Method for manufacturing a photovoltaic module with annealing for forming a photovoltaic layer and electrically conducting region

Abstract

The invention relates to a method for manufacturing a photovoltaic module comprising plurality of solar cells in a thin-layer structure, in which the following are formed consecutively in the structure: an electrode on the rear surface (41), a photovoltaic layer (43) obtained by depositing components including metal precursors and at least one element taken from Se and S and by annealing such as to convert said components into a semiconductor material, and another semiconductor layer (44) in order to create a pn junction with the photovoltaic layer (43); characterized in that the metal precursors form, on the electrode on the rear surface (41), a continuous layer, while said at least one element forms a layer having at least one break making it possible, at the end of the annealing step, to leave an area (430) of the layer of metal precursors in the metal state at said break.

Claims

1. A process for producing a photovoltaic module comprising a plurality of solar cells in a thin film structure, in which the following are successively produced in the structure: a rear-face electrode (41), a photovoltaic film (43) obtained by deposition of constituents comprising metal precursors and at least one element taken from Se and S and, by annealing, to convert these constituents into a semiconducting material, and another semiconducting film (44), in order to create a pn junction with the photovoltaic film (43), characterized in that the metal precursors form, on the rear-face electrode (41), a continuous film (420), whereas said at least one element forms a film (421) exhibiting at least one discontinuity (422), making it possible, on conclusion of the annealing, to leave a region (430) of the film (420) of metal precursors in the metal state at said discontinuity, said process further comprising: depositing said at least one element of S and Se to form the film (421) on the layer of metal precursors (420), the film (421) of the at least one element of S and Se is deposited so as to form the at least one discontinuity forming an opening (422) so that the film of precursors (420) is devoid of said at least one element of S and Se in a left free region (423) facing the opening (422), under the annealing, the metal precursors contained in the film of precursors (420) react with said at least one element of S and Se contained in the film (421) to form said photovoltaic film (43) and the metallic precursors of said film of precursors (420) at said left free region (423) remain in metallic form by being not in contact with the at least one element of S and Se and thereby form an electrically conducting region (430) through the photovoltaic film (43).

2. The process as claimed in claim 1, characterized in that the film (421) of said at least one element is deposited in localized fashion.

3. The process as claimed in claim 1, characterized in that the metal precursors are of the Cu, Ga and In or Cu, Zn and Sn type.

4. The process as claimed in claim 1, characterized in that the annealing is carried out at a temperature of between 400 and 600 C. and preferably of the order of 550 C.

5. The process as claimed in claim 1, characterized in that the other semiconducting film (44) exhibits a discontinuity (440) at said region (430).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a to 1f illustrate a series of stages to make a monolithic interconnection for a thin film photovoltaic module.

(2) FIGS. 2a to 2e illustrate a series of stages to make a monolithic interconnection for a thin film photovoltaic module according to one embodiment of the invention.

(3) FIG. 3 shows another embodiment of the invention wherein the thin film has discontinuities.

DETAILED DESCRIPTION OF THE INVENTION

(4) According to the invention, the metal precursors form, on the rear-face electrode, a continuous film, whereas said at least one element forms a film exhibiting at least one discontinuity, making it possible, on conclusion of the annealing, to leave a region of the film of metal precursors in the metal state at said discontinuity.

(5) The process thus makes it possible to avoid the etching stage P2 which is provided in conventional interconnection processes.

(6) In a first embodiment, said at least one element is deposited in localized fashion.

(7) Preferably, the metal precursors are of the Cu, Ga and In or Cu, Zn and Sn type.

(8) The annealing is advantageously carried out at a temperature of between 400 and 600 C. and preferably of the order of 550 C.

(9) Advantageously, the other semiconducting film exhibits a discontinuity at said region.

(10) The invention also relates to a photovoltaic module comprising a plurality of solar cells connected in series on a common substrate, each cell comprising a front-face electrode which is transparent to light and a rear-face electrode, separated from the front-face electrode by a photovoltaic film and another semiconducting film which makes it possible to create a pn junction.

(11) According to the invention, the front-face electrode of a cell is connected electrically to the rear-face electrode of the adjacent cell via a region of the photovoltaic film which is composed of metal precursors.

