Description

The present invention relates to a cyanide-free, pyrophosphate-free and phosphonic acid-free electrolyte and a process for the electrolytic deposition of a ternary alloy of the elements copper, tin and zinc. The electrolyte and the process are characterized in that not only zinc (II) ions and stannate anions but also copper ions are present in the electrolyte used. The copper and zinc ions are present in a particular molar ratio relative to one another.

The electrolytic deposition of brass (Cu-Zn alloy) and bronzes (Cu-Sn alloy) on consumer goods or decorative goods has been known for a long time. They serve, inter alia, as a replacement for nickel-containing finishing layers and are, for example, applied inexpensively to appropriate substrates in electrochemical barrel or rack plating processes.

In the production of brass and bronze layers for the electronics industry, the solderability of the resulting layer and possibly its mechanical strength are the critical properties of the layer to be produced. The appearance of the layers is generally less important than their functionality for use in this field. On the other hand, the decorative effect and also durability of the layer with an ideally unchanged appearance are the important target parameters in the production of bronze or brass layers on consumer goods.

In the production of brass or bronze layers, known processes are not only the conventional processes using cyanide-containing and thus highly toxic, alkaline baths but also various electrochemical processes which can mostly be assigned according to the composition of their electrolytes to one of two main groups found in the prior art: processes using electrolytes based on organosulphonic acid or processes using baths based on disphosphoric acid (pyrophosphoric acid) or phosphonic acid.

Cyanide-free electrolytic baths for the deposition of brass layers may be found, for example, in EP 790332. There, not only the copper and zinc, which can be added as pyrophosphate salts to the electrolyte, but also metal polyphosphates are added to the electrolyte. Possible metal polyphosphates are pyrophosphate salts of sodium, potassium, magnesium or calcium. The deposition of copper-zinc-tin layers is not described here.

EP 1 146148 describes a cyanide-free copper-tin electrolyte which contains the reaction product of an amine and an epichlorohydrin in a molar ratio of 1 :1 and also a cationic surfactant. The amine can be hexamethylene tetramine. JP 10102278 and US 6416571 describe baths for the deposition of copper-tin alloys. Cyanide-free electrolytic baths for the deposition of bronze layers are likewise adequately known. Thus, for example, WO 2009109271 reports that copper and tin can be deposited together from appropriate baths which have a large excess of pyrophosphate ions. All these teachings disclose exclusively the deposition of bronzes, i.e. copper-tin alloys.

The deposition of a ternary alloy consisting of copper, tin and zinc from a cyanide-free electrolyte is disclosed, for example, in EP 21 16634. There, not only a high concentration of pyrophosphate anions in the electrolyte but also a specific reaction product of hexamethylene tetramines and epichlorohydrin at a virtually neutral pH of the electrolyte are used. US20010014407 mentions in passing the deposition of a ternary alloy of Cu/Sn/Zn on copper surfaces as corrosion protection. Relatively low-tin alloys are obtained from the pyrophosphate-containing electrolyte.

US20100147696 discloses the deposition of Cu-Zn-Sn alloy from electrolytes containing phosphonic acid. The deposition processes described here give white coatings which are, however, relatively low in zinc.

A cyanide-free electrolyte for the deposition of ternary copper-zinc-tin alloys is described in Thin Solid Films, 517 (2009) 251 1 -2514. Here, a layer which is not defined in more detail is deposited from an alkaline electrolyte containing the metals copper in the oxidation state +2, zinc in the oxidation state +2 and tin in the oxidation state +4. The electrolyte described here is said to contain a ten-fold excess of tin and lead only to low-copper deposits.

EP 2037006 describes the electrolytic deposition of copper-tin-zinc alloys in a very particular atom ratio. The layers deposited have a composition which is said to be close to the formula Cu2ZnSn. The layers obtained in this way serve as base layer for the production of kesterite (CZTS or Cu2ZnSn(S,Se)4) which is a promising material for the production of photovoltaically active molecules (Solar Energy Materials & Solar Cells 201 1 , 95, 2136-2140; Chemical Physics Letters 201 1 , 501 , 619-622). In a production process for such modules, a correspondingly produced Cu2ZnSn layer is subsequently converted by reaction with sulphur or sulphur-containing compounds at elevated temperatures into the corresponding kesterite phase (e.g.: Thin Solid Films 2009, 517, 2465-2468). Such a procedure is likewise addressed in EP 2037006. There, the specific electrolytically produced Cu2ZnSn deposits are obtained from an electrolyte to which particular disubstituted benzene derivatives have been added. The copper and zinc ions can be added as pyrophosphates to the electrolyte. The tin is preferably used as stannate.

