TW202144510A - Conductive pastes for solar cells - Google Patents

Abstract

A conductive paste comprising: a solids portion comprising a silver powder and a crystalline metal compound powder; and an organic carrier medium in which the silver powder and crystalline metal compound powder are dispersed, wherein the solids portion has a glass content less than 1 wt%, wherein the crystalline metal compound powder has a lead content less than 0.5 wt% calculated as PbO; wherein the crystalline metal compound powder has a particle size with a D₉₀ ≤ 5 µm and a D₅₀ ≤ 2 µm, and wherein the crystalline metal compound powder has a composition comprising at least: 40 to 60 wt% Te calculated as TeO₂ ; 12 to 25 wt% Bi calculated as Bi₂ O₃ ; and 10 to 25 wt% Zn calculated as ZnO, expressed as percentage weights (wt%) relative to a total weight of the crystalline metal compound powder in the conductive paste excluding any silver compounds, and/or wherein the crystalline metal compound powder has a composition comprising at least: 20 to 40 mol% Te calculated as TeO₂ ; 2 to 6 mol% Bi calculated as Bi₂ O₃ ; and 14 to 29 mol% Zn calculated as ZnO, expressed as mole percentage (mol%) relative to a total of the crystalline metal compound powder in the conductive paste excluding any silver compounds, and/or wherein the crystalline metal compound powder has a composition comprising at least the following components expressed as a molar ratio defined by SUM(all sources of a particular element) / SUM(all sources of Ag): 0.0060 to 0.0090 Te/Ag; 0.0012 to 0.0026 Bi/Ag; and 0.0029 to 0.0074 Zn/Ag.

Description

Conductive Adhesives for Solar Cells

The present invention relates to conductive pastes particularly suitable for use in solar cells.

Screen-printed conductive (eg silver) pastes are commonly used as conductor tracks for solar cells such as silicon solar cells. These glues typically contain conductive (eg, silver) powder, glass frit, and sometimes one or more additional additives, all dispersed in an organic medium. Glass frits serve several functions. During firing, it becomes the molten phase and thus acts to bond the conductor rails to the semiconductor wafer. However, the frit is also critical in etching away an antireflection or passivation layer (usually silicon nitride) disposed on the surface of the semiconductor wafer to allow direct contact between the conductive tracks and the semiconductor. Glass frits are also often critical in forming ohmic contacts with semiconductor emitters.

The quality of the contact between the conductor rails and the semiconductor wafer helps determine the efficiency of the final solar cell. The best frits need to be optimized to flow at the proper temperature and provide the proper degree of etching of the antireflective layer. If too little etch is provided, there will be insufficient contact between the semiconductor wafer and the conductor tracks, resulting in high contact resistance. Conversely, over-etching can lead to the deposition of large silver islands in the semiconductor, destroying its p-n junction and thereby reducing its ability to convert solar energy into electricity.

There remains a need for conductive pastes such as for solar cells that provide a good balance of properties. In particular, there remains a need for conductive pastes for solar cells that provide excellent (reduced) contact resistance without adversely affecting the pn junction of the solar cell, and which conductive pastes include flow at a suitable temperature to The frit of the conductive paste is fired during the manufacture of solar cells. Much attention has recently been focused on improving the frit material included in conductive pastes for photovoltaic cells to provide a good balance of properties.

As outlined above, prior art conductive paste compositions for photovoltaic cell applications typically comprise three main components: (i) conductive (eg, silver) powder; (ii) glass frit; and (iii) an organic carrier medium in which the conductive Powder and frit are placed to print the conductive paste.

Applicants have unexpectedly discovered that the frit component of such conductive pastes can be replaced by a mixture of several crystalline metal compound (eg, metal oxide) particles. This is described in the applicant's earlier patent applications with publication numbers WO2017/081448 and WO2017/081449. These conductive pastes comprise: (i) conductive (eg silver) powder; (ii) a mixture of crystalline metal compounds in powder form (rather than glass frits); and (iii) an organic carrier medium in which the conductive powder and crystals are disposed Metal compound powder.

The present invention builds on Applicant's previous work on developing glass-free conductive compositions for photovoltaic cells and focuses in particular on providing conductive pastes that are: (i) environmentally friendly; (ii) low cost (iii) efficient in photovoltaic cell applications; and (iv) easier to optimize to new photovoltaic cell designs when compared to glass frit compositions, enabling faster innovation of next-generation photovoltaic cells and shorter R&D time to market.

(i) Environmental Considerations A particular advantage of using crystalline particles of different metal compounds in place of glass frit is that it removes the glass forming step from the process of making conductive pastes. The glass formation step typically has high energy demands because it requires heating the glass precursor to a temperature above the melting point of the crystalline material used to make the glass. Glass is commonly used in conductive adhesives due to its relatively low softening and melting point. Typically, glass used in conductive pastes flows at temperatures in the range of about 400-700°C. The inventors have surprisingly discovered that despite the relatively high melting points of at least some of the substantially crystalline metal compounds used in the glues of the present invention, these mixtures exhibit flow and melting characteristics similar to those of glass frits, making them Can be used with similar firing profiles and manufacturing methods for pastes containing frit.

In addition to the reduced energy requirements, the conductive adhesives described herein are also intended to be environmentally friendly from a chemical standpoint. In this regard, many prior art glass frits used in conductive pastes include lead (Pb) to provide glasses with suitable properties (eg, low melting point) for use in photovoltaic cells. However, lead is highly toxic. Accordingly, lead-free glass compositions have been developed. The compositions of the present invention seek to be crystalline and lead-free to combine the environmental advantages of low energy requirements and low chemical toxicity.

(ii) Cost Considerations By avoiding the glass formation step in the glue manufacturing process, energy costs can be reduced. Manufacturing time is also reduced, thus reducing operating time costs. In terms of capital investment costs, glass forming equipment is not required, thus reducing capital investment requirements for equipment. Furthermore, since large glass forming equipment is not required, the manufacturing method requires less space, thus reducing the manufacturing footprint and reducing capital investment for land and building space.

It should also be noted that the frit formation process inevitably introduces various impurities into the composition. Such contamination can be reduced by utilizing expensive specialized glass forming equipment. Nonetheless, some contamination is unavoidable, and this can affect consistency and eventual performance. By avoiding the requirement for frit in the conductive adhesive composition, the number of production steps in which contamination may occur is significantly reduced.

(iii) Performance Considerations While reducing cost and environmental impact is critical, conductive pastes must of course perform at least as well as prior art frit-based pastes for the manufacture of high-efficiency photovoltaic cells. Optimally, the conductive paste should be modified according to the performance characteristics of such prior art frit-based conductive pastes. In this regard, it has previously been established that providing tellurium in the frit results in better functional performance in photovoltaic cell applications. The applicant has found that it is also advantageous to provide tellurium in the composition when a crystalline metal compound is used in place of the glass frit. It is worth noting, however, that when a crystalline conductive paste is deposited and fired to form the conductive component, the crystalline component does not react to form tellurium-containing glasses of the type used in prior art pastes. Although a glass phase can be formed when the paste is fired, a tellurium alloy containing silver is included in the paste-forming composition rather than a tellurium glass of the type used in the formation of prior art conductive pastes. Thus, it should be appreciated that crystalline gums as described herein (ie, gums based on particle mixtures of different crystalline metal compounds) do not only comprise the same ratios as the starting materials of the glass frits used in prior art gum compositions Crystalline compounds. Furthermore, the mechanism by which the crystalline paste reacts to form the conductive component upon firing is different from the mechanism by which the frit-based paste reacts to form the conductive component. Therefore, crystalline conductive pastes have different compositional requirements for frit conductive pastes to optimize performance even though they share some common features, such as the use of tellurium.

In addition to the different compositional requirements, it has also been found that the crystalline metal compound particles should be processed to a smaller particle size than the glass frit particles typically used in prior art compositions. Therefore, in order to improve performance in photovoltaic cell applications, crystalline conductive pastes differ from frit-based pastes in two different ways: (i) crystalline metal compound particles are processed to be more compact than glass frit particles typically used in prior art compositions Small particle size; and (ii) crystalline conductive pastes have different chemical composition requirements than frit-based conductive pastes to achieve high performance in photovoltaic cells.

