CN111276553A - Method of forming a solar cell with a sintered multilayer thin film stack - Google Patents

Abstract

A method of forming a sintered multilayer stack is described. The method comprises the following steps: a) coating at least a portion of the surface of the base layer with a layer of wet metal particles, b) drying the layer of wet metal particles to form a dried metal particle layer, c) directly coating at least a portion of the dried metal particle layer with a wet intercalation layer to form a multilayer stack, d) drying the multilayer stack, and e) co-firing the multilayer stack to form a sintered multilayer stack. Intercalation can include one or more of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles. The wet metal particle layer may include: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel, or combinations thereof.

Description

Method of forming a solar cell with a sintered multilayer thin film stack

The present application is a divisional application filed on 2016, 24/11, under the reference 201611044889.9 entitled "method for forming a solar cell having a sintered multilayer thin film stack".

Cross Reference to Related Applications

Priority is claimed for us provisional patent application 62/259,636 filed on 24/11/2015, us provisional patent application 62/318,566 filed on 5/4/2016, us provisional patent application 62/371,236 filed on 5/8/2016, and us provisional patent application 62/423,020 filed on 16/11/2016, the entire contents of which are incorporated herein by reference.

Statement of government support

The present invention has been government supported based on the IIP-1430721 contract number awarded by NSF. In the present invention, the government may have certain rights.

Technical Field

The present invention relates to an intercalation slurry comprising noble metal particles, intercalation particles, and an organic vehicle.

An intercalation paste (intercalation paste) may be used to improve the power conversion efficiency of the solar cell. Silver-based embedding pastes are printed on the aluminum layer, which after firing have a moderate peel strength (peel strength) and are then soldered to a marking tape (tabbing ribbon). This paste is particularly well suited for silicon-based solar cells, which use aluminum Back Surface Field (BSF). Typically, 85-92% of the back surface area of silicon wafers of commercially produced mono-and poly-crystalline silicon solar cells is covered by a layer of aluminum particles, which forms the back surface field and makes ohmic contact (ohmic contact) with silicon. The remaining 5-10% of the back silicon surface is covered by the silver back marker layer, which does not generate a field and does not make ohmic contact with the silicon wafer. The rear marker layer is mainly used for soldering the marker strip to electrically connect the solar cells.

When the silver layer is in direct contact with a silicon-based layer (substrate) on the rear side of the solar cell instead of the aluminum particle layer contacting the substrate, it is estimated that the absolute baseline of the conversion efficiency of the solar cell is reduced by 0.1% to 0.2%. Therefore, it is highly desirable to cover the entire rear of the solar cell with a layer of aluminum particles and still be able to connect the solar cells together using the marker bands. In the past, researchers have attempted to print silver directly on top of the layer of aluminum particles, but during firing in air at high temperatures, the aluminum and silver layers interdiffuse (interdiffusion) and cause the layer surface to become oxidized and lose solderability. Some researchers have attempted to alter atmospheric conditions to reduce oxidation; however, the front side silver paste performs best in an oxidizing atmosphere, such as dry air, and the overall solar cell efficiency decreases after treatment in an inert atmosphere. Other researchers have attempted to lower the peak firing temperature of the wafer to reduce interdiffusion, but the front side silver paste requires a high peak firing temperature (i.e., greater than 650 ℃) to sinter the silicon oxynitride to make ohmic contact with the silicon base layer. Recently, researchers have used ultrasonic soldering of tin alloys directly on top of aluminum to create solderable surfaces. This technique has achieved adequate peel strength (i.e., 1-1.5N/mm), but requires additional equipment and uses large amounts of tin, which increases cost. In addition, the use of ultrasonic soldering on fragile materials such as aluminum and silicon wafers increases wafer breakage and reduces process yield.

There is a need to develop printable pastes that can improve (modify) the material properties of the underlying metal particle layer during firing. For example, noble metals (noble metals) containing pastes, which can be printed directly on aluminum and fired using standard solar cell processing conditions, can improve solar cell efficiency. These pastes reduce the interdiffusion of Ag/Al, thereby remaining solderable to the marker band. There is a need for a paste that is screen printable and functions as a drop-in replacement that does not incur additional significant expense and can be immediately integrated into existing production lines.

Disclosure of Invention

A sintered multilayer stack (fired multilayer stack) is disclosed. In one embodiment of the invention, a stack has a base layer, a metal particle layer on at least a portion of a surface of the base layer, a modified metal particle layer on at least a portion of a surface of the base layer, and a modified intercalation layer directly on at least a portion of the modified metal particle layer. The modified interposer has a solderable surface facing away from the base layer. The modified metal particle layer includes the same metal particles as the metal particle layer and at least one material from the modified intercalation. The improved intercalation comprises a noble metal and a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, indium, iron, lanthanum, hafnium, lead, lithium, magnesium, manganese, molybdenum, niobium, phosphorus, potassium, rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin, vanadium, zinc, zirconium, combinations thereof, and alloys thereof, oxides thereof, composites thereof, and other combinations thereof. In one arrangement, the modified intercalation comprises a noble metal and a material selected from the group consisting of: bismuth, boron, indium, lead, silicon, tellurium, tin, vanadium, zinc, combinations thereof and alloys thereof, oxides thereof, composites thereof, and other combinations thereof.

In one embodiment of the invention, the modified intercalation has two phases (phase): a noble metal phase (noble metal phase) and an intercalation phase (intercalation phase). Greater than 50% of the modified intercalated solderable surface may contain noble metal phases. The modified metal particle layer may include the metal particles discussed above and at least one material from the embedded phase. The embedded phase comprises a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, gallium, germanium, indium, iron, lanthanum, hafnium, lead, lithium, magnesium, manganese, molybdenum, niobium, phosphorus, potassium, rhenium, selenium, silicon, sodium, strontium, sulfur, tellurium, tin, vanadium, zinc, zirconium, combinations thereof, and alloys thereof, oxides thereof, composites thereof, and other combinations thereof. The noble metal phase comprises at least one material selected from the group consisting of: gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.

In another embodiment of the invention, the modified intercalation has two sublayers (sublayers): an intercalation (intercalation) layer directly on at least a portion of the modified metal particle layer, and a noble metal sublayer (noble metal sublayer) directly on at least a portion of the intercalation. The intercalated, solderable surface comprises a noble metal sublayer. The modified metal particle layer may include the metal particles discussed above and at least one material from the sub-intercalation. Possible materials for sub-intercalation are the same as described above for the embedding phase. Possible materials for the noble metal sub-layer are the same as described above for the noble metal phase.

In another embodiment of the invention, the sintered multilayer stack has as its modified metal particle layer a modified aluminum particle layer. It has an improved intercalation with two sublayers: a bismuth-rich (bismuth-rich) sublayer directly on the modified aluminum particle layer; and a silver-rich sublayer directly on the bismuth-rich sublayer. The intercalated, solderable surface is modified to include a silver-rich sublayer. The modified aluminum particle layer comprises aluminum particles and may further comprise at least one material selected from the group consisting of: aluminum oxide, bismuth, and bismuth oxide.

In one arrangement, at least one dielectric layer is directly on at least a portion of the surface of the substrate. The dielectric layer includes at least one material selected from the group consisting of: silicon, aluminum, germanium, gallium, hafnium, and oxides, nitrides, composites, and combinations thereof. In another arrangement, an alumina dielectric layer is directly on at least a portion of the surface of the base layer and a silicon nitride dielectric layer is directly on the alumina dielectric layer.

In one arrangement, a solid (e.g., eutectic) composite layer (compound layer) is directly on the surface of the substrate. The solid composite layer comprises one or more metals selected from the group consisting of: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, and one or more materials selected from the group consisting of: silicon, oxygen, carbon, germanium, gallium, arsenic, nitrogen, indium and phosphorus.

A portion of the substrate adjacent the substrate surface may be doped with at least one material selected from the group consisting of: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel, and combinations thereof.

In one embodiment of the invention, a portion of the sintered multilayer stack has a variable thickness. The sintered multilayer stack may have an average peak to valley height of greater than 12 μm.

At least 70 wt% (weight percent) of the intercalated, solderable surface can include a material selected from the group consisting of: silver, gold, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.

The base layer may comprise at least one material selected from the group consisting of: silicon, silicon dioxide, silicon carbide, aluminum oxide, sapphire, germanium, gallium arsenide, gallium nitride, and indium phosphide. Alternatively, the base layer may comprise a material selected from the group consisting of: aluminum, copper, iron, nickel, titanium, steel, zinc, and alloys, composites, and other combinations thereof. The metal particle layer may comprise a material selected from the group consisting of: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel and alloys, composites, and other combinations thereof. The noble metal may comprise a material selected from the group consisting of: silver, gold, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof.

The metal particle layer may have a thickness between 0.5 μm and 100 μm and/or a porosity between 1 and 50%. The modified intercalation layer may have a thickness of between 0.5 μm and 10 μm. The sintered multilayer stack may have a contact resistance between 0 and 5mOhm, as determined by power line measurements.

There may also be a marker band directly on at least a portion of the solderable surface that modifies intercalation. In one arrangement, the peel strength between the marker band and the modified insert is greater than 1N/mm.

In another embodiment of the invention, a sintered multilayer stack has a base layer, a metal particle layer on at least a portion of the base layer, a modified metal particle layer on at least a portion of the base layer, and a modified intercalation layer directly on at least a portion of the modified metal particle layer. The modified intercalation has two sublayers: a sub-intercalation directly on at least a portion of the modified metal particle layer, and a noble metal sub-layer directly on at least a portion of the sub-intercalation. The modified metal particle layer includes metal particles and at least one material from the sub-intercalation. Possible materials for sub-intercalation have been described above.

In another embodiment of the invention, a sintered multilayer stack has a silicon-based layer, an aluminum particle layer on at least a portion of the base layer, a modified aluminum particle layer on at least a portion of the base layer, and a modified intercalation layer directly on the modified aluminum particle layer. The modified intercalation has two sublayers: a bismuth-rich sublayer directly on the modified aluminum particle layer, and a silver-rich sublayer directly on the bismuth-rich sublayer. The modified aluminum particle layer comprises at least one material selected from the group consisting of: aluminum, aluminum oxide, bismuth, and bismuth oxide.

In one embodiment of the present invention, a solar cell has a silicon base layer, at least one front dielectric layer directly on at least a portion of a front surface of the silicon base layer, a plurality of fine grid lines (fine grid lines) on a portion of the front surface of the silicon base layer, at least one front bus layer (frontbus layer) in electrical contact with at least one of the plurality of fine grid lines, an aluminum particle layer on at least a portion of a rear surface of the silicon base layer, and a rear tabbing layer (rear tabbing layer) on a portion of the rear surface of the silicon base layer. The rear indicia layer includes a modified aluminum particle layer on a portion of the rear surface of the silicon base layer and a modified intercalation layer directly on at least a portion of the modified aluminum particle layer. The modified interposer has a solderable surface facing away from the silicon base layer. The modified aluminum particle layer includes aluminum particles and at least one material from modified intercalation. Possible materials for improved intercalation have been described above. The layer of aluminium particles may have a thickness between 1 μm and 50 μm and/or a porosity between 3 and 20%. The rear marker layer may have a thickness between 1 μm and 50 μm. The silicon base layer may be a single crystal silicon wafer, having either a p-type substrate or an n-type substrate. The silicon base layer may be a polysilicon wafer with a p-type substrate or an n-type substrate.

In one embodiment of the invention, the modified intercalation includes two phases: noble metal phase and embedded phase. More than 50% of the solderable surface can be made of noble metal phases. The modified aluminum particle layer includes aluminum particles and at least one material from the embedded phase. Possible materials for embedding the phase have been described above. Possible materials for the noble metal phase have been described above.

In another embodiment of the invention, the modified intercalation comprises two sublayers: a sub-intercalation directly on at least a portion of the modified metal particle layer, and a noble metal sub-layer directly on at least a portion of the sub-intercalation. The solderable surface includes a noble metal sublayer. The modified aluminum particle layer includes aluminum particles and at least one material from the sub-intercalation. Possible materials for sub-intercalation have been described above. Possible materials for the noble metal sublayer have been described above.

In another embodiment of the invention, the modified intercalation comprises two sublayers: a bismuth-rich sublayer directly on the modified aluminum particle layer, and a silver-rich sublayer directly on the bismuth-rich sublayer. The modified aluminum particle layer further comprises at least one material selected from the group consisting of: aluminum oxide, bismuth, and bismuth oxide. In one arrangement, the modified aluminum particle layer further comprises bismuth and/or bismuth oxide, and the weight ratio of bismuth to bismuth plus aluminum (Bi + Al)) is at least 20% higher in the modified aluminum particle layer than in the aluminum particle layer. The bismuth-rich sublayer may have a thickness between 0.01 μm and 5 μm or between 0.25 μm and 5 μm.

In one arrangement, the at least one back dielectric layer is directly on at least a portion of the back surface of the silicon-based layer. The back dielectric layer includes one or more of: silicon, aluminum, germanium, hafnium, gallium, and oxides, nitrides, composites, and combinations thereof. The back dielectric layer may comprise silicon nitride. In another arrangement, the aluminum oxide back dielectric layer is directly on at least a portion of the back surface of the silicon-based layer and the silicon nitride back dielectric layer is directly on the aluminum oxide back dielectric layer. In one arrangement, a solid aluminum-silicon eutectic layer is directly on a silicon base layer. In one arrangement, a portion of the silicon-based layers adjacent the back surface of the silicon-based layers further comprises a back surface field, and the back surface field is doped p-type to per cm³Is provided with 10¹⁷To 10²⁰Atoms (atoms).

In one embodiment of the invention, a portion of the rear indicia layer has a variable thickness and may have an average peak to valley height greater than 12 μm.

A marker band may be provided directly on at least a portion of the solderable surface that modifies intercalation. The solderable surface may be silver-rich. The solderable surface may comprise at least 75 wt% silver. A marker strip soldered to a silver-rich solderable surface may have a peel strength of greater than 1N/mm.

A portion of the modified aluminum particle layer may have a variable thickness. A portion of the modified aluminum particle layer may have an average peak to valley height greater than 12 μm. The contact resistance between the rear marker layer and the aluminium particle layer may be between 0 and 5mOhm as determined by powerline measurements.

In another embodiment of the invention, a solar cell has a silicon-based layer, at least one front dielectric layer directly on at least a portion of a front surface of the silicon-based layer, a plurality of fine gridlines on a portion of the front surface of the silicon-based layer, at least one front bus layer in electrical contact with at least one of the plurality of fine gridlines, a layer of aluminum particles on at least a portion of a back surface of the silicon-based layer, and a back logo layer on a portion of the back surface of the silicon-based layer. The rear indicia layer has a solderable surface. The rear indicia layer includes a modified aluminum particle layer on at least a portion of the rear surface of the silicon base layer, a bismuth-rich sublayer directly on at least a portion of the modified aluminum particle layer, and a silver-rich sublayer directly on at least a portion of the bismuth-rich sublayer. The modified aluminum particle layer includes aluminum particles and at least one material selected from the group consisting of: aluminum oxide, bismuth, and bismuth oxide.

In another embodiment of the present invention, a solar cell module has a front sheet, a front encapsulation layer on a rear surface of the front sheet, and a first silicon solar cell and a second silicon solar cell on the front encapsulation layer. Each silicon solar cell may be any silicon solar cell described herein. The solar cell module also has a first cell interconnect (first cell interconnect) comprising a first flag strip in electrical contact with both the front bus layer of the first silicon solar cell and the back flag layer of the second silicon solar cell, a back sheet, a back encapsulation layer (back encapsulating layer) on the back surface of the back sheet. A first portion of the back encapsulant layer is in contact with the first and second silicon solar cells, and a second portion of the back encapsulant layer is in contact with the front encapsulant layer.

The first battery interconnect may further include a junction box (junction box) in contact with the rear tab. The junction box may include at least one bypass diode (bypass diode). There may also be at least one bus bar connected to the first marker strip.

In one embodiment of the present invention, a slurry (paste) is disclosed. The slurry comprises between 10 wt% and 70 wt% noble metal particles, at least 10 wt% intercalating particles (intercalating particles) and organic vehicle (organic vehicle). The embedded particles include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles. The weight ratio of the embedded particles to the noble metal particles may be at least 1: 5.

the noble metal particles may include at least one material selected from the group consisting of: gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof. The noble metal particles may haveD50 between 100nm and 50 μm and 0.4 to 7.0m²Specific surface area between/g. A portion of the noble metal particles may have a shape such as spherical, platelet, and/or elongated. The noble metal particles can have a monomodal size distribution or a multimodal size distribution. In one embodiment, the noble metal particles are silver and have a D50 between 300nm and 2.5 μm and a D50 between 1.0 and 3.0m²Specific surface area between/g.

The embedded particles may have a D50 between 100nm and 50 μm and a D50 between 0.1 and 6.0m²Specific surface area between/g. A portion of the embedded particles may have a shape such as spherical, platelet, and/or elongated. The embedded particles may have a monomodal size distribution or a multimodal size distribution.

The low temperature base metal particles may comprise a material selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, and alloys, composites, and other combinations thereof. In one embodiment, the low temperature base metal particles comprise bismuth and have a D50 between 1.5 and 4.0 μm and a D50 between 1.0 and 2.0m²Specific surface area between/g.

In one embodiment of the invention, at least some of the low temperature base metal particles have a bismuth core particle surrounded by a single shell (single shell) comprising a material selected from the group comprising: silver, nickel-boron, tin, tellurium, antimony, lead, molybdenum, titanium, and alloys, composites, and other combinations thereof. In another embodiment of the present invention, at least some of the low temperature base metal particles have a bismuth core particle surrounded by a single shell comprising a material selected from the group consisting of: silicon oxide, magnesium oxide, boron oxide, and any combination thereof.

