EP1576209B1 - Method for regeneration of an electrolysis bath for the production of a compound i-iii-vi sb 2 /sb in thin layers - Google Patents

Description

The present invention relates to the manufacture of type I-III-VI ₂ semiconductors in thin layers, in particular for the design of solar cells.

The compounds I-III-VI ₂ of the CuIn _x Ga _(1-x) Se _y S _{(2-y) type} (where x is substantially between 0 and 1 and y is substantially between 0 and 2) are considered to be very low. promising and could be the next generation of thin-film photovoltaic cells. These compounds have a direct bandwidth of between 1.05 and 1.6 eV which allows a strong absorption of solar radiation in the visible.

The record yields of photovoltaic conversion were obtained by preparing thin films by evaporation on small surfaces. However, evaporation is difficult to adapt on an industrial scale because of problems of non-uniformity and low use of raw materials. Sputtering (cathode sputtering) is better suited to large areas but it requires vacuum equipment and very expensive precursor targets.

• dépôt à température et pression ambiantes dans un bain d'électrolyse,
• possibilité de traiter de grandes surfaces avec une bonne uniformité,
• facilité de mise en oeuvre,
• faible coût d'installation et des matières premières (pas de mise en forme particulière, taux d'utilisation élevé des matières), et
• grande variété des formes possibles de dépôt, due à la nature localisée du dépôt sur le substrat.

There is therefore a real need for alternative techniques at low cost and at atmospheric pressure. The technique of deposition of thin layers by electrochemistry, in particular by electrolysis, is presented as a very attractive alternative. The advantages of this filing technique are numerous and in particular the following:

• deposition at ambient temperature and pressure in an electrolysis bath,
• possibility to treat large areas with good uniformity,
• ease of implementation,
• low installation cost and raw materials (no particular shaping, high material utilization rate), and
• wide variety of possible forms of deposition, due to the localized nature of the deposit on the substrate.

Despite numerous studies in this field, the difficulties encountered concerned the control of the quality of electrodeposited precursors (composition and morphology) and the efficiency of the electrolysis bath after several successive deposits.

An object of the present invention is to provide a method of manufacturing thin layers of a compound I-III-VI _y (where y is close to 2) by electrolysis, which ensures the stabilization and reproducibility of the deposition conditions.

An underlying purpose is to be able to perform over large areas a large number of successive deposits of thin layers having the desired morphology and composition.

Another object of the present invention is to provide a process for the manufacture of thin layers of the compound I-III-VI _y , which ensures a satisfactory life of the electrolysis bath, as well as an efficient regeneration of the raw materials consumed during electrolysis.

Another object of the present invention is to provide a process for manufacturing thin layers of the compound I-III-VI _y , which ensures a regeneration of the raw materials consumed during the electrolysis, without unbalancing the composition of the electrolysis bath. and then reduce its life.

• a) prévoir un bain d'électrolyse comprenant du sélénium actif, de degré d'oxydation IV, ainsi qu'au moins deux électrodes, et
• b) appliquer une différence de potentiel entre les deux électrodes pour favoriser sensiblement une migration du sélénium actif vers l'une des électrodes et amorcer ainsi la formation d'au moins une couche mince de I-III-VI_y.

To this end, it proposes a process for producing a compound I-III-VI _y in thin layers by electrochemistry, where y is close to 2 and VI is an element comprising selenium, of the type comprising the following steps:

• a) providing an electrolysis bath comprising active selenium, of oxidation degree IV, and at least two electrodes, and
• b) applying a potential difference between the two electrodes to substantially promote a migration of active selenium to one of the electrodes and thereby initiate the formation of at least one thin layer of I-III-VI _y .

The process in the sense of the invention further comprises a step c) regeneration of selenium in active form in said bath, to increase a lifetime of said electrolysis bath.

Thus, within the meaning of the present invention, the regeneration of the active selenium bath is first carried out before regenerating it into element I (such as copper) and / or element III (such as indium or gallium). Indeed, it has been found that a low re-introduction of active selenium into the bath (preferably an excess of approximately 20% in molar concentration relative to the amount of selenium normally added) made it possible to obtain substantially again the same number and the same volumes of thin layers as those obtained at the end of step b).

Advantageously, at the end of step c), at least one new thin layer of I-III-VI _{y is formed} .

Thus, in a first embodiment, in step c), selenium is added to the bath to form an excess of active selenium in the bath.

In another embodiment, variant or complementary to the first embodiment mentioned above, in step c), a selenium oxidant is introduced into the bath to regenerate selenium in active form.

Usually, the electrolysis bath, as it ages during deposition, exhibits selenium colloids. This selenium in the form of colloids has a degree of oxidation 0 and, in the context of the present invention, is not likely to combine with the elements I and III. Advantageously, if the bath comprises selenium in the form of colloids in step b), the aforementioned oxidant is capable of regenerating the selenium in the form of colloids, into selenium in the active form.

