DE102013103495A1 - Method for producing a silicon substrate for solar cell production - Google Patents

Abstract

A method is described for producing a silicon substrate (1) for manufacturing a solar cell, the method comprising: providing an absorber layer (10) with a high-quality silicon layer (7) which has a high minority charge carrier life of, for example, more than 2 μs and a Layer thickness of, for example, less than 40 μm, and attaching a carrier layer (12) to the absorber layer, the carrier layer having an inferior quality polycrystalline silicon layer (11) which has a low minority carrier lifetime of, for example, less than 2 μs and a layer thickness of preferably more than 80 μm. The inferior quality polycrystalline silicon layer (11) can be deposited from a gas phase, similar to a Siemens process, with high deposition rates and, at the same time, very low impurities being possible. The silicon substrate (1) produced has sufficiently good electronic properties in its absorber layer in order to be able to produce a solar cell with high efficiency from it. The carrier layer with the inferior quality polycrystalline silicon layer is mainly used to mechanically stabilize the silicon substrate so that it can be further processed into a finished solar cell in a similar way to a conventional silicon wafer. In the production of the silicon substrate (1), no ingot needs to be crystallized and then sawed, which means that considerable costs can be saved.

Description

FIELD OF THE INVENTION

The present invention relates to a method for producing a silicon substrate, as can be used in the manufacture of solar cells. The invention further relates to a use of a silicon substrate produced according to the invention in the context of a solar cell production. In addition, the invention relates to a correspondingly producible silicon substrate and a solar cell formed with such a silicon substrate.

TECHNICAL BACKGROUND

Solar cells are used for photovoltaic conversion of light into electrical power. A solar cell is manufactured on the basis of a semiconductor substrate which is regularly formed from a semiconductor material provided with a base doping and on which an oppositely doped emitter region is provided so that charge carrier pairs generated by absorbed light are separated at a pn junction formed between the base region and the emitter region can be.

Silicon wafers with a thickness of 150 to 300 μm are currently used as the semiconductor substrate in the industrial production of solar cells. In order to produce this silicon wafer, silicon is usually freed from impurities using the so-called Siemens process and then deposited as polycrystalline silicon. In order to increase the quality of the silicon, the polycrystalline silicon is then crystallized into so-called ingots and the ingots then sawn into thin disk-shaped wafers.

Such conventional fabrication of silicon substrates for solar cell fabrication can result in significant costs for the silicon substrate, which may, for example, account for more than 40% of the total cost of a photovoltaic module constructed with such silicon substrates.

SUMMARY OF THE INVENTION

There may be a need for an alternative method of making a silicon substrate for solar cell fabrication. In particular, there may be a need for such a manufacturing method which is easy to handle and industrially implement and / or which permits a significant reduction of costs for the production of silicon substrates for solar cell production. There may also be a need for a method of manufacturing a solar cell in which silicon substrates produced in this way can be used. Furthermore, there may be a need for a silicon substrate or a solar cell, as can be produced by the proposed method.

Such needs may be met by the subject matter of any one of the independent claims of the present application. Advantageous embodiments of the invention are defined in the dependent claims.

According to one aspect of the present invention, a method for producing a silicon substrate for a production of a solar cell is described, which has at least the following method steps, preferably in the order given. First, an absorber layer having a high quality silicon layer is provided, wherein the high quality silicon layer has a high minority carrier lifetime of preferably more than 2 μs, preferably more than 10 μs and more preferably more than 100 μs, and a layer thickness of less than 100 μm, preferably less than 70 microns, and more preferably less than 40 microns. On a surface of this absorber layer, a carrier layer is deposited, which has a poor quality polycrystalline silicon layer. The low-quality polycrystalline silicon layer has a low minority carrier lifetime which is shorter than that of the high-quality silicon layer of the absorber layer, that is, a minority carrier lifetime of, for example, less than 2 μs. The inferior polycrystalline silicon layer can mainly serve for mechanical stabilization of the silicon substrate produced and for this purpose have a layer thickness of more than 30 .mu.m, preferably more than 80 .mu.m, and for example a layer thickness in the range of 80 to 160 .mu.m.

This aspect of the invention may, inter alia, be considered as being based on the findings and ideas described below.