The crystalline metal oxide particles may include oxygen and a metal selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites, and other combinations thereof.

The glass frit comprises a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, indium, hafnium, iodine, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxides thereof, composites thereof, and other combinations thereof.

The slurry may have a solids loading of between 30 wt% and 80 wt%. The embedded particles may constitute at least 15 wt% of the slurry. In one arrangement, the paste includes 45 wt% Ag particles, 30 wt% bismuth particles, and 25 wt% organic vehicle. In another arrangement, the paste includes 30 wt% Ag particles, 20 wt% bismuth particles, and 50 wt% organic vehicle. The slurry was heated at 25 ℃ for 4 seconds (sec)^-1May have a viscosity of between 10,000 and 200,000cP at shear rate (sheet rate).

In one embodiment of the present invention, a co-firing (co-firing) method of forming a sintered multilayer stack is described. The method comprises the following steps: a) coating at least a portion of the surface of the base layer with a layer of wet metal particles, b) drying the layer of wet metal particles to form a dried metal particle layer, c) directly coating at least a portion of the dried metal particle layer with a wet intercalation layer to form a multilayer stack, d) drying the multilayer stack, and e) co-firing the multilayer stack to form a sintered multilayer stack.

In another embodiment of the present invention, a sequential method of forming a sintered multilayer stack is described. The method comprises the following steps: a) coating at least a portion of the surface of the base layer with a layer of wet metal particles, b) drying the wet metal particle layer to form a dried metal particle layer, c) firing the dried metal particle layer to form a metal particle layer, d) directly coating at least a portion of the metal particle layer with a wet intercalation to form a multilayer stack, e) drying the multilayer stack, and f) firing the multilayer stack to form a sintered multilayer stack.

In one arrangement, the wet intercalation layer has between 10 wt% and 70 wt% noble metal particles, at least 10 wt% intercalating particles, and an organic vehicle for both the co-firing process and the sequential process. The embedded particles may include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles. The wet metal particle layer may comprise metal particles selected from the group comprising: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel and alloys, composites, and other combinations thereof.

In one arrangement, there is an additional step prior to step a) for both the co-firing method and the sequential method. An additional step includes depositing at least one dielectric layer on at least a portion of the surface of the base layer. In this arrangement, step a) comprises directly coating at least a portion of the dielectric layer with a layer of wet metal particles.

For both the co-firing process and the sequential process, each coating step may comprise a process selected from the group consisting of: screen printing, gravure printing, spray deposition, slot coating, 3D printing, and inkjet printing. In one arrangement, step a) comprises screen printing through a patterned screen to produce a layer of wet metal particles having a variable thickness.

For both the co-firing process and the sequential process, steps b) and d) may comprise drying at a temperature of less than 500 ℃ for between 1 second and 90 minutes, or at a temperature of between 150 ℃ and 300 ℃ for between 1 second and 60 minutes. Step e) may comprise rapidly heating to a temperature of greater than 600 ℃ in air for between 0.5 seconds and 60 minutes, or rapidly heating to a temperature of greater than 700 ℃ in air for between 0.5 and 3 seconds.

In one arrangement, for both the co-firing method and the sequential method, in an additional step f) comprises soldering a marker band on a portion of the sintered multilayer stack.

The low temperature base metal particles, crystalline metal oxide particles, glass frit particles, and metal particle layers are described in detail above.

In another embodiment of the present invention, a method of manufacturing a solar cell comprises the steps of: a) providing a silicon wafer, b) coating at least a portion of the back side of the silicon wafer with a wet aluminum particle layer, c) drying the wet aluminum particle layer to form an aluminum particle layer, d) directly coating at least a portion of the aluminum particle layer with a wet intercalation to form a multi-layer stack, e) drying the multi-layer stack, f) coating a plurality of fine gridlines and at least one front manifold layer on the front surface of the silicon wafer, g) drying the plurality of fine gridlines and the at least one front manifold layer to form a structure, and h) co-firing the structure to form a silicon solar cell.

The wet insert layer has been described above.

In one arrangement, there is an additional step between step a) and step b). An additional step comprises depositing at least one dielectric layer on at least a portion of the back surface of the silicon wafer. In this arrangement, step b) comprises directly coating at least a portion of the dielectric layer with a layer of wet aluminum particles.

Each coating step may comprise a method selected from the group comprising: screen printing, gravure printing, jet deposition, slot coating, 3D printing, and inkjet printing. In one arrangement, step b) comprises screen printing through a patterned screen to produce a layer of wet aluminium particles of variable thickness.

For both the co-firing process and the sequential process, steps e) and g) may comprise drying at a temperature of less than 500 ℃ for between 1 second and 90 minutes, or at a temperature of between 150 ℃ and 300 ℃ for between 1 second and 60 minutes. Step h) may comprise rapid heating in air to a temperature of greater than 600 ℃ for between 0.5 seconds and 60 minutes, or rapid heating in air to a temperature of greater than 700 ℃ for between 0.5 and 3 seconds.

The low temperature base metal, crystalline metal oxide particles and glass frit have been described in detail above.

Drawings

The foregoing and other aspects will become readily apparent to those skilled in the art when the following description of the illustrative embodiments is read in conjunction with the accompanying drawings. The figures are not drawn to scale. The drawings are only schematic and are not intended to be exhaustive or to limit the invention.

FIG. 1 is a schematic cross-sectional view of a multilayer stack prior to firing, in accordance with an embodiment of the present invention.

Fig. 2 is a schematic cross-sectional view of a sintered multilayer stack, in accordance with an embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view of a sintered multilayer stack in which an intercalation layer has a split phase.

Fig. 4 is a schematic cross-sectional view of a sintered multilayer stack in which the intercalation has a phase split into two sublayers.

Fig. 5 is a schematic cross-sectional view of a portion of the sintered multilayer stack shown in fig. 2, in accordance with an embodiment of the present invention.

FIG. 6 is a Scanning Electron Microscope (SEM) cross-sectional view of a co-sintered (co-fired) multi-layer stack, in accordance with an embodiment of the present invention.

Fig. 7 is a Scanning Electron Microscope (SEM) cross-sectional view of a co-sintered multi-layer stack with a silver-bismuth frit layer (frit layer).

FIG. 8 is a Scanning Electron Microscope (SEM) cross-sectional view (in SE2 mode) of a layer of aluminum particles on a silicon base layer.

FIG. 9 is a Scanning Electron Microscope (SEM) cross-sectional view (in InLens mode) of the aluminum particle layer on the silicon base layer shown in FIG. 8.

Fig. 10 is a Scanning Electron Microscope (SEM) cross-sectional view (in InLens mode) of a portion of a silicon solar cell comprising a co-sintered multi-layer stack.

Fig. 11 is a Scanning Electron Microscope (SEM) cross-sectional view (in SE2 mode) of the portion of the silicon solar cell comprising the co-sintered multi-layer stack shown in fig. 10.

Fig. 12 shows energy dispersive x-ray (EDX) spectra from a layer of aluminum particles and from a layer of modified aluminum particles, in accordance with an embodiment of the present invention.

Fig. 13 is an EDX spectrum of a surface of a post-marker layer comprising aluminum-bismuth intercalation, in accordance with an embodiment of the present invention.

Figure 14 shows the x-ray scattering pattern from a co-sintered multilayer film stacked on the back marker layer of a silicon solar cell.

Fig. 15 is a schematic cross-sectional view of a multilayer thin film stack including a dielectric layer (dielectric layer) prior to firing, in accordance with an embodiment of the present invention.

FIG. 16 is a schematic cross-sectional view of a sintered multilayer thin film stack including dielectric layers, in accordance with an embodiment of the present invention.

Fig. 17 is a plan view optical micrograph of a co-sintered multilayer thin film stack that has undergone bending.

Fig. 18 is a screen design (not drawn to scale) that may be used during deposition of a wet metal particle layer, in accordance with an embodiment of the present invention.

Figure 19 is a schematic cross-sectional view of a layer of dry metal particles having a variable thickness deposited using the screen shown in figure 18, in accordance with an embodiment of the present invention.

Fig. 20 is a schematic cross-sectional view of a modified metal particle layer having a variable thickness deposited using the screen of fig. 18 and then co-sintered, in accordance with an embodiment of the present invention.

Fig. 21 is a plan view optical micrograph of the co-sintered multi-layer stack shown in fig. 20.

Fig. 22 is a cross-sectional SEM image of a portion of a sintered multilayer stack having a variable thickness.

FIG. 23 is a cross-sectional SEM image of a portion of a thin film of aluminum particles on a silicon base layer having a flat thickness.

FIG. 24 is a surface topology scan of a sintered multilayer stack with variable thickness.

Fig. 25 is a surface topography scan of a layer of sintered aluminum particles.

Fig. 26 is a schematic diagram showing the front (or illuminated) side of a silicon solar cell.

Fig. 27 is a schematic diagram showing the back side of a silicon solar cell.

Fig. 28 is a schematic cross-sectional view of a solar cell module including a sintered multi-layer stack, in accordance with an embodiment of the present invention.

Fig. 29 is a Scanning Electron Microscope (SEM) cross-sectional view of the back side of a solar cell including a sintered multi-layer stacked and soldered flag tape, in accordance with an embodiment of the present invention.

Fig. 30 is a transmission line measurement plot of a silver post-marker layer on conventional silicon.

Fig. 31 is a transmission line measurement plot of silver-bismuth intercalation on an aluminum particle layer that can be used as a post-marker layer on silicon.

Detailed Description

A preferred embodiment has been shown in the context of sintering an embedded slurry on a metal particle layer. However, one skilled in the art will readily recognize that the materials and methods disclosed herein have application in a variety of contexts where good electrical contact to semiconductor or conductor materials is required, particularly where good adhesion, high performance, and low cost are important.

All publications referred to herein are incorporated herein by reference in their entirety for the same purpose as if fully set forth herein.

Disclosed herein are compositions and uses of an embedding paste comprising noble metal particles and embedding particles that can be printed on a metal particle layer to alter the properties of the metal particle layer after they are sintered into a sintered multilayer stack. In one embodiment of the invention, the embedding paste is used to provide a solderable surface on the metal particle layer that is not solderable by itself. The embedding paste may also be used to improve adhesion in firing the multilayer stack or to alter the interaction of the metal particle layer with the underlying base layer. The embedding paste is widely applicable to many applications including transistors (transistors), light emitting diodes and integrated circuits; however, the examples disclosed below will focus primarily on photovoltaic cells.

Definitions and methods

Scanning Electron Microscopy (SEM) and x-ray energy Dispersion Spectroscopy (EDX) (collectively SEM/EDX) As used herein Zeiss Gemini Ultra-55 analytical field emission scanning Electron microscopy, equipped with Bruker

6|60 detectors. Details regarding operating conditions are described for each analysis. Cross-sectional SEM images of the sintered multilayer stack were prepared by ion milling (ionimming). A thin epoxy layer is coated on top of the sintered multilayer stack and dried for at least 30 minutes. The sample was then transferred to a JEOL IB-03010CP ion mill operating at 5kV and 120uA for 8 hours to remove 80 microns from the sample edges. The milled samples were stored in a nitrogen box prior to SEM/EDX.

The term "drying" describes a heat treatment at a temperature of or below 500 ℃, or below 400 ℃, or below 300 ℃, for a period of between 1 second and 90 minutes or any range contained therein. The paste is typically applied to the base layer by screen printing or other deposition methods to produce a "wet" layer. The wet layer may be dried to reduce or remove volatile organic materials, such as solvents, to produce a "dried" layer.

The term "firing" describes heating at a temperature higher than 500 ℃, higher than 600 ℃ or higher than 700 ℃, for a period of between 1 second and 60 minutes or any range comprised therein. The term "sintered layer" describes a dried layer that has been sintered.

The term "multilayer stack" is used herein to describe a base layer having two or more layers of different materials thereon. A "sintered multilayer stack" is a multilayer stack whose layers have been dried and sintered. There are several ways to fire this multilayer stack. The term "co-sintering" is used to describe a process for sintering a multilayer stack only once. For example, during silicon solar cell fabrication, a layer of aluminum particle slurry is first applied to a substrate and dried. Subsequently, a post-marking paste layer is applied to a portion of the dried aluminum particle layer, followed by drying, resulting in a dried aluminum particle layer and a dried post-marking layer. During co-firing, the two dried layers are sintered simultaneously in one step. The term "sequential sintering" is used to describe the process of multiple sintering of a multilayer stack. During the sequential processing, a metal particle slurry is coated on a base layer, dried, and then sintered. The embedding paste is then coated on a portion of the dried and sintered metal particle paste (referred to as a metal particle layer). Subsequently, the entire multilayer stack is dried and sintered a second time. It should be noted that the embodiments of the invention describing the joint sintering of a multilayer stack or structure also apply to multilayer stacks or structures that have been sintered sequentially.

The term "intercalation" is used herein to describe the penetration of the porous material. In the context of the embodiments described herein, the term "embedding" describes the penetration of material from the embedded particles (intercalation particles) in an intercalation (intercalation layer) into an adjacent layer of porous dry metal particles during the firing process, which brings about a coating (partial or complete) of the embedded particle material on at least a portion of the metal particles. The term "modified metal particle layer" as used herein is used to describe this layer of sintered metal particles that has been infiltrated by the material from which the particles are embedded.

In describing relationships between adjacent layers, the preposition "on" as used herein means that the layers may or may not be in direct physical contact with each other. For example, a layer on a substrate means that the layer is positioned directly adjacent to the substrate or indirectly over or adjacent to the substrate. The particular layer is positioned above or adjacent to the base layer-to say, there may or may not be one or more additional layers between the particular layer and the base layer. In describing the relationship between adjacent layers, the preposition "directly on" as used herein means that the layers are in direct physical contact with each other. For example, a layer is positioned between layers on a substrate such that the layer is positioned directly adjacent to the substrate.

When the metal particle layer mainly contains metal a particles, it may be referred to as "metal a particle layer". For example, when the metal particle layer mainly contains aluminum particles, it may be referred to as an aluminum particle layer. When the modified metal particle layer mainly contains metal a particles, it may be referred to as "modified metal a particle layer". For example, when the modified metal particle layer comprises primarily aluminum particles, it may be referred to as a modified aluminum particle layer.

The term "solderable surface" is known in the art. "solderable surface" means a surface that can be soldered to a solder strip. Those having ordinary skill in the art are familiar with variations of solderable surfaces. Examples of materials that produce solderable surfaces include, but are not limited to, tin, cadmium, gold, silver, palladium, rhodium, copper, zinc, lead, nickel, alloys thereof, combinations thereof, compositions thereof, and mixtures thereof. In one embodiment, the surface is solderable when at least 70 wt% of the surface contains materials such as silver, gold, platinum, palladium, rhodium, alloys thereof, composites thereof, and other combinations.

The particles described herein may take on a variety of shapes, sizes, specific surface areas, and oxygen contents. The particles may be spherical, acicular, angular, dendritic, fibrous, platelet-shaped, particulate, irregular and nodular, as defined in ISO 3252. It should be understood that the term "spherical" as used herein means generally spherical in shape and may include spherical, granular, nodular, and sometimes irregular shapes. The term "flake" means flaked and sometimes angular, fibrous and irregular in shape. The term "elongated" means needle-like and sometimes angular, dendritic, fibrous and irregular in shape, as defined in ISO 3252: 1999. Particle shape, morphology, size and size distribution are generally dependent on the synthesis technique. A set of particles may include a combination of particles of different shapes and sizes.

Spherical or elongated particles are typically described by their D50, specific surface area and particle size distribution. The D50 value is defined as a value below which half of the number of particles have a diameter and above which half of the number of particles have a diameter. Measuring the particle diameter distribution is typically performed using a laser diffraction particle size analyzer, such as Horiba LA-950. For example, spherical particles are dispersed in a solvent where they separate well and the spread of transmitted light is directly related to the size distribution from smallest to largest diameter. The common approach is to show that the laser diffraction results are reported as D50 values based on the volume distribution. The statistical distribution of particle sizes can also be measured using a laser diffraction particle size analyzer. It is common for the noble metal particles to have a monomodal or multimodal particle size distribution. In a monomodal distribution, the particle size is monodisperse and D50 is in the center of the single distribution. A multimodal particle size distribution has more than one peak (or vertex) in the particle size distribution. The multimodal particle size distribution may increase the tap intensity of the powder, which typically leads to a higher green film density.

In some embodiments of the invention, the particles may have a flake or elongated shape as defined above. The flakes may have a diameter between 1 μm and 100 μm or between 1 μm and 15 μm and a thickness between 100nm and 500 nm. The elongated shape may have a diameter between 200nm and 100nm and a length greater than 1 μm. In another embodiment of the present invention, there is no limitation on the particle shape; any particle shape may be used as long as the maximum diameter thereof is not more than 50 μm, 5 μm or 1 μm.

