FR2498951A1 - Formation of thin films by spray pyrolysis - from solns. contg. film element salts and a reducer in large excess and at high concn. - Google Patents

Abstract

In the formation of a film by spraying onto a heated substrate an atomised soln. contg. a salt of a constituent element of the film and a reducing agent, the concn. of the reducing agent is greater than 1M and greater than 10X stoichiometric, pref. greater than 900X stoichiometric. Used esp. in deposition of CdTe films, but also in deposition of films of HgCdTe, Z CdS, CdS, CdSe, ZnS, ZnSe, GaAs, GaAlAs, InP, BN, NiCoCr, ZnP, ZnSnP, etc. Large excess of reducer at high concn. results in formation of films of high purity.

Description

Method for manufacturing a thin layer
The present invention relates to spray pyrolysis, which consists in forming a thin layer by spraying onto a heated substrate an atomized solution containing the appropriate salts of the constituent elements (for example Te) of the layer compound. The chemical reaction occurs due to spraying on the heated substrate, and the elements of the salts which are not constituents of the layer are removed by evaporation together with the solvent, which is typically water. For example, US Pat. No. 3,148,084 describes, among other examples, the formation of CdS by means of spray pyrolysis, in accordance with the following equation CdC12 + H2NC (= S) NH2 + # CdS with Cu in dispersion +
Cu ++> volatile products
The resistivity of the layer is then lowered by an annealing operation after spite.

Prior to the conception of the invention, Steven
A. Lis and Harvey B. Serreze proposed using 82 in ambient gas and / or spray gas to act as a reducing agent, to convert the impurities to a volatile form, and also to allow the use of Te in a different oxidation state in the starting material and in the layer. The attempts made by the inventors to implement such a suggestion have been unsuccessful.

 The inventors have covered that by adding an agent directly to the spray solution, various desirable oxidation / reduction reactions could be caused when the spray solution comes into contact with the heated substrate. Consequently, a characteristic of the invention consists in that the solution also contains an agent in an amount sufficient to change the oxidation state of at least one of the dissolved elements, after contact with the heated substrate.

Advantageous embodiments have the following characteristics: the reduction oxidation reaction involves the reduction of a component of the layer; this constituent element is tellurium, and the layer consists of cadmium telluride; the salts used in the spray solution are for example (NH4) 2TeO4 and Cd (OH) 2; the substrate is maintained at a temperature between 325 and 5500C (preferably between 370 and 4250C); inert gases are used for the spraying gas and the ambient gas a doping agent is also introduced into the spraying solution (the expression "doping agent" is used here in the sense which is defined on page 372 of the Kittel's work entitled Introduction to
Solid State Physics, 4th Edition, John Wiley & Sons "), and this doping agent changes its oxidation state after other contact with the heated substrate; and the doping agent is copper. In another mode embodiment, the layer consists of ZnCdS with a doping agent constituted by indium, and the doping agent is also added to the spraying solution and it changes oxidation state after coming into contact with the heated substrate. In still other embodiments, the oxidation / reduction reaction involves the conversion of an impurity to a volatile form, and in yet another embodiment, an amount of an element is used. constitutive of a layer which is greater than the amount necessary to govern with another constituent element of the layer, and the excess amount produces a modification of the conductivity of the layer. In addition, in embodiments, the reducing agent is an organic acid (preferably of formic acid or acetic acid) highly soluble (that is to say with a solubility greater than 1 M); and the solubility of these acids in the solvent makes it possible to increase the strength of the reducing agent up to levels which allow the formation of products of increased purity. Furthermore, in preferred embodiments, the concentration of the reducing agent is much more than 10 times and preferably more than 900 times the stoichiometric amount), which unexpectedly leads to the formation of a large layer. purity.

 An advantageous application of the layers manufactured in accordance with the invention consists in the use in photovoltaic devices, and a particularly advantageous use relates to a multilayer device comprising a layer of CdTe covered with graphite, on a layer of CdZnS placed in itself on a substrate coated with tin-indium oxide. Another very advantageous application relates to a solar heat absorption panel which comprises a layer of A1203-Ag, the oxidation / reduction reaction consisting in the reduction of silver to the metallic state, in which it absorbs heat.

