Formation Of A Transparent Conductive Oxide Film For Use In A Photovoltaic Structure

Abstract

A process for producing a photovoltaic structure that includes a thin coating film, generally made of a transparent conductive oxide, deposited on top of a sublayer having photovoltaic properties. The coating film is deposited electrochemically in an electrolysis bath and at least partly assisted by illumination coming from a light source.

Description

[0001] The present invention relates to the formation of a thin coating film that can be used within a photovoltaic device such as for example a solar cell.

[0002] This coating film may typically be a transparent conductive film serving as "front" electrode in the photovoltaic structure. The required properties for this film are that it be: [0003] transparent in the visible and [0004] conductive in the solar cell.

[0005] This coating film is often made of a material comprising predominantly an oxide such as zinc oxide (ZnO).

[0006] FIG. 1 shows schematically a photovoltaic structure that may have such a coating film.

[0007] A metal film MET (for example based on molybdenum) is deposited on a substrate SUB (for example by vacuum evaporation or by sputtering). The substrate may be made of conventional glass. This metal film MET has a thickness that may be between 0.5 and 1 micron.

[0008] Next, a photovoltaic film PHO is deposited on the metal film MET. The material of the film PHO may be silicon, CdTe or an I-III-VI.sub.2 alloy typically comprising: [0009] copper, [0010] indium, gallium and/or aluminum and [0011] selenium and/or sulfur.

[0012] The film PHO typically has a thickness of about 2 microns. It is generally deposited by vacuum evaporation, by cathode sputtering, by chemical vapor deposition, by screen printing or electrochemically.

[0013] Usually deposited on this photovoltaic film PHO is an interface film INT, called a "buffer layer", often based on cadmium sulfide, zinc sulfide or indium sulfide. It may be deposited chemically in solution (the chemical bath deposition (CBD) technique), under vacuum or in the vapor phase. Its thickness is generally between 5 and 100 nm.

[0014] The next two films, referenced ELE and TRA respectively, which are deposited on the buffer layer INT in the example illustrated in FIG. 1, form in particular the subject matter of the present invention. The film TRA itself is optional, but the surface film ELE acts as electrode in the photovoltaic structure.

[0015] The film TRA must be transparent so as to allow the light to reach the photovoltaic film PHO. It may be based on an oxide (for example based on lightly doped ZnO) and may have a thickness of about 50 to 100 nm. It is generally deposited by cathode sputtering or by chemical vapor deposition (CVD).

[0016] The film ELE may vary from 0.5 to 1 micron in thickness. It may be based on an oxide (for example zinc oxide) like the film TRA. The film ELE is itself doped for the purpose of making this film ELE conductive so as to serve as "front" electrode. The doping may be n-type doping, for example doped with aluminum. The film ELE must be transparent in the visible so that the photovoltaic film PHO can be illuminated. It must also be conductive in the solar cell.

[0017] Solar cells based on CuInSe.sub.2-type alloys using the addition of sulfur, gallium or aluminum form the basis of a new photovoltaic system the industrial exploitation of which is starting to grow. The performance of such cells ranges up to 19.9% in the laboratory, with in general standard values in the laboratory of 15-17%. Modules have efficiencies of: [0018] 11-14% for the system using gallium added to the I-III-VI.sub.2 photovoltaic alloy; and [0019] around 6-9% for the system using sulfur.

[0020] All these photovoltaic devices have, without exception, the structure illustrated in FIG. 1.

[0021] Usually, the surface film ELE is deposited by vacuum sputtering. In certain cases, it may be deposited by chemical vapor deposition and doped with boron.

[0022] However, for implementation on an industrial scale, it is preferable to use an atmospheric deposition method, such as electrochemical bath deposition. This technique is simple--it is unnecessary to apply a vacuum--and has a definite advantage in terms of production cost, investment cost and capability of treating large areas (as in the electroplating field in particular).

[0023] Moreover, as this deposition technique would continue on from the chemical deposition of the subjacent film TRA (for example based on CdS or ZnS), or even from the photovoltaic film PHO deposited electrochemically, the consistency of the production sequence is maintained, with a "front side" approach involving entirely wet processing. The advantage in terms of production cost is substantial.

