WO2010014105A1 - Composition and method of preparing nanoscale thin film photovoltaic materials - Google Patents

Composition and method of preparing nanoscale thin film photovoltaic materials Download PDF

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Publication number
WO2010014105A1
WO2010014105A1 PCT/US2008/071845 US2008071845W WO2010014105A1 WO 2010014105 A1 WO2010014105 A1 WO 2010014105A1 US 2008071845 W US2008071845 W US 2008071845W WO 2010014105 A1 WO2010014105 A1 WO 2010014105A1
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Prior art keywords
layer
metal alloy
photovoltaic cell
alloy nanoparticles
preparation
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PCT/US2008/071845
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French (fr)
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R. Douglas Carpenter
Kevin D. Maloney
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Quantumsphere, Inc.
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Priority to PCT/US2008/071845 priority Critical patent/WO2010014105A1/en
Publication of WO2010014105A1 publication Critical patent/WO2010014105A1/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
    • H01L31/0248—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 characterised by their semiconductor bodies
    • H01L31/0256—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 characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0322—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising only AIBIIICVI chalcopyrite compounds, e.g. Cu In Se2, Cu Ga Se2, Cu In Ga Se2
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • 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
    • H01L31/0248—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 characterised by their semiconductor bodies
    • H01L31/0352—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 characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • 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 potential barriers
    • H01L31/072—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 potential barriers the potential barriers being only of the PN heterojunction type
    • 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
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H01L31/02168—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells the coatings being antireflective or having enhancing optical properties for the solar cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/541—CuInSe2 material PV cells

Definitions

  • the inventions disclosed herein relate generally to the manufacture of materials for thin film photovoltaic cells. More specifically, the invention relates to an improved production process for making the active absorbing material containing metal alloy nanoparticles that allows for increased efficiency, reduced cost, and reduced weight.
  • Related Art
  • a photovoltaic cell is a device that converts light energy directly into electricity.
  • the high cost of polysilicon and resultant high cost of silicon solar cells has prevented widespread use of solar energy.
  • Recent advances in low cost, high efficiency, thin film polycrystalline solar cells based on copper-indium-gallium-selenium-sulfide (CIGS) absorption layers promises to make solar energy competitive with energy derived from fossil fuels. Although these materials have some of the highest efficiencies of all classes of solar cells, exceeding 15%, several steps in the production process of CIGS solar cells are toxic and/or expensive. Additionally, with thicker active CIGS layers in a photovoltaic device, there is an increase chanced of layer defects that could lower overall cell efficiency.
  • a photovoltaic cell has been prepared incorporating a photon-absorbing layer on an electrically conductive substrate in which the photon absorbing layer is comprised of metal alloy nanoparticles having the formula, for example, Cu 1 In ⁇ x Ga x . where x equals from 0 to 1.
  • the metal alloy nanoparticles are heated on the substrate in the presence of elements such as, for example, selenium and/or sulfur to a temperature sufficiently high to permit reaction, and preferably, become fused together to form a thin layer on the electrically conductive substrate to provide at least a portion of an electrical circuit that permits the flow of electrons.
  • this layer is less than 1 micron in thickness and, more preferably, the layer is less than about 500 nanometers.
  • a preparation suitable for use in a photonic-energy application comprises metal alloy nanoparticles configured to react with at least one material selected from Group Va or Group Via of the periodic table when the at least one material is in a non-gaseous state.
  • the metal alloy nanoparticles comprise at least one metal selected from Group Ib, Hb, or Ilia of the periodic table.
  • a substantial portion of the metal alloy nanoparticles can have a diameter less than about 50 nm. At least some of the nanoparticles can comprise a metal alloy or metal alloy oxide core and an oxide shell.
  • the metal alloy nanoparticles can be prepared by a vapor condensation process. At least a portion of the metal alloy nanoparticles can comprise at least one of copper, indium, and gallium. At least a portion of the metal alloy nanoparticles can have a composition according to the formula Cu 1 In] ⁇ Ga 1 , wherein x is a number from 0 to 1.
  • an assembly for a photovoltaic cell comprising an electrically-conductive substrate treated with at least one preparation as described herein and at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
  • the non-gaseous phase composition can comprise selenium or sulfur.
  • the electrically-conductive substrate can be a metal foil.
  • the electrically-conductive substrate can comprise molybdenum.
  • a photovoltaic cell comprising an assembly for a photovoltaic cell as described herein and an electrical contact.
  • the electrically-conductive substrate can be adjacent a base layer comprising glass, metal foil, or plastic.
  • the photovoltaic cell can further comprise an electron- transporting emission layer.
  • the emission layer can comprise cadmium sulfide.
  • the photovoltaic cell can further comprise an anti-reflective coating.
  • the anti-reflective coating can comprise zinc oxide.
  • the photovoltaic cell can further comprise an environmental protection layer.
  • a method of making photon-absorbing material comprising reacting at least one preparation as described herein with at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
  • the non-gaseous phase composition can comprise selenium or sulfur.
  • the method can comprise layering preparations as described herein, wherein each preparation layer has a different composition of metal alloy nanoparticles than its adjacent layer. Each layer can have a different gallium concentration than its adjacent layer.
  • a copper-indium-gallium (CIG) alloy was prepared utilizing facile manufacturing conditions.