As stated in the introduction, it is known that the costs associated with conventional silicon substrate fabrication can account for a significant portion of the total cost of a photovoltaic module. It has now been recognized that significantly more than half of the conventional manufacturing costs for the silicon substrate on the crystallization of the ingot and on the sawing in wafers account for only about 40% of the manufacturing costs for the formation of polycrystalline silicon, for example, using the Siemens process have to. For the crystallization of the ingot considerable amounts of energy for melting the polycrystalline silicon to be provided. When sawing the ingot into wafers, considerable amounts of material are lost through the saw cut. For example, if an ingot is sawed into 180 μm thick wafers, a total of about 320 μm silicon material must be used, since typically about 140 μm of the silicon material is lost due to the saw cut which is currently not technically feasible.

It has therefore been recognized that the cost of producing a silicon substrate can be significantly reduced if it is possible to develop industrially feasible manufacturing processes by which silicon substrates are produced without the need for crystallizing the ingot and / or the need for sawing the ingot into wafers can.

It is known that silicon layers with a layer thickness of a few micrometers or a few tens of micrometers can already be sufficient to absorb incident sunlight as required in order to serve as an absorber layer for a solar cell can. The use of thin silicon layers with a layer thickness of only between 1 and 100 .mu.m, preferably a layer thickness of less than 40 .mu.m, can thereby enable a significant reduction of the material costs in the production of solar cells. In this case, high efficiencies for the solar cell are possible if the thin silicon layer used as the absorber layer is of high electronic quality, that is, has a minority carrier lifetime τ which is greater than 2 microseconds, preferably greater than 100 microseconds, and typically less than 10 ms. A silicon layer with such high minority carrier lifetime is referred to herein as a high quality silicon layer.

Several methods have already been developed for producing thin, high-quality silicon layers. Among other things, so-called layer transfer processes have been developed by one of the inventors of the present invention, in which a high-quality silicon layer is generated on a carrier substrate and then separated from this carrier substrate mechanically or by etching processes. An overview of possible layer transfer processes is, for example, in R. Brendel: "Review Of Layer Transfer Processes For Crystalline Thin-Film Silicon Solar Cells", Japanese Journal of Applied Physics 40, 4431-4439 (2001). described. A high-quality silicon layer can be deposited, for example by epitaxy on a previously formed in a carrier substrate porous layer and then serve the porous layer as a predetermined breaking point to separate the deposited high-quality layer mechanically or by etching dissolving the porous layer of the carrier substrate. Alternatively, a high-quality silicon layer can be formed, for example, as a macroporous layer with low porosity on a carrier substrate and a highly porous layer can be generated below the low-pore layer, which can serve as a predetermined breaking point for separation, as described for example in US Pat DE 10 2009 024 613 A1 is described. Thin high-quality silicon layers can also be produced by ion implantation, as for example in M. Bruel: "Application Of Hydrogen Ion Beams To Silicon On Insulator Material Technology", Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions With Materials and Atoms 108, 313 (1996) is described. Another possibility for the formation of thin high-quality silicon layers offers the so-called Spalling, as for example in F. Dross et al .: "Stress-Induced Large-Area Lift-Off Of Crystalline Si Films", Applied Physics A 89, 149-152 (2007) is described.

However, the manufacturing methods described often have a problem in that the thin films of high quality silicon that are produced are very delicate and in particular very fragile. Therefore, such thin silicon films generally can not be used as a substitute for the silicon wafer conventionally used in solar cell manufacturing. In particular, these thin silicon layers can not be processed with currently common processing technologies used in today's factories for producing silicon solar cells from wafers. A possible cost advantage of such thin silicon layers for the production of silicon solar cells without the need for crystallization of the ingot or without the need for sawing operations can therefore generally not be used for this reason.

A possible solution was to deposit thin, high-quality silicon layers on a carrier substrate that is as cost-effective as possible. For example, it was considered to use substrates of crystalline silicon, since they could be processed together with the thin high-quality silicon layer in a conventional manner. Also, support substrates of ceramics such as sintered crystalline silicon particles have been considered.