The specific surface area of the particles (specific surface area) can be measured using the Brunauer-Emmett-Teller (BET) method in accordance with DIN ISO 9277, 2003-05. The specific surface areas of the particles disclosed herein, and in particular silver and bismuth particles, were determined by the following test methods: BET measurements were performed using TriStar 3000 (from Micromeritics instruments) operating on the basis of physisorption analysis techniques. Sample preparation includes degassing to remove adsorbed molecules. Nitrogen is the analyte gas and helium is used to determine the void volume of the sample tube. Micromeritics provides silica alumina (silica alumina) for use as a reference material, with preparation procedures and test conditions. The measurement begins by adding a known mass of reference material to the sample tube and mounting the sample tube on the BET device manifold. Thermally stable dosing manifold, sample tube and method for measuring saturation pressure (P)_o) The dedicated tube of (a) is evacuated. When a sufficient vacuum is reached, the manifold is filled with helium (a non-absorbing gas) and the sample port is opened to determine the warm free space of the sample at room temperature. The sample tube with the reference material was immersed in liquid nitrogen and cooled to around 77K, and free space analysis was performed again. Using P_oThe tube measures the saturation pressure of the adsorption, whereupon nitrogen is dosed into the manifold above atmospheric pressure. The pressure and temperature of the nitrogen are recorded and then the sample port is opened to allow the nitrogen to be absorbed onto the sample. After some time, the ports close, allowing the absorption to reach equilibrium. The amount absorbed is the amount of nitrogen removed from the manifold minus any residual nitrogen in the sample tube. The measurement points along the absorption isotherm are used to calculate the m for the reference material²Specific area in terms of/g; this procedure is repeated with any sample of interest, such as the particles described herein.

The particles described herein have significant thermal properties: melting point and/or softening point, both of which depend on the crystallinity of the material. The melting point of the particles can be determined by differential scanning calorimetry using a DSC2500 differential scanning calorimeter made by a TA instrument and using the method described in ASTM E794-06 (2012). The melting point of crystalline materials can also be determined using a heating stage and x-ray diffraction. As the crystalline material is heated above its melting point, the diffraction peak begins to disappear. The softening point is the temperature at which the amorphous or glassy particles begin to soften. The softening point of the glass particles can be determined using a dilatometer (dillatometer). The softening point can also be obtained by the fiber extension method described in ASTM C338-57.

Material for producing sintered multilayer stacks

In one embodiment of the invention, the base layer, the metal particle slurry and the embedding slurry form a sintered multilayer stack. The substrate may be a solid, planar, or rigid material. In one embodiment, the base layer comprises at least one material selected from the group consisting of: silicon, silicon dioxide, silicon carbide, aluminum oxide, sapphire, germanium, gallium arsenide, gallium nitride, and indium phosphide. Such substrates are commonly used for the deposition of layers that make up transistors, light emitting diodes, integrated circuits, and photovoltaic cells. The base layer may also be conductive and/or flexible. In another embodiment, the base layer comprises at least one material selected from the group consisting of: aluminum, copper, iron, nickel, titanium, steel, zinc, and alloys, composites, and other combinations thereof.

In one embodiment of the present invention, a metal particle slurry includes metal particles and an organic vehicle. In one arrangement, the metal particle paste further comprises an inorganic binder, such as a glass frit. In one arrangement, a commonly used, commercially available metal particle slurry is used. Metal pastes containing aluminum commonly used on silicon solar cells are sold by running Technology (e.g., RX8252H1), monocrytal (e.g., EFX-39), and GigaSolar Materials (e.g., M7). The metal particles may include at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, or alloys, composites, or other combinations thereof. In various arrangements, the metal particles have a D50 between 100nm and 100 μ ι η, between 500nm and 50 μ ι η, between 500nm and 200 μ ι η, or any range contained therein. The metal particles may have a spherical, elongated or flake shape and may have a monomodal or multimodal size distribution. The glass frit may be included in the metal particle paste in a small amount (i.e., less than 5 wt%). In one embodiment, the metal particle paste includes 70 wt% to 80 wt% aluminum particles, less than 2 wt% glass frit, and an organic vehicle.

In one embodiment of the present invention, an intercalation slurry includes noble metal particles, intercalation particles, and an organic vehicle. The term "solids loading" may be used in conjunction with the slurry to describe the amount and proportion of precious metal and embedded particulate solids in the slurry. The slurries described herein also include an organic vehicle, although this is not often explicitly stated.

Embedded slurry composition

In one embodiment of the present invention, as described herein, the noble metal particles comprise at least one material selected from the group consisting of: gold, silver, platinum, palladium, and rhodium, and alloys, composites, or other combinations thereof. In one embodiment, the noble metal particles comprise between 10 wt% and 70 wt% of the slurry. In various embodiments, the noble metal particles have a D50 of between about 100nm and 50 μm, between 300nm and 10 μm, between 300nm and 5 μm, or any range contained therein. In various embodiments, the noble metal particles have a particle size of from about 0.4 to 7.0m²In the range of from about 1 to 5m²A specific surface area in the range of/g or any range contained therein. The noble metal may have an oxygen content of up to 2 wt%; the oxygen may be homogeneously mixed throughout the particle, or the oxygen may be found in an oxide shell, which has a thickness of up to 500 nm. The noble metal particles may have a spherical, elongated or platelet shape and have a monomodal or multimodal size distribution. Silver particles are commonly used in metallization pastes in the solar industry. In one exemplary embodiment, at least some of the noble metal particles are silver, having a D50 between 300nm and 2.5 μm and a D of 1 to 3m²Specific surface area between/g.

The term "embedded particles" is used to describe particles that are deformable when heated and can at least partially sandwich a layer of porous metal particles when positioned adjacent to a porous layer of other metal particles and phase separate from the other metal particles based on the effects of the heating. In various arrangements, the embedded particles have a D50 between 50nm and 50 μ ι η, between 50nm and 10 μ ι η, between 300nm and 5 μ ι η, or any range contained therein. In one embodiment, the embedded particles have a D50 between 300nm and 3 μm. In various embodimentsThe embedded particles have a particle size of from about 0.1 to 6m²A/g, about 0.5 to 3m²G or 0.5 to 4m²A specific surface area in the range of/g or any range contained therein. According to one embodiment, the embedded particles are platelet-shaped and have a thickness of about 1.0 to 3.0m²Specific surface area in g. The embedded particles may have a spherical, elongated or platelet shape and may have a monomodal or multimodal size distribution.

There are three groups of particles that can be used as embedded particles: low temperature base metal particles (LTBM), crystalline metal oxide particles (crystalline metal oxide particles) and glass frit particles (glass frit particles). In some arrangements, the embedded particles comprise only low temperature base metal particles, or crystalline metal oxide particles or glass frit particles. In other arrangements, the embedded particles are a mixture of two or more particles from these groups. It is desirable that the element embedded in the particles has low solubility and does not alloy with the elements in the adjacent metal particle layer.

In one embodiment, the embedded particles are low temperature base metal particles. The term "low temperature substrate particles" (LTBM) as used herein is a description of particles which exclusively or essentially comprise any substrate metal or metal alloy, having a low temperature melting point, i.e. a melting point below 450 ℃. In some arrangements, the LTBM also contains up to 2 wt% oxygen; the oxygen may be homogeneously mixed throughout the particle, or the oxygen may be found in an oxide shell, having a thickness of up to 500nm, and coated or partially coated with the particle. In some arrangements, the melting point of LTMB is lower, for example, below 350 ℃ or below 300 ℃. In one embodiment of the invention, the LTBM is made exclusively or substantially of bismuth, tin, tellurium, antimony, lead, or alloys, composites or other combinations thereof. In one embodiment, the embedded particles comprise bismuth only and have a D50 between 1.5 and 4 μm and a D of 1 to 2m²Specific surface area between/g.

In another embodiment, the LTBM embedded particles are bismuth core particles surrounded by a metal or metal oxide shell. In another embodiment, the LTBM insert particles are bismuth core particles surrounded by a single shell made of silver, nickel alloys such as nickel boron, tin, tellurium, antimony, lead, molybdenum, titanium, composites and/or other combinations thereof. In another embodiment, the LTBM embedded particles are bismuth core particles, surrounded by a single shell, which is silicon oxide, magnesium oxide, boron oxide, or any combination thereof. Any of these shells may have a thickness ranging from 0.5nm to 1 μm, or 0.5nm to 200nm, or any range contained therein.

In another embodiment, the embedded particles are crystalline metal oxide particles. Metal oxides are compounds having at least one oxygen atom (the oxidation state of the anion being-2) and at least one metal atom. Many metal oxides contain multiple metal atoms, which may all be the same or may include multiple metals. A wide range of metal to oxygen atomic ratios is possible, as will be understood by those skilled in the art. Metal oxides are crystalline when they form ordered periodic structures. Such crystalline metal oxides can disperse x-ray radiation in a pattern of peaks of different intensity characteristics of their crystalline structure. In one embodiment, the crystalline metal oxide particles consist of or consist essentially of an oxide of at least one of the following metals: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, alloys, composites, or other combinations thereof.

For the structures disclosed herein and described in more detail below, as the crystalline metal oxide particles are heated, they begin to melt (i.e., reach their melting point (T) if at low temperatures below the temperature at which significant interdiffusion between metal particles or between structures of different compositions can occur)_M) This is useful). The melting point of the crystalline material in the mixed layer can be determined using thermal order and x-ray diffraction; as the sample is heated above its melting point, the diffraction peak decreases and subsequently disappears. In some exemplary embodiments, boron (III) oxide (B)₂O₃，T _M450 deg.c), vanadium (V) oxide (V)₂O₅，T_M690 ℃), tellurium (IV) oxide (TeO)₂，T_M733 ℃ C.) and bismuth (III) oxide (Bi)₂O₃，T_M817 c) may be deformed during the firing process and become embedded in the adjacent porous metal particle layer, resulting inAn improved metal particle layer is produced. In one exemplary embodiment, the embedded particles are crystalline bismuth oxide having a D50 between 50nm and 2 μm and 1 to 5m²Specific surface area between/g. In another embodiment, the crystalline metal oxide particles further comprise a minor amount (i.e., less than 10 wt%) of one or more additional elements that can adjust the melting point of the particles. Such additional elements may include, but are not limited to: silicon, germanium, lithium, sodium, potassium, magnesium, calcium, strontium, cesium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, iron, cobalt, rhenium, zinc, cadmium, gallium, indium, carbon, nitrogen, phosphorus, arsenic, antimony, sulfur, selenium, fluorine, chlorine, bromine, iodine, lanthanum, and cerium.

In another embodiment, the embedded particles are glass frit particles. In one embodiment, the glass frit consists of or consists essentially of oxygen alone and in combination with at least one of the following elements: silicon, boron, germanium, lithium, sodium, potassium, magnesium, calcium, strontium, cesium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, iron, cobalt, rhenium, zinc, cadmium, gallium, indium, tin, lead, carbon, nitrogen, phosphorus, arsenic, antimony, bismuth, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, lanthanum, cerium, oxygen, and alloys, composites, and other combinations thereof. This is useful if the glass frit has a softening point of less than 900 c or less than 800 c, so as to be effectively deformed during firing. In one exemplary embodiment, the embedded particles are bismuth silicate glass frit particles having a D50 between 50nm and 2 μm and 1 to 5m²Specific surface area between/g.

The term "organic carrier" describes a mixture or solution of organic chemicals or compounds that assists in dissolving, dispersing and/or suspending the solid components of a slurry. Many different organic vehicle mixtures may be used for the intercalation slurries described herein. Such organic vehicles may or may not contain thixotropic agents (thixotropes), stabilizers, emulsifiers, thickeners, plasticizers, surfactants, and/or other common additives.

The components of organic carriers are well known to those skilled in the art. The main constituents of the organic vehicle include one or more binders and one or more solvents. The binder may be a polymeric or monomeric organic component, or a "resin", or a mixture of both. The polymeric binder can have a variety of molecular weights and a variety of polydispersity indices. The polymeric binder may comprise a combination of two different monomer units, known as copolymers (copolymers), wherein the monomer units may be individually alternating or bulky (block copolymers). Polysaccharides are commonly used polymeric binders and include, but are not limited to, alkyl celluloses and alkyl derivatives such as methyl cellulose, ethyl cellulose, propyl cellulose, butyl cellulose, ethyl hydroxyethyl cellulose, cellulose derivatives, and mixtures thereof. Other polymeric binders include, but are not limited to, polyesters, polyethylenes, polypropylenes, polycarbonates, polyurethanes, polyacrylates (including polymethacrylates and polymethylmethacrylate), polyethylenes (including polyvinyl chloride, polyvinyl pyrrolidone, polyvinyl butyral, polyvinyl acetate), polyamides, polyglycols (including polyethylene glycol), phenolic resins, polyterpenes, derivatives thereof, and combinations thereof. The organic carrier binder may comprise between 1 and 30 wt% binder.

Solvents that may be used in the slurries described herein include, but are not limited to, alcohols, glycols (including ethylene glycol), polyols (including glycerol), mono-and polyethers, mono-and polyesters, alcohol ethers, alcohol esters, mono-and disubstituted adipates, mono-and polyacetates, ether acetates, ethylene glycol acetates, glycol ethers (including ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether), ethylene glycol ethyl ether acetates (including ethylene glycol monobutyl ether acetate), linear or branched saturated and unsaturated alkyl chains (including butane, pentane, hexane, octane, and decane), terpenes (including α -, β -, gamma-and 4-terpineol), 2,2, 4-trimethyl-1, 3-pentanediol monoisobutyrate (also known as notoxanil), and non-chlorinated solvents^TM) 2- (2-ethoxyethoxy) ethanol (also known as carbitol)^TM) Derivatives, combinations and mixtures thereof.

In aIn an arrangement, the organic vehicle comprises between 70-100 wt% solvent. The proportions and components of the binder, solvent, and any additives can be adjusted to achieve the desired dispersion or suspension of the slurry particles, the desired carbon content, and/or the desired rheological properties, as will be understood by those skilled in the art. For example, by adding thixotropic agents, e.g.

To change the slurry rheology. In another example, the carbon content of the organic vehicle may be increased or decreased by changing the binder and thixotropic agent and taking into account the peak firing temperature, firing profile (firing profile), and air flow that will occur during annealing. Minor additives may also be included. Such additives include, but are not limited to, thixotropic agents and surfactants. Such additives are well known in the art, and useful amounts of such ingredients can be determined by routine experimentation to maximize device efficiency and reliability. In one embodiment, the metalized paste has a viscosity between 10,000 and 200,000cP at 25 ℃ and at a shear rate of 4 sec-1, measured using a temperature controlled Brookfield RVDV-II + Pro viscometer.

Embedded slurry formulation

Exemplary ranges of ingredients for the embedding slurry according to some embodiments of the present invention are shown in table I. In various embodiments, the intercalation slurry has a solids loading between 30 wt% and 80 wt%, a noble metal particle composition between 10 wt% and 70 wt% of the intercalation slurry, an intercalation particle composition of at least 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, or 40 wt% of the intercalation slurry, and a weight ratio of intercalation particles to noble metal particles is at least 1: 5. In one exemplary embodiment, the noble metal particle content is 50 wt% and the embedded particle composition is at least 10 wt% of the embedded slurry. In various embodiments, the weight ratio of the intercalating particles to the noble metal particles in the intercalating slurry is at least 1:5, or 2:5, or 3:5, or 1:1, or 5: 2.

TABLE I

Formulation of the embedding paste in weight percent (wt%)

Type of slurry	Noble metal particles	Embedded particles	Organic vehicle
				Embedded paste (Range I)	10-70	10-50	20-70
Embedded paste (Range II)	20-50	10-35	30-60
				Embedding paste A	50	12.5	37.5
Embedding paste B	45	30	25
				Embedding paste C	45	30	25
Embedding paste D	30	20	50

In one embodiment of the invention, the intercalation paste contains between 20 and 50 wt% noble metal particles (i.e., intercalation paste range II in table I) and between 10 and 35 wt% intercalation particles, which may include LTBM, crystalline metal oxide, glass frit, or combinations thereof, for solar cell applications. In one embodiment, the embedded particles are metallic bismuth particles. Intercalation slurry A (Table I) may contain 50 wt% silver particles, 12.5 wt% bismuth particles and 37.5 wt% organic vehicle, giving a 1:4 (by weight) ratio of intercalation particles to noble metal particles. Intercalation slurry C (Table I) may contain 45 wt% silver particles, 30 wt% bismuth particles, and 25 wt% organic vehicle, giving a 1:1.5 (wt) ratio of intercalation particles to noble metal particles. When the embedding paste includes silver and bismuth particles, the notation Ag: and (4) Bi.

In another embodiment, the embedded particles are glass frit particles. Intercalation paste B (Table I) may contain 45 wt% silver particles, 30 wt% bismuth-based glass frit, and 25 wt% organic vehicle, giving a 1:1.5 (wt) ratio of intercalation particles to noble metal particles. In another embodiment, the embedded particles are a mixture of LTBM, crystalline metal oxide particles and glass frit particles. Embedding paste D (Table I) may comprise 30 wt% silver particles, 15 wt% metallic bismuth particles, 5 wt% high lead content glass frit particles, and 50 wt% organic vehicle. The formulation of the embedding paste may be adjusted to achieve the desired bulk resistance, contact resistance, layer thickness, and/or peel strength for a particular metal layer.

In another embodiment of the present invention, a method of forming an embedded slurry comprises the steps of: the method includes providing noble metal particles, providing embedded particles, and mixing the noble metal particles and the embedded particles together in an organic vehicle. In one arrangement, the embedded particles are added to an organic carrier and mixed in a planetary mixer (e.g., Thinky AR-100), followed by the noble metal particles (and additional organic carrier, if desired) being added and mixed in the planetary mixer. The embedment slurry may or may not be subsequently ground, for example, by using a three roll mill (e.g., Exakt 50I). In one arrangement, the intercalation slurry contains between 10 and 70 wt% noble metal particles and greater than 10 wt% intercalating particles.