A particular focus of this specification is to describe a conductive paste for solar cell applications having a modified crystalline composition for improved solar cell performance when compared to previous crystalline compositions.

(iv) Innovation Considerations When compared to glass frit compositions, crystalline conductive adhesives as described in this specification are easier to optimize to new photovoltaic cell designs, enabling more rapid innovation of next-generation photovoltaic cells and shortening solar energy R&D time for battery manufacturers to go to market. This is because the development and manufacture of new glass compositions for incorporation into conductive adhesive formulations is not required when developing new adhesives optimized for new cell designs. Instead, only the different metal compound compositions need to be mixed. As a result, the rate of optimizing glue for novel battery designs through iterations of different compositional variants increases significantly. Additionally, ratios of metal compounds that may not be considered and/or would not be suitable for making glass frits can be used. This has the potential to broaden the range of compositions that can be used in conductive adhesives. Therefore, it is contemplated that the use of the crystalline conductive paste method should help solar cell manufacturers in the scope and rate at which next-generation cells can be developed.

Conductive Adhesive Compositions With all of the above in mind, this specification describes conductive pastes that are: (i) based on finely crystalline metal compound powders rather than relatively coarse frit powders; (ii) lead (Pb) free; and ( iii) Having a crystalline metal compound powder chemical composition that provides improved functionality in photovoltaic cell applications when compared to previous crystalline conductive pastes. In particular, a conductive adhesive is provided, comprising: a solid portion comprising silver powder and crystalline metal compound powder; and an organic carrier medium in which the silver powder and the crystalline metal compound powder are dispersed, wherein the solid portion has less than 1 wt % glass content, wherein the crystalline metal compound powder has a lead content calculated as PbO of less than 0.5 wt%; wherein the crystalline metal compound powder has a _{particle size of D 90} ≤ 5 µm and D ₅₀ ≤ 2 µm, and wherein the crystalline metal compound powder The compound powder has a composition comprising at least the following: 40 to 60 wt % Te calculated _{as TeO 2 ; 12 to 25 wt % Bi calculated as Bi 2} O ₃ ; and 10 to 25 wt % Zn calculated as ZnO, expressed as The weight percentage (wt %) relative to the total weight of the crystalline metal compound powder not including any silver compound in the conductive paste, and/or wherein the crystalline metal compound powder has a composition comprising at least the following: Calculated _{as TeO 2} 20 to 40 mol % Te; _{2 to 6 mol % Bi calculated as Bi 2} O ₃ ; and 14 to 29 mol % Zn calculated as ZnO, expressed as the crystalline metal compound powder not including any silver compound in the conductive paste The molar percentage (mol%).

The conductive paste may comprise a solid portion of 85 to 95 wt % of the conductive paste and an organic vehicle of 5 to 15 wt % of the conductive paste. In addition, the solid portion may include 1 to 5 wt % of crystalline metal compound powder and 95 to 99 wt % of silver powder.

Preferably, _{Te, Bi and Zn are provided in the form of TeO 2} , Bi ₂ O ₃ and ZnO, in which case the wt% and mol% values correspond to the ratio of TeO ₂ , Bi ₂ O ₃ and ZnO in the crystalline metal compound powder. quantity. However, it should be noted, Te, Bi and Zn may be respectively provided with TeO _2, Bi ₂ O ₃ and ZnO different forms of the compounds. In this case, the wt % and mol % values of these different compounds should be converted to equivalents of _{TeO 2} , Bi ₂ O _{3 and ZnO depending on the amount of metal provided by the compounds.} It is conventional to define glass frit compositions in terms of their metal oxide content, even when the starting materials may include other types of compounds such as carbonates, phosphates, and the like. Therefore, specifying metal content in terms of its equivalent oxide content is a well-known method for defining compositional information for inorganic glass frits. In certain examples, the crystalline metal compounds each include only one type of metal (eg, a _{simple metal (M) oxide of the general formula M x} O _y ), although it is also contemplated that one or more of the crystalline compounds may include More than one type of metal (M1, M2), such as complex metal oxides of the _{general formula M1 x} M2 _y O _z.

Furthermore, it should be noted that the calculated value of the crystalline metal compound composition is defined as not including any silver compounds. This is because some silver content of the paste can be provided in the form of silver compounds rather than metallic silver. This will effectively dilute and change the values of the other components of the crystalline metal compound powder while effectively providing the corresponding gum composition. Therefore, when the crystalline metal compound powder includes one or more silver compounds, these compounds should be excluded from the calculation of the wt% or mol% values of the other components. Such silver compounds can decompose upon firing to yield silver metal and thus contribute to the metallic silver component of the fired product rather than the inorganic oxide component.

Another way to consider the possible presence of silver compounds is to use the concentration of silver (weight, atomic or molar) as a reference and use the ratio of other elements to silver content to define the composition of the glue. As an illustrative example, if the glue comprises 90 wt% of silver and 2 wt% crystalline metal oxides and crystalline metal oxide comprises 50 wt% TeO _2, the TeO ₂ / Ag weight ratio = 1:90. Similarly, molar/atomic ratio = 1/160 : 90/108 = 0.00625 : 0.83 = 0.625 : 83. Calculations can include all sources of a particular element. For example, metallic Te, TeO _{2 , AgTe 2} containing tellurium, and the like. That is, the ratio can be expressed as SUM(all sources of a specific element):SUM(all sources of Ag). This method enables the comparison and identification of equivalent gum compositions and it is intended that this specification covers such equivalent compositions. In this regard, a conductive paste as described herein includes a crystalline metal compound powder having a composition comprising at least the following components, expressed as SUM(all sources of specific elements)/SUM( All sources of Ag) defined molar ratios: Te/Ag from 0.0060 to 0.0090; Bi/Ag from 0.0012 to 0.0026; and Zn/Ag from 0.0029 to 0.0074. It should be understood that metal compound compositions may be defined in terms of such molar ratios and/or in terms of wt % and/or mol % values, and that all such values have been included in this specification for clarity and completeness middle.

The conductive adhesive compositions as defined herein represent improvements over those compositions disclosed in the applicant's earlier patent applications, publication numbers WO2017/081448 and WO2017/081449. In this regard, it should be noted that these earlier applications contain broad scope disclosures for the individual components of the glue, which may overlap with the scope specified for the individual components of the glue according to the present specification. However, it is not disclosed that the specific combination of features of the glues of the present invention or the region of the parameter space defined by this specification results in an improvement in performance. One major difference is that the compositions as described herein have significantly more zinc than all the exemplary compositions disclosed in WO2017/081448 and WO2017/081449. That is, the prior art compositions that have been exemplified include zinc oxide levels significantly below 10 wt%, whereas the compositions of the present specification have zinc oxide greater than 10 wt%, preferably greater than 13 wt%, and optionally greater than 15 wt% %. It has been found that compositions with tellurium, bismuth, and zinc contents combined in the currently defined ranges result in better functional performance of solar cells produced using these compositions.

The conductive paste as described herein is lead free. By lead free, we mean that the lead content (calculated as PbO) of the crystalline metal compound powder is less than 0.5 wt%, less than 0.1 wt%, less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt%.

The conductive paste as described herein is glass-free. By glass-free we mean that the glass content of the solid part is less than 1 wt%, less than 0.5 wt%, less than 0.25 wt%, less than 0.1 wt%, less than 0.05 wt%, less than 0.01 wt%, relative to the total weight of the solids part % or less than 0.005 wt%.

Of TeO to (as previously discussed silver compound is not included) calculation of wt% crystalline metal compound powder Te (tellurium) ₂ content: 48 wt% or at least 40, 42; 60,58,56,54 no more than , 52 or 50 wt%; or within a range defined by any combination of the above lower and upper limits. Alternatively, to TeO ₂ (not including silver compound) Te content of the metal compound powder of the mol% calculated as crystals: at least 20,22,24,26,28 or 30 mol%; no more than 40,38,36,34, 33 or 32 mol %; or within a range bounded by any combination of the above lower and upper limits. Still alternatively, the Te content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Te)/SUM(all sources of Ag), is: at least 0.0060, 0.0064, 0.0068, 0.0072, or 0.0074; not more than 0.0090, 0.0086, 0.0082, 0.0078, or 0.0076; or within a range defined by any combination of the above lower and upper limits.