  • copper-indium-gallium alloy can be selenized and/or sulfidized with elemental selenium and/or sulfur to form a photon-absorbing material, where by the resulting layer has a thickness of no more than about 1 micron, but preferably much less than 1 micron.
  • metal alloy nanoparticles and their oxides can be utilized for further alloying under favorable reaction conditions.
  • Metal alloy nanoparticles used in the described method in the preferred embodiments are from groups IB, HB, or JIIA on the periodic table have a diameter of less than 50 nm. More preferably, the metal alloy nanoparticles are comprised of copper and indium, and most preferably copper-indium-gallium (CIG).
  • a photovoltaic device comprises an emitting layer is applied to the photon-absorbing layer.
  • the emitting layer is comprised of a material that is highly efficient at electron transport from the photon- absorbing layer, and most preferably comprises cadmium sulfide or similar molecule.
  • an anti -reflective coating may be applied.
  • the anti -reflective coating is both optically and electrically conductive to permit sunlight to reach the emitting layer effectively.
  • the anti -reflective coating may preferably be zinc oxide.
  • an environmental protection layer is provided to provide weather-resistant properties to the device.
  • the environmental protection layer has optical and electrical conductive property, and may preferably comprise low-iron glass.
  • a method of preparing a photon-absorbing layer of nanosized material comprises heating metal alloy nanoparticles, prepared for example from a vapor condensation process, with at least one element selected from Groups VA and/or VIA on an electrically conductive substrate.
  • the nanosized material in the photon-absorbing layer is prepared by a vapor condensation process.
  • An example of such a process is described in U.S. Patent No. 7,282,167, which is incorporated herein in its entirety by reference.
  • Other methods for obtaining beneficial photon-absorbing layers for use in effective photovoltaic devices may be employed.
  • the composition is heated sufficiently high to permit reaction and create a substantially fused layer of nanosized particles. More preferably, the resulting layer is photon-absorbing for effective use in a photovoltaic device, and may comprise chalcopyrite.
  • a substantial portion of the metal alloy nanoparticles used in the method are less than 100 run, and most preferably less than 50 nm. Utilization of the preferred particle size increases uniformity of the resulting layer after the heating step.
  • the mixture be heated to a temperature such that there is sufficient reaction between the metal alloy nanoparticles and at least one element selected from groups VA and/or VIA. Most preferably, the temperature should be at least 250 0 C.
  • the temperature must be sufficient form a substantially fused layer of nanoparticles. It is more preferable that the layer be uniform and thin, most preferably less than 500 nm in thickness.
  • the composition of a photovoltaic absorbing chalcopyrite material prepared from metal alloy nanoparticles are comprised of at least copper and indium, and more preferably copper, indium, and gallium.
  • a composition comprising metal alloy nanoparticles prepared from a vapor condensation process can be prepared for use in an electronic device.
  • the metal alloy nanoparticles are preferably comprised from at least one metal from Groups IB, IIB, and/or MA, and are most preferably at least one of copper, indium, and/or gallium.
  • the metal alloy nanoparticles should have sufficiently high reactivity to permit reaction with elements from either Group VA and/or VIA in the gas, most preferably with selenium and/or sulfur in the liquid, or solid state.
  • the particles should have a size of less than 100 nm, and most preferably less than 50 nm. At least some of the metal alloy nanoparticles have an oxide shell.
  • a photovoltaic cell comprising a photon- absorbing layer, electronically conductive substrate, emitting layer, and anti-reflective coating.
  • the photon-absorbing layer is preferably comprised of copper-indium nanoparticles, and most preferably comprised of copper-indium -gallium nanoparticles.
  • the nanoparticles are substantially fused together, prepared by heating metal alloy nanoparticles sufficiently to permit reaction preferably with material from either Group VA and/or VIA, and most preferably with selenium and/or sulfur.
  • the photon-absorbing layer is supported on an electronically conductive substrate which provides a portion of an electrical circuit in combination with the photon- absorbing layer.
  • this layer is thin and continuous, and most preferably less than 500 nm thick.
  • An emitting layer is applied directly to the photon-absorbing layer.
  • the emitting layer is comprised of a material that is highly efficient at electron transport from the photon-absorbing layer, most preferably cadmium sulfide.
  • An anti- reflective coating is applied directly to the emitting layer.
  • an anti-reflective coating is both optically and electrically conductive to permit sunlight to enter the emitting layer, and most preferably is zinc oxide.
  • the composition may also comprise an environmental protection layer.
  • this layer is comprised of material that reduces damage cause by weathering, and is most preferably composed of low-iron glass.
  • Some of the preferred embodiments detail stratified layers of metal alloy nanoparticles, comprising the formula Cu]Ini. x Ga x wherein x can vary from 0 to 1.
  • the gallium concentration in at least one of the stratifications is different from the gallium concentration in another stratification, and most preferably the concentration of gallium is lower proximal to the emitting layer.
  • At least some of the preferred embodiments describe at least three and up to twenty stratifications.
  • the layer thickness of all stratifications combined should be thin and continuous, and most preferably the combined thickness is less than 500 nanometers.
  • Figure 1 is a schematic of a typical thin film solar cell.
  • Figure 2 is a schematic of a thin film solar cell described in some of the embodiments.
