CA1160515A

CA1160515A - Method to synthesize and produce thin films by spray pyrolysis

Info

Publication number: CA1160515A
Application number: CA000395405A
Authority: CA
Inventors: Peter M. Sienkiewicz; Richard L. Turcotte; Michael R. Squillante; Steven A. Lis; Harvey B. Serreze
Original assignee: Radiation Monitoring Devices Inc
Current assignee: Radiation Monitoring Devices Inc
Priority date: 1981-02-03
Filing date: 1982-02-02
Publication date: 1984-01-17
Also published as: FR2498951A1; FR2498951B1

Abstract

Abstract of the Disclosure Forming a film by spraying onto a heated substrate an atomized solution containing the appropriate salt of a constituent element of the film and an agent in sufficient amount to change the oxidation state of at least one solute element of the spray solution after contacting the heated substrate.
The film thus formed may be used in photovoltaic devices or in the formation of a solar-heat absorbing panel.

Description

Background of the Invention The invention relates to spray pyrolysis, which involves forming a film by spraying onto a heated substrate an atomized solution containing the appropriate salts of the constituent element (e.g., Te) of the film compound.
The chemical reaction occurs upon spraying on the heated substrate, and the non-constituent elements of the salts are removed by volatilization along with the Solvent, typically water. For example, United States Patent No. 3,148,084 discloses, along with other examples, the formation of CdS by spray pyrolysis according to the following equation:

