SEMI-TRANSPARENT THIN-FILM PHOTOVOLTAIC DEVICE PROVIDED WITH AN OPTIMIZED METAL/NATIVE OXIDE/METAL ELECTRICAL CONTACT

Abstract

A thin-film semi-transparent photovoltaic device comprising: a plurality of active photovoltaic zones, having a surface S.sub.5, formed of: a transparent substrate; a front electrode formed of a transparent electroconductive material arranged on the transparent substrate; an absorber made up of one or more photoactive thin layer(s); a rear electrode formed of a stack of at least: a conductive metal layer; and a native metal oxide layer having a nanometric thickness. The device additionally includes a plurality of transparent zones separating at least two active photovoltaic zones; and a metal reconnection layer having a contact surface S to the rear electrode, wherein the ratio R.sub.a=S/S.sub.5 between the contact surface S of the metal reconnection layer and the surface S.sub.5 of an active photovoltaic zone is such that 0.2%<R.sub.a<2%.

Claims

1. A semi-transparent thin-film photovoltaic device comprising: a plurality of active photovoltaic zones, having a surface S.sub.5, formed of: a transparent substrate; a front electrode formed of a transparent electroconductive material arranged on the transparent substrate; an absorber made up of one or more photoactive thin layers; and a rear electrode formed of a stack of at least: a conductive metal layer; and a native metal oxide layer having a nanometric thickness; a plurality of transparent zones separating at least of the two active photovoltaic zones; and a metal reconnection layer having a contact surface S to the rear electrode; wherein the ratio R.sub.a=S/S.sub.5 between the contact surface S of the metal reconnection layer and the surface S.sub.5 of an active photovoltaic zone is such that 0.2%<R.sub.a<2%.

2. The device of claim 1, wherein the ratio R.sub.a is such that 1.6%<R.sub.a<2%.

3. The device of claim 1, wherein the conductive metal layer is made of aluminum and the native oxide layer is made of alumina.

4. The device of claim 1, wherein the metal reconnection layer is made of aluminum.

5. The device of claim 1, wherein the contact surface of the metal reconnection layer includes a plurality of electrically interconnected patterns.

6. The device of claim 1, wherein the contact surface S between the metal reconnection layer and the rear electrode is rectangular in shape.

7. A semi-transparent thin-film photovoltaic device comprising: a plurality of active photovoltaic zones, having a surface S.sub.5, formed of: a transparent substrate; a front electrode formed of a transparent electroconductive material arranged on the transparent substrate; an absorber made up of one or more photoactive thin layers; and a rear electrode formed of a stack of at least: a conductive metal layer; and a native metal oxide layer having a nanometric thickness; a plurality of transparent zones separating at least of the two active photovoltaic zones; and a metal reconnection layer having a contact surface S to the rear electrode; wherein the ratio R.sub.a=S/S.sub.5 between the contact surface S of the metal reconnection layer and the surface S.sub.5 of an active photovoltaic zone is such that 1.6%<R.sub.a<2%.

8. The device of claim 7, wherein the conductive metal layer is made of aluminum and the native oxide layer is made of alumina.

9. The device of claim 7, wherein the metal reconnection layer is made of aluminum.

10. The device of claim 7, wherein the contact surface of the metal reconnection layer includes a plurality of electrically interconnected patterns.

11. The device of claim 7, wherein the contact surface S between the metal reconnection layer and the rear electrode is rectangular in shape.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0022] FIG. 1A is a cross section of a thin-film photovoltaic stack.

[0023] FIG. 1B is a plan view of the photovoltaic stack of FIG. 1A, within which a plurality of thin layers have been etched in regions in order to form transparent zones and active photovoltaic zones.

[0024] FIG. 1C is a plan view of the photovoltaic device of FIG. 1B, to which the metal reconnection zones according to the invention have been added.

[0025] FIG. 1D is a cross section according to the direction X of FIG. 1C.

[0026] FIG. 1E is a cross section taken from FIG. 1D, to which the insulation layer has been added.

[0027] FIG. 1F is a cross section taken from FIG. 1E, to which the metal reconnection layers have been added.

[0028] FIG. 2A is a diagram of a part of a semi-transparent photovoltaic cell according one embodiment of the invention.

[0029] FIG. 2B shows the development of the total electrical resistance R of photovoltaic devices having the same intrinsic characteristics, as a function of the length L of the rectangular contact surface.

[0030] FIG. 3A and FIG. 3B are diagrams of photovoltaic cells which are similar to FIG. 2A and correspond to other embodiments of the invention.

[0031] FIG. 3C shows another example photovoltaic cell.

