EP3398208A1 - Method for producing a bifacial solar cell and bifacial solar cell - Google Patents

Method for producing a bifacial solar cell and bifacial solar cell

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Publication number
EP3398208A1
EP3398208A1 EP15828816.7A EP15828816A EP3398208A1 EP 3398208 A1 EP3398208 A1 EP 3398208A1 EP 15828816 A EP15828816 A EP 15828816A EP 3398208 A1 EP3398208 A1 EP 3398208A1
Authority
EP
European Patent Office
Prior art keywords
type
wafer
solar cell
region
doped region
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15828816.7A
Other languages
German (de)
French (fr)
Inventor
Pirmin PREIS
Florian BUCHHOLZ
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
International Solar Energy Research Center Konstanz EV
Original Assignee
International Solar Energy Research Center Konstanz EV
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Publication of EP3398208A1 publication Critical patent/EP3398208A1/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • H01L31/02—Details
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • 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
    • 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

Definitions

  • Solar cells are well-known devices for conversion of light to electricity which use the inner photo-effect of a semiconduc ⁇ tor and control the transport of thus generated charges by transition regions between differently doped semiconductor re ⁇ gions, e.g. p-n transitions.
  • a number of terms that are used in this description have a well-defined meaning.
  • the side of the solar cell that faces the direct light source dur ⁇ ing operation of the solar cell is usually defined inde ⁇ pendently of the electrical current orientation of the solar cell when it is not installed for operation and called “front side” or "upper side".
  • the opposite side is the "back side” or “lower side” of the solar cell.
  • the per ⁇ son skilled in the art of solar cells typically uses the word “strong” in the context of doping, e.g. in terms like “strong doping” or “strongly doped region” for a doping of about 10 20 to 10 21 dopant atoms per cm 3
  • the word “weak” in this context means typically a doping of about 10 15 to 10 16 dopant atoms per cm 3
  • a strongly doped region of opposite doping type than the silicon substrate, on which the solar cell is fabri- cated, is typically named “emitter”, whereas the weakly doped said substrate is known as "base”.
  • a strongly doped region on the opposite side than the first strongly doped region, of the same doping as the base of the solar cell, represents the "front surface field", when present on the front side or “back surface field”, when present on the back side of the solar cell .
  • researchers and manufacturers have been working hard in order to increase efficiency and decrease pro ⁇ duction cost of solar cells.
  • one of the more promising roads towards these aims that has been followed so far mostly by researchers are bifacial solar cells, because they enable a significant relative increase in yearly energy yield that is much higher than the relative increase in pro ⁇ duction cost induced by the modifications of the production process required in order to create bifacial solar cells.
  • the main reason for the reduction in costs per produced kWh is the possibility to harvest solar radiation not only with the front side, but also with the back side. This is performed either by collecting diffuse radiation back-scattered from the sky and ground, when the module is mounted in the classical manner or by mounting the module vertically and collecting radiation via the front and the back side to a similar extend. As a conse ⁇ quence the back side of the solar cell gains in importance. In contrast to other high efficiency solar cells not only good passivation and internal reflection but also the optical prop- erties become predominant.
  • Bifacial PERT solar cells passivated emitter and rear, total ⁇ ly diffused solar cells
  • methods for their fabrication are known for example from H.M. Ohtsuka et al . , "Bifacial Silicon Solar Cells with 21.3% Front Efficiency and 19.8% Rear Effi ⁇ ciency", Progress in Photovoltaics : Research and Applications 8, no. 4 (2000), 385-390 or L. Yang et al . , "High Efficiency Screen Printed Bifacial Solar Cells on Monocrystalline Cz Sil ⁇ icon", Progress in Photovoltaics: Research and Applications 19, no. 3 (2011), 275-279.
  • CH3COOH CH3COOH
  • etching methods containing a sol ⁇ vent, HF and an oxidizing substance may be used in a polishing or texturing manner (e.g. see I. Rover, G. Roewer, K. Bohmham- mel and K. Wambach, "Reactivity of crystalline silicon in the system HF-HNO3-H2O (a novel study)", Proc. of the 19th European Photovoltaic Solar Energy Conference, Paris, France, 2004). Accordingly, it is possible to produce roughnesses (sdr, en ⁇ largement of the surface compared to the projected surface) in the range of 0,1 to 120%.
