THIN-FILM PHOTOVOLTAIC DEVICE AND ASSOCIATED METHOD OF FABRICATION

Abstract

A thin-film photovoltaic device is proposed having an optimized layout for one of the electrical contacts. The device comprises a substrate. A first thin film forming a first electrical contact of the photovoltaic device is arranged on the substrate. An absorber is arranged on the first electrical contact. A second thin film forming a second electrical contact of the photovoltaic device is arranged on the substrate. A transparent conductive layer is arranged on the absorber. In addition, the second electrical contact is spaced apart from the first electrical contact, and the transparent conductive layer is in contact with the absorber and the second electrical contact.

Claims

1: A thin-film photovoltaic device in the form of a semi-transparent photovoltaic panel, comprising: a substrate, at least a first electrically conductive thin layer forming a first electrical contact of the photovoltaic device on the substrate, an absorber arranged on the first electrical contact, at least a second electrically conductive thin layer forming a second electrical contact of the photovoltaic device on the substrate, a transparent conductive layer arranged on the absorber, the second electrical contact being spaced apart from the first electrical contact, and the transparent conductive layer being in contact with the absorber and the second electrical contact.

2: The device according to claim 1, wherein the second electrical contact and the first electrical contact are structured in an interleaved manner.

3: The device according to claim 1, wherein the second electrical contact comprises each of the layers of the first electrical contact.

4: The device according to claim 1, wherein the absorber covers side faces and an upper face of the first electrical contact.

5: The device according to claim 1, wherein the second electrical contact further comprises a material selected among: 316L stainless steel, an alloy of FeCrNiMo, NiMoP, MoO.sub.2, ZnO, SnO.sub.2, aluminum-doped ZnO, fluorine-doped tin oxide FTO, steels containing carbon and manganese, compounds based on cobalt and phosphorus.

6: The device according to claim 1, wherein the photovoltaic device is a light-concentrating device.

7: A method for fabricating a thin film photovoltaic device in the form of a semi-transparent photovoltaic panel, comprising: forming at least a first electrically conductive thin layer forming a first electrical contact of the photovoltaic device on a substrate, forming at least a second electrically conductive thin layer forming a second electrical contact of the photovoltaic device on the substrate, depositing an absorber on the first electrical contact, depositing a transparent conductive layer on the absorber, the second electrical contact being spaced apart from the first electrical contact, and the transparent conductive layer being in contact with the absorber and the second electrical contact.

8: The method according to claim 7, further comprising: forming the second electrically conductive thin layer by etching the first electrical contact.

9: The method according to claim 7, further comprising: forming the first and second electrical contacts such that the second electrical contact and the first electrical contact are structured in an interleaved manner.

10: The method according to claim 7, further comprising: forming the first and second electrical contacts such that the second electrical contact is spaced apart from the first electrical contact by a distance substantially equal to or greater than a thickness of the absorber.

11: The method according to claim 7, wherein: depositing the absorber on the first electrical contact further comprises: depositing a precursor material of the absorber on the first electrical contact by a technique selected among: electrodeposition, evaporation, ink printing, incorporating a material selected among selenium and sulfur into the precursor material.

12: The method according to claim 7, further comprising: forming a corrosion-resistant, electrically conductive material that is stable in selenization or sulfurization, on the second electrical contact.

13: The method according to claim 12, further comprising: forming the corrosion-resistant, electrically conductive material that is stable in selenization or sulfurization, by a technique selected among: electrodeposition, thermal oxidation in an oxidizing atmosphere of a conductive layer of the second electrical contact, electrochemical oxidation of a conductive layer of the second electrical contact.

14: The method according to claim 12, further comprising: treating the corrosion-resistant, electrically conductive material that is stable in selenization or sulfurization, by adding phosphorus.

15: The method according to claim 7, further comprising: depositing an electrically conductive layer on the second electrical contact, subsequent to an annealing step.

Description

DESCRIPTION OF FIGURES

[0065] The method of the invention will be better understood from reading the following description of some exemplary embodiments presented for illustrative purposes, which are in no way limiting, and from studying the following drawings in which:

[0066] FIG. 1 is a schematic side view of a thin-film photovoltaic device according to one embodiment;

[0067] FIG. 2 is a schematic top view of a thin-film photovoltaic device according to one embodiment;

[0068] FIG. 3 is a schematic representation of six steps of a method for fabricating a photovoltaic device according to one embodiment.

