WO2018174898A1 - Verres à faible gravure et sans contact pour compositions de pâte électroconductrice - Google Patents

Verres à faible gravure et sans contact pour compositions de pâte électroconductrice Download PDF

Info

Publication number
WO2018174898A1
WO2018174898A1 PCT/US2017/024055 US2017024055W WO2018174898A1 WO 2018174898 A1 WO2018174898 A1 WO 2018174898A1 US 2017024055 W US2017024055 W US 2017024055W WO 2018174898 A1 WO2018174898 A1 WO 2018174898A1
Authority
WO
WIPO (PCT)
Prior art keywords
electroconductive paste
glass frit
paste composition
silicon substrate
preferred
Prior art date
Application number
PCT/US2017/024055
Other languages
English (en)
Inventor
Devidas Raskar
Guang Zhai
Gerd Schulz
Yi Yang
Maryam KAZEMZADEH DEHDASHTI
Original Assignee
Heraeus Precious Metals North America Conshohocken Llc
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 Heraeus Precious Metals North America Conshohocken Llc filed Critical Heraeus Precious Metals North America Conshohocken Llc
Priority to PCT/US2017/024055 priority Critical patent/WO2018174898A1/fr
Priority to CN201780087366.9A priority patent/CN110337423A/zh
Priority to TW107103151A priority patent/TWI662560B/zh
Publication of WO2018174898A1 publication Critical patent/WO2018174898A1/fr

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
    • C03C8/14—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions
    • C03C8/18—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions containing free metals
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
    • C03C8/02—Frit compositions, i.e. in a powdered or comminuted form
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
    • C03C8/02—Frit compositions, i.e. in a powdered or comminuted form
    • C03C8/04—Frit compositions, i.e. in a powdered or comminuted form containing zinc
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
    • C03C8/14—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions
    • C03C8/16—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions with vehicle or suspending agents, e.g. slip
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/14—Conductive material dispersed in non-conductive inorganic material
    • H01B1/16—Conductive material dispersed in non-conductive inorganic material the conductive material comprising metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/20—Conductive material dispersed in non-conductive organic material
    • H01B1/22—Conductive material dispersed in non-conductive organic material the conductive material comprising metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells

Definitions

  • This invention relates to glass frits for use in electroconductive paste compositions.
  • the glass compositions have low etching and non-contact forming properties suitable for use in electroconductive paste compositions for dual or double-printing applications.
  • the glass frits may be used in electroconductive paste compositions for forming backside soldering tabs in passivated emitter rear solar cells.
  • Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products.
  • a solar cell When light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder is transmitted into the solar cell.
  • the photons of the transmitted light are absorbed by the solar cell, which is usually made of a semiconducting material such as silicon.
  • the energy from the absorbed photons excites electrons of the semiconducting material from their atoms, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes which are applied on the solar cell surface. In this way, electricity may be conducted between interconnected solar cells.
  • Solar cells may have a variety of different structures.
  • Conventional solar cells have an antireflection coating (ARC) applied to the front side of a semiconductor substrate, so as to reduce reflection of incident light.
  • Front side electrodes typically formed of electroconductive pastes, are applied on top of the antireflection coating in a grid-like pattern.
  • electroconductive paste (which may be the same or different from the front side
  • electroconductive paste is applied to the backside of the solar cell to form backside
  • the electroconductive paste compositions used to form these various components of the solar cell are specifically formulated for their particular application.
  • a conventional electroconductive composition contains metallic particles, an inorganic component, and an organic vehicle.
  • Front side electroconductive paste compositions used to form the front side electrodes are designed to have good etching and contact forming characteristics, so that they may etch through the front side layers, such as the antireflection layer, and make contact with the underlying silicon substrate.
  • the electroconductive paste used to form the front side electrodes is applied to the front surface of the solar cell by screen printing in a grid pattern, which is formed of thin, parallel finger lines and wider busbars (at least two) intersecting the finger lines perpendicularly.
  • the finger lines may be double printed, such that a first layer of finger lines is printed on the front surface of the solar cell, and then a second layer of finger lines is printed on top of the first layer of finger lines.
  • Other such methods are disclosed, for example, in U.S. Patent Application Publication No. 2012/0180862 and U.S. Patent No. 9,224,888. Such dual or double-printing techniques has been shown to improve the efficiency of the resulting solar cell.
  • the electroconductive paste used to print the first layer of finger lines and/or busbars is preferably different than the electroconductive paste used to print the second layer of finger lines and/or busbars.
  • the glass frit present in the electroconductive paste which forms the second layer of finger lines and/or busbars should be carefully formulated so as to have limited or no chemical reactivity with the solar cell substrate materials, such as the antireflection layer, to avoid damage to those layers. This reduces the occurrence of shunting and the loss of open circuit voltage (Voc) in the solar cell, which in turn improves solar cell efficiency.
  • Voc open circuit voltage
  • a different type of solar cell is a passivated emitter rear cell, or "PERC" solar cell.
  • the aluminum paste is applied directly to the back surface of the solar cell substrate to form the back surface field.
  • a rear passivation layer typically formed of a dielectric material such as alumina, is first applied to the back surface of the substrate. Additional rear surface layers, such as a silicon nitride capping layer, may be applied on top of the rear passivation layer.
  • portions of the rear passivation layer and additional rear layers are removed to expose areas of the underlying substrate. This step may be achieved by, for example, acid etching or laser drilling.
  • the aluminum electroconductive paste is then applied on top of the rear layers, thus filling in the "holes" created during the removal process. In these areas, a local back surface field is formed when the aluminum paste is fired.
  • the backside soldering tabs may be applied to the back surface either before or after the application of the aluminum paste.
  • FIGS. 1 and 2. PERC solar cells have been shown to have increased efficiency compared to solar cells that do not have a rear passivation layer.
  • the electroconductive pastes used to form the backside soldering tabs on a PERC cell must also have limited or no chemical reactivity with the underlying layers, such as the rear passivation layer and rear capping layer, in order to avoid damage to those layers which results in decreased electrical performance of the solar cell.
  • Non-contact forming and low etching glasses are known in the art. However, many of these types of glasses are lacking in that they require relatively high firing temperatures during processing due to their chemical composition, which is unsuitable for use with certain types of conventional solar cells and PERC solar cells. Further, improvements in the electrical performance of electroconductive pastes formulated with non-contact glasses are desired.
  • the invention provides an electroconductive paste composition which includes metallic particles, at least one glass frit, and an organic vehicle.
  • the at least one glass frit is lead-free and includes at least SiCh, B2O3, and B12O3 in a total amount of about 65-95 wt%, based upon 100% total weight of the at least one glass frit, about 5-20 wt% of at least one alkaline oxide, and about 3-10 wt% of M0O3.
  • the invention is further directed to a method of preparing a passivated emitter rear solar cell, including the steps of applying a backside electroconductive paste to a rear surface of a silicon substrate having at least one rear passivation layer formed thereon, the rear passivation layer having a plurality of open holes formed therein to expose areas of the silicon substrate, applying an aluminum electroconductive paste in the open holes of the rear passivation layer to contact the silicon substrate, and heating the silicon substrate.
  • the backside electroconductive paste includes the inventive pastes disclosed herein.
  • a passivated emitter rear solar cell which includes a silicon substrate having a front and back surface, a rear passivation layer on the back surface of the silicon substrate having a plurality of open holes formed therein, an aluminum back contact formed in the open holes of the rear passivation layer, at least one backside soldering tab on the back surface of the silicon substrate formed from the electroconductive paste compositions disclosed herein, and a front grid electrode formed on the front surface of the silicon substrate.
  • the invention is also directed to a method of forming a solar cell, including the steps of printing a first electroconductive paste onto a front surface of a silicon substrate to form a first set of electrodes, printing a second electroconductive paste on top of the first electroconductive paste to form a second set of electrodes, and heating the silicon substrate.
  • electroconductive paste is different from the second electroconductive paste, and the second electroconductive paste includes the inventive pastes disclosed herein.
  • the invention further provides a solar cell which includes a silicon substrate having an antireflection coating applied to a surface thereof, a first set of electrodes formed directly on the antireflection coating, and a second set of electrodes formed on top of the first set of electrodes, wherein the second set of electrodes are formed from the electroconductive paste compositions disclosed herein.
  • FIG. 1 is a side cross-sectional view of a standard solar cell
  • FIG. 2 is a side cross-sectional view of a PERC solar cell
  • FIGS. 3 A-3D are top views of various dual printing methods for forming front side electrodes on conventional solar cells using an electroconductive paste composition according to an embodiment of the invention.
  • FIG. 4 is a top view of a dual printing method used with an electroconductive paste composition according to an embodiment of the invention.
  • Electroconductive pastes used to form solar cell electrodes generally include conductive metallic particles, glass frit(s), an organic vehicle, and optional additive(s).
  • the glass frit of the invention exhibits low etching characteristics when fired, such that it reduces damage to the underlying surface layers on the substrate, thereby improving solar cell performance in either dual printed solar cells or PERC solar cells.
  • the glass frit acts as an adhesion media, facilitating the bonding between the conductive particles and the silicon substrate, and thus providing good electrical contact therebetween.
  • conventional glass frits etch through the surface layers (e.g., antireflective layer) of the silicon substrate, such that effective electrical contact can be made between the electroconductive paste and the silicon wafer.
  • the glass frits of the invention are designed to exhibit minimal etching characteristics. Such a glass frit is particularly useful in dual printing applications and in forming backside soldering tabs on a PERC solar cell. Specifically, the glass frits described herein exhibit minimized etching of the underlying surface layers during firing of the solar cell, which in turn minimizes damage to those underlying layers which would otherwise result in shunting and open circuit voltage losses and thus reduced solar cell efficiency. As such, the glass frits of the invention are designed to be low-etching and non-contact forming glasses.
  • the glass frit(s) of the invention preferably have a low lead content or are lead-free, as lead is known to have strong etching properties with respect to surface layers on solar cells.
  • the term "low lead content” refers to a glass frit having a lead content of at least 0.5 wt% and less than about 5 wt%, such as less than about 4 wt%, less than about 3 wt%, less than about 2 wt%, less than about 1 wt%, and less than about 0.8 wt%, based upon 100% total weight of the glass frit.
  • the term "lead-free” refers to a glass frit having a lead content of less than about 0.5 wt%, preferably less than about 0.4 wt%, more preferably less than about 0.3 wt%, more preferably less than about 0.2 wt%, and most preferably less than about 0.1 wt% lead, based upon 100% total weight of the glass frit.
  • the glass frit comprises less than about 0.01 wt% lead, which may be present as an incidental impurity from the other glass components.
  • the glass composition does not include any intentionally added lead.
  • the glass frit preferably comprises a relatively high total content of silicon oxide (S1O2), boron oxide (B2O3), and bismuth oxide (B12O3). These oxides typically function as glass forming oxides.
  • the glass frit comprises S1O2 in the range of about 40 to about 70 wt%, based upon 100% total weight of the glass frit, and preferably about 40 to about 65 wt%.
  • the glass frit comprises about 40 to about 50 wt% S1O2, or alternatively about 55 to about 65 wt% S1O2, based upon 100% total weight of the glass frit.
