WO2014171686A1 - Solar cell and manufacturing method thereof - Google Patents

Solar cell and manufacturing method thereof Download PDF

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Publication number
WO2014171686A1
WO2014171686A1 PCT/KR2014/003203 KR2014003203W WO2014171686A1 WO 2014171686 A1 WO2014171686 A1 WO 2014171686A1 KR 2014003203 W KR2014003203 W KR 2014003203W WO 2014171686 A1 WO2014171686 A1 WO 2014171686A1
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WIPO (PCT)
Prior art keywords
emitter layer
solar cell
impurity
substrate
present
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PCT/KR2014/003203
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French (fr)
Inventor
Sung Ho Hwang
Gang Il Kim
Young Kuk Kim
Jeong Eui Hong
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Hanwha Chemical Corporation
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Publication of WO2014171686A1 publication Critical patent/WO2014171686A1/en

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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
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar 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
    • H01L31/04—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
    • 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 present invention relates to a solar cell and a manufacturing method thereof.
  • the present invention relates to a solar cell showing low contact resistance and high efficiency, and a manufacturing method thereof.
  • FIG. 1 is a cross-sectional view illustrating a basic structure of a solar cell.
  • the solar cell includes a substrate 100; an emitter layer 200 that is positioned on the substrate 100 and has a conductivity type opposite to the substrate 100; an anti-reflection film 300 that is positioned on the emitter layer 200; a front electrode 400 that is contacted with the emitter layer; and a back electrode 500 that is positioned on the back side of the substrate 100.
  • the shallow emitter refers to an emitter layer having high sheet resistance of 60 to 120 ⁇ /sq. Such a shallow emitter is advantageous in that its recombination rate is low and short-wavelength sun light can be used.
  • Korean Patent Application No. 2010-0068987 discloses a method for increasing a potential difference at the p-n junction region by fomiing the selective emitter via the selective formation of a more heavily impurity-doped region on the top using a dopant paste of silicon substrate and for improving the short wavelength response to increase the efficiency of photovoltaic power.
  • No. 2010-0068987 discloses a method for increasing a potential difference at the p-n junction region by fomiing the selective emitter via the selective formation of a more heavily impurity-doped region on the top using a dopant paste of silicon substrate and for improving the short wavelength response to increase the efficiency of photovoltaic power.
  • the method includes the steps of incorporating and diffusing a second conductive type impurity into the silicon substrate to form a second conductive type semiconductor layer on the top of the silicon substrate; printing the silicon substrate surface with the dopant paste and heating it to form a more heavily doped region on the second conductive type semiconductor layer; etching the silicon substrate surface using the dopant paste as a barrier; removing the dopant paste which is printed on the silicon substrate surface and patterning a metal material to contact with the more heavily doped region, thereby forming an electrode; and progressing an additional diffusion process for extending the more heavily doped region.
  • an object of the present invention is to provide a solar cell having improved electrical characteristics.
  • Another object of the present invention is to provide a method for manufacturing the solar cell having improved electrical characteristics by precisely controlling the depth and doping concentration of an emitter layer formed inside a substrate in a simple manner.
  • the present invention provides a solar cell, including
  • a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity
  • a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
  • the present invention provides a method for manufacturing a solar cell, including the steps of:
  • the present invention provides a solar cell that is provided with low contact resistance to have improved efficiency of photovoltaic power.
  • the present invention provides a method for manufacturing a solar cell having improved electrical characteristics in a simple manner.
  • FIG. 1 is a cross-sectional view illustrating a basic structure of a solar cell
  • FIG. 2 is a graph showing a doping concentration according to a depth from the surface in the solar cell of the present invention or the conventional solar cell;
  • FIG. 3 is a graph showing an energy diagram of an emitter layer in the solar cell of the present invention or the conventional solar cell.
  • a solar cell includes a first conductive type substrate that is doped with a first impurity; a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity; a front electrode that is positioned on the emitter layer; and a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
  • a method for manufacturing the solar cell according to one embodiment of the present invention includes the steps of injecting a second impurity into a first conductive type substrate containing a first impurity to form a second conductive type emitter layer, and injecting the second impurity at a higher concentration inside the emitter layer than at the surface of the emitter layer; forming a front electrode on the emitter layer; and forming a back electrode on the back side of the substrate.
  • a solar cell of the present invention includes a first conductive type substrate that is doped with a first impurity
  • a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity
  • a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
  • the substrate is a silicon semiconductor substrate of a first conductive type.
  • the silicon may be a crystalline silicon such as a mono crystalline silicon or a polycrystalline silicon, or a non-crystalline silicon.
  • the first conductivity may be p-type, and in this regard, it may be doped with a trivalent element as the first impurity, such as boron (B), gallium (Ga), or indium (In).
  • a trivalent element such as boron (B), gallium (Ga), or indium (In).
  • the first conductivity may be N-type, and in this regard, it may be doped with a group V element as the first impurity, such as phosphorus (P), arsenic (As), and antimony (Sb).
  • a group V element as the first impurity, such as phosphorus (P), arsenic (As), and antimony (Sb).
  • the emitter layer is a second conductive type that is opposite to that of the substrate and contains the second impurity.
  • the emitter layer may be an N-type conductive type and doped with a group V element as the second impurity, such as phosphorus (P), arsenic (As), and antimony (Sb).