(12) As this module does not comprise etching of P2 type, it exhibits, between two adjacent cells, only a single opening resulting from a stage of P3 etching emerging on the substrate.

(13) The electrical connection between the front-face electrode of a cell and the rear-face electrode of the adjacent cell is additionally of good quality.

(14) Furthermore, the other semiconducting film advantageously exhibits a discontinuity at the metal region of the photovoltaic film.

(15) The photovoltaic film is advantageously made of CIGS or of CZTS.

(16) A better understanding of the invention will be obtained and other aims, characteristics and advantages of the invention will become more clearly apparent on reading the description which follows and which is made with regard to the appended figures, in which:

(17) FIGS. 2a to 2e are cross sectional views which represent different stages of the process according to the invention and

(18) FIG. 3 is also a cross sectional view which represents an alternative form of the process according to the invention.

(19) FIG. 2a represents a substrate 4, which can be made of various materials, conventionally of glass, of plastic or of metal, covered with a thin isolating film. Generally this substrate is made of soda/lime glass, the thickness of which is a few millimeters and, for example, 3 mm.

(20) A metal film 41 is deposited on this substrate 4, which film forms a rear-face electrode for the various cells of the photovoltaic module which will be obtained by the process according to the invention. This film is made, for example, of molybdenum and its thickness is between 100 nm and 2 m and in particular equal to approximately 1 m.

(21) The deposition of the molybdenum film can in particular be carried by cathode sputtering.

(22) It should be pointed out that the film 41 can also be made of a semiconductor material, such as ITO, or of a nitride-based conducting material, such as ZrN or TiN.

(23) FIG. 2a shows that an etching stage is carried out after the deposition of the film 41. As indicated above, this etching is generally carried out either mechanically or by laser ablation. It results in the formation of a groove 410, which is thus devoid of metal.

(24) This groove 410 makes it possible to define the rear-face electrodes 41a and 41b of the adjacent cells 5 and 6 illustrated in FIG. 2e.

(25) This etching stage corresponds to the abovementioned stage P1.

(26) The width of this groove 410 is generally between 10 and 100 m and it is preferably of the order of 50 m.

(27) FIG. 2b illustrates the stage of the process in which the film 42, comprising the constituents which will result in the formation of the photovoltaic film, is produced.

(28) This film 42 comprises a film 420 of metal precursors, for example Cu, Ga and In, and a film 421, for example of selenium.

(29) The metal precursors can be deposited on the film 41 by printing or cathode sputtering methods.

(30) These precursors can be deposited in the elemental form, that is to say in the form of Cu, Ga and In, according to successive films. They can also be deposited simultaneously in the form of alloys, for example CuGa, CuIn or InGa.

(31) Preferably and with these same examples of constituents, the metal precursors are deposited in the form of at least two films, with a first film composed of the CuGa alloy and a second film composed of indium.

(32) In this case, the homogeneity of the film of metal precursors can be improved by carrying out an alternating deposition of fine CuGa and indium films. These films can in particular exhibit a thickness of approximately 50 nm, the deposition of successive films being carried out until the thickness desired for the film 420 is obtained.

(33) This thickness is generally between 300 nm and 1 m and it is preferably equal to 750 nm.

(34) Still in the example under consideration, the metal precursors are preferably deposited so that the following ratios are observed:
0.75Cu/(In+Ga)0.95; 0.55In/(In+Ga)0.85 and 0.15Ga/(In+Ga)0.45.

(35) These ratios are conventionally chosen in order to retain the electrical and optical properties desired for this type of semiconductor.

(36) The selenium film 421 is subsequently deposited, for example via vacuum evaporation, on the film 420.

(37) As illustrated in FIG. 2b, this film 421 is not continuous. On the contrary, it exhibits a discontinuity 422. In other words, the film 420 of metal precursors is devoid of selenium in the region 423 left free.

(38) In order to avoid short circuits, the distance between the discontinuity 422 and the etching 410 will advantageously be between 50 and 150 m and preferably of the order of 100 m.

(39) This discontinuity in the film 421 can be obtained by different processes.