  • the glass frit comprises about 1 to about 30 wt% B2O3, preferably about 5 to about 25 wt%, based upon 100% total weight of the glass frit.
  • the glass frit may comprise about 5 to about 20 wt% B2O3, preferably about 5 to about 10 wt%, based upon 100%) total weight of the glass frit.
  • the glass frit may comprise about 15 to about
  • the glass frit further comprises about 5 to about 30 wt%> B12O3, preferably about 5 to about 25 wt%>, and more preferably about 10 to about 25 wt%>, based upon 100%> total weight of the glass frit.
  • the glass frit may comprise about 10 to about 15 wt%> B12O3, or about 15 to about 25 wt%> B12O3, based upon 100%> total weight of the glass frit.
  • the total content of S1O2, B2O3, and B12O3 in the glass frit is preferably at least 65 wt%> and no more than about 95 wt%>, based upon 100%> total weight of the glass frit.
  • the total content of S1O2, B2O3, and B12O3 is at least 70 wt%> and no more than about 90 wt%>.
  • the glass frit preferably includes at least one alkaline oxide, such as, for example, Na20, L12O, or K2O. These alkaline oxides typically function as glass modifiers which decrease the chemical reactivity of the glass frit at elevated firing temperatures.
  • the glass frit includes a combination of at least two of these alkaline oxide(s), such as Na20 and L12O.
  • the glass frit includes at least about 4 wt%>, preferably at least about 5 wt%>, total alkaline oxide(s), based upon 100%> total weight of the glass frit.
  • the glass frit includes no more than about 20 wt%> alkaline oxide(s), preferably no more than about 10 wt%>, and most preferably no more than about 8 wt%, based upon 100% total weight of the glass frit. In an alternative embodiment, the glass frit includes about 16-20 wt% total alkaline oxide(s).
  • the glass frit may also include zinc oxide (ZnO). If the glass frit includes ZnO, it is present in an amount of no more than 15 wt%, preferably no more than 12 wt%, and most preferably no more than 10 wt%, based upon 100% total weight of the glass frit.
  • ZnO zinc oxide
  • the glass frit of the invention also includes at least one oxide of molybdenum (e.g., M0O3), niobium, (e.g., >2 ⁇ 5 ), aluminum, sulfur, selenium, tellurium, vanadium, and tungsten.
  • the glass frit preferably includes at least about 2 wt% M0O3, preferably at least about 3 wt%, and more preferably at least about 4 wt%, based upon 100%) total weight of the glass frit.
  • the glass frit includes no more than about 15 wt%> M0O3, and preferably no more than about 10 wt%> M0O3.
  • the glass frit includes no more than about 9 wt%> M0O3. If present, the glass frit includes less than about 2 wt% of NbiOs.
  • the weight ratio of the content of (S1O2 + B2O3 + B12O3) to the total content of (M0O3 + >2 ⁇ 5 ) is at least about 8 and no more than about 25, based upon 100%) total weight of each of these components.
  • the weight ratio of ((S1O2 + B2O3 + B12O3) + (M0O3 + >2 ⁇ 5 )) to (alkaline oxide(s) + ZnO) is at least about 4 and preferably no more than about 18, based upon 100%> total weight of each of these components.
  • the glass frit may include other elements, oxides, compounds which generate oxides upon heating, and/or mixtures thereof.
  • the glass frit may include other oxides or compounds known to one skilled in the art, including, but not limited to, magnesium, titanium, zirconium, nickel, gadolinium, antimony, cerium, zirconium, titanium, manganese, tin, ruthenium, cobalt, iron, copper, germanium, indium, alkaline earth metals, rare earth metals, phosphorous, and chromium, or any combination of at least two thereof, compounds which can generate those metal oxides upon firing, or a mixture of at least two of the aforementioned metals, a mixture of at least two of the aforementioned oxides, a mixture of at least two of the aforementioned compounds which can generate those metal oxides on firing, or mixtures of two or more of any of the above mentioned.
  • Preferred glass frits according to the invention are powders of amorphous or partially crystalline solids which exhibit a glass transition.
  • the glass transition temperature T g is the temperature at which an amorphous substance transforms from a rigid solid to a partially mobile undercooled melt upon heating.
  • Another important characteristic of the glass frits of the invention is the glass softening point, which is typically lower than the glass transition temperature and which identifies the point at which the glass begins to soften beyond some arbitrary point. Methods for the determination of the glass softening and glass transition temperatures are well known to the person skilled in the art.
  • the glass transition temperature T g may be determined using a DSC apparatus SDT Q600 (commercially available from TA Instruments) which simultaneously records differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) curves.
  • the instrument is equipped with a horizontal balance and furnace with a platinum/platinum- rhodium (type R) thermocouple.
  • the sample holders used are aluminum oxide ceramic crucibles with a capacity of about 40-90 ⁇ .
  • the measurement software Q Advantage; Thermal Advantage Release 5.4.0 and Universal Analysis 2000, version 4.5A Build 4.5.0.5 is applied respectively.
  • As pan for reference and sample aluminum oxide pan having a volume of about 85 ⁇ is used.
  • An amount of about 10-50 mg of the sample is weighted into the sample pan with an accuracy of 0.01 mg.
  • the empty reference pan and the sample pan are placed in the apparatus, the oven is closed and the measurement started.
  • a heating rate of 10 K/min is employed from a starting temperature of 25 °C to an end temperature of 1000 °C.
  • the balance in the instrument is always purged with nitrogen (N 2 5.0) and the oven is purged with synthetic air (80% N2 and 20% O2 from Linde) with a flow rate of 50 ml/min.
  • the first step in the DSC signal is evaluated as glass transition using the software described above, and the determined onset value is taken as the temperature for T g .
  • the T g is below the desired firing temperature of the electroconductive paste.
  • preferred glass frits have a T g of at least about 400°C.
  • preferred glass frits have a T g of no more than about 900°C, preferably no more than about 800°C, and most preferably no more than about 700°C.
  • the glasses have a glass softening point of about 400-550°C, more preferably about 480-530°C.
  • the glasses have a glass softening point of about 650-800°C, preferably about 690-760°C.
  • glass frit particles can exhibit a variety of shapes, sizes, and surface area to volume ratios.
  • the glass particles may exhibit the same or similar shapes (including length:width:thickness ratio) as may be exhibited by the conductive metallic particles, as discussed herein.
  • Glass frit particles with a shape, or combination of shapes, which favor improved electrical contact of the produced electrode are preferred. It is preferred that the median particle diameter dso of the glass frit particles (as set forth above with respect to the conductive metallic particles) be at least about 0.1 ⁇ .
  • the dso of the glass frit be no more than about 10 ⁇ , more preferably no more than about 5 ⁇ , and most preferably no more than about 2.5 ⁇ .
  • the glass frit particles have a specific surface area of at least about 0.5 m 2 /g, preferably at least about 1 m 2 /g, and most preferably at least about 2 m 2 /g.
  • the specific surface area be no more than about 15 m 2 /g, preferably no more than about 10 m 2 /g.
  • the glass frit particles may include a surface coating. Any such coating known in the art and which is considered to be suitable in the context of the invention can be employed on the glass frit particles.
  • Preferred coatings according to the invention include those coatings which promote dispersion of the glass in the organic vehicle and improved contact of the electroconductive paste. If such a coating is present, it is preferred that the coating correspond to no more than about 10 wt%, preferably no more than about 8 wt%, most preferably no more than about 5 wt%, in each case based on the total weight of the glass frit particles.
  • the electroconductive paste includes at least about 0.5 wt% glass frit, preferably at least about 1 wt%, and most preferably at least about 2 wt%, based upon 100% total weight of the paste.
  • the paste preferably includes no more than about 10 wt% glass frit, preferably no more than about 8 wt%, more preferably no more than about 6 wt%, and most preferably no more than about 5 wt%, based upon 100% total weight of the electroconductive paste.
  • the electroconductive composition also comprises conductive metallic particles.
  • conductive metallic particles are those which exhibit optimal conductivity and which effectively sinter upon firing, such that they yield electrodes with high conductivity.
  • Conductive metallic particles known in the art suitable for use in forming solar cell electrodes are preferred.
  • Preferred metallic particles include, but are not limited to, elemental metals, alloys, metal derivatives, mixtures of at least two metals, mixtures of at least two alloys or mixtures of at least one metal with at least one alloy.
  • the electroconductive paste may comprise at least about 30 wt% metallic particles, preferably at least about 35 wt%, more preferably at least about 40 wt%, more preferably at least about 45 wt%, and most preferably at least about 50 wt%, based upon 100% total weight of the paste.
  • the electroconductive paste comprises at least about 55 wt% metallic particles, based upon 100% total weight of the paste.
  • the electroconductive paste preferably includes no more than about 99 wt% metallic particles, preferably no more than about 95 wt%, more preferably no more than about 90 wt%, more preferably no more than about 85 wt%, more preferably no more than about 80 wt%, more preferably no more than about 75 wt%, more preferably no more than about 70 wt%, and more preferably no more than about 65 wt%, based upon 100% total weight of the paste composition.
  • the electroconductive paste includes no more than about 60 wt%, based upon 100%) total weight of the paste composition.
  • Electroconductive pastes having this conductive metallic particle content are suitable for use in dual printing techniques, either as the electroconductive paste used to form the second layer of the front side finger lines and/or to form the front side busbars, as well as in forming backside soldering tabs on a PERC solar cell.
  • Metals which may be employed as the metallic particles include at least one of silver, copper, gold, aluminum, nickel, platinum, palladium, molybdenum, and mixtures or alloys thereof.
  • the metallic particles are silver.
  • the silver may be present as elemental silver, a silver alloy, or silver derivate. Suitable silver derivatives include, for example, silver alloys and/or silver salts, such as silver halides (e.g., silver chloride), silver oxide, silver nitrate, silver acetate, silver trifluoroacetate, silver orthophosphate, and combinations thereof.
  • the metallic particles may comprise a metal or alloy coated with one or more different metals or alloys, for example silver particles coated with aluminum or copper particles coated with silver.
  • the metallic particles may be present with a surface coating, either organic or inorganic. Any such coating known in the art, and which is considered to be suitable in the context of the invention, may be employed on the metallic particles.
  • Preferred organic coatings are those coatings which promote dispersion into the organic vehicle.
