by ion exchange, which provides short circuits between the Cu2S layer and the SnO , the latter constituting the negative electrode of the cell, rendering the cell inoperative. In order to convert solar energy to electrical enery on a large scale, square miles of solar cells may be required. Since Cd is a rare and expensive metal, it becames important to form photovoltaic cells with minimum quantities of CdS and hence with extremely thin CdS films, yet that the cell be reliably fabricated ?60Z81 and have long life. We have produced CdS photovoltaic cells in which the layer of CdS is about 2. or 4. microns thick, yet which show zero shorting permeation, the layers being highly impervious to Cu2S or Cu containing solution, in an ion exchange process. We have heretofore used the method of spraying a CdS forming solution on a glass coated with SnOx, intermittently and at a slow rate, while maintaining the surface of the glass at uniform and constant temperature in the range between 500F and 800F. According to the present invention, in one embodiment, plural sprays are re-quired to form the CdS layer. All sprays employ the known materials, i.e., a solution containing essentially CdC12 and thiourea, but to one spray is added AlC13 . 6 H20, in proportions by weight equal to between 10% and 50% of Al, adding thiourea as required to combine with the Al, and in a superposed spray op-tionally a small quantity of HCl, but no AlC13 . 6 H20, or a very much reduced quantity of AlC13 . 6 H20; or, instead of applying two discrete films, one containing a large quantity of aluminum com-pound and the other containing none or very little, the coating may be formed by gradually decreasing the concentration of aluminum in proceeding from the bottom of the layer adjacent to the SnOx to the top of the layer, so that the top of the layer contains no aluminum or very little aluminum while the bottom of the layer may have as much as 50% aluminum. The quantity of the sulphur employed must be proportioned to the quantity of aluminum, forming a compound or crystal therewith.
It is then found that that portion of the CdS layer which contains a large proportion of alumi-num is-extremely hard d ~ ~
so that it can only with difficulty be removed by application of acid or by scraping, is highly adherent to the Sn~ layer, so that it can only with difficulty be removed, and is highly im-pervious to materials involved in forming a Cu2S layer by ion exchange, or to Cu2S, and inhibits diffusion of Cu through a CdS
layer, where the Cu2S is formed by dipping or electroplating, i.e., by an ion exchange process. The inclusion of aluminum in CdS in the large quantities specified does not constitute a doping procedure, but is a new compound or alloy or a new crystalline form, having physical properties quite distinct from those of CdS
or CdS with small amounts of aluminum, in terms of hardness, toughness and imperviousness, but which remains photoconductive.
For example, it is known that A12S3 and CdS crystals are both hexagonal, and the Al may form A12S3 crystals or a composite crystal or alloy, of Al, Cd, S, or a ternary compound, though tests indicate the latter possibility to be unlikely. It has been found that if the entire film of CdS is heavily impregnated with Al and the cell remains operative, but at reduced efficiency.
SUMMARY OF THE INVFNTION
The method of making a CdS - Cu2S photovoltaic cell of the present invention comprises forming a layer of CdS containing aluminum on a transparent vitreous substrate, coated with SnO and providing a layer of Cu S over the layer of CdS with the aluminum being provided in the CdS layer in an amount to effectively in-hibit the penetration of Cu through the CdS layer to the SnO coating.
Preferably the method of fabricating a photovoltaic cell comprises spraying on a hot insulating transparent substrate coated with SnOx, a water solution of cadmium chloride, thiourea and an aluminum containing soluble compound in heavy concentration.
On the latter may be superposed a further sprayed layer of cadmium 1 .
-- ~0~V~
chloride and thirourea, having little or no included aluminum.
The aluminum concentration in the CdC12 layer may be concentrated most heavily in the underside of the CdS layer, i.e., be dispersed throughout the CdS layer with a much heavier concentration at the bottom of the layer than occurs at the top, where no Al may occur or only little, or the Al may be dispersed equally throughout, the process being conducted while the sprayed surface of the substrate is maintained at a constant temperature in the range - 4 a -' -~o~
about 500F to 800F, and thereafter heating the coated substrate to about 525C, cooling to approximately room temperature, and thereafter coating with Cu2S solution either by dipping or by electroplating to form a heterojunction, and forming an electrode over the heterojunction.
DETAILED DISCLOSURE
The process disclosed in co-pending application S.N. 215,901 is applied to a glass substrate as the glass sub-strate travels along a tank containing a molten salt or a molten metal. The sheet of glass may be supported solely by the molten liquid or partially by the molten liquid and partially by extraneous supports, so that its bottom surface is immersed in the liquid and the liquid continuously supplies heat to the substrate. In the course of spraying the substrate with solu-tions, the upper exposed surface of the substrate is cooled by the spray. It is then necessary to conduct the spraying operation intermittently and over so small a portion of the substrate surface at any instant of time, that the substrate surface can acquire from the molten material enough heat to recover the temperature of the exposed surface of the substrate between the sprays applied to any area. This process provides a uniform layer thickness, and also facilitates maintenance of constant temperature, or maintains the temperature more nearly constant than is otherwise feasible. -Subject to the considerations stated in the immediately preceding paragraph, glass is assumed to be at approximately the temperature of the heated liquid, the temperature of which is such as to maintain the exposed surface of the glass at a temperature in .
. ~ ., .
the range 500 F to 800F, and it is assumed for the purpose of the present invention that the glass has been coated with SnOx in a thin transparent layer, as by the methods taught in our parent application. Spraying is advantageously accomplished under intense ultraviolet light, which is absorbed by CdS and enhances the internal energy of the CdS crystals.
EXAMPLE I
In a first example of the invention, two solutions are prepared. The first solution may be in the proportion:
2 liters water 60. cc l Molar CdCl2 60. cc 1 Molar thiourea 1.95 gm AlC13 . 6 H2O tadd sufficient thiourea to satisfy quantity of AlC13 used) The second solution employed is in the proportion:
5 liters water 150. cc 1 Molar thiourea 150. cc 1 Molar CdC12 2 1/2 cc Hydrochloric acid While thiourea is specified as a component, its function is to produce sulphur. Other compounds which are soluble in water, and which give up sulphur, can be substituted. Specifically, NN DMthiourea has been employed, but thiourea is the least ex-pensive compound which has been found satisfactory. The specific quantity of AlCl3 . 6 H2O may be varied over a wide range, pro-vided that the quantity of sulphur available is proportionally varied. The purpose of Al inclusion is to provide at least a stratum of CdS by combination of CdCl2 with thiourea which is heavily impregnated with aluminum or Al2S3 microcrystals~ During ` 106~)Z~3~
l t~le coating the Cl and the H20 are removed due to the temperature at which the process is carried out, and aluminum remains. This aluminum may be included at concentrations in any proportion from 10 to 50 molar percent of the solution. Even higher percentages of aluminum may be employed, but higher percentages have not been found to produce superior performance. The layer of CdS, heavily impregnated with aluminum, is found to have physical properties quite different from CdS containing no aluminum or relatively small amounts. The layer is extremely hard, impervious to Cu2S or Cu, and is highly adherent to the SnOx. The reason for these physical properties is not known, but it appears that either aluminum is incorporated in the CdS crystal so that a new form of crystal is formed, or that a new tertiary compound is formed, or that Al2S3 microcrystals are formed.
The second solution contains no-aluminum, in the example cited, but in fact some aluminum may be included in the form of AlC13 . 6 H20. Inclusion of HCl is optional, and for reasons unknown increases slightly the output of the cell, but so far as is known, does not otherwise affect the operation of the cell.