The electrolytes described for the deposition of the ternary alloy of copper, tin and zinc obviously all have only a low ability to deposit a desired ternary alloy composition of this type when, for example, specific additional additives are not added to the electrolyte or extremely high tin-IV concentrations are not present in the electrolyte. The alkaline bath-stabilizing additives such as pyrophosphates, phosphonic acid or cyanides are also frequently used. The additional complexity caused thereby makes the production and processing of the electrolytes unattractive.

It was therefore an object of the present invention to provide an electrolyte and a corresponding process for the deposition of a ternary alloy of copper, tin and zinc, which are able to bring about the indicated deposition with a preferred stochiometry in a very optimal way. The electrolyte should ideally have a simple composition and be sufficiently stable. The process and the electrolyte according to the invention should also be superior to the processes and electrolytes known from the prior art from ecological and economic points of view.

These objects and further objects which can be derived in an obvious manner from the prior art by a person skilled in the art are achieved by an electrolyte having the features of the present Claim 1 and by a corresponding process according to Claim 9. Preferred embodiments of the present invention may be found in the claims dependent on these two claims.

The use of an aqueous, cyanide-free, pyrophosphate-free and phosphonic acid-free, basic electrolyte in which the proportion of copper in the alloy is in the range from 38 to 44% by weight, the proportion of tin is in the range from 34 to 42% by weight and the proportion of zinc is in the range from 16 to 26% by weight, which contains the metals copper and zinc to be deposited in ionically dissolved form and tin as water-soluble stannate salt and where the electrolyte has a molar ratio of copper ions to zinc ions in the range from 1 :1 to 1 :10, for the deposition of a ternary alloy leads, extremely surprisingly but no less advantageously, to achievement of the stated object. It has been found that the target alloy composition can also be achieved using the electrolyte described here when no stabilizing cyanides, pyrophosphates or phosphonic acid derivates are present in the electrolyte. The electrolyte is stable, despite expectations. Any turbidity occurring will be redissolved with the addition of the water-soluble stannate salt and according to the invention it is possible to produce an appropriate deposit. The electrolyte described here is further noteworthy for the fact that obviously no further substances which influence the deposition of the ternary alloy have to be added to the electrolyte in order to bring about an appropriately composed deposition of copper-zinc-tin. In particular, the addition of reaction products of amines with epichlorohydrin as proposed in EP 21 16634 (mentioned at the outset) and the addition of disubstituted benzene derivatives known from EP 2037006 can be dispensed with. The composition of the ternary alloy of copper, tin and zinc can obviously be controlled in a simple manner via the abovementioned features alone, as long as Sn is simultaneously present as Sn ⁴⁺ . This has not yet been proposed in the prior art.

In a preferred embodiment of the present invention, the concentration of tin ions in the electrolyte is in a particular ratio relative to the copper ions present. It has proven advantageous for the molar ratio of Cu to Sn ions in the electrolyte to be 1 :2 to 1 :6, preferably 1 :3 to 1 :5 and particularly preferably around 1 :4.

