WO2013142180A1 - Penetration through nitride-passivated silicon substrates - Google Patents

Penetration through nitride-passivated silicon substrates Download PDF

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Publication number
WO2013142180A1
WO2013142180A1 PCT/US2013/030757 US2013030757W WO2013142180A1 WO 2013142180 A1 WO2013142180 A1 WO 2013142180A1 US 2013030757 W US2013030757 W US 2013030757W WO 2013142180 A1 WO2013142180 A1 WO 2013142180A1
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Prior art keywords
silver
recited
silicon
chloride
haiide
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Application number
PCT/US2013/030757
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French (fr)
Inventor
Ovadia ABED
Valerie Kaye GINSBERG
James P. Novak
Richard Lee Fink
Original Assignee
Applied Nanotech Holdings, Inc.
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Publication date
Application filed by Applied Nanotech Holdings, Inc. filed Critical Applied Nanotech Holdings, Inc.
Publication of WO2013142180A1 publication Critical patent/WO2013142180A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • 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
    • 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 pertains to silicon nitride coated substrates, and more specifically to metallization paste for silicon nitride-coated silicon solar wafers,
  • FIGS, lA-iC illustrate cross-sections of silicon solar ceils
  • Silicon nitride is an insulating material, and may be used as an impervious surface coating for silicon-based devices. Silicon nitride may be used as an anti-.refieci.ing layer on the front (or top) side of silicon solar cells, and while a!so serving to passivate the surface of the silicon, FIG, LA illustrates a cross-section of a portion of an exemplary silicon solar cell configuration.
  • a silicon substrate 1.01 is coated with a silicon nitride layer 1.02.
  • Front electrodes 103 made of silver are in contact with the silicon substrate 101.
  • the silver contact 105 at the back may provide soldering contacts for connecting wires.
  • the challenge of making such solar cells is diffusing the metal through, the impervious nitride layer 102 and contacting the underlying silicon structure 101. Without such, a metal-silicon contact, the solar cell will not work.
  • the front contact electrodes 103 may be fabricated on the front side.
  • a method to fabricate these electrodes 103 involves printing a silver paste (or ink) containing silver particles, glass frit, and a vehicle solution. Other well-known additives, such as dispersants and rheology modifiers, may also be present.
  • the paste may be printed using a screen printer and then fired (heated) (e.g., using a belt furnace having a peak temperature of approximately 7O0--9O °C).
  • FIG. IB illustrates the front side of the silicon substrate 101 with the silicon nitride layer 102, and silver paste 106 after being deposited (e.g., primed) onto the nitride layer 102 hut prior to firing
  • FIG. IC illustrates the front side of the silicon substrate 101 , the silicon nitride layer .102, and .the silver electrode .103 formed after firing, wherein the silver forms a contact (i.e., ohmic or electrically conducting) with the silicon substrate 1 1.
  • the silver particles provide the eleetroconduetivUy of the electrode 103.
  • the particle size and the .range of distribution of the particles used may differ in various formulations from approximately 1 ran to several microns.
  • the vehicle solution may comprise an organic polymer, such as ethyl, cellulose in an organic solvent, (e.g., terpineoi), thickens the paste and also affords binding of the paste to the substrate.
  • the glass frit may.be a mixture of oxides of boron, bismuth, lead, zinc, silicon, thallium (or other metal oxides) that assist in burning through the nitride layer 102.
  • the capability to burn through the nitride layer 102 and form a contact between the silver particles 103 and the silicon substrate 101 underneath is a combined mechanism between the silver particles and the glass frit
  • the size distributions of the particles as well as the composition of the glass frit play a role in achieving a good electrochemical contact between the silver electrode 103 formed and the silicon.
  • Othe additives, such as dispersantS: and rheology modifiers, may also be present.
  • Embodiments of the present invention add halide, such as silver chloride ("AgCI”), particles to the silver pastes or inks, which results in a significant decrease in the contact resistance between the silver electrode 103 and the silicon substrate 101.
  • a glass frit may be used to assist in achieving an effective burnthrough.
  • the silver chloride reacts with the nitride layer 102 according to the following mechanism:
  • the silicon nitride SUN. ⁇ may only partiall convert to the silver chloride resulting in a mixed nitride chloride Si s N v CL;.
  • the reaction may result in a mixed oxide nitride Si x N y O 3 ⁇ 4 .
  • the silicon nitride will partially react to form an oxynitride derivative before -further or complete conversion to silicon oxide.
  • These material intermediates have a much less stable lattice Structure, which is now capable of dissolving in the glass frit and allowing silver particles to reach the silicon substrate 1 1 underneath.
  • chloride derivatives are much less stable. Note that oxygen has two available bonds and thus forms chains of type (0-Si-0-S.i ⁇ 0) Xj while chloride breaks the Si- -Si chains and caps them with chlorine atoms -O-Si-C! ⁇ C!-Si-O-. Such terminated chains are less stable and ca more easily dissolve and react.
  • This mechanism of actio is similar to chloride fluxes used for brazing.
  • the purpose of flux is to destabilize the oxide layers on the metals to be joined.
  • silver chloride has particular benefit, because during the reaction it forms fresh metallic silver that may dissolve easily in the glass frit and have facile transport properties through the nitride 102 to the silicon layer 101.
  • the halides may comprise any one or more of silver chloride, silver nitrate and tetramethylammonium chloride, silver tertafluoroborate, silver irichloroacetate, silver fluoride, silver bromide, silver iodide, and bismuth chloride. It can also be anticipated from Equation ! that the silver ca be replaced by other elements, such as gold, lead, or any element halide that satisfies the thermodynamic conditions for the equation.
  • metal halides do not have to be directly introduced. Instead, one can add metal salt and, for example, quaternary amine halide, which can form the metal halide in situ during mixing or during firing. One may also introduce other precursors, such as a halide containing organic materials, and organometallic compounds that can form metal halides when decomposed during the firing step.
  • the primary metal particles may be derivatked with an outer surface coating that introduces the halides.
  • This may be in the form of a metal haUde-surface coating or a halide-containiiig surface iigand.
  • silver pastes 1-5 were prepared according, to the compositions listed in Table 1.
  • Paste 5 served as a control and did not contain AgCl,
  • the particle sizes of the silver powders used were approximately 200-500 nm. Although a solvent concentration varies considerably, it does not cause major effects on contact resistance.
  • Table ⁇ shows results obtained.
  • the contact resistance values (last row) were significantly lower for the silver chloride containing formulations 1-4 compared to the control paste 5. This improvement canno be attributed to differences in solvent concentration between, pastes 1-4 and control, paste 5. Dispersant. concentration is also not relevant as can be seen from silver paste 1. Also, the effect is demonstrated across a wide range of frit to silver.
  • silver pastes may be prepared using silver particles that have been reacted with C3 ⁇ 4 to form AgCI on the surface rather than adding secondary AgCI. particles.

