DE102010025281A1 - Method for local removal of a surface layer and solar cell - Google Patents

Abstract

A method for the local removal of a surface layer (36) applied to a texture (30) of a textured substrate, the texture (30) having a plurality of structural elements (32) with structural tips (34) and / or structural edges, comprising the steps the local irradiation of the texture (30) through the surface layer (36) by means of a laser radiation which at least partially penetrates the surface layer (36) and is at least partially absorbed by the texture (30), the intensity of which is set so that the texture (30) by means of the laser radiation is locally melted and subsequently recrystallized (16), the surface layer (36) in the region of the structure tips (34) and / or the structure edges being opened locally so that the openings around the structure tips after the local irradiation of the texture (30) (34) and / or the structure edges around contiguous, unopened areas of the surface layer (36) completely are surrounded, and the removal (18) of the recrystallized areas (38) of the texture (30) in an etching step by means of an etching medium, the surface layer (36) apart from the recrystallized areas (38) being used as an etching mask against the etching medium, as well as a solar cell (70).

Description

The invention relates to a method for the local removal of a surface layer applied to a texture and to a solar cell according to the preamble of claim 10.

In various applications, it is necessary to locally remove a surface layer from a microstructure. For example, in DE 10 2008 056 456 A1 a method for producing a solar cell with a two-stage doping described, in which a deposited on a textured solar cell substrate silicon oxide layer is to be removed locally. Known, this can be realized by laser ablation. In this case, the solar cell substrate is irradiated in the corresponding areas with laser radiation and vaporized below the silicon oxide layer located solar cell substrate material so that the arranged on the evaporated solar cell substrate material silicon oxide layer is blasted off. The texture present on the textured solar cell substrate, which represents a microstructure, is damaged here. This has a negative effect on the efficiency of the solar cell. To avoid this, the laser damage can be removed by etching. Here, however, the texture is removed or at least leveled. For solar cells, this results in loss of efficiency.

In order to prevent this, texture etching solutions can be used in the etching of the laser damage, so that a texture is present even after the laser damage etching. However, the use of texture etching solutions increases manufacturing costs. In addition, not all or no textured etch solution can be used in many applications, since existing surfaces layers, in the case of the solar cell, for example, the silicon oxide layer, may not be removed.

In the case of laser ablation, lasers are usually used which emit laser pulses having a duration in the nanosecond range. The laser damage resulting from the laser ablation can basically be reduced by using laser pulses with very short pulse durations. For the described example of the solar cell, lasers having a pulse length in the picosecond or even femtosecond range would have to be used in order to reduce the resulting laser damage such that a leveling of the texture can be sufficiently avoided in a subsequent etching of the laser damage. However, lasers with such short pulse durations considerably increase the manufacturing outlay.

Against this background, the object of the present invention is to provide a method with which a low-cost surface layer arranged on the texture of a textured substrate can be locally removed by laser radiation while largely retaining the texture.

This object is achieved by a method having the features of claim 1.

Furthermore, the invention has for its object to provide a cost-effectively producible solar cell with improved efficiency available.

This object is achieved by a solar cell having the features of claim 10.

Advantageous developments are each the subject of dependent claims.

The method according to the invention provides for the local removal of a surface layer applied to a texture of a textured substrate, the texture having a plurality of structural elements with structure tips and / or structure edges. First, a local irradiation of the texture through the surface layer by means of a surface layer ( 36 ) at least partially penetrating and at least partially absorbed by the texture laser radiation whose intensity is adjusted so that the texture ( 30 ) is locally melted by means of the laser radiation and subsequently recrystallized, the surface layer in the region of the structure tips and / or structure edges being opened locally such that the openings after the local irradiation of the texture around the structure tips and / or structure edges are surrounded by coherent unopened regions Surface layer are completely surrounded, these non-open areas preferably extend between the structural peaks and / or structural edges of adjacent structural elements. Subsequently, the removal of the recrystallized regions of the texture in an etching step by means of an etching medium, wherein the surface layer is used off the recrystallized areas as Ätzmaskierung against the etching medium, so that no texture material is removed here.