Method of forming a sintered multilayer stack

In one embodiment of the invention, a sintered multilayer stack includes a base layer having at least one layer of metal particles and at least one intercalation layer thereon. In one embodiment, the sintered multilayer stack is formed using a co-firing process comprising the steps of: coating a metal particle layer on a surface of a base layer, drying the metal particle layer, directly coating an intercalation layer on a portion of the dried metal particle layer, drying the intercalation layer, and then co-firing the multilayer stack. In another embodiment, the sintered multilayer stack is formed using a sequential firing schedule comprising the steps of: coating a metal particle layer on the surface of a base layer, drying the metal particle layer, firing the metal particle layer, directly coating an intercalation layer on a portion of the sintered metal particle layer, drying the intercalation layer and then firing the multilayer stack. In one embodiment, during firing, a portion of the intercalation penetrates into the metal particle layer, thereby converting the metal particle layer into a modified metal particle layer. In some embodiments, each coating step comprises a method independently selected from the group comprising: screen printing, gravure printing, jet deposition, slot coating, 3D printing, and inkjet printing. In one embodiment, the metal particle layer is applied to a portion of the base layer by screen printing a metal particle paste, and the intercalation is applied directly to a portion of the metal particle layer by screen printing an intercalation paste after it is dried. In one embodiment, a portion of the surface of the base layer is covered by at least one dielectric layer, and the metal particle layer is coated on a portion of the dielectric layer.

Dried and sintered multilayer stack morphology

Fig. 1 is a schematic cross-sectional view illustrating a multilayer stack 100 prior to co-sintering, in accordance with an embodiment of the present invention. The layer of dry metal particles 120 is directly on a portion of the base layer 110. The intercalation layer 130, which is composed of the intercalation particles and noble metal particles, is directly on a portion of the dried metal particle layer 120, as described above. In various embodiments of the present invention, intercalation layer 130 has an average thickness of between 0.25 μm and 50 μm, between 1 μm and 25 μm, between 1 μm and 10 μm, or any range contained therein. In one embodiment of the present invention, intercalation 130 includes noble metal particles, intercalating particles, and optionally an organic binder (which may remain in intercalation 130 after drying). The noble metal particles and the embedded particles may be homogeneously distributed in intercalation layer 130 prior to co-firing. In one arrangement, the noble metal particles and the embedded particles do not deform after drying (and before firing), retaining their original size and shape.

In one embodiment of the present invention, the layer of dry metal particles 120 is porous and includes at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, and alloys, composites, or other combinations thereof. In one arrangement, prior to co-firing, the dried metal particle layer 120 comprises metal particles and may or may not comprise an organic binder and may or may not comprise non-metal particles, such as glass frit. The metal particles typically do not deform after drying (and before firing), retaining their original size and shape.

During firing, the embedded particles from intercalation 130 are embedded into a portion of dried metal particle layer 120 adjacent (as shown below in fig. 1) intercalation 130. The portion of the dried metal particle layer 120 adjacent to the intercalation layer 130 and into which the embedded particle material permeates is referred to as the "modified metal particle layer" for purposes of this disclosure. After firing, the remaining portion of the metal layer 120, which is not adjacent to the intercalation and into which no or only trace amounts of the intercalated metal material have infiltrated, is dried, referred to as a "metal particle layer," for purposes of this disclosure. In one arrangement, during firing, the particles in the dried metal particle layer 120 can sinter or melt such that the metal particle layer has a different morphology and a lower porosity than the dried metal particle layer 120. The change and sintering of the multilayer stack that occurs during firing will be discussed in more detail below.

Fig. 2 is a schematic cross-sectional view illustrating a sintered multilayer stack 200 (the structure 100 of fig. 1 after it has been sintered), in accordance with an embodiment of the present invention. The sintered multilayer stack 200 includes a modified (due to firing) metal particle layer 222 adjacent at least a portion of the base layer 210, and a modified (due to firing) intercalation 230 adjacent the modified metal particle layer 222. During firing, at least a portion of the noble metal particles and the embedded particles in the intercalation (shown as 130 in fig. 1 prior to firing) form phases that are phase separated from one another. The noble metal particles can sinter or melt, changing morphology and reducing the porosity of modified intercalation 230. At least a portion of the embedded particles melt and flow or embed into the adjacent modified metal particle layer 222 as at least a portion of the noble metal particles (which may sinter or melt) move toward the solderable surface 230S of modified intercalation 230. The modified metal particle layer 222 comprises metal particles that have penetrated therein from the material of the embedded particles in the intercalation (shown as 130 in fig. 1 prior to firing), changing the material properties of a portion of the dried metal layer (shown as 120 in fig. 1 prior to firing) to form the modified metal particle layer 222. The material from the embedded particles may loosely connect the metal particles filled in the modified metal particle layer 222, or it may coat the metal particles already in contact with each other in the modified metal particle layer 222.

In some arrangements, there is also a layer of metal particles 220 into which little or only trace amounts of embedded particle material have penetrated. In one arrangement, the metal particle layer 220, which is not in direct contact with the modified intercalation 230, does not contain an increased concentration of elements from the embedded particles. In some arrangements, the metal particle layer 220 and the modified metal particle layer 222 form a mixture with the base layer 210 or the doped base layer 210 during co-firing (not shown). Although fig. 2 indicates a sharp boundary between the metal particle layer 220 and the modified metal particle layer 222, it should be understood that the boundary is not generally sharp. In some arrangements, the boundary is determined by modifying the lateral spread of the intercalation 230 material into the metal particle layer 220 during co-firing.

In some embodiments of the invention, the material in modified intercalation 230 of fig. 2 is divided into a phase containing material from the intercalated particles and a phase containing the noble metal. Fig. 3 is a schematic cross-sectional view showing a sintered multilayer stack 390 (corresponding to structure 200 of fig. 2) and in which modified intercalation 330 has a separate phase. Sintered multilayer stack 390 (only in multilayer stack region 350) includes modified metal particle layers 322 (during firing) in multilayer stack region 350 between a portion of base layer 300 and modified (during firing) intercalation 330. A metal particle layer 320 comprising metal particles 392 is on the base layer 300 adjacent to the multilayer stack region 350.

Modified intercalation 330 contains two phases: noble metal phase 335 and embedded phase 333, and has solderable surface 335S. A majority (at least greater than 50%) of the solderable surface consists of noble metal phase 335. In some arrangements, the noble metal phase 335 and the embedded phase 333 are not completely phase separated during firing, so that there is also some embedded phase 333 at the solderable surface 335S. The modified metal particle layer 322 includes metal particles 392 and a portion of the material from the embedded phase 333. There is an interface 322I between the modified intercalation layer 330 and the adjacent metal particles 392 in the modified metal particle layer 322. The interface 322I may not be smooth and depends on the size and shape of the metal particles 392 and the firing conditions. In embodiments where the optional glass frit is already included in the dried metal particle layer (120 in fig. 1) prior to firing, the modified metal particle layer 322 and the metal particle layer 320 may also include a small amount of glass frit (not shown) that makes up less than 3 wt% of the layer. .

In other embodiments, the phase separation of the materials in intercalation 230 is modified in FIG. 2 to form a layered structure. Fig. 4 is a schematic cross-sectional view illustrating a sintered multilayer stack 400 (corresponding to structure 200 of fig. 2) including an intercalation having two sublayers. Sintered multilayer stack 400 (only in multilayer stack region 450) includes modified (during firing) metal particle layer 422 in multilayer stack region 450 between a portion of base layer 410 and modified (during firing) intercalation 430. A metal particle layer 420 containing metal particles 402 is on the base layer 410 of the adjacent multi-layer stack region 450.

Modified intercalation 430 includes two sublayers: sub-insertion layer 433 directly on modified metal particle layer 422, and noble metal sub-layer 435 directly on sub-insertion layer 433. Noble metal sublayer 435 has solderable surface 435S. Modified metal particle layer 422 comprises metal particles 402 and some material 403 from sub-intercalation layer 433. There is an interface 422I between modified intercalation 430 (or sub-intercalation 433) and the topmost metal particle 402 in modified metal particle layer 422. In embodiments where the optional glass frit is already included in the dried metal particle layer (120 in fig. 1) prior to firing, the modified metal particle layer 422 and the metal particle layer 420 may also include a small amount of glass frit (not shown) that makes up less than 3 wt% of the layer.

Cross-sectional SEM images are used to identify the layers and measure the layer thickness in the multilayer stack. The average layer thickness of the layers in the multilayer stack is obtained by averaging at least ten thickness measurements, each separated by at least 10 μm, across the cross-sectional image. In various embodiments of the present invention, the metal particle layer (e.g., 220 in FIG. 2) has an average thickness of between 0.5 μm and 100 μm, between 1 μm and 50 μm, between 2 μm and 40 μm, between 20 μm and 30 μm, or any range contained therein. This layer of metal particles on the base layer is typically smooth, having minimum and maximum layer thicknesses over an area of 1x1mm that are within 20% of the average metal particle layer thickness. In addition to cross-sectional SEM, layer thicknesses and changes in the described areas can be accurately measured using an Olympus LEXTOLS 40003D laser measurement microscope and/or profilometer (profilometer), such as Veeco Dektak 150.

In one exemplary embodiment, the metal particle layer (e.g., 220 in FIG. 2) is made of sinterable aluminum particles and has an average thickness of 25 μm. The porosity of the metal particle layer can be measured in the range between 0.01kPa and 2Mpa using a mercury porosimeter, for example a CE instrument Pascal 140 (low pressure) or Pascal 440 (high pressure). The layer of sintered metal particles may have a porosity between 1% and 50%, between 2% and 30%, between 3% and 20%, or any range contained therein. The layer of sintered metal particles made from aluminium particles and used in solar applications may have a porosity between 10% and 18%.

The thicknesses of the sub-intercalation and noble metal sub-layers, for example as schematically shown at 433 and 435 respectively in fig. 4, are measured in the actual multilayer stack using cross-sectional SEM/EDX. The sublayers are distinguished in the SEM by contrast differences between the intercalation and noble metal phases. The EDX map (mapping) is used to identify the interface location, shown in FIG. 4 as 432I. In various embodiments, the noble metal sub-layer has a thickness between 0.5 μm and 10 μm, between 0.5 μm and 5 μm, between 1 μm and 4 μm, or any range contained therein. In various embodiments, sub-intercalation has a thickness between 0.01 μm and 5 μm, between 0.25 μm and 5 μm, between 0.5 μm and 2 μm, or any range contained therein.

In one embodiment of the invention, the modified intercalation comprises two phases: noble metal phase and embedded phase. This structure is shown in detail in fig. 4. Typically, the embedded phase is not solderable, so if solderable surface 230S contains mostly noble metal phase, it is useful to ensure solderability. In various arrangements, the solderable surface includes greater than 50%, greater than 60%, or greater than 70% noble metal phases. In one arrangement, the intercalation-modified solderable surface contains mostly noble metal(s). The flat view EDX is used to determine the concentration of the element on the modified intercalation surface. SEM/EDX was performed using the apparatus disclosed above and at an acceleration voltage of 10kV, with a 7mm sample working distance and 500 x magnification. In various embodiments, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt%, or at least 98 wt% of solderable surface 230S of modified intercalation 230 comprises one or more of gold, silver, platinum, palladium, rhodium, and alloys, composites, and other combinations thereof. The firing conditions, the embedded particle and noble metal particle types and sizes all reflect the degree of phase separation in the modified intercalation morphology.

The modified metal particle layer (shown as 222 in fig. 2) contains a higher concentration of embedded particle material than the metal particle layer (shown as 220 in fig. 2). A comparison of EDX spectra obtained from a cross section of the modified metal particle layer and the metal particle layers in the actual multilayer stack can be used to determine the concentration of material from the modified intercalation that has embedded the modified metal particle layer. SEM/EDX apparatus as described above, operating at 20kV, with 7mm operationDistance, for measuring the ratio of metal (e.g., bismuth) from the embedded particles to total metal (e.g., bismuth plus aluminum) in a cross-sectional sample of the modified metal particle layer. The weight ratio (ratio of embedded metal to total metal) is referred to as the IM: M ratio. The baseline EDX analysis was performed in the region of the metal particle layer, which was at least 500 μm away from the modified metal particle layer to ensure reproducible measurements. A second EDX spectrum was obtained from the modified metal particle layer, and the spectra were compared. In the determination of the IM: M ratio, only the peaks of the metal elements (i.e., peaks from carbon, sulfur and oxygen are ignored) are considered. When the ratio is analyzed, the noble metal and any metallic elements from the substrate are excluded, thereby preventing unreliable results. In one embodiment, when the dried metal particle layer (shown as 120 in fig. 1) comprises aluminum particles and intercalation 130 comprises bismuth and silver particles, the metal particle layer (i.e., after firing) comprises approximately 1 wt% bismuth and greater than 98 wt% aluminum, having a 1: bi (Al + Bi) (IM: M) ratio of 99. The other insert metal comprises less than 0.25 wt% of the modified metal particle layer and is not considered in calculating the IM to M ratio. In various other embodiments, the IM: M ratio is 1:10⁶1:1000, 1:100, 1:50, 1:25 or 1: 10.

It should be noted that the base layer may have some surface roughness, which may result in the interface with them also being rough. Fig. 5 is a schematic cross-sectional view illustrating a portion of such a base layer 510, modified metal particle layer 522, and modified intercalation layer 530, in accordance with an embodiment of the present invention. There is a non-planar interface 501B between the base layer 510 and the modified metal particle layer 522. There is a non-planar interface 522B between modified metal particle layer 520 and modified intercalation layer 530. The line 502 indicates the deepest intrusion of the sub-layer 510 into the modified metal particle layer 522. Line 504 indicates the deepest intrusion of modified intercalation 530 into modified metal particle layer 522. The region of the modified metal particle layer 522 between the line 502 and the line 504 may be referred to as a sample region 522A. In determining the IM: M ratio in the modified metal particle layer 522, it is useful to limit this analysis to the sample region 522A, thereby avoiding spurious results due to interface roughness.

In illustrative embodiments, the IM: M in the modified metal particle layer is 20% higher, 50% higher, 100% higher, 200% higher, 500% higher, or 1000% higher than in the metal particle layer (in a region at least 500 μ M away from the modified metal particle layer). In one exemplary embodiment, the intercalation containing bismuth particles is on an aluminum particle layer, and the modified metal particle layer (as analyzed in a sample area, e.g., shown as 522A in FIG. 5) contains 4 wt% bismuth and 96 wt% aluminum, with a Bi (Al + Bi) (or IM: M) ratio of 1: 25. The ratio of Bi (Al + Bi) in the modified metal particle layer is 400% higher than that in the metal particle layer.

When the intercalation contains crystalline metal oxides and/or frits, which contain more than one metal, the IM: M ratio is determined quantitatively and additively by EDX for the embedded metal components. For example, if the frit contains both bismuth and lead, then the ratio is defined as (Bi + Pb) to (Bi + Pb + Al).

In various embodiments, the sintered multilayer stack further comprises a solid mixed layer formed by the interaction between the dried metal particle layer and the metal particles in the base layer during firing. The solid mixed layer may include, but is not limited to, an alloy, a eutectic, a composite, a mixture, or a combination thereof. In one arrangement, the modified metal particle layer and the base layer form a solid mixed (multi-) region at their interface. The solid hybrid (multi) region may comprise one or more alloys. The solid mixing (multi) zone may be continuous (one layer) or semi-continuous. Depending on the composition of the base and metal particle layers, the alloy(s) or other mixture formed may include one or more of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, silicon, oxygen, carbon, germanium, gallium, arsenic, indium, and phosphorus. For example, aluminum and silicon can form a eutectic above 660 ℃, which upon cooling brings a solid aluminum-silicon (Al-Si) eutectic layer at the silicon interface. In one exemplary embodiment, the solid mixture layer is a solid Al-Si eutectic layer formed on a portion of the silicon based layer. The formation and morphology of solid Al-Si eutectic layers is well known in silicon solar cells. In another embodiment, the base layer is doped with at least one of aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, and alloys, composites, and other combinations thereof. In one example, aluminum is a p-type dopant in silicon, and during firing, aluminum from the aluminum particle layer of the adjacent base layer provides more aluminum dopant to form highly p-type doped regions in the silicon-based layer, known as the back surface field.

Depending on atmospheric conditions, the embedded particles may experience multiple phase changes as they melt and become embedded in the modified metal particle layer in the sintered multilayer stack. Depending on the materials in the modified metal particle layer and the base layer, the embedded particles also form a crystalline mixture as they are embedded in the modified metal particle layer. This crystal mixture may improve the cohesion between the metal particles in the metal particle layer, prevent inter-diffusion of specific elements, and/or reduce the electrical contact resistance between the metal layers in the sintered multilayer stack. In one embodiment, the modified intercalation and modified metal particle layer comprises crystals composed of bismuth and at least one of oxygen, silicon and silver and alloys, composites and other combinations thereof.

In one embodiment, the noble metal phase comprises at least one material selected from the group consisting of: gold, silver, platinum, palladium, rhodium, alloys thereof, compositions thereof, and other combinations thereof. In one arrangement, the noble metal phase essentially comprises one or more of these materials. When one of these materials constitutes the bulk (majority) of the noble metal phase, the noble metal phase is described as being rich in this material. For example, if the noble metal phase, noble metal layer, or noble metal sublayer comprises a substantial portion of silver, it may be referred to as a silver-rich region, silver-rich layer, or silver-rich sublayer, respectively.