The Bi (bismuth) content of the crystalline metal compound powder, calculated as Bi ₂ O ₃ (excluding silver compounds) wt %: at least 12, 13, 14 or 15 wt %; not more than 25, 23, 21, 19 or 17 wt % %; or within a range defined by any combination of the above lower and upper limits. Alternatively, to Bi ₂ O ₃ (not including the silver compound) Bi metal compound powder crystallized mol% of the calculated amount of: at least 2.0,2.5,3.0 or 3.4 mol%; 6.0,5.0,4.0 or not more than 3.8 mol%; or within a range defined by any combination of the above lower and upper limits. Still alternatively, the Bi content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Bi)/SUM(all sources of Ag), is: at least 0.0012, 0.0014, 0.0016 or 0.0018; not more than 0.0026, 0.0024, 0.0022 or 0.0020; or within a range defined by any combination of the above lower and upper limits.

Zn (zinc) content of crystalline metal compound powder calculated as wt% of ZnO (excluding silver compound): at least 10, 12, 13, 14, 15 or 16 wt%; not more than 25, 23, 21, 19 or 18 wt%; or within a range defined by any combination of the above lower and upper limits. Alternatively, the Zn content of the crystalline metal compound powder, calculated as mol % of ZnO (excluding silver compounds), is: at least 14, 16, 18, 20 or 21 mol %; not more than 29, 27, 25, 24 or 23 mol % ; or within a range bounded by any combination of the above lower and upper limits. Still alternatively, the Zn content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Zn)/SUM(all sources of Ag), is: at least 0.0029, 0.0035, 0.0040, 0.0045, or 0.0050; not more than 0.0074, 0.0070, 0.0065, 0.0060 or 0.0055; or within a range defined by any combination of the above lower and upper limits.

The crystalline metal compound powder may further comprise _{the following Li (lithium) content calculated as LiO 2} (excluding silver compound) wt %: at least 5, 6, 7 or 8 wt %; not more than 15, 13, 11, 10 or 9 wt % %; or within a range defined by any combination of the above lower and upper limits. Alternatively, the crystalline metal compound powder may further comprise _{the following Li contents calculated as LiO 2} (excluding silver compounds) mol %: at least 25, 28, 29 or 30 mol %; not more than 45, 40, 35, 34, 33, 32 or 31 mol%; or within a range defined by any combination of the above lower and upper limits. Still alternatively, the Li content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Li)/SUM(all sources of Ag), is: at least 0.0080, 0.0100, 0.0120, 0.0140, or 0.0150; not more than 0.0240, 0.0220, 0.0200, 0.0180, or 0.0170; or within a range defined by any combination of the above lower and upper limits.

Li can be provided as two different compounds such as Li ₂ CO ₃ and Li ₃ PO ₄ . It has been found, although the use of other sources of phosphorus and lithium, but from the viewpoint of processing, Li ₂ CO _{3 3} PO and a mixture of Li ₄ is advantageous.

The crystalline metal compound powder may further comprise the following Mg (magnesium) content calculated as MgO (excluding silver compounds) wt%: at least 0.5, 1.0, 1.5 or 2.0 wt%; not more than 4.0, 3.5, 3.0 or 2.8 wt%; or Within the range defined by any combination of the above lower and upper limits. Alternatively, the crystalline metal compound powder may further comprise the following Mg content calculated as mol% of MgO (excluding silver compounds): at least 2, 3, 4 or 5 mol%; not more than 10, 9, 8 or 7 mol%; or Within the range defined by any combination of the above lower and upper limits. Still alternatively, the Mg content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Mg)/SUM(all sources of Ag), is: at least 0.0003, 0.0005, 0.0007, 0.009, or 0.0010; not more than 0.0024, 0.0020, 0.0018, 0.0016 or 0.0014; or within a range defined by any combination of the above lower and upper limits.

The crystalline metal compound powder may further comprise _{the following P (phosphorus) content calculated as P 2} O ₅ (excluding silver compound) wt %: at least 2.0, 3.0 or 4.0 wt %; not more than 7.0, 6.0 or 5.0 wt %; or at Within the range defined by any combination of the above lower and upper limits. Alternatively, the crystalline metal compound powder may further comprise _{the following P content calculated as P 2} O ₅ (excluding silver compound) mol %: at least 1, 2 or 3 mol %; not more than 6, 5 or 4 mol %; or at Within the range defined by any combination of the above lower and upper limits. Still alternatively, the P content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of P)/SUM(all sources of Ag), is: at least 0.0007, 0.0009, 0.0011, or 0.0013; not more than 0.0024, 0.0022, 0.0020, or 0.0018; or within a range defined by any combination of the above lower and upper limits. As previously discussed, it should be provided in a phosphate Li ₃ PO ₄ of the form.

Crystalline metal compound powder may further contains Na ₂ O (not including silver compound) wt% or less of the calculated Na (sodium) content: at least 0.1,0.3,0.5 or 0.8 wt%; not more than 1.5, 1.0 or 0.9 wt%; or Within the range defined by any combination of the above lower and upper limits. Alternatively, the crystalline metal compound powder may further contains Na ₂ O (not including silver compound) mol% of the content of Na is calculated: at least 0.2,0.5,0.8 or 1.0 mol%; 2.0,1.7,1.5 or not more than 1.3 mol% ; or within a range bounded by any combination of the above lower and upper limits. Still alternatively, the Na content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of Na)/SUM(all sources of Ag), is: at least 0.0001, 0.0003, 0.0005, or 0.0006; not more than 0.0012, 0.0010, 0.0008 or 0.0007; or within a range defined by any combination of the above lower and upper limits.

The crystalline metal compound powder may further comprise _{the following W (tungsten) content calculated as WO 3} (excluding silver compound) wt %: at least 1.0, 1.3 or 1.6 wt %; not more than 5.0, 3.5 or 2.0 wt %; Within the range defined by any combination of the lower limit and the upper limit. Alternatively, the crystalline metal compound powder may further comprise _{the following W content calculated as WO 3} (excluding silver compound) mol %: at least 0.2, 0.4 or 0.6 mol %; not more than 2.0, 1.5 or 1.0 mol %; Within the range defined by any combination of the lower limit and the upper limit. Still alternatively, the W content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM(all sources of W)/SUM(all sources of Ag), is: at least 0.0001, 0.0002, or 0.0003; not more than 0.0005 or 0.0004; or within a range defined by any combination of the above lower and upper limits.

The crystalline metal compound powder may be substantially free of boron. For example, the crystalline metal compound powder may have less than 0.1 wt% boron B ₂ O ₃ is calculated, is less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% of boron content.

As indicated previously, the conductive paste may comprise a solid portion of 85 to 95 wt% of the conductive paste and an organic vehicle of 5 to 15 wt% of the conductive paste. In addition, the solid portion may include 1 to 5 wt % of crystalline metal compound powder and 95 to 99 wt % of silver powder. These conductive paste compositions balance the printing properties of the paste and the electrical performance characteristics of the fired paste in solar cells. Likewise, the wt % of the silver powder and crystalline metal compound powder can be varied by providing at least a portion of the silver as a compound rather than the metallic element silver. Thus, the solid fraction may alternatively be defined as comprising 1 to 5 wt % crystalline metal compound powder (excluding silver compounds) and 95 to 99 wt % silver, including metallic element silver and silver compound powder. Still alternatively, the silver and crystalline metal compound powder contents of the solid fraction can be expressed as ratios as previously described.