EXAMPLE II
A single coating of water, CdC12, thiourea and AlC13 . 6 H20 is sprayed on glass coated with SnO , but as the spraying proceeds, the proportion of AlC13 . 6 H20 to CdC12 is gradually decreased, for example, logarithmically, so that, for example, as much as 50 molar percent aluminum may be present in the lowermost part of the final layer, and zero or substantially zero aluminum at the upper surface of the layer. As the quantity of AlCl3 is varied, the ;~
quantity of thiourea is varied proportionately, to satisfy both the Cd and the Al in forming the final compounds.
~ - 7 _ .: ' . - , , `~`` 106028:1 1 In either case, the preferred end result is that directly in contact with the SnO is a layer of CdS heavily impregnated with aluminum, usually about 30~ referred to CdS, and at the upper surface of the layer of CdS, on which is to be applied Cu2S to form a heterojunction, there is no aluminum or very little aluminum. Two distinct layers may be employed, or a gradually decreasing percentage of aluminum is proceeding from bottom to top of the layer.
The solutions are sprayed on the glass intermittently and slowly in successive passes over a considerab]e period of time, of the order of 10-40 minutes for the first coating and 10-40 minutes for the second coating, the Example I, and in the order of 20-60 minutes in the case of the graduated layer, and generally so slowly that the glass sheet always remains at the same average temper-ature during the spraying, despite the heat abstracted from the glass in the spraying process. The totla thickness of the layer formed in this way is between about 2. to 4. microns, or less, The coated plate is, after spraying is completed, heated to a temperature of about 500C to 550C for 15 minutes.
After the CdS layer has been formed, the bath is slowly cooled, and the coated product may then be removed. The SnOx layer employed is about 4 micron thick and has a resistivity per square of about 11-17 ohms.
To complete a photovoltaic cell, the CdS layer is coated with Cu2S by dipping the previously cooled cell into an appropriate ;
solution at room temperature or spraying with that solution at room temperature, or by forming the Cu2S from the solution by electro- -plating, in any case by ion exchange of Cu for Cd in CdS.
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1060;2 ~IL
The solution employed to form a Cu2S layer by dipping may be in proportions as follows:
100 cc Water 7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm H4Ce(SO4)4 1.5 gm NaCl The tartaric acid is a typical organic acid and for it may be substituted citric or lactic acid. The presence of H4Ce(SO4)4 appears to add an increment of about 50. millivolts of voltage to the final cell, but the manner in which it functions in the process is not understood. For NaCl may be substituted another halide, as NH4Cl. The HNO3 is optional. Thereafter a deposit of copper is applied over the Cu S to a thickness of about 7,000.A, and aluminum is then coated over the copper to protect the copper from oxidation and from the effects of moisture and airborne impurities.
A preferred solution employed in plating is:
EXAMPLE III
700 cc H2O
5 liters water 150. cc 1 Molar thiourea 150. cc 1 Molar CdC12 2 1/2 cc Hydrochloric acid While thiourea is specified as a component, its function is to produce sulphur. Other compounds which are soluble in water, and which give up sulphur, can be substituted. Specifically, NN DMthiourea has been employed, but thiourea is the least ex-pensive compound which has been found satisfactory. The specific quantity of AlCl3 . 6 H2O may be varied over a wide range, pro-vided that the quantity of sulphur available is proportionally varied. The purpose of Al inclusion is to provide at least a stratum of CdS by combination of CdCl2 with thiourea which is heavily impregnated with aluminum or Al2S3 microcrystals~ During ` 106~)Z~3~
l t~le coating the Cl and the H20 are removed due to the temperature at which the process is carried out, and aluminum remains. This aluminum may be included at concentrations in any proportion from 10 to 50 molar percent of the solution. Even higher percentages of aluminum may be employed, but higher percentages have not been found to produce superior performance. The layer of CdS, heavily impregnated with aluminum, is found to have physical properties quite different from CdS containing no aluminum or relatively small amounts. The layer is extremely hard, impervious to Cu2S or Cu, and is highly adherent to the SnOx. The reason for these physical properties is not known, but it appears that either aluminum is incorporated in the CdS crystal so that a new form of crystal is formed, or that a new tertiary compound is formed, or that Al2S3 microcrystals are formed.
The second solution contains no-aluminum, in the example cited, but in fact some aluminum may be included in the form of AlC13 . 6 H20. Inclusion of HCl is optional, and for reasons unknown increases slightly the output of the cell, but so far as is known, does not otherwise affect the operation of the cell.
EXAMPLE II
A single coating of water, CdC12, thiourea and AlC13 . 6 H20 is sprayed on glass coated with SnO , but as the spraying proceeds, the proportion of AlC13 . 6 H20 to CdC12 is gradually decreased, for example, logarithmically, so that, for example, as much as 50 molar percent aluminum may be present in the lowermost part of the final layer, and zero or substantially zero aluminum at the upper surface of the layer. As the quantity of AlCl3 is varied, the ;~
quantity of thiourea is varied proportionately, to satisfy both the Cd and the Al in forming the final compounds.
~ - 7 _ .: ' . - , , `~`` 106028:1 1 In either case, the preferred end result is that directly in contact with the SnO is a layer of CdS heavily impregnated with aluminum, usually about 30~ referred to CdS, and at the upper surface of the layer of CdS, on which is to be applied Cu2S to form a heterojunction, there is no aluminum or very little aluminum. Two distinct layers may be employed, or a gradually decreasing percentage of aluminum is proceeding from bottom to top of the layer.
The solutions are sprayed on the glass intermittently and slowly in successive passes over a considerab]e period of time, of the order of 10-40 minutes for the first coating and 10-40 minutes for the second coating, the Example I, and in the order of 20-60 minutes in the case of the graduated layer, and generally so slowly that the glass sheet always remains at the same average temper-ature during the spraying, despite the heat abstracted from the glass in the spraying process. The totla thickness of the layer formed in this way is between about 2. to 4. microns, or less, The coated plate is, after spraying is completed, heated to a temperature of about 500C to 550C for 15 minutes.
After the CdS layer has been formed, the bath is slowly cooled, and the coated product may then be removed. The SnOx layer employed is about 4 micron thick and has a resistivity per square of about 11-17 ohms.
To complete a photovoltaic cell, the CdS layer is coated with Cu2S by dipping the previously cooled cell into an appropriate ;
solution at room temperature or spraying with that solution at room temperature, or by forming the Cu2S from the solution by electro- -plating, in any case by ion exchange of Cu for Cd in CdS.
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1060;2 ~IL
The solution employed to form a Cu2S layer by dipping may be in proportions as follows:
100 cc Water 7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm H4Ce(SO4)4 1.5 gm NaCl The tartaric acid is a typical organic acid and for it may be substituted citric or lactic acid. The presence of H4Ce(SO4)4 appears to add an increment of about 50. millivolts of voltage to the final cell, but the manner in which it functions in the process is not understood. For NaCl may be substituted another halide, as NH4Cl. The HNO3 is optional. Thereafter a deposit of copper is applied over the Cu S to a thickness of about 7,000.A, and aluminum is then coated over the copper to protect the copper from oxidation and from the effects of moisture and airborne impurities.
A preferred solution employed in plating is:
EXAMPLE III
700 cc H2O
3.9 gm citric acid monohydrate 2.0 gm NH4C
7.5 gm cupric acetate pentahydrate The NH4Cl may be omittedbut improves the quality of the cells by a small factor. Plating is accomplished at a current ;
density of .5 ma per cm for 2-5 minutes, with the CdS layer negative with respect to a copper anode.
