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Ferroelectric solar cell.

Ferroelectric solar cell.

    US6639143B2. Solar cell using ferroelectric material(s). Google Patents

    Publication number US6639143B2 US6639143B2 US10/144,787 US14478702A US6639143B2 US 6639143 B2 US6639143 B2 US 6639143B2 US 14478702 A US14478702 A US 14478702A US 6639143 B2 US6639143 B2 US 6639143B2 Authority US United States Prior art keywords conductive type solar cell layer type semiconductor semiconductor substrate Prior art date 2001-08-24 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.) Expired. Lifetime Application number US10/144,787 Other versions US20030037815A1 ( en Inventor Jeong Kim Dong-seop Kim Soo-hong Lee 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.) Intellectual Keystone Technology LLC Original Assignee Samsung SDI Co Ltd Priority date (The priority date 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 date listed.) 2001-08-24 Filing date 2002-05-15 Publication date 2003-10-28 Priority claimed from KR2001-51440 external-priority 2002-05-15 Application filed by Samsung SDI Co Ltd filed Critical Samsung SDI Co Ltd 2002-05-15 Assigned to Samsung SDI CO., LTD. reassignment Samsung SDI CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KIM, DONG-SEOP, KIM, JEONG, LEE, SOO-HONG 2003-02-27 Publication of US20030037815A1 publication Critical patent/US20030037815A1/en 2003-10-28 Application granted granted Critical 2003-10-28 Publication of US6639143B2 publication Critical patent/US6639143B2/en 2015-05-14 Assigned to INTELLECTUAL KEYSTONE TECHNOLOGY LLC reassignment INTELLECTUAL KEYSTONE TECHNOLOGY LLC ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: Samsung SDI CO., LTD. 2022-05-15 Anticipated expiration legal-status Critical Status Expired. Lifetime legal-status Critical Current

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    • H — ELECTRICITY
    • H01 — ELECTRIC ELEMENTS
    • H01L — SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00 — Semiconductor devices sensitive to infra-red 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 infra-red 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
    • H — ELECTRICITY
    • H01 — ELECTRIC ELEMENTS
    • H01L — SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00 — Semiconductor devices sensitive to infra-red 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 infra-red 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 infra-red 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 infra-red 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 infra-red 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
    • H — ELECTRICITY
    • H01 — ELECTRIC ELEMENTS
    • H01L — SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00 — Semiconductor devices sensitive to infra-red 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 infra-red 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 infra-red 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
    • H — ELECTRICITY
    • H01 — ELECTRIC ELEMENTS
    • H01L — SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00 — Semiconductor devices sensitive to infra-red 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
    • 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

    Abstract

    A solar cell using a ferroelectric material(s) is provided with a ferroelectric layer at the front surface or the rear surface thereof, or at the front and the rear surfaces thereof. The ferroelectric layer is formed with a ferroelectric material such as BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3) and SBT(SrBi2Ta2O7).

    Description

    This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled SOLAR CELL USING FERROELECTRIC MATERIAL earlier filed in the Korean Industrial Property Office on Aug. 24, 2001 and there duly assigned Serial No. 2001-51440.

    The present invention relates to a solar cell and, more particularly, to a solar cell which makes use of a ferroelectric material or ferroelectric materials.

    Generally, a solar cell is provided with a structure where pairs of electrons and holes are generated in a semiconductor by way of the light from the outside, and electric fields are formed by way of a pn junction so that the electrons move to the n type semiconductor, and the holes move to the p type semiconductor, thereby producing the required electric power.

    In a buried contact solar cell (BCSC) that bears a high efficient solar cell structure, a groove is formed at the front surface of the solar cell, and a conductive material fills the internal space of the groove to thereby form a front metallic electrode of a buried type. In the BCSC structure, an SiO2-based layer is deposited onto the entire front surface of the cell except for the groove area to obtain a surface passivation effect and an anti-reflection effect.

    In addition to SiO2, an anti-reflection layer may be deposited with TiO2, MgF2, ZnS and SiNx and so on, while realizing the desired surface passivation effect through ion implantation, plasma hydrogenated treatment, SiNx deposition by plasma enhanced chemical vapor deposition (PECVD).

    Furthermore, in the BCSC structure, an Al-based layer is deposited, and heat-treated to thereby form a rear electrode with a heavily doped region. Consequently, the open-circuit voltage is increased by way of a rear surface field (BSF) effect, thereby enhancing the efficiency of the solar cell.

    However, with such a structure, damage is done to the cell structure during the process of heat-treating the Al-based layer for the rear electrode so that recombination of the electrons and the holes occurs increasingly at the surface area. In order to solve such a problem, a solar cell with a double side buried contact (DSBC) structure where the rear electrode is also buried in the form of a groove is introduced.

    However, in the DSBC structure, a shunt path is made between the rear electrode and a floating junction layer so that the desired BSF effect cannot be obtained.

    U.S. Pat. No. 6,081,017 discloses an improved technique of solving the above problems. A self-biased solar cell structure is introduced to reduce the degree of recombination of the electrons and the holes at the rear surface area while increasing the open-circuit voltage of the cell and enhancing the energy efficiency thereof. In the structure, a dielectric layer is deposited onto the entire surface of the substrate except for the rear electrode area, and a layer for a voltage application electrode is deposited thereon. The voltage application electrode is connected to the front electrode for the solar cell to thereby produce the desired self-biased voltage. The self-biased voltage is applied to the rear surface area while forming the desired rear surface field there. In this way, the loss of carrier recombination is reduced while enhancing the energy efficiency of the solar cell.

    However, in such a technique, the process of connecting the front electrode to the rear electrode should be additionally conducted, and this results in complicated processing steps. As the voltage from the solar cell is used for obtaining the desired rear surface field, the dimension of the rear surface field is limited to the value less than the open-circuit voltage.