The invention also makes it possible to use a starting material in an oxidation state different from the oxidation state in the layer, and this is particularly advantageous when it is desired to have in the layer an Oldmeut in a state which is strongly insoluble in spray solution. It also becomes possible to produce high purity layers by converting impurities into volatile form, so that they are not incorporated into the layer. It is easy to include doping agents in the layer to modify its electrical conductivity, without requiring an annealing operation after deposition, and it is also possible to modify the electrical characteristics of the layer by using an amount of a constituent element which is spiked to the amount necessary to react with the other constituent element. Finally, the invention allows spray pyrolysis to be used to produce a wide variety of layers, including the preparation of layers of the following materials:
CdTe, CdZnS, Ag, Cr, Bi2Te3, ZnTe, HgxCd1 ~ xTe, Zn # Cdî # 1S,
CdS, CdSe, ZnS, ZnSe, GaAs, GaxAll ~ x As, InP, BN, alloys
Ni-Co-Cr, Zn3P2, and ZnSnP2.

The invention will be better understood on reading the following description of embodiments, given without limitation. The following description refers to the accompanying drawings in which
Figure 1 is an elevational representation of a photovoltaic device manufactured in accordance with the invention.

 Figure 2 is a schematic vertical and partial section of a solar heat absorption panel manufactured in accordance with the invention.

 Figure 3 is a schematic representation of a spraying apparatus and heating means usable for manufacturing the devices of Figures 1 and 2, in accordance with the invention.

Structure and device
We will now consider the embodiment of FIG. 1 in which we see a photovoltaic device 10 which comprises a glass substrate 12, a layer of tin-indium oxide 14 (with a thickness of approximately 100 nm) on the substrate, a layer 16 of pulverized zinc-cadmium sulfide (100 to 200 nm thick) on the layer 12, a layer 17 of pulverized cadmium telluride (250 to 500 nm thick) on the layer 16, a layer 18 of Aquadag (dried suspension of graphite in water) on the layer 17, a metal wire 20 fixed to the layer of tinindium o.vvde 14 and a metal wire 24 connected to the 'Aquadag with a coat of silver paint 22.

 In the embodiment which is represented in FIG. 2, there is seen a solar heat absorption panel, 25, which comprises a glass substrate 26 on which is a layer 28 (100 to 200 nm thick) into an A1203-Ag cermet. Dots 29 represent regions of metallic silver dispersed throughout the volume of Au203.

We will now consider FIG. 3 in which we see a spraying and heating device 30 which includes a sealed enclosure 32. A spraying nozzle 34 (1/4 J with a cap for fluid 1050 SS from the firm
Spraying Systems Company) is supplied by a bottle of solution 36 and by a source of spray gas 38, consisting of nitrogen. The gas flow is regulated by a valve 40 and the pressure is controlled by a pressure gauge 42. The substrates 44 (for example the substrate 12 and the layer 14 or the substrate 26) are heated by a heating element 46, via of a tin bath 48. The nozzle 34 is located 30 cm above the substrates 44. the external means control the temperature of the heating element 46 and the latter is mounted in the enclosure 32 on an insulating block 50. Access to the heating element is provided by a passage 52 (shown diagrammatically), and a valve 54 regulates the supply of purge gas, consisting of nitrogen, which circulates inside the enclosure 32. uses heat shields 56 to protect the solution, the control gear and the nozzle, 34, 36, 40, from high temperatures and vapors from the heater below.

Manufacturing
To manufacture the photovoltaic device of FIG. 1, the layers 16 and 17 are sprayed on the glass coated with tin-indium oxide (the substrate 12 and the layer 14 of the device of FIG. 1, which can be supplied by the Pittsburg Plate Glass-Inc.) in of Figure 3. A spray solution I is first prepared for layer 16 and a spray solution II for layer 17, by making aqueous solutions comprising the ingredients indicated in the following table, with the concentrations indicated.

TABLE 1
Solution I
Ingredient Concentration in
solution solution tM)
CdCl2.2.5 H20 0.0167
ZnC12 0.0083
Thio-urea 0.025
In (OH) 3 0,00025
Formic acid 9.62
Solution II
Ingredient Concentration in
solution solution (M)
Cd (OH) 2 0.021
(NH4) 2Te 4 0.085
Cu ++ 0,00064
Formic acid 9.62
Cadmium hydroxide is prepared by dissolving 20 g of Cd (NO3) 2 of reactive quality in 100 cm3 of distilled water and- by titrating this solution with a solution of
Saturated NaOH, up to 1 complete precipitation of Cd (OH) 2. The precipitate is filtered, washed several times with water and dried at 8000 for 16 hours.