[0024] The electrochemical synthesis of a material widely used for the production of the surface film ELE or the film TRA, in particular zinc oxide ZnO, has been described in the document WO 96/31638. In that document, the supply of oxygen comes from the molecular oxygen dissolved in the electrolysis bath. In another document (M. Izaki, T. Omi and T. A. Pattinson, Appl. Phys. Lett, 68, 2439, 1996), the oxygen comes from nitrate ions.

[0025] The document by D. Lincot, B. Cavana, S. Quenet, S. Peulon and H. W. Schock, Proceedings 14.sup.th EC Photovoltaic Solar Energy Conference, Stephen and Ass. Ed., 2168, 1997, furthermore shows that it is possible to deposit zinc oxide surface films directly on a molybdenum/I-III-VI.sub.2 alloy stack with an efficiency close to 10%. Electrodeposition may therefore be used in the synthesis of ZnO conductive films as window films in photovoltaic cells based on an alloy of the CIS type (CIS standing for Cu(In,Ga)(Se,S).sub.2).

[0026] The document by D. Gal, G. Hodes, D. Lincot and H. W. Hodes, Thin Solid Films, 361-362, 79, 2000, deals with the direct deposition of transparent ZnO oxide films INT on CIS using a medium comprising dimethylsulfoxide (DMSO) heated to 130.degree. C. and in the presence of dissolved oxygen. The stack is then completed with an aluminum-doped ZnO film ELE. This film is therefore conductive and it is deposited by sputtering. Thus, in the above document, it is not a question of replacing the technique (sputtering) for depositing the conductive surface film ELE but of applying the electrodeposition technique to the synthesis of the subjacent (ZnO-based) film TRA and to possible other buffer layers.

[0027] Thus, the deposition of conductive transparent oxide films on top of the photovoltaic film, electrochemically, is a particularly advantageous technique. It has already been implemented in the field of photovoltaic cells. However, the results obtained can still be improved. In particular the adhesion of the films thus deposited, their homogeneity and their morphological quality merit further improvements.

[0028] The present invention aims to improve the situation.

[0029] The invention provides for this purpose a process for producing a photovoltaic structure comprising a thin coating film deposited on top of a sublayer having photovoltaic properties. According to the invention, the coating film is deposited electrochemically, the deposition being partly or completely assisted by illumination coming from a light source.

[0030] It has been observed that the illumination of the photovoltaic film induces an electrical effect that promotes uniform and homogeneous deposition of the coating film, at least at the start of growth of the coating film (called the "nucleation" step). In particular, it has been observed that the material deposited, right from nucleation, is laterally uniform and dense, this being unexpected, in particular for depositing a film of an oxide such as zinc oxide. The effect of the illumination may be explained by a charge density generated by the photovoltaic film which is uniformly distributed over the surface on which the coating film is deposited, a situation that favors good homogeneous morphology of the coating film. This satisfactory effect is also due to the suitable choice of deposition temperature, preferably in the 50.degree. C.-150.degree. C. range (in an aqueous, nonaqueous or hybrid medium, depending on the temperature chosen). In this temperature range specifically, zinc oxide formation is favored. In particular, it has been observed that at lower temperatures (room temperature for example), what are predominantly present are hydroxides (for example Zn(OH).sub.2), whereas at higher temperatures (for example around 70 to 80.degree. C.), oxides (ZnO for example) are predominantly present. Thus, by combining illumination with a suitable deposition temperature it is possible to achieve satisfactory results. Moreover, it has been found that the illumination also increases the coating film growth rate. The coating film may then have a thickness ranging up to a few microns, but it nevertheless exhibits good mechanical stability.

[0031] It has also been found that the coating film nucleation step carried out under given temperature and illumination conditions is generally improved in the presence of an interface film (INT) or INT and TRA films, compared with the use of a bare PHO film. This result leads to a spectacular increase in the photovoltaic performance, never observed hitherto. This has been achieved despite the brittleness of the INT and TRA films.

[0032] As indicated above, the coating film may be based on a transparent oxide, for example based on zinc oxide (ZnO). However, other alternative materials that can be deposited on a photovoltaic film, or on a photovoltaic film coated with a conductive film, are possible. Already, the coating film may be another form of oxide, but also sulfides or selenides. If the coating film is such an oxide film, it may be designed to act as transparent and/or conductive window in a photovoltaic cell. However, it is not necessary for the material of the coating film to be transparent in order to implement the invention since only the nucleation of the coating film requires to be assisted via illumination.

