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WO2015148637A1 - Cellules solaires à couches minces dotées de grilles de contact métalliques - Google Patents

Cellules solaires à couches minces dotées de grilles de contact métalliques Download PDF

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Publication number
WO2015148637A1
WO2015148637A1 PCT/US2015/022442 US2015022442W WO2015148637A1 WO 2015148637 A1 WO2015148637 A1 WO 2015148637A1 US 2015022442 W US2015022442 W US 2015022442W WO 2015148637 A1 WO2015148637 A1 WO 2015148637A1
Authority
WO
WIPO (PCT)
Prior art keywords
cell
electrode layer
layer
photovoltaic conversion
metallic grid
Prior art date
Application number
PCT/US2015/022442
Other languages
English (en)
Inventor
Xavier Multone
Jerome Steinhauser
Evelyne Vallat-Sauvain
Johannes Meier
Original Assignee
Tel Solar Ag
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.)
Filing date
Publication date
Application filed by Tel Solar Ag filed Critical Tel Solar Ag
Publication of WO2015148637A1 publication Critical patent/WO2015148637A1/fr

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/20—Electrodes
    • H10F77/206—Electrodes for devices having potential barriers
    • H10F77/211—Electrodes for devices having potential barriers for photovoltaic cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/10—Semiconductor bodies
    • H10F77/16—Material structures, e.g. crystalline structures, film structures or crystal plane orientations
    • H10F77/169—Thin semiconductor films on metallic or insulating substrates
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/20—Electrodes
    • H10F77/244—Electrodes made of transparent conductive layers, e.g. transparent conductive oxide [TCO] layers
    • 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