  • a photovoltaic (PV) cell is a device that converts solar energy directly into electricity. While there are several different classes of solar cells, the present invention has particular but not exclusive applicability to thin film solar cells made from materials such as copper-indium-gallium diselenide (CIGS) or copper-mdium-gallium-selenium sulfide (CIGSS). Unlike traditional Si-based solar cells, ClGS and CIGSS cells are flexible and are more acceptable for a wider variety of surface profiles, such as curved or contoured surfaces.
  • the diagram in Figure 1 shows at least some of the different layers in, for example, a ClGS- or CIGSS-based solar cell.
  • Base material 101 may be glass or metal foil, although a material having some plastic and/or elastic characteristic is preferable so that the cells permit increased flexibility.
  • substrate foil 102 may be deposited and can be used as a back contact.
  • the substrate foil 102 is preferably a metal foil and may preferably comprise molybdenum.
  • a photon-absorbing CIGS or CIGSS layer 103 may then be deposited onto foil 102.
  • the thickness of this layer is highly dependent on how CIGS is applied to the surface. While the thickness of a typical CIGS cell is about two or so microns, the present inventive photon-absorbing layer 103 has an average thickness of less than one micron and preferably less than about 500 nm on average and most preferably a maximum thickness of about 500 nanometers.
  • the CIGS layer is preferably formed as a p-type, photon- absorbing, layer based upon the particular arrangement of copper, indium, and gallium atoms.
  • an n-type electron transporting emission layer 104 can be applied to the photon-absorbing layer 103, preferably a layer comprising cadmium sulfide.
  • An anti-reflective coating of zinc oxide 105 may be applied to the emission layer 104.
  • the anti-reflective layer is both electrically and optically conductive, allowing photons to reach the photon- absorbing layer 103.
  • Electrical contact 106 may be applied to complete circuit 107 with foil 102 to collect and use the energy gained from light absorption.
  • an environmental protection layer 108 may be placed on top the anti-reflective coating 106 and electrical contact 105 to minimize the effects of weathering of the photovoltaic device.
  • the present invention benefits from increased surface area of the reactive metal alloy nanoparticles, as compared to the surface area of the metal substrate particles, primarily due to the large number of atoms on the surface of the nanoparticles.
  • a cube comprising a plurality of three nanometer nickel particles considered essentially as tiny spheres. As such, they would have about ten atoms on each side, about one thousand atoms in total. Of those thousand atoms, 488 atoms would be on the exterior surface and 512 atoms on the interior of the particle.
  • nanoparticles would have the energy of the bulk material and half would have higher energy due to the absence of neighboring atoms (nickel atoms in the bulk material have about twelve nearest neighbors while those on the surface has nine or fewer).
  • nickel atoms in the bulk material have about twelve nearest neighbors while those on the surface has nine or fewer.
  • a three micron sphere of nickel would have 10,000 atoms along each side for a total of one trillion atoms. There would be 999 A billion of those atoms in the bulk (low energy interior) material. That means that only 0.06% of the atoms would be on the surface of the three micron-sized material compared to the 48.8% of the atoms at the surface of the three nanometer nickel particles.
  • the metal alloy nanoparticles can be configured to have a surface energy sufficiently high to react with other elements under benign reaction conditions.
  • micron sized copper-indium-gallium (CIG) alloy particles have a lower surface energy density and would not react with elemental selenium or sulfur at temperatures below 750 0 C.
  • highly reactive and toxic H 2 Se or H 2 S gasses would be necessary to complete this reaction.
  • ClG alloy nanoparticles including those as small as 50 nanometers, can react with elemental materials such as selenium and/or sulfur at 25O 0 C to produce ClGS or ClGSS, both photon-absorbing materials.
  • CIGS or CIGSS material can be produced under more gentle, environmentally friendly conditions by virtue of the increased reactivity of nanoscale CIG.
  • Layers comprising CIGS and CIGSS materials may form chalcopyrites.
  • the nanoparticles can comprise a metal or metal oxide core and an oxide shell.
  • the materials When the CIG metal alloy nanoparticles are heated in the presence of selenium and/or sulfur on the conductive substrate, the materials combine to form a CIGS or CIGSS photon-absorbing layer. The resulting nanoparticles become partially fused or l 'necked' ⁇ Although the layer is uniform and continuous, the nanoparticles largely retain their discrete size and shape, and thus high surface area.
  • Photovoltaic cell efficiency is highly dependent on the cell ' s ability to efficiently absorb photons and transmit electrons, hi some cases, poor efficiency is caused by layer defects in CIGS or CIGSS photon absorbing material formed during the heating process and non-uniform distribution of material. Although thicker layers have the potential to absorb more photons, they are also more susceptible to these defects. However, when a highly active, thin, defect -free layer is applied, efficiency is highest. To optimize PV efficiency, the photovoltaic absorbing layer should be as thin as possible to decrease the likelihood of defects in the layer.
  • another aspect of at least one of the embodiments includes the idea that by using metal alloy nanoparticles as the starting materials, there is greater control over layer thickness and the potential to produce a thin layer, less than 500 nm in thickness.
  • the reactive metal alloy nanoparticles are preferably formed by a vapor condensation process such as that described in U.S. Patent No. 7,282,167, the entire contents of which is hereby expressly incorporated by reference.