The embedded phase contains elements from the embedded particles and may also contain elements from the external environment (e.g., oxygen) and small amounts of noble metal particles from adjacent metal particle layers and nearby base layers that have been integrated during firing. The broad arrangement of elements that can be in the embedded phase depends on whether a low temperature base metal, crystalline metal oxide and/or frit is used as the embedded particles. In one embodiment (when the embedded particles are only low temperature base metals), the embedded phase comprises at least one material selected from the group consisting of: bismuth, boron, tin, tellurium, antimony, lead, oxygen, and alloys, composites, and other combinations thereof. In another embodiment (when the embedded particles are only crystalline metal oxides), the embedded phase comprises at least one material selected from the group consisting of: bismuth oxides, tin, tellurium, antimony, lead, vanadium chromium, molybdenum, boron, manganese, cobalt and alloys, composites and other combinations thereof. In another embodiment (when the embedded particles are glass frits only), the embedded phase comprises oxygen and at least one of the following elements: silicon, boron, germanium, lithium, sodium, potassium, magnesium, calcium, strontium, cesium, barium, zirconium, hafnium, vanadium, niobium, chromium, molybdenum, manganese, iron, cobalt, rhenium, zinc, cadmium, gallium, indium, tin, lead, carbon, nitrogen, phosphorus, arsenic, antimony, bismuth, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, lanthanum, cerium, and alloys, composites, and other combinations thereof. When one of these materials constitutes the bulk of the embedded region, the embedded region is described as being rich in that material. For example, if the embedded region, intercalation or sub-intercalation comprises mostly bismuth, it may be referred to as a bismuth-rich region, bismuth-rich layer or bismuth-rich sub-layer, respectively.

Examples and applications of sintered multilayer stacks

Most metal particles containing aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, and titanium cannot be soldered using mild activated (RMA) fluxes and tin-based solders after firing. However, in solar cells and other devices, a solder strip is highly desirable to make electrical contact with a layer of metal particles, such as a layer of aluminum particles. As disclosed herein, the inventive embedding pastes containing noble metals, such as silver and gold, can be used on a layer of metal particles and sintered in air to produce a highly solderable surface. This, in contrast to other attempts, increases the solderability of the metal particle layer by adding a noble metal, which, due to the usual interdiffusion with the metal particle layer (e.g., aluminum) upon firing of the multilayer stack, results in a solderable surface containing very little noble metal, and thus very good soldering. For example, firing a commercially available silver-based post-marking paste containing less than 10 wt% glass frit onto a layer of aluminum particles does not result in a solderable surface. These layers undergo significant silver-aluminum interdiffusion during the firing step and result in a non-solderable silver-aluminum surface.

As disclosed herein, intercalation can be used to modify the material properties of the metal particle layer, thereby 1) blocking diffusion of the noble metal and providing a solderable surface, 2) mechanically strengthening the metal particle layer, and 3) assisting in etching the layers underlying the metal particle layer. In one embodiment of the invention, a multilayer stack is formed using an embedding paste that includes noble metal particles made of silver and embedded particles made of bismuth metal or a bismuth-based glass frit, and an adjacent metal particle layer that includes aluminum particles. The sintered multilayer stack is formed by: screen printing an aluminum paste on a bare silicon wafer (typically for solar cell applications), drying the sample at 250 ℃ for 30 seconds, screen printing an embedding paste on a portion of the dried aluminum particle layer, drying the sample at 250 ℃ for 30 seconds, and co-firing the sample using a spike ignition profile (spike fire profile) with a peak temperature between 700 ℃ and 820 ℃ and a ramp up and cooling rate of greater than 10 ℃/sec. All drying and firing steps are performed using a Despatch CDF7210 furnace, which is typically used in silicon solar manufacturing.

SEM/EDS analysis was used to determine the elemental composition of multiple regions in the fired multilayer stack of the polished cross section and to study the progress of intercalation. SEM/EDX was performed using the previously described apparatus using two different modes of operation. SEM micrographs were taken using a Zeiss Gemini Ultra-55 analytical field emission SEM using two modes known as SE2 and Inlens. The SE2 mode was operated at 5-10kV and 5-7mm working distance using the SE2 second electron detector and a scan cycle time of 10 seconds. The brightness and contrast are varied between 0 and 50% and between 0 and 60%, respectively, in order to maximize the contrast between the embedded region and the Al particles. The Inlens mode was operated at 1-3kV and a working distance of 3-7mm using an InLens second electron detector and a scan cycle time of 10 seconds. To capture the BSF in the lens mode, the brightness is set to 0% and the contrast is set to about 40%.

In one embodiment of the present invention, the intercalation paste including 10-15 wt% of the intercalation particles blocks interdiffusion between the noble metal (i.e., silver) and the metal particles (i.e., aluminum). Intercalation paste a (shown in table I) contains 12.5 wt% bismuth particles and 50 wt% Ag, giving a 1:4 weight ratio of intercalation particles to noble metal particles. The sintered multilayer stack is made as described above. SEM of sintered multilayer stacks was performed in SE2 mode using the equipment described above at an acceleration voltage of 5kV, a working distance of 7mm and a magnification of 4000 times.

FIG. 6 is a scanning electron micrograph cross-sectional view of a co-sintered multilayer stack. Modified intercalation layer 630 is directly on modified metal particle layer 622. Modified intercalation 630 includes bismuth-rich sublayer 632 (the intercalation phase), which includes bismuth oxide, and silver-rich sublayer 634 (the noble metal). The modified metal particle layer 622 comprises aluminum particles 621 and embedded phase material 623 that have diffused from the bismuth-rich sublayer 632. The bismuth-rich sublayer 632 is directly on the aluminum particles 621, at least near the interface region 631. The bismuth-rich sublayer 632 appears to prevent interdiffusion of silver from the modified intercalation 630 and aluminum from the modified metal particle layer 622 during the co-firing process. Fig. 6 is an example of the layered structure described above in fig. 4. The silver-rich sublayer 634 provides a highly solderable surface (away from the modified metal particle layer 622). The embedded phase material 623 does not penetrate far into the modified metal particle layer 622. The modified metal particle layer 622 mostly includes aluminum particles that weakly sinter together after co-firing and have poor mechanical strength. There is insufficient bismuth available to penetrate deep into the modified metal particle layer 622, and the bismuth-rich sublayer 632 can apply pressure to the modified metal particle layer 622, which can mechanically weaken the co-sintered multilayer stack. This co-sintered multilayer stack has a peel strength below 0.4N/mm (newtons per mm), with a major failure mechanism between the Al particles. Existing solar industry standards requiring peel strengths greater than 1N/mm are considered commercially viable.

Embedding paste B (shown in table I) used glass frit as the embedding particles to achieve a solderable surface. The intercalation paste B contained 30 wt% bismuth-based glass frit (intercalation) particles and 45 wt% Ag, giving a 1:1.5 weight ratio of intercalation particles to noble metal particles. The glass frit comprises mainly bismuth and has a glass transition temperature of 387 ℃ and a softening point of 419 ℃. SEM of sintered multilayer stacks was performed in SE2 mode using the equipment described above at an acceleration voltage of 5kV, a working distance of 7mm and a magnification of 4000 times. FIG. 7 is a scanning electron microscope cross-sectional view of such a co-sintered multi-layer stack, in accordance with an embodiment of the present invention. The modified aluminum particle layer 722 is a modified metal particle layer comprising aluminum particles 730. During co-firing, the bismuth-based glass frit does not completely phase separate from the Ag particles, leading to improved intercalation 750 with two phases: the noble metal phase 721 and the bismuth-based embedded phase 740 are similar to those shown in fig. 3 above. Surface 750S on modified intercalation 750 contains more than 50% of noble metal phase 721. Surface 750S may be soldered using fluxes commonly used in the solar cell industry (e.g., Kester 952S, Kester 951 and Alpha NR 205). The overall peel strength of the sintered multilayer stack is below 0.5N/mm, which may be due to the low permeability of the bismuth-based embedded phase 740 into the modified aluminum particle layer 722. In general, modifying the morphology of intercalation can be modified by varying the embedded particle composition and loading in the intercalation.

Embedded slurry barrier element interdiffusion and strengthening of underlying metal particle layer

The above examples show two paste formulations designed to block interdiffusion between the noble technology (i.e., silver) and the metal particles (i.e., aluminum), but their sintered layers lack sufficient mechanical strength when soldered. The intercalation paste C (shown in Table I) contains 30 wt% bismuth particles and 45 wt% silver particles (i.e., Ag: Bi intercalation paste), resulting in a 1:1.5 weight ratio of intercalation particles to noble metal particles. The increased content of embedded particles in the slurry results in a higher concentration of embedded material in the modified metal particle layer and results in a mechanically stronger sintered multilayer stack. The embedding paste C was used as a drop-in replacement for commercial post-silver marking paste during the manufacture of BSF, polycrystalline, p-type solar cells. The embedding paste C may also be referred to as silver on aluminum (Ag-on-Al), post-logo, post-float logo, or logo embedding paste. A set of characterization tools was used on the obtained sintered multilayer stack to assess IM: M (embedded metal: metal) ratio, noble metal surface coverage, and to determine if crystals were formed in the embedded region.

The effect of intercalation on the metal particle layer is best demonstrated by first illustrating the morphology of the sintered aluminum particle layer on the non-intercalated silicon base layer. Fig. 8 is a Scanning Electron Microscope (SEM) cross-sectional view of this sintered aluminum particle layer 822 on a silicon base layer 810 in SE2 mode, along the area of the silicon solar cell that does not contain intercalation. The sintered aluminum particle layer 822 is about 20 μm thick and includes aluminum particles 821 and a small amount of inorganic binder (i.e., frit) 840. An InLens mode scanning electron microscope of the same aluminum particle layer is shown in fig. 9. In the InLens model, the aluminum particle layer 922, aluminum particles 921, and silicon-based layer 910 are clearly visible, in addition to the back surface field area 970 and the solidified aluminum-silicon (Al-Si) eutectic layer 980.

The effect of intercalation on the resulting modified metal particle layer after co-firing can be understood with reference to fig. 10. Fig. 10 is an InLens SEM interface view of the same silicon solar cell used in the image shown in fig. 8, but along a region containing a co-sintered multilayer stack made using intercalation paste C (shown in table I). The co-sintered multilayer stack 1000 comprises a modified intercalation 1030, a modified aluminum particle layer 1022, a solidified Al-Si eutectic layer 1080, an aluminum doped Back Surface Field (BSF) region 1070, and a silicon based layer 1010. In one exemplary embodiment, the BSF in the silicon base layer is doped p-type to 10¹⁷To 10²⁰Per cm³In the meantime.

The SE2 mode scanning electron microscope of the co-sintered multi-layer stack of fig. 10 is shown in fig. 11. While the InLens mode clearly shows the BSF region, the SE2 mode is the preferred mode to reflect the bismuth (embedded phase) in the modified aluminum particle layer. The co-sintered multilayer stack 1100 comprises modified intercalation 1130, modified aluminum particle layer 1122, and silicon-based layer 1110. Silver sublayer 1134 and bismuth sublayer 1132 in modified interlayer 1130 are also seen. The BSF region and the solidified Al-Si eutectic layer are not clearly visible in this image. The modified aluminum particle layer 112 includes a substantial amount of bismuth embedding material 1103 that is embedded around the aluminum particles 1102 during co-firing. In some instances, the contrast between bismuth and silver may not be strong enough to clearly identify the sub-layer and the extent to which bismuth is embedded in the aluminum particle layer. Elemental maps of cross-sectional views in these examples can be obtained using SEM/EDX to fully determine the silver and bismuth positions in the co-fired multilayer stack.

The amount of embedded metal (i.e., bismuth) due to embedding in the modified aluminum particle layer can be determined by comparing EDX spectra, from the modified aluminum particle layer region and the aluminum particle layer region in the same cross-sectional sample. This is most useful if the regions are spaced more than 1 μm apart from each other. The manner in which this comparison is made has been described above as the IM: M or Bi (Bi + Al) ratio. This analysis is useful in determining whether the embedding paste is used in the manufacture of solar cells. Metallization layers in solar cells contain a limited subset of metals including, for example, aluminum, silver, bismuth, lead, and zinc. In commercial solar cells, the layer of aluminum particles contains almost entirely aluminum.

In one example, the embedded particles in the embedded paste C include only bismuth, and the metal particles in the metal particle layer are mostly aluminum. Comparing the ratio of bismuth to bismuth plus aluminum, Bi: (Bi + Al), in the aluminum particle layer (i.e., it does not interact with the intercalation slurry) with the modified aluminum particle layer is a useful metric in determining whether the intercalation slurry is incorporated into a solar cell. The EDX spectra for both layers were measured for approximately three minutes using the above described apparatus at an acceleration voltage of 20kV and a working distance of 7 mm. The EDX spectrum for the sintered aluminum particle layer 822 in fig. 8 was collected from region 898. The EDX spectrum for the modified aluminum particle layer 1122 in fig. 11 was collected from region 1199. Elemental quantification was performed on these spectra using Bruker QuantaxEsprit 2.0 software for automated element identification, background subtraction and peak fitting. The EDX spectrum is shown in figure 12. The aluminum and bismuth metal peak areas were quantified and the wt% for the two layers calculated from the EDX spectra in fig. 12 and summarized in table II below. No significant amount of any other metals can be identified in the EDX spectrum. The EDX spectrum of the aluminum particle layer shown in FIG. 12A yields a Bi (Bi + Al) wt% ratio of 1:244, and the EDX spectrum of the modified aluminum particle layer shown in FIG. 12B yields a Bi (Bi + Al) wt% ratio of 1:4, as shown in Table II. The modified aluminum particle layer 1122 has a Bi (Bi + Al) wt% ratio approximately equal to the ratio of Ag: the sintered aluminum particle layer 822 in Bi intercalation contact is 62 times higher. In various embodiments, the ratio of Bi (Bi + Al) in the sintered multilayer stack is at least 20%, or at least 50%, or at least 2x, or at least 5x or at least 10x or at least 50x higher than in the layer of sintered aluminum particles in the modified aluminum particle layer.

Table II:

quantitative determination of aluminum bismuth EDX and results Bi (Bi + Al) wt%)

	Al	Bi	Ratio of Bi (Bi + Al)
				Aluminum particle layer	40.290	0.166	1:244
Improved aluminum particle layer	43.641	14.974	1:3.91

The flat view EDX can be used to determine the concentration of elements on the surface of the rear marker layer in a silicon solar cell. In plan view, the EDX probe area surface is to a depth of about 4 μm or less, making this a useful technique for identifying inter-diffusivity in a jointly stacked multi-layer stack: a higher noble metal concentration means there is less interdiffusion and a lower noble metal concentration means there is more interdiffusion. Fig. 13 is a diagram of a process for making a glass substrate from Ag: flat view EDX spectroscopy performed on the surface of the Bi intercalated post-marker layer. EDX spectra were collected using SEM, operating at an acceleration voltage of 10kV, a working distance of 7mm and 500 x magnification. The main peak between 3.5 and 4keV and the smaller peak at 0.3keV are all identified as silver. The remaining small peaks in the spectrum are identified as follows: carbon at 0.3keV (small silver peaks with convolution); oxygen at 0.52 keV; aluminum at 1.48 keV; and bismuth at 2.4 keV. Elemental quantification was performed automatically using Bruker Quantax Esprit 2.0 software to subtract background, identify elemental peaks, and then adapt the peak intensities of the x-ray energies. The standard weight percentages of each element are shown in table III below. The overall silver coverage on the surface of the rear indicia layer was 96.3 weight percent (wt%).

TABLE III

Standard weight percent of elements on surface of rear marker layer

XRD can be used to distinguish between a sintered multilayer stack using bismuth particles in the intercalation and a sintered multilayer stack using a conventional silver-based marker strip, with less than 10 wt% frit as an inorganic binder XRD is performed using a Bruker ZXS D8Discover GADDS x-ray diffractometer equipped with a VANTEC-500 area detector and a cobalt x-ray source operating at 35kV and 40mA XRD.XRD is performed in FIG. 14 sintered multilayer stack XRD patterns (pattern) on the rear marker strip of a silicon solar cell are shown.diffractograms are measured in two 25 ° frames combined for a total window of 25-80 ° in 2 Θ.30 minutes under x-ray illumination using a cobalt K α wavelength.30 minutes are measured in each frame.background subtraction is not performed on the two diffraction patterns of FIG. 14.

The XRD diffraction pattern showed that, using Ag: the sintered metal stack formed by the Bi intercalation, or the rear marker layer in the solar cell, has a different pattern than one formed without the bismuth. XRD pattern a is from a co-sintered multilayer stack on the back logo layer of a silicon solar cell. Co-sintering the multilayer stack includes improved intercalation, formed using an intercalation paste, which contains approximately 45 wt% silver, 30 wt% Bi, and 25 wt% organic vehicle (as used above for paste C in table I). Peak(s)The value 1410 is identified as silver and the peak 1420 is bismuth oxide (Bi)₂O₃) And (4) crystals. XRD pattern B was from a co-sintered multilayer stack on a back mark layer of a silicon solar cell, formed using a commercially available back mark paste, which contains less than 10 wt% of glass frit, as was the intercalation on an aluminum particle layer. The co-sintered multilayer stack was dark indicating significant silver-aluminum interdiffusion. Peak 1450 is identified as the silicon-aluminum eutectic phase. Peak 1460 is identified as silver-aluminum alloy phase (i.e., Ag)₂Al). Silver peak 1410 was observed in pattern a, accompanied by bismuth oxide mixture, and absent in pattern B, where only silver was observed as part of the silver-aluminum alloy. This further demonstrates that bismuth prevents mutual diffusion in the sintered multilayer stack. In one embodiment, the rear indicia layer in a silicon solar cell comprises crystals of bismuth and at least one other element, such as silicon, silver, oxides thereof, alloys thereof, compositions thereof, or other combinations thereof. In another embodiment, the rear indicia layer comprises bismuth oxide crystals. In another embodiment, the embedded region undergoes multiple phase transitions during firing.