[0033] For example, the transparent conductive ZnO film may be deposited electrolytically in an aqueous medium or in a nonaqueous medium, for example dimethylsulfoxide, or in a mixture thereof, using dissolved zinc ions and an oxygen donor element such as dissolved oxygen, or another precursor (hydrogen peroxide, nitrate ions or any other soluble oxygen precursor), by a cathodic reaction.

[0034] The illumination may be in the visible, as monochromatic or polychromatic light. A spectral range between 100 and 1500 nm, for example between 200 and 1300 nm (solar illumination), has given good results with an incident power of between 0.1 mW/cm.sup.2 and 1 mW/cm.sup.2 (preferably between 1 mW/cm.sup.2 and 300 mW/cm.sup.2 in one embodiment).

[0035] The coating film may or may not be doped, for example with at least one element introduced in solution, taken from chlorine, fluorine, iodine, bromine, gallium, indium, boron and aluminum, and, in a standard embodiment, it may be doped in particular with chlorine.

[0036] As indicated above, the photovoltaic sublayer may be based on a I-III-VI.sub.2 alloy of the Cu(In,Ga,Al)(S,Se).sub.2 type deposited electrochemically or by any other technique, such as evaporation, sputtering, screen printing, chemical vapor deposition or inkjet deposition, whether partly or completely.

[0037] For example, I-III elements may be deposited by electrolysis, or by sputtering, and then a selenization and/or sulfurization operation may be carried out subsequently. A preferred embodiment is that described in document WO 03/094246 in which the I-III-VI.sub.2 alloy is entirely deposited electrochemically and then annealed by rapid thermal processing. However, other photovoltaic materials are conceivable (for example thin-film or bulk silicon, or the like).

[0038] The coating film may be deposited directly on the photovoltaic sublayer or, as a variant, it may be deposited on one or more interface films located above the photovoltaic sublayer.

[0039] Moreover, these other interface films may be deposited electrochemically or using other techniques. The materials constituting these films may for example be based on cadmium sulfide (CdS) and/or zinc sulfide (ZnS) and/or indium or gallium sulfide (In.sub.2S.sub.3 or Ga.sub.2S.sub.3 respectively). These films can be deposited by chemical vapor deposition or in solution, especially CBD, or by physical deposition, evaporation, cathode sputtering, or the like. However, it is particularly advantageous to deposit these films again by electrolysis, so that: [0040] the photovoltaic sublayer, [0041] the interface film(s), and [0042] the coating film (ZnO for example) are all deposited electrolytically, and therefore using a single treatment of the same type and without recourse to other techniques (sputtering or the like).

[0043] Here again, the ability to deposit a dense homogeneous coating film (in particular made of ZnO) on an interface film of the aforementioned type was unexpected since, without illumination, it was found that the coating film being formed was deposited inhomogeneously, in islands. Owing to the illumination during deposition, the coating film is adherent and exhibits both good lateral uniformity and good homogeneity, in particular in the presence of the interface film.

[0044] It should be pointed out in particular that the performance of the photovoltaic device based on CuInSe.sub.2-type photovoltaic films obtained by implementing the invention has already, with the illumination for assisting the deposition of the ZnO film on the photovoltaic sublayer, certainly increased the photovoltaic efficiency. However, this efficiency has also be increased owing to the presence of an interface film (CdS, ZnS, In.sub.2S.sub.3 or Ga.sub.2S.sub.3) on which the coating film (ZnO) can be deposited electrolytically with illumination assistance. To the knowledge of the inventors, the results achieved by implementing the invention are currently the best for devices based on transparent conductive films deposited electrolytically, with levels of efficiency comparable to those in the prior art using conventional processes.

[0045] The interface films may themselves be coated with an intermediate zinc oxide film or with a film of an alloy of zinc oxide with in particular magnesium (ZnMgO) deposited by sputtering, by CVD or in solution. The aim of the invention is in fact to form a coating film on a photovoltaic material whether or not coated with other films beforehand. For example, the following types of heterostructures may be envisaged: CIS/CdS, CIS/ZnS, CIS/CdS/ZnO, CIS/ZnS/ZnO, CIS/ZnO, CIS/In.sub.2S.sub.3, CIS/In.sub.2S.sub.3/ZnO or the like, in which the ZnO film may or may not be doped, or else divided into a doped film and an undoped complementary film. The ZnO-based interface film may, as indicated above, be based on (Zn,Mg)O.

