Patent Description:
In crystalline silicon solar cells, passivation of the contacts is very important to reduce recombination losses at the contacts and hence to obtain high conversion efficiency of light impinging on the cell into electrical current.

Typically, an ultrathin (<<NUM>) dielectric tunnel layer is deposited on the crystalline silicon substrate, e.g. of silicon oxide, upon which a doped polycrystalline silicon (poly-Si) capping layer is formed. Electrical contacts are then selectively applied upon this capping layer, e.g. by screen printing and subsequent curing of silver paste. During curing, which takes place at elevated temperatures to "set" the paste, the underlying poly-Si is often damaged and passivation is reduced as the silver material flows into the surface texture and defects.

The level of passivation attained ends up being a compromise, since a relatively thick poly-Si capping layer provides excellent passivation but absorbs light that would otherwise be converted in the active part of the solar cell. This parasitic light absorption reduces the conversion efficiency of the cell.

For this reason, poly-Si based passivating contacts metallized by screen printing and firing-through are mainly employed on the rear side of a solar cell with a conventional homo-junction on the front. In the quest of high efficiency solar cells, poly-Si passivating contacts are ideally applied on both sides of a solar cell. Therefore, to maximize photo-generation in the absorber layer, poly-Si based passivating contacts need to be arranged in such a way contact recombination is reduced, high passivation levels are maintained over the full wafer surface without creating excessive parasitic optical losses. It is noted that such losses are in the infrared response of the solar cell spectral response due to free carrier absorption (FCA) if the doped poly-Si is only on the rear side. If the poly-Si passivating contact is applied also on the front or on both sides of the solar cell, optical losses occur in the UV, visible and infrared regions of the solar cell spectral response.

Document <CIT> discloses a photovoltaic device with a back side layer stack comprising, disposed on a silicon substrate, the sequence of a full-surface first tunnel layer, a full-surface first polycrystalline silicon-based capping layer, a full-surface second tunnel layer, a full-surface second polycrystalline silicon-based capping layer, and metal contacts. Since all the layers in question other than the metal contacts are full-surface, the cell performance is far from optimal.

An aim of the present invention is thus to at least partially overcome the drawbacks of the prior art.

More precisely, the invention relates to a photovoltaic device comprising:.

According to the invention, said photovoltaic device further comprises:.

Doping (or lack thereof) of the silicon substrate and the various layers mentioned above can be arranged as required, and the invention is not limited to a particular dopant configuration. Furthermore, the layers mentioned above can be provided on one or both sides of the substrate.

As a result, the metal is kept away from the interface between the silicon substrate and the first tunnel layer (which forms the passivating contact interface), eliminating damage thereto e.g. during the thermal curing of silver paste used for the metal contact, or during deposition thereof if by PVD. The non-contacted areas outside of said zones are not provided with the second polycrystalline silicon-based capping layer, meaning that the thickness of the polycrystalline silicon-based material in these areas can be as thin as possible to minimise parasitic light absorption. The passivation of the contacts is hence maximised, and the parasitic light absorption is minimised, improving cell efficiency, fill factor, open source voltage and short-circuit current.

Advantageously, first tunnel layer is made of a dielectric material, and said second tunnel layer is made of a dielectric material or a semiconductor alloy, this latter not impeding vertical charge transport (i.e. charge transport in a direction perpendicular to the plane of the substrate).

Advantageously, at least one of said first polycrystalline silicon-based capping layer and said second polycrystalline silicon-based capping layer is made of polycrystalline silicon, with or without hydrogenation.

Advantageously, said second polycrystalline silicon-based capping layer has the same dopant type as said first polycrystalline silicon-based capping layer in the corresponding predetermined zone, that is to say in the underlying first polycrystalline silicon-based capping layer. Charge collection is hence optimised.

Advantageously, said second polycrystalline silicon-based capping layer has a dopant concentration which is greater than that of said first polycrystalline silicon-based capping layer in the corresponding predetermined zone.

Advantageously, said metal contact is of silver, silver/aluminium, aluminium or copper paste.