    U.S. Pat. No. 4,365,106 discloses a solar cell using a ferroelectric material. In the solar cell, the conversion of the optical energy to the electrical energy is made by way of the variation in polarization as a function of the temperature of the ferroelectric material. In the metal-insulator-semiconductor (MIS) structure, a ferroelectric material is used as an insulating material. The temperature of the ferroelectric material may be altered due to the light from the outside. In this case, the surface polarization of the ferroelectric material is varied while generating electric charge. The electric charge induces a strong electric field between the ferroelectric material and the semiconductor to thereby form an inversion layer. The inversion layer severs to make the desired pn junction. Therefore, the pairs of electrons and holes generated due to the light from the outside are separated from each other by way of the internal electric field to thereby produce the desired electrical energy as with the usual solar cell.

    However, in such a technique, hetero-junction is made at the pn junction interface between the ferroelectric material and the semiconductor so that the loss by the interfacial recombination of the electrons and the holes is increased. Furthermore, the electrons do not move about well due to the insulating effect of the ferroelectric material, and this lowers the efficiency of the solar cell.

    It is, therefore, an object of the present invention to provide a high efficiency solar cell which involves simplified structural components as well as simplified processing steps.

    It is another object of the present invention to provide a high efficiency solar cell which makes use of a ferroelectric material or ferroelectric materials.

    These and other objects may be achieved by a solar cell where the front surface or the rear surface thereof or the front and rear surfaces thereof are formed with a ferroelectric layer.

    Specifically, the solar cell has a pn structure with a semiconductor substrate of a first conductive type, a semiconductor layer of a second conductive type formed on the first conductive type semiconductor substrate, and a pn junction formed at the interface between the first conductive type semiconductor substrate and the second conductive type semiconductor layer. The first and the second conductive types are opposite to each other in polarity. A front electrode is placed over the pn structure while being connected to the second conductive type semiconductor layer. A rear electrode is placed below the pn structure while being connected to the first conductive type semiconductor substrate. A ferroelectric layer is formed on one of the front surface of the second conductive type semiconductor layer and the rear surface of the first conductive type semiconductor substrate. A poling electrode is formed on at least a part of the ferroelectric layer.

    The ferroelectric layer is formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), or SBT(SrBi2Ta2O7).

    The poling electrode placed on the ferroelectric layer at the front surface of the second conductive type semiconductor layer is formed with a transparent conductive oxide material selected from ITO (indium tin oxide), RuO2, SrRuO3, IrO2, or La1-xSrxCoO3.

    The poling electrode placed on the ferroelectric layer at the rear surface of the semiconductor substrate is formed with a metallic electrode material selected from Al, Cu, Ag, or Pt.

    A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or the similar components, wherein:

    FIG. 1 is a cross sectional view of a solar cell with a double side buried contact (DSBC) structure where a ferroelectric layer is formed at the front surface of the cell according to the present invention; and

    FIG. 2 is a cross sectional view of a solar cell with a DSBC structure where a ferroelectric layer is formed at the rear surface of the cell according to the present invention.

    Preferred embodiments of this invention will be explained with reference to the accompanying drawings. The first or second conductive type refers to the p or n type. First, the structure of a solar cell using a ferroelectric according to the present invention is described as below.

    FIG. 1 is a cross sectional view of a solar cell with a double side buried contact (DSBC) structure where a ferroelectric layer is formed at the front surface of the cell. As shown in FIG. 1, the DSBC solar cell has a pn structure with a semiconductor substrate 10 of a first conductive type, a semiconductor layer 11 of a second conductive type formed on the first conductive type semiconductor substrate 10 while being patterned by way of top grooves 17 a, and a pn junction 30 formed at the interface between the first conductive type semiconductor substrate 10 and the second conductive type semiconductor layer 11. The first and the second conductive types are opposite to each other in polarity

    A ferroelectric layer 13 is formed on the second conductive type semiconductor layer 11 with a ferroelectric material such as BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), and SBT(SrBi2Ta2O7). As the ferroelectric material involves spontaneous polarization in a microscopic scale even at the critical temperature or less, but does not involve the spontaneous polarization in a macroscopic scale due to the domain formation. Therefore, it is necessary to make poling in the polarized direction. In order to make the poling, it is required to provide an electrode for applying voltage from the outside. In this connection, a poling electrode 14 is formed on at least a part of the ferroelectric layer 13.

    As the poling electrode 14 covers the front surface of the ferroelectric layer 13, it is preferably formed with a transparent conductive oxide material such as indium tin oxide (ITO), RuO2, SrRuO3, IrO2, La1-xSrxCoO3.

    Furthermore, the ferroelectric layer 13 involves a surface passivation effect, and takes a role of an anti-reflection layer. Therefore, in the solar cell with such a ferroelectric layer, a layer based on CeO2, MgF2 or TiO2 for an anti-reflection and an layer based on SiO2 or SiNx for an anti-reflection and a surface passivation may be omitted.

    Another second conductive type semiconductor layer 15 is formed on the rear surface of the semiconductor substrate 10 while being patterned by way of bottom grooves 17 b. A surface oxide layer 16 is formed on the semiconductor layer 15 to make the desired rear surface reflection.

    The top and the bottom grooves 17 a and 17 b are patterned to have a predetermined depth from the front and the rear surface of the solar cell, and heavily doped regions 18 a and 18 b where impurities are doped at high concentration are formed at the inner wall of the top and the bottom grooves 17 a and 17 b. A conductive material fills the internal space of the grooves 17 a and 17 b to thereby form front electrodes 19 a and rear electrodes 19 b, respectively.

    FIG. 2 is a cross sectional view of a solar cell with a DSBC structure where a ferroelectric layer is formed at the rear surface of the cell.

    As shown in FIG. 2, the DSBC solar cell has a pn structure with a semiconductor substrate 20 of a first conductive type, a semiconductor layer 21 of a second conductive type formed on the first conductive type semiconductor substrate 20 while being patterned by way of top grooves 24 a, and a pn junction 40 formed at the interface between the first conductive type semiconductor substrate 20 and the second conductive type semiconductor layer 21 while being patterned by way of the top grooves 24 a. The first and the second conductive types are opposite to each other in polarity.