 The starting solution for copper is prepared by attacking copper shot (99% purity, supplied by the company Fisher Scientific Co.), by dissolving the product obtained in a slight excess of concentrated nitric acid, and diluting it to O, G5 M.

 All chemicals are reactive grade and are supplied by Fisher Scientific Comparu, with the exception of In (OH) 3, which is ultra-pure and is supplied by Ventron Corp., Alpha Division, Danvers , MA, EUA, and (NH4) 2TeO4 which is supplied in powder form by the firm Ventron.

Despite the fact that a large amount of reducing agent is used, the Te in the solution It remains in the +6 oxidation state, which allows a concentration of
Te higher than would be possible if the Te was in the ~ 2 state, very insoluble.

The spray chamber is purged inside the enclosure 32 using nitrogen. The volume of the chamber is evacuated three times with nitrogen, then purged continuously for five more minutes. The glass substrates re-sr8tus of indium tin oxide are cleaned by vapor degreasing using reagent grade trichlor4thylene, they are placed on the heated and molten tin bath, 48, and they are brought to a temperature between 325 and 55000 (preferably between 370 and 4250C, the temperature of 40,000 being advantageous). 35 cm3 of solution I is sprayed on the heated substrates, with the nozzle 34, the spray gas circulating at 2.0 cm3 / min under a pressure of 0.70 to 0.84 bar, and the liquid being pumped from the bottle 36 by siphoning. The spraying is stopped by stopping the circulation of the fluid from the bottle 36. This leads to the formation of the layer 16 in ZnCdS on the layer 14, and the reaction which occurs on the heated substrate is as follows CdC12 + ZnCl2 + ZnCdS doped with In +
H2NC <= S) NH2 +> reaction products (1)
In (OH) 3 + Volatile reducing agent
Indium is apparently reduced from the +3 state to metallic indium when it is sprayed onto the substrate, and this metallic doping agent makes the layer more conductive, without it being necessary to carry out an annealing operation after deposit.

Immediately following the spraying of solution I, 200 cm3 of solution II are sprayed on the substrates 44, under the same spraying and temperature conditions, and the following reaction occurs on the heated substrates Cd (OH) 2 + (NH4) 2TeO4 CdTe doped with Cu + + Or +> reaction products (2) volatile reducing agent
Tellurium is reduced to the -2 state, which allows ++ to combine it with cadmium. the Ou is apparently reduced to metallic copper, which makes the layer more conductive.

 Here again, an annealing operation after deposition is not necessary.

 The presence of the reducing agent in high concentration in the two solutions I and II also collects a removal of the impurities, converting them into volatile reaction products, and nitrogen, that is to say an inert element, is used. both as a spray gas and as an ambient gas, to avoid the parasitic introduction of impurities from the atmosphere.

 The substrates are removed from the heating element after a period not exceeding three minutes after spraying solution II. The substrates are cooled in the purged atmosphere for five minutes, then extracted from the chamber through passage 52.

A suspension of Aquadag-E (25% graphite in water; applied by the firm Acheson Colloids) is applied to the upper surface of the substrates subjected to the spraying.
Company), using a wooden applicator, and dried at room temperature. The silver paint 22 is applied to the surface of the Aquadag-E layer 18, and the copper wire 24 is fixed to the paint 22 to establish an external electrical connection. The silver paint should be dried for several hours at room temperature. A second copper conductor 20 is directly connected to the tin-indium oxide 14, using an indium solder, after having stripped part of the layers 16 and 17. The concentration of cadmium in the solution It is higher to what is necessary to react with tellurium, since it has been found that cadmium is more volatile than tellurium during spraying on the heated substrate.

 Formic acid is a particularly useful reducing agent since it is highly soluble in aqueous solutions, and this makes it possible to increase the concentration of the reducing agent, up to the high concentrations which have been found necessary to obtain the desired substantially complete oxidation / reduction reactions on the surface of the heated substrate. Acetic acid is a very advantageous alternative. In general, it is preferable that the reducing agent is an organic acid highly soluble in water.

