DE102010024835A1 - Method for fabrication of a backside contact solar cell - Google Patents

Abstract

A method for producing a back contact solar cell, comprising the steps a) providing a crystalline silicon substrate (101) with a front side and a rear side; b) simultaneous diffusion of a phosphorus dopant on at least a part of said front side and at least a part of said back side into said crystalline silicon substrate (101), whereby a front side phosphor diffusion area (201) with a first diffusion depth and a rear side phosphor diffusion area (203) is generated with the same first diffusion depth, a layer of phosphorus silicate glass (202) on the front-side phosphorus diffusion region (201) and a layer in an in situ step during the diffusion of said phosphorus dopant in an in situ step is formed from phosphorus silicate glass (204) on the back phosphor diffusion region (203); c) forming a first dielectric layer (305) on at least a portion of the back of the silicon substrate (101); d) removing at least part of the phosphorus silicate glass layer (202) on the front side of the silicon substrate (101) and e) heating the product obtained by carrying out steps a) to d), as described above, for a certain period of time and at a certain temperature wherein said time period and temperature are selected such that said front phosphor diffusion region (201) and said back phosphor diffusion region (203) expand further into the crystal to a second depth of diffusion, which after heating for the front phosphor diffusion region (203A) is different from that for the rear phosphor diffusion region (203B).

Description

The invention relates to a method for producing a back contact solar cell having the features according to the preamble of claim 1 and a solar cell produced using said method.

Solar cells are well-known components that sunlight, d. H. convert electromagnetic radiation into electrical energy. The front side of a solar cell or a substrate used to manufacture a solar cell is the light-facing side when the solar cell is in operation. The back surface of a solar cell or a substrate used for manufacturing a solar cell is the side opposite to the front side.

Generally speaking, a solar cell can be made by forming p- and n-doped regions in a semiconductor substrate, typically silicon. Boron is often used as a p-dopant and phosphorus is often used as an n-dopant.

Light striking the solar cell generates electron-hole pairs. The electrons and holes thus generated typically migrate into p-type and n-type regions due to an electric field generated whenever p- and n-type regions are in contact with each other. So that the solar cell can forward current to an external circuit, so as to produce an electrical coupling, the doped regions are coupled to contacts, which are usually made of metal.

In a back contact solar cell, these contacts are located on the back of the solar cell. Variations of this solar cell type and methods for their preparation are for example US Pat. No. 6,998,288 B1 . US Pat. No. 7,135,350 B1 and WO 2009/074469 A2 known.

In the case of back contact solar cells, the n-doped regions on the front side of the solar cell or on the back side of the solar cell must satisfy different conditions. On the front side, a high layer resistance of the "front surface field" (FSF) of the n-dopant leads to improved surface passivation. On the back side, a low layer resistance of the back surface field (BSF) of the n-dopant is to be preferred in order to reduce the contact resistance. The sheet resistance is largely determined by the dopant concentration.

For this reason, all known back contact solar cell fabrication methods use two separate process steps related to the diffusion of n-dopant atoms into the substrate, one to form the n-dopant FSF and the other to form the n-dopant BSF. However, each of these process steps takes time and costs money.

Therefore, it is the object of this invention to provide a more time and cost efficient method for making a back contact solar cell and a lower cost solar cell made by this method.

This problem is solved by a method for producing a back contact solar cell having the features of claim 1 and a solar cell produced according to this method. The advantageous embodiment variants of the method are each the subject of dependent claims.

The method for producing a back contact solar cell according to this invention comprises the steps of a) providing a crystalline silicon substrate having a front side and a back side and b) simultaneously diffusing a phosphorus dopant on at least a part of said front side and at least a part of said back side in FIG said silicon substrate being such as to produce a front phosphorus diffusion region having a first diffusion depth and a backside diffusion region having the same first diffusion depth, and that during the diffusion of said phosphorus dopant, a layer of phosphosilicate glass in an in situ step on the front phosphorus Diffusion region and a layer of phosphosilicate glass is formed in an in situ step on the back phosphorus diffusion region.