  • the etching is achieved by the acidic treatment with HF and HNO3.
  • the problem to improve the conversion efficiency of the solar cell and/or to reduce the production cost is an ongoing task, so that the problem remains to develop bifacial solar cells and methods for production of bifacial solar cells that are more cost-efficient and lead to more effective solar cells than known so far.
  • This problem is solved by the method for production of a bifacial solar cell according to claim 1 and the bifacial solar cell according to claim 7. Further advanta ⁇ geous features are claimed in the claims that are dependent thereon .
  • a high efficiency PERT cell can be obtained.
  • the solar cell can comprise an alkaline textured front side and an acidic textured back side.
  • the main ad- vantage of such a cell is that the properties of the back side can be chosen deliberately according to the makeup of the acidic etching solution that is used for preparing the back side .
  • the method starts with the step of providing a wafer. More specifically, as-cut p- or n-doped wafers can be used, prefer ⁇ ably Si wafers of Czochralski type, most preferably n-doped Si wafers of Czochralski type. Then, an acidic treatment is applied to the wafer, advanta ⁇ geously to both front side and back side of the wafer simulta ⁇ neously.
  • the etch depth achieved by the acidic treatment is within the range of 3, 5pm and 15pm. Good results have been achieved by use of mix ⁇ tures of HF in concentration range between 50g/l and 150g/l and HNO3 in the concentration range between 300g/l and 700g/l.
  • acidic treatment of the back side of the wafer may not appear to make sense, because it leads to increased surface roughness of the back side, which increases the sur ⁇ face recombination velocity and thus has an adverse effect.
  • the inventors have found that it still leads to an overall gain performance, which can tentatively be explained by an overcompensation of this adverse effect by lower contact resistance and higher fill factors that are achieved .
  • the prop ⁇ erties of the side with a thus obtained surface structure can be manipulated to obtain an optimum of surface roughness to yield good light coupling from the back side and to ensure good contacting, and a minimum in surface recombination veloc ⁇ ity .
  • a first doped re- gion or a dopant atoms containing layer on top of the wafer, serving as source in later high temperature step is formed on a first side of the wafer, preferably on the side that will later be the back side of the solar cell.
  • this region or layer will be used to form the highly doped region on the back side of the final product. Specifically, it can optionally be used to create a back sur ⁇ face field.
  • this first doped region is typically (but not necessarily) a strongly doped region, wherein the do- pant type of this region is identical to the dopant type of the -usually weakly doped- wafer.
  • the first side of the wa ⁇ fer, on which the first doped region or layer has been creat- ed, is coated using any of the known techniques, e.g. by for ⁇ mation of a silicon nitride, a silicon oxide layer, an alumin ⁇ ium oxide layer or any combination of such layers.
  • the second side of the wafer is textured by alkaline texturing, wherein any of the alkaline texturing processes known in the art can be used.
  • any of the alkaline texturing processes known in the art can be used.
  • the creation of the first doped region at the first side is performed before the alkaline texturing of the second side occurs.
  • se ⁇ quence surface contamination which has adverse effects on the total efficiency of the can be reduced. This effect is strongest when diffusion techniques are used for creation of the first doped region at the back side.
  • alka ⁇ line texturing (which can also be named alkaline etching)
  • a random pyramid structure is created on the second side of the solar cell.
  • Good alkaline texturing quality is obtained due to the acidic pre-treatment .
  • no additional etching step is re ⁇ quired that may degrade the surface properties of the front (or the back) side to remove the highly doped region on the other side, which is the inevitable consequence of many of the known techniques for the formation of highly doped regions.
  • the second doped region is created at the front side using any of the techniques known in the art . If tube furnace diffusion is used the resulting glassy layer may optionally be removed or used as part of a dielectric stack.
  • the dielectric stack may be fabricated according to methods known from the art, e.g. by formation of a silicon ni ⁇ tride, silicon oxide or aluminium oxide layer or a by applying stacks of such layers.
