WO2011160814A2 - Method for creating a passivated boron-doped region, especially during production of a solar cell, and solar cell with passivated boron-diffused region - Google Patents

Method for creating a passivated boron-doped region, especially during production of a solar cell, and solar cell with passivated boron-diffused region Download PDF

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WO2011160814A2
WO2011160814A2 PCT/EP2011/003060 EP2011003060W WO2011160814A2 WO 2011160814 A2 WO2011160814 A2 WO 2011160814A2 EP 2011003060 W EP2011003060 W EP 2011003060W WO 2011160814 A2 WO2011160814 A2 WO 2011160814A2
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boron
doped region
semiconductor substrate
glass layer
borosilicate glass
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French (fr)
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WO2011160814A3 (en
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Valentin Mihailetchi
Radovan Kopecek
Eckard Wefringhaus
Rudolf Harney
Johann Jourdan
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International Solar Energy Research Center Konstanz E. V.
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/068—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • H01L31/0682—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells back-junction, i.e. rearside emitter, solar cells, e.g. interdigitated base-emitter regions back-junction cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic System
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to a method for creating a passivated boron-doped region with the features of the preamble of claim 1 and a solar cell fabricated by application of said method.
  • a solar cell can be created by forming p-doped and n-doped regions in a Silicon substrate.
  • passivation of a boron-doped region has been achieved by addition of surface films of SiN, thermally or chemically grown Si02 or, preferably, stacks of Si02 and SiN, as de- scribed e.g. in the article "Surface passivation of silicon solar cells using plasma-enhanced chemical-vapour-deposited SiN films and thin thermal Si02/plasma SiN stacks" by J.
  • the problem solved by this invention is providing a method for providing a more effectively passivated boron-doped region in a semiconductor substrate, especially during production of a solar cell, and providing a solar cell with more effective passivation of the boron-doped region.
  • the method for fabrication of a passivated boron-doped region, especially in a solar cell, comprises the steps of a) providing a semiconductor substrate, b) forming a boron-doped region in said semiconductor substrate, wherein at least part of a surface of said semiconductor substrate belongs to said boron-doped region, and
  • borosilicate glass layer is formed on the surface of the boron-doped region and wherein at least one dielectric coating film is formed in direct contact with at least part of said borosilicate glass layer.
  • a borosilicate glass layer on the surface of the boron-doped region may lead either to a two-layer structure of boron-doped region and borosilicate glass layer or to a three-layer structure of boron-doped region, boron-rich layer and borosilicate glass layer.
  • the dielectric coating film may be applied to a semiconductor substrate that has been covered by a surface layer during the recited or ad- ditional steps of the method.
  • the term "silicon substrate” as used in this invention may relate to a silicon substrate whose properties have been changed by method steps. This includes addition of surface layers created thereon or added thereto, it does not only indicate changes in the silicon substrate body, as obtained e.g. by doping.
  • step a) a silicon substrate is provided, in step b) the forming of the boron-doped region is done by diffusion and the borosilicate glass layer is formed during formation of the boron-doped region in an in- situ step. This removes the need to create the borosilicate glass layer in a separate processing step and leads to an extremely time and cost efficient production press.
  • the method may be applied to both p-type silicon substrates and n-type silicon substrates.
  • a borosilicate glass layer may be generated automatically under standard conditions regarding temperature and duration of the boron diffusion step if the boron diffusion is performed in an atmosphere containing oxygen and either BBr3 or BC13 and a carrier gas.
  • Boron sources including B2H6 and BN .
  • a further improvement of the thus obtained passivation is achieved if after formation of said borosilicate glass layer the process continues by an in-situ thermal oxidation in an atmosphere containing 02 or H20.
  • the thickness of said borosilicate glass layer is reduced in such a way that after thinning the borosilicate glass layer retains a thickness of at least one nm.
  • Such a reduction of the thickness of said borosilicate layer can be performed by etching in a chemical solution comprising at least one of the group consisting of a solution containing hydrofluidic acid, a solution containing sodium hydroxide, a solution containing potassium hydroxide, a solution containing sulphuric acid, a solution containing hydrogen peroxide or a solution containing nitric acid.
  • the stack of borosilicate glass and dielectric coating film is especially effective if said dielectric coating film comprises silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amorphous silicon including hydrogen or silicon oxide.
