Contacts for a photovoltaic cell with two active surfaces

Abstract

Fabrication of a double-sided photovoltaic cell, with two opposite active surfaces, comprising a step of depositing, on each active surface, at least one electric contact. The deposition step comprises in particular a shared operation of depositing on each of the active surfaces, implemented by electrolysis in a shared electrolysis tank comprising: a first compartment for depositing a metal layer on a first active surface of the cell, for fabrication of a contact comprising said metal layer on the first active surface; and a second compartment for depositing, by oxidation, a metal oxide conductor layer on the second active surface of the cell, for the fabrication of a contact comprising said metal oxide layer on the second active surface.

Claims

1. A method for fabrication of a double-sided photovoltaic cell, with two opposite active surfaces, comprising: depositing, on each active surface, at least one electric contact, by electrolysis in a shared electrolysis tank, said shared electrolysis tank comprising: a first compartment for depositing a metal layer on a first active surface of the cell, for fabrication of a contact comprising said metal layer on the first active surface; and a second compartment for depositing, by oxidation, a metal oxide conductor layer on the second active surface of the cell, for the fabrication of a contact comprising said metal oxide layer on the second active surface.

2. The method according to claim 1, wherein the metal in the metal oxide layer is at least one element among Mn, Ni, Mo, Cd, Sn, In, Zn, Pb, Ag, Cu, W, Ta, Ga, Fe, Co, Cr and Ti.

3. The method according to claim 1, wherein, during said depositing: electrons are generated in the first compartment, opposite the first active surface, for enhancing the deposit of the metal layer; and holes are generated in the second compartment, opposite the second active surface, for enhancing the deposit by oxidation of the metal oxide layer.

4. The method according to claim 1, wherein the first and second compartments are separated by an ion exchange membrane that the electrolysis tank comprises.

5. The method according to claim 1, wherein, the deposition of at least one of the metal layer on the first active surface and of the metal oxide layer on the second active surface is assisted by lighting of at least one of the first and second active surfaces.

6. The method according to claim 1, wherein the depositing operation is assisted by application of a potential difference specific for generating at least negative charges on the first active surface.

7. The method according to claim 1, wherein the depositing operation is assisted by application of a potential difference specific for generating positive charges on the second active surface.

8. The method according to claim 1, wherein the photovoltaic cell is a double-sided homojunction with n.sup.+ type on the first active surface and p.sup.+ on the second active surface.

9. The method according to claim 1, wherein the photovoltaic cell is silicon-based.

10. The method according to claim 1, wherein the metal layer on the first active surface is nickel-based, and the step of depositing the electric contact on the first active surface is continued by depositing a layer comprising at least copper onto the metal layer, by electrolysis.

11. The method according to claim 1, wherein the step of depositing the electric contact on the second active surface is continued by depositing a layer comprising at least copper onto the metal oxide layer, by electrolysis.

12. The method according to claim 1, wherein depositing the contact is preceded by etching at least one electrically insulating layer, deposited on each active surface said etching exposes each active surface in a region in which the operation of depositing by electrolysis is applied.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics will appear upon reading the following detailed description of sample embodiments, and on examining the attached drawings on which:

(2) FIG. 1 shows an implementation for the fabrication of a cell by electrolysis according to an embodiment;

(3) FIG. 2 shows an example of a cell according to an embodiment;

(4) FIGS. 3 to 5 show alternative implementations to that from FIG. 1 for the fabrication of a cell by electrolysis according to an embodiment.

DETAILED DESCRIPTION

(5) In the sample embodiment presented below, the use of the semiconducting property of the P-N junction of the double-sided cell is proposed: the illumination of at least one of the two surfaces leads to the photogeneration of electron/hole pairs. Electrons are generated on the n.sup.+ surface and holes on the p.sup.+ surface. It is therefore possible to perform an electrochemical reaction of reduction on the n.sup.+ surface (and therefore deposit a metal), and in parallel to enhance an oxidative electrochemical reaction on the opposite p.sup.+ surface (and therefore deposit a metal oxide typically).

(6) Thus, without need for connection to an external electric circuit, if the photovoltaic material of the cell (for example the silicon) is in contact with two adequate electrolytes (one per surface) it is thus possible to implement a simultaneous deposit: on the n.sup.+ doped surface: electrodeposition of a metal (for example nickel); on the p.sup.+ doped surface: electrodeposition of a metal oxide (for example, MnO.sub.2, NiO, MoO.sub.3, CdO, SnO.sub.2, etc.).

(7) Thus, referring to FIG. 1, the substrate SUB (for example silicon), having photovoltaic properties and comprising opposite first and second active surfaces (respectively doped n.sup.+ and p.sup.+), is immersed in an electrolysis tank BA. An ion (anionic or cationic) exchange membrane ME is further provided in the extension of the substrate SUB for closing two respective compartments C1 and C2 in the tank.