  • material may be fed onto a heater element so as to vaporize the material, allowing the material vapor to flow upwardly from the heater element in a controlled substantially laminar manner under free convection, injecting a flow of cooling gas upwardly from a position below the heater element, preferably parallel to and into contact with the upward flow of the vaporized material and at the same velocity as the vaporized material, allowing the cooling gas and vaporized material to rise and mix sufficiently long enough to allow nano-scale particles of the material to condense out of the vapor, and drawing the mixed flow of cooling gas and nano-scale particles with a vacuum into a storage chamber.
  • Binary, tertiary, or ternary metal nanoparticle alloys of Groups IB, HB and/or Groups IHA on the periodic table preferably have a particle size of less than 50 nanometers, and can be so more reliably when prepared by a vapor condensation process.
  • the band gap energy of the photovoltaic absorbing layer can be modified by stratifying the amount of gallium, where a higher gallium concentration is located closer to the substrate and a lower concentration closer to the photon-absorbing and emission layer interfact (p-n junction). This can be accomplished via multiple layers of nano-scale metal alloy particles with a different gallium concentration in each layer. By applying these layers with subsequent selenization and sulfidization, a graded absorber layer is produced, and the sum of all layers in still less than 0.5 microns in thickness. This methodology has an added benefit in that surface contact is enhanced at the p-n junction, as cadmium sulfide and gallium repel each other. An example is shown in Figure 2.
  • Base material 201 is typically glass or metal foil, however plastic is most preferable so that the cells have increased flexibility.
  • substrate foil 202 is deposited and used as a back contact, and is preferably a metal foil and most preferably molybdenum.
  • gallium-rich CIG layer 211 is then deposited onto foil 202.
  • CIG layers are then deposited, each with decreased gallium concentration.
  • a final, gallium-free layer 212 is applied.
  • the total sum of layers 213 has a maximum thickness of 500 nm. These deposited layers are then heated and then reacted with elements from Group VA and/or IVA.
  • an n-type electron transporting cadmium sulfide emission layer 204 is then applied on top of photon- absorbing layers 213.
  • An anti-reflective coating of zinc oxide 205 is applied on top of emission layer 204. This layer is both electrically and optically conductive, allowing photons to reach photon- absorbing layers 213.
  • Electrical contact 206 is applied to complete circuit 207 with foil 202 to collect and use the energy gained from light absorption.
  • an environmental protection layer is placed on top of anti -reflective coating 208 and electrical contact 206 to prevent and protect against weathering.
  • Copper (19.278 g), indium (80.36 g), and gallium (20.916 g) were mixed in a graphite crucible under argon at 800 0 C, stirred to mix, and allowed to cool.
  • the resulting ingot was crushed into a powder.
  • This powder was further reacted in a vapor condensation reactor at 1400 0 C for one hour to yield copper-indium-gallium alloy nanoscale particles, with a final composition of CUiIn 0 7 Ga 0 3 .
  • a portion of the resulting nanoscale alloy (0.778 g) was placed in a graphite crucible and selenium (0.898 g) was added.
  • the crucible was covered with a graphite lid, then placed in an oven and heated to 500 0 C for 75 minutes in an inert atmosphere.
  • the resulting CIGS photovoltaic absorber material was allowed to cool to room temperature.

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Abstract

A photo-absorbing layer for use in an electronic device; the layer including metal alloy nanoparticles copper, indium and/or gallium made preferably from a vapor condensation process or other suitable process, the layer also including elemental selenium and/or sulfur heated at temperatures sufficient to permit reaction between the nanoparticles and the selenium and/or sulfur to form a substantially fused layer. The reaction may result in the formation of a chalcopyrite material. The layer has been shown to be an efficient solar energy absorber for use in photovoltaic cells.

Description

COMPOSITION AND METHOD OF PREPARING NANOSCALE THIN FILM
PHOTOVOLTAIC MATERIALS
BACKGROUND OF THE INVENTION Technical Field
[0001) The inventions disclosed herein relate generally to the manufacture of materials for thin film photovoltaic cells. More specifically, the invention relates to an improved production process for making the active absorbing material containing metal alloy nanoparticles that allows for increased efficiency, reduced cost, and reduced weight. Related Art
100021 A photovoltaic cell is a device that converts light energy directly into electricity. The high cost of polysilicon and resultant high cost of silicon solar cells has prevented widespread use of solar energy. Recent advances in low cost, high efficiency, thin film polycrystalline solar cells based on copper-indium-gallium-selenium-sulfide (CIGS) absorption layers promises to make solar energy competitive with energy derived from fossil fuels. Although these materials have some of the highest efficiencies of all classes of solar cells, exceeding 15%, several steps in the production process of CIGS solar cells are toxic and/or expensive. Additionally, with thicker active CIGS layers in a photovoltaic device, there is an increase chanced of layer defects that could lower overall cell efficiency.
[0003] These limitations present a roadblock to safe and cost-efficient mass- manufacture. The present invention is helpful in overcoming at least some of these deficiencies. For example, preparing a layer with reduced thickness is one key aspect to improve photovoltaic efficiency and to reduce materials cost. An additional benefit to using a thinner CIGS layer is a decreased weight contribution, which is critical in space applications.