[0069] For clarity, the dimensions of the various elements represented in these figures are not necessarily in proportion to their actual dimensions. In the figures, identical references correspond to identical elements.

DETAILED DESCRIPTION

[0070] The present invention relates to a photovoltaic device having electrical contacts arranged in an optimized manner relative to the absorber. The electrical contacts described below are also easier to construct and can be precisely positioned relative to an absorber.

[0071] In particular, the invention relates to thin-film solar cells which can be arranged in a solar panel possibly equipped with a light concentration system.

[0072] The invention proposes a unique arrangement for the two electrical contacts located near the absorber, which makes it possible to avoid obscuring the absorber and enables simple fabrication as close as possible to the absorber.

[0073] FIG. 1 schematically illustrates a sectional view of a photovoltaic device 100 according to an embodiment of the invention. This photovoltaic device comprises a substrate 10, on which rests a first stack of electrically conductive thin layers forming a first electrical contact 1. A second stack of electrically conductive thin layers forming a second electrical contact 2 rests on the same substrate 10.

[0074] Although FIG. 1 illustrates a stack of thin layers, it is possible to use a single layer to form the first and second electrical contacts.

[0075] As represented in FIG. 1, the entire lower surface of the two electrical contacts rests on the substrate 10.

[0076] In FIG. 1, the first and second electrical contacts are created from the same stack of layers. This stack consists of a conductive layer 11, generally made of metal such as molybdenum having a thickness comprised between 500 nm and 1 m. Alternatively, this conductive layer 11 may be a transparent conductive oxide such as aluminum-doped ZnO, boron or chlorine, fluorine-doped tin oxide FTO, indium-doped tin oxide ITO, or SnO.sub.2. A barrier layer 12 having a typical thickness of 50 nm of a metal alloy such as MoN or TiN for example serves to protect the conductive layer 11 from a selenization or sulfurization annealing. A metal overlayer 13 arranged on the barrier layer 12 serves to encourage deposition of the absorber on the first electrical contact. This overlayer 13 is usually of molybdenum or copper and has a thickness typically comprised between 50 nm and 300 nm. The Mo overlayer is sacrificial. During annealing, it assists m forming a layer of MoSe.sub.2 which ensures the ohmicity of the electrical contact.

[0077] As represented in FIG. 1, the first and second electrical contacts are both arranged on the same face of the substrate 10, on the same side of the substrate 10 as the absorber 3 possessing photovoltaic properties. Given this arrangement, the substrate 10 is electrically insulating according to the representation in FIG. 1, in order to prevent a short circuit between the first and second electrical contacts. However, as an alternative, it is possible to provide an insulating layer on the substrate 10 and below the first and second electrical contacts.

[0078] The second electrical contact 2 is separated from the first electrical contact 1 by a space 15. This space 15 makes it possible to electrically separate the second electrical contact 2 from the first electrical contact 1. During fabrication of the first and second electrical contacts, the width of this space 15 can be adjusted, as can the shape of the first and second electrical contacts.

[0079] The first electrical contact 1 comprises an absorber 3, on at least its upper surface. The absorber 3, as shown in FIG. 1, also covers the side faces of the first electrical contact 1. Such encapsulation of the first electrical contact 1 by the absorber 3 reduces leakage currents from the first electrical contact 1 towards the second electrical contact 2. However, it is possible that the absorber 3 only partially covers the side surfaces or does not cover them at all. Leakage currents can then be reduced for example by a deposition of an insulating layer in the space 15 between the first electrical contact 1 and the second electrical contact 2.

[0080] The second electrical contact 2 may comprise, on at least a portion of its surface, a corrosion-resistant, electrically conductive material 4 that is stable in selenization or sulfurization. This material protects the second electrical contact 2 from the corrosion caused by a possible subsequent step of electrodeposition, as well as from an annealing that could be damaging to metal layers. Annealing in a selenium or sulfur atmosphere typically takes place during fabrication of the absorber 3, subsequent to the deposition of the second electrical contact 2. Preferably, the corrosion-resistant, electrically conductive material 4 that is stable in selenization or sulfurization also has the property of being easy to deposit on a layer of molybdenum or copper, and of having good resistance to ammonia, as ammonia may be involved during the deposition of buffer layers covering the absorber 3.