  • Preferred inorganic coatings are those coatings which regulate sintering and promote adhesive performance of the resulting electroconductive paste. If such a coating is present, it is preferred that the coating correspond to no more than about 5 wt%, preferably no more than about 2 wt%, and most preferably no more than about 1 wt%, based on 100% total weight of the metallic particles.
  • the conductive particles can exhibit a variety of shapes, sizes, and specific surface areas. Some examples of shapes include, but are not limited to, spherical, angular, elongated (rod or needle like) and flat (sheet like). Conductive metallic particles may also be present as a combination of particles with different shapes, such as, for example, a combination of spherical metallic particles and flake-shaped metallic particles.
  • dso is the median diameter or the medium value of the particle size distribution. It is the value of the particle diameter at 50% in the cumulative distribution.
  • Particle size distribution may be measured via laser diffraction, dynamic light scattering, imaging, electrophoretic light scattering, or any other methods known in the art. Specifically, particle size according to the invention is determined in accordance with ISO 13317-3 :2001. As set forth herein, a Horiba LA-910 Laser Diffraction Particle Size Analyzer connected to a computer with an LA-910 software program is used to determine the median particle diameter. The relative refractive index of the metallic particle is chosen from the LA-910 manual and entered into the software program.
  • the test chamber is filled with deionized water to the proper fill line on the tank.
  • the solution is then circulated by using the circulation and agitation functions in the software program. After one minute, the solution is drained. This is repeated an additional time to ensure the chamber is clean of any residual material.
  • the chamber is then filled with deionized water for a third time and allowed to circulate and agitate for one minute. Any background particles in the solution are eliminated by using the blank function in the software. Ultrasonic agitation is then started, and the metallic particles are slowly added to the solution in the test chamber until the transmittance bars are in the proper zone in the software program. Once the transmittance is at the correct level, the laser diffraction analysis is run and the particle size distribution of the metallic component is measured and given as dso.
  • the median particle diameter dso of the metallic particles be at least about 0.1 ⁇ , and preferably at least about 0.5 ⁇ . At the same time, the dso is preferably no more than about 5 ⁇ , and more preferably no more than about 4 ⁇ .
  • the electroconductive paste composition includes a combination of spherical silver particles and flake- shaped silver particles, in which the dso of the spherical silver particles is preferably less than or equal to about 3 ⁇ and the dso of the flake-shaped silver particles is preferably less than or equal to about 5 ⁇ .
  • Specific surface area is a property of solids equal to the total surface area of the material per unit mass, solid, or bulk volume, or cross sectional area. It is defined either by surface area divided by mass (with units of m 2 /g) or surface area divided by volume (units of m "1 ).
  • the specific surface area may be measured by the BET (Brunauer-Emmett-Teller) method, which is known in the art. As set forth herein, BET measurements are made in accordance with DIN ISO 9277: 1995.
  • a Monosorb Model MS-22 instrument manufactured by Quantachrome Instruments
  • SMART method Standard Method with Adaptive dosing Rate
  • aluminum oxide available from Quantachrome Instruments as surface area reference material Cat. No. 2003
  • Samples are prepared for analysis in the built-in degas station. Flowing gas (30% N2 and 70% He) sweeps away impurities, resulting in a clean surface upon which adsorption may occur. The sample can be heated to a user-selectable temperature with the supplied heating mantle. Digital temperature control and display are mounted on the instrument front panel. After degassing is complete, the sample cell is transferred to the analysis station.
  • Quick connect fittings automatically seal the sample cell during transfer, and the system is then activated to commence the analysis.
  • a dewar flask filled with coolant is manually raised, immersing the sample cell and causing adsorption.
  • the instrument detects when adsorption is complete (2-3 minutes), automatically lowers the dewar flask, and gently heats the sample cell back to room temperature using a built-in hot-air blower.
  • the desorbed gas signal is displayed on a digital meter and the surface area is directly presented on a front panel display.
  • the entire measurement (adsorption and desorption) cycle typically requires less than six minutes.
  • the technique uses a high sensitivity, thermal conductivity detector to measure the change in concentration of an adsorbate/inert carrier gas mixture as adsorption and desorption proceed.
  • the detector When integrated by the on-board electronics and compared to calibration, the detector provides the volume of gas adsorbed or desorbed.
  • N2 5.0 with a molecular cross-sectional area of 0.162 nm 2 at 77K is used for the calculation.
  • a one-point analysis is performed and a built-in microprocessor ensures linearity and automatically computes the sample's BET surface area in m 2 /g.
  • the metallic particles may have a specific surface area of at least about 0.1 m 2 /g, preferably at least about 0.2 m 2 /g. At the same time, the specific surface area is preferably no more than 10 m 2 /g, and more preferably no more than about 5 m 2 /g. In one embodiment, the metallic particles have a specific surface area of between about 0.7 and 1.7 m 2 /g.
  • the electroconductive paste of the invention also comprises an organic vehicle.
  • the organic vehicle is present in the electroconductive paste in an amount of at least about 0.01 wt%, preferably at least about 0.5 wt%, more preferably at least about 1 wt%, more preferably at least about 5 wt%, more preferably at least about 10 wt%, more preferably at least about 15 wt%, more preferably at least about 20 wt%, more preferably at least about 25 wt%, and more preferably at least about 30 wt%, based upon 100% total weight of the paste.
  • the electroconductive paste includes at least about 35 wt% of organic vehicle, based upon 100%) total weight of the paste.
  • the electroconductive paste includes no more than about 60 wt%>, preferably no more than about 55 wt%>, preferably no more than about 50 wt%>, and preferably no more than about 45 wt%>, based upon 100%> total weight of the paste. In a preferred embodiment, the electroconductive paste includes no more than about 40 wt%>, based upon 100%> total weight of the paste.
  • Preferred organic vehicles in the context of the invention are solutions, emulsions or dispersions based on one or more solvents, preferably organic solvent(s), which ensure that the components of the electroconductive paste are present in a dissolved, emulsified or dispersed form.
  • Preferred organic vehicles are those which provide optimal stability of the components of the electroconductive paste and endow the paste with a viscosity allowing effective printability.
  • the organic vehicle comprises an organic solvent and optionally one or more of a binder (e.g., a polymer), a surfactant and a thixotropic agent.
  • a binder e.g., a polymer
  • the organic vehicle comprises one or more binders in an organic solvent.
  • Preferred binders in the context of the invention are those which contribute to the formation of an electroconductive paste with favorable stability, printability, viscosity and sintering properties. All binders which are known in the art, and which are considered to be suitable in the context of this invention, may be employed as the binder in the organic vehicle. Preferred binders (which often fall within the category termed "resins") are polymeric binders, monomeric binders, and binders which are a combination of polymers and monomers. Polymeric binders can also be copolymers wherein at least two different monomeric units are contained in a single molecule.
  • Preferred polymeric binders are those which carry functional groups in the polymer main chain, those which carry functional groups off of the main chain and those which carry functional groups both within the main chain and off of the main chain.
  • Preferred polymers carrying functional groups in the main chain are for example polyesters, substituted polyesters, polycarbonates, substituted polycarbonates, polymers which carry cyclic groups in the main chain, poly-sugars, substituted poly-sugars, polyurethanes, substituted polyurethanes, polyamides, substituted polyamides, phenolic resins, substituted phenolic resins, copolymers of the monomers of one or more of the preceding polymers, optionally with other co-monomers, or a combination of at least two thereof.
  • the binder may be polyvinyl butyral or polyethylene.
  • Preferred polymers which carry cyclic groups in the main chain are for example polyvinylbutylate (PVB) and its derivatives and poly-terpineol and its derivatives or mixtures thereof.
  • Preferred poly-sugars are for example cellulose and alkyl derivatives thereof, preferably methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, propyl cellulose, hydroxypropyl cellulose, butyl cellulose and their derivatives and mixtures of at least two thereof.
  • Other preferred polymers are cellulose ester resins, e.g., cellulose acetate propionate, cellulose acetate buyrate, and any combinations thereof.
  • Preferred polymers which carry functional groups off of the main polymer chain are those which carry amide groups, those which carry acid and/or ester groups, often called acrylic resins, or polymers which carry a combination of aforementioned functional groups, or a combination thereof.
  • Preferred polymers which carry amide off of the main chain are for example polyvinyl pyrrolidone (PVP) and its derivatives.
  • Preferred polymers which carry acid and/or ester groups off of the main chain are for example polyacrylic acid and its derivatives, polymethacrylate (PMA) and its derivatives or polymethylmethacrylate (PMMA) and its derivatives, or a mixture thereof.
  • Preferred monomeric binders are ethylene glycol based monomers, terpineol resins or rosin derivatives, or a mixture thereof.
  • Preferred monomeric binders based on ethylene glycol are those with ether groups, ester groups or those with an ether group and an ester group, preferred ether groups being methyl, ethyl, propyl, butyl, pentyl, hexyl, and higher alkyl ethers, the preferred ester group being acetate and its alkyl derivatives, preferably ethylene glycol monobutyl ether monoacetate or a mixture thereof.
  • Alkyl cellulose preferably ethyl cellulose, its derivatives and mixtures thereof with other binders from the preceding lists of binders or otherwise are the most preferred binders in the context of the invention.
  • the binder may be present in an amount of at least about 0.1 wt%, and preferably at least about 0.5 wt%, based upon 100% total weight of the organic vehicle.
  • the binder may be present in an amount of no more than about 10 wt%, preferably no more than about 8 wt%, and more preferably no more than about 7 wt%, based upon 100% total weight of the organic vehicle.
