DE102010030884A1 - Depositing a buffer layer on a copper indium selenide/sulfide (CIS) absorber layer of a CIS-thin layer solar cell comprises providing a solar cell slug, rinsing and treating it with an aqueous solution of a sulfur-donating substance - Google Patents

Abstract

Depositing a buffer layer having at least one sulfide of cadmium, indium, magnesium, zinc or tin on a copper indium selenide/sulfide (CIS) absorber layer comprises: (a) providing a solar cell slug; (b) treating the solar cell slug with an aqueous alkaline adsorber solution, at least one salt, a metal comprising cadmium, indium, magnesium, tin or zinc; (c) rinsing the solar cell slug with an aqueous alkaline rinsing solution; (d) treating the solar cell slug with an aqueous solution of a sulfur-donating substance; and (e) optionally multiple cyclic repeating of the steps (b)-(d). Depositing a buffer layer having at least one sulfide of cadmium, indium, magnesium, zinc or tin on a copper indium selenide/sulfide (CIS) absorber layer of a CIS-thin layer solar cell comprises providing a solar cell slug having a substrate deposited on a CIS-absorber layer; treating the solar cell slug at 30-90[deg] C with an aqueous alkaline adsorber solution, at least one salt, a metal comprising cadmium, indium, magnesium, tin or zinc of concentration 1-50 mmol/l; rinsing the solar cell slug with an aqueous alkaline rinsing solution; treating the solar cell slug at 30-90[deg] C with an aqueous solution of a sulfur-donating substance having a concentration of 30-120 mmol/l; and optionally multiple cyclic repeating of the steps (b)-(d) until full development of the buffer layer. An independent claim is also included for the CIS thin layer solar cell prepared by the above method.

Description

The invention relates to a method for depositing a buffer layer on a CIS thin-film solar cell and a CIS thin-film solar cell produced by the method.

Prior art and background of the invention

Although photovoltaics as a market has grown almost exponentially in recent years, its contribution to primary energy consumption worldwide is still less than 2%. One way to further increase this percentage and reduce the cost of solar power generation to the level of traditional, non-renewable electricity generation technologies is to use one of the three currently production-ready thin-film technologies instead of the usual crystalline silicon cells: aSi (amorphous Silicon), CdTe (cadmium telluride) or CIS (copper indium selenide / sulfide). However, the production of chalcopyrite cells (CIS solar cells) still needs some improvement in order to meet the requirements for large-scale and cost-effective cell production.

A CIS thin-film solar cell (or chalcopyrite thin-film solar cell) consists of the on a support material (glass, metal or plastic) applied semiconductor, also called absorber, quite comparable to the "wafer" in crystalline silicon cells. Silicon wafers are known to become silicon solar cells by creating a pn junction by diffusing certain elements. Analogously, photoelectrically active solar cells are formed from the CIS absorber layers of thin-film technology by applying a so-called buffer layer (buffer layer). It is understood that in this case, analogous to the doping of silicon wafers, the essential process does not consist in the deposition of the n-type buffer layer on the surface of the p-type absorber, but in the diffusion of dopants from the buffer layer into the semiconductor , In addition, since buffer layers tend to be extremely thin - in chalcopyrite cells 30-50 nanometers - occasionally, instead of a layer deposition, also spoken of a "dip doping".

The typical composition of a buffer layer consists of a mixture of metal hydroxide, metal hydroxysulfide and metal sulfide, whereby by appropriate variation of the deposition parameters (eg by increasing the temperature of the reaction solution from 20 ° C to 60 ° C), the buffer layer is provided with a Gradients, starting with the metal hydroxide directly on the absorber surface and ending with the sulfide as a contact to the subsequent layer, usually the window layer (also called TCO "transmissive conductive oxide"). Cadmium (Cd), indium (In), zinc (Zn) or tin (Sn) is optionally used as the metal of the buffer layer.

It becomes clear that the mechanism for depositing the buffer layer is very complex and requires the use of wet chemistry. The properties of the generated buffer layer also determine the efficiency of the entire solar cell, as well as the underlying semiconductor material.