 The concentration of formic acid must be greater than 1 M and it must also be greater than ten times the stoichiometric amount for the desired oxidation / reduction reactions to occur upon contact with the heated substrate.

 The Al203-Ag cermet of FIG. 2 is manufactured by the same procedure as for the device of FIG. 1, by spraying a solution III (table 2) on the glass substrate 26, using the same spraying and temperature conditions. and the same characteristics of the nozzle as in the previous description.

TABLE 2
Solution III
Ingredients Concentration in
solution (M) Al (NO3) 3.9H20 0.003 AgNO3 0.0015
Formic acid 9.62
Here again, all the chemicals are of reactive quality and are supplied by the firm Fisher
Scientific Company.

The following table shows examples of other solutions that can be sprayed with the device in Figure 3:
TABLE 3
Solution Ingredients Concentration in solution (M)
IV Bi (C2H302) 3 0.01
(NH4) 2TeO4 0.0150
Formic acid 9.62
V Zn (N3) 2 0.21
(NH4) 2Te04 0.21
Formic acid 9.62
VI AgNO3 0.1
Formic acid 9.62
VIII Cr (N03) 3 0.1
Formic acid 9.62
The reactions appear to be as follows 2 Bi (C2H302) 3 + Bi2Te3 + products 3 (NH4) 2TeO4 +> volatile reaction (4)
Reducing agent Zn (N3) 2 + (NH4) 2Te04 4 ZnTe + products of (5) + formic acid volatile reactions
AgNO3 + formic acid> AgO + products of
volatile reaction (6) Cr (NO3) + formic acid o CrO + products
volatile reactions (7)
A constituent element itself can be used as a source of metallic doping agent, using an excess of this element in the spray. For example, we can increase the Cd part of CdTe, as described by the following equation:
Excess Cd (OH) 2 + Cd1,01Te + (NH4) 2Te04 - * reaction products (8)
volatile
In the above reaction, to maintain an excess of Cd in the layer, 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 with a high concentration also has the function of converting the impurities into a volatile form.

Other embodiments
Other embodiments fall within the scope of the invention. For example, the principles set out above can be applied to the use of an oxidizing agent, when it is desired that an element of the layer is in a higher oxidation state in the layer than in the material departure. In addition, other doping agents can be used.

 The solvent need not be water; methanol is an interesting alternative.

 Finally, the principles set out above apply to the manufacture of a large number of other layers, such as for example HgxCd1 ~ xTe, ZnxCdi # xS, CdS, CdSe, ZnS, ZnSe, GaAs, GaxAl1 xAs, InP, BN, Ni-Co-Cr alloys, 2n3P2 and ZnSnP2.

 It goes without saying that many other modifications can be made to the method and to the device described and shown, without departing from the scope of the invention.

Claims (35)