However, the previous approaches for combining a thin high-quality silicon layer with a carrier substrate serving for mechanical stabilization usually suffer from considerable disadvantages. For example, some approaches do not avoid crystallizing the ingot or cutting the ingot into wafers, which is particularly low in achieving cost would appear advantageous. In addition, some of the contemplated carrier substrates do not meet the generally high demands on the purity of the components used for solar cell production for the manufacture of a solar cell. If, for example, the thin high-quality silicon layer is to be applied to the carrier substrate with thermal processes, impurities, in particular recombination-rich metallic impurities, can pass from the carrier substrate into the high-quality silicon layer and thereby reduce the minority carrier lifetime and ultimately the efficiency for the solar cell to be produced.

Embodiments of the invention proposed herein may provide a method of fabricating a silicon substrate, such as may be used to fabricate a solar cell, by means of which a high quality thin silicon layer, which may serve as an absorber layer for the solar cell, with a low quality polycrystalline silicon Silicon layer, which can serve as a mechanically stabilizing support layer can be combined in order to provide a silicon substrate in this way, on the one hand can ensure sufficient for a solar cell electronic quality in the absorber layer and on the other hand, a sufficient mechanical stability. Although the low-quality polycrystalline silicon layer of the carrier layer has a low electronic quality and would thus not be suitable as an absorber layer for a solar cell, but preferably includes a sufficiently low impurity concentration, so that there is no danger that harmful impurities from the carrier layer in the absorber layer can reach. In addition, the described production method makes it possible to dispense with the crystallization of ingots and the sawing into wafers, thereby enabling considerable cost-saving potentials.

According to one embodiment of the present invention, it is proposed to provide an absorber layer with a high-quality silicon layer for forming a silicon substrate and to attach a special carrier layer to a surface of this absorber layer. The carrier layer may consist of a low-quality polycrystalline silicon layer or at least have such a. The qualitatively inferior polycrystalline silicon layer can be deposited with sufficient purity at high deposition rates from a gas phase or possibly also from a liquid phase. It may be advantageous to deposit the inferior polycrystalline silicon layer directly from a gas phase, for example in a CVD process (chemical vapor deposition) at high temperatures and high deposition rates, similar to what can be done by means of the so-called Siemens process. Abscheidedrücke this example be in the range of 1000 to 10,000 hPa.

A deposition directly from the gas phase, for example analogously to the Siemens process, allows a simple and cost-effective generation of the carrier layer, wherein during the deposition from the gas phase it can simultaneously be ensured that the deposited polycrystalline silicon layer is electronically inferior, that is has a low minority carrier lifetime of, for example, less than 2 μs, but the low electronic quality is mainly due to a high density of defects within the polycrystalline material, but impurity contamination, particularly with recombination-active metal atoms, can be kept very low.

Since the carrier layer is not intended to take over the function of the light absorber in a solar cell which is manufactured on the basis of the silicon substrate to be produced according to the invention, but is intended primarily for mechanical stabilization of the absorber layer, the silicon layer used for the carrier layer need not be monocrystalline but can Since a high concentration of structural defects is unproblematic, instead of forming a carrier layer, a poor quality polycrystalline silicon layer is used. In this case, the fact that the qualitatively inferior polycrystalline silicon layer is deposited, for example, directly from the gas phase, precludes crystallization of an ingot and subsequent sawing of the ingot for the formation of the carrier layer.

The low quality polycrystalline silicon layer may be deposited from a gas phase at a high temperature of above 700 ° C, preferably above 1100 ° C. This enables high deposition rates, similar to the production of polycrystalline silicon in the Siemens process, for example 2.5 to 250 μm / min, typically 10 to 100 μm / min. Processing times for producing the carrier layer can thus be kept short.

In the formation of the low-quality polycrystalline silicon layer, care must be taken that the deposition from the gas phase used for this purpose is carried out sufficiently pure, so that impurities with recombination-strong metal atoms are less than 1 × 10 ¹³ cm ^-3 , preferably less than 1 × 10 ¹² cm ^{. 3} , stay.