  • a group V element such as phosphorus (P), arsenic (As), and antimony (Sb).
  • the thickness of the emitter layer is determined by a depth of the injected second impurity from the surface of the substrate. According to one embodiment of the present invention, the thickness of the emitter layer may be about 100 to about 500 nm. Further, according to one embodiment of the present invention, the emitter layer may have a sheet resistance of about 60 to about 120 ⁇ /sq.
  • a P-N junction is formed at an interface between the substrate and the emitter layer.
  • P-N junction of the solar cell When light is incident on P-N junction of the solar cell, electron-hole pairs are generated. The electric field accelerates the electrons toward the N-layer and the holes toward the P-layer. Thus, a photoelectromotive force is generated at the P-N junction.
  • a load or a system is connected to both ends of the solar cell, current flows and thus power may be generated.
  • the solar cell of the present invention is characterized in that the second impurity contained in the emitter layer shows a concentration peak, that is, the highest concentration inside the emitter layer rather than at the surface of the emitter layer, unlike the conventional solar cell having a concentration peak at the surface of the emitter layer.
  • the concentration peak of the second impurity may be formed at a depth of about 30 to about 70% from the surface of the emitter layer, with respect to the thickness of the emitter layer, or may be formed at a depth of about 50 to about 250 nm from the surface of the emitter layer.
  • FIG. 2 is a graph showing a doping concentration according to a depth from the surface in the solar cell of the present invention or the conventional solar cell.
  • x-axis represents the depth from the surface and y-axis represents the doping density.
  • the second impurity in the solar cell according to Example of the present invention shows a peak concentration at a depth of about 100 nm from the surface of the substrate, whereas the second impurity in the solar cell according to the conventional technology shows a peak concentration at the surface of the substrate.
  • the emitter layer forms an energy diagram of a potential well type, thereby showing low contact resistance and low ideality factor.
  • the conventional solar cell having a concentration peak at the surface of the emitter layer, in which the concentration decreases from the surface to the inside, has a low energy level at its surface.
  • recombination may occur at the surface of the substrate, which causes a reduction in the electrical efficiency of the solar cell.
  • FIG. 3 is a graph showing an energy diagram of the emitter layer in the solar cell of the present invention and the conventional solar cell.
  • X-axis represents from a depth from the surface
  • y-axis represents a conduction band.
  • the emitter layer forms a doping profile having an energy diagram of a potential well type, thereby maintaining low contact resistance and low ideality factor.
  • a relatively expensive silver (Ag) paste for the shallow emitter is not needed during formation of the front electrode, thereby achieving cost reduction and providing a solar cell having improved electrical characteristics.
  • the solar cell of the present invention may further include an anti-reflection film on the emitter layer.
  • the anti-reflection film plays a role of passivating a defect that exists on a surface of or in a bulk of the emitter layer and reducing reflectivity of incident solar light on a front surface of the substrate. If a defect of the emitter layer is passivated, a recombination site of a hydrophobic carrier is removed to increase an open-circuit voltage (Voc) of the solar cell. As solar reflectivity decreases, an amount of light reaching the P-N junction increases and then a short-circuit current (Isc) of the solar cell increases. Accordingly, a conversion efficiency of the solar cell increases as much as increases in the open-circuit voltage and the short-circuit current of the solar cell by the anti-reflective film.
  • Voc open-circuit voltage
  • Isc short-circuit current
  • the anti-reflection film may have, for example, a monolayered structure of any one selected from the group consisting of a silicon nitride film, a silicon nitride film including hydrogen, a silicon oxide film, a silicon oxide nitride film, MgF 2 , ZnS, Ti0 2 and Ce0 2 , or a multilayered structure in which two or more layers are combined, but is not limited thereto.
  • a thickness of the anti-reflective film may be about 30 to about 100 nm, but is not limited thereto.
  • the front electrode - is provided on the emitter layer.
  • the front electrode may contain silver (Ag).
  • the front electrode may have a width of about 60 to about 120 ⁇ and a height of about 10 to about 35 ⁇ , but the present invention is not limited thereto.
  • a back electrode is provided on the back side of the substrate.
  • the back electrode may be formed on the back side of the substrate, and may contain aluminum (Al).
  • the aluminum contained in the back electrode may diffuse through the back side of the substrate to form a back side field layer at the interface between the back electrode and the substrate. If the back side field layer is formed, it may be prevented that the carrier moves toward the back side of the substrate and recombine therewith. If the carrier recombination is prevented, the open circuit voltage may increase to improve the efficiency of the solar cell.
  • the solar cell according to the present invention includes the emitter layer that is doped with impurity having a concentration peak in the inside thereof rather than at the surface thereof, thereby having low contact resistance and improving efficiency of the solar cell.
  • the solar cell according to the present invention for example, a monocrystalline cell, may have an efficiency of 19.3% or more.
  • a method for manufacturing the solar cell of the present invention includes the steps of:
  • the first conductive type substrate is prepared.
  • the substrate may be doped with the Group III elements of B, Ga, In, etc. as the first impurity.
  • the surface of the substrate may be etched or a saw damage etching process may be performed.
  • the surface of the substrate may be further subjected to a texturing process.