  • contacts are created on front- and back side by met ⁇ allization. Again, any known technique for execution of this processing step can be used.
  • the method described is performed preferably using inline pro ⁇ cessing equipment for the etching of the silicon. This leads to a significant reduction of the production cost and is made possible by the choice and sequence of the processing steps that are used.
  • the method moreover allows for using existing inline pro ⁇ cessing equipment for solar cell production, e.g. for the fab- rication of multicrystalline solar cells. This may allow for upgrading of existing solar cell process lines with low costs for additional equipment. This is an important advantage of the method, which further helps with reducing the cost of thus obtained solar cells.
  • waste water resulting from ex- ecution of step b) and waste water resulting from execution of step e) are at least partly mixed in order to achieve partial pH balancing of the waste water.
  • the respec ⁇ tive waste waters have each negative properties for the envi ⁇ ronment that at least partly cancel each other if these waste waters are combined or mixed.
  • the production method for a bifacial solar cell in accordance with the invention offers a number of important ad ⁇ vantages in combination, especially:
  • the cell according to the invention comprises a p-type or re ⁇ type base, a first strongly doped region of p-type or n-type located on a side of the p- or n-type base, a second strongly doped n- or p-type region located on the side of the p- or re ⁇ type base that is opposite to the side on which the first strongly doped region is located, wherein said second strongly doped region is a p-type region if the first strongly doped region is an n-type region and said second strongly doped re ⁇ gion is an n-type region if said first strongly doped region is a p-type region, said second strongly doped n- or p-type region being arranged on a random pyramid textured surface and having a random pyramid structured surface; at least one me- tallic contact located on the first strongly doped p-type or n-type region; and at least one metallic contact located on the second strongly doped n-type or p-type region
  • said first strongly doped region is arranged on and/or has an acidic etched sur ⁇ face. It should be noted that the person skilled in the art can distinguish between wafer surfaces obtained by acidic treatment and alkaline treatment, respectively, because these procedures lead to different types of surfaces structures which can be easily recognized e.g. by application of a micro ⁇ scope .
  • said first strongly doped region is a p-type region if said base is a p-type base and said first strongly doped region is an n-type region if said base is an n-type base.
  • the first strongly doped surface which is arranged on the side of the wafer that remains acidic etched, can preferably serve as a back surface field and the strongly doped region on the opposite side of the cell can then act as a front side emitter.
  • the first side corresponds to the back side of the finished cell and the second side corresponds to its front side.
  • Fig. 1 the process flow of an embodiment of the fabrication method
  • Fig. 2 a schematic representation of a bifacial solar cell in accordance with the invention that can for example be cre ⁇ ated in accordance with the process flow shown in Fig. 1.
  • Fig. 1 shows the process flow of an embodiment of the fabrica ⁇ tion method.
  • Step 100 comprises providing a p- or n- doped wa ⁇ fer.
  • Step 200 comprises applying an acidic treatment to the wafer to remove sawing damage and contaminations.
  • Step 300 comprises creating a first doped region on a first side of the wafer, or creating a dopant containing layer, serving as source in later high temperature processing steps not explic ⁇ itly shown in Figure 1.
  • Step 400 comprises coating the first side of the wafer with a dielectric layer or a stack of die- lectric layers.
  • Step 500 comprises preparing a second side of the wafer by alkaline texturing.
  • Step 600 comprises creating a second doped region at the second side of the wafer.
  • Step 700 comprises coating the second side with a dielectric layer or a stack of dielectric layers of the wafer; and step 800 compris- es providing the bifacial solar cell (10) with contacts by metallization .
  • step 800 comprising- es providing the bifacial solar cell (10) with contacts by metallization .
  • Figure 1 in this process sequence bleeds from process steps 200 and 500, respectively, will neutralize each other, so that waste water management is much easier.
  • Figure 2 shows a bifacial solar cell 10 with a p- or n- doped base 1 that can be created in accordance with the process flow shown in Fig 1.
  • the surface 6 at the front side of the bifa ⁇ cial solar cell 10 is structured with a random pyramid tex ⁇ ture, whereas the surface 7 at the back side 10b of the bifa- cial solar cell 10 is an acidic surface, in this example an isotextured surface.