 1. Method for manufacturing a thin layer (16, 28) in which a first solution is prepared with first dissolved elements comprising a salt of a first component of the layer, and this solution is sprayed onto a substrate heated (12, 14; 26) to form a first layer on this substrate, the dissolved elements and the solvent elements not forming part of the layer forming volatile reaction products after coming into contact with the heated substrate, characterized in that that the solution also contains an agent in an amount sufficient to change the oxidation state of at least one of the dissolved elements, after contact with the heated substrate.  2. Method according to claim 1, characterized in that said dissolved element is a constituent element of the layer.  3. Method according to claim 1, characterized in that said agent is a reducing agent.  4. Method according to claim 3, characterized in that said dissolved element is the first constituent element and it consists of tellurium, the dissolved elements also comprises a salt of a second constituent element and this second constituent element is cadmium.  5. Method according to claim 4, characterized in that the salt of the first constituent element is (lGI4) 2TeO4 and the salt of the second constituent element is Cd (OH) 2.  6. Method according to claim 1, characterized in that the substrate (12, 14; 26) is maintained at a temperature between 325 and 5500C.  7. Method according to claim 6, characterized in that the substrate is maintained at a temperature between 370 and 4250C.  8. Method according to claim 2, characterized in that the layer consists of CdTe, Ag, Cr, Bi2Te3, CdZnS or ZnTe.  9. Method according to claim 1, characterized in that the spraying operation comprises spraying an inert gas into a chamber (32) which contains an ambient gas which is inert.  10. The method of claim 1, characterized in that said dissolved element is a doping agent and this doping agent changes oxidation state after coming into contact with the heated substrate.  11. Method according to claim 10, characterized in that the doping agent is In or Cu.  12. Method according to claim 10, characterized in that the layer consists of CdZnS and the doping agent consists of In.  13. Method according to claim 4, characterized in that another dissolved element is a doping agent this doping agent changes its oxidation state after coming into contact with the heated substrate, and this doping agent is Cu.  14. Method according to claim 1, characterized in that said dissolved element is an impurity which is converted into a volatile form with the change of oxidation state.  15. Method according to claim t, characterized in that the layer comprises a second component, the first component is in an amount greater than what is necessary to react with the second component, and the excess of the first component produced a change in the conductivity of the layer.  16. A method of manufacturing a photovoltaic device, characterized in that a spray layer is formed on a substrate, in accordance with the method of claim 4, and external electrical connections are fixed to this layer.  17. The method of claim 16, characterized in that the substrate consists of glass (12) coated with tin-indium oxide (14).  18. The method of claim 1, characterized in that, after spraying the first solution on the substrate, spraying a second solution containing dissolved elements which comprise a third salt different from the salt of a first constituent element, and an agent in quarx  if, ffis ## te to modify at least one of the second dissolved elements, after contact with the first layer (16) being on the heated substrate, in order to give a multilayer structure which comprises a second layer (17) of composition different from that of the first layer (16).  19. Method according to claim 1 #, characterized in that the first layer (16) consists of ZnCdS.  20. Method according to claim 19, characterized in that the second layer (17) consists of CdTe.  21 Method according to claim 2O, characterized in that the third salt consists of (NH4) TeO4.  22. Method according to claim 21, characterized in that the first solution comprises a first doping agent which changes the oxidation state after having come into contact with the heated substrate.  23. The method of claim 22, characterized in that the second solution comprises a second doping agent which changes the oxidation state after coming into contact with the first layer (16) sia the heated substrate.  24. The method according to claim 23, characterized in that the first doping agent consists of In and the second doping agent consists of Or.  25. A method of manufacturing a photovoltaic device, characterized in that a multilayer structure (16, 17) is prepared according to the method of any one of claims 18, 20 or 24, and a first external electrical connection is fixed on the first layer and a second external electrical connection on the second layer.  26. A method of manufacturing a photovoltaic device, characterized in that a multilayer structure (16, 17) is prepared according to the method of any one of claims 18, 20 or 24, the substrate comprising an oxide layer tin-indium (14) on the surface of which the first solution is sprayed; a layer (18) of graphite suspension in water is applied to the second layer; the suspension is allowed to dry; a first electrical connection is fixed on the first layer; and a second electrical connection (24) is fixed to the dried layer of graphite suspension.  27. The method of claim 3, characterized in that the layer (28) consists of Ail203 and said dissolved element consists of Ag.  28. Method according to claim 1, characterized in that the layer consists of Hg x Cd l-x Te, CdS, CdSe, ZnS, ZnSe, GaAs, GaxAli # xAs, InP, BN, Ni-Co-Cr alloys, Zn3P2, ZnSnP2 or ZnxCdi # xS.  29. Method according to claim 1, characterized in that said agent is an organic acid having a solubility greater than 1 M in the solvent, such as formic acid or acetic acid.  30. The method of claim 3, characterized in that the concentration of the reducing agent is greater than 1 M and greater than 10 times the stoichiometric quantity necessary to react with said dissolved element and advantageously greater s #oeE - # ': fots * - metric.  31. Method according to claim 30, characterized in that said dissolved element is a constituent element of the layer.  32. Method according to claim 30, characterized in that said dissolved element is the first constituent element and consists of tellurium, the dissolved elements comprise a soluble inorganic compound of a second constituent element, and this second constituent element is cadmium.  33. Method according to claim 30, characterized in that the salt of the first constituent element is (NH4) 2 TeO4 and the soluble inorganic compound of the second building block is Cd (OH) 2.  34. Method according to claim 30, characterized in that another dissolved element is a doping agent which changes the oxidation state after coming into contact with the heated substrate.  35. The method of claim 30, characterized in that said dissolved element is an impurity which is converted into a volatile form under the effect of the change of oxidation state.