  • the method comprises annealing after formation of said di- electric coating film, this leads to improved surface passivation which may be related to an improved release of hydrogen from the dielectric layer. Specifically, this can be achieved especially well if the annealing temperature is higher than the deposition temperature of said dielectric coating film and may advantageously be realized during a firing step that is performed to realize the metal contacts of the solar cell.
  • the solar cell according to this invention comprises a semiconductor substrate, at least one phosphorous doped region in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact, at least one boron doped region in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact and at least one dielectric coating film cover- ing at least part of a surface of said semiconductor substrate, wherein said semiconductor substrate comprises at least one borosilicate glass layer on the surface of said at least one boron doped region and wherein said at least one dielectric film is in direct contact with at least one surface of said borosilicate glass layer that is opposed to the surface of the borosilicate glass layer facing the at least one boron doped region.
  • This arrangement leads to an improved passivation of the boron-doped regions of the solar cell, which increases the effective lifetime of the minority charge carriers created during operation of the cell, leading to an improved yield of the solar cell according to this invention.
  • borosilicate layer on the surface of said at least one boron doped region includes a two-layer structure of boron- doped region and borosilicate glass layer as well as a three- layer structure of boron-doped region, intermediate boron-rich layer and borosilicate glass layer.
  • the preferred semiconductor substrate is silicon, specifically p-type silicon or n-type silicon. If the borosilicate glass layer has a thickness of lnm, 'it can act as part of an antire- flection coating.
  • Advantageous materials for the dielectric coating film comprise silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amor- phous silicon including hydrogen or silicon oxide.
  • Figure 1 measured effective recombination lifetime of boron diffused n-type silicon wafers treated with different passivation techniques
  • Figure 2 a first embodiment of a solar cell according to this invention, representing an n-type solar cell
  • FIG. 3 a second embodiment of a solar cell according to this invention, representing a p-type solar cell
  • FIG. 4 a third embodiment of a solar cell according to this invention, representing a back contacted n- type solar cell
  • Figure 5 a fourth embodiment of a solar cell according to this invention, representing a back contacted p- type solar cell.
  • silicon substrate includes a silicon substrate whose properties have been changed. This includes addition surface layers created thereon or added thereto, it does not only indicate changes in the silicon substrate body, as obtained e.g. by doping.
  • front and back are used in the context of the solar cells shown in Figures 2-5 in the following way:
  • the term “front” relates to the side of the solar cell that is facing the light during operation of the solar cell, the tern “back” to the side opposite to the front side.
  • Figure 1 shows measured effective recombination lifetime of boron diffused n-type silicon wafers treated with different passivation techniques for the case of a dielectric coating film of siliconnitride including hydrogen. It is evident from these measurements that the passivation method according to the arrangement according to this invention, i.e. in the presence of a surface passivation with stacked bo- rosilicate glass and SiNx:H, effective lifetimes can be increased notably compared to the ones obtained using previously known surface passivation techniques, stressing the significant advantage achieved by this invention.
  • Figure 2 shows a first embodiment of a solar cell 200 according to this invention, representing an n-type solar cell. What is displayed is a n-type silicon substrate 201.
  • n-type silicon substrate 201 On the back side of said n-type silicon substrate 201, it has a phosphorous-doped region 203 which is contacted by metal contacts 207, made of silver. The back surface of the phosphorous doped region 203 that is not covered by the metal contacts 207 is covered by a dielectric coating film 205, PECVD-deposited silicon nitride including hydrogen.
  • a boron-doped region 202 On the front side of said n-type silicon substrate 201, it has a boron-doped region 202, the front side of which is covered by a layer of borosilicate glass 202A.
  • Metal contacts 206 made of a mixture comprising silver and aluminum contact the boron-doped region 202.
  • the front surface of the borosilicate glass layer 202A is covered by a dielectric coating film 205, PECVD-deposited silicon ni- tride including hydrogen.
  • FIG. 3 shows a second embodiment of a solar cell 300 according to this invention, representing a p-type solar cell.
  • a p-type silicon substrate 301 On the front side of said p-type silicon substrate 301, it has a phosphorous-doped region 303 which is contacted by metal contacts 307, made of silver. The front surface of the phosphorous doped region 303 that is not covered by the metal contacts 307 is covered by a dielectric coating film 304, PECVD-deposited silicon nitride including hydrogen.