Thus, it will be understood that selenium in active form is understood to mean selenium at the oxidation state IV, which can be reduced to the ionic electrode Se ^2- and to naturally combine with the elements I and III. to form the thin films of I-III-VI _y , and differing from the selenium of oxidation state 0, for example in the form of colloids in the bath solution, which does not combine with elements I and III.

In a particularly advantageous embodiment, said oxidant is oxygenated water, preferably in concentration in the bath of an order of magnitude corresponding substantially to at least five times the initial concentration of selenium in the bath.

The addition of hydrogen peroxide in the bath then makes it possible to regenerate the electrolysis bath at very low cost. In addition, this regeneration is carried out without pollution of the bath since a simple degassing makes it possible to recover the initial constitution of the bath.

In this context, where the electrolysis bath is regenerated by limiting its pollution by the regenerating additives, a step subsequent to step c), of regeneration of the electrolysis bath by introduction of oxides and / or of hydroxides of elements I and III.

• la figure 1 représente schématiquement une couche mince obtenue par la mise en oeuvre du procédé selon l'invention, et
• la figure 2 représente schématiquement un bain d'électrolyse pour la mise en oeuvre du procédé selon l'invention.

Other advantages and characteristics of the invention will appear on reading the following detailed description of embodiments given by way of non-limiting examples, as well as on the examination of the drawings which accompany it and on which :

• the figure 1 schematically represents a thin layer obtained by the implementation of the method according to the invention, and
• the figure 2 schematically represents an electrolysis bath for the implementation of the method according to the invention.

Referring to the figure 1 copper and indium diselenide CO layers are obtained at ambient pressure and temperature by electrodeposition of a thin layer of precursors of suitable composition and morphology on a glass substrate S covered with molybdenum MO. The term " precursor layer" is intended to mean a thin layer of overall composition close to CuInSe ₂ and directly obtained after electrolysis deposition, without any subsequent treatment.

Electrodeposition is carried out from an acid bath B ( figure 2 ), stirred by M blades, containing an indium salt, a copper salt and dissolved selenium oxide. The concentrations of these precursor elements are between 10 ^-4 and 10 ^-2 M. The pH of the solution is set between 1 and 4.

• une électrode de molybdène Ca (pour cathode) sur laquelle se forme la couche mince par électrodéposition,
• et une électrode de référence au sulfate mercureux REF, sont immergées dans le bain B.

Three electrodes An, Ca and REF, of which:

• a molybdenum electrode Ca (for cathode) on which the thin layer is formed by electrodeposition,
• and a REF mercurous sulfate reference electrode are immersed in bath B.

The electric potential difference applied to the molybdenum electrode is between -0.8 and -1.2 V relative to the REF reference electrode.

Thickness layers between 1 and 4 microns are obtained, with current densities of between 0.5 and 10 mA / cm ² .

Under defined conditions of composition, agitation and potential difference, it is possible to obtain dense, adherent layers of homogeneous morphology whose composition is close to the stoichiometric composition: Cu (25%), In ( 25 + ε%) and Se (50%), with a composition slightly richer in indium, as shown in Table I below. We can thus make deposits on surfaces of 10x10 cm ² .

An exemplary embodiment of the invention is given below.

• [CuSO₄] =1,0.10^-3 M,
• [In₂(SO₄)₃]=3.0.10^-3 M,
• [H₂SeO₃]=1,7.10^-3 M,
• [Na₂SO₄] =0,1 M,

où la notation "M" correspond à l'unité "mole par litre", pour un pH de 2,2.A typical deposit is made from a bath whose initial formulation is as follows:

• [CuSO ₄ ] = 1.0 × 10 ^-3 M,
• [In ₂ (SO ₄ ) ₃ ] = _3.0 × 10 ^-3 M,
• [H ₂ SeO ₃ ] = 1.7 × 10 ^-3 M,
• [Na ₂ SO ₄ ] = 0.1 M,

where the notation "M" corresponds to the unit "mole per liter", for a pH of 2.2.

The precursors are deposited by an imposed voltage cathode reaction at -1 V relative to the REF electrode. The current density is -1 mA / cm ² .

After each electrolysis, the Cu, In and Se elements are recharged on the basis of the number of coulombs indicated by a detection cell (not shown), which thus counts the number of ions that have interacted in the bath solution. This refill makes it possible to keep constant the concentration of the elements during the successive electrodeposits. The pH can also be readjusted by addition of sodium hydroxide (such as NaOH, for a concentration such as 1M) but this measure is not systematically necessary here, as will be seen later.