[0046] Referring to FIG. 2, the present invention also relates to a facility for implementing the above process and comprising, in particular, an electrolysis bath 1 and a light source 2 illuminating this bath. For ease of representation, the illumination rail 2 is shown in FIG. 2 above the bath. However, in a preferred embodiment, the illumination comes from the side, substantially perpendicular to the substrate, and not from above. The film may be deposited in aqueous medium, in organic medium or in a mixture thereof. The oxygen may be supplied by bubbling (at 3) air or molecular oxygen O.sub.2 into the bath, which dissolves therein.

[0047] The invention also relates to the photovoltaic structure comprising a thin coating film deposited on top of a sublayer having photovoltaic properties, and in which in particular the coating film is a transparent oxide film deposited directly on the photovoltaic sublayer and has a uniform homogeneous morphology at least at the interface with the photovoltaic sublayer.

[0048] The coating film may be characterized by a controlled surface roughness, in the form of crystalline facets, columnar grains and/or needles, so as to be able to increase the photocurrent generated by the photovoltaic cell.

[0049] It will thus be understood that implementing the invention offers one possible way of synthesizing, in solution, the conductive and/or transparent oxide film (especially a zinc oxide film) under atmospheric conditions and not requiring the use of gaseous reactants. The invention makes it possible, as will be seen in the embodiment examples described below, to obtain transparent conductive zinc oxide films resulting in devices having an efficiency similar to that obtained with the conventional sputtering deposition technique. The invention therefore opens the way for replacing vacuum techniques, to produce the transparent conductive film, with a very simple, inexpensive and low-cost electrochemical method.

[0050] The invention provides the following advantages: [0051] the coating film may be deposited at low temperature (below 100.degree. C.), requiring no expensive vacuum deposition or vapor deposition equipment; [0052] it is possible to deposit coatings on large areas using an industrially reproducible coating technique, of great interest for producing low-cost large-scale photovoltaic panels; [0053] it is possible, as will be seen, to dispense with the TRA film in FIG. 1 (denoted hereafter by "i-ZnO", i.e. the "intrinsic" (undoped) transparent interface film on which the doped conductive film ELE is usually deposited), thereby making it possible to dispense with one step in the solar cell production process; and [0054] it is also possible, as will be seen below, to avoid the toxic element cadmium for producing the interface film INT.

[0055] Other features and advantages of the invention will become apparent on examining the following detailed description, together with the appended drawings in which, apart from FIGS. 1 and 2 described earlier:

[0056] FIG. 3A is a scanning electron micrograph in lateral cross section of a molybdenum/I-III-VI.sub.2 alloy/CdS interface/intrinsic ZnO window/ZnO stack electrodeposited according to the invention on glass;

[0057] FIG. 3B is an enlargement of the micrograph of FIG. 3A;

[0058] FIG. 3C is a top view of the stack of FIGS. 3A and 3B;

[0059] FIG. 3D is an enlargement of the micrograph of FIG. 3C;

[0060] FIG. 3E shows the J-V characteristic of the stack shown in FIGS. 3A to 3D;

[0061] FIG. 4A is a scanning electron micrograph in lateral cross section of a molybdenum/I-III-VI.sub.2 alloy/ZnS interface/ZnO stack electrodeposited directly on glass according to the invention;

[0062] FIG. 4B is a top view of the stack shown in FIG. 4A;

[0063] FIG. 4C is an enlargement of the micrograph shown in FIG. 4B;

[0064] FIG. 4D shows the J-V characteristic of the stack shown in FIGS. 4A to 4C;

[0065] FIG. 5A is a scanning electron micrograph in lateral cross section of a molybdenum/I-III-VI.sub.2 alloy/ZnO stack on glass, directly electrodeposited according to the invention on the photovoltaic alloy;

[0066] FIG. 5B is an enlargement of the micrograph shown in FIG. 5A;

[0067] FIG. 5C is a top view of the stack shown in FIGS. 5A and 5B;

[0068] FIG. 5D is an enlargement of the micrograph shown in FIG. 5C;

[0069] FIG. 5E shows the J-V characteristic of a molybdenum/I-III-VI.sub.2 alloy/ZnO stack on glass directly electrodeposited according to the invention on the photovoltaic alloy;

[0070] FIG. 6 shows the J-V characteristic of a molybdenum/I-III-VI.sub.2 alloy/CdS interface/ZnO stack on glass electrodeposited according to the invention; and

[0071] FIG. 7 shows the J-V characteristic of a molybdenum/I-III-VI.sub.2 alloy/intrinsic ZnO window/ZnO stack on glass electrodeposited according to the invention, over a large area (17.5 cm.sup.2), whereas the other J-V characteristics were recorded on small areas (0.1 cm.sup.2).