According to another aspect of the present invention, there is provided a method for fabricating a surface electrode for a solar cell, the method comprising applying a conductive paste as defined above to a semiconductor substrate and firing the coated conductive paste. The method may use a firing profile wherein the temperature of the surface of the coated conductive paste exceeds 500°C for two minutes or less, one minute or less, 30 seconds or less, 20 seconds or less, or 10 seconds or shorter period of time. The firing curve exceeds 400°C for a period of two minutes or less, one minute or less, 30 seconds or less, or 20 seconds or less, as appropriate. The firing curve exceeds 300°C for a period of two minutes or less, one minute or less, 30 seconds or less, or 20 seconds or less, as appropriate. The firing curve exceeds 200°C for a period of two minutes or less, one minute or less, 30 seconds or less, or 25 seconds or less, as appropriate. The firing curve exceeds 100°C for a period of two minutes or less, one minute or less, or 30 seconds or less, as appropriate.

The conductive adhesive of the present invention includes an organic medium and a solid portion. The solid portion includes a conductive material and a mixture of inorganic particles. Each of these will be discussed, as will various methods of making conductive pastes using them.

Inorganic Particle Mixture - Composition The solid portion of the conductive paste described herein contains a blend of substantially crystalline inorganic materials in particle form. This inorganic blend is sometimes referred to herein as a mixture of oxide particles. Oxides, carbonates, and other materials as described below can be mixed (eg, by co-grinding) and then incorporated into the conductive paste.

In general, in some aspects of the invention, inorganic particle mixtures are made from two or more different particulate inorganic materials, such as metal compounds, eg, metal oxides, metal carbonates, and the like. The particles are substantially crystalline. The mixture may contain non-oxide materials and may be formed from materials that are not oxides.

By particle nature is meant the presence of discrete, separate or individual particles of each inorganic component. These differ from the molten amorphous structure of glass frits previously used in conductive pastes for photovoltaic cell applications. Since the inorganic particles are substantially crystalline, they do not exhibit glass transition.

In the solid portion, there is a mixture of conductive material and inorganic particles. The conductive material and inorganic particle mixture may be the only components of the solid portion. The solid part may therefore consist only of the conductive material and the mixture of inorganic particles.

Therefore, in the solid fraction, the content of amorphous oxide material or glass is extremely low. The glass content of the solid fraction may be less than 1 wt%. For example, the glass content of the solids portion may be less than 0.5 wt%, less than 0.25 wt%, less than 0.1 wt%, less than 0.05 wt%, less than 0.01 wt%, or less than 0.005 wt% relative to the total weight of the solids portion. It is possible that the solid portion is substantially free of glass. In some embodiments, the solid portion does not include any intentionally added glass and/or any intentionally formed glass phase.

It will be understood by those skilled in the art that glass materials are not synonymous with amorphous materials, or even amorphous regions within crystalline materials. The glass material exhibits glass transfer. Although glasses may include some crystalline domains (which may not be completely amorphous), these glasses are still distinct from the discrete crystalline particles described herein.

Of course, those skilled in the art will recognize that due to the nature of the processing conditions used, some amorphous or glassy phases may form even when substantially crystalline starting materials are used. In aspects of the present invention, this is minimized. For example, there may be some surface reaction of oxide particles caused by grinding, or deposition of carbon from organic salts (eg, formate and/or acetate), metal organics, and/or organic solvents. However, lack of glass transfer (ie, no glass transfer exhibited) can characterize differences from glass materials.

The inorganic particle mixture includes substantially crystalline particles of different metal compounds. The individual metal compounds can be selected, for example, from metal oxides, metal carbonates or metal phosphates. As will be understood by those skilled in the art, the metal compound may include incidental impurities. Such incidental impurities will be present in the metal compound in very low amounts (eg, <1 mol% or <0.5 mol% relative to the metal compound). In addition, processing (eg, co-grinding) of the metal compounds can cause some surface modification or reaction of the compounds. In this case, however, the bulk of each particle remains as a crystalline metal compound and can still be identified by known techniques. Thus, crystalline particles are distinct from glass particles formed from glass materials that exhibit a glass transition temperature and/or are substantially amorphous. In contrast to crystalline materials that exhibit strong and narrow peaks associated with crystallographic planes, glasses exhibit broad and poorly defined features in X-ray diffraction (XRD) analysis. That is, it should be noted that some glass materials contain a proportion or composition of material in a crystalline phase within a bulk amorphous phase. Thus, in XRD, the glass exhibits broad and poorly defined features, but may also exhibit small, narrow peaks corresponding to small amounts of crystalline material within the glass. Glass can also be characterized by complex irregular networks containing more than one different type of metal, although in such irregular glass networks and complex crystalline metal oxides containing regular lattices containing oxygen and more than one metal distinguish between. Again, the differences between these materials are evident in the XRD analysis. In addition, other techniques can be used to confirm that the material is glass, these techniques include: Extended X-ray Absorption Fine Structure (EXAFS); X-ray Pair Distribution Function (X-ray PDF); Neutron Pair Distribution Function (Neutron PDF); Solid State NMR Resonance (NMR); Thermogravimetric Analysis (TGA); and Differential Thermal Analysis (DTA). These techniques are known in the art for analyzing glasses and crystalline materials and distinguishing between the two.

The detailed composition of the crystalline metal compound powder according to the present specification has been set forth in the Summary of the Invention section.

Mixtures of Inorganic Particles - Particle Size Mixtures of inorganic particles with certain particle size distributions are unexpectedly useful. Therefore, control of particle size distribution is important in the gum composition of the present invention.

The inorganic particle mixture may have a particle size distribution wherein (a) D ₁₀ ≤ 0.41 µm; (b) D ₅₀ ≤ 1.6 µm; (c) D ₉₀ ≤ 4.1 µm; (d) (D ₅₀ - D ₁₀ ) ≤ 1.15 µm; (e) (D ₉₀ - D ₅₀ ) ≤ 2.5 µm; (f) (D ₉₀ - D ₁₀ ) ≤ 3.7 µm; and/or (g) (D ₅₀ /D ₁₀ ) ≤ 3.85.

In an example of a glue according to this specification, one or more, two or more, three or more, four or more, five or more, or six or more.

In some instances, requirement (a) is satisfied. In some instances, requirement (b) is satisfied. In some instances, requirement (c) is satisfied. In some instances, requirement (d) is satisfied. In some instances, requirement (e) is satisfied. In some instances, requirement (f) is satisfied. In some instances, requirement (g) is satisfied. Any combination of these requirements can be met in the present examples.

With respect to requirement (a), D ₁₀ is 0.41 µm or less, e.g. 0.4 µm or less, 0.39 µm or less, 0.35 µm or less, 0.32 µm or less, 0.3 µm or less, 0.28 µm or less Low, 0.25 µm or less or 0.24 µm or less.

The value of D ₁₀ is preferably 0.4 µm or less.

Typically, the D ₁₀ particle size may be at least 0.1 µm, at least 0.12 µm, at least 0.14 µm, at least 0.17 µm, or at least 0.2 µm.

Thus, in some embodiments, D _{10 is} in the range of 0.2 μm ≤ D ₁₀ ≤ 0.4 μm.

Requests for (b), a mixture of the inorganic particles D ₅₀ is preferably less than or equal to 1.6 μm. D ₅₀ can be 1.55 µm or less, 1.5 µm or less, 1.45 µm or less, 1.4 µm or less, 1.35 µm or less, 1.3 µm or less, 1.25 µm or less, 1.2 µm or less Low, 1.15 µm or lower, 1.1 µm or lower, 1.05 µm or lower, 1 µm or lower, or 0.95 µm or lower.

The D ₅₀ value is preferably 1.05 μm or less.

Typically, D ₅₀ particle size may be at least 0.1 μm, at least 0.3 μm, 0.5 μm, or at least at least 0.8 μm.

Thus, in some instances, D _{50 is} in the range of 0.3 μm ≤ D ₅₀ ≤ 1.05 μm.

Requests for (c), a mixture of the inorganic particles D ₉₀ is preferably less than or equal to 4.1 μm. D ₉₀ can be 4 µm or less, 3.8 µm or less, 3.6 µm or less, 3.4 µm or less, 3.2 µm or less, 3 µm or less, 2.8 µm or less, 2.6 µm or less Low, 2.4 µm or lower, 2.2 µm or lower, 2.1 µm or lower, 2 µm or lower, or 1.9 µm or lower.