  • the second conductive type emitter layer opposite to the first conductive type is formed on the top of the substrate.
  • the substrate is P type conductive type
  • the emitter layer is N type conductive type
  • N type impurity such the Group V elements of phosphorus (P), arsenic (As), antimony (Sb), etc. may be doped as the second impurity.
  • the second impurity may be doped at a depth of about 100 to about 500 nm from the surface of the substrate. That is, the emitter layer may be formed to have a thickness of about 100 to about 500 nm.
  • the second impurity contained in the emitter layer is injected to have a concentration peak, that is, the highest concentration inside the emitter layer rather than at the surface of the emitter layer.
  • the concentration peak of the second impurity may be formed at a depth of about 30 to about 70% from the surface of the emitter layer, with respective to the entire thickness of the emitter layer, or may be formed at a depth of about 50 to about 250 nm from the surface of the emitter layer.
  • the doping of the second impurity may be performed by an ion implantation process.
  • the ion implantation process makes it possible to obtain the desired depth by controlling the doping concentration, thereby achieving high uniformity at a low doping concentration. Meanwhile, as the total concentration of impurity decreases, the electron-hole recombination rate decreases, resulting in high efficiency of photovoltaic power. However, the substrate resistance increases, and a difference in the contact resistance between the substrate and the electrode occurs, and therefore, the efficiency is reduced when electrons during photovoltaic power generation are collected.
  • An ion concentration profile of the general ion implantation process is similar to that of the conventional diffusion method. That is, the concentration gradually decreases from the surface to the inside.
  • the emitter layer has the highest concentration in the inside thereof rather than at the surface thereof. Therefore, the emitter layer forms a doping profile having an energy diagram of a potential well type, thereby maintaining low contact resistance and low ideality factor.
  • the emitter layer having a concentration peak of the second impurity in the inside thereof can be formed by properly controlling the ion injection amount and injection energy of ion implantation equipment.
  • the ion concentration peak can be formed in the inside rather than at the surface by using the injection energy of about 50 to about 400 keV.
  • the conventional solar cell including the emitter layer having a concentration peak at the surface, a low energy diagram is formed at the surface. Therefore, when the excited electrons generated in the first conductive type substrate are diffused into the second conductive type emitter layer, recombination may occur on the surface of the substrate. In contrast, in the present invention, movement of the generated electrons toward the surface of the substrate can be prevented by an electric field due to a potential barrier, thereby improving surface recombination velocity.
  • the method may further include the step of forming an anti-reflection film on the top of the emitter layer after forming the emitter layer.
  • the anti-reflection film may be formed by vacuum deposition, chemical vapor deposition, spin coating, screen printing or spray coating, but is not limited thereto.
  • the anti-reflection film may have, for example, a monolayered structure of any one selected from the group consisting of a silicon nitride film, a silicon nitride film containing hydrogen, a silicon oxide film, a silicon oxide nitride film, MgF 2 , ZnS, Ti0 2 and Ce0 2 , or a multilayered structure in which two or more layers are combined, but is not limited thereto.
  • the anti-reflection film may be formed to have a thickness of about 30 to about
  • the opening may be formed via patterning using any known method such as photolithography, an optical scribing method, a mechanical scribing method, an etching method using plasma, a wet-type etching method, a dry-type etching method, a lift-off method, and a wire mask method.
  • the opening may be formed by removing a constant size using a laser ablation method, but is not limited thereto.
  • the paste for forming the front electrode is screen-printed while filling the opening, and then heated to form the front electrode.
  • the paste for forming the front electrode may be a silver (Ag) paste.
  • the silver paste is not a paste for forming the shallow emitter which is an expensive paste having a high silver content, but a relatively inexpensive paste having a low silver content.
  • the step of screen-printing the silver paste and then heating it may be carried out in the manner that the printing is done by using a screen printer before it is sintered in a belt firing under nitrogen atmosphere.
  • the front electrode may be fonned to have a width of about 60 to about 120 ⁇ and a height of about 15 to about 35 ⁇ .
  • the method for manufacturing the solar cell of the present invention includes the step of forming a back electrode. More particularly, the step of forming the back electrode may be carried out by printing a paste for the back electrode on the back side of the substrate and then heat treatment.
  • the paste for the back electrode may be an aluminium (Al) paste.
  • the step of forming the back electrode may be carried out either prior to the step of forming the front electrode or after the step of forming the front electrode, and it is not affected by this order.
  • the aluminum paste may contain aluminum, quartz silica, binder, etc.
  • the aluminum may diffuse through the back side of the substrate to form the back side field layer at the interface between the back electrode and the substrate. If the back side field layer is formed, it may be prevented that the carrier moves toward the back side of the substrate and recombine therewith. If the carrier recombination is prevented, the open circuit voltage may increase to improve the efficiency of the solar cell.
  • the step of heat treatment of the aluminum past may be performed by sintering it in a belt firing under nitrogen atmosphere.
  • the back electrode may be formed simultaneously with the step of forming the front electrode.
  • the silver paste for forming the front electrode is screen-printed
  • the aluminum paste for forming the back electrode is screen-printed on the back side, and then the front electrode and the back electrode may be formed simultaneously by the firing process.
  • the silicon surface of 156 mm monocrystalline silicon wafer was subjected to a saw damage removal process, and then a texturing process was carried out using KOH and IPA to reduce surface reflectance.