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

Abstract

Silver chloride is added to a metallization paste containing silver particles, glass frit, organic binder, and solvent. This formulation, when applied on a silicon nitride-passivated silicon substrate, such as used for solar cells, achieves better electrical contact than pastes lacking silver chloride. Silver chloride and other halides can be used to assist penetration through silicon nitride-passivated substrates.

Description

PENETRATIO THROUGH NiTRlDE-PASSIVATED SILICON SUBSTRATES
This application claims priority to U.S. Provisional Patent Application No, 61/613,156, which is hereby incorporated by reference herein.
Technical Field
The present invention pertains to silicon nitride coated substrates, and more specifically to metallization paste for silicon nitride-coated silicon solar wafers,
Brief Description of the Dra wi ngs
FIGS, lA-iC illustrate cross-sections of silicon solar ceils,
Detai led Description
Silicon nitride is an insulating material,, and may be used as an impervious surface coating for silicon-based devices. Silicon nitride may be used as an anti-.refieci.ing layer on the front (or top) side of silicon solar cells, and while a!so serving to passivate the surface of the silicon, FIG, LA illustrates a cross-section of a portion of an exemplary silicon solar cell configuration. A silicon substrate 1.01 is coated with a silicon nitride layer 1.02. Front electrodes 103 made of silver are in contact with the silicon substrate 101. On the back (or bottom) side of the silicon substrate 101, there is a second electrode comprising an aluminum layer 104 and a silver contact 1.05. The silver contact 105 at the back, may provide soldering contacts for connecting wires. The challenge of making such solar cells is diffusing the metal through, the impervious nitride layer 102 and contacting the underlying silicon structure 101. Without such, a metal-silicon contact, the solar cell will not work.
I order to collect the electrical current produced by the solar cell, the front contact electrodes 103 may be fabricated on the front side. A method to fabricate these electrodes 103 involves printing a silver paste (or ink) containing silver particles, glass frit, and a vehicle solution. Other well-known additives, such as dispersants and rheology modifiers, may also be present. The paste may be printed using a screen printer and then fired (heated) (e.g., using a belt furnace having a peak temperature of approximately 7O0--9O °C). Upon firing, the silver particles are joined together (sintered) while penetrating through the nitride layer 102 with the assistance of the glass -frit to form the final sliver electrode 103 in contact with the silicon 101. FIG, IB illustrates the front side of the silicon substrate 101„ the silicon nitride layer 102, and silver paste 106 after being deposited (e.g., primed) onto the nitride layer 102 hut prior to firing, FIG. IC illustrates the front side of the silicon substrate 101 , the silicon nitride layer .102, and .the silver electrode .103 formed after firing, wherein the silver forms a contact (i.e., ohmic or electrically conducting) with the silicon substrate 1 1.
The silver particles provide the eleetroconduetivUy of the electrode 103. The particle size and the .range of distribution of the particles used may differ in various formulations from approximately 1 ran to several microns, The vehicle solution may comprise an organic polymer, such as ethyl, cellulose in an organic solvent, (e.g., terpineoi), thickens the paste and also affords binding of the paste to the substrate. The glass frit may.be a mixture of oxides of boron, bismuth, lead, zinc, silicon, thallium (or other metal oxides) that assist in burning through the nitride layer 102. The capability to burn through the nitride layer 102 and form a contact between the silver particles 103 and the silicon substrate 101 underneath is a combined mechanism between the silver particles and the glass frit The size distributions of the particles as well as the composition of the glass frit play a role in achieving a good electrochemical contact between the silver electrode 103 formed and the silicon. 1 1. Othe additives, such as dispersantS: and rheology modifiers, may also be present.