~, ~060;2~1 EXAMPLE IV
700 cc H2O
10 gm cupric acetate pentahydrate 15 cc acetic acid (20~) .
The present method can be employed with Nesa glass as the starting material, obviating the need for coating with SnO , but Nesa glass has a resistivity per square of about 50.-75. ohms per square, whereas by our methods very low res-istivity coatings may be produced, i.e., of the order of 8-17 ohms per square. The use of low resistivity coatings of SnO
increases the efficiency of the cell by decreasing the amount of energy which is lost in the SnO .
Reference has been made hereinabove to the use of very intense ultraviolet light while spraying the cadmium sulphide forming solution. The precise intensity of the irradiation has not been measured, but four 150. watt ultraviolet lamps have been placed within a few inches of a small surface being sprayed, and the ultraviolet light is advantageous. If there is no irradiation during spraying, the cells sometimes have relatively lower outputs than occurs in the presence of intense ultraviolet light.
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It also appears to be important to the process that the spraying occur by depositing droplets whlch are as uniform as possible. If the spray consists of many small and many large droplets, the very small droplets are evaporated by the intense heat, approximate to the exposed surface of the substrate, and only the larger droplets reach the substrate. This causes some wastage of CdS and it implies that the rate at which the spray is applied must take into account the non-uniformity of the droplets. While an air spray has been employed in the pro-duction of photovoltaic cells in accordance with the process,the use of electrostatic spraying is known to produce more uniform droplets than can be produced by an air spray and is therefore preferred. In accordance with the method of co-pending application S.N. 215,~01, the glass substrate will be moving longitudinally along a trough or tank containing molten material, and the spray will occur by transverse passes across the sub-strate as the glass moves, so that the total quantity of spray reaching any given small area of the substrate will be uniform.
The method allows for the fact that spray out of an air gun or out of an electrostatic spray gun does not have a uniform pattern, the continuous motion of the spray gun and of the glass relative to the spray gunenforcmg an averaging of the total spray deposited on any small unit area of the substrate, so that the final layer will be of uniform thickness.
It has been found that the cell remains operative if the entire CdS film is heavily impregnated with Al, say to between 10~ and 50~ by weight of Cd.
`
" 1060Z~31 After a cell is completed, including electrodes, it is heated to 400F to 500F.
The present cell is to be exposed to solar radiation via its glass substrate. The presence of Al in the CdS does not materially affect transparency of the CdS-Al layer, so that the heterojunction may be exposed via the latter.
SUPPLEMENTARY DISCLOSURE
The Cu2S layer of the photovoltaic cell may be more broadly referred to as a Cu S layer because as it is appreciated by those skilled in the art, there may be impurities present in the compound which will give a stoichiometric ratio for the com-pound of something other than Cu2S; therefore the method for making the photovoltaic cell having a transparent vitreous sub-strate provided with a first conductive layer, a second CdS
containing layer and a third, Cu S layer forming the heterojunction with the second layer, comprises providing aluminum in the second layer in an amount effective to inhibit penetration of Cu through the second~layer, to the conductive layer.
As previously described, the Cu S layer may be formed by dipping or by electroplating at or near room temperature, however, it is also possible to form the Cu S layer by a combination of dipping and electroplating at or near room temperature.
We have produced CdS photovoltaic cells in which the layer of CdS is about 2 to 4 microns thick, yet which show zero shorting mermeation, the layers being highly impervious to Cu S
or Cu containing solution, in an ion exchange process. We have heretofore used the method of spraying a CdS forming solution on a glass coated with SnO , intermittently and covering only a small portion of the glass at a given point in time, while maintaining the surface of the glass at uniform and constant temperature in the range between 500F and 1100F. According to the present invention, in one embodimenti multiple sprays are required to form the CdS layer. Each spray comprises a solution containing a cadmium compoun~ and a sulphur containing compound.
~ - 13 -- r Eiowever, to one spray is added AlC13 . 6 H20, in proportions such that the Al content is 10 to 50 molar percent of the total metal ion content of the solution, and the quantity of sulphur containing compound is increased as required to combine with the Al. In a superposed spray solution, - 13 a -~ 10602~1 optionally no AlC13 . 6H2O is included, or a very much reduced quantity of AlC13 . 6H2O, such that the layer formed by the superposed spray contains little or no impregnated Al. In-stead of applying two discrete films, one formed from a spray containing a large quantity of aluminum com~ound and the other containing little or none, a single film may be formed by grad-ually decreasing the aluminum compound content of the solution being sprayed in proceeding from the stratum of the film adjacent to the SnO layer to the stratum of the film adjacent to the exposed surface of the CdS film. In this manner the stratum of the CdS film adjacent to the SnO layer is impreg-nated with significantly greater quantities of Al than the stratum of the film adjacent to the exposed surface of the CdS
film. The quantity of the sulphur containing compound is pro-portioned to the quantity of the aluminum compound used.
After heat treating at a temperature in the range 450C to 550C, it is found that the portion of the CdS layer in which Al is impregnated is extremely hard and highly adherent to the SnO layer, so that it can only with difficulty be removed by application of acid or by scraping and is highly impervious to chemicals involved in forming a Cu S layer by ion exchange, or to Cu S, and inhibits diffusion of Cu through a CdS layer. The impregnation of Al in the CdS film in the rela-tively large quantities resulting from use of solutions con-taining Al in a quantity representing 10 to 50 molar percent of the total metal ion content of the solution does not constitute a doping procedure, such as disclosed in Middleton, et al., U.S. Patent No. 3,411,050. Rather it comprises a new compound or material, or a new crystalline form, having properties quite _ la _ B
` 106~)Z~
distinct from those of CdS or CdS impregnated with only small amounts of Al. It has been found that if the entire film of CdS is heavily impregnated with Al the cell remains operative, but at reduced efficiency.
A method of fabricating a photovoltaic cell comprises spraying on a hot insulating transparent substrate coated with a transparent conducting film, a solution comprising a solvent, a cadmium salt, a sulphur containing com~ound and an aluminum containing soluble compound in heavy concentration. Thereafter a further solution comprising a cadmium salt and a sulphur containing compound, having little or no included aluminum may be sprayed onto the substrate. The aluminum concentration -in the resulting CdS layer may be concentrated most heavily in the underside of the CdS layer, i.e., be dispersed throughout the CdS layer ! but with a much heavier concentration at the bottom of the layer than occurs at the top, where no Al may occur or only little, or the Al may be dispersed equally throughout. The spray process is conducted while the sprayed surface of the substrate is maintained at a constant temperature in the range about 500F to 1100F. After the sprav process is completed, the coated substrate is heated in an oxygen con-taining atmosphere to about 525C, then cooled to approximately room temperature. Thereafter, the exposed surface of the CdS
film is converted to Cu S, either by dipping, electroplating, or a combination of both to form a heterojunction. The exposed surface of the CdS film may also be converted to Cu S by spraying thereon a suitable copper containing solution. Thereafter, an electrode is formed over the Cu S layer.
B
.
. . ~.
- ~0602Bl EXAMPLE V
In a flrst example of the invention, two solutions are prepared. The first solution may be in the proportion:
2 liters Water 60 cc 1 Molar CdC12 solution ; 74 cc 1 Molar thiourea solution 1.95 gm AlC13 . 6 H2O
The second solution employed is in the proportion:
5 liters Water 150 cc 1 Molar thiourea solution 150 cc 1 Molar CdC12 solution 2.5 cc Hydrochloric acid While thiourea is specified as a component, its function is to produce sulphur. Other compounds which are soluble in water, and which give up sulphur, can be substituted.