(40) First of all, the deposition of selenium can be carried out in localized fashion by carrying out a vacuum evaporation through a mask.

(41) This mask can, for example, exhibit the following characteristics: be made of electrodeposited nickel and comprise one or more slits (the number of slits depending on the number of photovoltaic cells provided in the module to be produced), each slit exhibiting a width of between 50 and 150 m and preferably equal to substantially 100 m.

(42) The film 421 can also be deposited by printing methods (screen printing or inkjet printing, for example), using in particular an ink based on selenium nanoparticles dispersed in an organic solvent. This process exhibits the advantage of being less expensive as the printing methods can be carried out at atmospheric pressure.

(43) Furthermore, the thickness of the film 421 depends on the thickness of the film 420 of metal precursors.

(44) By way of example, for a film 420, the thickness of which is between 500 nm and 1 m, the selenium film 421 will exhibit a thickness of between 1 and 2 m. Furthermore, for a film 420 exhibiting a thickness of 775 nm, the selenium film 421 will exhibit a thickness of 1.5 m. This is because it is necessary to observe an excess Se stoichiometry of approximately 40%, it being known that the theoretical stoichiometry of the Cu(In, Ga)Se.sub.2 is such that Se/Cu=2. It is the same when the metal precursors chosen result in the formation of CZTS.

(45) FIG. 2c illustrates another stage of the process in which the film of constituents 42 is converted into a film of semiconductor material 43.

(46) The conversion is carried out conventionally by a high temperature annealing carried out under a neutral atmosphere, for example of nitrogen or argon.

(47) This annealing can be carried out at a temperature of between 400 and 600 C. and preferably at a temperature of approximately 550 C.

(48) The duration of the annealing is generally between 30 s and 5 min and it is preferably of a duration of approximately 1 min.

(49) Once the annealing has been carried out, the metal precursors and the selenium react to form a film of CIGS semiconductor 43 which is localized.

(50) This is because, in the region 423, the metal precursors were not in contact with the selenium and they will thus remain in metallic form.

(51) The region of the film 43 in which the precursors remain in metallic form is identified by the reference 430 in FIG. 2c.

(52) In this region 430 and as a function of the annealing temperature, the metal precursors will be in the form of InGa alloy, CuGA alloy, CuIn alloy, elemental indium, elemental Ga or elemental Cu, or also a mixture of a portion or all of these elements.

(53) It should be noted that, during the annealing, a portion of the selenium also diffuses close to the discontinuity 422 and thus reacts with the metal precursors present in the film 420, at the region 423. Consequently, the width of the region 430 is less than the width of the opening 422.

(54) By way of example, under annealing conditions such that the temperature rise gradient is 10 C./s with a maximum at 550 C. for 1 min and at an argon pressure of 900 mbar, and with a thickness of the selenium film 421 of 1.5 m, the width of the region 430 will be of the order of 85 m for an opening 422 with a width of 100 m.

(55) Generally, it is found that, the greater the thickness of the selenium film, the smaller the width of the metal region 430.

(56) Likewise, the greater the duration of the annealing, the smaller the width of the region 430.

(57) This metal region 430 will make it possible to ensure the electrical connection between the front-face electrode of a cell of the photovoltaic module with the rear-face electrode of the adjacent cell, without it being necessary to carry out a stage of etching in the photovoltaic film, which etching corresponds to the stage P2 according to the state of the art.

(58) Generally, the width of the region 430 will be between 50 and 150 m, whereas the width of the discontinuity 422 will be between 60 and 160 m.

(59) FIGS. 2d and 2e describe the other stages of the process according to the invention, which are similar to those described with reference to FIGS. 1c, 1e and 1f.

(60) Thus, with reference to FIG. 2d, a film 44 of n-type semiconductor is deposited on the film 46, in order to form the pn junction. As indicated from the viewpoint of FIG. 1c, the material used can be CdS, ZnS or ZnOS, in particular deposited with a chemical bath.

(61) FIG. 2d illustrates another stage of the process which consists in depositing a film of a transparent and conducting oxide 45 on the film 44.

(62) It may be noted that an intermediate ZnO film can be deposited between the films 44 and 45. This intermediate film is optional. Its role has been described above with reference to FIG. 1e.