CdC12 + H2NC(=S)NH2 + CdS with dispersed Cu +
Cu+~ -~ volatile products The resistivity of the film is then lowered by a post-deposition annealing step.
Before the conception of the invention claimed herein, Messrs. Steven A. Lis and Harvey B. Serreze proposed using H2 in the ambient gas and/or spray gas to act as a reducing agent to convert impurities to a volatile form and to also allow the use of Te in a different oxidation state in the starting material than it is in the film. Our attempts to implement that proposal were unsuccess-ful.
Summary of the Invention We have discovered that by adding an agent directly to the spray solu-tion we could cause a variety of desirable oxidation/reduction reactions to oc-cur when the spray solution contacted the heated substrate.
In accordance with the present invention, there is provided in the process of making a thin film comprising preparing a first solution with first solute elements including a salt of a first constituent element of said film and spraying said solution onto a heated substrate to form a first film on said sub-strate, the solute and solvent elements not constituting said film forming vola-tile reaction products after contacting said heated substrate, the improvement wherein said solution also includes an agent in sufficient amount to change the oxidation state of at least one said solute element after contacting said heated substrate.
The invention also further comprises attaching external electrical connections to said substrate to form a photovoltaic device.
Furthermore the invention provides after said spraying of said first solution onto said substrate, spraying a second solution with second solute ele-ments including a third salt different from said salt of a first constituen~
element and an agent in sufficient amount to change at least one said second solute element after contacting said first film on said heated substrate, to result in a multilayered structure with a second film of composition different from said first film.
In preferred embodiments the oxidation/reduction reaction involves reducing a constituent element of the film; this constituent element is telluri-um, and the film is cadmium telluride; the salts used in the spray solution are ~NH4)2TeO4 and Cd(OH)2; the substrate is maintained at a temperature between 325 and 550C (preferably between 370 and 425C); inert spray and ambient gasses are used; a dopant ("dopant" being used herein in the sense defined at p. 372 of Kittel, Introduction to Solid State Physics, 4th ed., John Wiley ~ Sons) material is also placed in the spray solution, and it changes in oxidation state after contacting the heated substrate; and the dopant is copper. In another preferred embodiment the film is ZnCdS with an indium dopant, and the dopant is also added to the spray solution and changes in oxidation state after contacting the heated substrate. In other preferred embodiments the oxidation/reduction reaction in-volves converting an impurity to a volatile form, and in yet another preferred embodiment, more of one constituent element of a film is used than is necessary 116~515 to react with another constituent element of the film, and the excess causes an alteration in the conductivity of the film. Further, in preferred embodiments, the reducing agent is a highly soluble (i.e., greater than 1 M) organic acid (preferably formic or acetic acid); the solubility of these acîds in the solvent allows increasing the strength of reducing agent to levels allowing the forma-tion of products of increased purity. Moreover, in preferred embodiments, the concentration of the reducing agent is much greater than a stoichiometric amount (i.e., greater than 10 times and preferably greater than 900 times the stoichio-metric amount), unexpectedly resulting in the formation of a high purity film.
A preferred application for films made according to the invention is used in photovoltaic devices, and a most preferred use is a multilayer device having a graphite covered CdTe layer on a CdZnS layer on an indium tin oxide coated substrate. Another highly preferred application is a solar-heat absorb-ing panel with an A1203-Ag film, the oxidation/reduction reaction being the re-duction of the Ag to the metallic, heat-absorbing state.
The invention allows using a starting material in a different oxidation state than it is in the film, and this is particularly advantageous when it is desired to have a film element in a state that is highly insoluble in the spray solution. Also, the production of high purity films is made possible by convert-ing impurities to volatile form so that they do not become part of the film.
Dopants can be easily included in the film to alter its electrical conductivity without the requirement of a post-deposition annealing step, and the film's electrical characteristics can also be changed by using an amount of one consti-tuent element that is greater than the amount necessary to react with another constituent element. Finally, the invention permits the use of spray pyrolysis to produce a wide variety of films including preparing CdTe, CdZnS, Ag, Cr, i2Te3, ZnTe) HgxCdl_xTe, ZnxCdl_xS, CdS, CdSe, ZnS, ZnSe, GaAs, Ga All As, 1 1605~5 InP, BN, Ni-Co-Cr alloys, Zn3P2, and ZnSnP2 films.
Preferred Embodiments The preferred embodiments of the invention will now be described.
Drawings Figure 1 is an elevation of a photovoltaic device made according to the invention.
Figure 2 is a diagrammatic fragmentary vertical sectional view of a solar-heat absorbing panel made according to the invention.
Figure 3 is a diagrammatic representation of the spraying apparatus and heating means useful for making the devices of Figures 1 and 2 according to the invention.
Structure and Apparatus Referring to the embodiment of Figure 1, the~e is shown photovoltaic device 10 comprising glass substrate 12, layer of indium tin oxide 14 (approxi-ma~ely 1000 Angstroms thick) thereon, layer 16 of sprayed cadmium zinc sulfide (1000 to 2000 Angstroms thick~ thereon, layer 17 of sprayed cadmium telluride (2500 to 5000 Angstroms thick) thereon, layer 18 of Aquadag (a dried graphite-in-water suspension) thereon, wire 20 attached to indium tin oxide layer 14 and wire 24 connected to the Aquadag by silver paint patch 22.
In the embodiment of Figure 2 there is shown solar-heat absorbing panel 25 comprising glass substrate 26 and layer 28 ~1000-2000 Angstroms thick) of A1203-Ag cermet thereon. Dots 29 represent regions of metallic silver dis-persed throughout the A1203.
Referring to Figure 3, there is shown spraying and heating apparatus 30 having air-tight enclosure 32. Spray nozzle 34 (1/4 J with a 1050 SS fluid cap available from Spraying Systems Company~ is supplied by solution bottle 36 and source of nitrogen spray gas 38. The gas flow is regulated by valve 40, and l 1 60515 the pressure is monitored from gauge 42. Substrates 44 (e.g., substrate 12 and layer 14 or substrate 26) are heated by heater 46 through tin bath 48. Nozzle 34 is located 12" above substrates 44. External means controls the temperature of heater 46, and the heater is mounted within enclosure 32 on insulating block 50. Access to the heater is provided by pass-through 52 (shown diagrammatical-ly), and valve 54 regulates supply of nitrogen purge gas through the interior of box 32. Heat shields 56 are used tG shield the solution and nozzle control apparatus 34, 36, 40 from high temperatures and the vapors from the heating apparatus below it.
Manufacture _, In making the photovoltaic device of Figure 1, films 16 and 17 are sprayed onto indium tin oxide coated glass (substrate 12 and layer 14 of the Figure 1 device, available from Pittsburg Plate Glass Inc.) using the Figure 3 apparatus. First spray solution I ror layer 16 and spray solution II for layer 17 are prepared by making aqueous solutions including the ingredients set forth in the following table at the indicated concentrations.
Table 1 Solution I