Under these conditions, it is usually found that after an indication of 500 ± 100 Coulombs in a solution of one liter (corresponding to the electrodeposition of 4 to 5 thin layers of 25 cm ² having a thickness of 2 μm ), a Partial or total detachment of CuInSe ₂ layers appears systematically.

According to the invention, this detachment disappears by regeneration of the selenium bath, even before regenerating the elements Cu and In.

It is appropriate here to distinguish the active selenium of degree of oxidation IV, usually denoted Se (IV), of the inactive selenium, of degree of oxidation 0, which is generally observed in the form of colloids in the electrolysis bath. and usually noted Se (0).

It is pointed out that the active selenium Se (IV) is the only one capable of being reduced to the Ca electrode in the Se ² ion form and of combining, in this form, with the elements Cu and In to form the CuInSe thin layers. ₂ .

• soit en Se^2- favorable à la formation des couches minces comme indiqué ci-avant,
• soit en Se(0) sous forme de colloïdes, ce qui est défavorable à la formation des couches minces, notamment parce que les colloïdes posent des problèmes à l'interface entre le substrat (ou la couche MO de molybdène ici) et la couche mince de Cu-In-Se en formation.

It is also indicated that there are two competitive reactions during the electrolysis: the selenium introduced into the bath can be transformed at the electrode:

• or in ^2- favorable to the formation of thin layers as indicated above,
• or in Se (0) in the form of colloids, which is unfavorable for the formation of thin layers, in particular because the colloids pose problems at the interface between the substrate (or the MO layer of molybdenum here) and the thin layer Cu-In-Se in formation.

Advantageously, an excess regeneration of Se (IV) is carried out in the bath. For this purpose, dissolved selenium oxide is added to the electrolysis bath to retard the aging of the bath. In practice, for a thin layer formed and 115 Coulombs passed into the solution, it is theoretically necessary to add 1.8 × 10 ^-4 M of [H ₂ SeO ₃ ] to the solution to recover an initial selenium concentration of 1.7 × 10 ^-3 M A double addition of this quantity (ie 3.6 × 10 ^-4 M and therefore an excess of 1.8 × 10 ^-4 M of [H ₂ SeO ₃ ]), at the fifth deposit, makes it possible to obtain adhering layers again. These thin layers have the composition (Table I) and the desired morphology. Overregeneration of 3.6 × 10 ^-4 M thus makes it possible to obtain a cycle of 4 to 5 layers of satisfactory adhesion before observing new debonding problems. After each peeling cycle, the renewal of this operation makes it possible to obtain adherent layers.

Alternatively or in addition to this operation, an oxidizer is used to re-oxidize the selenium in the form Se (0), to obtain selenium in the form Se (IV). For this purpose, hydrogen peroxide H ₂ O ₂ is preferably used, by bringing a large excess of H ₂ O ₂ into the solution (concentration of the order of 10 ^-2 M, preferably close to 4.10 ^-2 M). The layers become adherent for 4 to 5 successive deposits of thin layers, then come off again. The renewal of this operation also makes it possible to obtain adherent layers again. Advantageously, it has been observed that the addition of hydrogen peroxide also makes it possible to obtain thin layers of relatively more smooth morphology.

We thus note a great similarity of the effects that a regeneration in Se (IV) provides and the addition of H ₂ O ₂ in the solution. It is further indicated that other types of oxidant than oxygenated water, especially ozone O ₃ , can be used to increase the life of baths.

The composition (Table I) and the morphology of the layers is substantially the same, whether hydrogen peroxide has been added to the bath or that regeneration of selenium (IV) has been carried out. <u> Table I </ u>: Comparative analysis of the composition of CuInSe <sub> 2 </ sub> thin films electrodeposited as a function of excess selenium Se (IV) overregeneration and an addition of oxygenated water. Cu (%) In (%) Se (%) First deposit 21.4 27.5 51 Addition of H ₂ O ₂ 22.9 25 52 Regeneration in excess of Se (IV) 21.4 28.8 49.7

The addition of hydrogen peroxide or the regeneration in excess of Se (IV) considerably increase the number of layers that can be deposited with a bath. Such a recycling of the bath makes it possible to consume integrally, by electrolysis, the introduced elements, and more particularly the indium, which makes it possible to reduce in a particularly advantageous manner the precursor manufacturing costs, in particular with respect to the methods of evaporation or sputtering.

It is indicated that, according to the advantageous aspect of the regeneration of the bath within the meaning of the invention, oxides or hydroxides of copper and / or indium are added to regenerate the CuInSe ₂ electrolysis bath in copper and / or indium.