The texture of the textured substrate may be formed by a microstructure. The textured substrate is preferably a textured silicon substrate. Texture material should always be understood as the material of which the texture is made.

The texture has structure tips and / or structure edges, whereby, below the structure edges, the common boundary line of two adjacent, a different orientation of the Surface-normal surface elements of the textured surface is to be understood.

As explained above, in the case of a laser ablation, the texture material would be vaporized as a result of absorbed laser radiation. Due to the resulting vapor pressure of the vaporized material, the surface layer would be blasted off. This occurs only from a certain laser intensity, which allows a sufficiently large energy input. For the purposes of the present invention, the intensity of the laser radiation is below this certain laser intensity, the so-called ablation threshold, when no or only so little texture material is vaporized that the surface layer is not blasted off.

In. In the method according to the invention, the surface is sent open locally during local melting and / or recrystallization. This is to be understood in that the recrystallized areas are at least no longer completely covered by the surface layer. As a result, they may subsequently be attacked and removed by the etchant.

As an etching medium, alkaline etching solutions, for example an aqueous KOH or an aqueous NaOH solution, can be used without texturizing additives.

As a surface layer, for example, a silicon oxide layer can be used. Its thickness is preferably 2 to 70 nm, particularly preferably 10 to 70 nm. Alternatively, inter alia, a silicon nitride layer may be used as the surface layer. Its thickness is preferably 20 to 150 nm.

The method according to the invention has proven useful in the removal of a surface layer from a texture whose structural elements have substantially a structural diameter of less than 100 μm, preferably less than 50 μm and particularly preferably less than 15 μm. The height of the structural elements is advantageously 3 to 15 microns.

Preferably, texture peaks of the texture are melted and recrystallized. Structure peaks are to be understood as meaning the furthest protruding areas of the individual structural elements of the texture. Melting of the structure tips can be achieved, for example, by utilizing heat accumulation effects or poor heat dissipation, and / or optical effects during the irradiation of the texture with laser radiation. Advantageously, the structure tips are fused over a cross-sectional area of less than 5 μm ² , preferably over a cross-sectional area of less than 1 μm ² .

Alternatively, texture edges of the texture may be at least partially fused and recrystallized. If the structural elements of the texture are designed, for example, as pyramids, edges of these pyramids can be completely or partially melted and subsequently recrystallized.

In local smelting, melt drops of molten texture material may be formed. Under a melt drop is to understand a drop-like structure of molten texture material. In particular, the melted drop may have a spherical shape or a spherical shape. That the melted drop is formed from molten texture material is not to be understood as meaning that no other ingredients may be included. For example, portions of the opened surface layer may be contained in the melt drop.

Particularly preferred melt droplets are formed whose largest diameter is 0.5 to 2 microns. Since a melt droplet does not necessarily have spherical shape, its diameter can not be determined unambiguously. The diameter varies depending on the spatial direction in which the diameter is determined. If all possible diameters of a melting drop are determined, then the largest specific diameter is 0.5 to 2 μm.

Advantageously, the texture in parts of the irradiated texture surface is locally melted and recrystallized, which together extend over a maximum of 15 of the irradiated texture surface. More preferably, they extend only over 10 of the irradiated texture surface. All subareas of the irradiated texture surface in which the texture is locally melted and recrystallized thus cover a maximum of 15 and a maximum of 10 of the total irradiated texture surface.

In a further development of the invention, after the removal of the recrystallized regions, etching recesses in the texture are formed in areas not covered by the surface layer. This is done by means of an etching medium, for example by means of an etching solution. Advantageously, the surface layer continues to serve as Ätzmaskierung. The etching medium used is chosen accordingly. The etch wells are thus formed in areas not covered by the surface layer. Etch rate and exposure time of the etching medium determine the depth of the etch wells formed and thus the size of the surface layer-free surface. The deeper the etch pits are made, the larger the proportion of the texture surface which is free from the surface layer.