Specifically, in chalcopyrite solar cells, the buffer layer deposition is almost uniform worldwide by a method which is abbreviated as CdS-CBD (cadmium sulfide - chemical bath deposition). The classical buffer layer deposition process proceeds in principle as follows:
An aqueous, ammoniacal solution of a cadmium salt, for example cadmium acetate, is mixed with a likewise alkaline, aqueous solution of a sulfur carrier, for example of thiourea. A container is filled with as high a surface / volume ratio as possible with chalcopyrite absorber cells and with the aforementioned mixture of liquids and heated to about 60 ° C. over a period of about 10 minutes. During this period, metal hydroxide, then metal hydroxysulfide and finally metal sulfide are deposited on the absorber but also on all other surfaces in a temperature-controlled surface reaction. In parallel, a volume reaction takes place in the reaction solution of the classical CBD, which leads to the formation of metal sulfide particles in the solution ("flocculation") and thus to their consumption. After the process, the liquid mixture must therefore be discarded or processed, it is not about multiple times usable.

The material yield of the described CdS-CBD process is usually less than 10%, ie over 90% of the toxic heavy metal cadmium must be disposed of as waste. Typically, the amount of unusable reaction solution is about 15 l / m ² , which results in a solar cell production of 100 MWp / a 22.8 million liters of heavy metal-containing liquid per year, not counting the amounts of water or solvent to clean the cell backs or for regular cleaning of the vessel walls.

Furthermore, because of the "wear" of the reaction liquid and the required temperature rise during the course of the process, the described classical CdS-CBD process can only be made poorly suitable for the production of ribbon cells, ie for cells whose absorbers are based on endless metal or plastic Foil tapes is deposited.

As far as a classic buffer layer deposition on tapes roll-to-roll process to take place, the CIS band is guided in an oblique groove upwards, the bottom of the channel is heated to increasing from 20 to 60 ° C, while the CIS tape hereby is in thermal contact and the already mixed reaction liquid flows towards it ("channel process"). The consumption amounts are here with 50 l / m ² Cd-containing liquid, thus again considerably higher than with the stationary CdS CBD. Since the process does not take place in a defined volume of liquid, a spatially resolved control of the CBD reaction process is almost impossible in the channel process.

• – Die Chalkopyrit-Zelle, fertig prozessiert inklusive Absorberschicht (CIS-Halbleiter), wird durch Tauchen oder Besprühen mit einer Metallsalzlösung benetzt (Schritt 1, Adsorption).
• – Die feuchte Zelle wird unter Schutzgas getrocknet (Schritt 2, Trocknung).
• – Die Schwefelverbindung entsteht, indem die in Schritt 2 getrocknete Zelle einige Sekunden bei erhöhter Temperatur einer Schwefelwasserstoff-Atmosphäre ausgesetzt wird (Schritt 3, Sulfurisierung).

In principle much better suitable for tape and with much less pollutant attack works the so-called ILGAR method (Ion Layer Gas Reaction):

• - The chalcopyrite cell, finished processed including absorber layer (CIS semiconductor), is wetted by dipping or spraying with a metal salt solution (step 1, adsorption).
• - The moist cell is dried under inert gas (step 2, drying).
• The sulfur compound is formed by exposing the cell dried in step 2 to a hydrogen sulfide atmosphere at elevated temperature for a few seconds (step 3, sulfurization).

Since the resulting thickness of the metal sulfide layer is only a few nm, the described three-stage process for producing the buffer layer in CIS cells must be repeated up to 20 times, which limits the applicability in the large-scale production of CIS cells. Apart from that, the ILGAR process, in particular in its spray ILGAR variant, can certainly be regarded as suitable for use as a tape.

It is advantageous that no mixing of the cation- and sulfur-containing reaction component takes place. The enormous wastewater volumes of the classic CBD at ILGAR are therefore eliminated.

Another advantage of the ILGAR is that instead of Cd compounds mainly other, less toxic metals, eg. As indium or zinc, sulfurized or oxidized. Understandably, there are efforts in the CIS cell production basically to dispense with cadmium in the buffer layer. Although the amounts used in the modules are negligibly small, the layer thickness is only about 50 nm.

Apart from the disadvantage of numerous process cycles, the use of hydrogen sulfide, because of its toxicity, also disagrees with the use of ILGAR as a large-scale method for depositing the buffer layer on chalcopyrite cells.

For the deposition of thin metal sulfide compounds, the so-called SILAR process (Successive Ionic Layer Adsorption and Reaction) has recently been proposed. Like ILGAR, this process is carried out in three stages, which are usually run about 20 times, but for the sulfur reaction in step 3, no gas, but an aqueous solution of the sulfur carrier is used.

