WO2003079438A1

WO2003079438A1 - Multijunction photovoltaic device with shadow-free independent cells and the production method thereof

Info

Publication number: WO2003079438A1
Application number: PCT/FR2003/000844
Authority: WO
Inventors: Claude Jaussaud; Eric Jalaguier; Pierre Gidon; Marc Pirot
Original assignee: Commissariat A L'energie Atomique
Priority date: 2002-03-19
Filing date: 2003-03-17
Publication date: 2003-09-25
Also published as: AU2003227840A1; FR2837625A1; FR2837625B1; EP1485951A1

Abstract

The invention relates to a multijunction photovoltaic device with independent cells (20, 21, 22, 23), comprising contact pick-ups from the cells (20, 21, 22, 23) which are provided on the front and/or rear face by means of metallisted wells (24), the sides of said wells being insulated from the materials forming the different semi-conducting layers. The invention also relates to a method of producing a multijunction photovoltaic device.

Description

MULTI-JUNCTION PHOTOVOLTAIC DEVICE WITH CELLS

INDEPENDENT WITHOUT SHADING EFFECT AND METHOD OF

REALIZATION OF SUCH A DEVICE

DESCRIPTION

TECHNICAL AREA

The present invention relates to a multi-junction photovoltaic device with independent cells without shading effect and a method for producing such a device.

STATE OF THE PRIOR ART

In a multi-junction photovoltaic device, multi-junction cells are produced by stacking layers of materials deposited by epitaxy. These cells being connected in series, the current flowing through them therefore remains the same, which does not allow optimum performance to be obtained. Such an optimum yield, in fact, would be obtained with different currents flowing in the different cells. The use of a growth method by epitaxy imposes, moreover, an agreement of parameter of crystal lattice between the cells, which limits the possibilities of optimizing the forbidden bands of the junctions. A patent application WO 99 52 155, which relates to a semiconductor structure of a photovoltaic component, makes it possible to alleviate this problem. This application describes, in fact, multi-junction cells produced by oxide bonding ("wafer bonding") of two cells. The cells thus stuck are electrically independent. They can have a significant mesh disagreement between them. Contact is made by metallization grids located on either side of each cell. This patent application specifies that the upper and buried metallization grids have the same geometry so as to limit the shading effect to the single upper grid.

Thus, as illustrated in FIG. 1, a multi-junction cell, according to this patent application WO 99/52151, comprises a first cell 10 provided with contacts 11, a second cell 12 provided with contacts 13, and an insulating bonding layer 14 disposed between these two cells 10 and 12. Such an embodiment reduces the shading effect but does not eliminate it. A residual shading effect always limits the efficiency of such cells.

This problem relating to the existence of a shading effect is further accentuated in the case of concentrated cells. Indeed, for such cells, the current densities passing through the metallizations are very high. To limit the series resistance, the size of the metallizations could be increased, but this would cause even greater shading. We must therefore, in general, limit the cell surfaces.

In a cell with a concentration of 1 square millimeter of surface, with a conversion efficiency of 20%, under a concentration of 300, for example, the current is 15 A / cm ² , and the resistance metallization is 6 mΩ / square, which, for a metal surface of 10% of the cell surface leads to a resistance of 60 mΩ / square and to a voltage drop of 9 mV. Such a voltage remains low if it is compared to the cell voltage of the order of 1 V.

On the other hand, for a cell of 1 square centimeter, the voltage drop is 100 times greater, and comparable to the voltage of the cell. All the power of the cell is thus lost in metallization.

In practice, such a difficulty can be resolved in two different ways.

One can, on the one hand, use cells of small area, of the order of a square millimeter (case of cells on III-V materials). A large number of cells is then necessary to cover a given surface. This results in a significant assembly cost.

On the other hand, in the case of a single cell, place the contacts on the rear face of the cell. These contacts can therefore be very wide, without creating any shading effect. However, such a solution is not applicable to multi-junction cells. In fact, the lower cells would undergo, in this case, the shading effects of the contacts of the upper cells.

Furthermore, the method proposed in patent application WO 99 52155 considered above, does not describe how the resumption of contact is then carried out on the buried grids. However, such a recovery is difficult to achieve. The present invention aims to provide a multi-junction photovoltaic device with independent cells and a method for producing such a photovoltaic cell making it possible to eliminate the effects of shading and to facilitate contact resumption.