The diameter D ₉₀ of 2.2 μm or less is preferred.

Typically, the D ₉₀ particle size may be at least 1 µm, at least 1.2 µm, at least 1.4 µm, or at least 1.5 µm.

Thus, in some instances, D _{90 is} in the range of 1.4 μm ≤ D ₉₀ ≤ 2.2 μm.

Regarding requirement (d), (D ₅₀ - D ₁₀ ) is 1.15 µm or less, e.g. 1.1 µm or less, 1 µm or less, 0.8 µm or less, 0.6 µm or less, 0.59 µm or less , 0.58 µm or lower, 0.57 µm or lower, 0.56 µm or lower, 0.55 µm or lower, 0.54 µm or lower, or 0.53 µm or lower.

The value of (D ₅₀ - D ₁₀ ) is preferably 0.6 µm or less.

Typically, _{the difference between D 50} and D ₁₀ may be at least 0.1 µm, at least 0.2 µm, at least 0.3 µm, or at least 0.35 µm.

Thus, in some examples, (D ₅₀ - D ₁₀ ) is in the range of 0.3 μm ≤ (D ₅₀ - D ₁₀ ) ≤ 0.6 μm.

Regarding requirement (e), (D ₉₀ - D ₅₀ ) is 2.5 µm or less, e.g. 2 µm or less, 1.75 µm or less, 1.5 µm or less, 1.25 µm or less, 1.15 µm or less , 1.1 µm or less, 1.05 µm or less, 1 µm or less, or 0.95 µm or less.

The value of (D ₉₀ - D ₅₀ ) is preferably 1.15 µm or less.

Typically, _{the difference between D 90} and D ₅₀ may be at least 0.5 µm, at least 0.6 µm, at least 0.7 µm, or at least 0.75 µm.

Thus, in some instances, (D ₉₀ - D ₅₀ ) is in the range of 0.6 µm ≤ (D ₉₀ - D ₅₀ ) ≤ 1.15 µm.

Regarding requirement (f), (D ₉₀ - D ₁₀ ), that is _{, the difference between D 90} and D ₁₀ is preferably less than or equal to 3.7 µm. (D ₉₀ - D ₁₀ ) can be 3.5 µm or less, 3 µm or less, 2.5 µm or less, 2 µm or less, 1.8 µm or less, 1.6 µm or less, 1.5 µm or less lower, 1.45 µm or lower, 1.4 µm or lower, or 1.35 µm or lower.

The value of (D ₉₀ - D ₁₀ ) is preferably 1.8 µm or less.

Typically, _{the difference between D 90} and D ₁₀ may be at least 1 µm, at least 1.1 µm, at least 1.2 µm, or at least 1.3 µm.

Thus, in some instances, (D ₉₀ - D ₁₀ ) is in the range of 1.1 µm ≤ (D ₉₀ - D ₁₀ ) ≤ 1.8 µm.

Regarding requirement (g), (D ₅₀ /D ₁₀ ), that is, the value obtained by dividing D _{50 by D 10} is less than or equal to 3.85. Value (D ₅₀ / D ₁₀₎ of 3.8 or less may be 3.7 or less, 3.6 or less, 3.5 or less, 3.4 or less, 3.3 or less, 3.2 or less, 3.1 or less , 3 or lower, 2.8 or lower, or 2.6 or lower.

The value of (D ₅₀ /D ₁₀ ) is preferably 3.6 or less.

Typically, the ratio between D ₅₀ and D ₁₀ may be at least 1, at least 1.5, at least 2, or at least 2.3 μm.

Thus, in some examples, (D ₅₀ /D ₁₀ ) is in the range of 2.2≦(D ₅₀ /D ₁₀ )≦3.6.

The particle sizes and distributions described herein can be determined using laser diffraction methods, eg, using a Malvern Mastersizer 2000.

Inorganic Particle Mixtures - Preparation Inorganic particle mixtures can be prepared by mixing the raw materials of the desired metal compound. These raw materials may be oxides, carbonates, etc. discussed above. Blending can be performed in a known manner. Typically, the inorganic particle mixture is not melted, quenched or other glass making techniques.

Mixing or blending the above materials can produce inorganic particle mixtures suitable for use in the present invention. These starting materials can be used in substantially crystalline form.

Mixing or blending techniques are well known in the art. The inventors have found that co-milling techniques are particularly effective in preparing suitable inorganic (eg oxide) particle mixtures. Without wishing to be bound by theory, it is believed that this is due to its effect on reducing particle size and/or providing a narrow particle size distribution. Alternatively, the components of the inorganic particle mixture may be milled separately (or otherwise processed to provide the desired particle size and/or particle size distribution, if desired), and then combined to provide the inorganic particle mixture.

Mixing (eg, co-milling) the raw materials of the inorganic particle mixture can be followed by mixing the resulting blend with the organic medium and conductive material, eg, in any order. Co-milling may be the only processing performed on the feedstock of the inorganic particle mixture. For example, the glass manufacturing method may not be performed. It should be understood that, alternatively, each component of the inorganic particle mixture can be separately added to the conductive material and the organic medium, so as to obtain the conductive adhesive of the present invention.

For example, the oxides, carbonates, etc. discussed above can be blended. Next, the resulting mixture may or may not be milled. When milling, the process can be carried out, for example, in a planetary mill to provide the desired particle size distribution as discussed above. Wet milling can be carried out in an organic solvent such as diethylene glycol butyl ether. The resulting blended powder can then be dried. Sieving can be performed to further adjust the particle size distribution.

Conductive Paste Conductive paste is suitable for forming conductive tracks or coatings on substrates. It is particularly suitable for forming surface electrodes on semiconductor substrates, such as in solar cells. The conductive paste is also suitable for forming electrodes on thin film solar cells. The conductive adhesive may be the front-side conductive adhesive.

The conductive paste may comprise a solid portion of 85 to 95 wt % of the conductive paste and an organic vehicle of 5 to 15 wt % of the conductive paste. In addition, the solid portion may include 1 to 5 wt % of crystalline metal compound powder and 95 to 99 wt % of silver powder. These conductive paste compositions balance the printing properties of the paste and the electrical performance characteristics of the fired paste in solar cells. As described in the Summary of the Invention, a portion of the silver content may be provided in the form of compounds rather than elemental metallic silver powder.

Silver is used as the conductive material in the glue. This is especially preferred in solar cell applications, such as where it is intended to bring the glue into contact with the n-type emitter of the solar cell. In some examples, especially where the glue is intended to be in contact with the p-type emitter of the solar cell, the conductive material may comprise aluminum, for example, it may be a blend of silver and aluminum.

The conductive material may be provided in the form of particles, such as metal particles. The particle form is not particularly limited, but may be in the form of flakes, spherical particles, granules, crystals, powders, or other irregular particles or mixtures thereof.

Typically, D ₅₀ particle size of the electrically conductive material is at least 0.1 μm, 0.5 μm, or at least at least 1 μm. D ₅₀ particle size may be 15 μm or less, 10 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less. Particle size can be determined using laser diffraction methods (eg using a Malvern Particle Size Analyzer 2000).

The conductive material may have a surface area of at least 0.1 m ² /g, at least 0.2 m ² /g, at least 0.3 m ² /g, at least 0.4 m ² /g, or at least 0.5 m ² /g. For example, it may be 5 m ² /g or less, 3 m ² /g or less, 2 m ² /g or less, 1 m ² /g or less, 0.8 m ² /g or less Small or 0.7 m ² /g or less.

Organic Medium The solid portion of the conductive adhesive is dispersed in the organic medium. The solid portion may account for 85 to 95 wt% of the conductive paste and the organic vehicle may account for 5 to 15 wt% of the conductive paste. The organic medium typically contains an organic solvent in which one or more additives are dissolved or dispersed. The components of the organic medium are typically selected to provide suitable consistency and rheological properties to permit printing of conductive pastes onto semiconductor substrates and to stabilize the paste during shipping and storage.