    A ferroelectric layer 22 is formed on the rear surface of the semiconductor substrate 20 while being patterned by way of bottom grooves 24 b. A poling electrode 23 for poling the ferroelectric layer 22 is formed on at least a part of the ferroelectric layer 22.

    The ferroelectric layer 22 is formed with a ferroeletric material such as BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), and SBT(SrBi2Ta2O7).

    In case the ferroelectric layer 22 is positioned at the rear surface of the solar cell, it is not necessary to form such a ferroelectric layer with a transparent conductive oxide material. Therefore, the poling electrode 23 for the ferroelectric layer 22 may be formed with a metallic material such as Al, Cu, Ag and Pt.

    The top and the bottom grooves 24 a and 24 b are formed at the front and the rear surface of the solar cell, and heavily doped regions 25 a and 25 b where impurities are doped at high concentration are formed at the inner wall of the grooves 24 a and 24 b, respectively. A conductive material fills the internal space of the grooves 24 a and 24 b to thereby form front electrodes 26 a, and rear electrodes 26 b, respectively.

    As the ferroelectric layer 22 positioned at the rear surface of the solar cell takes a role of preventing a rear surface reflection, an additional surface oxide layer for preventing the rear surface reflection may be omitted.

    Furthermore, as the ferroelectric layer 22 positioned at the rear surface of the solar cell forms a rear surface field, an additional rear surface field layer based on Al may be omitted. It is also possible that a ferroelectric layer is additionally deposited onto the existent Al-based layer to thereby further enhance the rear surface field effect.

    Such a ferroelectric layer may be formed at the front and the rear surface of the solar cell, and this structure may be applied for use in various kinds of DSBC or BCSC solar cells.

    The operational effects of the ferroelectric layer positioned at the front or the rear surface of the solar cell will now be explained in detail.

    Generally, the ferroelectric layer generates polarized electric charge at its surface by way of spontaneous polarization, and a strong electric field is formed at the inside of the semiconductor due to the polarized electric charge. Therefore, in case a ferroelectric layer is formed at the front surface of the solar cell, a surface passivation effect can be obtained.

    Specifically, pairs of electron and hole are formed at the inside of the semiconductor by way of the light from the outside, the electrons and the holes are separated from each other due to the electric potential difference made at the pn junction. In case a p type semiconductor is used as the substrate while making the pn junction through diffusion, the electric field at the pn junction is directed from the n type semiconductor to the p type semiconductor. Under the influence of the biased electric field, the electrons are flown to the n type semiconductor, whereas the holes are flown to the p type semiconductor, thereby producing the desired electric power.

    However, dangling bonds or impurities are much present at the surface of the semiconductor while serving to be a recombination center for the pairs of electron and hole.

    In case a ferroelectric layer is deposited onto the semiconductor substrate while forming a strong electric field at the surface thereof, the pairs of electron and hole inclined to reach the semiconductor surface are separated from each other due to the electric field formed by way of the ferroelectric layer, and hence, prevented from being recombined with each other. In this way, the ferroelectric layer enhances the surface passivation effect. Furthermore, with the deposition of the ferroelectric layer, the open-circuit voltage of the solar cell can be increased to a great scale while enhancing the energy efficiency thereof.

    For the same reason, in case a ferroelectric layer is positioned at the rear surface of the solar cell, the desired rear surface field effect can be obtained while increasing the open-circuit voltage of the solar cell and enhancing the energy efficiency thereof.

    ferroelectric, solar, cell

    The ferroelectric layer may further take a role of controlling the reflection of sunlight depending upon variation in the thickness thereof. When the thickness of the ferroelectric layer deposited onto the front surface of the solar cell is controlled to be 1000-1500 Å (Angstroms), it can take a role of an anti-reflection layer for reducing the light reflection rate to a large scale at the front of the solar cell. Similarly, the ferroelectric layer deposited onto the rear surface of the solar cell takes a role of rear surface reflection (BSR) for reflecting the light not absorbed in the semiconductor to the inside of the semiconductor again. In either case, the short circuit current is increased, thereby enhancing the energy efficiency of the solar cell.

    A second conductive type semiconductor layer 11 is formed on a first conductive type semiconductor substrate 10. This results in a pn structure where a pn junction is formed at the interface between the first conductive type semiconductor substrate 10 and the second conductive type semiconductor layer 11.

    Thereafter, a ferroelectric layer 13 is formed on the second conductive type semiconductor layer 11 through depositing a ferroelectric material. The ferroelectric material is selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), or SBT(SrBi2Ta2O7). The deposition is made through chemical vapor deposition, sol-gel process, sputtering, or pulse laser deposition. In order to crystallize the ferroelectric material, heat-treatment may be made after the deposition of the ferroelectric layer 13.

    In the case of the sol-gel process, the deposited ferroelectric layer is kept to be in an amorphous state. Therefore, in order to obtain a ferroelectric crystalline structure, it is necessary to heat-treat the deposited ferroelectric layer at 450-800° C. (Celsius). In the case of the chemical vapor deposition, the sputtering and the pulse laser deposition, the deposited ferroelectric layer bears a crystalline structure so that it is not necessary to make heat-treatment with respect to the deposited layer. But the heat-treatment may be made to enhance the performance characteristic of the ferroelectric layer.

    Thereafter, a poling electrode 14 is formed on at least a part of the ferroelectric layer 13. The target material for the poling electrode 14 may be determined depending upon the position of the ferroelectric layer. As shown in FIG. 1, when the ferroelectric layer 13 is positioned at the front surface of the solar cell, it is formed with a transparent conductive oxide material such as ITO, RuO2, SrRuO3, IrO2, and La1-xSrxCoO3.

    The ferroelectric layer 13 positioned at the front surface of the solar cell involves a surface passivation effect, and takes a role of an anti-reflection layer. Therefore, in the solar cell with such a ferroelectric layer, a layer based on CeO2, MgF2, TiO2, SiO2 or SiNx for an anti-reflection or a surface passivation may be omitted.