The intercalation layer may be etched through the dielectric layer during firing

In some device applications, a dielectric layer is deposited on the surface of the base layer prior to deposition of the metal layer, thereby passivating the surface of the base layer and improving the electronic properties. The dielectric layer may also prevent inter-diffusion of species between the base layer and the adjacent metal particle(s) layer. In some cases, it may be highly desirable to etch through the dielectric layer to form a mixture between the base layer and the metal particle layer to improve electrical conduction between the base layer and the metal particle layer. Frits containing bismuth and lead are known to etch through various dielectric layers (e.g., silicon nitride) during co-firing of silicon solar cells. In one illustrative embodiment, intercalation paste D (from table I above) contains approximately 30 wt% silver, 20 wt% intercalation particles (15 wt% metallic bismuth particles, 5 wt% high lead content glass frit), and 50 wt% organic vehicle. This embedded paste is particularly useful if etching through the dielectric layer is desired.

Fig. 15 shows a schematic cross-sectional view of a multi-layer stack 1500 including a base layer 1510 coated with at least one dielectric layer 1513, prior to firing, in accordance with an embodiment of the invention. A layer of dry metal particles 1520 is on a portion of the dielectric layer 1513. Intercalation 1530, consisting of embedded particles and noble metal particles, as described above, directly on a portion of the dried metal particle layer 1520. The noble metal particles and the embedded particles may be homogeneously distributed in intercalation 1530 prior to firing. The dielectric layer includes at least one of silicon, aluminum, germanium, gallium, hafnium, oxides thereof, nitrides thereof, composites thereof, and combinations thereof. In one arrangement, the dielectric layer 1513 is a 75nm thick silicon nitride layer. In another embodiment, there is a second dielectric layer (not shown) between dielectric layer 1513 and base layer 1510. In one arrangement, the second dielectric layer is a 10nm thick aluminum oxide layer directly on base layer 1510, and dielectric layer 1513 is a 75nm thick silicon nitride layer directly on the aluminum oxide layer. The dried metal particle layer 1520 is formed by depositing a metal particle slurry on the dielectric layer 1513 and then drying. In one arrangement, the dried metal particle layer 1520 is 20 μm thick and includes aluminum particles. The intercalation 1530 contains embedded particles, such as glass frit, which contains lead or bismuth, deposited on the dried metal particle layer 1520, covering at least a portion of the dried metal particle layer 1520, and then dried.

Fig. 16 is a schematic cross-sectional view illustrating a sintered multi-layer stack 1600 (after the structure 1500 of fig. 15 has been sintered), in accordance with an embodiment of the invention. A portion of base layer 1610 is coated with at least one dielectric layer 1613. During co-firing, at least some of the embedded particles in modified intercalation 1630 (which include the glass frit described with reference to fig. 15) melt and begin to flow, embedding into modified metal particle layer 1622. In one arrangement, material from the glass frit in modified intercalation 1630 penetrates into and through the metal particles in modified metal particle layer 1622 and etches into dielectric layer 1613 (1513 before firing), allowing some of the metal from modified metal particle layer 1622 to chemically and electrically interact with base layer 1610, forming one or more new mixtures 1614. Other embedded particles (e.g., bismuth particles) from modified intercalation 1630 may also be embedded in modified metal particle layer 1622 and may provide structural support. In one arrangement, at least a portion of the noble metal particles and the embedded particles in modified intercalation 1630 form phases that are phase separated from one another, as described in more detail above with reference to fig. 2. In some arrangements, there is also a region 1620 of metal particles (on dielectric layer 1613) into which little or only trace amounts of embedded particle material penetrate. In one exemplary embodiment, the embedded particles are bismuth particles and glass frit particles, and the metal particles are aluminum.

Introducing thickness variation of metal particle layer to reduce bowing

Intercalation can result in stresses in the underlying modified metal particle layer during firing that can lead to bowing or wrinkling and thus poor layer strength and electrical communication between layers. For example, intercalation can have a different coefficient of thermal expansion than the adjacent modified metal particle layers, resulting in different expansion or contraction of the layers during firing. Another source of pressure in the adjacent modified metal particle layer may be the intercalation of molten embedded particle material between the metal particles. These stresses can result in improved metal particle layers and/or improved intercalation bowing or wrinkling. Bowing or wrinkling can be described as large, periodic or aperiodic deviations in layer thickness. Typically, this results in delamination between layers. For example, the initial thickness of the stack containing the intercalated and dried metal particle layers is approximately the same throughout before the intercalation on the dried metal particle layers is sintered. After co-firing, the thickness of the sintered multilayer stack comprising the modified intercalation and modified metal particle layer may be up to three times the initial thickness in some regions.

Fig. 17 is a plan view optical micrograph of a co-sintered multi-layer stack in which bending has occurred. Modified intercalation 1730 is visible. Modified insertion layer 1730 has been bent; some peak regions 1712 are indicated in fig. 17. Adjacent metal particle layer 1720 is not curved and remains smooth or approximately flat. Even though modified intercalation 1730 has deformed, the mechanical integrity of the co-sintered multilayer stack remains strong through a peel strength of greater than 1N/mm. However, bending can make it challenging to make good, strong contact between modified insertion layer 1730 and the flag strip (not shown) when they are soldered together. Modifying the curved surface of interposer 1730 may result in incomplete solder wetting in the area of interposer 1730, which may reduce peel strength and solder bonding reliability. It is useful to reduce or eliminate the bowing in co-firing multi-layer stacks to ensure successful soldering to the marker bands.

Variable thicknesses may be combined into sintered multilayer stacks to significantly reduce bowing and/or wrinkling of the layers. When one or more of the layers has a variable thickness, an uneven interface is created between the layers. One indication of variable thickness is a non-planar interface between the sintered multilayer thin film stack. The variable thickness is created by patterning a portion of the first layer and then printing the second layer directly on the patterned portion of the first layer, thereby creating a non-planar interface between the two layers. In one arrangement, the layer having a variable thickness is a result of having been printed using a patterned screen. After firing, the thickness of each layer may be reduced, but firing does not result in a layer with a variable thickness becoming a layer with a uniform thickness. The variable thickness in one layer can be measured and quantified before and after firing using cross-sectional SEM and surface topology techniques. In various embodiments, a layer can be described as having a variable thickness when it has a thickness variation of at least 20% greater than or at least 20% less than the average thickness of the layer, as measured in a 1x1mm area.

Fig. 18 is a screen that may be used during deposition of a metal particle slurry to achieve a variable thickness of a dried metal particle layer, in accordance with an embodiment of the present invention. The screen 1800 has an open mesh area 1810, and a patterned area 1820. Patterned area 1820 includes closed area 1821 and open area 1822. When the screen is used during printing of a wet metal particle layer, the slurry flows through the open areas 1822 and open mesh areas 1810 and is blocked by the closed areas 1821, which results in a deposited wet metal particle layer having a variable thickness. In one embodiment, the wet metal particle layer is then dried to form a variable thickness dried metal particle layer, and the embedded slurry is deposited directly on the variable thickness dried metal particle layer.

There are several factors that can affect the variable thickness in the dried metal particle layer, such as mesh count, wire diameter and shape, wire angle relative to the frame, emulsion (emulsion) thickness, and screen design. The mesh size and wire diameter determine the minimum pattern shape and openings that can be printed. Thickness variations in the dried metal particle layer are also affected by the flow of the metal particle slurry, which affects layer slip. The slurries can be designed with high viscosity and thixotropy to precisely control where they are deposited on the substrate. It is also possible to vary the magnitude of the thickness variation in the metal particle layer by adjusting the emulsion thickness of the screen. The screen can be designed to ensure a continuous layer of dry metal particles on the surface of the base layer, with a variable layer thickness, either entirely or only in specific areas. In one illustrative embodiment, the metal particle paste is screen printed using a 230 mesh screen having an emulsion thickness of 5 μm. In one arrangement, the patterned area 1820 has a 100 μm series of 100 μm open area 1822 of 3mm adjacent closed area 1821 of 3 mm. There is no limitation on the pattern type, period (or lack thereof), or size. Many patterns can result in variable thickness and the pattern can be tailored for a variety of printing conditions and slurry formulations.

Fig. 19 is a schematic cross-sectional view of a layer of dry metal particles having a variable thickness deposited on a base layer 1910 using the screen 1800 of fig. 18, in accordance with an embodiment of the present invention. An outer region 1925 of the dried metal particle layer 1920 is formed by depositing a slurry of metal particles through the open mesh areas 1810 of the screen 1800 and then drying the slurry of metal particles. The layer of variable thickness dry metal particles 1922 in regions 1925 is deposited through patterned regions 1820 of the screen 1800 and has a variable thickness. The embedding paste is then printed directly onto the variable thickness dried metal particle layer 1922 in region 1925 and dried to form intercalation 1930.

Fig. 20 is a schematic cross-sectional view of the structure of fig. 19 deposited on a base layer 2010 using the screen 1800 of fig. 18 (the structure of fig. 19) with a layer of dry metal particles of variable thickness after the structure has been co-sintered, in accordance with an embodiment of the invention. Outside the region 2025, there is a metal particle layer 2020 (formed from the dried metal particle layer 1920 of fig. 19). As described above, co-firing results in the embedding of material from the intercalation 1930 (fig. 19) into the underlying variable thickness dried metal particle layer 1922 (fig. 19), converting the variable thickness metal particle layer 1922 to the variable thickness modified metal particle layer 2022 and converting the intercalation 1930 to the modified intercalation 2030. In one arrangement, the modified metal particle layer 2022 has patterned thickness variations including, but not limited to, periodic ridges, edges, and other distinctive shapes. It should be noted that the thickness of modified intercalation 2030 is generally uniform, and that the non-planar interface between the modified intercalation and the modified metal particle layer (due to its variable thickness) can be inferred by measuring the change in the total layer thickness of the multilayer stack.

Fig. 21 is a plan view optical micrograph of a co-sintered multilayer stack in which a metal particle slurry is printed with variable thickness (in some areas) using a screen such as that shown in fig. 18. The intercalation is printed directly onto the variable thickness regions of the metal particle layer and the multi-layer stack is co-sintered to form modified intercalation 2121 on the top surface, with edges on each side of the approximately flat metal particle layer 2120. The metal particle layer 2120 has a flat top surface. The surface of modified intercalation 2121 is uneven with a pattern reflecting the thickness variation in the underlying modified metal particle layer. The surface of modified insert 2121 does not exhibit a curved or wrinkled character, as is clearly visible in modified insert 1730 of fig. 17. In one embodiment of the invention, a portion of the co-sintered multilayer stack has a variable thickness.

A useful unit of measure to describe the variable thickness is to compare the peak and valley thicknesses to the average layer thickness. There may be some unintentional thickness variation in any layer, but these variations are typically less than 20% of the average layer thickness. A layer can be considered flat (having a uniform thickness) if its thickness varies by less than 20% of the average layer thickness. By careful design of the screen used for printing the metal particle paste, it is possible to produce a layer with a variable thickness having a thickness variation measured in an area of 1x1mm of at least 20% greater or at least 20% less than the average thickness of the layer.

The variable thickness in the sintered multilayer stack can be measured from SEM images of polished cross-sectional samples. FIG. 22 is a cross-sectional SEM image of a portion of a sintered multilayer stack 2210 with variable thickness, in accordance with an embodiment of the invention. Cross-sectional samples were prepared and drawn using the method described above. The sintered multilayer stack 2210 comprises a modified intercalation 2211, a modified aluminum particle layer 2212, and a silicon base layer 2213. Two interfaces on each side of the modified aluminum particle layer 2212 are identified in the image: the interface 2218 between the silicon base layer 2213 and the modified aluminum particle layer 2212, and the interface 2217 between the modified aluminum particle layer 2212 and the modified intercalation layer 2211. Solderable surface 2216 is also shown. For comparison, FIG. 23 shows a silicon base layer 2322 having a flat aluminum particle thin film 2321 that does not have a variable thickness.

The average thickness of the modified aluminum particle layer 2212 in fig. 22 is calculated from the average thickness measurements. The thickness between the two interfaces 2217 and 2218 in fig. 22 is measured at regular intervals (e.g., 10 microns) across the sample. The thickness is also measured at local maxima and local minima. Software, such as ImageJ 1.50a, can be used to obtain the average thickness as well as the minimum and maximum thicknesses. The peaks and valleys seen in a single cross-sectional sample may not represent the entire sintered multilayer stack. It is therefore useful to perform this measurement on multiple cross-sectional samples, thus ensuring a measurement and many peaks and valleys. These methods are known to those skilled in the art.

For the sample shown in fig. 22, modified aluminum particle layer 2212 has an average thickness of 11.3 μm, a peak thickness of 18.4 μm, and a valley thickness of 5.2 μm. The peak thickness is 64% greater than the average thickness and the valley is 54% less than the average thickness. In various embodiments, the layer having a variable thickness has a peak thickness that is at least 20%, at least 30%, at least 40%, or at least 50% greater than the average layer thickness. In various embodiments, the layer having a variable thickness has a valley thickness that is at least 20%, at least 30%, at least 40%, or at least 50% less than the average layer thickness.

When modified interlayer 2211 is continuous and approximately uniform in thickness, solderable surface 2216 of modified interlayer 2211 is approximately parallel to interface 2217. In one embodiment of the invention, all of the measurements described for modifying aluminum particle layer 2212 can be taken for modifying aluminum particle layer 2212 and the combined thickness of modified interlayer 2211 between solderable surface 2216 and interface 2217. The comparison of the thickness measurements for the two combined layers is a good approximation for comparing the thickness measurements for the modified aluminum particle layer 2212 only. For the combined layer in fig. 22, the peak thickness was 44% greater than the average bulk thickness of 13.2 μm and the valley was 43% less than the average bulk thickness. This alternative method can systematically measure the thickness variation in the sintered stacked multilayer underneath.

For some applications, only a portion of the sintered multilayer stack needs to have a variable thickness. For example, the aluminum particle layer on the back side of a silicon solar cell is typically flat. It is useful to introduce variable thickness in the back logo layer portion (which includes improved intercalation) on the back side of such a cell. The comparison of the thickness variation in a portion of the indicia layer with the thickness variation in a portion of the surrounding aluminum particle layer can be used to determine whether a layer having a variable thickness is used on the back side of the solar cell.

Another useful metric for determining variable thickness in a sintered multilayer thin film stack is the average valley to peak height, which is the difference between the average of local maxima and the average of local minima. In cross-sectional SEM images, local maxima and local minima are not guaranteed to be in the image, so surface topology metrology methods such as profilometers, coherence scanning interferometers, and zoom microscopes are more useful. An example of a surface photometer is Bruker or Veeco Dektak150 or equivalent. A coherent scanning interferometer can be performed using an Olympus LEXT OLS 40003D measurement microscope. The software accompanying these methods can automatically calculate the average peak-to-valley difference.

In one example embodiment, a profilometer is used to determine the average peak to valley height in the same sample, both for sintered multilayer stacks with variable thickness and for layers of aluminum particles with uniform thickness. Veeco Dektak150 was used to measure surfaces in a 1x1mm area using a 12.5mm radius probe to generate a 3D topological surface map. Fig. 24 is a 3D surface topology map of a sintered multilayer stack with variable thickness, and fig. 25 is a 3D surface topology map of a (adjacent) aluminum particle layer with uniform thickness. The brightest areas in the figure indicate local maxima and the darkest areas indicate local minima. Fig. 24 shows the thickness variation (from-20.2 μm to 15.9 μm) that would be expected for a sintered multilayer stack comprising layers of variable thickness modified metal particles. Fig. 25 shows the thickness variation (from-4.9 μm to 5.5 μm) that would be expected for an aluminum particle layer with a uniform thickness. The average peak-to-valley height was calculated using the program Veeco Vision v4.20, which automatically identified and averaged the local maxima and minima, and then subtracted the difference. The average peak to valley height is 35.54 μm for the sintered multilayer stack of fig. 24 and 9.51 μm for the aluminum layer of fig. 25. In various embodiments, the layers each have a variable thickness when the average peak to valley height is greater than 10 μm, greater than 12 μm, or greater than 15 μm, and the layers have a uniform thickness when the average peak to valley height is less than 10 μm, less than 12 μm, or less than 15 μm.

In one embodiment of the present invention, when the co-sintered variable thickness multilayer stack is modified for intercalation, one of which is shown in fig. 20, is soldered to the flag tape, its peel strength is twice that of the sintered multilayer stack without variable thickness. In one arrangement, the modified intercalation on the surface of this variable thickness sintered multilayer stack is soldered to tin-based flag tapes and they have a peel strength of greater than 1.5N/mm, or greater than 2N/mm, or greater than 3N/mm. The thickness variation can be optimized to provide a continuous metal particle layer and back surface field on a base layer for a silicon solar cell. The thickness variation may be optimized such that the contact resistance of this co-sintered variable thickness multilayer stack is equal to or lower than the contact resistance of an approximately flat co-sintered multilayer stack. In one exemplary embodiment, the thickness variation in the engineered and modified metal particle layer includes regions of less than 20 μm, 10 μm, 5 μm, or 2 μm thickness when using an embedding paste to etch through the dielectric layer.

The variable thickness (multi) layer described above may be used as the (multi) component in any of the sintered multilayer stacks described herein. Variable thickness (multi) layers, such as variable thickness dried and modified metal particle layers, can be used on any silicon solar cell to reduce bowing of the rear indicia layer.