Specifically NN DMthiourea has been employed, but thiourea is the least expensive compound which has been found satisfactory.
The specific quantity of AlC13 . 6 H2O may be varied over a wide range, provided that the quantity of sulphur available is proportionally varied. The purpose of Al inclusion is to pro-vide at least a stratum of CdS by combination of CdC12 with thiourea which is heavily impregnated with an aluminum com- ;
pound, the exact nature of which is unknown. The aluminum content of the solution may be varied from 10 to 50 molar percent of the total metal ion content of the solution. Even higher molar percentages of aluminum may be employed, but higher percentages have not been found to produce superior performance. The CdS film having Al impregnated therein formed -with a solution having an Al content of from 10 to 50 molar .
~ ~)6~
percent of the total metal ion content of the solution is ~ -found to have properties quite different from CdS films contain-ing no aluminum or relatively small amounts of aluminum. The layer is extremely hard, impervious to Cu S or Cu, and is highly adherent to the SnO . The reason for these physical properties is not known.
The second solution contains no aluminum, in the example cited, but in fact some aluminum may be included in the form of AlC13 . 6 H2O. Inclusion of HCl is optional, and for reasons unknown increases slightly in the output of the cell, but so far as is known does not otherwise affect the operation of the cell.
EXAMPLE VI
Similar to EXAMPLE IV but with the solutions being differently comprised. The first solution may be in the proportion:
8 liters Water 18.63 gm CdC12 . 2 1/2 H2O
8.77 gm Thiourea 6.96 gm AlC13 6 H2O
2 cc Hydrochloric acid The second solution may be in the proportion:
7.5 gm cupric acetate pentahydrate The NH4Cl may be omittedbut improves the quality of the cells by a small factor. Plating is accomplished at a current ;
density of .5 ma per cm for 2-5 minutes, with the CdS layer negative with respect to a copper anode.
~, ~060;2~1 EXAMPLE IV
700 cc H2O
10 gm cupric acetate pentahydrate 15 cc acetic acid (20~) .
The present method can be employed with Nesa glass as the starting material, obviating the need for coating with SnO , but Nesa glass has a resistivity per square of about 50.-75. ohms per square, whereas by our methods very low res-istivity coatings may be produced, i.e., of the order of 8-17 ohms per square. The use of low resistivity coatings of SnO
increases the efficiency of the cell by decreasing the amount of energy which is lost in the SnO .
Reference has been made hereinabove to the use of very intense ultraviolet light while spraying the cadmium sulphide forming solution. The precise intensity of the irradiation has not been measured, but four 150. watt ultraviolet lamps have been placed within a few inches of a small surface being sprayed, and the ultraviolet light is advantageous. If there is no irradiation during spraying, the cells sometimes have relatively lower outputs than occurs in the presence of intense ultraviolet light.
,:, :
,:
A~
.. . .. .. . . .
` ` ~06~
It also appears to be important to the process that the spraying occur by depositing droplets whlch are as uniform as possible. If the spray consists of many small and many large droplets, the very small droplets are evaporated by the intense heat, approximate to the exposed surface of the substrate, and only the larger droplets reach the substrate. This causes some wastage of CdS and it implies that the rate at which the spray is applied must take into account the non-uniformity of the droplets. While an air spray has been employed in the pro-duction of photovoltaic cells in accordance with the process,the use of electrostatic spraying is known to produce more uniform droplets than can be produced by an air spray and is therefore preferred. In accordance with the method of co-pending application S.N. 215,~01, the glass substrate will be moving longitudinally along a trough or tank containing molten material, and the spray will occur by transverse passes across the sub-strate as the glass moves, so that the total quantity of spray reaching any given small area of the substrate will be uniform.
The method allows for the fact that spray out of an air gun or out of an electrostatic spray gun does not have a uniform pattern, the continuous motion of the spray gun and of the glass relative to the spray gunenforcmg an averaging of the total spray deposited on any small unit area of the substrate, so that the final layer will be of uniform thickness.
It has been found that the cell remains operative if the entire CdS film is heavily impregnated with Al, say to between 10~ and 50~ by weight of Cd.
`
" 1060Z~31 After a cell is completed, including electrodes, it is heated to 400F to 500F.
The present cell is to be exposed to solar radiation via its glass substrate. The presence of Al in the CdS does not materially affect transparency of the CdS-Al layer, so that the heterojunction may be exposed via the latter.
SUPPLEMENTARY DISCLOSURE
The Cu2S layer of the photovoltaic cell may be more broadly referred to as a Cu S layer because as it is appreciated by those skilled in the art, there may be impurities present in the compound which will give a stoichiometric ratio for the com-pound of something other than Cu2S; therefore the method for making the photovoltaic cell having a transparent vitreous sub-strate provided with a first conductive layer, a second CdS
containing layer and a third, Cu S layer forming the heterojunction with the second layer, comprises providing aluminum in the second layer in an amount effective to inhibit penetration of Cu through the second~layer, to the conductive layer.
As previously described, the Cu S layer may be formed by dipping or by electroplating at or near room temperature, however, it is also possible to form the Cu S layer by a combination of dipping and electroplating at or near room temperature.
We have produced CdS photovoltaic cells in which the layer of CdS is about 2 to 4 microns thick, yet which show zero shorting mermeation, the layers being highly impervious to Cu S
or Cu containing solution, in an ion exchange process. We have heretofore used the method of spraying a CdS forming solution on a glass coated with SnO , intermittently and covering only a small portion of the glass at a given point in time, while maintaining the surface of the glass at uniform and constant temperature in the range between 500F and 1100F. According to the present invention, in one embodimenti multiple sprays are required to form the CdS layer. Each spray comprises a solution containing a cadmium compoun~ and a sulphur containing compound.
~ - 13 -- r Eiowever, to one spray is added AlC13 . 6 H20, in proportions such that the Al content is 10 to 50 molar percent of the total metal ion content of the solution, and the quantity of sulphur containing compound is increased as required to combine with the Al. In a superposed spray solution, - 13 a -~ 10602~1 optionally no AlC13 . 6H2O is included, or a very much reduced quantity of AlC13 . 6H2O, such that the layer formed by the superposed spray contains little or no impregnated Al. In-stead of applying two discrete films, one formed from a spray containing a large quantity of aluminum com~ound and the other containing little or none, a single film may be formed by grad-ually decreasing the aluminum compound content of the solution being sprayed in proceeding from the stratum of the film adjacent to the SnO layer to the stratum of the film adjacent to the exposed surface of the CdS film. In this manner the stratum of the CdS film adjacent to the SnO layer is impreg-nated with significantly greater quantities of Al than the stratum of the film adjacent to the exposed surface of the CdS
film. The quantity of the sulphur containing compound is pro-portioned to the quantity of the aluminum compound used.
After heat treating at a temperature in the range 450C to 550C, it is found that the portion of the CdS layer in which Al is impregnated is extremely hard and highly adherent to the SnO layer, so that it can only with difficulty be removed by application of acid or by scraping and is highly impervious to chemicals involved in forming a Cu S layer by ion exchange, or to Cu S, and inhibits diffusion of Cu through a CdS layer. The impregnation of Al in the CdS film in the rela-tively large quantities resulting from use of solutions con-taining Al in a quantity representing 10 to 50 molar percent of the total metal ion content of the solution does not constitute a doping procedure, such as disclosed in Middleton, et al., U.S. Patent No. 3,411,050. Rather it comprises a new compound or material, or a new crystalline form, having properties quite _ la _ B
` 106~)Z~
distinct from those of CdS or CdS impregnated with only small amounts of Al. It has been found that if the entire film of CdS is heavily impregnated with Al the cell remains operative, but at reduced efficiency.