    Thereafter, another second type semiconductor layer 15 is formed on the rear surface of the semiconductor substrate 10, and a surface oxide layer 16 is formed on the semiconductor layer 15 to make the desired rear surface reflection.

    A plurality of grooves 17 a and 17 b are formed at the front and the rear surface of the solar cell with a predetermined depth. Heavily doped regions 18 a and 18 b where impurities are doped at high concentration are formed at the inner wall of the grooves 17 a and 17 b, and a conductive material fills the internal space of the grooves 17 a and 17 b to thereby form front electrodes 19 a and rear electrodes 19 b. Consequently, a solar cell is completed with a DSBC structure shown in FIG. 1.

    In operation, after the ferroelectric layer 13 is poled using the poling electrode 14 and the rear electrode 19 b, the electric power is extracted from the solar cell via the front electrode 19 a and the rear electrode 19 b. In case such a ferroelectric layer is placed at the rear surface of the solar cell, the ferroelectric layer 22 is poled using the poling electrode 23 and the front electrode 26 a, and the electric power is extracted from the solar cell via the front electrode 26 a and the rear electrode 26 b.

    As described above, when a ferroelectric layer is formed at the front or the rear surface of the solar cell, an internal electric field is formed at the surface of the semiconductor by way of the spontaneous polarization of the ferroelectric layer. Consequently, the pairs of electron and hole are separated from each other due to the electric field so that recombination of the electrons and holes is prevented, and the desired surface passivation effect is obtained due to the formation of a rear surface field.

    Accordingly, it becomes possible to omit an Al-based layer for forming the rear surface field while simplifying the structural components and the processing steps. Furthermore, even with the presence of the rear surface field formation layer, a ferroelectric layer maybe additionally formed on the rear surface field formation layer to maximize the surface passivation effect.

    When an Al-based layer is deposited to form the desired rear surface field in a conventional art, heat treatment at 800-900° C. should be made with respect to the Al-based layer. In case the Al-based layer is replaced by a PZT-based layer, the temperature of heat treatment for the PZT-based layer may be lowered to be 600° C. In this case, it can be prevented that the lifetime of the minority carriers is reduced due to the high temperature processing while deteriorating the efficiency of the solar cell.

    As described above, in case the internal electric field is formed at the surface of the semiconductor, the open-circuit voltage of the solar cell is increased so that the efficiency of the solar cell can be enhanced.

    As the ferroelectric layer conducts its function of controlling the reflection of light by varying the thickness thereof, the light reflection at the front surface of the solar cell is prohibited while re-directing the non-used light to the semiconductor at the rear surface of the cell.

    Accordingly, it becomes possible to omit an anti-reflection layer based on SiO2, Si3N4, CeO2 or MgF2 while simplifying the structural components and the processing steps.

    Furthermore, when the light reflection is controlled by way of the ferroelectric layer, the short-circuit current is increased while enhancing the efficiency of the solar cell.

    While the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and substitutions can be made thereto without departing from the spirit and scope of the present invention as set forth in the appended claims.

    Claims ( 20 )

    a semiconductor layer of a second conductive type formed on a front surface of said first conductive type semiconductor substrate, the first and the second conductive types being opposite to each other in polarity, a pn junction formed at the interface between said first conductive type semiconductor substrate and said second conductive type semiconductor layer;

    a ferroelectric layer formed on at least one of a front surface of said second conductive type semiconductor layer and a rear surface of said first conductive type semiconductor substrate; and

    The solar cell of claim 1. wherein said ferroelectric layer is formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), and SBT(SrBi2Ta2O7).

    The solar cell of claim 1. wherein said poling electrode is placed on said ferroelectric layer at the front surface of said second conductive type semiconductor layer and is formed with a transparent conductive oxide material.

    The solar cell of claim 3. wherein said transparent conductive oxide material for said poling electrode is selected from ITO (indium tin oxide), RuO2, SrRuO3, IrO2, and La1-xSrxCoO3.

    The solar cell of claim 1. wherein said poling electrode is placed on said ferroelectric layer at the rear surface of said first conductive semiconductor substrate and is formed with a metallic electrode material selected from Al, Cu, Ag, and Pt.

    a semiconductor layer of a second conductive type formed on a front surface of said first conductive type semiconductor substrate, the first and the second conductive types being opposite to each other in polarity, a pn junction formed at the interface between said first conductive type semiconductor substrate and said second conductive type semiconductor layer;

    a surface oxide layer formed on a rear surface of said first conductive type semiconductor substrate;

    a plurality of grooves formed at a front surface of said ferroelectric layer and a rear surface of said surface oxide layer, with a predetermined depth to expose said semiconductor substrate; and

    a heavily doped region of a second conductive type between said front electrodes and an internal surface of the plurality of grooves formed at said front surface of said ferroelectric layer; and

    a heavily doped region of a first conductive type between said rear electrodes and an internal surface of the plurality of grooves formed at the rear surface of said surface oxide layer.

    The solar cell of claim 6. wherein the ferroelectric layer is formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3) and SBT(SrBi2Ta2O7).

    The solar cell of claim 6. wherein said poling electrode is formed with a transparent conductive oxide material.

    The solar cell of claim 9. wherein said transparent conductive oxide material for said poling electrode is selected from ITO (indium tin oxide), RuO2, SrRuO3, IrO2, and La1−xSrxCoO3.

    a semiconductor layer of a second conductive type formed on a front surface of said first conductive type semiconductor substrate, the first and the second conductive types being opposite to each other in polarity, a pn junction formed at the interface between said first conductive type semiconductor substrate and said second conductive type semiconductor layer;

    a ferroelectric layer formed on a rear surface of said first conductive type semiconductor substrate;

    a plurality of grooves formed at a front surface of said second conductive type semiconductor layer and a rear surface of said ferroelectric layer, with a predetermined depth to expose said semiconductor substrate; and

    a heavily doped region of a second conductive type between said front electrodes and an internal surface of the plurality of grooves formed at the front surface of said second conductive type semiconductor layer; and

    a heavily doped region of a first conductive type between said rear electrodes and an internal surface of the plurality of grooves formed at the rear surface of said ferroelectric layer.