[0072] In the embodiment examples described below, the coating film is based on zinc oxide deposited electrochemically, using a zinc salt dissolved in an aqueous solvent, in the presence of an oxygen donor species, which may very advantageously be dissolved gaseous oxygen. The electrolyte also contains a support salt so as to make it conductive. The deposition is carried out by applying a cathodic potential to the electrode according to the following electrochemical reaction:

Zn(II)+1/2O.sub.2+2e.sup.-.fwdarw.ZnO

in which it will be understood that the electrons, denoted by 2e.sup.-, are favorably supplied by the photocurrent that the photovoltaic film PHO, or the film covered with its interface film, produces during the illumination thereof.

[0073] The ZnO oxide is deposited on the electrode in thin-film form, the thickness of which is controlled by the amount of electricity exchanged during the reaction. Again referring to FIG. 1, the electrode receiving the deposit may be formed from the following stack, on the glass substrate SUB: [0074] the molybdenum (Mo) film MET; [0075] the film PHO of I-III-VI.sub.2 alloy such as Cu(In,Ga,Al)(S,Se).sub.2 (called CIS hereafter); [0076] the front film INT (based on CdS or ZnS); and [0077] the front film TRA (i-ZnO), which advantageously may be dispensed with. It should also be recalled that the ZnO may be deposited electrolytically directly on the film of photovoltaic material based on the I-III-VI.sub.2 alloy.

[0078] The deposition is typically carried out in water at a temperature of about 70.degree. C. It lasts about 1 hour or less, and leads to the formation of a dense film with a thickness between 500 and 800 nm.

[0079] Deposition Conditions are Described by Way of Example Below.

[0080] The electrolyte used (called a "chloride bath") contains Zn.sup.2+ ions of 5 mM concentration introduced into demineralized water (18 M.cm.sup.-3) in the form of ZnCl.sub.2 salt and Cl.sup.- ions of 0.1 M concentration introduced in the form of KCl salt. The bath is saturated with oxygen by bubbling gaseous oxygen thereinto. The bath is maintained at a temperature of 70.degree. C. throughout the duration of the electrodeposition. Stirring is provided by using a bar magnet. The reference potential is a K.sub.2SO.sub.4-saturated mercurous sulfate electrode or MSE (K.sub.2SO.sub.4-saturated Hg/Hg.sub.2SO.sub.4 electrode). It is placed in a compartment separated from the electrolysis solution by an alumina frit of low porosity filled with a saturated potassium sulfate solution. An electrolytic bridge is used to maintain it at room temperature so as to avoid any fluctuation in potential that may appear due to the effect of the temperature when the bath is heated. The deposition potential is set at -1.4 V/MSE. A zinc plate is used as counterelectrode and makes it possible both to pass the current into the external circuit and to regenerate the electrolyte with Zn.sup.2+ ions, thus preventing it from being exhausted by consumption at the working electrode.

[0081] Film conductivity is obtained by adding doping impurities to the solution, for example, and as described above, chloride ions incorporated into the film, which then has a high conductivity stable over time, this constituting an advantageous property for the operation of solar devices.

[0082] According to the invention, a light source is used to assist the deposition, at least during the nucleation phase.

[0083] The light flux is delivered by a solar simulator, the homogeneous light flux of which is about 100 mW/cm.sup.2 at 15 cm distance. This flux may be adjusted by modifying the distance between the lamp and the film, up to 5 m in the configuration used, thereby making it possible to reduce the flux arriving at the surface of the specimen down to a few mW/cm.sup.2. Under the conditions of this example, the current densities used during the deposition are between 0.1 and 0.4 mA/cm.sup.2. To promote light transmission through the electrochemical reactor and increase illumination homogeneity, a parallelepipedal glass cell is used, the specimen being placed vertically in this configuration and the illumination being horizontal. The deposition of a ZnO film generally lasts between half an hour and one hour.