SUMMARY OF THE INVENTION
[0004] hi some embodiments of the current invention, a photovoltaic cell has been prepared incorporating a photon-absorbing layer on an electrically conductive substrate in which the photon absorbing layer is comprised of metal alloy nanoparticles having the formula, for example, Cu1In^xGax. where x equals from 0 to 1. In an inventive method of manufacture, the metal alloy nanoparticles are heated on the substrate in the presence of elements such as, for example, selenium and/or sulfur to a temperature sufficiently high to permit reaction, and preferably, become fused together to form a thin layer on the electrically conductive substrate to provide at least a portion of an electrical circuit that permits the flow of electrons. Preferably, this layer is less than 1 micron in thickness and, more preferably, the layer is less than about 500 nanometers.
[0005] In one embodiment, a preparation suitable for use in a photonic-energy application is provided. The preparation comprises metal alloy nanoparticles configured to react with at least one material selected from Group Va or Group Via of the periodic table when the at least one material is in a non-gaseous state. The metal alloy nanoparticles comprise at least one metal selected from Group Ib, Hb, or Ilia of the periodic table.
[0006] A substantial portion of the metal alloy nanoparticles can have a diameter less than about 50 nm. At least some of the nanoparticles can comprise a metal alloy or metal alloy oxide core and an oxide shell. The metal alloy nanoparticles can be prepared by a vapor condensation process. At least a portion of the metal alloy nanoparticles can comprise at least one of copper, indium, and gallium. At least a portion of the metal alloy nanoparticles can have a composition according to the formula Cu1In]^Ga1, wherein x is a number from 0 to 1.
[0007] In another embodiment, an assembly for a photovoltaic cell is provided comprising an electrically-conductive substrate treated with at least one preparation as described herein and at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
[0008] The non-gaseous phase composition can comprise selenium or sulfur. The electrically-conductive substrate can be a metal foil. The electrically-conductive substrate can comprise molybdenum.
|0009] In another embodiment, a photovoltaic cell is provided comprising an assembly for a photovoltaic cell as described herein and an electrical contact.
[0010] The electrically-conductive substrate can be adjacent a base layer comprising glass, metal foil, or plastic. The photovoltaic cell can further comprise an electron- transporting emission layer. The emission layer can comprise cadmium sulfide. The photovoltaic cell can further comprise an anti-reflective coating. The anti-reflective coating can comprise zinc oxide. The photovoltaic cell can further comprise an environmental protection layer.
|0011] In another embodiment, a method of making photon-absorbing material is provided comprising reacting at least one preparation as described herein with at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
[0012] The non-gaseous phase composition can comprise selenium or sulfur. The method can comprise layering preparations as described herein, wherein each preparation layer has a different composition of metal alloy nanoparticles than its adjacent layer. Each layer can have a different gallium concentration than its adjacent layer.
[0013] In accordance with at least one of the preferred embodiments disclosed herein, a copper-indium-gallium (CIG) alloy was prepared utilizing facile manufacturing conditions. For example, but without limitation, copper-indium-gallium alloy can be selenized and/or sulfidized with elemental selenium and/or sulfur to form a photon-absorbing material, where by the resulting layer has a thickness of no more than about 1 micron, but preferably much less than 1 micron.
[0014] At least some of the embodiments of the present invention benefit from the presence of an increased number of reactive atoms exist at the surface of a nanoparticle. As such, metal alloy nanoparticles and their oxides can be utilized for further alloying under favorable reaction conditions. Metal alloy nanoparticles used in the described method in the preferred embodiments are from groups IB, HB, or JIIA on the periodic table have a diameter of less than 50 nm. More preferably, the metal alloy nanoparticles are comprised of copper and indium, and most preferably copper-indium-gallium (CIG).
[0015] In at least one embodiment of the present invention, a photovoltaic device comprises an emitting layer is applied to the photon-absorbing layer. Preferably, the emitting layer is comprised of a material that is highly efficient at electron transport from the photon- absorbing layer, and most preferably comprises cadmium sulfide or similar molecule. On top of the emitting layer, an anti -reflective coating may be applied. In some of the preferred embodiments, the anti -reflective coating is both optically and electrically conductive to permit sunlight to reach the emitting layer effectively. The anti -reflective coating may preferably be zinc oxide.
[0016} In other preferred embodiments, an environmental protection layer is provided to provide weather-resistant properties to the device. Preferably, the environmental protection layer has optical and electrical conductive property, and may preferably comprise low-iron glass.
[0017] In one application of the present inventive process, a method of preparing a photon-absorbing layer of nanosized material is contemplated. One such method comprises heating metal alloy nanoparticles, prepared for example from a vapor condensation process, with at least one element selected from Groups VA and/or VIA on an electrically conductive substrate. Preferably, the nanosized material in the photon-absorbing layer is prepared by a vapor condensation process. An example of such a process is described in U.S. Patent No. 7,282,167, which is incorporated herein in its entirety by reference. Other methods for obtaining beneficial photon-absorbing layers for use in effective photovoltaic devices may be employed. The composition is heated sufficiently high to permit reaction and create a substantially fused layer of nanosized particles. More preferably, the resulting layer is photon-absorbing for effective use in a photovoltaic device, and may comprise chalcopyrite.