Advantageously, one or more of the following is true:.

The invention also relates to a method of manufacturing a photovoltaic device as described above, the method comprising steps of:.

According to the invention, the method further comprises:.

As a result, the metal is kept away from the interface between the silicon substrate and the first tunnel layer (which forms the passivating contact interface), eliminating damage thereto e.g. in the case thermal curing of silver paste used for the metal contact. The non-contacted areas outside of said zones are not provided with the second polycrystalline silicon-based capping layer, meaning that the thickness of the polycrystalline silicon-based material in these areas can be as thin as possible to minimise parasitic light absorption. The passivation of the contacts is hence maximised, and the parasitic light absorption is minimised, improving cell efficiency, fill factor, open source voltage and short-circuit current.

Advantageously, e) further comprises substeps of:.

This is a particularly efficient and simple method to pattern the second polycrystalline silicon-based capping layer. However, in an alternative method, second polycrystalline silicon-based capping layer can be deposited in a patterned fashion, e.g. by masking followed by deposition.

Advantageously, said first tunnel layer is made of a dielectric material, and said second tunnel layer is made of a dielectric material or a semiconductor alloy.

Advantageously, at least one of said first polycrystalline silicon-based capping layer and said second polycrystalline silicon-based capping layer is made of polycrystalline silicon.

Advantageously, said metal contact is of silver or aluminium or silver/aluminium or Cu paste.

Advantageously, one of the following is true:.

Further details of the invention will become apparent upon reading the detailed description below, in reference to the annexed figures in which:.

<FIG> illustrates schematically part of a photovoltaic device <NUM> according to the invention, only the features necessary for the understanding of the invention having been represented, namely the configuration of passivated contacts forming the core of the invention and all other layers having been omitted. The various layers are represented schematically, and relative thicknesses are not to be imputed from the drawings.

Photovoltaic device <NUM> comprises a silicon substrate <NUM>, typically monocrystalline silicon in the form of a wafer or a multicrystalline substrate as is generally known. Depending on the type of photovoltaic device, this substrate <NUM> may be undoped, uniformly doped, doped in various zones, either on one side, or both sides of the wafer, or similar. Since the principle of the invention applies to many types of solar cells based on a monocrystalline silicon wafer whether back side contacted, front side contacted or bifacially contacted, no further discussion on this point is required.

On a first face 3a of said wafer is provided a first tunnel layer <NUM>, made for instance of a dielectric material such as SiOx, SiNx, SiONx, AlNx, or AlONx of any convenient stoichiometry. This layer typically has a thickness of less than <NUM>, preferably less than <NUM> or even less than <NUM> but typically more than <NUM>, and is deposited over substantially the whole surface of the substrate <NUM>. This thickness is sufficiently small to enable charge carriers (electrons or holes, depending on the case) to pass through by tunnelling.

Upon a face of the first tunnel layer <NUM> which faces away from the substrate <NUM> is provided a first polycrystalline silicon-based capping layer <NUM>, which may be doped or undoped. In the present embodiment, the first polycrystalline silicon-based capping layer <NUM> is provided upon substantially the entirety of said face of the first tunnel layer, but as can be seen from <FIG> (see below), under certain circumstances in function of the cell configuration, the first polycrystalline silicon-based capping layer <NUM> can be patterned. The term "polycrystalline silicon-based layer" should be understood as designating a polycrystalline layer formed of a silicon-based material such as Si itself, SiOx, SiCx or SiNx, this material optionally being hydrogenated. In terms of the level of crystallinity, a Raman crystallinity of at least <NUM>%, preferably at least <NUM>% is typically adequate. Thickness is typically between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

Upon substantially the entirety of a face of the first polycrystalline silicon-based capping layer <NUM> which faces away from the substrate <NUM> is provided a second tunnel layer <NUM>, which is either a dielectric tunnel layer subject to the same constraints in terms of materials and thickness as the first tunnel layer <NUM>, or a semiconductor alloy tunnel layer. In the case in which both tunnel layers <NUM>, <NUM> are dielectrics, they do not have to be identical. In terms of semiconductor alloys, Si-O, Si-N, Si-C and AlONx alloy layers can be used, with a thickness less than <NUM>, preferably less than <NUM>, further preferably less than <NUM>, and greater than <NUM>.