  • a boron-doped region 302 On the back side of said p-type silicon substrate 301, it has a boron-doped region 302, the back side of which is covered by a layer of borosilicate glass 302A.
  • Metal contacts 306 made of a mixture comprising silver and aluminum contact the boron-doped region 302.
  • the back surface of the borosilicate glass layer 302A is covered by a dielectric coating film 305, PECVD-deposited silicon ni- tride including hydrogen.
  • Figure 4 shows a third embodiment of a solar cell 400 according to this invention, representing a back contacted n-type solar cell. What is displayed is a n-type silicon substrate 401. On the front side and parts of the back side of said n- type silicon substrate 401, it has phosphorous-doped regions 403. The phosphorous-doped regions 403 that are located on the back side of the n-type silicon substrate 401 is contacted by metal contacts 407, made of silver. The surfaces of the phosphorous doped regions 403 that are not covered by the metal contacts 407 are covered by dielectric coating films 404,405, PECVD-deposited silicon nitride including hydrogen.
  • boron-doped regions 402 On the remaining parts of the back side of said n-type silicon substrate 401, there are boron-doped regions 402, the back sides of which are covered by layers of borosilicate glass 402A.
  • Metal contacts 406 made of a mixture comprising silver and aluminum contact the boron-doped regions 402.
  • the back surface of the borosilicate glass layer 402A is covered by a dielectric coating film 405, PECVD-deposited silicon nitride including hydrogen.
  • Figure 5 shows a fourth embodiment of a solar cell 500 according to this invention, representing a back contacted p-type solar cell. What is displayed is a p-type silicon substrate 501. On the front side and parts of the back side of said p- type silicon substrate 501, it has boron-doped regions 502. The boron-doped regions 502 that are located on the back side of the p-type silicon substrate 501 are contacted by metal contacts 506, made of a mixture comprising silver and alumi- num.
  • the surfaces of the boron doped regions 502 that are not covered by the metal contacts 506 are covered by layers of borosilicate glass 502A that, in turn, are covered by dielectric coating films 504,505, PECVD-deposited silicon nitride includ ⁇ ing hydrogen.
  • dielectric coating films 504,505 On the remaining parts of the back side of said p-type silicon substrate 501, there are phosphorous-doped re ⁇ gions 503.
  • Metal contacts 506 made of a mixture comprising silver and aluminum contact the phosphorous-doped regions 503.
  • the back surface of phosphorous-doped regions 503 that is not covered by the metal contacts 506 is also covered by the dielectric coating film 505, PECVD-deposited silicon nitride including hydrogen.
  • all types of solar cells can be fabricated according to the principles of this invention, e.g. conventional solar cells, interdigitated back-contact back-j unction solar cells, a metal wrap through or an emitter wrap through having boron diffusion regions on the light receiving side ore on the opposite side.

Abstract

The invention relates to a method for fabrication of a passivated boron-doped region, especially in a solar cell, comprising the steps of a) providing a semiconductor substrate, b) forming a boron-doped region (202,302,402,502) in said semiconductor substrate (201, 201, 301, 401), wherein at least part of a surface of said semiconductor substrate belongs to said boron-doped region (202,302,402, 502) and c) forming at least one dielectric coating film (204,205,304,305,404,405, 504,505) on at least part of the surface of the semiconductor substrate (201,301, 401,501) as modified by previous process-ing steps, wherein a borosilicate glass layer (202A, 302A, 402A, 502A) is formed on the surface of the boron-doped region (202, 302,402,502) and that at least one dielectric coating film (204,304,404,504) is formed in direct contact with at least part of said borosilicate glass layer (202A,302A, 402A,502A) and a solar cell (200,300,400,500) comprising a semiconductor substrate, at least one phosphorous doped region (203,303,403, 503) in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact (207, 307,407,507), at least one boron doped region (202,302,402, 502) in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact (206, 306,406,506) and at least one dielectric coating film (204,205, 304,305, 404,405,504,505) covering at least part of a surface of said semiconductor substrate, wherein said semiconductor substrate comprises at least one borosilicate glass layer (202A, 302A, 402A, 502A) on the surface of said at least one boron doped region (202,302,402,502) and that said at least one dielectric film (204,304,404, 504) is in direct contact with at least one surface of said borosilicate glass layer (202A, 302A, 402A, 502A) that is opposed to the surface of the borosilicate glass layer (202, 302, 402, 502) facing the at least one boron doped region.