In further process steps according to this invention, as step c), a dielectric layer is formed on said phosphorosilicate glass layer on at least a part of the back side of the silicon substrate, as step d) at least part of the phosphosilicate glass layer on the front side of the silicon substrate is removed and as step e) after heating the product produced by the above-mentioned steps for a certain period of time and at a certain temperature, wherein said period and said temperature are selected such that said front phosphorus diffusion region and said rear phosphorus diffusion region further penetrate into the crystal to a second diffusion depth, which is different for the front phosphorus diffusion region after heating than for the backside phosphor diffusion region. The usual for heating Temperatures and durations are 850-1050 ° C or 10-300 minutes.

Preferably, the steps are to be performed in the order mentioned above and by the order of the assigned letters of the alphabetical order.

By causing the first diffusion step to cause formation of a phosphosilicate glass layer, a source of dopant atoms is provided on the surface of the silicon substrate. The removal of at least a portion of said phosphorosilicate glass layer on the front side of the substrate causes the amount of additionally available dopant atoms from this source to be different than to the backside. The heating results in the different amount of additionally available dopant atoms resulting in different diffusion profiles.

Consequently, both a phosphorus-FSF and a phosphorus-BSF can be formed simultaneously with the aid of the process according to this invention, which obviates the time-consuming and cost-intensive requirement of two separate process steps.

In an advantageous embodiment variant, an n-type silicon substrate is provided in step a) because it has a longer service life.

In a further advantageous embodiment variant, the phosphorosilicate glass layer is completely removed from the front side in step d), before a second dielectric layer is formed on the front side of the silicon substrate in step e).

Generally speaking, it is advantageous if a phosphosilicate glass layer of at least 1 nm is present on the backside of said silicon substrate. Parameters that can be used to control the thickness are, for. As the diffusion temperature, the diffusion duration, the O2 gas flow, the N2 gas flow and the amount or the flow of the phosphorus diffusion source used.

In a further advantageous embodiment variant, the process conditions for step d), in particular the etching time and the concentration of the etchant in the etching solution, are selected such that the front side of the silicon substrate resulting from the implementation of step d) is hydrophobic.

In a further advantageous embodiment variant, the first dielectric layer on at least one part of the rear side of the silicon substrate and / or the said second dielectric layer on the front side of the silicon substrate are used to provide a diffusion mask in a further production step of the solar cell. In this way, cross-contamination of dopants can be reliably avoided.

In a further advantageous embodiment variant, between the execution of step c) and the implementation of step e) at least the step of removing the dielectric layer on the back side of said silicon substrate and the removal of the phosphorus diffusion region on at least a part of the backside of said silicon substrate but not all over the back side of said silicon substrate and the step of cleaning the parts from which the dielectric layer on the back side of said silicon substrate and on the back side of the phosphorus diffusion region has been removed, because this is a simple and practical possibility to define the areas in which boron-doped material is to be provided. It is particularly advantageous if under these conditions step e) takes place in an atmosphere comprising O 2, N 2 and boron, so that a boron-diffused region is formed in those parts in which the dielectric layer on the back side of said silicon substrate and on the back side of the phosphorus region was removed. This leads to a further optimization of the manufacturing process, especially in terms of duration, because the heating necessary to form a boron-diffused region, i. H. p-doped portions of the silicon substrate, may be used simultaneously to achieve those diffusion profiles that result in the desired properties of the phosphorus-FSF and the phosphorus-BSF.

In a further advantageous embodiment variant, the process conditions are selected such that, after step e), the second diffusion depth for the front phosphorus diffusion region is smaller than the second diffusion depth for the rearward phosphorus diffusion region.

In a further advantageous embodiment variant, the process conditions are selected such that, after step e), the sheet resistance of the backside phosphorus diffusion region obtained is lower than the sheet resistance of the front phosphorus diffusion region.