Examples of suitable solvents for organic media include one or more solvents selected from the group consisting of diethylene glycol butyl ether, diethylene glycol butyl ether acetate, terpineol, dialkylene glycol alkyl ethers such as , diethylene glycol dibutyl ether and tripropylene glycol monomethyl ether), ester alcohols (such as Texanol®), 2-(2-methoxypropoxy)-1-propanol, and mixtures thereof.

Examples of suitable additives include dispersants, viscosity/rheology modifiers, thixotropy modifiers, wetting agents, thickeners, stabilizers and surfactants to aid in the dispersion of the solid portion in the gum.

For example, the organic medium may comprise one or more components selected from the group consisting of: rosin (kollophonium resin), acrylic resin (eg, Neocryl®), alkylammonium polycarboxylate polymer Salts (eg, Dysperbik® 110 or 111), polyamide waxes (such as Thixatrol Plus® or Thixatrol Max®), nitrocellulose, ethylcellulose, hydroxypropylcellulose, and lecithin.

Typically, the conductive paste is prepared by mixing together the conductive material, the components of the inorganic particle mixture, and the components of the organic medium in any order.

Fabrication of Surface Electrodes and Solar Cells Those skilled in the art are familiar with suitable methods for fabricating surface electrodes for solar cells. Similarly, those skilled in the art are familiar with suitable methods for fabricating solar cells.

Methods for fabricating surface electrodes for solar cells typically include applying a conductive paste onto the surface of a semiconductor substrate and firing the coated conductive paste. The conductive adhesive can be applied by any suitable method. For example, the conductive paste can be applied by printing, such as by screen printing or ink jet printing. The conductive paste can be coated on the semiconductor substrate to form the light-receiving surface electrode of the solar cell. Alternatively, a conductive paste can be coated on the semiconductor substrate to form the backside surface electrode of the solar cell. The solar cells can be n-type or p-type solar cells. The glue can be applied to the n-type emitter (in p-type solar cells) or to the p-type emitter (in n-type solar cells). Some solar cells are called back junction cells. In this case, the conductive adhesive of the present invention can preferably be applied to the back surface of the semiconductor substrate of the solar cell. Similar to anti-reflective coatings applied to the light-receiving surfaces of solar cells, such backside surfaces are typically covered with an insulating passivation layer (eg, a SiN layer). Alternatively, the conductive paste can be applied to thin film solar cells or the conductive paste can be applied to substrates of electronic devices other than solar cells.

Those skilled in the art know suitable techniques for firing the coated conductive paste. Example firing curves are shown in FIG. 1 . A typical firing process lasts about 30 seconds, with the electrode surface reaching a peak temperature of about 800°C. Typically, the furnace temperature will be higher to achieve this surface temperature. Firing the coated conductive paste may use a firing profile in which the surface temperature of the coated conductive paste exceeds 500°C for a period of two minutes or less. Preferred characteristics of the firing profiles of the glues described herein are shown in Figure 1 and described in the Summary of the Invention section.

The semiconductor substrate of the electrode may be a silicon substrate. For example, it may be a single crystal semiconductor substrate or a polycrystalline semiconductor substrate. Alternative substrates include CdTe. The semiconductor may be, for example, a p-type semiconductor or an n-type semiconductor.

The semiconductor substrate may include an insulating layer on its surface. Typically, the conductive paste of this specification is applied on top of the insulating layer to form the electrodes. Typically, the insulating layer will be non-reflective. A suitable insulating layer is SiNx (eg, SiN). The other suitable insulating layer comprises _{Si 3 N 4, SiO 2,} Al 2 O 3 and TiO _2.

Methods for fabricating p-type solar cells may include applying a backside conductive paste (eg, comprising aluminum) to the surface of a semiconductor substrate and firing the backside conductive paste to form a backside electrode. The backside conductive paste is usually applied to the opposite side of the semiconductor substrate and the light-receiving surface electrode.

In the manufacture of p-type solar cells, a backside conductive paste is usually applied to the backside (non-light receiving side) of a semiconductor substrate and dried on the substrate, after which the frontside conductive paste is applied to the front side (light receiving side) of the semiconductor substrate side) and dried on the substrate. Alternatively, the front side glue can be applied first, followed by the back side glue. The conductive paste is typically cofired (ie, the substrate with the front and back pastes coated thereon is fired) to form solar cells that include front and back conductive tracks.

The efficiency of solar cells can be improved by providing a passivation layer on the backside of the substrate. Suitable materials include, of an SiNx _{(e.g., SiN), Si 3 N 4} , SiO 2, Al 2 O 3 and TiO _2. Typically, regions of the passivation layer are partially removed (eg, by laser ablation) to permit contact between the semiconductor substrate and the backside conductive rails. Alternatively, where the glue of this specification is applied to the backside, the glue can be used to etch the passivation layer to enable electrical contact to be made between the semiconductor substrate and the conductive tracks.

In the case where the conductive track is formed on a substrate other than the semiconductor substrate of the solar cell, the manner of applying the conductive paste to the substrate is not particularly limited. For example, the conductive paste can be printed (eg, ink jet printing or screen printing) onto the substrate, or it can be coated (eg, dip-coated) onto the substrate. The firing conditions are also not particularly limited, but may be similar to those described above with respect to forming the surface electrodes of the solar cell.

Where ranges are specified herein, it is intended that each endpoint of the range be independent. Thus, each such upper endpoint of an expressly contemplated range can be independently combined with each such lower endpoint, and vice versa.

example Inorganic blends

Crystalline metal compound powders were prepared by a series of compositions according to the present specification. Compositional information for each example is provided in the table below. Tables 1 and 2 give the composition of each crystalline metal compound component calculated in terms of its oxide equivalent wt% (Table 1) or mol% (Table 2). In the example, the Te, Bi, Zn and Ce components are provided as their crystalline oxides, the Li is provided as a mixture of carbonate and phosphate (which also provides the phosphorus component), and the Mg, K, and na, and to provide a W component in the form of H ₂ WO _4. component Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% TeO2 53.9 51.0 51.2 49.6 49.2 49.0 49.1 48.9 50.6 50.8 48.9 Li2O 8.6 8.1 8.2 9.3 9.0 9.0 9.0 8.8 9.2 9.2 8.8 ZnO 12.1 11.4 10.4 15.0 14.9 14.8 14.8 14.8 15.3 15.4 17.2 Bi2O3 15.8 20.9 21.1 17.1 17.0 16.9 17.0 16.9 16.2 16.2 15.6 P2O5 4.4 4.1 4.2 4.3 4.2 4.2 4.2 4.2 4.7 4.7 4.6 MgO 1.8 1.2 1.7 1.0 1.0 1.0 1.0 1.0 1.3 1.3 2.6 WO3 3.1 2.9 2.9 3.3 3.3 3.3 3.3 3.3 1.7 1.7 1.6 K2O 0.0 0.0 0.0 0.0 0.0 0.9 0.7 1.2 1.0 0.0 0.0 Ce2O3 0.0 0.0 0.0 0.0 0.9 0.9 0.9 0.9 0.0 0.0 0.0 Na2O 0.3 0.3 0.3 0.3 0.6 0.0 0.0 0.0 0.0 0.7 0.7 Table 1 : Crystalline Metal Compound Powder Composition in wt% of Corresponding Oxides component Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% mol% TeO2 37.4 37.4 37.4 33.9 33.9 33.9 33.9 33.9 34.1 34.1 31.9 Li2O 31.8 31.9 31.9 33.8 33.0 33.1 33.4 32.8 32.9 32.9 30.8 ZnO 16.4 16.4 14.9 20.1 20.1 20.1 20.1 20.1 20.2 20.2 22.0 Bi2O3 3.8 5.3 5.3 4.0 4.0 4.0 4.0 4.0 3.7 3.7 3.5 P2O5 3.4 3.4 3.4 3.3 3.3 3.3 3.3 3.3 3.6 3.6 3.4 MgO 5.1 3.5 5.0 2.7 2.7 2.7 2.7 2.7 3.5 3.5 6.6 WO3 1.5 1.5 1.5 1.6 1.6 1.6 1.6 1.6 0.8 0.8 0.7 K2O 0.0 0.0 0.0 0.0 0.0 1.1 0.8 1.4 1.2 0.0 0.0 Ce2O3 0.0 0.0 0.0 0.0 0.3 0.3 0.3 0.3 0.0 0.0 0.0 Na2O 0.6 0.6 0.6 0.6 1.1 0.0 0.0 0.0 0.0 1.2 1.1 Table 2 : Crystalline Metal Compound Powder Composition in mol% of Corresponding Oxides

Inorganic blends were prepared by mixing the crystalline metal compounds using a laboratory mixer to produce a mixed material, followed by wet milling the mixed material in dipropylene glycol monomethyl ether to produce a co-milled material. The resulting dispersion was then dried in a tray dryer and sieved to yield a blended powder.