Embodiments of the present invention add halide, such as silver chloride ("AgCI"), particles to the silver pastes or inks, which results in a significant decrease in the contact resistance between the silver electrode 103 and the silicon substrate 101. A glass frit may be used to assist in achieving an effective burnthrough.
The silver chloride reacts with the nitride layer 102 according to the following mechanism:
AgCl + S13N ~ Ag + SiCU + N2 (Equation 1}
The silicon nitride SUN.}) may only partiall convert to the silver chloride resulting in a mixed nitride chloride SisNvCL;.
Glass frits based on lead oxide react according to the following equation:
PbO + SijRj ~» Pb Si(>2 + N2 (Equation 2)
Again, the reaction may result in a mixed oxide nitride SixNyO¾. In other words, the silicon nitride will partially react to form an oxynitride derivative before -further or complete conversion to silicon oxide. These material intermediates have a much less stable lattice Structure, which is now capable of dissolving in the glass frit and allowing silver particles to reach the silicon substrate 1 1 underneath. Unlike oxide derivatives of silicon, chloride derivatives are much less stable. Note that oxygen has two available bonds and thus forms chains of type (0-Si-0-S.i~0)Xj while chloride breaks the Si- -Si chains and caps them with chlorine atoms -O-Si-C! ÷ C!-Si-O-. Such terminated chains are less stable and ca more easily dissolve and react.
This mechanism of actio is similar to chloride fluxes used for brazing. The purpose of flux is to destabilize the oxide layers on the metals to be joined.
According to Equatio 1, silver chloride has particular benefit, because during the reaction it forms fresh metallic silver that may dissolve easily in the glass frit and have facile transport properties through the nitride 102 to the silicon layer 101. From Equation 1 , it is seen that the reaction is not confined to chlorides, but can be applied to other halides, such as fluorides, bromides, and iodides. The halides may comprise any one or more of silver chloride, silver nitrate and tetramethylammonium chloride, silver tertafluoroborate, silver irichloroacetate, silver fluoride, silver bromide, silver iodide, and bismuth chloride. It can also be anticipated from Equation ! that the silver ca be replaced by other elements, such as gold, lead, or any element halide that satisfies the thermodynamic conditions for the equation.
The metal halides do not have to be directly introduced. Instead, one can add metal salt and, for example, quaternary amine halide, which can form the metal halide in situ during mixing or during firing. One may also introduce other precursors, such as a halide containing organic materials, and organometallic compounds that can form metal halides when decomposed during the firing step.
Additionally, the primary metal particles may be derivatked with an outer surface coating that introduces the halides. This may be in the form of a metal haUde-surface coating or a halide-containiiig surface iigand.
In accordance with the foregoing, five silver pastes 1-5 were prepared according, to the compositions listed in Table 1. Paste 5 served as a control and did not contain AgCl, The particle sizes of the silver powders used were approximately 200-500 nm. Although a solvent concentration varies considerably, it does not cause major effects on contact resistance.
Sets of parallel lines of 250 micron wide and 5 mm long were printed for each of the silver pastes 1-5 using a screen printer onto silicon nitride-coated silicon wafers. The wafers were then dried at approximately 300°C for approximately 30 minutes. The dried wafers were then fired on a belt furnace at a peek temperature of approximately 800°C The patterns were checked under an optical microscope to confirm having comparable pattern quality, and the contact resistances listed in Table 1 were measured using a four probe setup.
Figure imgf000005_0001
Table ί shows results obtained. The contact resistance values (last row) were significantly lower for the silver chloride containing formulations 1-4 compared to the control paste 5. This improvement canno be attributed to differences in solvent concentration between, pastes 1-4 and control, paste 5. Dispersant. concentration is also not relevant as can be seen from silver paste 1. Also, the effect is demonstrated across a wide range of frit to silver.
In alternative embodiments, silver pastes may be prepared using silver particles that have been reacted with C¾ to form AgCI on the surface rather than adding secondary AgCI. particles.