In a further advantageous embodiment variant, after step f) a third dielectric layer ( 601 ) on the backside of the silicon substrate ( 101 ) applied. In this way, the boron diffusion from step f) can be passivated.

In a further advantageous embodiment variant comprises at least one of said dielectric layers comprise one or more or all of the materials hydrogen-containing PECVD silicon nitride, hydrogen-containing silicon carbide, hydrogen-containing silicon oxynitride, hydrogen-containing amorphous silicon or silicon oxide. These materials allow passivation of the surface and the antireflection layers. They are also suitable as diffusion boundary layers.

In a further advantageous embodiment variant, at least one backside phosphorus diffusion region and at least one boron diffusion region are contacted, said contacting being achieved with simultaneous metallization using the same metal paste or metal source within a single process step. This leads to a further optimization of the manufacturing process.

The invention will now be explained in more detail with reference to the following figures, which show:

A crystalline silicon substrate as provided in step a) according to the method of this invention,

: the crystalline silicon substrate after step b) has been performed,

: the crystalline silicon substrate after performing steps c) and d),

: The crystalline silicon substrate after portions of the dielectric layer have been removed from the back surface, a second dielectric layer has been deposited on the front side of the silicon substrate on the phosphorus diffusion region and portions of the backside phosphorus diffusion region have been removed,

: The crystalline silicon substrate after step e) has been carried out in a boron atmosphere,

: The crystalline silicon substrate after a third dielectric layer has been deposited on the backside of the silicon substrate,

: The crystalline silicon substrate after the backside phosphorus diffusion regions and the boron diffusion regions have been contacted and

: Data obtained in experiments showing the phosphorus diffusion profiles, including the diffusion depth, obtained by using steps a) and b) of the process and after applying steps a) to e) of the process and forming a second dielectric layer ,

to refer to a single embodiment variant of the method, which is why identical reference numbers are used here. The relative thickness of the layers and / or regions as shown in the figures is sometimes exaggerated to illustrate the effect of using certain process steps. The phrase "modified crystalline silicon substrate" as used below refers to a silicon substrate whose properties have been altered, including surface layers formed on or added thereto, and not merely indicative of variations in the silicon substrate base.

shows a crystalline silicon substrate 101 as provided in step a) according to the method of this invention. Next, a phosphorus dopant becomes the crystalline silicon substrate 101 diffused at the same time on the front and on the back using known methods, for example in a tube furnace.

As in As shown, a modified crystalline silicon substrate is produced during this process, including a front phosphorus diffusion region 201 with a first diffusion depth and a backside phosphorus diffusion region 203 with the same first diffusion depth. In addition, the process conditions of the diffusion process, in particular the phosphorus source, temperature, duration, O 2 gas flow and N 2 gas flow, are selected such that during the diffusion of said phosphorus dopant a layer of phosphosilicate glass 202 in an in situ step on the front phosphorus diffusion region 201 is formed and a layer of phosphosilicate glass 204 in an in situ step on the backside phosphorus diffusion region 203 is formed, as well as in shown.

As in Next, the phosphosilicate glass layer is shown 202 on the front side of the modified crystalline silicon substrate 101 removed, preferably by means of etching. In addition, a first dielectric layer 305 on at least a part of the back side of the modified crystalline silicon substrate 101 formed and a second dielectric layer 306 becomes on the front side of the modified crystalline silicon substrate 101 educated. For this purpose, any method known in the art for forming such a layer on a solar cell can be used.

The second dielectric layer serves as a barrier which protects the diffused phosphor layer against later diffusion with boron. Alternatively, such protection can also be provided by charging the wafers in a front-to-front configuration for boron diffusion.