Preferably, the etch wells are formed by continuing the etch step in which the recrystallized regions are removed after removal of the recrystallized regions. The etch wells are formed in this case with the same etching medium with which also the recrystallized areas have been removed. This allows a cost-effective process management.

Advantageously, etch wells are formed, which taper with increasing etch depth.

Preferably, by forming the etch wells, a surface portion free of the surface layer is created, the entire surface of which is 5 to 80 of the total surface of the texture irradiated with laser radiation. This surface portion is hereinafter referred to as the opening area. Particularly preferably, the opening area is 20 to 60 of the total surface of the texture irradiated with laser radiation.

In practice, it has been proven to locally remove the surface layer from a texture designed as a pyramidal structure. The structural elements of the texture are pyramids in this case. A pyramid structure can be formed, for example, by alkaline or acidic textured silicon substrates, in particular those with a so-called isotype. Preferably, textured monocrystalline silicon substrates with a <100> crystal orientation are used. In particular, the silicon substrates mentioned can be silicon solar cell substrates.

Particularly preferably, the etchant wells formed after removal of the recrystallized regions are designed as inverted pyramids. This has proved particularly useful in the manufacture of solar cells.

The texture can be irradiated, for example, with laser radiation having a wavelength in the green spectral range. Preferably, and most preferably when the texture material is silicon, laser radiation having a wavelength of 515 nm or 532 nm is used. Appropriately, pulsed laser radiation is used. In practice, pulse lengths between 20 ns and 50 ns have proven successful.

The laser beam should be as homogeneous as possible in terms of power distribution. As a result, a more uniform melting of the texture over the irradiated surface can be realized. Suitable are multimode lasers, in which the laser beam is coupled out over a several meters long optical fiber. This is the case, for example, with the Rofin Powerline L 100 laser source.

Preferably, the surface layer is locally removed from a texture which is formed on a semiconductor substrate and consists of semiconductor material, preferably of silicon. Such a texture may be formed by a textured semiconductor substrate, such as a textured silicon substrate. Particularly preferably, a monocrystalline silicon substrate with a <100> crystal orientation is used.

Depending on the application, it may be advantageous to leave the surface layer still on the texture even after carrying out the method according to the invention, in particular after the formation of etch wells. For example, in solar cell fabrication, a surface layer of silicon oxide may be left on the texture to improve surface passivation.

The method according to the invention and its developments can be used for locally removing a surface layer from a texture of a solar cell substrate in regions of a two-stage emitter structure to be doped more heavily.

A solar cell according to the invention has a solar cell substrate, which is at least partially provided with a texture, wherein structural elements of the texture have etching troughs which extend into the structural elements.

A texture of solar cells serves to increase the coupling of light into the solar cell substrate and represents a microstructure. Its structural elements essentially have a structural diameter of less than 100 μm, preferably less than 50 μm and particularly preferably less than 15 μm. Furthermore, the structural elements are preferably 3 μm to 15 μm high. Such microstructures can be prepared in a conventional manner by alkaline or acid Texturätzlösungen. Accordingly, the texture on the solar cell substrate may be an alkaline or an acidic texture, in particular an isotexture.

The etching pits can additionally improve the reflection behavior of the solar cell, in particular in comparison to a solar cell, in which instead of the etching pits, planar surfaces are present in partial regions of the texture. This enables better efficiency of the solar cell.

To further improve the reflection behavior, etch wells are provided, which increasingly taper as they extend into the structure elements.

The solar cell substrate may be implemented as a silicon solar cell substrate. Preferably, it is a monocrystalline silicon solar cell substrate, and more preferably one with a <100> crystal orientation.

The etching pits are preferably arranged on structural tips of the structural elements. In accordance with the above, structural peaks are to be understood as the most prominent regions of the individual structural elements of the texture. Alternatively, the etch wells could be arranged on edges of the structural elements, for example on edges of pyramids.