According to the invention, the metals copper and zinc are present in ionically dissolved form in the present electrolyte. The copper can be added in the form of divalent copper salts to the electrolyte. Zinc will be present in the form of 2-valent ions in the electrolyte. The molar ratio according to the invention of copper ions to zinc ions is preferably in the range from 1 :1 to 1 :6. Very particular preference is given to a value of about 1 :2. The tin is added as stannate salt, i.e. in the 4-valent form, to the electrolyte. Such stannate salts are well known to those skilled in the art. Particularly suitable stannate salts here are, for example, sodium stannate and potassium stannate. The ratios of the concentrations of copper and zinc ions relative to one another are critical in determining the composition of the alloy deposited. It is naturally also advantageous for the tin used to be present in an appropriate ratio to the copper and zinc ions in total. The molar ratio of stannate salt used to the sum of copper and zinc ions should be 1 :1 - 5:1 , preferably 1 .5:1 - 3:1 and particularly preferably from 2:1 to 2.5:1 in each case based on the metals. As regards the metals copper and zinc to be deposited, which are, as indicated, present in ionically dissolved form in the electrolyte, and the tin which is present in dissolved form as stannate, the concentration ranges of the metal in the electrolyte can be selected by a person skilled in the art. It has been found to be advantageous for the ion concentration of copper to be in the range from 0.1 to 5 g/L of electrolyte, the concentration of tin to be in the range from 0.5 to 20 g/L of electrolyte and the ion concentration of zinc to be in the range from 0.2 to 15 g/L of electrolyte. The concentration of copper is particularly preferably 0.3 - 2 g/L, very preferably 0.5 - 1 .0 g/L. The concentration of zinc is particularly preferably 0.3 - 5 g/L, very preferably 0.5 - 2.0 g/L. The concentration of tin is particularly preferably 2 - 10 g/L, very preferably 3.5 - 7.0 g/L. In a particularly advantageous embodiment, use is made of an electrolyte in which:

Copper is present in a concentration of 0.5 - 1.0 g/L,

Zinc is present in a concentration of 1.0 - 2.0 g/L,

Tin is present in a concentration of 3.5 - 7.0 g/L, and

in each case based on the metal.

As indicated above, the copper and zinc ions are present in dissolved form in the electrolyte. As compounds of these metals to be deposited which are soluble in water under the reaction conditions indicated, it is possible to employ compounds selected from the group consisting of, carbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides, oxide-hydroxides, oxides and combinations thereof. Preference is given to using carbonate, hydrogencarbonates or sulphates. Very particular preference is given to the addition in the form of sulphate salt in this context. The electrolyte is operated in the slightly to strongly alkaline range. The pH of the electrolyte is preferably in the range from 8 to 13, more preferably from 8.5 to 12 and very particularly preferably from 9 to 1 1 .5. The pH of the electrolyte according to the invention is especially preferably about 10-1 1. A person skilled in the art will know how to set such pH values in the electrolyte using appropriate buffer substances. Preferred buffer substances are salts of weak organic or inorganic acids selected from the group consisting of phosphoric acid and citric acid.

Further additives (brighteners, wetting agents) selected from the group consisting of monocarboxylic and dicarboxylic acids, alkanesulphonic acids, betaines and aromatic nitro compounds can be added to the electrolyte. Such additives are adequately known for the present type of baths, in particular in the field of deposition of brass or bronze. As regards the amount and the materials used, reference may be made to the literature. Such additives are particularly preferably selected from the group consisting of oxalic acid, tartaric acid, citric acid and salts thereof.

The present invention likewise provides a process for the electrolytic deposition of Cu- Zn-Sn alloy layers, in which the substrate to be coated is dipped as cathode into an electrolyte according to the invention and a flow of current is established between the anode and the cathode. It goes without saying that the embodiments of the electrolyte mentioned as preferred are analogously likewise preferred for the process.

It is advantageous for the proportion of copper in the ternary alloy deposited to be in the range from 38 to 44% by weight, the proportion of tin to be in the range from 34 to 42% by weight and the proportion of zinc to be in the range from 16 to 26% by weight. Preference is given to alloys containing 39 - 42% by weight of Cu, very preferably about 40 - 41 % by weight. Preference is also given to alloys containing 19 - 23% by weight of Zn, very preferably about 21 % by weight. Preference is likewise given to alloys containing 36 - 40% by weight of Sn, very preferably about 38% by weight. The sum of the alloy constituents should be 100% by weight. The alloy deposited should have a thickness of 0.4 - 5 μηι, preferably 0.5 - 3 μηι and very particularly preferably of 1 - 2 μπι.