Description

Method for creating a passivated boron-doped region, especially during production of a solar cell, and solar cell with passivated boron-diffused region The invention relates to a method for creating a passivated boron-doped region with the features of the preamble of claim 1 and a solar cell fabricated by application of said method.
One of the standard processes used in semiconductor technology is the doping of crystalline silicon substrates. For example, a solar cell can be created by forming p-doped and n-doped regions in a Silicon substrate.
One of the most important types of p-dopants is Boron. How- ever, for a number of important applications, it is necessary to achieve passivation, more specifically surface passivation, of created boron-doped regions. Using again solar cells as an example, passivation increases notably the effective life time of electrons and holes generated by light impinging on the so- lar cell, thus increasing the efficiency of the solar cell.
So far, passivation of a boron-doped region has been achieved by addition of surface films of SiN, thermally or chemically grown Si02 or, preferably, stacks of Si02 and SiN, as de- scribed e.g. in the article "Surface passivation of silicon solar cells using plasma-enhanced chemical-vapour-deposited SiN films and thin thermal Si02/plasma SiN stacks" by J.
Schmidt et al., Semicond. Sci. Rechnol. 16 (2001), 164-170) or the article "Surface passivation of boron diffused emitters for high efficiency solar cells" by J. Benick et al. from the proceedings of IEEE Electron Devices Society: 33rd IEEE Photovoltaic Specialists Conference 2008, San Diego, CA, Mayll- 16,2008. However, none of these known approaches provides a perfect passivation of the boron-doped regions, further opti- mization of the passivation is needed. The problem solved by this invention is providing a method for providing a more effectively passivated boron-doped region in a semiconductor substrate, especially during production of a solar cell, and providing a solar cell with more effective passivation of the boron-doped region.
This problem is solved by a method for fabrication of a solar cell with the features of claim 1 and a solar cell according to claim 11. Advantageous embodiments of the method are de- scribed in the dependent claims.
The method for fabrication of a passivated boron-doped region, especially in a solar cell, according to this invention comprises the steps of a) providing a semiconductor substrate, b) forming a boron-doped region in said semiconductor substrate, wherein at least part of a surface of said semiconductor substrate belongs to said boron-doped region, and
c) forming at least one dielectric coating film on at least part of the surface of the semiconductor substrate as modified by previous processing steps, wherein a borosilicate glass layer is formed on the surface of the boron-doped region and wherein at least one dielectric coating film is formed in direct contact with at least part of said borosilicate glass layer. As a function of the reaction conditions that are em- ployed, the formation of a borosilicate glass layer on the surface of a boron-doped region can lead to an intermediate surface layer known in the art as "boron-rich layer". For this reason, the claimed formation of a borosilicate glass layer on the surface of the boron-doped region may lead either to a two-layer structure of boron-doped region and borosilicate glass layer or to a three-layer structure of boron-doped region, boron-rich layer and borosilicate glass layer.
As indicated by the use of the phrase "semiconductor substrate as modified by previous processing steps", the dielectric coating film may be applied to a semiconductor substrate that has been covered by a surface layer during the recited or ad- ditional steps of the method. In general, the term "silicon substrate" as used in this invention may relate to a silicon substrate whose properties have been changed by method steps. This includes addition of surface layers created thereon or added thereto, it does not only indicate changes in the silicon substrate body, as obtained e.g. by doping.
The key concept of this invention resides in providing surface passivation of a boron-doped layer by means of a stack of bo- rosilicate glass and dielectric coating film. It has been found that surprisingly this type of stack leads to a notably improved passivation as indicated, e.g., by a significant increase of minority carrier life time. In an advantageous embodiment, in step a) a silicon substrate is provided, in step b) the forming of the boron-doped region is done by diffusion and the borosilicate glass layer is formed during formation of the boron-doped region in an in- situ step. This removes the need to create the borosilicate glass layer in a separate processing step and leads to an extremely time and cost efficient production press. The method may be applied to both p-type silicon substrates and n-type silicon substrates. Specifically but not exclusively, a borosilicate glass layer may be generated automatically under standard conditions regarding temperature and duration of the boron diffusion step if the boron diffusion is performed in an atmosphere containing oxygen and either BBr3 or BC13 and a carrier gas. There are numerous other possible Boron sources, including B2H6 and BN .