    The solar cell of claim 11. wherein said ferroelectric layer is formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3) and SBT(SrBi2Ta2O7).

    The solar cell of claim 11. wherein said poling electrode is formed with a metallic electrode material selected from Al, Cu, Ag, and Pt.

    a semiconductor layer of a second conductive type formed on a front surface of said first conductive type semiconductor substrate, the first and the second conductive types being opposite to each other in polarity, a pn junction formed at the interface between said first conductive type semiconductor substrate and said second conductive type semiconductor layer; and

    a ferroelectric layer formed on at least one of a front surface of said second conductive type semiconductor layer and a rear surface of said first conductive type semiconductor substrate.

    The solar cell of claim 15. further comprising a poling electrode formed on at least a portion of said ferroelectric layer.

    The solar cell of claim 16. further comprised of said ferroelectric layer being formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), and SBT(SrBi2Ta2O7).

    The solar cell of claim 16. further comprised of said poling electrode being placed on said ferroelectric layer at the front surface of said second conductive type semiconductor layer and being formed with a transparent conductive oxide material.

    The solar cell of claim 18. with said transparent conductive oxide material for said poling electrode being selected from ITO (indium tin oxide), RuO2, SrRuO3, IrO2, and La1−xSrxCoO3.

    The solar cell of claim 16. said poling electrode being placed on said ferroelectric layer at the rear surface of said first conductive semiconductor substrate and being formed with a metallic electrode material selected from Al, Cu, Ag, and Pt.

    US10/144,787 2001-08-24 2002-05-15 Solar cell using ferroelectric material(s) Expired. Lifetime US6639143B2 ( en )

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    Press Release: Lead halide perovskites are not ferroelectric: further research is needed to discover why they are so good for solar cells

    In a solar cell, when the sunlight impacts the material, a charge is generated. Specifically, this charge corresponds to an electron-hole pair, where an electron is excited to the conduction Band, leaving a hole in the valence Band. For the cells to be efficient, this pair of charges has to be separated and extracted as efficiently as possible (electron and hole must be directed to opposite electrodes to be captured) to generate an electric current. This is where ferroelectricity comes into play: this property would generate a built-in electric field in the material that could assist charge separation.

    In the particular case of lead halide perovskites, ferroelectricity could help to understand why they work so well as active material in solar cells, and in fact, that was a plausible explanation so far. However, the study published in Energy Environmental Science by researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC) and the Helmholtz-Zentrum Berlin für Materialien und Energie (Germany) demonstrate, for the first time, that the fact that these materials are optimal for solar cells is not due to ferroelectricity. This work is very interesting for understanding why these cells are so efficient, says Andrés Gómez, researcher at the ICMAB-CSIC and first author of the article. We will have to keep looking for the final answer.

    Figure 1: Scan direction dependence of the DPFM signals. (a) Scheme of the DPFM measurement of a ferroelectric sample (top left panel), with an antiparallel domain configuration, in which “Pdw” stands for “polarisation down” and “Pup” for “polarisation up”. Upon application of a suitable mechanical load, a negative charge is built up by the piezoelectric effect on the left side while a positive charge is built up at the right side (bottom left panel). The signal recorded, that is the current, is the derivative of the charge and it reverses its sign when the tip crosses different domains, depending upon the scan direction: the tip going from left to right (top right panel) and from right to left (bottom right panel). (b) DPFM images obtained for periodically poled lithium niobate (PPLN) with an antiparallel domain configuration. Scale bar: 5 μm. For PPLN, the current sign is reversed as the scan direction changes, exactly as expected for a ferroelectric domain structure. (c) DPFM images of the CsFAMA perovskite scanned under similar conditions to those of PPLN. Scale bar: 5 μm. The CsFAMA perovskite does not show a sign reversal of the DPFM signal, but rather the images for the two scan directions (left and right panels) are quite similar, which resemble typical current-sensing AFM mapping. (d) Random profiles extracted from PPLN (top panel) and the CsFAMA perovskite (bottom panel).

    The secret: the new technique used

    The technique used to elucidate the non-ferroelectricity of lead halide perovskites is the DPFM (direct piezoelectric force microscopy) technique. A patent application describing the characterization technique was filed in 2017 by ICMAB-CSIC researchers. Until now there was only one advanced mode of atomic force microscopy (AFM) called piezoresponse force microscopy (PFM) to study the ferroelectricity of these samples. However, this mode has caused a lot of controversy, as it is not reliable enough to distinguish between a ferroelectric material and one which is not. Although it is possible to measure ferroelectricity with PFM, other effects can give a false signal, obtaining erroneous results, explains Gómez.

    However, the DPFM technique, introduced in 2017 at the ICMAB-CSIC, complementary to PFM, measures the piezoelectric effect in a direct way and allows to clearly discern if a sample is ferroelectric or not. The technique does not produce spurious signals, since it excludes many measuring artefacts because via the piezoelectric effect a mechanical energy is directly converted into electrical energy in a strictly proportional way. This fact is fundamental to be able to examine the existence of ferroelectricity in lead halide perovskites, an issue that has been under debate for several years.

    For this study, polycrystalline samples of lead halide perovskites and samples of other materials with known ferroelectricity used as control were analyzed, and experiments were conducted with perovskites with different properties (grain size, layer thickness, different substrates, different textures. ) using PFM and DPFM, and even EFM (electrostatic force microscopy).

    This is the first time that the DPFM technique is used in lead halide perovskite solar cells. No other research group has been able, with nanometer-scale resolution, to elucidate whether these cells are really ferroelectric or not, says Gómez. Now we know.

    Figure Cover: We use the sun cream to indicate the Ferroelectricity free perovskites. The percentage is the efficiency of the perovskites, and the acronym is the type of perovskites. The crystal structure of each perovskite is represented.