For example, by adding copper oxides CuO and indium In ₂ O ₃ in the bath, the following reactions (1) and (2) are formed:

CuO + H2O → Cu ²⁺ + 2OH ^- (1)

(1/2) In ₂ O ₃ + (3/2) H ₂ O → In ³⁺ + 3OH ^- (2)

On the other hand, if the CuSO ₄ and In ₂ (SO ₄ ) ₃ compounds were added, the bath would be polluted with SO ₄ ^2- sulfate ions.

In addition, the formation reaction CuInSe ₂ to the cathode is written:

Cu ²⁺ + In ³⁺ + 2H ₂ SeO ₃ + 8H ⁺ + 13e ^- → CuInSe ₂ + 6H ₂ O (3)

where e- corresponds to the notation of an electron, while at the anode, we have the following reaction:

(13/2) H ₂ O → 13H ⁺ + (13/4) O ₂ + 13th ^- (4)

to respect the balance of loads.

It is then found, according to another advantage provided by the addition of Cu and In oxides, that the difference of five excess H ⁺ ions according to equations (3) and (4) is compensated by the five OH ions. - introduced by reactions (1) and (2). It will thus be understood that the addition of Cu and In oxides also makes it possible to stabilize the pH of the solution and to dispense with the addition of sodium hydroxide as indicated above.

It is further indicated that the addition of Cu (OH) ₂ and In (OH) ₃ hydroxides produces the same effects, reactions (1) and (2) becoming simply:

Cu (OH) ₂ → Cu ²⁺ + 2OH ^- (1 ')

In (OH) ₃ → In ³⁺ + 3OH ^- (2 ')

This ensures a durability and a stability of the electroplating baths of compounds I-III-VI _y such that Cu-In-Se _y (with y close to 2) by adding agents which do not affect the quality of the layers . The electrodeposited precursor layer contains the composition elements close to the stoichiometry I-III-VI ₂ . The compositions and the morphology are controlled during the electrolysis. These agents (excess of Se (IV) or H ₂ O ₂ ) can easily be used for any type of electrolysis bath allowing the electrodeposition of I-III-VI systems such as Cu-In-Ga-Al-Se- S.

The conversion yields obtained (9% without anti-reflective surface layer) attest to the quality of the deposits obtained by the process according to the invention.

Of course, the present invention is not limited to the embodiment described above by way of example; it extends to other variants.

Thus, it will be understood that the elements I and III initially introduced into the solution in the form of CuSO ₄ and In ₂ (SO ₄ ) ₃ may advantageously be introduced in the form of oxides or hydroxides of copper and indium for limit the pollution of the bath.

Claims (10)

1. A method of producing a I-III-VI_y compound in thin film form by electrochemistry, in which y is close to 2 and VI is an element comprising selenium, of the type comprising the following steps:

   a) of providing an electrolysis bath comprising active selenium, in oxidation state IV, and at least two electrodes; and

   b) of applying a potential difference between the two electrodes in order to substantially promote migration of the active selenium toward one of the electrodes and thus initiate the formation of at least one thin film of I-III-VIy,

   characterized in that it furthermore includes a step c) of regenerating the selenium in active form in said bath, in order to increase the lifetime of said electrolysis bath.
2. The method as claimed in claim 1, characterized in that, at step c), an oxidizing agent for selenium (Se(0)) is introduced into the bath in order to regenerate selenium in active form (Se(IV)).
3. The method as claimed in claim 2, characterized in that, when the bath contains selenium in colloid form (Se(0)) at step b), said oxidizing agent is designed to regenerate the selenium in colloid form (Se(0)) to selenium in active form (Se(IV)).
4. The method as claimed in either of claims 2 and 3, characterized in that said oxidizing agent is hydrogen peroxide (H₂O₂).
5. The method as claimed in claim 4, characterized in that the concentration of hydrogen peroxide added to the bath is of the order of magnitude corresponding substantially to at least five times the initial selenium concentration in the bath.
6. The method as claimed in one of claims 1 to 5, characterized in that, at step c), selenium is added to the bath in order to form an excess of active selenium in the bath.
7. The method as claimed in claim 6, characterized in that, for substantially one tenth of the concentration of selenium at step a) consumed by producing at least one thin film at step b), substantially twice the consumed concentration is added to the bath at step c).
8. The method as claimed in one of the preceding claims, characterized in that, after step c), at least one new thin film of I-III-VI_y is formed.
9. The method as claimed in one of the preceding claims, characterized in that, to produce thin CuInSe_y films, the bath comprises, at step a), for one unit of concentration of copper in the bath, about 1.7 units of concentration of active selenium.
10. The method as claimed in one of the preceding claims, characterized in that it includes a step after step c), of regenerating the electrolysis bath by introducing oxides and/or hydroxides of elements I (CuO; Cu(OH)₂) and III (In₂O₃; In (OH)₃).

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