A further improvement of the thus obtained passivation is achieved if after formation of said borosilicate glass layer the process continues by an in-situ thermal oxidation in an atmosphere containing 02 or H20.
It is also possible to make use of the borosilicate glass layer as part of an antireflection coating layer. To do so, prior to forming said at least one dielectric coating film, the thickness of said borosilicate glass layer is reduced in such a way that after thinning the borosilicate glass layer retains a thickness of at least one nm. Such a reduction of the thickness of said borosilicate layer can be performed by etching in a chemical solution comprising at least one of the group consisting of a solution containing hydrofluidic acid, a solution containing sodium hydroxide, a solution containing potassium hydroxide, a solution containing sulphuric acid, a solution containing hydrogen peroxide or a solution containing nitric acid.
The stack of borosilicate glass and dielectric coating film is especially effective if said dielectric coating film comprises silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amorphous silicon including hydrogen or silicon oxide.
If the method comprises annealing after formation of said di- electric coating film, this leads to improved surface passivation which may be related to an improved release of hydrogen from the dielectric layer. Specifically, this can be achieved especially well if the annealing temperature is higher than the deposition temperature of said dielectric coating film and may advantageously be realized during a firing step that is performed to realize the metal contacts of the solar cell.
The solar cell according to this invention comprises a semiconductor substrate, at least one phosphorous doped region in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact, at least one boron doped region in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact and at least one dielectric coating film cover- ing at least part of a surface of said semiconductor substrate, wherein said semiconductor substrate comprises at least one borosilicate glass layer on the surface of said at least one boron doped region and wherein said at least one dielectric film is in direct contact with at least one surface of said borosilicate glass layer that is opposed to the surface of the borosilicate glass layer facing the at least one boron doped region. This arrangement leads to an improved passivation of the boron-doped regions of the solar cell, which increases the effective lifetime of the minority charge carriers created during operation of the cell, leading to an improved yield of the solar cell according to this invention.
It is pointed out that the formation of a borosilicate glass layer on the surface of a boron-doped region can lead to an intermediate surface layer known in the art as "boron-rich layer". For this reason, the term "borosilicate layer on the surface of said at least one boron doped region" in the context of this document includes a two-layer structure of boron- doped region and borosilicate glass layer as well as a three- layer structure of boron-doped region, intermediate boron-rich layer and borosilicate glass layer.
The preferred semiconductor substrate is silicon, specifically p-type silicon or n-type silicon. If the borosilicate glass layer has a thickness of lnm, 'it can act as part of an antire- flection coating.
Advantageous materials for the dielectric coating film comprise silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amor- phous silicon including hydrogen or silicon oxide.
The invention is now explained further using the following figures that show: Figure 1: measured effective recombination lifetime of boron diffused n-type silicon wafers treated with different passivation techniques, Figure 2 a first embodiment of a solar cell according to this invention, representing an n-type solar cell,
Figure 3 a second embodiment of a solar cell according to this invention, representing a p-type solar cell,
Figure 4 a third embodiment of a solar cell according to this invention, representing a back contacted n- type solar cell, and
Figure 5 a fourth embodiment of a solar cell according to this invention, representing a back contacted p- type solar cell.
The relative thickness of layers and/or regions displayed in the Figures is partly represented in an exaggerated way in or- der to illustrate the effect of the application of respective steps of the method more clearly.
The term "silicon substrate" as used in this invention includes a silicon substrate whose properties have been changed. This includes addition surface layers created thereon or added thereto, it does not only indicate changes in the silicon substrate body, as obtained e.g. by doping.
The terms "front" and "back" are used in the context of the solar cells shown in Figures 2-5 in the following way: The term "front" relates to the side of the solar cell that is facing the light during operation of the solar cell, the tern "back" to the side opposite to the front side.