(63) This film 45 can be composed of Al-doped ZnO, this film being deposited by cathode sputtering.

(64) The thickness of the film 45 is between 100 and 800 nm and preferably equal to approximately 500 nm.

(65) Finally, FIG. 2e illustrates a final stage of the process, in which another etching is carried out in the stack of films.

(66) This etching stage corresponds to the stage P3.

(67) The opening 431 obtained makes it possible to define two adjacent cells 5 and 6 and to isolate them electrically, at their front-face electrodes 45a and 45b.

(68) Generally, the information given for the implementation of the stages illustrated in FIGS. 1c, 1e and 1f is also valid for the stages illustrated from the viewpoint of FIGS. 2d and 2e.

(69) FIG. 2e also illustrates the pathway of the charges between two adjacent cells 5 and 6.

(70) Thus, the front-face electrode 45a of the first cell 5 makes it possible to collect, on the front face, the electric charges generated in this cell 5 and convey them to the rear-face electrode 41b of the adjacent cell 6, through the metal region 430 of the film 43.

(71) While on this subject, it should be noted that the thickness of the film 44 is sufficiently small to allow the electric charges to pass from the electrode 45a to the region 430.

(72) Generally, the thickness of the film 44 is less than 100 nm and it is preferably of the order of 10 nm.

(73) Thus, the process which has just been described exhibits the advantage of eliminating one of the etching stages conventionally provided in monolithic interconnection processes, in this case the stage P2, and thus to be freed from the disadvantages related to this etching stage.

(74) As an alternative form, the selenium film 421 can be replaced by a film of sulfur or of a mixture of sulfur and selenium.

(75) The sulfur can be deposited on the film 420 by the same deposition processes as those mentioned above for the selenium.

(76) However, in order to deposit a mixture of sulfur and selenium, it will be possible to use a printing method, such as inkjet printing, starting from ink based on sulfur and selenium.

(77) The deposition of sulfur and selenium can also be carried out under vacuum while alternating the selenium film or films and the sulfur film or films in order to obtain a distribution according to an S/Se bilayer or an S/Se multilayer, the sulfur and the selenium being alternated. The formation of a multilayer makes it possible to improve the distribution of the two components. Thus, the annealing will then result in the formation of Cu(In, Ga) (S, Se).sub.2.

(78) The metal constituents forming the film 420 can also be of the Cu, Zn and Sn type.

(79) The metal precursors are preferably deposited so that the following ratios are observed:
0.75Cu/(Zn+Sn)0.95 and 1.05Zn/Sn1.35.

(80) The process according to the invention then results in solar cells being obtained, the photovoltaic film of which is made of a material of the CZTS type and in particular Cu.sub.2ZnSnSe.sub.4, Cu.sub.2ZnSnS.sub.4 or Cu.sub.2ZnSn (S, Se).sub.4, according to whether the film 421 comprises selenium, sulfur or a mixture of the two components.

(81) Germanium can also be incorporated in the unit cell of the CZTS to form a material of the Cu.sub.2Zn(Sn, Ge)(S, Se).sub.4 type, when the film 421 comprises a mixture of selenium and sulfur.

(82) The metal constituents forming the film 420 can also be of the Cu, Al and In type.

(83) The process according to the invention then results in solar cells being obtained, the photovoltaic film of which is made of a material of the Cu(In, Al)(S, Se).sub.2 type.

(84) Another alternative embodiment of the process is described with reference to FIG. 3.

(85) FIG. 3 shows that the film 44, in particular made of ZnOS, is not deposited continuously over the photovoltaic film 43. On the contrary, the film 44 comprises discontinuities 440 at the regions 430 of the photovoltaic film 43.

(86) This localized deposition can, for example, be carried out by inkjet printing. It exhibits the advantage, in comparison with the embodiment illustrated in FIG. 2e, of avoiding the addition of a resistance which would originate from the presence of the film 44 between the electrode 45a and the region 430.

(87) The sole aim of the reference signs inserted after the technical characteristics appearing in the claims is to facilitate understanding of the latter and should not limit the scope thereof.