    Article

    Andrés Gómez, Qiong Wang, Alejandro R. Goñi, Mariano Campoy-Quiles and Antonio Abate. Ferroelectricity-free lead halide perovskites. Energy Environ. Sci., 2019, Advance Article. DOI: 10.1039/C9EE00884E

    Patent

    Device and method for mapping ferroelectric and/or piezoelectric samples. EP3285075A1. European Patent Office. Andrés Gómez, Martí Gich, Teresea Puig, Xavier Obradors, Adrián Carretero.

    Could ferroelectrics improve solar energy conversion efficiencies?

    To complement her seminar “Could ferroelectrics improve solar energy conversion efficiencies?” Madeleine Morris has written a blog post for us on her work and around the topics she discussed. You can download the slides from her talk [PDF] as well.

    Solar energy – Where are we at?

    Solar is becoming an increasingly viable source of renewable energy thanks in part to both increasing device efficiencies and a Rapid decrease in production costs of photovoltaic (PV) panels. Despite the drop in price, however, fabrication of silicon-based solar technologies remains a highly energy intensive and relatively expensive process. Furthermore, the efficiencies of these mature technologies, although impressive, has somewhat plateaued in the last 20 or so years. [1] The field of solar energy research, on the other hand, has not. A number of ‘emerging’ PV technologies have appeared as researchers across the world attempt to harness the Sun’s vast energy source in cost-effective, scalable and efficient ways.

    Emerging PV technologies

    ‘Plastic electronics’ is one area which has received much attention, particularly at Imperial College London. In fact there’s a whole research centre dedicated to it: the Centre for Plastic Electronics (CPE). So-called because of the organic (that is, carbon-based) semiconducting polymers and molecules it utilises, this field concentrates on solution-processable materials. The aim is to fabricate electronic devices, such as solar cells, which are low-cost and can be produced via large-scale processing techniques, unlike their silicon-based counterparts. And it’s going reasonably well – improvements have been and are still being made. However, efficiencies of organic photovoltaics (OPVs) are on the order of 10% and so it’s just not quite efficient enough yet to allow large-scale market penetration.

    Solar Fuels: the ‘other’ solar energy

    Even disregarding these low efficiencies, photovoltaics alone cannot solve the world’s energy crisis – they generate electricity when the sun is up and it must either be used immediately or stored until needed. In addition, less than 20% of the world’s final energy consumption is in the form of electricity, [2] so in addition to cleaning up our electricity generation, we must consider the source of the rest of our energy consumption. Around 20% of the world’s energy usage is in transport, [3] and thus if we are to have any hope of meeting carbon emission targets we must find a clean, storable and portable alternative to fossil fuels. Electric vehicles are making excellent progress, but may not be able to cope with long-distance or flight travel. Our current infrastructure relies on burnable fuels – it could therefore be much easier and more sensible to find a way to generate a clean chemical fuel which we can integrate into our economy the way it is. Hydrogen is considered by some as the ‘ultimate’ clean fuel, and is already being utilised as a fuel in some vehicles. The problem, however, is that the overwhelming majority (~96%!) of this hydrogen is derived from fossil fuels sources, mostly steam reforming of natural gas. [4] This means that hydrogen production is currently a net contributor to carbon emissions, [5] and thus is far from being a sustainable fuel source. So, not only do we need to find a fuel that is clean upon burning, but it needs to be able to be produced in a sustainable way.

    The field of solar fuels is dedicated to just that. Also known as ‘artificial photosynthesis,’ it aims to find a way to use just sunlight, water and air to make carbon-free or.neutral fuels by mimicking the solar to chemical energy conversion process which plants have been utilising for millions of years. Solar water splitting, for example, uses the Sun’s energy to break water into its elements: oxygen and hydrogen.

    And there we have it – a zero-carbon fuel produced in a clean way. Alternatively, the hydrogen could be used to convert CO2 into useful carbon based molecules, which could be utilised as net carbon-neutral fuels. Unfortunately (or rather fortunately for life on Earth!), the splitting of water is a chemically challenging process: it doesn’t occur spontaneously and so we need catalysts to make it possible. Solar fuels researchers are dedicated to finding materials which are cheap, earth abundant and non-toxic which can facilitate the chemical reactions we need. We can do it, but like the organic PV field, efficiencies of this technology have much room for improvement. In fact, solar fuels conversion efficiencies are typically lower than 1%.

    Fatal attraction

    So why are efficiencies in both OPV and solar fuels devices so low? One of the biggest problems is a process called ‘recombination.’ In order harness the energy from solar irradiation, a photon of light must first be absorbed by a semiconductor. This creates an electron and a hole (the positive counterpart of a negative electron), the charge carriers required to convert the solar energy into electricity or chemical energy. Next comes the hard bit: we need to successfully separate the electron and hole and get them to opposite sides of a device before can we actually utilise the solar energy. The trouble, though, is that the oppositely charged carriers are coulombically attracted to each other – if they ‘feel’ each other they will recombine, and we will have lost the energy of the absorbed photon.

    So in both OPVs and water splitting systems we need a driving force to pull charge carriers apart in order to make use of them. Clever ways of modifying device architecture have been developed, but tend to make fabrication processes more complex. We could also just apply an external electrical bias across the device which can be very effective for reducing recombination. But this poses logistical problems when it comes to commercial applications of these devices. And anyway, aren’t we trying to get energy out of our devices, rather than putting it in?

    How could ferroelectrics help?

    This is where a certain class of materials called ‘ferroelectrics’ come into the picture, which are the FOCUS of my PhD research. These materials possess a permanent internal electric field within the material. A bit like a battery, one side will therefore be slightly positive and the other slightly negative. This internal electric field will drive electrons and holes in opposite directions in a material and should therefore reduce the likelihood of them recombining.

    This could have big implications for PV devices as it would allow us to generate both larger currents and voltages. And for solar fuels it could be even more significant – an added difficulty of water splitting is the fact that we generate oxygen and hydrogen on the surface of a material, which will readily react with each other in a rather explosive manner. So, if we have electrons and holes going to opposite sides of the material, we’ll generate the two products in spatially distinct areas. This has the potential not only to increase efficiencies, but also make gas collection much simpler.