  • the first strongly doped region 3 comprises a single dielectric layer or a stack of dielectric layers, which is not shown in figure 1 because of its signifi ⁇ cantly smaller thickness.
  • the second strongly doped region 2 of n-type if the base 1 is a p-type base and of p-type if the base 1 is an n-type base (thus of opposite type than the first strongly doped region 3), which forms the emitter.
  • the second strongly doped region 2 also comprises a single dielectric layer or a stack of dielectric layers, which is not shown in figure 1 because of its signifi ⁇ cantly smaller thickness.
  • Reference numerals are :

Abstract

A method for producing a bifacial solar cell (10) comprising the steps of applying an acidic treatment to both sides of a wafer to remove sawing damage; creating a first doped region on a first side (10b) of the wafer; coating the first side with a dielectric layer; preparing a second side (10a) of the wafer by alkaline texturing; creating a second doped region at the second side; coating the second side with a dielectric layer; and providing the bifacial solar cell with contacts, wherein these steps are executed in the described order. A bifacial solar cell (10), comprising a base (1); a first strongly doped region (3) of p-type or n-type located on a side of the base (1) and arranged on an acidic etched surface (7); a second strongly doped n- or p-type region (2) located on the opposite side of the base, arranged on a random pyramid textured surface and having a random pyramid structured surface (6); metallic contacts (5, 4) located on the first and second strongly doped regions.

Description

Method for producing a bifacial solar cell and bifacial solar cell
Solar cells are well-known devices for conversion of light to electricity which use the inner photo-effect of a semiconduc¬ tor and control the transport of thus generated charges by transition regions between differently doped semiconductor re¬ gions, e.g. p-n transitions. In the art of solar cells, a number of terms that are used in this description have a well-defined meaning. For example, the side of the solar cell that faces the direct light source dur¬ ing operation of the solar cell is usually defined inde¬ pendently of the electrical current orientation of the solar cell when it is not installed for operation and called "front side" or "upper side". Conversely, the opposite side is the "back side" or "lower side" of the solar cell. Also, the per¬ son skilled in the art of solar cells typically uses the word "strong" in the context of doping, e.g. in terms like "strong doping" or "strongly doped region" for a doping of about 1020 to 1021 dopant atoms per cm3, whereas the word "weak" in this context means typically a doping of about 1015 to 1016 dopant atoms per cm3. A strongly doped region of opposite doping type than the silicon substrate, on which the solar cell is fabri- cated, is typically named "emitter", whereas the weakly doped said substrate is known as "base". A strongly doped region on the opposite side than the first strongly doped region, of the same doping as the base of the solar cell, represents the "front surface field", when present on the front side or "back surface field", when present on the back side of the solar cell . Since many years, researchers and manufacturers have been working hard in order to increase efficiency and decrease pro¬ duction cost of solar cells. In the last years, one of the more promising roads towards these aims that has been followed so far mostly by researchers are bifacial solar cells, because they enable a significant relative increase in yearly energy yield that is much higher than the relative increase in pro¬ duction cost induced by the modifications of the production process required in order to create bifacial solar cells. The main reason for the reduction in costs per produced kWh is the possibility to harvest solar radiation not only with the front side, but also with the back side. This is performed either by collecting diffuse radiation back-scattered from the sky and ground, when the module is mounted in the classical manner or by mounting the module vertically and collecting radiation via the front and the back side to a similar extend. As a conse¬ quence the back side of the solar cell gains in importance. In contrast to other high efficiency solar cells not only good passivation and internal reflection but also the optical prop- erties become predominant.
Bifacial PERT solar cells (passivated emitter and rear, total¬ ly diffused solar cells) and methods for their fabrication are known for example from H.M. Ohtsuka et al . , "Bifacial Silicon Solar Cells with 21.3% Front Efficiency and 19.8% Rear Effi¬ ciency", Progress in Photovoltaics : Research and Applications 8, no. 4 (2000), 385-390 or L. Yang et al . , "High Efficiency Screen Printed Bifacial Solar Cells on Monocrystalline Cz Sil¬ icon", Progress in Photovoltaics: Research and Applications 19, no. 3 (2011), 275-279.