According to one embodiment, the low-quality polycrystalline silicon layer may be deposited with a relatively high doping concentration of more than 1 × 10 ¹⁸ cm ^-3 , preferably more than 5 × 10 ¹⁸ cm ^-3 . Since such a heavily doped silicon layer has a relatively high electrical conductivity, it can take over the function of an electrical contact for the adjacent absorber layer in the context of a subsequent production of a solar cell based on the silicon substrate produced. Depending on the type of doping either for electrons or for holes, the carrier layer can form an electrical connection between the absorber layer and a solar cell metallization.

According to one embodiment of the invention, the low-quality polycrystalline silicon layer may be deposited from the gas phase directly onto a surface of the absorber layer. In other words, firstly the absorber layer can be produced and provided with its high-quality silicon layer and then this absorber layer can serve as a substrate, on which subsequently the qualitatively inferior polycrystalline silicon layer of the carrier layer can be deposited directly from the gas phase. During the vapor deposition, polycrystalline silicon deposits on the surface of the absorber layer, so that the carrier layer formed therewith is mechanically fixed and preferably electrically conductively connected to the absorber layer.

In the embodiment described above, it may be advantageous to form on one surface of the high-quality silicon layer of the absorber layer a functional layer which can bring about a function advantageous as a passivation layer, mirror layer, insulating layer and / or protective layer, and subsequently the qualitatively inferior polycrystalline silicon layer from the gas phase to deposit on a surface of the functional layer opposite the high-quality silicon layer. This functional layer can be formed as a single layer or as a layer system with several partial layers. For example, the functional layer may be formed as a dielectric layer or dielectric layer stack. Alternatively, the functional layer may be formed as a stack of a plurality of, for example, differently porous silicon layers. The functional layer can fulfill different functions for a solar cell to be produced. For example, it can act as a mirror for increasing the optical absorption or serve as a surface passivation for a solar cell to be produced. Possible materials for a functional layer usable as a dielectric layer include silicon oxide, silicon carbide, silicon oxycarbide, silicon oxynitride and aluminum oxide, in various stoichiometries. Optionally, the dielectric layer can have openings which can serve, for example, as contact holes for an electrical connection between the absorber layer which adjoins on one side and the carrier layer present on its other side.

As an alternative to the embodiment described above, in a further embodiment, the low-quality polycrystalline silicon layer from the gas phase can first be deposited on a separate substrate and then attached to the surface of the absorber layer and mechanically fixed to it. By means of such a separate formation of the qualitatively inferior polycrystalline silicon layer for the carrier substrate, it can be avoided, for example, that the absorptive layer or a previously applied additional layer, such as, for example, a dielectric layer serving as a functional layer, are damaged by the high temperatures occurring.

When the inferior polycrystalline silicon layer is formed by deposition on a separate substrate, a so-called connector layer may be formed between the surface of the absorber layer and an adjacent surface of the support layer, which may serve to mechanically bond the absorber layer to the support layer. In other words, the absorber layer and the carrier layer can first be produced separately from each other and then brought into planar contact with the provided between these two layers connector layer with suitable process control for a mechanically strong and possibly also electrically conductive connection between the absorber layer on the one hand and the Carrier layer on the other hand can provide. The connector layer may be chosen such that the desired mechanically strong connection can be achieved already at process temperatures that are well below those temperatures, as used during the deposition of the low-quality polycrystalline silicon layer from the gas phase. Damage to the adjacent absorber and carrier layer can be avoided. As a connector layer, for example, a layer of a silicon-aluminum alloy can be used, which is liquid at low temperatures, for example, well below 600 ° C.

According to an embodiment of the present invention, the absorber layer may be provided on a porous layer formed on a base substrate. The base substrate can during the formation of the absorber layer to it serve to mechanically hold the absorber layer. For example, the absorber layer may be formed on the base substrate adjacent to the porous layer formed thereon by epitaxial methods. Alternatively, the absorber layer may be formed as a low-porous macroporous layer over an underlying highly porous layer. After the absorber layer has been finished, it can be separated from the base substrate, wherein the porous layer formed underneath can serve as a predetermined breaking point or can be dissolved in a further etching process. It may be advantageous to deposit the carrier layer on the absorber layer or to connect it to the absorber layer before the absorber layer is detached from the base substrate in order to be able to provide sufficient mechanical stability for the absorber layer in this way. Alternatively, the high-quality silicon layer to be used for the absorber layer can also be formed by means of ion implantation or spalling. In principle, it is also conceivable to provide the absorber layer as a very thinly sawn wafer and then to provide this for mechanical stabilization with a thicker carrier layer.