Advantageously, etching pits are provided, which extend from a base surface into the structural elements, said base area being less than 5 μm ² , preferably less than 1 μm ² . Such bases have been proven, for example, in solar cells with a two-stage doping, especially in selective emitters.

The structural elements of the texture are preferably designed as pyramids. As stated above, this can be realized by the choice of suitable texture etching solutions.

The etch wells preferably have the form of inverted pyramids. As a result, the reflection behavior of the solar cell can be further improved.

In one embodiment variant of the solar cell according to the invention, the structural elements of the texture are covered with a surface which has openings in the regions of the etching pits. Advantageously, the surface layer is embodied as a silicon oxide layer, preferably with a thickness of 2 to 70 nm, particularly preferably with a thickness of 10 to 70 nm. Alternatively, the surface layer may be embodied as a silicon nitride layer, preferably with a thickness of 20 to 150 nm.

Advantageously, structure elements of the texture which have etch-wells are arranged in more heavily doped regions of a two-stage doping, preferably in more heavily doped regions of a selective emitter structure.

If etching elements having etching elements are arranged in the more heavily doped regions, their etch wells are consequently also largely arranged in more heavily doped regions of the two-stage doping. Particularly preferably, a part of the etchant wells arranged in the more heavily doped regions of the two-stage doping is not covered by a metallization of the solar cell, for example front side contacts.

Furthermore, the invention will be explained in more detail with reference to figures. Where appropriate, elements having equivalent effect are provided with like reference numerals. Show it:

1 Schematic representation of an embodiment of the method according to the invention

2 Top view and perspective view of a part of a texture during different phases of the embodiment of the 1 .

3 Schematic representation of an embodiment of the solar cell according to the invention

4 Enlarged partial view of a plan view of the embodiment of 3 ,

The schematic diagram in 1 illustrated together with the schematic representations of 2 a first embodiment of the method according to the invention. This is based on a silicon substrate, which is first textured 10 , As a result of this texturing 10 becomes a texture on the silicon substrate 30 formed, which represents a microstructure and consists of the same material as the silicon substrate.

As 2 Illustrated on the left edge of the picture is the texture 30 formed in the present embodiment as a pyramidal structure. structural elements 32 are therefore pyramids whose height h varies in the range of 3 to 15 microns.

Furthermore, on the silicon substrate and thus on the texture 30 a silicon oxide layer 36 educated. This can be done, for example, by means of chemical vapor deposition. Alternatively, there is the possibility of thermal oxidation of the silicon substrate or plasma oxidation. In the presentation of the 2 is the education 12 the silicon oxide layer 36 through points on the texture 30 and the structural elements 32 shown schematically. After training 12 the silicon oxide layer on the silicon substrate constitutes the silicon oxide layer 36 one on texture 30 applied surface layer.

Below is the texture 30 irradiated with a laser radiation 14 the intensity of which is below an ablation threshold of the texture material. Because the texture 30 is formed of silicon substrate material, the texture material is silicon. The intensity of the laser radiation is thus below the ablation threshold of silicon.

As a result of irradiation 14 the texture 30 With laser radiation surface structure peaks are in subregions of a total irradiated texture surface 34 the texture 30 locally molten and subsequently recrystallized 16 , The recrystallization can be initiated simply by switching off the laser radiation and thus ending the energy supply.

As 2 can be removed in the present embodiment during the melting 16 the structure tips 34 Melting drops of molten texture material, ie silicon, formed, which subsequently as recrystallized melt droplets 38 remain on the structure tips. By local melting and recrystallization 16 the structure tips 34 In the area of the structure peaks, the silicon oxide layer is opened. The recrystallized melt droplets 38 are therefore no longer of the silicon oxide layer 36 covered. Off the melting drops 38 is the texture 30 but still from the silicon oxide layer 36 covered. This silicon oxide layer 36 becomes at a subsequent etching 18 of the silicon substrate in a KOH etching solution used as Ätzmaskierung, so from the silicon oxide layer 36 covered areas of texture 30 can not be etched by the KOH etching solution. The recrystallized melt drops 38 On the other hand, they are attacked and removed by the KOH etching solution 18 ,

After removing 18 the recrystallized areas 38 the KOH etching is continued 20 , Here are etchings 40 not through the silicon oxide layer 36 protected areas. The silicon oxide layer 36 So also serves as Ätzmaskierung. As 2 shows are etch wells 4 0 formed, which taper with increasing etch depth. In the present case, the etch wells 40 the shape of inverted pyramids.