Definitions

  • the present invention relates to solar cells having metallic electrical contacts, and a method for forming such devices.
  • Photovoltaic devices also known as solar cells
  • solar cells are entering the mainstream as a reliable and cost-effective source of renewable energy.
  • One of the main drivers of adoption of photovoltaic technology has been the decrease of price per watt (or kilowatt) that each generation of devices has been able to reach, along with increased performance manifested primarily as power generated per unit area. Therefore, active research and development efforts are directed at achieving ongoing cost reductions in all the layers that comprise a thin film solar cell and all the processes of forming thereof.
  • TCO transparent conductive oxide
  • ZnO zinc oxide
  • Sn0 2 tin oxide
  • ITO indium tin oxide
  • TCO materials are used in the form of a transparent layer as electrical contacts.
  • a particularly critical layer is the front electrical contact which faces the light source, i.e the sun.
  • TCO materials are their relatively high cost, particularly if they contain indium (In), but zinc-containing LPCVD precursors, like diethyl-zinc can be also be expensive. Besides having a relatively high cost, TCO materials also in some cases limit the current density
  • zinc oxide in particular has a favorable crystal structure with an approximately 50° angle between crystal facets, and is thus a very effective scatterer of light. It is deposited typically via a low pressure chemical vapor deposition (LPCVD) or sputtering process in a layer 1 to 2 ⁇ thick, and is highly transparent (>95%) in the 400 to 800nm light wavelength range, thus suitable for solar energy capture. Every alternative electrical contact solution would thus need to address light scattering as well as electrical performance.
  • LPCVD low pressure chemical vapor deposition
  • sputtering process in a layer 1 to 2 ⁇ thick, and is highly transparent (>95%) in the 400 to 800nm light wavelength range, thus suitable for solar energy capture. Every alternative electrical contact solution would thus need to address light scattering as well as electrical performance.
  • An aspect of the invention includes a thin film solar cell, comprising a substrate and a front electrode layer adjacent the substrate, the front electrode layer comprising a metallic grid.
  • the substrate can comprise glass, which may be diffusive, or textured, or nanotextured.
  • the glass may comprise a
  • the solar cell further comprises a current-generating layer stack, which may comprise multiple photovoltaic conversion units, such as p-i-n or n-i-p photovoltaic conversion units, which may comprise amorphous and/or crystalline silicon absorbers.
  • a current-generating layer stack may comprise multiple photovoltaic conversion units, such as p-i-n or n-i-p photovoltaic conversion units, which may comprise amorphous and/or crystalline silicon absorbers.
  • Alternative embodiments may include photovoltaic conversion units comprising copper indium selenide (CIS), copper indium gallium selenide (CIGS), dye solar cells, or may comprise a hybrid of multiple aforementioned types of photovoltaic conversion units.
  • a back electrode layer is formed on the opposite side of the current-generating layer stack, wherein the back electrode layer can also comprise a metallic grid.
  • the front electrode layer has an optical transmission greater than about 95%, and a sheet resistance of less than about 30 ⁇ per square, preferably less than about 20 ⁇ per square.
  • Another aspect of the invention includes a front, or back, or both electrode layers that comprise a transparent conductive oxide (TCO) layer and/or a p-oxide contact layer in addition to the metallic grids.
  • TCO transparent conductive oxide
  • Yet another aspect of the invention includes the process of forming front and back electrode layer metallic grids using rolling mask lithography (RML), to pattern the metallic grid, wherein the patterning step is followed by an etch and/or liftoff step to pattern a metallic layer from which the metallic grid is formed.
  • RML rolling mask lithography
  • FIG. 1 shows a cross section of an exemplary photovoltaic device in accordance with an embodiment of the invention.
  • FIG. 2 shows a view of the metallic grid contact layer in accordance with an embodiment of the invention.
  • FIG. 3 shows a schematic of the rolling mask lithography (RML) process.
  • FIG. 4 shows a comparison of optical and electrical performance
  • FIG. 5 shows photovoltaic device current density for various types of contact structures.
  • Embodiments of the present invention relate to design of and method of forming a photovoltaic device, i.e. a solar cell.
  • photovoltaic device which represents a device capable of conversion of incoming light energy into electrical energy
  • terms such as cell, solar cell, solar panel, etc.
  • the invention is also applicable to other types of devices that require or can optionally have optically transparent or semi-transparent electrical contacts, such as for example various display devices, liquid crystal display (LCD) devices, light emitting diodes (LED) and similar lighting devices, etc.
  • LCD liquid crystal display
  • LED light emitting diodes
  • embodiment means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but do not denote that they are present in every embodiment.
  • appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same
  • FIG. 1 shows a cross section of a thin film solar cell 100 in accordance with an embodiment of the invention.
  • the solar cell is formed on a transparent substrate 1 10, typically made of glass or other suitable transparent material.
  • the lower side of the substrate is the side from which light is incident onto the solar cell (not shown) and may be coated with an a nti reflective coating 105, to reduce reflections and maximize coupling of the incident irradiation.
  • a current-generating layer stack 155 is formed comprising (in this example) a tandem of two p-i-n silicon
  • the photovoltaic conversion units 150 and 160 The first p-i-n photovoltaic conversion unit
  • the 150 may be an amorphous silicon photovoltaic conversion unit.
  • the second p-i-n photovoltaic conversion unit 160 may be a crystalline photovoltaic conversion unit.
  • the current-generating layer stack 155 may comprise additional, i.e. 3 or more photovoltaic conversion units, and the photovoltaic conversion units themselves do not need to comprise silicon, but may comprise copper indium selenide (CIS), copper indium gallium selenide (CIGS), dye photovoltaic conversion units, etc.
  • CIS copper indium selenide
  • CGS copper indium gallium selenide
  • dye photovoltaic conversion units etc.
  • the photovoltaic conversion units may be n-i-p photovoltaic conversion units and/or the current-generating layer stack 155 may comprise a hybrid of multiple types of photovoltaic conversion units, e.g. a silicon and a CIS/CIGS photovoltaic conversion unit.
  • the makeup of the current-generating layer stack will not be discussed in further detail here, and the reader is directed to copending U.S. Patent Application No. 14/159,002, entitled "SYSTEM AND METHOD FOR TRAPPING LIGHT IN A SOLAR CELL", and copending U.S. Provisional Patent Application No.
  • a front electrode layer is formed between the substrate 1 10 and the first photovoltaic conversion unit 150.
  • the front electrode layer comprises a metallic grid 140 formed on the surface 120 of substrate 1 10.
  • the surface 120 of substrate 1 10 may be polished flat or it may be textured to increase light scattering into the photovoltaic conversion units 150 and 160.
  • the glass can be textured by etching, scribing, or any other suitable method known to those skilled in the art.
  • the surface 120 may be nanotextured or may comprise a nanoimprinted layer to form a texture or features for increased light scattering.
  • the metallic grid may be formed from a metal such as silver (Ag), aluminum (Al), copper (Cu), or other suitable metal.