By means of embodiments of the manufacturing method described herein, silicon substrates can be formed, which on the one hand can have advantageous electrical and optical properties for the production of solar cells, and on the other hand, in particular due to the absence of crystallization of an ingot or on a sawing of an ingot, produced very inexpensively can be. In this case, a silicon substrate produced in this way has an at least two-layer structure with a high-quality silicon layer serving as the absorber layer and a qualitatively inferior polycrystalline silicon layer serving as a carrier layer. The absorber layer and the carrier layer are firmly and flatly connected to each other. The low-quality polycrystalline silicon layer of the carrier layer has preferably been deposited from a gas phase and therefore has the typical structural and electrical properties for such a vapor deposition.

A solar cell can be produced with a silicon substrate produced according to the invention, wherein appropriate processing methods can be carried out on the silicon substrate to form features necessary for a function as a solar cell. Since the silicon substrate has similar mechanical and electrical properties to silicon wafers conventionally used in solar cell production, the processing methods used in conventional silicon solar cell production can be used in particular. Accordingly, the silicon substrates described herein can be advantageously used as a replacement for expensive silicon wafers and processed in conventional industrial productions. In particular, the silicon substrates may be processed to form, for example, further layers, such as dielectric layers, which may serve, inter alia, as antireflection layers, passivation layers, protective layers, or the like. For example, different types of chemical vapor deposition (CVD) can be used for this purpose. Additionally, the silicon substrates may be suitably processed to form electrical contact structures thereon. For this purpose, for example, screen printing technologies, vapor deposition technologies or the like can be used.

It should be noted that the foregoing description has described mainly the method steps and their features which are essential for the production and production methods presented, as they are also defined in the appended claims. A person skilled in the art will recognize that further process steps may be necessary or advantageous for producing a silicon substrate for solar cell production or for manufacturing a solar cell with the aid of such a silicon substrate. For example, a complete manufacturing sequence may comprise further etching steps, diffusion steps, texturing steps, steps for depositing surface layers, metallization steps, etc.

In addition, one of ordinary skill in the art will recognize that the method steps and features described hereinbefore in connection with embodiments of the invention may be appropriately interchanged or combined and supplemented by further processing steps to thereby provide further embodiments of the invention and possibly synergy effects reach. In addition, the method steps described can lead to corresponding structural features for silicon substrates according to the invention and solar cells produced with their help.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent to those skilled in the art from the following description of exemplary embodiments and with reference to the accompanying drawings, in which neither the description nor the drawings are to be construed as limiting the invention.

1 (a) to (f) illustrates a processing sequence of a method of manufacturing a silicon substrate according to an embodiment of the present invention.

2 (a) to (g) illustrates a processing sequence of a method of manufacturing a silicon substrate according to an alternative embodiment of the present invention.

The details shown in the figures are each only schematically illustrated and not reproduced to scale. Like reference numerals refer to like or equivalent features throughout the several parts of the figures.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In the 1 (a) to 1 (f) For example, a processing sequence of a method of manufacturing a silicon substrate according to an embodiment of the present invention is illustrated. In this processing sequence, a low-quality polycrystalline silicon layer is deposited from the gas phase directly onto a surface of an absorber layer provided with a high-quality silicon layer.

With reference to the 1 (a) to 1 (d) For example, an exemplary fabrication and provision of an absorber layer having a high quality silicon layer using a layer transfer method will be described. It should be noted, however, that the absorber layer can also be made by any other method which allows an absorber layer having a thin, eg 20 μm, thick layer of high quality silicon with a high minority carrier lifetime of more than 2 μs, preferably more to provide as 100 μs. For example, the absorber layer can also be formed with a macroporous silicon layer, as shown in FIG DE 10 2009 024 613 A1 is described.

It should also be noted that in the framework of the 1 shown processing sequence also parts and features are already co-produced, which can take over certain functions in a later to be produced from the silicon substrate solar cell, but this partial processing of a solar cell need not necessarily be part of a method for manufacturing a silicon substrate.