  • Preferred solvents are components which are removed from the paste to a significant extent during firing. Preferably, they are present after firing with an absolute weight reduced by at least about 80% compared to before firing, preferably reduced by at least about 95% compared to before firing. Preferred solvents are those which contribute to favorable viscosity, printability, stability and sintering characteristics. All solvents which are known in the art, and which are considered to be suitable in the context of this invention, may be employed as the solvent in the organic vehicle.
  • Preferred solvents are those which exist as a liquid under standard ambient temperature and pressure (SATP) (298.15 K, 25 °C, 77 °F), 100 kPa (14.504 psi, 0.986 atm), preferably those with a boiling point above about 90 °C and a melting point above about -20 °C.
  • SATP standard ambient temperature and pressure
  • Preferred solvents are polar or non-polar, protic or aprotic, aromatic or non-aromatic.
  • Preferred solvents are mono-alcohols, di-alcohols, poly-alcohols, mono-esters, di-esters, poly-esters, mono- ethers, di-ethers, poly-ethers, solvents which comprise at least one or more of these categories of functional group, optionally comprising other categories of functional group, preferably cyclic groups, aromatic groups, unsaturated bonds, alcohol groups with one or more O atoms replaced by heteroatoms, ether groups with one or more O atoms replaced by heteroatoms, esters groups with one or more O atoms replaced by heteroatoms, and mixtures of two or more of the aforementioned solvents.
  • Preferred esters in this context are di-alkyl esters of adipic acid, preferred alkyl constituents being methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups or combinations of two different such alkyl groups, preferably dimethyladipate, and mixtures of two or more adipate esters.
  • Preferred ethers in this context are diethers, preferably dialkyl ethers of ethylene glycol, preferred alkyl constituents being methyl, ethyl, propyl, butyl, pentyl, hexyl and higher alkyl groups or combinations of two different such alkyl groups, and mixtures of two diethers.
  • Preferred alcohols in this context are primary, secondary and tertiary alcohols, preferably tertiary alcohols, terpineol and its derivatives being preferred, or a mixture of two or more alcohols.
  • Preferred solvents which combine more than one different functional groups are 2,2,4-trimethyl-l,3-pentanediol monoisobutyrate, often called texanol, and its derivatives, 2- (2-ethoxyethoxy)ethanol, often known as carbitol, its alkyl derivatives, preferably methyl, ethyl, propyl, butyl, pentyl, and hexyl carbitol, preferably hexyl carbitol or butyl carbitol, and acetate derivatives thereof, preferably butyl carbitol acetate, or mixtures of at least two of the aforementioned.
  • the organic solvent may be present in an amount of at least about 60 wt%, and more preferably at least about 70 wt%, and most preferably at least about 80wt%, based upon 100% total weight of the organic vehicle. At the same time, the organic solvent may be present in an amount of no more than about 99 wt%, more preferably no more than about 95 wt%, based upon 100% total weight of the organic vehicle.
  • the organic vehicle may also comprise one or more surfactants and/or additives.
  • Preferred surfactants are those which contribute to the formation of an electroconductive paste with favorable stability, printability, viscosity and sintering properties. All surfactants which are known in the art, and which are considered to be suitable in the context of this invention, may be employed as the surfactant in the organic vehicle.
  • Preferred surfactants are those based on linear chains, branched chains, aromatic chains, fluorinated chains, siloxane chains, polyether chains and combinations thereof.
  • Preferred surfactants include, but are not limited to, single chained, double chained or poly chained polymers.
  • Preferred surfactants may have non-ionic, anionic, cationic, amphiphilic, or zwitterionic heads.
  • Preferred surfactants may be polymeric and monomeric or a mixture thereof.
  • Preferred surfactants may have pigment affinic groups, preferably hydroxyfunctional carboxylic acid esters with pigment affinic groups (e.g., DISPERBYK®-108, manufactured by BYK USA, Inc.), acrylate copolymers with pigment affinic groups (e.g., DISPERBYK®-116, manufactured by BYK USA, Inc.), modified poly ethers with pigment affinic groups (e.g., TEGO® DISPERS 655, manufactured by Evonik Tego Chemie GmbH), other surfactants with groups of high pigment affinity (e.g., TEGO® DISPERS 662 C, manufactured by Evonik Tego Chemie GmbH).
  • pigment affinic groups preferably hydroxyfunctional carboxylic acid esters with pigment affinic groups (e.
  • polymers not in the above list include, but are not limited to, polyethylene oxide, polyethylene glycol and its derivatives, and alkyl carboxylic acids and their derivatives or salts, or mixtures thereof.
  • the preferred polyethylene glycol derivative is poly(ethyleneglycol)acetic acid.
  • Preferred alkyl carboxylic acids are those with fully saturated and those with singly or poly unsaturated alkyl chains or mixtures thereof.
  • Preferred carboxylic acids with saturated alkyl chains are those with alkyl chains lengths in a range from about 8 to about 20 carbon atoms, preferably C9H19COOH (capric acid), C11H23COOH (Laurie acid), C13H27COOH (myristic acid) C15H31COOH (palmitic acid), C17H35COOH (stearic acid), or salts or mixtures thereof.
  • Preferred carboxylic acids with unsaturated alkyl chains are C18H34O2 (oleic acid) and C18H32O2 (linoleic acid).
  • the preferred monomeric surfactant is benzotriazole and its derivatives.
  • the surfactant may be at least about 0.01 wt%, based upon 100% total weight of the organic vehicle. At the same time, the surfactant is preferably no more than about 10 wt%, preferably no more than about 8 wt%, and more preferably no more than about 6 wt%, based upon 100% total weight of the organic vehicle.
  • Preferred additives in the organic vehicle are those materials which are distinct from the aforementioned components and which contribute to favorable properties of the electroconductive paste, such as advantageous viscosity, printability, stability and sintering characteristics.
  • Additives known in the art, and which are considered to be suitable in the context of the invention, may be used.
  • Preferred additives include, but are not limited to, thixotropic agents, viscosity regulators, stabilizing agents, inorganic additives, thickeners, emulsifiers, dispersants and pH regulators.
  • Preferred thixotropic agents include, but are not limited to, carboxylic acid derivatives, preferably fatty acid derivatives or combinations thereof.
  • Preferred fatty acid derivatives include, but are not limited to, C9H19COOH (capric acid), C11H23COOH (Laurie acid), C13H27COOH (myristic acid) C15H31COOH (palmitic acid), C17H35COOH (stearic acid) C18H34O2 (oleic acid), C18H32O2 (linoleic acid) and combinations thereof.
  • a preferred combination comprising fatty acids in this context is castor oil.
  • Preferred additives are components added to the paste, in addition to the other components explicitly mentioned, which contribute to increased electrical performance of the paste, of the electrodes produced thereof, or of the resulting solar cell.
  • additives can also be present in the electroconductive paste separately.
  • Preferred additives include, but are not limited to, thixotropic agents, viscosity regulators, emulsifiers, stabilizing agents or pH regulators, inorganic additives, thickeners and dispersants, or a combination of at least two thereof.
  • Preferred inorganic organometallic additives include, but are not limited to, Mg, Ni, Te, W, Zn, Mg, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Rh, V, Y, Sb, P, Cu and Cr or a combination of at least two thereof, preferably Zn, Sb, Mn, Ni, W, Te, Rh, V, Y, Sb, P and Ru, or a combination of at least two thereof, oxides thereof, compounds which can generate those metal oxides on firing, or a mixture of at least two of the aforementioned metals, a mixture of at least two of the aforementioned oxides, a mixture of at least two of the aforementioned compounds which can generate those metal oxides on firing, or mixtures of two or more of any of the above mentioned.
  • the electroconductive paste comprises an additive that promotes adhesion, such as an oxide of manganese (e.g., MnCh), zinc (e.g., ZnO), aluminum (e.g., AI2O3), or bismuth (e.g., B12O3).
  • an additive that promotes adhesion such as an oxide of manganese (e.g., MnCh), zinc (e.g., ZnO), aluminum (e.g., AI2O3), or bismuth (e.g., B12O3).
  • the paste may include at least about 0.1 wt% additive(s).
  • the paste preferably includes no more than about 10 wt% additive(s), preferably no more than about 5 wt%, and most preferably no more than about 2 wt%, based upon 100% total weight of the paste.
  • the electroconductive paste comprises no more than about 1 wt% additive(s), based upon 100% total weight of the paste.
  • the glass frit materials are combined with the conductive metallic particles and organic vehicle using any method known in the art for preparing a paste composition.
  • the method of preparation is not critical, as long as it results in a homogenously dispersed paste.
  • the components can be mixed, such as with a mixer, then passed through a three roll mill, for example, to make a dispersed uniform paste.
  • the raw glass frit materials can be co-milled with silver particles, for example, in a ball mill for 2-24 hours to achieve a homogenous mixture of glass frit and silver particles, which are then mixed with the organic vehicle.
  • the invention also relates to solar cells.
  • the solar cell comprises a semiconductor substrate (e.g., a silicon wafer) and an electroconductive paste composition according to any of the embodiments described herein.
  • the invention relates to a solar cell prepared by a process which includes applying an electroconductive paste composition according to any of the embodiments described herein to a semiconductor substrate (e.g., a silicon wafer) and firing the semiconductor substrate.
  • a semiconductor substrate e.g., a silicon wafer
  • Preferred wafers according to the invention have regions, among other regions of the solar cell, capable of absorbing light with high efficiency to yield electron-hole pairs and separating holes and electrons across a boundary with high efficiency, preferably across a p-n junction boundary.
  • Preferred wafers according to the invention are those comprising a single body made up of a front doped layer and a back doped layer.
  • the wafer comprises appropriately doped tetravalent elements, binary compounds, tertiary compounds or alloys.
  • Preferred tetravalent elements in this context include, but are not limited to, silicon, germanium, or tin, preferably silicon.
  • Preferred binary compounds include, but are not limited to, combinations of two or more tetravalent elements, binary compounds of a group III element with a group V element, binary corn-pounds of a group II element with a group VI element or binary compounds of a group IV element with a group VI element.
  • Preferred combinations of tetravalent elements include, but are not limited to, combinations of two or more elements selected from silicon, germanium, tin or carbon, preferably SiC.
  • the preferred binary compounds of a group III element with a group V element is GaAs.
  • the wafer is silicon. The foregoing description, in which silicon is explicitly mentioned, also applies to other wafer compositions described herein.