STATEMENT OF THE INVENTION

The invention relates to a multi-junction photovoltaic device with independent cells, characterized in that it comprises contact resumption of the cells produced on the front and / or rear face by metallized wells whose sides are isolated from the materials constituting the different layers. semiconductor.

In an advantageous embodiment, the metallized lines for collecting the current coming from the metallized wells and situated on the front face are of triangular section. Advantageously, the metallized wells are made of aluminum. Their sides are isolated from the materials constituting the various semiconductor layers of said device by a deposit of insulating material, such as an oxide or a nitride.

The invention also relates to a method for producing a multi-junction photovoltaic device characterized in that it comprises the following steps: - production of all or part of a first cell, - production of all or part of a second cell from a support substrate, this step further comprising the production of an area capable of allowing the withdrawal of the support substrate, - deposition on at least one of the two cells an insulating layer,

- bonding of the two assemblies thus formed, an insulating layer thus separating the two cells, - removal of the support substrate from the second cell,

- etching of the wells allowing contact to be taken up on the front and / or rear face of the device, insulation of the sides of the wells, filling of the wells with metal deposition

- production of metallized lines allowing the collection of current from the wells on the front and / or rear of the device.

Advantageously, the method comprises a step of finishing the cells carried out prior to the etching of the wells.

Advantageously, a passivation of the sides of the wells is carried out before their isolation. The metallized lines located on the front face of the device are produced so that they have a triangular section. These triangular section lines can be produced by metallic deposition, then lithography and finally etching.

To make these lines of rectangular section, you can have the following steps: a step of depositing a layer of resin on a semiconductor substrate,

a step of lithography of this resin layer, a step of shaping trapezoidal metal studs having a determined slope,

- a wet etching step allowing the tip of these metal studs to be formed,

- a step of removing the resin layer.

Advantageously, the bonding of the two sets can be done by molecular adhesion.

Advantageously, the production of the area able to allow the withdrawal of the support substrate consists in the production between the support substrate and the active layers of the cell of a layer for stopping the chemical etching of the support substrate. The withdrawal of the support substrate can be carried out by mechanical abrasion then chemical etching of said support substrate. The production of the zone capable of allowing the withdrawal of the support substrate may consist in the production in the support substrate of a weakened zone capable of allowing the separation of this support substrate. The weakened zone can be produced in the support substrate by ion implantation. The support substrate can be removed by heat treatment and / or application of mechanical forces.

Advantageously, the production of the zone capable of allowing the withdrawal of the support substrate consists of the production between the support substrate and the active layers of the cell of a sacrificial layer having a selective dissolution with respect to the support substrate and to these said layers. The withdrawal of the support substrate can be carried out by selective dissolution of the sacrificial layer.

The device of the invention can in particular be used in concentration cells.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an assembly of multi-junction cells of the known art.

FIG. 2 illustrates the multi-junction photovoltaic device according to the invention.

FIG. 3 illustrates an advantageous characteristic of the device of the invention.

FIGS. 4A to 4E illustrate the steps of a first embodiment of a method for producing tips,

FIGS. 5A to 5G illustrate the steps of a second embodiment of a method for producing tips,

FIGS. 6A to 6E illustrate an example of a method for producing a cell of the device of the invention. FIGS. 7A to 7D illustrate an alternative embodiment of the method of the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

As illustrated in FIG. 2, the multi-junction photovoltaic device of the invention comprises multi-junction cells independent from a physical point of view (their mesh parameter can be very different) and electric, here four cells 20, 21, 22 and 23 separated from each other by insulating layers 26, 27 and 28, some of whose contacts are reported in front face, which receives the light, the others being transferred to the rear face by metallized wells 24. Each of these cells 20, 21, 22 and 23 comprises several semiconductor layers, here three layers. These wells 24 can have a small section without creating a large series resistance, because their length is short.

Typically, these wells 24 have a depth of between 5 to 20 micrometers, which depends on the number and thickness of the layers forming the device of the invention, and a section of approximately 10 × 10 square micrometers.

These wells 24 are dug by etching. Their sides are isolated from the semiconductor materials constituting the different layers by deposits of insulating material 25, such as oxides or nitrides.

A passivation treatment of the surfaces can be carried out before the deposition of such insulating materials.

The metallized lines allowing the collection of the current coming from these metallized wells 24 can advantageously have a triangular section 29 on the front face of the device of the invention, as illustrated in FIG. 3. Such a feature eliminates the shading effects due to metallization on this face.