In predetermined zones <NUM> of the surface of the second dielectric tunnel layer <NUM> facing away from the substrate <NUM>, a second polycrystalline silicon-based capping layer <NUM> is provided, which may be doped or undoped, may be hydrogenated and preferably has the same dopant type (or lack thereof) as the first polycrystalline silicon-based capping layer <NUM>, although this does not have to be the case. Advantageously, both the polycrystalline silicon-based capping layers <NUM>, <NUM> are doped with the same type of dopant (P-type or N-type), and the second polycrystalline silicon-based capping layer <NUM> has a higher dopant concentration than the first polycrystalline silicon-based capping layer <NUM>, which helps maximise charge carrier extraction. This layer is thicker than <NUM>, typically between <NUM> and <NUM>, preferably <NUM> to <NUM>.

Finally, metal contacts <NUM> are situated directly or indirectly upon the second polycrystalline silicon-based capping layer <NUM>, again on a surface thereof facing away from the substrate <NUM>. For instance, it is not to be excluded that a further tunnel layer and/or a further capping layer (or several) could be provided between the second polycrystalline silicon-based capping layer <NUM> and the metal contacts <NUM>.

In areas <NUM> situated outside of said predetermined zones <NUM>, the second dielectric tunnel layer <NUM> is left exposed, which ensures a minimum of parasitic light absorption.

The layers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> can be applied to either the front side (i.e. the light-incident side) of the photovoltaic device <NUM>, to the back side (i.e. the shaded side), or to both sides, depending on the type of solar cell being implemented. Furthermore, each of said layers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are preferably provided directly upon the underlying layer (or substrate in the case of the first tunnel layer <NUM>), although the presence of intervening layers is not to be excluded.

The structure of the photovoltaic device <NUM> according to the invention having been described in reference to <FIG>, a particularly advantageous but non-limiting method of its fabrication will now be described in reference to <FIG>.

<FIG> represents the sequence of steps to form the portion of the layer stack comprising silicon substrate <NUM>, first tunnel layer <NUM>, first polycrystalline silicon-based capping layer <NUM>, and second dielectric tunnel layer <NUM>.

Firstly, silicon substrate <NUM> is provided, as described above.

Subsequently, first tunnel layer <NUM> is formed upon a surface of the silicon substrate, for example by means of growth through oxidation, nitridation or similar of the surface, or by deposition such as a chemical vapour deposition process (e.g. APCVD, LPCVD etc., with or without plasma enhancement), physical vapor deposition (sputtering) or similar. This layer <NUM> is typically undoped, but a dopant can be included during deposition, as is generally known.

Subsequently, first polycrystalline silicon-based capping layer <NUM> is formed upon the first tunnel layer <NUM>, preferably directly thereupon without deposition or formation of an intervening layer. This is typically carried out by a physical vapour deposition (PVD, with or without plasma enhancement) or a chemical vapour deposition process (PECVD, LPCVD), depending on the material chosen, as is generally known.

Subsequently, second tunnel layer <NUM> is formed upon substantially the entirety of the first polycrystalline silicon-based capping layer <NUM>, preferably directly thereupon without deposition or formation of an intervening layer, either by being grown thereupon (e.g. by a hot nitric acid process, ozonate deionised water, UV-Ozone, or other oxidation process), or is deposited thereupon by PVD, CVD, atomic layer deposition (ALD) or similar, depending on the material chosen. This is ideally carried out in the same deposition tool as for the first polycrystalline silicon-based capping layer <NUM>.

Moving now to <FIG>, the second polycrystalline silicon-based capping layer <NUM> is then formed on the surface of the second tunnel layer <NUM>, preferably directly thereupon without deposition or formation of an intervening layer, typically by means of PVD, LPCVD or PECVD, with an optional dopant being incorporated during deposition.