Embedded paste as plug-in replacement in silicon solar cells

In one embodiment, an intercalation paste comprising 45 wt% noble metal particles, 30 wt% intercalating particles, and 25 wt% organic vehicle (paste C in table I above) can be used as a drop in replacement (drop in replacement) to form a rear marker layer in a silicon solar cell. The fabrication of p-n bonded silicon solar cells is well known in the art. Goodrich et al provide a complete process flow to fabricate back surface field silicon solar cells, which are referred to as "standard c-Si solar cells". See Goodrich et al, "wafer-based single crystal silicon photovoltaic power generation road maps: opportunities for further reduction of manufacturing costs using known techniques are improved, solar materials and solar cells (2013), page 110-135, which is incorporated herein by reference. In one embodiment, a method for fabricating a solar cell electrode includes the steps of: providing a silicon wafer having a portion of its front surface covered in at least one dielectric layer, coating the back surface of the silicon wafer with a layer of aluminum particles, drying the layer of aluminum particles, coating a layer of embedded paste (back logo) on a portion of the layer of aluminum particles, drying the layer of embedded paste, coating the dielectric layer on the front surface of the silicon wafer with a plurality of fine gridlines and at least one front manifold layer, drying and co-firing the silicon wafer. Methods such as screen printing, gravure printing, jet deposition, slot coating, 3D printing, and/or inkjet printing may be used to apply the multiple layers. As one example, an Ekra or Baccini screen printer may be used to deposit the aluminum particle layer, the embedding paste layer, and the front side grid lines and the manifold layer. In another embodiment, the solar cell has at least one dielectric layer covering at least a portion of the back surface of the silicon wafer. For the PERC (passivated emitter rear cell) architecture, two dielectric layers (i.e., aluminum oxide and silicon nitride) are applied to the rear side of the silicon solar cell prior to application of the aluminum particle layer. Drying of the multiple layers can be accomplished in a belt furnace (belt furnace) at a temperature between 150 ℃ and 300 ℃ for 30 seconds to 15 minutes. In one arrangement, a Despatch CDF7210 belt furnace is used for drying and co-firing silicon solar cells, which includes the sintered multilayer stack described herein.In one arrangement, the completion of the co-firing uses a rapid heating technique and heating in air to a temperature greater than 760 ℃ for between 0.5 and 3 seconds, which is a common temperature profile for aluminum back surface field silicon solar cells. Temperature profile of a wafer is typically achieved using a thermocouple having a thermocouple attached to the bare wafer

And (5) calibrating the system.

Fig. 26 is a schematic diagram showing the front (or illuminated) side of a silicon solar cell 2600. Silicon solar cell 26-having a silicon wafer 2610 with at least one dielectric layer (not shown) on top of which are fine grid lines 2620 and front bus lines 2630. In one embodiment, the dielectric layer on the front side of the silicon wafer comprises at least one material selected from the group consisting of silicon, nitrogen, oxygen, aluminum, gallium, germanium, hafnium, composites, and combinations thereof. In another embodiment, the dielectric layer on the front side of the silicon wafer is silicon nitride and is less than 200nm thick. Commercially available front side silver metallization pastes known in the art may be used to form the fine grid lines 2620 and the front bus lines 2630. It should be noted that the front side silver layer (i.e., the fine grid lines 2620 and front bus lines 2630 made from silver metallization paste) may be etched through the dielectric layer and in direct contact with the silicon wafer 2610 during co-firing. In one embodiment, the silicon wafer 2610 is monocrystalline and doped n-type or p-type. In another embodiment, the silicon wafer 2610 is polycrystalline and doped n-type or p-type. In one exemplary embodiment, the base layer is a polycrystalline p-type silicon wafer having an n-type emitter.

Fig. 27 is a schematic diagram showing the back side of a silicon solar cell 2700. The back side is coated with a layer of aluminum particles 2730 and has a back marker layer 2740 distributed over the silicon wafer 2710. In one embodiment, the dielectric layer on the back side comprises at least one material selected from the group consisting of: silicon, nitrogen, aluminum, oxygen, germanium, gallium, hafnium, composites, and combinations thereof on the front surface of the silicon wafer. In another exemplary embodiment, the dielectric layer on the front surface of the silicon wafer is silicon nitride and is less than 200nm thick. In one embodiment, there is no dielectric layer on the backside of the silicon wafer. Commercially available aluminum pastes known in the art may be printed on at least 85%, or at least 90%, or at least 95%, or at least 97% of the total surface area of the back side of the silicon wafer prior to firing, which may be described as an overall Al coverage. The aluminum particle layer (after co-firing) 2730 has an average thickness of between 20 and 30 μm. In various embodiments, the aluminum particle layer 2730 has a porosity between 3 and 20%, between 10 and 18%, or any range contained therein. For conventional BSF (back surface field) solar cell architectures, the back marker layer is applied directly to the silicon wafer. However, in order to improve the power conversion efficiency of the solar cell, it is useful to print a rear logo layer on the aluminum particle layer. In one embodiment, the intercalation is coated directly onto a portion of the dried aluminum particle layer to form the post marker layer 2740. Fig. 27 shows one possible pattern for the rear marker layer 2740. The intercalated and underlying aluminum particle layers are finally co-sintered to form a sintered multilayer stack as described herein. In various embodiments, the post marker layer (or modified intercalation) 2740 has a thickness of between 1 μm and 20 μm, or between 2 μm and 10 μm, or between 2.5 μm and 8 μm.

The variable thickness metal (aluminum) particle layer, previously described above, can be used on the back side of a silicon solar cell to reduce bowing of the back marker layer and improve adhesion and electrical contact. In one embodiment of the invention, a portion of the rear indicia layer has a variable thickness. In another embodiment of the invention, a portion of the modified aluminum particle layer has a variable thickness. In one arrangement, the rear marker layer on the surface of this variable thickness modified aluminum particle layer is soldered to a tin-based marker tape, resulting in a peel strength of greater than 0.7N/mm, greater than 1.5N/mm, greater than 2N/mm, or greater than 3N/mm. The thickness variation can be optimized to provide a continuous metal particle layer and back surface field on a base layer for a silicon solar cell. In another embodiment, in the area of the rear marker layer, a portion of the combined layers (the modified aluminum particle layer and the rear marker layer) in that area has a thickness that is at least 20%, 30%, or 40% greater than the average combined layer thickness measured over a 1x1mm area. In another embodiment, in the area of the rear marker layer, a portion of the combined layers (the modified aluminum particle layer and the rear marker layer) in that area has a thickness that is at least 20%, 30%, or 40% less than the average combined layer thickness measured over a 1x1mm area.

In one embodiment of the invention, a solar cell comprising any of the sintered multilayer stacks discussed herein may be incorporated into a solar module. There are many possible solar module designs in which such solar cells are used, as will be known to those skilled in the art. The number of solar cells in the module is not intended to be limiting. Typically, 60 or 72 individual solar cells are consolidated into commercially available modules, but it is possible to consolidate more or less depending on the application (i.e., consumer electronics, residential, commercial, public facilities, etc.). The module typically contains bypass diodes (not shown), a junction box (not shown) and a support frame (not shown) that does not directly contact the solar cells. The bypass diode and junction box may also be considered components of the battery interconnection.

Fig. 28 is a schematic cross-sectional view illustrating a portion of a solar cell module, in accordance with an embodiment of the present invention. The solar cell module includes at least one silicon solar cell 2840. The front side 2840F of silicon solar cell 2840 is connected to first flag tape 2832 (which enters and leaves the page), on which there is front encapsulant layer 2820 and front sheet 2810. The back side 2840B of the silicon solar cell 2840 is connected to a second marker band 2834, on which there is a back encapsulant layer 2850 and a back plate 2860. The marker bands 2832, 2834 adjacent solar cells are electrically contacted to the front side of one cell (i.e., the front bus bar on the front side) and the back side of the adjacent solar cell (i.e., the back marker band on the back side) by solder connections. A large number of solar cells in a solar module can be electrically connected together as cell interconnects using a flag tape.

Typical battery interconnects include metal bus ribbons (metal bus bars) soldered to metal marker ribbons on the solar cells and connecting the marker ribbons. In one embodiment of the invention, the marker band is a metal band with a solder coating. This solder-coated marking tape may have a thickness in the range of 20 to 1000 μm, 100 to 500 μm, 50 to 300 μm, or any range contained therein. The width of the solder-coated marker band may be between 0.1 to 10mm, 0.2 to 1.5mm, or any range contained therein. The length of the tape is determined by the application, design and substrate size. The solder coating may have a thickness between 0.5 and 100 μm, between 10 and 50 μm, or any range contained therein. The solder coating may comprise tin, lead, silver, bismuth, copper, zinc, antimony, manganese, indium, or alloys, composites, or other combinations thereof. The metallic marker band may have a thickness of between 1 μm and 1000 μm, between 50 and 500 μm, between 75 and 200 μm, or any range contained therein. The metallic marker band may comprise copper, aluminum, silver, gold, carbon, tungsten, zinc, iron, tin, or alloys, composites, or other combinations thereof. The width of the metallic marker band may be between 0.1 to 10mm, 0.2 to 1.5mm, or any range contained therein. In one embodiment, the marker band is a copper band 200 μm thick and 1mm wide coated with 20 μm thick tin on each side: lead (60:40 wt%) solder coating.

The front sheet 2810 in fig. 28 provides some mechanical support for the module and has good optical transmission properties over the portion of the solar spectrum that the silicon solar cells 2840 are designed to absorb the solar module is positioned such that the front sheet 2810 faces the source of illumination, e.g., sunlight 2890. the front sheet 2810 is typically made of low iron content soda-lime glass (soda-lime glass). the front and back encapsulant layers 2820 and 2850 protect the silicon solar cells 2840 from electrical, chemical, and physical stimuli during operation.

The back plate 2860 provides protection for the silicon solar cells 2840 from the back side and may or may not be optically transparent. The solar module is positioned such thatThe back plate 2860 faces away from the illumination source, such as sunlight 2890. The back panel 2860 may be a multi-layer structure made of three polymeric films. DuPont^TM

Polyvinyl fluoride (PVF) films are typically used for the backsheet. Fluoropolymers (fluoropolymers) and polyethylene terephthalate (PET) may also be used in the backsheet. The glass sheet may also be used as a backsheet, which may assist in providing structural support to the solar module. A support frame (not shown) may also be used to improve structural support; the support frame is typically made of aluminum.

In one embodiment of the present invention, a method for forming a solar cell module is provided. The tabs are applied to the individual solar cells (which contain any of the sintered multilayer stacks described herein) either manually or by using an automated tabbing or drawing machine. The individual cells are then electrically connected in series by soldering them directly to the flag tape. The resulting structure is called a "cell string". Typically, a plurality of battery strings are arranged on a front encapsulant layer that has been applied to the front sheet. These multiple battery strings are connected to each other using a bus tape to create a circuit. The bus strips are wider than the flag strips used in the battery string. When the circuit between all the battery strings is completed, a rear encapsulant is applied to the back of the connected battery strings and a rear sheet is placed on the rear encapsulant. The assembly is then sealed using a vacuum lamination process and heated (typically below 200 ℃) to polymerize the encapsulant. The frame is typically joined around the front panel to provide structural support. Finally, a junction box is connected to the battery interconnect and to the solar module. The bypass diode may be connected within the junction box or may be connected within the module during the battery interconnection process.

In one embodiment of the present invention, a method of forming a solar module is provided, comprising: a) providing at least one solar cell having a front surface and a back surface; wherein the rear surface comprises a sintered multi-layer stack, b) a portion of a solder flag tape over a portion of the rear flag layer and the front bus bar to create a battery string, c) optionally, a solder flag tape to the bus bar tape to complete the circuit, d) arranging the battery string on the front encapsulant layer that has been applied to the front sheet, e) applying the rear encapsulant layer to the battery string and connecting the rear sheet to the rear encapsulant layer to form a module assembly, f) laminating the module assembly; g) electrically connecting and physically engaging the terminal block.

It is possible to decompose the solar module using the following steps to determine if the multi-layer stacks as described above have been consolidated. The back sheet and the back encapsulant are removed to expose the back surface of the solar cell's logo. A fast curing epoxy is applied to the solar cell's logo strip and surrounding back surface. After the epoxy has cured the cells are removed from the module and a diamond saw is used to cut away the sections of the logo strip/solar cell. The previously described ion mill was used to mill sections and SEM/EDX was performed to determine if the structure was as described in the embodiments of the present invention. Fig. 29 is a polished cross-sectional SEM image of the back (non-illuminated) side of a solar cell. The samples were from solar cells (which included a novel sintered multilayer stack) that had been incorporated into a solar module and subsequently removed as described above. The image shows a metallic flag strip 2932 and its solder coating 2931, which is soldered to the sintered multi-layer stack 2902. The build-up layers of the sintered multilayer stack 2902 are clearly visible. Just below the solder coating 2931 are a modified interposer 2945, a modified metal particle layer 2944, and a silicon base layer 2941. The layers identified in the figures may be more easily identified using EDX.

Other PV cell architectures

The embedding paste can be used to create a variety of sintered multilayer stacks that can be used as metallization layers on the front and back sides of many different solar cell architectures. As disclosed herein, the embedded paste and sintered multilayer stack may be used in solar cell architectures including, but not limited to, BSF silicon solar cells, Passivated Emitter and Rear Contact (PERC) solar cells, and bifacial interdigitated back contact (solar cells).

The PERC solar cell architecture is improved based on the BSF solar architecture, reducing back contact surface recombination by using a dielectric gate (dielectric barrier) between the silicon base layer and the back contact. In a PERC cell, a portion of the backside (i.e., not illuminated) of the silicon wafer is passivated with at least one dielectric layer to reduce current carrier recombination. The novel sintered multilayer stack disclosed herein can be used in PERC solar cells. In one embodiment, the dielectric layer on the backside of the silicon wafer comprises at least one of silicon, nitrogen, aluminum, oxygen, germanium, hafnium, gallium, composites, and combinations thereof. In another embodiment, the dielectric layer on the backside of the silicon wafer comprises a 10nm thick layer of aluminum oxide on the silicon surface and a 75nm thick layer of silicon nitride on the aluminum oxide layer. Commonly used aluminum pastes designed for PERC cells (e.g., single crystal EFX-39, EFX-85) cannot penetrate through the dielectric layer. In order to chemically react the aluminum particle layer and make ohmic contact with the silicon, a small region of the dielectric layer is removed by laser ablation prior to deposition of the aluminum particle layer.

PERL (passivated emitter with rear locally diffused) and PERT (passivated emitter with rear fully diffused) are two PERC cell architectures, which further improve device performance. Both types rely on doping the back of the si-based layer to further inhibit back contact recombination, which acts like a back surface field in BSF cells. In a PERL cell, the backside of the silicon base layer is doped around an opening in the dielectric that makes contact with the back aluminum layer. Doping is typically achieved by propagating the dopant through the dielectric opening using a boron mixture or aluminum from the aluminum particle layer making the back contact, similar to the BSF fabrication process. The PERT cell is similar to the PERL, but all of the silicon in contact with the back dielectric layer is doped except for the silicon adjacent to the dielectric opening of the back contact.

In one embodiment, an intercalation paste, containing intercalating particles that are not etched through a dielectric layer, is used as a post-marker layer on a PERC, PERL, or PERT cell. A "non-etching" embedding paste is used to provide a solderable silver surface and a mechanically strengthened underlying (modified) aluminum particle layer. The resulting sintered multilayer stack comprises a silicon wafer covered with at least one dielectric layer, a modified aluminum particle layer, and a modified intercalation layer; for PERL or PERT, silicon is doped only at the dielectric openings or also through the dielectric interface, respectively. The use of a non-etching embedding paste can further reduce etching of the dielectric layer and reduce surface recombination. For example, bus bar paste traditionally used for the post-marker layer in a PERC cell is printed directly on and partially etched through the dielectric layer, which increases surface recombination during co-firing.

For cells using a back dielectric layer (i.e., PERC, PERL, PERT), the embedding paste may be modified to etch through the dielectric layer and aid in propagating doping of the silicon region at the dielectric opening, in accordance with one embodiment of the present invention. An "etching" embedding paste (e.g., embedding paste D in table I) is used to provide a solderable silver surface, mechanically strengthen the underlying (modified) aluminum particle layer, and etch through the dielectric layer, exposing the silicon surface to aluminum particles, which may result in aluminum doping to the exposed silicon. The resulting sintered multilayer stack comprises silicon wafers, modified aluminum particle layers and modified intercalation. The sintered multilayer stack may further comprise Al-doped regions near the silicon surface (similar to the back surface field in BSF cells), and a solid silicon-aluminum eutectic layer at the interface between the silicon wafer and the modified aluminum particle layer. The use of an embedding paste to etch through the dielectric layer(s) has various advantages. First, it is an inexpensive alternative to laser ablation procedures that have proven expensive and unreliable in the past. Second, when the wafer is co-fired, laser ablation often removes tens to hundreds of microns of the silicon base layer material and can lead to the formation of large voids between the silicon base layer and the aluminum particle layer. The sintered embedding paste does not cause alteration of the wafer surface prior to co-sintering, which leads to better bond formation, reduced void formation and better reproducibility than when laser ablation is used.