It may be pointed out that the alloy composition can likewise change with the temperature prevailing during the electrolysis. The electrolysis is therefore carried out in the range from 20 to 90°C, preferably from 30 to 60°C and very preferably about 45°C. The composition of the ternary alloy of copper, tin and zinc can likewise change with the current density set during the electrolysis. It is advantageous to set a current density in the range from 0.1 to 5 ampere per square decimetre. The current density is preferably from 0.2 to 1 .0 ampere per square centimetre, very preferably from 0.3 to 0.8 ampere per square centimetre. As anode, it is possible to use any electrode which comes into question for this purpose to a person skilled in the art. Preference is given to using insoluble anodes (e.g. platinated titanium anodes or mixed metal oxide anodes). In this context, soluble anodes composed of a material selected from the group consisting of electrolytic copper, phosphorus-containing copper, tin, tin-copper alloy, zinc-copper alloy and zinc- tin-copper alloy or combinations of these anodes are likewise advantageous.

It is known from EP 2037006 cited at the outset that, inter alia, copper-tin-zinc alloys having a particular composition are suitable for the production of photovoltaic cells. As regards the production of thin film solar cells, reference may be made to the information given there and in EP 2336394. As regards the production of the solar cells, reference may likewise be made to the following literature:

Solar Energy Materials & Solar Cells, 95 (201 1 ) 2136-2140;

Chemical Physics Letters, 501 (201 1 ) 619-622;

Solar Energy Materials & Solar Cells, 93 (2009) 996-999;

Thin Solid Films, 517 (2009) 2465-2468;

Phys. Stat. Sol., 9 (2008) 1772-1778;

Phys. Stat. Sol., 5 (2009) 1266-1268;

Thin Solid Films, 517 (2009) 251 1 -2514.

The present invention therefore likewise provides a process for producing a thin film solar cell having a p-type absorption layer based on a CuxZnySnzSaSeb compound, where x = 1.5 - 2.5, y = 0.9 - 1 .5, z = 0.5 - 1 .1 , a = 0 - 4.2 and b = 0 - 4.2 and x + y + z and a + b are each about 4 (±0.2), wherein a CuxZnySnz alloy is produced by a process according to the invention and this layer is subsequently reacted with sulphur, a sulphur compound and/or a selenium compound in such a way that the corresponding compound is formed.

The alloy composition achieved by means of electrolysis preferably very closely approximates that corresponding to the alloy base material in the material kesterite (Cu2ZnSnS4). The layer produced by the process of the invention very preferably consists of a composition close to the formula Cu2ZnSn. From this, the desired compound Cu2ZnSn(SeS)4 (CZTS) is produced by action of sulphur, selenium and/or appropriate compounds using appropriate processes as discussed in the literature.

In the light of the prior art, it was not possible to see that a process configured like the present process with the electrolyte according to the invention makes it possible to produce corresponding ternary alloy compositions by electrolytic deposition in such a simple way. In particular, it is surprising that the presence of particular ratios of copper and zinc ions relative to one another are responsible for this. In view of this, it appears to be particularly advantageous that the simple setting of the appropriate ratios obviously makes it possible to set the alloy composition deposited in an advantageous way. In particular, it is not necessary to add stabilizing substances such as cyanides, phosphonic acid derivatives or pyrophosphoric acids or disubstituted benzene derivatives and reaction products of amines with epichlorohydrin to the alkaline electrolyte in order to produce an advantageous alloy composition. It appears that the stannate salt itself serves as an adequate stabilizer for preventing zinc or copper compounds from precipitating out in the alkaline range. This was not to be expected in the light of the background of the known prior art.

Examples:

General procedure:

The ingredients mentioned below are dissolved in water and set to the appropriate pH. Any initial turbidity in the electrolyte disappears again after addition of the stannates. An electrolysis is subsequently carried out under the conditions indicated (45°C, pH=1 1 , 0.5 A/dm ² ).

Example

90 g/l K ₂ HP0 ₄

15 g/l Di-K- Oxalate

1 .5 g/I Na ₂ S0 ₃

0.5 g/l Cu (as Cu sulphate solution) 0.0078 mol

1 .0 g/l Zn (as Zn sulphate solution) 0.015 mol

+3.5 g/l Sn (as Na stannate) 0.03 mol

Adjust to pH 1 1.0 with KOH

45°C

0.5 A/dm ²

The result is a ternary copper-tin-zinc alloy from a stable electrolyte.