Figure 1 shows measured effective recombination lifetime of boron diffused n-type silicon wafers treated with different passivation techniques for the case of a dielectric coating film of siliconnitride including hydrogen. It is evident from these measurements that the passivation method according to the arrangement according to this invention, i.e. in the presence of a surface passivation with stacked bo- rosilicate glass and SiNx:H, effective lifetimes can be increased notably compared to the ones obtained using previously known surface passivation techniques, stressing the significant advantage achieved by this invention. Figure 2 shows a first embodiment of a solar cell 200 according to this invention, representing an n-type solar cell. What is displayed is a n-type silicon substrate 201. On the back side of said n-type silicon substrate 201, it has a phosphorous-doped region 203 which is contacted by metal contacts 207, made of silver. The back surface of the phosphorous doped region 203 that is not covered by the metal contacts 207 is covered by a dielectric coating film 205, PECVD-deposited silicon nitride including hydrogen. On the front side of said n-type silicon substrate 201, it has a boron-doped region 202, the front side of which is covered by a layer of borosilicate glass 202A. Metal contacts 206 made of a mixture comprising silver and aluminum contact the boron-doped region 202. The front surface of the borosilicate glass layer 202A is covered by a dielectric coating film 205, PECVD-deposited silicon ni- tride including hydrogen.
Figure 3 shows a second embodiment of a solar cell 300 according to this invention, representing a p-type solar cell. What is displayed is a p-type silicon substrate 301. On the front side of said p-type silicon substrate 301, it has a phosphorous-doped region 303 which is contacted by metal contacts 307, made of silver. The front surface of the phosphorous doped region 303 that is not covered by the metal contacts 307 is covered by a dielectric coating film 304, PECVD-deposited silicon nitride including hydrogen. On the back side of said p-type silicon substrate 301, it has a boron-doped region 302, the back side of which is covered by a layer of borosilicate glass 302A. Metal contacts 306 made of a mixture comprising silver and aluminum contact the boron-doped region 302. The back surface of the borosilicate glass layer 302A is covered by a dielectric coating film 305, PECVD-deposited silicon ni- tride including hydrogen.
Figure 4 shows a third embodiment of a solar cell 400 according to this invention, representing a back contacted n-type solar cell. What is displayed is a n-type silicon substrate 401. On the front side and parts of the back side of said n- type silicon substrate 401, it has phosphorous-doped regions 403. The phosphorous-doped regions 403 that are located on the back side of the n-type silicon substrate 401 is contacted by metal contacts 407, made of silver. The surfaces of the phosphorous doped regions 403 that are not covered by the metal contacts 407 are covered by dielectric coating films 404,405, PECVD-deposited silicon nitride including hydrogen. On the remaining parts of the back side of said n-type silicon substrate 401, there are boron-doped regions 402, the back sides of which are covered by layers of borosilicate glass 402A. Metal contacts 406 made of a mixture comprising silver and aluminum contact the boron-doped regions 402. The back surface of the borosilicate glass layer 402A is covered by a dielectric coating film 405, PECVD-deposited silicon nitride including hydrogen.
Figure 5 shows a fourth embodiment of a solar cell 500 according to this invention, representing a back contacted p-type solar cell. What is displayed is a p-type silicon substrate 501. On the front side and parts of the back side of said p- type silicon substrate 501, it has boron-doped regions 502. The boron-doped regions 502 that are located on the back side of the p-type silicon substrate 501 are contacted by metal contacts 506, made of a mixture comprising silver and alumi- num. The surfaces of the boron doped regions 502 that are not covered by the metal contacts 506 are covered by layers of borosilicate glass 502A that, in turn, are covered by dielectric coating films 504,505, PECVD-deposited silicon nitride includ¬ ing hydrogen. On the remaining parts of the back side of said p-type silicon substrate 501, there are phosphorous-doped re¬ gions 503. Metal contacts 506 made of a mixture comprising silver and aluminum contact the phosphorous-doped regions 503. The back surface of phosphorous-doped regions 503 that is not covered by the metal contacts 506 is also covered by the dielectric coating film 505, PECVD-deposited silicon nitride including hydrogen.
In general, all types of solar cells can be fabricated according to the principles of this invention, e.g. conventional solar cells, interdigitated back-contact back-j unction solar cells, a metal wrap through or an emitter wrap through having boron diffusion regions on the light receiving side ore on the opposite side.