    I’ve just told you that the internal fields in ferroelectric materials have the potential to massively reduce recombination rates in semiconductors and improve device efficiencies. But does it really work like that?

    How can I help?

    In my PhD research I aim to answer this question. I use a spectroscopic technique to monitor how long electrons and holes stick around for in ferroelectric materials before the recombine. By manipulating the internal fields inside the materials whilst doing this, I can investigate what effect they have on the recombination rates. The aim is to understand what goes on in these materials when we shine light on them, and use this knowledge to design more efficient devices.

    ferroelectric, solar, cell

    So far I’ve found that recombination rates in barium titanate, an archetypal ferroelectric semiconducting material, are vastly slower than in non-ferroelectric materials with similar structures – on the order of hundreds of milliseconds rather than microseconds. What’s more, I’ve found that if I eliminate the internal fields in the barium titanate, the recombination gets several orders of magnitude faster. This strongly indicates that internal fields make a huge impact on the behaviour of photogenerated charges. [6]

    What’s the catch?

    Have I made a super-efficient solar cell out of barium titanate, then? Unfortunately, it’s not as simple as that. We have to consider several other factors when choosing the material(s) for our devices. Such as do they absorb visible light? Can electrons and holes move through the material easily so that they can be collected? In the case of barium titanate, and many other ferroelectric semiconductors, the answer to both of these questions is a resounding no. But it’s not all doom and gloom. Some interesting studies have shown that if you put a thin layer of a non-ferroelectric material on top of barium titanate, you can make it behave like a ferroelectric itself. So we could take a light absorbing material which usually exhibits fast recombination and put it on top of something ferroelectric and slow down the recombination. There’s also work being put into finding, or even creating, new ferroelectric materials which tick all the boxes – visible light absorbing, allow efficient transport of charges through the material and exhibit long carrier lifetimes.

    Conclusion

    In summary, the fields of both emerging PV and solar fuels technologies are extremely active right now. It’s an exciting time, but also a critical one. I don’t think I need to convince anyone here of the urgency of eliminating fossil fuel usage. There is a huge effort across the globe to realise this goal. To do this we must find ways to push device efficiencies up, and soon. Understanding and utilising ferroelectric materials for solar energy conversion could help to do this.

    References

    [2] International Energy Agency, Key World Energy Statistics 2015, 2015.

    [4] M. Pagliaro, A. G. Konstandopoulos, in Solar Hydrogen: Fuel of the Future, Royal Society Of Chemistry, Cambridge, 2012, pp. 1–39.

    [6] M. R. Morris, S. R. Pendlebury, J. Hong, S. Dunn, J. R. Durrant, Adv. Mater. 2016, Accepted. Return to top

    Review on Ferroelectrics in Developing Novel Solar Cells

    The study in ferroelectric photovoltaic (PV) effect has become an important topic in fields of condensed matter physics and materials sciences in recent years. For the rapidly growing needs in solar energy, the interest in developing ferroelectric PV devices with specific properties has drastically increased. This paper firstly focuses on the fundamental aspects of PV effect in ferroelectric materials, and presents a review on the latest developments in ferroelectric PV cells. For the application of ferroelectrics on solar cells, relevant patents are also presented and discussed, to describe the main characteristics of ferroelectric PV cells and achieve the understanding on their future development.

    Recent Patents on Materials Science

    Title:Review on Ferroelectrics in Developing Novel Solar Cells

    Volume: 5 Issue: 2

    Author(s): Jingzhong Xiao, Jose. A. Paixiao, Maria M. R. Costa and Dunlu Sun

    Affiliation:

    Abstract: The study in ferroelectric photovoltaic (PV) effect has become an important topic in fields of condensed matter physics and materials sciences in recent years. For the rapidly growing needs in solar energy, the interest in developing ferroelectric PV devices with specific properties has drastically increased. This paper firstly focuses on the fundamental aspects of PV effect in ferroelectric materials, and presents a review on the latest developments in ferroelectric PV cells. For the application of ferroelectrics on solar cells, relevant patents are also presented and discussed, to describe the main characteristics of ferroelectric PV cells and achieve the understanding on their future development.

    About this article

    Cite this article as:

    Xiao Jingzhong, A. Paixiao Jose., M. R. Costa Maria and Sun Dunlu, Review on Ferroelectrics in Developing Novel Solar Cells, Recent Patents on Materials Science 2012; 5(2). https://dx.doi.org/10.2174/1874464811205020159

    DOIhttps://dx.doi.org/10.2174/1874464811205020159 Print ISSN1874-4648
    Publisher NameBentham Science Publisher Online ISSN1874-4656

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    Scientists Grow Lead-Free Solar Material With Built-In Switch

    Solar panels, also known as photovoltaics, rely on semiconductor devices, or solar cells, to convert energy from the sun into electricity.

    To generate electricity, solar cells need an electric field to separate positive charges from negative charges. To get this field, manufacturers typically dope the solar cell with chemicals so that one layer of the device bears a positive charge and another layer a negative charge. This multilayered design ensures that electrons flow from the negative side of a device to the positive side — a key factor in device stability and performance. But chemical doping and layered synthesis also add extra costly steps in solar cell manufacturing.

    Light microscopy image of nanowires, 100 to 1,000 nanometers in diameter, grown from cesium germanium tribromide (CGB) on a mica substrate. The CGB nanowires are samples of a new lead-free halide perovskite solar material that is also ferroelectric. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

    Now, a research team led by scientists at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with UC Berkeley, has demonstrated a unique workaround that offers a simpler approach to solar cell manufacturing: A crystalline solar material with a built-in electric field — a property enabled by what scientists call “ferroelectricity.” The material was reported earlier this year in the journal Science Advances.

    The new ferroelectric material — which is grown in the lab from cesium germanium tribromide (CsGeBr3 or CGB) — opens the door to an easier approach to making solar cell devices. Unlike conventional solar materials, CGB crystals are inherently polarized, where one side of the crystal builds up positive charges and the other side builds up negative charges, no doping required.