[0081] However, it should noted that the presence of material 4 in the second electrical contact is not necessary. For example when layer 13 is of MoN or TiN, the second electrical contact can withstand annealing without suffering substantial damage.

[0082] As a non-limiting example, the corrosion-resistant, electrically conductive material 4 that is stable in selenization or sulfurization can be selected among: 316L stainless steel, an alloy of FeCrNiMo, NiMoP, MoO.sub.2, ZnO. SnO.sub.2, aluminum-doped ZnO, fluorine-doped tin oxide FTO, steels containing carbon and manganese, compounds based on cobalt and phosphorus 316L stainless steel is a particularly suitable material for protecting the second electrical contact 2, 316L stainless steel typically consists of 0.02% carbon C, 16% to 18% chromium Cr, 10.5% to 13% nickel Ni, 2% to 2.5% molybdenum Mo, and 2% manganese Mn. The presence of nickel and chromium typically contributes to corrosion resistance in an electrodeposition bath. Chromium reacts with oxygen in the air and forms a chromium oxide layer. Nickel integrates into the oxide layer and improves the properties of the passive layer. The presence of metals such as molybdenum, titanium, or copper can further improve the chemical resistance of the second electrical contact 2, particularly in non-oxidizing environments.

[0083] In addition, it is possible to further improve the corrosion resistance of the corrosion-resistant, electrically conductive material 4 that is stable in selenization or sulfurization, by the addition of phosphorus P. Phosphorus promotes the formation of an amorphous structure in an alloy such as 316L stainless steel.

[0084] FTO has the characteristic of resistance to selenization or sulfurization for about an hour at temperatures of 600 C. and may also be suitable as material 4. Similarly, aluminum-doped ZnO only partially reacts with seleniun and sulfur at a temperature of 600 C. and may also be suitable as material 4.

[0085] As represented in FIG. 1, the second electrical contact 2 may comprise more layers than the first electrical contact 1. In addition to the possible presence of a corrosion-resistant electrically conductive material that is stable in selenization or sulfurization, the second electrical contact may comprise, on its upper surface, a conductive overlayer of an electrically conductive material 14, typically zinc. Metals other than zinc may also be considered. This conductive overlayer allows adjusting the electrical conduction properties of the second electrical contact 2.

[0086] Electrical contact between the absorber 3 and the second electrical contact 2 is provided by the transparent electrically conductive layer 5 arranged on the absorber, in contact with the absorber and the second electrical contact. This transparent layer 5 may typically be the window layer of aluminum-doped zinc oxide of the photovoltaic device 100. Alternatively, the transparent layer 5 may for example be chlorine-doped ZnO, boron-doped, ZnMgO, indium-doped tin oxide ITO, or FTO.

[0087] This original arrangement of the first and second electrical contacts in the photovoltaic device 100 enables accurate and maximized closeness between the first and second electrical contacts, in order to carry away the high current densities that typically can occur in a light-concentrating solar cell.

[0088] In addition to minimizing the space 15, the efficiency with which the electric current generated in the absorber is collected by the first electrical contact 1 and second electrical contact 2 is dependent on the shape of these electrical contacts.

[0089] FIG. 2 schematically represents a top view of an exemplary arrangement between the first electrical contact 1 and the second electrical contact 2. The device shown consists of substantially rectangular electrical contacts, arranged so as to form interleaved combs. The structure shown in FIG. 2 is particularly suitable for discharging high current densities in a light-concentrating photovoltaic device 100. The system for focusing the light on the absorber 3 is not shown in FIG. 2.

[0090] Given the fact that the first electrical contact 1 is interleaved with the second electrical contact 2, the portion of the second electrical contact 2 facing the first electrical contact is large, while maintaining a small space 15. This allows effectively discharging high current densities while allowing the use of a second electrical contact of reduced dimensions. Thus, as illustrated in FIG. 2, most of the upper surface of the substrate 10 is not covered either by the first electrical contact 1 or the second electrical contact 2. Free space 20 allows incident light to reach the substrate directly and possibly to pass through it. Thus, it is possible to conceive of using the photovoltaic device of the invention in window glass, particularly when the substrate 10 is translucent or transparent.