  • FIG. 2 shows a cut-away view of thin film solar cell 100 of FIG. 1 with just the substrate 1 10 and metallic grid 140 shown formed thereupon, for clarity.
  • the metallic grid 140 comprises grid elements 142 that enclose openings 145, wherein the size of the openings 145, the cross sections of grid elements 142, are selected such that a high optical transmission of the metallic grid 140 is achieved, in the range of 90% or higher, typically around 95%, and to preserve the current carrying capability of the metallic grid (i.e. having a low resistance).
  • the front electrode layer may further comprise an optional front contact layer 130 in addition to the metallic grid 140.
  • the front contact layer 130 may comprise a thin layer of transparent conductive oxide (TCO) or a p- doped oxide layer, wherein the front contact layer 130 serves to provide additional scattering of light (as in the case of a TCO) and/or to protect the first photovoltaic conversion unit 150 from diffusion of species from metallic grid 140 and substrate 1 10.
  • Suitable transparent conductive oxides include but are not limited to zinc oxide (ZnO), tin oxide (Sn0 2 ), and indium tin oxide (ITO).
  • the p-oxide layer may comprise a ⁇ - ⁇ 3 ⁇ / ⁇ - ⁇ 3 ⁇ layer stack, for example.
  • the thickness of the front contact layer 130 may be greater than about 5nm, for example.
  • a similar contact structure may be formed on the back side of the current- generating layer stack 155. While it is not critical for a metallic grid to be used as a back contact, because less light coupling therethrough, it is still preferable from the standpoint of maximizing solar cell performance, and from a cost perspective.
  • a back electrode layer may be formed atop the second photovoltaic conversion unit 160 (or atop a stack comprising more than two photovoltaic conversion units), the back electrode layer comprising a metallic grid 170.
  • Metallic grid 170 may be similar to metallic grid 140, of FIGs. 1 and 2, or it may be of a different geometry and comprising a different material. With the back electrode layer formed, a reflector 190 is typically added to the solar cell 100, to reflect transmitted light back into solar cell so it can be fully utilized in energy conversion.
  • the back electrode layer may also comprise a back contact layer 180 comprising a thin layer of transparent conductive oxide (TCO), to protect the second photovoltaic conversion unit 160 from diffusion of species from metallic grid 170.
  • TCO transparent conductive oxide
  • An effective thickness of the back contact layer 180 may be about 5nm, or higher.
  • FIG. 4 shows the achievable range 410 of optical transmission and sheet resistance for metallic grids formed by a rolling mask lithography (RML) process commercialized by Rolith, Inc., of 5880 W. Las Positas Blvd #51 , Pleasanton, CA 94588, United States.
  • Optical transmission in excess of 95% can be achieved with metallic grid geometries and dimensions similar to those shown in FIGs. 1 and 2, with a sheet resistance of less than 10 ⁇ per square - all marked improvements over traditional TCOs.
  • Typical performance may include an optical transmission in the 95% range, a wavelength passband from 350nm to 1200nm, and a sheet resistance of 10 to 30 ⁇ per square.
  • the total photocurrent can be significantly increased due to reduced absorption losses in wavelength ranges inaccessible with the use of typical TCOs, i.e. blue, violet, and UV with wavelengths less than about 400nm, and red and infrared with wavelengths greater than about 800nm. All that may be achieved at a lower per-area cost, provided an inexpensive patterning and metal deposition solution is used, compared to traditional TCO layer forming.
  • FIG. 5 shows actual measured current densities vs total thickness for traditional TCOs, in a smooth state (plot 520) and rough condition for improved light trapping (plot 510).
  • a theoretical current density curve 530 is plotted for the case with no TCO, and a plot 540 is provided calculated based on anticipated performance of aforementioned metallic grids 140 and 170 used in lieu of TCO contacts, without the use of optional additional front and back contact layers 130 and 180.
  • a photocurrent improvement in the range of 2.5 to 3 mA/cm 2 is readily apparent over traditional TCOs, allowing a higher conversion efficiency to be achieved in a thin film solar cell.
  • the metallic grid 140 does provide some light scattering into the current- generating layer stack 155, it is advantageous to use additional methods of inducing light scattering, particularly in the case where a TCO front contact layer 130 is not used.
  • Methods to induce light scattering may include using a diffuse glass substrate 1 10, or a substrate 1 10 where the surface 120 is textured or nanotextured, or where it has a pattern formed thereupon, for example using nanoimprinting or other patterning methods.
  • a higher solar cell performance can be achieved, manifested by a higher open circuit voltage Voc and a higher fill factor FF.
  • the conditioning of the substrate 1 10 and its surface 120 also can be tailored to improve index matching, to reduce unwanted internal reflection losses.
  • a design of a thin film solar cell with metallic grids 140 and 170, and in particular the decision whether to include TCO front and back contact layers 130 and 180 will involve making design tradeoffs.
  • a TCO-free thin film solar call would be expected to exhibit a higher level of reliability, due to absence of TCO-related problems, such as moisture-, or UV-induced degradation, or anodic degradation.
  • the use of metallic grids only allows the decoupling of electrical and optical functions of the contact scheme.
  • a TCO front and back contact layers introduce additional light scattering, which is beneficial, and they also serve as diffusion barriers.
  • the introduction of metallic grids may require changes in bus bar design and scribing processes. Overall, the design will necessarily become a result of an optimization process, with many parameters taken into account.
  • FIG. 3 depicts a schematic of the patterning process 300, in which a substrate 1 10 is moved through a patterning process that begins with dispensing photoresist from a nozzle (or set of nozzles) 310, to form a photoresist layer 320.
  • Patterning of the photoresist layer 320 is accomplished by exposure to UV light from a UV source 340 placed inside a rolling mask 330.
  • the rolling mask 330 comprises a pattern that is to be formed in the photoresist layer 320.
  • the RML process proceeds at a continuous high rate, forming a latent image of the pattern inside photoresist layer 320.
  • the exposed photoresist layer 320 is then subjected to a development process wherein developer is dispensed onto the photoresist layer 320 from nozzle(s) 350 to form a pattern 370.
  • the process completes with a rinse step in which a rinse liquid is dispensed from rinse nozzle(s) 360, to clean the pattern 370 atop substrate 1 10, and prepare it for further processing steps.
  • the pattern 370 can be formed as a negative of the metallic grid pattern, and the metallic grid can be formed by simple metal deposition into the pattern openings and across the negative pattern 370. This step is followed by a liftoff etch process in which the negative pattern 370 is lifted off along with the overlying metal layer, to expose a metallic grid.
  • a more traditional approach can be taken, in which a metallic layer is formed prior to RML patterning, the pattern 370 being a positive pattern formed thereupon, and used as a mask for a subsequent etch step in which openings 145, for example, of metallic grid 140 are formed by etching the metal. Both processes can be followed by further cleaning steps to remove the lifted-off or etched material and photoresist from the substrate 1 10.
  • a metallic grid for example metallic grid 140
  • the usual process of forming a current-generating layer stack 155, the back electrode layer, and reflector 190, can proceed, as known to those skilled in the art.