  • the p-n junction boundary is located where the front doped layer and back doped layer of the wafer meet.
  • the back doped layer is doped with an electron donating n-type dopant and the front doped layer is doped with an electron accepting or hole donating p- type dopant.
  • the back doped layer is doped with p-type dopant and the front doped layer is doped with n-type dopant.
  • a wafer with a p-n junction boundary is prepared by first providing a doped silicon substrate and then applying a doped layer of the opposite type to one face of that substrate.
  • the doped silicon substrate can be prepared by any method known in the art and considered suitable for the invention.
  • Preferred sources of silicon substrates according to the invention include, but are not limited to, mono-crystalline silicon, multi-crystalline silicon, amorphous silicon and upgraded metallurgical silicon, most preferably mono-crystalline silicon or multi-crystalline silicon.
  • Doping to form the doped silicon substrate can be carried out simultaneously by adding the dopant during the preparation of the silicon substrate, or it can be carried out in a subsequent step. Doping subsequent to the preparation of the silicon substrate can be carried out by gas diffusion epitaxy, for example.
  • Doped silicon substrates are also readily commercially available. According to one embodiment, the initial doping of the silicon substrate may be carried out simultaneously to its formation by adding dopant to the silicon mix.
  • the application of the front doped layer and the highly doped back layer, if present, may be carried out by gas-phase epitaxy.
  • This gas phase epitaxy is preferably carried out at a temperature of at least about 500 °C, preferably at least about 600°C, and most preferably at least about 650°C.
  • the temperature is preferably no more than about 900°C, preferably no more than about 800°C, and most preferably no more than about 750°C.
  • the gas phase epitaxy is preferably carried out at a pressure of at least about 2 kPa, preferably at least about 10 kPa, and most preferably at least about 40 kPa.
  • the pressure is preferably no more than about 100 kPa, preferably no more than about 80 kPa, and most preferably no more than about 70 kPa.
  • silicon substrates can exhibit a number of shapes, surface textures and sizes.
  • the shape of the substrate may include cuboid, disc, wafer and irregular polyhedron, to name a few.
  • the wafer is a cuboid with two dimensions which are similar, preferably equal, and a third dimension which is significantly smaller than the other two dimensions.
  • the third dimension may be at least 100 times smaller than the first two dimensions.
  • silicon substrates with rough surfaces are preferred.
  • One way to assess the roughness of the substrate is to evaluate the surface roughness parameter for a sub-surface of the substrate, which is small in comparison to the total surface area of the substrate, preferably less than about one hundredth of the total surface area, and which is essentially planar.
  • the value of the surface roughness parameter is given by the ratio of the area of the sub-surface to the area of a theoretical surface formed by projecting that sub-surface onto the flat plane best fitted to the sub-surface by minimizing mean square displacement.
  • a higher value of the surface roughness parameter indicates a rougher, more irregular surface and a lower value of the surface roughness parameter indicates a smoother, more even surface.
  • the surface roughness of the silicon substrate is preferably modified so as to produce an optimum balance between a number of factors including, but not limited to, light absorption and adhesion to the surface.
  • the two larger dimensions of the silicon substrate can be varied to suit the application required of the resultant solar cell. It is preferred according to the invention for the thickness of the silicon wafer to be below about 0.5 mm, more preferably below about 0.3 mm, and most preferably below about 0.2 mm. Some wafers have a minimum thickness of 0.01 mm or more.
  • the front doped layer be thin in comparison to the back doped layer. It is also preferred that the front doped layer have a thickness of at least about 0.1 ⁇ , and preferably no more than about 10 ⁇ , preferably no more than about 5 ⁇ , and most preferably no more than about 2 ⁇ .
  • Preferred dopants are those which, when added to the silicon wafer, form a p-n junction boundary by introducing electrons or holes into the band structure. It is preferred that the identity and concentration of these dopants is specifically selected so as to tune the band structure profile of the p-n junction and set the light absorption and conductivity profiles as required.
  • Preferred p- type dopants include, but are not limited to, those which add holes to the silicon wafer band structure. All dopants known in the art and which are considered suitable in the context of the invention can be employed as p-type dopants.
  • Preferred p-type dopants include, but are not limited to, trivalent elements, particularly those of group 13 of the periodic table. Preferred group 13 elements of the periodic table in this context include, but are not limited to, boron, aluminum, gallium, indium, thallium, or a combination of at least two thereof, wherein boron is particularly preferred.
  • Preferred n-type dopants are those which add electrons to the silicon wafer band structure.
  • Preferred n-type dopants are elements of group 15 of the periodic table.
  • Preferred group 15 elements of the periodic table in this context include, but are not limited to, nitrogen, phosphorus, arsenic, antimony, bismuth or a combination of at least two thereof, wherein phosphorus is particularly preferred.
  • the various doping levels of the p-n junction can be varied so as to tune the desired properties of the resulting solar cell. Doping levels are measured using secondary ion mass spectroscopy.
  • the semiconductor substrate i.e., silicon wafer
  • the device "GP4- Test Pro” equipped with software package “GP-4 Test 1.6.6 Pro” available from GP Solar GmbH
  • the four point measuring principle is applied.
  • the two outer probes apply a constant current and two inner probes measure the voltage.
  • the sheet resistance is deduced using the Ohmic law in ⁇ / To determine the average sheet resistance, the measurement is performed on 25 equally distributed spots of the wafer.
  • the "GP-Test.Pro” is equipped with a 4-point measuring head (Part Number 04.01.0018) with sharp tips in order to penetrate the anti -reflection and/or passivation layers. A current of 10 mA is applied. The measuring head is brought into contact with the non- metallized wafer material and the measurement is started. After measuring 25 equally distributed spots on the wafer, the average sheet resistance is calculated in ⁇ /
  • a solar cell obtainable from a process according to the invention.
  • Preferred solar cells according to the invention are those which have a high efficiency, in terms of proportion of total energy of incident light converted into electrical energy output, and those which are light and durable.
  • a solar cell includes: (i) front electrodes, (ii) a front doped layer, (iii) a p-n junction boundary, (iv) a back doped layer, and (v) soldering tabs.
  • the solar cell may also include additional layers, particularly PERC solar cells, as set forth herein.
  • an antireflective layer may be applied as the outer layer before the electrode is applied to the front face of the solar cell.
  • an antireflective layer may also be applied to the back surface of the solar cell as well. All antireflective layers known in the art and which are considered to be suitable in the context of the invention can be employed. Preferred antireflective layers are those which decrease the proportion of incident light reflected by the front face and increase the proportion of incident light crossing the front face to be absorbed by the wafer.
  • Antireflective layers which give rise to a favorable absorption/reflection ratio, are susceptible to etching by the electroconductive paste, are otherwise resistant to the temperatures required for firing of the electroconductive paste, and do not contribute to increased recombination of electrons and holes in the vicinity of the electrode interface, are preferred.
  • Preferred antireflective layers include, but are not limited to, SiNx, S1O2, AI2O3, T1O2 or mixtures of at least two thereof and/or combinations of at least two layers thereof.
  • the antireflective layer is SiNx, in particular where a silicon wafer is employed.
  • the thickness of antireflective layers is suited to the wavelength of the appropriate light.
  • the antireflective layers have a thickness of at least 20 nm, preferably at least 40 nm, and most preferably at least 60 nm.
  • the thickness is preferably no more than about 300 nm, more preferably no more than about 200 nm, and most preferably no more than about 90 nm.
  • One or more passivation layers may be applied to the front and/or backside of the silicon wafer as an outer layer.
  • the front passivation layer(s) may be applied before the front electrode is formed, or before the antireflective layer is applied (if one is present).
  • PERC solar cells have a rear passivation layer applied to the backside of the silicon wafer during wafer production, such as through plasma vapor deposition techniques. During production of a PERC solar cell, the front antireflective layer is first applied, then the rear passivation layer is applied, and lastly a rear capping layer is applied.
  • Preferred passivation layers are those which reduce the rate of electron/hole recombination in the vicinity of the electrode interface. Any passivation layer which is known in the art and which is considered to be suitable in the context of the invention can be employed. Preferred passivation layers according to the invention include, but are not limited to, aluminum oxide, silicon nitride, silicon dioxide and titanium dioxide. With respect to PERC solar cells, aluminum oxide (e.g., AI2O3) is preferred for forming the rear passivation layer. It is preferred for the passivation layer to have a thickness of at least 0.1 nm, preferably at least 10 nm, and most preferably at least 30 nm. As the same time, the thickness is preferably no more than about 2 ⁇ , preferably no more than about 1 ⁇ , and most preferably no more than about 200 nm. Additional Protective Layers
  • the cell can be encapsulated to provide chemical protection.
  • transparent polymers often referred to as transparent thermoplastic resins, are used as the encapsulation material, if such an encapsulation is present.
  • Preferred transparent polymers in this context are silicon rubber and polyethylene vinyl acetate (PVA).
  • a transparent glass sheet may also be added to the front of the solar cell to provide mechanical protection to the front face of the cell.
  • a back protecting material may be added to the back face of the solar cell to provide mechanical protection. Preferred back protecting materials are those having good mechanical properties and weather resistance.
  • the preferred back protection material according to the invention is polyethylene terephthalate with a layer of polyvinyl fluoride. It is preferred for the back protecting material to be present underneath the encapsulation layer (in the event that both a back protection layer and encapsulation are present).
  • a frame material can be added to the outside of the solar cell to give mechanical support.