A method of fabricating a photovoltaic cell comprises spraying on a hot insulating transparent substrate coated with a transparent conducting film, a solution comprising a solvent, a cadmium salt, a sulphur containing com~ound and an aluminum containing soluble compound in heavy concentration. Thereafter a further solution comprising a cadmium salt and a sulphur containing compound, having little or no included aluminum may be sprayed onto the substrate. The aluminum concentration -in the resulting CdS layer may be concentrated most heavily in the underside of the CdS layer, i.e., be dispersed throughout the CdS layer ! but with a much heavier concentration at the bottom of the layer than occurs at the top, where no Al may occur or only little, or the Al may be dispersed equally throughout. The spray process is conducted while the sprayed surface of the substrate is maintained at a constant temperature in the range about 500F to 1100F. After the sprav process is completed, the coated substrate is heated in an oxygen con-taining atmosphere to about 525C, then cooled to approximately room temperature. Thereafter, the exposed surface of the CdS
film is converted to Cu S, either by dipping, electroplating, or a combination of both to form a heterojunction. The exposed surface of the CdS film may also be converted to Cu S by spraying thereon a suitable copper containing solution. Thereafter, an electrode is formed over the Cu S layer.
B
.
. . ~.
- ~0602Bl EXAMPLE V
In a flrst example of the invention, two solutions are prepared. The first solution may be in the proportion:
2 liters Water 60 cc 1 Molar CdC12 solution ; 74 cc 1 Molar thiourea solution 1.95 gm AlC13 . 6 H2O
The second solution employed is in the proportion:
5 liters Water 150 cc 1 Molar thiourea solution 150 cc 1 Molar CdC12 solution 2.5 cc Hydrochloric acid While thiourea is specified as a component, its function is to produce sulphur. Other compounds which are soluble in water, and which give up sulphur, can be substituted.
Specifically NN DMthiourea has been employed, but thiourea is the least expensive compound which has been found satisfactory.
The specific quantity of AlC13 . 6 H2O may be varied over a wide range, provided that the quantity of sulphur available is proportionally varied. The purpose of Al inclusion is to pro-vide at least a stratum of CdS by combination of CdC12 with thiourea which is heavily impregnated with an aluminum com- ;
pound, the exact nature of which is unknown. The aluminum content of the solution may be varied from 10 to 50 molar percent of the total metal ion content of the solution. Even higher molar percentages of aluminum may be employed, but higher percentages have not been found to produce superior performance. The CdS film having Al impregnated therein formed -with a solution having an Al content of from 10 to 50 molar .
~ ~)6~
percent of the total metal ion content of the solution is ~ -found to have properties quite different from CdS films contain-ing no aluminum or relatively small amounts of aluminum. The layer is extremely hard, impervious to Cu S or Cu, and is highly adherent to the SnO . The reason for these physical properties is not known.
The second solution contains no aluminum, in the example cited, but in fact some aluminum may be included in the form of AlC13 . 6 H2O. Inclusion of HCl is optional, and for reasons unknown increases slightly in the output of the cell, but so far as is known does not otherwise affect the operation of the cell.
EXAMPLE VI
Similar to EXAMPLE IV but with the solutions being differently comprised. The first solution may be in the proportion:
8 liters Water 18.63 gm CdC12 . 2 1/2 H2O
8.77 gm Thiourea 6.96 gm AlC13 6 H2O
2 cc Hydrochloric acid The second solution may be in the proportion:
4 liters Water 24.70 gm CdC12 . 2 1/2 H2O
10.96 gm Thiourea EXAMPLE VII
A solution of water, CdC12, thiourea and AlC13 . 6 H2O
is sprayed on glass coated with SnO , but as the spraying pro-ceeds the proportion of AlC13 . 6 H2O to CdC12 is gradually B
. . :
. . .
106~28~l decreased, for example, logarithmically. For example, as muchas 50 molar percent aluminum may he present in the solution forming the lowermost part of the CdS film, and zero or sub-stantially zero molar percent aluminum in the solution forming the upper surface of the film. As the quantity of AlC13 is varied, the quantity of thiourea is varied proportionately, to satisfy both the Cd and the Al in forming the final compounds.
In all cases, the preferred end result is that directly in contact with the SnO is a layer of CdS heavily impregnated with aluminum and at the upper surface of the layer of CdS, which is to be converted to Cu S to form a heterojunction, there is no aluminum or very little aluminum. Two distinct layers may be employed, or a decreasing percentage of aluminum in proceeding from bottom to top of the layer.
The solutions are sprayed on the glass intermittently and slowly in successive passes over a considerable period of time, of the order of 10-40 minutes for the first coating and 10-40 minutes for the second coating, in Examples V and VI, and in the order of 20-60 minutes in the case of the graduated layer, and intermittently and over only a small portion of the glass at any one instant so that the glass sheet always remains at the same average temperature during the spraying, despite the heat removed from the glass in the spraying process.
The total thickness of the layer formed in this way is between about 2 to 4 microns, or less. The coated plate is, after spraying is completed, heated to a temperature of about 450C `
to 550C for approximately 15 minutes.
It should be noted that although the preferred method of forming a CdS film which is impregnated with Al is via the B
.. ;............ .. ....... ~. . . . .
:
~L06~2~1 spray method above described, other methods of forming such a film may be employed. Thus, the CdS film, which is impregnated with Al, may be formed by vacuum depositicn, dipping, electro-plating, or any other suitable means by which films are deposited on a substrate, as known in the art. Although we are of the belief that the best such films are formed using the spray method, these other well-known methods of forming a film may also be utilized.
After the CdS layer has been formed, the bath is slowly cooled, and the coated product may then be removed.
To complete a photovoltaic cell, the exposed surface of the CdS layer is converted to Cu S by dipping the previously cooled cell into an appropriate solution at room temperature, -~
by electroplating, by a combination of dipping and electro-plating, or by spraying thereon a suitable copper containing solution.
The solution employed to form a Cu S layer by dipping may be in proportions as follows: ~ ~
EXAMPLE VIII ~ -100 cc Water 7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm 4 4)4 1.5 gm NaCl The tartaric acid is a typical organic acid and for it may be substituted citric or lactic acid. The presence of H4Ce(SO4)4 appears to add an increment of about 50 millivolts of voltage to the final cell, but the manner in which it functions 10602~
in the process is not understood. For NaCl may be substituted another halide, as NH4Cl. The HNO3 is optional.
Another solution which may be utilized in the dipping process is:
EXAMPLE IX
700 cc Water 7 gm Citric acid monohydrate 30 gm NH4Cl 3.5 gm CuCl As with the EXAMPLE VIII solution, another halide may be substituted for NH4Cl.
A preferred solution employed in electroplating to form CuxS is:
EXAMPLE X
700 cc Water 3.9 gm Citric acid monohydrate 2.0 gm NH4C
7.5 gm Cupric acetate pentahydrate The NH4Cl may be omitted, but seems to improve the quality of the cells by a small factor. Plating is accomplished at a current density of .5 ma per cm for 2-5 minutes, with the CdS layer negative with respect to a copper anode.