Comparing the most common bifacial devices, it is found that two common methods for the preparation of the back sides are employed: alkaline polishing (Yang et al . ) and alkaline tex¬ turing (Song et al . ) resulting in shiny flat and random pyra¬ mid structures, respectively. While the advantage of the first is good passivation quality, the disadvantages are poor light trapping and challenging contact formation, when contacts are formed by common screen printing. The advantages of the latter are very good light trapping features of the back side and easy contact formation, on the cost of increased recombination of charge carriers due to the strong increase in surface area.
It is a well-known fact that HF / HN03 / (H20 or H2S04 or
CH3COOH) mixtures or other etching methods containing a sol¬ vent, HF and an oxidizing substance may be used in a polishing or texturing manner (e.g. see I. Rover, G. Roewer, K. Bohmham- mel and K. Wambach, "Reactivity of crystalline silicon in the system HF-HNO3-H2O (a novel study)", Proc. of the 19th European Photovoltaic Solar Energy Conference, Paris, France, 2004). Accordingly, it is possible to produce roughnesses (sdr, en¬ largement of the surface compared to the projected surface) in the range of 0,1 to 120%. Preferably, the etching is achieved by the acidic treatment with HF and HNO3.
The problem to improve the conversion efficiency of the solar cell and/or to reduce the production cost is an ongoing task, so that the problem remains to develop bifacial solar cells and methods for production of bifacial solar cells that are more cost-efficient and lead to more effective solar cells than known so far. This problem is solved by the method for production of a bifacial solar cell according to claim 1 and the bifacial solar cell according to claim 7. Further advanta¬ geous features are claimed in the claims that are dependent thereon . By the method for production of a bifacial solar cell accord¬ ing to the invention, a high efficiency PERT cell can be obtained. The solar cell can comprise an alkaline textured front side and an acidic textured back side. The main ad- vantage of such a cell is that the properties of the back side can be chosen deliberately according to the makeup of the acidic etching solution that is used for preparing the back side . The method starts with the step of providing a wafer. More specifically, as-cut p- or n-doped wafers can be used, prefer¬ ably Si wafers of Czochralski type, most preferably n-doped Si wafers of Czochralski type. Then, an acidic treatment is applied to the wafer, advanta¬ geously to both front side and back side of the wafer simulta¬ neously. By this processing step sawing damage and contamina¬ tions, more specifically contaminations residing from the wa¬ tering process, can be reliably removed. Preferably, the etch depth achieved by the acidic treatment is within the range of 3, 5pm and 15pm. Good results have been achieved by use of mix¬ tures of HF in concentration range between 50g/l and 150g/l and HNO3 in the concentration range between 300g/l and 700g/l. At first sight, acidic treatment of the back side of the wafer may not appear to make sense, because it leads to increased surface roughness of the back side, which increases the sur¬ face recombination velocity and thus has an adverse effect. Surprisingly, however, the inventors have found that it still leads to an overall gain performance, which can tentatively be explained by an overcompensation of this adverse effect by lower contact resistance and higher fill factors that are achieved . By varying the composition of the acidic etch bath, the prop¬ erties of the side with a thus obtained surface structure can be manipulated to obtain an optimum of surface roughness to yield good light coupling from the back side and to ensure good contacting, and a minimum in surface recombination veloc¬ ity .
Another benefit from the acidic treatment is the low metal surface contamination after the etching compared to alkaline etched wafers so that need for post etching cleaning is re¬ duced or may fall away completely when typical inline etching tools are used.
After application of the acidic treatment, a first doped re- gion or a dopant atoms containing layer on top of the wafer, serving as source in later high temperature step is formed on a first side of the wafer, preferably on the side that will later be the back side of the solar cell. Preferably, this region or layer will be used to form the highly doped region on the back side of the final product. Specifically, it can optionally be used to create a back sur¬ face field. In this case, this first doped region is typically (but not necessarily) a strongly doped region, wherein the do- pant type of this region is identical to the dopant type of the -usually weakly doped- wafer. In other words, in this op¬ tional case if an n-type wafer is used, in this processing step a strongly n-doped layer is formed, whereas if a p-type wafer is used a strongly p-doped layer is formed. For for- mation of this first doped region any of the known techniques, especially diffusion, implantation and sputtering and any do¬ pant atoms of the required type (i.e. p or n) may be used. It should be noted that according to the claimed method it is also possible to use a double-sided diffusion method at this point of the process, as later in the processing sequence a resulting doping layer at the second side of the wafer is re- moved by alkaline etching/texturing anyway, which contributes to low production cost.