  • an emitter layer was formed by ion implantation with energy of 200 keV. At this time, a concentration peak of phosphorus (P) was formed at a depth of 100 nm from the surface of the emitter layer. The total depth of the emitter layer was 200 nm.
  • a silicon nitride film was deposited on the emitter layer by PECVD at a thickness of 80 nm to form an anti-reflection film.
  • Al paste (ALSOLAR manufactured by Toyo Aluminium K. K) was screen-printed on the back side, and subsequently, dried in a belt firing of 300°C for 60 seconds and sintered in a belt firing of 900°C for 60 seconds.
  • the back electrode formed after sintering had a thickness of about 30 ⁇ .
  • the anti-reflection film was removed in a size of 40 ⁇ by laser ablation to form an opening.
  • an inexpensive Ag paste (17A, Dupont) was screen-printed, and then sintered in a belt firing of 900°C for 20 seconds so as to form the front electrode.
  • a solar cell was manufactured in the same manner as in Example 1 , except that during formation of the emitter layer, phosphorus was doped by a diffusion process using POCL 3 in a tube furnace at 900°C to form an emitter layer having a sheet resistance of 70 ⁇ 90 ⁇ /sq and an expensive Ag paste (17F, Dupont) was used during formation of the front electrode.
  • Example 1 The electric performances of the solar cells manufactured in Example 1 and Comparative Example 1 were measured using an I-V tester and compared using a PC ID program. The results are shown in the following Table 1.
  • Jsc means the short-circuit current density measured at zero output voltage
  • Voc means the open circuit voltage measured at zero output current
  • FF[%] means the fill factor
  • Eta [%] means the efficiency.
  • the solar cell of the present invention has improved electric perfomiance, compared to the solar cell manufactured by the conventional method.

Abstract

The present invention relates to a solar cell and a manufacturing method thereof. The solar cell of the present invention includes a first conductive type substrate that is doped with a first impurity; a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity; a front electrode that is positioned on the emitter layer; and a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer. According to the present invention, provided is a solar cell that is provided with low contact resistance to have improved efficiency of photovoltaic power.

Description

[DESCRIPTION]
[Invention Title]
Solar Cell and Manufacturing Method Thereof
[Technical Field]
The present invention relates to a solar cell and a manufacturing method thereof.
More particularly, the present invention relates to a solar cell showing low contact resistance and high efficiency, and a manufacturing method thereof.
This application claims priority benefits from Korean Patent Application No. 10-2013-0041768, filed on April 16, 2013, the entire contents of which are fully incorporated herein by reference.
[Background Art]
In recent years, with prediction of exhaustion of existing energy resources such as oil and coal, interests in alternative energy source are increasing. Among the alternative energy resources, solar cells have been drawing attention as next-generation batteries that directly convert solar energy into electrical energy by using semiconductor devices. Solar cells are largely classified into a silicon solar cell, a compound semiconductor solar cell, and a tandem solar cell. Among these solar cells, the silicon solar cell leads the market.
FIG. 1 is a cross-sectional view illustrating a basic structure of a solar cell.
Referring to FIG. 1 , the solar cell includes a substrate 100; an emitter layer 200 that is positioned on the substrate 100 and has a conductivity type opposite to the substrate 100; an anti-reflection film 300 that is positioned on the emitter layer 200; a front electrode 400 that is contacted with the emitter layer; and a back electrode 500 that is positioned on the back side of the substrate 100.
On the other hand, for high efficiency of silicon solar cells, there have been developed a variety of technologies, such as shallow emitters, selective emitters, or the like. The shallow emitter refers to an emitter layer having high sheet resistance of 60 to 120 Ω/sq. Such a shallow emitter is advantageous in that its recombination rate is low and short-wavelength sun light can be used.
Korean Patent Application No. 2010-0068987 discloses a method for increasing a potential difference at the p-n junction region by fomiing the selective emitter via the selective formation of a more heavily impurity-doped region on the top using a dopant paste of silicon substrate and for improving the short wavelength response to increase the efficiency of photovoltaic power. In the case of No. 2010-0068987, the method includes the steps of incorporating and diffusing a second conductive type impurity into the silicon substrate to form a second conductive type semiconductor layer on the top of the silicon substrate; printing the silicon substrate surface with the dopant paste and heating it to form a more heavily doped region on the second conductive type semiconductor layer; etching the silicon substrate surface using the dopant paste as a barrier; removing the dopant paste which is printed on the silicon substrate surface and patterning a metal material to contact with the more heavily doped region, thereby forming an electrode; and progressing an additional diffusion process for extending the more heavily doped region.
However, this method has problems of complicated process and high costs. Moreover, in the case of diffusing the second conductive type impurity to form an emitter region, impurity is deposited on the substrate surface by a screen printing process, and then penetrated into the substrate by heat treatment. Therefore, it is difficult to precisely control impurity diffusion and thus it is impossible to form the emitter region with an accurate size inside the substrate. Problematically, electrical characteristics of solar cells are deteriorated.
[Disclosure]
I Technical Problem]
In order to solve the problems of the conventional technologies described above, an object of the present invention is to provide a solar cell having improved electrical characteristics.