First, a base substrate 3 For example, in the form of a p ⁺ doped, 600 micron thick silicon wafer provided ( 1 (a) ). On a surface of the base substrate 3 which may be flat or textured, becomes a porous layer, for example, by anodic etching 5 generated ( 1 (b) ). The porous layer may advantageously consist of two partial layers of different porosity, wherein a highly porous partial layer is, for example 200 nm thick and has a porosity of about 50% and above a low-porous layer with a thickness of for example 1 .mu.m and a porosity of 20% is formed ,

On the porous layer 5 then becomes a high quality thin silicon layer 7 isolated ( 1 (c) ). This can have a layer thickness of between 1 and 100 .mu.m, preferably between 3 and 40 .mu.m, and be deposited by epitaxy, for example, as a phosphorus-doped n-type layer. This high quality thin silicon layer 7 may have a boron or phosphorus doping with, for example, a doping concentration of between 1 × 10 ¹⁴ and 5 × 10 ¹⁷ cm ^-3 and z. B. are deposited by means of CVD techniques.

Subsequently, an exposed surface 8th the high-quality silicon layer 7 with a functional layer 9 coated, for example, in the form of a dielectric layer ( 1 (d) ). This functional layer 9 is optional and may for example consist of silicon oxide (SiO _x ), silicon carbide (SiC _x ), silicon oxy carbide (SiO _x C _y ), silicon oxynitride (SiO _x N _y ) or aluminum oxide (Al _x O _y ) or a stack of different porous silicon layers and have a layer thickness of, for example, between 20 and 200 nm. It is also possible to deposit a plurality of dielectric layers as a layer stack. The functional layer can fulfill various functions for a solar cell to be produced later from the silicon substrate. For example, it can serve as a mirror for increasing an optical absorption within the absorber layer of the solar cell or as surface passivation. In one as a functional layer 9 serving, electrically insulating dielectric layer, local point-shaped openings 25 are provided, which can serve as contact holes in the solar cell, for example.

The high quality silicon layer 7 together with the optional functional layer 9 and possibly further layers not described here can be used as the absorber layer for the entire silicon substrate to be produced and for the solar cell to be produced therefrom 10 serve.

Directly on the exposed surface of the absorber layer 10 , ie in the case described on the outer functional layer 9 , is then from a gas phase, the z. Example, a silane such as SiH _x Cl _4-x (where _x = 1, 2, 3 or 4) contains a polycrystalline silicon layer 11 isolated ( 1 (e) ). The deposition can be carried out at high temperatures of over 700 ° C, for example, over 1100 ° C, take place. At such high temperatures, the polycrystalline silicon layer 11 be deposited very quickly, for example, with deposition rates of more than 20 microns / min. In this case, the polycrystalline silicon layer is formed 11 with very many crystal defects, ie it consists of many very small crystallites with a high density of intervening grain boundaries and offsets. Due to these crystal defects, the minority carrier lifetime is in the polycrystalline silicon layer 11 very short, in particular less than 2 μs, whereby it is not suitable as an absorber layer for a solar cell. A mechanical stabilizing effect for the adjacent absorber layer 10 can the poor quality polycrystalline silicon layer 11 but certainly meet, provided that it has a layer thickness of at least 30 microns, preferably between 80 and 180 microns.

In the example described, the polycrystalline silicon layer 11 formed as an n ⁺ -type layer with a strong phosphorus doping concentration. Due to the high deposition temperatures, phosphorus can be partially made of the polycrystalline silicon layer 11 in the area of the openings 25 in the dielectric layer 9 in the adjacent high-quality silicon layer 7 diffuse. This can be done in areas 21 these openings to a passivation of the back of the absorber layer 10 come so that in later use as a solar cell generated charge carriers can not recombine on structural defects. Because of the high temperature during the deposition of the polycrystalline silicon layer 11 Boron may also come from near-surface areas of the base substrate 3 or the porous layer formed thereon 5 in the high quality silicon layer 7 diffuse and there a near-surface p-doped region 13 form, which can later act as an emitter for the solar cell.