The transition from the removal 18 recrystallized areas by KOH etching to form 20 Etching wells by continued KOH etching are preferably carried out without interruption of the KOH etching process. In principle, however, it is possible to interrupt the etching process and to continue it at a later time, for example in another etching bath. In this way, different etching solutions could be used. Preferably, however, the same etching solution is used.

3 shows in a simplified schematic representation of an embodiment of the solar cell according to the invention 70 , This has a solar cell substrate 71 which is presently embodied as a silicon substrate. The solar cell is provided at its front with a selective emitter, which consists of a more heavily doped region 76 and a weaker doped area 74 is formed. In the more heavily doped area 76 is a metallization 72 on the solar cell substrate 71 arranged, which forms a front side contact. At the back is a back contact 73 provided, which may be formed in any manner known per se, for example, as a surface screen print contact with aluminum back panel or as a local back contact with local back panel.

4 shows in a partial display area A an enlarged partial view of a plan view of the solar cell 70 out 3 , As can be seen herein, the solar cell is 70 on its front with a texture 78 Mistake. This represents a microstructure. The structural elements 79 are again executed as pyramids.

In the partial view of the 4 are recognizable by vertical, dashed lines separated areas. In the left, less heavily doped area 74 is an intact texture 78 in front. The structural elements 79 are also with a silicon oxide layer 86 covered. The structure tips 80 are intact in this area.

To the less heavily doped area 74 joins the more heavily doped area to the right 76 at. This more heavily doped area 76 points to the structure tips 80 the structural elements 79 Ätzmulden 82 in the form of inverted pyramids. The etch wells extend from a base 84 , which in the representation of the 4 runs approximately parallel to the plane and is indicated by a hatched area in the structural elements 79 into it. The base area 84 is less than 5 microns ² , preferably less than 1 micron ² in the present embodiment.

In the more heavily doped area 76 are the structural elements 79 and thus the etch wells 82 mostly from the metallization 72 covered. As 4 however, is part of the more heavily doped region 76 arranged etch wells 82 not from metallization 72 covered. Corresponding, not from the metallization 72 covered etchings 82 find yourself in 3 right from the metallization 72 ,

The structural elements 79 the texture 30 are in the embodiment of 4 with the silicon oxide layer 86 covered. However, this points in areas of etching pits 82 openings 88 so that the etch wells 82 not from the silicon oxide layer 86 are covered. They are therefore not provided with dots. In the production of the solar cell 70 could through the etch wells, or openings 88 Through, dopant enhances into the solar cell substrate 71 diffused and in this way the more heavily doped region 76 be formed. There is no laser damage. It is also more heavily doped and not metallized 72 covered areas the reflection of the solar cell due to the etch wells low, which allows for improved efficiency.

LIST OF REFERENCE NUMBERS

10

Texturing silicon substrate

12

Form silicon oxide layer on silicon substrate

14

Irradiate the texture

16

Local melting and recrystallization of the structure tips

18

Removing recrystallized areas

20

Forming Etching pits

30

texture

32

structural element

34

structure tips

36

silicon oxide

38

Recrystallized melt drop

40

Ätzmulde

70

solar cell

71

solar cell substrate

72

metallization

73

back contact

74

Weakly doped area

76

Heavily doped area

78

texture

79

structural elements

80

structure tip

82

Ätzmulde

84

Floor space

86

silicon oxide

88

opening

H

Height of the structure element

A

Part display area

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008056456 A1 [0002]