    In addition to being ferroelectric, CGB is also a lead-free “halide perovskite,” an emerging class of solar materials that have intrigued researchers for their affordability and ease of synthesis compared to silicon. But many of the best-performing halide perovskites naturally contain the element lead. According to other researchers, lead remnants from perovskite solar material production and disposal could contaminate the environment and present public health concerns. For these reasons, researchers have sought new halide perovskite formulations that eschew lead without compromising performance.

    Peidong Yang, Faculty Senior Scientist, Berkeley Lab, adjusts the settings on a probe station, which tests nano wire connectivity, at his lab at Hildebrand Hall, UC Berkeley campus, Berkeley, California, 08/24/2022. Yang recently led a research team in the development of a lead-free perovskite solar material with a built-in electric field. The advance offers a more sustainable approach to solar cell manufacturing. Yang is a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of chemistry and materials science and engineering at UC Berkeley.

    CGB could also advance a new generation of switching devices, sensors, and super-stable memory devices that respond to light, said co-senior author Ramamoorthy Ramesh, who held titles of senior faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of materials science and engineering at UC Berkeley at the time of the study and is now vice president of research at Rice University.

    Perovskite solar films are typically made using low-cost solution-coating methods, such as spin coating or ink jet printing. And unlike silicon, which requires a processing temperature of about 2,732 degrees Fahrenheit to manufacture into a solar device, perovskites are easily processed from solution at room temperature to around 300 degrees Fahrenheit — and for manufacturers, these lower processing temperatures would dramatically reduce energy costs.

    But despite their potential boost to the solar energy sector, perovskite solar materials won’t be market-ready until researchers overcome long-standing challenges in product synthesis and stability, and material sustainability.

    Pinning down the perfect ferroelectric perovskite

    Perovskites crystallize from three different elements; and each perovskite crystal is delineated by the chemical formula ABX3.

    Most perovskite solar materials are not ferroelectric because their crystalline atomic structure is symmetrical, like a snowflake. In the past couple of decades, renewable energy researchers like Ramesh and Yang have been on the hunt for exotic perovskites with ferroelectric potential — specifically, asymmetrical perovskites.

    A few years ago, first author Ye Zhang, who was a UC Berkeley graduate student researcher in Yang’s lab at the time, wondered how she could make a lead-free ferroelectric perovskite. She theorized that placing a germanium atom in the center of a perovskite would distort its crystallinity just enough to engender ferroelectricity. On top of that, a germanium-based perovskite would free the material of lead. (Zhang is now a postdoctoral researcher at Northwestern University.)

    Scanning electron microscopy image of CGB nanowires, 100 to 1,000 nanometers in diameter, grown on a silicon substrate via a technique called chemical vapor transport. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

    But even though Zhang had honed in on germanium, there were still uncertainties. After all, conjuring up the best lead-free, ferroelectric perovskite formula is like finding a needle in a haystack. There are thousands of possible formulations.

    So Yang, Zhang, and team partnered with Sinéad Griffin, a staff scientist in Berkeley Lab’s Molecular Foundry and Materials Sciences Division who specializes in the design of new materials for a variety of applications, including quantum computing and microelectronics.

    With support from the Materials Project, Griffin used supercomputers at the National Energy Research Scientific Computing Center (NERSC) to perform advanced theoretical calculations based on a method known as density-functional theory.

    Through these calculations, which take atomic structure and chemical species as input and can predict properties such as the electronic structure and ferroelectricity, Griffin and her team zeroed in on CGB, the only all-inorganic perovskite that checked off all the boxes on the researchers’ ferroelectric perovskite wish list: Is it asymmetrical? Yes, its atomic structure looks like a rhombohedran, rectangle’s crooked cousin. Is it really a perovskite? Yes, its chemical formula — CeGeBr3 — matches the perovskite’s telltale structure of ABX3.

    The researchers theorized that the asymmetric placement of germanium in the center of the crystal would create a potential that, like an electric field, separates positive electrons from negative electrons to produce electricity. But were they right?

    Measuring CGB’s ferroelectric potential

    To find out, Zhang grew tiny nanowires (100 to 1,000 nanometers in diameter) and nanoplates (around 200 to 600 nanometers thick and 10 microns wide) of single-crystalline CGB with exceptional control and precision.

    “My lab has been trying to figure out how to replace lead with less toxic materials for many years,” said Yang. “Ye developed an amazing technique to grow single-crystal germanium halide perovskites — and it’s a beautiful platform for studying ferroelectricity.”

    X-ray experiments at the Advanced Light Source revealed CGB’s asymmetrical crystalline structure, a signal of ferroelectricity. Electron microscopy experiments led by Xiaoqing Pan at UC Irvine uncovered more evidence of CGB’s ferroelectricity: a “displaced” atomic structure offset by the germanium center.

    Meanwhile, electrical measurement experiments carried out in the Ramesh lab by Zhang and Eric Parsonnet, a UC Berkeley physics graduate student researcher and co-author on the study, revealed a switchable polarity in CGB, satisfying yet another requirement for ferroelectricity.

    But a final experiment — photoconductivity measurements in Yang’s UC Berkeley lab — yielded a delightful result, and a surprise. The researchers found that CGB’s light absorption is tunable — spanning the spectrum of visible to ultraviolet light (1.6 to 3 electron volts), an ideal range for coaxing high energy conversion efficiencies in a solar cell, Yang said. Such tunability is rarely found in traditional ferroelectrics, he noted.

    Yang says there is still more work to be done before the CGB material can make its debut in a commercial solar device, but he’s excited by their results so far. “This ferroelectric perovskite material, which is essentially a salt, is surprisingly versatile,” he said. “We look forward to testing its true potential in a real photovoltaic device.”

    This research was supported by the U.S. Department of Energy (DOE) Office of Science.

    The Advanced Light Source, Molecular Foundry, and NERSC are DOE Office of Science user facilities at Berkeley Lab.

    Lawrence Berkeley National Laboratory and its scientists have been recognized with 14 Nobel Prizes.

    DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.

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