In accordance with one embodiment of the present invention, an embedding paste may be used to provide a solderable surface for a cell construction that relies on a layer of aluminum particles to make ohmic contact with p-type silicon. Examples of these configurations include interdigitated back contact solar cells, n-type BSF cell architectures, and bifacial solar cells. In one embodiment, slurry C (from Table I) is embedded) Is applied to the Al layer of an interdigitated back contact solar cell structure, such as a Zebra cell. For an n-type BSF architecture, which has achieved 20% power conversion efficiency for Al-fully covered cells, the embedding paste can replace the traditional post-flag Ag paste that directly contacts silicon, thus lowering the V of the solar cell_oc. In various n-type wafer based solar cell architectures, an embedding paste may be used on the front side (i.e., illumination side). The embedding paste can also be used in combination with Al paste to reduce the cost of the bifacial solar cell. Existing bifacial solar cell architectures use Ag pastes that contain small amounts of aluminum (e.g., less than 5 wt% Al) to make ohmic contact with the p-type silicon layer. Existing two-sided architectures use nearly twice the amount of silver for BSF architectures, which is prohibitive in terms of cost. It is useful to use pure aluminum paste in a two-sided architecture, but Al is not solderable. An embedding paste comprising silver (e.g., paste C in table I) can be printed on the Al paste in a two-sided design and provide mechanical stability and sinterable surface while reducing the amount of Ag used.

Material properties of the sintered multilayer stack and the effect on the silicon solar cell.

Material properties of interest in sintered multilayer stacks for solar cells and other electronic devices include solderability, peel strength, and contact resistance.

Solderability is the ability to form a strong physical bond between two metal layers at temperatures below 400 ℃ by the flow of molten metal solder between the two metal surfaces. Soldering on modified intercalation of a sintered multilayer stack may be performed after heating in air to above 650 ℃. Soldering involves the use of a flux, which is a chemical agent that cleans or etches one or both surfaces prior to reflow of the molten solder. Solder fluxes typically used in solar cells, denoted RMA (e.g.,

186) or R: (

952) Deposited on the marking tape and at 7Drying at 0 ℃. These fluxes etch many metal oxides, such as alumina (Al), that form on aluminum particles when sintered in air₂O₃) Is not effective.

Peel strength is a measure of solder bond strength and an indication of reliability for integrated circuits, light emitting diodes, and solar applications. Solder coated with 0.8 to 20mm wide and 100-300um thick metal strips can be dipped into flux and dried. It is placed on the modified insert and soldered using a soldering iron (solder iron) at a temperature between 200 ℃ and 400 ℃. Peel strength is the force required to peel the solder strip at a given peel speed at 180 ° angle to the solder direction, separated by the width of the solder strip. The soft solder joint (solder joint) formed during the soldering process has an average peel strength at 1mm/sec of greater than 1N/mm (e.g., a 2mm flag strip requires a peel force of greater than 2N to remove the solder strip). The solar cells are electrically connected by a flag, which is soldered to the front bus bar of one cell and the rear flag layer of an adjacent cell. Typically, the peel strength for contact of the marker strip in commercially available solar cells is between 1.5 and 4N/mm. When using a sintered multilayer stack as the post-marker layer, the primary failure mode would be near the Al-Si interface, which can be determined using plan view SEM/EDX. In one exemplary embodiment, the peel strength is greater than 1N/mm when the layer of (intercalated modified) silver rich sub-layer is soldered with a tin based flag tape.

Meier et al describe how four-point probe electrical measurements can be used to determine the resistance of each metallization layer on a complete solar cell. See Meier et al, "determining the component of series resistance from measurements on finished batteries," IEEE (2006), page 2615, which is incorporated herein by reference. The bulk resistance (bulk resistance) of a metallization layer is directly related to the bulk resistance of the material from which it is made. In one embodiment of the invention, the bulk resistance of pure Ag is 1.5x10^-8Omega-m; pure Ag metallization layers used on industrial solar cells have a bulk resistance 1.5 to 5 times higher than pure Ag bulk resistance. The bulk resistance is important for fine gridlines, which must carry current over a relatively long (i.e., greater than 1cm) length. When the battery is marked in the module, the front busThe resistance of the flow strip and the post marker layer is less important.

In most integrated circuit, LED and solar cell architectures, current from the metal particle layer flows through the modified metal particle layer and into the modified intercalation. For sintered multilayer stacks, the contact resistance between these three layers plays an important role in device performance. The measurement of the contact resistance between these layers in a sintered multilayer stack can be measured using Transmission Line Measurement (TLM) (ref: Meier et al, "copper backside collector strip: Ag elimination and full aluminum coverage in crystalline silicon solar cells and modules", IEEE PVSC (2015), pages 1-6). TLM is plotted as the resistance versus distance between the electrodes. TLM is particularly useful for measuring contact resistance, 1) between a layer of metal particles and a layer of modified metal particles, and 2) between a layer of modified metal particles and a layer of modified intercalation. The contact resistance of the sintered multilayer stack is the sum of the contact resistances 1) and 2) described above. The contact resistance of the sintered multilayer stack is half the y-intercept value of a linear fit of the resistance versus distance measurement. The resistance between the busbars was measured using a Keithley 2410 digital source meter (Sourcemeter) set with a four-point probe, with a source current between-0.5A to +0.5A and a voltage measured. In various embodiments, the contact resistance of the sintered multilayer stack is between 0 to 5mOhm, 0.25 to 3mOhm, 0.3 to 1mOhm, or any range contained therein. The sheet resistance of the metal particle layer is determined by multiplying the line slope by the electrode length. The contact resistance and sheet resistance are used to digitally determine the transmission length and consequently the contact resistivity. The change in series resistance is determined by the contact resistivity divided by the partial area coverage of the modified insertion layer. In various embodiments, the change in series resistance is less than 0.200 Ω -cm²Less than 0.100 omega-cm²Less than 0.050 ohm-cm²Less than 0.010 ohm-cm²Or less than 0.001 ohm-cm²。

The contact resistance between the rear marker layer and the aluminum particle layer affects the series resistance and the power conversion efficiency of the solar cell. Such contact resistance can be measured by transmission line measurements. A transmission line drawing of a conventional post-silver marker layer on silicon with a 300 μm layer of overlapping aluminum particles is shown in fig. 30. A transmission line drawing of a modified intercalation on an aluminum particle layer, used as a post-marker layer, is shown in fig. 31. The y-intercept value in FIG. 31 is 1.11mOhm compared to the y-intercept value of 0.88 in FIG. 30. The contact resistance between the rear logo (insert) layer and the aluminum particle layer was 0.56 mOhm. The contact resistance for the conventional post-flag architecture is 0.44 mOhm. In various embodiments, the contact resistance between the post-logo (interleaf) layer and the aluminum particle layer is between 0 and 5mOhm, between 0.25 and 3mOhm, or between 0.3 and 1mOhm or any range contained therein. The sheet resistance of the aluminum layer is determined by the slope of the line times the length of the electrode and is approximately 9 mOhm/square (square) in fig. 30 and 31.

Although TLM is the preferred method for accurately extracting the contact resistance of the sintered multilayer stack (i.e., the back marker layer and the aluminum particle layer), it is possible to determine the contact resistance on the complete intra-solar cell using a four-point probe method. The method is used by first measuring two rear indicia layers (R)_Ag-to-Ag) And then moving the probe over the Al particle layer (within 1mm of the rear marker layer) to obtain R_Al-to-Al. Contact resistance through R_Al-to-AlSubtracting R_Ag-to-AgAnd then divided by 2 to obtain. This is not as accurate as the TLM measurement, but when averaging measurements from multiple solar cells, it may be approximately in 0.50 mOhm.

The resistance and sheet resistance are used to digitally determine the transmission length and consequently the contact resistivity. In fig. 31, the transmission length of the co-sintered multilayer stack is 5mm and the contact resistance is 2.2m Ω. The change in series resistance is estimated by dividing this number by the partial area coverage of the modified insertion layer. In FIG. 31, the estimated change in series resistance is 0.023 Ω -cm²Which is equal to the 0.020 ohm-cm calculated for the conventional rear marker layer measured in figure 30²Change in series resistance. The change in series resistance can be measured directly by making a control BSF (back surface field) silicon solar cell with full Al coverage and no back marker layer and making a control BSF with full Al coverage and Ag: BSF silicon solar cell with Bi intercalation. The series resistance of the cell can be obtained from the current-voltage curves at various light intensities, and the difference in series resistance can be summarized as a post-cursorIncreased contact resistance between the label layer and the sintered aluminum particle layer. In various embodiments, the change in series resistance in the solar cell is less than 0.200 Ω -cm²Less than 0.100 omega-cm²Less than 0.050 ohm-cm²Less than 0.010 ohm-cm²Or less than 0.001 ohm-cm²。

One benefit of using intercalation on silicon solar cells is that an open-circuit voltage (V) is brought about by a continuous back surface field formed on the silicon wafer_oc) Improvement of (1). V_ocGain can be obtained by comparing conventional BSF solar cells with solar cells containing Ag: the Bi embedded in the slurry BSF solar cell was measured directly, as described herein, when both devices had the same back bus surface area. Conventional BSF silicon solar cells are fabricated using silver-based post-marking pastes printed directly on a silicon wafer and surrounded by a layer of aluminum particles. Intercalation (e.g., fabricated using intercalation paste C) may be used on silicon solar cells with full Al surface coverage. V of two kinds of solar cells_ocMeasured by a current-voltage test under sunlight intensity. For a wave having a length greater than 5cm²Solar cells with post-mark surface areas, when intercalated, have a V-mark as compared to the post-mark layer on a conventional silicon architecture_ocCan increase by at least 0.5mV, at least 1mV, at least 2mV, or at least 4 mV. Finally, when using the interposer fabric instead of the traditional post-flag design, the short-circuit current density (J)_sc) And fill factor (fill factor) were also improved. Silver does not make ohmic contact with p-type silicon. The silicon marker layer reduces current collection directly on p-type silicon, which can be estimated by performing electroluminescence or photoluminescence measurements on complete or incomplete solar cells. J. the design is a square_scThe increase in cell performance can also be measured by testing cells with embedded construction versus a back marker layer directly on silicon. Another benefit is an increase in fill factor, which may depend on V_ocIncrease in contact resistance, and/or change in recombination dynamics on the back side of the solar cell.

It is to be understood that the invention described herein may be carried out by different equipment, materials and devices, and that various modifications, both as to the equipment and operating procedures, may be accomplished without departing from the scope of the invention itself.

Claims (37)

1. A method of forming a sintered multilayer stack, the method comprising the steps of:

a) coating at least a portion of the surface of the base layer with a layer of wet metal particles;

b) drying the wet metal particle layer to form a dried metal particle layer;

c) directly coating at least a portion of the dried metal particle layer with wet intercalation to form a multilayer stack;

wherein, wet inserting layer includes:

comprising between 10 wt% and 70 wt% noble metal particles;

at least 10 wt% of embedded particles; and

an organic vehicle;

wherein the embedded particles include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles;

d) drying the multilayer stack; and

e) co-firing the multi-layer stack to form a sintered multi-layer stack.

2. The method of claim 1, wherein the wet metal particle layer comprises metal particles comprising a material selected from the group consisting of: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel, and alloys, composites, and other combinations thereof.

3. The method of claim 1, further comprising: a step of depositing at least one dielectric layer on at least a portion of the surface of the base layer prior to step a), and wherein step a) comprises directly coating at least a portion of the dielectric layer with a layer of wet metal particles.

4. The method of claim 1, wherein each coating step comprises a method independently selected from the group consisting of: screen printing, gravure printing, jet deposition, slot coating, 3D printing, and inkjet printing.

5. The method of claim 1, wherein step a) comprises screen printing through a patterned screen to produce a layer of wet metal particles having a variable thickness.

6. The method of claim 1, wherein steps b) and d) comprise drying at a temperature of less than 500 ℃ for 1 second to 90 minutes.

7. The method of claim 1, wherein steps b) and d) comprise drying at a temperature between 150 ℃ and 300 ℃ for 1 second to 60 minutes.

8. The method of claim 1, wherein step e) comprises rapidly heating in air to a temperature greater than 600 ℃ for 0.5 seconds to 60 minutes.

9. The method of claim 1, wherein step e) comprises rapidly heating in air to a temperature greater than 700 ℃ for 0.5 to 3 seconds.

10. The method of claim 1, further comprising: step f) soldering a marker band on a portion of the sintered multilayer stack.

11. The method of claim 1, wherein the low temperature base metal particles comprise a material selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, and alloys, composites, and other combinations thereof.

12. The method of claim 1, wherein the crystalline metal oxide particles comprise oxygen and a metal selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites, and other combinations thereof.

13. The method of claim 1, wherein the glass frit comprises a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, indium, hafnium, iodine, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxides thereof, composites thereof, and other combinations thereof.

14. A method of forming a sintered multilayer stack, the method comprising the steps of:

a) coating at least a portion of the surface of the base layer with a layer of wet metal particles;

b) drying the wet metal particle layer to form a dried metal particle layer;

c) firing the dried wet metal particle layer to form a metal particle layer;

d) directly coating at least a portion of the metal particle layer with wet intercalation to form a multilayer stack;

wherein, wet inserting layer includes:

between 10 wt% and 70 wt% noble metal particles;

at least 10 wt% of embedded particles; and

an organic vehicle;

wherein the embedded particles include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles;

e) drying the multilayer stack; and

f) firing the multilayer stack to form a sintered multilayer stack.

15. The method of claim 14, wherein the metal particle layer comprises metal particles comprising a material selected from the group consisting of: aluminum, copper, iron, nickel, molybdenum, tungsten, tantalum, titanium, steel, and alloys, composites, and other combinations thereof.

16. The method of claim 14, further comprising: a step of depositing at least one dielectric layer on at least a portion of the surface of the base layer prior to step a), and wherein step a) comprises directly coating at least a portion of the dielectric layer with a layer of wet metal particles.

17. The method of claim 14, wherein each coating step comprises a method selected from the group consisting of: screen printing, gravure printing, jet deposition, slot coating, 3D printing, and inkjet printing.

18. The method of claim 14, wherein step a) comprises screen printing through a patterned screen to produce a layer of wet metal particles having a variable thickness.

19. The method of claim 14, wherein steps b) and e) comprise drying at a temperature of less than 500 ℃ for 1 second to 90 minutes.

20. The method of claim 14, wherein steps b) and e) comprise drying at a temperature between 150 ℃ and 300 ℃ for 1 second to 60 minutes.

21. The method of claim 14, wherein steps c) and f) comprise rapid heating to a temperature greater than 600 ℃ for 0.5 seconds to 60 minutes.

22. The method of claim 14, wherein steps c) and f) comprise rapidly heating in air to a temperature greater than 700 ℃ for 0.5 to 3 seconds.

23. The method of claim 1, further comprising: step g) soldering a marker band on a portion of the sintered multilayer stack.

24. The method of claim 14, wherein the low temperature base metal particles comprise a material selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, and alloys, composites, and other combinations thereof.

25. The method of claim 14, wherein the crystalline metal oxide particles comprise oxygen and a metal selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites, and other combinations thereof.

26. The method of claim 14, wherein the glass frit comprises a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, indium, hafnium, iodine, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxides thereof, composites thereof, and other combinations thereof.

27. A method of fabricating a solar cell, the method comprising the steps of:

a) providing a silicon wafer having a front surface and a back surface;

b) coating at least a portion of the rear surface of the silicon wafer with a layer of wet aluminum particles;

c) drying the wet aluminum particle layer to form an aluminum particle layer;

d) directly coating at least a portion of the aluminum particle layer with wet intercalation to form a multilayer stack;

wherein, wet inserting layer includes:

between 10 wt% and 70 wt% noble metal particles;

at least 10 wt% of embedded particles; and

an organic vehicle;

wherein the embedded particles include one or more selected from the group consisting of low temperature base metal particles, crystalline metal oxide particles, and glass frit particles;

e) drying the multilayer stack;

f) applying a plurality of fine gridlines and at least one front manifold layer on a front surface of a silicon wafer;

g) drying the plurality of fine gridlines and the at least one front manifold layer to form a structure; and

h) co-firing the structure to form a silicon solar cell.

28. The method of claim 27, further comprising: a step of depositing at least one dielectric layer on at least a portion of the rear surface of the silicon wafer between step a) and step b), and wherein step b) comprises directly coating the dielectric layer with a layer of wet aluminum particles.

29. The method of claim 27, wherein each coating step comprises a method selected from the group consisting of: screen printing, gravure printing, jet deposition, slot coating, 3D printing, and inkjet printing.

30. The method of claim 27, wherein step b) comprises screen printing through a patterned screen to produce a layer of wet metal particles having a variable thickness.

31. The method of claim 27, wherein steps e) and g) comprise drying at a temperature of less than 500 ℃ for 1 second to 90 minutes.

32. The method of claim 27, wherein steps e) and g) comprise drying at a temperature between 150 ℃ and 300 ℃ for 1 second to 60 minutes.

33. The method of claim 27, wherein co-firing comprises rapid heating in air to a temperature greater than 600 ℃ for 0.5 seconds to 60 minutes.

34. The method of claim 27, wherein co-firing comprises rapid heating in air to a temperature greater than 700 ℃ for 0.5 to 3 seconds.

35. The method of claim 27, wherein the low temperature base metal particles comprise a material selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, and alloys, composites, and other combinations thereof.

36. The method of claim 27, wherein the crystalline metal oxide particles comprise oxygen and a metal selected from the group consisting of: bismuth, tin, tellurium, antimony, lead, vanadium, chromium, molybdenum, boron, manganese, cobalt, and alloys, composites, and other combinations thereof.

37. The method of claim 27, wherein the glass frit comprises a material selected from the group consisting of: antimony, arsenic, barium, bismuth, boron, cadmium, calcium, cerium, cesium, chromium, cobalt, fluorine, gallium, germanium, indium, hafnium, iodine, iron, lanthanum, lead, lithium, magnesium, manganese, molybdenum, niobium, potassium, rhenium, selenium, silicon, sodium, strontium, tellurium, tin, vanadium, zinc, zirconium, alloys thereof, oxides thereof, composites thereof, and other combinations thereof.

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