Further, another object of the present invention is to provide a method for manufacturing the solar cell having improved electrical characteristics by precisely controlling the depth and doping concentration of an emitter layer formed inside a substrate in a simple manner.
[Technical Solution] In order to achieve the above objects, the present invention provides a solar cell, including
a first conductive type substrate that is doped with a first impurity;
a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity;
a front electrode that is positioned on the emitter layer; and
a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
Further, the present invention provides a method for manufacturing a solar cell, including the steps of:
injecting a second impurity into a first conductive type substrate containing a first impurity to form a second conductive type emitter layer, and injecting the second impurity at a higher concentration inside the emitter layer than at the surface of the emitter layer;
forming a front electrode on the emitter layer; and
forming a back electrode on the back side of the substrate.
[Advantageous Effects]
The present invention provides a solar cell that is provided with low contact resistance to have improved efficiency of photovoltaic power.
Further, the present invention provides a method for manufacturing a solar cell having improved electrical characteristics in a simple manner.
Further, an expensive paste for a shallow emitter is not needed, when the front electrode is formed, and therefore, the cost reduction will be achieved.
[Brief Description of Drawings]
FIG. 1 is a cross-sectional view illustrating a basic structure of a solar cell;
FIG. 2 is a graph showing a doping concentration according to a depth from the surface in the solar cell of the present invention or the conventional solar cell; and
FIG. 3 is a graph showing an energy diagram of an emitter layer in the solar cell of the present invention or the conventional solar cell.
[Detailed Description of the Invention] A solar cell according to one embodiment of the present invention includes a first conductive type substrate that is doped with a first impurity; a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity; a front electrode that is positioned on the emitter layer; and a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
Further, a method for manufacturing the solar cell according to one embodiment of the present invention includes the steps of injecting a second impurity into a first conductive type substrate containing a first impurity to form a second conductive type emitter layer, and injecting the second impurity at a higher concentration inside the emitter layer than at the surface of the emitter layer; forming a front electrode on the emitter layer; and forming a back electrode on the back side of the substrate.
It will be understood that, although the terms "first", "second", etc. may be used herein to describe various elements, they are only used to distinguish one element from another.
It will be also understood that when a layer or an element is referred to as being "on" or "upon" another layer or element, it can be directly formed on the other layer or element, or intervening layers or elements may be present therebetween.
While the present invention is susceptible to various modifications and alternative forms, specific embodiments thereof will herein be described in detail. It should be understood, however, that there is no intent to limit the embodiments of the present invention to the particular forms disclosed, but conversely, the embodiments of the present invention are to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.
Hereinafter, a detailed description will be given as to a solar cell of the present invention and a manufacturing method thereof with reference to drawings.
Solar cell
A solar cell of the present invention includes a first conductive type substrate that is doped with a first impurity;
a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity;
a front electrode that is positioned on the emitter layer; and
a back electrode that is positioned on the back side of the substrate, in which the second impurity has a concentration peak inside the emitter layer.
First, the substrate is a silicon semiconductor substrate of a first conductive type. The silicon may be a crystalline silicon such as a mono crystalline silicon or a polycrystalline silicon, or a non-crystalline silicon.
According to one embodiment of the present invention, the first conductivity may be p-type, and in this regard, it may be doped with a trivalent element as the first impurity, such as boron (B), gallium (Ga), or indium (In).
According to another embodiment of the present invention, the first conductivity may be N-type, and in this regard, it may be doped with a group V element as the first impurity, such as phosphorus (P), arsenic (As), and antimony (Sb).
The emitter layer is a second conductive type that is opposite to that of the substrate and contains the second impurity.
For example, when the substrate is a p-type conductive type, the emitter layer may be an N-type conductive type and doped with a group V element as the second impurity, such as phosphorus (P), arsenic (As), and antimony (Sb).
In the present invention, the thickness of the emitter layer is determined by a depth of the injected second impurity from the surface of the substrate. According to one embodiment of the present invention, the thickness of the emitter layer may be about 100 to about 500 nm. Further, according to one embodiment of the present invention, the emitter layer may have a sheet resistance of about 60 to about 120 Ω/sq.
As such, when the substrate and the emitter layer are doped with impurities of opposite conductive type, a P-N junction is formed at an interface between the substrate and the emitter layer. When light is incident on P-N junction of the solar cell, electron-hole pairs are generated. The electric field accelerates the electrons toward the N-layer and the holes toward the P-layer. Thus, a photoelectromotive force is generated at the P-N junction. At this time, when a load or a system is connected to both ends of the solar cell, current flows and thus power may be generated.
The solar cell of the present invention is characterized in that the second impurity contained in the emitter layer shows a concentration peak, that is, the highest concentration inside the emitter layer rather than at the surface of the emitter layer, unlike the conventional solar cell having a concentration peak at the surface of the emitter layer.
For example, according to one embodiment of the present invention, the concentration peak of the second impurity may be formed at a depth of about 30 to about 70% from the surface of the emitter layer, with respect to the thickness of the emitter layer, or may be formed at a depth of about 50 to about 250 nm from the surface of the emitter layer.
FIG. 2 is a graph showing a doping concentration according to a depth from the surface in the solar cell of the present invention or the conventional solar cell. In FIG. 2, x-axis represents the depth from the surface and y-axis represents the doping density.