(8) The illumination ECL of at least one of the surfaces enhances the photogeneration of electrons ne− on the first surface (in the compartment C1) and holes TR on the second surface (in the compartment C2). With the electrons in the compartment C1, metal (for example nickel, Ni) can be deposited on the free surface of the substrate (between the silicon nitride masks SiN.sub.x). In parallel and in particular simultaneously, in the compartment C2, holes (positive charges) are created enhancing metal oxide deposition, and in particular deposition of a conducting oxide (for example, MnO.sub.2 or another as presented later in the examples).

(9) In particular, the metal oxide electrochemically deposited on the p.sup.+ surface preferably has the following properties. It involves: An electrical conductor equivalent within an order of magnitude to nickel (14.Math.10.sup.6 S/m) Forming an ohmic contact with the silicon And a barrier to the diffusion of copper in the silicon Without degrading under thermal treatment (range of 350 to 550° C.) for the subsequent steps of fabrication of the cell, While also minimizing the level of impurities (for example of carbon) in the resulting layer.

(10) Further, if this oxide is transparent and conducting (like titanium oxide or ITO), electricity can be collected while allowing incident light to pass (reduced shadowing rate).

(11) Preferably, on this oxide, copper can advantageously be deposited by electrolysis for increasing the conductivity of the electrical contacts. Next, a fine layer of silver can be deposited on the copper to protect it from oxidation and improve the weldability in order to fabricate assembled photovoltaic modules. Finally, a thermal treatment can be done so that NiSi can form on the n.sup.+ surface, since NiSi has a role of ohmic contact on the silicon and in prevention of the diffusion of copper.

(12) Thus, the solar cell can have a stack of the type illustrated in FIG. 2 (in a sample implementation relating to a double-sided cell, here of N-PERT type, for “Passivated Emitter, Rear Totally Diffused”), in which: the reference MO.sub.x designates generally a metal oxide with M preferably, but without limitation, at least one of the elements: Mn, Cr, Fe, Co, Ni, Cu, Zn, Cd, W, Mo, In, Ga, Ta, and V. The noble metals (Pd, Pt, Au, and Ag) can also be considered, with however a higher fabrication cost; the reference SiN.sub.x designates one or more silicon nitrides (and SiO.sub.2 from the silica).

(13) Using an electrochemical technique for the formation of the electric contacts has many advantages (compared to the usual technique, screen printing): reduction of the fabrication cost (in particular by reduction of the quantity of silver to be deposited); and increased performance (contacts for very resistive emitters, improvement of shadowing rate, better conductivity).

(14) Further, illuminating a photovoltaic substrate for depositing base metal layers (including an oxide) on both surfaces, simultaneously and without contact with an external circuit, by wet route also has advantages: photocurrent generated by the diode can be used for electrodeposition of the metal and metal oxide. connection to an external circuit is not necessary because the oxidation-reduction pair on each of the surfaces is directly at the photovoltaic substrate/electrolyte interface.

(15) More precisely, again referring to FIG. 1, the phenomena used are: photogeneration of electrons on the n.sup.+ surface in contact with a reducible compound in the C1 compartment, with a reaction of the type: Ox.sub.1+ne.sup.−.fwdarw.Red.sub.1 photogeneration of holes on the p.sup.+ surface in contact with an oxidizable compound in the C2 compartment, with a reaction of the type: Red.sub.2.fwdarw.Ox.sub.2+ne.sup.−

(16) In order to avoid acidification or alkalization of the compartments, an ion exchange membrane (Nafion type, anionic or cationic) can separate the two components and maintain the ionic equilibrium.

(17) The illuminating device may comprise a row of diodes or halogen lamps having a maximum electromagnetic irradiation in the red visible or near infrared region. The radiation can preferably include the domain of ultraviolet optical wavelengths for activating and/or catalyzing the electrolyte/silicon interface in order to enhance nucleation at the silicon surface of the first deposition sites.

(18) Depending on the electrochemical reactions brought into play, the illumination can advantageously be applied on both surfaces, so that the electrochemical potentials on both surfaces can be adjusted if needed.

(19) Deposition speeds at least 10 times greater than those obtained by an autocatalytic type method (electroless method, the most effective currently known) can be achieved with the photogenerated current (of order of magnitude of several tens of mA.Math.cm.sup.−2). The activation step from the electroless method (starting of the nucleation of the nickel Ni contact for example using palladium particles) is not needed. The electrolytic baths implemented are simpler and more stable than the electroless baths and purer deposits can be achieved (less carbon type impurities in the resulting film). The overall method is therefore faster and less costly than the electroless method, with equivalent photovoltaic performance.

(20) More generally, the implementation in the meaning of the present disclosure presents many advantages compared to known techniques, such as the screen printing, electroless and LIP/FBP techniques. Already, the processing from the disclosure is simultaneous, quick and without electrical contact for both surfaces of the double-sided solar cells for the formation of electrical collection grids by preferably using illumination of one or both surfaces.

(21) More specifically, compared to the electroless method, it is possible with the technique from the disclosure: to incorporate less impurities (such as carbon or phosphorus) in the deposits; to save fabrication time by simultaneously treating both surfaces at a high deposition rate; to manage the bath more easily in terms of life of the bath, recharging, and deposition temperature.