After this step has been performed, the first side of the wa¬ fer, on which the first doped region or layer has been creat- ed, is coated using any of the known techniques, e.g. by for¬ mation of a silicon nitride, a silicon oxide layer, an alumin¬ ium oxide layer or any combination of such layers.
Only after the first doped region at the first side has been formed, the second side of the wafer is textured by alkaline texturing, wherein any of the alkaline texturing processes known in the art can be used. In other words, it is an im¬ portant aspect of this invention that the creation of the first doped region at the first side is performed before the alkaline texturing of the second side occurs. Surprisingly, it has been found that as a consequence of this processing se¬ quence surface contamination, which has adverse effects on the total efficiency of the can be reduced. This effect is strongest when diffusion techniques are used for creation of the first doped region at the back side. By means of the alka¬ line texturing (which can also be named alkaline etching) , a random pyramid structure is created on the second side of the solar cell. Good alkaline texturing quality is obtained due to the acidic pre-treatment . Furthermore, in contrast to other PERT cell process methods, no additional etching step is re¬ quired that may degrade the surface properties of the front (or the back) side to remove the highly doped region on the other side, which is the inevitable consequence of many of the known techniques for the formation of highly doped regions.
Another benefit from applying alkaline texturing after pre- processing, is the fact that the metal surface contamination of the surface after the alkaline texturing step is smaller than when accordingly processing as-cut or saw-damage etched wafers. The reason for this is the very low initial surface contamination before the alkaline texturing step.
After the alkaline texturing step, the second doped region is created at the front side using any of the techniques known in the art . If tube furnace diffusion is used the resulting glassy layer may optionally be removed or used as part of a dielectric stack. The dielectric stack may be fabricated according to methods known from the art, e.g. by formation of a silicon ni¬ tride, silicon oxide or aluminium oxide layer or a by applying stacks of such layers.
Finally, contacts are created on front- and back side by met¬ allization. Again, any known technique for execution of this processing step can be used.
The method described is performed preferably using inline pro¬ cessing equipment for the etching of the silicon. This leads to a significant reduction of the production cost and is made possible by the choice and sequence of the processing steps that are used.
The method moreover allows for using existing inline pro¬ cessing equipment for solar cell production, e.g. for the fab- rication of multicrystalline solar cells. This may allow for upgrading of existing solar cell process lines with low costs for additional equipment. This is an important advantage of the method, which further helps with reducing the cost of thus obtained solar cells.
Another advantage of the method resides in the fact that sig¬ nificant reduction of the cost for waste water processing can be obtained. In order to do so, waste water resulting from ex- ecution of step b) and waste water resulting from execution of step e) are at least partly mixed in order to achieve partial pH balancing of the waste water. In other words, the respec¬ tive waste waters have each negative properties for the envi¬ ronment that at least partly cancel each other if these waste waters are combined or mixed.
Therefore, the production method for a bifacial solar cell in accordance with the invention offers a number of important ad¬ vantages in combination, especially:
- adjustability of back side roughness by varying the etch¬ ing bath composition used for the removal of the saw dam¬ age ;
- reduced contact resistance and higher fill factors of the metallic contacts at the back side of the solar cell com¬ pared to PERT cells with alkaline polished rear sides, leading to increased total efficiency of the cell; veloc¬ ity compared to fully alkaline textured PERT cells;
- reduction of surface contamination by avoiding alkaline processing steps before creation of the first strongly doped layers including a significant reduction of metal contamination compared to processing sequences in which high temperature diffusion is applied after alkaline etching of as-cut wafers;
- a processing sequence which is adapted to be performed in inline processing facilities originally designed to run known production processes such as fabrication of mul- ticrystalline solar cells rather than in batch mode; and
- improved conditions for waste water reusal.