List of reference numerals
200, 300, 400, 500 solar cell
201, 301 n-type crystalline silicon substrate 202,302,402,502 boron doped region
202A, 302A, 402A, 502A borosilicate glass layer
203, 303, 403, 503 phosphorous doped region
204, 205, 304, 305,
404, 05, 504, 505 dielectric coating film
206,306,406,506 metal contact
207, 307, 407, 507 metal contact

Claims

Method for fabrication of a passivated boron-doped re¬ gion, especially in a solar cell, comprising the steps of a) providing a semiconductor substrate
b) forming a boron-doped region (202,302,402,502) in said semiconductor substrate (201 , 201 , 301 , 401 ) , wherein at least part of a surface of said semiconductor substrate belongs to said boron-doped region (202,302, 02,502), and
c) forming at least one dielectric coating film (204,205, 304,305,404, 405,504,505) on at least part of the surface of the semiconductor substrate (201,301,401,501) as modified by previous processing steps
c h a r a c t e r i z e d i n t h a t
a borosilicate glass layer ( 202A, 302A, 402A, 502A) is formed on the surface of the boron-doped region (202,302, 402,502) and that at least one dielectric coating film (204,304,404,504) is formed in direct contact with at least part of said borosilicate glass layer (202A, 302A, 402A, 502A) .
Method according to claim 1, wherein in step a) a silicon substrate (201,301,401,501) is provided, in step b) the forming of the boron-doped region (202,302,402,502) is done by diffusion and the borosilicate glass layer (202A, 302A, 402A, 502A) is formed during formation of the boron- doped region (202,302,402,502).
Method according to claim 2, wherein said silicon substrate (201,301,401,501) is of p-type material or n-type material .
Method according to one of claims 2 to 3, wherein the boron diffusion is performed in an atmosphere containing oxygen and either BBr3 or BC13 and a carrier gas. Method according to one of claims 2 to 4, after formation of said borosilicate glass layer the process continues by an in-situ thermal oxidation in an atmosphere containing 02 or H20.
Method according to one of claims 1 to 5, wherein prior to forming said at least one dielectric coating film (204,205,304,305,404,405,504,505) the thickness of said borosilicate glass layer (202A, 302A, 02A, 502A) is reduced, wherein after thinning the borosilicate glass layer (202A, 302A, 402A, 502A) retains a thickness of at least one nm.
Method according to claim 6, wherein said reduction of the thickness of said borosilicate layer (202A, 302A, 02A, 502A) is performed by etching in a chemical solution comprising at least one of the group consisting of a solution containing hydrofluidic acid, a solution containing sodium hydroxide, a solution containing potassium hydroxide, a solution containing sulphuric acid, a solution containing hydrogen peroxide or a solution containing nitric acid.
Method according to one of claims 1 to 7, wherein said dielectric coating film (204,205,304,305,404, 05,504,505) comprises silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amorphous silicon including hydrogen or silicon oxide .
Method according to one of claims 1 to 8, wherein the method comprises annealing after formation of said dielectric coating film (204,205,304,305,404,405,504,505).
Method according to claim 9, wherein the annealing temperature is higher than the deposition temperature of said dielectric coating film (204,205,304,305,404,405, 504, 505) .
Solar cell (200,300,400,500) comprising
a semiconductor substrate,
at least one phosphorous doped region (203,303,403,503) in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact (207, 307, 407, 507) ,
at least one boron doped region (202,302,402,502) in said semiconductor substrate that is connectable to an electric circuit by means of at least one metal contact
(206,306,406,506) and
at least one dielectric coating film (204,205,304,305, 404,405,504,505) covering at least part of a surface of said semiconductor substrate
c h a r a c t e r i z e d i n t h a t
said semiconductor substrate comprises at least one boro- silicate glass layer (202A, 302A, 402A, 502A) on the surface of said at least one boron doped region (202,302,402,502) and that said at least one dielectric film (204,304,404, 504) is in direct contact with at least one surface of said borosilicate glass layer (202A, 302A, 402A, 502A) that is opposed to the surface of the borosilicate glass layer (202, 302, 402, 502) facing the at least one boron doped region .
Solar cell (200,300,400,500) according to claim 11, wherein said semiconductor substrate is silicon, specifically p-type silicon or n-type silicon.
Solar cell (200,300,400,500) according to claim 11 or 12, wherein said borosilicate glass layer (202A, 302A, 402A, 502A) has a thickness of at least lnm. Solar cell (200,300,400,500) according to one of claims 11 to 13, wherein said dielectric coating film (204, 205,304,305,404,405,504,505) comprises silicon nitride including hydrogen, silicon carbide including hydrogen, silicon oxynitride including hydrogen, amorphous silicon including hydrogen or silicon oxide.
PCT/EP2011/003060 2010-06-23 2011-06-21 Method for creating a passivated boron-doped region, especially during production of a solar cell, and solar cell with passivated boron-diffused region WO2011160814A2 (en)

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