(22) Compared to the screen-printing method, the disclosure provides in particular an improvement of the performance of the photovoltaic devices by: reducing the shadowing rate of the cell by reducing the size of the metallized pattern (width of the collecting fingers can be reduced over 50%); improving contact with very resistive (>120Ω) emitters; improving the electrical conductivity and reducing the contact resistance with the emitter.

(23) and also fabrication savings by: eliminating (or greatly reducing) silver by replacing it with copper; replacing the screen printing by an electrochemical method (less burdensome).

(24) Below, some specific implementation examples are presented:

(25) In a first sample implementation presented below, MnO.sub.2 and nickel are deposited simultaneously on the p.sup.+ and n.sup.+ surface respectively.

(26) In a tank with two compartments separated by a Nafion cation exchange member, using a system of joints ensuring the sealing of both surfaces, a nickel metallization bath is placed in the n.sup.+ compartment (C1) and an MnO.sub.2 metallization bath in the p.sup.+ compartment (C2). The substrate (silicon) is illuminated with a row of white LED lights on one surface.

(27) For depositing Ni, a temperature of 60° C. is applied for two minutes in a Watts bath. The nickel is deposited according to the following general reaction:
Ni.sup.2++2e.sup.−.fwdarw.Ni
2H.sup.++2e.sup.−.fwdarw.H.sub.2

(28) In parallel, for depositing MnO.sub.2, in a bath comprising 0.01 M Mn (C.sub.2H.sub.3O.sub.2).sub.2 (called Mn acetate), 0.02 M NH.sub.4 (C.sub.2H.sub.3O.sub.2) (called ammonium acetate) and 10% C.sub.2H.sub.6OS (dimethyl sulfoxide (DMSO)), a temperature of 60° C. is applied for two minutes. The manganese oxide is then deposited at the anode according to the following general reaction:
Mn.sup.2++2H.sub.2O.fwdarw.MnO.sub.2+4H.sup.++2e.sup.−

(29) Advantageously, the protons (H.sup.+) can be balanced between the two compartments by the ion exchange membrane.

(30) On both surfaces simultaneously, copper can next be deposited by direct electrolysis for increasing the conductivity of the electrical contacts. Next, a fine layer of silver can be deposited on the copper to protect it from oxidation and improve the weldability in order to fabricate photovoltaic modules. Finally, a thermal treatment (range from 350 to 550° C.) can be done so that a NiSi alloy can be formed on the n.sup.+ surface, which has a role of ohmic contact on the silicon and prevention of diffusion of the copper.

(31) In a second embodiment, cadmium oxide CdO is deposited on the p.sup.+ surface, and nickel is deposited on the n.sup.+ surface.

(32) CdO can be deposited at ambient temperature in 0.05 M cadmium acetate [Cd (C.sub.2H.sub.3O.sub.2).sub.2] and 0.1 M Na.sub.2SO.sub.4.

(33) The same operating mode is the one described in the first embodiment can next be applied (nickeling on the n.sup.+ surface, simultaneous coppering on both surfaces, silvering and then thermal treatment).

(34) In a third embodiment, tin oxide SnO.sub.2 is deposited on the p.sup.+ surface and nickel on the n.sup.+ surface, by providing 0.1 M SnCl.sub.2 and 0.5 M NaNO.sub.3 in DMSO.

(35) In a fourth embodiment, manganese oxide MnO.sub.2 is deposited on the p.sup.+ surface and nickel on the n.sup.+ surface with a single deposition bath because in this case, both surfaces of the substrate are exposed to the same electrolyte which contains at least one nickel salt (for example sulfate) and one manganese salt (for example an acetate). By application of standard potentials for electrochemical reactions, the reduction reaction of the nickel salt takes place at the cathode and the formation of the manganese oxide at the anode. In the case of a single bath, the ion exchange membrane is no longer necessary (ionic equilibrium is established by the exchange of protons between the reactions at the anode and the cathode).

(36) Of course the present disclosure is not limited to the embodiments presented above as examples; it extends to other variants.

(37) The principle of photogeneration of the current for example allows implementation variants. Typically, it can additionally be assisted by application of a potential between the p.sup.+ and n.sup.+ terminals of the substrate (preferred range of 0 to 2 V). The direction of polarization is shown in FIG. 3.

(38) Again, alternatively, relating to this implementation, it is possible to provide an electrode specific to the reaction and different from the substrate. The principle of photogeneration of the current remains the same but is assisted by application of a potential between the n.sup.+ terminal of the substrate and an anode (preferred range from 0 to 2 V). The direction of polarization is described in FIG. 4. The anode can be soluble (for example nickel, advantageously) or insoluble (for example an oxide, TiO.sub.2/IrO.sub.2).

(39) Alternatively again, the direction of polarization can be such as described in FIG. 5. In this case, the cathode is preferably insoluble (for example an oxide, TiO.sub.2/IrO.sub.2).

(40) Further, the use of lightning for enhancing the reactions in both compartments of the electrolytic tank was described above. This implementation is advantageous in the case of a silicon substrate in particular. Just the same, it is not absolutely necessary for example for a substrate other than silicon. Typically, in some substrate cases, the electrical stimulation shown in one of FIGS. 3 to 5 could suffice for example.