• – Schritt 1 (Adhäsion) entspricht weitgehend dem Schritt 1 des ILGAR, indem der fertige Absorber einer CIS-Zelle in eine wässrige Lösung eines Metallsalzes (z. B. Cd oder Zn-Acetat) getaucht wird. Das Bad 1 ist üblicherweise stark alkalisch eingestellt (pH 10 bis 12) und wird bei ca. 50°C betrieben. Die übliche Verweildauer im Bad liegt dabei bei mehr als 5 min, die bekannt gewordene Konzentration von Cd-Salz bei 12,5 mmol/l.
• – Schritt 2 besteht in einer gründlichen Spülung der Probe, wobei in aller Regel destilliertes Wasser Verwendung findet. Der Grund für eine Spülung ist, anders als die Trocknung in Schritt 2 des ILGAR, alle Metallverbindungen von der Zelle abzuwaschen, die nicht durch Adsorption fest und gleichmäßig verteilt mit der Absorber-Oberfläche verbunden sind.
• – Schritt 3 des SILAR besteht im Eintauchen der Probe in eine wässrige Lösung eines Schwefelträgers, üblicherweise Natriumsulfid. Die übliche Konzentration ist hierbei 50 mmol/l, die übliche Bad-Temperatur 30°C, der pH-Wert 5 und die übliche Verweildauer mehr als 10 min.

In contrast to the "classical CBD procedure", there are only a limited number of publications and no practical reports for the SILAR procedure, so that there can currently be no talk of a "SILAR standard procedure". As far as previously published, the following procedure for buffer layer deposition should usually be marked with "SILAR":

• Step 1 (adhesion) is broadly similar to step 1 of the ILGAR in that the final absorber of a CIS cell is immersed in an aqueous solution of a metal salt (eg, Cd or Zn acetate). The bath 1 is usually highly alkaline (pH 10 to 12) and is operated at about 50 ° C. The usual residence time in the bath is more than 5 minutes, the known concentration of Cd salt at 12.5 mmol / l.
• - Step 2 consists in a thorough rinsing of the sample, distilled water is usually used. The reason for a rinse, unlike the drying in step 2 of the ILGAR, is to wash off any metal compounds that are not firmly and evenly distributed to the absorber surface by adsorption.
• Step 3 of the SILAR consists of immersing the sample in an aqueous solution of a sulfur carrier, usually sodium sulfide. The usual concentration here is 50 mmol / l, the usual bath temperature 30 ° C, the pH 5 and the usual residence time more than 10 min.

• – Durch die Trennung der reaktiven Komponenten ist die Nutzungsdauer der Flüssigkeitsbäder nicht kurzzeitlich begrenzt, es fallen also, im Gegensatz zum klassischen CBD-Verfahren keine großen Mengen verbrauchter, cadmiumhaltiger Reaktionsflüssigkeit an.
• – Die Parameter (Temperatur, Konzentration, pH-Wert, etc.) der aufeinander folgenden Zyklen können unterschiedlich eingestellt werden, so dass in Anlehnung an den Reaktionsverlauf beim klassischen CBD zunächst Metallhydroxid, dann Metallhydroxysulfid und schließlich Metallsulfid auf dem durchlaufenden Zellen-Band abgeschieden/erzeugt wird.

The SILAR process has two advantages:

• - Due to the separation of the reactive components, the useful life of the liquid baths is not limited in the short term, so it fall, in contrast to the classical CBD process no large amounts of spent, cadmium-containing reaction liquid.
• - The parameters (temperature, concentration, pH, etc.) of the successive cycles can be set differently, so that based on the course of reaction in the classical CBD first metal hydroxide, then metal hydroxysulfide and finally metal sulfide deposited / generated on the continuous cell band.

However, as with the ILGAR, the three process steps mentioned in SILAR are repeated until a sufficiently thick layer of the metal sulfide has formed. With the aforementioned concentrations and temperatures, as described in the literature, a total residence time of about 20 minutes in the baths and about 20 cycles is needed. At belt speeds of 2 m / min, this would require an installation of 800 m length. It should be noted that it is not the thickness but homogeneity and phase purity of the deposited metal sulfide layer which determines the suitability of the layer produced as a buffer layer of a CIS solar cell, ie. H. in particular the influence of the buffer layer on the photoelectric efficiency of the solar cell produced.