[0084] Once the deposition has been carried out, the cells are complete and are characterized conventionally.

[0085] The deposition of ZnO films by electrolysis according to the process of the invention has made it possible to achieve photovoltaic conversion efficiencies of more than 12%, reaching in certain cases a record value of 16.3%. These efficiencies are comparable to those obtained using the sputtering technique for depositing ZnO.

[0086] FIGS. 3A to 3D show the morphology of a stack of the Mo/CIS (or CIGS, for a slight incorporation of gallium)/CdS/i-ZnO and ZnO type by electrodeposition according to the invention. Thus, here the ZnO conductive film conventionally formed by sputtering is replaced with a ZnO film deposited by electrolysis according to the invention.

[0087] The images are obtained by scanning electron microscopy. These show good, compact and homogeneous, coverage of the electrodeposited ZnO (FIG. 3A) on the subjacent, especially CIGS, films. In particular, the electrodeposited ZnO film is highly covering and of uniform thickness. Referring to FIG. 3D, the ZnO film has, in the example shown, a surface with crystalline facets.

[0088] The characteristic plot of current density per unit area versus voltage (hereafter termed the "J-V characteristic") for this type of stack is shown in FIG. 3E, which indicates an open-circuit voltage Voc of 704 mV and a short-circuit photocurrent density Jsc of -29 mA/cm.sup.2 for a form factor FF of 75.8% and an efficiency .eta. of 15.8%. This efficiency is very comparable to that of 16.2% for the same stack produced under the same conditions but with a surface ZnO film obtained by sputtering according to the prior art. It should be pointed out that the electrodeposition of ZnO according to the invention may be carried out on CIGS/CdS/i-ZnO structures produced conventionally, with no adaptation. Moreover, a ZnO film was electrodeposited by the process according to the invention on CIGS/CdS/i-ZnO specimens coming from another test center, and the results are very similar: [0089] Voc=649 mV, [0090] Jsc=-30.1 mA/cm.sup.2, [0091] FF=71.7%, [0092] .eta.=14.7%, which clearly demonstrates the flexibility of the electrolysis deposition technique.

[0093] The J-V characteristic in FIG. 3E is labeled REF and the J-V characteristics of the stacks described below will be compared with this characteristic REF.

[0094] Referring now to FIGS. 4A to 4D, in the case of a stack of the Mo/CIGS/ZnS and ZnO type by electrodeposition according to the invention, again a very compact surface film of ZnO with crystalline facets may be seen (FIG. 4C). The ZnS film is deposited by CBD (chemical bath deposition).

[0095] The J-V characteristic of this type of stack is shown in FIG. 4D, which indicates the following parameters:

Voc=656 mV and Jsc=-30.8 mA/cm.sup.2 for a form factor FF=73.1% and an efficiency .eta. of 15.5%, notwithstanding the fact that the CdS is replaced with ZnS and the undoped ZnO film is omitted.

[0096] Referring now to the case of a stack of the Mo/CIGS/CdS and ZnO type by electrodeposition according to the invention, again a homogeneous and compact surface film of ZnO is observed. The J-V characteristic of this type of stack is shown in FIG. 6, which indicates the following parameters:

Voc=652 mV and Jsc=-29.3 mA/cm.sup.2 for a form factor FF=63% and an efficiency .eta. of 12.1%, notwithstanding the fact that the undoped ZnO film has been omitted. However, it should be pointed out here that replacement of cadmium by zinc in the interface film INT gave better results in the previous example shown in FIGS. 4A to 4D.

[0097] It may therefore be seen that the range of efficiencies obtained is comparable to that in which a film of intrinsic ZnO is sputtered before the ZnO electrodeposition. The process according to the invention can therefore be applied directly after the conventional step of CdS or ZnS deposition without requiring to recreate a vacuum to deposit the i-ZnO film. Furthermore, the results appear to be better when the interface film is based on ZnS rather than CdS.

[0098] Referring now to FIGS. 5A to 5E in the case of a stack of the Mo/CIGS and ZnO type deposited directly by electrodeposition according to the invention, again a surface film of ZnO with crystalline facets is seen (FIG. 5D) the homogeneity and compactness of which are satisfactory (FIG. 5A). FIG. 5E shows the J-V characteristic of a stack of the type in which ZnO is electrodeposited directly on CIGS and indicates the following results: [0099] Voc=580 mV, [0100] Jsc=-29.8 mA/cm.sup.2, [0101] FF=67.5%, and [0102] efficiency .eta.=11.7%, which results nevertheless remain satisfactory.