Claims

WHAT IS CLAIMED IS:
1. A metallization paste containing a haiide.
2. The metallization paste as recited in claim 1. in electrical contact with silicon of a solar eel],
3. The metallization paste as recited in claim 1, wherein the metallization paste comprises silver.
4. "The metallization paste as recited in claim 1 , wherein the haiide is silver chloride.
5. The metallization paste as recited i claim 1, wherein the haiide is selected from the group consisting of silver fluoride, silve bromide, silver iodide, and bismuth chloride.
6. A method for burning through a silicon nitride layer comprising depositing a. metal mixture containing a haiide and heating the metal mixture.
7. The method as recited in claim 6, wherein the haiide is selected from the group consisting of silver nitrate and tetntraethyiammomum chloride, sil ver tertafiuoroborate, and silver tric oroacetate,
8. The method as recited in claim 6, wherein the haiide is selected from the group consisting of silver fluoride, silver bromide, silver iodide, and bismuth chloride.
9. The method as recited in. claim 6, wherein the metal mixture further comprises glass forming materials selected from the group consisting of boron oxide, bismuth oxide, lead oxide, and silicon oxide.
10. The method as recited in claim 6, wherein the silicon nitride layer is on a silicon substrate of a solar cell,
1 1. The method as recited in claim 10, wherein the metal mixture bums through the nitride layer to form an electrical contact with the silicon substrate.
12. The method as recited in claim 1 1 , wherein the metal mixture further comprises silver.
13. The method as recited in claim 12? wherein the hali.de further comprises silver chloride.
PCT/US2013/030757 2012-03-20 2013-03-13 Penetration through nitride-passivated silicon substrates WO2013142180A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US201261613156P 2012-03-20 2012-03-20
US61/613,156 2012-03-20

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9818889B2 (en) 2012-12-29 2017-11-14 Cheil Industrial, Inc. Composition for solar cell electrodes and electrode fabricated using the same

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH11213754A (en) * 1998-01-30 1999-08-06 Sharp Corp Conductive paste
JP2005347276A (en) * 2005-06-22 2005-12-15 Sharp Corp Electric conductive paste, electrode, solar cell, and method for manufacturing solar cell
US20060289055A1 (en) * 2005-06-03 2006-12-28 Ferro Corporation Lead free solar cell contacts
US20080108218A1 (en) * 2001-10-05 2008-05-08 Cabot Corporation Low viscosity precursor compositions and methods for the deposition of conductive electronic features

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH11213754A (en) * 1998-01-30 1999-08-06 Sharp Corp Conductive paste
US20080108218A1 (en) * 2001-10-05 2008-05-08 Cabot Corporation Low viscosity precursor compositions and methods for the deposition of conductive electronic features
US20060289055A1 (en) * 2005-06-03 2006-12-28 Ferro Corporation Lead free solar cell contacts
JP2005347276A (en) * 2005-06-22 2005-12-15 Sharp Corp Electric conductive paste, electrode, solar cell, and method for manufacturing solar cell

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9818889B2 (en) 2012-12-29 2017-11-14 Cheil Industrial, Inc. Composition for solar cell electrodes and electrode fabricated using the same

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