Claims (15)

Method for locally removing a surface layer ( 36 ) on a texture ( 30 ) of a textured substrate, wherein the texture ( 30 ) a plurality of structural elements ( 32 ) with structure tips ( 34 ) and / or structural edges, comprising the following steps: - local irradiation of the texture ( 30 ) through the surface layer ( 36 ) by means of a surface layer ( 36 ) at least partially penetrating and of the texture ( 30 ) at least partially absorbed laser radiation whose intensity is adjusted so that the texture ( 30 ) is locally melted by means of laser radiation and subsequently recrystallized ( 16 ), the surface layer ( 36 ) in the area of the structural peaks ( 34 ) and / or the structural edge is opened locally so that the openings after the local irradiation of the texture ( 30 ) around the structure tips ( 34 ) and / or the structural edges around contiguous unopened areas of the surface layer ( 36 ) are completely surrounded, - remove ( 18 ) of the recrystallized regions ( 38 ) of the texture ( 30 ) in an etching step by means of an etching medium, wherein the surface layer ( 36 ) away from the recrystallized areas ( 38 ) is used as etch masking over the etching medium. Method according to claim 1, characterized in that the unopened regions between the structural tips ( 34 ) and / or structural edges of adjacent structural elements ( 32 ). Method according to one of the preceding claims, characterized in that the structural tips ( 34 ) and / or the texture edges of the texture ( 30 ) are melted and recrystallized ( 16 ). Method according to one of the preceding claims, characterized in that in the local melting ( 16 ) Melt drops ( 38 ) are formed from molten texture material whose largest diameter is 0.5 to 2 microns. Method according to one of the preceding claims, characterized in that the texture ( 30 ) is locally melted and recrystallized in subregions of the irradiated texture ( 16 ), wherein the portions extend over at most 15 of the surface of the irradiated texture. Method according to one of the preceding claims, characterized in that after removal ( 18 ) of the recrystallized regions ( 38 ) in not from the surface layer ( 36 ) covered areas etching pits ( 40 ) in the texture ( 30 ) be formed ( 20 ). Method according to claim 6, characterized in that one of the surface layer ( 36 ), the entire surface of which is provided with 5% to 80%, preferably 20% to 60%, of the total surface of the texture irradiated with laser radiation ( 30 ) is. Method according to one of the preceding claims, characterized in that the surface layer ( 36 ) locally from a texture executed as a pyramid structure ( 30 ) Will get removed ( 16 ). Method according to one of the preceding claims, characterized in that the surface layer ( 36 ) locally of a texture ( 30 ) is removed, which is formed on a semiconductor substrate ( 12 ) and consists of semiconductor material, preferably of silicon. Solar cell ( 70 ) having at least partially a texture ( 30 ) provided solar cell substrate ( 71 ), characterized in that structural elements ( 79 ) of the texture ( 78 ) Etching pits ( 82 ), which are in the structural elements ( 79 ) extend into it. Solar cell ( 70 ) according to claim 10, characterized in that the etching pits ( 82 ) at structural tips ( 80 ) of the structural elements ( 79 ) are arranged. Solar cell ( 70 ) according to any one of claims 10 to 11, characterized in that etching pits ( 82 ) are provided, extending from a base ( 84 ) starting in the structural elements ( 79 ), and that this area ( 84 ) is less than 5 μm ² , preferably less than 1 μm ² . Solar cell ( 70 ) according to one of claims 10 to 12, characterized in that the structural elements ( 79 ) of the texture ( 78 ) are designed as pyramids. Solar cell according to one of claims 10 to 13, characterized in that the etch wells ( 82 ) have the shape of inverted pyramids. Solar cell according to one of claims 10 to 14, characterized in that the structural elements ( 79 ) of the texture ( 78 ) with a surface layer ( 86 ), which are in the areas of etchant ( 82 ) Openings ( 88 ) having.

Images