EXAMPLE XI
700 cc Water 7 gm Citric acid monohydrate 1.2 gm NH4Cl
10.96 gm Thiourea EXAMPLE VII
A solution of water, CdC12, thiourea and AlC13 . 6 H2O
is sprayed on glass coated with SnO , but as the spraying pro-ceeds the proportion of AlC13 . 6 H2O to CdC12 is gradually B
. . :
. . .
106~28~l decreased, for example, logarithmically. For example, as muchas 50 molar percent aluminum may he present in the solution forming the lowermost part of the CdS film, and zero or sub-stantially zero molar percent aluminum in the solution forming the upper surface of the film. As the quantity of AlC13 is varied, the quantity of thiourea is varied proportionately, to satisfy both the Cd and the Al in forming the final compounds.
In all cases, the preferred end result is that directly in contact with the SnO is a layer of CdS heavily impregnated with aluminum and at the upper surface of the layer of CdS, which is to be converted to Cu S to form a heterojunction, there is no aluminum or very little aluminum. Two distinct layers may be employed, or a decreasing percentage of aluminum in proceeding from bottom to top of the layer.
The solutions are sprayed on the glass intermittently and slowly in successive passes over a considerable period of time, of the order of 10-40 minutes for the first coating and 10-40 minutes for the second coating, in Examples V and VI, and in the order of 20-60 minutes in the case of the graduated layer, and intermittently and over only a small portion of the glass at any one instant so that the glass sheet always remains at the same average temperature during the spraying, despite the heat removed from the glass in the spraying process.
The total thickness of the layer formed in this way is between about 2 to 4 microns, or less. The coated plate is, after spraying is completed, heated to a temperature of about 450C `
to 550C for approximately 15 minutes.
It should be noted that although the preferred method of forming a CdS film which is impregnated with Al is via the B
.. ;............ .. ....... ~. . . . .
:
~L06~2~1 spray method above described, other methods of forming such a film may be employed. Thus, the CdS film, which is impregnated with Al, may be formed by vacuum depositicn, dipping, electro-plating, or any other suitable means by which films are deposited on a substrate, as known in the art. Although we are of the belief that the best such films are formed using the spray method, these other well-known methods of forming a film may also be utilized.
After the CdS layer has been formed, the bath is slowly cooled, and the coated product may then be removed.
To complete a photovoltaic cell, the exposed surface of the CdS layer is converted to Cu S by dipping the previously cooled cell into an appropriate solution at room temperature, -~
by electroplating, by a combination of dipping and electro-plating, or by spraying thereon a suitable copper containing solution.
The solution employed to form a Cu S layer by dipping may be in proportions as follows: ~ ~
EXAMPLE VIII ~ -100 cc Water 7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm 4 4)4 1.5 gm NaCl The tartaric acid is a typical organic acid and for it may be substituted citric or lactic acid. The presence of H4Ce(SO4)4 appears to add an increment of about 50 millivolts of voltage to the final cell, but the manner in which it functions 10602~
in the process is not understood. For NaCl may be substituted another halide, as NH4Cl. The HNO3 is optional.
Another solution which may be utilized in the dipping process is:
EXAMPLE IX
700 cc Water 7 gm Citric acid monohydrate 30 gm NH4Cl 3.5 gm CuCl As with the EXAMPLE VIII solution, another halide may be substituted for NH4Cl.
A preferred solution employed in electroplating to form CuxS is:
EXAMPLE X
700 cc Water 3.9 gm Citric acid monohydrate 2.0 gm NH4C
7.5 gm Cupric acetate pentahydrate The NH4Cl may be omitted, but seems to improve the quality of the cells by a small factor. Plating is accomplished at a current density of .5 ma per cm for 2-5 minutes, with the CdS layer negative with respect to a copper anode.
EXAMPLE XI
700 cc Water 7 gm Citric acid monohydrate 1.2 gm NH4Cl
5 gm Cupric acetate pentahydrate As indicated above, a combination of dipping and electroplating may be used in forming the Cu S layer.
106~
Alternatively, a suitable solution may be sprayed onto the CdSfilm to form the Cu S layer.
After the Cu S formation process is completed, a deposit of copper may be applied over the Cu S to a thickness of about 7,000 A and aluminum or another suitable metal is then coated over the copper to protect the copper from oxidation and from the effects of moisture and airborne impurities.
The present method can be employed with Nesa glass as the starting material, obviating the need for coating with SnO , but Nesa glass has a sheet resistivity per square of about 50-75 ohms per square, whereas by our methods very low resistivity coatings may be produced, i.e., of the order of 1-20 ohms per square. The use of low resistivity coatings of SnO increases the efficiency of the cell by decreasing the amount of energy which is lost in the SnO .
It also appears to be important to the process that the spraying occur by depositing droplets which are as uniform as possible. If the spray consists of many small and many large droplets, the very small droplets are evaporated by the intense heat, approximate to the exposed surface of the sub-strate, and only the larger droplets reach the substrate.
This causes some wastage of CdS and it implies that the rate at which the spray is applied must take into account the non-uniformity of the droplets.
In accordance with the method of our parent applica-tion, supra, the glass substrate will be moving longitudinally along a trough or tank containing molten material, and the spray will occur by transverse passes across the substrate as the glass moves, so that the total quantity of spray reaching `- -` 1060Z13~
any given small area of the substrate will be unlform. The method allows for the fact that spray out of an air gun or out of an electrostatic spray gun does not have a uniform pattern.
After a cell is completed, including electrodes, it is heated to 400F to 500F.
The present cell is to be exposed to solar radiation via its glass substrate. The presence of Al in the CdS does not materially affect transparency of the CdS-Al layer, so that the heterojunction may be exposed via the latter.
It should be understood that various changes, modi-fications, and variations in the structure and function of the present invention may be effected without departing from the spirit and scope of the present invention as defined in the appended claims.
. ~
106~
Alternatively, a suitable solution may be sprayed onto the CdSfilm to form the Cu S layer.
After the Cu S formation process is completed, a deposit of copper may be applied over the Cu S to a thickness of about 7,000 A and aluminum or another suitable metal is then coated over the copper to protect the copper from oxidation and from the effects of moisture and airborne impurities.
The present method can be employed with Nesa glass as the starting material, obviating the need for coating with SnO , but Nesa glass has a sheet resistivity per square of about 50-75 ohms per square, whereas by our methods very low resistivity coatings may be produced, i.e., of the order of 1-20 ohms per square. The use of low resistivity coatings of SnO increases the efficiency of the cell by decreasing the amount of energy which is lost in the SnO .
It also appears to be important to the process that the spraying occur by depositing droplets which are as uniform as possible. If the spray consists of many small and many large droplets, the very small droplets are evaporated by the intense heat, approximate to the exposed surface of the sub-strate, and only the larger droplets reach the substrate.
This causes some wastage of CdS and it implies that the rate at which the spray is applied must take into account the non-uniformity of the droplets.
In accordance with the method of our parent applica-tion, supra, the glass substrate will be moving longitudinally along a trough or tank containing molten material, and the spray will occur by transverse passes across the substrate as the glass moves, so that the total quantity of spray reaching `- -` 1060Z13~
any given small area of the substrate will be unlform. The method allows for the fact that spray out of an air gun or out of an electrostatic spray gun does not have a uniform pattern.
After a cell is completed, including electrodes, it is heated to 400F to 500F.