Although this deposition can be carried out selectively, e.g. by masking off the areas <NUM> which will not ultimately be provided with the second polycrystalline silicon-based capping layer <NUM>, in the illustrated embodiment this layer is provided over substantially the entirety of the second tunnel layer <NUM>, and then as shown in figure <NUM>, is selectively removed from the areas <NUM>.

This can be carried out my any known means, such as applying an etching mask in the zones <NUM> (not illustrated) followed by dry or wet etching, laser ablation, laser crystallisation of the polysilicon layer <NUM> in the areas <NUM> followed by etching of the crystallised region.

Advantageously, the polysilicon layer <NUM> is masked and the etching is carried out by means of a wet process, in an etching solution of deionized water and ammonia, potassium hydroxide, sodium hydroxide, tetramethylammonium hydroxide, or tetraethylammonium hydroxide.

More preferably the etching solution in potassium hydroxide at a concentration of <NUM> to <NUM>, preferably <NUM> to <NUM>.

The etching step is advantageously performed at temperature between <NUM> and <NUM>, for instance at room temperature (<NUM>-<NUM>).

Under some conditions, the wet etching process may result in the formation of a porous silicon layer, which can be subsequently etched in wet solutions such as hydrofluoric acid.

None of the etchants mentioned above are capable of etching the material of the second tunnel layer <NUM>, which hence acts as an etch stop leaving a clean and transparent surface, without requiring precise process control.

An annealing process can be carried out either before or after patterning of the second polycrystalline silicon-based capping layer <NUM>, at a temperature of above <NUM>, typically between <NUM> and <NUM>. Indeed, it is possible to carry out two annealing steps, one after deposition of the first polycrystalline silicon-based capping layer <NUM>, and one after deposition of the second polycrystalline silicon-based capping layer <NUM>.

Finally, subsequent to the formation and/or patterning of the second polycrystalline silicon-based capping layer <NUM>, the metal contacts <NUM> are applied thereto, either in the form of silver paste which is subsequently cured, PVD of any convenient form, metal plating or similar.

As a result, the metal electrode is only in contact with the second polysilicon layer <NUM>, which eliminates damage induced by the metallization process. Since such damage reduces passivation, keeping the metal away from the substrate / passivating contact interface (i.e. the substrate <NUM> / first tunnel layer <NUM> interface) eliminates this and hence improves the overall cell efficiency, while keeping the first polycrystalline silicon-based capping layer <NUM> as thin as possible in the areas <NUM> not used for contacting (i.e. the areas outside the zones <NUM>).

After patterning of the second polycrystalline silicon-based capping layer <NUM> and application of the metal contacts <NUM>, the structure of <FIG> is obtained, which can then be subjected to other manufacturing steps, interconnection, encapsulation etc. as is generally known in the art. For instance, a dielectric layer such as SiN can be provided in the areas <NUM> not used for contacting. Other layers required for forming the complete photovoltaic device <NUM> can be formed as is generally know, at any convenient point in the process.

In the case of a bifacially-contacted photovoltaic device <NUM>, the steps can be carried out sequentially on each side of the substrate <NUM>, or if appropriate, at least certain steps can be carried out simultaneously.

Other modifications of the above-described method are also possible. For instance, in the case in which any of the layers <NUM>, <NUM>, <NUM>, <NUM> are doped, rather than providing a dopant precursor in the deposition process, dopants may be applied in a supplementary processing step. Such a supplementary processing step may be ion implantation or deposition of a supplemental layer containing dopant which is then diffused into the underlying layer, followed by removal of said supplemental layer may be carried out.

Furthermore, it is not excluded that other intervening layers are present in the layer stack, particularly (but not exclusively) between the substrate <NUM> and the first tunnel layer <NUM>, however it is preferable that the layers are provided in the sequence described, without extra layers being present therebetween.