[0103] Admittedly the performance is substantially inferior to that with CdS or ZnS buffer layers, but it remains, however, well above 10%, thereby validating this use of the invention in photovoltaic applications.

[0104] The deposition technique according to the invention may be applied homogeneously on large areas, with efficiency results ranging up to 9.5% for the following stack: glass/Mo/CIS(EVAP)/CdS(CBD)/i-ZnO(SPUT)/ZnO(ED) on plates of 17.5 cm.sup.2 area. FIG. 7 shows the J-V characteristic of such a structure.

[0105] The use of light to assist the electrodeposition according to the invention results in a spectacular improvement in the lateral homogeneity of the deposition. This advantage is a key point in the application of the process according to the invention on large areas. Typically the deposition could be carried out on areas ranging up to the order of 1 m.sup.2 or more, by virtue of an appropriate electrolysis device.

[0106] Overall, the efficiencies obtained are high for CIS-based cells with a CdS-based window film. The use of CIS substrates with ZnS-based interface films is novel and very promising. Moreover, the deposition of the ZnO surface film by electrolysis permanently obviates the subjacent intrinsic ZnO (or ZnMgO) film usually deposited by sputtering (reference TRA in FIG. 1).

[0107] Of course, the present invention is not limited to the embodiment described above by way of example; rather it extends to other variants.

[0108] For example, the transparent and/or conductive oxide coating film may be electrodeposited by fixing the potential at the electrode (deposition in what is called "potentiostatic" mode in a preferred embodiment) or by fixing the current flowing through the electrode (in what is called "galvanostatic" deposition mode).

Claims

1. A process for producing a photovoltaic structure, said structure comprising a thin coating film deposited on top of a sublayer having photovoltaic properties, wherein the coating film is deposited electrochemically, the deposition being at least partly assisted by illumination coming from a light source.

2. The process as claimed in claim 1, wherein the illumination of the photovoltaic sublayer causes an electrical effect conducive to uniform and homogeneous deposition of the coating film, at least at the start of growth of the coating film.

3. The process as claimed in claim 1, wherein the coating film is based on a transparent oxide and the coating film is intended to act as at least one of transparent and conductive window of a photovoltaic cell.

4. The process as claimed in claim 3, wherein the coating film is based on zinc oxide.

5. The process as claimed in claim 4, wherein the film is deposited by electrolysis with zinc ions dissolved in an electrolysis bath, by a cathodic reaction.

6. The process as claimed in claim 5, wherein the cathodic reaction is performed with addition of an oxygen donor element taken from at least one of oxygen, hydrogen peroxide and nitrates.

7. The process as claimed in claim 1, wherein the coating film is made conductive by doping with at least one element introduced in solution, taken from chlorine, fluorine, iodine, bromine, gallium, indium, boron and aluminum.

8. The process as claimed in claim 1, wherein the illumination is within the spectral range between 100 and 1500 nm and with an incident power between 0.1 mW/cm.sup.2 and 1 W/cm.sup.2.

9. The process as claimed in claim 1, wherein the photovoltaic sublayer consists of a I-III-VI.sub.2 alloy of the Cu(In,Ga,Al)(S,Se).sub.2 type.

10. The process as claimed in claim 1, wherein the coating film is deposited directly on the photovoltaic sublayer.

11. The process as claimed in claim 1, wherein the coating film is deposited on at least one interface film located above the photovoltaic sublayer.

12. The process as claimed in claim 11, wherein the interface film is a film based on cadmium sulfide.

13. The process as claimed in claim 11, wherein the interface film is a film based on zinc sulfide.

14. The process as claimed in claim 11, wherein the interface film is a film based on indium sulfide or gallium sulfide.

15. The process as claimed in claim 12, wherein the interface film is covered with a film of undoped zinc oxide or an undoped zinc oxide alloy.

16. The process as claimed in claim 1, wherein the coating film is deposited electrochemically at a temperature in the 50.degree. C. to 150.degree. C. range.

17. A facility for implementing the process as claimed in claim 1, comprising an electrolysis bath and a light source illuminating the photovoltaic film.