[0018] In addition, it is preferable that a substantial portion of the metal alloy nanoparticles used in the method are less than 100 run, and most preferably less than 50 nm. Utilization of the preferred particle size increases uniformity of the resulting layer after the heating step.
|0019] During the heating step, it is preferred that the mixture be heated to a temperature such that there is sufficient reaction between the metal alloy nanoparticles and at least one element selected from groups VA and/or VIA. Most preferably, the temperature should be at least 2500C.
[0020] Ln some of the embodiments, the temperature must be sufficient form a substantially fused layer of nanoparticles. It is more preferable that the layer be uniform and thin, most preferably less than 500 nm in thickness.
[0021] Some of the preferred embodiments detail the composition of a photovoltaic absorbing chalcopyrite material prepared from metal alloy nanoparticles. Preferably, the nano-scale metal alloy particles are comprised of at least copper and indium, and more preferably copper, indium, and gallium.
[0022] According to some of the embodiments in the current invention, a composition comprising metal alloy nanoparticles prepared from a vapor condensation process can be prepared for use in an electronic device. The metal alloy nanoparticles are preferably comprised from at least one metal from Groups IB, IIB, and/or MA, and are most preferably at least one of copper, indium, and/or gallium.
[0023] Preferably, the metal alloy nanoparticles should have sufficiently high reactivity to permit reaction with elements from either Group VA and/or VIA in the gas, most preferably with selenium and/or sulfur in the liquid, or solid state. Thus, to permit this reaction, the particles should have a size of less than 100 nm, and most preferably less than 50 nm. At least some of the metal alloy nanoparticles have an oxide shell.
[0024] hi other preferred embodiments, a photovoltaic cell comprising a photon- absorbing layer, electronically conductive substrate, emitting layer, and anti-reflective coating is described. The photon-absorbing layer is preferably comprised of copper-indium nanoparticles, and most preferably comprised of copper-indium -gallium nanoparticles. The nanoparticles are substantially fused together, prepared by heating metal alloy nanoparticles sufficiently to permit reaction preferably with material from either Group VA and/or VIA, and most preferably with selenium and/or sulfur.
[0025] The photon-absorbing layer is supported on an electronically conductive substrate which provides a portion of an electrical circuit in combination with the photon- absorbing layer. Preferably, this layer is thin and continuous, and most preferably less than 500 nm thick.
[0026] An emitting layer is applied directly to the photon-absorbing layer. Preferably, the emitting layer is comprised of a material that is highly efficient at electron transport from the photon-absorbing layer, most preferably cadmium sulfide. An anti- reflective coating is applied directly to the emitting layer. Preferably, an anti-reflective coating is both optically and electrically conductive to permit sunlight to enter the emitting layer, and most preferably is zinc oxide. [0027] Additionally, the composition may also comprise an environmental protection layer. Preferably, this layer is comprised of material that reduces damage cause by weathering, and is most preferably composed of low-iron glass.
{0028} Some of the preferred embodiments detail stratified layers of metal alloy nanoparticles, comprising the formula Cu]Ini.xGax wherein x can vary from 0 to 1. Preferably, the gallium concentration in at least one of the stratifications is different from the gallium concentration in another stratification, and most preferably the concentration of gallium is lower proximal to the emitting layer.
[0029] At least some of the preferred embodiments describe at least three and up to twenty stratifications. The layer thickness of all stratifications combined should be thin and continuous, and most preferably the combined thickness is less than 500 nanometers.
BRIEF DESCRIPTION QF THE DRAWINGS
[0030] The features mentioned above in the summary of the invention, along with other features of the inventions disclosed herein, are described below with reference to the drawings of the preferred embodiments. The illustrated embodiments in the figures listed below are intended to illustrate, but not to limit the inventions.
[0032] Figure 1 is a schematic of a typical thin film solar cell.
[0033] Figure 2 is a schematic of a thin film solar cell described in some of the embodiments.
DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS
[0034] The features mentioned above in the summary of the invention, along with other features of the inventions disclosed herein, are described below with reference to the drawings of the preferred embodiments. The illustrated embodiments in the figures listed below are intended to illustrate, but not to limit the inventions.
[0035] A photovoltaic (PV) cell is a device that converts solar energy directly into electricity. While there are several different classes of solar cells, the present invention has particular but not exclusive applicability to thin film solar cells made from materials such as copper-indium-gallium diselenide (CIGS) or copper-mdium-gallium-selenium sulfide (CIGSS). Unlike traditional Si-based solar cells, ClGS and CIGSS cells are flexible and are more acceptable for a wider variety of surface profiles, such as curved or contoured surfaces. The diagram in Figure 1 shows at least some of the different layers in, for example, a ClGS- or CIGSS-based solar cell. Base material 101 may be glass or metal foil, although a material having some plastic and/or elastic characteristic is preferable so that the cells permit increased flexibility. Upon the base material, substrate foil 102 may be deposited and can be used as a back contact. The substrate foil 102 is preferably a metal foil and may preferably comprise molybdenum. A photon-absorbing CIGS or CIGSS layer 103 may then be deposited onto foil 102. The thickness of this layer is highly dependent on how CIGS is applied to the surface. While the thickness of a typical CIGS cell is about two or so microns, the present inventive photon-absorbing layer 103 has an average thickness of less than one micron and preferably less than about 500 nm on average and most preferably a maximum thickness of about 500 nanometers. The CIGS layer is preferably formed as a p-type, photon- absorbing, layer based upon the particular arrangement of copper, indium, and gallium atoms.