Summary of the invention

The inventive method for depositing a buffer layer on a CIS thin-film solar cell attempts to overcome or at least mitigate one or more of the existing problems of the prior art. In particular, the number of cycles should be kept as low as possible compared to the described SILAR method with a view to a tape suitability of the process. However, the properties of the buffer layer should not be degraded.

• a) Bereitstellen eines Solarzellenrohlings mit einer auf einem Substrat aufgebrachten CIS-Absorber-Schicht;
• b) Behandeln des Solarzellenrohlings bei 30°C bis 90°C mit einer wässrigen, alkalischen Adsorberlösung, die wenigstens ein Salz eines Metalls aus der Gruppe Cd, In, Mg, Sn und Zn mit einer Konzentration im Bereich von 1 bis 50 mmol/l enthält;
• c) Spülen des Solarzellenrohlings mit einer wässrigen, alkalischen Spüllösung;
• d) Behandeln des Solarzellenrohlings bei 30°C bis 90°C mit einer wässrigen Lösung einer schwefelspendenden Substanz mit einer Konzentration im Bereich von 30 bis 120 mmol/l; und
• e) gegebenenfalls mehrfaches zyklisches Wiederholen der Abfolge der Schritte b) bis d) bis sich die Pufferschicht vollständig ausgebildet hat.

A first aspect of the invention is to provide a method for depositing a buffer layer comprising one or more sulfides from the group of the metals Cd, In, Mg, Sn and Zn, on a CIS absorber layer of a CIS thin-film solar cell, contains. The method comprises the following steps:

• a) providing a solar cell blank with a CIS absorber layer applied to a substrate;
• b) treating the solar cell blank at 30 ° C to 90 ° C with an aqueous, alkaline adsorber solution containing at least one salt of a metal from the group Cd, In, Mg, Sn and Zn in a concentration in the range of 1 to 50 mmol / l contains;
• c) rinsing the solar cell blank with an aqueous, alkaline rinse solution;
• d) treating the solar cell blank at 30 ° C to 90 ° C with an aqueous solution of a sulfur-donating substance having a concentration in the range of 30 to 120 mmol / l; and
• e) optionally multiple cyclic repeating the sequence of steps b) to d) until the buffer layer has completely formed.

Accordingly, the method according to the invention initially provides a conventional provision of a solar cell blank coated with the CIS absorber in step a). This blank is preferably a continuous belt coated with the CIS absorber layer. In other words, the method according to the invention is particularly suitable for the buffer deposition on tapes in a roll-to-roll process.

In step b), the provided solar cell blank is brought into contact with an aqueous alkaline adsorber solution. At temperatures of 30-90 ° C, in particular 50-90 ° C, and concentrations of the metal salts in the range of 1-50 mmol / l, in particular 10-30 mmol / l, a deposition of metal salts takes place at the surface of the CIS absorber Layer mainly by adsorption. According to a preferred variant, the adsorbent solution from step b) additionally contains a wetting agent, in particular a surfactant, in order to achieve the most homogeneous possible deposition. The solar cell blank can be dipped directly into a bath of the aqueous, alkaline adsorber solution in step b) or it is applied to the CIS absorber layer by means of a spray or mist process. Optionally, the solar cell blank can be heated before or during step b). The adsorbent solution preferably contains metal salts selected from a group comprising formates, acetates, citrates, acetonates, carbamates, p-toluenesulfonates and tartrates.

Subsequent to step b), in step c) is washed with an aqueous, alkaline rinse solution instead of the otherwise usual distilled water. As a result, the formation of metal hydroxides can be prevented, which are much more difficult to convert into their sulfides in the subsequent process step. In particular, the rinse solution is an aqueous ammoniacal solution, i. H. the rinse solution from step b) contains ammonia.

In step d), the sulfurization of the previously applied metal is carried out by bringing it into contact with an aqueous solution of a sulfur-donating substance. The reaction is carried out at 30-90 ° C, in particular 50-90 ° C, and a concentration of the sulfur-donating substance in the range of 30-120 mmol / l, in particular 60-120 mmol / l. Both temperature and concentration are therefore clearly above the comparable SILAR process. Preferably, in step d) a treatment with ultrasound, in particular in the megahertz area, takes place at the same time.

The sulfur-donating substances are preferably 2-mercaptobenzothiazole, 3-mercaptobenzimidazole, 3-mercapto-1,2-propanediol, thioacetamide, polysulfides, thiophene derivatives, diisobutylsulfide, ditetrabutylsulfide, dichlorobenzenethiol, Diethyldithiocarbamat, thiocyanate, thioglycolic acid, thiocarbamide or thiourea used.