After these steps must be in the modified crystalline silicon substrate 101 still p-doped regions are generated. To achieve this, first in shown situation generated. Parts of the dielectric layer 305 on the back are removed such that they no longer cover the entire back of the device produced in the previous process steps to parts of the back phosphorosilicate glass layer 204 to make accessible. The exposed parts are also removed, preferably by etching, around parts of the phosphorus diffusion region 203 to make accessible. Again, the so accessible parts are removed. Method for removing parts of a dielectric layer 305 and sharing a backside phosphorus diffusion layer are well known in the art. This ablation becomes areas 401 with worn dielectric layer 305 , eroded phosphorosilicate glass layer 204 and ablated phosphorus diffusion region 203 on the back of the thus modified crystalline silicon substrate 101 generated. In other words, in the fields 401 were all modifications of the crystalline silicon substrate 101 , located on the back of the crystalline silicon substrate 101 were removed.

Subsequently, the in produced situation by the thus modified crystalline silicon substrate 101 for a period of time, usually between 10 and 300 minutes, at a high temperature, usually 850-1050 ° C, in an atmosphere comprising O2, N2 and boron. On the one hand, this leads to the diffusion of boron into the crystalline silicon substrate 101 on the other hand, to form boron diffusion regions 501 in the fields of 401 , Usual process parameters for the generation of suitable boron diffusion regions 501 are known in the art. These parameters, especially those for duration and temperature, are also suitable to ensure that the front phosphorus diffusion region 201 and the remaining parts of the backside phosphorus diffusion region 203 continue to expand into the crystal to a second diffusion depth, distinct from the front phosphorus diffusion region 203A on the one hand and the backside phosphor diffusion area 203B on the other hand, after heating. This difference is due to the amount of phosphosilicate glass present on the front side or rear side of the modified crystalline silicon substrate 101 which acts as dopant source during diffusion.

Next is the in produced situation by a third dielectric layer on the back of the thus modified crystalline silicon substrate 101 is applied using known methods. Finally, the contacting of the modified crystalline silicon substrate 101 out that is, the contacting of the backside phosphorus diffusion region and the boron diffusion region to that in shown situation, including the metal contacts 701 ; 702 which can be generated according to any method known in the art. However, it is important to note that products made by a process according to this invention can be contacted by simultaneous metallization using the same metal paste or metal source in a single process step.

FIG. 3 illustrates the phosphorus diffusion profiles, including the diffusion depth, obtained on the one hand by the use of steps a) and b) of the method and on the other hand after application of steps a) to f) of the method, using diffusion data obtained in experiments. Triangles mark the initial P-diffusion in both the front and back phosphor diffusion areas, circles mark the phosphorus diffusion after heating on the back, and squares mark the phosphorus diffusion after heating on the front. These data demonstrate that the approach outlined above, based on concurrent diffusion of the n-type dopant into the front and back of the crystalline silicon substrate, indeed produces a significant difference in diffusion profiles for the current state of the art known in the art always two separate diffusion processes are required.

LIST OF REFERENCE NUMBERS

101

Crystalline silicon substrate

201

Front phosphorus diffusion area with first diffusion depth

202

Phosphorus silicate glass layer

203

Backside phosphor diffusion area with first diffusion depth

204

Phosphorus silicate glass layer

305

First dielectric layer

306

Second dielectric layer

401

Part with eroded dielectric layer and removed phosphorus diffusion region

501

Boron diffusion region

201A

Front phosphorus diffusion area after heating

203A

backside phosphor diffusion area after heating

601

Third dielectric layer

701

Contact to the back. Phosphorus diffusion region

702

Contact to the boron diffusion region

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

• US 6998288 B1 [0005]
• US 7135350 B1 [0005]
• WO 2009/074469 A2 [0005]

Claims (12)