Concentration in Ingredient Solution (M) CdC12.2.5 H2O 0.0167 ZnC12 0.0083 Thiourea 0.025 In(OH)3 0.00025 Formic Acid 9.62 l 16~515 Solution II
Concentration in Ingredient Cd~OH)2 0.021 ( 4)2 e4 0.085 Cu++ 0.00064 Formic Acid 9.62 The cadmium hydroxide is prepared by dissolving 20 gm of reagent grade Cd~N03)2 in 100 ml distilled H20 and *itrating this with a saturated NaOH solu-tion to complete precipitation of Cd(OH)2. The precipitate is filtered, washed several times with H20 and dried at 80C for 16 hours.
The copper stock solution is prepared by etching copper shot (99% pure, available from Fisher Scientific Co), dissolving it in a slight excess of con-centrated nitric acid, and diluting it to 0.05 M.
All chemicals are reagent grade and obtained from Fisher Scientific Company, except for In(OH)3, which is ultra pure and obtained from the Alpha Division of Ventron Corp., Danvers, MA, and the (NH4)2TeO4, which is obtained inpowdered form from Ventron.
In spite of the fact that a large amount of reducing agent is used, the Te in Solution II remains in the +6 oxidation state, allowing a higher con-centration of Te than would be possible if Te were in the highly insoluble -2 state.
The spray chamber within box 32 is purged using nitrogen gas. The volume of the chamber is displaced three times with the nitrogen, and then the chamber is flushed continuously for five minutes more. The indium tin oxide coated glass substrates are cleaned by vapor degreasing with reagent grade tri-chloroethylene, and placed On the heated molten tin bath 48 and brought to a - 116~351S

temperature between 325 and 550CJ (preferably between 370 and ~2~C, and most preferably of 400C). Thirty-five ml of solution I is sprayed onto the heated substrates with nozzle 34, the spray gas flowing at 2.0 ml/min at a pressure of 10 to 12 psi, the liquid being pumped from bottle 30 by siphoning. The spray is discontinued by stopping the fluid flow from container 36. This results in the formation of layer 16 of ZnCdS on top of layer 14, the following reaction occur-ring on the heated substrate:

CdCl2 + ZnCl2 + ZnCdS doped with NC~=S)NH2 + -- ~ In + volatile reaction (1 In(OH)3 + reducing products agent The indium is apparently reduced from the +3 state to metallic indium upon spraying on the substrate, and this metallic dopant makes the layer more conduc-tive without the necessity of a post-deposition annealing step.
Immediately following the spraying of solution I, 200 ml of solution II is sprayed onto substrates 44 under the same spray and temperature conditions, the following reaction occurring on the heated substrate:

Cd(OH)2 + (NH4)2TeO4 CdTe doped with Cu +
+ Cu++ + ~ volatile reaction (2) Reducing agent products The tellurium is reduced to the -2 state, allowing it to combine with the cadmi-um. The Cu is apparently reduced to metallic copper, thereby making the layer more conductive. Again, a post-deposition annealing step is not required.
The presence of the reducing agent in high concentration in both solu-tions I and II also induces removal of impurities by converting them to volatile reaction products, and nitrogen, an inert element, is used as both the spray and ambient gas to a~oid the extraneous introduction of impurities from the at-mosphere.
The substrates are removed from the heater within three minutes after spraying of solution II. The substrates are cooled in the purged atmosphere for l 1 60515 five minutes, and then removed from the chamber through pass-through 52.
The Aquadag-E suspension (25% graphite in water; available from Acheson Golloids Company) is applied to the upper surface of the sprayed sub-strates with a wood applicator and dried at room temperature. Silver paint 25 is applied to the surface of the Aquadag-E layer 18, and copper wire lead 24 is attached to the paint 25 for external electrical connection. The silver paint should be dried for several hours at room temperature. A second copper lead 20 is coTmected directly to the indium tin oxide 12 using indium solder after scraping off some of layers 16 and 17.
The cadmium concentration in the solution II is greater than that necessary to react with the tellurium because cadmium has been found to be more volatile than the tellurium during spraying onto the heated substrate.
Formic acid is a particularly useful reducing agent, as it is highly soluble in aqueous solutions, and this allows increasing the concentration of the reducing agent to the high concentrations that have been found necessary to obtain the desired, virtually complete oxidation/reduction reactions occurring on the heated substrate surface. Acetic acid is a highly preferred alternative.
In general, it is preferred that the reducing agent be a highly-water-soluble organic acid.
The formic acid concentration should be greater than 1 M and also greater than ten times the stoichiometric amolmt to cause the desired oxidation/
reduction reactions to occur upon contacting the heated substrate.
The A1203-Ag cermet of Figure 2 is made by the same procedure as the Figure 1 device by spraying solution III (Table 2~ onto glass substrate 26, us-ing the same spray, temperature, and nozzle conditions described above.