DETAILED DESCRIPTION

[0032] FIG. 1A is a cross section of an example photovoltaic stack. In this example, the stack is made up of: [0033] a glass substrate (1); [0034] a front electrode (2) formed of a transparent conductive oxide, for example aluminum-doped zinc oxide (ZnO:Al); [0035] an absorber (3) made up of a plurality of layers based on amorphous silicon (a_Si) forming a p-i-n junction; and [0036] a rear electrode (4) formed of: [0037] a layer of aluminum (41); [0038] a native oxide layer (42) of alumina.

[0039] It is possible to transform said stack, by means of photolithographic etching methods and deposition methods known to a person skilled in the art, in order to obtain a semi-transparent photovoltaic module. The first step of this method consists in forming transparent zones (6.sub.T), and in electrically insulating the collector buses (7+, 7−) by means of insulation zones (6.sub.I). The transparent and insulation zones (6.sub.T and 6.sub.I) are formed by successive etching of the thin layers forming the rear electrode, the absorber and the front electrode.

[0040] FIG. 1B is a plan view of semi-transparent photovoltaic cells, the transparent zones (6.sub.T) of which are in the shape of mutually parallel horizontal strips which separate, in pairs, the opaque active photovoltaic zones (5) having a critical dimension CD.sub.5 (corresponding in this case to the width thereof) and a length L.sub.5. The surface S.sub.5 of the photovoltaic strip is thus equal to the product of the critical dimension CD.sub.5 times the length L.sub.5. The vertical opaque strips are the collector buses (7.sup.+ and 7.sup.−) which are electrically insulated from the active photovoltaic zones (5). The collector buses (7.sup.+ and 7.sup.−, respectively) have a critical dimension denoted CD.sup.+ and CD.sup.−, respectively. The transparent zones (6.sub.T) electrically insulate the active photovoltaic strips (5), each of said strips forming unitary photovoltaic cells. The transparent zones (6.sub.T) have a critical dimension denoted CD.sub.T.

[0041] In order to electrically connect (in series and/or parallel) said insulated active photovoltaic zones to the collector buses (7.sup.+ and 7.sup.−) so as to obtain a photovoltaic module, it is necessary to implement electrical contact between the front electrode (2) and one of the collector buses (7.sup.+), and electrical contact between the rear electrode (4) and the other collector bus (7.sup.−).

[0042] The implementation of a reconnection of the VIA (8) type and of the rear electrode (4) type comprises a plurality of successive steps which may be implemented simultaneously.

[0043] Step 1: Reconnection zones of the VIA (8) type are etched within the active photovoltaic zones (5). An active photovoltaic zone (4A) close to the collector bus (7.sup.−) is left without a VIA. It is precisely within this zone that the reconnection between the rear electrode (4) and the metal layer (14) takes place, the dimensioning of which is addressed by the invention. FIG. 1D is a cross section of FIG. 1C according to the direction X, where the reconnection zones of the VIA (8) type appear and the active photovoltaic zone (4A) in the vicinity of the collector bus (7.sup.−) is left without a VIA.

[0044] Step 2: An electrical insulation layer (9) is introduced in order to electrically insulate the front electrode (2) from the rear electrode (4). FIG. 1E is a cross section taken from FIG. 1D, to which the insulation layer (9) has been added. This electrical insulation layer is for example a transparent, permanent and photosensitive resin. Rear reconnection zones (4B) are left vacant within the active photovoltaic zones (4A) available for the reconnection of the rear electrode, in order to implement the reconnection on the metal (4).

[0045] Step 3: A metal reconnection layer is thus deposited and etched. It is thus split into two distinct zones (18 and 14), as shown in FIG. 1F. It may be etched for example, by way of a new photolithography step, in order to connect the front electrode (2) to the collector bus (7.sup.+) and the rear electrode (4) to the collector bus (7.sup.−), in order to make the semi-transparent photovoltaic module functional.

[0046] Example embodiments of the invention aims to improve the reconnection between the rear electrode (4) and the collector bus (7.sup.−) by optimizing the contact surface S between the rear electrode (4) and the metal reconnection layer (14).

[0047] In order to determine the optimal characteristics of said contact surface, a plurality of semi-transparent thin-film photovoltaic devices have been produced, an example of which is described in FIG. 2A. Said devices have the same physical and architectural characteristics. The manufacturing process is entirely identical for all the versions of said devices produced. Said devices differ merely by the contact surface S between the rear electrode (4) and the metal reconnection layer (14). In said devices, the surface S is rectangular in shape, has a critical dimension CD of 8 μm, and a length L. Said length L varies from 80 μm to 960 μm, depending on the device in question.