Paste Preparation A conductive silver paste comprising a mixture of substantially crystalline inorganic particles was prepared using commercially available silver powder (inorganic blend as described herein), with the remainder being a standard organic medium. The glue was prepared by premixing all components and passing several times in a three-roll mill, resulting in a homogeneous glue.

Solar cell formation Monocrystalline PERC wafers with sheet resistance of 130 Ohm/sq, size 6 inches were screen printed on their backsides with commercially available aluminum, dried in an IR quality belt dryer and randomly grouped . Each of these sets is screen printed with a front-side silver paste, which is one of the conductive pastes described herein and described in more detail above.

The stencil used for the front glue had a finger opening of 50 µm. After printing the front side, the cells were dried in an IR quality belt dryer and fired in a Despatch belt oven. The Despatch furnace has six firing zones with upper and lower heaters. The first three zones were programmed to about 500°C to burn the adhesive from the glue, the fourth and fifth zones were at higher temperatures, with a maximum temperature of 840°C (furnace temperature) in the final zone. The belt speed for this experiment was 490 cm/min. The recorded temperature was determined by measuring the temperature of the surface of the solar cell during the firing process using a thermocouple. The temperature of the surface of the solar cell does not exceed 800°C. This is typical of the firing temperatures employed for pastes comprising glass, which typically has a softening point of about 600°C. Surprisingly, such good flow behavior and contact formation were observed for the crystalline inorganic particle mixtures of the present specification.

After cooling, the fired solar cells were tested in an I-V curve tracer model cetisPV-CTL1 from Halm. Results are provided by the I-V curve tracer by direct measurements or calculations using its internal software.

To minimize the effect of contact area, the cells prepared in each experimental group (Experiments A, B, C and D in Table 3 below) were prepared using the same printing screen and the same glue rheology. This ensures that the line widths of the glues being compared are substantially the same and have no effect on the measurements.

Solar Cell Performance The fill factor (FF) indicates the performance of a solar cell relative to a theoretically ideal (0 resistance) system. Fill factor is related to contact resistance - the lower the contact resistance, the higher the fill factor will be. However, if the inorganic additive of the conductive adhesive is too aggressive, it may damage the pn junction of the semiconductor. In this case, the contact resistance will be lower, but a lower fill factor will occur due to damage to the pn junction (recombination effects and lower shunt resistance). Thus, a high fill factor indicates that there is a low contact resistance between the silicon wafer and the conductive tracks, and the firing of the paste on the semiconductor does not adversely affect the pn junction of the semiconductor (ie, higher shunt resistance).

The quality of the pn junction can be determined by measuring the pseudo fill factor (SunsVocFF). This is the fill factor independent of losses due to resistance in the battery. Therefore, the lower the contact resistance and the higher the SunsVoc FF, the higher the resulting fill factor will be. Those skilled in the art are familiar with methods for determining SunsVoc FF, eg as described in ref. SunsVoc FF is measured under open circuit conditions and is independent of series resistance effects.

The efficiency of the solar cell is calculated based on the comparison of solar energy input and electrical energy output. It should be noted that even very small changes in efficiency can still be extremely valuable in commercially available solar cells.

Perform other tests to measure open circuit voltage (Uoc) and series resistance.

Solar cell test results for several examples are summarized in Table 3: composition maximum efficiency Uoc (V) FF (%) SunsVoc_FF (%) Efficiency (%) Series resistance (Ohm•cm ² ) Experiment A Example 1 20.63 0.6724 76.03 83.41 20.03 0.00547 Example 2 19.61 0.6713 73.23 83.68 19.52 0.00783 Example 3 19.47 0.6691 71.12 83.72 18.77 0.00943 Example 4 20.63 0.6718 76.42 83.47 20.22 0.00536 Example 5 20.99 0.6731 78.25 83.42 20.89 0.00388 Example 6 20.93 0.6740 78.33 83.49 20.84 0.00384 Example 7 21.11 0.6734 76.85 83.59 20.57 0.00502 Example 8 17.81 0.6706 66.96 83.77 17.79 0.01236 Experiment B Example 5 21.14 0.6686 79.18 83.29 21.05 0.00300 Example 9 21.35 0.6710 78.95 83.32 21.20 0.00316 Experiment C Example 9 21.51 0.6689 79.80 83.00 21.23 0.00230 Example 10 21.48 0.6707 79.70 83.23 21.31 0.00254 Experiment D Example 10 21.51 0.6706 80.11 83.32 21.41 0.00232 Example 11 21.51 0.6726 80.07 83.48 21.45 0.00252 Table 3 : Solar Cell Test Results

It may be noted that although these compositions have not been fully optimized for current cell types, they still exhibit functional performance characteristics equivalent to current frit-based conductive pastes.

Although the present invention has been particularly shown and described with reference to certain embodiments, it will be understood by those skilled in the art that changes in form and detail may be made without departing from the scope of the invention as defined by the scope of the appended claims. Various changes.

Reference 1. A. McEvoy, T. Markvart, L. Castaner. Solar cells: Materials, Manufacture and Operation. Academic Press, second edition, 2013.

For a better understanding of the invention and to demonstrate how the invention may be practiced, certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 shows an example of a firing profile for a glue composition as described herein.

Claims (17)