Experiments were carried out by forming a number of identical cells <NUM> as illustrated in <FIG> on the same wafer, with different thicknesses of second tunnel layer <NUM>, and as a comparison, with cells in which the second tunnel layer <NUM> was omitted. In each case, the second polycrystalline silicon-based capping layer <NUM> covers the full area of the cell <NUM>, and as a result, this cell <NUM> does not correspond to the invention but rather forms a proof of concept to prove that the second tunnel layer <NUM> does not impede charge extraction, and indeed improves the performance of the cell. Where appropriate, the doping types of the various layers has been indicated.

The substrate <NUM> was a float zone crystalline silicon wafer with a thickness of <NUM> and doped with phosphorus with a resistivity of <NUM>Ω·cm. First tunnel layer <NUM> is of SiOx with a thickness of approximately <NUM>. First polycrystalline silicon-based capping layer <NUM> is of Poly-Si, with a thickness of approximately <NUM> and is doped phosphorus doped. Second tunnel layer <NUM> is the same as first tunnel layer <NUM>, except that the thickness has been varied between <NUM> and <NUM>-<NUM> (<NUM> second deposition time). Second polycrystalline silicon-based capping layer <NUM> is of the same material, doping type and dopant concentration as the first polycrystalline silicon-based capping layer <NUM>, with a thickness of approximately <NUM>. In each case, the cells were annealed at <NUM> for <NUM> minutes. Metal contacts <NUM> are of screen printed silver paste, and the areas of the surface of the second capping layer <NUM> outside of the metal contacts <NUM> is covered with a dielectric layer <NUM> of silicon nitride. The SiNx <NUM> improves the optical and electrical properties (since it provides passivating atoms) of the solar cell. It is also possible to apply the dielectric layer <NUM> over the whole surface, and when the metal contacts <NUM> are fired, spikes of metal are driven through the dielectric layer <NUM> making direct electrical contact with the underlying layer <NUM>, rendering the dielectric layer <NUM> non-functional under the metal contacts <NUM>. This simplifies production compared to depositing the dielectric layer <NUM> in a patterned fashion, or compared to full area deposition and subsequent local removal where the contacts <NUM> are intended to be situated. This applies to every instance of the dielectric layer <NUM>, and also to dielectric layer <NUM> (see below).

On the opposite side of the substrate <NUM> is provided a substantially full-area layer stack comprising, in a direction moving away from the substrate <NUM>, a back side tunnel layer <NUM> of SiOx, a p-type doped capping layer of polysilicon <NUM>, a transparent conductive oxide layer <NUM> of indium tin oxide, and a back contact <NUM> of silver.

The results are reported in the following tables, in which Voc is the open circuit voltage, FF is fill factor, and Jsc is the short circuit current.

As can be seen from the foregoing, a second tunnel layer <NUM> deposited for <NUM> seconds gives the best cell performance on all metrics, whereas a thicker layer, when deposited for <NUM> seconds, gives a wider range of results, which are poorer, possibly due to impeding charge carrier extraction.

In respect of why the thickness of the second tunnel layer <NUM> corresponding to a <NUM> second deposition time gives the best results, possible explanations are that:.

<FIG> illustrate various non-limiting structures of portions of photovoltaic cells <NUM> according to the invention, incorporating the passivating contacts forming the core of the invention. The doping types of various layers having been indicated where appropriate, and as is generally known, the dopant types can be inverted. Furthermore, the direction of incident light has been indicated by a sun symbol and a large arrow.

<FIG> illustrates a portion of a photovoltaic cell <NUM> which, on the front side of its substrate <NUM>, i.e. the side intended to face the direction of incident light, comprises the layers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> as described in the context of <FIG>, together with a dielectric layer <NUM> (such as SiN or other) provided on areas of the surface of the second tunnel layer <NUM> not covered by the second capping layer <NUM>. On the opposite, shaded side of the substrate <NUM> (rear side) is provided the sequence of layers <NUM>, <NUM>, <NUM>, <NUM> as disclosed in the context of <FIG>.