  • Frame materials are well known in the art and any frame material considered suitable in the context of the invention may be employed.
  • the preferred frame material according to the invention is aluminum.
  • a dual-printed solar cell may be prepared by applying a first electroconductive paste in a first defined pattern, such as, for example, a busbar pattern, to the antireflection coating on the front side of the silicon wafer to form a first layer.
  • a second electroconductive paste i.e., the inventive pastes disclosed herein, may be applied on top of the first layer to form a different pattern, such as a layer of finger lines and/or another layer of busbars to reinforce the first pattern.
  • a backside electroconductive paste is then applied to the backside of the solar cell to form soldering tabs.
  • FIGS. 3 A-3D Various means of dual printing the first and second electroconductive pastes are illustrated in FIGS. 3 A-3D.
  • a first electroconductive paste is used to print only busbars
  • a second electroconductive paste is used to print only finger lines.
  • a first electroconductive paste is used to print only finger lines
  • a second electroconductive paste (the inventive paste) is used to print finger lines (on top of the first layer of finger lines) and busbars.
  • the first step is the same as in FIG. 3B, but in a second step, the second electroconductive paste is used to print only busbars.
  • the first electroconductive paste may be used to print both a first layer of finger lines and busbars
  • the second electroconductive paste may be used to print only a second layer of finger lines.
  • the electroconductive pastes may be applied in any manner known in the art and considered suitable in the context of the invention. Examples include, but are not limited to, impregnation, dipping, pouring, dripping on, injection, spraying, knife coating, curtain coating, brushing or printing or a combination of at least two thereof. Preferred printing techniques are ink-jet printing, screen printing, tampon printing, offset printing, relief printing or stencil printing or a combination of at least two thereof. It is preferred according to the invention that the electroconductive paste is applied by printing, preferably by screen printing. Specifically, the screens preferably have mesh opening with a diameter of about 40 ⁇ or less (e.g., about 35 ⁇ or less, about 30 ⁇ or less). At the same time, the screens preferably have a mesh opening with a diameter of at least 10 ⁇ .
  • a PERC solar cell is prepared as follows. First, the front side diffusion layer, as set forth above, is formed on the silicon substrate. On the front side of the substrate, an antireflection coating, such as those disclosed herein, is then applied. Next, the rear passivation layer, such as an alumina layer, is applied to the back surface of the substrate, such as by plasma vapor deposition. Additional rear surface layers, such as a silicon nitride capping layer, may then be applied on top of the rear passivation layer. Next, portions of the rear passivation layer and additional rear layers are removed to expose areas of the underlying substrate. This step may be achieved by, for example, acid etching or laser drilling.
  • An aluminum electroconductive paste is then applied on top of the rear layers, thus filling in the "holes" created by the removal process. In these areas, a local back surface field is formed when the aluminum paste is fired.
  • the backside soldering tabs may be applied to the back surface either before or after the application of the aluminum paste.
  • the front side electrodes are also formed concurrently with the backside soldering tabs according to conventional methods known in the art.
  • FIG. 2 An exemplary PERC solar cell is illustrated in FIG. 2.
  • AR refers to the antireflection layer
  • BSF refers to the back surface field
  • AlOx refers to the alumina rear passivation layer
  • SiNx refers to the rear capping layer
  • Rear-Al refers to the rear aluminum paste
  • Rear Ag refers to the backside soldering tabs.
  • the substrate is then subjected to one or more thermal treatment steps, such as, for example, conventional over drying, infrared or ultraviolet curing, and/or firing.
  • the substrate may be fired according to an appropriate profile. Firing sinters the printed electroconductive paste so as to form solid electrodes. Firing is well known in the art and can be effected in any manner considered suitable in the context of the invention. It is preferred that firing be carried out above the T g of the glass frit materials.
  • the maximum temperature set for firing is below about 1,000°C, preferably below about 900°C.
  • Furnace set temperatures as low as about 800°C have been employed for obtaining solar cells. Firing set temperatures should also allow for effective sintering of the metallic particles to be achieved.
  • the firing temperature profile is typically set so as to enable the burnout of organic materials from the electroconductive paste composition.
  • the firing step is typically carried out in air or in an oxygen-containing atmosphere in a belt furnace. It is preferred for firing to be carried out in a fast firing process with a total firing time of at least 30 seconds, and preferably at least 40 seconds.
  • the firing time is preferably no more than about 3 minutes, more preferably no more than about 2 minutes, and most preferably no more than about 1 minute.
  • the time that the wafer temperature is above 600°C is most preferably in a range from about 3 to 7 seconds.
  • the substrate may reach a peak temperature in the range of about 700 to 975°C for a period of about 1 to 5 seconds.
  • the firing may also be conducted at high transport rates, for example, about 100-700 cm/min, with resulting hold-up times of about 0.5 to 3 minutes. Multiple temperature zones, for example 3-12 zones, can be used to control the desired thermal profile.
  • Firing of electroconductive pastes on the front and back faces can be carried out simultaneously or sequentially. Simultaneous firing is appropriate if the electroconductive pastes applied to both faces have similar, preferably identical, optimum firing conditions. Where appropriate, it is preferred for firing to be carried out simultaneously. Where firing is carried out sequentially, it is preferable for the back electroconductive paste to be applied and fired first, followed by application and firing of the electroconductive paste to the front face of the substrate.
  • the electrical performance of a solar cell is measured using a commercial IV-tester "cetisPV-CTLl" from Halm Elektronik GmbH. All parts of the measurement equipment as well as the solar cell to be tested are maintained at 25°C during electrical measurement. This temperature should be measured simultaneously on the cell surface during the actual measurement by a temperature probe.
  • the Xe Arc lamp simulates the sunlight with a known AMI .5 intensity of 1000 W/m 2 on the cell surface. To bring the simulator to this intensity, the lamp is flashed several times within a short period of time until it reaches a stable level monitored by the "PVCTControl 4.313.0" software of the IV-tester.
  • the Halm IV tester uses a multi-point contact method to measure current (I) and voltage (V) to determine the solar cell's TV-curve. To do so, the solar cell is placed between the multi-point contact probes in such a way that the probe fingers are in contact with the bus bars (i.e., printed lines) of the solar cell. The numbers of contact probe lines are adjusted to the number of bus bars on the cell surface. All electrical values were determined directly from this curve automatically by the implemented software package. As a reference standard, a calibrated solar cell from ISE Freiburg consisting of the same area dimensions, same wafer material, and processed using the same front side layout, was tested and the data was compared to the certificated values.
  • a plurality of solar cells according to the invention can be arranged spatially and electrically connected to form a collective arrangement called a module.
  • Preferred modules according to the invention can have a number of arrangements, preferably a rectangular arrangement known as a solar panel.
  • a large variety of ways to electrically connect solar cells, as well as a large variety of ways to mechanically arrange and fix such cells to form collective arrangements, are well known in the art.
  • Preferred methods according to the invention are those which result in a low mass to power output ratio, low volume to power output ration, and high durability.
  • Aluminum is the preferred material for mechanical fixing of solar cells according to the invention.
  • multiple solar cells are connected in series and/or in parallel and the ends of the electrodes of the first cell and the last cell are preferably connected to output wiring.
  • the cell connectors are bonded to the area where the backside soldering tabs have been formed.
  • the solar cells are typically encapsulated in a transparent thermal plastic resin, such as silicon rubber or ethylene vinyl acetate.
  • a transparent sheet of glass is placed on the front surface of the encapsulating transparent thermal plastic resin.
  • a back protecting material for example, a sheet of polyethylene terephthalate coated with a film of polyvinyl fluoride, is placed under the encapsulating thermal plastic resin.
  • These layered materials may be heated in an appropriate vacuum furnace to remove air, and then integrated into one body by heating and pressing.
  • solar cells are typically left in the open air for a long time, it is desirable to cover the circumference of the solar cell with a frame material consisting of aluminum or the like.
  • a set of exemplary glass frit compositions (G1-G4) were each prepared according to the formulations set forth in Table 1 below.
  • Two reference glass frits were also prepared (G- REF1 and G-REF2). All amounts are provided in weight percent, based upon 100% total weight of the glass frit composition.
  • the glasses were formed using a melting and quenching process, whereby the starting materials were mixed at predetermined amounts in powder form. The mixture was then heated in air or in an oxygen-containing atmosphere to form a melt which was then quenched. The quenched glass was then ground, ball milled, and screened in order to provide a mixture with the desired particle size.
  • the pastes were then mixed to a uniform consistency.
  • a standard front side paste (SOL9621H manufactured by Heraeus Precious Metals North America Conshohocken LLC of West Conshohocken, Pennsylvania) was screen printed onto the front side of a blank multicrystalline silicon wafer with 90 ⁇ /D sheet resistance, using a screen with 360 mesh stainless steel wire, at about 16 ⁇ wire diameters and 15 ⁇ EOM, to form the finger lines.
  • the reference and exemplary pastes were then screen printed over the finger lines to form the busbars, as illustrated in FIG. 4.
  • a commercially available backside paste was used to form the backside soldering tabs, which extended across the full length of the cell and were about 4 mm wide.
  • Example 1 The same exemplary and reference pastes of Example 1 (with the exception of P3) were then used to form the backside soldering tabs on a PERC solar wafer (manufactured by SolarWorld Americas Inc. of Hillsboro, Oregon).
  • a standard front side paste (SOL9620A manufactured by Heraeus Precious Metals North America Conshohocken LLC of West Conshohocken, Pennsylvania) was screen printed onto the front side of PERC solar cell wafer (commercially available from