Referring to FIG. 2, the second impurity in the solar cell according to Example of the present invention shows a peak concentration at a depth of about 100 nm from the surface of the substrate, whereas the second impurity in the solar cell according to the conventional technology shows a peak concentration at the surface of the substrate.
When the second impurity has a concentration peak at a depth of the above range, the emitter layer forms an energy diagram of a potential well type, thereby showing low contact resistance and low ideality factor.
The conventional solar cell having a concentration peak at the surface of the emitter layer, in which the concentration decreases from the surface to the inside, has a low energy level at its surface. Thus, when excited electrons formed in the first conductive type substrate are diffused into the second conductive type emitter layer, recombination may occur at the surface of the substrate, which causes a reduction in the electrical efficiency of the solar cell.
In contrast, in the solar cell of the present invention, movement of the generated electrons toward the surface is prevented by an electric field due to a potential barrier of the potential well, thereby improving surface recombination velocity.
FIG. 3 is a graph showing an energy diagram of the emitter layer in the solar cell of the present invention and the conventional solar cell. In FIG. 3, X-axis represents from a depth from the surface, and y-axis represents a conduction band.
Referring to FIG. 3, the emitter layer forms a doping profile having an energy diagram of a potential well type, thereby maintaining low contact resistance and low ideality factor. In addition, a relatively expensive silver (Ag) paste for the shallow emitter is not needed during formation of the front electrode, thereby achieving cost reduction and providing a solar cell having improved electrical characteristics.
According to one embodiment of the present invention, the solar cell of the present invention may further include an anti-reflection film on the emitter layer.
The anti-reflection film plays a role of passivating a defect that exists on a surface of or in a bulk of the emitter layer and reducing reflectivity of incident solar light on a front surface of the substrate. If a defect of the emitter layer is passivated, a recombination site of a hydrophobic carrier is removed to increase an open-circuit voltage (Voc) of the solar cell. As solar reflectivity decreases, an amount of light reaching the P-N junction increases and then a short-circuit current (Isc) of the solar cell increases. Accordingly, a conversion efficiency of the solar cell increases as much as increases in the open-circuit voltage and the short-circuit current of the solar cell by the anti-reflective film.
The anti-reflection film may have, for example, a monolayered structure of any one selected from the group consisting of a silicon nitride film, a silicon nitride film including hydrogen, a silicon oxide film, a silicon oxide nitride film, MgF2, ZnS, Ti02 and Ce02, or a multilayered structure in which two or more layers are combined, but is not limited thereto. Furthermore, a thickness of the anti-reflective film may be about 30 to about 100 nm, but is not limited thereto.
The front electrode -is provided on the emitter layer.
According to one embodiment of the present invention, the front electrode may contain silver (Ag). According to one embodiment of the present invention, the front electrode may have a width of about 60 to about 120 μιη and a height of about 10 to about 35 μη , but the present invention is not limited thereto.
A back electrode is provided on the back side of the substrate.
The back electrode may be formed on the back side of the substrate, and may contain aluminum (Al). The aluminum contained in the back electrode may diffuse through the back side of the substrate to form a back side field layer at the interface between the back electrode and the substrate. If the back side field layer is formed, it may be prevented that the carrier moves toward the back side of the substrate and recombine therewith. If the carrier recombination is prevented, the open circuit voltage may increase to improve the efficiency of the solar cell.
The solar cell according to the present invention includes the emitter layer that is doped with impurity having a concentration peak in the inside thereof rather than at the surface thereof, thereby having low contact resistance and improving efficiency of the solar cell.
The solar cell according to the present invention, for example, a monocrystalline cell, may have an efficiency of 19.3% or more.
Manufacturing method of solar cell
A method for manufacturing the solar cell of the present invention includes the steps of:
injecting a second impurity into a first conductive type substrate containing a first impurity to form a second conductive type emitter layer, and injecting the second impurity at a higher concentration inside the emitter layer than at the surface of the emitter layer;
forming a front electrode on the emitter layer; and
forming a back electrode on the back side of the substrate.
First, the first conductive type substrate is prepared.
If the first conductive type substrate is P type, the substrate may be doped with the Group III elements of B, Ga, In, etc. as the first impurity.
According to one embodiment of the present invention, to remove an oxide film or binding that is formed on the surface of the substrate, the surface of the substrate may be etched or a saw damage etching process may be performed. In addition, in order to improve light absorption rate by increasing surface area and reducing light reflection rate, the surface of the substrate may be further subjected to a texturing process.
Next, the second conductive type emitter layer opposite to the first conductive type is formed on the top of the substrate. For example, if the substrate is P type conductive type, the emitter layer is N type conductive type, and N type impurity such the Group V elements of phosphorus (P), arsenic (As), antimony (Sb), etc. may be doped as the second impurity.
The second impurity may be doped at a depth of about 100 to about 500 nm from the surface of the substrate. That is, the emitter layer may be formed to have a thickness of about 100 to about 500 nm.
According to the method for manufacturing the solar cell of the present invention, the second impurity contained in the emitter layer is injected to have a concentration peak, that is, the highest concentration inside the emitter layer rather than at the surface of the emitter layer.