As mentioned above, in order to realize the method of the in- vention and make use of the full advantages offered by it, the above-mentioned steps have to be executed in the above- described order. For the sake of completeness, it is stated explicitly that in general this does not rule out insertion of additional processing steps that do not correspond to men- tioned processing steps between processing steps mentioned above .
The cell according to the invention comprises a p-type or re¬ type base, a first strongly doped region of p-type or n-type located on a side of the p- or n-type base, a second strongly doped n- or p-type region located on the side of the p- or re¬ type base that is opposite to the side on which the first strongly doped region is located, wherein said second strongly doped region is a p-type region if the first strongly doped region is an n-type region and said second strongly doped re¬ gion is an n-type region if said first strongly doped region is a p-type region, said second strongly doped n- or p-type region being arranged on a random pyramid textured surface and having a random pyramid structured surface; at least one me- tallic contact located on the first strongly doped p-type or n-type region; and at least one metallic contact located on the second strongly doped n-type or p-type region. Furthermore, according to the invention said first strongly doped region is arranged on and/or has an acidic etched sur¬ face. It should be noted that the person skilled in the art can distinguish between wafer surfaces obtained by acidic treatment and alkaline treatment, respectively, because these procedures lead to different types of surfaces structures which can be easily recognized e.g. by application of a micro¬ scope .
More specifically, it has an isotextured or acidic polished surface, or an intermediate structure in between these ex¬ tremes, depending on the choice of etching mixtures. At first sight, using structures obtained by acidic treatment on the back side of the wafer and the layers arranged thereon may not appear to make sense, because it leads to increased surface roughness of the back side, which increases the surface recom¬ bination velocity and thus has an adverse effect. Surprising¬ ly, however, the inventors have found that it still leads to an overall gain in performance, which can tentatively be ex¬ plained by an overcompensation of the adverse roughness effect by lower contact resistance and higher fill factors that are achieved when the contacts are formed on the thus treated sur¬ face . In a preferred embodiment of the cell, said first strongly doped region is a p-type region if said base is a p-type base and said first strongly doped region is an n-type region if said base is an n-type base. Accordingly, the first strongly doped surface, which is arranged on the side of the wafer that remains acidic etched, can preferably serve as a back surface field and the strongly doped region on the opposite side of the cell can then act as a front side emitter. In a preferred embodiment of both method and cell, the first side corresponds to the back side of the finished cell and the second side corresponds to its front side. The invention is next explained in more detail using figures that show specific embodiments of the invention as examples. The figures show:
Fig. 1: the process flow of an embodiment of the fabrication method, and
Fig. 2: a schematic representation of a bifacial solar cell in accordance with the invention that can for example be cre¬ ated in accordance with the process flow shown in Fig. 1.
Fig. 1 shows the process flow of an embodiment of the fabrica¬ tion method. Step 100 comprises providing a p- or n- doped wa¬ fer. Step 200 comprises applying an acidic treatment to the wafer to remove sawing damage and contaminations. Step 300 comprises creating a first doped region on a first side of the wafer, or creating a dopant containing layer, serving as source in later high temperature processing steps not explic¬ itly shown in Figure 1. Step 400 comprises coating the first side of the wafer with a dielectric layer or a stack of die- lectric layers. Step 500 comprises preparing a second side of the wafer by alkaline texturing. Step 600 comprises creating a second doped region at the second side of the wafer. Step 700 comprises coating the second side with a dielectric layer or a stack of dielectric layers of the wafer; and step 800 compris- es providing the bifacial solar cell (10) with contacts by metallization . As illustrated in Figure 1, in this process sequence bleeds from process steps 200 and 500, respectively, will neutralize each other, so that waste water management is much easier. Figure 2 shows a bifacial solar cell 10 with a p- or n- doped base 1 that can be created in accordance with the process flow shown in Fig 1. The surface 6 at the front side of the bifa¬ cial solar cell 10 is structured with a random pyramid tex¬ ture, whereas the surface 7 at the back side 10b of the bifa- cial solar cell 10 is an acidic surface, in this example an isotextured surface.