The present cell is to be exposed to solar radiation via its glass substrate. The presence of Al in the CdS does not materially affect transparency of the CdS-Al layer, so that the heterojunction may be exposed via the latter.
It should be understood that various changes, modi-fications, and variations in the structure and function of the present invention may be effected without departing from the spirit and scope of the present invention as defined in the appended claims.
. ~
Claims (60)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of making a photovoltaic cell having a film of CdS microcrystals over a coating of SnOx on a glass plate and an outer coating of Cu2S forming a heterojunction with the CdS film which inhibits penetration of Cu to said coating of SnOx comprising maintaining said glass plate uniformly at a temperature in the range 500°F.-800°F while spraying CdS forming solutions on said coating of SnOx, in sequence, said solutions including the proportions, first:
2 liters water 60 cc 1 Molar CdCl2 68.cc 1 Molar thiourea 1.95 gm AlCl3 . 6 H2O
and second:
5 liters water 150. cc 1 Molar CdCl2 150. cc 1 Molar thiourea
2 liters water 60 cc 1 Molar CdCl2 68.cc 1 Molar thiourea 1.95 gm AlCl3 . 6 H2O
and second:
5 liters water 150. cc 1 Molar CdCl2 150. cc 1 Molar thiourea
2. A method of making a photovoltaic cell having a film of CdS microcrystals over a coating of SnOx on a glass plate and an outer Cu2S coating forming a heterojunction with the CdS film which inhibits penetration of Cu to said coating of SnOx comprising the steps of maintaining said plate at a constant temperature in the range 500°F-800°F while spraying over said coating of SnOx a water solution of CdCl2, a soluble aluminum containing salt and thiourea in an effective amount, to provide the CdS film in which the content of aluminum is 10-50 molar percent of the metal content of the water solution.
3. The method according to claim 2, wherein said aluminum containing salt is AlCl3 . 6 H2O.
4. The method according to claim 2, wherein said film is sprayed to a thickness of about 2.-4. microns.
5. The method according to claim 2, wherein is deposited on said film a further film consisting essentially of CdS.
6. The method according to claim 2, wherein is sprayed on said film an aqueous solution including substantially only CdCl2 and thiourea to form a further film of CdS
7. The method according to claim 6, wherein is included the further step of post heating said substrate to a temperature in the range 500°C to 550°C.
8. The method according to claim 7, wherein is included the further step of coating said further film with Cu2S to provide the outer coating.
9. The method according to claim 7, wherein is included the further step of coating the last named film of CdS with an aqueous solution including CuCl.
10. The method according to claim 7, wherein is included the further step of coating the last named film of CdS with a solution essentially in the proportions:
700. cc water 1.4 cc HNO3 (5-1) 1.5 gm CuCl 1.5 gm H4Ce(SO4)4 1.5 gm NaCl 1.5 gm (L+)-Tartaric acid.
700. cc water 1.4 cc HNO3 (5-1) 1.5 gm CuCl 1.5 gm H4Ce(SO4)4 1.5 gm NaCl 1.5 gm (L+)-Tartaric acid.
24 The process according to claim 10, wherein is included the further step of applying an electrode to said cell, and heating said cell to a temperature in the range 400°F to 500°F.
12. The method of fabricating a photovoltaic cell comprising providing a layer of SnOx on a glass sheet, said layer of SnOx having a resistivity of about 8-17 ohms per square, coating on said SnOx an aqueous solution forming a film of microcrystalline CdS including at least a stratum containing a quantity of Al in the proportion 10 to 50 molar percent of the total metal ion content in the solution, coating on said film of microcrystalline CdS containing Al a further film of CdS which is substantially free of said Al, and providing over said further film of CdS
a layer of Cu2S by ion exchange.
a layer of Cu2S by ion exchange.
13. The method according to Claim 12, wherein said layer of Cu2S is stoichiometrically Cu2S.
14. The process of fabricating a photovoltaic cell, com-prising applying to a transparent vitreous substrate coated with a transparent film of SnOx, an aqueous solution containing aluminum in the proportion of 10-50 molar percent of the total metal ion content of the solution to form a film of CdS containing aluminum in a least a lower part of the cross section of said film and substantially no Al in a further part of the cross section of said film, said film having a total thickness of about 2.-4. microns, and providing a layer of Cu2S over the applied film.
15. The process according to claim 14, wherein said Cu2S
is stoichiometrically pure.
is stoichiometrically pure.
16. The process according to claim 14, wherein said Cu2S
is provided at approximately room temperature.
is provided at approximately room temperature.
The method of making a CdS-Cu2S photovoltaic cell which comprises providing by ion-exchange a layer of Cu2S on a layer of CdS formed by an aqueous solution coated over a transparent vitreous substrate provided with a conductive layer said layer of CdS being about 2 to 4 microns in thickness and including at least through a portion of its thickness Al in the range at least 10 molar percent of the total metal ion content of the aqueous solution.
18. The method of making a CdS-Cu2S photovoltaic cell which comprises forming a layer of CdS containing Al on a trans-parent vitreous substrate coated with SnOx, and providing a layer of Cu2S over said layer of CdS at approximately room temperature by applying to said layer of CdS an aqueous solution capable of forming Cu2S, the aluminum in the CdS layer being provided in an amount to effectively inhibit the penetration of Cu and Cu2S
through the CdS layer to the SnOx coating.
through the CdS layer to the SnOx coating.
19. The process according to claim 18, wherein said layer of Cu2S is applied by dipping in an aqueous solution containing a Cu salt for ion exchange of Cu for Cd.
20. The method according to claim 2, wherein is included the further step of irradiating said water solution while in spray form with intense ultraviolet light.
21. The method according to claim 12, wherein is included the step of irradiating said film of microcrystalline CdS during said coating with intense ultraviolet light.
22. The process according to claim 14, wherein is included the step of irradiating said film of CdS during said applying with intense ultraviolet light.
23. The method of making a photovoltaic cell, comprising forming a layer of CdS impregnated with aluminum on a transparent ?eous substrate coated with a conducting layer, and forming a layer of Cu2S over said layer of CdS by applying to said layer of CdS an aqueous solution capable of forming Cu2S, the aluminum being provided in an amount to effectively inhibit the penetra-tion of Cu and Cu2S through the CdS layer to the conducting layer.
24. The process according to claim 23, wherein said layer of Cu2S is applied by dipping said substrate in an aqueous solution containing a copper containing compound for ion exchange of Cu for Cd.
25. A method of making a photovoltaic cell of the type having a layer of CdS impregnated with aluminum over a transparent vitreous substrated coated with a conducting layer comprising forming a layer of Cu2S over said layer of CdS by electroplating in a copper containing aqueous solution to accomplish ion exchange of Cu for Cd, the aluminum in the CdS layer being provided in an amount to effectively inhibit penetration of Cu and Cu2S there-through.
26. The process according to claim 25, wherein said step of forming a layer of Cu2S by ion exchange is conducted in an electrolyte containing cupric acetate from a copper anode.