To complete the silicon substrate, then the composite of absorber layer 10 and carrier layer 12 from the base substrate 3 to be disconnected ( 1 (f) ). In this case, the porous layer 5 serve as a release layer, which can act as a predetermined breaking point in mechanical detachment of the composite or which can be dissolved in an additional etching step or can be resolved by rearrangement of atoms at high temperatures.

The silicon substrate thus prepared 1 can then be further processed like a normal wafer to form a solar cell. The high quality silicon layer 7 should form the solar facing absorber layer of the solar cell. It may be advantageous to realize many elements of a high efficiency solar cell in solar cell processing, such as backside passivation with high diffusion in the regions 21 of point contacts on the back and a diffused emitter on the front. The heavy doping of the low-quality polycrystalline silicon layer 11 the carrier layer 12 allows easy electrical contact with industry-standard techniques such. B. screen printing.

An alternative embodiment of a method according to the invention for producing a silicon substrate will be described below with reference to FIGS 2 (a) to (g) described. This embodiment differs from the previously described embodiment in particular in that the high-quality silicon layer 7 and the low quality polycrystalline silicon layer 11 on two separate base substrates 3 . 15 separated and then joined together. As a result, the risk can be avoided that z. B. the functional layer 9 , which will later serve in the solar cell for passivation, during the deposition of the low-quality polycrystalline silicon layer 11 z. B. is damaged due to the prevailing high temperatures.

On two separate base substrates 3 . 15 in the form of thick p ⁺ -type silicon wafers ( 2 (a) and 2 B) ) are each a porous layer 5 . 17 generated ( 2 (c) and 2 (d) ). In the one base substrate 3 is then on the porous layer 5 a high quality p-type silicon layer 7 epitaxially deposited and above one with local openings 25 provided functional layer 9 deposited ( 2 (f) ). For the other base substrate 15 is on the porous layer 17 a poor quality polycrystalline p ^- -type silicon layer 11 deposited directly from the gas phase and above this one later as a connector layer 19 deposited layer of a silicon-aluminum alloy deposited ( 2 (e) ).

The as absorber layer 10 Serving composite of high-quality silicon layer 7 and functional layer 9 on the one hand and as a carrier layer 12 serving composite of low-quality polycrystalline silicon layer 11 and connector layer 19 On the other hand, then mechanically fixed and electrically conductive are then connected together. For this, the connector layer 19 the carrier layer 12 in contact with the dielectric layer 9 the absorber layer 10 brought. Then the connector layer becomes 19 heated to a temperature of, for example, about 600 ° C, at which it liquefies, so as to subsequently solidify the absorber layer 10 with the carrier layer 12 firmly put together.

Again, the elevated temperatures during the joining process can be used to dopants in areas 23 the openings 25 in as a functional layer 9 serving dielectric layer in the high quality silicon layer 7 collect.

The absorber layer 10 and the carrier layer 12 can each before, during or after the joining process of the base substrates 3 . 15 be solved, the porous layers 5 . 17 can serve as predetermined breaking points again.

Alternatively, the absorber layer 10 and the carrier layer 12 may also be otherwise provided or deposited on other base substrates and / or otherwise separated from the base substrates.

After any removal of remnants of the porous layers 5 . 17 can the produced silicon substrate 1 are considered equivalent in every respect to a conventional wafer for the manufacture of a solar cell and further processed in industrially proven way.

Finally, it should be noted that the terms "comprise", "exhibit" etc. are not intended to exclude the presence of other elements. The term "a" does not exclude the presence of a plurality of objects. The reference signs in the claims are merely for ease of understanding and are not intended to limit the scope of the claims in any way.

LIST OF REFERENCE NUMBERS

1

silicon substrate

3

base substrate

5

porous layer

7

high quality silicon layer

8th

Surface of the high quality silicon layer

9

functional layer

10

absorber layer

11

low quality polycrystalline silicon layer

12

backing

13

diffused emitter layer

15

base substrate

17

porous layer

19

connector layer

21

Areas of punctual indiffusion

23

Areas of punctiform connections

25

point-shaped openings in functional layer

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 102009024613 A1 [0012, 0037]