The steps b) -d) are repeated cyclically sequentially until a buffer layer of the desired thickness and quality has formed.

• – das Metall im ersten Zyklus oder den ersten Zyklen überwiegend als Hydroxid abgeschieden wird;
• – das Metall im sich anschließenden Zyklus oder in den sich anschließenden Zyklen überwiegend als Hydroxysulfid abgeschieden wird; und
• – das Metall im abschließenden Zyklus oder den abschließenden Zyklen überwiegend als Sulfid abgeschieden wird.

According to a preferred variant of the method, at least 3 cycles of the sequence of steps b) to d) are traversed, wherein the process parameters in the individual cycles are set such that

• - The metal is predominantly deposited as hydroxide in the first cycle or the first cycles;
• - The metal is deposited in the subsequent cycle or in the subsequent cycles predominantly as hydroxysulfide; and
• - The metal is predominantly deposited as a sulfide in the final cycle or cycles.

According to the aforementioned process variant, it is therefore provided that initially the reaction conditions are adjusted so that mainly metal hydroxide is deposited. As the number of cycles progresses, the equilibrium is shifted toward the formation of predominantly metal hydroxides by parameter control. Finally, in the last or last cycles, the reaction conditions are predetermined so that predominantly metal sulfide precipitates. It should be noted that a temperature increase as well as an increase in concentration leads to an increase in the sulfide separation. Furthermore, with increasing pH, the reaction can also be shifted to the sulfide. Finally, as a further control parameter for the deposition, the residence time in the individual cycles is available, whereby the equilibrium with the metal sulfides is usually shifted with increasing residence time. Accordingly, according to the preferred variant of the method, at least 3 cycles are provided, which together lead to a very homogeneous buffer layer. According to the first investigations, the process is even superior to buffer layers deposited by conventional alternative methods. According to the procedure according to the invention, the number of cycles is less than 10, preferably limited to 3.

According to a further preferred variant of the method, the solar cell blank is moved in the bath during the performance of step b). In particular, the solar cell blank in step b) and / or step d) is guided horizontally downwards on the surface of the adsorber solution or of the solution of the sulfur-donating substance with the CIS absorber layer. The adsorber solution or thiourea solution is flowed from below against the CIS absorber layer and at the same time the top of the solar cell blank is kept dry by blowing air through it.

Due to the uniform bath and / or sample movement in step b) of the process, the adhesion process can be accelerated and thus a very uniform structure can be forced on the entire CIS absorber surface. The special process control in steps b) and / or d) ensures that a particularly uniform coating is formed on the underside of the belt, but on the other hand an undesired coating on the upper side of the belt is avoided.

A further aspect of the invention is the provision of a CIS thin-film solar cell, which was produced by the previously described method.

The invention will be explained in more detail with reference to an embodiment and the accompanying drawings. The figures show:

Brief description of the figures

1 A schematic representation of the structure of a solar cell; and

2 - An illustration of the process sequence of the invention.

Detailed description of the invention

Solar cell blanks 4 , consisting of sections 35 × 100 mm ² stainless steel foil of 0.15 mm thickness, which were first in a high vacuum with a molybdenum layer and subsequently coated with copper, indium, gallium and selenium ("precursor"), then undergo an annealing process. The blanks with the overlying, dark gray, about 2 microns thick CIGS semiconductor layer ("absorber") were provided immediately after their production for the wet-chemical process described below.

The construction of a solar cell blank 4 is exemplary in 1 illustrated. The blank has a buffer layer 1 , a window layer 2a (usually ZnO (Al)), an intrinsic window layer 2 B , a grid 3 (usually Ni / Al), a chalcogenic precursor layer 4a (S, Se), a metallic precursor layer 4b (Cu, In, Ga), a back contact 5 (usually Mo (PVD)), a matching layer 6a (Cr), a barrier layer 6b (Ti), a first carrier material 7a (ES) and a second carrier material 7b on.

The solar cell blanks 4 were immersed in an aqueous solution of cadmium acetate with the constant movement of the samples and the reaction liquid, which was adjusted by the addition of ammonia to a strongly basic pH value and a small amount of a commercially available surfactant was added. In the first cycle, the concentration was kept at 10 mmol / l, the pH at 9 and the temperature constant at 50 ° C, the treatment time was 1.5 min ( 2 : first bath 1.1 ).