The concentration peak of the second impurity may be formed at a depth of about 30 to about 70% from the surface of the emitter layer, with respective to the entire thickness of the emitter layer, or may be formed at a depth of about 50 to about 250 nm from the surface of the emitter layer.
The doping of the second impurity may be performed by an ion implantation process.
The ion implantation process makes it possible to obtain the desired depth by controlling the doping concentration, thereby achieving high uniformity at a low doping concentration. Meanwhile, as the total concentration of impurity decreases, the electron-hole recombination rate decreases, resulting in high efficiency of photovoltaic power. However, the substrate resistance increases, and a difference in the contact resistance between the substrate and the electrode occurs, and therefore, the efficiency is reduced when electrons during photovoltaic power generation are collected. An ion concentration profile of the general ion implantation process is similar to that of the conventional diffusion method. That is, the concentration gradually decreases from the surface to the inside.
However, according to the method for manufacturing the solar cell of the present invention, the emitter layer has the highest concentration in the inside thereof rather than at the surface thereof. Therefore, the emitter layer forms a doping profile having an energy diagram of a potential well type, thereby maintaining low contact resistance and low ideality factor.
During the process of injecting impurity, a large number of substrates are generally loaded at a time for mass-production. It is difficult to inject a sufficient amount of impurity between the substrates, because the gap between the substrates is narrow. Thus, the impurity concentration is low in the middle of the substrate to form high sheer resistance. As a result, shunts can be formed during electrode formation to decrease fill factor (FF) and voltage (Voc). Ultimately, there is a problem that cell efficiency of the solar cell is deteriorated. According to the manufacturing method of the present invention, the above problems can be solved.
Further, if a thin emitter layer is formed, contact resistance is increased by sintering the paste for forming the front electrode in the subsequent process of forming the front electrode, problematically resulting in a reduction in fill factor. In order to deal with this problem, a silver (Ag) paste for forming a shallow emitter is used, but it is expensive to cause an increase in production costs of the solar cell. However, according to the manufacturing method of the present invention, a relatively expensive paste for the shallow emitter is not needed during formation of the front electrode, and an inexpensive paste can be used to achieve cost reduction.
As described above, the emitter layer having a concentration peak of the second impurity in the inside thereof can be formed by properly controlling the ion injection amount and injection energy of ion implantation equipment. For example, the ion concentration peak can be formed in the inside rather than at the surface by using the injection energy of about 50 to about 400 keV.
In the conventional solar cell including the emitter layer having a concentration peak at the surface, a low energy diagram is formed at the surface. Therefore, when the excited electrons generated in the first conductive type substrate are diffused into the second conductive type emitter layer, recombination may occur on the surface of the substrate. In contrast, in the present invention, movement of the generated electrons toward the surface of the substrate can be prevented by an electric field due to a potential barrier, thereby improving surface recombination velocity.
According to one embodiment of the present invention, the method may further include the step of forming an anti-reflection film on the top of the emitter layer after forming the emitter layer.
The anti-reflection film may be formed by vacuum deposition, chemical vapor deposition, spin coating, screen printing or spray coating, but is not limited thereto. Also, the anti-reflection film may have, for example, a monolayered structure of any one selected from the group consisting of a silicon nitride film, a silicon nitride film containing hydrogen, a silicon oxide film, a silicon oxide nitride film, MgF2, ZnS, Ti02 and Ce02, or a multilayered structure in which two or more layers are combined, but is not limited thereto.
The anti-reflection film may be formed to have a thickness of about 30 to about
100 mil.
There are formed an opening which penetrates the anti-reflection film with removing the same and at the same time removes a part of the emitter layer with exposing a part thereof.
The opening may be formed via patterning using any known method such as photolithography, an optical scribing method, a mechanical scribing method, an etching method using plasma, a wet-type etching method, a dry-type etching method, a lift-off method, and a wire mask method. According to one embodiment of the present invention, the opening may be formed by removing a constant size using a laser ablation method, but is not limited thereto.
The paste for forming the front electrode is screen-printed while filling the opening, and then heated to form the front electrode. The paste for forming the front electrode may be a silver (Ag) paste. The silver paste is not a paste for forming the shallow emitter which is an expensive paste having a high silver content, but a relatively inexpensive paste having a low silver content. According to one embodiment of the present invention, the step of screen-printing the silver paste and then heating it may be carried out in the manner that the printing is done by using a screen printer before it is sintered in a belt firing under nitrogen atmosphere.
The front electrode may be fonned to have a width of about 60 to about 120 μιη and a height of about 15 to about 35 μιη.
The method for manufacturing the solar cell of the present invention includes the step of forming a back electrode. More particularly, the step of forming the back electrode may be carried out by printing a paste for the back electrode on the back side of the substrate and then heat treatment. The paste for the back electrode may be an aluminium (Al) paste.
According to one embodiment of the present invention, the step of forming the back electrode may be carried out either prior to the step of forming the front electrode or after the step of forming the front electrode, and it is not affected by this order.
The aluminum paste may contain aluminum, quartz silica, binder, etc. During the heat treatment of the aluminum paste, the aluminum may diffuse through the back side of the substrate to form the back side field layer at the interface between the back electrode and the substrate. If the back side field layer is formed, it may be prevented that the carrier moves toward the back side of the substrate and recombine therewith. If the carrier recombination is prevented, the open circuit voltage may increase to improve the efficiency of the solar cell.