[0036] To enhance the flow of electrons through the cell, an n-type electron transporting emission layer 104 can be applied to the photon-absorbing layer 103, preferably a layer comprising cadmium sulfide. An anti-reflective coating of zinc oxide 105 may be applied to the emission layer 104. Preferably, the anti-reflective layer is both electrically and optically conductive, allowing photons to reach the photon- absorbing layer 103. Electrical contact 106 may be applied to complete circuit 107 with foil 102 to collect and use the energy gained from light absorption. If desired, an environmental protection layer 108 may be placed on top the anti-reflective coating 106 and electrical contact 105 to minimize the effects of weathering of the photovoltaic device.
[0037] The present invention benefits from increased surface area of the reactive metal alloy nanoparticles, as compared to the surface area of the metal substrate particles, primarily due to the large number of atoms on the surface of the nanoparticles. As an example, a cube comprising a plurality of three nanometer nickel particles considered essentially as tiny spheres. As such, they would have about ten atoms on each side, about one thousand atoms in total. Of those thousand atoms, 488 atoms would be on the exterior surface and 512 atoms on the interior of the particle. This means that roughly half of the nanoparticles would have the energy of the bulk material and half would have higher energy due to the absence of neighboring atoms (nickel atoms in the bulk material have about twelve nearest neighbors while those on the surface has nine or fewer). A three micron sphere of nickel would have 10,000 atoms along each side for a total of one trillion atoms. There would be 999 A billion of those atoms in the bulk (low energy interior) material. That means that only 0.06% of the atoms would be on the surface of the three micron-sized material compared to the 48.8% of the atoms at the surface of the three nanometer nickel particles.
[0038J Depending upon the process of manufacture, the metal alloy nanoparticles can be configured to have a surface energy sufficiently high to react with other elements under benign reaction conditions. For example, micron sized copper-indium-gallium (CIG) alloy particles have a lower surface energy density and would not react with elemental selenium or sulfur at temperatures below 7500C. Typically, highly reactive and toxic H2Se or H2S gasses would be necessary to complete this reaction. However, ClG alloy nanoparticles, including those as small as 50 nanometers, can react with elemental materials such as selenium and/or sulfur at 25O0C to produce ClGS or ClGSS, both photon-absorbing materials. As such, metal alloy nanoparticles have been shown to have exponentially higher surface area-to-volume ratio than that of a micron-scale metal alloy particle. Thus, CIGS or CIGSS material can be produced under more gentle, environmentally friendly conditions by virtue of the increased reactivity of nanoscale CIG. Layers comprising CIGS and CIGSS materials may form chalcopyrites. The nanoparticles can comprise a metal or metal oxide core and an oxide shell.
[0039] When the CIG metal alloy nanoparticles are heated in the presence of selenium and/or sulfur on the conductive substrate, the materials combine to form a CIGS or CIGSS photon-absorbing layer. The resulting nanoparticles become partially fused or l'necked'\ Although the layer is uniform and continuous, the nanoparticles largely retain their discrete size and shape, and thus high surface area.
[0040] Photovoltaic cell efficiency is highly dependent on the cell's ability to efficiently absorb photons and transmit electrons, hi some cases, poor efficiency is caused by layer defects in CIGS or CIGSS photon absorbing material formed during the heating process and non-uniform distribution of material. Although thicker layers have the potential to absorb more photons, they are also more susceptible to these defects. However, when a highly active, thin, defect -free layer is applied, efficiency is highest. To optimize PV efficiency, the photovoltaic absorbing layer should be as thin as possible to decrease the likelihood of defects in the layer. Thus, another aspect of at least one of the embodiments includes the idea that by using metal alloy nanoparticles as the starting materials, there is greater control over layer thickness and the potential to produce a thin layer, less than 500 nm in thickness.
[0041] The reactive metal alloy nanoparticles are preferably formed by a vapor condensation process such as that described in U.S. Patent No. 7,282,167, the entire contents of which is hereby expressly incorporated by reference. With such a process, material may be fed onto a heater element so as to vaporize the material, allowing the material vapor to flow upwardly from the heater element in a controlled substantially laminar manner under free convection, injecting a flow of cooling gas upwardly from a position below the heater element, preferably parallel to and into contact with the upward flow of the vaporized material and at the same velocity as the vaporized material, allowing the cooling gas and vaporized material to rise and mix sufficiently long enough to allow nano-scale particles of the material to condense out of the vapor, and drawing the mixed flow of cooling gas and nano-scale particles with a vacuum into a storage chamber. Binary, tertiary, or ternary metal nanoparticle alloys of Groups IB, HB and/or Groups IHA on the periodic table preferably have a particle size of less than 50 nanometers, and can be so more reliably when prepared by a vapor condensation process.