These triangular points or sections 29, due to their shape and the material used, typically aluminum, make it possible to redirect the incident light 30 towards the interior of the cells.

Typical dimensions of such points 31 are given below:

- distance between the corresponding walls of two points 31: p≡50 μm,

- distance between two points 31: d = 35 μm,

- width of a point 31: 1 = 15 μm,

- height of each point 31: h≡28 μm. A 10 mm ² cell has approximately 6000 pads. 6% of the surface is thus covered by studs.

Example of a method for producing such tips

In a process for producing such triangular points or sections, a layer of resin is deposited on a semiconductor substrate, and a lithography of this layer of resin is carried out. Then, in the form of trapezoidal metal studs having a determined slope, a wet etching step is used to form the tip of these metal studs. The resin layer can then be removed.

In a first embodiment, this method comprises a step of depositing a full plate with a layer of metal, for example of aluminum, and a layer of resin. The thicknesses of these layers are for example around 20 micrometers. The resin is then exposed using techniques well known to those skilled in the art. After lithography, with a suitable light profile, trapezoidal or pointed resin studs are obtained, the slope of which is precisely defined above the places where the metal tracks must be located.

A dry etching step is then carried out through such a resin mask. It makes it possible to obtain metal studs with a trapezoidal profile, with very rough sides. Indeed, there is transfer of the slope of each resin pad at the metal pad located below, via a proportionality coefficient which depends on the etching parameters.

A chemical etching step with the resin in place makes it possible to smooth the sides of the metal studs and to form the tip of each stud. In fact, chemical etching is isotropic: it therefore retains the slope obtained in the metal while smoothing the surface of the stud. Depending on the nature of the metal deposited, wet etching may include one or more steps. For example, in the case of an AlSi deposit, a first wet etching makes it possible to etch the aluminum. It is followed by a second wet etching intended to remove the silicon grains remaining after the first etching.

Resin residues can then be removed by conventional techniques, well known to those skilled in the art. Thus, as illustrated in FIGS. 4A to 4E, this method therefore comprises, more precisely:

a step of depositing a full plate with a layer of metal 60 and a layer of resin 61 on a semiconductor substrate 62, as illustrated in FIG. 4A,

a lithography step of the resin layer 61 which makes it possible to obtain, as illustrated in FIG. 4B, pointed resin pads 63 of slope α, or also trapezoidal pads of slope α,

a step of dry etching carried out through the resin mask thus formed which makes it possible to obtain metal studs 64 of trapezoidal shape of slope αa with rough sides, situated under the resin studs, as illustrated in FIG. 4C,

a wet etching step which makes it possible to smooth the sides of the metal studs 64 and to form the tip thereof, while retaining the slope αa imposed during the dry etching, as illustrated in FIG. 4D,

a step of removing the resin which makes it possible to obtain the desired result, as illustrated in FIG. 4E.

After the wet etching step, it is possible to obtain metal tracks having, as illustrated in FIG. 4D, approximately the dimensions, height h, period p, 1 width: h = 22 μm 1 = 12 μm p = 35 μm The profile of each metal track is determined, for given dry etching conditions, by the profile of the resin: the slopes of the resin pads and of the metal pads are proportional, with a coefficient which depends on the conditions of dry etching.

For example, with a slope in the resin layer of 56 ° and an etching by a mixture of C12, BC13 and N2 under a pressure of 200 mTorr and a power of 200 W, studs are obtained for aluminum metal tracks having a slope of 70 ° to 75 °.

The resin profile is obtained by controlling the distance between the mask and the plate to be etched: when the mask is in contact with the plate, a vertical resin profile is obtained. When the mask is moved away from the plate, a sloping resin profile is obtained, the profile deviating all the more from the vertical as the mask is more distant. Other parameters are also involved such as the sunshine time and the light density used.

The smoothing of the sides of the metal studs can then be obtained by chemical etching in a solution of 75% of H ₃ P0 ₄ , 3.5% of HN0 ₃ , 15% of CH ₃ C0 ₂ H, and 6.5 % of H ₂ 0.