27. The process according to claim 25, wherein said aqueous solution is in the proportions:
700 cc H2O
gm Cupric acetate pentahydrate cc Acetic acid (20%)
700 cc H2O
gm Cupric acetate pentahydrate cc Acetic acid (20%)
28. The process according to claim 25, wherein said aqueous solution is in the proportions:
700 cc H2O
3.9 gm Citric acid monohydrate 2.0 gm NH4Cl 7.5 gm Cupric acetate pentahydrate
700 cc H2O
3.9 gm Citric acid monohydrate 2.0 gm NH4Cl 7.5 gm Cupric acetate pentahydrate
29. The process according to claim 25, wherein said aqueous solution is in the proportions:
700 cc H2O
3.9 gm Citric acid monohydrate 7.5 gm Cupric acetate pentahydrate
700 cc H2O
3.9 gm Citric acid monohydrate 7.5 gm Cupric acetate pentahydrate
30. The process according to claim 25, wherein said aqueous solution is in the proportions:
700 cc H2O
7 gm Citric acid monohydrate 1.2 gm NH4Cl gm Cupric acetate pentahydrate
700 cc H2O
7 gm Citric acid monohydrate 1.2 gm NH4Cl gm Cupric acetate pentahydrate
31. The process according to claim 25, wherein said ion exchange is accomplished at a current density of about .5 ma per cm2 for 2-5 minutes.
32. A method of making a photovoltaic cell as claimed in claim 17, wherein said layer of CdS containing aluminim is deposited thereon by a spray process using a spray comprised of essentially uniform droplet sizes.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
33. A method of making a photovoltaic cell of the type having a transparent vitreous substrate; a first conductive layer, a second layer comprised of CdS, and a third CuxS layer forming a heterojunction with said second layer, comprising the steps of: spraying said conductive layer with an aqueous solu-tion comprising a cadmium salt, an aluminum containing compound, and a sulphur containing compound in an effective amount to form the CdS layer; the aluminum content of the solution being at least 10 molar per cent of its total metal content of the solution;
said solution reacting on said conductive layer to form at least one stratum of said second layer of CdS with at least a portion containing aluminum in an amount effective to inhibit penetration of Cu to said conductive layer.
said solution reacting on said conductive layer to form at least one stratum of said second layer of CdS with at least a portion containing aluminum in an amount effective to inhibit penetration of Cu to said conductive layer.
34. The method according to claim 33, further comprising the step of heat treating the cell in an oxygen containing atmosphere at a temperature in the range of 450°C to 550°C.
35. The method according to claim 33, wherein said aluminum containing compound is AlCl3 . 6 H2O
36. The method according to claim 33, wherein said second layer is sprayed to a thickness of about 2 to 4 microns.
37. The method according to claim 33, wherein the cadmium salt is CdCl2.
38. The method according to claim 33, wherein the sulphur containing compound is thiourea.
39. The method according to claim 33, wherein the method includes the further step of irradiating said CdS layer with ultraviolet light during its formation.
40. The method according to claim 33, further including the step of spraying a second aqueous solution comprising a cadmium salt and a sulphur containing compound, to form a second stratum of said second layer of CdS.
41. The method according to claim 40, wherein said solution forming said first stratum is formed in the proportions:
2 liters Water 60 cc 1 Molar CdCl2 Solution 74 cc 1 Molar Thiourea Solution 1.95 gm AlCl3 . 6 H2O
and, said second solution forming said second stratum is formed in the proportions:
5 liters Water 150 cc 1 Molar CdCl2 Solution 150 cc 1 Molar Thiourea Solution 2.5 cc HCl
2 liters Water 60 cc 1 Molar CdCl2 Solution 74 cc 1 Molar Thiourea Solution 1.95 gm AlCl3 . 6 H2O
and, said second solution forming said second stratum is formed in the proportions:
5 liters Water 150 cc 1 Molar CdCl2 Solution 150 cc 1 Molar Thiourea Solution 2.5 cc HCl
42. The method according to claim 33, wherein forming said heterojunction comprises the step of contacting said second layer with a solution containing copper ions to affect an exchange of cadmium in said second layer with said copper ions to form a layer of CuxS over said second layer.
43. The method according to claim 42, wherein contacting said second layer with said solution containing copper ions com-prises the step of immersing said second layer in said solution containing copper ions.
44. The method according to claim 43, wherein said solution of copper ions comprises:
100 cc H2O
7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm H4Ce (SO4)4 1.5 gm NaCl
100 cc H2O
7 cc HNO3 (5-1) 1.5 gm (L+)-Tartaric Acid 1.5 gm CuCl 1.5 gm H4Ce (SO4)4 1.5 gm NaCl
45. The method according to claim 42, including the further step of electroplating said second layer using an aqueous solution of copper ions.
46. The method according to claim 45, wherein said solution of copper ions comprises:
700 cc H2O
gm Cupric Acetate pentahydrate cc Acetic acid (20%)
700 cc H2O
gm Cupric Acetate pentahydrate cc Acetic acid (20%)
47. The method according to claim 45, wherein said solution of copper ions comprises:
700 cc H2O
3.9 gm Citric acid monohydrate 2.0 gm NH4Cl 7.5 gm Cupric acetate pentahydrate
700 cc H2O
3.9 gm Citric acid monohydrate 2.0 gm NH4Cl 7.5 gm Cupric acetate pentahydrate
48. The method according to claim 45, wherein said solution of copper ions comprises:
700 cc H2O
7 gm Citric acetate monohydrate 1.2 gm NH4Cl gm Cupric acetate pentahydrate
700 cc H2O
7 gm Citric acetate monohydrate 1.2 gm NH4Cl gm Cupric acetate pentahydrate
49. The method according to any of the claims 45-47, wherein said electroplating is accomplished at a current density of about 0.5 ma per CM2 for 2-5 minutes.
31 ? The method according to claim 33, further including the step of applying an electrode to said third layer.
51. A photovoltaic cell including a transparent vitreous substrate, a first film of transparent conductive material deposited on said substrate, a second film of CdS microcrystals deposited on said first film said second film having an aluminum content of a least 10 molar percent of the total metal content of an aluminum containing aqueous solution reacted on the first film to form at least part of the second film in at least a portion of the thickness of said second film, a third film of CuxS
deposited on said second film and forming a heterojunction there-with, and an electrode deposited on said third film.
deposited on said second film and forming a heterojunction there-with, and an electrode deposited on said third film.
52. The cell according to claim 51, wherein said second film is about 2-4 microns thick.
53. The cell according to claim 51, wherein said first film has a sheet resistivity of about 1-20 ohms per square.
54. The cell according to claim 51, wherein said first film is SnOx and said substrate is glass.
55. A photovoltaic cell according to claim 51, wherein said electrode is copper.
56. A photovoltaic cell according to claim 51, wherein said electrode is copper and wherein a protective material is applied over said copper.
57. The cell according to claim 51, wherein the quantity of said Al is continuously graded in progressively decreasing proportions in proceeding from said first film through said second film.
58. The cell according to claim 57, wherein said progres-sively decreasing proportions are exponentially decreasing proportions.
59. The cell according to claim 58, wherein said decreasing proportions extend throughout said second film.
60. A method for making a photovoltaic cell of the type having a transparent vitreous substrate provided with a first conductive layer, a second CdS containing layer, and a third CuxS layer forming a heterojunction with the second layer comprising providing aluminum in the second layer in an amount effective to inhibit penetration of Cu through the second layer to the conductive layer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US50857074A | 1974-09-23 | 1974-09-23 | |
| US05/631,815 US4086101A (en) | 1974-09-23 | 1975-11-14 | Photovoltaic cells |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1060281A true CA1060281A (en) | 1979-08-14 |
Family
ID=27056226
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA235,472A Expired CA1060281A (en) | 1974-09-23 | 1975-09-15 | Photovoltaic cell containing cds layer impregnated with aluminum |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1060281A (en) |
-
1975
- 1975-09-15 CA CA235,472A patent/CA1060281A/en not_active Expired
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