According to one embodiment of the present invention, the step of heat treatment of the aluminum past may be performed by sintering it in a belt firing under nitrogen atmosphere.
According to another embodiment of the present invention, the back electrode may be formed simultaneously with the step of forming the front electrode. In other words, the silver paste for forming the front electrode is screen-printed, and the aluminum paste for forming the back electrode is screen-printed on the back side, and then the front electrode and the back electrode may be formed simultaneously by the firing process. [Mode for Invention]
Hereinafter, the present invention will be explained in more detail with reference to Examples according to the present invention. However, these Examples are only for the illustration of the present invention and it is not intended that the scope of the present invention is limited thereby.
<Example>
Example 1
The silicon surface of 156 mm monocrystalline silicon wafer was subjected to a saw damage removal process, and then a texturing process was carried out using KOH and IPA to reduce surface reflectance.
After the texturing process, an emitter layer was formed by ion implantation with energy of 200 keV. At this time, a concentration peak of phosphorus (P) was formed at a depth of 100 nm from the surface of the emitter layer. The total depth of the emitter layer was 200 nm.
A silicon nitride film was deposited on the emitter layer by PECVD at a thickness of 80 nm to form an anti-reflection film.
Al paste (ALSOLAR manufactured by Toyo Aluminium K. K) was screen-printed on the back side, and subsequently, dried in a belt firing of 300°C for 60 seconds and sintered in a belt firing of 900°C for 60 seconds. The back electrode formed after sintering had a thickness of about 30 μπι.
The anti-reflection film was removed in a size of 40 μιη by laser ablation to form an opening.
In order to form a front electrode, an inexpensive Ag paste (17A, Dupont) was screen-printed, and then sintered in a belt firing of 900°C for 20 seconds so as to form the front electrode.
Comparative Example 1
A solar cell was manufactured in the same manner as in Example 1 , except that during formation of the emitter layer, phosphorus was doped by a diffusion process using POCL3 in a tube furnace at 900°C to form an emitter layer having a sheet resistance of 70 ~ 90 Ω/sq and an expensive Ag paste (17F, Dupont) was used during formation of the front electrode.
<Experimental Example>
Evaluation of electric performance of solar cell
The electric performances of the solar cells manufactured in Example 1 and Comparative Example 1 were measured using an I-V tester and compared using a PC ID program. The results are shown in the following Table 1. Here, Jsc means the short-circuit current density measured at zero output voltage, Voc means the open circuit voltage measured at zero output current, and FF[%] means the fill factor and Eta [%] means the efficiency.
[Table 1 ]
Figure imgf000015_0001
As shown in the results of Table 1 , it can be seen that the solar cell of the present invention has improved electric perfomiance, compared to the solar cell manufactured by the conventional method.
[Reference Numeral]
100: substrate
200: emitter layer
300: anti-reflection film
400: front electrode
500: back electrode

Claims

[CLAIMS]
[Claim 1 ]
A solar cell, comprising:
a first conductive type substrate that is doped with a first impurity;
a second conductive type emitter layer that is positioned on the substrate and doped with a second impurity;
a front electrode that is positioned on the emitter layer; and
a back electrode that is positioned on the back side of the substrate, wherein the second impurity has a concentration peak inside the emitter layer.
[Claim 2]
The solar cell according to claim 1, wherein the second impurity has a concentration peak at a depth of 30 to 70% from the surface of the emitter layer.
[Claim 3 ]
The solar cell according to claim 1 , wherein the emitter layer has a thickness of 100 to 500 nm.
[Claim 4]
The solar cell according to claim 1 , wherein the front electrode contains silver
(Ag).
[Claim 5]
The solar cell according to claim 1, wherein the back electrode contains aluminum (Al).
[Claim 6]
The solar cell according to claim 1, wherein the emitter layer has an energy diagram of a potential well type.
[Claim 7]
A method for manufacturing a solar cell, comprising the steps of:
injecting a second impurity into a first conductive type substrate containing a first impurity to form a second conductive type emitter layer, and injecting the second impurity at a higher concentration inside the emitter layer than at the surface of the emitter layer; forming a front electrode on the emitter layer; and
forming a back electrode on the back side of the substrate.
[Claim 8]
The method according to claim 7, wherein the second impurity is injected to have a concentration peak at a depth of 30 to 70% of the emitter layer.
[Claim 9]
The method according to claim 7, wherein the emitter layer is formed to have a thickness of 100 to 500 nm.
[Claim 10]
The method according to claim 7, further comprising the step of forming an anti-reflection film on the emitter layer.
[Claim 11 ]
The method according to claim 7, wherein the step of injecting the second impurity is carried out by an ion implantation process.
[Claim 12]
The method according to claim 11, wherein the injection energy of the ion implantation process is 50 to 400 keV.
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KR20080046439A (en) * 2006-11-22 2008-05-27 주식회사 엘지화학 Method of preparing solar cell and solar cell prepared thereby
KR101139443B1 (en) * 2009-09-04 2012-04-30 엘지전자 주식회사 Hetero-junction solar cell and fabrication method thereof
KR101139459B1 (en) * 2009-08-27 2012-04-30 엘지전자 주식회사 Sollar Cell And Fabrication Method Thereof
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