In a second embodiment, as illustrated in FIGS. 5A to 5G, the method of the invention comprises the following steps: a step of depositing a full plate with a layer of a first resin 70 on a semiconductor substrate 71, as illustrated in FIG. 5A,

- A step of insolation and development of this layer of a first resin to form a mold 72 for a subsequent metal deposition, as illustrated in FIG. 5B. The slope of this mold in this first resin is obtained by lithography with a light profile of adequate shape, using a negative resin,

a step of depositing metal, for example copper, by electrolysis, the metal filling the mold 72 previously formed by the first resin, by forming trapezoidal metal studs 73, as illustrated in FIG. 5C,

- A step of depositing a second resin 74 and lithography so that this second resin covers the top of the trapezoidal metal studs, as illustrated in FIG. 5D (the slope of these studs is therefore perfectly controlled because imposed by the slope of the mold ),

a step of removing the first resin, as illustrated in FIG. 5E,

a step of wet etching of the metal studs 73 making it possible to form the point of the latter, as illustrated in FIG. 5F,

- A step of removing the second resin, as illustrated in Figure 5G. Example of a method for making a cell

In a GalnP stacking example

(gap 1.7 ev) / Si (gap 1.1 ev) allowing a potential conversion efficiency of 43%, as illustrated in FIGS. 6A to 6E, such a process can comprise the following steps:

- Production of a silicon cell (except metal) as illustrated in FIG. 6A, this cell comprising a silicon support substrate 41 lightly doped (for example of type p) and two zones 42 and 43 heavily doped respectively of type n and p in the example considered in order to make the junction and allow the formation of ohmic contacts,

- Production of a GalnP cell: on a support substrate 44, for example in AsGa, formation of a barrier layer 50 in the chemical etching of the support substrate, for example in AlGaAs and epitaxy of the useful layers forming the AlInP cell, then N-doped GalnP, p-doped GalnP and GalnP (layers 45, 46, 47 and 48 respectively), as illustrated in FIG. 6B,

- deposition of an oxide layer on at least one of the two cells, forming in the example considered layer 49 of the second cell,

- Bonding of the two assemblies thus formed by conventional bonding techniques, for example by molecular adhesion. The insulating layer thus separates the two cells,

- Removal of the support substrate 44 for example by mechanical then chemical abrasion with stopping of the chemical etching on the stop layer. The structure illustrated in FIG. 6C is thus obtained, - formation of the contacts: etching of the wells, passivation and electrical insulation of the sides of the wells (deposits 52), filling of the wells with metallic deposit (typically aluminum), for example a CVD 51 deposit ("Chemical Vapor Deposition"), as illustrated in FIG. 6D,

- production of metallizations for the collection of triangular section current 53 on the front face: deposition of aluminum by spraying, lithography, dry etching of aluminum (chlorinated gases: BC13, C12; neutral gas: argon), as illustrated in the figure 6E.

Variants are possible for this process in particular in order to allow the reuse of the support substrate 44. Instead of a stop layer, a weakened zone is created in the support substrate 44 for example by implantation of hydrogen ions through the support substrate ("smart-cut" process described in US Pat. No. 5,374,564). After the active layers of the cell have been formed and bonded to the first cell, the support substrate is then separated at the level of this weakened area by heat treatment and / or application of mechanical forces. A third variant consists in using a sacrificial layer produced between the support substrate 44 and the active layers 45 to 48, for example made of AlAs. To separate the support substrate, this layer is subsequently selectively dissolved. A fourth variant, which relates to a solution with elimination of an AsGa substrate by dissolution, is illustrated in FIGS. 7A to 7D.

As illustrated in FIG. 7A, on the AsGa substrate 60, an epitaxy of an etching stop layer 61 is carried out, then of a layer 62 serving for the resumption of epitaxy (AsGa), and the deposition of a layer 63 of oxide or nitride. On the other hand, an oxide layer 65 is deposited on a silicon substrate 64. For this cell Si, all or part of the technology can be carried out before the bonding of the two substrates, which is illustrated on the Figure 7B.

In this FIG. 7B are illustrated the bonding of the two substrates 60 and 64 via the two oxide layers 63 and 65, and the elimination of the substrate AsGa 60, by chemical dissolution or abrasion + chemical dissolution.

In FIG. 7C are illustrated the elimination of the stop layer 61, and the epitaxy of four layers 67, 68, 69 and 70 forming the cell (p layers GalnP; GalnP; n GalnP; n AlInP).

In FIG. 7D is illustrated the implementation of the technology according to the invention for such an AsGa cell 72, with metallized wells 72 whose sides are isolated from semiconductor materials by deposits of insulating material 73, and deposits 74 on areas of doping 75 and 76. For the cell Si, there is then realization of the technological stages which were not carried out before the bonding of the two substrates. The cell thus produced is lit from the AsGa cell side (light 77).