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  • Photovoltaic Devices (AREA)

Abstract

L'invention concerne une cellule solaire améliorée ayant des grilles de contact métalliques seules ou en association avec une couche de contact d'oxyde conducteur transparent (TCO) ou une couche de contact de p-oxyde. L'invention concerne également un procédé de formation de telles cellules solaires. Des grilles de contact métalliques, avec ou sans couche d'oxyde conducteur transparent (TCO) ou couche de contact de p-oxyde, peuvent être utilisées avec divers types de piles de couches de génération de courant de cellule solaire à couches minces, et éventuellement combinées avec des substrats de verre imprimés ou à motifs, de diffusion ou texturés pour obtenir des caractéristiques électriques et optiques améliorées de cellule solaire.
PCT/US2015/022442 2014-03-25 2015-03-25 Cellules solaires à couches minces dotées de grilles de contact métalliques WO2015148637A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US201461970264P 2014-03-25 2014-03-25
US61/970,264 2014-03-25

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20230268452A1 (en) * 2020-06-26 2023-08-24 Evolar Ab Photovoltaic top module

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20100096004A1 (en) * 2006-10-25 2010-04-22 Unidym, Inc. Solar cell with nanostructure electrode(s)
US20100197068A1 (en) * 2008-10-30 2010-08-05 Hak Fei Poon Hybrid Transparent Conductive Electrode
US20120103669A1 (en) * 2009-05-26 2012-05-03 Institucio Catalana De Recerca I Estudis Avancats Metal transparent conductors with low sheet resistance
US20120227794A1 (en) * 2009-09-18 2012-09-13 Applied Materials, Inc. Threshold adjustment implants for reducing surface recombination in solar cells
US20130340817A1 (en) * 2010-09-03 2013-12-26 Oerlikon Solar Ag, Trubbach Thin film silicon solar cell in tandem junction configuration on textured glass

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20100096004A1 (en) * 2006-10-25 2010-04-22 Unidym, Inc. Solar cell with nanostructure electrode(s)
US20100197068A1 (en) * 2008-10-30 2010-08-05 Hak Fei Poon Hybrid Transparent Conductive Electrode
US20120103669A1 (en) * 2009-05-26 2012-05-03 Institucio Catalana De Recerca I Estudis Avancats Metal transparent conductors with low sheet resistance
US20120227794A1 (en) * 2009-09-18 2012-09-13 Applied Materials, Inc. Threshold adjustment implants for reducing surface recombination in solar cells
US20130340817A1 (en) * 2010-09-03 2013-12-26 Oerlikon Solar Ag, Trubbach Thin film silicon solar cell in tandem junction configuration on textured glass

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20230268452A1 (en) * 2020-06-26 2023-08-24 Evolar Ab Photovoltaic top module

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