Cited non-patent literature

• R. Brendel: "Review Of Layer Transfer Processes For Crystalline Thin-Film Silicon Solar Cells", Japanese Journal of Applied Physics 40, 4431-4439 (2001) [0012]
• M. Bruel: "Application Of Hydrogen Ion Beams To Silicon On Insulator Material Technology", Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions With Materials and Atoms 108, 313 (1996) [0012]
• F. Dross et al .: "Stress-Induced Large-Area Lift-Off Of Crystalline Si Films", Applied Physics A 89, 149-152 (2007) [0012]

Claims (15)

Method for producing a silicon substrate ( 1 ) for manufacturing a solar cell, the method comprising: providing an absorber layer ( 10 ) with a high-quality silicon layer ( 7 ), which has a high minority carrier lifetime and a layer thickness of less than 100 μm, attaching a carrier layer ( 12 ) to a surface of the absorber layer ( 10 ), wherein the carrier layer ( 12 ) an inferior polycrystalline silicon layer ( 11 ), wherein the qualitatively inferior polycrystalline silicon layer ( 11 ) has a low minority carrier lifetime shorter than the high minority carrier lifetime and a layer thickness greater than 30 μm. The method of claim 1, wherein the high minority carrier lifetime is greater than 2 μs and the low minority carrier lifetime is less than 2 μs. Method according to claim 1 or 2, wherein the qualitatively inferior polycrystalline silicon layer ( 11 ) is deposited at a high temperature of over 700 ° C. Method according to one of claims 1 to 3, wherein the low-quality polycrystalline silicon layer ( 11 ) is separated from a gas phase. Method according to one of claims 1 to 4, wherein the low-quality polycrystalline silicon layer ( 11 ) is deposited with a doping concentration of more than 1 × 10 ¹⁸ cm ^-3 . Method according to one of claims 1 to 5, wherein the qualitatively inferior polycrystalline silicon layer ( 11 ) has a contamination with recombination-strong metal atoms of less than 1 × 10 ¹³ cm ^-3 . Method according to one of claims 1 to 6, wherein the low-quality polycrystalline silicon layer ( 11 ) from the gas phase directly onto a surface of the absorber layer ( 10 ) is deposited. Method according to claim 7, wherein a functional layer ( 9 ), which can bring about a function that is advantageous for a solar cell as a passivation layer, mirror layer, insulation layer and / or protective layer, on a surface of the high-quality silicon layer ( 7 ) of the absorber layer ( 10 ) and the low quality polycrystalline silicon layer ( 11 ) from the gas phase to a high-quality silicon layer ( 7 ) opposite surface of the functional layer ( 9 ) is deposited. Method according to one of claims 1 to 8, wherein the qualitatively inferior polycrystalline silicon layer ( 11 ) from the gas phase first on a separate base substrate ( 15 ) is deposited and then to the surface of the absorber layer ( 10 ) is attached and mechanically connected to this. Method according to claim 9, wherein between the surface of the absorber layer ( 10 ) and an adjacent surface of the carrier layer, a connector layer ( 19 ) for the mechanically strong bonding of the absorber layer ( 10 ) with the carrier layer ( 12 ) is formed. Method according to one of claims 1 to 10, wherein the absorber layer ( 10 ) on a base substrate ( 3 ) formed porous layer ( 5 ) provided. Method according to one of claims 1 to 11, wherein the high-quality silicon layer ( 7 ) is formed by a method selected from the group consisting of epitaxy, layer transfer processes, ion implantation and spalling. A method for manufacturing a solar cell, comprising: producing a silicon substrate ( 1 ) by a method according to any one of claims 1 to 12; Processing the silicon substrate ( 1 ) for the formation of features necessary for a function as a solar cell. Silicon substrate for manufacturing solar cells, wherein the silicon substrate ( 1 ): an absorber layer ( 10 ) with a high-quality silicon layer ( 7 ), which has a high minority carrier lifetime and a layer thickness of less than 100 μm, one with a surface of the absorber layer ( 10 ) fixed and surface-connected support layer ( 12 ), wherein the carrier layer ( 12 ) an inferior polycrystalline silicon layer ( 11 ) having a low minority carrier lifetime shorter than the high minority carrier lifetime and a layer thickness greater than 30 μm. Solar cell with a silicon substrate ( 1 ) according to claim 14.

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