This variant can be carried out with recovery of the substrate, by replacing the barrier layer with a weakened area in the substrate and then separation.

Examples of stacking junctions can be: • Cell with two junctions.

- first semiconductor: GalnP (gap 1.8 eV),

- second semiconductor: silicon (1.1 eV). • Cell with four junctions.

- first semiconductor: GalnP, - second semiconductor: GaAs,

- third semiconductor: silicon,

- fourth semiconductor: GalnAs.

The device according to the invention therefore makes it possible to overcome the drawbacks of the prior art by allowing a simple contact resumption on the independent multi-junction cells and this, without shading effect of the metallizations. The gain is important. It can be estimated at around 10% additional incident light for a two-cell device whose contacts of the first cell are taken up on the front of the device.

Claims

1. Multi-junction photovoltaic device with independent cells (20, 21, 22, 23), characterized in that it includes contact resumption of the cells produced on the front and / or rear face by metallized wells (24), the sidewalls are isolated from the materials constituting the various semiconductor layers.

2. Device according to claim 1, in which the metallized lines for collecting the current coming from the metallized wells and situated on the front face are of triangular section (29).

3. Device according to claim 1, wherein the metallized wells (24) are made of aluminum.

4. Device according to claim 1, in which the metallized wells (24) have their sides isolated from the materials constituting the various semiconductor layers of said device by a deposit of insulating material (25).

5. Device according to claim 4, in which this insulating material is an oxide or a nitride.

6. Method for producing a multi-junction photovoltaic device, characterized in that it comprises the following steps:

- realization of all or part of a first cell (41,42,43),

- production of all or part of a second cell from a support substrate (44), this step further comprising the production of an area capable of allowing the withdrawal of the support substrate (44),

- depositing on at least one of the two cells an insulating layer (49),

- bonding of the two assemblies thus formed, an insulating layer thus separating the two cells,

- removal of the support substrate (44) from the second cell,

- etching of the wells (51) allowing contact to be taken up on the front and / or rear face of the device, isolation of the sides of the wells (52), filling of the wells with metal deposition,

7. The method of claim 6, wherein a cell finishing step is performed prior to the etching of the wells.

8. The method of claim 6, wherein one carries out a passivation of the sides of the wells (51) before the insulation thereof.

9. The method of claim 6, wherein the metallized lines located on the front face of the device are produced so that they have a triangular section (53).

10. The method of claim 9, wherein these triangular section lines are produced by metallic deposition, then lithography and finally etching.

11. The method as claimed in claim 9, which, in order to produce these lines of triangular section, comprises the following steps:

a step of depositing a resin layer (41; 70) on a semiconductor substrate, a step of lithography of this resin layer,

a step of forming trapezoidal metal studs (44; 73) having a determined slope,

a wet etching step making it possible to form the tip of these metal studs (44; 73),

- a step of removing the resin layer (41; 70).

12. Method according to any one of claims 6 to 11, wherein the bonding of the two sets is done by molecular adhesion.

13. Method according to any one of claims 6 to 12, in which the production of the zone capable of allowing the withdrawal of the support substrate (44) consists in the production between the support substrate and the active layers of the cell of a barrier layer for chemical etching of the support substrate.

14. The method of claim 13, wherein the removal of the support substrate is effected by mechanical abrasion then chemical etching of said support substrate.

15. Method according to any one of claims 6 to 12, in which the production of the zone capable of allowing the withdrawal of the support substrate (44) consists in the production in the support substrate (44) of a weakened zone capable of allow the separation of this support substrate.

16. The method of claim 15, wherein the production in the support substrate of the weakened area is carried out by ion implantation.

17. Method according to any one of claims 14 or 15, in which the withdrawal of the support substrate is carried out by heat treatment and / or application of mechanical forces.

18. Method according to any one of Claims 6 to 12, in which the production of the zone capable of allowing the withdrawal of the support substrate (44) consists in the production between the support substrate and the active layers of the cell of a sacrificial layer having a selective dissolution with respect to the support substrate and to these said layers.

19. The method of claim 18, wherein the removal of the support substrate is carried out by selective dissolution of the sacrificial layer.

20. Use of the device according to any one of claims 1 to 5 in a concentration cell.