<FIG> illustrates another portion of a photovoltaic cell <NUM> which differs from that of <FIG> in that, on the rear side, the transparent conductive oxide layer <NUM> has been omitted, and the back contacts <NUM> are a patterned metal layer e.g. of silver paste. A dielectric layer <NUM>, similar to dielectric layer <NUM>, is provided on the areas of the layer <NUM> which are not covered by the back contacts <NUM>. Alternatively, the dielectric layer <NUM> can be provided over the whole surface, and when the patterned metal layer <NUM> e.g. of silver paste is fired, metal spikes are driven through the dielectric layer, making an electrical connection therethrough and rendering it redundant under the metal layer <NUM>. However, this simplifies production as noted above in the context of dielectric layer <NUM>.

<FIG> illustrates another portion of a photovoltaic cell <NUM> which differs from that of <FIG> in that, on the rear side, a similar configuration of passivating contact is provided as that on the front side. Hence, capping layer <NUM> is coated with a further tunnel layer <NUM> similar to the front side tunnel layers <NUM>, <NUM>, and then in certain zones, a further capping layer <NUM> is provided which is analogous to second capping layer <NUM> but of opposite doping, mutatis mutandis.

Finally, <FIG> illustrates a portion of a back-contacted photovoltaic device <NUM> incorporating passivating contacts according to the invention. On the light-incident side of the substrate <NUM> is simply provided a dielectric layer <NUM> similar to that above. On the back side, a substantially full area first tunnel layer <NUM> is provided, and then in the zones <NUM>, first polycrystalline silicon-based capping layer <NUM> is provided in a patterned fashion, and then the remainder of the layer stack <NUM>, <NUM>, <NUM> is provided thereupon and topped with metal contacts <NUM> as above, the doped layers <NUM> and <NUM> having p-type doping of a first type. The modifications to the process described above in order to pattern the first capping layer <NUM> are known to the skilled person (e.g. deposition followed by masking and etching, laser ablation or similar, or masking followed by deposition and mask removal) and need not be explained in detail here.

Between each zone <NUM>, and out of contact with the layer stacks in the zones <NUM>, is provided a patterned polycrystalline silicon layer <NUM> with n-type doping. This polycrystalline silicon layer <NUM> is topped with further metal contact layer <NUM>. The areas of the first tunnel layer <NUM> not covered by layer <NUM> or <NUM> is coated with a dielectric layer <NUM> as above, and it is also possible to deposit dielectric layer <NUM> over the entirety of the surface after deposition of the second polycrystalline silicon-based capping layer <NUM>, including the vertical surfaces of the layer stacks in the zone <NUM>, followed by local removal in the zones <NUM> or simply by allowing the metal o the contacts <NUM> to penetrate therethrough during firing. Furthermore, it is possible for the contacts <NUM> to cover less than the entirety of the zones <NUM>. This global cell configuration is well known in the art, and further description of its functioning need not be required, however it is clear how the layer stack comprising layer <NUM>, <NUM>, <NUM>, <NUM> is applied with this configuration so as to obtain a photovoltaic device according to the invention.

Claim 1:
Photovoltaic device (<NUM>) comprising:
- a silicon substrate (<NUM>);
- a first tunnel layer (<NUM>) situated upon at least a first side (3a) of said silicon substrate (<NUM>);
- a first polycrystalline silicon-based capping layer (<NUM>) situated upon said first tunnel layer (<NUM>);
- a second tunnel layer (<NUM>) situated upon substantially the entirety of said first polycrystalline silicon-based capping layer (<NUM>);
characterised in that said photovoltaic device (<NUM>) further comprises:
- a second polycrystalline silicon-based capping layer (<NUM>) situated upon predetermined zones (<NUM>) of said second tunnel layer (<NUM>), areas of said second tunnel layer (<NUM>) situated outside of said predetermined zones being free of said second polycrystalline silicon-based capping layer; and
- a metal contact (<NUM>) situated upon at least part of said second polycrystalline silicon-based capping layer (<NUM>).