1 1 6t~515 Table 2 Solution III
Concentration in Ingredients Solution (M) Al(NO3)3.9H2O 0.003 AgNO3 0.0015 Formic acid 9.62 Once again the chemicals are all reagent grade and are available from Fisher Sientific Company.
Examples of other solutions which can be sprayed with the Figure 3 apparatus are presented in the following table:
Table 3 Concentration in Solution Ingredients Solution (M) IV Bi(C2H3O2)3 0.01 (N 4)2Te4 0.0150 Formic acid 9.62 V Zn(N3)2 0.21 ( 4)2 e ~ 0.21 Formic acid 9.62 VI AgNO3 0.1 Formic acid 9.62 VII Cr(NO3)3 0.1 Formic acid 9.62 The reactions appear to be as follows:

2 Bi(C2H3O2)3 + Bi2Te3 + volatile

3(NH4)2Teo4 + reaction products (4) reducing agent Zn(N3)2 ~ (NH4)2TeO4 ~ ZnTe ~ volatile + Formic acid reaction products (5) l 16~S15 AgNO3 + Formic acid ~ Ag -~ volatile reaction products (6) Cr(NO3)3 ~ Formic acid ~, Cr -~ volatile reaction products (7) A constituent element may itself be used as a source of metallic dopant by using an excess of the material in the spray. For example, the Cd portion of CdTe can be increased as is described in the following equation:

excess cd(OH)2 + -- ~ Cdl.olTe +
(NH4)2TeO4 volatile reaction (8) products In the above reaction, in order to maintain an excess of Cd in the film, a much larger excess of Cd salt must be used in the spray solution.
In all of the above examples, the presence of a reducing agent in a high concentration also serves to convert impurities to a volatile form.
Other Embodiments Other embodiments are within the following claims. For example the principles outlined above will find application in using an oxidizing agent when it is desired to have a film element in a higher oxidation state in the film than in the starting material. Also, other dopants will work. The solvent need not be water; methanol is a preferred alternative.
Finally, the principles outlined above will apply to making a large number of other films, e.g., HgXCdl xTe, ZnxCdl xS, CdS, CdSe, ZnS, ZnSe, GaAs, GaxAll xAs, InP, Bn, Ni-Co-Cr alloys, Zn3P2, and ZnSnP2.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the process of making a thin film comprising preparing a first solution with first solute elements including a salt of a first constituent element of said film and spraying said solution onto a heated substrate to form a first film on said substrate, the solute and solvent elements not constituting said film forming volatile reaction products after contacting said heated substrate, the improvement wherein said solution also includes an agent in sufficient amount to change the oxidation state of at least one said solute element after contacting said heated substrate.

2. The process of claim 1 wherein said one solute element is a con-stituent element of said film.

3. The process of claim 1 wherein said agent is a reducing agent.

4. The process of claim 3 wherein said one solute element is said first constituent element and is tellurium, said solute element further comprising a salt of a second constituent element, said second constituent element being cadmium.

5. The process of claim 4 wherein said salt of said first constituent element is (NH4)2TeO4 and said salt of said second constituent element is Cd(OH)2.

6. The process of claim 1 wherein said substrate is maintained at a temperature between 325 and 550°C.

7. The process of claim 6 wherein said substrate is maintained at a temperature between 370 and

8. The process of claim 2 wherein said film comprises CdTe, Ag, Cr, Bi2Te3, CdZnS or ZnTe.

9. The process of claim 1 wherein said spraying step comprises spraying with an inert gas in a chamber having an ambient gas that is inert.

10. The process of claim 1 wherein said one solute element is a dopant said dopant changing in oxidation state after contacting said heated substrate.

11. The process of claim 10, wherein said dopant is In or Cu.

12. The improvement of claim 10 wherein said film is CdZnS and said dopant is In.

13. The process of claim 4 wherein another said solute element is a dopant, said dopant changing in oxidation state after contacting said heated substrate, said dopant being Cu.