[0042] For further efficiency optimization, the band gap energy of the photovoltaic absorbing layer can be modified by stratifying the amount of gallium, where a higher gallium concentration is located closer to the substrate and a lower concentration closer to the photon-absorbing and emission layer interfact (p-n junction). This can be accomplished via multiple layers of nano-scale metal alloy particles with a different gallium concentration in each layer. By applying these layers with subsequent selenization and sulfidization, a graded absorber layer is produced, and the sum of all layers in still less than 0.5 microns in thickness. This methodology has an added benefit in that surface contact is enhanced at the p-n junction, as cadmium sulfide and gallium repel each other. An example is shown in Figure 2. Base material 201 is typically glass or metal foil, however plastic is most preferable so that the cells have increased flexibility. Upon the base material, substrate foil 202 is deposited and used as a back contact, and is preferably a metal foil and most preferably molybdenum. First, gallium-rich CIG layer 211 is then deposited onto foil 202. Subsequent CIG layers are then deposited, each with decreased gallium concentration. A final, gallium-free layer 212 is applied. The total sum of layers 213 has a maximum thickness of 500 nm. These deposited layers are then heated and then reacted with elements from Group VA and/or IVA. To permit the flow of electrons through the cell, an n-type electron transporting cadmium sulfide emission layer 204 is then applied on top of photon- absorbing layers 213. An anti-reflective coating of zinc oxide 205 is applied on top of emission layer 204. This layer is both electrically and optically conductive, allowing photons to reach photon- absorbing layers 213. Electrical contact 206 is applied to complete circuit 207 with foil 202 to collect and use the energy gained from light absorption. Furthermore, an environmental protection layer is placed on top of anti -reflective coating 208 and electrical contact 206 to prevent and protect against weathering.
EXAMPLE - PREPARATION OF CIGS
Copper (19.278 g), indium (80.36 g), and gallium (20.916 g) were mixed in a graphite crucible under argon at 8000C, stirred to mix, and allowed to cool. The resulting ingot was crushed into a powder. This powder was further reacted in a vapor condensation reactor at 14000C for one hour to yield copper-indium-gallium alloy nanoscale particles, with a final composition of CUiIn0 7Ga0 3. A portion of the resulting nanoscale alloy (0.778 g) was placed in a graphite crucible and selenium (0.898 g) was added. The crucible was covered with a graphite lid, then placed in an oven and heated to 5000C for 75 minutes in an inert atmosphere. The resulting CIGS photovoltaic absorber material was allowed to cool to room temperature.
The foregoing description is that of preferred embodiments having certain features, aspects, and advantages in accordance with the present inventions. Various changes and modifications also may be made to the above-described embodiments without departing from the spirit and scope of the inventions.

Claims

WHAT IS CLAIMED IS:
3. A preparation suitable for use in a photonic-energy application, the preparation comprising metal alloy nanoparticles configured to react with at least one material selected from Group Va or Group Via of the periodic table when the at least one material is in a non-gaseous state, wherein the metal alloy nanoparticles comprise at least one metal selected from Group Ib, 0b, or Ilia of the periodic table.
2. The preparation according to Claim I3 wherein a substantial portion of the metal alloy nanoparticles has a diameter less than about 50 ran.
3. The preparation according to any of Claims 1 and 2, wherein at least some of the nanoparticles comprise a metal alloy or metal alloy oxide core and an oxide shell.
4. The preparation according to any preceding claim wherein at least a portion of the metal alloy nanoparticles comprise at least one of copper, indium, and gallium.
5. The preparation according to Claim 4, wherein at least a portion of the metal alloy nanoparticles has a composition according to the formula CuiIn]-ΛGaλ, wherein x is a number from 0 to 1.
6. The preparation according to any preceding claim, wherein the metal alloy nanoparticles are prepared by a vapor condensation process.
7. An assembly for a photovoltaic cell comprising an electrically-conductive substrate treated with at least one preparation according to any preceding claim and at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
8. The assembly according to Claim 7, wherein the non-gaseous phase composition comprises selenium or sulfur.
9. The assembly according to any of Claims 7 and 8, wherein the electrically- conductive substrate is a metal foil.
10. The assembly according to any of Claims 7-9, wherein the electrically- conductive substrate comprises molybdenum.
1 1. A photovoltaic cell comprising the assembly according to any of Claims 7-10 and an electrical contact.
12. The photovoltaic cell according to Claim 11, wherein the electrically- conductive substrate is adjacent a base layer comprising glass, metal foil, or plastic.
13. The photovoltaic cell according to any of Claims 1 1 and 12 further comprising an electron-transporting emission layer.
14. The photovoltaic cell according to Claim 13, wherein the emission layer comprises cadmium sulfide.
15. The photovoltaic cell according to any of Claims 11—14 further comprising an anti -reflective coating.
16. The photovoltaic cell according to Claim 15, wherein the anti-reflective coating comprises zinc oxide.
17. The photovoltaic cell according to any of Claims 1 1-16, further comprising an environmental protection layer.
18. A method of making photon-absorbing material comprising reacting at least one preparation according to any of Claims 1-6 with at least one non-gaseous phase composition comprising material selected from Group Va or Group Via of the periodic table.
19. The method according to Claim 18, wherein the non-gaseous phase composition comprises selenium or sulfur.
20. The method according to any of Claims 18 and 19, comprising layering preparations according to any of Claims 1-6, wherein each preparation layer has a different composition of metal alloy nanoparticles than its adjacent layer.
21. The method according to Claim 20, wherein each layer has a different gallium concentration than its adjacent layer.
PCT/US2008/071845 2008-07-31 2008-07-31 Composition and method of preparing nanoscale thin film photovoltaic materials WO2010014105A1 (en)

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