A conductive adhesive comprising: a solid portion comprising silver powder and crystalline metal compound powder; and an organic carrier medium in which the silver powder and the crystalline metal compound powder are dispersed, wherein the solid portion has a glass content of less than 1 wt %, wherein the crystalline metal compound powder has a lead content calculated as PbO of less than 0.5 wt%; wherein the crystalline metal compound powder has a _{particle size of D 90} ≤ 5 µm and D ₅₀ ≤ 2 µm, and wherein the crystalline metal compound powder has at least a The following composition: 40 to 60 wt % Te calculated _{as TeO 2 ; 12 to 25 wt % Bi calculated as Bi 2} O ₃ ; and 10 to 25 wt % Zn calculated as ZnO, expressed relative to the conductive paste weight percent (wt%) does not include any of the crystalline metal compound is a silver compound of powder of the total weight of, and / or wherein the crystalline metal compound powder having at least the following of the composition: 20 to 40 mol to TeO ₂ calculation of% Te; _{2 to 6 mol % Bi calculated as Bi 2} O ₃ ; and 14 to 29 mol % Zn calculated as ZnO, expressed as the molar percentage of the crystalline metal compound powder in the conductive paste excluding any silver compound ( mol%), and/or wherein the crystalline metal compound powder has a composition comprising at least the following components expressed as moles defined by SUM(all sources of specific elements)/SUM(all sources of Ag) Ratios: Te/Ag of 0.0060 to 0.0090; Bi/Ag of 0.0012 to 0.0026; and Zn/Ag of 0.0029 to 0.0074. The conductive adhesive of claim 1, wherein _{the Te content of the crystalline metal compound powder calculated as TeO 2} wt % is: at least 42, 44, 46 or 48 wt %; not more than 58, 56, 54, 52 or 50 wt % ; or within a range defined by any combination of the above lower and upper limits, and/or wherein _{the Te content of the crystalline metal compound powder, calculated as TeO 2} mol %, is: at least 22, 24, 26, 28 or 30 mol %; Not more than 38, 36, 34, 33 or 32 mol%; or within the range defined by any combination of the above lower and upper limits, and/or where the sum is determined by SUM(all sources of Te)/SUM(all sources of Ag) The Te content of the crystalline metal compound powder, calculated as a defined molar ratio, is: at least 0.0064, 0.0068, 0.0072, or 0.0074; not more than 0.0086, 0.0082, 0.0078, or 0.0076; or within a range defined by any combination of the above lower and upper limits . The conductive adhesive of claim 1 or 2, wherein _{the Bi content of the crystalline metal compound powder calculated as Bi 2} O ₃ wt % is: at least 13, 14 or 15 wt %; not more than 23, 21, 19 or 17 wt % ; or within the range defined by any combination of the above lower and upper limits, and/or wherein _{the Bi content of the crystalline metal compound powder, calculated as Bi 2} O ₃ mol %, is: at least 2.5, 3.0 or 3.4 mol %; not more than 5.0, 4.0 or 3.8 mol%; and/or within the range defined by any combination of the above lower and upper limits, and/or in which the molars defined by SUM(all sources of Bi)/SUM(all sources of Ag) The ratio-calculated Bi content of the crystalline metal compound powder is: at least 0.0014, 0.0016, or 0.0018; not more than 0.0024, 0.0022, or 0.0020; or within a range defined by any combination of the above lower and upper limits. If the conductive adhesive of claim 1 or 2, wherein the Zn content of the crystalline metal compound powder calculated as ZnO wt% is: at least 12, 13, 14, 15 or 16 wt%; not more than 23, 21, 19 or 18 wt%; within the bounds of any combination, and/or wherein the Zn content of the crystalline metal compound powder calculated as ZnO mol % is: at least 16, 18, 20 or 21 mol %; not more than 27, 25, 24 or 23 mol %; or within any of the above lower and upper limits within the bounds of the portfolio, and/or wherein the Zn content of the crystalline metal compound powder, calculated as the molar ratio defined by SUM(all sources of Zn)/SUM(all sources of Ag), is: at least 0.0035, 0.0040, 0.0045 or 0.0050; not more than 0.0070, 0.0065, 0.0060 or 0.0055; or within the range defined by any combination of the above lower and upper limits. The conductive paste of claim 1 or 2, wherein the crystalline metal compound powder further comprises _{the following Li content calculated as LiO 2} wt %: at least 5, 6, 7 or 8 wt %; not more than 15, 13, 11, 10 or 9 wt %; or within the range defined by any combination of the above lower and upper limits, and/or wherein the crystalline metal compound powder further comprises _{the following Li content calculated as LiO 2} mol %: at least 25, 28, 29 or 30 mol %; not more than 45, 40, 35, 34, 33, 32, or 31 mol%; or within the range defined by any combination of the above lower and upper limits, and/or wherein, to be determined by SUM (all sources of Li)/ The Li content of the crystalline metal compound powder, calculated as a molar ratio defined by SUM (all sources of Ag), is: at least 0.0080, 0.0100, 0.0120, 0.0140, or 0.0150; not more than 0.0240, 0.0220, 0.0200, 0.0180, or 0.0170; or Within the range defined by any combination of the above lower and upper limits. If the conductive adhesive of claim 5, Wherein the Li system is provided in the form of two different compounds. The conductive adhesive of claim 6, wherein the Li is provided in the form of Li ₂ CO ₃ and Li ₃ PO ₄ . If the conductive adhesive of claim 1 or 2, wherein the crystalline metal compound powder further comprises the following Mg content calculated as MgO wt%: at least 0.5, 1.0, 1.5 or 2.0 wt%; not more than 4.0, 3.5, 3.0 or 2.8 wt%; or at any of the lower and upper limits set forth above within the bounds of the portfolio, and/or wherein the crystalline metal compound powder further comprises the following Mg content calculated as MgO mol %: at least 2, 3, 4 or 5 mol %; not more than 10, 9, 8 or 7 mol %; or at any of the above lower and upper limits within the bounds of the portfolio, and/or wherein the Mg content of the crystalline metal compound powder, calculated as the molar ratio defined by SUM(all sources of Mg)/SUM(all sources of Ag), is: at least 0.0003, 0.0005, 0.0007, 0.009 or 0.0010; not more than 0.0024, 0.0020, 0.0018, 0.0016 or 0.0014; or within a range defined by any combination of the above lower and upper limits. The conductive paste of claim 1 or 2, wherein the crystalline metal compound powder further comprises _{the following P content calculated as P 2} O ₅ wt %: at least 2.0, 3.0 or 4.0 wt %; not more than 7.0, 6.0 or 5.0 wt %; or within a range defined by any combination of the above lower and upper limits, and/or wherein the crystalline metal compound powder further comprises _{the following P content, calculated as P 2} O ₅ mol %: at least 1, 2 or 3 mol %; not more than 6, 5 or 4 mol %; or within the range defined by any combination of the above lower and upper limits, and/or where calculated as a molar ratio defined by SUM(all sources of P)/SUM(all sources of Ag) The P content of the crystalline metal compound powder is: at least 0.0007, 0.0009, 0.0011 or 0.0013; not more than 0.0024, 0.0022, 0.0020 or 0.0018; or within a range defined by any combination of the above lower and upper limits. The conductive paste of claim 1 or 2, wherein the crystalline metal compound powder further comprises _{the following Na content calculated as Na 2} O wt %: at least 0.1, 0.3, 0.5 or 0.8 wt %; not more than 1.5, 1.0 or 0.9 wt % ; or within a range defined by any combination of the above lower and upper limits, and/or wherein the crystalline metal compound powder further comprises _{the following Na content, calculated as Na 2} O mol %: at least 0.2, 0.5, 0.8 or 1.0 mol %; Not more than 2.0, 1.7, 1.5 or 1.3 mol%; or within the range defined by any combination of the above lower and upper limits, and/or within the range defined by SUM(all sources of Na)/SUM(all sources of Ag) The Na content of the crystalline metal compound powder, calculated on a molar ratio, is: at least 0.0001, 0.0003, 0.0005, or 0.0006; not more than 0.0012, 0.0010, 0.0008, or 0.0007; or within a range defined by any combination of the foregoing lower and upper limits. The conductive paste of claim 1 or 2, wherein the crystalline metal compound powder further comprises _{the following W content calculated as WO 3} wt %: at least 1.0, 1.3 or 1.6 wt %; not more than 5.0, 3.5 or 2.0 wt %; or in Within the range defined by any combination of the above lower and upper limits, and/or wherein the crystalline metal compound powder further comprises _{the following W content calculated as WO 3} mol %: at least 0.2, 0.4 or 0.6 mol %; not more than 2.0, 1.5 or 1.0 mol%; or within the range defined by any combination of the above lower and upper limits, and/or in which the crystalline metal is calculated as a molar ratio defined by SUM (all sources of W)/SUM (all sources of Ag) The W content of the compound powder is: at least 0.0001, 0.0002, or 0.0003; not more than 0.0005 or 0.0004; or within a range defined by any combination of the above lower and upper limits. If the conductive adhesive of claim 1 or 2, wherein the lead content of the crystalline metal compound powder calculated as PbO is less than 0.1 wt %, less than 0.05 wt %, less than 0.01 wt % or less than 0.005 wt %. If the conductive adhesive of claim 1 or 2, wherein the solid portion has a glass content of less than 0.5 wt%, less than 0.25 wt%, less than 0.1 wt%, less than 0.05 wt%, less than 0.01 wt%, or less than 0.005 wt% relative to the total weight of the solids portion. If the conductive adhesive of claim 1 or 2, Wherein the solid portion accounts for 85 to 95 wt % of the conductive adhesive and the organic vehicle accounts for 5 to 15 wt % of the conductive adhesive. If the conductive adhesive of claim 1 or 2, Wherein the solid portion includes 1 to 5 wt % of crystalline metal compound powder and 95 to 99 wt % of silver powder. A method for making a surface electrode for a solar cell, The method comprises applying a conductive paste as defined in any one of claims 1 to 15 to a semiconductor substrate and firing the applied conductive paste. As in the method of claim 16, Wherein firing the coated conductive paste uses a firing profile in which the surface temperature of the coated conductive paste exceeds 500°C for a period of two minutes or less.

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