[0091] The interleaved structure shown in FIG. 2 can have many variations. The shapes of the first and second electrical contacts can vary, as can the space 15 between these contacts. One should, however, prefer structures in which the first contact comprises protrusions arranged facing portions of complementary shape created in the second contact, maintaining a substantially constant gap between the first and second contacts along their mutually facing portions. According to an advantageous variant, the ends of the interleaved structure of the first and second electrical contacts may be of trapezoidal shape. The base of the trapezoids of the contacts, near the terminals of the cell, serves to carry away larger amounts of current.

[0092] In addition, the shape of the electrical contacts may be chosen so as to give a solar panel a more aesthetic form, particularly when the substrate is transparent and comprises a light concentration system.

[0093] The photovoltaic device 100 according to the invention also has the advantage of benefiting from a fabrication method that is particularly simple and inexpensive to implement.

[0094] For example, FIG. 3 shows six steps of fabricating a photovoltaic device 100 according to the invention.

[0095] A first step S1 may comprise the deposition of at least one metal layer on a substrate 10. In the example of FIG. 3, the substrate is electrically insulating, and a stack of three layers as described above is deposited on its upper surface.

[0096] In a second step S2, the stack created in step S1 is etched so that two separate networks forming the first electrical contact 1 and the second electrical contact 2 are exposed in the stack, before the addition of supplemental layers. This etching may be accomplished by laser ablation, electrical discharge machining, or any other known etching process, to create two networks whose spacing is controlled and selected.

[0097] As an alternative to these two steps, it is also possible to consider a direct deposition of two networks of distinct metal layers to form the two electrical contacts on the same substrate 10.

[0098] Next, in a step S3, an absorber precursor 30 is deposited on the first electrical contact 1. This deposition may be carried out by electrodeposition, a particularly advantageous selective process which uses a current applied to the first electrical contact in order to selectively deposit indium and gallium only on this electrical contact. Other selective processes can be considered, such as ink printing. A selective evaporation process may also be considered. Alternatively, non-selective processes such as sputtering or co-evaporation may be implemented, with the use of a mask.

[0099] In addition, an electrically conductive material 14 may be deposited on the upper surface of the second electrical contact 2, preferably after a selenization or sulfurization annealing step. This electrically conductive material 14 may for example be zinc. It may be deposited on at least a portion of the upper surface of the second electrical contact 2, or may cover the upper surface of the second electrical contact 2 and side faces of the second electrical contact.

[0100] In a step S4, the corrosion-resistant electrically conductive material 4 that is stable in selenization or sulfurization is formed on the second electrical contact 2. This deposition may occur in different ways. Preferably, it is carried out by selective electrodeposition. This step S4 of formation of material 4 is optional.

[0101] Alternatively, it may also be carried out by thermal oxidation in an oxidizing atmosphere, such as air or oxygen, by causing current to flow in the second electrical contact 2 which heats said contact. Thermal oxidation allows oxidizing the molybdenum of the second electrical contact so as to create MoO.sub.2, while controlling various parameters of the oxidation such as the current applied to the second electrical contact 2, the duration of the oxidation, and the concentration of oxidizing agents.

[0102] Another variant consists of carrying out an electrochemical oxidation of the molybdenum of the second electrical contact 2, by controlling the electric potential applied to the second electrical contact and the pH of an electrolytic solution into which the sample being fabricated is immersed.

[0103] It should be noted that steps S3 and S4 described above can be swapped.

[0104] Next, the precursor 30 is thermally annealed under a selenium or sulfur atmosphere in a step S5. The second electrical contact 2 is then protected by the material 4 forming a protective layer around metals otherwise susceptible to damage during this annealing step.

[0105] Finally, the fabrication of the photovoltaic device 100) is completed by the possible deposition of a buffer layer and a window layer. The electrically conductive transparent layer 5 ensuring an electrical contact between the absorber 3 and the second electrical contact 2 may be the window layer of the photovoltaic device or another layer. Step S6 schematically represents the photovoltaic device 100 so obtained.

[0106] The thin-film photovoltaic device described above and its method of fabrication offer electrical contacts that are accurately positioned as close as possible to an absorber without obscuring the absorber. In addition, the fabrication of this device is facilitated and is less expensive in comparison to the fabrication of devices of the prior art. The photovoltaic device described above is particularly suitable for applications concerning a light-concentrating photovoltaic device.