At the rear side 10b of the bifacial solar cell 10, there is a first strongly doped region 3 of p-type if the base 1 is a p- type base and of n-type if the base 1 is an n-type base, which forms the back surface field. The first strongly doped region 3 comprises a single dielectric layer or a stack of dielectric layers, which is not shown in figure 1 because of its signifi¬ cantly smaller thickness.
At the front side 10a, there is a second strongly doped region 2 of n-type if the base 1 is a p-type base and of p-type if the base 1 is an n-type base (thus of opposite type than the first strongly doped region 3), which forms the emitter. It should be noted that the second strongly doped region 2 also comprises a single dielectric layer or a stack of dielectric layers, which is not shown in figure 1 because of its signifi¬ cantly smaller thickness. Metallization fingers 4, 5, provided on the first strongly doped region 3 and the second strongly doped region 2, respec¬ tively, form the electrical contacts for the bifacial solar cell 10. Reference numerals :
1 base
2 second strongly doped region
3 first strongly doped region
4 metallization finger
5 metallization finger
6 surface with random pyramid texture
7 acidic etched surface
10 bifacial solar cell
10a front side
10b back side

Claims

Claims
Method for producing a bifacial solar cell (10) comprising the steps :
a) providing a p- or n- doped wafer (100);
b) applying an acidic treatment to the wafer to remove sawing damage and contaminations (200);
c) creating a first doped region on a first side (10b) of the wafer, or creating a dopant containing layer, serving as source in later high temperature step (300) on a first side (10b) of the wafer;
d) coating the first side (10b) of the wafer with a dielec¬ tric layer or a stack of dielectric layers (400);
e) preparing a second side (10a) of the wafer by alkaline texturing (500) ;
f) creating a second doped region at the second side (10a) of the wafer (600) ;
g) coating the second side (10a) with a dielectric layer or a stack of dielectric layers of the wafer (700); and h) providing the bifacial solar cell (10) with contacts by metallization (800);
wherein the steps a to h are executed in the above- described order.
Method according to claim 1, wherein a glass layer is re¬ moved from the second side (10a) as a part of step e) .
Method according to any of claims 1 or 2, wherein a glass layer resulting from step f) is either removed or used as part of a dielectric stack in step g) .
4. Method according to one of claims 1 to 3, wherein steps b) and e) of the method are performed using inline pro- cessing equipment.
Method according to claim 4, wherein the acidic treatment of step b) is performed in a part of the inline pro¬ cessing facilities that is designed for an acidic pro¬ cessing step.
Method according to any of claims 1 to 5, wherein waste water resulting from execution of step b) and waste water resulting from execution of step e) are at least partly mixed in order to achieve partial pH balancing of the waste water.
Bifacial solar cell (10), comprising
- a p-type or n-type base (1);
- a first strongly doped region (3) of p-type or n-type located on a side of the p- or n-type base (1);
- a second strongly doped n- or p-type region (2) lo¬ cated on the side of the p- or n- type base (1) that is opposite to the side on which the first strongly doped region (3) is located, wherein said second strongly doped region (2) is a p-type region if the first strongly doped region (3) is an n-type region and said second strongly doped region (2) is an re¬ type region if said first strongly doped region (3) is a p-type region, said second strongly doped n- or p-type region (2) being arranged on a random pyramid textured surface and having a random pyramid struc¬ tured surface (6);
- at least one metallic contact (5) located on the
first strongly doped p-type or n-type region (3) ; and
- at least one metallic contact (4) located on the sec¬ ond strongly doped n-type or p-type region (2); and c h a r a c t e r i z e d i n that
said first strongly doped region (3) is arranged on and/ or has an acidic etched surface (7) . 8. Bifacial solar cell (10),
c h a r a c t e r i z e d i n that the acidic etched surface (7) is an isotextured surface (7) or acidic pol¬ ished surface (7) or a surface (7) with a structure in between theses extremes.
EP15828816.7A 2015-12-28 2015-12-28 Method for producing a bifacial solar cell and bifacial solar cell Withdrawn EP3398208A1 (en)

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