[0048] The total surface of said devices is 2.5 cm by 2.5 cm, i.e. 6.26 cm.sup.2, having an area ratio of transparent zones of 50%. Said devices comprise: [0049] a glass transparent substrate (1); [0050] a front electrode (2) formed of aluminum-doped zinc oxide (ZnO:Al); [0051] an absorber (3) consisting substantially of amorphous silicon (a_Si); [0052] a metal rear electrode (4) formed of: [0053] a layer of aluminum (40) of 500 nm and having a mean square surface roughness of 15 nm; [0054] a native oxide layer (41) of alumina of 4 nm; [0055] two collector buses (7.sup.+ and 7.sup.−) having a critical dimension CD.sup.+ and CD.sup.− of 1 mm; [0056] active photovoltaic zones (5), the critical dimension CD.sub.5 of which is 15 μm and the length L.sub.5 of which is 23 mm, thus having a surface S.sub.5 of 345,000 μm.sup.2; [0057] transparent zones (6.sub.T), the critical dimension CD.sub.T of which is 15 μm, [0058] a metal reconnection layer (14) of metal/native oxide/metal made of aluminum of 500 nm thickness, deposited by spraying, in equipment which does not make it possible to carry out plasma etching of the native alumina.

[0059] The theoretical total electrical resistance was calculated on the basis of modeling of resistances of different materials making up said devices, known to a person skilled in the art. Thus, the interface resistances, including the metal/native oxide/metal contact resistance, are not taken into account in this calculation. The theoretical total resistance R.sub.TH is estimated at 120Ω.

[0060] Current/voltage (I-V) measurements have made it possible to determine the real values of the total electrical resistances of each device. The curve 9.sub.TH of FIG. 2B shows the development of the theoretical total electrical resistance R.sub.TH as a function of the length L. In reality, the curve 9.sub.TH is a horizontal straight line, which means that the contact resistance does not depend on the length L and is negligible with respect to the resistance of the different materials which make up the device, in accordance with the prior art. The curve 9 of FIG. 2B shows the development of the total electrical resistance R of the devices produced, as a function of the length L. Surprisingly, said curve exhibits an exponential decay as a function of the length L. For a length of 80 μm, the total electrical resistance is 2,500Ω, i.e. 20 times greater than the theoretical total electrical resistance estimated at 120Ω for said devices. The greater the length L, the smaller the total electrical resistance R. The curve profile tends towards a horizontal asymptote around 150Ω. The difference of 30Ω compared with the theoretical value can be explained by errors in the estimation of the resistivities and thicknesses of materials and/or the failure to take into account the interface resistances.

[0061] It is considered that the contact is optimized when the total electrical resistance R reaches 125% of the value obtained by virtue of the asymptote of the curve. In this case, this corresponds to a value L=800 μm, and thus to a surface S of 6,400 μm.sup.2. The ratio of the surface is R.sub.a=S/S.sub.5=6,400/345,000=0.018=1.8%. Now, a person skilled in the art would have used a metal reconnection layer consisting of metal/native oxide/metal of the order of magnitude of the critical dimension of the photovoltaic strip but slightly smaller than said strip, i.e. for example a surface of the order of 14*14 μm.sup.2=196 μm.sup.2, i.e. a ratio of barely R.sub.a=196/345,000=0.057%. In this example, between the optimization according to the invention and the predictable selection of a person skilled in the art, there is a ratio of 32 between the two surfaces of the metal reconnection layer consisting of metal/native oxide/metal.

[0062] Although the example of FIG. 2A, set out above, relates to a rectangular shape, the contact surface may be of any desired shape. For reasons of photolithographic methods, patterns different from those shown in FIG. 2A may be selected for overcoming the problems of definition of patterns for the lithography or the etching for example. An example is shown in FIG. 3A. The contact surface is thus formed by small rectangles of the same dimension as the VIAs, which are electrically interconnected. However, any continuous shape may be suitable for meeting the criteria of the invention, even a heart-shaped pattern as proposed in FIG. 3B.

[0063] According to the invention, the optimization of the contact surface is achieved only if all the different parts of said surface are in electrical contact. For example, in the embodiment of [FIG. 3C], the surface S is made up of the surfaces S.sub.1+S.sub.2. However, S.sub.1 and S.sub.2 are not electrically interconnected, and therefore this arrangement does not allow for the optimization according to the invention.

TABLE-US-00001 TABLE 1  1 Substrate  2 Front electrode  3 Absorber  4A Active photovoltaic zone available for the reconnection of the rear electrode  4B Reconnection zone of the rear electrode 40 Metal layer of the rear electrode 41 Native oxide of the metal layer of the rear electrode  5 Active photovoltaic zones L.sub.5 Length of a photovoltaic strip CD.sub.5 Critical dimension of a photovoltaic strip CD.sub.T Critical dimension of a transparent strip CD Critical dimension of the metal reconnection layer (14) CD.sup.+, CD.sup.− Critical dimension of the collector buses S.sub.5 Surface of a photovoltaic strip  6.sub.T Transparent zones  6.sub.I Insulation zone 7.sup.+, 7.sup.− Collector bus  8 Reconnection of the metal grid, VIA  9 Insulation layer 10 Ambient air surrounding the photovoltaic device 14 Metal reconnection layer consisting of metal/native oxide/metal 18 Metal reconnection layer of the VIA type