14. The process of claim 1 wherein said one solute element is an im-purity that converts to a volatile form with said change in oxidation state.

15. The process of claim 1 wherein said film has a second constituent element, said first constituent element being greater in amount than necessary to react with said second constituent element, the excess of said one con-stituent element causing an alteration in conductivity of said film.

16. The process according to claim 4 which further comprises attaching external electrical connections to said substate to form a photovoltaic device.

17. The process of claim 16 wherein said substrate is glass coated with indium tin oxide.

18. The process of claim 1 further comprising, after said spraying of said first solution onto said substrate, spraying a second solution with second solute elements including a third salt different from said salt of a first constituent element and an agent in sufficient amount to change at least one said second solute element after contacting said first film on said heated substrate, to result in a multilayered structure with a second film of com-position different from said first film.

19. The process of claim 18 wherein said first film is ZnCdS.

20. The process of claim 19 wherein said second film is CdTe.

21. The process of claim 20 wherein said third salt is (NH4)TeO4.

22. The process of claim 21 wherein said first solution includes a first dopant which changes in oxidation state after contacting said heated substrate.

23. The process of claim 22 wherein said second solution includes a second dopant which changes in oxidation state after contacting said first film on said heated substrate.

24. The process of claim 23 wherein said first dopant is In and said second dopant is Cu.

25. The process according to claim 18 further comprising attaching a first external electrical connection to said first film and a second external electrical connection to said second film to form a photovoltaic device.

26. The process according to claim 20 further comprising attaching a first external electrical connection to said first film and a second external electrical connection to said second film to form a photo-voltaic device.

27. The process according to claim 24 further comprising attaching a first external electrical connection to said first film and a second external electrical connection to said second film to form a photovoltaic device.

28. The process according to claim 18 further comprising applying a layer of graphite-in-water suspension on top of said second film, allowing said suspension to dry, attaching a first electrical connection to said first film, and attaching a second electrical connection to said dried layer of graphite suspension to form a photovoltaic device, said substrate having a layer of indium tin oxide on the surface on which said first solution is sprayed.

29. The process according to claim 20 further comprising applying a layer of graphite-in-water suspension on top of said second film, allowing said suspension to dry, attaching a first electrical connection to said first film, and attaching a second electrical connection to said dried layer of graphite suspension to form a photovoltaic device, said substrate having a layer of indium tin oxide on the surface on which said first solution is sprayed.

30. The process according to claim 24 further comprising applying a layer of graphite-in-water suspension on top of said second film, allowing said suspension to dry, attaching a first electrical connection to said first film, and 14 .

attaching a second electrical connection to said dried layer of graphite suspension to form a photovoltaic device, said substrate having a layer of indium tin oxide on the surface on which said first solution is sprayed.

31. The process of claim 3 wherein said film is A12O3 and said one solute element is Ag.

32. The process of claim 1 wherein said film comprises HgXCd1-xTe, ZnxCD1-xS, CdS, CdSe, ZnS, ZnSe, GaAs, GaxAl1 As, InP, BN, Ni-Co-Cr alloys, Zn3P2, or ZnSnP2.

33. The process of claim 1 wherein said agent is an organic acid with a solubility greater than 1 M in the solvent.

34. The process of claim 33 wherein said organic acid is formic acid.

The process of claim 33 wherein said organic acid is acetic acid.

36. The process of claim 3 wherein the concentration of said reducing agent is greater than 1 M and greater than 10 times the stoichiometric amount necessary to react with said one solute element.

37. The process of claim 36 wherein the concentration of said reducing agent is greater than 900 times the stoichiometric amount.

38. The process of claim 36 wherein said one solute element is a constituent element of said film.

39. The process of claim 36 wherein said one solute element is said first constituent element and is tellurium, said solute elements comprising a soluble inorganic compound of a second constituent element, said second constituent element being cadmium.

40. The process of claim 36 wherein said salt of said first constituent element is (NH4)2TeO4 and said soluble inorganic compound of said second constituent is Cd(OH)2.

41. The process of claim 36 wherein another said solute element is a dopant, said dopant changing in oxidation state after contacting said heated substrate.

42. The process of claim 36 wherein said one solute element is an im-purity that converts to a volatile form with said change in oxidation sate.