The sample was then rinsed thoroughly in mild ammonia, deionized water at 30 ° C. ( 2 : second bath 2.1 ).

Subsequently, the sample was mounted horizontally in a fixture mechanically connected to a 12.5 MHz ultrasonic generator. At the same time, the top of the cell was blown over the entire surface with oil-free compressed air. Then, the sample was placed on the surface of the liquid and the liquid flowed from below laminar. The reaction liquid consisted of an aqueous solution of thiourea whose pH had been basified by adding ammonia. In the first cycle, the pH was 8, the concentration was 60 mmol / l, the temperature of the bath was controlled to 50 ° C constant, the residence time was 3 minutes ( 2 : third bath 3.1 ).

Then, the sample was thoroughly rinsed in deionized water and dried by blowing with inert gas.

The samples were then subjected to a second and third cycle as described above with the temperature, pH and residence time respectively increased and extended, respectively ( 2 : Bath 1.2 ).

Finally, an examination of the generated buffer layer was carried out with laboratory-standard agents. For this purpose, layer thicknesses between 40 and 50 nm were determined by RFA. In fact, however, these tests on the layer do not say much about the fitness of the generated buffer layer or, better, the generated p / n junction. The buffer-coated blanks were therefore processed as usual to the complete solar cell finished by applying a TCO window layer by magnetron sputtering and a substantially consisting of silver grid was applied.

Subsequently, efficiencies were determined under STC (standard test conditions) and compared with cells whose buffer layer was prepared by the generally customary CBD method. The success of the procedure according to the invention was demonstrated by measuring equal, sometimes even slightly better, cell efficiencies on the cells produced according to the exemplary embodiment.

Claims (10)

A method of depositing a buffer layer containing one or more sulfides selected from the group of metals Cd, In, Mg, Sn and Zn on a CIS absorber layer of a CIS thin film solar cell comprising the steps of: a) providing a solar cell blank with a CIS absorber layer applied to a substrate; b) treating the solar cell blank at 30 ° C to 90 ° C with an aqueous, alkaline adsorber solution containing at least one salt of a metal from the group Cd, In, Mg, Sn and Zn in a concentration in the range of 1 to 50 mmol / l contains; c) rinsing the solar cell blank with an aqueous, alkaline rinse solution; d) treating the solar cell blank at 30 ° C to 90 ° C with an aqueous solution of a sulfur-donating substance having a concentration in the range of 30 to 120 mmol / l; and e) optionally multiple cyclic repeating the sequence of steps b) to d) until the buffer layer has completely formed. The method of claim 1, wherein at least 3 cycles of the sequence of steps b) to d) are traversed, wherein the process parameters are set in the individual cycles such that - The metal is predominantly deposited as hydroxide in the first cycle or the first cycles; - The metal is deposited in the subsequent cycle or in the subsequent cycles predominantly as hydroxysulfide; and - The metal is predominantly deposited as a sulfide in the final cycle or cycles. The method of claim 1 or 2, wherein the adsorbent solution from step b) contains a wetting agent. Method according to one of the preceding claims, wherein the solar cell blank is moved in the bath during the performance of step b). Method according to one of the preceding claims, wherein the solar cell blank in step b) and / or step d) is guided horizontally on the surface of the adsorber solution or the thiourea solution with the CIS absorber layer down, the adsorber solution or solution of a sulfur-donating substance of At the bottom of the CIS absorber layer is flowing and at the same time the top of the solar cell blank is kept dry by blowing air. The method of any one of the preceding claims, wherein the solar cell blank is a continuous belt coated with the CIS absorber layer. A process according to any one of the preceding claims, wherein the rinse solution from step c) contains ammonia. Method according to one of the preceding claims, in which a treatment with ultrasound in the MHz range takes place simultaneously in step d). Process according to one of the preceding claims, in which sulfur-donating substances are selected from the group comprising 2-mercaptobenzothiazole, 3-mercaptobenzimidazole, 3-mercapto-1,2-propanediol, thioacetamide, polysulfides, thiophene derivatives, diisobutylsulfide, ditetrabutylsulfide, dichlorobenzenethiol, diethyldithiocarbamate, thiocyanate , Thioglycolic acid, thiocarbamide and thiourea. CIS thin-film solar cell produced by a process according to one of claims 1 to 9.

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