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Geochemistry & Mineralogy (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Dispersion Chemistry (AREA)
  • Sustainable Energy (AREA)
  • Sustainable Development (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Power Engineering (AREA)
  • Inorganic Chemistry (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Conductive Materials (AREA)

Abstract

L'invention concerne une composition de pâte électroconductrice présentant des caractéristiques de gravure réduites. La composition de pâte électroconductrice comprend des particules métalliques, au moins une fritte de verre, et un véhicule organique. La ou les frittes de verre sont exemptes de plomb et comprennent environ 65 à 95 % en poids d'au moins SiO2, B2O3, and Bi2O3, sur la base de 100 % du poids total de la ou des fritte de verre, environ 5 à 20 % en poids d'au moins un oxyde alcalin, et environ 3 à 10 % en poids d'oxyde de molybdène.
PCT/US2017/024055 2017-03-24 2017-03-24 Verres à faible gravure et sans contact pour compositions de pâte électroconductrice WO2018174898A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
PCT/US2017/024055 WO2018174898A1 (fr) 2017-03-24 2017-03-24 Verres à faible gravure et sans contact pour compositions de pâte électroconductrice
CN201780087366.9A CN110337423A (zh) 2017-03-24 2017-03-24 用于导电膏组合物的低蚀刻和非接触式玻璃
TW107103151A TWI662560B (zh) 2017-03-24 2018-01-30 用於導電膠組合物之低蝕刻及非接觸式玻璃

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/US2017/024055 WO2018174898A1 (fr) 2017-03-24 2017-03-24 Verres à faible gravure et sans contact pour compositions de pâte électroconductrice

Publications (1)

Publication Number Publication Date
WO2018174898A1 true WO2018174898A1 (fr) 2018-09-27

Family

ID=58548877

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US2017/024055 WO2018174898A1 (fr) 2017-03-24 2017-03-24 Verres à faible gravure et sans contact pour compositions de pâte électroconductrice

Country Status (3)

Country Link
CN (1) CN110337423A (fr)
TW (1) TWI662560B (fr)
WO (1) WO2018174898A1 (fr)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114283963B (zh) * 2021-12-20 2023-05-26 江苏索特电子材料有限公司 导电浆料组合物及其制备方法和应用、晶硅太阳能电池

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2009029738A1 (fr) * 2007-08-31 2009-03-05 Ferro Corporation Structure de contact en couches pour des cellules solaires
US20090104461A1 (en) * 2007-10-18 2009-04-23 E. I. Du Pont De Nemours And Company CONDUCTIVE COMPOSITIONS AND PROCESSES FOR USE IN THE MANUFACTURE OF SEMICONDUCTOR DEVICES: Mg-CONTAINING ADDITIVE
WO2010135496A1 (fr) 2009-05-20 2010-11-25 E. I. Du Pont De Nemours And Company Procédé de réalisation d'une électrode de grille sur la face avant d'une plaquette de silicium
US20120180862A1 (en) 2011-01-13 2012-07-19 Henry Hieslmair Non-contacting bus bars for solar cells and methods of making non-contacting bus bars
WO2012119157A1 (fr) * 2011-03-03 2012-09-07 E. I. Du Pont De Nemours And Company Procédé pour la formation d'une électrode arrière d'argent d'un émetteur passivé et cellule solaire de silicium à contact arrière
EP2848657A1 (fr) * 2013-09-16 2015-03-18 Heraeus Precious Metals North America Conshohocken LLC Pâte électroconductrice à verre favorisant l'adhésion
US9224888B2 (en) 2010-12-06 2015-12-29 Shin-Etsu Chemical Co., Ltd. Solar cell and solar-cell module

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP3772194B2 (ja) * 2000-08-31 2006-05-10 独立行政法人産業技術総合研究所 光制限材料
WO2006107052A1 (fr) * 2005-04-04 2006-10-12 Matsushita Electric Industrial Co., Ltd. Composition de verre pour le recouvrement d’electrode et pate de verre contenant ladite composition
US9947809B2 (en) * 2009-11-11 2018-04-17 Samsung Electronics Co., Ltd. Conductive paste and electronic device and solar cell including an electrode formed using the conductive paste
US8252204B2 (en) * 2009-12-18 2012-08-28 E I Du Pont De Nemours And Company Glass compositions used in conductors for photovoltaic cells
TWI474342B (zh) * 2014-05-29 2015-02-21 China Steel Corp 鋁膠、太陽能電池之背面電極的製作方法與太陽能電池
TWI541833B (zh) * 2015-02-03 2016-07-11 Conductive aluminum adhesives for use in local backlash solar cells and solar cells using the conductive aluminum
CN106396416A (zh) * 2016-08-29 2017-02-15 海宁市瑞银科技有限公司 一种硅基太阳电池背场钝化银浆用无铅玻璃粉及其制备方法

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2009029738A1 (fr) * 2007-08-31 2009-03-05 Ferro Corporation Structure de contact en couches pour des cellules solaires
US20090104461A1 (en) * 2007-10-18 2009-04-23 E. I. Du Pont De Nemours And Company CONDUCTIVE COMPOSITIONS AND PROCESSES FOR USE IN THE MANUFACTURE OF SEMICONDUCTOR DEVICES: Mg-CONTAINING ADDITIVE
WO2010135496A1 (fr) 2009-05-20 2010-11-25 E. I. Du Pont De Nemours And Company Procédé de réalisation d'une électrode de grille sur la face avant d'une plaquette de silicium
US9224888B2 (en) 2010-12-06 2015-12-29 Shin-Etsu Chemical Co., Ltd. Solar cell and solar-cell module
US20120180862A1 (en) 2011-01-13 2012-07-19 Henry Hieslmair Non-contacting bus bars for solar cells and methods of making non-contacting bus bars
WO2012119157A1 (fr) * 2011-03-03 2012-09-07 E. I. Du Pont De Nemours And Company Procédé pour la formation d'une électrode arrière d'argent d'un émetteur passivé et cellule solaire de silicium à contact arrière
EP2848657A1 (fr) * 2013-09-16 2015-03-18 Heraeus Precious Metals North America Conshohocken LLC Pâte électroconductrice à verre favorisant l'adhésion

Also Published As

Publication number Publication date
TW201835942A (zh) 2018-10-01
CN110337423A (zh) 2019-10-15
TWI662560B (zh) 2019-06-11

Similar Documents

Publication Publication Date Title
EP2913140B1 (fr) Fritte de verre contenant du molybdène pour composition de pâte électro-conductrice
US10217876B2 (en) Poly-siloxane containing organic vehicle for electroconductive pastes
WO2015039023A1 (fr) Pâte électroconductrice dotée d'un verre favorisant l'adhérence
US20150194546A1 (en) Low-silver electroconductive paste
TWI577742B (zh) 用於導電性漿料之有機載體
US20150340115A1 (en) Coated conductive metallic particles
US20160190361A1 (en) Glass compositions for electroconductive paste compositions
US20150075597A1 (en) Electroconductive paste with adhension promoting glass
US20150206993A1 (en) Lead-bismuth-tellurium inorganic reaction system for electroconductive paste composition
EP3040320A1 (fr) Composition de verre pour des compositions de pâte électroconductrice
US20170092788A1 (en) Poly-siloxane containing organic vehicle for electroconductive pastes
EP2848657A1 (fr) Pâte électroconductrice à verre favorisant l'adhésion
US20150243811A1 (en) Silver-lead-silicate glass for electroconductive paste composition
US10115836B2 (en) Lead-bismuth-tellurium-silicate inorganic reaction system having improved adhesion properties
US10224438B2 (en) Lead-tellurium inorganic reaction systems
EP2913313A1 (fr) Fritte de verre contenant du tungstène pour composition de pâte électro-conductrice
WO2018175167A1 (fr) Véhicule organique contenant du polysiloxane pour pâtes électroconductrices destinées à des cellules solaires perc
EP2896602B1 (fr) Pâte électroconductrice à faible teneur en argent
TWI662560B (zh) 用於導電膠組合物之低蝕刻及非接觸式玻璃
US20190280134A1 (en) Confined contact area on a silicon wafer
WO2019173053A1 (fr) Couche de germe pour un contact amélioré sur une tranche de silicium
EP2921533A1 (fr) Particules d'oxyde inorganique présentant un revêtement organique

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 17717946

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 17717946

Country of ref document: EP

Kind code of ref document: A1