Method for producing a back contact solar cell, comprising the steps of a) providing a silicon substrate ( 101 ) with a front and a back; b) simultaneously diffusing a phosphorus dopant on at least part of said front side and at least part of said backside into said crystalline silicon substrate ( 101 ), whereby a front phosphorus diffusion region ( 201 ) having a first diffusion depth and a backside phosphorus diffusion region ( 203 ) is produced with the same first diffusion depth, wherein during the diffusion of said phosphorus dopant in an in situ step, a layer of phosphosilicate glass ( 202 ) on the front phosphorus diffusion region ( 201 ) and in an in situ step a layer of phosphosilicate glass ( 204 ) on the backside phosphorus diffusion region ( 203 ) is formed; c) forming a first dielectric layer ( 305 ) on at least a part of the backside of the silicon substrate ( 101 ); d) removal of at least parts of the phosphosilicate glass layer ( 202 ) on the front side of the silicon substrate ( 101 ); e) heating the product obtained by carrying out steps a) to d) as described above for a certain period of time and at a specific temperature, wherein said period and said temperature are to be selected such that said front phosphorus diffusion region ( 201 ) and said backside phosphorus diffusion region ( 203 ) continue to expand into the crystal to a second depth of diffusion which, after heating, for the front phosphorus diffusion region (FIG. 203A ) is other than for the backside phosphorus diffusion region ( 203B ). Process according to claim 1, wherein in step a) an n-type silicon substrate ( 101 ) provided. Method according to claim 1 or 2, in which in step d) the phosphorosilicate glass layer is completely removed from the front side of the silicon substrate ( 101 ) and before step e) a second dielectric layer ( 306 ) on the front side of the silicon substrate ( 101 ) is formed. Method according to one of claims 1 to 3, in which the process conditions for step d) are selected such that the front side of the silicon substrate ( 101 ) obtained after performing step d) is hydrophobic. Method according to one of claims 1 to 4, in which the first dielectric layer ( 305 ) on at least a part of the rear side of the silicon substrate and / or the said second dielectric layer ( 306 ) on the front side of the silicon substrate ( 101 ) are used to provide a diffusion mask in a subsequent production step of the solar cell. Method according to claim 1, in which between the execution of step c) and the execution of step e) at least the following steps - removal of the dielectric layer ( 305 ) on the back side of said silicon substrate and the backside phosphorus diffusion region ( 203 ) on at least a part of the rear side of said silicon substrate ( 101 ), but not on the entire surface of said silicon substrate ( 101 ), and - cleaning the parts ( 401 ), of which the dielectric layer ( 305 ) on the back side of said silicon substrate and the backside phosphorus diffusion region ( 203 ) were carried out. Method according to claim 6, in which step e) takes place in an atmosphere comprising O 2, N 2 and boron, so that a boron-diffused region ( 501 ) arises in those parts in which the dielectric layer ( 305 ) on the backside of said silicon substrate ( 101 ) and was ablated on the back phosphorus diffusion region. Method according to one of claims 1 to 7, in which the process conditions are selected such that after step e) the second diffusion depth for the front phosphorus diffusion region (FIG. 201A ) is smaller than the second diffusion depth for the backside phosphorus diffusion region ( 203A ). Method according to one of Claims 1 to 8, in which the process conditions are selected such that after step e) the sheet resistance of the backside phosphorus diffusion region ( 201A ) is lower than the sheet resistance of the front phosphorus diffusion region ( 203A ). Method according to one of claims 1 to 9, wherein after step e) a third dielectric layer ( 601 ) on the backside of the silicon substrate ( 101 ) is applied. Method according to one of claims 1 to 10, in which at least one of said dielectric layers ( 305 ; 306 ; 601 ) one, several or all of the materials hydrogen-containing PECVD silicon nitride, hydrogen-containing silicon carbide, hydrogen-containing Silicon oxynitride, hydrogen-containing amorphous silicon or silicon oxide. Method according to one of claims 1 to 11, in which at least one phosphorus diffusion region ( 203A ) and at least one boron diffusion region ( 501 ), wherein said contacting is achieved with simultaneous metallization using the same metal paste or metal source within a single process step.

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